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GEOLOGY AND REVELATION.

Sicut Augustinus docet, in hujusmodi quæstionibus duo sunt observanda. Primo quidem ut Veritas Scripturæ inconcusse teneatur. Secundo, cum Scriptura Divina multipliciter exponi possit, quod nulli expositioni aliquis ita præcise inhæreat, ut si certa ratione constiterit hoc esse falsum quod aliquis sensum Scripturæ esse credebat, id nihilominus asserere præsumat; ne Scriptura ex hoc ab infidelibus derideatur, et ne eis via credendi præcludatur.

S. Thomas, De Opere Secundæ Diei; Summa, Pars 1, Quæst. 68, Art. 1.

As Augustine teacheth, there are two things to be observed in questions of this kind. First, that the truth of Scripture be inviolably maintained. Secondly, since Divine Scripture may be explained in many ways, that no one cling to any particular exposition with such pertinacity that, if what he supposed to be the teaching of Scripture should turn out to be plainly false, he would nevertheless presume to put it forward; lest thereby Sacred Scripture should be exposed to the derision of unbelievers, and the way of salvation should be closed to them.

Saint Thomas, On the Work of the Second Day.

Geology and Revelation:
OR THE
Ancient History of the Earth,
CONSIDERED IN THE LIGHT OF
GEOLOGICAL FACTS AND REVEALED RELIGION.
WITH ILLUSTRATIONS.
BY THE
Rev. GERALD MOLLOY, D. D.,
PROFESSOR OF THEOLOGY IN THE ROYAL COLLEGE OF ST. PATRICK, MAYNOOTH.
WITH AN INTRODUCTION
To the American edition; and a chapter on Cosmogony, [by permission]
from the Manual of Geology, by Prof. J. D. Dana.
NEW YORK:
G. P. PUTNAM & SONS,
1870.

Stereotyped by Little, Rennie & Co., 645 and 647 Broadway, N. Y.
Press of The New York Printing Company, 81, 83, and 85 Centre St., N. Y.

To the Very Reverend
CHARLES WILLIAM RUSSELL, D. D.
PRESIDENT OF SAINT PATRICK’S COLLEGE, MAYNOOTH,
This Volume is Inscribed,
WITH EVERY SENTIMENT OF AFFECTION AND RESPECT.

PREFACE.

The progress of modern Science has given rise to not a few objections against the truths of Revelation. And of these there is none which seems to have taken such a firm hold of the public mind in England, and, indeed, throughout Europe generally, as that which is derived from the interesting and startling discoveries of Geology. Accordingly, when I was engaged, some years ago, in explaining and defending the Evidences of Revealed Religion, I found myself brought face to face with Geological phenomena and Geological speculations.

It was plainly impossible to consider, in a candid and philosophical spirit, the argument with which I had to deal, so long as I remained ignorant of the evidence on which it was based. I resolved, therefore, to make myself familiar with the leading principles and the leading facts of Geology. And thus I was drawn insensibly into the study of this science; to which I have devoted, for some years, the greater part of my leisure hours.

Impressed with the conviction that no fact can be really at variance with Revealed Truth, I determined, in the first place, to ascertain the facts which have been brought to light by the researches of Geologists. The general principles, which might afterward appear to be clearly involved in these facts when duly classified and arranged, I was fully prepared to admit. And I hoped, in the end, to search out and discover the harmony which, I was satisfied, must exist between conclusions thus established and the Inspired Word of God.

While occupied in working out this problem for myself, it was suggested to me that others, who had not time or opportunity to pursue the same line of inquiry, would, perhaps, be glad to share in the fruits of my studies. In deference to this suggestion I consented, not without misgivings, to write a series of papers on Geology in its relations with Revealed Religion, which have appeared, from time to time, in the Irish Ecclesiastical Record. From the attention these papers attracted, crude and fragmentary as they were, it soon became evident that the question was not without interest for a large class of readers. And I have been led to believe that a more full and mature, but at the same time a popular, Treatise on the subject would be a welcome accession to ecclesiastical literature, and would supply a want that has long been felt. Such a Treatise I have proposed to myself in the present Volume.

In Geology I wish to disclaim at the outset, all pretension to original researches; which my opportunities did not permit, nor the scope of my Work demand. It was not my object to enlarge the bounds of Geological knowledge; but rather to ascertain what that knowledge is, and to set it before my readers in plain and simple words. For this purpose I have had recourse to the great masters of the science: and have endeavored to gather into a systematic form the phenomena upon which they are all agreed; to sketch in outline the general theory about which there is practically no dispute; and to draw out the line of reasoning by which, as it seems to me, this theory may be most effectively demonstrated.

Exact references are given to the original authorities on all questions of importance, and on many points even of minor detail: partly that I might not seem to claim as my own what belongs to others; partly that I might consult for the convenience of those who should wish to investigate more minutely what I have but lightly touched. And here it may be well to observe, with regard to the two classic works of Sir Charles Lyell, his Elements and his Principles, which have been reproduced so many times and in so many forms, that I have uniformly referred to the latest edition of each.

The Woodcuts which illustrate the Volume will, I venture to hope, help to convey a clear and distinct impression of many natural objects which can be represented but imperfectly in words. Some of the most striking and effective are taken from the admirable Manual of Geology brought out some years ago by the Reverend Doctor Haughton, of Trinity College, Dublin. My best thanks are due to the learned author for the kindness with which he placed his Woodblocks at my disposal. I have also to express my acknowledgments to Sir Charles Lyell, who has allowed me to reproduce some of the drawings that embellish his works; and to the eminent publishers, Messrs. Bell and Daldy of London, and Mr. Nimmo of Edinburgh, who have, with great courtesy, furnished me with electrotypes of several figures from the works of Doctor Mantell and Mr. Hugh Miller.

To my colleagues in Maynooth I am much indebted for their judicious suggestions and friendly assistance during the progress of the Work. In particular I desire to testify my obligations to our distinguished Professor of Scripture, the Reverend Doctor M’Carthy, for the unwearied kindness with which he has allowed me to draw at pleasure on his profound and extensive knowledge of the Sacred Text.

G. M.

Saint Patrick’s College, Maynooth,
December 1st, 1869.

PREFACE TO THE AMERICAN EDITION.

Dr. Molloy has, in the present work, made an important contribution to a department of scientific and theologic literature, which has already been enriched by the labors of several other Catholic Fathers, among whom must be mentioned Cardinal Wiseman,[1] Father Perrone,[2] and Father Pianciani,[3] who, in Italy, maintain substantially, the same ground which, in England, has been sustained by Dr. Chalmers, Dr. Buckland, Pye Smith, and Hugh Miller, and we may now add with pleasure, by Dr. Molloy. Names which, in the United States, find their counterparts in Dr. Hitchcock, Prof. Silliman, Prof. A. Guyot, Dr. Thompson, and J. D. Dana.

Reviewing the progress of opinion touching the relations of Science to Revealed Religion, it is noteworthy that while many Protestant theologians and writers on both sides of the Atlantic have, until a recent period, treated the discoveries of science, and especially of Geology, so far as they affect theological dogmas, in a manner, if not of contempt, at least of distrust or unfairness: on the contrary, the Romanist writers who have discussed these themes, have done so, generally, in a spirit of broad catholicity well calculated to command the respect it merits. They have shown no sensitiveness or timidity lest, perchance, their exegesis might be disturbed by candidly admitting the changes demanded by the discoveries of Science.

The author’s discussion of the principles of Geology evinces much familiarity both with the science and what is equally important, the necessities of the unscientific reader. He has presented, in the second part of his book, an interesting review, infused by copious quotations from the Christian Fathers, from the time of St. Augustine, showing that long before Geology had any existence as a science, and of course, when the discussions and doubts it has excited were unknown, the essential points respecting Time and the order of Creation had received careful attention from devout thinkers, and that the conclusions at which they arrived, on purely theological grounds, were, in most cases, much the same as those which the best writers of our time deduce from Geological evidence.

It is now thirty-five years since (1835) Cardinal, then Dr. Wiseman, delivered in Rome, before the English College, of which he was the head, his Lectures, already referred to, on the connection between Science and Religion, in the fifth and sixth of which he considers more particularly the Geological argument. The spirit of these lectures was a just rebuke to the narrow bigotry of such writers as Mr. Croly, Fairholm, and Granville Penn, as well as certain American theologians, who, by means of arrogance and denunciation, sought to silence the voice of truth, as proclaimed in the language of discovery, announcing the nature and the extent of those changes in life and in physical development which are recorded in the Genesis of the Rocks, because they conceived these immutable truths must of necessity conflict with the Genesis of Moses; the real conflict being only with their narrow interpretations. With rare moral courage Dr. Wiseman grappled with the great questions discussed so well in his lectures, at a time when there prevailed, with reference to such themes, a very wide-spread distrust, even among men of moderate opinions. In fact, the candor and courtesy displayed by Dr. Wiseman in his lectures, presents an enviable contrast to the acrimony of many theologians, and worthy of all praise, and in harmony with the learning and good taste which characterize his writings.

Dr. Molloy is a worthy disciple of the same school, and we are glad to find in him the same candor and liberality which it is certainly to be hoped he will receive at the hands of those who may differ from him. His geological arguments and illustrations are very naturally drawn, chiefly from British authorities. It is evident that the condition of opinion upon these matters among religious teachers and readers in Great Britain is less advanced than it is in this country or in continental Europe. Our author has obviously but little familiarity with the American literature of this subject. The similarity in some parts of his book both in thought and style with the writings on this subject of the late Professor Silliman, of Yale College, is quite noticeable. He has obviously not seen the writings of Dr. Hitchcock, of Guyot, of Dana, and of other American writers. We have therefore by the kind permission of the author reproduced in this edition the chapter on Cosmogony from Professor Dana’s Manual of Geology.[4] The views set forth, in a very condensed form, in this chapter, embrace also the ideas of Professor Arnold Guyot, of Princeton, as presented by him in his unpublished lecture upon the same subject.

American readers will remember also that Professor Dana has discussed this subject much more at length in a series of papers published in the Bibliotheca Sacra,[5] in a review of Dr. Tayler Lewis’s Six Days of Creation.[6] It is greatly to be desired that Professor Dana should soon make a revised edition of his various writings upon this subject, a work which would be received with interest on both sides of the Atlantic.

We do not propose here to present the bibliography of this subject with any completeness, but we desire to mention, to those who have not seen it, a little volume of excellent spirit by Dr. Jos. P. Thompson, of New York, entitled Man in Genesis and Geology,[7] which discusses chiefly the relations of man to creation, in seven lectures, the first of which is an “Outline of Creation in Genesis.” Even as we write another small volume on this subject comes to hand under the title of Chemical History of the Six Days of Creation,[8] by Mr. John Phin, which also contains the substance of a series of lectures delivered by the author, who handles his theme in a spirit equally reverential and scientific, and well calculated to do good.

Those who desire to know the best exposition of this subject at the hands of a modern theologian will read the first part of Dr. Lange’s Genesis, or the First Book of Moses,[9] in Dr. Tayler Lewis’s translation, pp. 159-177. The candid and scholarly spirit of the learned authors of this work indicates a marked change in discussions of this nature when compared with similar literature of the last generation.

These few suggestions, chiefly on the American literature of this subject, are offered in the belief that some readers may be glad to know where to turn for similar discussions, while Dr. Molloy will certainly not misinterpret our kindly intentions in suggesting to him some contemporary sources of information to most of which he very probably had no means of access when his excellent work was prepared.

July, 1870.

CONTENTS.

LIST OF ILLUSTRATIONS.

LIST OF TABLES.

GEOLOGY AND REVELATION.

INTRODUCTORY CHAPTER.

Scope of the work explained—Geology looked on with suspicion by Christians—Hailed with triumph by Unbelievers—No contradiction possible between the works of Nature and the Word of God—Author not jealous of progress in Geological Discoveries—Points of contact between Geology and Revelation—The question stated—The answer—Division of the work.

Among the various pursuits that engage the human mind, there are few so attractive as Geology, none so important as Revelation. Each of these two studies has an interest peculiar to itself. The one is chiefly concerned about the world in which we are living: the other about the world to which we are hastening. Geology leads us down into the depths of the Earth, and there, unfolding to our view a long series of strange unwritten records impressed on lasting monuments by the hand of Nature, it proceeds to trace back the history of our Globe through myriads of ages into the distant past. Revelation, on the other hand, comes to us from above; and setting forth the far more wonderful records of God’s dealings with man, holds out the hope of another world “everlasting in the heavens”[10] which shall still remain when this earth and all the works that are therein shall have melted away with fervent heat.[11]

But, it may be asked, why should two such incongruous topics be set down for discussion side by side? To answer this question is to explain the scope and design of the present work. We are not going to write a Manual of Geology; nor yet a Treatise on Revelation. Taken separately, these two subjects have been handled with eminent skill and ability; the one by the votaries of Science, the other by the friends of Theology. It is our purpose to consider them not so much in themselves as in their mutual relations: to compare the conclusions of Geology with the truths of Revelation; and to inquire if it be possible to accept the one and yet not to abandon the other.

An uneasy apprehension has long prevailed among devout Christians, and a declared conviction among a large class of unbelievers, that the discoveries of Geology are at variance with the facts recorded in the Book of Genesis. Now, the historical narrative of Genesis lies at the very foundation of all Revealed Religion. Hence the science of Geology, has come to be looked on with suspicion by the simple-minded faithful, and to be hailed with joy, as a new and powerful auxiliary, by that infidel party which, in these latter days, has assumed a position so bold and defiant. It is now confidently asserted that we cannot uphold the teaching of Revelation, unless we shut our eyes to the evidence of Geology; and that we cannot pursue the study of Geology, if we are not prepared to renounce our belief in the doctrines of Revelation.

Vet surely this cannot be. Truth cannot be at variance with truth. If God has recorded the history of our Globe, as Geologists maintain, on imperishable monuments within the Crust of the Earth, we may be quite sure He has not contradicted that Record in His Written Word. There may be for a time, indeed, a conflict between the student of Nature and the student of Revelation. Each is liable to error when he undertakes to interpret the record that is placed in his hands. Many a brilliant Geological theory, received at first with unbounded applause, has been dissipated by the progress of discovery even within the lifetime of its author. On the other hand, it cannot be denied that Theologians have sometimes imputed to the Bible that which the Bible does not teach. Learned and pious men—Protestants and Catholics alike—once believed that the Book of Joshua represents the succession of day and night as produced by the revolution of the Sun around the Earth: whereas it is now considered quite plain that the Book of Joshua, properly understood, teaches nothing of the kind; but that the Inspired Writer, in describing a wonderful phenomenon of Nature, simply employs the language of men according to the established usage of his time. We need not wonder, therefore, that a conflict of opinion should sometimes arise between the Geologist and the Theologian; but a conflict there cannot be between the story which God has described on His works and the story He has recorded in His Written Word.

Though we come forward, therefore, among those whose duty and whose glory it is to uphold Revelation, we are by no means jealous of the wonderful ardor, and we may add, the wonderful success, with which the study of Geology has been lately pursued. We have too much confidence in the truth of our cause to apprehend that it can suffer in any way from the progress of Natural Science. It is our conviction, rather, that the more thoroughly the works of Nature are understood, the more perfectly they will be found to harmonize with the truths of Revelation. We are not afraid, therefore, to venture into the realms of Geology and to come face to face with its discoveries. Too long, perhaps, has this interesting and popular science been neglected by those who are ranged under the banner of Religion. Let it be ours to show that the study of God’s works is not incompatible with the belief in God’s Word; and that it is quite possible to investigate the ancient history of the world we inhabit without forfeiting our right to a better.

The points of contact between Geology and Revelation are chiefly these two:—First, the Antiquity of the Earth; Secondly, the Antiquity of the Human Race. In the present Volume we shall confine our attention to the Antiquity of the Earth. The subject that offers itself for discussion may be stated in a few words. Geologists maintain that the Crust of the Earth has been slowly built up by means of a long series of operations which would require hundreds of thousands, perhaps millions of years for their accomplishment: whereas the Bible narrative, it is alleged, allows but the short lapse of six or eight thousand years from the creation of the world to the present time. The Geological record, then, seems to contradict the Mosaic; and the question is, how this apparent contradiction is to be explained.

Some have ventured to solve the problem by rejecting the historical narrative of the Bible: others by ignoring the plain facts of Geology. But there is a third class of writers, including many names of the highest eminence and authority, who contend that we may admit the extreme Antiquity of our Globe, which Geology so imperatively demands, without compromising in the smallest degree the truthfulness of the Mosaic story. They say that the Chronology of the Bible stops short with Adam, and does not go back to the beginning of the world. By means of the data which the Bible supplies we may calculate, at least roughly, the lapse of time from the Creation of Adam to the Birth of Christ. But from the first beginning of all created things, when God made the Heavens and the Earth, to the close of the Sixth Day when Adam was introduced upon the scene, that is an interval which, in the Bible narrative, is left altogether undefined and uncertain. This is the view which we hope to develop and to illustrate in the course of the following pages.

Our task naturally divides itself into two parts. First, it will be our duty to consider the received theory of Geology, and to examine in detail some of the interesting and wonderful phenomena on which it is founded. This course of investigation, while it is plainly indispensable for the intelligent appreciation of our subject, cannot fail at the same time to unfold many new and striking views of the Power, and the Goodness, and the Providence of God. “For the invisible things of Him from the creation of the world are clearly seen, being understood by the things that are made; even His eternal Power and Godhead.”[12]

In the Second Part we shall consider the Antiquity of the Earth in reference to the History of Genesis. It will be our purpose to show that, as far as the Bible narrative is concerned, an interval of countless ages may have elapsed between the first creation of the Heavens and the Earth and the beginning of the Six Mosaic Days. Furthermore, we shall contend that, without any prejudice to the Sacred History, we may suppose these Days themselves to have been, not days in the ordinary sense of the word, but long and indefinite Periods of Time. If we succeed in establishing these views, it will be obvious to infer that, while the Bible enables us to determine, at least by approximation, the Age of the Human Race, it allows time without limit for the past history of the Earth.

PART I.
GEOLOGICAL THEORY AND THE EVIDENCE BY WHICH IT IS SUPPORTED.

CHAPTER I.
THEORY OF GEOLOGISTS.

Geology defined—Facts and Theories—Recent progress of Geology—Stratification of Rocks—Aqueous Rocks; of Mechanical Origin—of Chemical Origin—of Organic Origin—Igneous Rocks, Plutonic and Volcanic—Metamorphic Rocks—Summary of the Rocks that compose the Crust of the Earth—Relative order of position—Internal condition of the Globe—Movements of the Earth’s Crust—Subterranean disturbing force—Uplifting and bending of Strata—Denudation and its Causes—Fossil Remains—Their Value in Geological Theory.

The object of Geology is to examine and record the appearances presented by the Crust of the Earth; and by the aid of these appearances, to trace out the long series of events by which it has been brought into its present condition. Geology, therefore, like all other natural sciences, is made up partly of fact, and partly of theory. It belongs to the Geologist first to investigate the phenomena which the Crust of the Earth exhibits to the eye. For this purpose he descends into the mine and the quarry; he visits the lofty cliff by the sea-shore, the deep ravine on the mountain side, the cutting of a railway; in a word, every spot where a section of the Earth’s Crust is exposed to view, either by the action of Nature or by the hand of man. He then retires into the silence of his closet, with his note-book and his specimens; and there, having arranged and classified the various phenomena which he has already examined with his eyes in the outer world, he proceeds to make his deductions, and to build up his theory. He seeks to explain how materials, so diverse in their composition, have come to be piled up together, with such admirable order, and yet with such endless variety; and how the solid rocks have come to be the repository of petrified trees and plants and bones and shells, which seem, as it were, to start up from their graves, and to tell strange stories of a bygone world.

In the early days of Geology there were comparatively few who devoted themselves with patient industry to the collection and classification of facts: while the number was legion of those who, with a very meagre knowledge of facts, set themselves to build up systems. A vast multitude of different and conflicting theories were, in this way, brought into existence, and attracted for a time much public attention, each one being vehemently defended by its friends and as vehemently assailed by its enemies. These theories resting on no solid foundation, could not hold their ground against the advancing tide of new discoveries. They flourished for a brief space, and then gave way to others scarcely more substantial, which were destined in their turn to be likewise rejected and forgotten. Thus it came to pass, from the manifest instability of its principles, that Geology was long held in light repute, and practical men set little store by its boasted discoveries and startling revelations.

But it would be unjust and unphilosophical to condemn the modern theory of Geologists because of their past errors. We must judge of this science, not according to what it once was in the feebleness of its infancy, but according to what it now is in the growing strength of its mature years. It seems to be in the nature of things that groundless speculations and wild conjectures go before, and sober Science follows in their wake. The visionary dreams of the Alchemist led the way to the science of Chemistry, and the idle fancies of the Astrologist have given place to the marvellous discoveries of Astronomy. So, too, amidst the confused mass of conflicting arguments and opinions, by which the phenomena of Geology were for a long time enveloped and obscured, the seeds of a new science were slowly germinating. New facts were eagerly sought after to support or to impugn the favorite theory of the hour; and though theory after theory passed away, yet the facts remained. In course of time this accumulation of facts became broad and deep and solid enough to form a sound basis for inductive reasoning; and thus almost within our own days Geology may be fairly said to have assumed the rank and dignity of a science.

