The Project Gutenberg eBook, Earth Features and Their Meaning, by William Herbert Hobbs
| Note: | Images of the original pages are available through Internet Archive. See [ https://archive.org/details/cu31924004975763] |
EARTH FEATURES AND THEIR MEANING
THE MACMILLAN COMPANY
NEW YORK · BOSTON · CHICAGO DALLAS · SAN FRANCISCO
MACMILLAN & CO., Limited
LONDON · BOMBAY · CALCUTTA MELBOURNE
THE MACMILLAN CO. OF CANADA, Ltd.
TORONTO
Plate 1.
Mount Balfour and the Balfour Glacier in the Selkirks.
EARTH FEATURES AND
THEIR MEANING
AN INTRODUCTION TO GEOLOGY
FOR THE STUDENT AND THE GENERAL READER
BY
WILLIAM HERBERT HOBBS
PROFESSOR OF GEOLOGY IN THE UNIVERSITY OF MICHIGAN
AUTHOR OF “EARTHQUAKES. AN INTRODUCTION TO
SEISMIC GEOLOGY”; “CHARACTERISTICS OF
EXISTING GLACIERS”; ETC.
New York THE MACMILLAN COMPANY 1921
All rights reserved
Copyright, 1912, By THE MACMILLAN COMPANY.
Norwood Press J. S. Cushing Co.—Berwick & Smith Co. Norwood, Mass., U.S.A.
TO THE MEMORY
OF
GEORGE HUNTINGTON WILLIAMS
PREFACE
The series of readings contained in the present volume give in somewhat expanded form the substance of a course of illustrated lectures which has now for several years been delivered each semester at the University of Michigan. The keynote of the course may be found in the dominant characteristics of the different earth features and the geological processes which have been betrayed in the shaping of them. Such a geological examination of landscape is replete with fascinating revelations, and it lends to the study of Nature a deep meaning which cannot but enhance the enjoyment of her varied aspects.
That there is a real place for such a cultural study of geology within the University is believed to be shown by the increasing number of students who have elected the work. Even more than in former years the American travels afar by car or steamship, and the earth’s surface features in all their manifold diversity are thus one after the other unrolled before him. The thousands who each year cross the Atlantic to roam over European countries may by historical, literary, or artistic studies prepare themselves to derive an exquisite pleasure as they visit places identified with past achievement of one form or another. Yet the Channel coast, the gorge of the Rhine, the glaciers of Switzerland, and the wild scenery of Norway or Scotland have each their fascinating story to tell of a history far more remote and varied. To read this history, the runic characters in which it is written must first of all be mastered; for in every landscape there are strong individual lines of character such as the pen artist would skillfully extract for an outline sketch. Such character profiles are often many times repeated in each landscape, and in them we have a key to the historical record.
An object of the present readings has thus been to enable the student to himself pick out in each landscape these more significant lines and so read directly from Nature. In the landscapes which have been represented, the aim has been to draw as far as possible upon localities well known to travelers and likely to be visited, either because of their historical interest or their purely scenic attractions. It should thus be possible for a tourist in America or Europe to pursue his landscape studies whenever he sets out upon his travels. The better to aid him in this endeavor, some suggestions concerning the itinerary of journeys have been supplied in an appendix.
Regarded as a textbook of geology, the present work offers some departures from existing examples. Though it has been customary to combine in a single text historical with dynamical and structural geology, a tendency has already become apparent to treat the historical division apart from the others. Again, a desire to treat the science of geology comprehensively has led some authors into including so many subjects as to render their texts unnecessarily encyclopedic and correspondingly uninteresting to the general reader. It is the author’s belief that there is a real need for a book which may be read intelligently by the general public, and it must be recognized that the beginner in the subject cannot cover the entire field by a single course of readings. The present work has, therefore, been prepared with a view to selecting for study those dominant geological processes which are best illustrated by features in northern North America and Europe. It is this desire to illustrate the readings by travels afield, which accounts for the prominence given to the subject of glaciation; for the larger number of colleges and universities in both America and Europe are surrounded by the heavy accumulations that have resulted from former glaciations.
Emphasis has also been placed upon the dependence of the dominant geological processes of any region upon existing climatic conditions, a fact to which too little attention has generally been given. This explains the rather full treatment of desert regions, of which, in our own country particularly, much may be illustrated upon the transcontinental railway journeys.
More than in most texts the attempt has here been made to teach directly through the eye with the efficient aid of apt illustrations intimately interwoven with the text. For such success as has been reached in this endeavor, the author is greatly indebted to two students of the University of Michigan,—Mr. James H. Meier, who has prepared the line drawings of landscapes, and Mr. Hugh M. Pierce, who has draughted the diagrams. Though credit has in most cases been given where illustrations have been made from another’s photographs, yet especial mention should here be made of the debt to Dr. H. W. Fairbanks of Berkeley, California, whose beautiful and instructive photographs are reproduced upon many a page.
As given at the University of Michigan, the lectures reflected in the present volume are supplemented by excursions and by so much laboratory practice as is necessary to become familiar with the more common minerals and rocks, and to read intelligently the usual topographical and geological maps. In the appendices the means for carrying out such studies, in part with newly devised apparatus, have been indicated.
The scope of the book precludes the possibility of furnishing the reader with the sources for the body of fact and theory which is presented, although much may be inferred from the names which appear beneath the illustrations, and more definite knowledge will be found in the references to literature supplied at the ends of chapters. A large amount of original and unpublished material is for a similar reason unlabeled, and it has been left for the professional geologist to detect these new strands which have been drawn into the web.
WILLIAM HERBERT HOBBS.
Ann Arbor, Michigan, October 25, 1911.
CONTENTS
| CHAPTER I | |
| The Compilation of Earth History | |
| PAGE | |
| The sources of the history—Subdivisions of geology—The study of earthfeatures and their significance—Tabular recapitulation—Geologicalprocesses not universal—Change, and not stability, the order of nature—Observationalgeology versus speculative philosophy—The scientificattitude and temper—The value of the hypothesis—Heading references | [1] |
| CHAPTER II | |
| The Figure of the Earth | |
| The lithosphere and its envelopes—The evolution of ideas concerning theearth’s figure—The oblateness of the earth—The arrangement ofoceans and continents—The figure toward which the earth is tending—Astronomicalversus geodetic observations—Changes of figure duringcontraction of a spherical body—The earlier figures of the earth—Thecontinents and oceans at the close of the Paleozoic era—Theflooded portions of the present continents—The floors of the hydrosphereand atmosphere—Reading references | [8] |
| CHAPTER III | |
| The Nature of the Materials in the Lithosphere | |
| The rigid quality of our planet—Probable composition of the earth’s core—Theearth a magnet—The chemical constitution of the earth’s surfaceshell—The essential nature of crystals—The lithosphere a complexof interlocking crystals—Some properties of natural crystals,minerals—The alterations of minerals—Reading references | [20] |
| CHAPTER IV | |
| The Rocks of the Earth’s Surface Shell | |
| The processes by which rocks are formed—The marks of origin—Themetamorphic rocks—Characteristic textures of the igneous rocks—Theclassification of rocks—Subdivisions of the sedimentary rocks—Thedifferent deposits of ocean, lake, and river—Special marks oflittoral deposits—The order of deposition during a transgression ofthe sea—The basins of deposition of earlier ages—The deposits of thedeep sea—Reading references | [30] |
| CHAPTER V | |
| Contortions of the Strata within the Zone of Flow | |
| The zones of fracture and flow—Experiments which illustrate the fractureand flow of solid bodies—The arches and troughs of the foldedstrata—The elements of folds—The shapes of rock folds—The overthrustfold—Restoration of mutilated folds—The geological map andsection—Measurement of the thickness of formations—The detectionof plunging folds—The meaning of an unconformity—Reading references | [40] |
| CHAPTER VI | |
| The Architecture of the Fractured Superstructure | |
| The system of the fractures—The space intervals of joints—The displacementsupon joints: faults—Methods of detecting faults—Thebase of the geological map—The field map and the areal geologicalmap—Laboratory models for study of geological maps—The methodof preparing the map—Fold vs. fault topography—Reading references | [55] |
| CHAPTER VII | |
| The Interrupted Character of Earth Movements: Earthquakesand Seaquakes | |
| Nature of earthquake shocks—Seaquakes and seismic sea waves—Thegrander and the lesser earth movements—Changes in the earth’ssurface during earthquakes: faults and fissures—The measure ofdisplacement—Contraction of the earth’s surface during earthquakes—Theplan of an earthquake fault—The block movements of thedisturbed district—The earth blocks adjusted during the Alaskanearthquake of 1899 | [67] |
| CHAPTER VIII | |
| The Interrupted Character of Earth Movements: Earthquakesand Seaquakes (concluded) | |
| Experimental demonstration of earth movements—Derangement of waterflow by earth movement—Sand or mud cones and craterlets—Theearth’s zones of heavy earthquake—The special lines of heavy shock—Seismotectoniclines—The heavy shocks above loose foundations—Constructionin earthquake regions—Reading references | [81] |
| CHAPTER IX | |
| The Rise of Molten Rock to the Earth’s Surface; VolcanicMountains of Exudation | |
| Prevalent misconceptions about volcanoes—Early views concerning volcanicmountains—The birth of volcanoes—Active and extinct volcanoes—Theearth’s volcano belts—Arrangement of volcanic ventsalong fissures, and especially at their intersections—The so-calledfissure eruptions—The composition and the properties of lava—Thethree main types of volcanic mountain—The lava dome—The basalticlava domes of Hawaii—Lava movements within the caldron of Kilauea—Thedraining of the lava caldrons—The outflow of the lava floods | [94] |
| CHAPTER X | |
| The Rise of Molten Rock to the Earth’s Surface; VolcanicMountains of Ejected Materials | |
| The mechanics of crater explosions—Grander volcanic eruptions of cindercones—The eruption of Volcano in 1888—The eruption of Taalvolcano on January 30, 1911—The materials and the structure of cindercones—The profile lines of cinder cones—The composite cone—Thecaldera of composite cones—The eruption of Vesuvius in 1906—Thesequence of events within the chimney—The spine of Pelé—Theaftermath of mud flows—The dissection of volcanoes—Theformation of lava reservoirs—Character profiles—Reading references | [115] |
| CHAPTER XI | |
| The Attack of the Weather | |
| The two contrasted processes of weathering—The rôle of the percolatingwater—Mechanical results of decomposition: spheroidal weathering—Exfoliationor scaling—Dome structure in granite masses—Theprying work of frost—Talus—Soil flow in the continued presence ofthaw water—The splitting wedges of roots and trees—The rock mantleand its shield in the mat of vegetation—Reading references | [149] |
| CHAPTER XII | |
| The Life Histories of Rivers | |
| The intricate pattern of river etchings—The motive power of rivers—Oldland and new land—The earlier aspects of rivers—The meshesof the river network—The upper and lower reaches of a river contrasted—Thebalance between degradation and aggradation—Theaccordance of tributary valleys—The grading of the flood plain—Thecycles of stream meanders—The cut-off of the meander—Meanderscars—River terraces—The delta of the river—The levee—Thesections of delta deposits | [158] |
| CHAPTER XIII | |
| Earth Features shaped by Running Water | |
| The newly incised upland and its sharp salients—The stage of adolescence—Thematurely dissected upland—The Hogarthian line of beauty—Thefinal product of river sculpture: the peneplain—The river crosssections of successive stages—The entrenchment of meanders withrenewed uplift—The valley of the rejuvenated river—The arrest ofstream erosion by the more resistant rocks—The capture of one river byanother—Water and wind gaps—Character profiles—Reading references | [169] |
| CHAPTER XIV | |
| The Travels of the Underground Water | |
| The descent within the unsaturated zone—The trunk channels of descendingwater—The caverns of limestones—Swallow holes and limestonesinks—The sinter deposits—The growth of stalactites—Formationof stalagmites—The Karst and its features—A desert from thedestruction of forests—The ponore and the polje—The return of thewater to the surface—Artesian wells—Hot springs and geysers—Thedeposition of siliceous sinter by plant growth—Reading references | [180] |
| CHAPTER XV | |
| Sun and Wind in the Lands of Infrequent Rains | |
| The law of the desert—The self-registering gauge of past climates—Somecharacteristics of the desert waste—Dry weathering: the red andbrown desert varnish—The mechanical breakdown of the desert rocks—Thenatural sand blast—The dust carried out of the desert | [197] |
| CHAPTER XVI | |
| The Features in Desert Landscapes | |
| The wandering dunes—The forms of dunes—The cloudburst in thedesert—The zone of the dwindling river—Erosion in and about thedesert—Characteristic features of the arid lands—The war of duneand oasis—The origin of the high plains which front the RockyMountains—Character profiles—Reading references | [209] |
| CHAPTER XVII | |
| Repeating Patterns in the Earth Relief | |
| The weathering processes under control of the fracture system—Thefracture control of the drainage lines—The repeating pattern in drainagenetworks—The dividing lines of the relief patterns: lineaments—Thecomposite repeating patterns of the higher orders—Readingreferences | [223] |
| CHAPTER XVIII | |
| The Forms carved and molded by Waves | |
| The motion of a water wave—Free waves and breakers—Effect of thebreaking wave upon a steep, rocky shore: the notched cliff—Coves,sea arches, and stacks—The cut rock terrace—The cut and builtterrace on a steep shore of loose materials—The work of the shorecurrent—The sand beach—The shingle beach—Bar, spit, and barrier—Theland-tied island—A barrier series—Character profiles—Readingreferences | [231] |
| CHAPTER XIX | |
| Coast Records of the Rise or Fall of the Land | |
| The characters in which the record has been preserved—Even coast linethe mark of uplift—A ragged coast line the mark of subsidence—Slowuplift of the coasts; the coastal plain and cuesta—The sudden upliftsof the coast—The upraised cliff—The uplifted barrier beach—Coastterraces—The sunk or embayed coast—Submerged river channels—Recordsof an oscillation of movement—Simultaneous contrary movementsupon a coast—The contrasted islands of San Clemente andSanta Catalina—The Blue Grotto of Capri—Character profiles—Readingreferences | [245] |
| CHAPTER XX | |
| The Glaciers of Mountain and Continent | |
| Conditions essential to glaciation—The snow-line—Importance of mountainbarriers in initiating glaciers—Sensitiveness of glaciers to temperaturechanges—The cycle of glaciation—The advancing hemicycle—Continentaland mountain glaciers contrasted—The nourishmentof glaciers—The upper and lower cloud zones of the atmosphere | [261] |
| CHAPTER XXI | |
| The Continental Glaciers of Polar Regions | |
| The inland ice of Greenland—The mountain rampart and its portals—Themarginal rock islands—Rock fragments which travel with theice—The grinding mill beneath the ice—The lifting of the grindingtools and their incorporation within the ice—Melting upon the glaciermargins in Greenland—The marginal moraines—The outwash plainor apron—The continental glacier of Antarctica—Nourishment ofcontinental glaciers—The glacier broom—Field and pack ice—Thedrift of the pack—The Antarctic shelf ice—Icebergs and snowbergsand the manner of their birth—Reading references | [271] |
| CHAPTER XXII | |
| The Continental Glaciers of the “Ice Age” | |
| Earlier cycles of glaciation—Contrast of the glaciated and nonglaciatedregions—The “driftless area”—Characteristics of the glaciatedregions—The glacier gravings—Younger records over older: theglacier palimpsest—The dispersion of the drift—The diamonds ofthe drift—Tabulated comparison of the glaciated and nonglaciatedregions—Unassorted and assorted drift—Features into which thedrift is molded—Marginal or “kettle” moraines—Outwash plains—Pittedplains and interlobate moraines—Eskers—Drumlins—Theshelf ice of the ice age—Character profiles | [297] |
| CHAPTER XXIII | |
| Glacial Lakes which marked the Decline of the Last Ice Age | |
| Interference of glaciers with drainage—Temporary lakes due to ice blocking—The“parallel roads” of the Scottish glens—The glacial LakeAgassiz—Episodes of the glacial lake history within the St. LawrenceValley—The crescentic lakes of the earlier stages—The early LakeMaumee—The later Lake Maumee—Lakes Arkona and Whittlesey—LakeWarren—Lakes Iroquois and Algonquin—The NipissingGreat Lakes—Summary of lake stages—Permanent changes ofdrainage effected by the glacier—Glacial Lake Ojibway in the Hudson’sBay drainage basin—Reading references | [320] |
| CHAPTER XXIV | |
| The Uptilt of the Land at the Close of the Ice Age | |
| The response of the earth’s shell to its ice mantle—The abandoned strandsas they appear to-day—The records of uplift about Mackinac Island—Thepresent inclinations of the uplifted strands—The hinge lines ofuptilt—Future consequences of the continued uptilt within the lakeregion—Gilbert’s prophecy of a future outlet of the Great Lakes tothe Mississippi—Geological evidences of continued uplift—Drowningof southwestern shores of Lakes Superior and Erie—Reading references | [340] |
| CHAPTER XXV | |
| Niagara Falls a Clock of Recent Geological Time | |
| Features in and about the Niagara gorge—The drilling of the gorge—Thepresent rate of recession—Future extinction of the American Fall—Thecaptured Canadian Fall at Wintergreen Flats—The WhirlpoolBasin excavated from the St. David’s gorge—The shaping of theLewiston Escarpment—Episodes of Niagara’s history and their correlationwith those of the glacial lakes—Time measures of the Niagaraclock—The horologe of late glacial time in Scandinavia—Readingreferences | [352] |
| CHAPTER XXVI | |
| Land Sculpture by Mountain Glaciers | |
| Contrasted sculpturing of continental and mountain glaciers—Wind distributionof the snow which falls in mountains—The niches whichform on snowdrift sites—The augmented snowdrift moves down thevalley: birth of the glacier—The excavation of the glacial amphitheateror cirque—Life history of the cirque—Grooved and fretteduplands—The features carved above the glacier—The features shapedbeneath the glacier—The cascade stairway in glacier-carved valleys—Thecharacter profiles which result from sculpture by mountain glaciers—Thesculpture accomplished by ice caps—The Norwegian tind orbeehive mountain—Reading references | [367] |
| CHAPTER XXVII | |
| Successive Glacier Types of a Waning Glaciation | |
| Transition from the ice cap to the mountain glacier—The piedmontglacier—The expanded-foot glacier—The dendritic glacier—Theradiating glacier—The horseshoe glacier—The inherited-basin glacier—Summaryof types of mountain glacier—Reading references | [383] |
| CHAPTER XXVIII | |
| The Glacier’s Surface Features and the Deposits upon its Bed | |
| The glacier flow—Crevasses and séracs—Bodies given up by the Glacierdes Bossons—The moraines—Selective melting upon the glaciersurface—Glacier drainage—Deposits within the vacated valley—Marksof the earlier occupation of mountains by glaciers—Readingreferences | [390] |
| CHAPTER XXIX | |
| A Study of Lake Basins | |
| Fresh water and saline lakes—Newland lakes—Basin-range lakes—Rift-valleylakes—Earthquake lakes—Crater lakes—Coulée lakes—Morainallakes—Pit lakes—Glint or colk lakes—Ice-dam lakes—Glacier-lobelakes—Rock-basin lakes—Valley moraine lakes—Landslidelakes—Border lakes—Ox-bow lakes—Saucer lakes—Crescenticlevee lakes—Raft lakes—Side-delta lakes—Delta lakes—Barrierlakes—Dune lakes—Sink lakes—Karst lakes: poljen—Playa lakes—Salines—Alluvial-damlakes—Résumé—Reading references | [401] |
| CHAPTER XXX | |
| The Ephemeral Existence of Lakes | |
| Lakes as settling basins—Drawing off of water by erosion of outlet—Thepulling in of headlands and the cutting off of bays—Lake extinctionby peat growth—Extinction of lakes in desert regions—The rôle oflakes in the economy of nature—Ice ramparts on lake shores—Readingreferences | [426] |
| CHAPTER XXXI | |
| The Origin and the Forms of Mountains | |
| A mountain defined—The festoons of mountain arcs—Theories of originof the mountain arcs—The Atlantic and Pacific coasts contrasted—Theblock type of mountain—Mountains of outflow or upheap—Domedmountains of uplift; laccolites—Mountains carved fromplateaus—The climatic conditions of the mountain sculpture—Theeffect of the resistant stratum—The mark of the rift in the erodedmountains—Reading references | [435] |
| APPENDICES | |
| A. The quick determination of the common minerals | [449] |
| B. Short descriptions of some common rocks | [462] |
| C. The preparation of topographical maps | [467] |
| D. Laboratory models for study in the interpretation of geological maps | [472] |
| E. Suggested itineraries for pilgrimages to study earth features | [475] |
| Index | [489] |
LIST OF PLATES
| PLATE | |||
| 1. | Mount Balfour and the Balfour Glacier in the Selkirks | [Frontispiece] | |
| FACING PAGE | |||
| 2. | A. | Layers compressed in experiments and showing the effect of a competent layer in the process of folding | [44] |
| B. | Experimental production of a series of parallel thrusts within closely folded strata | [44] | |
| C. | Apparatus to illustrate shearing action within the overturned limb of a fold | [44] | |
| 3. | A. | An earthquake fault opened in Formosa in 1906 with vertical andlateral displacements combined | [72] |
| B. | Earthquake faults opened in Alaska in 1889 on which verticalslices of the earth’s shell have undergone individual adjustments | [72] | |
| 4. | A. | Experimental tank to illustrate the earth movements which aremanifested in earthquakes. The sections of the earth’s shell arehere represented before adjustment has taken place | [82] |
| B. | The same apparatus after a sudden adjustment | [82] | |
| C. | Model to illustrate a block displacement in rocks which are intersectedby master joints | [82] | |
| 5. | A. | Once wooded region in China now reduced to desert through deforestation | [156] |
| B. | “Bad Lands” in the Colorado Desert | [156] | |
| 6. | A. | Barren Karst landscape near the famous Adelsberg grottoes | [188] |
| B. | Surface of a limestone ledge where joints have been widened throughsolution | [188] | |
| 7. | A. | Ranges of dunes upon the margin of the Colorado Desert | [210] |
| B. | Sand dunes encroaching upon the oasis of Oued Souf, Algeria | [210] | |
| 8. | A. | The granite needles of Harney Peak in the Black Hills of SouthDakota | [216] |
| B. | Castellated erosion chimneys in El Cobra Cañon, New Mexico | [216] | |
| 9. | Map of the High Plains at the eastern front of the Rocky Mountains | [220] | |
| 10. | A. | View in Spitzbergen to illustrate the disintegration of rock underthe control of joints | [228] |
| B. | Composite pattern of the joint structures within recent alluvialdeposits of the Syrian Desert | [228] | |
| 11. | A. | Ripple markings within an ancient sandstone | [232] |
| B. | Wave breaking as it approaches the shore | [232] | |
| 12. | A. | V-shaped cañon cut in an upland recently elevated from the sea,San Clemente Island, California | [256] |
| B. | A “hogback” at the base of the Bighorn Mountains, Wyoming | [256] | |
| 13. | A. | Precipitous front of the Bryant Glacier outlet of the Greenlandinland ice | [272] |
| B. | Lateral stream beside the Benedict Glacier outlet, Greenland | [272] | |
| 14. | View of the margin of the Antarctic continental glacier in KaiserWilhelm Land | [282] | |
| 15. | A. | An Antarctic ice foot with boat party landing | [290] |
| B. | A near view of the front of the Great Ross Barrier, Antarctica | [290] | |
| 16. | A. | Incised topography within the “driftless area” | [300] |
| B. | Built-up topography within the glaciated region | [300] | |
| 17. | A. | Soled glacial bowlders which show differently directed striæ uponthe same facet | [306] |
| B. | Perched bowlder upon a striated ledge of different rocktype, Bronx Park, New York | [306] | |
| C. | Characteristic knob and basin surface of a moraine | [306] | |
| 18. | A. | Fretted upland of the Alps seen from the summit of Mount Blanc | [372] |
| B. | Model of the Malaspina Glacier and the fretted upland above it | [372] | |
| 19. | A. | Contour map of a grooved upland, Bighorn Mountains, Wyoming | [372] |
| B. | Contour map of a fretted upland, Philipsburg Quadrangle, Montana | [372] | |
| 20. | Map of the surface modeled by mountain glaciers in the Sierra Nevadasof California | [376] | |
| 21. | A. | View of the Harvard Glacier, Alaska, showing the characteristicterraces | [394] |
| B. | The terminal moraine at the foot of a mountain glacier | [394] | |
| 22. | A. | Model of the vicinity of Chicago, showing the position of theoutlet of the former Lake Chicago | [400] |
| B. | Map of Yosemite Falls and its earlier site near Eagle Peak | [400] | |
| 23. | A. | View of the American Fall at Niagara, showing the accumulationof blocks beneath | [414] |
| B. | Crystal Lake, a landslide lake in Colorado | [414] | |
| 24. | A. | Apparatus for exercise in the preparation of topographic maps | [468] |
| B. | The same apparatus in use for testing the contours of a map | [468] | |
| C. | Modeling apparatus in use | [468] | |
ILLUSTRATIONS IN THE TEXT
| FIG. | PAGE | |
| 1. | Diagram to show the measure of the earth’s surface irregularities | [11] |
| 2. | Map to show the reciprocal relation of areas of land and sea | [11] |
| 3. | The tetrahedral form toward which the earth is tending | [12] |
| 4. | A truncated tetrahedron to show the reciprocal relation of projection and depression upon the surface | [13] |
| 5. | Approximations to earlier and present figures of the earth | [15] |
| 6. | Diagrams for comparison of coasts upon an upright and upon an inverted tetrahedron | [17] |
| 7. | The continents, including submerged portions | [18] |
| 8. | Diagram to indicate the altitude of different parts of the lithosphere surface | [18] |
| 9. | Diagram to show how the terrestrial rocks grade into the meteorites | [22] |
| 10. | Comparison of a crystalline with an amorphous substance | [24] |
| 11. | “Light figure” seen upon etched surface of calcite | [25] |
| 12. | Battered sand grains which have developed crystal faces | [26] |
| 13. | Unassimilated grains of quartz within a garnet crystal | [28] |
| 14. | New minerals developed about the core of an augite crystal | [28] |
| 15. | A common rim of new mineral developed by reaction where earlier minerals come into contact | [28] |
| 16. | Laminated structure of a sedimentary rock | [30] |
| 17. | Characteristic textures of igneous rocks | [33] |
| 18. | Diagram to show the order of sediments laid down during a transgression of the sea | [37] |
| 19. | Fractures produced by compression of a block of molder’s wax | [41] |
| 20. | Apparatus to illustrate the folding of strata | [41] |
| 21. | Diagrams of fold types | [42] |
| 22. | Diagrams to illustrate crustal shortening | [42] |
| 23. | Anticlinal and synclinal folds | [43] |
| 24. | Diagrams to illustrate the shapes of rock folds | [44] |
| 25. | Secondary and tertiary flexures superimposed upon the primary ones | [44] |
| 26. | A bent stratum to illustrate tension and compression upon opposite sides | [45] |
| 27. | A geological section with truncated arches restored | [47] |
| 28. | Diagram to illustrate the nature of strike and dip | [47] |
| 29. | Diagram to show the use of T symbols for strike and dip observation | [48] |
| 30. | Diagram to show how the thickness of a formation is determined | [49] |
| 31. | A plunging anticline | [50] |
| 32. | A plunging syncline | [50] |
| 33. | An unconformity upon the coast of California | [51] |
| 34. | Series of diagrams to illustrate the episodes involved in the production of an angular unconformity | [52] |
| 35. | Types of deceptive or erosional unconformities | [53] |
| 36. | A set of master joints in shale | [55] |
| 37. | Diagram to show the manner of replacement of one set of joints by another | [56] |
| 38. | Diagram to show the different combinations of joint series | [56] |
| 39. | View of the shore in West Greenland | [57] |
| 40. | View in Iceland which shows joint intervals of more than one order | [57] |
| 41. | Faulted blocks of basalt near Woodbury, Connecticut | [58] |
| 42. | A fault in previously disturbed strata | [59] |
| 43. | Diagram to show the effect of erosion upon a fault | [60] |
| 44. | A fault plane exhibiting drag | [60] |
| 45. | Map to show how a fault may be indicated by abrupt changes in strike and dip | [61] |
| 46. | A series of parallel faults revealed by offsets | [61] |
| 47. | Field map prepared from the laboratory table | [64] |
| 48. | Areal geological map based upon the field map | [64] |
| 49. | A portion of the ruins of Messina | [67] |
| 50. | Ruins of the Carnegie Palace of Peace at Cartaga, Costa Rica | [68] |
| 51. | Overturned bowlders from Assam earthquake of 1897 | [69] |
| 52. | Post sunk into ground during Charleston earthquake | [69] |
| 53. | Map showing localities where shocks have been reported at sea off Cape Mendocino, California | [70] |
| 54. | Effect of seismic water wave in Japan | [70] |
| 55. | A fault of vertical displacement | [71] |
| 56. | Escarpment produced by an earthquake fault in India | [72] |
| 57. | A fault of lateral displacement | [72] |
| 58. | Fence parted and displaced by lateral displacement on fault during California earthquake | [72] |
| 59. | Fault with vertical and lateral displacements combined | [72] |
| 60. | Diagram to show how small faults may be masked at the earth’s surface | [73] |
| 61. | “Mole hill” effect above buried earthquake fault | [73] |
| 62. | Post-glacial earthquake faults | [74] |
| 63. | Earthquake cracks in Colorado desert | [74] |
| 64. | Railway tracks broken or buckled at time of earthquake | [75] |
| 65. | Railroad bridge in Japan damaged by earthquake | [75] |
| 66. | Diagrams to show contraction of earth’s crust during an earthquake | [76] |
| 67. | Map of the Chedrang fault of India | [76] |
| 68. | Displacements along earthquake fault in Alaska | [77] |
| 69. | Abrupt change in direction of throw upon an earthquake fault | [77] |
| 70. | Map of faults in the Owens Valley, California, formed during earthquake of 1872 | [78] |
| 71. | Marquetry of the rock floor in the Tonopah district, Nevada | [79] |
| 72. | Map of Alaskan coast to show adjustments of level during an earthquake | [79] |
| 73. | An Alaskan shore elevated seventeen feet during the earthquake of 1899 | [80] |
| 74. | Partially submerged forest from depression of shore in Alaska during earthquake | [80] |
| 75. | Effect of settlement of the shore at Port Royal during earthquake of 1907 | [80] |
| 76. | Diagrams to illustrate the draining of lakes during earthquakes | [83] |
| 77. | Diagram to illustrate the derangements of water flow during an earthquake | [84] |
| 78. | Mud cones aligned upon an earthquake fissure in Servia | [84] |
| 79. | Craterlet formed near Charleston, South Carolina, during the earthquake of 1886 | [85] |
| 80. | Cross section of a craterlet | [85] |
| 81. | Map of the island of Ischia to show the concentration of earthquake shocks | [87] |
| 82. | A line of earth fracture revealed in the plan of the relief | [87] |
| 83. | Seismotectonic lines of the West Indies | [88] |
| 84. | Device to illustrate the different effects of earthquakes in firm rock and in loose materials | [88] |
| 85. | House wrecked in San Francisco earthquake | [90] |
| 86. | Building wrecked in California earthquake by roof and upper floor battering down the upper walls | [91] |
| 87. | Breached volcanic cone in New Zealand showing the bending down of the strata near the vent | [96] |
| 88. | View of the new Camiguin volcano formed in 1871 in the Philippines | [97] |
| 89. | Map to show the belts of active volcanoes | [98] |
| 90. | A portion of the “fire girdle” of the Pacific | [98] |
| 91. | Volcanic cones formed in 1783 above the Skaptár fissure in Iceland | [99] |
| 92. | Diagrams to illustrate the location of volcanic vents upon fissure lines | [100] |
| 93. | Outline map showing the arrangement of volcanic vents upon the island of Java | [100] |
| 94. | Map showing the migration of volcanoes along a fissure | [101] |
| 95. | Basaltic plateau of the northwestern United States due to fissure eruptions of lava | [102] |
| 96. | Lava plains about the Snake River in Idaho | [102] |
| 97. | Characteristic profiles of lava volcanoes | [103] |
| 98. | A driblet cone | [104] |
| 99. | Leffingwell Crater, a cinder cone in the Owens Valley, California | [104] |
| 100. | Map of Hawaii and its lava volcanoes | [106] |
| 101. | Section through Mauna Loa and Kilauea | [106] |
| 102. | Schematic diagram to illustrate the moving platform in the crater of Kilauea | [107] |
| 103. | View of the open lava lake of Halemaumau | [108] |
| 104. | Map to show the manner of outflow of the lava from Kilauea in the eruption of 1840 | [109] |
| 105. | Lava of Matavanu flowing down to the sea during the eruption of 1906 | [110] |
| 106. | Lava stream discharging into the sea from a lava tunnel | [111] |
| 107. | Diagrammatic representation of the structure of lava volcanoes as a result of the draining of frozen lava streams | [112] |
| 108. | Diagram to show the formation of mesas by outflow of lava in valleys and subsequent erosion | [112] |
| 109. | Surface of lava of the Pahoehoe type | [113] |
| 110. | Three successive views to show the growth of the island of Savaii, from lava outflow in 1906 | [113] |
| 111. | View of the volcano of Stromboli showing the excentric position of the crater | [116] |
| 112. | Diagrams to illustrate the eruptions within the crater of Stromboli | [117] |
| 113. | Map of Volcano in the Æolian Islands | [118] |
| 114. | “Bread-crust” lava projectile from the eruption of Volcano in 1888 | [119] |
| 115. | “Cauliflower cloud” of steam and ash rising above the cinder cone of Volcano | [120] |
| 116. | Eruption of Taal volcano in 1911 seen from a distance of six miles | [120] |
| 117. | The thick mud veneer upon the island of Taal (after a photograph by Deniston) | [121] |
| 118. | A pear-shaped lava projectile | [121] |
| 119. | Artificial production of a cinder cone | [122] |
| 120. | Diagram to show the contrast between a lava dome and a cinder cone | [123] |
| 121. | Mayon volcano on the island of Luzon, Philippine Islands | [123] |
| 122. | A series of breached cinder cones due to migration of the eruption along a fissure | [124] |
| 123. | The mouth upon the inner cone of Mount Vesuvius from which flowed the lava of 1872 | [124] |
| 124. | A row of parasitic cones raised above a fissure opened on the flanks of Etna in 1892 | [125] |
| 125. | View of Etna, showing the parasitic cones upon its flanks | [125] |
| 126. | Sketch map of Etna to show the areas covered by lava and tuff respectively | [126] |
| 127. | Panum crater showing the caldera | [126] |
| 128. | View of Mount Vesuvius before the eruption of 1906 | [127] |
