Chap. X.] PAL^ONTOLOGICAL COLLECTIONS. 61 



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 de- 

 posits 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 ter- 

 tiary 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 

 continually worn away, as soon as they are brought up 

 by the slow and gradual rising of the land within the 

 grmding 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 withstand the incessant action of 

 the waves, when first upraised and during successive 

 oscillations of level, as well as the subsequent subaerial 

 de""radation. Such thick and extensive accumulations 

 of sediment may be formed in two ways ; either in pro- 

 found 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 



