10 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



From the principle already adduced, namely, the regular and gradual eleva- 

 tion of this part of the continent, I should have judged from the small altitude of 

 the beds at Punta Alta, that the formation had not been very ancient. The con- 

 clusion here arrived at, concerning the age of these fossil mammalia, is nearly the 

 same, with that, inferred respecting those entombed in the Pampas ; and it will 

 hereafter be shown, that some of the species are common to the two districts. 

 We may suppose, that whilst the ancient rivers of the Plata occasionally carried 

 down the carcasses of animals existing in that country, and deposited them in 

 the mud of the estuary; other animals inhabited the plains round the Sierra de la 

 Ventana, and that lesser streams, acting together with the currents of a large 

 bay, drifted their remains towards a point, where sand and shingle were accumu- 

 lating into a shoal. The whole area has since been elevated : the estuary mud 

 of the former rivers has been converted into wide and level plains ; and the shoals 

 of the ancient Bahia Blanca now form low headlands on the present coast. 



The third locality, which I have to specify, is Port St. Julian, in latitude 49° 

 15' on the coast of Southern Patagonia. The tertiary plains of that country 

 are modelled into a succession of broad and level terraces, which abut one above 

 the other ; and where they approach the coast, are generally cut off by a line of 

 precipitous cliff. The whole surface is thickly covered by a bed of gravel, com- 

 posed of various kinds of porphyries, and probably originating from rocks situ- 

 ated within the Cordillera. The lower part of the formation consists of several 

 varieties of sandstone, and contains many fossil shells, the greater number of 

 which are not found in a living state. 



The south side of Port St. Julian is formed by a spit of flat land, of nearly a 

 hundred feet in height ; and on its surface existing species of littoral shells are 

 abundantly scattered. The gravel is there covered (a circumstance which I did 

 not observe in scarcely any other locality) by a thin but irregular bed of a sandy 

 or loamy soil, which likewise fills up hollows or channels worn through it. In the 

 largest of these channels the remains of the single fossil quadruped, which was 

 here discovered, were embedded. The skeleton probably was at first perfect ; 

 but the sea having washed away part of the cliff, has removed many of the 

 bones, — the remaining ones, however, still occupying their proper relative position 

 to each other. I am inclined to attribute the origin of this earthy matter, to the 





