AMERICAN SCIENCE SERIES—ADVANCED COURSE

GEOLOGY

BY
THOMAS C. CHAMBERLIN AND ROLLIN D. SALISBURY

Heads of the Departments of Geology and Geography, University of Chicago
Members of the United States Geological Survey
Editors of the Journal of Geology

IN THREE VOLUMES

VOL. I.—GEOLOGIC PROCESSES AND THEIR RESULTS

SECOND EDITION, REVISED

NEW YORK
HENRY HOLT AND COMPANY
1909

Copyright, 1904,
BY
HENRY HOLT AND COMPANY

PRINTED IN THE U. S. A.

PREFACE.

In the preparation of this work it has been the purpose of the authors to present an outline of the salient features of geology, as now developed, encumbered as little as possible by technicalities and details whose bearings on the general theme are unimportant. In common with most writers of text-books on geology, the authors believe that the subject is best approached by a study of the forces and processes now in operation, and of the results which these forces and processes are now bringing about. Such study necessarily involves a consideration of the principles which govern the activities of geologic agencies. These topics are presented in Volume I, and prepare the way for the study of the history of past ages, which is outlined in Volume II.

The general plan of the work has been determined by the experience of the authors as instructors. Little emphasis is laid on the commonly recognized subdivisions of the science, such as dynamic geology, stratigraphic geology, physiographic geology, etc. The treatment proceeds rather from the point of view that the science is a unit, that its one theme is the history of the earth, and that the discussions of dynamic geology, physiographic geology, etc., apart from their historical bearing, lose much of their significance and interest. The effort has, therefore, been to emphasize the historical element, even in the discussion of special themes, such as the work of rivers, the work of snow and ice, and the origin and descent of rocks. This does not mean that phases of geology other than historical have been neglected, but it means that an effort has been made to give a historical cast to all phases of the subject, so far as the topics permit.

Throughout the work the central purpose has been not merely to set forth the present status of knowledge, but to present it in such a way that the student will be introduced to the methods and spirit of the science, led to a sympathetic interest in its progress, and prepared to receive intelligently, and to welcome cordially, its future advances. Where practicable, the text has been so shaped that the student may follow the steps which have led to present conclusions. To this end the working methods of the practical geologist have been implied as frequently as practicable. To this end also there has been frankness of statement relative to the limitations of knowledge and the uncertainty of many tentative conclusions. In these and in other respects, the purpose has been to take the student into the fraternity of geologists, and to reveal to him the true state of the development of the science, giving an accurate and proportionate view of the positive knowledge attained, of the problems yet unsolved, or but partially solved, and of solutions still to be attained.

The theoretical and interpretative elements which enter into the general conceptions of geology have been freely used, because they are regarded as an essential part of the evolution of the science, because they often help to clear and complete conceptions, and because they stimulate thought. The aim has been, however, to characterize hypothetical elements as such, and to avoid confusing the interpretations based on hypothesis, with the statements of fact and established doctrines. Especial care has been taken to recognize the uncertain nature of prevalent interpretations when they are dependent on unverified hypotheses, especially if this dependence is likely to be overlooked. If this shall seem to give prominence to the hypothetical element, it should also be regarded as giving so much the more emphasis to that which is really trustworthy, in that it sets forth more frankly that which is doubtful. Hypothetical and unsolved problems have been treated, so far as practicable, on the multiple basis; that is, alternative hypotheses and alternative interpretations are frequently presented where knowledge does not warrant positive conclusions.

In many cases the topics discussed will be found to be presented in ways differing widely from those which have become familiar. In some cases, fundamentally new conceptions of familiar subjects are involved; in others, topics not usually discussed in text-books are stated with some fullness; and in still others, the emphasis is laid on points which have not commonly been brought into prominence. Whether the authors have been wise in departing to this extent from beaten paths, the users of the volumes must decide.

The work is intended primarily for mature students, and is designed to furnish the basis for a year’s work in the later part of the college course. By judicious selection of material to be presented and omitted, the volumes will be found useful for briefer courses, and by the use of the numerous references to the fuller discussions of special treatises, they may be made the basis for more extended courses than are commonly given in undergraduate work. The attempt has also been made to make the volumes readable, in the belief that many persons not in colleges or universities will be interested in following a connected account of the earth’s history, and of the means by which that history is recorded and read. Antecedent elementary courses in geology will not be necessary to the use of these volumes, though such courses may be helpful.

The arrangement of themes adopted is such as to bring to the fore processes with which all students are immediately in contact, and which are available for study at all seats of learning. The commoner geologic agents, such as the atmosphere and running water, have been elaborated somewhat more fully than is customary, and the common rather than the exceptional phases of the work of these agents have been emphasized, both because of their greater importance and their universal availability. The text has been so shaped as to suggest field work in connection with these topics especially, since work of this sort is everywhere possible.

After the preliminary outline, which is intended to give some idea of the scope of the science, and of its salient features, and to show the relations of the special subjects which follow, the order of treatment is such as to pass from the commoner and more readily apprehended portions of the subject to those which are less readily accessible and more obscure. Following the same general conception, the treatment of the topics is somewhat graded, the earlier chapters being developed with greater simplicity and fullness, while the later are somewhat more condensed.

Many acknowledgments are made in the text and foot-notes, but it is impossible to adequately acknowledge all the sources which have been drawn upon, since the whole body of literature has been laid under contribution. The authors especially acknowledge the generous assistance of Professor J. P. Iddings in connection with the chapter on The Origin and Descent of Rocks; of Dr. F. R. Moulton, Professor C. S. Slichter, Professor L. M. Hoskins, Mr. A. C. Lunn, and Mr. W. H. Emmons in connection with mathematical problems; of Professor C. R. Barnes in connection with the geologic functions of life; and of Professor Julius Stieglitz in connection with chemical subjects.

The illustrations have been selected from numerous sources, which are usually acknowledged in the text. Especial acknowledgment is due to the U. S. Geological Survey for the use of numerous photographs and maps, and to Mr. G. A. Johnson, who has made many of the drawings reproduced in Volume I. The authors are under even larger obligations for assistance in the preparation of Volume II, for which acknowledgment will be made in the proper place.

University of Chicago, January, 1904.

CONTENTS.

PLATES.

TABLES.

GEOLOGY

CHAPTER I.

PRELIMINARY OUTLINE.

Geology treats of the structure of the earth, of the various stages through which it has passed, and of the living beings that have dwelt upon it, together with the agencies and processes involved in the changes it has undergone. Geology is essentially a history of the earth and its inhabitants. It is one of the broadest of the sciences, and brings under consideration certain phases of nearly all the other sciences, particularly those of astronomy, physics, chemistry, zoology, and botany. It also embraces the earlier expressions of mental development and of life-relationships, chiefly as found in the lower animals.

Subdivisions.—Naturally so broad a science has many special aspects which constitute subdivisions, in a sense, though they are rather dominant phases than independent sections. That phase which treats of the outer relations of the earth is Cosmic or Astronomic geology; that which treats of the constituent parts of the earth and its material is Geognosy, of which the most important branch is Petrology, the science of rocks. That branch which investigates the structural arrangement of the material, or “the architecture of the earth,” is Geotectonic, or Structural geology; while that which deals with the surface changes and topographic forms, that is, with the face of the earth, is Physiographic geology. The study of the fossils that have been preserved in the rocks, and of the faunas and floras that these imply, constitutes Paleontologic geology, or Paleontology. The treatment of the succession of events forms Historical geology. This is chiefly worked out by the succession of beds laid down in the progress of the ages, which constitutes Stratigraphic geology. The treatment of causes, agencies, and processes is the function of Dynamic or Philosophic geology.

Besides these there are special applications which give occasion for other terms, as Economic geology, which is concerned with the industrial applications of geologic knowledge; Mining geology, which is a sub-section of economic geology, relating to the application of geologic facts and principles to mining operations; Atmospheric geology, Glacial geology, and others that define themselves, and are for the greater part but limited aspects of the broad science.

Dominant processes.—Three sets of processes, now in operation on the surface of the lithosphere, have given rise to most of the details of its configuration, and even many of its larger features. These processes have been designated diastrophism, vulcanism, and gradation. Diastrophism includes all crustal movements, whether slow or rapid, gentle or violent, slight or extensive. Many parts of the land, especially along coasts, are known to be slowly sinking relative to the sea-level, while other parts are known to be rising. The fact that rocks originally formed beneath the sea now exist at great elevations, and the further fact that areas which were once land are now beneath the sea, are sufficient evidence that similar changes have taken place in the past. Vulcanism includes all processes connected with the extrusion of lava and other volcanic products, and with the rise of lava from lower to higher levels, even if not extruded. Vulcanism and diastrophism may be closely associated, for local movements at least are often associated with volcanic eruptions, and more considerable movements may be connected with the movements of subsurface lavas, even when the connection is not demonstrable. Gradation includes all those processes which tend to bring the surface of the lithosphere to a common level. Gradational processes belong to two categories—those which level down, degradation, and those which level up, aggradation. The transportation of material from the land, whether by rain, rivers, glaciers, waves, or winds, is degradation and the deposition of material, whether on the land or in the sea, is aggradation. Degradation affects primarily the protuberances of the lithosphere, while aggradation affects primarily its depressions.

Astronomic Geology.

The earth as a planet.—Though supremely important to us, the earth is but one of the minor planets attendant upon the sun, and is in no very special way distinguished as a planetary body. Of the eight planets, four, Jupiter, Saturn, Uranus, and Neptune, are much larger than the earth, while three, Mars, Venus, and Mercury, are smaller. There are a host of asteroids, but all together they do not equal the mass of the smallest planet. The average mass of the eight planets is more than fifty times that of the earth, while the largest, Jupiter, is more than three hundred times as massive as the earth. The earth’s position in the group is in no sense distinguished. It is neither the outer nor the inner, nor even the middle planet. Even in the minor group to which it belongs, it is neither the outermost nor the innermost member, though in this group it is the largest. Its average distance from the sun is about 92.9 million miles, and this fixes its revolution at 365¼ days, for its period of revolution is directly dependent on its distance from the sun, and is necessarily longer than the revolutions of the inner planets and shorter than those of the outer planets. Its rotation in twenty-four hours is not far different from that of its neighbor Mars, but is much slower than the more distant and larger planets, Jupiter and Saturn, which rotate in about ten hours. Comparison cannot be made with the innermost and outermost planets, because their rotations are not yet satisfactorily determined. The plane of the earth’s revolution lies near the common plane of the whole system, but this is not peculiar, as all of the planets revolve in nearly the same plane. Only a few of the small asteroids depart notably from this common plane. This has an important bearing on theories of the origin of the system, since this close coincidence of the planes of the orbits is not consistent with any haphazard aggregation of the material. Of similar importance is the fact that all of the planets revolve in the same direction and in ellipses that do not depart widely from circles. The eccentricity of the earth’s orbit is only about ¹⁄₆₀. This eccentricity varies somewhat, due to the disturbing influences of the other planets, and this variation has been regarded by some geologists as an influential cause of climatic changes, but its adequacy to produce great effects has been doubted by others. The inclination of the earth’s axis, now about 23½°, holds an intermediate position, some of the planets having axes more inclined, as Saturn, 26⅚°, and others less inclined, as Jupiter, 3°. The inclination of the axis is subject to trivial variations at present, and in the long periods of the past has possibly changed more notably. This possible change has also been thought to be a cause of climatic variation, but its efficiency has not been demonstrated.

Its satellite.—The earth is peculiar in having one unusually large satellite, which has a mass ¹⁄₈₁ of its own. The great planets have several satellites whose combined mass exceeds that of the moon, and perhaps in some few cases the individual satellites may be larger than the moon, but they do not sustain so large a ratio to their planets, for Titan, probably the largest, is only ¹⁄₄₆₀₀ of the mass of Saturn. There is little doubt that the moon has played an important part in the history of the earth. It is the chief agency in developing oceanic tides, and it possibly also develops a body tide in the earth itself. These tides act as a brake on the rotation of the earth and tend to reduce its rate, and thereby to lengthen the day. While this may have been counteracted in some measure by the shrinkage of the earth, which tends to increase its rate of rotation, it has been held by eminent physicists and geologists that the rotation of the earth has been greatly lessened during its history, and that a long train of important consequences has resulted. If the contraction of the earth has been sufficient to offset this lessening, the tidal brake must be credited with the prevention of the excessive speed of rotation which would otherwise have been developed. The tides are efficient agencies in the shore wear of the oceans, and in the distribution of marine sediments, and these, it will be seen later, are important elements in the formation of strata.