During the last quarter of a century it has been studied with a more ardent enthusiasm than, perhaps, any other science in England, in France, in Germany, and in America. It has been studied, too, upon better principles than before: less attention has been paid to the building up of theories, and far more pains and labor have been expended on the careful investigation of natural phenomena. There are still, no doubt, different schools of Geologists which are divided among themselves as regards many important details of theory; but there are some general conclusions upon which all Geologists are substantially agreed, and which, they assure us, are established by evidence that is absolutely irresistible. It is to these conclusions we wish to invite the attention of our readers; for they bear very closely on the question of the Antiquity of the Earth.

Geologists tell us, then, that the materials of which the Earth’s Crust is composed, are not heaped together in a confused mass, but are disposed with evident marks of definite and systematic arrangement. This is an important truth, of which many examples are familiar to us all, though perhaps we do not all attend to their significance. Thus in a quarry, we see commonly enough first a bed of limestone, then above that a bed of gravel, and higher still a bed of clay: and even the limestone itself is not usually a compact mass, but is arranged in successive layers, something like the successive courses of masonry in a building. Now it appears that a very large proportion of the Earth’s Crust is made up in this way of successive layers, or strata, as they are called by Geologists. These strata are composed of various substances, such as clay, chalk, sand, lime, and coal; and they present everywhere the same general appearances. They are known under the common name of Aqueous Rocks,[13] because it is believed that they were originally formed under water; and here it is that the professors of Geology first come into collision with the popular notions that formerly prevailed.

They hold that these stratified rocks were not arranged as we see them now, when the Earth first came from the hands of its Creator, but have been formed, during the lapse of unnumbered ages, by the operation of natural causes. Nay more, they have divided the rocks into sundry classes, and they undertake to explain the particular process by which each several variety has been produced. First in order and importance are those which derive their existence from the mechanical force of moving water. The materials of which they are composed first existed in the form of minute particles, which were transported by the action of water from one place to another; then they were spread out over a given surface, just as we now see layers of sand, or mud, or gravel deposited near the mouths of rivers, or in the estuaries of the sea, or even upon the land itself during temporary inundations. Lastly, after a long interval came the slow but certain process of consolidation. The fine sand was cemented together and became sandstone; the loose gravel by a similar process was transformed into a solid mass, known by the name of Conglomerate or Pudding-stone; while the soft mud by simple pressure was converted into a kind of slaty clay, called Shale. Thus from age to age Nature was ever building up new strata, and consolidating the old.

Next in order are the Aqueous Rocks, which owe their origin to the agency of chemical laws. To this class belong many of our limestone formations. Large quantities of carbonate of lime are held in solution by water charged with carbonic acid gas: when the carbonic acid, in course of time, passes off, the carbonate of lime can no longer be held in solution, and it is accordingly precipitated in a solid form to the bottom. In this manner was formed that peculiar kind of limestone called Travertine, which abounds in Italy, and which is well known to all who have visited Rome, as the stone of which the Coliseum was built. A still more familiar example, on a small scale, is seen in the case of Stalactites and Stalagmites. Water saturated with carbonic acid trickles down the sides, or drops from the roof of a limestone cavern. In its course it dissolves carbonate of lime, and holds it in solution; afterward, reaching the floor of the cavern, it slowly evaporates and leaves behind it a thin sheet of limestone which is called a Stalagmite; while the icicle-like pendants that are formed by a similar process, on the roof of the cavern, are called Stalactites.

There is a third class of Aqueous Rocks which are supposed to be made up almost exclusively of the fragmentary remains of plants and animals, and are therefore called Organic. The well-known coral reefs, so dreaded by the sailor in tropical seas, are believed to be nothing more than a mass of stony skeletons belonging to the minute marine animalcules known among zoologists as Polyps or Zoophytes. These little creatures, existing together in countless multitudes, extract carbonate of lime from the waters of the ocean in which they dwell, and by the action of their living organs, convert it into a solid frame or skeleton, which is called coral. From generation to generation the same process has been going on during the long succession of Geological ages; and huge masses of coral rock, hundreds of miles in length, have thus been slowly built up from fathomless depths of the ocean to within a few feet of its surface. Our vast coal formations, on the other hand, afford a ready example of rocks which are chiefly composed of vegetable remains.

So much for the Aqueous or Stratified Rocks. Geology next brings before us another and a very different group, of which the origin is ascribed to fire, and which are consequently designated by the title of Igneous Rocks. In their general appearance they are chiefly distinguished from the former by the absence of regular stratification; but they are, nevertheless, intersected by numerous planes of division, or joints, as they are called, and thus divided into blocks of various size and form. Geologists believe that these rocks were at one time reduced to a molten state by the action of intense heat, and afterward allowed slowly to cool and to crystallize. They are divided into two classes, the Plutonic and the Volcanic. The Plutonic Rocks are chiefly granite of some kind or another; and though they now often appear at the surface, they are supposed to have been produced originally at a considerable depth within the crust of the Earth, “or sometimes, perhaps, under a certain weight of incumbent ocean.”[14] The Volcanic Rocks have been formed at or near the surface of the Earth, and, as the name implies, they are usually ejected, in a state of fusion, from the fissures of an active volcano; though not unfrequently they assume the more imposing form of basaltic columns, as at the Giant’s Causeway in Ireland, or on the island of Staffa near the coast of Argyleshire in Scotland.

One group of rocks yet remains to be noticed. They have been called by various names at different times, but are now generally designated by the term Metamorphic. In some respects they resemble the Aqueous Rocks, while, in others, they are more nearly allied to the Igneous. Like the former, they are stratified in their outward arrangement; like the latter, they are more or less crystalline in their internal texture. As to their origin, we are told that they were first deposited under water, like the Aqueous Rocks, and that afterward their internal structure was altered by the agency of subterranean heat. Hence the name Metamorphic, first suggested by Sir Charles Lyell, which conveys the idea that these rocks have undergone a change of form. To this group belong many varieties of slate, and also the far-famed statuary marble of Italy.

Our readers will perceive from this brief outline that, if we follow the theory of Geologists, the rocks which compose the Crust of the Earth may be conveniently divided, according to their origin, into three leading groups, the Aqueous, the Igneous, and the Metamorphic. The Aqueous are formed under water, either by the mechanical force of the water itself when in motion, or by the agency of chemical laws, or by the intervention of organic life. Hence they are naturally subdivided into three classes, the Mechanical, the Chemical, the Organic. The Igneous Rocks are produced by heat, being first melted and then allowed to cool. When this process takes place under great pressure in the depths of the Earth, the result is granite; and the granite Rocks are called Plutonic: when near the surface, through the agency of a volcano, the Rocks so formed are called Volcanic. Lastly, the Metamorphic Rocks are nothing else than Aqueous Rocks, of which the texture has been altered by the action of intense heat.

As regards the relative order of position amongst these various classes of rocks, the lowest place seems uniformly to belong to the granitic or Plutonic group. It is true that the granite will often appear at the surface of the Earth; but wherever there is a series of rocks piled one above the other, the granite will always be the lowest. This assertion is based on two broad facts; first, whenever we get to the bottom of the other rocks, they are always found to rest on granite; and secondly, no other rock has ever yet been found beneath it. From this circumstance granite is conceived to be the solid foundation of the Earth’s Crust, and so is often called fundamental granite. Above the granite the Aqueous Rocks have been slowly spread out layer by layer during the long lapse of ages, now in this part of the world, now in that, according as each in its turn was exposed to the action of water. The Volcanic Rocks do not occur in any fixed order of succession. They are distributed irregularly over almost every country of the globe, occurring sometimes in the form of cone-shaped mountains, sometimes in the form of stately pillars, and sometimes in the form of massive solid walls, called Dykes, forced right through the softer Aqueous Rocks, which were deposited on the surface of the Earth before the eruption. As to the Metamorphic Rocks, which are supposed to owe their peculiar character to the contact of molten mineral matter, wherever they occur, they are found in the immediate neighborhood of some Igneous Rock.

The condition of the Earth beneath its thin external crust has never been the subject of direct observation; for Geologists have never yet been able to penetrate below the granite rocks. Nevertheless, this subject has been often discussed, and has offered a wide field for philosophical speculation. Upon one point all are agreed, that within the Crust of the Earth an intense heat very generally prevails;—a heat so intense that it would be quite sufficient, acting under ordinary circumstances, to reduce all known rocks to a state of igneous fusion. Hence it was a common opinion among the older Geologists that the condition of our globe is that of a vast central nucleus composed of molten mineral, and covered over with a comparatively thin external shell of solid rock. The most eminent Geologists, however, of the present day, hesitate to accept this opinion. They observe: (1) That we have not yet learned what the material is of which the interior of the Earth is composed; therefore we cannot tell for certain what degree of heat is sufficient to reduce that material to a liquid state. (2) It is uncertain how far the immense pressure at great depths may operate to keep matter in a solid state, even when raised to a very high degree of temperature. (3) There are certain astronomical and physical difficulties involved in this theory, which have not yet been fully cleared up. Modern Geologists, therefore, proceeding with more caution than their predecessors, while they regard the opinion as probable, refuse to set it down as conclusively demonstrated. But, that a very high temperature prevails in the interior of our globe, is a conclusion, they say, which is established by abundant evidence, and which may be regarded as morally certain.

It may be asked how the various strata of Aqueous Rocks, which constitute the chief portion of the Earth’s Crust, have been lifted up above the level of the sea; for, according to our theory, they were all first deposited under water. This is a question that must inevitably occur to the mind of every reader, and Geologists are ready with an answer. They tell us that from the earliest ages the Crust of the Earth has been subject to disturbance and dislocation. At various times and in various places it was upheaved, and what had been before the bed of the ocean became dry land; again it sunk below its former level, and what had been before dry land became the bed of the ocean. Thus, in the former case a new stratum which had been deposited at the bottom of the sea, with all its varied remains of a bygone age, was converted for a season into the surface of the Earth, and became the theatre of animal and vegetable life: while in the latter case, the old surface of the Earth with its countless tribes of animals and plants,—its fauna and flora as they are called,—was submerged beneath the waters, there to receive in its turn the broken up fragments of a former world, deposited in the form of mud, or sand, or pebbles, or minute particles of lime. Nor is this all; it is but a single link in the chain of Geological chronology. We are asked to believe that, in many parts of the globe, this upward and downward movement has been going on alternately for unnumbered ages; so that the very same spot which was first the bed of the ocean, was afterward dry land, then the bottom of an estuary or inland lake, then perhaps once more the floor of the sea, and then dry land again: and furthermore we are assured that, while it remained in each one of these various conditions, thousands and thousands of years may have rolled away.

But from what source does that mighty power come which can thus upheave the solid Earth, and banish the ocean from its bed? We are told in reply that this giant power dwells in the interior of the Earth itself, and is no other than the subterranean heat of which we have already spoken. This vast internal fire acts with unequal force upon different parts of the shell or Crust of the Earth, uplifting it in one place, and in another allowing it to subside. Now it is violent and convulsive, bursting asunder the solid rocks, and shaking the foundations of the hills: again it is gentle and harmless, upheaving vast continents with a scarcely perceptible undulation, not unlike the long, silent swell of the ocean. So it has been from the beginning, and so it is found to be even now, in this last age of the Geological Calendar. For even within historic times mountains have been suddenly upheaved from the level plain; and many parts of the Earth’s Crust have been subject to a slow, wave-like movement, rising here and subsiding there, at the rate of perhaps a few feet in a century. Sometimes, too, the fiery liquid itself has burst its barriers, and poured its destructive streams of molten rock far down into the peaceful, smiling valleys.

This theory of an internal disturbing force, which from time to time produces elevations and depressions of the Earth’s Crust, serves to explain another phenomenon, that cannot fail to have struck even the least observant eye. The Aqueous Rocks of mechanical formation are said to have been composed of minute fragments, which were first held suspended in water, and afterward fell to the bottom. If this be true, it follows that these rocks, in the first period of their existence, must have been arranged in beds parallel to the horizon, or nearly so. But we now find them, as everybody knows, in a great variety of positions: sometimes they are parallel to the horizon, sometimes inclined to it, sometimes at right angles to it; sometimes, too, they are broken right across, sometimes curved and twisted after a very fantastic fashion. Now, all these appearances are the natural results of an upheaving force acting irregularly from below on the solid shell of the Earth. When the subterranean fire is brought to bear equally at the same time on a broad extent of surface, then the overlying strata are bodily lifted up, and preserve their horizontal position. But when the whole force acts with local intensity on a very contracted area, then, at that particular spot, the rocks above will be tilted up, and their position entirely changed. Sometimes they will be only bent and crushed together, sometimes dislocated and turned over; sometimes, perhaps, a mountain will be formed, and the rocks before parallel to the horizon, will afterward remain parallel to the slopes of the mountain.

There is another process known by the name of Denudation, which we cannot pass over in silence, for it occupies a very important place in the Natural History of our globe. Since time first began Denudation has been ever going on at the surface of the Earth, and it has left its mark more or less distinctly upon every group of rocks, from the lowest to the highest. It includes all the various operations by which the old existing rocks are broken up into fragments, or ground into powder, or worn away by friction, or dissolved by chemical action, and then transported from their former site to become the elements of new strata. Hence the name Denudation; since by these operations the former surface of the Earth is carried away and a surface before covered is laid bare. The amount of destruction effected by this process in each successive age is always equal to the bulk of Aqueous Rocks formed within the same time. This will be at once understood when we remember that the Aqueous Rocks are produced, for the most part, by the deposition of sediment; and sediment is nothing else than the fragments, more or less minute, of pre-existing rocks. What is deposited on the bed of the ocean has been taken from the surface of the land; and the new strata are built up from the ruins of the old. When we see a great building of stone towering aloft to the sky, we are certain that somewhere else on the Earth a quarry has been opened, and that the amount of excavation in the quarry is exactly represented by the bulk of solid masonry in the building. Just in the same way, the mass of Aqueous Rocks is at once the monument and the measure of previous Denudation.

The process of Denudation is the work of many and various natural causes. Heat and cold, rain, hail, and snow, chemical affinities, the atmosphere itself, all have a share in it; but the largest share belongs to the mechanical action of moving water. Every little rill that flows down the mountain side is charged with finely-powdered sediment which it is ever wearing away from the surface of its own bed. Every great stream, besides the immense quantities of mud and sand which in times of flood it carries along in its turbulent course, has its channel strewn over with pebbles at which it never ceases to work, rounding off the angles and polishing the surfaces; and these pebbles, what are they but the fragments of old rocks and the elements of new,—the rubble-stone of Nature’s edifice on its way from the quarry to the building? Then there are those mighty rivers, such as the Amazon, the Orinoco, the Mississippi, the Nile, the Ganges, discharging into the sea day by day their vast freight of mineral matter, millions of cubic feet in bulk, and thousands upon thousands of tons in weight. Often this ponderous volume of mud or sand is carried far out to sea by the action of currents, but sometimes it is deposited near the shore, forming what is called a Delta, and exhibiting an admirable example of stratified rock in the earliest stage of its existence. Lastly, we have to notice the giant power of the great ocean itself, acting with untiring energies on the coasts of continents and islands all over the world, excavating and undermining cliffs, rolling huge rocks hither and thither, and spreading out the divided fragments in a new order at the bottom of the sea.

To apprehend fully the magnitude of the effects which may fairly be ascribed to this last-mentioned power, we must remember that, according to Geological theory, almost every portion of the Earth’s Crust has been more than once lifted up above the surface of the ocean, and afterward depressed below it. It is believed that this alternate rising and sinking was effected very often, perhaps most commonly, not by sudden convulsions, but rather by slow or gradual movements. Now, during this process, as the land was emerging from the waters or sinking beneath them, new surfaces would be presented in each succeeding century to the force of the ocean currents and the erosive action of the breakers; and it is not difficult to conceive that the accumulated ruins produced, in a long lapse of time, by destructive agents so powerful, so untiring, so universal, may have readily furnished the materials for a very large proportion of the Aqueous Rocks now in existence.

Hitherto we have considered the Crust of the Earth as a great structure slowly reared up by the hand of Nature; we have spoken of the Rocks that compose it, of their origin and history, of the order in which they are disposed, and of the various agencies that have been at work to mould them into their present form and feature. We have now to contemplate this marvellous structure under a new aspect; for we are told by Geologists that it is a vast sepulchre, within which lie entombed the remains of life that has long since passed away. Each series of strata is but a new range of tombs; and each tomb has a story of its own. Here a gigantic monster is disclosed to view, compared to which the largest beast that now roams through the forest is puny in form and contemptible in strength: there, within a narrow space, millions of minute animal frames are found closely compacted together, each so small that its existence can be detected only by the aid of a powerful microscope. In one place whole skeletons are found almost entire, embedded in the bosom of the solid rock; in another, we have a boundless profusion of bones and shells; and again in another, neither the skeleton itself appears, nor yet its scattered bones, but simply the imprint of footsteps once left upon the sandy beach, and still remaining engraved on the stone into which the fine sand has been converted chiefly by the agency of pressure. There is no scarcity of relics in this wonderful charnel-house of Nature. For half a century the work of plunder has been going on without relaxation or remorse; the tombs have been yielding up their dead; every city in the civilized world has filled its museums, and the cabinets of private collectors are overflowing: but the spoils that have hitherto been carried away seem to bear a very small proportion to those which yet remain behind.

These remains of animals and plants embedded in the Crust of the Earth are called Fossils; and Geologists maintain that the Fossils preserved in each group of strata represent the animals and plants that flourished on the surface of the Earth, or in the waters of the ocean, when that group of strata was in process of formation. There they lived, and there they died, and there they were buried, in the sand, or the shingle, or the mud that came down from the waters above. Their descendants, however, still lived on, and new forms of life were called into being by the voice of the Omnipotent Creator, making, as it were, a connecting link between the new age of the world that was coming in and the old one that was passing away. But they, too, died and found a tomb beneath the waters; for Nature, with unexhausted energies, was still busy collecting materials from the old rocks, and building up the new. And so that age passed away like the former, and another came; and every age was represented by its own group of strata; and each group of strata was, in its turn, covered over with a new deposit; and the tombs were all sealed up, with their countless legions of dead, their massive monuments of stone, their strange hieroglyphic inscriptions. At length came the last stage of the world’s history, and man appeared upon the scene; and it is his privilege to descend into this wonderful sepulchre, and to wander about amidst the monuments, and to strive to read the inscriptions. In our own days more especially, eager and enthusiastic students are abroad over the whole face of the globe, and are gathering together from every country the Fossil Remains of extinct worlds. By the aid of Natural History they seek to assign to each its own proper place in the ranks of creation; to trace the rise, the progress, and the extinction of every species in its turn; and even to describe the nature and the character of all the various forms of life that have dwelt upon the Earth from the beginning.

Such is the theory of Geology as expounded at the present day by its most able and popular advocates. We have passed over a multitude of minor details that we might not weary our readers, and we have kept aloof from disputed points that we might not get entangled in a purely scientific controversy. Our object has simply been to gather together into a systematic form those more general conclusions which, however startling they may seem to practical men of the world, and even to many of those whose minds have been accustomed to the pursuit of science in other departments, are nevertheless regarded as certain by all who have devoted their lives to the study of Geology. It now remains to investigate the facts on which these conclusions are based, and to consider the line of argument by which so many able and earnest men have been led to accept them. In this vast field of inquiry we shall chiefly direct our attention to those points that bear upon the Antiquity of the Earth; and in attempting to bring home to our readers the nature and the force of Geological reasoning, we shall confine ourselves altogether to simple and familiar illustrations.

CHAPTER II.
THEORY OF DENUDATION ILLUSTRATED BY FACTS.

Principle of reasoning common to all the physical sciences—This principle applicable to Geology—Carbonic acid an agent of denudation—Vast quantity of lime dissolved by the waters of the Rhine and borne away to the German ocean—Disintegration of rocks by frost—Professor Tyndall on the Matterhorn—Running water—Its erosive power—An active and unceasing agent of denudation—Mineral sediment carried out to sea by the Ganges and other great rivers—Solid rocks undermined and worn away—Falls of the Clyde at Lanark—Excavating power of rivers in Auvergne and Sicily—Falls of Niagara—Transporting power of running water—Floods in Scotland—Inundation in the valley of Bagnes in Switzerland.

In the physical sciences it is a common principle of reasoning to account for the phenomena that come before us in nature, by the operation of natural causes which we know to exist. Nay, this principle seems to be almost an instinct of our nature, which guides even the least philosophical amidst us, in the common affairs of life. When we stand amongst the ruins of an ancient castle, we feel quite certain that we have before us, not alone the monument of Time’s destroying power, but also the monument of human skill and labor in days gone by. We entertain no doubt that ages ago the sound of the mason’s hammer was heard upon these walls, now crowned with ivy; that these moss-grown stones were once hewn fresh in the quarry, and piled up one upon another by human hands; and that the building itself was designed by human skill, and intended for the purposes of human habitation and defence. Or, if we see a footprint in the sand, we conclude that a living foot has been there; and from the character of the traces it has left, we judge what was the species of animal to which it belonged, whether man, or bird, or beast. It is true that God is Omnipotent. He might, if it had so pleased Him, have built the old castle at the creation of the world, and allowed it to crumble slowly into ruins: or he might have built it yesterday, and made a ruin begin to be where no castle had stood before; and covered the stones with moss, and mantled the walls in ivy. And as to the footprint in the sand, it were as easy for Him to make the impress there, as to make the foot that left the impress. All this is true: but yet if any one were to argue in this style against us, he would fail to shake our convictions; we should still unhesitatingly believe that human hands once built the castle, and that a living foot once trod the shore.

