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.