Dependence on the sun.—By far the most important external relation of the earth, however, is its dependence on the sun. The earth is a mere satellite of the sun, less than ¹⁄₃₀₀₀₀₀ of its mass, and hence under its full gravitative control. The earth is dependent on the sun for nearly all its heat and light, and, through these, for nearly all of the activities that have given character to its history. It is too much to say that all activities on the surface of the earth are solely dependent on those of the sun, for a certain measure of heat and light and other energy is derived from other bodies, and a certain not inconsiderable source of energy is found in the interior of the earth itself; yet all of these are so far subordinate to that great flood of energy which comes from the sun that they are quite inconsequential. The history of the earth in the past has been intimately dependent upon that of the sun, and its future is locked up with the destiny of that great luminary. Geology in its broadest phases can therefore scarcely be separated from the study of the sun, but this falls within the function of the astronomer rather than the geologist.

Meteorites.—There are a multitude of small bodies passing through space in varying directions and with varying velocities and occasionally encountering the earth, to which they add their substance. Some of these meteorites revolve about the sun much as if they were minute planets, but some of them come from such directions and with such velocities as to show that they do not belong to the solar family. Some consist almost wholly of metal, chiefly iron alloyed with a small percent. of nickel (holosiderites); some consist of metal and rock intimately mixed (syssiderites and sporadosiderites); and some consist wholly of rock (asiderites). The rock is usually composed of the heavier basic minerals, though some meteorites consist largely of carbonaceous material. Besides meteorites, there is little doubt that wandering gaseous particles strike the earth, but this is beyond the reach of present demonstration. The amount of substance added to the earth by these meteorites and gases in recent times is relatively slight compared with the whole body of the earth. What contribution may have come to the earth in earlier times from such sources is a matter of hypothesis which will be discussed later.

Geognosy.

The constitution of the earth.—Turning from its external relations to the earth itself, a natural threefold division is presented: (1) the atmosphere, (2) the hydrosphere, and (3) the lithosphere.

I. The Atmosphere.

The atmosphere is an intimate mixture of (1) all those substances that cannot take a liquid or solid state at the temperatures and pressures which prevail at the earth’s surface, together with (2) such transient vapors as the various substances of the earth throw off. The first class form the permanent gases of the atmosphere, and consist of nitrogen about 79 parts, oxygen about 21 parts, carbon dioxide about .03 part, together with small quantities of argon, neon, xenon, krypton, helium, and other rare constituents. The second class are the transient and fluctuating constituents of the atmosphere, chief among which is aqueous vapor, which varies greatly in amount according to temperature, pressure, and other conditions. To this are to be added volcanic emanations and a great variety of volatile organic substances. Theoretically, every substance, however solid, discharges particles which may transiently become constituents of the atmosphere. Practically, only a few of these exist in such quantity as to be appreciable. Dust and other suspended matter are usually regarded as impurities rather than constituents of the atmosphere, but they play a not unimportant part by affecting its temperature and luminosity, and by facilitating the condensation of moisture.

Mass and extent.—The total mass of the atmosphere is estimated at five quadrillion tons, or ¹⁄₁₂₀₀₀₀₀ of the mass of the earth. It is relatively dense at the surface of the earth and decreases in density outwards in a manner difficult of absolute determination, so that the actual height of the appreciable atmosphere is not positively known. The true conception of the atmosphere is perhaps that of a tenuous envelope exerting a pressure of about fifteen pounds per square inch at the sea-level, and thinning gradually upwards until it reaches a tenuity which is inappreciable, but perhaps not ceasing absolutely until the sphere of gravitative control of the earth is passed, about 620,000 miles from the lithosphere. In the lower portion, according to the kinetic theory of gases, the molecules fly to and fro, colliding with each other with almost inconceivable frequency, and with very short paths between successive collisions, but in the upper rare portion some of the molecules bound outwards, and do not strike other molecules, and hence pursue long elliptical paths until the gravity of the earth overcomes their momentum, when they return, perhaps to bound off again or to force other molecules to do so. This fountain-like nature of the outer part of the atmosphere makes any sharp definition of its limit impracticable. Some molecules are believed to be shot away at such speed that they do not return. Beyond about 620,000 miles from the surface of the lithosphere, the differential attraction of the sun is greater than that of the earth, and if the attraction of the earth does not turn the molecules back before reaching this distance, they are almost certain to be lost to the earth.

The measurement of heights by the aneroid barometer, which is much used in practical geology, is dependent on the lessening of pressure as the instrument is carried upward.

Geologic activity.—The atmosphere is the most mobile and active of the three great subdivisions of the earth, and when its indirect effects through the agency of water, as well as its direct effects, are considered, it is to be regarded as one of the most effective agencies of change. It acts chemically upon the rock substance of the earth, causing induration in some instances, but more often inducing disintegration and change of composition by means of which rock is reduced to soil, or soil-like material, and rendered susceptible of easy removal by winds and waters. When in motion the atmosphere acts mechanically on the surface of the earth, transporting dust and sand, and by the friction of these it abrades the surface. It is chiefly effective, however, in furnishing the conditions for water action. Partly by its mechanical aid, but chiefly by securing the right temperature, it is a necessary factor in the action of rains, streams, glaciers, and the various forms of moving water upon land. So also, on the ocean, wave action is essentially dependent on the winds. In the absence of atmospheric propulsion, wave action would be chiefly confined to the tides and to occasional earthquake impulses, and would lose nearly all its efficiency. Stream action and wave action, which are the most declared of the geological agencies, are therefore to be credited as much to the atmosphere as to the hydrosphere, since the action is a joint one to which both envelopes are essential.

A thermal blanket.—A function of the atmosphere of supreme importance is the thermal blanketing of the earth. In its absence the heat of the sun would reach the surface with full intensity, and would be radiated back from the surface almost as rapidly as received, and only a transient heating would result. During the night an intensity of cold would intervene scarcely less severe than the temperature of space. In penetrating the atmosphere certain portions of the radiant energy of the sun are absorbed. Of the remainder which reaches the surface of the earth, a part is transformed into vibrations of lower intensity, which are then more effectively retained by the atmosphere. The air thus distributes and equalizes the temperature. The two constituents of the atmosphere which are most efficient in this work are aqueous vapor and carbon dioxide, and the climate of the earth is believed to have been very greatly affected by the varying amounts of these constituents in the atmosphere, as well as by the total mass of the atmosphere.

The function of the atmosphere in sustaining life and promoting all that depends on life is too obvious to need comment.

The special geological action of the atmosphere will be discussed in the next chapter.

II. The Hydrosphere.

About 1300 quadrillion tons of water lie upon the surface of the solid earth. This equals about ¹⁄₄₅₄₀ part of the earth’s mass. Were the surface of the solid earth perfectly spheroidal, this would constitute a universal ocean somewhat less than two miles deep. Owing to the inequalities of the rock surface, the water is chiefly gathered into a series of great basins or troughs occupying about three-fourths (72%) of the earth’s surface. These basins are all connected with each other and act as a unit, so that anything which changes the level of the water in one changes the level of all. This helps to make a common record of all great movements of the earth’s body, for the level of the ocean determines where the detritus from the land shall lodge, and hence where the edge of the marine beds shall be formed. This will appear more clearly when the formation of marine strata is discussed.

Oceanic dimensions.—The surface area of the ocean is estimated by Murray at 143,259,300 square miles. Of this, somewhat more than 10,000,000 square miles lie on the continental shelf, i.e., lap up on the borders of the continental platforms. This shows that the great basins are somewhat more than full. If about 600 feet of the upper part of the ocean were removed, the true ocean basins would be just full, and the surfaces of the true continental platforms would be dry land. The area of the true oceanic basins is about 133,000,000 square miles, and that of the true continental platforms about 64,000,000 square miles. Under about 20% of the ocean area, the bottom sinks to depths between 6000 and 12,000 feet; under about 53% it sinks to depths between 12,000 and 18,000 feet; and under the remaining 4% it ranges from 18,000 feet down to about 30,000. The last includes those singular sunken areas known as “deeps,” and sometimes called anti-plateaus, as they extend downward from the general ocean bottom much as the plateaus protrude upwards from the general land surface.

Besides the ocean, the hydrosphere includes all the water which constitutes the surface streams and lakes, together with that which permeates the pores and fissures of the outer part of the solid earth; but altogether these are small in amount compared with the great ocean mass.

Geologic activity.—Of all geological agencies water is the most obvious and apparently the greatest, though its efficiency is conditioned upon the presence of the atmosphere, upon the relief of the land, and upon the radiant energy of the sun. Through the agency of rainfall, of surface streams, of underground waters, and of wave action, the hydrosphere is constantly modifying the surface of the lithosphere, while at the same time it is bearing into the various basins the wash of the land and depositing it in stratified beds. It thereby becomes the great agency for the degradation of the land and the building up of the basin bottoms. It works upon the land partly by dissolving soluble portions of the rock substance, and partly by mechanical action. The solution of the soluble part usually loosens the insoluble, and renders it an easy prey of the surface waters. These transport the loosened material to the valleys and at length to the great basins, meanwhile rolling and grinding it and thus reducing it to rounder forms and a finer state, until at length it reaches the still waters or the low gradients of the basins and comes to rest. The hydrosphere is therefore both destructive and constructive in its action. As the beds of sediment which it lays down follow one another in orderly succession, each later one lying above each earlier one, they form a time record. And as relics of the life of each age become more or less imbedded in these sediments, they furnish the means of following the history of life from age to age. The historical record of geology is therefore very largely dependent upon the fact that the waters have thus buried in systematic order the successive life of the ages. Aside from this, the means of determining the order of events of the earth’s history are limited and more or less uncertain.

The special processes of the hydrosphere in its various phases will be the subject of discussion hereafter (Chaps. [III], [IV], [VI]). Suffice it here to recognize its great function in the constant degradation of the land, and in the deposition of the derived material in orderly succession in the basins.

Chief horizons of activity.—The great horizons of geological activity are (1) the contact zone between the atmosphere and the hydrosphere, chiefly the surface of the ocean, (2) the contact zone between the hydrosphere and the lithosphere, chiefly the shore belts, and (3) the contact zone of the atmosphere and surface waters, with the face of the continents. It is in these three zones that the greatest external work is being done and has been done in all the known ages.

III. The Lithosphere.

The atmosphere and hydrosphere are rather envelopes or shells than true spheres, though in some degree both penetrate the lithosphere. The lithosphere, on the other hand, is a nearly perfect oblate spheroid with a polar diameter of 7899.7 miles, and an equatorial diameter of about 26.8 miles more. Its equatorial circumference is 24,902 miles, its meridional circumference 24,860 miles, its surface area 196,940,700 square miles, its volume 260,000,000,000 cubic miles, and its average specific gravity about 5.57. The oblateness of the spheroid is an accommodation to the rotation of the earth, the centrifugal force at the equator being sufficient to cause the specified amount of bulging there. Computations seem to indicate that the accommodation is very nearly what would take place if the earth were in a liquid condition, from which the inference has been drawn that it must have been in that condition when it assumed this form, and must have continued essentially liquid until it attained its present rate of rotation, since, if the earth once rotated at a much higher speed, the flattening at the poles and the bulging at the equator must have been correspondingly greater. It is thought by others, however, that the plasticity of the earth is such that it would at all times assume a close degree of approximation to the demands of rotation, even if the interior were in a solid condition. By still others it is thought that the contraction of the earth has tended to accelerate the rotation about as much as the tides have tended to retard it, and that it has undergone little change of form.

Irregularities.—It is only in a general view, however, that there is a close approximation to a perfect spheroidal surface. In detail there are very notable variations from it. Geodetic surveys seem to have shown that the equatorial diameters are not all equal, even when the measurements are reduced to sea-level, but research along this line has not reached a sufficient stage of completeness to permit satisfactory discussion. It is, however, highly probable that the ocean surface as well as the average land surface is warped out of the perfect spheroidal form to some notable degree. This is very likely due to inequalities in the density of the earth’s interior. The fact that the larger portion of the water is gathered on one side of the globe, while the land chiefly protrudes on the opposite side, is very possibly due to unequal specific gravity in the interior of the earth.

The most obvious departure from a spheroidal form is found in the protrusion of the continents and in the sinking away of the earth surface under the oceans. As these inequalities present themselves to-day, they are known as continental platforms and ocean basins. These do not correspond accurately with the present land and water surfaces. About the continental lands there is a submerged border extending some distance out from the shore, and constituting a sea-shelf beyond which the surface descends rapidly to the great depths of the ocean. This slightly submerged portion, known as the continental shelf, belongs as properly to the continent as the adjacent low lands which are not submerged. The submergence of the edge of this shelf at present is usually about 100 fathoms, so that if the upper 600 feet of the ocean were removed, the outlines of the land would correspond quite closely with the border of the true continental platform.