Now, this principle of reasoning is the foundation on which the ablest modern Geologists claim to build their science. The untiring hand of Nature is ever busy around us: they ask us to come and look at her works, and to judge of what she has done in past ages, by that which she is now doing before our eyes. She is still, they say, building up her strata all over the globe, of limestone, and sandstone, and clay; she is still lifting up in one place the bed of the ocean, and in another submerging the dry land; she is still bursting open the Crust of the Earth by the action of internal fire, disturbing and tilting up the horizontal strata; she is still upheaving her mountains and scooping out her valleys. All these operations are open to our inspection; we may go forth and study them for ourselves; we may examine the works that are wrought, and we may discover, too, the causes by which they are produced. And if it should appear that a very close analogy exists between these works that are now coming into existence, and the long series of works that are piled up in the Crust of the Earth, it is surely not unreasonable to refer the latter class of phenomena to the action of the same natural causes which we know to have produced the former.

It cannot be denied that this argument is deserving of a fair and candid consideration. Let us proceed, then, to examine how far it is founded on fact, and how far it can be justly applied to the various heads of Geological theory. We will commence with the origin and history of Stratified Rocks; for this constitutes, in a manner, the framework on which the whole system of Geology is supported and held together. It is alleged that the elements of which Stratified Rocks are composed are but the broken fragments and minute atoms of pre-existing rocks, carried off by the agents of Denudation, and spread out over some distant area in regular beds or layers; which, in progress of ages, were slowly consolidated into rocks of various quality and texture. With the view of testing this theory by the light of the principle just explained, we purpose, in the first place, to exhibit some examples of the many forms in which the process of Denudation is going on at the present day all over the world; and afterward, to show that out of the materials thus obtained Stratified Rocks of every description—Mechanical, Chemical, Organic—are being regularly built up in sundry places; and that these correspond in every essential feature with the Stratified Rocks in the Crust of the Earth.

Among the chemical agents of Denudation, there is none more widely diffused than Carbonic acid gas. It is everywhere given out by dead animal and vegetable matter during the process of putrefaction; it is plentifully evolved from springs in every country; and it is emitted in enormous quantities from the earth in all volcanic districts, as well those in which the volcanoes are now extinct as those in which they are active. Now, it is well known from observation, that carbonic acid has the property of decomposing many of the hardest rocks, especially those in which felspar is an ingredient. This phenomenon is exhibited on a large scale in the ancient volcanic district of Auvergne, in central France. The carbonic acid, which is abundantly evolved from the earth, penetrates the crevices and pores of the solid granite, which being unable to resist its decomposing action, is rapidly crumbling to pieces. This mysterious decay of hard rock has been happily called by Dolomieu, “la maladie du granite.”[15]

Again, all the water which flows over the surface of the land is highly charged with carbonic acid. The rain imbibes it in falling through the atmosphere; and the rivers receive still further accessions from the earth as they pursue their course to the sea. In this combination we discover a powerful agent of Denudation; for limestone rock will be dissolved by water which is impregnated with carbonic acid. Thus all the rivers and streams in the world, when they flow through a limestone channel, are constantly dissolving the solid rock and bearing away the elements of which it is composed. A single example will be sufficient to show the magnitude of the results which are thus produced. It has been calculated by Bischof, a celebrated German chemist, that the carbonate of lime which is carried each year to the sea by the waters of the Rhine, is sufficient for the formation of 32,000,000,000 of oyster shells; or, to view the matter in another light, it would be sufficient to produce a stratum of limestone one foot thick, and four square miles in extent.[16] If such be the yearly produce of one river, how great must be the accumulated effects of all the rivers in the world since our planet first came from the hand of its Creator!

Passing from the chemical to the mechanical agents of Denudation, it is worth while to notice the immense power which is often generated by the agency of frost, especially in those countries that are subject to great vicissitudes of heat and cold. During a thaw, water finds its way into the clefts and joints by which all rocks are traversed, and when it is afterward converted into ice, it expands with a mechanical force that is almost irresistible. The hardest rocks are burst asunder, great blocks are detached from the mountain side, and sent rolling down its slopes, or tumbling over crags and precipices, until at length they come to rest in shattered fragments at the bottom of the valley. In this condition they await but the coming of the winter’s torrent to be borne still further on their long journey to the sea.

The fearful havoc done in this way by the alternate action of sun and frost contributes in no small degree to the fantastic and picturesque forms assumed by the mountain peaks of Switzerland. Huge masses of rock have been literally hewn away, until nothing has remained behind but those splintered obelisks and tapering pinnacles so familiar to the eye amidst the sublime scenery of the Alps. Indeed one of the greatest perils encountered by the adventurous spirits whose ambition it is to rival one another in the danger of their exploits, and to climb whatever was before regarded as inaccessible, arises from the enormous fragments of rock which are rent almost unceasingly from the overhanging crags and hurled into the abysses below them. The following incident related by Professor Tyndall is very much to the point. “We had gathered up our things, and bent to the work before us, when suddenly an explosion occurred overhead. Looking aloft, in mid-air was seen a solid shot from the Matterhorn describing its proper parabola through the air. It split to pieces as it hit one of the rock-towers below, and its fragments came down in a kind of spray, which fell wide of us, but still near enough to compel a sharp look out. Two or three such explosions occurred afterward, but we crept along the back fin of the mountain, from which the falling boulders were speedily deflected right and left.”

This occurred in 1862, on the occasion of an unsuccessful attempt to reach the highest peak of the Matterhorn. Six years later, when Professor Tyndall at length actually accomplished the object on which he seems to have set his heart, he found the work of destruction still going on. “We were now,” he says in his narrative, “beside a snow-gully, which was cut by a deep furrow along its centre, and otherwise scarred by the descent of stones. Here each man arranged his bundle and himself so as to cross the gully in the minimum of time. The passage was safely made, a few flying shingle only coming down upon us. But danger declared itself where it was not expected. Joseph Maquignas led the way up the rocks. I was next, Pierre Maquignas next, and last of all the porters. Suddenly a yell issued from the leader: ‘Cachez vous!’ I crouched instinctively against the rock, which formed a by no means perfect shelter, when a boulder buzzed past me through the air, smote the rocks below me, and with a savage hum flew down to the lower glacier.”[17]

Even in our own country, every one is familiar with the efficacy of frozen water in producing landslips. The rain which soaks into the ground in winter, is converted into ice when frost sets in; and upon steep slopes or precipices, its expansive power bursts open the earth, and causes large masses of stones and clay to tumble headlong to the bottom.

But moving water constitutes the most powerful, and, at the same time, the most universal agent of Denudation. And it is chiefly to the effects of moving water that we mean to direct attention; because its action is more striking to the eye, and more easily understood by the general reader. Every one is aware that the waters of the ocean are constantly passing off by evaporation into the higher regions of the atmosphere, and are there condensed into clouds. These clouds in course of time descend upon all parts of the earth, but especially on the high and mountainous districts. Then rivulets are formed which flow smoothly down the gentle slopes of the undulating country, or plunge headlong over the rocky mountain cliffs; and the rivulets uniting form streams, and the streams, receiving new tributaries as they advance, become rivers; and the rivers flow on to the sea, and discharge each day and each hour their enormous volumes of water back again into the ocean from which they came. Thus all the water of the world is constantly in motion, ever hurrying on, as it were, in one unending round of duty. This is the teaching of daily experience and observation. And we may add, it is the teaching of Sacred Scripture as well. The Wise Man said long ago: “All the rivers run into the sea, yet the sea doth not overflow: unto the place from whence the rivers come, thither they return to flow again.”[18]

Now, the power of this moving water is a mighty wide-spread agent of change in the physical condition of the globe. For wherever water is in motion over the surface of the land, whether it be a rippling stream, or a mountain torrent, or a majestic river, it is surely wearing away the channel through which it flows, and carrying along in its course particles of clay, or sand, or gravel. This subject is illustrated with great force and great simplicity by Mr. Page. “Every person,” he says, “must have observed the rivers in his own district, how they become muddy and turbid during floods of rain, and how their swollen currents eat away the banks, deepen the channels, and sweep away the sand and gravel down to some lower level. And if, during this turbid state, he will have the curiosity to lift a gallon of the water, and allow it to settle, he will be astonished at the amount of sediment or solid matter that falls to the bottom. Now, let him multiply this gallon by the number of gallons daily carried down by the river, and this day by years and centuries, and he will arrive at some faint idea of the quantity of matter worn from the land by rivers, and deposited by them in the ocean. In the same way as one river grinds and cuts for itself a channel, so does every stream and rill and current of water. The rain as it falls washes away what the winds and frosts have loosened; the rill takes it up, and, mingling it with its own burden, gives it to the stream; the stream takes it up and carries it to the river, and the river bears it to the ocean.”[19]

When the current is feeble, the greater part of this earthy material is thrown down upon the way, and forms a stratum of alluvial soil in the bed of the river, and also in the adjoining lowlands, during the time of temporary floods. But when several streams unite, then the carrying power of the current is enormously increased: huge stones are rolled along, and dashed one against another, and broken into fragments, and the fragments are rounded by friction, and become pebbles, and the pebbles become gravel, and the gravel, mud; and the mud is carried on to the mouth of the river, and there falling to the bottom, it forms a tongue of land which is called a delta; or else perhaps it chances to meet with some great ocean current, and then it begins a new journey, and is borne far away to be deposited in the profound and tranquil depths of the sea. It is not, however, mineral matter alone that is transported by the action of rivers. Trees that once were growing on the banks of the stream, and the bones of animals, and human remains, and works of art, are seen floating down with the current, and are found embedded in the sand and mud of the delta at the river’s mouth.

These are some of the actual realities which all may witness, who will go and study for themselves the history of this wonderful element, from the time when it first soars aloft as vapor to the sky, until it returns to the bosom of its parent ocean laden with the spoils of the land. To some of our readers, perhaps, results of this kind may appear insignificant, when considered in relation to the enormous bulk of the stratified rocks. But it should be remembered that the force of which we speak is unceasing in its operation over the whole surface of the earth; and even though the work were small which is accomplished in each successive year, the accumulated effects produced in a lengthened period of time must be immensely great. Besides, it would be a very serious error to form our ideas on this subject, as many would seem to do, from the examples which are to be found within the narrow limits of our own island. We should rather seek for our illustrations among those mighty rivers that drain the vast continents of the world, and exhibit the erosive and transporting power of running water on the grandest scale.

It happens, fortunately for our purpose, that an attempt has been made by scientific men to compute the amount of matter discharged into the sea, by some particular rivers within a given time. For such a computation it is necessary, in the first place, to calculate the volume of water that passes down the channel during that time; and then, by repeated experiments, to ascertain the average proportion of earthy matter which is held suspended in the water. This has been done with the greatest care by the Rev. Mr. Everest, in the case of the river Ganges; and it appears that during the rainy season, which lasts four months every year, from June to September, about 6,000,000,000 cubic feet of mud are carried along by the stream past the town of Ghazepoor, near which the observations were made. Now this enormous bulk of mineral matter would be sufficient to form a stratum of rock one foot in height, and two hundred and eighteen square miles in extent. Or, to adopt the computation of Sir Charles Lyell, the amount which passes by every day is equal to that which might be transported by 2000 Indiamen, each freighted with a cargo of mud 1400 tons in weight. And it is important to remember that this estimate represents but a portion of the sediment which passes into the sea through the channel of the Ganges; for the observations of Mr. Everest were taken at a point which is 500 miles from the sea, and at which the river has not yet received the contributions of its largest tributaries.

We are able, therefore, with some degree of confidence, to estimate the amount of Denudation which is every year effected by the Ganges. And, although the same calculations have not yet been applied with equal care to other great rivers, there is no reason to suppose that the Ganges is an exception. It is asserted on good grounds that the Brahmapootra, which unites with the Ganges close to the Bay of Bengal, carries with it an equal amount of earthy sediment. According to Sir Charles Lyell, the quantity of solid matter brought down each year by the Mississippi amounts to 3,702,758,400 cubic feet. And it is said that 48,000,000 cubic feet of earth are daily discharged into the sea by the Yellow River in China, called by the natives the Hoang Ho.[20] Thus year after year the waste of the land is carried away by rivers, to be spread out over wide areas of the ocean, and perhaps to furnish the materials of future continents.

The effects of running water in wearing away and transporting masses of solid rock are not less deserving of our notice. Every one who has followed the course of a great river when it flows through a rocky channel, must have observed large blocks projecting from the cliffs above, which, having been undermined by the action of the water, seem ready to tumble headlong into the stream; and others lying below, which had fallen before; and others again which had been already carried a considerable distance by the winter’s torrent. Even where the rocks are not displaced, they are gradually being worn away, partly by the friction of the water, but much more by the grinding action of the gravel which the water holds in suspension. Not only is the surface of the rocks thus rounded and polished, but large circular pits, called pot-holes, are formed by the whirling waters of an eddy carrying round and round a few grains of hard sand.

At the falls of the Clyde near Lanark in Scotland, these various phenomena may be seen to great advantage. Good illustrations are to be found also in many volcanic regions. Some of the larger streams in Auvergne have in course of time forced their way through the solid lava rock, cutting out for themselves channels broad and deep. In Sicily too, we are told, the river Simeto, whose course was blocked up by a current of lava about the beginning of the seventeenth century, has since that time eaten its way through this compact and hardened mass, and now flows on to the sea through a rocky passage forty feet in depth and from fifty to several hundred feet in width.[21]

But there is no part of the world yet explored where these effects are exhibited on the same gigantic scale as at the far-famed Falls of Niagara. The massive limestone rock from which the waters are precipitated is slowly but certainly disappearing. An enormous volume of water, more than a third of a mile in breadth, plunges in a single bound over a sheer precipice of one hundred and sixty-five feet. The soft slaty rocks upon which the limestone rests are soon eaten away by the action of the spray which rises from the pool below; and then the overhanging cliffs, left without any support, topple over, and are carried off by the torrent. The position of the Falls, therefore, is not stationary, but is receding by very sensible degrees in the direction of Lake Erie, from which the river flows. Speaking of this phenomenon, Sir Charles Lyell observes with much show of reason: “The idea of perpetual and progressive waste is constantly present to the mind of every beholder: and as that part of the chasm which has been the work of the last hundred and fifty years resembles precisely in depth, width, and character the rest of the gorge, which extends seven miles below, it is most natural to infer, that the entire ravine has been hollowed out in the same manner, by the recession of the cataract. It must at least be conceded, that the river supplies an adequate cause for executing the whole task thus assigned to it, provided we grant sufficient time for its completion.”[22]

With a view to enable our readers to understand more fully the prodigious force which rivers have been known to exert in the transportation of rocks, it may be useful to draw attention to one or two principles of physical science. First, we have the well-known law of Archimedes, that a solid body immersed in a liquid loses a part of its weight equal to the weight of the liquid displaced. Now solid rock as compared with water, bulk for bulk, is rarely more than three times, and often not more than twice as heavy. Consequently, according to this law, almost all rocks will lose a third of their weight, and many will lose one-half, when immersed in water. Again, it has been established that the power of water to move bodies that are in it increases as the sixth power of the velocity of the current. Hence, if the velocity of a current is increased two-fold, its moving power will be increased sixty-four fold; if the velocity is increased three-fold, the moving power will be increased seven hundred and twenty fold; and so on.

From these principles it follows, first, that a much smaller power is required to move a block of stone lying in the bed of a river, than if it were lying on the surface of the land; and secondly, that a very slight increase in the velocity of a current effects a very great increase in its moving power. We need not wonder, then, when we hear of the enormous masses of rocks and trees and mason-work which are carried away even by small rivers in times of flood.[23]

Here are a few examples. In August, 1829, a fragment of sandstone, fourteen feet long, three feet wide, and one foot thick, was carried by the river Nairn, in Scotland, a distance of two hundred yards. On the same occasion the river Dee swept away a bridge of five arches, built of solid granite, which had stood uninjured for twenty years; the whole mass of masonry sunk into the bed of the stream and was seen no more. And the river Don, as we are assured on the authority of Mr. Farquharson, forced a mass of stones four or five hundred tons in weight up a steep inclined plane, leaving them in a great rectangular heap on the summit. A small rivulet called the College, in Northumberland, when swollen by a flood in August, 1827, “tore away from the abutment of a mill-dam a large block of greenstone-porphyry weighing nearly two tons, and transported it to the distance of a quarter of a mile.”[24] But it is needless to multiply examples of phenomena which are occurring every day around us, and of which many among our readers have probably been eye-witnesses.

The transporting power of rivers must not always be estimated by the bulk and velocity of the current; for it is often greatly increased by some accidental obstruction, which for a time blocks up the channel through which the river flows. An instructive illustration is afforded by the river Dranse, which flows through the valley of Bagnes, in Switzerland, and empties itself into the Rhone above the lake of Geneva. In the year 1818 the avalanches which fell down from the mountain side formed a barrier across the valley, and thus effectually blocked up the course of the stream. The upper part of the valley was, in consequence, soon converted into a lake which gradually increased in size as the season advanced. When summer came, and the melting of the snows began, the ice barrier suddenly gave way with a tremendous crash, and the lake was emptied in half an hour. The mass of water, thus in a moment disengaged, burst with destructive violence over the lower valley, sweeping away rocks, forests, houses, bridges, and cultivated lands. Thousands of trees were torn up by the roots, fragments of granite as large as houses were rolled along, and the whole flood presented the appearance of a moving mass of ruins.

CHAPTER III.
THEORY OF DENUDATION—FURTHER ILLUSTRATIONS.

The breakers of the ocean—Caverns and fairy bridges of Kilkee—Italy and Sicily—The Shetland Islands—East and south coast of Britain—Tracts of land swallowed up by the sea—Island of Heligoland—Northstrand—Tides and currents—South Atlantic current—Equatorial current—The Gulf Stream—Its course described—Examples of its power as an agent of transport.

While the rain, the rivers, and the streams, are thus wasting away the mountains and plains of the interior country, the waves of the sea are exerting a power no less destructive on the coasts of islands and of continents. The breakers dashing against the foot of a lofty cliff, dissolve and decompose and wear away the lower strata; and the overhanging rocks, thus undermined, fall down in course of time by their own weight. With the next returning wave these rocks are themselves hurled back against the cliff; and so, as some one has happily remarked, the land would seem to supply a powerful artillery for its own destruction. The effects of the breakers are often very unequal, even on the same line of cliffs. Some parts of the rock are more yielding than others, or perhaps they are more exposed to the action of the waves, or perhaps they are divided by larger joints and more freely admit the destructive element. These parts will be the first to give way, while the harder and less exposed rock will be left standing: and in this way forms the most capricious and fantastic are produced.

No finer examples could be wished for than those which are seen in the neighborhood of Kilkee, and along the promontory of Loop Head, in the county of Clare. Sometimes the ground is undermined with caverns, into which, when the tide is coming in, the waves of the Atlantic rush with resistless force, making new additions each day to the accumulated ruins of ages. Sometimes lofty pinnacles of rock are left standing in the midst of the waters, like giant sentinels stationed there by Nature to guard the coast. In one or two instances these isolated fragments are connected with the main land by natural arches of rock, which are called fairy bridges by the people; but more commonly they appear as rocky islets, and answer exactly to the poet’s description—

“The roaring tides The passage broke that land from land divides; And where the lands retired the rushing ocean rides.”

It is interesting to observe in passing, that, in the original verses of the Æneid, of which these lines are Dryden’s translation, Virgil has recorded a belief which prevailed in his time, and which, upon scientific grounds, is now regarded as highly probable by Geologists, that the island of Sicily had been once connected by land with Italy, and was separated from it by the action of the waves:

“Hæc loca, vi quondam et vasta convulsa ruina, Tantum ævi longinqua valet mutare vetustas! Dissiluisse ferunt, quum protenus utraque tellus Una foret; venit medio vi pontus et undis Hesperium Siculo latus abscidit, arvaque et urbes Litore deductas angusto interluit æsta.”

Æneid, iii., 414-19.

But whatever may be thought of this opinion thus rendered immortal by the genius of the poet, we shall not stop to discuss its merits. For in the present stage of our argument, it is our object to deal, not with vague and uncertain traditions, nor even with philosophical speculations, but rather with the facts which are actually going on in nature, and which any one of our readers may examine for himself. With this object in view, we shall take a few examples from the Eastern and Southern coasts of Great Britain, which have been carefully explored by scientific men for the purpose of observing and recording the amount of destruction accomplished by the waves within recent times.

Fig. 1.—Granitic rocks to the south of Hillswick Ness, Shetland. From Lyell’s Principles of Geology.

