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. [Illustration] [Illustration] _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.