| 129. | Sketches of the summit of the Vesuvian cone to bring out the changes in its outline | [128] |
| 130. | Night view of Vesuvius from Naples before the outbreak of 1906, showing a small lava stream descending the central cone | [129] |
| 131. | Scoriaceous lava encroaching upon the tracks of the Vesuvian railway | [130] |
| 132. | Map of Vesuvius, showing the position of the lava mouths opened upon its flanks during the eruption of 1906 | [131] |
| 133. | The ash curtain over Vesuvius lifting and disclosing the outlines of the mountain | [132] |
| 134. | The central cone of Vesuvius as it appeared after the eruption of 1906 | [132] |
| 135. | A sunken road upon Vesuvius filled with indrifted ash | [133] |
| 136. | View of Vesuvius from the southwest during the waning stages of the eruption | [133] |
| 137. | The main lava stream advancing upon Boscotrecase | [133] |
| 138. | A pine snapped off by the lava and carried forward upon its surface | [133] |
| 139. | Lava front pushing over and running around a wall in its path | [134] |
| 140. | One of the ruined villas in Boscotrecase | [134] |
| 141. | Three diagrams to illustrate the sequence of events during the cone-building and crater-producing periods | [135] |
| 142. | The spine of Pelé rising above the chimney of the volcano after the eruption of 1902 | [136] |
| 143. | Successive outlines of the Pelé spine | [137] |
| 144. | Corrugated surface of the Vesuvian cone due to the mud flows which followed the eruption of 1906 | [138] |
| 145. | View of the Kammerbühl near Eger in Bohemia | [139] |
| 146. | Volcanic plug exposed by natural dissection of a volcanic cone in Colorado | [140] |
| 147. | A dike cutting beds of tuff in a partly dissected volcano of southwestern Colorado | [140] |
| 148. | Map and general view of St. Paul’s rocks, a volcanic cone dissected by waves | [141] |
| 149. | Dissection by explosion of Little Bandai-san in 1888 | [141] |
| 150. | The half-submerged volcano of Krakatoa before and after the eruption of 1883 | [142] |
| 151. | The cicatrice of the Banat | [142] |
| 152. | Diagram to illustrate a probable cause of formation of lava reservoirs and the connection with volcanoes upon the surface | [143] |
| 153. | Effect of relief of load upon rocks by arching of a competent formation | [144] |
| 154. | Character profiles connected with volcanoes | [146] |
| 155. | Diagrams to show the effect of decomposition in producing spheroidal bowlders | [150] |
| 156. | Spheroidal weathering of an igneous rock | [151] |
| 157. | Dome structure in granite mass | [152] |
| 158. | Talus slope beneath a cliff | [153] |
| 159. | Striped ground from soil flow | [154] |
| 160. | Pavement of horizontal surface due to soil flow | [154] |
| 161. | Tree roots prying rock apart on fissure | [154] |
| 162. | Bowlder split by a growing tree | [155] |
| 163. | Rock mantle beneath soil and vegetable mat | [155] |
| 164. | Diagram to show the varying thickness of mantle rock upon the different portions of a hill surface | [156] |
| 165. | Gullies from earliest stage of a river’s life | [160] |
| 166. | Partially dissected upland | [160] |
| 167. | Longitudinal sections of upper portion of a river valley | [161] |
| 168. | Map and sections of a stream meander | [163] |
| 169. | Tree undermined on the outer bank of a meander | [164] |
| 170. | Diagrams to show the successive positions of stream meanders | [164] |
| 171. | An ox-bow lake in the flood plain of a river | [165] |
| 172. | Schematic representation of a series of river terraces | [165] |
| 173. | “Bird-foot” delta of the Mississippi River | [167] |
| 174. | Diagrams to show the nature of delta deposits as exhibited in sections | [168] |
| 175. | Gorge of the River Rhine near St. Goars | [169] |
| 176. | Valley with rounded shoulders characteristic of the stage of adolescence | [170] |
| 177. | View of a maturely dissected upland | [170] |
| 178. | Hogarth’s line of beauty | [171] |
| 179. | View of the oldland of New England, with Mount Monadnock rising in the distance | [171] |
| 180. | Comparison of the cross sections of river valleys of different stages | [172] |
| 181. | The Beavertail Bend of the Yakima River | [173] |
| 182. | A rejuvenated river valley | [174] |
| 183. | Plan of a river narrows | [174] |
| 184. | Successive diagrams to illustrate the origin of “trellis drainage” | [175] |
| 185. | Sketch maps to show the earlier and present drainage near Harper’s Ferry | [176] |
| 186. | Section to illustrate the history of Snickers Gap | [177] |
| 187. | Character profiles of landscapes shaped by stream erosion in humid climates | [177] |
| 188. | Diagram to show the seasonal range in the position of the water table | [180] |
| 189. | Diagram to show the effect of an impervious layer upon the descending water | [181] |
| 190. | Sketch map to illustrate corrosion of limestone along two series of vertical joints | [181] |
| 191. | Diagram to show the relation of limestone caverns to the river system of the district | [182] |
| 192. | Plan of a portion of Mammoth Cave, Kentucky | [183] |
| 193. | Trees and shrubs growing upon the bottoms of limestone sinks | [183] |
| 194. | Diagrams to show the manner of formation of stalactites and stalagmites | [185] |
| 195. | Sinter formations in the Luray caverns | [186] |
| 196. | Map of the dolines of the Karst region | [187] |
| 197. | Cross section of a doline formed by inbreak | [187] |
| 198. | Sharp Karren of the Ifenplatte | [188] |
| 199. | The Zirknitz seasonal lake | [189] |
| 200. | Fissure springs arranged at intersections of rock fractures | [190] |
| 201. | Schematic diagrams to illustrate the different types of artesian wells | [191] |
| 202. | Cross section of Geysir, Iceland | [192] |
| 203. | Apparatus for simulating geyser action | [193] |
| 204. | Cone of siliceous sinter about the Lone Star Geyser | [194] |
| 205. | Former shore lines in the Great Basin | [198] |
| 206. | Map of the former Lake Bonneville | [199] |
| 207. | Borax deposits in Death Valley, California | [201] |
| 208. | Hollowed forms of weathered granite in a desert of Central Asia | [201] |
| 209. | Hollow hewn blocks in a wall in the Wadi Guerraui | [202] |
| 210. | Smooth granite domes shaped by exfoliation | [203] |
| 211. | Granite blocks rent by diffission | [204] |
| 212. | “Mushroom Rock” from a desert in Wyoming | [205] |
| 213. | Windkanten shaped by sand blast in the desert | [205] |
| 214. | The “stone lattice” of the desert | [206] |
| 215. | Shadow erosion in the desert | [206] |
| 216. | Cliffs in loess with characteristic vertical jointing | [207] |
| 217. | A cañon in loess worn by traffic and wind | [207] |
| 218. | Diagrams to illustrate the effects of obstructions in arresting wind-driven sand | [209] |
| 219. | Sand accumulating on either side of a firm and impenetrable obstruction | [210] |
| 220. | Successive diagrams to illustrate the history of the town of Kunzen upon the Kurische Nehrung | [210] |
| 221. | View of desert barchans | [211] |
| 222. | Diagrams to show the relationships of dunes to sand supply and wind direction | [211] |
| 223. | Ideal section showing the rising mountain wall about a desert and the neighboring slope | [212] |
| 224. | Dry delta at the foot of a range upon the borders of a desert | [213] |
| 225. | Map of distributaries of streams which issue at the western base of the Sierra Nevadas | [213] |
| 226. | A group of “demoiselles” in the “bad lands” | [214] |
| 227. | Amphitheater at the head of the Wadi Beni Sur | [215] |
| 228. | Mesa and outlier in the Leucite Hills of Wyoming | [216] |
| 229. | Flat-bottomed basin separating dunes | [216] |
| 230. | Billowy surface of the salt crust on the central sink of the desert of Lop | [217] |
| 231. | Schematic diagram to show the zones of deposition in their order from the margin to the center of a desert | [217] |
| 232. | Mounds upon the site of the buried city of Nippur | [218] |
| 233. | Exhumed structures in the buried city of Nippur | [218] |
| 234. | Section across the High Plains | [219] |
| 235. | Section across the lenticular threads of alluvial deposits of the High Plains | [220] |
| 236. | Distributaries of the foot hills superimposed upon an earlier series | [220] |
| 237. | Character profiles in the landscapes of arid lands | [220] |
| 238. | Rain sculpturing under control by joints | [224] |
| 239. | Sagging of limestone above joints | [224] |
| 240. | Map of the joint-controlled Abisko Cañon in Northern Lapland | [225] |
| 241. | Map of the gorge of the Zambesi River below Victoria Falls | [225] |
| 242. | Controlled drainage network of the Shepaug River in Connecticut | [226] |
| 243. | A river network of repeating rectangular pattern | [226] |
| 244. | Squared mountain masses which reveal a distribution of joints in block patterns of different orders | [228] |
| 245. | Island groups of the Lofoten Archipelago | [229] |
| 246. | Diagrams to illustrate the composite profiles of the islands on the Norwegian coast | [229] |
| 247. | Diagram to show the nature of the motions within a free water wave | [231] |
| 248. | Diagram to illustrate the transformation of a free wave into a breaker | [232] |
| 249. | Notched rock cliff and fallen blocks | [233] |
| 250. | A wave-cut chasm under control by joints | [233] |
| 251. | Grand Arch upon one of the Apostle Islands in Lake Superior | [234] |
| 252. | Stack near the shore of Lake Superior | [234] |
| 253. | The Marble Islands, stacks in a lake of the southern Andes | [235] |
| 254. | Squared stacks revealing the position of the joint planes on which they were carved | [235] |
| 255. | Ideal section cut by waves upon a steep rocky shore | [236] |
| 256. | Map showing the outlines of the island of Heligoland at different stages in its history | [236] |
| 257. | Ideal section carved by waves upon a steep shore of loose materials | [237] |
| 258. | Sloping cliff and boulder pavement at Scituate, Massachusetts | [237] |
| 259. | Map to show the nature of the shore current and the forms which are molded by it | [238] |
| 260. | Crescent-shaped beach in the lee of a headland | [239] |
| 261. | Cross section of a beach pebble | [239] |
| 262. | A storm beach on the northeast shore of Green Bay | [240] |
| 263. | Spit of shingle on Au Train Island, Lake Superior | [240] |
| 264. | Barrier beach in front of a lagoon | [241] |
| 265. | Cross section of a barrier beach with lagoon in its rear | [242] |
| 266. | Cross section of a series of barriers and an outer bar | [242] |
| 267. | A barrier series and an outer bar on Lake Mendota at Madison, Wisconsin | [242] |
| 268. | Series of barriers at the western end of Lake Superior | [243] |
| 269. | Character profiles resulting from wave action upon shores | [243] |
| 270. | The even shore line of a raised coast | [246] |
| 271. | The ragged coast line produced by subsidence | [246] |
| 272. | Portion of the Atlantic coastal plain at the base of the oldland | [246] |
| 273. | Ideal form of cuestas and intermediate lowlands carved from a coastal plain | [247] |
| 274. | Uplifted sea cave on the coast of California | [248] |
| 275. | Double-notched cliff near Cape Tiro, Celebes | [248] |
| 276. | Uplifted stacks on the coast of California | [249] |
| 277. | Uplifted shingle beach across the entrance to a former bay upon the coast of California | [250] |
| 278. | Raised beach terraces near Elie, Fife, Scotland | [250] |
| 279. | Uplifted sea cliffs and terraces on the Alaskan coast | [250] |
| 280. | Diagrams to show how excessive sinking upon the sea floor will cause the shore to migrate landward | [251] |
| 281. | A drowned river mouth or estuary upon a coastal plain | [251] |
| 282. | Archipelago of steep rocky islets due to submergence | [252] |
| 283. | The submerged Hudsonian channel which continues the Hudson River across the continental shelf | [252] |
| 284. | Marine clay deposits near the mouths of the Maine rivers which preserve a record of earlier subsidence and later elevation | [253] |
| 285. | View of the three standing columns of the Temple of Jupiter Serapis, at Pozzuoli | [254] |
| 286. | Three successive views to set forth the recent oscillations of level on the northern shore of the Bay of Naples | [255] |
| 287. | Relief map of San Clemente Island, California | [256] |
| 288. | Relief map of Santa Catalina Island, California | [257] |
| 289. | Cross section of the Blue Grotto, on the island of Capri | [258] |
| 290. | Character profiles of coast elevation and subsidence | [259] |
| 291. | Map showing the distribution of existing glaciers and the two important wind poles of the earth | [263] |
| 292. | An Alaskan glacier spreading out at the foot of the range which nourishes it | [264] |
| 293. | Surface of a glacier whose upper layers spread with but slight restraint from retaining walls | [265] |
| 294. | Section through a mountain glacier | [267] |
| 295. | Profile across the largest of the Icelandic ice caps | [267] |
| 296. | Ideal section across a continental glacier | [267] |
| 297. | View of the Eyriks Jökull, an ice cap of Iceland | [268] |
| 298. | The zones of the lower atmosphere as revealed by recent kite and balloon exploration | [269] |
| 299. | Map of Greenland, showing the area of inland ice and the routes of explorers | [271] |
| 300. | Profile in natural proportions across the southern end of the continental glacier of Greenland | [272] |
| 301. | Map of a glacier tongue with dimple above | [273] |
| 302. | Edge of the Greenland inland ice, showing the nunataks diminishing in size toward the interior | [274] |
| 303. | Moat surrounding a nunatak in Victoria Land | [274] |
| 304. | A glacier pavement of Permo-Carboniferous age in South Africa | [276] |
| 305. | Diagrams to illustrate the manner of formation of scape colks | [277] |
| 306. | Marginal moraine now forming at the edge of the continental glacier of Greenland | [279] |
| 307. | Small lake between the ice front and a moraine which it has recently built | [279] |
| 308. | View of a drained lake bottom between the ice front and an abandoned moraine | [280] |
| 309. | Diagrams to show the manner of formation and the structure of an outwash plain and fosse | [280] |
| 310. | Map of the ice masses of Victoria Land, Antarctica | [282] |
| 311. | Sections across the inland ice and the shelf ice of Antarctica | [283] |
| 312. | Diagram to show the nature of the fixed glacial anticyclone above continental glaciers | [284] |
| 313. | Snow deltas about the margins of a glacier tongue in Greenland | [285] |
| 314. | View of the sea ice of the Arctic region | [286] |
| 315. | Map of the north polar regions, showing the area of drift ice and the tracks of the Jeannette and the Fram | [288] |
| 316. | The shelf ice of Coats Land with surrounding pack ice | [290] |
| 317. | Tidewater cliff on a glacier tongue from which icebergs are born | [290] |
| 318. | A Greenlandic iceberg after a long journey in warm latitudes | [291] |
| 319. | Diagram showing one way in which northern icebergs are born from the glacier tongue | [291] |
| 320. | A northern iceberg surrounded by sea ice | [292] |
| 321. | Tabular Antarctic iceberg separating from the shelf ice | [293] |
| 322. | Map of the globe, showing the areas covered by continental glaciers during the “ice age” | [297] |
| 323. | Glaciated granite bowlder weathered out of a moraine of Permo-Carboniferous age, South Australia | [298] |
| 324. | Map to show the glaciated and nonglaciated regions of North America | [298] |
| 325. | Map of the glaciated and nonglaciated areas of northern Europe | [299] |
| 326. | An unstable erosion remnant characteristic of the “driftless area” | [300] |
| 327. | Diagram showing the manner in which a continental glacier obliterates existing valleys | [301] |
| 328. | Lake and marsh district in northern Wisconsin | [302] |
| 329. | Cross section in natural proportion of the latest North American continental glacier | [303] |
| 330. | Diagram showing the earlier and the later glacier records together upon the same limestone surface | [304] |
| 331. | Map to show the outcroppings of peculiar rock types in the region of the Great Lakes, and some localities where “drift copper” has been collected | [305] |
| 332. | Map of the “bowlder train” from Iron Hill, Rhode Island | [306] |
| 333. | Shapes and approximate natural sizes of some of the diamonds from the Great Lakes region | [307] |
| 334. | Glacial map of a portion of the Great Lakes region | [308] |
| 335. | Section in coarse till | [310] |
| 336. | Sketch map of portions of Michigan, Ohio, and Indiana, showing the distribution of moraines | [312] |
| 337. | Map of the vicinity of Devil’s Lake, Wisconsin, partly covered by the continental glacier | [313] |
| 338. | Moraine with outwash apron in front | [313] |
| 339. | Fosse between an outwash plain and a moraine | [314] |
| 340. | View along an esker in southern Maine | [315] |
| 341. | Outline map of moraines and eskers in Finland | [315] |
| 342. | Sketch maps showing the relationships of drumlins and eskers | [316] |
| 343. | View of a drumlin, showing an opening in the till | [317] |
| 344. | Outline map of the front of the Green Bay lobe to show the relationships of drumlins, moraines, outwash plains, and ground moraine | [317] |
| 345. | Character profiles referable to continental glacier | [318] |
| 346. | View of the flood plain of the ancient Illinois River near Peoria | [320] |
| 347. | Broadly terraced valleys which mark the floods that once issued from the continental glacier of North America | [321] |
| 348. | Border drainage about the retreating ice front south of Lake Erie | [321] |
| 349. | The “parallel roads” of Glen Roy in the Scottish Highlands | [322] |
| 350. | Map of Glen Roy and neighboring valleys of the Scottish Highlands | [322] |
| 351. | Three successive diagrams to set forth the late glacial lake history of the Scottish glens | [324] |
| 352. | Harvesting time on the fertile floor of the glacial Lake Agassiz | [325] |
| 353. | Map of Lake Agassiz | [325] |
| 354. | Map showing some of the beaches of Lake Agassiz and its outlet | [326] |
| 355. | Narrows of the Warren River where it passed between jaws of granite and gneiss | [327] |
| 356. | Map of the valley of the Warren River near Minneapolis | [327] |
| 357. | Portion of the Herman beach on the shore of the former Lake Agassiz | [328] |
| 358. | Map of the continental glacier of North America when it covered the entire St. Lawrence basin | [329] |
| 359. | Outline map of the early Lake Maumee | [330] |
| 360. | Map to show the first stages of the ice-dammed lakes within the St. Lawrence basin | [330] |
| 361. | Outline map of the later Lake Maumee and its outlet | [332] |
| 362. | Outline map of lakes Whittlesey and Saginaw | [333] |
| 363. | Map of the glacial Lake Warren | [333] |
| 364. | Map of the glacial Lake Algonquin | [334] |
| 365. | Outline map of the Nipissing Great Lakes | [335] |
| 366. | Probable preglacial drainage of the upper Ohio region | [337] |
| 367. | Diagrams to illustrate the episodes in the recent history of a Connecticut river | [338] |
| 368. | The notched rock headland of Boyer Bluff on Lake Michigan | [341] |
| 369. | View of Mackinac Island from the direction of St. Ignace | [342] |
| 370. | The “Sugar Loaf”, a stack of Lake Algonquin upon Mackinac Island | [342] |
| 371. | Beach ridges in series on Mackinac Island | [343] |
| 372. | Notched stack of the Nipissing Great Lakes at St. Ignace | [343] |
| 373. | Series of diagrams to illustrate the evolution of ideas concerning the uplift of the lake region since the Ice Age | [344] |
| 374. | Map of the Great Lakes region to show the isobases and hinge lines of uptilt | [345] |
| 375. | Series of diagrams to indicate the nature of the recovery of the crust by uplift when unloaded of an ice mantle | [346] |
| 376. | Portion of the Inner Sandusky Bay, for comparison of the shore line of 1820 with that of to-day | [350] |
| 377. | Ideal cross section of the Niagara Gorge to show the marginal terrace | [353] |
| 378. | View of the bed of the Niagara River above the cataract where water has been drained off | [353] |
| 379. | View of the Falls of St. Anthony in 1851 | [354] |
| 380. | Ideal section to show the nature of the drilling process beneath the cataract | [355] |
| 381. | Plan and section of the gorge, showing how the depth is proportional to the width | [355] |
| 382. | Comparative views of the Canadian Falls in 1827 and 1895 | [356] |
| 383. | Map to show the recession of the Canadian Fall | [357] |
| 384. | Comparison of the present with the future falls | [358] |
| 385. | Bird’s-eye view of the captured Canadian Fall at Wintergreen Flats | [358] |
| 386. | Map of the Whirlpool Basin | [360] |
| 387. | Map of the cuestas which have played so important a part in fixing the boundaries of the lake basins | [361] |
| 388. | Bird’s-eye view of the cuestas south of Lakes Ontario and Erie | [362] |
| 389. | Sketch map of the greater portion of the Niagara Gorge to illustrate Niagara history | [363] |
| 390. | Snowdrift hollowing its bed by nivation | [368] |
| 391. | Amphitheater formed upon a drift site in northern Lapland | [369] |
| 392. | The marginal crevasse on the highest margin of a glacier | [370] |
| 393. | Niches and cirques in the Bighorn Mountains of Wyoming | [371] |
| 394. | Subordinate cirques in the amphitheater on the west face of the Wannehorn | [371] |
| 395. | “Biscuit cutting” effect of glacial sculpture in the Uinta Mountains of Wyoming | [372] |
| 396. | Diagram to show the cause of the hyperbolic curve of cols | [372] |
| 397. | A col in the Selkirks | [373] |
| 398. | Diagrams to illustrate the formation of comb ridges, cols, and horns | [374] |
| 399. | The U-shaped Kern Valley in the Sierra Nevadas of California | [375] |
| 400. | Glaciated valley wall, showing the sharp line which separates the abraded from the undermined rock surface | [375] |
| 401. | View of the Vale of Chamonix from the séracs of the Glacier des Bossons | [376] |
| 402. | Map of an area near the continental divide in Colorado | [377] |
| 403. | Gorge of the Albula River in the Engadine cut through a rock bar | [378] |
| 404. | Idealistic sketch, showing glaciated and nonglaciated side valleys | [378] |
| 405. | Character profiles sculptured by mountain glaciers | [379] |
| 406. | Flat dome shaped under the margin of a Norwegian ice cap | [379] |
| 407. | Two views which illustrate successive stages in the shaping of tinds | [380] |
| 408. | Schematic diagram to bring out the relationships of the various types of mountain glaciers | [383] |
| 409. | Map of the Malaspina Glacier of Alaska | [384] |
| 410. | Map of the Baltoro Glacier of the Himalayas | [385] |
| 411. | View of the Triest Glacier, a hanging glacieret | [385] |
| 412. | Map of the Harriman Fjord Glacier of Alaska | [386] |
| 413. | Map of the Rotmoos Glacier, a radiating glacier of Switzerland | [386] |
| 414. | Outline map of the Asulkan Glacier in the Selkirks, a horseshoe glacier | [387] |
| 415. | Outline map of the Illecillewaet Glacier of the Selkirks, an inherited-basin glacier | [388] |
| 416. | Diagram to illustrate the surface flow of glaciers | [390] |
| 417. | Diagram to show the transformation of crevasses into séracs | [391] |
| 418. | View of the Glacier des Bossons, showing the position of accidents to Alpinists | [392] |
| 419. | Lines of flow upon the surface of the Hintereisferner Glacier in the Alps | [393] |
| 420. | Lateral and medial moraines of the Mer de Glace and its tributaries | [393] |
| 421. | Ideal cross section of a mountain glacier | [394] |
| 422. | Diagrams to illustrate the melting effects upon glacier ice of rock fragments of different sizes | [394] |
| 423. | Small glacier table upon the Great Aletsch Glacier | [395] |
| 424. | Effects of differential melting and subsequent refreezing upon a glacier surface | [396] |
| 425. | Dirt cone with its casing in part removed | [396] |
| 426. | Schematic diagram to show the manner of formation of glacier cornices | [397] |
| 427. | Superglacial stream upon the Great Aletsch Glacier | [398] |
| 428. | Ideal form of the surface left on the site of a piedmont glacier apron | [399] |
| 429. | Map of the site of the earlier piedmont glacier of the Upper Rhine | [399] |
| 430. | Diagram and map to bring out the characteristics of newland lakes | [402] |
| 431. | View of the Warner Lakes, Oregon | [402] |
| 432. | Schematic diagram to illustrate the characteristics of basin-range lakes | [403] |
| 433. | Schematic diagram of rift-valley lakes and the valley of the Jordan | [403] |
| 434. | Map of the rift-valley lakes of East Central Africa | [404] |
| 435. | Earthquake lakes formed in 1811 in the flood plain of the Lower Mississippi | [404] |
| 436. | View of a crater lake in Costa Rica | [405] |
| 437. | Diagrams to illustrate the characteristics of crater lakes | [406] |
| 438. | View of Snag Lake, a coulée lake in California | [406] |
| 439. | Diagrams to illustrate the characteristics of morainal lakes | [407] |
| 440. | Diagram to show the manner of formation of pit lakes | [408] |
| 441. | Diagrams to illustrate the characteristics of pit lakes | [408] |
| 442. | Diagram to show the manner of formation of glint lakes | [409] |
| 443. | Map of a series of glint lakes on the boundary of Sweden and Norway | [409] |
| 444. | Map of ice-dam lakes near the Norwegian boundary of Sweden | [410] |
| 445. | Wave-cut terrace of a former ice-dam lake in Sweden | [410] |
| 446. | View of the Márjelen Lake from the summit of the Eggishorn | [411] |
| 447. | Diagrams to illustrate the arrangement and the characters of rock-basin lakes | [412] |
| 448. | Convict Lake, a valley-moraine lake of California | [413] |
| 449. | Lake basins produced by successive slides from the steep walls of a glaciated mountain valley | [414] |
| 450. | Lake Garda, a border lake upon the site of a piedmont apron | [414] |
| 451. | Diagrams to bring out the characteristics of ox-bow lakes | [415] |
| 452. | Diagrammatic section to illustrate the formation of saucer-like basins between the levees of streams on a flood plain | [415] |
| 453. | Saucer lakes upon the bed of the former river Warren | [416] |
| 454. | Levee lakes developed in series within meanders in a delta plain | [417] |
| 455. | Raft lakes along the banks of the Red River in Arkansas and Louisiana | [418] |
| 456. | Map of the Swiss lakes Thun and Brienz | [419] |
| 457. | Delta lakes formed at the mouth of the Mississippi | [419] |
| 458. | Delta lakes at the margin of the Nile delta | [420] |
| 459. | Diagrams to illustrate the characteristics of barrier lakes | [420] |
| 460. | Dune lakes on the coast of France | [421] |
| 461. | Sink lakes in Florida, with a schematic diagram to illustrate the manner of their formation | [421] |
| 462. | Map of the Arve and the Upper Rhone | [426] |
| 463. | View of the Arve and the Rhone at their junction | [427] |
| 464. | A village in Switzerland built upon a strath at the head of Lake Poschiavo | [428] |
| 465. | View of the floating bog and surrounding zones of vegetation in a small glacial lake | [429] |
| 466. | Diagram to show how small lakes are transformed into peat bogs | [430] |
| 467. | Map to show the anomalous position of the delta in Lake St. Clair | [431] |
| 468. | A bowlder wall upon the shore of a small lake | [432] |
| 469. | Diagrams to show the effect of ice shove in producing ice ramparts upon the shores of lakes | [433] |
| 470. | Various forms of ice ramparts | [433] |
| 471. | Map of Lake Mendota, showing the position of the ridge which forms from ice expansion and the ice ramparts upon the shores | [434] |
| 472. | The great multiple mountain arc of Sewestan, British India | [436] |
| 473. | Diagrams to illustrate the theories of origin of mountain arcs | [437] |
| 474. | Festoons of mountain arcs about the borders of the Pacific Ocean | [438] |
| 475. | The interrupted Armorican Mountains common to western Europe and eastern North America | [438] |
| 476. | A zone of diverse displacement in the western United States | [439] |
| 477. | Section of an East African block mountain | [439] |
| 478. | Tilted crust blocks in the Queantoweap valley | [440] |
| 479. | View of the laccolite of the Carriso Mountain | [441] |
| 480. | Map of laccolitic mountains | [441] |
| 481. | Ideal sections of laccolite and bysmalite | [442] |
| 482. | The gabled façade largely developed in desert landscapes | [443] |
| 483. | Balloon view of the Mythen in Switzerland | [444] |
| 484. | The battlement type of erosion mountain | [445] |
| 485. | Symmetrically formed low islands repeated in ranks upon Temagami Lake, Ontario | [445] |
| 486. | Forms of crystals of a number of minerals | [454] |
| 487. | Forms of crystals of a number of minerals | [457] |
| 488. | A student’s contour map | [469] |
| 489. | Models to represent outcrops of rock | [472] |
| 490. | Special laboratory table set with a problem in geological mapping which is solved in [Figs. 47] and [48] | [472] |
| 491. | Three field maps to be used as suggestions in arranging laboratory table for problems in the preparation of areal geological maps | [473] |
| 492. | Sketch map of Western Scotland and the Inner Hebrides to show location of some points of special geological interest | [481] |
| 493. | Outline map of a geological pilgrimage across the continent of Europe | [483] |
EXPLANATORY LIST OF ABBREVIATIONS FOR JOURNAL NAMES IN READING REFERENCES
Am. Geol.: American Geologist.
Am. Jour. Sci.: American Journal of Science, New Haven.
Ann. de Géogr.: Annales de Géographie, Paris.
Ann. Rept. Geol. and Geogr. Surv. Ter.: Annual Report of the Geological and Geographical Survey of the Territories (Hayden), Washington.
Ann. Rept. Geol. and Nat. Hist. Surv. Minn.: Annual Report of the Geological and Natural History Survey of Minnesota, Minneapolis.
Ann. Rept. Mich. Geol. Surv.: Annual Report of the Michigan Geological Survey, Lansing.
Ann. Rept. U. S. Geol. Surv.: Annual Report of the United States Geological Survey, Washington.
Bull. Am. Geogr. Soc.: Bulletin of the American Geographical Society, New York.
Bull. Earthq. Inv. Com. Japan: Bulletin of the Earthquake Investigation Committee of Japan, Tokyo.
Bull. Geogr. Soc. Philadelphia: Bulletin of the Geographical Society of Philadelphia.
Bull. Geol. Soc. Am.: Bulletin of the Geological Society of America.
Bull. Mus. Comp. Zoöl.: Bulletin of the Museum of Comparative Zoölogy, Harvard College, Cambridge.
Bull. N. Y. State Mus.: Bulletin of the New York State Museum, Albany.
Bull. Soc. Belge d’Astronomie: Bulletin de la Société Belge d’Astronomie, Brussels.
Bull. Soc. Belge Géol.: Bulletin de la Société Belge de Géologie, Brussels.
Bull. Soc. Sc. Nat. Neuchâtel: Bulletin de la Société des Sciences Naturelles de Neuchâtel.
Bull. Univ. Calif. Dept. Geol.: Bulletin of the University of California, Department of Geology, Berkeley.
Bull. U. S. Geol. Surv.: Bulletin of the United States Geological Survey, Washington.
Bull. Wis. Geol. and Nat. Hist. Surv.: Bulletin of the Wisconsin Geological and Natural History Survey, Madison.
C. R. Cong. Géol. Intern.: Comptes Rendus de la Congrès Géologique Internationale.
Dept. of Mines, Geol. Surv. Branch, Canada: Department of Mines, Geological Survey Branch, Canada.
Geogr. Abh.: Geographische Abhandlungen.
Geogr. Jour.: Geographical Journal, London.
Geol. Folio U. S. Geol. Surv.: Geological Folio of the United States Geological Survey.
Geol. Mag.: Geological Magazine, London (sections designated by decades).
Jour. Am. Geogr. Soc.: Journal of the American Geographical Society, New York.
Jour. Coll. Sci. Imp. Univ. Tokyo: Journal of the College of Science of the Imperial University, Tokyo, Japan.
Jour. Geol.: Journal of Geology, Chicago.
Jour. Sch. Geogr.: Journal of School Geography.
Livret Guide Cong. Géol. Intern.: Livret Guide Congrès Géologique Internationale.
Mem. Geol. Surv. India: Memoirs of the Geological Survey of India, Calcutta.
Mitt. Geogr. Ges. Hamb.: Mitteilungen der Geographische Gesellschaft, Hamburg.
Mon. U. S. Geol. Surv.: Monograph of the United States Geological Survey, Washington.
Nat. Geogr. Mag.: National Geographic Magazine, Washington.
Nat. Geogr. Mon.: National Geographic Monographs, American Book Company, New York.
Naturw. Wochenschr.: Naturwissenschaftliche Wochenschrift.
Pet. Mitt.: Petermanns Mittheilungen aus Justus Perthes’ Geographischer Anstalt, Gotha.
Pet. Mitt., Ergänzungsh. or Erg.: Petermanns Mittheilungen, Gotha (Ergänzungsheft or Supplementary Paper).
Phil. Jour. Sci.: Philippine Journal of Science, Manila.
Phil. Trans.: Philosophical Transactions of the Royal Society, London.
Proc. Am. Acad. Arts and Sci.: Proceedings of the American Academy of Arts and Sciences.
Proc. Am. Assoc. Adv. Sci.: Proceedings of the American Association for the Advancement of Science.
Proc. Am. Phil. Soc.: Proceedings of the American Philosophical Society, Philadelphia.
Proc. Bost. Soc. Nat. Hist.: Proceedings of the Boston Society of Natural History, Boston.
Proc. Ind. Acad. Sci.: Proceedings of the Indiana Academy of Science.
Proc. Linn. Soc. New South Wales: Proceedings of the Linnean Society of New South Wales.
Proc. Ohio State Acad. Sci.: Proceedings of the Ohio State Academy of Science.
Prof. Pap. U. S. Geol. Surv.: Professional Paper of the United States Geological Survey, Washington.
Pub. Carneg. Inst.: Publication of the Carnegie Institution of Washington.
Pub. Mich. Geol. and Biol. Surv.: Publication of the Michigan Geological and Biological Survey, Lansing.
Quart. Jour. Geol. Soc. Lond.: Quarterly Journal of the Geological Society, London.
Rept. Brit. Assoc. Adv. Sci.: Report of the British Association for the Advancement of Science.
Rept. Geol. Surv. Mich.: Report of the Geological Survey of Michigan, Lansing.
Rept. Mich. Acad. Sci.: Report of the Michigan Academy of Science, Lansing.
Rept. Nat. Conserv. Com.: Report of the National Conservation Commission, Washington.
Rept. Smithson. Inst.: Report of the Smithsonian Institution, Washington.
Sci. Bull. Brooklyn Inst. Arts and Sci.: Science Bulletin of the Brooklyn Institute of Arts and Sciences.
Scot. Geogr. Mag.: Scottish Geographic Magazine, Edinburgh.
Smith. Cont. to Knowl.: Smithsonian Contributions to Knowledge, Washington.
Tech. Quart.: Technology Quarterly of the Massachusetts Institute of Technology, Boston.
Trans. Am. Inst. Min. Eng.: Transactions of the American Institute of Mining Engineers, New York.
Trans. Roy. Dublin Soc.: Transactions of the Royal Dublin Society.
Trans. Seis. Soc. Japan: Transactions of the Seismological Society of Japan, Tokyo.
Trans. Wis. Acad. Sci.: Transactions of the Wisconsin Academy of Sciences, Arts, and Letters, Madison.
U. S. Geogr. and Geol. Surv. Rocky Mt. Region: United States Geographical and Geological Survey of the Rocky Mountain Region (Powell), Washington.
Zeit. d. Gesell. f. Erdk. z. Berlin: Zeitschrift der Gesellschaft für Erdkunde zu Berlin.
Zeit. f. Gletscherk: Zeitschrift für Gletscherkunde, Berlin.
EARTH FEATURES AND THEIR MEANING
CHAPTER I
THE COMPILATION OF EARTH HISTORY
The sources of the history.—The science which deals with the chapters of earth history that antedate the earliest human writings is geology. The pages of the record are the layers of rock which make up the outer shell of our world. Here as in old manuscripts pages are sometimes found to be missing, and on others the writing is largely effaced so as to be indistinct or even illegible. An intelligent interpretation of this record requires a knowledge of the materials and the structure of the earth, as well as a proper conception of the agencies which have caused change and so developed the history. These agencies in operation are physical and chemical processes, and so the sciences of physics and chemistry are fundamental in any extended study of geology. Not only is geology, so to speak, founded upon chemistry and physics, but its field overlaps that of many other important sciences. The earliest earth history has to do with the form, size, and physical condition of a minor planet in the solar system. The earliest portion of the story belongs therefore to astronomy, and no sharp line can be drawn to separate this chapter from those later ones which are more clearly within the domain of geology.