The Shetland Islands, exposed to the whole fury of the Atlantic, present many phenomena not unlike those of Kilkee and Loop Head, but upon a far grander scale. Whole islands have been swept away by the resistless power of the waters, and of others nothing remains but massive pillars of hard rock, which have been well described as rising up “like the ruins of Palmyra in the desert of the ocean.” Passing to the mainland, it is recorded that in the year 1795 a village in Kincardineshire was carried away in a single night, and the sea advanced a hundred and fifty yards inland, where it has ever since maintained its ground. In England, almost the whole coast of Yorkshire is undergoing constant dilapidation. On the south side of Flamborough Head the cliffs are receding at an average rate of two yards and a quarter in the year, for a distance of thirty-six miles along the coast. This would amount to a mile since the Norman Conquest, and to more than two miles since the occupation of York by the Romans. It is not surprising, therefore, to learn that many spots marked in the old maps of the country as the sites of towns or villages, are now sandbanks in the sea. Even places of historic name have not been spared. The town of Ravenspur, from which, in 1332, Edward Baliol sailed for the invasion of Scotland, and at which Henry the Fourth landed in 1399, to claim the throne of England, has long since been swallowed up by the devouring element.

On the coast of Norfolk it was calculated, at the beginning of the present century, that the mean loss of the land was something less than one yard in the year. The inn at Sherringham was built on this calculation in 1805, and it was expected to stand for seventy years. But unfortunately the actual advance of the sea exceeded the calculation. Sir Charles Lyell, who visited this spot in 1829, relates that during the five preceding years seventeen yards of the cliff had been swept away, and nothing but a small garden was then left between the building and the sea. The same distinguished writer tells us that in the harbor of this town there was at that time water sufficient to float a frigate where forty-eight years before had stood a cliff fifty feet in height with houses built upon it. And remarking upon these facts, he says, that “if once in half a century an equal amount of change were produced suddenly by the momentary shock of an earthquake, history would be filled with records of such wonderful revolutions of the earth’s surface; but if the conversion of high land into deep sea be gradual, it excites only local attention.”

In the neighborhood of Dunwich, once the most considerable seaport on the coast of Suffolk, the cliffs have been wasting away from an early period of history. “Two tracts of land which had been taxed in the time of King Edward the Confessor, are mentioned in the Conqueror’s survey, made but a few years afterward, as having been devoured by the sea.” And the memory of other losses in the town itself—including a monastery, several churches, the town-hall, the jail, and many hundred houses—together with the dates of their occurrence, is faithfully preserved in authentic records. In 1740 the sea reached the churchyard of Saint Nicholas and Saint Francis, so that the graves, the coffins, and the skeletons, were exposed to view on the face of the cliffs. Since that time the coffins, and the tombstones, and the churchyard itself, have disappeared beneath the waves. Nothing now remains of this once flourishing and populous city but the name alone, which is still attached to a little village of about twenty houses. The spot on which the Church of Reculver stands, near the mouth of the Thames, was a mile inland in the reign of Henry the Eighth; in the year 1834 it was overhanging the sea; and it would long ago have been demolished, but for an artificial causeway of stones constructed with a view to break the force of the waves. It is estimated that the land on the northeast coast of Kent is receding at the rate of about two feet in the year. The promontory of Beachy Head in Sussex is also rapidly falling away. In the year 1813 an enormous mass of chalk, three hundred feet in length and eighty in breadth, came down with a tremendous crash; and slips of the same kind have often occurred, both before and since.

To these examples from Great Britain we may add one or two from the German Ocean. Seven islands have completely disappeared within a very narrow area since the time of Pliny; for he counted twenty-three between Texel and the mouth of the Eider, whereas now there are but sixteen. The island of Heligoland, at the mouth of the Elbe, has been for ages subject to great dilapidation. Within the last five hundred years three-fourths of it have been carried away; and since 1770 the fragment that remains has been divided into two parts by a channel which is at present navigable for large ships. A still more remarkable instance of destruction effected by the waves of the sea occurred in the island of Northstrand, on the coast of Schleswig. Previous to the thirteenth century it was attached to the mainland, forming a part of the continent of Europe, and was a highly cultivated and populous district about ten miles long, and from six to eight broad. In the year 1240 it was cut off from the coast of Schleswig by an inroad of the sea, and it gradually wasted away up to the seventeenth century, when its entire circumference was sixteen geographical miles. Even then the industrious inhabitants,—about nine thousand in number,—endeavored to save what remained of their territory by the erection of lofty dykes; but on the eleventh of October, 1634, the whole island was overwhelmed by another invasion of the sea, in which 6000 people perished, and 50,000 head of cattle. Three small islets are all that now remain of this once fertile district.[25]

The breakers of the ocean receive no small aid in their work of destruction from the action of tides and currents which co-operate with the winds to keep the waters of the sea in constant motion. And though the winds may sleep for a time, the tides and currents are always actively at work, and never for a moment cease to wear away the land. But they are even more powerful auxiliaries as agents of transport. If it were not for them, the ruins which fall from the rocks to-day would to-morrow form a barrier against the waves, and the work of destruction would cease. But Nature has ordained it otherwise. When the tide advances, it rolls the broken fragments toward the land, and when it recedes, it carries them back to the deep; and so by unceasing friction these fragments are worn away to pebbles, and then, being more easily transported, they are carried off to sea and deposited in the bed of the ocean: or else, perhaps, they are cast up on the sloping shore, to form what is so familiar to us all under the name of a shingle-beach.

This is a subject on which it is needless to enlarge. Every one knows that the tides have the power of transporting solid matter; though most of us, perhaps, do not fully appreciate the magnitude of their accumulated effects, working as they do with untiring energies upon the coasts of islands and continents all over the world. It is not, however, so generally known that the ocean is traversed in all directions by powerful currents, which, from their regularity, their permanence, and their extent, have been aptly called the rivers of the ocean. We do not mean here to inquire into the causes of these currents, upon which the progress of physical science has thrown considerable light: neither can we hope to describe even the principal currents that prevail over the vast tracts of water which constitute about three-fourths of the entire surface of our globe. We shall content ourselves with tracing the course of one great system, which may serve to give some idea of their general character and enormous power.

This system would seem to have its origin with a stream that flows from the Indian Ocean toward the southwest, and then doubling the Cape of Good Hope, turns northward along the African coast. It is here called the South Atlantic Current. When it encounters the shores of Guinea, it is diverted to the west, and stretches across the Atlantic, traversing forty degrees of longitude until it reaches the projecting promontory of Brazil in South America. In this part of its course it is known as the Equatorial Current, because it follows pretty nearly the line of the Equator: it varies in breadth from two hundred to five hundred miles, and it travels at the mean rate of thirty miles a day, though sometimes its velocity is increased to seventy or eighty. Next, under the name of the Guyana Current, it pursues a northwesterly direction, following the line of the coast; and passing close to the island of Trinidad, becomes diffused, and almost seems to be lost, in the Caribbean Sea. Nevertheless, it again issues with renewed energy from the Gulf of Mexico, and rushing through the Straits of Florida at the rate of four and five miles an hour, it issues once more into the broad waters of the Atlantic. From this out it is called the Gulf Stream, and is well known to all who are concerned in Transatlantic navigation; for it sensibly accelerates the speed of vessels which are bound from America to Europe, and sensibly retards those sailing from Europe to America.

The Gulf Stream, however, does not set out on its Transatlantic voyage directly that it issues from the Straits of Florida. It keeps at first a northeasterly course, following the outline of the American continent, passing by New York and Nova Scotia, and brushing the southern extremity of the great Newfoundland Bank. Then taking leave of the land, it sweeps right across the Atlantic. After a time it seems to divide into two branches, one inclining to the south, and losing itself among the Azores, the other bending toward the north, washing the shores of Ireland, Scotland, Norway, and reaching even to the frozen regions of Spitzbergen. The breadth of the Gulf Stream, when it issues from the Straits of Florida, is about fifty miles, but it afterward increases to three hundred. Its color is a dark indigo blue, which, contrasting sharply with the green waters of the Atlantic, forms a line of junction distinctly visible for some hundreds of miles: afterward, when this boundary line is no longer sensible to the eye, it is easily ascertained by the thermometer; for the temperature of the Gulf Stream is everywhere from eight to ten degrees higher than that of the surrounding ocean.[26]

We leave our readers to infer from this brief description how immense must be the power of transport which belongs to such currents as these. They sweep along the shores of continents, and carry away the accumulated fragments of rock, which had first been rent from the cliffs by the waves of the sea, and then borne out to a little distance by the tides: they pass by the mouths of great rivers, and receiving the spoils of many a fertile and populous country, and the ruins of many an inaccessible mountain ridge, they hurry off to deposit this vast and varied freight in the deep abysses of the ocean. There is one circumstance, however, which we ought not to pass over in silence; for it is of especial importance to the Geologist, and might easily escape the notice of the general reader. It is a well ascertained fact that plants and fruits and other objects from the West Indian Islands are annually washed ashore by the Gulf Stream on the northwestern coasts of Europe. The mast of a man-of-war burnt at Jamaica was after some months found stranded on one of the Western Islands of Scotland;[27] and General Sabine tells us that when he was in Norway, in the year 1823, casks of palm-oil were picked up on the shore near the North Cape, which belonged to a vessel that had been wrecked the previous year at Cape Lopez on the African coast.[28] It seems most probable that these casks of oil must first have crossed the Atlantic from east to west in the Equatorial Current, then described the circuit of the West Indian Islands, and finally coming in with the Gulf Stream, recrossed the Atlantic, performing altogether a journey of more than eight thousand miles. From these facts it is clear that, by the agency of ocean currents, the productions of one country may be carried to another that is far distant. And Geologists do not fail to make use of this important conclusion when they find the animal and vegetable remains of different climates associated together in the same strata of the Earth.

CHAPTER IV.
THEORY OF DENUDATION—CONCLUDED.

Glaciers—Their nature and composition—Their unceasing motion—Powerful agents of denudation—Icebergs—Their number and size—Erratic blocks and loose gravel spread out over mountains, plains, and valleys, at the bottom of the sea—Characteristic marks of moving ice—Evidence of ancient glacial action—Illustrations from the Alps—From the mountains of the Jura—Theory applied to northern Europe—To Scotland, Wales, and Ireland—The fact of denudation established—Summary of the evidence—This fact the first step in geological theory.

The next agent of Denudation to which we invite the attention of our readers, is one of which our own country affords us no example, but which may be seen in full operation amidst the wild and impressive scenery of Switzerland. And we know not how we can better introduce the subject than by the solemn address of a great poet, in whom an ardent love of nature was blended with a deep sense of religion. As he stood in the midst of the snow-clad mountains that shut in the valley of Chamouni, his spirit, “expanded by the genius of the spot,” soared away from the scenes before him to the Great Invisible Author of all that is beautiful and sublime in nature, and he poured forth that well-known hymn of praise and worship in which he thus apostrophizes the massive glaciers of Mont Blanc:—

“Ye ice-falls! ye that from the mountain’s brow
Adown enormous ravines slope amain—
Torrents, methinks, that heard a mighty voice,
And stopped at once amid their maddest plunge!
Motionless torrents! silent cataracts!
Who made you glorious as the gates of Heaven
Beneath the keen full moon? Who bade the sun
Clothe you with rainbows? Who with living flowers
Of loveliest blue, spread garlands at your feet? God! let the torrents, like a shout of nations,
Answer! and let the ice-plains echo, God!
God! sing ye meadow-streams with gladsome voice!
Ye pine-groves, with your soft and soul-like sounds!
And they too have a voice, yon piles of snow,
And in their perilous fall shall thunder, God!”[29]

A Glacier is an enormous mass of solid ice filling up a valley, and stretching from the eternal snows which crown the summits of the mountains, down to the smiling cornfields and rich pastures of the plains. It is constantly fed by the accumulated snows of winter, which, slipping and rolling down the slopes of the mountains, lodge in the valleys below, and are there converted into ice. For it must be remembered that the Glacier properly so called does not commonly extend much higher than 9000 feet above the level of the sea. Beyond that elevation the compact and massive ice gradually passes into frozen snow, called by the French Nevé, and by the Germans Firn. The change which takes place in the condition of the snow as it descends into the valley is chiefly owing to these two circumstances: first, it is closely compacted together by the weight of the snowy masses pressing down upon it from above; and secondly, in the summer months it is thawed upon the surface during the day by the heat of the sun, and frozen again at night. On a small scale this process is practically familiar to every school-boy. When he makes a snow-ball he is practically converting a mass of snow into ice, and that by a series of operations very closely resembling those which Nature employs in the manufacture of a Glacier.

In Switzerland the Glacier is often two or three miles in breadth, from twenty to thirty miles in length, and five or six hundred feet in depth. Though so vast in its bulk and so solid in its character, it is not, as might be supposed, a fixed, immovable mass. On the contrary, it is moving incessantly, but slowly, down the valley which it occupies, at the rate of several inches—sometimes one or two feet, and even more—in the day. In Greenland a Glacier explored by Doctor Hayes, in his expedition to the North Pole, was found to move for a whole year at the average rate of a hundred feet a day. It may be thought, perhaps, that this fact requires further confirmation; but at all events it is certain that the language of the poet, when he addresses the Glaciers as “motionless torrents,” though it conveys an accurate and beautiful idea of the appearance they present to the eye, is not rigorously true in a scientific sense. Indeed, it is just because the Glaciers are not motionless that they serve as instruments of Denudation.

Their agency in this respect “consists partly in their power of transporting gravel, sand, and huge stones, to great distances, and partly in the smoothing, polishing, and scoring of their rocky channels, and the boundary walls of the valleys through which they pass. At the foot of every steep cliff or precipice in high Alpine regions, a sloping heap is seen of rocky fragments detached by the alternate action of frost and thaw. If these loose masses, instead of accumulating on a stationary base, happen to fall upon a Glacier, they will move along with it, and, in place of a single heap, they will form in the course of years a long stream of blocks. If a Glacier be twenty miles long, and its annual progression about five hundred feet, it will require about two centuries for a block thus lodged upon its surface to travel down from the higher to the lower regions, or to the extremity of the icy mass. This terminal point usually remains unchanged from year to year, although every part of the ice is in motion, because the liquefaction by heat is just sufficient to balance the onward movement of the Glacier, which may be compared to an endless file of soldiers, pouring into a breach, and shot down as fast as they advance.

“The stones carried along on the ice are called in Switzerland the moraines of the Glacier. There is always one line of blocks on each side or edge of the icy stream, and often several in the middle, where they are arranged in long ridges or mounds of snow and ice, often several yards high. The reason of their projecting above the general level, is the non-liquefaction of the ice in those parts of the surface of the Glacier which are protected from the rays of the sun, or the action of the wind, by the covering of the earth, sand, and stones. The cause of medial moraines was first explained by Agassiz, who referred them to the confluence of tributary Glaciers. Upon the union of two streams of ice, the right lateral moraine of one of the streams comes in contact with the left lateral moraine of the other, and they afterward move on together, in the centre, if the confluent Glaciers are equal in size, or nearer to one side if unequal.

“Fragments of stone and sand which fall through crevasses in the ice, and get interposed between the moving Glacier and the fundamental rock, are pushed along so as to have their angles more or less worn off, and many of them are entirely ground down into mud. Some blocks are pushed along between the ice and the steep boundary rocks of the valley, and these, like the rocky channel at the bottom of the valley, often become smoothed and polished, and scored with parallel furrows, or with lines and scratches produced by hard minerals, such as crystals of quartz, which act like the diamond upon glass. The effect is perfectly different from that caused by the action of water, or a muddy torrent forcing along heavy stones; for these not being held like fragments of rock in ice, and not being pushed along under great pressure, cannot scoop out long rectilinear furrows or grooves parallel to each other. The discovery of such markings at various heights far above the surface of existing Glaciers, and for miles beyond their present terminations, affords geological evidence of the former extension of the ice beyond its present limits in Switzerland and other countries.”[30]

Fig. 2.—Iceberg seen in mid-ocean 1400 miles from any known land.

Sometimes, however, it happens, especially in extreme northern and southern latitudes, that the glacier valley leads down to the sea. In such cases, huge masses of ice are floated off, and, with their ponderous burden of gravel, mud, and rocks, are carried away by currents toward the equator. Immense numbers of these floating islands of ice, or Icebergs, as they are called, are seen by mariners drifting along in the Northern and Southern oceans. In 1822 Scoresby counted five hundred between the latitudes 69° and 70° N., many of which measured a mile in circumference, and rose two hundred feet above the surface of the sea.[31] The annexed drawing, copied by kind permission of the author from Sir Charles Lyell’s Principles of Geology, affords a good idea of the appearance that such Icebergs present to the eye. The one represented in the fore-ground was supposed to reach a height of nearly three hundred feet, and was observed with many others floating about in the Southern Ocean at a distance of 1400 miles from any known land. An angular mass of rock was visible on the surface. The part exposed was twelve feet high and from five to six broad: but it was conjectured, from the color of the surrounding ice, that the greater part of the stone was concealed from view.

How enormous must be the magnitude of those ponderous masses may be learned from the fact that the bulk of ice below the level of the water is about eight times as great as that above: and in point of fact, Captain Sir John Ross saw several of them aground in Baffin’s Bay, where the water was 1500 feet deep. It has been calculated that the beds of earth and stones which they carry along cannot be less than from 50,000 to 100,000 tons in weight. Sir Charles Lyell, writing in 1865 from the results of the latest investigations on this subject, says: “Many had supposed that the magnitude commonly attributed to icebergs by unscientific navigators was exaggerated; but now it appears that the popular estimate of their dimensions has rather fallen within than beyond the truth. Many of them, carefully measured by the officers of the French exploring expedition of the Astrolabe, were between 100 and 225 feet high above water, and from two to five miles in length. Captain d’Urville ascertained one of them, which he saw floating, to be thirteen miles long, and a hundred feet high, with walls perfectly vertical.”[32]

They have been known to drift from Baffin’s Bay to the Azores, and from the South Pole to the Cape of Good Hope.[33] As they approach the milder climate of the temperate zones, the ice gradually melts away, and thus the moraines of arctic and antarctic glaciers are deposited at the bottom of the deep sea. In this way, submarine mountains and valleys and table-lands are strewn over with scattered blocks of foreign rocks, and gravel, and mud, which have been transported hundreds of miles across the unfathomable abysses of the ocean.

Though we are chiefly concerned with Glaciers and Icebergs as agents of Denudation, yet we cannot pass away from the subject without referring to the Geological theory of an ancient Glacial Period. This little digression from the main purport of our present argument will not be unacceptable, we hope, to our readers. The theory is in itself interesting and ingenious; and it offers an admirable illustration of the kind of reasoning by which Geologists are guided in their speculations.

It is well known that the action of moving ice leaves a very peculiar and characteristic impress on the surface of the rocks, and even on the general aspect of the country over which it passes. This is no mystery of science, but a plain fact which any one that chooses may observe for himself. Every Glacier carries along in its course a vast quantity of loose gravel, hard sand, and large angular stones. A considerable proportion of these materials in course of time fall through crevasses in the ice, and become firmly embedded in the under surface of the Glacier. Then, as the moving mass slowly descends the valley, they are shoved along under enormous pressure, and the surface of the rocks beneath is furrowed, scratched, and polished, in a remarkable and unmistakable manner. The furrows and scratches are rectilinear and parallel to an extent never seen in the marks produced by any other natural agency: and they always coincide more or less in their direction with the general course of the valley. A reciprocal action often takes place: the large blocks of stone, frozen into the under surface of the Glacier, are themselves scored and polished by friction against the floor and sides of the valley.

Fig. 3.—Block of Limestone furrowed, scratched, and polished, from the Glacier of Rosenlaui, Switzerland. (Lyell.)

aa, White streaks or scratches. bb, Furrows.

Similar effects are produced by Icebergs; not of course when drifting about in the deep sea, but when they come into contact with a gently-shelving coast and grate along the bottom. These mountains of ice, laden with the débris of the land, are often carried along with the velocity of from two to three miles an hour; and before their enormous momentum can be entirely destroyed, an extensive surface of rock must have been rounded, grooved, and scarred, pretty much in the same way as by the action of a Glacier. There can be no failure of the grinding materials. During the process of melting, the Iceberg is constantly turning over according as the centre of gravity shifts its position; and thus a new part of its surface, with fresh angular blocks of stone, together with fresh masses of sand and gravel, is constantly brought into contact with the floor of the ocean. And this is not mere theory. All these phenomena may be witnessed any day on the shores of Baffin’s Bay and Hudson’s Bay, and along the coast of Labrador.

Again, the evidence of glacial action may be discovered in the materials themselves which have been transported by ice. Many of the large erratic blocks, after having travelled immense distances, exhibit the same sharp angular appearance as if they had only just fallen down from the cliff on the mountain side. By this circumstance they are at once distinguished from blocks of stone transported by running water; for in these the angles are sure to be rounded off by friction. Sometimes, too, they are deposited not only far away from the same rock, but in regions where no rock of the same kind exists. In the case of Icebergs, they are not unfrequently carried many hundreds of miles before being dropped into the depths of the ocean, and, in the course of their long journey, borne over the lofty ridges of submarine mountain chains.

