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.