543 



ing extinct Mammalia in the neighbourhood of the Plata;" by Charles 

 Darwin, Esq., F.G.S. 



Mr. Darwin premised his account of the geological features of the 

 district in which the remains of the Toxodon, described at the meet- 

 ing on the 19th of April by Mr. Owen, (p. 541) were found, by re- 

 marking that as the other mammalia and the fossil shells had not yet 

 been accurately examined, the notice was necessarily imperfect. 



To the westward and southward of the great estuary of La Plata, 

 extend those level and almost boundless plains which are known by 

 the name of the Pampas. Their geological constitution over many 

 hundred square miles does not vary. It consists of a reddish argil- 

 laceous earth, which generally contains irregular concretions of a white 

 aluminous limestone, or indurated marl, often passing irregularly into 

 a compact calcareous stone, traversed by small linear cavities, si- 

 milar to those which occur in many of the freshwater limestones of 

 Europe. In the province of Entre Rios, the formation which 

 composes the surface of the Pampas overlies and passes into a 

 series of beds of sand, clay, and crystalline cellular limestone ; 

 containing sharks' teeth, gigantic oysters, and other shells belong- 

 ing to the genera area, Venus and pecten. These shells, with the 

 exception of the oyster, have a general resemblance to existing 

 species. To the northward and eastward of the Plata, the province 

 of Banda Oriental, though very low and level, consists of gneiss, 

 granite and primary slate. These rocks are generally concealed by a 

 considerable thickness of a reddish earth, which, though at first sight 

 like ordinary detritus, belongs to the same formation with that 

 composing the Pampas. This deposit, extending over so wide an 

 area on both sides of the Plata, abounds with very numerous remains 

 of various extinct mammalia j among which the Toxodon, Mega- 

 therium, Mastodon, an animal covered with an armadillo-like case, and 

 as Mr. Darwin believes, the horse, co-existed in the same district. 



Proofs of the elevation of the land within a recent period, occur in 

 several parts. Mr. Darwin stated that he had seen in the possession 

 of Sir W. Parish, marine shells which occur near Buenos Ayres in 

 great beds, elevated several yards above the level of the river; and 

 these same species the author had found living on the mud banks on 

 another part of the coast. He, therefore, inferred, that at no very 

 remote period a great bay occupied the area both of the Pampas and 

 of the lower parts of Banda Oriental ; and that into this bay the 

 several rivers, which now unite to form the Plata, poured down red- 

 dish sediment, resulting, as at the present day, from the decompo- 

 sition of the granites of Brazil, and charged with carbonate and sul- 

 phate of lime, perhaps derived from the Cordillera. On the cliff-formed 

 shores of Entre Rios, the line can be distinguished where the estuary 

 mud first encroached on the deposits of the ocean. The author also 

 supposed that the ancient rivers, like those of the present day, car- 

 ried down the carcases of land animals, which thus became entombed 

 in the accumulating sediment. Since that period, by the gradual 

 rising of the land, the bottom of the great bay has been converted 

 into plains, almost as level as the surface of the former sea ; and the 