BATHYMETRICAL CHART OF THE OCEANS
SHOWING THE “DEEPS” ACCORDING TO SIR JOHN MURRAY

It is customary to look upon the protrusions of the continents as the great features of the earth’s surface, but in reality the oceanic depressions are the master phenomena. In breadth, depth, and capacity they much exceed the continental protrusions, and if the earth be regarded as a shrunken body, the settling of the ocean bottoms has doubtless constituted its greatest surface movement. From the estimates of Murray, Gilbert has derived the following tables, showing the relative areas of the lithosphere above, below, and between certain levels.[1]

From these estimates it appears that if the surface were graded to a common level by cutting away the continental platforms and dumping the matter in the abysmal basins, the average plane would lie somewhere near 9000 feet below the sea-level. The continental platform may be conceived as rising from this common plane rather than from the sea-level.

Epicontinental seas.—Those shallow portions of the sea which lie upon the continental shelf, and those portions which extend into the interior of the continent with like shallow depths, such as the Baltic Sea and the Hudson Bay, may be called epicontinental seas, for they really lie upon the continent, or at least upon the continental platform; while those other detached bodies of water which occupy deep depressions in the surface are to be regarded as true abysmal seas, as, for example, the Mediterranean and Caribbean seas and the Gulf of Mexico, whose bottoms are as profound as many parts of the true ocean basin itself.

Diversities of surface.—The bottoms of the oceanic basins are diversified by broad undulations which range through many thousands of feet, but they are not carved into the diversified forms that give variety to land surfaces. The ocean bottoms are also diversified by volcanic peaks, many of which rise to the surface and constitute isolated islands. Some of them have notable platforms at or near the surface, cut by the waves or built up by the accumulation of sediment and of coralline and other growths about them. Aside from these encircling platforms, the solid surface usually shelves rapidly down to abysmal depths, so that the islands constitute peaks whose heights and slopes would seem extraordinary if the ocean were removed.

The surface of the land is diversified in a similar way by broad undulations and volcanic peaks, and also by narrower wrinklings and foldings of the crust; but all of these irregularities have been carved into diversified and picturesque forms by subaërial erosion. In this respect the surface of the land differs radically from the bed of the sea. The agencies which have produced the continental platforms and abysmal basins, and the great undulations and foldings, as well as the volcanic extrusions that mark them, are yet subjects of debate. Here lie some of the most difficult problems of geology, but these cannot be stated with sufficient brevity to find a place here.

The surface mantle of the lithosphere.—The surface of the lithosphere is very generally mantled by a layer of loose material composed of soil, clay, sand, gravel, and broken rock. This loose material is sometimes known as mantle rock, and sometimes as rock waste. On the land, mantle rock is often composed of the disintegrated products of underlying rock formations. It represents the results of the recent action of the atmosphere, of water, of changes of temperature, and of other physical agencies acting on the outer part of the rock sphere. The surface of this mantle is being constantly removed by wind and water, but as constantly renewed by continued decomposition of the rock below. In some areas, especially in the northern part of North America and the northwestern part of Europe, the soil graduates down into an irregular sheet of mixed clay, sand, gravel, and bowlders, known as drift. From this and other evidence it is inferred that at a time not greatly antedating our own, ice, chiefly in the form of glaciers, spread extensively over the high latitudes of the northern hemisphere. In some parts of the earth the surface is still covered by fields of snow and ice, comparable to those which formed the drift. In still other places, especially along the flood plains of streams, the mantle rock consists of deposits made by streams which were unable to carry their loads of sediment to the sea.

The crust of the lithosphere.—Much of the detritus washed down from the land finds its way to bodies of standing water, and beneath lakes and seas the mantle of loose material is made up largely of the gravel, sand, and mud derived from the land. Before deposition these materials are more or less assorted and arranged in layers by waves and currents. When consolidated they constitute rock. The weathering of the rocks of the land, the wearing away of the resulting detritus, and its deposition beneath standing water, are among the most important processes of geologic change.

On the land, the mantle of loose material is sometimes absent, and in such places the surface of solid rock of the crust appears. Bare surfaces of rock are most commonly seen where the topography is rough, especially on the slopes of steep-sided valleys and mountains, and on the slopes of cliffs which face seas or lakes. Solid rock, without covering of soil or loose material of any sort, is also frequently seen in the channels of streams, especially where there are falls or rapids.

We have but to note the effects of a vigorous shower on a steep slope, or of a swift stream on its channel, or of waves on the cliffs which face lakes and seas, to understand at least one of the reasons why loose materials are frequently absent from steep slopes. The very general exposure of solid rock where conditions favor surface erosion suggests that rock is everywhere present beneath the soil or subsoil. Fortunately there is an easy way of testing the universality of the crust beneath the mantle. In all lands inhabited by civilized peoples there are numerous wells and other excavations ranging from a few feet to several hundred feet in depth, and occasional wells and mine-shafts reach depths of several thousand feet. Even in shallow excavations rock is often encountered, and in most regions excavations as much as two or three hundred feet deep usually reach rock, and no really deep boring has ever failed to find it. It may, therefore, be accepted as a fact that the upper surface of the solid rock is nowhere far below the surface.

Concerning the thickness of the crust, if there be any true crust at all, little is known by direct observation. The deepest valleys, such as the canyon of the Colorado, and the shafts and borings of the deepest mines and wells, give knowledge of nothing but rock. The deepest excavations extend rarely more than a mile below the surface. It is certain that rock of known kinds extends to far greater depths.

The interior.—Concerning the great interior of the earth, little is known except by inference. From the weight of the earth,[2] it is inferred that its interior is much more dense than its surface. From its behavior under the attraction of other bodies, it is believed to be at least as rigid as steel, and its interior cannot, therefore, be liquid, in the usual sense of that term. From the phenomena of volcanoes, and from observations on temperature in deep borings, it is inferred that its interior is very hot. Further inferences concerning its character are less simply stated, and will be referred to later.

The solid part of the earth is therefore composed of (1) a thin layer of unconsolidated or earthy material, a few feet to a few hundred feet in thickness, covering (2) a layer or zone many thousands of feet, and probably many miles, thick, composed of solid rock comparable to that exposed at numerous points on the surface, and (3) a central mass, to which the preceding layers are but a shell, composed of hot, dense, and rigid rock, the real nature of which is not known by observation.

Varieties of rock in crust.—If the mantle of soil, subsoil, and glacial rubbish were stripped from the land, the surface beneath would be found to be made up of a great variety of rocks, all of which may be grouped into two great classes. About four-fifths of the land surface would be of rock arranged in layers, and the other fifth would be of crystalline rock, generally without distinct stratification, and often bearing evidence of the effects of high temperature.

Stratified rocks.—The composition of most stratified rocks corresponds somewhat closely with the composition of sediments now being carried from the land and being deposited in the sea. Their arrangement in layers is the same, and the markings on the surfaces of the layers, such as ripple-marks, rill-marks, wave-marks, etc., are identical. Furthermore, the stratified rocks of the land, like the recent sediments of the sea, frequently contain the shells and skeletons of animals, and sometimes the impressions of plants. Most of the relics of life found in the stratified rocks belonged to animals or plants which lived in salt water. Because of their structure, their composition, their distinctive markings, and the remains of life which they contain, it is confidently inferred that most of the stratified rocks which lie beneath the mantle rock of the land were originally laid down in beds beneath the sea, and that the familiar processes of the present time furnish the key to their history.

Fig. 1.—Beds of (Cambrian) sandstone, a, are conformable with one another, but unconformable on beds of (Huronian) quartzite, b, Near Ableman, Wis.

Conformability.—When the stratified rocks exposed by the removal of the mantle rock are examined, the successive beds are sometimes found to lie on one another in regular succession, showing that they were laid down one after another, without change in the attitude of the surface on which they were deposited. Such rocks are conformable (the beds of series a, [Fig. 1]). In other cases it would be seen that certain beds overlie the worn surfaces of lower beds, the layers of which may have a different angle of inclination (series a, [Fig. 1], is unconformable on series b). Such relations show that the lower series of beds was disturbed and eroded before the overlying beds were deposited on them. Such series of rocks are unconformable.

Relative ages.—The structure and relations of rocks lead to inferences as to their relative ages. In the case of stratified rocks it is obvious that overlying beds were deposited later than those below, and where there is unconformity it is evident that an interval of time elapsed between the deposition of the unconformable series. Another and in some respects more important means of telling their order of formation is found in the remains of life entrapped in the water-laid sediments. Whatever life existed in the waters in which the sediments were deposited was liable to burial, and if it was possessed of hard parts, such as bones, teeth, shells, hard integuments, etc., these parts, or at least their impressions, were likely to be preserved in the sediments. Even tracks and imprints of perishable parts are sometimes preserved. All these relics, which we call fossils, give indications of the kinds of life which existed when the beds were formed. The fossils of the youngest beds show that the life which existed when they were deposited was quite like that of the present time. The fossils of the next older and lower beds show greater departure from present types. This series of changes continues downward as lower and lower beds are studied, until beds at considerable depths contain no relics of existing species but, in lieu thereof, forms of more primitive types. Some of these earlier types are clearly the ancestors of more modern forms, while others seem to have no living descendants. Going still deeper, the fossils indicate life of more and more primitive types, until they depart very widely from the living forms, and seem to be but remotely ancestral. So the beds may be followed downward until the lowest, which contain distinct evidences of life, are reached.

It should be understood that it is not possible to proceed directly downward through the whole succession of bedded rocks, but that the edges of the various beds may be found here and there where they have been brought to the surface by warpings or tiltings, or exposed by the wearing away of the beds which once overlay them. The full series of strata is made out only by putting together the data gathered throughout all lands, and even when this is done an absolutely complete series cannot yet be made out or, at least, has not been.

The crystalline rocks.—The crystalline rocks which would appear if the mantle rock were removed are of two types, igneous and metamorphic. Igneous rocks may be loosely defined as hardened lavas. Metamorphic rocks are those which are greatly changed from their original condition. Either stratified or igneous rocks may become metamorphic.

Igneous rocks sustain various relations to the stratified rocks, as illustrated by [Fig. 2]. From these relations it is possible to tell something of the order of their formation. Where the stratified rocks are broken through by lavas, it is obvious that the stratified rocks were formed first, and the lavas intruded later. Lava sheets intruded between beds of stratified rock can be told from those which flowed out on the surface and were subsequently buried, for in the former case the sedimentary rocks, both above and below the igneous rock, were affected by the heat, while in the latter case only those below were so affected.

Fig. 2.—Diagrammatic representation of the relations of igneous rock to stratified rock. The igneous rocks, represented in black, have been forced up from beneath.

More commonly than otherwise the metamorphic rocks ([Fig. 3]) lie beneath the sedimentary beds and are often broken through by the igneous rocks. From their position in many places their great age may be inferred, but locally, especially where dynamic action has been severe, relatively young rocks are metamorphic.

Fig. 3. The figure represents a section of the earth about 1000 miles Long. The unequally thick black line at the top represents on something like its proper scale the depth of the stratified rock. The area below represents crystalline rock, largely metamorphic.

Four great sedimentary eras.—The water-laid series represents four great eras in the history of the earth, as shown by the relics of life imbedded in them. Beginning with the latest, these are the Cenozoic (recent life), during which the life took on its modern aspect; the Mesozoic (middle life), during which the life bore a mediæval aspect; the Paleozoic (ancient life), during which the life belonged to older types; and the Proterozoic (earlier life), during which it is inferred that much life prevailed, though its record is very imperfect. It may safely be assumed to have been more primitive than that of the Paleozoic, as it was earlier. Each of these great divisions embraced several lesser periods or epochs, and these again are subdivided more and more closely according to the degrees of refinement to which studies are carried. The chief of these subdivisions are given in the table on [page 19], and others will come under consideration in the historical chapters.

In these four great series of sedimentary rocks there are, here and there, intrusions of igneous rocks, and in some places the sedimentary beds have been metamorphosed into crystalline rocks by heat and pressure. This is particularly true in the lowest of these series, the Proterozoic, where a large part of the sediment is metamorphosed, and where there is much igneous rock, but it is still clear that the main portion of this series was originally water-laid sediment, and so it belongs to the sedimentary series rather than the Archean, in which the sediments are the minor rather than the main factor. It has, however, usually been classed with the Archean, and it is certainly not always easy to draw the dividing line. In a sense it may be regarded as a transition series.