Furthermore, it often happens that a Glacier shrinks backward up the valley, and sometimes even disappears altogether. When the melting of the ice at the lower extremity exactly balances its onward progress, then the Glacier seems stationary to the eye, and occupies from year to year the same position. But, when a number of hot seasons follow one another in immediate succession, the ice is melted more rapidly than the Glacier advances, and in consequence it gradually becomes shorter, and seems to the eye to recede toward the upper parts of the valley. In this case the long lines of moraines, which before had rested on the ice, are left spread out on the plains or deposited on the slopes of the mountain. Immense blocks of stone are by this means frequently set down on the summits of lofty crags, and in such like positions to which they could not be brought by any other natural agency. These Perched Blocks, as they are called, and also those long regular mounds of earth and stones abound in several of the Swiss valleys, and constitute a very striking feature of Alpine scenery.

Now, it appears that all these various characteristic marks of glacial operations can be distinctly traced in many countries where the action of moving ice has been unknown within the period of history. And on this fact is founded the Geological theory of an ancient Glacial Period. We are confidently assured that a great part of Northern Europe, including even our own islands, not to speak of America and other countries as well in the northern as in the southern hemisphere, were, in some far distant age, the scene of those same phenomena which are witnessed at the present day amid the solemn grandeur of the Alps, and in the frozen wastes of the Arctic regions. In that age enormous Glaciers moved slowly downward from the snow-clad heights over innumerable valleys now rich with the fruits of the earth; ponderous Icebergs floated over wide areas of the ocean, where now the dry land appears; and vast piles of promiscuous rubbish, with great angular blocks of stone, were deposited on the slopes and crests of submarine mountains that now tower hundreds of feet above the level of the sea.

To illustrate this theory, we would begin with a country where the vestiges of glacial operations in past times may be studied side by side with the glacial phenomena of the present day. In Switzerland it needs but little skill to discern many marks and tokens of moving ice where moving ice is no longer found. In descending, for example, the valley of the Hasli or the valley of the Rhone, the intelligent traveller can hardly fail to observe how the rocks all around are scarred and furrowed, precisely after the same fashion as the rocks in the higher parts of the same valleys are now being scarred and furrowed by the Glacier of the Aar and the Glacier of the Rhone. At intervals, too, may be seen long mounds of unstratified gravel and mud, with large fragments of rock, in every way resembling the terminal moraines now daily accumulating at the extremities of existing Glaciers. When these facts are once distinctly brought home to the mind, it is impossible to resist the conclusion that several of the Alpine Glaciers once extended far beyond their present limits down the valleys of Switzerland.

If we proceed a little distance to the mountains of the Jura, now wholly devoid of Glaciers, we shall find that the same glacial phenomena with which we have become so familiar in the Alps, are still everywhere presented to the eye. And we feel instinctively impelled to pursue the same line of inductive reasoning. Moving ice, we know from abundant observation, is capable of producing these effects: nor have we ever seen effects of this kind produced by any other cause: nay, there is no other natural agent known that is capable of producing such effects: it is therefore reasonable to infer that moving ice was the cause of these effects; and that, in some bygone age, great masses of ice moved slowly over the valleys of the Jura as they now move slowly over the valleys of the Alps.

Another circumstance may here be noticed which is well worthy of consideration. The Alps are composed of granite, gneiss, and such like crystalline rocks: the Jura, of limestone and various other formations, altogether different from those of the Alps. Now, scattered loosely over the valleys of the Jura, and perched upon its lofty crests, we find immense angular blocks—some of them as large as cottages—of the Alpine rocks. The question naturally arises, how have they been transported to their present site. Certainly not by the action of water; for in that case the projecting angles would have been rounded off, and the sharp edges worn away. But the work might have been easily accomplished by the power of moving ice, and could not have been accomplished by any other natural agency with which we are acquainted. Thus we are led to conclude that the Glaciers of the Alps must, by some means or another, have once made their way northward across the great valley of Switzerland, fifty miles wide, and deposited their ponderous burdens of gravel, sand, and erratic blocks on the mountains of the Jura.

It would carry us too far from our present purpose to draw out this theory in all its details. But we cannot for-bear briefly to touch upon some of the bold and startling conclusions to which it has led. The Geologist having, by patient and varied exercise, in the regions of existing Glaciers, trained his eye and his judgment in the observation of those phenomena that mark the action of moving ice, soon begins to discover that they are not wanting in other countries. They are not to be found, indeed, beneath the burning sun of Africa, nor on the borders of the Mediterranean Sea. But as he travels northward they begin by degrees to appear; and when at length he reaches the shores of the Baltic, they are spread out profusely before him as they were in the bosom of the Alps. All this had puzzled Geologists for years; but the clue has been found at last. What is going on to-day in Switzerland, and in Greenland, and on the shores of Labrador, must have been going on, ages ago, in Germany, and in Denmark, and on the shores of the Baltic. We may argue from the effect to the cause. Here are the moraines, the erratics, the perched blocks, and the surfaces of rock furrowed and scratched with ice: at some past time there must have been the moving Glaciers and the floating Icebergs.

Following out this line of argument, and applying it to countries nearer home, Geologists have come to the conclusion that the Grampian Hills in Scotland, the mountains of Kerry in Ireland, the Snowdonian heights in Wales, and many other ranges of hills in these islands, were in former times subjected to the action of moving ice. Nay, it is contended, with much show of reason, that these islands must have been, for a considerable time, in great part submerged beneath the sea, and traversed by floating Icebergs. When large erratic blocks are found in the immediate neighborhood of the formation from which they have been derived, then it is easy to explain their origin and to trace their course. But it often happens that the nearest rock of the same mineral composition, and therefore, the nearest rock from which they can possibly have been derived, is separated from the site which they now occupy by a lofty chain of mountains. By what means, then, have they been transported hither? Not by moving water, for their sharp edges and projecting angles are still preserved. Not by Glaciers; for a Glacier cannot climb a steep mountain ridge. It would seem, indeed, that in the present geographical distribution of land and water, there is no natural cause which could carry them from the parent rocks to their present position. But if we suppose that in some long past age of the world, Great Britain and Ireland were submerged beneath the sea, and that Icebergs floated in the waters above, the problem is solved at once. The fragments of far distant rocks frozen into the Icebergs might then have been carried over the summits of what are now lofty mountains, and as the ice melted away, might have been deposited all along their slopes and even on their highest crests.

The presence of marine shells, belonging chiefly to species which now exist only in the arctic seas, affords a strong confirmation of this hypothesis. For they are found intimately associated with the erratic blocks, not merely in valleys, to which the sea might be supposed to have had access in times of extraordinary flood, but upon lofty mountains at a height of five hundred, six hundred, and even thirteen hundred feet above the level of the sea. There is no difficulty in accounting for this phenomenon if we suppose the country to have been at one time submerged, and the glacial drift in which the shells are found embedded to have been deposited by Icebergs on the floor of the ocean. If we refuse to make this supposition the difficulty is simply insurmountable.[34]

But it is somewhat beside our purpose to wander so far into the region of theory and speculation. Our main object in these chapters has been to establish the fact that Denudation is actually taking place to an almost incredible extent, in the present age of the world. For this purpose we have enumerated the principal agents by which this process is carried on; and we have endeavored to show from the authenticated researches of travellers and scientific men that they have been at work within the period of history, and are still at work around us. Our summary is, indeed, brief; but it is still sufficient to demonstrate that, even during the present age, the whole surface of the Globe has been ever in a constant state of change; that mountain heights have been worn away, and valleys have been scooped out, and lofty cliffs have disappeared, and bold headlands have been rent in twain, and rocks and earths have day by day been broken up and dissolved and decomposed, by the never ceasing operation of natural causes; and that the broken fragments are at every moment moving along over the surface of the land or through the depths of the sea.

Now Geologists tell us that these are the raw materials of a new building which is going on in these latter times under the guiding hand of Nature. Indeed, they say it is not so much a new building as the uppermost story of an old building. If we descend into the Crust of the Earth we may trace this building even from the foundations, which are laid upon the solid granite, up through each successive stage of limestone, and sandstone, slate, conglomerate, and clay, until we come to the surface, where new strata, composed of the same elements, and exhibiting the same general characteristics, are slowly growing up before our eyes. Thus will the idea gradually steal upon the mind, that the works of ages long gone by are reproduced once again in our own days, and that we may study the history of the past in the mirror of the present which nature holds up to our view.

This is the branch of Geological argument upon which we are now about to enter. We have visited Nature, as it were, in her quarry, and we have seen how she collects her materials, how she fashions them to her purpose, how she transports them to the place for which they are designed. If it be true, as alleged, that with these materials she is actually engaged, at the present moment, in building upon the existing surface of our Globe a new series of stratified rocks, which are the exact counterpart of those beneath, this fact affords at least a very strong presumption in favor of one very important principle in the theory of Geologists. Let us, then, follow the course of her operations and judge for ourselves.

CHAPTER V.

STRATIFIED ROCKS OF MECHANICAL ORIGIN—THEORY DEVELOPED AND ILLUSTRATED.

Formation of stratified rocks ascribed to the agency of natural causes—This theory supported by facts—The argument stated—Examples of mechanical rocks—Materials of which they are composed—Origin and history of these materials traced out—Process of deposition—Process of consolidation—Instances of consolidation by pressure—Consolidation perfected by natural cements—Curious illustrations—Consolidation of sandstone in Cornwall—Arrangement of strata explained by intermittent action of the agents of Denudation.

The Stratification of Rocks is one of the most remarkable features which the Crust of the Earth presents to our notice; and the principles by which this phenomenon is explained belong to the very foundation of Geological theory. It is now universally agreed that the successive layers or strata, which constitute such a very large proportion of the Earth’s Crust, and which cannot fail to attract the notice even of the most careless observer, have been slowly built up during a long series of ages by the action of natural causes. In support of this bold and comprehensive theory, geologists appeal to the operations which are going on in nature at the present day, or which have been observed and recorded within historic times. There is a vast machinery, they say, even now at work all over the world, breaking up the rocks that appear at the surface of the Earth, transporting the materials to different sites, and there constructing new strata, just the counterpart of those which we see piled up one above the other, wherever a section of the Earth’s Crust is exposed to view. It is given to us, therefore, on the one hand to contemplate the finished work as it exists in the Crust of the Earth, and on the other, to examine the work still in progress upon its surface; and if both are found to agree in all their most remarkable characteristics, it is not unreasonable to infer that the one was produced in bygone ages by the very same causes that are now busy in the production of the other.

In the examination of this argument we first turned our attention to the numerous and powerful agents that are now employed in the breaking up and transporting of existing rocks. It was impossible within our narrow limits to enumerate them all. But we selected those which are at the same time the most familiar in their operations, and the most striking in their results:—mighty rivers discharging daily and hourly into the sea the accumulated spoils of vast continents; the breakers of the ocean dashing with unceasing energy against all the cliffs and coasts of the world; the tides and currents of the sea taking up the ruins which the breakers have made, and carrying them far away to the lonely depths of the ocean; the frozen rain bursting massive rocks asunder with its expansive force, and sending the fragments over lofty cliffs and steep precipices to become the prey of roaring mountain torrents, or perhaps, more fortunate, to find a place of tranquil rest on the bosom of the glittering Glacier; then this wondrous Glacier itself, a moving sea of ice, bearing along its ponderous burden from the summits of lofty mountains far down into the smiling plains, and meanwhile, with tremendous power, grinding, and furrowing, and wearing away the floor of the valley, and leaving behind it an impress which even time cannot efface; and lastly, the massive Icebergs which stud the northern and southern seas, drifting along like floating islands above the fathomless abysses of the ocean, and scattering their huge boulders over the surface of submarine mountains and valleys.

All these phenomena have been learned from actual and repeated observation. They are not philosophical speculations, but ascertained facts. We cannot doubt, therefore, that the work of demolition is going on; it remains for us now to inquire about the work of reconstruction.

The reader will remember that Geologists divide the stratified rocks into three distinct classes, Mechanical, Chemical, and Organic. This distinction, they say, is founded on the actual operations of Nature. From a close examination of the natural agents now at work in the world, it appears that some strata are being formed chiefly by the action of mechanical force; others chiefly by the influence of chemical laws; and others again chiefly by the intervention of organic life. Thus we have three distinct classes of rock at present coming into existence, each exhibiting its own peculiar characteristics, and each, moreover, having its counterpart among the strata that compose the Crust of the Earth. We shall now proceed to set forth some of the evidence that may be advanced in favor of these important conclusions, beginning with those rocks that are called Mechanical.

And first it is important to have, at least, a general idea of the appearance which Mechanical Rocks present to the eye. We shall take three familiar examples, Conglomerate, Sandstone, and Clay. Conglomerate, or Pudding-stone as it is sometimes called, is composed of pebbles, gravel, and sand, more or less compacted together, and generally forming a hard and solid mass. The various materials of which it is composed, though united in the one rock, nevertheless remain their own external forms, and may be distinctly recognized even by the unpractised eye. Sandstone, as the name implies, is made up of grains of sand closely compressed and cemented together. The quality and appearance of this rock vary very much according to the size and character of its constituent particles. Often the grains of sand are as large as peas, or even larger; sometimes they are so minute that they cannot be distinguished without the aid of a lens. For the most part they consist of quartz, with grains of limestone intermixed; and they are usually rounded, as if by the action of running water. Clay is a rather vague and general term, now commonly employed to denote any finely-divided mineral matter which contains from ten to thirty per cent. of Alumina, and is thereby rendered plastic, and capable, when softened with water, of being moulded like paste with the hand. It occurs in many different forms among the strata of the Earth, according to the different minerals that enter into its composition and the different influences to which it has been subjected. Marl and Loam may be taken as well-known illustrations: the former is a clay in which there is a large proportion of calcareous matter; the latter is a mixture of clay and sand. Sometimes by pressure clay is condensed into a kind of slaty rock called Shale, which has the property of being easily split up into an immense number of thin plates or laminæ.

It should be remembered that there is not always a perfect uniformity in the structure of these rocks. In Conglomerate, for example, the pebbles may be as large as cannon balls, or they may be only the size of walnuts. So, too, we have every variety of fineness and coarseness in the quality of Sandstone. Again, both Conglomerate and Sandstone are often largely adulterated with clay, and on the other hand, clay will sometimes contain more than its usual proportion of sand or lime. Lastly, these materials are in one place compacted into hard and solid rock, in another they are found in a loose and incoherent condition.

But amidst all these varieties of form and texture, the rocks we have been describing generally preserve their peculiar characteristics, and with a little experience can be easily recognized. They are found to constitute a very large part, perhaps we might say the larger part, of the stratified rocks in every country that has hitherto been explored by Geologists. Wherever we go we are met by the same familiar appearances;—beds of Conglomerate, Sandstone, Clay, Marl, Shale, recurring again and again through a series of many hundred strata, sometimes in one order, and sometimes in another; sometimes without any formation of a different kind intervening, and sometimes alternating with limestone or other rocks of which we shall speak hereafter.

Such is the general character and appearance of those strata which are known among Geologists as Aqueous Rocks of Mechanical origin. Now, it must at once strike the reader, that these rocks are made up of just those very materials—the same both in kind and in form—that we have already shown to be daily prepared and fashioned by a vast and complex machinery in the great workshop of Nature. He will remember how enormous blocks are detached from the mountain side, or from the cliffs on the seashore, and broken up into fragments; how the fragments in time become pebbles, sand, and mud; and how these are caught up by rivers, tides, and currents, and carried far away to sea. Here we have certainly all the materials that are necessary for the building up of Conglomerate, Sandstone, and Shale. We have seen how they are prepared by the hand of Nature, how they are moulded into shape, how they are transported from place to place. Let us now pursue the sequel of their history, and follow them on to the end.

It is plain they cannot remain forever suspended in water; sooner or later they must fall to the bottom. Yet they will not all fall together. For though all are carried downward by the one force of gravity, those materials that are smaller and lighter will be more impeded by the resistance of the water. The pebbles and coarse gravel will be the first to reach the bottom, then the sand, and last of all the fine, impalpable mud. Thus, as the current sweeps along in its course, the sediment which it bears away from the land will be in a manner sorted, and three distinct layers of different materials will be deposited in the bed of the ocean;—first, nearest to the shore, a layer of pebbles and coarse gravel, then a layer of sand, and last of all a layer of fine mud or clay. This is the first step in the construction of stratified rock. To complete the work nothing more is necessary than the consolidation of these loose and incoherent materials. If this could be accomplished, then we should have a solid stratum of Conglomerate, a solid stratum of Sandstone, and a solid stratum of Shale formed in the bed of the ocean.

With regard to this operation, however, we cannot hope for the advantage we have hitherto enjoyed, of actual observation. The process of consolidation, if it take place at all, is going on in the depths of the Sea. But though it is thus removed beyond the reach of our senses, it is not beyond the reach of our intelligence. We may borrow the torch of Science, and search even into the hidden recesses of Nature’s secret laboratory.

In the first place, a partial consolidation of clay and sand, and even of gravel, may take place under the influence of pressure alone. Many of us are familiar with this truth, but few, perhaps, are aware how extensively it is illustrated in the practical arts of life. Here are some curious and interesting examples. The minute fragments of coal which are produced by the friction of larger blocks against one another, and which may be obtained abundantly in the neighborhood of every coal mine, are now manufactured into a solid patent fuel by the simple process of forcible compression. Again, the dust and rubble of black lead, formerly cast aside as useless, are now carefully collected, and by no other force than pressure are converted into a solid mass, fit to be employed in the manufacture of lead-pencils. “The graphite or black lead of commerce,” says Sir Charles Lyell, “having become very scarce, Mr. Brockedon contrived a method by which the dust of the purer portions of the mineral found in Borrowdale might be recomposed into a mass as dense and as compact as native graphite. The powder of graphite is first carefully prepared and freed from air, and placed under a powerful press on a strong steel die, with air-tight fittings. It is then struck several blows, each of a power of a thousand tons; after which operation the powder is so perfectly solidified that it can be cut for pencils, and exhibits, when broken, the same texture as native graphite.”[35] An instance yet more to our purpose occurs in the experiments made to try the force of gunpowder. Leathern bags filled with sand are put into the mortar that is to receive the cannon-ball at a distance of fifty feet from the mouth of the gun; and the sand is often compressed by the percussion of the ball into a solid mass of Sandstone.[36] Now the deposits of which we are speaking cannot fail to be subjected to a very powerful and a very constant compressing force. For, since the process of deposition is always going on, the matter which is deposited to-day will to-morrow be covered with a new layer, and in the course of ages it may lie beneath an immense pile of mineral matter, hundreds or even thousands of feet in thickness.

But in fact there is another and more important agent at work. When the harder and more compact blocks of Conglomerate and Sandstone are subjected to a close analysis in the laboratory of the chemist, it is found that they are strongly cemented together, sometimes by a solution of lime filling up the interstices between the grains or pebbles, sometimes by a solution of silica, sometimes by a solution of iron. Now this discovery affords a useful clue when we come to study the present operations of Nature. It is to the agency of a mineral cement we must look for the perfect consolidation of Mechanical Rocks. Let us see if such a cement can be found.

It is well known that the water of rivers, lakes, and springs, is more or less charged with carbonic acid gas; and therefore, when it comes in contact with limestone, it dissolves a portion of the lime and holds it in solution. Hence it follows that in every part of the world there exists an abundant store of calcareous cement. Again, our readers must have observed the brownish, rusty color sometimes produced by streams on the surface of rocks and herbage. This is the result of the iron with which the streams are impregnated: and we are informed by scientific inquirers that water containing a solution of iron prevails very generally in almost all countries. The solution of silica in water is not so common; because pure silica cannot be dissolved by water except at a very high temperature. Nevertheless, it has been clearly demonstrated by observation, that silica, where it occurs in certain combinations with other mineral substances, may be dissolved readily enough: for instance, in the decomposition of felspar, and of all rocks in which felspar is an ingredient, silica is carried off in a state of solution.[37] And since these rocks are very numerous, and distributed over every part of the earth, we may fairly conclude that a solution of silica exists very abundantly in nature.

Now when we bear in mind that we have on the one hand in the Crust of the Earth, solid strata of Conglomerate and Sandstone, exhibiting the evident operation of these mineral cements; and on the other hand, near the surface, the loose materials of Conglomerate and Sandstone as if ready to be cemented, and close at hand the cementing mineral itself in a convenient form, it is not unreasonable to assume that the process should actually take place;—that water highly charged with iron, or lime, or silica, should filter through the loose gravel and sand, depositing its mineral cement as it passes along, and converting the newly-formed strata into compact and solid rock.

