322 



THE POORNESS OF 



eral successive and peculiar marine faunas will probably be 

 preserved to a distant age. A little reflection will explain 

 why, along the rising coast of the western side of South 

 America, no extensive formations with recent or tertiary 

 remains can anywhere be found, though the supply of sedi- 

 ment must for ages have been great, from the enormous 

 degradation of the coast rocks and from the muddy streams 

 entering the sea. The explanation, no doubt, is that the 

 littoral and sublittoral 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 



v/aves. 



"We may, I think, conclude that sediment must be ac- 

 cumulated in extremely thick, solid, 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 extensive accumulations 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 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 neighborhood 

 during the period of its accumulation. Or sediment may 

 be deposited to any thickness and extent over a shallow 

 bottom, if it continue slowly to subside. In this latter case, 

 as long as the rate of subsidence and the supply of sedi- 

 ment nearly balance each other, the sea will remain shallow 

 and favorable for many and varied forms, and thus a rich 

 fossiliferous formation, thick enough, when upraised, to 

 resist a large amount of denudation, 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 formation, 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 de- 

 gradation as it has as yet suffered, but which will hardly 