Subdivisions of geology.—The terms “cosmic geology” and “astronomic geology” have sometimes been used to cover the astronomy of the earth planet. The later earth history develops, among other things, the varied forms of animal and vegetable life which have had a definite order of appearance. Their study is to a large extent zoölogy and botany, though here considered from an essentially different viewpoint. This subdivision of our science is called paleontological geology or paleontology, which in common usage includes the plant as well as the animal world, or what is sometimes called paleobotany. In order to fix the order of events in geological history, these biological studies are necessary, for the pages of the record have many of them been misplaced as a result of the vicissitudes of earth history, and the remains of life in the rock layers supply a pagination from which it is possible to correctly rearrange the misplaced pages. As compiled into a consecutive history of the earth since life appeared upon it, we have the division of historical geology; though this differs but little from stratigraphical geology, the emphasis in the case of the former being placed on the history itself and in the latter upon the arrangement of events—the pagination of the record.
So far as they are known to us, the materials of which the earth is composed are minerals grouped into various characteristic aggregates known as rocks. Here the science is founded upon mineralogy as well as chemistry, and a study of the rock materials of the earth is designated petrographical geology or petrography. The various rocks which enter into the composition of the earth’s outer shell—the only portion known to us from direct observation—are built into it in an architecture which, when carefully studied, discloses important events in the earth’s history. The division of the science which is concerned with earth architecture is geotectonic or structural geology.
The study of earth features and their significance.—The features upon the surface of the earth have all their deep significance, and if properly understood, a flood of light is thrown, not only upon present conditions, but upon many chapters of the earth’s earlier history. Here the relation of our study to topography and geography is very close, so that the lines of separation are but ill defined. The terms “physiographical geology”, “physiography”, and “geomorphology” are concerned with the configuration of the earth’s surface—its physiognomy—and with the genesis of its individual surface features. It is this genetical side of physiography which separates it from topography and lends it an absorbing interest, though it causes it to largely overlap the division of dynamical geology or the study of geological processes. In fact, the difference between dynamical geology and physiography is largely one of emphasis, the stress being laid upon the processes in the former and upon the resultant features in the latter.
Under dynamical geology are included important subdivisions, such as seismic geology, or the study of earthquakes, and vulcanology, or the study of volcanoes. Another large subject, glacial geology, belongs within the broad frontier common to both dynamical geology and physiography. A relatively new subdivision of geological science is orientational geology, which is concerned with the trend of earth features, and is closely related both to physiography and to dynamical and structural geology.
Tabular recapitulation.—In a slightly different arrangement from the above order of mention, the subdivisions of geology are as follows:—
Subdivisions of Geology
| Petrographical Geology. | Materials of the earth. | |
| Geotectonic Geology. | Architecture of the earth’s outer shell. | |
| Dynamical Geology. | Earth processes. | |
| Seismic Geology—earthquakes.Vulcanology—volcanoes. GlacialGeology—glaciers, etc. | ||
| Physiographical Geology. | Earth physiognomy and its genesis. | |
| Orientational Geology. | The arrangement and the trend of earth features. |
In one way or another all of the above subdivisions of geology are in some way concerned in the genesis of earth physiognomy, and they must therefore be given consideration in a work which is devoted to a study of the meaning of earth features. The compiled record of the rocks is, however, something quite apart and without pertinence to the present work. As already indicated its subdivisions are:—
| Astronomic Geology. | Planetary history of the earth. |
| Statigraphic Geology. | The pagination of earth records. |
| Historical Geology. | The compiled record and its interpretation. |
| Paleontological Geology. | The evolution of life upon the earth. |
In every attempt at systematic arrangement difficulties are encountered, usually because no one consideration can be used throughout as the basis of classification. Such terms as “economic geology” and “mining geology” have either a pedagogical or a commercial significance, and so would hardly fit into the system which we have outlined.
Geological processes not universal.—It is inevitable that the geology of regions which are easily accessible for study should have absorbed the larger measure of attention; but it should not be forgotten that geology is concerned with the history of the entire world, and that perspective will be lost and erroneous conclusions drawn if local conditions are kept too often before the eyes. To illustrate by a single instance, the best studied regions of the globe are those in which fairly abundant precipitation in the form of rain has fitted the land for easy conditions of life, and has thus permitted the development of a high civilization. In degree, and to some extent also in kind, geologic processes are markedly different within those widely extended regions which, because either arid or cold, have been but ill fitted for human habitation. Yet in the historical development of the earth, those geologic processes which obtain in desert or polar regions are none the less important because less often and less carefully observed.
Change, and not stability, the order of nature.—Man is ever prone to emphasize the importance of apparent facts to the disadvantage of those less clearly revealed though equally potent. The ancient notion of the terra firma, the safe and solid ground, arose because of its contrast with the far more mobile bodies of water; but this illusion is quickly dispelled with the sudden quaking of the ground. Experience has clearly shown that, both upon and beneath the earth’s surface, chemical and physical changes are going on, subject to but little interruption. “The hills rock-ribbed and ancient as the sun” is a poetical metaphor; for the Himalayas, the loftiest mountains upon the globe, were, to speak in geological terms, raised from the sea but yesterday. Even to-day they are pushing up their heads, only to be relentlessly planed down through the action of the atmosphere, of ice, and of running water. Even more than has generally been supposed, the earth suffers change. Often within the space of a few seconds, to the accompaniment of a heavy earthquake, many square miles of territory are bodily uplifted, while neighboring areas may be relatively depressed. Thus change, and not stability, is the order of nature.
Observational geology versus speculative philosophy.—There appears to be a more or less prevalent notion that the views which are held by scientists in one generation are abandoned by those of the next; and this is apt to lead to the belief that little is really known and that much is largely guessed. Some ground there undoubtedly is for such skepticism, though much of it may be accounted for by a general failure among scientists, as well as others, to clearly differentiate that which is essentially speculative from what is based broadly upon observed facts. Even with extended observation, the possibility of explaining the facts in more than one way is not excluded; but the line is nevertheless a broad one which separates this entire field of observation from what is essentially speculative philosophy. To illustrate: the mechanics of the action which goes on within volcanic craters is now fairly well understood as a result of many and extended observations, and it is little likely that future generations of geologists will discredit the main conclusions which have been reached. The cause of the rise of the lava to the earth’s surface is, on the other hand, much less clearly demonstrated, and the views which are held express rather the differing opinions than any clear deductions from observation. Again, and similarly, the physical history of the great continental glaciers of the so-called “ice age” is far more thoroughly known than that of any existing glacier of the same type; but the cause of the climatic changes which brought on the glaciation is still largely a matter for speculation.
In the present work, the attempt will be, so far as possible, to give an exposition of geologic processes and the earth features which result from them, with hints only at those ultimate causes which lie hidden in the background.
The scientific attitude and temper.—The student of science should make it his aim, not only clearly to separate in his studies the proximate from the ultimate causes of observed phenomena, but he should keep his mind always open for reaching individual conclusions. No doctrines should be accepted finally upon faith merely, but subject rather to his own reasoning processes. This should not be interpreted to mean that concerning matters of which he knows little or nothing he should not pay respect to the recognized authorities; but his acceptance of any theory should be subject to review so soon as his own horizon has been sufficiently enlarged. False theories could hardly have endured so long in the past, had not too great respect been given to authorities, and individual reasoning processes been held too long in subjection.
The value of the hypothesis.—Because all the facts necessary for a full interpretation of observed phenomena are not at one’s hand, this should not be made to stand in the way of provisional explanations. If science is to advance, the use of hypothesis is absolutely essential; but the particular hypothesis adopted should be regarded as temporary and as indicating a line of observation or of experimentation which is to be followed in testing it. Thus regarded with an open mind, inadequate hypotheses are eventually found to be untenable, whereas correct explanations of the facts by the same process are confirmed. Most hypotheses of science are but partially correct, for we now “see through a glass darkly”; but even so, if properly tested, the false elements in the hypothesis are one after the other eliminated as the embodied truth is confirmed and enlarged. Thus “working hypothesis” passes into theory and becomes an integral part of science.
Reading References for Chapter I
The most comprehensive of general geological texts written in English is Chamberlin and Salisbury’s “Geology” in three volumes (Henry Holt, 1904-1906), the first volume of which is devoted exclusively to geological processes and their results. An abridged one-volume edition of the work intended for use as a college text was issued in 1906 (College Geology, Henry Holt). Other standard texts are:—
Sir Archibald Geikie. Text-book of Geology, 4th ed. 2 vols. London, 1902, pp. 1472.
W. B. Scott. An Introduction to Geology. 2d ed. Macmillan, 1907, pp. 816.
J. D. Dana. Manual of Geology. New edition. American Book Company, 1895, pp. 1087.
Joseph LeConte. Elements of Geology. (Revised by Fairchild.) Appleton, 1905, pp. 667.
A very valuable guide to the recent literature of dynamical and structural geology is Branner’s “Syllabus of a Course of Lectures on Elementary Geology” (Stanford University, 1908).
On the relation of geology to landscape, a number of interesting books have been written:—
James Geikie. Earth Sculpture or the Origin of Land-Forms. New York and London, 1896, pp. 397.
John E. Marr. The Scientific Study of Scenery. Methuen, London, 1900, pp. 368.
Sir A. Geikie. The Scenery of Scotland. 3d ed. Macmillan, London, 1901, pp. 540.
Sir John Lubbock. The Scenery of Switzerland and the Causes to which it is Due. Macmillan, London, 1896, pp. 480.
Lord Avebury. The Scenery of England. Macmillan, London, 1902, pp. 534.
Sir A. Geikie. Landscape in History, and Other Essays. Macmillan, London, 1905, pp. 352.
N. S. Shaler. Aspects of the Earth. Scribners, New York, 1889, pp. 344.
G. de La Noe et Emm. de Margerie. Les Formes du Terrain, Service Géographique de l’Armée. Paris, 1888, pp. 205, pls. 48.
W. M. Davis. Practical Exercises in Physical Geography, with Accompanying Atlas. Ginn and Co., Boston, 1908, pp. 148, pls. 45.
John Muir. The Mountains of California. Unwin, London, 1894, pp. 381.
Upon the use and interpretation of topographic maps in illustration of characteristic earth features, the following are recommended:—
R. D. Salisbury and W. W. Atwood. The Interpretation of Topographic Maps, Prof. Pap., 60 U.S. Geol. Surv., pp. 84, pls. 170.
D. W. Johnson and F. E. Matthes. The Relation of Geology to Topography, in Breed and Hosmer’s Principles and Practice of Surveying, vol. 2. Wiley, New York, 1908.
Général Berthaut. Topologie, Étude du Terrain, Service Géographique de l’Armée. Paris, 1909, 2 vols., pp. 330 and 674, pls. 265.
The United States Geological Survey issues free of charge a list of 100 topographic atlas sheets which illustrate the more important physiographic types. In his “Traité de Géographie Physique”, Professor E. de Martonne has given at the end of each chapter the important foreign maps which illustrate the physiographic types there described.
“The Principles of Geology”, by Sir Charles Lyell, published first in three volumes, appeared in the years 1830-1833, and may be said to mark the beginning of modern geology. Later reduced to two volumes, an eleventh edition of the work was issued in 1872 (Appleton) and may be profitably read and studied to-day by all students of geology. Those familiar with the German language will derive both pleasure and profit from a perusal of Neumayr’s “Erdgeschichte” (2d ed. revised by Uhlig. Leipzig and Vienna, 2 vols., 1895-1897), and especially the first volume, “Allgemeine Geologie.” A recent French work to be recommended is Haug’s “Traité de Géologie” (Paris, 1907).
Some texts of physical geography may well be consulted, especially Emm. de Martonne’s “Traité de Géographie Physique.” Colin, Paris, 1909, pp. 910, pls. 48, and figs. 396.
Note. An explanatory list of abbreviations used in the reading references follows the List of Illustrations.
CHAPTER II
THE FIGURE OF THE EARTH
The lithosphere and its envelopes.—The stony part of the earth is known as the lithosphere, of which only a thin surface shell is known to us from direct observation. The relatively unknown central portion, or “core”, is sometimes referred to as the centrosphere. Inclosing the lithosphere is a water envelope, the hydrosphere, which comprises the oceans and inland bodies of water, and has a mass 1/4540 that of the lithosphere. If uniformly distributed, the hydrosphere would cover the lithosphere to the depth of about two miles, instead of being collected in basins as it now is. Though apparently not continuous, if we take into account the zone of underground water upon the continents, the hydrosphere may properly be considered as a continuous film about the lithosphere. It is a fact of much significance that all the ocean basins are connected, so that the levels are adjusted to furnish a common record of deposits over the entire surface that is sea-covered.
Enveloping the hydrosphere is the gaseous envelope, the atmosphere, with a mass 1/1200000 that of the lithosphere. The atmosphere is a mixture of the gases oxygen and nitrogen in parts by volume of one of the former to four of the latter, with a relatively small percentage of carbon dioxide. Locally, and at special seasons, the atmosphere may be charged with relatively large percentages of water vapor; and we shall see that both the carbon dioxide and the vapor contents are of the utmost importance in geological processes and in the influence upon climate. Unlike the water which composes the hydrosphere, the gases of the atmosphere are compressible. Forced down by the weight of superincumbent gas, the layers of the atmosphere at the level of the sea sustain a pressure of about fifteen pounds to the square inch; but this pressure steadily decreases in ascending to higher levels. From direct instrumental observation, the air has now been investigated to a height of more than twelve miles from the earth’s surface.
The evolution of ideas concerning the earth’s figure.—The ideas which in all ages have been promulgated concerning the figure of the earth have been many and varied. Though among them are not wanting the purely speculative and fantastic, it will be interesting to pass in review such theories as have grown directly out of observation.
The ancient Hebrews and the Babylonians were dwellers of the desert, and in the mountains which bounded their horizon they saw the confines of the earth. Pushing at last westward beyond the mountains, they found the Mediterranean, and thus arrived at the view that the earth was a disk with a rim of mountains which was floated upon water. The rare but violent rainfalls to which they were accustomed—the desert cloudburst—further led them to the belief that the mountain rim was continued upward in a dome or firmament of transparent crystal upon which the heavenly bodies were hung and from which out of “windows of heaven” the water “which is above the earth” was poured out upon the earth’s surface. Fantastic as this theory may seem to-day, it was founded upon observation, and it well illustrates the dangers of reasoning from observation within too limited a field.
As soon as men began to sail the sea, it was noticed that the water surface is convex, for the masts of ships were found to remain visible long after their hulls had disappeared below the horizon. It is difficult to say how soon the idea of the earth’s rotundity was acquired, but it is certainly of great antiquity. The Dominican monk Vincentius of Beauvais, in a work completed in 1244, declared that the surfaces of the earth and the sea were both spherical. The poet Dante made it clear that these surfaces were one, and in his famous address upon “The Water and the Land”, which was delivered in Verona on the 20th of January, 1320, he added a statement that the continents rise higher than the ocean. His explanation of this was that the continents are pulled up by the attraction of the fixed stars after the manner of attraction of magnets, thus giving an early hint of the force of gravitation.
The earth’s rotundity may be said to have been first proven when Magellan’s ships in 1521 had accomplished the circumnavigation of the globe. Circumnavigation, soon after again carried out by Sir Francis Drake, proved that the earth is a closed body bounded by curving surfaces in part enveloped by the oceans and everywhere by the atmosphere. The great discovery of Copernicus in 1530 that the earth, like Venus, Mars, and the other planets, revolves about the sun as a part of a system, left little room for doubt that the figure of the earth was essentially that of a sphere.
The oblateness of the earth.—Every schoolboy is to-day familiar with the fact that the earth departs from a perfect spherical figure by being flattened at the ends of its axis of rotation. The polar diameter is usually given as 1/299 shorter than the equatorial one. This oblateness of the spheroid was proven by geodesists when they came to compare the lengths of measured degrees of arc upon meridians in high and in low latitudes.
Fig. 1.—Diagrams to afford a correct impression of the measure of the inequalities upon the earth’s surface compared to the earth’s radius. The shell represented in b is 1/100 of the earth’s radius, and in a this zone is magnified for comparison with surface inequalities.
The oblateness of the geoid is well understood from accepted hypotheses to be the result of the once more rapid rotation of the planet when its materials were more plastic, and hence more responsive to deformation. An elastic hoop rotating rapidly about an axis in its plane appears to the eye as a solid, and becomes flattened at the ends of its axis in proportion as the velocity of rotation is increased. Like the earth, the other planets in the solar system are similarly oblate and by amounts dependent on the relative velocities of rotation.
The departure of the geoid from the spherical surface, owing to its oblateness, is so small that in the figures which we shall use for illustration it would be less than the thickness of a line. Since it is well recognized and not important in our present consideration, we shall for the time being speak of the figure of the earth in terms of departures from a standard spherical surface.
The arrangement of oceans and continents.—There are other departures from a spherical surface than the oblateness just referred to, and these departures, while not large, are believed to be full of significance. Lest the reader should gain a wrong impression of their magnitude, it may be well to introduce a diagram drawn to scale and representing prominent elevations and depressions of the earth ([Fig. 1]).
Wrong impressions concerning the figure of the lithosphere are sometimes gained because its depressions are obliterated by the oceans. The oceans are, indeed, useful to us in showing where the depressions are located, but the figure of the earth which we are considering is the naked surface of the rock. In a broad way, the earth’s shape will be given by the arrangement of the oceans and the continents. As soon as we take up the study of this arrangement, we find that quite significant facts of distribution are disclosed.
Fig. 2.—Map on Mercator’s projection to show the reciprocal relation of the land and sea areas (after Gregory and Arldt).
One of the most significant facts involved in the distribution of land and sea, is a concentration of the land areas within the northern and the seas within the southern hemisphere. The noteworthy exception is the occurrence of the great and high Antarctic continent centered near the earth’s south pole; and there are extensions of the northern continent as narrowing land masses to the southward of the equator. Hardly less significant than the existence of land and water hemispheres is the reciprocal or antipodal distribution of land and sea ([Fig. 2]). A third fact of significance is a dovetailing together of sea and land along an east-and-west direction. While the seas are generally A-shaped and narrow northward, the land masses are V-shaped and narrow southward, but this occurs mainly in the southern hemisphere. Lastly, there is some indication of a belt of sea dividing the land masses into northern and southern portions along the course of a great circle which makes a small angle with the earth’s equator. Thus the western continent is nearly divided by a mediterranean sea,—the Caribbean,—and the eastern is in part so divided by the separation of Europe from Africa.
Fig. 3.—The form toward which the figure of the earth is tending, a tetrahedron with symmetrically truncated angles.
The figure toward which the earth is tending.—Thus far in our discussion of the earth’s figure we have been guided entirely by the present distribution of land and water. There are, however, depressions upon the surface of the land, in some cases extending below the level of the sea, which are not to-day occupied by water. By far the most notable of these is the great Caspian Depression, which with its extension divides central and eastern Asia upon the east from Africa and Europe upon the west. This depression was quite recently occupied by the sea, and when added to the present ocean basins to indicate depressions of the lithosphere, it shows that the earth’s figure departs from the standard spheroid in the direction of the form represented in [Fig. 3]. This form approximates to a tetrahedron, a figure bounded by four equal triangular faces, here with symmetrically truncated angles. Of all regular figures with plane surfaces the tetrahedron has the smallest volume for a given surface, and it presents moreover a reciprocal relation of projection to depression. Every line passing through its center thus finds the surface nearer than the average distance upon one side and correspondingly farther upon the other ([Fig. 4]).
Astronomical versus geodetic observations.—Confirmation of the conclusions arrived at from the arrangement of oceans and continents has been secured in other fields. It was pointed out that the earth’s oblateness was proven by comparison of the measured degrees of latitude upon the earth’s surface in lower and higher latitudes, the degree being found longer as the pole is approached. Any variation from the spherical surface must obviously increase the size of the measured degree of latitude in proportion to the departure from the standard form, and so the tetrahedral figure with one of its angles at the south pole will require that the degrees of latitude be longer in the southern than they are in the northern hemisphere. This has been found by measurement to be the case, and the result is further confirmed by pendulum studies upon the distribution of the earth’s attraction or gravity. If less of the mass of the earth is concentrated in the southern hemisphere, its attraction as measured in vibrations of the pendulum should be correspondingly smaller.
Fig. 4.—A truncated tetrahedron, showing how the depression upon one side of the center is balanced by the opposite projection.
Other confirmations of the tetrahedral figure of the earth have been derived from a comparison of astronomical data, which assume the earth to be a perfect spheroid, with geodetic measurements, which are based upon direct measurements. Thus the arc measured in an east-and-west direction across Europe revealed a different curvature near the angle of the tetrahedral figure from what was found farther to the eastward.
Changes of figure during contraction of a spherical body.—If we inquire why the earth in cooling should tend to approach the tetrahedral figure, an answer is easily found. When formed, the earth appears to have been a but slightly oblate spheroid, or practically a sphere—the shape which of all incloses the most space for a given surface. Cooled and solidified at the surface to the temperature of the surrounding air, and the core still hot and continuing to lose heat, the core must continue to contract though the outer shell is no longer able to do so. The superficial area being thus maintained constant while the volume continues to diminish, the figure must change from the initial one of greatest bulk to others of smaller volume, and ultimately, if the process should continue indefinitely, to the tetrahedron, which of all regular figures has the minimum volume for a given surface.
That a contracting sphere does indeed pass through such a series of changes has been shown by the behavior of contracting soap bubbles and of rubber balloons, as well as by experiments upon the exhaustion of air contained in hollow metal spheres of only moderate strength. In all these instances, the ultimate form produced indicates an indenting of four sides of the sphere which have the positions of the faces of a tetrahedron. The late Professor Prinz of Brussels secured some extremely interesting results in which he obtained intermediate forms with six angles, but unfortunately these studies were not prepared for publication at the time of his death.
The earth’s departure from the spheroid in the direction of the modified tetrahedron is, as we have seen, no hypothesis, but observed fact revealed in (1) the concentration of the land about a central ocean in the northern hemisphere; in (2) the antipodal relation of the land to the water areas, and in (3) the threefold subdivision of the surface into north and south belts by the two greater oceans and the Caspian Depression.
The earlier figures of the earth.—The manner in which continent and ocean are dovetailed into each other in an east-and-west direction has been generally adduced as additional evidence for the tetrahedral figure as above described. Closer examination shows that instead of being in harmony with this figure, it indicates a departure from it, and, as we shall see, a significant departure which undoubtedly has its origin in the earlier history of the planet. The mediterranean seas of both the eastern and the western hemispheres likewise interfere with the perfection of the tetrahedral figure and require an explanation.
Let us then examine in outline the past history of the world with reference especially to the evolution of the continents and to the times and the manners of surface change. It is now well known that there have been three major periods of great deformation of the earth’s shell. The first of these of which we have record came at the end of the first great era of geologic history, the so-called Eozoic era; a second great transformation came at the close of the second or Paleozoic era; and a third began at the end of the next or Mesozoic era, an adjustment which is apparently continuing to-day. Each of these great surface deformations was accompanied by great volcanic eruptions of which we have the evidence in the lavas remaining for our inspection, and each was followed by the formation of great glaciers which spread over large areas of the existing continents.
Before the earliest of these great changes, the earth appears to have approximated in its figure somewhat closely to the ideal spheroid, for it was everywhere enveloped in the hydrosphere as a universal ocean. Toward the close of this period came the adjustments which brought the lithosphere to protrude through the hydrosphere in shield-like continents whose distribution, as shown by the rocks of this period, is of great significance. Within the northern hemisphere rose three land shields spaced at nearly equal intervals and at nearly equal distances from the northern pole. One of these was centered where now is Hudson Bay, another about the present Baltic Sea, and the relics of the third are found in northeastern Siberia. These earliest continents have been referred to as the Laurentian, Baltic, and Angara shields. Within the southern hemisphere shields appear to have developed in somewhat similar grouping, namely, in South America, in South Africa, and in Australia ([Figs. 3] and [5]).
Fig. 5.—Approximations to earlier and present figures of the earth.
These coigns or angles of a form into which the earlier spheroid of the earth was being transformed have persisted through the greater part of subsequent geologic time, and have been enlarged by the growth of sediments about them as well as by the later elevation and wrinkling of these deposits into marginal mountain ranges.
The continents and oceans which arose at the close of the Paleozoic era.—At the close of the second great era in the recorded history of the earth, the now somewhat enlarged continents were profoundly altered during a series of convulsive movements within the surface shell of the lithosphere. When these convulsions were over, there was a new disposition of land and sea, but one quite different from the present arrangement. Instead of being extended in north-south belts, as they are at present, the continents stretched out in broad east-west zones, one in the northern and the other in the southern hemisphere. To the broad southern continent of which so little now remains, the name “Gondwana Land” has been given, and to the sea which divided the northern from the southern continent the name “Ocean of Tethys.” The northern continent stretched across the site of the present Atlantic Ocean as the “North Atlantis”, its northern shore to the westward being somewhat farther south than the present northern coast of North America, since life forms migrated in the northern ocean from the site of Behring Sea to that of the present North Atlantic.
This arrangement of land and water during the middle period of the earth’s recorded history, when considered with reference both to its earlier and to its later evolution, may perhaps be best accounted for by the assumption that the lithosphere was then shaped like [Fig. 5] (middle). In this figure two truncated tetrahedrons are joined in a common plane of contact which may be described as the twin plane. This medial depression upon the lithosphere was occupied by the intercontinental sea, the Ocean of Tethys.
Near the close of this second great era of the earth’s continental history, crustal convulsions, which were perhaps the most remarkable in the entire record, resulted in the almost complete disappearance of the southern continent and a concentration of the land within the northern hemisphere as a somewhat interrupted belt surrounding a central polar ocean ([Figs. 3] and [5]).
Upon the assumption of twin tetrahedrons in the intermediate era of continental evolution, both the Ocean of Tethys of that time and its present remnants, the Caribbean and Mediterranean seas, are accounted for. The V-shaped continent extensions and the A-shaped oceans of the southern hemisphere ([Fig. 2]) may likewise be considered as relics of the now largely submerged tetrahedron of the southern hemisphere, since this had its apex to the northward ([Fig. 6]).
Fig. 6.—Diagrams for comparison of shore lines upon tetrahedrons which have an angle, the first at the south and the second at the north.
Thus we see that the lithosphere can scarcely be regarded as a perfect spheroid, since in the course of geologic ages it has undergone successive departures from this original form. In its present state it has been described as tetrahedral, though we must keep in mind that the sharp angles of that figure are deeply truncated. The soundings first by Nansen and more recently by Peary in the Arctic basin, far to the north of the continental border, showed that this depression is characterized by profound depths, and so have afforded confirmation of the tetrahedral figure. To match this depression at the northern extremity of the earth’s axis, a high continent reaching to elevations in excess of 10,000 feet has been penetrated by Sir Ernest Shackleton at the opposite extremity of this polar diameter. Considering its size and its elevation, the Antarctic continent with its glacier mantle is the largest protuberance upon the surface of the lithosphere.
In our study of the departures of the earth from the standard spheroidal surface, we might even go a step farther and show how the tetrahedron, which best represents the symmetry of the present figure, is somewhat deformed by a flattening perpendicular to the Pacific Ocean. To draw attention to this flattening of the earth, it has sometimes been described as “potato-shaped”, since the outline perpendicular to this face is imperfectly heart-shaped or like a flattened “peg top.”
Fig. 7.—The continents with submerged portions added (after Gilbert).
The flooded portions of the present continents.—We are accustomed to think of the continents as ending at the shores of the oceans. If, however, we are to regard them as platforms which rise from the ocean depressions, their margins should be considerably extended, for a submerged shelf now practically surrounds all the continents to a nearly uniform depth of 100 fathoms or 600 feet. The oceans thus more than fill their basins and may be thought of as spilling over upon the continents. In [Fig. 7], the submerged portions of the continents have been joined to those usually represented, thus adding about 10,000,000 square miles to their area, and giving them one third, instead of one fourth, of the lithosphere surface.
Fig. 8.—Diagram to indicate the altitude of different parts of the lithosphere surface.
The floors of the hydrosphere and atmosphere.—The highest altitudes upon the continents and the profoundest deeps of the ocean are each removed about 30,000 feet, or nearly 6 miles, from the level of the sea. In comparison with the entire surface of the lithosphere, these extremes of elevation represent such small areas as to be almost inappreciable. Only about 1/80 of the lithosphere surface rises more than 6000 feet above sea level, and about the same proportion lies deeper than 18,000 feet below the same datum plane ([Fig. 8]). Almost the entire area of the lithosphere is included either in the so-called continental plateau or platform, in the oceanic platform, or in the slope which separates the two. The continental platform includes the continental shelf above referred to, and represents about one third of the entire area of the planet. This platform has a range of elevation from 6000 feet above to 600 feet below sea level and has an average altitude of about 2300 feet. The oceanic platform slopes more steeply, ranges in depth from 12,000 to 18,000 feet below sea level, and comprises about one half the lithosphere surface. The remaining portion of the surface, something less than one eighth of all, is included in the steep slopes between the two platforms, between 600 and 12,000 feet below sea. The two platforms and the slope between them must not, however, be thought of as continuous features upon the surface, but merely as representing the average elevations of portions of the lithosphere.
Reading References for Chapter II
On the evolution of ideas concerning the earth’s figure:—
Suess. The Face of the Earth (Clarendon Press, 1906), vol. 2, Chapter 1.
v. Zittel. History of Geology and Paleontology (Walter Scott, London, 1901), Chapters 1-2.
The departure of the spheroid toward the tetrahedron:—
W. Lowthian Green. Vestiges of the Molten Globe, Part 1. London, 1875. (Now a rare work, but it contains the original statement of the idea.)
J. W. Gregory. The Plan of the Earth and Its Causes, Geogr. Jour., vol. 13, 1899, pp. 225-251 (the best general statement of the arguments for a tetrahedral form).
W. Prinz. L’échelle reduite des expériences géologiques, Bull. Soc. Belge d’Astronomie, 1899.
B. K. Emerson. The Tetrahedral Earth and Zone of the Intercontinental Seas, Bull. Geol. Soc. Am., vol. 11, 1911, pp. 61-106, pls. 9-14.
M. P. Rudski. Physik der Erde (Tauchnitz, Leipzig, 1911), Chapters 1-3 (the best discussion of the geoid from the purely mathematical standpoint, so far as the spheroid is concerned).
The earlier figures of the earth:—
Th. Arldt. Die Entwicklung der Kontinente und ihrer Lebewelt. Engelmann, Leipzig, 1907. (Contains a valuable series of map plates, showing the probable boundaries of the continents in the different geological periods).
CHAPTER III
THE NATURE OF THE MATERIALS IN THE LITHOSPHERE
The rigid quality of our planet.—For a long time it was supposed that the solid earth constituted a crust only which was floated upon a liquid interior. This notion was clearly an outgrowth of the then generally accepted Laplacian hypothesis of the origin of the universe, which assumed fluid interiors for the planets, the crust being suggested by the winter crust of frozen water upon the surface of our inland lakes. To-day the nebular hypothesis in the Laplacian form is fast giving place to quite different conceptions, in which solid particles, and not gaseous ones, are conceived to have built up the lithosphere. The analogy with frozen water has likewise been abandoned with the discovery that frozen rock, instead of floating, sinks in its molten equivalent.
Yet even more cogent arguments have been brought forward to show that whatever may be the state of aggregation within the earth’s core—and it may be different from any now known to us—it nevertheless has many of the properties recognized as belonging to solid and rigid bodies. Provisionally, therefore, we may regard the earth’s core as rigid and essentially solid. It was long ago pointed out by the late Lord Kelvin that if our lithosphere were not more rigid than a ball of glass of the same size, it would be constantly passing through periodic six-hourly distortions of great amplitude in response to the varying attractions of the moon. An equally striking argument emanating from the same high authority is furnished by the well-known egg-spinning demonstration. For illustration, Kelvin was accustomed to take two eggs, one boiled and the other raw, and attempt to spin them upon their ends. For the boiled, and essentially solid, egg this is easily accomplished, but internal friction of the liquid contents of the raw egg quickly stops any rotary motion which is imparted to it. Upon the same grounds it is argued that had the earth’s interior possessed the properties of a liquid, rotation must long since have ceased.
A stronger proof of earth rigidity than either of these has been lately furnished by the instrumental study of earthquakes. With the delicate apparatus which is now installed for the purpose, heavy earthquakes may be sensed which have occurred anywhere upon the earth’s surface, the earth movement sending its own message by the shortest route through the core of the earth to the observing station. A heavy shock which occurs in New Zealand is recorded in England, almost diametrically opposite, in about twenty-one minutes after its occurrence. The laws of wave propagation and their relation to the properties of the transmitting medium are well known, and in order to explain such extraordinary velocity it is necessary to assume that for such impulses the earth’s interior is much more rigid than the finest tool steel.
Probable composition of the earth’s core.—In deriving views concerning the nature of the earth’s interior we are greatly aided by astronomical studies. The common origin long ago indicated for the planets of the solar system and the sun has been confirmed by the analysis of light with the aid of the spectroscope. It has thus been found that the same chemical elements which we find in the earth are present also in the sun and in the other stellar bodies. Again, the group of planets of the solar system which are nearest to the sun—Mercury, Venus, the Earth, and Mars—have each a high density, all except Mars, the most distant, having specific gravities very closely 5½, that of Mars being about 4. This average specific gravity is also that of the solid bodies, the so-called meteorites, which reach the surface of our planet from the surrounding space. Yet though the earth as a whole is thus found to have a specific gravity five and a half times that of water, its surface shell has an average density of less than half this value, or 2.7.
The study of meteorites has given us a possible clew to the nature of the earth’s interior; for when both terrestrial and celestial rock types are classified and placed in orderly arrangement, it is found that the chemical elements which compose the two groups are identical, and that these are united according to the same physical and chemical laws. No new element has been discovered in the one group that has not been found in the other, and though some compounds of these elements, the minerals, occur in the earth’s crust that have not been found in meteorites, and though some occur in meteorites which are not known from the earth, yet of those which are common to both bodies there is agreement, even to the minor details ([Fig. 9]). It is found, however, that the commonest of the minerals in the earth’s shell are absent from meteorites, as the commoner constituents of meteorites are wanting in the earth’s crust. This observation would go far to show that we may in the two cases be examining different portions of quite similar bodies; and this view is strikingly confirmed when the rocks of the two groups are arranged in the order of their densities ([Fig. 9]).
Fig. 9.—Diagram to show how terrestrial rocks grade into those of the meteorites. 1, oxygen; 2, silicon; 3, aluminium; 4, alkali metals; 5, alkaline earth metals; 6, iron, nickel, cobalt, etc.; a, granites and rhyolites; b, syenites and trachytes; c, diorites and andesites; d, gabbros and basalts; e, ultra-basic rocks; f, basic inclosures in basalt, etc.; g, iron basalts of west Greenland; h, iron masses of Ovifak, west Greenland; a’-d’, meteorites in order of density (after Judd).