The Archean complex.—Beneath the dominantly sedimentary but partly metamorphic and igneous series there is a very complex group of rocks largely of metamorphosed igneous origin, though containing some metamorphosed sediments. These extend downwards to unknown depths. While all the great formations are occasionally bent and broken, these lowest ones are almost everywhere warped, folded, and contorted, often in the most intricate way. They have been very generally mashed and sheared by enormous pressure, so that they have become foliated, and their original character is much masked. They therefore form a series of great obscurity and complexity. As they are at the bottom of the known series, they have been called the “Fundamental gneiss” and the “Basement complex,” but as the part which we see is not the true base nor the true foundation, it is safer to call them simply the Archean (very ancient) complex. As life appears to have been present during a part at least of the period of its formation is referred to the Archeozoic era.

Fig. 4.—Diagram to illustrate the relations of the five great groups of formations.
AR = Archean, Pr = Proterozoic, P = Paleozoic, M = Mesozoic, C = Cenozoic.

Beyond and below this series, the structure of the earth is a matter of inference. Vast as are the preceding series, they together form relatively but a thin shell on the outer surface of the globe.

The foregoing series are diagrammatically expressed in [Fig. 4], and systematically presented to the eye in the following table.

GENERAL TABLE OF GEOLOGIC DIVISIONS.

Cenozoic		Present.
		Pleistocene.
		Pliocene.
		Miocene.
		Oligocene.
		Eocene.
		Transition (Arapahoe and Denver).
Mesozoic		Upper Cretaceous.
		Lower Cretaceous (Comanche or Shastan).
		Jurassic.
		Triassic.
Paleozoic		Permian.
		Coal Measures or Pennsylvanian.
		Subcarboniferous, or Mississippian.
		Silurian.
		Devonian.
		Ordovician.
		Cambrian.
		Great interval.
Proterozoic		Keweenawan.
		Interval.
		Animikean or Penokean. (Upper Huronian of some authors).
		Interval.
		Huronian.
		Great interval.
Archeozoic		Archean Complex.		Great Granitoid Series. (Intrusive in the main, Laurentian.)
				Great Schist Series. (Mona, Kitchi, Lower Keewatin, Coutchiching, Lower Huronian of some authors.)

The purpose of this general survey is to bring the salient features of the earth’s structure into view preparatory to entering in more detail into the study of particular processes and special formations and to lay a foundation for the fuller apprehension of the successive stages of the history of the earth, which constitutes the chief purpose of geological study. It is now advisable to turn to the detailed consideration of individual processes and specific structures. The complexity of the actions involved in the history of the earth is so great that such separate consideration at the outset is helpful.

CHAPTER II.

THE ATMOSPHERE AS A GEOLOGICAL AGENT.

While it is convenient to regard the lithosphere as the earth proper, and the atmosphere as its envelope, the latter is as truly a part of the planet as the former, and its activities and its history are as truly subjects of geological study as the formation of the rocks. This view is in no way vitiated by the fact that the special study of the atmosphere is set apart under the name Meteorology, for in the same way the special study of rocks is set apart under the name Petrology, that of ancient life under the name Paleontology, and that of other phases of the subject under other names. The atmosphere is one of the three great formations of the earth, and as a geological factor takes its place beside the hydrosphere and the lithosphere. It has played a part in the history of the earth comparable to that of the water, though its mass is less and its record more elusive. Unsubstantial as the atmosphere seems when contrasted with the liquid and solid portions of the earth, its extreme mobility and its chemical activity compensate for its lightness and tenuity, and give it a function of the first order of importance.

The atmosphere plays a direct part as (1) a mechanical and (2) a chemical agent, and at the same time serves an indirect function in furnishing favorable conditions under which (3) solar radiation produces temperature effects, and (4) evaporation gives origin to precipitation and stream effects, and furnishes the necessary conditions for land plants and animals, and the important influences that spring from them.

This chapter is devoted to the work of the atmosphere in these and some less notable phases. The consideration of the origin and history of the atmosphere will receive attention later.

A. THE ATMOSPHERE AS A DIRECT AGENCY.

I. Mechanical Work.

The mechanical work of the atmosphere is accomplished chiefly through its movement. A feeble breeze is competent to move particles of dust, and winds of moderate velocity to shift sand. Exceptionally strong winds sometimes move small pebbles, but winds of sufficient force to move larger pieces of rock are rare. It follows, therefore, that the impact of the wind has little direct effect except on surfaces covered with dust and dry sand.

The transportation of material by the wind is limited by the size of the particles to which it has access. Dust particles expose more surface to the wind relative to their mass than sand grains. Winds which are unable to carry sand may still carry dust, and winds which are able to shift sand no more than a trivial distance may blow dust great distances.

The common conception of wind as a horizontal movement of some part of the atmosphere is not altogether accurate. Every obstacle against which wind blows causes deflections of its currents, and some of these deflections are upward. Furthermore, there are exceptional winds, in which the vertical element predominates. Particles of dust are often involved in these upward currents, and by them carried to great heights, and in the upper air are transported great distances.

Transportation and deposition of dust.[3]—The universality of the transportation of dust by the wind is well known. No house, no room, and scarcely a drawer can be so tightly closed but that dust enters it, and the movements of dust in the open must be much more considerable. The visible dustiness of the atmosphere in dry regions during wind-storms is adequate and familiar proof of the efficiency of the wind as a transporter of dust.

Under special circumstances, opportunity is afforded for rough determinations of the distance and height to which wind-blown dust is transported. Snow taken from snow-fields in high mountain regions is found to contain a small amount of earthy matter. Its particles are often found to be in part volcanic, even when the place whence the snow was taken is scores or even hundreds of miles from the nearest volcano. There is probably no snow-field so high, or so far from volcanoes, but that volcanic dust reaches it. If this be true of all snow-fields, it is probably true of all land surfaces. In the great Krakatoa[4] eruptions of 1883 large quantities of volcanic ash (pulverized lava) were projected to great heights into the atmosphere. The coarser particles soon settled; but, caught by the currents of the upper atmosphere, many of the finer particles were transported incredible distances. Through all their long journey, the particles of dust were gradually settling from the atmosphere, but not until the dust had traveled repeatedly round the earth did its amount become so small as to cease to make its influence felt in the historic red sunsets which it occasioned.[5] Some of this dust completed the circuit of the earth in 15 days.

In various parts of Kansas and Nebraska[6] there are very considerable beds of volcanic dust, locally as much as 30 feet thick, which must have been transported from volcanic vents by the wind, though there are no known centers of volcanic action, past or present, within some hundreds of miles of some of the localities where the dust occurs. These beds of volcanic dust, so far from its source, may serve as an illustration of the importance of atmospheric movements as a geological force.

Volcanic dust is shot into the atmosphere rather than picked up by it. Dust picked up by the wind is perhaps transported not less widely than volcanic dust, but, after settling, its point of origin is less readily determined. It would perhaps be an exaggeration to say that every square mile of land surface contains particles of dust brought to it by the wind from every other square mile, but such a statement would probably involve much less exaggeration than might at first be supposed.

Examples of extensive deposits of dust other than volcanic are also known. In China there is an extensive earthy formation, the loess, sometimes reaching 1,000 feet in thickness, which von Richtofen believes to have been deposited by the wind.[7] This conclusion has, however, not passed unchallenged.[8] The loess of some other regions has been referred to the same origin, and some of it is quite certainly eolian.[9]

The transportation of dust is important wherever strong winds blow over dry surfaces, free or nearly free of vegetation, and composed of earthy matter. Its effects may be seen in such regions as the sage-brush plains of western North America. The roots of the sage-brush hold the soil immediately about them, but between the clumps of brush, where there is little other vegetation, the wind has often blown away the soil to such an extent that each clump of brush stands up several inches, or even a foot or two, above its surroundings ([Fig. 5]). Such mounds are often partly due to the lodgment of dust about the bushes.

Fig. 5.—This figure shows the effect of sage-brush or other similar vegetation in holding sand or earth, or in causing its lodgment, in dry regions.

Where the earthy matter is moist, the cohesion of the particles is great, and the wind cannot pick them up. Furthermore, if the surface is generally moist, it is likely to be covered with vegetation which protects it against the wind. But even where vegetation is prevalent the wind finds many a vulnerable point. Thus on the edges of plains or plateaus facing abrupt valleys, the wind attacks the soil from the side, and in such situations all earthy matter may be stripped from the underlying rock for considerable distances from the edge of the cliff ([Fig. 6]). This may be seen at numerous points on the lava plateaus of Washington.

Fig. 6.—Diagram to illustrate the way in which the wind sometimes strips the soil from the edge of a bluff. This phenomenon is not rare in the basin of the Columbia River in Washington.

The presence of dust in the upper atmosphere during a rain-storm is sometimes the occasion of phenomena which are often misinterpreted. If there be abundant dust in the atmosphere through which rain-drops or snowflakes fall, much of it is gathered up by them, and the water is thereby rendered turbid and the snow discolored. Here is to be found the explanation of “mud-rain,” “blood-rain” (red dust), etc.

Since dust is carried to a considerable extent in the upper atmosphere, its movements and its deposition are little affected by obstacles on the surface of the land. A building or a hedge can only affect the lodgment of that part of the atmospheric dust which comes in contact with it or is swept into its lee. Since most obstacles on the surface of the solid part of the earth reach up but slight distances into the atmosphere, the dust of the greater part of the air settles without especial reference to them, and is spread more or less uniformly over the surface on which it falls.

Fig. 7.—Diagram to illustrate the effect of an obstacle on the transportation and deposition of sand. The direction of the wind is indicated by the upper arrow. The lower arrows represent the direction of eddies in the air occasioned by the obstacle. If the surface in which the obstacle was set was originally flat (dotted line), the sand would tend to be piled up on either side at a little distance from the obstacle, but more to leeward. At the same time, depressions would be hollowed out near the obstacle itself (see full line). (After Cornish.)

Much of the dust transported by the wind is carried out over seas or lakes and falls into them. By this means, sedimentation is doubtless going on at the bottom of the whole ocean, and at the bottoms of all lakes. While means of determining the amount of dust blown into the sea are not at hand, it is safe to say that, were such determinations possible, the result, if stated in terms of weight, would be surprising.

Transportation and deposition of sand.—In its transportation by the wind, sand is not commonly lifted far above the surface of the land, and its movement is therefore more generally interfered with by surface obstacles than is the movement of dust. A shrub, a tree, a fence, a building, or even a stone may occasion the lodgment of sand in considerable quantity, though it has little effect on the lodgment of dust. The effect of obstacles is illustrated by [Fig. 7] (see also [Fig. 5]). If the obstacle which occasions the lodgment of sand presents a surface which the wind cannot penetrate, such as a wall, sand is dropped abundantly on its windward side as well as on the leeward; but if it be penetrable, like an open fence, the lodgment takes place chiefly on its leeward side. In cultivated regions cases are known where, in a few weeks of dry weather, sand has been drifted into lanes in the lee of hedges to the depth of two or three feet, making them nearly impassable to vehicles.

Formation of dunes.—In contrast with dust deposited from the atmosphere, wind-blown sand is commonly aggregated into mounds and ridges in the process of lodgment. These mounds and ridges are dunes. Once a dune is started, it occasions the further lodgment of sand, and is a cause of its own growth. Dunes sometimes reach heights of 200 or 300 feet, but they are much more commonly no more than 10 or 20 feet in height. On plane surfaces, there is a limit in height above which they do not rise, though the limit is different under different conditions. The velocity of wind at the bottom of the air is not so great as that higher up, and as a dune is built up, a level is presently reached where the stronger upper winds sweep away as much sand as is brought to the top. The very even crests of many dune ridges are probably to be accounted for in this way. Wind-blown or eolian sand, not piled up in heaps or ridges, is somewhat widespread, but does not constitute dunes.

Shapes of dunes.[10]—Dunes may assume the form of ridges or of hillocks. The ridges may be transverse to the direction of the prevailing wind or parallel with it. Where dunes assume the form of hillocks rather than ridges, a group of them may be elongate in a direction parallel to the dominant wind, or at right angles thereto. The shape assumed by a dune or a group of dunes depends on the abundance of the sand, the strength and direction of the wind, and the shape of the obstacle which occasions the lodgment.

Fig. 8.—Dune ridges parallel to the direction of the wind. Southwest part of India. Scale about 3 miles to the inch. (Cornish.)

