chap. xii. C one hiferous Deposits. 415 



the lower plains of Patagonia probably belong to this 

 same period, but neither are fossiliferous : it also so 

 happens that the great Pampean formation does not in- 

 clude, with the exception of the Azara, any mollusca. 

 There cannot be the smallest doubt that the upraised 

 shells along the shores of the Atlantic and Pacific, 

 whether lying on the bare surface, or embedded in mould 

 or in sand-hillocks, will in the course of ages be destroyed 

 by alluvial action : this probably will be the case even 

 with the calcareous beds of Coquimbo, so liable to dis- 

 solution by rain-water. If we take into consideration 

 the probability of oscillations of level and the conse- 

 quent action of the tidal waves at different heights, 

 their destruction will appear almost certain. Looking 

 to an epoch as far distant in futurity as we now are 

 from the past Miocene period, there seems to me scarcely 

 a chance, under existing conditions, of the numerous 

 shells now living in those zones of depths most fertile 

 in life, and found exclusively on the western and south- 

 eastern coasts of South America, being preserved to this 

 imaginary distant epoch. A whole conchological series 

 will in time be swept away, with no memorials of their 

 existence preserved in the earth's crust. 



Can any light be thrown on this remarkable absence 

 of recent conchiferous deposits on these coasts, on which, 

 at an ancient tertiary epoch, strata abounding with 

 organic remains were extensively accumulated ? I think 

 there can, namely, by considering the conditions neces- 

 sary for the preservation of a formation to a distant 

 age. Looking to the enormous amount of denudation 

 which on all sides of us has been effected, — as evi- 

 denced by the lofty cliffs cutting off on so many coasts 

 horizontal and once far-extended strata of no great 

 antiquity (as in the case of Patagonia), — as evidenced 

 by the level surface of the ground on both sides of great 



