6 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 





skeletons would thus become entombed in the estuary mud which was then tran- 

 quilly accumulating. Nothing less than a long succession of such accidents can 

 account for the vast number of remains now found buried. As their exposure 

 has invariably been due to the intersection of the plain by the banks of some 

 stream, it is not making an extravagant assertion, to say, that any line whatever 

 drawn across the Pampas would probably cross the skeleton of some extinct 



animal. 



At Bajada, a passage, as I have stated, may be traced upwards from the beds 

 containing marine shells, to the estuary mud with the bones of land animals. 

 In another locality a bed of the same mineralogical nature with the Pampas depo- 

 sit, underlies clay containing large oysters and other shells, apparently the same 

 with those at Bajada. We may, therefore, conclude that at the period when the 

 Area, Venus, and Oyster were living, the physical condition of the surrounding 



I 



country was nearly the same, as at the time when the remains of the mammalia 

 were embedded ; and therefore that these shells and the extinct quadrupeds pro- 

 bably either co-existed, or that the interval between their respective existences 



was, in a geological point of view, extremely short. In this part of South 



America there is reason to believe that the movements of the land have been so 

 regular, that the period of its elevation may be taken as an element in consider- 

 ing the age of any deposit. The circumstance, therefore, that the beds immedi- 



* 



ately bordering the Plata, contain very nearly the same species of molluscs, with 



- 



those now existing in the neighbouring sea, harmonizes perfectly with the more 

 ancient (though really modern) tertiary character of the fossils underlying the 

 Pampas deposit at Bajada, situated at a greater height, and at a considerable dis- 

 tance in the interior. I feel little doubt that the final extinction of the several 

 large quadrupeds of La Plata did not take place, until the time when the sea was 

 peopled with all, or nearly all, its present inhabitants. 



Bahia Blanca, situated in latitude 39°, and about 250 miles south of the 

 Plata, constitutes the second district, in which I found the remains of quadrupeds. 

 This large bay is nearly surrounded by very low land, on which successive lines 

 of sand dunes mark in many parts the retreat of the water. At some distance 

 inland a formation of highly indurated marl, passing into limestone, forms an 

 escarpment. Beyond this, rocks of the same character extend over a wide and 