Fig. 9.—Dune ridges transverse to the direction of the wind. Scale about 3 miles to the inch. (Cornish.)

The incipient stages of dune formation are readily seen in many dry, sandy regions. The dune is likely to start in the lee of some obstacle, and to be elongate in the direction of the wind, especially if the wind be strong relative to the supply of sand. This shape is permanently preserved if the proper relations between the supply of sand and strength and direction of wind are preserved. In the dune region of the Indian desert[11] the prevailing winds are alternately the southwest and northeast monsoons, the former being the stronger. The supply of sand comes from the southwest. Near the southwest coast the dune ridges are parallel to the direction of the wind ([Fig. 8]); in the interior, where the winds are less strong, the dunes are transverse to it ([Fig. 9]); while between the districts where these two types prevail intermediate forms occur. The transverse dune ridges ([Fig. 9]) are said to be the result of the lateral growth and erosion of longitudinal dunes.[12] In regions of changeable winds the shape of the dunes is subject to great variation. Dunes are sometimes crescentic, the convexity facing the wind ([Fig. 10]).

Fig. 10.—Crescentic dunes in ground-plan, the convexities facing the wind. (Bokhara.) (Walther.)

Along coasts, dune ridges are often transverse to the wind, and groups of dune hillocks are frequently elongate in the same direction. Here the source of supply of the sand is itself an elongate belt, often transverse to the dominant wind, and the resulting dunes often have great length transverse to the wind. Where the wind has strong mastery over the sand, the longitudinal tendency is seen, even along coasts.[13]

Fig. 11.—Section of a dune showing, by the dotted line, the steep leeward (bc) and gentler windward (ab) slope. By reversal of the wind the cross-section may be altered to the form shown by the line adc. (Cornish.)

Fig. 12.—Cross-section of a dune showing the profiles developed by scour of the wind on both flanks. (Cornish.)

The shapes of dunes in section, like the shapes in ground-plan, depend on the relative strength and constancy of the winds and the supply of sand. With constant winds and abundant drifting sand, dunes are steep on the lee side (bc, [Fig. 11]), where the angle of slope is the angle of rest for the sand. It rarely exceeds 23° or 24°.[14] Under the same conditions the windward slope is relatively gentle (ab, [Fig. 11]). If the winds be variable so that the windward slope of one period becomes the leeward slope of another, and vice versa, this form is not preserved. Thus, by reversal of the wind, the section abc, [Fig. 11], may be changed to adc. If the winds and the supply of sand be equal, on the average, from opposite directions, the slopes should, on the average, be equal, though perhaps unequal after any particular storm. The steep slopes of new-made dunes are lost after the sand has ceased to be blown. At some points where the winds erode (scour) more than they deposit, new profiles are developed (Figs. [12] and [13]). The erosion profiles may be very irregular if the dunes are partially covered with vegetation. The effect of vegetation in restraining wind erosion is shown in [Fig. 14], where plants have preserved a remnant of a dune.

The topographic map.—Since dunes as well as other topographic features are conveniently represented on contour maps, and since such maps will be used frequently in the following pages, a general explanation of them is here introduced.

“The features represented on the topographic map are of three distinct kinds: (1) inequalities of surface, called relief, as plains, plateaus, valleys, hills, and mountains; (2) distribution of water, called drainage, as streams, lakes, and swamps; (3) the works of man, called culture, as roads, railroads, boundaries, villages, and cities.

Fig. 13.—Diagram showing the outline of dunes in process of destruction. Seven Mile Beach, N. J. (N. J. Geol. Surv.)

Fig. 14.—Illustrates the protective effect of vegetation against wind erosion. Dune Park, Ind. (Cowles)

Relief.—All elevations are measured from mean sea-level. The heights of many points are accurately determined, and those which are most important are given on the map in figures. It is desirable, however, to give the elevation of all parts of the area mapped, to delineate the horizontal outline, or contour, of all slopes, and to indicate their grade or degree of steepness. This is done by lines connecting points of equal elevation above mean sea-level, the lines being drawn at regular vertical intervals. These lines are called contours, and the uniform vertical space between each two contours is called the contour interval. On the maps of the United States Geological Survey the contours and elevations are printed in brown (see [Plate II]).

Fig. 15.—Sketch and map of the same area to illustrate the representation of topography by means of contour lines (U. S. Geol. Surv.)

The manner in which contours express elevation, form, and grade is shown in the following sketch and corresponding contour map, [Fig. 15].

The sketch represents a river valley between two hills. In the foreground is the sea, with a bay which is partly closed by a hooked sand bar. On each side of the valley is a terrace. From the terrace on the right a hill rises gradually, while from that on the left the ground ascends steeply in a precipice. Contrasted with this precipice is the gentle descent of the slope at the left. In the map each of these features is indicated, directly beneath its position in the sketch, by contours. The following explanation may make clearer the manner in which contours delineate elevation, form, and grade:

1. A contour indicates approximately a certain height above sea-level. In this illustration the contour interval is 50 feet; therefore the contours are drawn at 50, 100, 150, 200 feet, and so on, above sea-level. Along the contour at 250 feet lie all points of the surface 250 feet above sea; and similarly with any other contour. In the space between any two contours are found all elevations above the lower and below the higher contour. Thus the contour at 150 feet falls just below the edge of the terrace, while that at 200 feet lies above the terrace; therefore all points on the terrace are shown to be more than 150 but less than 200 feet above sea. The summit of the higher hill is stated to be 670 feet above sea; accordingly the contour at 650 feet surrounds it. In this illustration nearly all the contours are numbered. Where this is not possible, certain contours—say every fifth one—are accentuated and numbered; the heights of others may then be ascertained by counting up or down from a numbered contour.

2. Contours define the forms of slopes. Since contours are continuous horizontal lines conforming to the surface of the ground, they wind smoothly about smooth surfaces, recede into all reëntrant angles of ravines, and project in passing about prominences. The relations of contour curves and angles to forms of the landscape can be traced in the map and sketch.

3. Contours show the approximate grade of any slope. The vertical space between two contours is the same, whether they lie along a cliff or on a gentle slope; but to rise a given height on a gentle slope one must go farther than on a steep slope, and therefore contours are far apart on gentle slopes and near together on steep ones.

For a flat or gently undulating country a small contour interval is used; for a steep or mountainous country a large interval is necessary. The smallest interval used on the atlas sheets of the Geological Survey is 5 feet. This is used for regions like the Mississippi delta and the Dismal Swamp. In mapping great mountain masses, like those in Colorado, the interval may be 250 feet. For intermediate relief contour intervals of 10, 20, 25, 50, and 100 feet are used.

Drainage.—Watercourses are indicated by blue lines. If the streams flow the year round the line is drawn unbroken, but if the channel is dry a part of the year the line is broken or dotted. Where a stream sinks and reappears at the surface, the supposed underground course is shown by a broken blue line. Lakes, marshes, and other bodies of water are also shown in blue, by appropriate conventional signs.

Culture.—The works of man, such as road, railroads, and towns, together with boundaries of townships, counties and states, and artificial details, are printed in black.”[15]

Topography of dune areas.—From what has been said, it is clear that the topography of dune regions may vary widely, but it is always distinctive. Where the dunes take the form of ridges ([Fig. 1, Pl. II]), the ridges are often of essentially uniform height and width for considerable distances. If there are parallel ridges, they are often separated by trough-like depressions. Where dunes assume the form of hillocks (Figs. [2] and [3], Pl. II), rather than ridges, the topography is even more distinctive. In some regions, depressions (basins) are associated with the dune hillocks. Occasionally they are hardly less notable than the dunes themselves. A somewhat similar association of hillocks and basins is locally developed by other means, but dunes are made up of sand and usually of sand only, while the composition of similarly shaped hillocks and depressions shaped by other agencies is notably different.

In [Fig. 1, Plate II] (Five Mile Beach, 8 miles northeast of Cape May, N. J.), the contour interval is 10 feet. There is here but one contour line (the 10-foot contour), though this appears in several places. Since this line connects places 10 feet above sea-level, all places between it and the sea (or marsh) are less than 10 feet above the water, while all places within the lines have an elevation of more than 10 feet. None of them reaches an elevation of 20 feet, since a 20-foot contour does not appear. It will be seen that some of the elevations in [Fig. 1] are elongate, while others have the form of mounds.

[Fig. 2] ([Pl. II]) shows dune topography along the Arkansas River in Kansas (near Larned); [Fig. 4], dune topography in Nebraska (Lat. 42°, Long. 103°), not in immediate association with a valley or shore; and [Fig. 3] shows irregular ridge-like dunes at the head of Lake Michigan. In [Fig. 2] the contour interval is 20 feet. All the small hillocks southeast of the river are dunes. Some of them are represented by one contour and some by two. The altitude of the region is considerable, the heavy contour representing an elevation of 2100 feet; but the dunes themselves are rarely more than 20 feet above their surroundings. In [Fig. 4], where the contour interval is also 20 feet, there are, besides the numerous hillocks, several depressions (basins). These are represented by hachures inside the contour lines. In some cases there are intermittent lakes (blue) in the depressions. The heavy contour at Spring Lakes in this figure is the contour of 4300 feet. There are two depression contours (4280 and 4260) below it. The bottom of the depression is therefore lower than 4260, but not so low as 4240. In [Fig. 3] the contour interval is 10 feet, and the dune ridges north of Miller are more than 50 feet high. The dune ridges here have helped to determine the position of this branch of the Calumet River, and have blocked its former outlet. The present drainage is to the westward.

Migration of dunes.—By the continual transfer of sand from its windward to its leeward side, a dune may be moved from one place to another, though continuing to be made up, in large part, of the same sand. In their migration dunes sometimes invade fertile lands, causing so great loss that means are devised for stopping them. The simplest method (employed in France and Holland) is to help vegetation to get a foothold in the sand. The effect of the vegetation is to pin the sand down. As a dune ridge along a coast travels inland, another may be formed behind it. Successions of dune ridges are thus sometimes formed.

Fig. 16.—Diagram illustrating the migration of dunes on the Kurische Nehrung. (Credner.)

A remarkable instance of the migration of a sand dune is recorded on the Kurische Nehrung on the north coast of Germany. The Nehrung consists of a long narrow neck of land composed of sand, lying off the main coast. At the beginning of this century there was a notable dune ridge on one side. Since that time it has migrated a considerable distance, and in its migration it has been brought into the relationships illustrated in the accompanying diagrams ([Fig. 16]). In 1800 the dune ridge was on one side of a church, which was then in use. In 1839 the ridge had been so far shifted to the leeward as to completely bury the church, and in 1869, its migration had progressed so far as to again discover the building.[16]

Fig. 17.—Migration of dunes into a timbered region. Dune Park, Ind. Head of Lake Michigan. (Meyers.)

When dunes migrate into a timbered region they bury and kill the trees ([Fig. 17]). In one instance on the coast of Prussia a tall pine forest, covering hundreds of acres, was destroyed during the brief period between 1804 and 1827.[17] At some points in New Jersey orchards have been so far buried within the lifetime of their owners that only the tops of the highest trees are exposed. Trees and other objects once buried may be again discovered by farther migration of the sand (Figs. [18] and [19]).[18]

Fig. 18.—A resurrected forest. The dune sand after burying and killing the timber has been shifted beyond it. Dune Park. Ind. (Meyers.)

Eolian sand, not aggregated into distinct dunes, is often destructive. Even valleys and cities are sometimes buried by it. Drifting sands had so completely buried Nineveh two centuries after its destruction that its site was unknown.

Distribution of dunes.—Dunes are likely to be developed wherever dry sand is exposed to the wind. Their favorite situations are the dry and sandy shores of lakes and seas, sandy valleys, and arid sandy plains.

Along coasts, dunes are likely to be extensively developed only where the prevailing winds are on shore. Thus about Lake Michigan, where the prevailing winds are from the west, dunes are abundant and large on the east shore, and but few and small on the west. In shallow water, shore currents and storm waves often build up a reef of sand a little above the normal level of the water. When the waves subside, the sand dries and the wind heaps it up into dunes. This sequence of events is in progress at many points on the Atlantic Coast. Sandy Hook, New Jersey, and the “beaches” farther south started as barrier ridges. When the waves had built them above normal water-level, the wind re-worked the sand, piling it up into mounds and hillocks ([Fig. 1, Pl. II]). Such dune belts a little off shore are sometimes turned to good account. They are usually separated from the mainland by a shallow lagoon. Where land is valuable, the lagoon is sometimes filled in, making new land, thus anticipating the result which nature would achieve more slowly. This has been done at some points on the western coast of Europe.