But this conclusion does not rest upon antecedent probability alone. We have proof unquestionable that a process such as we have described is actually going on. In the dredging of the river Thames large masses of solid Conglomerate are found from time to time, firmly compacted together by a ferruginous cement. And there is internal evidence that the process of solidification has been effected by natural causes within historic times; for it happens not unfrequently that Roman coins and fragments of pottery are found embedded in the solid block of stone. Similar discoveries were made in deepening the bed of the river Dove in Derbyshire, about the year 1832. Thousands of silver coins were found about ten feet under the surface, firmly cemented into a hard Conglomerate. Several of these coins bear dates of the thirteenth and fourteenth centuries; and therefore the pebbles which form the rock must have been deposited and converted into a solid mass since that time. But we must not suppose that so long an interval is necessary for the consolidation of rocks. In the early part of the present century a vessel called the Thetis was wrecked off cape Frio on the coast of Brazil. A few months afterward, when an attempt was successfully made to recover the dollars and other treasures which had gone to the bottom with the wreck, they were found completely enveloped in solid masses of quartzose Sandstone. The materials of the newly-formed stone were in this case manifestly derived from the granite rocks of the Brazilian coast.[38]

In many parts of the Mediterranean, and along its shores, this process is known to be going on with equal rapidity. “The new-formed strata of Asia Minor,” writes Sir Charles Lyell, “consists of stone, not of loose, incoherent materials. Almost all the streamlets and rivers, like many of those in Tuscany and the south of Italy, hold abundance of carbonate of lime in solution, and precipitate Travertine, or sometimes bind together the sand and gravel into solid Sandstones and Conglomerates; every delta and sandbar thus acquires solidity, which often prevents streams from forcing their way through them, so that their mouths are constantly changing their position.”[39] In the Museum at Montpelier is exhibited a cannon embedded in a crystalline calcareous rock which was taken up from the bed of the Mediterranean near the mouth of the Rhone.[40]

To these examples of the solidification of rock within recent times we are tempted to add one more, taken from a Memoir published by the late Dr. Paris in the Transactions of the Royal Geological Society of Cornwall. “A sandstone occurs in various parts of the northern coast of Cornwall, which affords a most instructive example of a recent formation, since we here actually detect Nature at work in converting loose sand into solid rock. A very considerable portion of the northern coast of Cornwall is covered with calcareous sand, consisting of minute particles of comminuted shells, which in some places has accumulated in quantities so great, as to have formed hills of from forty to fifty feet in elevation. In digging into these sand-hills, or upon the occasional removal of some part of them by the winds, the remains of houses may be seen; and in places where the churchyards have been overwhelmed, a great number of human bones may be found. The sand is supposed to have been originally brought from the sea by hurricanes, probably at a remote period. It first appears in a state of slight but increasing aggregation on several parts of the shore in the Bay of St. Ives; but on approaching the Gwythian River it becomes more extensive and indurated.... It is around the promontory of New Kaye that the most extensive formation of Sandstone takes place. Here it may be seen in different stages of induration, from a state in which it is too friable to be detached from the rock on which it reposes, to a hardness so considerable that it requires a very violent blow from a sledge to break it. Buildings are constructed of it; the church of Cranstock is entirely built with it; and it is also employed for various articles of domestic and agricultural uses.”

No reasonable doubt can therefore remain that the loose beds of gravel, sand, and clay, which, as we have already seen, are deposited from day to day, and from year to year, and from century to century, beneath the waters of the ocean, may be converted in the course of time by natural agents into solid rocks of Conglomerate, of Sandstone, and of Shale. But this is not enough. It yet remains for us to explain how these solid rocks come to be arranged in a series of distinct layers or strata. The reader will remember that the supply of materials in any given area of the ocean is not fixed and continuous, but, on the contrary, variable and intermittent. During the periodical rains within the tropics, and during the melting of the snows in high latitudes or in mountain regions, the rivers become enormously swollen, and carry down a far greater quantity of sediment than at other seasons. The waste of cliffs, too, by the action of the waves, is much greater in winter than in summer. Thus, while at one season a particular river or current may be comparatively free from sediment, at another it will carry along in its turbid course an almost incredible freight of mineral matter. We have a notable example in the case of the Ganges. The bulk of earthy matter which this river discharges into the sea during the four months of rain, averages about 50,000,000 of cubic feet per day; whereas the daily discharge during the three months of hot weather is considerably less than one hundredth part of that amount.[41]

Besides this variety in the quantity of materials carried, there is also a great variety in the velocity both of rivers and of currents; and therefore they will not always carry the same materials to the same distance; for the less rapid the stream, the sooner will the sediment fall to the bottom. We may add that currents, as is well known, often change their direction from various causes, and thus at different times they will carry the waste of the land to different parts of the ocean.

From these considerations two conclusions may be fairly deduced: First, that the process of deposition may often go on very rapidly for a time over a given area, and then altogether cease, and after an interval begin again. In this way time may be allowed for one deposit to acquire more or less consistency before the next is superimposed; and thus a succession of distinct beds will be produced. Secondly, we may infer that the same precise materials will not always be deposited over the same area; at one time it will be sand, at another gravel, at another clay, at another some combination of these or other mineral substances. And thus it may happen that the strata deposited in successive periods of time shall not only be distinct one from the other, but composed of different materials;—that there shall be, in fact, as we so often see that there are, beds of Conglomerate, Sandstone, Clay, Marl, and other rocks, succeeding one another in every variety of order.

CHAPTER VI.
STRATIFIED ROCKS OF MECHANICAL ORIGIN—FURTHER ILLUSTRATIONS.

Impossible to witness the formation of stratified rocks in the depths of the ocean—On a small scale examples are exhibited by rivers and lakes—Alluvial plains—Their extraordinary fertility—Great basin of the Nile—Experiments of the Royal Society—The Mississippi and the Orinoco—Some rivers fill up their own channels—Case of the river Po—Artificial embankments—Large tract of alluvial soil deposited by the Rhone in the Lake of Geneva—Deltas—The delta of the Ganges and Brahmapootra—Delta of the Nile.

The argument set forth in the last chapter is simple, ingenious, and persuasive. Nay, we must fairly confess that to us it seems conclusive. We do not mean to say that it amounts to a rigorous demonstration. But it affords at least a strong presumption that the process of deposition, the process of consolidation, and the process of stratification, are going on to a vast extent beneath the waters of the ocean; and that, in these latter ages of the world’s history, Aqueous Rocks are slowly growing up under the influence of natural causes, which resemble in every important feature those that are now attracting so much attention within the Crust of the Earth. We are therefore prepared to accept this conclusion, if it be not found at variance with any well-established fact, or with any known and certain truth. But in matters of physical science the evidence of our senses is, after all, the most satisfactory argument. And our readers, no doubt, would like to witness, if possible, with their eyes, the building up of Stratified Rocks. Now, though it is not given to us to see this process in all its colossal magnitude as it goes on within the depths of the mighty ocean, it is yet possible to behold it exhibited, as it were, in miniature, in certain cases where the sediment of rivers is deposited within reach of observation.

Every one is familiar with the fact that many rivers overflow their banks at certain seasons, and spread themselves out over a wide area, sometimes reaching to the foot of the hills that bound the valleys through which they flow. This is the origin of those Alluvial Plains so remarkable for their surpassing richness and fertility. In each successive year a thin film of sediment is deposited on the surface of the land; and thus in the course of ages a soil is formed capable of producing, season after season, the most luxuriant crops without manifesting any symptoms of exhaustion. The soil of the Alluvial Plain near St. Louis, on the Mississippi, is thus spoken of by a modern traveller: “As to the quality of the land, any given number of crops might be grown off it. Corn has been raised on it for a hundred years together—as far back as the settlement is known. To inquire about the system of farming in the West is not productive of information which would be of service on the continent of Europe. There is no system: the farmer scratches the ground and throws in the seed, and his bountiful harvests come up year after year without further thought or trouble. Thousands of centuries have made the soil for him, and it defies him to make too heavy demands upon it. It gives him all he asks, and is never known to disappoint or fail.”[42]

The great basin of the Nile offers an admirable example of an Alluvial Plain on a scale of considerable magnitude. Even in the days of Herodotus, Egypt was regarded as the “gift of the Nile:” and the correctness of this opinion has been placed beyond all reasonable doubt by the investigations of modern science. The river bears along in its current, especially during the flood season, a large quantity of fine earthy sediment obtained by the process of Denudation from the mountains of central Africa. Once a year, between the months of July and November, it overflows its banks, and this sediment is deposited on the adjoining plains. Thus a new layer of rich soil is spread out every year over the existing surface; and the whole country is, in a manner, growing upward at the average rate, according to a rough estimate, of about six inches in the century. Near Cairo, where excavations have been made, the successive layers of annual deposit are distinctly visible to the eye. And it is worthy of remark that, although each one of these is no thicker than a sheet of paste-board, the stratum of alluvial soil which overlies the sands of the desert, and which to all appearance has come into existence by the very same process, is often forty, fifty, and even sixty feet in depth.

A series of interesting observations and experiments have been recently made under the auspices of the Royal Society, which afford some useful information on this subject. The colossal statue of Rameses, near Memphis, was found to be partly embedded in a stratum of mud which had gradually accumulated around it. Upon sinking a shaft, it was discovered that from the present surface of the plain to the base of the pedestal is a distance of nearly ten feet. Now, Rameses flourished, according to Lepsius, about one thousand three hundred and sixty years before the Christian Era; and therefore, since that time, or within a space of 3200 years, it is pretty clear that a thickness of ten feet has been added at this spot to the Alluvial Plain of the Nile. It is hard to resist the conclusion that the next stratum of ten feet as we proceed downward, which, in every respect, resembles the first, must have been produced in the same way by natural causes; and so on till we reach the barren sand of the desert, which is here just forty-two feet below the present level of the plain.[43]

It should seem, therefore, that Egypt is nothing more than a great Alluvial Plain, slowly built up in the long lapse of ages, by the annual inundations of the Nile. Vast tracts of the same kind are to be found in other parts of the world. The Mississippi, which drains about one-seventh of the whole North American continent, has formed an Alluvial Plain more than a thousand miles in length, and from thirty to eighty in breadth. And in South America, the Orinoco once a year spreads out its swollen and turbid waters over an area not unfrequently seventy miles broad; leaving behind, when it subsides, a substantial layer of muddy sediment to enrich the soil.[44] It would be easy to accumulate examples. But we shall be content with having referred the reader to the Great Basin of the Nile, which affords special opportunities for the study of alluvial phenomena; being illustrated at once by the historical monuments of remote antiquity and the scientific researches of recent times.

There is another process by which Alluvial Plains are formed. It often happens that a river fills up the channel in which it has been moving for years, and is forced to shift its course and seek a new passage to the sea. In progress of time this channel is filled up like the former and deserted, and then a third, and then a fourth. At each change a new stratum is formed, almost always distinguished for its extraordinary fertility. This phenomenon is chiefly to be looked for when an extensive and almost level plain lies between some lofty range of mountains and the sea. In such a case, the river which bears away the waste of the mountains, will move onward in its course with a sluggish current, and will, of necessity, deposit the greater part of its burden on the way. There is scarcely a country in the world that does not abound in formations of this kind; and we could point to many notable instances in which herds of cattle are now grazing on the very spot where, within quite recent times, the turbid waters of some great stream flowed sullenly along.

The river Po, which receives through a thousand mountain torrents an enormous quantity of mineral sediment from the Alps, affords an instructive example. Since the beginning of the fifteenth century it has many times changed its course, often committing great devastations, and always leaving behind unmistakable traces of its movements. Several towns that once stood on the left bank of the river are now on the right. In some instances parish churches and religious houses were pulled down when the devouring stream was seen slowly to approach, and then rebuilt with the same materials at a greater distance. An old channel may be easily recognized at the present day near Cremona, which bears the name of Po Morto, and another called Po Vecchio, in the territory of Parma.

It may be interesting to our readers to learn that these movements have been checked in modern times. By a system of artificial embankment the waters of the river are now confined within definite and narrow limits: thus the velocity of the current is increased and a very considerable portion of the sediment is carried on to the sea. Nevertheless, much is still deposited in the bed of the river, which is, in consequence, raised higher and higher each successive year. Hence it has become necessary, in order to prevent inundations, to add every season to the height of the embankments, so that the river now presents the appearance of an enormous aqueduct, of which some idea may be formed from the fact that, in the neighborhood of Ferrara, the surface of the stream is higher than the roofs of the houses. This system of embankment is carried on very extensively in Northern Italy to check the overflowing of rivers, and to prevent them from changing their courses. It is as old as the time of Dante, who tells us that the inhabitants of Padua erected barriers along the Brenta when the snows began to melt and the season of the floods was approaching,

“Per difender lor ville e lor castelli,
Anzi che Chiarentana il caldo senta.”

Inferno, Canto xv.

As a river sometimes fills up its own channel, so too may it fill up a lake through which it flows, and convert it likewise into a great Alluvial Plain. Thus it is said several extensive lakes have been transformed into dry land in modern times near Parma, Piacenza, and Cremona. Elsewhere the process may be seen in actual operation. The Rhone when it enters the lake of Geneva is a turbid discolored stream; the natural consequence of the immense quantity of earthy sediment with which it is charged. But as it slowly moves along, the sediment falls to the bottom, and when, at length, “by Leman’s waters washed,” it emerges at the town of Geneva, and shoots beneath the magnificent bridge that joins the opposite shores, it has already assumed that beautiful azure blue which travellers love to gaze on, and poets love to sing. The sediment left behind goes to form a great alluvial tract which is slowly but steadily advancing into the lake. An ancient town called Port Vallais, which, eight centuries ago, stood at the water’s edge, is now a mile and a half inland. And if the world were to last long enough, and the natural agents at present in operation were to remain unchanged, the time would come, we can scarcely doubt, when the whole lake of Geneva would have been converted into an Alluvial Plain of vast extent and inexhaustible fertility.

This last example leads us on to the phenomenon of Deltas, which afford, perhaps, the best opportunity of observing the actual formation of stratified rocks. Some large rivers, as we have already seen, enter the sea with such extreme velocity as to bear away their sediment to a distance of several hundred miles from the land. But in other cases the onward rush of the stream is much sooner arrested, and the sediment, if it be not caught up by ocean currents, is deposited near the mouth of the river, and forms a triangular tract of alluvial land. This kind of deposit is called a Delta, from the resemblance it bears to the letter (Δ) of that name in the Greek Alphabet. The apex of the triangle points up the stream, the base is toward the sea. Hence, when a Delta is formed the river naturally divides into two branches, one flowing to the right, the other to the left. In progress of time new channels are almost always made, and the great stream empties itself into the sea by many mouths.

The Delta formed in the Bay of Bengal by the two great rivers of India, the Ganges and the Brahmapootra, offers an illustration of this phenomenon on a scale of unusual magnitude. Indeed, strictly speaking, it is not one Delta only, but rather two Deltas lying side by side; the one deriving its origin from the Ganges, the other from the Brahmapootra. This double Delta extends its base for two hundred and fifty miles along the Bay of Bengal, and stretches inward into the continent of India to an almost equal distance. Here, then, is a vast tract of country manifestly composed of earthy sediment, obtained by the process of Denudation from the Himalayan mountains, and afterward transported to its present site by the agency of moving water. But the deposition of earthy matter does not suddenly come to an end when we reach the present line of the coast. The sea is visibly discolored by the sediment far beyond the actual base of the Delta; and a sloping bank of mud is found to stretch beneath the waters of the Bay to a distance of a hundred miles.

Even within the short period of a man’s life the domain of dry land is often visibly enlarged. Sandbanks are first formed in some of those numerous winding channels through which the two rivers find their way to the sea. The sandbanks, receiving fresh accessions during each succeeding flood, in a short time become islands; and the islands have been known, in a few years, to attain a superficial extent of many square miles. Then begins to appear a wild and luxuriant vegetation—reeds, long grass, shrubs, and trees; and those impenetrable thickets are formed, to which the buffalo, the rhinoceros, and the tiger soon resort for shelter. A very extensive tract of this kind, adjoining the sea-coast, and known as the Sunderbunds, is said to be as large as the principality of Wales.

The Delta of the Nile, though not quite one-half as large as the Delta of the Ganges, presents nevertheless some features of peculiar interest. In many places where a vertical section is exposed to view, the phenomenon of stratification may be distinctly recognized. The upper part of the deposit belonging to each year is composed of earth of a lighter color than the lower part; and the whole forms a distinct layer of hardened clay, which may be easily separated from those above and below. This formation, therefore, corresponds exactly with those strata of shale which we so often meet with in the Crust of the Earth. Again, many of the old channels through which the Nile made its way to the sea in ancient times, have been since filled up and converted into solid land. The two extreme arms of the river, which formerly enclosed the Delta, were two hundred miles apart where they entered the Mediterranean. But these channels are now Alluvial Plains, and the base of the Delta is but ninety miles in length. Hence, though the quantity of land which has been formed by the sediment of the Nile is much greater now than it formerly was, the size of the Delta properly so called has not been increased but diminished.

If we turn to the great continent of America, we are met by results not less striking and important. The Delta of the Mississippi is two hundred miles in length, and one hundred and forty in breadth. This vast stratum of mud is between five and six hundred feet thick, and covers an area twelve thousand square miles in extent. Each year it receives from the great Father of Rivers a new accession of sediment which is computed at 3,700,000,000 of cubic feet. And besides this annual deposit of inorganic matter, we must not omit from our estimate the countless trees of various species and of gigantic size, which are torn up by the floods, carried along by the impetuous stream, and buried at last with the bones of animals, and works of human art, and other spoils of the land, in the mud of the Delta at the river’s mouth.[45]

CHAPTER VII.
STRATIFIED ROCKS OF CHEMICAL ORIGIN.

Chemical agency employed in the formation of mechanical rock—But some rocks produced almost exclusively by the action of chemical laws—Difference between a mixture and a solution—A saturated solution—Stalactites and Stalagmites—Fantastic columns in limestone caverns—The grotto of Antiparos in the Grecian Archipelago—Wyer’s cave in the Blue Mountains of America—Travertine rock in Italy—Growth of limestone in the Solfatara Lake near Tivoli—Incrustations of the Anio—Formation of travertine at the baths of San Filippo and San Vignone.

The Aqueous Rocks of which we have spoken in the last two chapters are called by Geologists Mechanical; inasmuch as they owe their existence chiefly to the agency of Mechanical force. It should be observed, however, that a very considerable share in the production of these rocks must be ascribed, not unfrequently, to Chemical influence. Chemical action helps to prepare the materials of which they are composed; and Chemical action likewise furnishes the calcareous, siliceous, and other mineral cements by which they are, in a great measure, consolidated. There is, however, a second class of Aqueous Rocks which are produced almost exclusively by the operation of Chemical laws, and which we have accordingly denominated Stratified Rocks of Chemical Origin. It is of these that we purpose to speak in the present chapter. They constitute a much smaller proportion of the Earth’s Crust than either the Mechanical or the Organic Rocks. But the history of their formation is curious and instructive. We shall confine ourselves to one or two simple and familiar illustrations.

In the course of these illustrations we shall have a good deal to say about Carbonate of Lime in a state of solution; and it may perhaps be useful to explain, first of all, what is meant by a solution, in the technical language of Chemistry. If a spoonful of salt is put into a tumbler of water, the particles of salt, after a little time, cease to cohere together, and become so diffused through the water as to be no longer visible to the eye, although their presence in every part may be easily discerned by the taste. The salt is then said to be dissolved, and the water in which it is dissolved is called a solution of salt. It is important to distinguish the case of a solution from the case of a mere mechanical mixture. If, instead of the salt, we were to put into the tumbler of water a spoonful of very fine sand, then we should have a mixture but not a solution. By stirring briskly the contents of the tumbler we might, indeed, effect a very close union between the particles of water and the particles of sand: but this union would be altogether different in kind from the union that was observed in the former case between the particles of water and the particles of salt. First, the sand would remain visible to the eye, making the water turbid and discolored; whereas the salt entirely disappeared, leaving the water limpid and transparent as before. Again, if the water be allowed to rest, the sand will in time fall to the bottom, whereas the salt will not.

But there is a limit to the capacity of water for holding salt in solution. If spoonful after spoonful be added, it will be found, when a certain point has been reached, that the water can at length dissolve no more. It is then called a saturated solution of salt. If, in this case, a portion of the water were to pass away by evaporation, it is clear, we should have the same quantity of salt as before, in a smaller quantity of water. The consequence would be that all the salt could not then be held in solution, and some of it would fall to the bottom; or, in chemical language, a precipitate of salt would be formed on the bottom of the tumbler. Now, according to the theory of Geologists, many rocks, hundreds of feet thick, and solid enough to form the walls of our palaces, our churches, and our castles, have been produced in the Crust of the Earth by just such a process as this. In support of their theory we are about to show that the process is actually going on in our own time, and is open to the examination of all who may desire to study it for themselves.

We shall begin with the formation of Stalactites and Stalagmites. The mode in which these singular masses of rock are brought into existence is very clearly explained, and the picturesque appearance they so often present to the eye is very graphically described, by Dr. Mantell, in his Wonders of Geology, from which the following passages are taken:—“One of the most common appearances in limestone caverns is the formation of what are called Stalactites, from a Greek word signifying distillation or dropping. Whenever water filters through a limestone rock it dissolves a portion of it; and on reaching any opening, such as a cavern, oozes from the sides or roof, and forms a drop, the moisture of which is soon evaporated by the air, and a small circular plate or ring of calcareous matter remains; another drop succeeds in the same place, and adds, from the same cause, a fresh coat of incrustation. In time, these successive additions produce a long, irregular, conical projection from the roof, which is generally hollow, and is continually being increased by the fresh accession of water, loaded with calcareous or chalky matter: this is deposited on the outside of the Stalactite already formed, and, trickling down, adds to its length by subsiding to the point, and evaporating as before; precisely in the same manner as, during frosty weather, icicles are formed on the edges of the eaves of a roof. When the supply of water holding lime in solution is too rapid to allow of its evaporation at the bottom of the Stalactite, it drops on the floor of the cave, and drying up gradually, forms in like manner a Stalactite rising upward from the ground, instead of hanging from the roof; this is called for the sake of distinction Stalagmite.

