

4 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



the nature of the soil. The formation consists of a reddish argillaceous earth, 

 generally containing irregular concretions of a pale brown, indurated marl. This 

 stone, where most compact, is traversed by small linear cavities, and in several 

 respects resembles the less pure fresh-water limestones of Europe. The concre- 

 tions not unfrequently become so numerous, that they unite and form a continu- 

 ous stratum, or even the entire mass. 



* 



At Bajada de St a . Fe, in the Province of Entre Rios, beds of sand, limestone, 

 and clay of different qualities, containing sharks' teeth and sea-shells, underlie 

 the Pampas deposit. The shells, although numerous, are few in kind. Mr. 

 George B. Sowerby informs me that they appear to belong to one of the less ancient 

 tertiary epochs ; they consist of Venus nov. spec, near to V. cancellata ; Area 

 nov. spec, near to A. antiquata ; a very large oyster, probably an extinct species; 

 an imperfect specimen of a second species of oyster near to O. edulis; and a 

 Pecten near to P. opercularis. These beds pass upwards into an indurated marl, 

 and this again into the red argillaceous earth of the Pampas, containing the re- 

 mains of those extinct quadrupeds, which every where characterize that deposit c 

 To the southward of the Plata level plains of an uniform composition, inter- 

 rupted only at wide intervals by hills of crystalline rock, extend to a distance of 

 about three hundred miles ; and to the northward for at least an equal space, and 

 probably much further. As might have been expected from the perfectly level 

 surface, wherever a continuous section is presented on the banks of the great 

 rivers, very slight changes of colour show, that the deposit has been accumulated 

 in strata as horizontal as the land, or as the water-line at the base of the cliffs. 



In the province of Banda Oriental (to the N. and N. E. of the Plata), and in 

 part of that of Entre Rios, the land, though very low and level, has a foundation 

 of granitic and other primary rocks. These older formations are partially covered, 

 in most parts, by a reddish earthy mass containing a few small calcareous con- 

 cretions ; while in other parts, they are concealed by more regular strata, of 

 indurated marl passing into limestone, of conglomerates, and ferruginous sand- 

 stone. The entire formation probably belongs to the same epoch with that of the 

 Pampas deposit. In the earthy mass, even where it is of little thickness, and 

 where it might readily be mistaken for detritus produced from the underlying 

 granites, remains of large quadrupeds have several times been discovered. 



