chap. xi. Buenos Ay res to St. Fd Bajada* 331 



M. d'Orbigny's supposition, and admit as certain, that 

 the Sceiidotherium and the large Dasypoid quadruped, 

 and as highly probable, that the Toxodon, Megatherium, 

 &c, some of the bones of which are perfectly preserved, 

 were embedded for the first time, and in a fresh condi- 

 tion, in the strata in which they were found entombed. 

 These gigantic quadrupeds, therefore, though belonging 

 to extinct genera and families, co-existed with the 

 twenty above-enumerated Mollusca, the barnacle and 

 two corals, still living on this coast. From the rolled 

 fragment of black bone, and from the plain of Punta 

 Alta being lower than that of Monte Hermoso, I con- 

 clude that the coarse sub-littoral deposits of Punta Alta, 

 are of subsequent origin to the Pampean mud of Monte 

 Hermoso ; and the beds at this latter place, as we have 

 seen, are probably of subsequent origin to the high 

 tosca-plain round the Sierra Ventana : we shall, how- 

 ever, return, at the end of this chapter, to the considera- 

 tion of these several stages in the great Pampean 

 formation. 



Buenos Ayres to St. Fe Bajada in Bntre Bios. — 

 For some distance northward of Buenos Ayres, the 

 escarpment of the Pampean formation does not approach 

 very near to the Plata, and it is concealed by vegeta- 

 tion : but in sections on the banks of the Rios Luxan, 

 Areco, and Arrecifes, I observed both pale and dark 

 reddish Pampean mud, with small, whitish concretions 

 of tosca ; at all these places mammiferous remains have 

 been found. In the cliffs on the Parana, at San Nicolas, 

 the Pampean mud contains but little tosca; here M. 

 d'Orbigny found the remains of two rodents (Gtenomys 

 Bonariemis and Kerodon antiquum) and the jaw of a 

 Canis : when on the river I could clearly distinguish in 

 this fine line of cliffs, ' horizontal lines of variation 