“It frequently happens, where these processes are uninterrupted, that a Stalactite hanging from the roof, and a Stalagmite formed immediately under it from the super-abundant water, increase until they unite, and thus constitute a natural pillar, apparently supporting the roof of the grotto. It is to the grotesque forms assumed by Stalactites and these natural columns, that caverns owe the interesting appearances described in such glowing terms by those who witness them for the first time. One of the most beautiful stalactitic caverns in England is at Clapham, near Ingleborough. In the Cheddar Cliffs, Somersetshire, there has been discovered a similar cave richly incrusted with sparry concretions. There are others in Derbyshire.

“The grotto of Antiparos in the Grecian Archipelago, not far from Paros, has long been celebrated. The sides and roof of its principal cavity are covered with immense incrustations of calcareous spar, which form either Stalactites depending from above or irregular pillars rising from the floor. Several perfect columns reaching to the ceiling have been formed and others are still in progress, by the union of the Stalactite from above with the Stalagmite below. These, being composed of matter slowly deposited, have assumed the most fantastic shapes; while the pure, white, and glittering spar beautifully catches and reflects the light of the torches of the visitors to this subterranean palace, in a manner which causes all astonishment to cease at the romantic tales told of the place—of its caves of diamonds and of its ruby walls; the simple truth, when deprived of all exaggeration, being sufficient to excite admiration and awe.

“Sometimes a linear fissure in the roof, by the direction it gives to the dropping of the lapidifying water, forms a perfectly transparent curtain or partition. A remarkable instance of this kind occurs in a cavern in North America called Wyer’s Cave. This cave is situated in a ridge of limestone hills running parallel to the Blue Mountains. A narrow and rugged fissure leads to a large cavern, where the most grotesque figures, formed by the percolation of water through beds of limestone, present themselves, while the eye, glancing onward, watches the dim and distant glimmers of the lights of the guides—some in the recess below, and others in the galleries above. Passing from these recesses, the passage conducts to a flight of steps that leads into a large cavern of irregular form and of great beauty. Its dimensions are about thirty feet by fifty. Here the incrustations hang just like a sheet of water that was frozen as it fell; there they rise into a beautiful stalactite pillar; and yonder compose an elevated seat, surrounded by sparry pinnacles. Beyond this room is another more irregular, but more beautiful; for besides having sparry ornaments in common with the others, the roof overhead is of the most admirable and singular formation. It is entirely covered with Stalactites, which are suspended from it like inverted pinnacles; and they are of the finest material, and most beautifully shaped and embossed. In another apartment an immense sheet of transparent Stalactite, which extends from the floor to the roof, emits, when struck, deep and mellow sounds like those of a muffled drum.

“Farther on is another vaulted chamber, which is one hundred feet long, thirty-six wide, and twenty-six high. Its walls are filled with grotesque concretions. The effect of the lights placed by the guides at various elevations, and leaving hidden more than they reveal, is extremely fine. At the extremity of another range of apartments, a magnificent hall, two hundred and fifty feet long, and thirty-three feet high, suddenly appears. Here is a splendid sheet of rock-work running up the centre of the room, and giving it the aspect of two separate and noble galleries. This partition rises twenty feet above the floor, and leaves the fine span of the arched roof untouched. There is here a beautiful concretion, which has the form and drapery of a gigantic statue; and the whole place is filled with stalagmitical masses of the most varied and grotesque character. The fine perspective of this room, four times the length of an ordinary church, and the amazing vaulted roof spreading overhead, without any support of pillar or column, produce a most striking effect. In another apartment, which has an altitude of fifty feet, there is at one end an elevated recess ornamented with a group of pendant Stalactites of unusual size and singular beauty. They are as large as the pipes of a full-sized organ, and ranged with great regularity: when struck they emit mellow sounds of various keys, not unlike the tones of musical glasses. The length of this extraordinary group of caverns is not less than one thousand six hundred feet.”

In the case of Stalactites and Stalagmites the actual formation of limestone by the influence of Chemical action is brought home forcibly to the mind, and, in a manner, made palpable to the senses. We shall now pass to other examples in which the process is scarcely less open to observation, and in which the limestone assumes a somewhat more massive and rock-like form. Every one who has been in Italy is familiar with the limestone rock called Travertine. It is seen in the ancient walls and the venerable temples of Pæstum, which have withstood unharmed the wasting hand of time for upward of twenty centuries. In Rome, too, this stone is associated in our minds as well with the enduring monuments of antiquity, as with the imposing splendor of Christian art. The Coliseum, the most stupendous of ruins, and St. Peter’s, the most sublime of temples, are built of Travertine. In fact it seems to have been, in every age, the chief building stone employed in the architecture of the Eternal City; and the quarries from which it was taken in ancient times may still be seen at Ponte Lucano, near Tivoli. Now it is an interesting fact, that close to this very spot, at the Solfatara lake on the one side, and at Tivoli itself on the other, the formation of Travertine is going on in our own time, by the precipitation of lime from a state of solution.

The Solfatara lake, situated about fourteen miles from Rome, on the road to Tivoli, is supplied with an unfailing stream of tepid water, impregnated with carbonic acid gas and saturated with carbonate of lime. The amount of carbonate of lime which the water is capable of holding in solution depends chiefly on three things: first, on the presence of carbonic acid; secondly, on the high temperature of the water; and thirdly, on its quantity. Now the carbonic acid is ever rising in bubbles to the surface and passing away; the temperature of the water is lowered by contact with the cooler atmosphere; and its quantity is diminished by evaporation. Thus the capacity which the water at first had for holding the carbonate of lime in solution is notably diminished, and a part of the lime is precipitated to the bottom in a solid form, or clings to the vegetable matter with which it comes in contact.

A very simple and interesting experiment, made in the early part of the present century by Sir Humphrey Davy, will illustrate the rapidity with which the formation of solid stone is even now taking place. In the month of May he fixed a stick in the bed of the lake, and left it standing until the following April, when he found that it was covered with an incrustation of limestone several inches thick.[46] In precisely the same way new layers of Travertine are annually deposited in the bed of the lake, and incrusted on its rocky margin; and so the lake itself is becoming smaller and smaller from year to year. We are told that in the middle of the seventeenth century it was a mile in circuit, and now it is a little more than a quarter of a mile.[47] Here, therefore, we have an immense mass of compact limestone rock, built up by natural agents within the last two centuries.

At Tivoli, about four miles beyond the Solfatara, and two miles from the quarries of Ponte Lucano, phenomena of the same kind are exhibited. The waters of the Anio, which are saturated with carbonate of lime, form incrustations of Travertine on the banks of the river; and at the celebrated falls, where the whole volume of the stream leaps at a bound from a height of three hundred and twenty feet, the most beautiful stalactites are formed by the foam.

The formation of Travertine is going on with no less activity in other parts of the Italian Peninsula. At the baths of San Filippo, in Tuscany, there are three warm springs which contain a very large amount of mineral matter in solution. The water which supplies the baths falls into a pond, where it has been known to deposit a solid stratum of rock thirty feet thick in twenty years. In the same neighborhood are the mineral baths of San Vignone. The source from which the water flows is situated on the summit of a hill not more than a few hundred yards from the high road between Sienna and Rome; and so rapid is the formation of stone, that half a foot of solid Travertine is deposited every year in the pipe that conducts the water to the baths. At this spot we have a very good illustration of the argument we are now considering. As the stream of water flows down the slopes of the hill, a thin layer of Travertine rock is produced on the surface of the earth, almost before our eyes; and so it was previous to our own time, and so it has been for ages, as history and tradition testify. The quantity produced in each year and in each century is comparatively small, but we can have no doubt that it has been produced by the means described. Now, beneath the surface of the Earth, immediately below these modern formations, of which we have so clearly ascertained the origin, we find strata of the same kind, composed of the same materials, and arranged in the same way, layer resting upon layer, down to a depth of two hundred feet: and the Geologist accounts for the formation of the one according to the same laws which he has seen at work in the production of the other.[48]

CHAPTER VIII.
STRATIFIED ROCKS OF ORGANIC ORIGIN—ILLUSTRATIONS FROM ANIMAL LIFE.

Nature of organic rocks—Carbonate of lime extracted from the sea by the intervention of minute animalcules—Chalk rock—Its vast extent—Supposed to be of organic origin—A stratum of the same kind now growing up on the floor of the Atlantic ocean—Coral reefs and islands—Their general appearance—Their geographical distribution—Their organic origin—Structure of the zoophyte—Various illustrations—Agency of the zoophyte in the construction of coral rock—How the sunken reef is converted into an island and peopled with plants and animals—Difficulty proposed and considered—Hypothesis of Mr. Darwin—Coral limestone in the solid crust of the earth.

We now pass to the third division of Aqueous Rocks, those, namely, which are believed to have come into existence chiefly through the agency of animal and vegetable life, and are therefore called Organic. The study of these rocks has been prosecuted with no inconsiderable ardor during the last thirty years; and the facts which have been brought to light are certainly amongst the most curious and interesting in the whole range of physical science. Indeed we are convinced that a simple narrative of the researches which have recently been made upon this subject, and the discoveries to which these researches have led, would be no less attractive, and scarcely less wonderful, than a fairy tale. But it is not for us to wander at large over this vast and tempting field of inquiry. We must be content with one or two examples, which may help to illustrate the process of inductive reasoning upon which the general principles of geological science are founded.

It is argued, then, that the present operations of Nature afford the best key for the interpretation of her works in bygone times. We observe various beds of rocks now in course of formation on the surface of the Earth; and within the Crust of the Earth we discover corresponding strata of the self-same rock already complete, and laid by, as it were, in Nature’s storehouse. Side by side, therefore, we may study and compare the finished work and the work that is yet in progress; and if, on a close examination, they are found to agree in all essential characters, we have doubtless a strong presumption, that the same causes which are now producing the one, must in former times have produced the other. This line of argument we have already considered in reference to those two classes of Aqueous Rocks, which are said to be respectively of Mechanical and of Chemical origin. We now proceed to show that it is no less applicable to those which are called Organic. And although we may not hope to unfold all the secret wonders of Nature’s laboratory, that have come to light in recent times, yet we may afford a passing glimpse at her operations, which can scarcely fail to be interesting and instructive.

We have shown how strata of solid rock are sometimes formed in lakes by the precipitation of lime from a state of solution. Now this process cannot take place in the sea; for though lime is present in the sea, the quantity of carbonic acid with which it is there associated, is far more than sufficient to render its precipitation impossible.[49] But Nature has another contrivance for gathering together the solid elements of her building. The depths of the ocean are teeming with life; and countless tribes of minute animals are furnished with the power of extracting the lime from the waters they inhabit, and of reproducing it under a new form. Sometimes, through this mysterious operation of organic life, the lime is converted into a calcareous shell, like that of the oyster; sometimes into a stony skeleton, as in the case of the numerous families of coral-producing animalcules. After death the soft, fleshy substance of these animals melts away and disappears; but the limestone shells and skeletons remain, accumulating during the long course of ages to an almost incredible extent. And, if we are to believe Geologists, out of these accumulated materials, sometimes preserving their original form and structure, sometimes altered more or less by chemical action, sometimes broken up into fragments by mechanical force, has been produced a very large proportion of the limestone rocks which occur so abundantly in the Crust of the Earth.

No better illustration can be found than the white earthy limestone, familiar to every one under the name of chalk. An undulating stratum of Chalk Rock, attaining not unfrequently a thickness of one thousand feet, may be said, speaking roughly, to underlie the southeastern half of England. Sometimes it appears at the surface: sometimes it dips downward, and forms a kind of great basin, over which are regularly spread out various other groups of Stratified Rocks. On the southern coast it rises to a height of several hundred feet above the level of the sea in a line of perpendicular cliffs, conspicuous from a distance by their dazzling whiteness. But the White Chalk of England is only an insignificant part of a great rock-formation, which may be traced over extensive areas throughout all Europe, from Ireland to the Crimea, from the Baltic Sea to the Bay of Biscay; and which everywhere preserves in a remarkable degree the same mineral character, and presents to the eye the same general appearance.

Now it had often been suggested by Geologists that this wide-spread formation derived its existence chiefly from the accumulated remains of organic life. For in many instances the broken shells of minute animalcules could be distinctly observed to constitute a part of the rock. And even where the organic structure could not be so clearly traced, the carbonate of lime composing the Chalk presented just that appearance which would naturally result from the decomposition of such shells. This theory, however, was long put forward with diffidence and received with incredulity. Even scientific men found it hard to persuade themselves that a solid rock of such great extent and thickness could have been the work of agents apparently so insignificant. But it has been confirmed and illustrated in a very interesting and unexpected manner within the last few years.

When the project of connecting Europe and America by a telegraph cable was first set on foot, it became necessary to ascertain, as far as possible, the general configuration of the ocean bottom and the exact nature of the bed on which the cable was to lie. Accordingly in the year 1857 an expedition was fitted out for this purpose under the command of Captain Dayman; and a careful series of soundings was taken between Valentia, on the West Coast of Kerry, and Trinity Bay on the shores of Newfoundland. It was found that the floor of the ocean between Ireland and America is a vast irregular plain, and that by far the greater part is covered over with a kind of soft mud or ooze. Samples of this ooze were scooped up, even at the most profound depths, by means of an ingenious apparatus attached to the sounding-lines, and brought undisturbed to the surface. Afterward they were carried home to England and submitted for examination to Professor Huxley. The result has been to show that the materials of a limestone rock, resembling in every essential feature the White Chalk of Europe, are being spread out at the present day over an area of immense extent on the floor of the Atlantic Ocean.

With the permission of our readers we shall allow Professor Huxley, as far as may be, to tell his own story.[50] As to the ocean floor itself, “It is,” he says, “a prodigious plain—one of the widest and most even plains in the world. If the sea were drained off, you might drive a wagon all the way from Valentia to Trinity Bay. And, except upon one sharp incline about two hundred miles from Valentia, I am not quite sure that it would even be necessary to put the skid on, so gentle are the ascents and descents upon that long route. From Valentia the road would lie down hill for about two hundred miles to the point at which the bottom is now covered by 1700 fathoms of sea-water. Then would come the central plain, more than a thousand miles wide, the inequalities of the surface of which would be hardly perceptible, though the depth of water upon it now varies from 10,000 to 15,000 feet; and there are places in which Mont Blanc might be sunk without showing its peak above water. Beyond this the ascent on the American side commences, and gradually leads for about three hundred miles, to the Newfoundland shore.”

The central plain here described, which has been since found to extend many hundred miles north and south of the cable line, is covered almost everywhere by that soft, mealy sort of mud of which we have already spoken; and this, it is now confidently believed, is nothing else than a stratum of Chalk Rock in an early stage of formation. When thoroughly dried it assumes a whitish color, and exhibits a texture which even to the superficial observer appears closely to resemble fine chalk. Nay, we are told that if so disposed, one may take a bit of it in his fingers and write with it upon a blackboard. Like chalk, too, when chemically analyzed it is found to be almost pure carbonate of lime.

But there is a yet more striking analogy between the mud of the Atlantic and the White Chalk of Europe. Both have been submitted to the magnifying power of the Microscope; and, after an examination conducted with scrupulous care, a wonderful and almost startling identity of mineral, or rather we should say of organic, composition has been established between them. To the naked eye Chalk is simply a soft, earthy sort of stone. But when a thin transparent slice is placed under the Microscope, the general mass is found to be made up of very minute particles, in which are embedded a vast number of other bodies possessing a well-defined form and structure. These are of various sizes, but on a rough average may be said not to exceed a hundredth of an inch in diameter. Hundreds of thousands of them are sometimes contained in a cubic inch of Chalk, together with countless millions of the more minute granules.

Professor Huxley succeeded in separating these bodies from the mass of granules in which they were embedded, and by examining them apart, he has ascertained still more fully their exact structure and composition. “Each one of them,” he says, “is a beautifully constructed calcareous fabric, made up of a number of chambers communicating freely with one another. They are of various forms. One of the commonest is something like a badly-grown raspberry, being formed of a number of nearly globular chambers of different sizes congregated together. It is called Globigerina; and some specimens of Chalk consist of little else than Globigerinæ and granules.”

Previous to 1857 the Globigerinæ of the Chalk were a matter of no small controversy among Geologists and Naturalists. Some contended that they were the organic remains—the shells or skeletons—of ancient animalcules. Others were disposed to regard them simply as aggregations of lime, which, so to speak, chanced to assume the form of these little chambered bodies; though it was not easy to explain, on this hypothesis, how these chance concretions, however much they varied in size, preserved over the whole of Europe the same exact form and structure. But the controversy is now at an end. The specimens of the Atlantic ooze brought home by Captain Dayman, when examined under the higher powers of the Microscope, are found, like Chalk, to be composed almost entirely of Globigerinæ. And that no doubt may remain as to their organic origin, a portion of the fleshy integument of the little animalcules is seen, in many cases, still adhering to the calcareous skeleton.

“Globigerinæ of every size,” we are told, “from the smallest to the largest, are associated together in the Atlantic mud, and the chambers of many are filled by a soft animal matter. This soft substance is, in fact, the remains of the creature to which the Globigerina shell, or rather skeleton, owes its existence—and which is an animal of the simplest imaginable description. It is, in fact, a mere particle of living jelly, without defined parts of any kind—without a mouth, nerves, muscles, or distinct organs; and only manifesting its vitality to ordinary observation by thrusting out and retracting, from all parts of its surface, long filamentous processes which serve for arms and legs. Yet this amorphous particle, devoid of everything which, in the higher animals we call organs, is capable of feeding, growing, and multiplying; of separating from the ocean the small proportion of carbonate of lime which is dissolved in sea-water; and of building up that substance into a skeleton for itself, according to a pattern which can be imitated by no other known agency.”

That the same process is going on in other parts of the ocean appears by observations made by Sir Leopold M’Clintock during the cruise of the Bulldog in 1860. He discovered that a calcareous ooze having the consistency of putty is spread out over extensive areas between the Faroe Islands and Iceland, and also between Iceland and Greenland. Of this mud about ninety-five per cent. is composed of Globigerinæ, which in some instances were brought up actually living to the surface, and busily engaged in secreting, by their vital powers, carbonate of lime from the waters of the sea.[51]

Professor Huxley goes yet one step further in following out the resemblance between the Chalk Rock that exists in the Crust of the Earth and the stratum of Chalk that is now growing up in the depths of the Atlantic. Not only are the Globigerinæ, of which the one is in great part composed, identical with the animalcules that make up about nine-tenths of the other, but even the minute granules that constitute the residue of each formation, correspond in a very remarkable manner. “In working over the soundings collected by Captain Dayman, I was surprised to find that many of what I have called the Granules of that mud were not, as one might have been tempted to think at first, the mere powder and waste of Globigerinæ, but they had a definite form and size. I termed these bodies Coccoliths, and doubted their organic nature. Doctor Wallich verified my observation, and added the interesting discovery that, not unfrequently, bodies similar to these Coccoliths were aggregated together into spheroids, which he termed Coccospheres. So far as we knew, these bodies, the nature of which is extremely puzzling and problematical, were peculiar to the Atlantic soundings.

“But a few years ago Mr. Sorby, in making a careful examination of the Chalk by means of thin sections and otherwise, observed, as Ehrenberg had done before him, that much of its granular basis possesses a definite form. Comparing these formed particles with those in the Atlantic soundings, he found the two to be identical; and thus proved that the Chalk, like the soundings, contains these mysterious Coccoliths and Coccospheres. Here was a further and a most interesting confirmation, from internal evidence, of the essential identity of the Chalk with modern deep-sea mud.”

We may, therefore, set it down as certain, first, that the formation of Chalk Rock is going on very extensively at the present day; and secondly, that the chief agency employed in its production is no other than the vital action of minute animalcules. This is no longer merely a plausible theory or an ingenious hypothesis: it is simply a matter of fact ascertained by direct observation. If then it is just and philosophical to ascribe like effects to like causes, the conclusion is plain that the White Chalk of Europe came into existence in some far distant age by just such a process as that which is now in operation on the bed of the Atlantic Ocean.

From the Chalk mud of the Atlantic we will now pass to the Coral Reefs that are growing up beneath the waters of the Pacific and the Indian Oceans. Every one has heard of Coral Reefs and Coral Islands; yet we fancy many persons have but vague and indefinite notions about them. We shall, therefore, in the first place, give a brief account of their general appearance, their extent, and their geographical distribution. Afterward we shall give some of the evidence which goes to show that these huge masses of rock owe their existence to the organic powers of minute living animalcules.