In a broad way, density, structure, and chemical composition are all similarly involved in the gradations illustrated by the diagram; and it is significant that while there are terrestrial rocks not represented by meteorites, the densest and the most unusual of the terrestrial rocks are chemically almost identical with the less dense of the celestial bodies.
The earth a magnet.—The denser, and likewise the more common, of the meteorite rocks—the so-called meteoric irons—are composed almost entirely of the elements iron, nickel, and cobalt. Such aggregates are not known as yet from terrestrial sources, although transitional types appear to exist upon the island of Disco off the west coast of Greenland. If it were possible to explore the earth’s interior, would such combinations of the iron minerals be encountered? Apart from the surprising velocity of transmission of earthquake waves, the strongest argument for an iron core to the lithosphere is found in the magnetic property of the earth as a whole. The only magnetic elements known to us are those of the heavy meteorites—iron, nickel, and cobalt,—and the earth is, as we know, a great magnet whose northern pole in British America and whose southern pole in Antarctica have at last been visited by Amundsen and David, respectively. The specific gravity of iron is 7.15, and those of nickel and cobalt, which in the meteorites are present in relatively small amounts, are 7.8 and 7.5, respectively. Considering that the surface shell of the earth has a specific gravity of 2.7, these values must be regarded as agreeing well with the determined density of the earth (5.6) and the other planets of its group (Mercury 5.7, Venus 5.4, Mars 4).
The chemical constitution of the earth’s surface shell.—The number of the so-called chemical elements which enter into the earth’s composition is more than eighty, but few of these figure as important constituents of the portion known to us. Nearly one half of the mass of this shell is oxygen, and more than a quarter is silicon. The remaining quarter is largely made up of aluminium, iron, calcium, magnesium, and the alkalies sodium and potassium, in the order named. These eight constituent elements are thus the only ones which play any important rôle in the composition of the earth’s surface shell. They are not found there in the free condition, but combined in the definite proportions characteristic of chemical compounds, and as such they are known as minerals.
The essential nature of crystals.—A crystal we are accustomed to think of as something transparent bounded by sharp edges and angles, our ideas having been obtained largely from the gem minerals. This outward symmetry of form is, however, but an expression of a power which resides, so to speak, in the heart or soul of the crystal individual—it has its own structural make-up, its individuality. No more correct estimates of the comparison of crystal individualities would be obtained by the study of outward forms alone of two minerals than would be gained by a judgment of persons from the cut of their clothing. Too often this outward dress tells only of the conditions by which both men and crystals have been surrounded, and but little of the power inherent in the individual. Many a battered mineral fragment with little beauty to recommend it, when placed under suitable conditions for its development, has grown into a marvel of beauty. Few minerals are so mean that they have not within them this inherent power of individuality which lifts them above the world of the amorphous and shapeless.
Fig. 10.—Comparison of a crystalline with an amorphous substance when expanded by heat and when attacked by acid.
Just as the real nature of a person is first disclosed by his behavior under trying circumstances, so of a crystal it is its conduct under stress of one sort or another which brings out its real character. By way of illustration let us prepare a sphere from the mineral quartz—it matters not whether we destroy the beautiful outlines of the crystal or employ a battered fragment—and then prepare a sphere of similar size and shape from a noncrystalline or amorphous substance like glass. If now these two spheres be introduced into a bath of oil and raised to a higher temperature, the glass globe undergoes an enlargement without change of its form; but the crystal ball reveals its individuality by expanding into a spheroid in which each new dimension is nicely adjusted to this more complex figure ([Fig. 10]).
We may, instead of submitting the two balls to the “trial by fire”, allow each to be attacked by the powerful reagent, hydrofluoric acid. The common glass under the attack of the acid remains as it was before, a sphere, but with shrunken dimensions. The crystal, on the other hand, is able to control the action of the solvent, and in so doing its individuality is again revealed in a beautifully etched figure having many curving outlines—it is as though the crystal had possessed a soul which under this trial has been revealed. This glimpse into the nature of the crystal, so as to reveal its structural beauty, is still more surprising when the crystal is taken from the acid in the early stages of the action and held close beneath the eye. Now the little etchings upon the surface display each the individuality of the substance, and joining with their neighbors they send out a beautifully symmetrical and entirely characteristic picture ([Fig. 11]).
Fig. 11.—“Light figure” seen upon an etched surface of a crystal of calcite (after Goldschmidt and Wright).
The lithosphere a complex of interlocking crystals.—To the layman the crystal is something rare and expensive, to be obtained from a jeweler or to be seen displayed in the show cases of the great museums. Yet the one most striking quality of the lithosphere which separates it from the hydrosphere and the atmosphere is its crystalline structure,—a structure belonging also to the meteorite, and with little doubt to all the planets of the earth group. A snowflake caught during its fall from the sky reveals all the delicate tracery of crystal boundary; collected from a thick layer lying upon the ground, it appears as an intricate aggregate of broken fragments more or less firmly cemented together. And so it is of the lithosphere, for the myriads of individuals are either the ruins of former crystals, or they are grown together in such a manner that crystal facets had no opportunity to develop.
Such mineral individuals as once possessed the crystal form may have been broken and their surfaces ground away by mutual attrition under the rhythmic beating of the waves upon a shore or in the continuous rolling of pebbles on a stream bed, until as battered relics they are piled away together in a bed of sand. Yet no amount of such rough handling is sufficient to destroy the crystal individuality, and if they are now surrounded with conditions which are suitable for their growth, their individual nature again becomes revealed in new crystal outlines. Many of our sandstones when turned in the bright sunlight send out flashes of light to rival a bank of snow in early spring. These bright flashes proceed from the facets of minute crystals formed about each rounded grain of the sand, and if we examine them under a lens, we may note the beauty of line formed with such exactness that the most delicate instruments can detect no difference between the similar angles of neighboring crystals ([Fig. 12]).
Fig. 12.—Battered sand grains which have taken on a new lease of life and have developed a crystal form. a, a single grain grown into an individual crystal; b, a parallel growth about a single grain; c, growth of neighboring grains until they have mutually interfered and so destroyed the crystal facets—the common condition within the mass of a rock (after Irving and Van Hise).
This individual nature of the crystal is believed to reside in a symmetrical grouping of the chemical molecules of the substance into larger and so-called “crystal molecules.” The crystal quality belongs to the chemical elements and to their compounds in the solid condition, but not to ordinary mixtures of them.
Some properties of natural crystals, minerals.—No two mineral species appear in crystals of the same appearance, any more than two animal species have been given the same form; and so minerals may be recognized by the individual peculiarities of their crystals. Yet for the reason that crystals have so generally been prevented from developing or retaining their characteristic faces, in the vast number of instances it is the behavior, and not the appearance, of the mineral substance which is made use of for identification.
When a mineral is broken under the blow of a hammer, instead of yielding an irregular fracture, like that of glass, it generally tends to part along one or more directions so as to leave plane surfaces. This property of cleavage is strikingly illustrated for a single direction in the mineral mica, for two directions in feldspar, and for three directions in calcite or Iceland spar. Other properties of minerals, such as hardness, specific gravity, luster, color, fusibility, etc., are all made use of in rough determinations of the minerals. Far more delicate methods depend upon the behavior of minerals when observed in polarized light, and such behavior is the basis of those branches of geological science known as optical mineralogy and as microscopical petrography. An outline description of some of the common minerals and the means for identifying them will be found in appendix A.
The alterations of minerals.—By far the larger number of minerals have been formed in the cooling and consequent consolidation of molten rock material such as during a volcanic eruption reaches the earth’s surface as lava. Beginning their growth at many points within the viscous mass, the individual crystals eventually may grow together and so prevent a development of their crystal faces.
Another class of minerals are deposited from solution in water within the cavities and fissures of the rocks; and if this process ceases before the cavities have been completely closed, the minerals are found projecting from the walls in a beautiful lining of crystal—the Krystallkeller or “crystal cellar.” It is from such pockets or veins within the rocks that the valuable ores are obtained, as are the crystals which are displayed in our mineral cabinets.
Fig. 13.—Crystal of garnet developed in a schist with grains of quartz included because not assimilated.
There is, however, a third process by which minerals are formed, and minerals of this class are produced within the solid rock as a product of the alteration of preëxisting minerals. Under the enormous pressures of the rocks deep below the earth’s surface, they are as permeable to the percolating waters as is a sponge at the surface. Under these conditions certain minerals are dissolved and their material redeposited after traveling in the solution, or solution and redeposition of mineral matter may go on together within the mass of the same rock. One new mineral may have been produced from the dissolved materials of a number of earlier species, or several new minerals may be the result of the alteration of a preëxisting mineral with a more complex chemical structure. Where the new mineral has been formed “in place”, it has sometimes been able to utilize the materials of all the minerals which before existed there, or it may have been obliged to inclose within itself those earlier constituents which it could not assimilate in its own structure ([Fig. 13]).
Fig. 14.—A crystal of augite within the mass of a rock altered in part to form a rim of the minerals hornblende and magnetite. Note the original outline of the augite crystal.
At other times a crystal which is imbedded in rock has been attacked upon its surface by the percolating solutions, and the dissolved materials have been deposited in place as a crown of new minerals which steadily widens its zone until the center is reached and the original crystal has been entirely transformed ([Fig. 14]). It is sometimes possible to say that the action by which these changes have been brought about has involved a nice adjustment of supply of the chemical constituents necessary to the formation of the new mineral or minerals. In rocks which are aggregates of several mineral species, a newly formed mineral may appear only at the common margin of certain of these species, thus showing that they supply those chemical elements which were necessary to the formation of the new substance ([Fig. 15]). Thus it is seen that below the earth’s surface chemical reactions are constantly going on, and the earlier rocks are thus locally being transformed into others of a different mineral constitution.
Fig. 15.—A new mineral (hornblende) forming as an intermediate “reaction rim” between the mineral having irregular fractures (olivine) and the dusty white mineral (lime-soda feldspar).
Near the earth’s surface the carbon dioxide and the moisture which are present in the atmosphere are constantly changing the exposed portions of the lithosphere into carbonates, hydrates, and oxides. These compounds are more soluble than are the minerals out of which they were formed, and they are also more bulky and so tend to crack off from the parent mass on which they were formed. As we are to see, for both of these reasons the surface rocks of the lithosphere succumb to this attack from the atmosphere.
In connection with those wrinklings of the surface shell of the lithosphere from which mountains result, the underlying rocks are subjected to great strains, and even where no visible partings are produced, the rocks are deformed so that individual minerals may be bent into crescent-shaped or S-shaped forms, or they are parted into one or more fragments which remain imbedded within the rock.
Reading References for Chapter III
Theories of origin of the earth:—
Thomson and Tait. Natural Philosophy. 2d ed. Cambridge, 1883, pp. 422.
T. C. Chamberlin. Chamberlin and Salisbury’s Geology, vol. 2, pp. 1-81.
Rigidity of the earth:—
Lord Kelvin. The Internal Condition of the Earth as to Temperature, Fluidity, and Rigidity, Popular Lectures and Addresses, vol. 2, pp. 299-318; Review of evidence regarding the physical condition of the earth, ibid., pp. 238-272.
Hobbs. Earthquakes (Appleton, New York, 1907), Chapters xvi and xvii.
Composition of the earth’s core and shell:—
O. C. Farrington. The Preterrestrial History of Meteorites, Jour. Geol., vol. 9, 1901, pp. 623-236.
E. S. Dana. Minerals and How to Study Them (a book for beginners in mineralogy). Wiley, New York, 1895.
On the nature of crystals:—
Victor Goldschmidt. Ueber das Wesen der Krystalle, Ostwalds Annalen der Naturphilosophie, vol. 9, 1909-1910, pp. 120-139, 368-419.
CHAPTER IV
THE ROCKS OF THE EARTH’S SURFACE SHELL
The processes by which rocks are formed.—Rocks may be formed in any one of several ways. When a portion of the molten lithosphere, so-called magma, cools and consolidates, the product is igneous rock. Either igneous or other rock may become disintegrated at the earth’s surface, and after more or less extended travel, either in the air, in water, or in ice, be laid down as a sediment. Such sediments, whether cemented into a coherent mass or not, are described as sedimentary or clastic rocks. If the fluid from which they were deposited was the atmosphere, they are known as subaërial or eolian sediments; but if water, they are known as subaqueous deposits. Still another class are ice-deposited and are known as glacial deposits.
Fig. 16.—Laminated structure of sedimentary rock, Western Kansas (after a photograph by E. S. Tucker).
But, as we have learned, rocks may undergo transformations through mineral alteration, in which case they are known as metamorphic rocks. When these changes consist chiefly in the production of more soluble minerals at the surface, accompanied by thorough disintegration, due to the direct attack of the atmosphere, the resulting rocks are called residual rocks.
The marks of origin.—Each of the three great classes of rocks, the igneous, sedimentary, and metamorphic, is characterized by both coarser and finer structures, in the examination of which they may be identified. The igneous rocks having been produced from magmas, which are essentially homogeneous, are usually without definite directional structures due to an arrangement of their constituents, and are said to have a massive structure. Sedimentary rocks, upon the other hand, have been formed by an assorting process, the larger and heavier fragments having been laid down when there was high velocity of either wind or water current, and the smaller and lighter fragments during intermediate periods. They are therefore more or less banded, and are said to have a bedded or laminated structure ([Fig. 16]).
Again, igneous rocks, being due to a process of crystallization, are composed of mineral individuals which are bounded either by crystal planes or by irregular surfaces along which neighboring crystals have interfered with each other; but in either case the grains possess sharply angular boundaries. Quite different has been the result of the attrition between grains in the transportation and deposition of sediments, for it is characteristic of the sedimentary rocks that their constituent grains are well rounded. Eolian sediments have usually more perfectly rounded grains than subaqueous deposits.
Glacial deposits, if laid down directly by the ice, are unstratified, relatively coarse, and contain pebbles which are both faceted and striated. Such deposits are described as till or tillite. If glacier-derived material is taken up by the streams of thaw water and is by them redeposited, the sediments are assorted or stratified, and they are described as fluvio-glacial deposits.
The metamorphic rocks.—Both the coarser structures and the finer textures of the metamorphic rocks are intermediate between those of the igneous and the sedimentary classes. A metamorphosed sedimentary rock, in proportion to its alteration, loses the perfect lamination and the rounded grain which were its distinguishing characters; while an igneous rock takes on in the process an imperfect banding, and the sharp angles of its constituent grains become rounded off by a sort of peripheral crushing or granulation. Metamorphic rocks are therefore characterized by an imperfectly banded structure described as schistosity or gneiss banding, and the constituent grains may be either angular or rounded. If the metamorphism has not been too intense or too long continued, it is generally possible to determine, particularly with the aid of the polarizing microscope, whether the original rock from which it was derived was of igneous or of sedimentary origin. There are, however, many examples which have defied a reliable verdict concerning their origin.
Characteristic textures of the igneous rocks.—In addition to the massiveness of their general aspect and the angular boundaries of their constituents, there are many additional textures which are characteristic of the igneous rocks. While those that have consolidated below the earth’s surface, the intrusive rocks, are notably compact, the magmas which arrive at the surface of the lithosphere before their consolidation reveal special structures dependent either upon the expansion of steam and other gases within them, or upon the conditions of flow over the earth’s surface. Magmas which thus reach the surface of the earth are described as lavas, and the rocks produced by their consolidation are extrusive or volcanic rocks. The steam included in the lava expands into bubbles or vesicles which may be large or small, few or many. According to the number and the size of these cavities, the rock is said to have a vesicular, scoriaceous, or pumiceous texture.
Most lavas, when they arrive at the earth’s surface, contain crystals which are more or less disseminated throughout the molten mass. The tourist who visits Mount Vesuvius at the time of a light eruption may thrust his staff into the stream of lava and extract a portion of the viscous substance in which are seen beautiful white crystals of the mineral leucite, each bounded by twenty-four crystal faces. It is clear that these crystals must have developed by a slow growth within the magma while it was still below the surface, and when the inclosing lava has consolidated, these earlier crystals lie scattered within a groundmass of glassy or minutely crystalline material. This scattering of crystals belonging to an earlier generation within a groundmass due to later consolidation is thus an indication of interruption in the process of crystallization, and the texture which results is described as porphyritic ([Fig. 17 b]). Should the lava arrive at the surface before any crystals have been generated and consolidate rapidly as a rock glass, its texture is described as glassy ([Fig. 17 c]).
When the crystals of the earlier generation are numerous and needle-like in form, as is very often the case, they arrange themselves “end on” during the rock flow, so that when consolidation has occurred, the rock has a kind of puckered lamination which is the characteristic of the fluxion or flow texture. This texture has sometimes been confused with the lamination of the sedimentary rocks, so that wrong conclusions have been reached regarding origin. At other times the same needle-like crystals within the lava have grouped themselves radially to form rounded nodules called spherulites. Such nodules give to the rock a spherulitic texture, which is nowhere better displayed than in the beautiful glassy lavas of Obsidian Cliff in the Yellowstone National Park.
Fig. 17.—Characteristic textures of igneous rocks. a, granitic texture characteristic of the deep-seated intrusive rocks; b, porphyritic texture characteristic of the extrusive and of the near-surface intrusive rocks; c, glassy texture of an extrusive rock.
Those intrusive rocks which consolidate deep below the earth’s surface, part with their heat but slowly, and so the process of crystallization is continued without interruption. Starting from many centers, the crystals continue to grow until they mutually intersect in an interlocking complex known as the granitic texture ([Fig. 17 a]).
Classification of rocks.—In tabular form rocks may thus be classified as follows:—
Subdivisions of the sedimentary rocks.—While the eolian sediments are all the product of a purely mechanical process of lifting, transportation, and deposition of rock particles, this is not always the case with the subaqueous sediments, since water has the power of dissolving mineral substance, as it has also of furnishing a home for animal and vegetable life. Deposited materials which have been in solution in water are described as chemical deposits, and those which have played a part in the life process as organic deposits. The organic deposits from vegetable sources are peat and the coals, while limestones and marls are the chief depositories of the remains of the animal life of the water. The tabular classification of the sediments is as follows:—
Classification of Sediments.
| Mechanical |
| Subaqueous Deposited by water. | Conglomerate, sandstone and shale. |
| Subaërial or Eolian Deposited by wind. | Sandstone and loess. | ||
| Glacial Deposited by ice. | Till and tillite. | ||
| Fluvio-glacial Glacier-water deposits. | Sands and gravels. | ||
| Chemical |
| Calcareous tufa | Deposited in springs and rivers. |
| Oölitic limestone | Deposited at the mouths of riversbetween high and low tide. | ||
| Organic |
| Formed of plant remains. | Peats and coals. |
| Formed of animal remains. | Limestones and marls. |
Winds are under favorable conditions capable of transporting both dust and sand, but not the larger rock fragments. The dust deposits are found accumulating outside the borders of deserts as the so-called loess ([Fig. 216]), though the sand is never carried beyond the desert border, near which it collects in wide belts of ridges described as dunes. When this sand has been cemented into a coherent mass, it is known as eolian sandstone. A section of the appendix (B) is devoted to an outline description of some of the commoner rock types.
The different deposits of ocean, lake, and river.—Of the subaqueous sediments, there are three distinct types resulting: (1) from sedimentation in rivers, the fluviatile deposits; (2) from sedimentation in lakes, the lacustrine deposits; and (3) from sedimentation in the ocean, marine deposits. Again, the widest range of character is displayed by the deposits which are laid down in the different parts of the course of a stream. Near the source of a river, coarse river gravels may be found; in the middle course the finer silts; and in the mouth or delta region, where the deposits enter the sea or a lake, there is found an assortment of silts and clays. Except within the delta region, where the area of deposition begins to broaden, the deposits of rivers are stretched out in long and relatively narrow zones, and are so distinguished from the far more important lacustrine and marine deposits.
Lakes and oceans have this in common that both are bodies of standing as contrasted with flowing water; and both are subject to the periodical rhythmic motions and alongshore currents due to the waves raised by the wind. About their margins, the deposits of lake and ocean are thus in large part wrested by the waves from the neighboring land. Their distribution is always such that the coarsest materials are laid down nearest to the shore, and the deposits become ever finer in the direction of deeper water. Relatively far from shore may be found the finest sands and muds or calcareous deposits, while near the shore are sands, and, finally, along the beach, beds of beach pebbles or shingle. When cemented into coherent rocks, these deposits become shales or limestones, sandstones, and conglomerates, respectively.
As regards the limestones, their origin is involved in considerable uncertainty. Some, like the shell limestone or coquina of the Florida coast, are an aggregation of remains of mollusks which live near the border of the sea. Other limestones are deposited directly from carbonate of lime in solution in the water. A deposit of this nature is forming in southern Florida, both as a flocculent calcareous mud and as crystals of lime carbonate upon a limestone surface. Again, there is the reef limestone which is built up of the stony parts of the coral animal, and, lastly, the calcareous ooze of the deep-sea deposits.
The marine sediments which are derived from the continents, the so-called terrigenous deposits, are found only upon the continental shelf and upon the continental slope just outside it. Of these terrigenous deposits, it is customary to distinguish: (1) littoral or alongshore deposits, which are laid down between high and low tide levels; (2) shoal water deposits, which are found between low-water mark and the edge of the continental shelf; and (3) aktian or offshore deposits, which are found upon the continental slope. The littoral and shoal water deposits are mainly gravels and sands, while the offshore deposits are principally muds or lime deposits.
Special marks of littoral deposits.—The marks of ripples are often left in the sand of a beach, and may be preserved in the sandstone which results from the cementation of such deposits (pl. 11 A). Very similar markings are, however, quite characteristic of the surface of wind-blown sand. For the reason that deposits are subject to many vicissitudes in their subsequent history, so that they sometimes stand at steep angles or are even overturned, it is important to observe the curves of sand ripples so as to distinguish the upper from the lower surface.
In the finer sands and muds of sheltered tidal flats may be preserved the impressions from raindrops or of the feet of animals which have wandered over the flat during an ebb tide. When the tide is at flood, new material is laid down upon the surface and the impressions are filled, but though hardened into rock, these surfaces are those upon which the rock is easily parted, and so the impressions are preserved. In the sandstones of the Connecticut valley there has been preserved a quite remarkable record in the footprints of animals belonging to extinct species, which at the time these deposits were laid down must have been abundant upon the neighboring shores.
Between the tides muds may dry out and crack in intersecting lines like the walls of a honeycomb, and when the cracks have been filled at high tide, a structure is produced which may later be recognized and is usually referred to as “mud-crack” structure. This structure is of special service in distinguishing marine deposits from the subaërial or continental deposits.
A variation in the direction of winds of successive storms may be responsible for the piling up of the beach sand in a peculiar “plunge and flow” or “cross-bedded” structure, a structure which is extremely common in littoral deposits, though simulated in rocks of eolian origin.
The order of deposition during a transgression of the sea.—Many shore lines of the continents are almost constantly migrating either landward or seaward. When the shore line advances over the land, the coast is sinking, and marine deposits will be formed directly above what was recently the “dry land.” Such an invasion of the land by the sea, due to a subsidence of the coast, is called a transgression of the sea, or simply a transgression. Though at any moment the littoral, shoal water, and offshore deposits are each being laid down in a particular zone, it is evident that each must advance in turn in the direction of the shore and so be deposited above the zones nearer shore. Thus there comes to be a definite series of continuous beds, one above the other, provided only that the process is continued ([Fig. 18]). At the very bottom of this series there will usually be found a thin bed of pebbly beach materials, which later will harden into the so-called basal conglomerate. If the size of the pebbles is such as to make possible an identification, it will generally be found that these represent the ruins of the rock over which the sea has advanced upon the land.
Fig. 18.—Diagram to show the order of the sediments laid down during a transgression of the sea.
Next in order above the basal conglomerate, will follow the coarser and then the finer sands, upon which in turn will be laid down the offshore sediments—the muds and the lime deposits. Later, when cemented together, these become in order, coarser and finer sandstones, shales, and limestones. The order of superposition, reading from the bottom to the top, thus gives the order of decreasing age of the formations.
A subsequent uplift of the coast will be accompanied by a recession of the sea, and when later dissected by nature for our inspection, the order of superposition and the individual character of each of the deposits may be studied at leisure. From such studies it has been found that along with the inorganic deposits there are often found the remains of life in the hard parts of such invertebrate animals as the mollusks and the crustacea. These so-called fossils represent animals which were gradually developed from simpler to more and more complex forms; and they thus serve the purpose of successive page numbers in arranging the order of disturbed strata, at the same time that they supply the most secure foundation upon which rests the great doctrine of evolution.
The basins of earlier ages.—It was the great Viennese geologist, Professor Suess, who first pointed out that in mountain regions there are found the thickest and the most complete series of the marine deposits; whereas outside these provinces the formations are separated by wide gaps representing periods when no deposits were laid down because the sea had retired from the region. The completeness of the series of deposits in the mountain districts can only be interpreted to mean that where these but lately formed mountains rise to-day, were for long preceding ages the basins for deposition of terrigenous sediments. It would seem that the lithosphere in its adjustment had selected these earlier sea basins with their heavy layers of sediment for zones of special uplift.
The deposits of the deep sea.—Outside the continental slope, whose base marks the limit of the terrigenous deposits, lies the deeper sea, for the most part a series of broad plains, but varied by more profound steep-walled basins, the so-called “deeps” of the ocean. As shown by the dredgings of the Challenger expedition and others of more recent date, the deposits upon the ocean floor are of a wholly different character from those which are derived from the continents. Except in the great deeps, or between depths of five hundred and fifteen hundred fathoms, these deposits are the so-called “ooze”, composed of the calcareous or chitinous parts of algæ and of minute animal organisms. The pelagic or surface waters of the ocean are, as it were, a great meadow of these plant forms, upon which the minute crustacea, such as globigerina, foraminifera, and the pteropods, feed in countless myriads. The hard parts of both plant and animal organisms descend to the bottom and there form the ooze in which are sometimes found the ear bones of whales and the teeth of sharks.
In the deeps of the ocean, none of these vegetable or animal deposits are being laid down, but only the so-called “red clay”, which is believed to represent decomposed volcanic material deposited by the winds as fine dust on the surface of the ocean, or the product of submarine volcanic eruption. From the absence of the ooze in these profound depths, the conclusion is forced upon us that the hard parts of the minute organisms are dissolved while falling through three or four miles of the ocean water.
Reading References for Chapter IV
J. S. Diller. The Educational Series of Rock Specimens collected and distributed by the United States Geological Survey, Bull. 150 U. S. Geol. Surv., 1898, pp. 1-400.
L. V. Pirsson. Rocks and Rock Minerals. Wiley, New York, 1908.
Sir John Murray. Deep-sea Deposits, Reports of the Challenger expedition, Chapter iii.
L. W. Collet. Les dépôts marins. Doin, Paris, 1907 (Encyclopédie Scientifique).
CHAPTER V
CONTORTIONS OF THE STRATA WITHIN THE ZONE OF FLOW
The zones of fracture and flow.—It is easy to think of the atmosphere and the hydrosphere as each sustaining at any point the load of the superincumbent material. At the sea level the weight of air upon each square inch of surface is about fifteen pounds, whereas upon the floor of the hydrosphere in the more profound deeps the load upon the square inch must be measured in tons. Near the lithosphere surface the rocks support by their strength the load of rock above them, but at greater depths they are unable to do this, for the load bears upon each portion of the rock with a pressure equivalent to the weight of a rock column which extends upward to the surface. The average specific gravity of rock is 2.7, and it is thus easy to calculate the length of the inch square column which has a weight equivalent to the crushing strength of any given rock. At the depth represented by the length of such a column, rocks cannot yield to pressure by fracture, for the opening of a crack implies that the rock upon either side is strong enough to prevent the walls from closing. At this depth, rock must therefore yield to pressure not by fracture, as it would at the surface, but by flow after the manner of a liquid; and so the zone below this critical level is referred to as the zone of flow.
Fig. 19.—Two intersecting parallel series of fractures produced upon each free surface of a prismatic block of stiff molders’ wax when broken by compression from the ends (after Daubrée and Tresca).
In contrast, the near-surface zone is called the zone of fracture. But different rocks possess different strengths, and these are subject to modifications from other conditions, such, for example, as the proximity of an uncooled magma. The zone of flow is therefore joined to the zone of fracture, not upon a definite surface, but in an intermediate zone described as the zone of fracture and flow.
Experiments which illustrate the fracture and flow of solid bodies.—A prismatic block prepared from stiff molders’ wax, if crushed between the jaws of a testing machine, yields a system of intersecting fractures which are perpendicular to the free surfaces of the block and take two directions each inclined by half of a right angle to the direction of compression ([Fig. 19]). This experiment may illustrate the manner in which fractures are produced by the compression within the zone of fracture of the lithosphere, as its core continues to contract.
To reproduce the conditions within the zone of flow, it will be necessary to load the lateral surfaces of the block instead of leaving them unconstrained as in the above-described experiment. The experiment is best devised as in [Fig. 20]. Here a series of layers having varying degrees of rigidity is prepared from beeswax as a base, either stiffened by admixture of varying proportions of plaster of Paris, or weakened by the use of Venice turpentine. Such a series of layers may represent rocks of as widely different characters as limestone and shale. The load which is to represent superincumbent rock is supplied in the experiment by a deep layer of shot.
Fig. 20.—Apparatus to illustrate the folding of strata within the zone of flow (after Willis).
When compression is applied to the layers from the ends, these normally solid materials, instead of fracturing, are bent into a series of folds. The stiffer, or more competent, layers are found to be less contorted than are the weaker layers, particularly if the latter have been protected under an arch of the more competent layer (pl. 2 A).
The arches and troughs of the folded strata.—Every series of folds is made up of alternating arches and troughs. The arches of the strata the geologist calls anticlines or anticlinal folds, and the troughs he calls synclines or synclinal folds ([Fig. 21]). When a stratum is merely dropped in a bend to a lower level without producing a complete arch or a complete trough, this half fold is termed a monocline.
Fig. 21.—Diagrams representing a, an anticline; b, a syncline; and c, a monocline.
Any flexuring of the strata implies a reduction of their surface area, or, considering a single section, a shortening. If the arches and troughs are low and broad, the deformation of the strata is slight, the shortening is comparatively small, and the folds are described as open ([Fig. 22 b]). If they be relatively both high and narrow, the deformation is considerable, a larger amount of crustal shortening has gone on, and the folds are described as close ([Fig. 22 c]). This closing up of the folds may continue until their sides have practically the same slope, in which case they are said to be isoclinal ([Fig. 22 d]).
Fig. 22.—A comparison of folds to express increasing degrees of crustal shortening or progressive deformation within the zone of flow: a, stratum before folding; b, open folds; c, close folds; d, isoclinal folds.
The elements of folds.—Folds must always be thought of as having extension in each of the three dimensions of space ([Fig. 23]), and not as properly included within a single plane like the cross sections which we so often use in illustration. A fold may be conceived of as divided into equal parts by a plane which passes along the middle of either the arch or the trough, and is called the axial plane. The line in which this plane intersects the arch or the trough is the axis, which may be called the crestline in an anticline, and the troughline in a syncline.
In the case of many open folds the axis is practically horizontal, but in more complexly folded regions this is seldom true. The departure of the axis from the horizontal is called the pitch, and folds of this type are described as pitching folds or plunging folds. The axis is in reality in these cases thrown into a series of undulations or “longitudinal folds”, and hence pitch will vary along the axis.
Fig. 23.—Anticlinal and synclinal folds in strata (after Willis).
Fig. 24.—Diagrams to illustrate the different shapes of rock folds.
The shapes of rock folds.—By the axial plane each fold is divided into two parts which are called its limbs, which may have either the same or different average inclinations. To describe now the shapes of rock folds and not the degree of compression of the district, some additional terms are necessary. Anticlines or synclines whose limbs have about the same inclinations are known as upright or symmetrical folds. The axial plane of the symmetrical fold is vertical ([Fig. 24]). If this plane is inclined to the vertical, the folds are unsymmetrical. So soon as the steeper of the two limbs has passed the vertical position and inclines in the same direction as the flatter limb, the fold is said to be overturned. The departure from symmetry may go so far that the axial plane of the fold lies at a very flat angle, and the fold is then said to be recumbent. The observant traveler by train along any of the routes which enter the Alps may from his car window find illustrations of most of these types of rock folds, as he may also, though generally less easily, in passing through the Appalachian Mountains.
Fig. 25.—Secondary and tertiary flexures superimposed upon the primary ones.
In regions which have been closely folded the larger flexures of the strata may be found with folds of a smaller order of magnitude superimposed upon them, and these in turn may show crumplings of still lower orders. It has been found that the folds of the smaller orders of magnitude possess the shapes of the larger flexures, and much is therefore to be learned from their careful study ([Fig. 25]). It is also quite generally discovered that parallel planes of ready parting, which are described as rock cleavage, take their course parallel to the axial plane within each minor fold. As was long ago shown by the pioneer British geologists, these planes of cleavage are essentially parallel and follow the fold axes throughout large areas.
Plate 2.
A. Layers compressed in experiments and showing the effect of a competent layer in the process of folding (after Willis).
B. Experimental production of a series of parallel thrusts within closely folded strata (after Willis).
C. Apparatus to illustrate shearing action within the overturned limb of a fold.
The overthrust fold.—Whenever a stratum is bent, there is a tendency for its particles to be separated upon the convex side of the bend, at the same time that those upon the concave side are crowded closer together—there is tension in the former case and compression in the latter ([Fig. 26]). Within an unsymmetrical or an overturned fold, the peculiar distortions in the different sections of the stratum are less simple and are best illustrated by [pl. 2 C]. This apparatus shows two similar piles of paper sheets, upon the edges of each of which a series of circles has been drawn. When now one of the piles is bent into an unsymmetrical fold, it is seen that through an accommodation by the paper sheets sliding each over its neighbor large distortions of the circles have occurred. In that steeper limb which with closer folding will be overturned the circles have been drawn out into long and narrow ellipses, and this indicates that those rock particles which before the bending were included in the circle have been moved past each other in the manner of the blades of a pair of shears. Such extreme “shearing” action is thus localized in the underturned limb of the fold, and a time must come with continuation of the compression when the fold will rupture at this critical place along a plane parallel to the longest axis of the ellipses or nearly parallel to the axial plane of the anticline. Such structures probably occur in the zone of combined fracture and flow, up into which the beds are forced in cases of close compression. Relief thus being found upon this plane of fracture, the upper portion of the fold will now ride over the lower, and the displacement is described as a thrust or overthrust.
Fig. 26.—A bent stratum to illustrate tension upon the convex and compression upon the concave side (after Van Hise).
In the long series of experiments conducted by Mr. Bailey Willis of the United States Geological Survey, all the stages between the overturned fold and the overthrust fold were reproduced. Where a series of folds was closely compressed, a parallel series of thrusts developed ([pl. 2 B]), so that a series of slices cutting across neighboring strata was slid in succession, each over the other, like the scales upon a fish or the shingles upon a roof. Quite remarkable structures of this kind have been discovered in rocks of such closely folded districts as the Northwest Highlands of Scotland, where the overriding is measured in miles. Near the thrust planes the rocks show a crushing of the grains, and the planes themselves are sometimes corrugated and polished by the movement.
Restoration of mutilated folds.—Since flexuring of the rocks takes place within the zone of flow at a distance of several miles below the earth’s surface, it is quite obvious that the results of the process can be studied only after some thousands of feet of superincumbent strata have been removed. We are a little later to see by what processes this lowering of the surface is accomplished, but for the present it may be sufficient to accept the fact, realizing that before foldings in the strata can reach the surface, they must have passed through the upper zone of fracture.