Fig. 19.—Migration of dune sand exposing bones in a cemetery. Hatteras Island, N. C. (Collier Cobb.)

Dunes are likely to occur along stream valleys ([Fig. 2, Pl. II]), if their bottoms or slopes are of sand, and not covered by vegetation. Dunes along valleys are usually on the side toward which the prevailing winds blow. Thus they are more common on the east side of the Mississippi than on the west. Dunes may be formed in the valley bottoms, but the sand is often blown up out of the valley and lodged on the bluffs above.

Apart from these special classes of situations, any sandy region the surface of which is dry is likely to have its surface material shifted by the wind and piled up into dune ridges or hillocks ([Fig. 4, Pl. II]). Dunes probably reach their greatest development in the Sahara, where some of them take the form of hillocks, and some the form of ridges. Travelers in that region report that dune ridges are sometimes encountered the faces of which are so high and steep as to be difficult of ascent, and that parties have been obliged to travel miles along their bases before finding a break where crossing was practicable.

Fig. 20.—Wind-ripples. (Cross, U. S. Geol. Surv.)

Wind-ripples.—The surface of the dry sand over which the wind has blown for a few hours is likely to be marked with ripples ([Fig. 20]) similar to those made on a sandy bottom beneath shallow water, under the influence of waves. Like ripple marks made by the water, wind-ripples have one side (the lee) steeper than the other. While the ripples are, as a rule, but a fraction of an inch high, they throw much light on the origin of the great dune ridges. If the ripples be watched closely during the progress of a wind-storm, they are found gradually to shift their position. Sand is blown up the gentler windward slope to the crest of the ridge and falls down on the other side. The moment it falls below the crest of the ridge to leeward, it is protected against the wind, and is likely to lodge. Wear on the windward side is about equal to deposition on the leeward, and the result is the orderly progression of the ripples in the direction in which the wind is blowing, just as in the case of dune ridges.

Abrasion by the wind.—While the effect of the wind on sandy and dusty surfaces may be considerable, its effect on solid rock is relatively slight and accomplished, not by its own impact, but by that of the material it carries. The effect of blown sand on rock surfaces over and against which it is driven is perhaps best understood by recalling the effects of artificial sand-blasts, by means of which glass is etched. In a region where sand is blowing, exposed surfaces of rock suffer from a multitude of blows struck by the sand grains in transit. The result is that such rock surfaces are worn, and worn in a way peculiar to the agency accomplishing the work. If the rock be made up of laminæ which are of unequal hardness, the blown sand digs out the softer ones, leaving the harder projecting as ridges between them. Adjacent masses of harder and softer rock of whatever thickness are similarly affected. The sculpturing thus effected on projecting masses of rock is often picturesque and striking (Figs. [21] and [22]), and is most common in arid regions. Details of wind-carving are shown in [Fig. 23].

Fig. 21.—Wind-carved rock. (Green.)

PLATE II.

U. S. Geol. Surv.

Scale, 1+ mile per inch.

Fig. 1. NEW JERSEY.

U. S. Geol. Surv.

Scale, 2+ mile per inch.

Fig. 2. KANSAS.

U. S. Geol. Surv.

Scale, 1+ mile per inch.

Fig. 3. INDIANA.

U. S. Geol. Surv.

Scale, 2+ mile per inch.

Fig. 4. NEBRASKA.

Sand drifted over loose stones lying on the surface often develops flat or flattish faces or facets on them. These facets are likely to be three in number, and the exposed portion of the stone is likely to develop a sort of pyramidal shape, the three flattish surfaces being mutually limited by tolerably well-defined lines ([Fig. 24]). Thus arise the three-faceted stones (Dreikanter of the Germans) commonly seen where sands have been long in movement.

Fig. 22.—Wind-carved hillock of cross-bedded sandstone. Missouri River, Montana. (Calhoun, U. S. G. S.)

Not only does the drifting sand wear the surface over which it passes and against which it strikes, but the grains themselves are worn in the process. They are liable to be broken as they strike rock surfaces, and they are likely to strike one another in the atmosphere. In both cases they are subject to wear, and so to reduction to a finer and finer state.

The erosion accomplished by the wind is therefore of various sorts. The impact of the wind itself picks up the fine materials which are already loosened, thus wearing down the surface from which they are removed; the materials picked up wear the rock surfaces against which they are blown, and the transported materials themselves suffer reduction in transit.

Effects of wind on plants.—Another effect of wind work is seen in the uprooting of trees ([Fig. 25]). The uprooting disturbs the surface in such a way as to make loose earth more readily accessible to wind and water. The uprooting of trees on steep slopes often causes the descent of considerable quantities of loose rock and soil. Again, organisms of various sorts (certain types of seeds, germs, etc.), as well as dust and sand, are extensively transported by the wind. While this is important biologically its geological effects are remote.

Fig. 23.—Figure showing details of wind-carving on rock surface (rhyolite). Mono Valley, California.

Fig. 24.—Wind-worn stones (Dreikanter).

Fig. 25.—Shows the disturbance of surface earth and rocks by upturning of trees. (Darton, U. S. Geol. Surv.)

Indirect effects of the wind.—Other dynamic processes are called into being or stimulated by the atmosphere. Winds generate both waves and currents, and both are effective agents in geological work. The results of their activities are discussed elsewhere.

II. The Chemical Work of the Atmosphere.

The chemical work of the atmosphere (including solution and precipitation from solution) is principally accomplished in connection with water, a dry atmosphere having relatively little direct chemical effect on rock or soils.

Precipitation from solution.—The water in the soil is constantly evaporating. Such substances as it contains in solution are deposited where the water evaporates, and where evaporation is long continued without re-solution of the substances deposited, the surface becomes coated with an efflorescence of mineral matter. Conspicuous examples are found in the alkali plains of certain areas in the western part of the United States. Since the alkaline efflorescence is the result of evaporation it is connected with the atmosphere, but the material of the efflorescence was brought to its present position by water. The principle involved is illustrated by the white efflorescence which frequently appears on brick walls during the dry days which follow a drenching rain. The water penetrates the brick and mortar and dissolves something of their substance, and when it is evaporated from the surface the material in solution is left behind.

In arid regions the deposition of substances other than alkali is common. The percolating waters dissolve whatever is soluble, and when they evaporate their mineral content is left. The pebbles and stones of the arid plains have in many places become heavily coated with mineral matter deposited in this way, and not infrequently cemented into conglomerate. One of the commonest mineral substances found in such situations is lime carbonate. In some cases it was doubtless derived by solution from limestone beds beneath the surface, but this is not always the case. It often encrusts the bits of lava on lava plains where it can hardly have been derived from limestone. The faces of cliffs of granite or gneiss, hundreds and even thousands of feet above all other sorts of rock,[19] are sometimes spotted with patches of lime carbonate. In the first case the lime carbonate was derived by chemical change from the lava, and in the second, from the granite or gneiss (see [Carbonation] below), but its present position is the result of evaporation.

Oxidation.—In the presence of moisture the oxygen of the air enters into combination with various elements of the soil and rocks. This is oxidation. No other common mineral substance shows the results of oxidation so quickly and so distinctly as iron. The oxidized portion is loose and friable, and a mass of iron exposed to a moist atmosphere will ultimately crumble away. This change is comparable to other less obvious changes taking place in many minerals at and below the surface. Oxidation generally involves the disintegration of the rock concerned. Its effects in this direction will be referred to in other connections.

Carbonation.—The production of lime carbonate from rock containing calcium compounds, but not in the form of carbonates, is known as carbonation, and is one of the important chemical changes effected by the carbon dioxide of the atmosphere in coöperation with water. In the process of carbonation the original minerals of complex composition are decomposed and simpler ones usually formed. Volumetric changes are involved, which often lead to the disruption of the rock (see Ground water). Furthermore, carbonates are among the more soluble minerals, and their production therefore brings some of the rock materials into a soluble condition, and their extraction through solution tends still further to disintegrate the rock. The carbonation of crystalline rocks is therefore a disintegrating process, and will be considered further in its many concrete applications.

Other chemical changes.—A third chemical process which often accompanies oxidation and carbonation is hydration. This is effected by water rather than by air, and will be considered in that connection. In general it leads to the disintegration of the minerals and rocks affected. The chemical effects of nitric acid, etc., developed through the agency of atmospheric electricity, and the corresponding effects of the gases and vapors which issue from volcanoes, many of them chemically active, are to be mentioned in this connection.

Conditions favorable for chemical changes.—Conditions are not everywhere equally favorable for the chemical work of the atmosphere. In general, high temperatures facilitate chemical action, and, other things being equal, rocks are more readily decomposed by atmospheric action in warm than in cold regions. Chemical activity is probably greater where the climate is continuously warm than where there are great changes of temperature. Changes of temperature, on the other hand, tend to disrupt rock, and thus increase the amount of surface exposed to chemical change. Since nearly all the chemical changes worked by the atmosphere on the rocks are increased by the presence of moisture, the chemical activity of the atmosphere is greater in moist than in dry regions.

B. THE ATMOSPHERE AS A CONDITIONING AGENCY.

The most obvious mechanical work of the atmosphere is effected by the wind, but mechanical results of great importance, conditioned by the atmosphere, are also effected when the air is still.

I. Temperature Effects.

When the sun shines on bare rock its surface is heated and expanded, and the expanded particles crowd one another with great force. Since rock is a poor conductor of heat its surface is heated and expanded notably more than parts beneath the surface. It follows that strains are set up between the expanded outer portion and the cooler and less expanded parts within. In the cooling of the same rock mass it is the outermost portion which cools first and fastest, and, contracting as it cools, strains are again set up between the outer part, which is cooled more, and the inner part, which is cooled less. The result may be illustrated by the effect of cold water on hot glass, or of hot water on cold glass. In either case the fracture is the result of the sudden and considerable differential expansion or contraction. Since the heating and cooling of rock are much slower than the heating and cooling of glass under the conditions mentioned, the rupturing effects are less conspicuous, but none the less real. The actual effects of temperature changes are illustrated by familiar phenomena. The surface portions of bowlders exposed to the sun are frequently seen to be shelling off ([Fig. 26]). The loosened concentric shells may be a fraction of an inch, or sometimes even several inches in thickness. This process of exfoliation affects not only bowlders, but bare rock surfaces wherever exposed to the sun (Figs. [27], [28]). It is often conspicuous on the faces of cliffs.

Fig. 26.—Exfoliation. A bowlder of weathering, the rock being granite. Wichita Mountains, Oklahoma.

Fig. 27.—A weathered summit of granite in the Wichita Mountains. Oklahoma. (Willis, U. S. Geol. Surv.)

Several conditions, some of which are connected with the atmosphere and some with the rock, determine the efficiency of this process. Since the breaking of the rock results from the expansion and contraction due to its changes of temperature, it follows that, other things being equal, the greater the change, the greater the breaking; but the suddenness of the temperature change is even more important than its amount. It follows that great daily, rather than great annual, changes of temperature[20] favor rock-breaking, though with changes of a given frequency their effectiveness is greater the greater their range. A partial exception to this generalization should be noted. If abundant moisture is present in the pores and cracks of the rock a change of temperature from 45° to 35° (Fahr.) might be far less effective in breaking the rock than a change from 35° to 25° in the same time, for in the latter case the sudden and very considerable expansion (about one-tenth) which water undergoes on freezing is brought into play. This may be called the wedge-work of ice. The daily range of temperature is influenced especially by latitude, altitude, and humidity. Other things being equal, the greatest daily ranges of temperature occur in high-temperate latitudes, though to this general statement there are local exceptions, depending on other conditions. High altitudes favor great daily ranges of temperature, so far as the rock surface is concerned (see Figs. [29], [30]), for though the rock becomes heated during the sunny day, the thinness and dryness of the atmosphere allow its heat to radiate rapidly at night. Here, too, the daily range of temperature is likely to bring the wedge-work of ice into play. Since the south side of a mountain (in the northern hemisphere) is heated more than the north, it is subject to the greater daily range of temperature, and the rock on this side suffers the greater disruption. Similarly, rock surfaces on which the sun shines daily are subject to greater disruption than those much shielded by clouds. Isolated peaks, because of their greater exposure, are subject to rather greater daily ranges of temperature than plateaus of the same elevation.

Fig. 28.—Exfoliation on a mountain slope. Mt. Starr-King (Cal.) from the north.

The daily range of temperature is also influenced by humidity. Because of the effect of water vapor in the atmosphere on insolation and radiation, a rock surface becomes hotter in the day and cooler at night beneath a dry atmosphere than beneath a moist one. Aridity therefore favors the disruption of rock by changing temperatures.

