PAL^ONTOLOGICAL COLl.WTIONS J43 



age. A little reflection will explain why, along the rising 

 coast of the western side of South Ameiica, no extensive 

 formations with recent or tertiary remains can anywhere be 

 found, though the supply of sediment must for ages havQ 

 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 tha t sediment must be accumu- 

 lat ed in ext remely thick, soird^or extensive m asses, in order 

 to withstand the incessant act ion "oF'the waves, when first 

 upraised and during successive oscillations of level, as well as 

 the subsequent subaerial degradation. Such thick and ex- 

 tensTve^'"Sx:"cumulations 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 _J 

 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 thick- 

 ness and extent over a shallow bottom, if it continue slowly 

 to subside. In this latter case, as long as the rate of subsi- 

 dence 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 denuda- 

 tion, 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 forma- 

 tion, 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 degradation as it has as yet 

 suffered, but which will hardly last to a distant geological 