It might perhaps be supposed that the anticlines would appear as the mountains upon the surface, and occasionally this is true; as, for example, in the folded Jura Mountains of western Europe. More generally, the mountains have a synclinal structure and the valleys an anticlinal one; but as no general rule can be applied, it is necessary to make a restoration of the truncated folds in each district before their character can be known.
The geological map and section.—The earth’s surface is in most regions in large part covered with soil or with other incoherent rock material, so that over considerable areas the hard rocks are hidden from view. Each locality at which the rock is found at the earth’s surface “in place” is described as an outcropping or exposure. In a study of the region each such exposure must be examined to determine the nature of the rock, especially for the purpose of correlation with neighboring exposures, and, in addition, both the probable direction in which it is continued along the surface—the strike—and the inclination of its beds—the dip. If the outcroppings are sufficiently numerous, and rock type, strike and dip, may all be determined, the folds of the district may be restored with almost as much accuracy as though their curves were everywhere exposed to view. A cross section through the surface which represents the observed outcrops with their inclinations and the assumed intermediate strata in their probable attitudes is described as a geological section ([Fig. 27]). A map upon which the data have been entered in their correct locations, either with or without assumptions concerning the covered areas, is known as a geological map.
Fig. 27.—A geological section based upon observations at outcrops, but with the truncated arches restored.
If the axes of folds are absolutely horizontal, and the surface of the earth be represented as a plain, the lines of intersection of the truncated strata with the ground, or with any horizontal surface, will give the directions of continuation of the individual strata. This strike direction is usually determined at each exposure by use of a compass provided with a spirit level. When that edge of the leveled compass which is parallel to the north-south line upon the dial is held against the sloping rock stratum, the angle of strike is measured in degrees by the compass needle. If the cardinal directions have been placed in their correct positions upon the compass dial, the needle will point to the northwest when the strike is northeast, and vice versa ([Fig. 28 a]). Upon the geologist’s compass it is therefore customary to reverse the initials which represent the east and west directions, in order that the correct strike may be read directly from the dial ([Fig. 28 b]).
Fig. 28.—Diagram to illustrate the manner of determining the strike of rock beds at an outcropping. a, a compass which has the cardinal directions in their natural positions; b, a compass with the east and west initials reversed upon the dial; c, home-made clinometer in position to determine the dip.
By the dip is meant the inclination of the stratum at any exposure, and this must obviously be measured in a vertical plane along the steepest line in the bedding plane. The dip angle is always referred to a horizontal plane, and hence vertical beds have a dip of 90°. The device for measuring this angle of dip, the clinometer, is merely a simple pendulum which serves as an indicator and is centered at the corner of a graduated quadrant. A home-made variety is easily constructed from a square piece of board and an attached paper quadrant ([Fig. 28 c]), but the geologist’s compass is always provided with a clinometer attachment to the dial.
Fig. 29.—Diagram to show the use of T symbols to indicate the dip and strike of outcroppings.
Since the strike is the intersection of the bedding plane with a horizontal surface, and the dip is the intersection with that particular vertical plane which gives the steepest inclination, the strike and dip are perpendicular to each other. To represent them upon maps, it is more or less customary to use the so-called T symbols, the top of the T giving the direction of the strike and the shank that of the dip. If meridians are drawn upon the map, the direction or attitude of the T can be found by the use of a simple protractor; and when entered upon the map, the exact angle of the strike may be supplied by a figure near the top of the T, and the dip angle by a figure at the end of the shank. It is the custom, also, to make the length of the shank inversely proportional to the steepness of the dip, so that in a broad way the attitudes of the strata may be taken in at a glance ([Fig. 29]). It is further of advantage to make the top of the T a double line, so that some symbol or color may show the correlations of the different exposures. To illustrate, in [Fig. 29], the symbol marked a represents an outcrop of limestone, the strike of which is 50° east of north (N. 50° E.), and the dip of which is 45° southeast. In the same figure b represents a shale outcrop in horizontal beds, which have in consequence a universal strike and a dip of 0°. An exposure of limestone in vertical beds which strike N. 60° E. is shown at c, etc.
Fig. 30.—Diagram to show how the thickness of a formation may be obtained from the angle of the dip and the width of the exposures.
Measurement of the thickness of formations.—When formations still lie in horizontal beds, we may sometimes learn their thickness directly either from the depth of borings to the underlying rock, or by measurements upon steep cañon walls. If the beds stand vertically, the matter is exceedingly simple, for in this case the thickness is the width of the outcrops of the formation between the beds which bound it upon either side. In the general case, in which the beds are neither horizontal nor vertical, the thickness must be obtained indirectly from the width of the exposures and the angle of the dip. The factor by which the exposure width must be multiplied is known as the sine of the dip angle ([Fig. 30]), which is given with sufficient accuracy for most purposes in the following table. It is obvious that in order to obtain the full thickness of a formation it is necessary to measure from the contact with the adjacent formation upon the one side to a similar contact with the nearest formation upon the other.
Natural Sines
| 0° | .00 | 35° | .57 | 70° | .94 | ||
| 5° | .09 | 40° | .64 | 75° | .97 | ||
| 10° | .17 | 45° | .71 | 80° | .98 | ||
| 15° | .26 | 50° | .77 | 85° | 1.00 | ||
| 20° | .34 | 55° | .82 | 90° | 1.00 | ||
| 25° | .42 | 60° | .87 | ||||
| 30° | .50 | 65° | .91 |
Fig. 31.—Combined surface and sectional views of a plunging anticline (after Willis).
Fig. 32.—Combined surface and sectional views of a plunging syncline (after Willis).
The detection of plunging folds.—When the axis of a fold is horizontal, its outcrops upon a plain will continue to have the same strike until the formation comes to an end. Upon a generally level surface, therefore, any regular progressive variation in the strike direction is an indication that the folds have a plunging or pitching character. Many serious mistakes of interpretation have been made because of a failure to recognize this evidence of plunging folds. The way in which the strikes are progressively modified will be made clear by the diagrams of [Figs. 31] and [32], the first representing a pitching anticline and the second a pitching syncline. In both these reciprocal cases the strikes of the beds undergo the same changes, and the dip directions serve to distinguish which of the two structures is present in a given case. There is, however, one further difference in that the hard layers of the plunging anticline, where they disappear below the surface in the axis, will present a domed surface sloping forward like the back of a whale as it rises above the surface of the sea. Plunging folds in series will thus appear in the topography as a series of sharply zigzagging ranges at those localities where the harder layers intersect the surface. Such features are encountered in eastern Pennsylvania, where the hard formations of the Appalachian Mountain system plunge northeastward under the later formations. The pitch of the larger fold is often disclosed by that of the minor puckerings superimposed upon it.
Fig. 33.—Unconformity between a lower and an upper series of beds upon the coast of California. Note how the hard layer stands in relief upon the connecting surface (after Fairbanks).
The meaning of an unconformity.—The rock beds, which are deposited one above the other during a transgression of the sea, are usually parallel and thus represent a continuous process of deposition. Such beds are said to be conformable. Where, upon the other hand, two series of deposits which are not parallel to each other are separated by a break, they are said to form unconformable series, and the break or surface of junction is an unconformity ([Fig. 33]).
Here it is evident that the sediments which compose the lower series of beds have been folded in the zone of flow, though the upper series has evidently escaped this vicissitude. Furthermore, the surface which delimits the lower series from the upper is somewhat irregular and shows a hard layer standing in relief, as it would if it had opposed greater resistance to the attacks of the atmosphere upon it.
Fig. 34.—Series of diagrams to illustrate in succession the episodes involved in the historical development of an angular unconformity. The vertical arrows indicate direction of movement of the land, and the horizontal arrows the direction of shore migration.
In reality, an unconformity between formations must be interpreted to mean that the lower series is not only older than the upper, as shown by the order of superposition, but that the time of its deposition was separated from that of the upper by a hiatus in which important changes took place in the lower series. The stages or episodes in the history of the beds represented in [Fig. 33] may be read as follows (see [Fig. 34 a-e]):—
(a) Deposition of the lower series during a transgression of the sea.
(b) Continued subsidence and burial of the lower series beneath overlying sediments, and flexuring in the zone of flow.
(c) Elevation of the combined deposits to and far above sea level and removal by erosion of vast thicknesses of the upper sediments.
(d) A new subsidence of the truncated lower series and deposition of the upper series across its eroded surface.
(e) A new elevation of the double series to its present position above sea level.
Fig. 35.—Types of deceptive or erosional unconformities.
From this succession of episodes it is seen that a break of this kind between two series of deposits involves a double oscillation of subsidence followed by elevation—a large depression followed by a large elevation, a smaller subsidence followed by elevation. The time interval which must have been represented by these repeated operations is so vast as at first to stagger the mind in contemplating it. When, as in this instance, the dips of the lower series of beds differ from those of the upper, we have to do with an angular unconformity. It may be, however, that the lower series was not so far depressed as to enter the zone of flow, and that its beds meet those of the upper series with apparent conformity. Such an unconformity is often extremely difficult to recognize, and it is described as a deceptive or erosional unconformity.
With a deceptive unconformity the clew to its real nature is usually some fact which indicates that the lower series of sediments had been raised above the level of the sea before the upper series was deposited upon it. This may be apparent either in the irregularity of the surface on which the two series are joined, in some evidence of the action of waves such as would be furnished by a basal conglomerate in the upper series, or some indication of different resistance of different rocks of the lower series to attacks of the atmosphere upon them ([Figs. 33] and [35 a-c]).
In most cases, at least, the lowest member of the upper series will be a different type of rock from the uppermost member of the lower series, hence the frequent occurrence of the discordant cross bedding in sandstone should not deceive even the novice into the assumption of an unconformity.
Reading References to Chapter V
The zones of fracture and flow:—
C. R. Van Hise. Principles of North American Precambrian Geology, 16th Ann. Rept. U.S. Geol. Surv., 1895, Pt. I, pp. 581-603.
Bailey Willis. Mechanics of Appalachian Structure, 13th Ann. Rept. U.S. Geol. Surv., 1893, Pt. II, pp. 217-253.
A. Daubrée. Études Synthétiques de Géologie Expérimentale. Paris, 1879, pp. 306-328, pl. II.
W. Prinz. Quelques remarques générales à propos de l’essai de carte tectonique de la belgique, etc., Bull. Soc. Belge Geol., vol. 18, 1904, p. 143, pl. V.
Analysis of folds:—
Van Hise and Willis as above; de Margerie et Heim; Les dislocations de l’écorce terrestre (in French and German languages). Zurich, 1888.
Geological maps:—
Wm. H. Hobbs. The Mapping of the Crystalline Schists, Jour. Geol., vol. 10, 1902, pp. 780-792, 858-890.
CHAPTER VI
THE ARCHITECTURE OF THE FRACTURED SUPERSTRUCTURE
Fig. 36.—A set of master joints developed in shale upon the shores of Cayuga Lake near Ithaca, New York (after U. S. G. S.).
Fig. 37.—Diagram to show how sets of master joints differing in direction by half a right angle may abruptly replace each other.
Fig. 38.—Diagram to show the different combinations of the series composing two double sets of master joints, and in a, a, a additional disorderly fractures.
The system of the fractures.—In referring to experiments made upon the fracture of solid blocks under compression ([p. 41]), it was shown that two series of parallel fractures develop perpendicular to each free surface of the block, and that these series are each of them inclined by half of a right angle to the direction of compression, and thus perpendicular to each other. The fragments into which a block with one free surface would thus tend to be divided should be square prisms perpendicular to the free surface. It would be interesting, if it were practicable, to learn from experiment how these prisms would be further fractured by a continuation of the compression. From mechanical considerations involving the resolution of forces with reference to the ready-formed fractures, it seems probable that the next series of fractures to form would bisect the angles of the first double series or set. Wherever rocks are found exposed in their original attitudes, they are, in fact, seen to be intersected by two parallel series of fractures which are perpendicular to the earth’s surface and to each other and are described as joints. In many cases more than two series of such fractures are found, yet even in these cases two more perfectly developed series are prominent and almost exactly perpendicular to each other as well as to the earth’s surface. This omnipresent double series or set of joints is the well-known set of master joints, and very often it is found developed practically alone ([Fig. 36]). Over large areas, the direction of the set of master joints may remain practically constant, or this set may quite suddenly give place to a similar set which is, however, turned through half a right angle from the first ([Fig. 37]). Not infrequently two such sets of master joints are found together bisecting each other’s angles within the same rocks, and to them are sometimes added additional though less perfect series of joint planes.
Studied throughout a considerable district, the various series which make up these two sets of master joints may be seen locally developed in different combinations as well as in association with additional fissure planes which are not easily reduced to any simple law of arrangement ([Fig. 38 a, a, a]). Only rarely are regular joint series observed which do not stand perpendicular to the original attitude of the rock beds. In a few localities, however, rectangular joint sets have been discovered which divide the rock into prisms parallel to the earth’s surface and with the joint series inclined to it each by half a right angle. Where the rock beds have been much disturbed, the complex of joints may be such as to defy all attempts at orderly arrangement.
Fig. 39.—View on the shore at Holstensborg, West Greenland, to show the subequal spacing of the joints (after Kornerup).
Fig. 40.—View of an exposed hillside in Iceland upon which the snow collected in crannies along the joints brings out to advantage both the larger and the smaller intervals of the joint system (after Thoroddsen).
The space intervals of joints.—The same kind of subequal spacing which characterizes the fractures near the surface of the block in Daubrée’s experiment ([Fig. 19], [p. 41]) is found simulated by the rock joints ([Fig. 39]). Such unit intervals between fractures may be grouped together into larger units which are separated by fractures of unusual perfection. We may think of such larger space units as having the smaller ones superimposed upon them ([Fig. 40]).
The displacements upon joints—faults.—In the vast majority of cases, the joint fractures when carefully examined betray no evidence of any appreciable movement of the two walls upon each other. Generally the rock layers are seen to cross the joints without apparent displacement. Joints are therefore planes of disjunction only, and not planes of displacement.
Fig. 41.—Faulted blocks of basalt divided by joints near Woodbury, Connecticut. To show the structure of the rock, some of the foliage has been removed in preparing the sketch from a photograph.
Within many districts, however, a displacement may be seen to have occurred upon certain of the joint planes, and these are then described as faults. Such displacements of necessity imply a differential movement of sections or blocks of the earth’s crust, the so-called orographic blocks, which are bounded by the joint planes and play individual rôles in the movement. A simple case of such displacements in rocks intersected by a single set of master joints is represented in the model of plate 4 C. The most prominent fault represented by this model runs lengthwise through the middle, and the displacement which is measured upon it not only varies between wide limits, but is marked by abrupt changes at the margins of the larger blocks. This vertical displacement upon the fault is called its throw. Though not illustrated by the model, horizontal displacements may likewise occur, and these will be more fully discussed when the subject of earthquakes is considered in the following chapter. An actual example of blocks displaced by vertical adjustment is represented in [Fig. 41], a simple type of faulting which has taken place in rocks but slightly disturbed from their original attitude, but intersected by a relatively simple system of master joints. In those regions where the beds have been folded and perhaps overthrust before their elevation into the zone of fracture, and which are further intersected by disorderly fissure planes, the results are far more complex. In such cases the planes of individual displacement may not be vertical, though they are generally steeper than 45°. For their description it is necessary to make use of additional technical terms ([Fig. 42]). The inclination of a sloping fault plane measured against the vertical is called the hade of the fault. The total displacement is measured along the plane of the fault from a point upon one limb to the point from which it was separated in the other. The additional terms are made sufficiently clear by the diagram.
Fig. 42.—A fault in previously disturbed strata. AB, displacement; AC, throw; BD, stratigraphic throw; BC, heave; angle CAB, hade.
Methods of detecting faults.—The first effect of a fault is usually to produce a crack at the surface of the earth; and, provided there is a vertical displacement or throw, an escarpment which rises upon the upthrown side of the fault. In general it may be said that escarpments which appear at the earth’s surface as plane surfaces probably represent planes of fracture, though not necessarily planes of faulting. In many cases the actual displacements lie buried under loose rock débris near to and paralleling the escarpment, and in some cases as a result of the erosional processes working upon alternately hard and soft layers of rock, the escarpment may later appear upon the downthrown side or limb of the fault ([Fig. 43]). As an illustration of a fault escarpment, the façade of El Capitan and many other rock faces of the Yosemite valley may be instanced.
Fig. 43.—Diagrams to show how an escarpment originally on the upthrown side of the fault may, through erosion, appear upon the downthrown side.
Fig. 44.—A fault plane exhibiting “drag.” The opening is artificial (after Scott).
When we have further studied the erosional processes at the earth’s surface, it will be appreciated that faults tend to quickly bury themselves from sight, whereas fold structures will long remain in evidence. Many faults will thus be overlooked, and too great weight is likely to be ascribed to the folds in accounting for the existing attitudes and positions of the rock masses. Faults must therefore be sought out if mistakes of interpretation are to be avoided.
The most satisfactory evidence of a fault is the discovery of a rock bed which may be easily identified, and which is actually seen displaced on a plane of fracture which intersects it ([Fig. 42], [p. 59]). When such an easily recognizable layer is not to be found, the plane of displacement may perhaps be discovered as a narrow zone composed of angular fragments of the rock cemented together by minerals which form out of solution in water. Such a fractured rock zone which follows a plane of faulting is a fault breccia. If the fault breccia, or vein rock, is much stronger than the rock on either side, it may eventually stand in relief at the surface like a dike or wall. At other times the displacement produces little fracture of the walls, but they slide over each other in such a manner as to yield either a smoothly corrugated or an evenly polished surface which is described as “slickensides.” It may be, however, that during the movement either one or both of the walls have “dragged”, and so are curled back in the immediate neighborhood of the fault plane ([Fig. 44]).
When, as is quite generally the case, the actual plane of displacement of a fault is not open to inspection, the movement may be proven by the observation of abrupt, as contrasted with gradual, changes in the strikes and dips of neighboring exposures ([Fig. 45]); or by noting that some easily recognized formation has been sharply offset in its outcrops ([Fig. 46]).
Fig. 45.—Map to show how a fault may be indicated in abrupt changes of the strike and dip of neighboring exposures.
Fig. 46.—A series of parallel faults indicated by successive offsets in the course of an easily recognizable rock formation.
There are in addition many indications rather than proofs of the presence of faults, which must be taken account of in every general study of the geology of a district. Thus the outcrops of all neighboring formations may terminate abruptly upon a straight line which intersects all alike. Deep-seated fissure springs may be aligned in a striking manner, and so indicate the course of a prominent fracture, though not necessarily of a fault. Much the same may be said of the dikes of cooled magma which have been injected along preëxisting fractures.
The base of the geological map.—Modern topographic maps form an important part of the library of the serious student of physiography; they are the gazetteer of this branch of science. Every civilized nation has to-day either completed a topographic atlas of its territory, or it is vigorously prosecuting a survey to furnish maps which represent the relief with some detail, and publishing the results in the form of an atlas of quadrangles. Thus a relief map will erelong be obtainable of any part of the civilized world, and may be purchased in separate sections. Nowhere is this work being taken up with greater vigor than in the United States, where a vast domain representing every type of topographic peculiarity is being attacked from many centers. Here and elsewhere the relief of the land is being expressed by so-called contours or lines of equal altitude upon the earth’s surface. It is as though a series of horizontal planes, separated by uniform intervals of 20 or 40 or 100 feet, had been made to intersect the surface, and the intersection curves, after consecutive numeration, had been dropped into a single plane for printing.
Where the slopes are steep, the contour lines in the topographic map will appear crowded together and so produce a deep shade upon the map; whereas with relatively flat surfaces white patches will stand out prominently upon the map. More and more the topographic map is coming into use, and for the student of nature in particular it is important to acquire facility in interpreting the relief from the topographic map. To further this end, a special model has been devised, and its use is described in appendix C. Usually before any satisfactory geological map can be prepared, a contoured topographic map of the district to be studied must be available.
The field map and the areal geological map.—As the atlas of topographic maps is the physiographic gazetteer, so geological maps together constitute the reference dictionary of descriptive geology. Not only are topographic maps of many districts now generally available, but more and more it has become the policy of governments to supply geological maps in the same quadrangle form which is the unit of the topographic map. The geological map is, however, a complex of so many conventional symbols, that without some practical experience in the actual preparation of one, it is exceedingly difficult for the student to comprehend its significance. A modern geological map is usually a rectangular sheet printed in color, upon which are many irregular areas of individual hue joined to each other like the parts of a child’s picture puzzle.
The colored areas upon the geological map are each supposed to indicate where a certain rock type or formation lies immediately below the surface, and this distribution represents the best judgment of the geologist who, after a study of the district, has prepared the map. Unfortunately the conventions in use are such that his observation and his theory have been hopelessly intermingled in the finished product. Armed with the geological map, the student who visits the district finds spread out before him, it may be, a landscape of hill and valley, of green forest and brown farming land, which is as different as may be from the colored puzzle which he holds in his hand. Hidden under the farm vegetation or masked by the woods are scattered outcroppings of rock which have been the basis of the geologist’s judgment in preparing the map. Experience shows that in order to bridge the wide gap between the geology in the landscape and the patches of color upon the map something more than mere examination of the colored sheet is necessary. We shall therefore describe, with the aid of laboratory models, the various stages necessary to the preparation of a geological map, and every student should be advised to follow this by practical study of some small area where rocks are found in outcrop.
Though the published areal geological map represents both fact and theory, the map maker retains an unpublished field map or map of observations, upon which the final map has been based. This field map shows the location of each outcrop that has been studied, with a record of the kind of rock and of such observations as strike, dip, and pitch. Our task will therefore be to prepare: (1) a field map; (2) an areal geological map; and (3) some typical geological sections.
Laboratory models for the study of geological maps.—In order to represent in the laboratory the disposition of rock outcrops in the field, special laboratory tables are prepared with removable covers and with fixed tops, which are divided into squares numbered like the township sections of the national domain ([Fig. 47]). To represent the rock outcrops, blocks are prepared which may be fixed in any desired position by fitting a pin into a small augur hole bored through the table. The outcrop blocks for the sedimentary rock types are so constructed as to show the strike and dip of the beds. (See [Appendix D].)
Fig. 47.—Field map prepared from a laboratory table.
The method of preparing the map.—To prepare the map, use is made of a geological compass with clinometer attachment, a protractor, and a map base divided into sections like the top of the table, and on the scale of one inch to the foot. Each exposure represented upon the table is “visited” and then located upon the base map in its proper position and attitude. The result is the field map ([Fig. 47]), which thus represents the facts only, unless there have been uncertainties in the correlation of exposures or in determining the position of the bedding plane.
Fig. 48.—Areal geological map constructed from the field map of [Fig. 47], with two selected geological sections.
To prepare the areal geological map from the field map, it is first necessary to fix the boundaries which separate formations at the surface; and now perhaps for the first time it is realized how large an element of uncertainty may enter if the exposures were widely separated. It is clear that no two persons will draw these lines in the same positions throughout, though certain portions of them—where the facts are more nearly adequate—may correspond. In [Fig. 48] is represented the areal geological map constructed from the field map, with the doubtful area at one side left blank.
Some conclusions from this map may now be profitably considered. The complexly folded sandstone formation at the left of the map appears as the oldest member represented, since its area has been cut through by the intrusive granite which does not intrude other formations, and is unconformably overlaid by the limestone and its basal layer of conglomerate. The limestone in turn is unconformably overlaid by the merely tilted sandstone beds at the right of the map. These three sedimentary formations clearly represent decreasing amounts of close folding, from which it is clear that each earlier formation has passed through an episode not shared by that of next younger age. Of the other intrusive rocks, the dike of porphyry is younger than all the other formations, with the possible exception of the upper sandstone. Offsetting of the formations has disclosed the course of a fault, and from its relations to the dikes we may learn that of these the porphyry is younger and the basalt older than the date of the faulting.
The dashed lines upon the map (AB and CD) have been selected as appropriate lines along which to construct geological sections ([Fig. 48], below map), and from these sections the exposed thicknesses of the different formations may be calculated. In one instance only, that of the conglomerate, can we be sure that this exposed thickness measures the entire formation.
Fold versus fault topography.—The more resistant or “stronger” rock beds, as regards attacks of the atmosphere, in the course of time come to stand in relief, separated by depressions which overlie the “weaker” formations. Simple open folds which are not plunging exercise an influence upon topography by producing generally long and straight ridges. More complex flexures, since they generally plunge, make themselves apparent by features which in the map are represented by curves. Fracture structures, and especially block displacements, are differentiated from these curving features by the dominance of straight or nearly rectilinear lines upon the map. The effect of erosion is to reduce the asperity of features and to mold them with flowing curves. The fracture structures are for this reason much more likely to be overlooked, and if they are not to elude the observer, they must be sought out with care. Fold and fracture structures may both be revealed upon the same map.
Reading References to Chapter VI
Joint systems:—
John Phillips. Observations made in the Neighborhood of Ferrybridge in the Years 1826-1828, Phil. Mag., 2d ser., vol. 4, 1828, pp. 401-409; Illustrations of the geology of Yorkshire, Pt. II, The Limestone District. London, 1836, pp. 90-98.
Samuel Haughton. On the Physical Structure of the Old Red Sandstone of the County of Waterford, considered with reference to cleavage, joint surfaces, and faults, Trans. Roy. Soc. London, vol. 148, 1858, pp. 333-348.
W. C. Brögger. Spaltenverwerfungen in der Gegend Langesund-Skien, Nyt Magazin for Naturvidernskaberne, vol. 28, 1884, pp. 253-419.
Wm. H. Hobbs. The Newark System of the Pomperaug Valley, Connecticut, 21st Ann. Rept. U. S. Geol. Surv., Pt. III, 1901, pp. 85-143.
Geological map:—
Wm. H. Hobbs. The Interpretation of Geological Maps, School Science and Mathematics, vol. 9, 1909, pp. 644-653.
CHAPTER VII
THE INTERRUPTED CHARACTER OF EARTH MOVEMENTS: EARTHQUAKES AND SEAQUAKES
Nature of earthquake shocks.—Man’s belief in the stability of Mother Earth—the terra firma—is so inbred in his nature that even a light shock of earthquake brings a rude awakening. The terror which it inspires is no doubt largely to be explained by this disillusionment from the most fundamental of his beliefs. Were he better advised, the long periods of quiet which separate earthquakes, and not the lighter shocks which follow all grander disturbances, would occasion him concern.
Fig. 49.—View of a portion of the ruins of Messina after the earthquake of December 28, 1908.
Earthquakes are the sensible manifestations of changes in level or of lateral adjustments of portions of the continents, and the seismic disturbances upon the sea—seaquakes and seismic sea waves—relate to similar changes upon the floor of the ocean.
During the grander or catastrophic earthquakes, the changes are indeed terrifying, and have usually been accompanied by losses to life and property, which are only to be compared with those of great conflagrations or of inundations on thickly populated plains. The conflagration has all too frequently been an aftermath of the great historic earthquakes. The earthquake of December 28, 1908, in southern Italy, destroyed almost the entire population of a great city, and left of its massive buildings only a confused heap of rubble ([Fig. 49]). Two years later a heavy earthquake resulted in great damage to cities in Costa Rica ([Fig. 50]), while two years earlier our own country was first really awakened to the danger in which it stands from these convulsive earth throes; though, as we shall see, these dangers can be largely met through proper methods of construction.
Fig. 50.—Ruins of the Carnegie Palace of Peace at Cartago, Costa Rica, destroyed when almost completed by the great earthquake of May 4, 1910 (after a photograph by Rear-Admiral Singer, U.S.N.).
Earthquakes are usually preceded for a brief instant by subterranean rumblings whose intensity appears to bear no relation to the shocks which follow. The ground then rocks in wavelike motions, which, if of large amplitude, may induce nausea, prevent animals from keeping upon their feet, and wreck all structures not specially adapted to withstand them. Heavy bodies are sometimes thrown up from the ground ([Fig. 51]), and at other times similar heavy masses are, apparently because of their inertia, more deeply imbedded in the earth. Thus gravestones and heavy stone posts are often sunk more deeply in the ground and are surrounded by a hollow and perhaps by small open cracks in the surface ([Fig. 52]). When bodies are thrown upward, it would imply that a quick upward movement of the ground had been suddenly arrested, while the burial of heavy bodies in the earth is probably due to a movement which begins suddenly and is less abruptly terminated.
Fig. 51.—Bowlders thrown into the air and overturned during the Assam earthquake of 1897 (after R. D. Oldham).
Fig. 52.—Heavy post sunk deeper into the ground during the Charleston earthquake of August 31, 1886 (after Dutton).
Seaquakes and seismic sea waves.—Upon the ocean the quakes which emanate from the sea floor are felt on shipboard as sudden joltings which produce the impression that the ship has struck upon a shoal, though in most instances there is no visible commotion in the water. The distribution of these shocks, as indicated either by the experiences of neighboring ships at the time of a particular shock, or by the records of vessels which at different times have sailed over an area of frequent seismic disturbance, appears to be limited to narrow zones or lines ([Fig. 53]). The same tendency of under-sea disturbances to be localized upon definite straight lines has been often illustrated by the behavior of deep-sea cables which are laid in proximity to one another and which have been known to part simultaneously at points ranged upon a straight line.
Fig. 53.—Map showing the localities at which shocks have been reported at sea off Cape Mendocino, California.
Far grander disturbances upon the floor of the ocean have been revealed by the great sea waves—the so-called “tidal waves”, properly referred to as tsunamis—which recur in those sea districts which adjoin the special earthquake zones upon the continents (p. 86). The forerunner of such a sea wave approaching the shore is usually a sudden withdrawal of the water so as to lay bare a portion of the bottom, but this is well-recognized to be the premonition of a gigantic oncoming wave which sweeps all before it and is only halted when it has rolled over all the low-lying country and encountered a mountain wall. Such seismic waves have been especially common upon the Pacific shore of South America and upon the Japanese littoral ([Fig. 54]). These waves proceed from above the great deeps upon the ocean bottom, and clearly result from the grander earth movements to which these depressions owe their exceptional depth. The withdrawal of the water from neighboring shores may be presumed to be connected with a descent of the floor of the depression and the consequent drawing-in of the ocean surface above. The later high wave would thus represent the dispersion of the mountain of water which is raised by the meeting of the waters from the different sides of the depression.
Fig. 54.—Effect of a seismic water wave at Kamaishi, Japan, in 1896 (after E. R. Scidmore).
Fig. 55.—A fault of vertical displacement.
The grander and the lesser earth movements.—Upon the land the grander and so-called catastrophic earthquakes are usually the accompaniment of important changes in the surface of the ground that will be discussed in later sections. Those shocks which do little damage to structures produce no visible changes in the earth’s surface, except, it may be, to shake down some water-soaked masses of earth upon the steeper slopes. Still other movements, and these too slight to be felt even in the night when the animal world is at rest, may yet be distinguished by their sounds, the unmistakable rumblings which are characteristic alike of the heaviest and the lightest of earthquake shocks.
Fig. 56.—Escarpment produced by an earthquake fault of vertical displacement which cut across the Chedrang River and thus produced a waterfall, Assam earthquake of 1897 (after R. D. Oldham).
Changes in the earth’s surface during earthquakes—faults and fissures.—Each of the grander among historic earthquakes has been accompanied by noteworthy changes in the configuration of the earth’s surface within the district where the shocks were most intense. A section of the ground is usually found to have moved with reference to another upon the other side of a vertical plane which is usually to be seen; we have here to do with the actual making of a fault or displacement such as we find the fossil examples of within the rocks. The displacement, or throw, upon the fault plane may be either upward or downward or laterally in one direction or the other, or these movements may be combined. A movement of adjacent sections of the ground upward or downward with reference to each other ([Fig. 55]) has been often observed, notably at Midori after the great Japanese earthquake of 1891, and in the Chedrang valley of Assam after the earthquake of 1897 ([Fig. 56]).
Fig. 57.—A fault of lateral displacement.
Fig. 58.—Fence parted and displaced fifteen feet by a transverse fault formed during the California earthquake of 1906 (after W. B. Scott).
—————
Fig. 59.—Fault with vertical and lateral displacements combined.
A lateral throw, unaccompanied by appreciable vertical displacement ([Fig. 57]), is especially well illustrated by the fault in California which was formed during the earthquake of 1906 ([Fig. 58]). A combination of the two types of displacement in one ([Fig. 59]) is exemplified by the Baishiko fault of Formosa at the place shown in plate 3 A.
The measure of displacement.—To afford some measure of the displacements which have been observed upon earthquake faults, it may be stated that the maximum vertical throw measured upon the fault in the Neo valley of Japan (1891) was 18 feet, in the Chedrang valley of Assam (1897) 35 feet, and of the Alaskan coast (1899) 47 feet. Large sections of land were bodily uplifted in these cases within the space of a few seconds, or at most a few minutes, by the amounts given. The largest recorded lateral displacement measured upon an earthquake fault is about 21 feet upon the California rift after the earthquake of 1906; though an amount only slightly less than this is indicated in the shifting of roads and arroyas dating from the earthquake of 1872 in the Owens valley, California. Fault lines once established are planes of special weakness and become later the seat of repeated movements of the same kind.
Plate 3.
A. An earthquake fault opened in Formosa in 1906, with vertical and lateral displacements combined (after Omori).
B. Earthquake faults opened in Alaska in 1889, on which vertical slices of the earth’s shell have undergone individual adjustments (after Tarr and Martin).
Fig. 60.—Diagram to show how small faults in the rock basement may be masked at the surface through adjustments within the loose rock mantle.
The greater number of earthquake faults are found in the loose rock cover which so generally mantles the firmer rock basement, and it is almost certain that the throws within the solid rock are considerably larger than those which are here measured at the surface, owing to the adjustments which so readily take place in the looser materials. Those lighter shocks of earthquake which are accompanied by no visible displacements at the surface do, however, in some instances affect in a measure the flow of water upon the surface, and thus indicate that small changes of surface level have occurred without breaks sufficiently sharp to be perceived ([Fig. 60]). Intermediate between the steep escarpment and the masked displacement just described is the so-called “mole-hill” effect,—a rounded and variously cracked slope or ridge above the position of a buried fault ([Fig. 61]).
Fig. 61.—Diagram to show the appearance of a “mole hill” above a buried earthquake fault (after Kotô).
The escarpments due to earthquake faults in loose materials at the earth’s surface can obviously retain their steepness for a few years or decades at the most; for because of their verticality they must gradually disappear in rounded slopes under the action of the elements. Smaller displacements within a rock which rapidly disintegrates under the action of frost and sun will likewise before long be effaced. In those exceptional instances where a resistant rock type has had all altered upper layers planed away until a fresh and hard surface is exposed, and has further been protected from the frost and sun beneath a thin layer of soil, its original surface may be retained unaltered for many centuries. Upon such a surface the lightest of sensible shocks, or even the smaller earth movements which are not perceived at the time, may leave an almost indelible record. Such records particularly show that the movements which they register occur upon the planes of jointing within the rock, and that these ready formed cracks have probably been the seats of repeated and cumulative adjustments ([Fig. 62]).
Fig. 62.—Post-glacial earthquake faults of small but cumulative displacement, eastern New York (after Woodworth).
Fig. 63.—Earthquake cracks in Colorado desert (after a photograph by Sauerven).