The Coral Reef is familiar to the navigator of tropical seas under a great variety of forms, and in many different stages of development. In one case it is a chain of hidden rocks rising not quite to the level of the sea; in another it appears just above the waters, but is washed over by each returning tide; while in another it rises up beyond the reach of the waves, is clothed with luxuriant vegetation, and inhabited by various species of animals, even by man himself. Again there is great diversity of outline among these rocks, whether they are sunk beneath the surface of the waters or lifted above them. But all may be reduced to four classes, of which we propose to give a short description.

First is the Atoll, or lagoon island. It is a circular strip of limestone rock enclosing a shallow lake within, and surrounded by a deep and often unfathomable ocean without. The scene presented by some of these circular reefs is described by travellers as equally striking for its singularity and its beauty. “A strip of land a few hundred yards wide is covered by lofty cocoa-nut trees, above which is the blue vault of heaven. This band of verdure is bounded by a beach of glittering white sand, the outer margin of which is encircled with a ring of snow-white breakers, beyond which are the dark heaving waters of the ocean. The inner beach encloses the still clear water of the lagoon, resting in its greater part on white sand, and, when illuminated by a vertical sun, of a most vivid green.”

These lagoon islands are often found in groups stretching, with little interruption, for many hundred miles across the ocean. The Maldives, for example, which lie a little distance to the southwest of Hindostan, form a continuous chain, running due north and south, four hundred and seventy miles in length and fifty miles in breadth. Each successive link in this chain does not consist, as might be supposed, of a single circular reef, but it is rather a ring of small coral islets, sometimes more than a hundred in number, each of which is itself a perfect Atoll or lagoon island such as we have just described. Of these miniature islets many are from three to five miles in diameter; while the larger rings of which they form a part are from thirty to fifty. The Laccadive islands, a little more to the north, exhibit a similar arrangement, and indeed would seem to be a continuation of the same group. In the Pacific are found some chains of coral islands yet more extensive; as for instance the Dangerous Archipelago, which is upward of eleven hundred miles in length, and from three to four hundred in breadth; but the islands within these spaces are thinly scattered, and insignificant in size.

Sometimes the annular strip of coral rock encloses within itself a lofty island, which rises up from the centre of the lagoon. In this case it is called an Encircling Reef; the lagoon being simply a broad channel surrounding the island in the centre, and encompassed itself by the coral rock. An example occurs in the island of Vanikoro, celebrated for the shipwreck of La Peyrouse, where the Encircling Reef runs at a distance of two or three miles from the shore, the channel between it and the land having a general depth of between two and three hundred feet. The well-known mountainous island of Tahiti in the South Pacific Ocean is also encompassed by an Encircling Reef, from which it is separated by a broad belt of tranquil water.

A third class of Coral Reefs consists of those which run parallel to the shores of continents or great islands, from which they are cut off by a broad channel, to which the sea has free access through certain open passages in the rock. They are called Barrier Reefs; and differ from the former only in this, that they do not surround the land, but run parallel to it at a distance of some miles. The Great Barrier Reef of Australia offers a noble example. It has been described as a huge, massive, submarine wall or terrace, fronting the northeastern coast of that continent, varying from ten to ninety miles in breadth, and extending, with some trifling interruptions, to a length of 1250 miles. Another reef of the same kind, 400 miles in length, faces the western coast of the long narrow island of New Caledonia.

When a chain of Coral rocks approaches close to the shore, so as to leave no intervening channel of deep water, they are called Fringing Reefs; and these constitute the fourth and last class of the Coral formation. They prevail everywhere in tropical regions, and appear as banks of Coral encrusting the rocky shores of islands and continents.

As regards the geographical distribution of Coral Reefs, the first circumstance that claims our notice, is that they are exclusively confined to the warmer regions of the globe. They exist in great profusion within the tropics, and are rarely to be found beyond the thirtieth parallels of latitude on each side of the Equator. The only remarkable exception is in the case of the Bermuda Islands in 32° north latitude; but here, it is to be observed, the ocean is warmed by the waters of the Gulf Stream. Another singular fact is the almost total absence of Coral Reefs from the Atlantic Ocean. In fact, the Bermudas, we believe, constitute here again the only exception. The Pacific, on the contrary, is wonderfully productive of coral; also the Indian Ocean, the Persian and Arabian Gulfs, and the Red Sea.

It may gratify, perhaps, the curiosity of some readers, if we add a word on the Red Coral which is now so favorite an ornament in the fashionable world. Though it never attains to the magnitude of those reefs and islands we have been describing, it partakes nevertheless of the same peculiar structure; and no doubt is entertained that, like them, it derives its existence from animal life, in the manner we shall presently explain. It is produced chiefly in the Mediterranean, in the Red Sea, and in the Persian Gulf; and is brought up from the great depths by means of a grappling apparatus attached to boats. The largest pieces have a shrub-like branching form, and are supposed to grow to the height of one foot in about eight years.[52]

So much for the existence of the Coral Formation. Next comes the question of its origin, with which, of course, we are chiefly concerned. It is now the received belief of all distinguished Naturalists, that these huge and wide-spread masses of limestone rock, against which the breakers of the ocean are ever thundering in vain, are the work of tiny marine animalcules, and chiefly of those seemingly insignificant creatures known by the name of Polyps or Zoophytes. The Zoophyte, they tell us, is a mason who himself produces the stones that he employs in his building. “He has neither plane, nor chisel, nor trowel; there is no sound of hammer in his city. He erects mighty and enduring edifices, yet has no mechanical power by which to raise his rocks to their summits. He can answer thee nothing—no tongue, no eyes, no hands, no brains has he—yet from the caves of old ocean has he raised that which fills you with admiration.”[53] Surely if all this be true, these countless myriads of animalcules call aloud to us from the depths of the ocean in language that cannot be mistaken: “Know ye that the Lord He is God; it is He that hath made us, and not we ourselves.”[54]

The Zoophyte belongs to the simplest form of the animal creation. Its body consists merely of a pouch or stomach, with tentacles arranged round the margin, which it can extend at pleasure to supply itself with food. In many species the individuals grow together on a common stem, from which new members are constantly shooting forth like buds from the branches of a tree. Hence the origin of the name Zoophyte, which literally means a plant-like animal. The common stem on which they grow is sometimes composed of a horny substance, but more generally it is pure carbonate of lime, which they secrete by the powers of organic action from the waters of the sea. It forms, therefore, a kind of internal skeleton or framework, to which the soft, gelatinous parts of the animal adhere, pretty much as, in the case of other animals, the flesh adheres to the bones. Thus we have, as it were, a community of living creatures, growing together upon one common stony framework, called a Polypidom or Polyp edifice, which they themselves build by the very fact of living.

Fig. 4.—Campanularia Gelatinosa.

Fig. 5.—Gorgonia Patula.

The peculiar structure of these wonderful little communities may perhaps be made more intelligible by the aid of a few illustrations. Figure 4 exhibits the branching skeleton and, at the extremities of the branches, the several Polyps by whose vital action the skeleton has been constructed. Some of the animalcules are shown in a state of activity, with their tiny arms spread out in search of food: others are withdrawn within their cells, and appear in a state of repose. This species of Zoophyte, which is highly magnified in the figure, flourishes abundantly on the shores of Ireland and England. It has received the name of Campanularia, from the bell-like form of its cells. Our next cut represents a Gorgonia from the Mediterranean, which is also considerably magnified. The fleshy integument of this specimen is of a brilliant red color: the Polyps are arranged in rows on each side of the stem, and are shown in a state of expansion.

Fig. 6.—Frustra Pilosa.

Fig. 7.—Madrepora Plantaginea.

A mass of Coral animalcules, which are known by the name of Frustra Pilosa, is represented of the natural size in Figure 6. To the naked eye it seems like a piece of fine net-work, disposed around a fragment of sea-weed, which may be observed protruding in the upper part of our illustration. With the aid of an ordinary magnifier the net-like surface is seen to abound in minute pores arranged with much regularity. Each of these pores is the cell of a Zoophyte. And if a fragment of Frustra be examined with a powerful microscope, when immersed in sea-water, the curious little inhabitants themselves may be seen darting in and out of their cells, expanding and contracting their long feelers, and exhibiting altogether a wonderful activity. In the adjoining woodcut, Figure 7, is shown another interesting species of the arborescent Zoophyte. It belongs to the family of Madrepores, and abounds in almost all Coral Reefs. Alive under water it appears clothed in a gelatinous coating of rich and varied hues. But when removed from its native element this gelatinous coating, which is the living animal substance, quickly melts away; and, in some instances, runs off from the calcareous skeleton in a kind of watery slime.

Fig. 8.—Corallium Rubrum.

A good idea of the celebrated red and pink Coral of commerce, so much admired for its brilliant color, and the high polish of which it is susceptible, may be gathered from our next illustration. As in the other species to which we have referred, the calcareous skeleton is enveloped in a living gelatinous substance, from which the Zoophytes seem to shoot out like buds from the bark of a tree. Several of these animalcules are exhibited in our figure, in the active enjoyment of life; gathering in, with their expanded tentacles, the elements of their stony edifice from the surrounding waters. After death the fleshy integument is wasted away by the action of the sea; and the framework that remains behind, washed ashore by the waves, or hooked up by the coral fisherman, is wrought into brooches, bracelets, necklaces, and other ornaments of various kinds.

Not a few varieties of the Coral-producing Zoophytes are to be found in actual living reality on our own coasts, where the curious student may examine for himself their habits and general structure. But it is in the warmer regions of the Earth that they are developed in the greatest numbers, and decked in the brightest hues. Those who have seen them through the crystal waters of tropical seas, swarming in countless multitudes on the clear white sand below, speak with enthusiasm of their luxuriant profusion and of their striking beauty. Combining to a picturesque elegance of form a rich variety and pleasing harmony of colors, they present to the eye a scene which has been compared to a magnificent garden, laid out in diverse beds of rare and splendid flowers.

So far we have spoken only of the Polypidom, that is to say, the community of Polyps living together on a common stem of their own construction. Now this Polypidom is the first element of the Coral Reef. In some species of Zoophytes, the Red Coral for instance, the calcareous stem never attains a size greater than that of a diminutive shrub. But in others, and they are very numerous, especially in tropical seas, there seems to be no limit to the growth of the solid stony framework. As the existing generation of Zoophytes is dying out, new individuals are ever budding forth, which continue unceasingly to secrete carbonate of lime, as their predecessors had done before them, from the waters of the ocean; and thus the tree-like form spreads its branching arms on every side, growing upward and outward day by day. The soft gelatinous parts of those generations that have passed away are, in a short time, dissolved, and the stony skeleton alone remains behind. Ages roll on: the calcareous framework, ever increasing in size, becomes at length a formidable rock; and this rock is the Coral Reef.

Let it not be supposed we are here advancing a theory: we are only stating a fact that has been established by close and repeated observations. All the phenomena exhibited in the development of the Polypidom, are exhibited no less plainly in every Coral Reef that has yet been examined. On the surface of the Reef are the living Zoophytes, clinging to the calcareous skeleton which is ever growing larger through the unconscious action of their vital functions; while immediately beneath may be seen the same stony skeleton, already divested of its fleshy integument, and beginning to assume the appearance of compact and massive rock. We can behold, therefore, the mason at work on the upper story of his building, and the structure already finished below. And so we have little less than ocular demonstration that the Coral Reef is the work of the Zoophyte.

It must not be supposed, however, that in every part of the Coral Reef, the form and outline of the stony skeleton are exactly preserved. Fragments of the rock are broken off by the force of the waves, and mixed up with the comminuted shells of oysters, mussels, and other crustaceous animals inhabiting the same waters. In this way a sort of calcareous gravel, sometimes a calcareous paste, is formed, which fills up the interstices, and connects the tree-like coral into a compact rock.

We have yet to explain how the Coral Reefs come, in many cases, to rise above the surface of the ocean, and to form dry land: for it has been found that the reef-building Zoophytes require to be continually immersed in salt water, and therefore, by their own efforts, they cannot raise their structure above the ordinary level of the sea. This question was for a long time involved in obscurity; but it has been cleared up by the actual observations of Naturalists in modern times. The following description, which is given to us by Chamisso, the companion of Kotzebue on his voyages, will convey a good idea of the process by which a sunken reef is often converted into a smiling, fruitful island. “When the reef is of such a height that it remains almost dry at low water, the corals leave off building. Above this line a continuous mass of solid stone is seen, composed of the shells of mollusks and echini, with their broken-off prickles and fragments of coral, united by calcareous sand, produced by the pulverization of shells. The heat of the sun often penetrates the mass of stone when it is dry, so that it splits in many places, and the force of the waves is thereby enabled to separate and lift blocks of coral, frequently six feet long and three or four in thickness, and throw them upon the reef, by which means the ridge becomes at length so high that it is covered only during some seasons of the year by spring tides. After this the calcareous sand lies undisturbed, and offers to the seeds of trees and plants cast upon it by the waves, a soil upon which they rapidly grow, to overshadow its dazzling white surface. Entire trunks of trees, which are carried by the rivers from other countries and islands, find here at length a resting place after their long wanderings: with these come some small animals, such as insects and lizards, as the first inhabitants. Even before the trees form a wood, the sea-birds nestle here; stray land-birds take refuge in the bushes; and, at a much later period, when the work has been long since completed, man appears and builds his hut on the fruitful soil.”[55]

Another question that seems to call for some explanation is suggested by the well-known habits of the Zoophytes themselves. From the observations of Kotzebue and Darwin it appears that those species which are most effective in the construction of Reefs cannot flourish at a greater depth than twenty or thirty fathoms; whereas the coral rocks rise up in many cases from the bottom of an unfathomable ocean. How, then, it may be asked, have the foundations of these wonderful structures been laid? This question opens a wide field for philosophical speculation; and we freely admit that no theory of Coral Reefs can be regarded as complete and satisfactory, which does not furnish a reasonable answer. But so far as the purpose of our argument is concerned, it is quite sufficient if a stratum of solid limestone, twenty fathoms thick, has been formed mainly through the agency of these minute animalcules. And this conclusion, so abundantly demonstrated by facts, is left quite untouched by the difficulty to which we now refer.

It will be interesting, however, to notice in passing the explanation of this phenomenon first suggested by Mr. Darwin, and now very generally accepted. He maintains that the whole Coral Reef—foundations and superstructure alike—is, in most cases, the result entirely of organic agency. The reef-building Zoophyte always begins his labors in water that is comparatively shallow. But as he is building upward, it often happens that the bed of the sea is sinking downward in pretty nearly the same proportion; and thus the reef is ever increasing in height from its original base, while the living mass of Zoophytes on its upper surface remains in about the same depth of water as when the building first began.

This theory is supported by a vast amount of curious and ingenious reasoning. In the first place, there is nothing more remarkable in the physical conformation of the Globe, than the immense predominance of water over land throughout those extensive tracts of ocean where Coral Reefs abound. Now this is just what we should naturally expect if the hypothesis of Mr. Darwin were admitted; for wherever the Crust of the Earth has been subsiding for many ages on a large scale, the domain of the sea must of necessity have been considerably enlarged, and that of the land contracted in proportion. Again, this hypothesis will be found to harmonize most perfectly with all the phenomena of Fringing Reefs, Barrier Reefs, Encircling Reefs, and Lagoon Islands. The Fringing Reef represents, as it were, the first stage of progress. The building operations have just commenced near the shore of some island or continent, and but little space intervenes between the land and the incrusting wall of coral. Then, as the Crust of the Earth gradually subsides, the water encroaches on the land, and forms a channel between it and the reef. Meanwhile the Zoophytes are at work, and the coral rock is growing upward as the foundation on which it rests is sinking downward: each year it is higher from the bed of the sea, and yet no nearer to the surface of the waters. And when at length the channel, which is ever growing wider and wider, has reached a certain limit, the Fringing Reef becomes a Barrier Reef, or if it encompasses an island, an Encircling Reef. Lastly, the Encircling Reef will finally become a Lagoon Island, when the highest peaks of the land it encloses have slowly disappeared beneath the surface of the waters.

In confirmation of this reasoning Mr. Darwin has pointed out numerous examples to illustrate each intermediate stage through which, according to his hypothesis, the Coral Reef must pass in the progress of its construction. He traces the gradual transition from the low bank of coral incrusting a rocky shore to the Encircling Reef that compasses round a lofty island, like Tahiti, with a broad channel between. Then he shows how this channel insensibly becomes wider and wider, encroaching more and more upon the land, until at length only a few high peaks remain above water. Finally he leads us on to the case of a perfect Atoll, within which no trace of land remains to be seen; and the channel, now become a lagoon, is encompassed by a Reef of Coral Rock that rises steeply from an unfathomed ocean.

We do not mean to dwell upon this ingenious speculation, which would carry us too far from the object at which we are aiming. It seems to us, however, that the arguments in its favor are at least deserving of careful consideration; and we may add that they receive new strength from the facts we shall have occasion hereafter to bring forward, when we come to speak of the undulating movements to which the Crust of the Earth has been subject at many different times, and in many different localities, even within the historic period.

The formation and structure of existing Coral Reefs being once fairly established, Geologists have little difficulty in ascribing a similar origin to many of the limestone strata that are found in the Crust of the Earth. For though the internal texture has been considerably modified in the long course of ages, by chemical and other influences, nevertheless the stony skeletons of the reef-building Zoophytes can be distinctly recognized in great abundance. Indeed it is not an uncommon thing to meet with limestone rock exhibiting plainly to the eye all the appearance of Coral Reefs lifted up from the bed of the ocean. “The Oolite,” says Doctor Mantell, “abounds in corals, and contains beds of limestone which are merely coral reefs that have undergone no change but that of elevation from the bottom of the deep, and the consolidation of their materials. The Coral-rag of Wilts presents in fact all the characters of modern reefs: the polypifera belong chiefly to the Astræidæ, the genera of which family principally contribute to the formations now going on in the Pacific. Shells, echinoderms, teeth, and bones of fishes, and other marine exuviæ, occupy the interstices between the corals, and the whole is consolidated by sand and gravel, held together in some instances by calcareous, in others by siliceous infiltrations. Those who have visited districts where the Coral-rag forms the immediate subsoil, and is exposed to view in the quarries or in natural sections, must have been struck with the resemblance of these rocks to modern coral banks.”[56]

Even in many of our finest marbles the coral skeletons may be traced distinctly enough, and contribute not a little to that variegated color which is so much admired. Nay, it is recorded by Mr. Parkinson that he discovered in a piece of solid marble, the animal membrane itself by which the lime was originally abstracted from the sea. He immersed the marble in dilute muriatic acid; and he relates with delight how, as the calcareous earth dissolved, and the carbonic acid gas escaped, he observed the animal tissue begin distinctly to appear in the form of light, elastic membranes.[57]

CHAPTER IX.
STRATIFIED ROCKS OF ORGANIC ORIGIN—ILLUSTRATIONS FROM VEGETABLE LIFE.

Origin of coal—Evident traces of plants and trees in coal-mines—Coal made up of the same elements as wood—Beds of coal found resting upon clay in which are preserved the roots of trees—Insensible transition from wood to coal—Forest-covered swamps—Accumulations of drift-wood in lakes and estuaries—Peat bogs—Beds of Lignite—Seams of pure coal with half-carbonized trees, some lying prostrate, some standing erect—Summary of the argument hitherto pursued—Objection to this argument from the Omnipotence of God—Answer to the objection.

As animals, by organic action, extract lime from the waters of the ocean they inhabit, which, being converted in the first instance into minute shells, or stony skeletons, afterward passes into a compact and solid rock, so in like manner do plants and trees extract carbon from the atmosphere in which they vegetate, and convert it into coal. No reasonable doubt can now be entertained that coal derives its existence, almost entirely, from the woody tissue of sunken swamps and forests. Though the nature of the process by which this transformation takes place, is yet but imperfectly understood, and is, indeed, at the present moment a subject of much discussion and controversy, nevertheless the fact that the change has taken place is fully accepted by all as an established truth, and is supported by an accumulation of evidence which it is not easy to resist.

The first circumstance to which we shall call attention, is the wonderful profusion of vegetable life that is always associated with coal. Every one who has descended at any time into a coal mine, or who has examined the specimens usually exhibited in a well-furnished museum, must have been struck by the countless forms of trees and plants, which still remain vividly impressed on this black and unsightly mineral. Dr. Buckland has described this phenomenon with much vigor and beauty in his celebrated Bridgewater Treatise: “The finest example I have ever witnessed is that of the coal mines of Bohemia just mentioned. The most elaborate imitations of living foliage upon the painted ceilings of Italian palaces, bear no comparison with the beauteous profusion of extinct vegetable forms with which the galleries of these instructive coal mines are overhung. The roof is covered as with a canopy of gorgeous tapestry, enriched with festoons of most graceful foliage, flung in wild irregular profusion over every portion of its surface. The effect is heightened by the contrast of the coal-black color of these vegetables with the light ground-work of the rock to which they are attached. The spectator feels himself transported, as if by enchantment, into the forests of another world; he beholds trees of forms and characters now unknown upon the surface of the earth, presented to his senses almost in the beauty and vigor of their primeval life; their scaly stems and bending branches, with their delicate apparatus of foliage, are all spread forth before him, little impaired by the lapse of countless ages, and bearing faithful records of extinct systems of vegetation, which began and terminated in times of which these relics are the infallible historians.”