114 Geology of South America. 



hKj of Monte Video itself, but identical with those of the 

 maritime coasts, at a distance of 74 miles nearer the mouth of 

 the river. M. D'Orbigny observed in the neighbourhood of 

 San-Pedro, on the tosca plains, about thirty yards above the 

 coasts of the Parana, small mounds, about two or three yards 

 high, having an elongated form, and, generally speaking, the 

 same direction as the course of the Parana. These mounds 

 are composed of very fine sand, and are so filled with shells 

 that they have received from the inhabitants the name of 

 concMllas, These shells belong to the species Azara labiata, 

 which no longer lives near San-Pedro, and is not met with, 

 descending the river, sooner than at Riacho-de-las-Palmas, not 

 far from Buenos Ayres ; it abounds in the fresh and brackish 

 water of the mouth of the Plata. These mounds, whose thick- 

 ness and extent are so considerable that they have been used 

 for the manufacture of hydraulic lime, cannot have been accu- 

 mulated by man. If, on the one hand, the state of preserva- 

 tion of the shells proves that they belong to a deposit contem- 

 poraneous with the human epoch, the fact of their two valves 

 being united, and their perfect preservation, forbid, on the 

 other hand, the idea of their having been transported, and 

 prove that they lived at no great distance, if not upon the 

 very spot where they are now found. These deposits are evi- 

 dently connected with the cause which has given rise to the 

 fonnation of the medanos^ or ancient downs, which are found 

 distributed very far from the sea in the centra of the Pampas, 

 towards the south. 



To the west of the Cordillera, analogous mounds, containing 

 the shells which occur on the present coasts, have been observed 

 at Talcahuano, at Coquimbo, at Cobija, at Arica, and at Lima, 

 over an extent of upwards of 1600 miles. 



The recent shells observed by M. D'Orbigny in the raised 

 beaches of the two shores of South America, have given rise 

 to two very interesting remarks. The first is, that all these 

 shells have their analogues in the neighbouring seas, and that, 

 as a whole, they exhibit on the two sides of the Andes as great 

 a difference as is presented by the existing faunas of these 

 two seas. Whence it necessarily results, that at the epoch 

 when they lived the two seas were already separated. The 



