CiiAP. IX. PALEONTOLOGICAL COLLECTIONS. 075 



elapsed lictwoL'u Iho consecutive formations, we may infer that 

 this could nowhere be ascertained. The frequent and great 

 changes in the miiieralogieal composition of consecutive forma- 

 tions, generall}^ implying great changes in the geography of 

 the surronnding lands, whence the sediment has been derived, 

 accords with tlie belief of vast intervals of time having elapsed 

 between each formation. 



But we can, I thinlc, see wliy 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 uj)raised several hun- 

 dred feet Avithin the recent period, than the absence of any 

 recent deposits sufliciently extensive to last for even a short 

 geological period. Along the whole west coast, Avhich is in- 

 liabited by a peculiar marine fauna, tertiary beds are so poorly 

 cTeveloped, that no 'record of several successive and peculiar 

 marine faunas will probably be preserved to a distant age. A 

 little refl(^ction will explain Avhy, 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 exjilanation, no doubt, is, that 

 the littoral and sub-littoral deposits are continually worn away 

 as soon as they nro, lirought up by the slow and gradual rising 

 of the land witliin the grinding action of the coast-waves. 



We niay, I think, conclude that sediment must be accumu- 

 lated in extremely thick, solid, or extensive masses, in order 

 to withstand the incessant action of tlie waves, wlien lirst up- 

 raised and during successive oscillations of level, as well as 

 the subsequent subaerial degradation. Such thick and exten- 

 sive accumulations of sediment may be formed in two waj-s; 

 either in profound depths of the sea, in which case the bottom 

 will not be inhabited by so many and such varied f(jrms of life 

 as the more shallow seas; and the* mass Avhen upraised will 

 give an imperfect record of the organisms which existed 

 tliroughout the world during the period of its accunuilation; 

 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 hmg as the rate of subsidence and the suj> 

 ply of sediment nearly balance each other, the sea will remain 

 shallow and favorable for many and varied fonns, and thus a 