Turning from the conditions of the atmosphere which affect the disruption of rock to the conditions of the rock which influence the same process, several points are to be noted. In the first place, the disrupting effects of changes of temperature are slight or nil where the solid rock is protected by soil, clay, sand, gravel, snow, or other incoherent material. If the constituent parts of the loose material are coarse, like bowlders, their surfaces are affected like those of larger bodies of rock. The color of rock, its texture and its composition, also influence its range of daily temperature by influencing absorption and conduction. Dark-colored rocks absorb more heat than light-colored ones, and compact rocks are better conductors than porous ones. Great absorption and slow conduction favor disruption. A given range of temperature is unequally effective on rocks of different mineral composition. In general crystalline rocks (igneous and metamorphic) are more subject to disruption by this means than sedimentary rocks, partly because they are more compact, but especially because they are made up of aggregates of crystals of different minerals which, under changes of temperature, expand and contract at different rates, while the common sedimentary rocks are made up largely of numerous particles of one mineral.

Fig. 29.—Top of Notch Peak, Bighorn Mountains, Wyo. Shows the thoroughly broken character of the rock on the summit, the absence of soil, vegetation, etc. (Kümmel.)

Fig. 30.—A detail from [Fig. 29] showing the size of the rock blocks. (Kümmel.)

Fig. 31.—Peak north of Kearsarge Pass, the Sierras. Shows the way in which serrate peaks break up into angular blocks.

The freezing of water in the pores of rock is effective in disrupting them only when the pores are essentially full at the time of freezing. Otherwise there is room for the expansion attending the freezing. If the pores of the rock are large, the expansion on freezing may force out sufficient water to balance the increase of volume, even though the rock was completely saturated. If the pores be very small the water passes out less readily, and if the rock is saturated, freezing is more likely to be attended with disruption.[21]

In view of these considerations the breaking of rock by changes of temperature should be greatest on the bare slopes of isolated elevations of crystalline rock, where the temperature conditions of temperate latitudes prevail, and where the atmosphere is relatively free from moisture. All these conditions are not often found in one place, but the disrupting effects of changing temperatures are best seen where several of them are associated (Figs. [29], [30], and [31]).

The importance of this method of rock-breaking has rarely been appreciated except by those who have worked in high and dry regions. Climbers of high mountains know that almost every high peak is covered with broken rock to such an extent as to make its ascent dangerous to the uninitiated. High serrate peaks, especially of crystalline rock, are, as a rule, literally crumbling to pieces ([Fig. 31]). The piles of talus which lie at the bases of steep mountain slopes are often hundreds of feet in height, and their materials are often in large part the result of the process here under discussion. In mountain regions where atmospheric conditions favor sudden changes of temperature, the sharp reports of the disruption of rock masses are often heard. Masses of rock, scores and even hundreds of pounds in weight, are frequently thus detached and started on their downward course.[22] Small pieces of rock are of course much more commonly broken off than large ones. The disruption of rock by changes of temperature is not usually the result of a single change of temperature, but rather of many successive expansions and contractions.

The sharp needle-like peaks which mark the summits of most high mountain ranges ([Fig. 32]) are largely developed by the process here outlined. The altitude at which the serrate topography appears varies with the latitude, being, as a rule, higher in low latitudes and lower in high. But even in the same latitude it varies notably with the isolation of the mountains and with the aridity of the climate. Thus within the United States the sharply serrate summits appear in some places, as in Washington and Oregon, at altitudes of 6000 to 10,000 feet, while in the isolated Wichita range of Oklahoma, much farther south, but in a much drier climate, the same sort of topography is developed at altitudes of 2500 to 3000 feet.

Even in low latitudes and moist climates the effects of temperature changes are often seen. Thin beds of limestone at the bottom of quarries are sometimes so expanded by the heat of the sun as to arch up and break.[23] In desert and arid regions,[24] whatever the altitude, the effects of temperature changes are often striking.

Fig. 32.—Serrate peaks of granitic rock in Black Hills. (Darton, U. S. Geol. Surv.)

The disruption of rock by changes of temperature is one phase of weathering. It tends to the formation of a mantle of rock waste, which, were it not removed, would soon completely cover the solid rock beneath and protect it from further disruption by heating and cooling; but the loose material thus produced becomes an easy prey to running water, so that the work of the atmosphere prepares the way for that of other eroding agencies.

II. Evaporation and Precipitation.

Perhaps the most important work of the atmosphere as a dynamic agent lies in its function as the medium for the circulation and distribution of water. Atmospheric temperature is the primary factor governing evaporation, an important factor in the circulation of the vapor after it is formed, and controls its condensation and precipitation.

The average amount of annual precipitation on the land is variously estimated at from forty to sixty inches, the lesser figure being probably more nearly correct. Since much of this water falls at high altitudes, the work which it accomplishes in getting back to the sea is great. The water which falls on the land, if withdrawn wholly from the ocean, would exhaust that body of water in 10,000 to 15,000 years if none of it returned. The work of evaporation is of course not done by the atmosphere, though the atmosphere determines the effect of the solar energy which does the work.[25]

The precipitation is distributed with great inequality, and this inequality affects both the rain and the snow. Some regions have heavy precipitation and some light; some regions have much rain and little snow; others have much snow and little rain; others have rain and no snow, and still others have snow and little or no rain. The amount and distribution of rain and snow determine the size and distribution of streams and glaciers, and streams and glaciers are the most important agencies modifying the surface of the land.

It is impossible to separate sharply the geologic work of the water of the atmosphere from that of other waters; but so long as moisture is in the atmosphere (including the time of its precipitation) its effects are best considered in connection with the atmosphere.

The mechanical work of the rain.—In falling the rain washes the atmosphere, taking from it much of the dust, spores, etc., which the winds have lifted from the surface of the dry land. Not only this, but in passing through the atmosphere the water dissolves some of its gases, and perhaps particles of soluble solid matter. When therefore the falling water reaches the surface of the land it is no longer pure, and some of the gases it has taken up in its descent enable it to dissolve various mineral matters on which pure water has little effect.

As it falls on the surface of the land the rain produces various effects of a mechanical nature. In the first place, it leaves on the surface the solid matter taken from the air. The amount of material, thus added to any given region in any particular shower is trivial, but in the course of long periods of time the total amount of material washed out of the air must be very great.

Every rain-drop strikes a blow. If the drops fall on vegetation, they have little effect, but if they fall on sand or unprotected earthy matter they cause movements of the particles on one another, and this movement involves friction and wear. While the results thus effected are inconsiderable in any brief period of time, they are not so insignificant when the long periods of the earth’s history are considered.

Clayey soils contract and often crack on drying. Falling on such a soil when it is dry the rain causes it to expand, and the cracks are healed by lateral swelling. The same soils are baked under the influence of the sun, and when in this condition are softened and made more mobile by the falling of rain. Under the influence of the expansion and contraction occasioned by wetting and drying, the soils and earths on slopes creep slowly downward. When rain falls on dry sand or dust the cohesion is at once increased, and shifting by the wind is temporarily stopped.

After the water has fallen on the land its further work cannot be looked upon as a part of the work of the atmosphere; but any conception of the geological work of the atmosphere which did not recognize the fact that the waters of the land have come through the atmosphere would be inadequate. The work of the water after it has been precipitated from the atmosphere must be considered in another chapter.

III. Effects of Electricity.

Another dynamic effect conditioned by the atmosphere is that produced by lightning. In the aggregate this result is inconsequential; yet instances are known where large bodies of rock have been fractured by a stroke of lightning, and masses many tons in weight have sometimes been moved appreciable distances. Incipient fusion in very limited spots is also known to have been induced by lightning. Where it strikes sand it often fuses the sand for a short distance, and, on cooling, the partially fused material is consolidated, forming a little tube or irregular rod (a fulgurite) of partially glassy matter. Fulgurites are usually only a few inches in length, and more commonly than otherwise a fraction of an inch in diameter. Strictly speaking these results are the effect of the electricity of the atmosphere rather than of the atmosphere itself, but they are best mentioned in this connection.

Allusion has already been made to the chemical changes in the atmosphere occasioned by electric discharges.

Fig. 33.—Stratified jointed rock in process of weathering. (Cross, U. S. Geol. Surv.)

Fig. 34.—Represents a later stage of the processes illustrated by [Fig. 33]. (Darton, U. S. Geol. Surv.)

SUMMARY.

Weathering.—The result of all atmospheric processes, whether physical or chemical, by which surface rock is disrupted, decomposed or in any way loosened, is weathering. This convenient term also includes similar results effected by ground water, plants, etc. The tendency of weathering is to produce a mantle of residuary earth over solid rock. Weathering by mechanical means tends to produce material which, though in a finer state of division, is still like the original rock in chemical composition. Weathering by chemical means tends to produce a mantle made up chiefly of the less soluble parts of the rock from which it was derived. All processes of weathering prepare material for transportation by wind and water.

Fig. 35.—Details of a weathered rock surface, due partly to wind work and partly to solution. The particular phase of weathering illustrated by this figure is known as “honeycomb” weathering. (Gilbert, U. S. Geol. Surv.)

Many considerations determine the thickness which the mantle of weathered rock (mantle rock) attains. Some of these considerations have to do with the atmosphere, and some with drainage. Since the latter are, on the whole, more important, this matter will be discussed in connection with the work of water (Chapters [III] and [IV]).

CHAPTER III.

THE WORK OF RUNNING WATER.

Familiar phenomena, both of land and sea, reveal the constant activity and importance of water as a geologic agent. Even when there is no precipitation the moisture in the air influences its activity in certain ways. Just as iron “rusts” more readily in moist air than in dry, so changes in other mineral substances are influenced by atmospheric humidity. Where precipitation takes place the results are more obvious. The passing shower works changes in the surface of the land, striking in proportion to the rate and amount of precipitation. The rains feed the streams, and every stream is modifying its bed, and with increasing rapidity as its current is swollen. Even the moisture which is precipitated as snow works its appropriate results. Before it melts it protects the surface against other agents of change; but if it accumulates in sufficient quantity in appropriate situations, it may give rise to avalanches and glaciers, which, like running water, degrade the surface over which they pass.

A part of the water which falls as rain, and a part of that which results from the melting of snow and ice, sinks into the soil and into the rock below, becoming ground water. It is this ground water which especially justifies the name hydrosphere, often applied to the waters of the earth, for it literally forms a spherical layer in the outer portion of the solid part of the earth. During the stay of the water beneath the surface it effects changes in the rocks through which it passes, dissolving mineral matter here and depositing it there, substituting one substance for another in this place, and effecting new chemical combinations in that. Slow as these processes are, they have worked wondrous changes in the course of the earth’s history.

When the waters are gathered together in ponds, lakes, and oceans, they are still active, and the results of their activity are seen along the shores, where winds and waves produce their chiefest effects. Even the ocean currents, far from land, and the processes of the deep sea, are not without their effect on the course of geological history.

The work of the surface waters, ground (underground) waters, standing waters, and ice will be considered in order.

RAIN AND RIVER EROSION.

Rain and river erosion began when the first rains fell on land surfaces. Neither the location nor the nature of the first land surface is known. There is little reason to believe that the ocean was ever universal, but there is reason to believe that most land areas have at some time or other been covered by the sea. The prevalent conception that land areas which were once submerged came into existence by being elevated above sea-level, should be supplemented by the alternative conception that submerged areas may have become land by the depression of the ocean basins, thus drawing off the water from the areas where it was shallow. Thus in [Fig. 36] the sinking of the sea-bottom from a to b would lower the surface of the water from cc′, to dd′, and draw off the water from the surfaces cd and c′d′.

Fig. 36.—Diagram to illustrate the origin of lands by the lowering of the sea-level due to depression of the sea bottom. If the bottom is depressed from a to b the surface will be drawn down from cc′ to dd′, and the surfaces cd and c′d′ will become land.

Without attempting to picture the character of the original land our study of subaërial erosion may begin with an area which has just been changed from sea bottom to land. What is the nature of such a land surface? Of what material is it composed, and what is the character of its topography? Concerning its constitution something may be inferred from the nature of the deposits now found at the bottom of the sea. Near the shore and in shallow water they often consist of gravel and sand, though other materials are not wanting. Far from shore and in deep water they consist for the most part of fine sediments, some of which were washed or blown from the land, some of which came from the shells and other secretions of marine animals, some from volcanoes, and some from various other sources. The topography of the newly emerged land may have had some likeness to the topography of the sea bottom. The numerous soundings which have been made over large areas of the sea have shown that its bottom is, as a rule, free from the numerous small irregularities which affect the surface of the land. They seem to show that a large part of the ocean bottom is so nearly flat that, if the water were removed, the eye would hardly detect irregularities in the surface. This statement does not lose sight of the fact that the ocean bottom is, in certain places, markedly irregular. Volcanic peaks and striking irregularities of other sorts abound in some places. Nevertheless if the bottom of the sea could be seen as the land is, its most striking feature, taken as a whole, would be its apparent flatness.

