"chap. xii. Recent Conchiferous Formations. 417 



ordeal of the beach ; and on most coasts, the waves on 

 the beach tend to wear down and disperse every object 

 exposed to their action. Now, both on the south-eastern 

 and western shores of South America, we have had clear 

 proofs that the land has been slowly rising, and in the 

 long lines of lofty cliffs, we have seen that the tendency 

 of the sea is almost everywhere to eat into the land. 

 Considering these facts, it ceases, I think, to be sur- 

 prising, that extensive recent conchiferous deposits are 

 entirely absent on the southern and western shores of 

 America. 



Let us take the one remaining case, of the bed of 

 the sea slowly subsiding during a length of time, whilst 

 sediment has gone on being deposited. It is evident 

 that strata might thus accumulate to any thickness, each 

 stratum being deposited in shallow water, and conse- 

 quently abounding with those shells which cannot live 

 at great depths : the pressure, also, I may observe, of 

 each fresh bed would aid in consolidating all the lower 

 ones. Even on a rather steep coast, though such must 

 ever be unfavourable to widely extended deposits, the 

 formations would always tend to increase in breadth 

 from the water encroaching on the land. Hence we 

 may admit that periods of slow subsidence will com- 

 monly be most favourable to the accumulation of 

 conchiferous deposits, of sufficient thickness, extension, 

 and hardness, to resist the average powers of denudation. 



We have seen that at an ancient tertiary epoch, 

 fossiliferous deposits were extensively deposited on the 

 coasts of South America ; and it is a very interesting 

 fact, that there is evidence that these ancient tertiary 

 beds were deposited during a period of subsidence. 

 Thus, at Navidad, the strata are about 800 feet in thick- 

 ness, and the fossil shells are abundant both at the level 

 of the sea and some way up the cliffs ; having sent a 