————
Contraction of the earth’s surface during earthquakes.—The wide variations in the amount of the lateral displacement upon earthquake faults, like those opened in California in 1906, show that at the time of a heavy earthquake there must be large local changes in the density of the surface materials. Literally, thousands of fissures may appear in the lowlands, many of them no doubt a secondary effect of the shaking, but others, like the quebradas of the southern Andes or the “earthquake cracks” in the Colorado desert ([Fig. 63]), may have a deeper-seated origin. Many facts go to show, however, that though local expansion does occur in some localities, a surface contraction is a far more general consequence of earth movement. In civilized countries of high industrial development, where lines of metal of one kind or another run for long distances beneath or upon the surface of the ground, such general contraction of the surface may be easily proven. Comparatively seldom are lines of metal pulled apart in such a way as to show an expansion of the surface; whereas bucklings and kinkings of the lines appear in many places to prove that the area within which they are found has, as a whole, been reduced.
Fig. 64.—Diagrams to show how railway tracks are either broken or buckled locally within the district visited by an earthquake.
Fig. 65.—The Biwajima railroad bridge in Japan after the earthquake of 1891 (after Milne and Burton).
Fig. 66.—Diagrams to show how the compression of a district and its consequent contraction during an earthquake may close up the joint spaces within the rock basement and concentrate the contraction of the overlying mantle where this is partially cut through and so weakened in the valley sections.
Water pipes laid in the ground at a depth of some feet may be bowed up into an arch which appears above the surface; lines of curbing are raised into broken arches, and the tracks of railways are thrown into local loops and kinks which imply a very considerable local contraction of the surface ([Fig. 64]). With unvarying regularity railway or other bridges which cross rivers or ravines, if the structures are seriously damaged, indicate that the river banks have drawn nearer together at the time of the disturbance. In such cases, whenever the bridge girder has remained in place upon its abutments, these have either been broken or back-tilted as a whole in such a manner as to indicate an approach of the foundations which was prevented at the top by the stiffness of the girder ([Fig. 65]).
Fig. 67.—Map of the Chedrang fault which made its appearance during the Assam earthquake of 1897. The figures give the amounts of the local vertical displacement measured in feet (after R. D. Oldham).
The simplest explanation of such an approach of the banks at the sides of the valleys cut in loose surface material is to be found in a general closing up of the joint spaces within the underlying rock, and an adjustment of the mantle upon the floor mainly in the valley sections ([Fig. 66]).
Fig. 68.—Map giving the displacements in feet measured along an earthquake fault formed in Alaska in 1899 (after Tarr and Martin).
The plan of an earthquake fault.—In our consideration of earthquake faults we have thus far given our attention to the displacement as viewed at a single locality only. Such displacements are, however, continued for many miles, and sometimes for hundreds of miles; and when now we examine a map or plan of such a line of faulting, new facts of large significance make their appearance. This may be well illustrated by a study of the plan of the Chedrang fault which appeared at the time of the Assam earthquake of 1897 ([Fig. 67]). From this map it will be noticed that the upward or downward displacement upon the perpendicular plane of the fault is not uniform, but is subject to large and sudden changes. Thus in order the measurements in feet are 32, 0, 18, 35, 0, 8, 25, 12, 8, 2, 0. The fault formed in 1899 upon the shores of Russell Fjord in Alaska ([Fig. 68]) reveals similar sudden changes of throw, only that here the direction of the movement is often reversed; or, otherwise expressed, the upthrow is suddenly transferred from one side of the fault to the other. Such abrupt changes in the direction of the displacement have been observed upon many earthquake faults, and a particularly striking one is represented in [Fig. 69].
Fig. 69.—Abrupt change in the direction of throw upon an earthquake fault which was formed in the Owens valley, California, in 1872. The observer looks directly along the course of the fault from the left foreground to the cliff beyond and to the left of the impounded water (after a photograph by W. D. Johnson).
The block movements of the disturbed district.—The displacements upon earthquake faults are thus seen to be subdivided into sections, each of which differs from its neighbors upon either side and is sharply separated from them, at least in many instances. These points of abrupt change of displacement are, in many cases at least, the intersection points with transverse faults ([Fig. 69]). Such points of abrupt change in the degree or in the direction of the displacement may be, when looked at from above, abrupt turning points in the direction of extension of the fault, whose course upon the map appears as a zigzag line made up of straight sections connected by sharp elbows ([Fig. 70]).
Fig. 70.—Map of the faults within an area of the Owens valley, California, formed in part during the earthquake of 1872, and in part due to early disturbances, In the western portions the displacements cut across firm rock and alluvial deposits alike without deviation of direction (after a map by W. D. Johnson).
Such a grouping of surface faults as are represented upon the map is evidence that the area of the earth’s shell, which is included, has at the time of the earthquake been subject to adjustments as a series of separate units or blocks, certain of the boundaries of which are the fault lines represented. The changes in displacement measured upon the larger faults make it clear that the observed faults can represent but a fraction of the total number of lines of displacement, the others being masked by variations in the compactness of the loose mantling deposits. Could we but have this mantle removed, we should doubtless find a rock floor separated into parts like an ancient Pompeiian pavement, the individual blocks in which have been thrown, some upward and some downward, by varying amounts. Less than a hundred miles away to the eastward from the Owens Valley, a portion of this pavement has been uncovered in the extensive operations of the Tonapah Mining District, so that there we may study in all its detail the elaborate pattern of earth marquetry ([Fig. 71]) which for the floor of the Owens valley is as yet denied us.
Fig. 71.—Marquetry of the rock floor of the Tonapah Mining District, Nevada (after Spurr).
Fig. 72.—Map of a portion of the Alaskan coast to show the adjustments in level during the earthquake of 1899 (after Tarr and Martin).
The earth blocks adjusted during the Alaskan earthquake of 1899.—For a study of the adjustments which take place between neighboring earth blocks during a great earthquake, the recent Alaskan disturbance has offered the advantage that the most affected district was upon the seacoast, where changes of level could be referred to the datum of the sea’s surface. Here a great island and large sections of the neighboring shore underwent movements both as a whole in large blocks and in adjustments of their subordinate parts among themselves ([Fig. 72]). Some sections of the coast were here elevated by as much as 47 feet, while neighboring sections were uplifted by smaller amounts ([Fig. 73]), and certain smaller sections were even dropped below the level of the sea.
Fig. 73.—View on Haencke Island, Disenchantment Bay, Alaska, revealing the shore that rose seventeen feet above the sea during the earthquake of 1899, and was found with barnacles still clinging to the rock (after Tarr and Martin).
The amount of such subsidence is, however, difficult to ascertain, for the reason that the former shore features are now covered with water and thus removed from observation. In favorable localities the minimum amount of submergence may sometimes be measured upon forest trees which are now flooded with sea water. In [Fig. 74] a portion of the coast is represented where the beach sand is now extended back into the spruce forest, a distance of a hundred feet or more, and where sedgy beach grass is growing among trees whose roots are now laved in salt water. At the front of this forest the great storm waves overturn the trees and pile the wreckage in front of those that still remain standing.
Fig. 74.—Partially submerged forest upon the shore of Knight Island, Alaska, due to the sinking of a section of the coast during the earthquake of 1899 (after Tarr and Martin).
Fig. 75.—Settlement of a section of the shore at Port Royal, Jamaica, during the earthquake of January 14, 1907, adjacent to a similar but larger settlement of the near shore during the earthquake of 1692 (after a photograph by Brown).
————
Upon the glaciated rock surfaces of the Alaskan coast, exceptionally favorable opportunities are found for study of the intricate pattern of the earth mosaic which is under adjustment at the time of an earthquake. Upon Gannett Nunatak the surface was found divided by parallel faults into distinct slices which individually underwent small changes of level ([plate 3 B]).
CHAPTER VIII
THE INTERRUPTED CHARACTER OF EARTH MOVEMENTS: EARTHQUAKES AND SEAQUAKES (Concluded)
Experimental demonstration of earth movements.—The study of the Alaskan earthquake of 1899 showed that during this adjustment within the earth’s shell some of the local blocks moved upward and by larger amounts than their neighbors, and that still others were actually depressed so that the sea flowed over them. It must be evident that such differential vertical movements of neighboring blocks at the earth’s surface can only take place if lateral transfers of material are made beneath it. From under those strips of coast land which were depressed, material must have been moved so as to fill the void which would otherwise have formed beneath the sections that were uplifted. If we take into consideration much larger fractions upon the surface of our planet, we are taught by the great seaquakes which are now registered upon earthquake instruments at distant stations that large downward movements are to-day in progress beneath the sea much more than sufficient to compensate all extensions of the earth’s surface within those districts where the land is rising in mountains. From under the offshore deeps of the ocean to beneath the growing mountains upon the shore, a transfer of earth material must be assumed to take place when disturbances are registered.
Within the time interval that separates the sudden adjustments of the surface which are manifested in earthquakes, the condition of strain which brings them about is steadily accumulating, due, as we generally assume, to earth contraction through loss of its heat. It seems probable that the resistance to an immediate adjustment is found in the rigidity of the shell because of the compression to which it is subjected. To illustrate: a row of blocks well fitted to each other may be held firmly as a bridge between the jaws of a vice, because so soon as each block starts to fall a large resistance from friction upon its surface is called into existence, a force which increases with the degree of compression.
It is thus possible upon this assumption crudely to demonstrate the adjustment of earth blocks by the simple device represented in [plate 4 A]. The construction of this experimental tank is so simple that little explanation is necessary. Wooden blocks of different heights are supported in water within a tank having a glass front, and are kept in a strained condition at other than their natural positions of flotation by the compression of a simple vice at the top. Held firmly in this position, they may thus represent the neighboring blocks within the earth’s outer shell which are supported upon relatively yielding materials beneath, and prevented from at once adjusting themselves to their natural positions through the compression to which they are subjected. Held as they now are, the water near the ends of the tank is forced up beneath the blocks to higher than its natural level, and thus tends to flow from both ends toward the center. Such a movement would permit the end blocks to drop and force the middle ones to rise. The end blocks are, let us say, the sections of Alaskan coast line which sunk during the earthquake, as the center blocks are the sections which rose the full measure of 47 feet. Upon a larger scale the end blocks may equally well be considered as the floor of the great deeps off the Alaskan coast, whose sinking at the time of the earthquake was the cause of the great sea wave. Upon this assumption the center blocks would represent the Alaskan coast regarded as a whole, which underwent a general uplift.
Though we may not, in our experiment, vary the tendency to adjustment by any contractional changes in either the water or the blocks, we may reduce the compression of the vice, which leads to the same general result. As the compression of the vice is slowly relaxed, a point is at last reached at which friction upon the block surfaces is no longer sufficient to prevent an adjustment taking place, and this now suddenly occurs with the result shown in [plate 4 B]. In the case of the earth blocks, this sudden adjustment is accompanied by mass movements of the ground separated by faults, and these movements produce successional vibrations that are particularly large near the block margins, and other frictional vibrations of such small measure as to be generally appreciated by sounds only. The jolt of the adjustments has thrown some blocks beyond their natural position of rest, and these sink and rise subsequently in order to readjust themselves with lighter vibrations, which may be repeated and continued for some time. In the case of the earth these later adjustments are the so-called aftershocks which usually continue throughout a considerable period following every great earthquake. Gradually they fall off in intensity and frequency until they can no longer be felt, and are thereafter continued for a time as rumblings only.
Plate 4.
A. Experimental tank to illustrate the earth movements which are manifested in earthquakes. The sections of the earth’s shell are here represented before adjustment has taken place.
B. The same apparatus after a sudden adjustment.
C. Model to illustrate a block displacement in rocks which are intersected by master joints.
Derangement of water flow by earth movement.—The water which supported the blocks in our experiment has represented the more mobile portion of the earth’s substance beneath its outer zone of fracture. The surface water layers in the tank may, however, be considered in a different way, since their behavior is remarkably like that of the water within and upon the earth’s surface during an earth adjustment. At the instant when adjustment takes place in the tank, water frequently spurts upward from the cracks between the sinking end blocks; and if in place of one of the higher center blocks we insert one whose top is below the level of the water in the tank, a “lake” will be formed above it. When the adjustment occurs, this lake is immediately drained by outflow of the water at its bottom along one of the cracks between the blocks ([Fig. 76]).
Fig. 76.—Diagrams to illustrate the draining of lakes during earthquakes.
Such derangements of water flow as have been illustrated by the experiment are among the commonest of the phenomena which accompany earthquakes. Lakes and swamp lands have during earthquakes been suddenly drained, fountains of water have been seen to shoot up from the surface and have played for some minutes or hours before their sudden disappearance in a sucking down of the water with later readjustment. During the great earthquake of the lower Mississippi valley in 1811, known as the New Madrid earthquake, the earlier Lake Eulalie was completely drained, and upon the now exposed bed there appeared parallel fissures on which were ranged funnel-like openings down which the water had been sucked. In other sections of the affected region the water shot up in sheets along fissures to the tops of high trees. Areas where such spurting up of the water has been observed have in most cases been shown to correspond to areas of depression, and such areas have sometimes been left flooded with water. During the Indian earthquake of 1819 an area of some 200 square miles suddenly sank and was transformed into a lake.
Fig. 77.—Diagram to illustrate-the derangements of flow of water at the time of an earthquake; water issuing at the surface over downthrown rocks, and being sucked down in upthrown blocks.
Fig. 78.—Mud cones aligned upon a fissure opened at Moraza, Servia, during the earthquake of April 4, 1904 (after Michailovitch).
Sand or mud cones and craterlets.—From a very moderate depth below the surface to that of several miles, all pore spaces and all larger openings within the rock are completely filled with water, the “trunk lines” of whose circulation is by way of the joints or along the bedding planes of the rocks. The principal reservoirs, so to speak, of this water inclosed within the rock are the porous sand formations. When, now, during an earthquake a block of the earth’s shell is suddenly sunk and as suddenly arrested in its downward movement, the effect is to compress the porous layers and so force the contained water upward along the joints to the surface, carrying with it large quantities of the sand ([Fig. 77]).
Fig. 79.—One of the many craterlets formed near Charleston, South Carolina, during the earthquake of August 31, 1886. The opening is twenty feet across, and the leaves about it are encased in sand as were those upon the branches of the overhanging trees to a height of some twenty feet (after Dutton).
Fig. 80.—Cross section of a craterlet to show the trumpet-like form of the sand column.
Ejected at the surface this water appears in fountains usually arranged in line over joints, or even in continuous sheets, and the sand collecting about the jets builds up lines of sand or mud cones sometimes described as “mud volcanoes” ([Fig. 78]). The amount of sand thus poured out is sometimes so great that blankets of quicksand are spread over large sections of the country. Most frequently, however, the sand is not built above the general level of the surface, but forms a series of craterlets which are largely shaped as the water is sucked down at the time of the readjustment with which the play of such earthquake fountains is terminated ([Fig. 79]). Subsequent excavations made about such craterlets have shown them to have the form of a trumpet, and that in the sand which so largely fills them there are generally found scales of mica and such light bodies as would be picked out from the heterogeneous materials of the sand layers and carried upward in the rush of water to the surface ([Fig. 80]).
The earth’s zones of heavy earthquake.—Since earthquakes give notice of a change of level of the ground, the special danger zones from this source are the growing mountain systems which are usually found near the borders of the sea. Such lines of mountains are to-day rising where for long periods in the past were the basins of deposition of former seas. They thus represent the zones upon the earth’s surface which are the most unstable—which in the recent period have undergone the greatest changes of level.
Fig. 81.—Map of the island of Ischia to show how the shocks of recent earthquakes have been concentrated at the crossing point of two fractures (after Mercalli and Johnston-Lavis).
By far the most unstable belt upon the earth’s surface is the rim surrounding the Pacific Ocean, within which margin it has been estimated that about 54 per cent of the recorded shocks of earthquake have occurred. Next in importance for seismic instability is the zone which borders both the Mediterranean Sea and the Caribbean—the American Mediterranean—and is extended across central Asia through the Himalayas into Malaysia. Both zones approximate to great circles upon the earth’s surface and intersect each other at an angle of about 67°. It has been estimated that about 95 per cent of the recorded continental earthquakes have emanated from these belts.
Fig. 82.—A line of earth fracture indicated in the plan of the relief, which may at any time become the seat of movement and resultant shock.
The special lines of heavy shock.—Within any earthquake district the shocks are not felt with equal severity at all places, but there are, on the contrary, definite lines which the disturbance seems to search out for special damage. From their relations to the relief of the land these lines would appear to be lines of fracture upon the boundaries of those sections of the crust that play individual rôles in the block adjustment which takes place. More or less masked as these lines are beneath the rounded curves of the landscape, they are given an altogether unenviable prominence with each succeeding earthquake. At such times we may think of the earth’s surface as specially sensitized for laying bare its hidden structure, as is the sensitized plate under the magical influence of the X rays.
When, at the time of an earthquake, blocks are adjusted with reference to their neighbors, the movements of oscillation are greatest in those marginal portions of direct contact. Corners of blocks—the intersecting points of the important faults—should for the same reason be shaken with a double violence, and this assumption appears to be confirmed by observation. Upon the island of Ischia, off the Bay of Naples, the shocks from recent earthquakes have been strangely concentrated near the town of Casamicciola, which was last destroyed in 1883. This unfortunate city lies at the crossing point of important fractures whose course upon the island is marked by numerous springs and suffioni ([Fig. 81]).
Seismotectonic lines.—The lines of important earth fractures, as will be more clearly shown in the sequel ([p. 227]), are often indicated with some clearness by straight lines in the plan of the surface relief ([Fig. 82]). Lines of this nature are easily made out upon the map of the West Indies, and if we represent upon it by circles of different diameters the combined intensities of the recorded earthquakes in the various cities, it appears that the heavily shaken localities are ranged upon lines stamped out in the relief, with the most severely damaged places at their intersections ([Fig. 83]). These lines of exceptional instability are known as seismotectonic lines—earthquake structure lines.
Fig. 83.—Seismotectonic lines of the West Indies.
The heavy shocks above loose foundations.—It is characteristic of faults that they soon bury themselves from sight under loose materials, and are thus made difficult of inspection. The escarpment which is the direct consequence of a vertical displacement upon a fault tends to migrate from the place of its formation, rounding the surface as it does so and burying the fault line beneath its deposits ([Fig. 43], [p. 60]).
This is not, however, the sole reason why loose foundations should be places of special danger at the time of earth shocks, for the reason that earthquake waves are sent out in all directions from the surfaces of displacement through the medium of the underlying rock. These waves travel within the firm rock for considerable distances with only a gradual dissipation of their energy, but with their entry into the loose surface deposits their energy is quickly used up in local vibrations of large amplitude, and hence destructive to buildings.
Fig. 84.—Device to illustrate the different effects upon the transmission and the character of shocks which are produced by firm rock and by loose materials.
The essential difference between firm rock and such loose materials as are found upon a river bottom or in the “made land” about our cities may be illustrated by the simple device which is represented in [Fig. 84]. Two similar metal pans are suspended from a firm support by bands of steel and “elastic” braid of similar size and shape, and carry each a small block of wood standing upon its end. Similar light blows are now administered directly to the pans with the effect of upsetting that block which is supported by the loose braid because of the large range or amplitude of movement that is imparted to the pan. The “elastic” braid, because of these large vibrations of which it is susceptible, may represent the loose materials when an earthquake wave passes into them. In the case of the steel support, the energy of the blow, instead of being dissipated in local swingings of the pan, is to a large extent transmitted through the elastic metal to materials beyond. The steel thus resembles in its high elasticity the firmer rock basement, which receives and transmits the earthquake shocks, but except when ruptured in a fault is subject to vibrations of small amplitude only.
Construction in earthquake regions.—Wherever earthquakes have been felt, they are certain to occur again; and wherever mountains are growing or changes of level are in progress, there no record of past earthquakes is required in order to forecast the future seismic history. Although the future earthquakes may be predicted, the time of their coming is, fortunately or unfortunately, still hidden from us. If one’s lot is to be cast in an earthquake country, the only sane course to pursue is to build with due regard to future contingencies.
The danger, from destructive fires may to-day be largely met by methods of construction which levy an additional burden of cost. Though the danger from seismic disturbances can hardly be met as fully as that from fire, yet it is true that buildings may be so constructed as to withstand all save those heaviest shocks in the immediate vicinity of the lines of large displacement. Here, also, a considerable additional expense is involved in the method of construction, in the case of residences particularly.
From what has been said, it is obvious that much of the danger from earthquakes can be met by a choice of site away from lines of important fracture and from areas of relatively loose foundation. The choice of building materials is next of importance. Those buildings which succumb to earthquakes are in most cases racked or shaken apart, and thus they become a prey to their own inherent properties of inertia. Each part of a structure may be regarded as a weight which is balanced upon a stiff rod and pivoted upon the ground. When shocks arrive, each part tends to be thrown into vibration after the manner of an inverted pendulum. In proportion, therefore, as the weights are large and rest upon long supports, the danger of overthrow and of tearing apart is increased. In general, structures are best constructed of light materials whose weight is concentrated near the ground. Masonry structures, and especially high ones, are, therefore, the least suited for resisting earthquakes, of which the late complete destruction of the city of Messina is a grewsome reminder. Despite repeated warnings in the past, the buildings of that stricken city were generally constructed of heavy rubble, which in addition had been poorly cemented ([Fig. 49], [p. 67]). Such structures are usually first ruptured at the edges and corners, since here the vibrations which tend to tear the building asunder are resisted by no supports and are reënforced from neighboring walls.
Fig. 85.—House wrecked in San Francisco earthquake of 1906 because the floors and partitions were not securely fastened to the walls (after R. L. Humphrey).
An advantage of the first importance is evidently secured if the rods of the pendulum, of which the building is conceived to be composed, have sufficient elasticity to be considerably distorted without rupture and to again recover their original position. This is the supreme advantage of structural steel for all large buildings, which is coupled, however, with the disadvantage that the riveted fastenings are apt to be quickly sheered off under the vibrations. Large and high buildings, when sufficiently elastic, have fortunately the property of destroying the earth waves by interference before they have traveled above the lower stories.
For large structures in which wood cannot be used, strongly reënforced concrete is well adapted, for it has in general the same advantages as steel with somewhat reduced elasticity, but with a more effective binding together of the parts. This requirement of thorough bracing and tying together of the several parts of a building causes it to vibrate, not as many pendulums, but as one body. If met, it removes largely the danger from racking strains, and for small structures particularly it is the requirement which is most easily complied with. For such buildings it is therefore necessary that the framework should be built in a close network with every joint firmly braced and with all parts securely tied together. Especial attention should be given to the fastenings of floor and partition ends. The house shown in [Fig. 85] could not have been subjected to heavy shocks, for though the walls are thrown down, the floors and partitions have been left near their original positions.
Fig. 86.—Building wrecked at San Mateo, California, during the late earthquake. The heavy roof and upper floor, acting as a unit, have battered down the upper walls (after J. C. Branner).
This tendency of the walls, floors, partitions, and roof to act as individual units in the vibration, is one that must be reckoned with and be met by specially effective bracing and tying at the junctions. Otherwise these larger parts of the structure may act like battering rams to throw over the walls or portions of them ([Fig. 86]).
Reading References for Chapters VII and VIII
General works:—
John Milne. Seismology. London, 1898, pp. 320.
C. E. Dutton. Earthquakes in the Light of the New Seismology. Putnam, New York, 1904, pp. 314.
A. Sieberg. Handbuch der Erdbebenkunde. Braunschweig, 1904, pp. 362.
Count F. de Montessus de Ballore. Les Tremblements de Terre, Géographie Séismologique. Paris, 1906, pp. 475; La Science Séismologique. Paris, 1907, pp. 579.
William Herbert Hobbs. Earthquakes, an Introduction to Seismic Geology. Appleton, New York, 1907, pp. 336.
C. G. Knott. The Physics of Earthquake Phenomena. Clarendon Press, Oxford, 1908, pp. 283.
E. Rudolph. Ueber Submarine Erdbeben und Eruptionen, Beiträge zur Geophysik, vol. 1, 1887, pp. 133-365; vol. 2, 1895, pp. 537-666; vol. 3, 1898, pp. 273-536.
Descriptive reports of some important earthquakes:—
C. E. Dutton. The Charleston Earthquake of August 31, 1886, 9th Ann. Rept. U. S. Geol. Surv., 1889, pp. 203-528.
B. Kotô. On the Cause of the Great Earthquake in Central Japan, 1891, Jour. Coll. Sci. Imp. Univ., Tokyo, Japan, vol. 5, 1893, pp. 295-353, pls. 28-35.
John Milne and W. K. Burton. The Great Earthquake of Central Japan. 1891, pp. 10, pls. 30.
R. D. Oldham. Report on the Great Earthquake of 12th June, 1897, Mem. Geol. Surv. India. Vol. 29, 1899, pp. 379, pls. 42.
A. C. Lawson, and others. The California Earthquake of April 18, 1906, Report of the State Earthquake Investigation Commission, three quarto vols. (Carnegie Institution of Washington); many plates and figures.
Italian Photographic Society, Messina and Reggio before and after the Earthquake of December 28, 1908 (an interesting collection of pictures). Florence, 1909.
R. S. Tarr and L. Martin. Recent Changes of Level in the Yakutat Bay Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64, pls. 12-23.
William Herbert Hobbs. The Earthquake of 1872 in the Owens Valley, California, Beiträge zur Geophysik, vol. 10, 1910, pp. 352-385, pls, 10-23.
Faults in connection with earthquakes:—
William H. Hobbs. On Some Principles of Seismic Geology, Beiträge zur Geophysik, vol. 8, 1907, Chapters iv-v.
Expansion or contraction of the earth’s surface during earthquakes:—
William H. Hobbs. A Study of the Damage to Bridges during Earthquakes, Jour. Geol., vol. 16, 1908, pp. 636-653; The Evolution and the Outlook of Seismic Geology, Proc. Am. Phil. Soc., vol. 48, 1909, pp. 27-29.
Earthquake construction:—
John Milne. Construction in Earthquake Countries, Trans. Seis. Soc., Japan, vol. 14, 1889-1890, pp. 1-246.
F. de Montessus de Ballore. L’art de bâtir dans les pays à tremblements de terre (34th Congress of French Architects), L’Architecture, 193 Année, 1906, pp. 1-31.
Gilbert, Humphrey, Sewell, and Soulé. The San Francisco Earthquake and Fire of April 18, 1906, and their Effects on Structures and Structural Materials, Bull. 324, U. S. Geol. Surv., 1907, pp. 1-170, pls. 1-57.
William H. Hobbs. Construction in Earthquake Countries, The Engineering Magazine, vol. 37, 1909, pp. 1-19.
Lewis Alden Estes. Earthquake-proof Construction, a discussion of the effects of earthquakes on building construction with special reference to structures of reënforced concrete, published by Trussed Concrete Steel Company. Detroit, 1911, pp. 46.
CHAPTER IX
THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE
VOLCANIC MOUNTAINS OF EXUDATION
Prevalent misconceptions about volcanoes.—The more or less common impression that a volcano is a “burning mountain” or a “smoking mountain” has been much fostered by the school texts in physical geography in use during an earlier period. The best introduction to a discussion of volcanoes is, therefore, a disillusionment from this notion. Far from being burning or smoking, there is normally no combustion whatever in connection with a volcanic eruption. The unsophisticated tourist who, looking out from Naples, sees the steam cap which overhangs the Vesuvian crater tinged with brown, easily receives the impression that the material of the cloud is smoke. Even more at night, when a bright glow is reflected to his eye and soon fades away, only to again glow brightly after a few moments have passed, is it difficult to remove the impression that one is watching an intermittent combustion within the crater. The cloud which floats away from the crest of the mountain is in reality composed of steam with which is admixed a larger or smaller proportion of fine rock powder which gives to the cloud its brownish tone. The glow observed at night is only a reflection from molten lava within the crater, and the variation of its brightness is explained by the alternating rise and fall of the lava surface by a process presently to be explained.
Not only is there no combustion in connection with volcanic eruptions, but so far as the volcano is a mountain it is a product of its own action. The grandest of volcanic eruptions have produced no mountains whatever, but only vast plains or plateaus of consolidated molten rock, and every volcanic mountain at some time in its history has risen out of a relatively level surface.
When the traditional notions about volcanoes grew up, it was supposed that the solid earth was merely a “crust” enveloping still molten material. As has already been pointed out in an earlier chapter, this view is no longer tenable, for we now know that the condition of matter within the earth’s interior, while perhaps not directly comparable to any that is known, yet has properties most resembling known matter in a solid state; it is much more rigid than the best tool steel. While there must be reservoirs of molten rock beneath active volcanoes, it is none the less clear that they are small, local, and temporary. This is shown by the comparative study of volcanic outlets within any circumscribed district.
It is perhaps not easy to frame a definition of a volcano, but its essential part, instead of being a mountain, is rather a vent or channel which opens up connection between a subsurface reservoir of molten rock and the surface of the earth. An eruption occurs whenever there is a rise of this material, together with more or less steam and admixed gases, to the surface. Such molten rock arriving at the surface is designated lava. The changes in pressure upon this material during its elevation induce secondary phenomena as the surface is approached, and these manifestations are often most awe inspiring. While often locally destructive, the geological importance of such phenomena is by reason of their terrifying aspect likely to be greatly exaggerated.
Early views concerning volcanic mountains.—As already pointed out, a volcano at its birth is not a mountain at all, but only, so to speak, a shaft or channel of communication between the surface and a subterranean reservoir of molten rock. By bringing this melted rock to the surface there is built up a local elevation which may be designated a mountain, except where the volume of the material is so large and is spread to such distances as to produce a plain (see fissure eruptions below).
In the early history of geology it was the view of the great German geologist von Buch and his friend and colleague von Humboldt, that a volcanic mountain was produced in much the same manner as is a blister upon the body. The fluids which push up the cuticle in the blister were here replaced by fluid rock which elevated the sedimentary rock layers at the surface into a dome or mound which was open at the top—the so-called crater. This “elevation-crater” theory of volcanoes long held the stage in geological science, although it ignored the very patent fact that the layers on the flanks of volcanic cones are not of sedimentary rock at all, but, on the contrary, of the volcanic materials which are brought up to the surface during the eruption. The observational phase of science was, however, dawning, and the English geologists Scrope and Lyell were able to show by study of volcanic mountains that the mound about the volcanic vent was due to the accumulation of once molten rock which had been either exuded or ejected. Making use of data derived from New Zealand, Scrope showed that, instead of being elevated during the formation of a volcanic mountain, the sedimentary strata of the vicinity may be depressed near the volcanic vent ([Fig. 87]).
Fig. 87.—Breached volcanic cone near Auckland, New Zealand, showing the bending down of the sedimentary strata in the neighborhood of the vent (after Heaphy and Scrope).
The birth of volcanoes.—To confirm the impression that the formation of the volcanic mountain is in reality a secondary phenomenon connected with eruptions, we may cite the observed birth of a number of volcanoes. On the 20th of September, 1538, a new volcano, since known as Monte Nuovo (new mountain), rose on the border of the ancient Lake Lucrinus to the westward of Naples. This small mountain attained a height of 440 feet, and is still to be seen on the shore of the bay of Naples. From Mexico have been recorded the births of several new volcanoes: Jorullo in 1759, Pochutla in 1870, and in 1881 a new volcano in the Ajusco Mountains about midway between the Gulf of Mexico and the Pacific Ocean. The latest of new volcanoes is that raised in Japan on November 9, 1910, in connection with the eruption of Usu-san. This “New Mountain” reached an elevation of 690 feet.
Fig. 88.—View of the new Camiguin volcano from the sea. It was formed in 1871 over a nearly level plain. The town of Catarman appears at the right near the shore (after an unpublished photograph by Professor Dean C. Worcester).
As described by von Humboldt, Jorullo rose in the night of the 28th of September, 1759, from a fissure which opened in a broad plain at a point 35 miles distant from any then existing volcano. The most remarkable of new volcanoes rose in 1871 on the island of Camiguin northward from Mindanao in the Philippine archipelago. This mountain was visited by the Challenger expedition in 1875, and was first ascended and studied thirty years later by a party under the leadership of Professor Dean C. Worcester, the Secretary of the Interior of the Philippine Islands, to whom the writer is indebted for this description and the accompanying illustration of this largest and most interesting of new-born volcanoes. As in the case of Jorullo, the eruption began with the formation of a fissure in a level plain, some 400 yards distant from the town of Catarman ([Fig. 88]). The eruption continued for four years, at the end of which time the height of the summit was estimated by the Challenger expedition to be 1900 feet. At the time of the first ascent in 1905, the height was determined by aneroid as 1750 feet, with sharp rock pinnacles projecting some 50 or 75 feet higher.
Active and extinct volcanoes.—The terms “active” and “extinct” have come into more or less common use to describe respectively those volcanoes which show signs of eruptive activity, and those which are not at the time active. The term “dormant” is applied to volcanoes recently active and supposed to be in a doubtfully extinct condition. From a well-known volcano in the vicinity of Naples, volcanoes which no longer erupt lava or cinder, but show gaseous emanations (fumeroles) are said to be in the solfatara condition, or to show solfataric activity.
Experience shows that the term “extinct”, while useful, must always be interpreted to mean apparently extinct. This may be illustrated by the history of Mount Vesuvius, which before the Christian era was forested in the crater and showed no signs of activity; and in fact it is known that for several centuries no eruption of the volcano had taken place. Following a premonitory earthquake felt in the year 63, the mountain burst out in grand explosive eruption in 79 A.D. This eruption profoundly altered the aspect of the mountain and buried the cities of Pompeii, Stabeii, and Herculaneum from sight. Once more, this time during the middle ages, for nearly five centuries (1139 to 1631) there was complete inactivity, if we except a light ash eruption in the year 1500. During this period of rest the crater was again forested, but the repose was suddenly terminated by one of the grandest eruptions in the mountain’s history.
Fig. 89.—Map showing the location of the belts of active volcanoes.
The earth’s volcano belts.—The distribution of volcanoes is not uniform, but, on the contrary, volcanic vents appear in definite zones or belts, either upon the margins of the continents or included within the oceanic areas ([Fig. 89]). The most important of these belts girdles the Pacific Ocean, and is represented either by chains or by more widely spaced volcanic mountains throughout the Cordilleran Mountain system of South and Central America and Mexico, by the volcanoes of the Coast and Cascade ranges of North America, the festooned volcanic chain of the Aleutian Islands, and the similar island arcs off the eastern coast of the Eurasian continent. The belt is further continued through the islands of Malaysia to New Zealand, and on the Pacific’s southern margin are found the volcanoes of Victoria Land, King Edward Land, and West Antarctica.
Fig. 90.—A portion of the “fire girdle” of the Pacific, showing the relation of the chains of volcanic mountains to the deeps of the neighboring ocean floor.
This volcano girdle is by no means a perfect one, for in addition to the principal festoons of the western border there are many secondary ones, and still other arcs are found well toward the center of the oceanic area. Another broad belt of volcanoes borders the Mediterranean Sea, and is extended westward into the Atlantic Ocean. Narrower belts are found in both the northern and southern portions of the Atlantic Ocean, on the margins of the Caribbean Sea, etc. The fact of greatest significance in the distribution seems to be that bands of active volcanoes are to be found wherever mountain ranges are paralleled by deeps on the neighboring ocean floor ([Fig. 90]). As has been already pointed out in the chapter upon earthquakes, it is just such places as these which are the seat of earthquakes; these are zones of the earth’s crust which are undergoing the most rapid changes of level at the present time. Thus the rise of the land in mountains is proceeding simultaneously with the sinking of the sea floor to form the neighboring deeps.
Fig. 91.—Volcanic cones formed in 1783 above the Skaptár fissure in Iceland (after Helland).
