IMPERFECTION OF GEOLOGICAL RECORD 67 



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 ex- 

 tensive 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 for- 

 mations 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 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 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 

 jea, 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 neighbor- 

 hood during the period of its accumulation. Or, sedi- 

 ment may be deposited to any thickness and extent over 

 a shallow bottom, if it continue slowly to subside. In 



