Seite - 343 - in The Origin of Species

PAL^ONTOLOGICAL COLl.WTIONS J43 age. A little reflection will explain why, along the rising coast of the western side of South Ameiica, no extensive formations with recent or tertiary remains can anywhere be found, though the supply of sediment must for ages havQ been great, from the enormous degradation of the coast-rocks and from muddy streams entering the sea. The explanation,' no doubt, is, that the littoral and sub-littoral deposits are continually worn away, as soon as they are brought up by the slow and gradual rising of the land within the grinding action of the coast-waves. We may, I think, conclude that sediment must be accumu- lated in extremely thick, soird^or extensive masses, in order to withstand the incessant action "oF'the waves, when first upraised and during successive oscillations of level, as well as the subsequent subaerial degradation. Such thick and ex- tensTve^'"Sx:"cumulations of sediment may be formed in two ' ways ; either in profound depths of the sea, in which case~~] the bottom will not be inhabited by so many and such varied_J forms of life, as the more shallow seas; and the mass when upraised will give an imperfect record of the organisms which existed in the neighbourhood during the period of its accumulation. Or, sediment may be deposited to any thick- ness and extent over a shallow bottom, if it continue slowly to subside. In this latter case, as long as the rate of subsi- dence and the supply of sediment nearly balance each other, ^ the sea will remain shallow and favourable for many and varied forms, and thus a rich fossiliferous formation, thick enough, when upraised, to resist a large amount of denuda- tion, may be formed. I am convinced that nearly all our ancient formations, which are throughout the greater part of their thickness rich in fossils, have thus been formed during subsidence. Since publishing my views on this subject in 1845, I have watched the progress of Geology, and have been surprised to note how author after author, in treating of this or that great forma- tion, has come to the conclusion that it was accumulated during subsidence. I may add, that the only ancient tertiary formation on the west coast of South America, which has been bulky enough to resist such degradation as it has as yet suffered, but which will hardly last to a distant geological