With the topography of the sea bottom the topography of the land is, in its details, in sharp contrast. In order to get at the history of the latter, we may study the sequence of events which would follow the emergence of a portion of the former.

Subaërial Erosion without Valleys.

For the sake of emphasizing the fundamental principles involved in the work of running water, a hypothetical case will first be studied in some detail, even at the risk of elaborating processes already understood. The principles themselves will find application later in relations which are much less simple.

Let it be assumed that the area of newly emerged land is a circular dome-shaped island. The simplest possible condition is represented by assuming its slope to be the same in all directions from the center, and its materials to be absolutely homogeneous. Such an island would be subject to all the forces ordinarily operating on land surfaces. The chief agency tending to modify land surfaces is atmospheric precipitation. It will be assumed that the rain falls on the surface of the island with absolute equality at all points, and that all other forces which affect it operate equally everywhere.

The rain falling on a land area disappears in various ways; part of it evaporates, part of it sinks, and part of it runs off over the surface. If the island be composed of fine and unconsolidated materials, such as clay, the water which runs off over the surface will carry sediment down to the sea. If the island be composed of solid rock instead, exposure to the air will cause it to decay, and the products of decay, such as sand and mud, will suffer a like fate.

For the sake of a clear understanding of the processes involved, two cases may be postulated; one in which the waters of the sea remove the sediment washed down from the hypothetical island as fast as it reaches the shore, and one in which they allow it to accumulate without let or hindrance. In both cases the wear of the waves will be neglected.

1. In the first case the water flowing off over the surface (the run-off) will descend equally in all directions. It will constitute a continuous sheet of surface-water, and both its volume and its velocity will be the same at all points equally distant from the summit. Erosion accomplished by sheets of running water, as distinct from streams, is sheet (or sheet-flood) erosion.[26] Since the material of the surface is homogeneous, the wear effected by the water will be equal at all points where its velocity and volume are equal. For obvious reasons the depth of the run-off will increase from summit to base. The gradient (slope) also increases in the same direction, and the increase of volume and of gradient conspire to augment the velocity of the water, and therefore of the wear effected by it. If the thin sheet of water starting from the top of the island with relatively low velocity be able to wash off even a little fine material from the surface, the thicker sheet farther down the slope, moving with greater velocity, will be able to carry away more of the same sort of material, and the increase will be progressive from summit to base. It follows, therefore, that the surface will be worn equally at points equally distant from the summit, but unequally at points unequally distant from it. The first shower which falls on the island may be conceived to wash off from its surface a very thin sheet of material, but a sheet which increases in thickness from top to bottom. The run-off will not be stopped immediately on reaching the sea, but will displace the sea-water to some slight depth, and wear the surface some trivial distance below the normal level of the sea. The result of successive showers working in the same way through a long period of time will be to diminish the area of the island and to steepen its slopes. The results of a considerable period of erosion under these conditions are shown diagrammatically in [Fig. 37], which illustrates both the diminution in area which the island has suffered, and the increase in the angle of its slopes. Immediately about it, at the stage represented by aa, [Fig. 37], there is a narrow marginal platform, or submerged terrace, in place of the land area which has been worn away at or just below the level of the sea. Long successions of rains working in the same way will give the island steeper slopes, a smaller area, and a wider marginal terrace. Successive stages are shown by the lines bb and cc, [Fig. 37].

Fig. 37.—Diagram to illustrate the effect of rain erosion on an island where there is no deposition or wave erosion about its borders. The uppermost curve represents the original surface, while aa, bb, and cc represent successive surfaces developed by sheet erosion, on the supposition that no material is deposited along the shores.

If rain falls on such an island until it completes the work which it is possible for running water to do, the island will be reduced essentially to the level of the sea, and in its place there will be a plain, the area of which will be equal to that of the original island. Its central point will be at the level of the sea, and its borders a trifling distance below it ([Fig. 38]). The island is gone, and in its place there is a plain as low as running water can wear it. Other agencies might come in to defeat the result just outlined, but if the island did not rise or sink after its formation, rain falling upon it would, under the conditions specified, finally bring about the result which has been sketched. The plain ([Fig. 38]) which succeeds the island is a base-level of erosion, though this term is also used in other ways. Under these conditions the slope of the land would remain convex at all stages, but the convex erosion profile of the land would meet a nearly straight line just below sea-level. The relative lengths of these two elements of the profile, the curve above and the straight line below, vary as erosion progresses, the convex portion becoming shorter and the other longer, The two parts of the profile taken together are concave upward at the lower end all the time, and for a greater distance from its lower end in all the advanced stages of erosion ([Fig. 37]).

Fig. 38.—Diagram to illustrate the final effect of rain erosion under the conditions specified in the text. The diagram expresses the final result of the processes suggested by [Fig. 37].

In the destruction of the land under these conditions neither valleys nor hills would be developed, nor would the topography of the land be fashioned to correspond with the surfaces with which we are familiar.

It is to be distinctly borne in mind that the foregoing is a hypothetical case; it is not probable that such an island ever existed, or ever will; but that does not diminish the value of the illustration, since the principles involved are operating on every land mass, though in less simple relations.

2. The second case differs from the first in that the sediment washed down from the land is deposited about its borders. This results in the building up of a marginal platform, as shown in [Figs. 39–41]. As erosion goes on more sediment is washed down and deposited, partly on the narrow marginal shelf which has already been developed, and partly on its outer slope, as shown in the figures. The marginal flat is thus extended beyond the original shores of the island on the one hand, and toward its center on the other. As it develops, its inner portion, and indeed all except its outer edge (ab, Figs. [40] and [41]), will be gradually built up above the level of the water. This marginal lowland is developed at a level as low as running water, under the conditions then and there present, can reduce the land. Such a surface may be said to be at grade, since running water neither wears it down nor builds it up. Its angle of slope is a function of (1) the volume of the water running over it, and (2) of the load which the water carries.

Fig. 39–41.—Diagrams to illustrate the effect of rain erosion on an island when all the eroded material is deposited about the shore. The black portions represent deposition. The dotted lines represent the original surface. The several diagrams represent successive stages in the process.

Since the marginal plain of the above illustration extends beyond the original shore of the island, the area of land is increased, though both its average elevation and its mass (above water) are reduced. In case destructive processes did not operate on the marginal graded plain the spreading and lowering suggested by Figs. [39] and [40] would go on until the central mass of the island was brought down to a gradient in harmony with that of the gently sloping border, as shown in [Fig. 41]. When this had been accomplished there would be a relatively large land area with low slopes ([Fig. 41]) in place of the smaller area with steeper ones (compare Figs. [39] and [40]). The basal part of the larger island from the center to the original margin would be made up of the original material in its original position (unshaded part of [Fig. 41]). Its surface would be covered, least deeply near its center and most deeply near the original margin, with débris gradually shifted from higher levels, as shown in [Fig. 41].

Were such an island as that shown in [Fig. 41] once formed, the rain falling on it, and flowing off over its surface, would carry off its surface soil and spread it about the shores. Though the surface of the marginal flat of [Fig. 40] was as low as running water could bring it at the time it was developed, the conditions of erosion have changed by the time the land reaches the conditions shown in [Fig. 41], and the same amount of rainfall may now be effective in erosion. In the first case ([Fig. 40]) the water descending from the higher part of the land brought down sediment and started across the flat with a load. Its energy was consumed in transporting what it had, not in getting new material. In the second case ([Fig. 41]) the water flowing over the gently sloping surface has no initial load, and its energy is therefore available for erosion. Under continued rainfall, the area of the land shown in [Fig. 41] would be increased as before by successive marginal deposits (see [Fig. 42]), and at the same time its average height would be reduced. The lowering and enlarging of the island would continue until the whole surface was brought so nearly to the level of the sea that water would cease to run over it with sufficient velocity to carry away even the fine material of its surface. Such a surface, brought down as low as running water can degrade it, is also (see [p. 57]) a base-level. It will be seen from the foregoing illustrations that a graded surface may pass into a base-level, with no sharper line of demarkation than that which separates a mature man from an old one. In this case, as in the preceding, the island has been base-leveled, but still without the formation of valleys or hills.

Fig. 42.—Diagram to illustrate the result of the continuation of the processes shown in Figs. 39–41.

Both the preceding hypothetical cases make it clear that, from the point of view of erosion, every drop of water which runs off over the surface of the land has for its mission the getting of the land into the sea. Under ordinary conditions surface drainage must fail to bring a land area altogether to sea-level, the absolute base-level of subaërial forces; but it is not simply the water which runs off over the surface which degrades the land. That which sinks beneath the surface contributes to the same end by slowly dissolving mineral matter below the surface, and finally carrying it to the sea. In this way the reduction of land areas to sea-level may be completed.

The rain-water which evaporates from the surface without sinking beneath it does not effect much wear; but the water thus evaporated is subject to reprecipitation, so that, in the long run, it may assist in the work which has been sketched. Thus it is not simply the waters which run off over the surface of the land, but all which fall upon it, which unite to compass its destruction.

The Development of Valleys.

By the growth of gullies.—Had the slopes of the hypothetical island not been absolutely uniform the processes of erosion would have been different. Let the departure from uniformity be supposed to consist of a single slight meridional depression near the base of the island ([Fig. 43]). As the rain falls it will no longer run off equally in all directions. A greater volume will flow through the depression than over other parts of the surface having the same altitude, and the greater volume of water along this line will give greater velocity, greater velocity will occasion greater erosion, and greater erosion will deepen the depression. The immediate result is a gully or wash ([Fig. 44]). So soon as the gully is started it tends still further to concentrate drainage in itself, and is thereby enlarged. The water which enters it from the sides widens it; that which enters at its head lengthens it by causing its upper end to recede; and all which flows through it, so long as its bottom is above base-level, deepens it. The enlarged gully will gather more water to itself, and, as before, increased volume means increased velocity, and increased velocity increased erosion. As the gully grows, therefore, its increased size becomes the occasion of still further enlargement.

Fig. 43.—Diagram showing a slight meridional depression in the surface of an otherwise even-sloped island.

Continued growth transforms the gully into a ravine, though between a gully and a ravine there is no distinct line of demarkation. But growth does not stop with ravine-hood. Water from every shower gathers in the ravine, and, flowing through it, increases its length, width, and depth, until it reaches such proportions that the term ravine is laid aside, as childhood names are, and the depression becomes a valley.

Fig. 44.—Diagram illustrating the development of a gully, starting from the condition shown in [Fig. 43].

It was assumed in the preceding paragraphs that the single depression in the slope was meridional and low on the slope, but almost any sort of depression in almost any position would bring about a similar result, since it would lead to concentration of the run-off. Had the original surface been interrupted by ridges instead of depressions, the effect on valley development would have been much the same, for a ridge, like a depression, would, in almost any position, occasion the concentration of the run-off, and so the development of valleys. Under the conditions represented in [Fig. 44] the lengthening of the drainage depression is effected chiefly at its upper end, the head of the valley working its way farther and farther back into the land. This method of elongation is known as head erosion. But the lengthening of the valley is not always wholly by head erosion. The gully normally begins where concentration of run-off begins, and if this were not at sea-level, the gully might be lengthening at both ends at the same time. This would have been the case, for example, had the original depression of [Fig. 43] been half-way up the slope of the island.

If while the slopes of the island were absolutely uniform its surface material failed of homogeneity, the result would be much the same as if the slopes were unequal. If the material lying along a certain meridian of the island be slightly softer than that over the rest of the surface, the run-off, which would at the outset be equal on all sides, would effect more erosion along the line of the less resistant material than elsewhere. The result would be a depression along this line, and, once started, the depression would be a cause of its own growth. If the soft material were disposed in any way other than that indicated, the final result would be much the same, for it would quickly give origin to a depression which would lead to the concentration of the surface-waters, and this is the condition for the development of a gully, a ravine, and finally a valley.

Fig. 45.—Diagram to illustrate the effect of sheet and stream erosion on the outline of an island when no deposition takes place about its borders. The dotted line represents the original outline of the island, the full line its border at a later time. The stream develops a reëntrant (bay) in the outline.