Becent Alluvial Deposits. 115 



second observation made by M. D'Orbigny is, that the recent 

 shells of the raised beaches of the two coasts of South Ame- 

 rica, are all in the natural position in which they lived, the 

 acephala having their two valves united and placed vertically. 

 This fact supports the idea of a sudden movement, and not a 

 gradual elevation of the coast, as has been supposed by some 

 authors. The study of the present coasts proves, that when 

 the sea gradually abandons a shore, it leaves, everywhere on 

 the uncovered portion, shells, which are exposed for a long 

 period to the incessant movement of the waves, and which 

 soon become more or less rounded, no one remaining in its 

 original position. Nothing of this kind presenting itself in 

 the elevated deposits visited by M. D'Orbigny, it seems to him 

 evident that these shells had been suddenly and instantaneously 

 raised from the bottom of the sea to the height which they 

 now occupy. This leads him to conclude that a sudden 

 movement has taken place on the surface of America, whose 

 traces are preserved, on the one hand, in the terrestrial allu- 

 via J and, on the other, in the elevation of the marine beds of 

 the coasts of the two oceans. 



The terrestrial alluvia and the marine beds which cover the 

 Pampean tertiary formation, would thus be contemporaneous 

 with the species which now live on the globe ; while the Pam- 

 pean formation itself, from its terrestrial fauna being very dif- 

 ferent from the fauna of the present day, would belong to a 

 very different anterior epoch, characterised by large animals 

 of a lost race. Thus, while on the one hand, the Pampean 

 formation seems to refer to a great event which destroyed the 

 megatherium and the mylodon, it would seem equally pro- 

 bable, that since the existence of the present fauna there 

 have been general and transient causes which, at the same 

 time that they elevated a portion of the shore, as well of the 

 Atlantic as of the Pacific Ocean, containing organised bodies 

 identical with those living at the present day, have also de- 

 nuded the plateaux and mountains, and have transported to 

 the Pampas and to the plains of Moxos those immense masses 

 of alluvial matter which are there observed, and whose mo- 

 dern origin is indicated, as we have already mentioned, by 



