In the third place, all the stratified rocks appear to have been formed out of the fragments of other rocks, worn down by the action of water and atmospheric agencies. This is particularly true of that large proportion of these rocks which contain the remains of animals and plants. The mud, sand, and gravel of which these are mostly composed, must have been worn from rocks previously existing, and have been transported into lakes, and the ocean, as the same process is now going on. There the animals and plants, which died in the waters, and were transported thither by rivers, must have been buried; next, the rocks must have been hardened into stone, by admixture with lime, or iron, or by internal heat; and, finally, have been raised above the waters, so as to become dry land. Beds of limestone are interstratified with those of shale, sandstone, and conglomerate; but these form only a small proportion of the whole, and, besides, were mostly formed in an analogous manner, though by agencies more decidedly chemical. Now, for the most part, this process of forming rocks by the accumulation of mud, sand, and gravel is very slow. In general, such accumulations, at the bottom of lakes and the ocean, do not increase more than a few inches in a century. During violent floods, indeed, and in a few limited spots, the accumulation is much more rapid; as in the Lake of Geneva, through which the Rhone, loaded with detritus from the Alps, passes, where a delta has been formed two miles long and nine hundred feet thick, within eight hundred years.[7] And occasionally such rapid depositions probably took place while the older rocks were in the course of formation. But in general, the work seems to have gone on as slowly as it usually does at present. Yet, in the fourth place, there must have been time enough since the creation to deposit at least ten miles of rocks in perpendicular thickness, in the manner that has been described. For the stratified rocks are at least of that thickness in Europe, and in this country much thicker; or, if we regard only the fossiliferous strata as thus deposited, (since some geologists might hesitate to admit that the non-fossiliferous rocks were thus produced,) these are six and a half miles thick in Europe, and still thicker in this country. How immense a period was requisite for such a work! Some do, indeed, contend that the work, in all cases, as we have allowed it in a few, may have been vastly more rapid than at the present day. But the manner in which the materials are arranged, and especially the preservation of the most delicate parts of the organic remains, often in the very position in which the animals died, show the quiet and slow manner in which the process went on.