CHAP. X.] PAL^ONTOLOGICAL COLLECTIONS. 255 



derived, accord with the belief of vast intervals of time having 

 elapsed between each formation. 



We can, I think, see why the geological formations of each 

 region are almost invariably intermittent ; that is, have not 

 followed each other in close sequence. Scarcely any fact struck 

 me more when examining many hundred miles of the South 

 American coasts, which have been upraised several hundred feet 

 within the recent period, than the absence of any recent deposits 

 sufficiently extensive to last for even a short geological period. 

 Along the whole west coast, which is inhabited by a peculiar 

 marine fauna, tertiary beds are so poorly developed, that no 

 record of several 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 sediment must for 

 ages have 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 con- 

 tinually 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 accumulated 

 in extremely thick, solid, or extensive masses, in order to with- 

 stand 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 neighbourhood 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 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 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 con- 

 clusion that it was accumulated during subsidence. I may add, 