Fig. 92.—Diagrams to illustrate the location of volcanic vents upon fissure lines, a, openings caused by lateral movement of fissure walls; b, openings formed at fissure intersections.
Arrangement of volcanic vents along fissures and especially at their intersections.—Within those districts in which volcanoes are widely separated from their neighbors, the law of their arrangement is difficult to decipher, but the view that volcanic vents are aligned over fissures is now supported by so much evidence that illustrations may be supplied from many regions. An exceptionally perfect line of small cones is found along the Skaptár cleft in Iceland, upon which stands the large volcano of Laki. This fissure reopened in 1783, and great volumes of lava were exuded. Over the cleft there was left a long line of volcanic cones ([Fig. 91]). There are in Iceland two dominating series of parallel fissures of the same character which take their directions respectively northeast-southwest and north-south. Many such fissures are traceable at the surface as deep and nearly straight clefts or gjás, usually a few yards in width, but extending for many miles. The Eldgjá has a length of more than 18 English miles and a depth varying from 400 to 600 feet. On some of these fissures no lava has risen to the surface, whereas others have at numerous points exuded molten rock. Sometimes one end only of a fissure, the more widely gaping portion, has supplied the conduits for the molten lava. This is well illustrated by the cratered monticules raised by the common ant over the cracks which separate the blocks of cement sidewalk, the hillocks being located where the most favorable channel was found for the elevation of the materials.
Fig. 93.—Outline map of the eastern portion of the island of Java, displaying the arrangement of volcanic vents in alignment upon fissures with the larger mountains at fissure intersections (after Verbeek).
Those places upon fissures which become lava conduits appear to be the ones where the cleft gapes widest so as to furnish the widest channel. Wherever a differential lateral movement of the walls has occurred, openings will be found in the neighborhood of each minor variation from a straight line ([Fig. 92 a]). Wherever there are two or more series of fissures, and this would appear to be the normal condition, places favorable for lava conduits occur at fissure intersections. Within such veritable volcano gardens as are to be found in Malaysia, the law of volcano distribution became apparent so soon as accurate maps had been prepared. Thus the outline map of a portion of the island of Java ([Fig. 93]) shows us that while the volcanoes of the island present at first sight a more or less irregular band or zone, there are a number of fissures intersecting in a network, and that the volcanoes are aligned upon the fissures with the larger cones located at the intersections. So also in Iceland, the great eruption of Askja in 1875 occurred at the intersection of two lines of fissure.
Outside these closely packed volcanic regions, similar though less marked networks are indicated; as, for example, in and near the Gulf of Guinea. If now, instead of reducing the scale of our volcano maps, we increase it, the same law of distribution is no less clearly brought out. The monticules or small volcanic cones which form upon the flanks of larger volcanic mountains are likewise built up over fissures which on numerous occasions have been observed to open and the cones to form upon them.
Fig. 94.—Map of the Puy Pariou in the Auvergne of central France. The seat of eruption has migrated along the fissure upon which the earlier cone had been built up (after Scrope).
Still further reducing now the area of our studies and considering for the moment the “frozen” surface of the boiling lava within the caldron of Kilauea, this when observed at night reveals in great perfection the sudden formation of fissures in the crust with the appearance of miniature volcanoes rising successively at more or less regular intervals along them.
It not infrequently happens that after a volcanic vent has become established above some conduit in a fissure, the conduit migrates along the fissure, thus establishing a new cone with more or less complete destruction of the old one ([Fig. 94]).
The so-called fissure eruptions.—The grandest of all volcanic eruptions have been those in which the entire length and breadth of the fissures have been the passageway for the upwelling lava. Such grander eruptions have been for the most part prehistoric, and in later geologic history have occurred chiefly in India, in Abyssinia, in northwestern Europe, and in the northwestern United States. In western India the singularly horizontal plateaus of basaltic lava, the Dekkan traps, cover some 200,000 square miles and are more than a mile in depth. The underlying basement where it appears about the margins of the basalt is in many places intersected by dikes or fissure fillings of the same material. No cones or definite vents have been found.
Fig. 95.—Basaltic plateau of the northwestern United States due to fissure eruptions of lava.
The larger portion of the northwestern British Isles would appear to have been at one time similarly blanketed by nearly horizontal beds of basaltic lava, which beds extended northwestward across the sea through the Orkney and Faroe islands to Iceland. Remnants of this vast plateau are to-day found in all the island groups as well as in large areas of northeastern Ireland, and fissure fillings of the same material occur throughout large areas of the British Isles. In many cases these dikes represent once molten rock which may never have communicated with the surface at the time of the lava outpouring, yet they well illustrate what we might expect to find if the basalt sheets of Iceland or Ireland were to be removed.
The floods of basaltic lava which in the northwestern United States have yielded the barren plateau of the Cascade Mountains ([Fig. 95]) would appear to offer another example of fissure eruption, though cones appear upon the surface and perhaps indicate the position of lava outlets during the later phases of the eruptive period. The barrenness and desolation of these lava plains is suggested by [Fig. 96].
Fig. 96.—Lava plains about the Snake River in Idaho.
Though the greater effusions of lava have occurred in prehistoric times, and the manner of extrusion has necessarily been largely inferred from the immense volume of the exuded materials and the existence of basaltic dikes in neighboring regions, yet in Iceland we are able to observe the connection between the dikes and the lava outflows. Professor Thoroddsen has stated that in the great basaltic plateau of Iceland, lava has welled out quietly from the whole length of fissures and often on both sides without giving rise to the formation of cones. At three wider portions of the great Eld cleft, lava welled out quietly without the formation of cones, though here in the southern prolongation of the fissure, where it was narrower, a row of low slag cones appeared. Where the lava outwellings occurred, an area of 270 square miles was flooded.
Fig. 97.—Characteristic profiles of lava volcanoes. 1, basaltic lava mountain; 2, mountain of siliceous lava (after Judd).
The composition and the properties of lava.—In our study of igneous rocks (Chapter IV) it was learned that they are composed for the most part of silicate minerals, and that in their chemical composition they represent various proportions of silica, alumina, iron, magnesia, lime, potash, and soda. The more abundant of these constituents is silica, which varies from 35 to 70 per cent of the whole. Whenever the content of silica is relatively low,—basic or basaltic lava,—the cooled rock is dark in color and relatively heavy. It melts at a relatively low temperature, and is in consequence relatively fluid at the temperatures which lavas usually have on reaching the earth’s surface. Furthermore, from being more fluid, the water which is nearly always present in large quantity within the lava more readily makes its escape upon reaching the surface. Eruptions of such lava are for this reason without the violent aspects which belong to extrusions of more siliceous (more “acidic”) lavas. For the same reason, also, basaltic lava flows more freely and can spread much farther before it has cooled sufficiently to consolidate. This is equivalent to saying that its surface will assume a flatter angle of slope, which in the case of basaltic lava seldom exceeds ten degrees and may be less than one degree ([Fig. 97]).
Fig. 98.—A driblet cone (after J. D. Dana).
Siliceous lavas, on the other hand, are, when consolidated, relatively light both in color and weight and melt at relatively high temperatures. They are, therefore, usually but partly fused and of a viscous consistency when they arrive at the earth’s surface. Because of this viscosity they offer much resistance to the liberation of the contained water, which therefore is released only to the accompaniment of more or less violent explosions. The lava is blown into the air and usually falls as consolidated fragments of various degrees of coarseness.
Fig. 99.—View of Leffingwell crater, a cinder cone in the Owens valley, California (after an unpublished photograph by W. D. Johnson).
It must not, however, be assumed that the temperature of lava is always the same when it arrives at the surface, and hence it may happen that a siliceous lava is exuded at so high a temperature that it behaves like a normal basaltic lava. On the other hand, basaltic lavas may be extruded at unusually low temperatures, in which case their behavior may resemble that of the normal siliceous lavas. If, however, as is generally the case, the energy of explosion of a basaltic lava is relatively small, any ejected portions of the liquid lava travel to a moderate height only in the air, so that on falling they are still sufficiently pasty to adhere to rock surfaces and thus build up the remarkably steep cones and spines known as “spatter cones” or “driblet cones” ([Fig. 98]). When, on the other hand, the energy of explosion is great, as is normally the case with siliceous lavas, the portions of ejected lava have been fully consolidated before their fall to the surface, so that they build up the same type of accumulation as would sand falling in the same manner. The structures which they form are known as tuff, cinder, or ash cones ([Fig. 99]).
Whenever the contained water passes off from siliceous lavas without violent explosions, the lava may flow from the vent, but in contrast to basaltic lavas it travels a short distance only before consolidating. The resulting mountain is in consequence proportionately high and steep ([Fig. 97]). Eruptions characterized by violent explosions accompanied by a fall of cinder are described as explosive eruptions. Those which are relatively quiet, and in which the chief product is in the form of streams of flowing lava, are spoken of as convulsive eruptions.
The three main types of volcanic mountain.—If the eruptions at a volcanic vent are exclusively of the explosive type, the material of the mountain which results is throughout tuff or cinder, and the volcano is described as a cinder cone. If, on the other hand, the vent at every eruption exudes lava, a mountain of solid rock results which is a lava dome. It is, however, the exception for a volcano which has a long history to manifest but a single kind of eruption. At one time exuding lava comparatively quietly, at another the violence with which the steam is liberated yields only cinder, and the mountain is a composite of the two materials and is known as a composite volcanic cone.
The lava dome.—When successive lava flows come from a crater, the structure which results has the form of a more or less perfect dome. If the lava be of the basaltic or fluid type, the slopes are flat, seldom making an angle of as much as ten degrees with the horizon and flatter toward the summit ([Fig. 101], [p. 106]). If of siliceous or viscous lava, on the other hand, the slopes are correspondingly steep and in some cases precipitous. To this latter class belong some of the Kuppen of Germany, the puys of central France, and the mamelons of the Island of Bourbon.
Fig. 100.—Map of Hawaii and the lava volcanoes of Mokuaweoweo (Mauna Loa) and Kilauea (after the government map by Alexander).
The basaltic lava domes of Hawaii.—At the “crossroads of the Pacific” rises a double line of lava volcanoes which reach from 20,000 to 30,000 feet above the floor of the ocean, some of them among the grandest volcanic mountains that are known. More than half the height and a much larger proportion of the bulk of the largest of these are hidden beneath the ocean’s surface. The two great active vents are Mokuaweoweo (on Mauna Loa) and Kilauea, distinct volcanoes notwithstanding the fact that their lava extravasations have been merged in a single mass. The rim of the crater of Mauna Loa is at an elevation of 13,675 feet above the sea, whereas that of Kilauea is less than 4000 feet and appears to rest upon the flank of the larger mountain ([Figs. 100] and [101]). Although one crater is but 20 miles distant from the other and nearly 10,000 feet lower, their eruptions have apparently been unsympathetic. Nowhere have still active lava mountains been subjected to such frequent observations extending throughout a long period, and the dynamics of their eruptions are fairly well understood. To put this before the reader, it will be best to consider both mountains, for though they have much in common, the observations from one are strangely complementary to those of the other. The lower crater being easily accessible, Kilauea has been often visited, and there exists a long series of more or less consecutive observations upon it, which have been assembled and studied by Dana and Hitchcock. The place of outflow of the Kilauea lavas has not generally been visible, whereas Mokuaweoweo has slopes rising nearly 14,000 feet above the sea and displays the records of outflow of many eruptions, some of which were accompanied by the grandest of volcanic phenomena.
Fig. 101.—Section through Mauna Loa and Kilauea.
Lava movements within the caldron of Kilauea.—The craters of these mountains are the largest of active ones, each being in excess of seven miles in circumference. In shape they are irregularly elliptical and consist of a series of steps or terraces descending to a pit at the bottom, in which are open lakes of boiling lava. Enough is known of the history of Kilauea to state that the steep cliffs bounding the terraces are fault walls produced by inbreak of a frozen lava surface. The cliff below the so-called “black ledge” was produced by the falling in of the frozen lava surface at the time of the outflow of 1840, the lava issuing upon the eastern flank of the mountain and pouring into the sea near Nanawale. Since that date the floor of the pit below the level of this ledge has been essentially a movable platform of frozen lava of unknown and doubtless variable thickness which has risen and descended like the floor of an elevator car between its guiding ways ([Fig. 102]). The floor has, however, never been complete, for one or more open lakes are always to be seen, that of Halemaumau located near the southwestern margin having been much the most persistent. Within the open lakes the boiling lava is apparently white hot at the depth of but a few inches below the surface, and in the overturnings of the mass these hotter portions are brought to the surface and appear as white streaks marking the redder surface portions. From time to time the surface freezes over, then cracks open and erupt at favored points along the fissures, sending up jets and fountains of lava, the material of which falls in pasty fragments that build up driblet cones. Small fluid clots are shot out, carrying a threadlike line of lava glass behind them, the well-known “Pelé’s hair.” Sometimes the open lakes build up congealed walls, rising above the general level of the pit, and from their rim the lava spills over in cascades to spread out upon the frozen floor, thus increasing its thickness from above ([Fig. 103]). At other times a great dome of lava has been pushed up from the pit of Halemaumau under a frozen shell, the molten lava shining red through cracks in its surface and exuding so as to heal each widely opened fissure as it forms.
Fig. 102.—Schematic diagram to illustrate the moving platform of frozen lava which rises and falls in the crater of Kilauea.
At intervals of from a few years to nine or ten years the crater has been periodically drained, at which times the moving platform of frozen lava has sunk more or less rapidly to levels far below the black ledge and from 900 to 1700 feet below the crater rim. Following this descent a slow progressive rise is inaugurated, which has sometimes gone on at a rate of more than a hundred feet per year, though it is usually much slower than this. When the platform has reached a height varying from 700 to 350 feet below the crater rim, another sudden settlement occurs which again carries the pit floor downward a distance of from 300 to 700 feet.
Fig. 103.—View of the open lava lake of Halemaumau within the crater of Kilauea, the molten lava shown cascading over the raised lava walls on to the floor of the pit (after Pavlow).
The draining of the lava caldrons.—The changes which go on within the crater of Mokuaweoweo, though less studied than those of Kilauea, appear to be in some respects different. Here every eruption seems to be preceded by a more or less rapid influx of melted lava to the pit of the crater, this phenomenon being observed from a distance as a brilliant light above the crater—the reflection of the glow from overhanging vapor clouds. The uprising of the lava has often been accompanied by the formation of high lava fountains upon the surface, and the molten lava sometimes appears in fissures near the crater rim at levels well above the lava surface within the pit.
Although in many cases the lava which has thus flooded the crater has suddenly drained away without again becoming visible, it is probable that in such cases an outlet has been found to some submarine exit, since under-ocean discharge effects have been observed in connection with eruptions of each of the volcanoes.
Fig. 104.—Map showing the manner of outflow of lava from Kilauea during the eruption of 1840. The outflowing lava made its appearance successively at the points A, B, C, m, n, and finally at a point below n, from whence it issued in volume and flowed down to the sea at Nanawale (after J. D. Dana).
Inasmuch as no earthquakes are felt in connection with such outflows as have been described, it is probable that the hot lava fuses a passageway for itself into some open channel underneath the flanks of the mountain. Such a course is well illustrated by the outflow of Kilauea in 1840, when, it will be remembered, occurred the great down-plunge of the crater that yielded the pit below the black ledge. At this time the lava first made its appearance upon the flanks of the mountain at the bottom of a small pit or inbreak crater which opened five miles southeast of the main crater of Kilauea ([Fig. 104]). Within this new crater the lava rose, and small ejections soon followed from fissures formed in its neighborhood. Some time after, the lava sank in the first new crater, only to reappear successively at other small openings ([Fig. 104, B, C, m, n]) and finally to issue in volume at a point eleven miles from the shore and flow thereafter upon the surface of the mountain until it had reached the sea. Only the slightest earth tremors were felt, and as no rumblings were heard, it is evident that the lava fused its way along a buried channel largely open at the time (see below, [p. 112]).
In a majority of the eruptions of Mokuaweoweo, when the outflowing lavas have become visible, the molten rock has apparently fused its way out to the surface of the mountain at points from 1000 to 3000 feet below the bottom of the crater, and this discharge has corresponded in time to the lowering of the lava surface within the crater. There are, however, three instances upon record in which the lava issued from definite rents which were formed upon the mountain flanks at comparatively low levels. In contrast to the formation of fused outlets, these ruptures of a portion of the mountain’s flank were always accompanied by vigorous local earthquakes of short duration. In one instance (the eruption of 1851) such a rent appeared under the same conditions but at an elevation of 12,500 feet, or near the level of the lava in the crater.
The outflow of the lava floods.—In order to properly comprehend these and many otherwise puzzling phenomena connected with volcanoes, it is necessary to keep ever in mind the quite remarkable heat-insulating property of congealed lava. So soon as a thin crust has formed upon the surface of molten rock, the heat of the underlying fluid mass is given off with extreme slowness, so that lava streams no longer connected with their internal lava reservoirs may remain molten for decades.
Fig. 105.—Lava of Matavanu upon the Island of Savaii flowing down to the sea during the eruption of 1906. The course may be followed by the jets of steam escaping from the surface down to the great steam cloud which rises where the fluid lava discharges into the sea (after H. I. Jensen).
We have seen that for Mokuaweoweo and Kilauea, lava either quietly melts its way to the surface at the time of outflow, or else produces a rent for its egress to the accompaniment of vigorous local earthquakes. In either case if the lava issues at a point far below the crater, gigantic lava fountains arise at the point of outflow, the fluid rock shooting up to heights which range from 250 to 600 or more feet above the surface. A certain proportion of this fluid lava is sufficiently cooled to consolidate while traveling in the air, and falling, it builds up a cinder cone which is left as a location monument for the place of discharge. From this outlet the molten lava begins its journey down the slope of the mountain, and quickly freezes over to produce a tunnel, beneath the roof of which the fluid lava flows with comparatively slow further loss of heat. Save for occasional steam jets issuing from its surface, it may give little indication of its presence until it has reached the sea ([Fig. 105]).
Fig. 106.—Lava stream discharging into the sea from beneath the frozen roof of a lava tunnel. Eruption of Matavanu on Savaii in 1906 (after Sapper).
If sufficient in volume and the shore be not too distant, the stream of lava arrives at the sea, where, discharging from the mouth of its tunnel, it throws up vast volumes of steam and induces ebullition of the water over a wide area ([Fig. 106]). Professor Dana, who visited Hawaii a few months only after the great outflow of 1840, states that the lava, upon reaching the ocean, was shivered like melted glass and thrown up in millions of particles which darkened the sky and fell like hail over the surrounding country. The light was so bright that at a distance of forty miles fine print could be read at midnight.
Fig. 107.—Diagrammatic representation of the structure of the flanks of lava volcanoes as a result of the draining of frozen lava streams.
Protected from any extensive consolidation by its congealed cover, the lava within a stream may all drain away, leaving behind an empty lava tunnel, which in the case of the Hawaiian volcanoes sometimes has its roof hung with beautiful lava stalactites and its floor studded with thin lava spines. Later lava outflows over the same or neighboring courses bury such tunnels beneath others of similar nature, giving to the mountain flanks an elongated cellular structure illustrated schematically in [Fig. 107]. These buried channels may in the future be again utilized for outflows similar in character to that of Kilauea in 1840.
Fig. 108.—Diagram to show the manner of formation of mesas or table mountains by the outflow of lava in valleys and the subsequent more rapid erosion of the intervening ridges. R, earlier river valley; R’R’, later valleys.
While the formation of lava stalactites of such perfection and beauty is peculiar to the Hawaiian lava tunnels, the formation of the tunnel in connection with lava outflow is the rule wherever a dissipation at the end has permitted of drainage. A few hours only after the flow has begun, the frozen surface has usually a thickness of a few inches, and this cover may be walked over with the lava still molten below. At first in part supported by the molten lava, the tunnel roof sometimes caves in so soon as drainage has occurred.
Fig. 109.—Surface of lava of the Pahoehoe type.
Wherever basaltic lava has spread out in valleys on the surface of more easily eroded material, either cinder or sedimentary formations, the softer intervening ridges are first carried away by the eroding agencies, leaving the lava as cappings upon residual elevations. Thus are derived a type of table mountain or mesa of the sort well illustrated upon the western slopes of the Sierra Nevadas in California ([Fig. 108]).
Fig. 110.—Three successive views to illustrate the growth of the Island of Savaii from the outflow of lava at Matavanu in the year 1906. a, near the beginning of the outflow; b, some weeks later than a; c, some weeks later than b (after H. I. Jensen).
The surface which flowing lava assumes, while subject to considerable variation, may yet be classified into two rather distinct types. On the one hand there is the billowy surface in which ellipsoidal or kidney-shaped masses, each with dimensions of from one to several feet, lie merged in one another, not unlike an irregular collection of sofa pillows. This type of lava has become known as the Pahoehoe, from the Hawaiian occurrence ([Fig. 109]). A variation from this type is the “corded” or “ropy” lava, the surface of which much resembles rope as it is coiled along the deck of a vessel, the coils being here the lines of scum or scoriæ arranged in this manner by the currents at the surface of the stream ([Fig. 123], [p. 124]). A quite different type is the block lava (Aa type) which usually has a ragged scoriaceous surface and consists of more or less separate fragments of cooled lava ([Fig. 131], [p. 130]).
Wherever lava flows into the sea in quantity, it extends the margin of the shore, often by considerable areas. The outflow of Kilauea in 1840 extended the shore of Hawaii outward for the distance of a quarter of a mile, and a more recent illustration of such extension of land masses is furnished by [Fig. 110].
CHAPTER X
THE RISE OF MOLTEN ROCK TO THE EARTH’S SURFACE
VOLCANIC MOUNTAINS OF EJECTED MATERIALS
The mechanics of crater explosions.—If we now turn from the lava volcano to the active cinder cone, we encounter an entire change of scene. In place of the quiet flow and convulsive movements of the molten lava, we here meet with repeated explosions of greater or less violence. If we are to profitably study the manner of the explosions, considering the volcanic vent as a great experimental apparatus, it would be well to select for our purpose a volcano which is in a not too violent mood. The well-known cinder cone of Stromboli in the Eolian group of islands north of Sicily has, with short and unimportant interruptions, remained in a state of light explosive activity since the beginning of the Christian era. Rising as it does some three thousand feet directly out of the Mediterranean, and displaying by day a white steam cap and an intermittent glow by night, its summit can be seen for a distance of a hundred miles at sea and it has justly been called the “Lighthouse of the Mediterranean.” The “flash” interval of this beacon may vary from one to twenty minutes, and it may show, furthermore, considerable variation of intensity.
For the reason that the crater of the mountain is located at one side and at a considerable distance below the actual summit, the opportunity here afforded of looking into the crater is most favorable whenever the direction of the wind is such as to push aside the overhanging steam cloud ([Fig. 111]). Long ago the Italian vulcanologist Spallanzani undertook to make observations from above the crater, and many others since his day have profited by his example.
Within the crater of the volcano there is seen a lava surface lightly frozen over and traversed by many cracks from which vapor jets are issuing. Here, as in the Kilauea crater, there are open pools of boiling lava. From some of these, lava is seen welling out to overflow the frozen surface; from others, steam is ejected in puffs as though from the stack of a locomotive. Within others lava is seen heaving up and down in violent ebullition, and at intervals a great bubble of steam is ejected with explosive violence, carrying up with it a considerable quantity of the still molten lava, together with its scumlike surface, to fall outside the crater and rattle down the mountain’s slope into the sea. Following this explosion the lava surface in the pool is lowered and the agitation is renewed, to culminate after the further lapse of a few minutes in a second explosion of the same nature. The rise of the lava which precedes the ejection appears at night as a brighter reflection or glow from the overhanging steam cloud—the flash seen by the mariner from his vessel.
Fig. 111.—The volcano of Stromboli, showing the excentric position of the crater (after a sketch by Judd).
What is going on within the crater of Stromboli we may perhaps best illustrate by the boiling of a stiff porridge over a hot fire. Any one who has made corn mush over a hot camp fire is fully aware that in proportion as the mush becomes thicker by the addition of the meal, it is necessary to stir the mass with redoubled vigor if anything is to be retained within the kettle. The thickening of the mush increases its viscosity to such an extent that the steam which is generated within it is unable to make its escape unless aided by openings continually made for it by the stirring spoon. If the stirring motion be stopped for a moment, the steam expands to form great bubbles which soon eject the pasty mass from the kettle.
For the crater of Stromboli this process is illustrated by the series of diagrams in [Fig. 112]. As the lava rises toward the surface, presumably as a result of convectional currents within the chimney of the volcano, the contained steam is relieved from pressure, so that at some depth below the surface it begins to separate out in minute vesicles or bubbles, which, expanding as they rise, acquire a rapidly accelerating velocity. Soon they flow together with a quite sudden increase of their expansive energy, and now shooting upward with further accelerated velocity, a layer of liquid lava with its cover of scum is raised on the surface of a gigantic bubble and thrown high into the air. Cooled during their flight, the quickly congealed lava masses become the tuff or volcanic ash which is the material of the cinder cone.
Fig. 112.—Diagrams to illustrate the nature of eruptions within the crater of Stromboli.
Grander volcanic eruptions of cinder cones.—Most cinder and composite cones, in the intervals between their grander eruptions, if not entirely quiescent, lapse into a period, of light activity during which their crater eruptions appear to be in all essential respects like the habitual explosions within the Strombolian crater. This phase of activity is, therefore, described as Strombolian. By contrast, the occasional grander eruptions which have punctuated the history of all larger volcanoes are described in the language of Mercalli as Vulcanian eruptions, from the best studied example.
Just what it is that at intervals brings on the grander Vulcanian outburst within a volcano is not known with certainty; but it is important to note that there is an approach to periodicity in the grander eruptions. It is generally possible to distinguish eruptions of at least two orders of intensity greater than the Strombolian phase; a grander one, the examples of which may be separated by centuries, and one or more orders of relatively moderate intensity which recur at intervals perhaps of decades, their time intervals subdividing the larger periods marked off by the eruptions of the first order.
Fig. 113.—Map of Volcano in the Eolian group of islands. The smaller craters partially dissected by the waves belong to Vulcanello (after Judd).
The eruption of Volcano in 1888.—In the Eolian Islands to the north of Sicily was located the mythical forge of Vulcan. From this locality has come our word “volcano”, and both the island and the mountain bear no other name to-day ([Fig. 113]). There is in the structure of the island the record of a somewhat complex volcanic history, but the form of the large central cinder cone was, according to Scrope, acquired during the eruption of 1786, at which time the crater is reported to have vomited ash for a period of fifteen days. Passing after this eruption into the solfatara condition, with the exception of a light eruption in 1873, the volcano remained quiet until 1886. So active had been the fumeroles within the crater during the latter part of this period that an extensive plant had been established there for the collection especially of boracic acid. In 1886 occurred a slight eruption, sufficient to clear out the bottom of the crater, though not seriously to disturb the English planter whose vineyards and fig orchards were in the valley or atrio near the point d upon the map ([Fig. 113]), nearly a mile from the crater rim. On the 3d of August, 1888, came the opening discharge of an eruption, which, while not of the first order of magnitude, was yet the greatest in more than a century of the mountain’s history, and may serve us to illustrate the Vulcanian phase of activity within a cinder cone. During the day, to the accompaniment of explosions of considerable violence, projectiles fell outside the crater rim and rolled down the steep slopes toward the atrio. These explosions were repeated at intervals of from twenty to thirty minutes, each beginning in a great upward rush of steam and ash, accompanied by a low rumbling sound. During the following night the eruptions increased in violence, and the anxious planter remained on watch in his villa a mile from the crater. Falling asleep toward morning, he was rudely awakened by a rain of projectiles falling upon his roof. Hastily snatching up his two children he ran toward the door just as a red hot projectile, some two feet in diameter, descended through the roof, ceiling, and floor of the drawing room, setting fire to the building. A second projectile similar to the first was smashed into fragments at his feet as he was emerging from the house, burning one of the children. Making his escape to Vulcanello at the extremity of the island, the remainder of the night and the following day, until rescue came from Lipari, were spent just beyond the range of the falling masses.
Fig. 114.—“Bread-crust” lava projectile from the eruption of Volcano in 1888 (after Mercalli).
When the writer visited the island some months later, the eruption was still so vigorous that the crater could not be reached. The ruined villa, smashed and charred, stood with its walls half buried in ash and lapilli, among which were partly smashed pumiceous lava projectiles. The entire atrio about the mountain lay buried in cinder to the depth of several feet and was strewn with projectiles which varied in size from a man’s fist to several feet in diameter ([Fig. 114]). The larger of these exhibited the peculiar “bread-crust” surface and had generally been smashed by the force of their fall after the manner of a pumpkin which has been thrown hard against the ground. One of these projectiles fully three feet in diameter was found at the distance of a mile and a half from the crater. Though diminished considerably in intensity, the rhythmic explosions within the crater still recurred at intervals varying from four minutes to half an hour, and were accompanied by a dull roar easily heard at Lipari on a neighboring island six miles away. Simultaneously, a dark cloud of “smoke”, the peculiar “cauliflower cloud” or pino mounted for a couple of miles above the crater ([Fig. 115]), and the rise was succeeded by a rain of small lava fragments or lapilli outside the crater rim.
Fig. 115.—Peculiar “cauliflower cloud” or pino composed of steam and ash, rising above the cinder cone of Volcano during the waning phases of the explosive eruption of 1888 (after a photograph by B. Hobson).
There seems to be no good reason to doubt that Vulcanian cinder eruptions of this type differ chiefly in magnitude from the rhythmic explosion within the crater of Stromboli, if we except the elevation of a considerable quantity of accessory and older tuff which is derived from the inner walls of the crater and carried upward into the air together with the pasty cakes of fresh lava derived from the chimney. It is this accessory material which gives to the pino its dark or even black appearance.
Fig. 116.—Double explosive eruption of Taal volcano on the morning of January 30, 1911.
The eruption of Taal volcano on January 30, 1911.—The recent eruption of the cinder cone known as Taal volcano is of interest, not only because so fresh in mind, but because two neighboring vents erupted simultaneously with explosions of nearly equal violence ([Fig. 116]). This Philippine volcano lies near the center of a lake some fifteen miles in diameter and about fifty miles south of the city of Manila. After a period of rest extending over one hundred and fifty years, the symptoms of the coming eruption developed rapidly, and on the morning of January 30 grand explosions of steam and ash occurred simultaneously in the neighboring craters, and the condensed moisture brought down the ash in an avalanche of scalding mud which buried the entire island. Almost the entire population of the island, numbering several hundreds, was literally buried in the blistering mud ([Fig. 117]); and the gases from the explosions carried to the distant shores of the lake added to this number many hundred victims.
Fig. 117.—The thick mud veneer upon the island of Taal (after a photograph by Deniston).
Fig. 118.—A pear-shaped lava projectile.
The shocks which accompanied the explosions raised a great wave upon the surface of the lake, which, advancing upon the shores, washed away structures for a distance of nearly a half mile.
The materials and the structure of cinder cones.—Obviously the materials which compose cinder cones are the cooled lava fragments of various degrees of coarseness which have been ejected from the crater. If larger than a finger joint, such fragments are referred to as volcanic projectiles, or, incorrectly, as “volcanic bombs.” Of the larger masses it is often true that the force of expulsion has not been applied opposite the center of mass of the body. Thus it follows that they undergo complex whirling motions during their flight, and being still semiliquid, they develop curious pear-shaped or less regular forms ([Fig. 118]). When crystals have already separated out in the lava before its rise in the chimney of the volcano, the surrounding fluid lava may be blown to finely divided volcanic dust which floats away upon the wind, thus leaving the crystals intact to descend as a crystal rain about the crater. Such a shower occurred in connection with the eruption of Etna in 1669, and the black augite crystals may to-day be gathered by the handful from the slopes of the Monti Rossi ([Fig. 125], [p. 125]).
Fig. 119.—Artificial production of the structure of a cinder cone with use of colored sands carried up in alternation by a current of air (after G. Linck).
The term lapilli, or sometimes rapilli, is applied to the ejected lava fragments when of the average size of a finger joint. This is the material which still partially covers the unexhumed portions of the city of Pompeii. Volcanic sand, ash, and dust are terms applied in order to increasingly fine particles of the ejected lava. The finest material, the volcanic dust, is often carried for hundreds and sometimes even for thousands of miles from the crater in the high-level currents of the atmosphere. Inasmuch as this material is deposited far from the crater and in layers more or less horizontal, such material plays a small rôle in the formation of the cinder cone. The coarser sands and ash, on the other hand, are the materials from which the cinder cone is largely constructed.
The manner of formation and the structure of cinder cones may be illustrated by use of a simple laboratory apparatus ([Fig. 119]). Through an opening in a board, first white and then colored sand is sent up in a light current of air or gas supplied from suitable apparatus. The alternating layers of the sand form in the attitudes shown; that is to say, dipping inward or toward the chimney of the volcano at all points within the crater rim, and outward or away from it at all points outside ([Fig. 119]). If the experiment is carried so far that at its termination sand slides down the crater walls into the chimney below, the inward dipping layers will be truncated, or even removed entirely, as shown in [Fig. 119 b].
Fig. 120.—Diagram to show the contrast between a lava dome and a cinder cone. AAA, cinder cone; BabC, lava dome; DE, line of low cinder cones above a fissure (after Thoroddsen).
The profile lines of cinder cones.—The shapes of cinder cones are notably different from those of lava mountains. While the latter are domes, the mountains constructed of cinder are conical and have curves of profile that are concave upward instead of convex ([Fig. 120]). In the earlier stages of its growth the cinder cone has a crater which in proportion to the height of the mountain is relatively broad ([Fig. 99], [p. 104]).
Fig. 121.—Mayon volcano on the island of Luzon, P.I. A remarkably perfect high cinder cone.
Speaking broadly, the diameter of the crater is a measure of the violence of the explosions within the chimney. A single series of short and violent explosive eruptions builds a low and broad cinder cone. A long-continued succession of moderately violent explosions, on the other hand, builds a high cone with crater diameter small if compared with the mountain’s altitude, and the profile afforded is a remarkably beautiful sweeping curve ([Fig. 121]). Toward the summit of such a cone the loose materials of which it is composed are at as steep an angle as they can lie, the so-called angle of repose of the material; whereas lower down the flatter slopes have been determined by the distribution of the cinder during its fall from the air. When one makes the ascent of such a mountain, he encounters continually steeper grades, with the most difficult slope just below the crest.
Fig. 122.—A series of breached cinder cones where the place of eruption has migrated along the underlying fissure. The Puys Noir, Solas, and La Vache in the Mont Dore Province of central France (after Scrope).
The composite cone.—The life histories of volcanoes are generally so varied that lava domes and the pure types of cinder cones are less common than volcanoes in which paroxysmal eruptions have alternated with explosions, and where, therefore, the structure of the mountain represents a composite of lava and cinder. Such composite cones possess a skeleton of solid rock upon which have been built up alternate sloping layers of cinder and lava. In most respects such cones stand in an intermediate position between lava domes and cinder cones.
Fig. 123.—The bocca or mouth upon the inner cone of Mount Vesuvius from which flowed the lava stream of 1872. This lava stream appears in the foreground with its characteristic “ropy” surface.
Regarded as a retaining wall for the lava which mounts in the chimney, the cinder cone is obviously the weakest of all. Should lava rise in a cinder cone without an explosion occurring, the cone is at once broken through upon one side by the outwelling of the lava near the base. Thus arises the characteristic breached cone of horseshoe form ([Fig. 122]).
