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.