Fig. 124.—A row of parasitic cones raised above a fissure which was opened upon the flanks of Mount Etna during the eruption of 1892 (after De Lorenzo).
Quite in contrast with the weak cinder cone is the lava dome with its rock walls and relatively flat slopes. Considered as a retaining wall for lava it is much the strongest type of volcanic mountain, and it is likely that the hydrostatic pressure of the lava within the crater would seldom suffice to rupture the walls, were it not that the molten rock first fuses its way into old stream tunnels buried under the mountain slopes (see ante, p. 112). Composite cones have a strength as retaining walls for lava which is intermediate between that of the other types. Their Vulcanian eruptions of the convulsive type are initiated by the formation of a rent or fissure upon the mountain flanks at elevations well above the base, the opening of the fissure being generally accompanied by a local earthquake of greater or less violence.
From one or more such fissures the lava issues usually with sufficient violence at the place of outflow to build up over it either an enlarged type of driblet cone, referred to as a “mouth”, or bocca[1] ([Fig. 123]), or one or more cinder cones which from their position upon the flanks of the larger volcano are referred to as parasitic cones ([Fig. 124]). The lava of Vesuvius more frequently yields bocchi at the place of outflow, whereas the flanks of Etna are pimpled with great numbers of parasitic cinder cones, each the monument to some earlier eruption ([Fig. 125]).
Fig. 125.—View looking toward the summit of Etna from a position upon the southern flank near the village of Nicolosi. The two breached parasitic cones seen behind this village are the Monti Rossi which were thrown up in 1669 and from which flowed the lava which overran Catania (after a photograph by Sommer).
It is generally the case that a single eruption makes but a relatively small contribution to the bulk of the mountain. From each new cone or bocca there proceeds a stream of lava spread in a relatively narrow stream extending down the slopes ([Fig. 126]).
Fig. 126.—Sketch map of Etna, showing the individual surface lava streams (in black) and the tuff covered surface (stippled).
The caldera of composite cones.—Because of the varied episodes in the history of composite cones, they lack the regular lines characteristic of the two simpler types. The larger number of the more important composite cones have been built up within an outer crater of relatively large diameter, the Somma cone or caldera, which surrounds them like a gigantic ruff or collar. This caldera is clearly in most cases at least the relic of an earlier explosive crater, after which successive eruptions of lesser violence have built a more sharply conical structure. This can only be interpreted to mean that most larger and long-active volcanoes have been born in the grandest throes of their life history, and that a larger or smaller lateral migration of the vent has been responsible for the partial destruction of the explosion crater. Upon Vesuvius we find the crescent-like rim of Monte Somma; on Etna it is the Val del Bove, etc. It is this caldera of composite cones which gave rise to the theory of the “elevation crater” of von Buch (see ante, [p. 95], and [Fig. 127]).
Fig. 127.—Panum crater, showing the caldera and the later interior cones (after Russell).
The eruption of Vesuvius in 1906.—The volcano Vesuvius rises on the shores of the beautiful bay of Naples only about ten miles distant from the city of Naples. The mountain consists of the remnant of an earlier broad-mouthed explosion crater, the Monte Somma, and an inner, more conical elevation, the Monte Vesuvio. Before the eruption of 1906 this central cone was sharply conical and rose to a height of about 4300 feet above the surface of the bay, or above the highest point of the ancient caldera. The base of this inner cone is at an elevation of something less than half that of the entire mass, and is separated from the encircling ring wall of the old crater by the atrio, to which corresponds in height a perceptible shelf or piano upon the slope toward the bay of Naples ([Fig. 128]).
Fig. 128.—View of Mount Vesuvius as it appeared from the Bay of Naples shortly before the eruption of 1906. The horn to the left is Monte Somma.
An active composite cone like that of Vesuvius is for the greater part of the time in the Strombolian condition; that is to say, light crater explosions continue with varying intensity and interval, except when the mountain has been excited to the periodic Vulcanian outbreaks with which its history has been punctuated. The Strombolian explosions have sufficient violence to eject small fragments of hot lava, which, falling about the crater, slowly built up a rather sharp cone. The period of Strombolian activity has, therefore, been called the cone-producing period. Just before each new outbreak of the Vulcanian type, the altitude of the mountain has, therefore, reached a maximum, and since the larger explosive eruptions remove portions of this cone at the same time that they increase the dimensions of the crater, the Vulcanian stage in contrast to the other has been called the crater-producing period. In this period, then, the material ejected during the explosions does not consist solely of fresh lava cakes, but in part of the older débris derived from the crater walls, whence it is avalanched upon the chimney after each larger explosion. The overhanging cloud, which during the Strombolian period has consisted largely of steam and is noticeably white, now assumes a darker tone, the “smoke” which characterizes the Vulcanian eruption.
Fig. 129.—A series of consecutive sketches of the summit of the Vesuvian cone, showing the modifications in its outline (after Sir William Hamilton).
On several historical occasions the cone of Vesuvius has been lowered by several hundred feet, the greatest of relatively recent truncations having occurred in 1822 and in 1906. Between Vulcanian eruptions the Strombolian activity is by no means uniform, and so the upward growth of the cone is subject to lesser interruptions and truncations ([Fig. 129]).
The Vesuvian eruption of 1906 has been selected as a type of the larger Vulcanian eruption of composite cones, because it combined the explosive and paroxysmal elements, and because it has been observed and studied with greater thoroughness than any other. The latest previous eruption of the Vulcanian order had occurred in 1872. Some two years later the period of active cone building began and proceeded with such rapidity that by 1880 the new cone began to appear above the rim of the crater of 1872. From this time on occasional light eruptions interrupted the upbuilding process, and as the repairs were not in all cases completed before a new interruption, a nest of cones, each smaller than the last, arose in series like the outdrawn sections of an old-time spyglass. At one time no less than five concentric craters were to be seen.
For a brief period in the fall of 1904 Vesuvius had been in almost absolute repose, but soon thereafter the Strombolian crater explosions were resumed. On May 25, 1905, a small stream of lava began to issue from a fissure high up upon the central cone, and from this time on the lava continued to flow down to the valley or atrio, separating the inner cone from the caldera remnant of Monte Somma. Seen in the night, this stream of lava appeared from Naples like a red hot wire laid against the mountain’s side ([Fig. 130]). With gradual augmentation of Strombolian explosions and increase in volume of the flowing lava stream, the same condition continued until the first days of April in 1906. The flowing lava had then overrun the tracks of the mountain railway and accumulated in considerable quantity within the atrio ([Fig. 131]).
Fig. 130.—Night view of Vesuvius from Naples before the outbreak of 1906. A small lava stream is seen descending from a high point upon the central cone (after Mercalli).
On the morning of April 4, a preliminary stage of the eruption was inaugurated by the opening of a new radial fissure about 500 feet below the summit of the cone ([Fig. 132 a]), and by early afternoon the cone-destroying stage began with the rise of a dark “cauliflower cloud” or pino to replace the lighter colored steam cloud. The cone was beginning to fall into the crater, and old lava débris was mingled in the ejections with the lava clots blown from the still fluid material within the chimney. From now on short and snappy lightning flashes played about the black cloud, giving out a sharp staccato “tack-a-tack.” The volume and density of the cloud and the intensity of the crater explosions continued to increase until the culmination on April 7. On April 5 at midnight a new lava mouth appeared upon the same fissure which had opened near the summit, but now some 300 feet lower ([Fig. 132 b]). The lava now welled out in larger volume corresponding to its greater head, and the stream which for ten months had been flowing from the highest outlet upon the cone now ceased to flow. The next morning, April 6, at about 8 o’clock, lava broke out at several points some distance east of the opening b, and evidently upon another fissure transverse to the first ([Fig. 132 c]). The lava surface within the chimney must still have remained near its old level,—effective draining had not yet begun,—since early upon the following morning a small outflow began nearly at the top of the cone upon the opposite side and at least a thousand feet higher.
Fig. 131.—Scoriaceous lava encroaching upon the tracks of the Vesuvian railway (after a photograph by Sommer).
Fig. 132.—Map of Vesuvius, showing the position and order of formation of the lava mouths upon its flanks during the eruption of 1906 (after Johnston-Lavis).
The culmination of the eruption came in the evening of April 7, when, to the accompaniment of light earthquakes felt as far as Naples, lava issued for the first time in great volume from a mouth more than halfway down the mountain side ([Fig. 132 f]), and thus began the drainage of the chimney. At about the same time with loud detonations a huge black cloud rose above the crater in connection with heavy explosions, and a rain of cinder was general in the region about the mountain but especially within the northeast quadrant. Those who were so fortunate as to be in Pompeii had a clear view of the mountain’s summit where red hot masses of lava were thrown far into the air. The direction of these projections was reported to have been not directly upward, but inclined toward the northeast quadrant of the mountain; but since with a northeast surface wind the heaviest deposit of ash and dust should have been upon the southwestern quadrant of the mountain, it is evident that the material was carried upward until it reached the contrary upper currents of the atmosphere, to be by them distributed.
Fig. 133.—The ash curtain which had overhung Vesuvius lifting and disclosing the outlines of the mountain on April 10, 1911 (after De Lorenzo).
Fig. 134.—The central cone of Vesuvius as it appeared after the eruption of 1906, but with the earlier profile indicated. The truncation represents a lowering of the summit by some five hundred feet, with corresponding increase in the diameter of the crater (after Johnston-Lavis).
When the heavy curtain of ash, which now for a number of succeeding days overhung all the circum-Vesuvian country, began to lift ([Fig. 133]), it was seen that the summit of the cone had been truncated an average of some 500 feet ([Fig. 134]). All the slopes and much of the surrounding country had the aspect of being buried beneath a cocoa-colored snow of a depth to the northeastward of several feet, where it had drifted into all the hollow ways so as almost to efface them ([Fig. 135]). More than thrice as heavy as water, the weak roof timbers of the houses at the base of the mountain gave way beneath the added load upon them, thus making many victims. Inasmuch, however, as the ash-fall partakes of the same general characters as in eruptions from cinder cones, we may here give our attention especially to the streams of lava which issued upon the opposite flank of the mountain ([Fig. 136]).
Fig. 135.—A sunken road filled with indrifted cocoa-colored ash from the Vesuvian eruption of 1906.
Fig. 136.—View of Vesuvius taken from the southwest during the waning stages of the eruption of 1906. In the middle distance may be discerned the several lava mouths aligned upon a fissure, and the courses of the streams which descend from them. In the foreground is the main lava stream with scoriaceous surface (after W. Prinz).
————
The main lava stream descended the first steep slopes with the velocity of a mile in twenty-five minutes, about the strolling speed of a pedestrian, but this rate was gradually reduced as the stream advanced farther from the mouth. Taking advantage of each depression of the surface, the black stream advanced slowly but relentlessly toward the cities at the southwest base of the mountain. With a motion not unlike that of a heap of coal falling over itself down a slope, the block lava advances without burning the objects in its path ([Fig. 137]).
Fig. 137.—The main lava stream of 1906 advancing upon the village of Boscotrecase.
Fig. 138.—An Italian pine snapped off by the lava and carried forward upon its surface as a passenger (after Haug).
————
The beautiful pines are merely charred where snapped off and are carried forward upon the surface of the stream ([Fig. 138]). When a real obstruction, such as a bridge or a villa, is encountered, the stream is at first halted, but the rear crowding upon the van, unless a passage is found at the side, the lava front rises higher and higher until by its weight the obstruction is forced to give way ([Figs. 139] and [140]).
Fig. 139.—Lava front both pushing over and running around a wall which lies athwart its course (after Johnston-Lavis).
Fig. 140.—One of the villas in Boscotrecase which was ruined by the Vesuvian lava flow of 1906. The fragments of masonry from the ruined walls traveled upon the lava current, where they sometimes became incased in lava.
The sequence of events within the chimney.—The thorough study of this Vesuvian eruption has placed us in a position to infer with some confidence in our conclusions the sequence of events within the chimney and crater of the volcano, both before and during the eruption. Anticipating some conclusions derived from the observed dissection of volcanoes, which will be discussed below, it may be stated that what might be termed the core of the composite cone—the chimney—is a more or less cylindrical plug of cooled lava which during the active period of the vent has an interior bore of probably variable caliber. This plug in its lower section appears in solid black in all the diagrams of [Fig. 141]. During the cone-building period ([Fig. 141 a and b]) the plug is obviously built upward along with the cone, for lava often flows out at a level a few hundred feet only below the crater rim. By what process this chimney building goes on is not well understood, though some light is thrown upon it by the post-eruption stage of Mont Pelé in 1902-1903 (see below).
Fig. 141.—Three diagrams to illustrate the sequence of events within the crater of a composite cone during the cone-building and crater-producing periods. a and b, two successive stages of the cone building or Strombolian period; c, enlargement of the crater, truncation of the cone, and destruction of the upper chimney during the relatively brief crater-producing or Vulcanian period.
Both the older and newer sections of this plug or chimney are furnished some support against the outward pressure of the contained lava by the surrounding wall of tuff; and they are, therefore, in a condition not unlike that of the inner barrel of a great gun over which sleeves of metal have been shrunk so as to give support against bursting pressures. On the other hand, when not sustaining the hydrostatic pressure of the liquid lava within, the chimney would tend to be crushed in by the pressure of the surrounding tuff. Its strength to withstand bursting pressures is dependent not alone upon the thickness of its rock walls, but also upon its internal diameter or caliber. A steam cylinder of given thickness of wall, as is well known, can resist bursting pressures in proportion as its internal diameter is small. So in the volcanic chimney, any tendency to remelt from within the chimney walls must weaken them in a twofold ratio.
We are yet without accurate temperature observations upon the lava in volcanic chimneys, but it seems almost certain that these temperatures rise as the Vulcanian stage is approaching, and such elevation of temperature must be followed by a greater or less re-fusion of the chimney walls. The sequence of events during the late Vesuvian eruption is, then, naturally explained by progressive re-fusion and consequent weakening of the chimney walls, thus permitting a radial fissure to open near the top and gradually extend downwards. Thus at first small and high outlets were opened insufficient to drain the chimney, but later, on April 7, after this fissure had been much extended and a new and larger one had opened at a lower level, the draining began and the surface of lava commenced rapidly to sink.
Fig. 142.—The spine of Pelé rising above the chimney of the volcano after the eruption of 1902 (after Hovey).
When the rapid sinking of the lava surface occurred, the lower lava layers were almost immediately relieved of pressure, thus causing a sudden expansion of the contained steam and resulting in grand crater explosions. The partially refused and fissured upper chimney, now unable to withstand the inward pressure of the surrounding tuff walls, since outward pressures no longer existed, crushed in and contributed its materials and those of the surrounding tuff to the fragments of fresh lava rising in volume in the grand explosions ([Fig. 141 c]). In outline, then, these seem to be the conditions which are indicated by the sequence of observed events in connection with the late Vesuvian outbreak.
Fig. 143.—Outlines of the Pelé spine upon successive dates. The full line represents its outline on December 26, 1902; the dotted-dashed line is a profile of January 3, 1902; while the dotted line is that of January 9, 1903. The dark line is a fissure (after E. O. Hovey).
The spine of Pelé.—The disastrous eruption of Mont Pelé upon Martinique in the year 1902 is of importance in connection with the interesting problem of the upward growth of volcanic chimneys during the cone-building period of a volcano. After the conclusion of this great Vulcanian eruption, a spine of lava grew upward from the chimney of the main crater until it had reached an elevation of more then a thousand feet above its base, a figure of the same order of magnitude as the probable height of the upper section of the Vesuvian chimney previous to the eruption of 1906. The Pelé spine ([Fig. 142]) did not grow at a uniform rate, but was subject to smaller or larger truncations, but for a period of 18 days the upward growth was at the rate of about 41 feet per day. Later, the mass split upon a vertical plane revealing a concave inner surface, and was somewhat rapidly reduced in altitude to 600 feet ([Fig. 143]), only to rise again to its full height of about 1000 feet some three months later.
While apparently unique as an observed phenomenon, and not free from uncertainty as to its interpretation, the growth of this obelisk has at least shown us that a mass of rock can push its way up above the chimney of an active volcano even when there are no walls of tuff about it to sustain its outward pressures.
Fig. 144.—Corrugated surface of the Vesuvian cone after the mud flows which followed the eruption in 1906 (after Johnston-Lavis).
The aftermath of mud flows.—When the late Vulcanian explosions of Vesuvius had come to an end, all slopes of the mountain, but especially the higher ones, were buried in thick deposits of the cocoa-colored ash, included in which were larger and smaller projectiles. As this material is extremely porous, it greedily sucks up the water which falls during the first succeeding rains. When nearly saturated, it begins to descend the slopes of the mountain and soon develops a velocity quite in contrast with that of the slow-moving lava. The upper slopes are thus denuded, while the fields and even the houses about the base are invaded by these torrents of mud (lava d’acqua). Inasmuch as these mud flows are the inevitable aftermath of all grander explosive eruptions, the Italian government has of late spent large sums of money in the construction of dikes intended to arrest their progress in the future. It was streams of this sort that buried the city of Herculaneum after the explosive eruption of 79 A.D.
After the mud flows have occurred, the Vesuvian cone, like all similar volcanic cones under the same conditions, is found with deep radial corrugations ([Fig. 144]), such as were long ago described as “barrancoes” and supposed to support the “elevation crater” theory of volcano formation.
The dissection of volcanoes.—To the uninitiated it might appear a hopeless undertaking to attempt to learn by observation the internal structure of a volcano, and especially of a complex volcano of the composite type. The earliest successful attempt appears to have been made by Count Caspar von Sternberg in order to prove the correctness of the theory of his friend, the poet Goethe. Goethe had claimed that a little hill in the vicinity of Eger, on the borders of Bohemia, was an extinct volcano, though the foremost geologist of the time the famous Werner, had promulgated the doctrine that this hill, in common with others of similar aspect, originated in the combustion of a bed of coal. The elevation in question, which is known as the Kammerbühl, consists mainly of cinder, and Goethe had maintained that if a tunnel were to be driven horizontally into the mountain from one of its slopes, a core or plug of lava would be encountered beneath the summit. The excavations, which were completed in 1837, fully verified the poet’s view, for a lava plug was found to occupy the center of the mass and to connect with a small lava stream upon the side of the hill ([Fig. 145]).
Fig. 145.—The Kammerbühl near Eger, showing the tunnel completed in 1837 which proved the volcanic nature of the mountain (after Judd).
It is not, however, to such expensive projects that reference is here made, but rather to processes which are continually going on in nature, and on a far grander scale. The most important dissecting agent for our purpose is running water, which is continually paring down the earth’s surface and disclosing its buried structures. How much more convincing than any results of artificial excavation, as evidence of the internal structure of a volcano, is the monument represented in [Fig. 146], since here the lava plug stands in relief like a gigantic thumb still surrounded by a remnant of cinder deposits. Such exposed chimneys of former volcanoes are found in many regions, and have become known as volcanic necks, pipes, or plugs.
Fig. 146.—Volcanic plug exposed by natural dissection of a volcanic cone in Colorado (U. S. G. S.).
Fig. 147.—A dike cutting beds of tuff in a partly dissected volcano of southwestern Colorado (after Howe, U. S. G. S.).
Not infrequently the beds of tuff composing the flanks of the volcano, upon dissection by the same process, bring to light walls of cooled lava standing in relief ([Fig. 147])—the filling of the fissure which gave outlet to the flanks of the mountain at the time of the eruption. Study of exposed dikes formed in connection with recent eruptions of Vesuvius has shown that in many instances they are still hollow, the lava having drained from them before complete consolidation.
Another agent which is effective in uncovering the buried structures of volcanoes is the action of waves on shores. Always a relatively vigorous erosive agency, the softer structures of volcanic cones are removed with especial facility by this agent. On the shores of the island of Volcano, the little cone of Vulcanello has been nearly half carried away by the waves, so as to reveal with especial perfection the structure of the cinder beds as well as the internal rock skeleton of the mass. Here the characteristic dips of lava streams, intercalated as they now are between tuff deposits and the lava which consolidated in fissures, are both revealed.
Fig. 148.—Map and general view of St. Paul’s Rocks, a volcanic cone dissected by waves.
In mid-Atlantic a quite perfect crater, the St. Paul’s Rocks, has been cut nearly in half so as to produce a natural harbor ([Fig. 148]).
In still other instances we may thank the volcano itself for opening up the interior of the mountain for our inspection. The eruption in 1888 of the Japanese volcano of Bandai-san, by removing a considerable part of the ancient cone, has afforded us a section completely through the mountain. The summit and one side of the small Bandai was carried completely away, and there was substituted a yawning crater eccentric to the former mountain and having its highest wall no less than 1500 feet in height ([Fig. 149]). In two hours from the first warning of the explosion the catastrophe was complete and the eruption over.
Fig. 149.—Dissection by explosion of Little Bandai-san in 1888 (after Sekiya).
The eruption of Krakatoa in 1883, probably the grandest observed volcanic explosion in historic times, left a volcanic cone divided almost in half and open to inspection ([Fig. 150]). Rakata, Danan, and Perbuatan had before constituted a line of cones built up round individual craters subsequent to the partial destruction of an earlier caldera, portions of which were still existent in the islands Verlaten and Lang. By the eruption of 1883 all the exposed parts and considerable submerged portions of the two smaller cones were entirely destroyed, and the larger one, known as Rakata, was divided just outside the plug so as to leave a precipitous wall rising directly from the sea and showing lava streams in alternation with somewhat thicker tuff layers, the whole knit together by numerous lava dikes.
Fig. 150.—The half-submerged volcano of Krakatoa in the Sunda Straits before and after the eruption of 1883 (after Verbeek).
In order to carry our dissecting process down to levels below the base of the volcanic mountain, it is usually necessary to inspect the results of erosion by running water. Here the plug or chimney, instead of being surrounded by tuff, is inclosed by the country rock of the region, which is commonly a sedimentary formation. Such exposed lower sections of volcanic chimneys are numerous along the northwestern shores of the British Isles. Where aligned upon a dislocation or noteworthy fissure in the rocks, the group of plugs has been referred to as a scar or cicatrice ([Fig. 151]). Associated with the plugs of the cicatrice are not infrequently dikes, or, it may be, sheets of lava extended between layers of sediment and known as sills.
Fig. 151.—The cicatrice of the Banat (after Suess).
If we are able to continue the dissection process to still greater depths, we encounter at last igneous rock having a texture known as granitic and indicating that the process of consolidation was not only exceedingly slow but also uninterrupted. This rock is found in masses of larger dimensions, and though generally of more or less irregular form, no one dimension is of a different order of magnitude from the others. Such masses are commonly described as bosses, or, if especially large, as batholites ([Fig. 152]). Wherever the rock beds appear as though they had been forced up by the upward pressure of the igneous mass, the latter takes the form of a mushroom and has been described as a laccolite ([Figs. 479-481], [pp. 441-442]). Evidence seems, however, to accumulate that in the greater number of cases the molten rock has fused its way upward, in part assimilating and in part inclosing the rock which it encountered. This process of upward fusion has been likened to the progress of a red hot iron burning its way through a board.
The formation of lava reservoirs.—The discarding of the earlier notion that the earth has a liquid interior makes it proper in discussing the subject of volcanoes to at least touch upon the origin of the molten rock material. As already pointed out, such reservoirs as exist must be local and temporary, or it would be difficult to see how the existing condition of earth rigidity could be maintained. From the rate at which rock temperatures rise, at increasing depths below the surface, it is clear that all rocks would be melted at very moderate depths only, if they were not kept in a solid state by the prodigious loads which they sustain. Any relief from this load should at once result in fusion of the rock.
Fig. 152.—Diagram to illustrate a probable cause of formation of lava reservoirs, and to show the connection between such reservoirs and the volcanoes at the surface.
Now the restriction of active volcanoes to those zones of the earth’s surface within which mountains are rising, and where in consequence earthquakes are felt, has furnished us at least a clew to the origin of the lava. Regarded as a structure capable of sustaining a load, the competency of an arch is something quite remarkable, so that the arching up of strong rock formations into anticlines within the upper layers of the zone of flow, or of combined fracture and flow, would be sufficient to remove the load from relatively weak underlying beds, which in consequence would be fused and form local reservoirs of lava ([Figs. 152] and [153]).
It has been further quite generally observed that lines of volcanoes, in so far as they betray any relation in position to neighboring mountain ranges, tend to appear upon the rear or flatter limb of unsymmetrical arches, or where local tension would favor the opening of channels toward the surface. Moreover, wherever recent block movements of surface portions of the earth’s shell have been disclosed in the neighborhood of volcanoes, the latter appear to be connected with downthrown blocks, as though the lava had, so to speak, been squeezed out from beneath the depressed block or blocks.
Fig. 153.—Result of experiment with layers of composition to illustrate the effect of relief of load upon rocks by arching of competent formation (after Willis).
We must not, however, forget that the igneous rocks are greatly restricted in the range of their chemical composition. No igneous rock type is known which could be formed by the fusion of any of the carbonate rocks such as limestone or dolomite, or of the more siliceous rocks, such as sandstone or quartzite. There remains only the argillaceous class of sediments, the shales and slates, and so soon as we examine the composition of these rocks we are struck by the remarkable resemblance to that of the class of igneous rocks. For purposes of comparison there is given below the composite or average constitution of igneous rocks in parallel column, with the average attained by combining the analyses of 56 slates and shales, the latter recalculated with water excluded:
| Average Igneous Rock | Average Shale | ||||||||
| (Clark) | (Washington) | ||||||||
| SiO2 | 61.25 | 61.69 | 63.34 | ||||||
| Al2O3 | 15.81 | 15.94 | 15.56 | ||||||
| Fe2O3 | 2.70 | } | 6.31 | 1.88 | } | 4.53 | 4.41 | } | 7.89 |
| FeO | 3.61 | 2.65 | 3.48 | ||||||
| MgO | 4.47 | 4.90 | 3.54 | ||||||
| CaO | 5.03 | 5.02 | 3.33 | ||||||
| Na2O | 3.64 | 4.09 | 1.29 | ||||||
| K2O | 2.87 | 3.35 | 3.52 | ||||||
| TiO2 | .62 | .48 | .53 | ||||||
| 100.00 | 100.00 | 100.00 | |||||||
This close resemblance is probably of deep significance, for the reason that shales and slates are structurally the weakest of all rocks and for the further reason that they rather generally directly underlie the carbonate rocks, which are by contrast the strongest (see ante, [p. 37]). For these reasons shales and slates are the only rocks which are likely to be fused by relief from load through the formation of anticlinal arches within the earth’s zone of flow. If this view is well founded, lavas and other igneous rocks are in large part fused argillaceous sediments formed in connection with the process of folding, or are refused rocks of igneous origin and similar composition.
Character profiles.—The character profiles of features connected in their origin with volcanoes are particularly easy to recognize, and in a few cases in which they might be confused with others of a different origin, an examination of the materials of the features should lead to a definitive judgment.
The lava plains which result from massive outflows of basalt might perhaps strictly be regarded as lack of feature, so great may be their continuous extent. Wherever definite vents exist, a broad flat dome is the usual result of the extravasation of a basaltic lava. The puys of France and many of the Kuppen of Germany, being formed from less fluid lava, have afforded profiles with relatively small radius of curvature.
In its youthful stage, the cinder cone usually presents a broad summit sag and relatively short side slopes, whereas the cone of later stages is apt to present long sweeping and upwardly concave curves with both the gradient and the radius of curvature increasing rapidly toward the summit. In contrast, too, with the earlier stage, the crest is relatively small. A marked reduction in the high symmetry of such profiles is noted wherever a breaching by lava outflow has occurred ([Fig. 154]).
With the composite cone, complexity and corresponding lack of symmetry is introduced, especially in the partially ruined caldera, and by the more or less accidental distribution of parasitic cones, as well as by migrations of the central cone. Peculiarly similar acuminated profiles result from spatter-cone formation, from the formation of a superchimney spine, and by the uncovering of the chimney through denudational processes—the volcanic neck.
Fig. 154.—Character profiles connected with volcanoes.
Another important feature resulting from denudation is the Mesa or table mountain with its protecting basalt cap above softer rocks. Its profile most resembles that of table mountains due to differential erosion of alternately strong and weak horizontally bedded rocks, such as compose the upper portion of the section in the Grand Cañon of the Colorado. Here, however, in place of a single unusually strong top layer there are found several strong layers in alternation with weaker ones so as to produce additional steps in the profile.
Reading References to Chapters IX and X
General works:—
Paulett Scrope. The Geology of the Extinct Volcanoes of Central France. John Murray, London, 1858, pp. 258. (An epoch-making work of early date which, like the following reference, may be studied to advantage to-day.)
Sir Charles Lyell. Principles of Geology, vol. 1, Chapters xxiii-xxv.
Melchior Neumayr. Erdgeschichte, vol. 1, Allgemeine Geologie, revised edition by v. Uhlig, 1897, pp. 133-277 (a storehouse of valuable information clearly presented).
J. D. Dana. Characteristics of Volcanoes, with Contributions of Facts and Principles from the Hawaiian Islands. Dodd, Mead, and Company, New York, 1890, pp. 397.
Tempest Anderson. Volcanic Studies in Many Lands, being reproductions of photographs by the author with explanatory notes. John Murray, London, 1903, pp. 200, pls. 105.
T. G. Bonney. Volcanoes, their Structure and Significance. John Murray, London, 1899, pp. 331.
I. C. Russell. Volcanoes of North America. Macmillan, New York, 1897, pp. 346.
Elisée Réclus. Les volcans de la terre, Belgian Society of Astronomy, Meteorology, and Physics of the Globe, 1906-1910 (a valuable descriptive geographical and bibliographical work of reference).
G. Mercalli. I vulcani attivi della terre. Hoepli, Milan, 1907, pp. 421. (A most valuable work, beautifully illustrated, but in the Italian language.)
Arrangement of volcanic vents:—
Th. Thoroddsen. Die Bruchlinien und ihre Beziehungen zu den Vulkanen, Pet. Mitt., vol. 51, 1905, pp. 1-5, pl. 5.
R. D. M. Verbeek. Various volumes and atlases of maps covering the Dutch East Indies and fully cited in the following reference (p. 21).
William H. Hobbs. The Evolution and the Outlook of Seismic Geology, Proc. Am. Phil. Soc., vol. 48, 1909, pp. 17-27.
Birth of volcanoes:—
F. Omori. The Usu-san Eruption and Earthquake and Elevation Phenomena, Bull. Earthq. Inv. Com., Japan, vol. 5, No. 1, 1911, pp. 1-37, pls. 1-13.
Fissure eruptions:—
Th. Thoroddsen. Island, IV, Vulkane, Pet. Mitt., Ergänzungsh. 153, 1906, pp. 108-111.
A. Geikie. Text-book of Geology, 4th ed., pp. 342-346.
Lava domes of Hawaii:—
J. D. Dana. Characteristics of Volcanoes (as above).
C. H. Hitchcock. Hawaii and Its Volcanoes. Honolulu, 1909, pp. 314.
Eruption of Matavanu volcano in 1906:—
Karl Sapper. Der Matavanu-Ausbruch auf Savaii, 1905-1906, Zeit. d. Gesell. f. Erdk. z. Berlin, vol. 19, 1906, pp. 686-709, 4 pls.
H. J. Jensen. The Geology of Samoa, and the Eruptions in Savaii, Proc. Linn. Soc., New South Wales, vol. 31, 1906, pp. 641-672, pls. 54-64.
Tempest Anderson. The Volcano of Matavanu in Savaii, Quart. Jour. Geol. Soc., London, vol. 66, 1910, pp. 621-639, pls. 45-52.
Eruption of Volcano in 1888:—
H. J. Johnston-Lavis. The South Italian Volcanoes. Naples, 1891, pp. 342, pls. 16.
Eruption of Taal volcano in 1911:—
W. E. Pratt. The Eruption of Taal Volcano, January 30, 1911, Phil. Jour. Sci., vol. 6, No. 2, Sec. A, 1911, pp. 63-86, pls. 1-14.
F. H. Noble. Taal Volcano, album of views of 1911 eruption, Manila, 1911, pp. 1-48.
The volcano of Etna:—
G. vom Rath. Der Aetna. Bonn, 1872, pp. 1-33. (A beautiful piece of descriptive writing from both the geological and scenic standpoints.)
Sartorius von Waltershausen. Der Aetna. Leipzig, 1880, 2 quarto vols., pp. 371 and 548.
The eruption of Vesuvius in 1906:—
H. J. Johnston-Lavis. Geological Map of Monte Somma and Vesuvius, with a short and concise account, etc. Geo. Philip & Son, London, 1891.
H. J. Johnston-Lavis. The Eruption of Vesuvius in April, 1906, Trans. Roy. Dublin Soc., vol. 9, 1909, Pt. VIII, pp. 139-200, pls. 3-23 (the most authoritative work upon the subject).
T. A. Jaggar, Jr. The Volcano Vesuvius in 1906, Tech. Quart., vol. 19, 1906, pp. 105-115.
W. Prinz. L’éruption du Vesuv d’avril, 1906, Ciel et Terre, 27e Année, 1906, pp. 1-49.
Frank A. Perret. Notes on the Electrical Phenomena of the Vesuvian Eruption, April, 1906, Sci. Bull., Brooklyn Inst. Arts and Sci., vol. 1, No. 11, pp. 307-312; Vesuvius, Characteristics and Phenomena of the Present Repose Period, Am. Jour. Sci., vol. 28, 1909, pp. 413-430.
William H. Hobbs. The Grand Eruption of Vesuvius in 1906, Jour. Geol., vol. 14, 1906, pp. 636-655.
The spine of Pelée:—
E. O. Hovey. The New Cone of Mont Pelée and the Gorge of the Rivière Blanche, Martinique, Am. Jour. Sci., vol. 16, 1903, pp. 269-281, pls. 11-14.
A. Heilprin. The Tower of Pelée. Philadelphia, 1904, pp. 62, pls. 22.
A. Lacroix. La montagne Pelée et ses éruptions, Acad. des Sciences, Paris, 1904, Chapter iii.
Karl Sapper. In den Vulkangebieten Mittelamerikas und Westindiens, Stuttgart, 1905, pp. 172-178.
A. C. Lane. Absorbed Gases of Vulcanism, Science, N.S., vol. 18, 1903, p. 760.
G. K. Gilbert. The Mechanism of the Mont Pelée Spine, ibid., vol. 19, 1904, pp. 927-928.
I. C. Russell. Pelée Obelisk once More, ibid., vol. 21, 1905, pp. 924-931.
The dissection of volcanoes:—
J. W. Judd. Volcanoes, Chapter v.
S. Sekya and Y. Kikuchi. The Eruption of Bandai-San, Trans. Seis. Soc., Japan, vol. 13, Pt. 2, 1890, pp. 140-222, pls. 1-9.
R. D. M. Verbeek. Krakatau. Batavia, 1885, pp. 557, pls. 25.
Royal Society. The Eruption of Krakatoa and Subsequent Phenomena. London, 1888, pp. 494.
G. K. Gilbert. Report on the Geology of the Henry Mountains, U.S. Geogr. and Geol. Surv., Rocky Mt. Region, Washington, 1877, pp. 22-60.
Sir A. Geikie. Ancient Volcanoes of Great Britain, vol. 2 especially.
D. W. Johnson. Volcanic Necks of the Mount Taylor Region, New Mexico, Bull. Geol. Soc. Am., vol. 18, 1907, pp. 303-324, pls. 25-30.