chap, xl Sierra Ventana. 317 



southern part is generally formed of the harder and 

 more calcareous varieties. I will briefly describe my 

 route : about twenty-five miles SSW. of the capital, 

 in a well forty yards in depth, the upper part, and, as I 

 was assured, the entire thickness, was formed of dark 

 red Pampean mud without concretions. North of the 

 R. Salado, there are many lakes ; and on the banks of 

 one (near the Guardia) there was a little cliff similarly 

 composed, but including many nodular and stalactiform 

 concretions : I found here a large piece of tesselated 

 armour, like that of the Glyptodon, and many fragments 

 of bones. The cliffs on the Salado consist of pale- 

 coloured Pampean mud, including and passing into 

 great masses of tosca-rock : here a skeleton of the 

 Megatherium and the bones of other extinct quadrupeds 

 (see the list at the end of this chapter) were found. 

 Large quantities of crystallized gypsum (of which 

 specimens were given me) occur in the cliffs of this 

 river ; and likewise (as I was assured by Mr. Lumb) in 

 the Pampean mud on the R. Chuelo, seven leagues 

 from Buenos Ayres : I mention this because M. d'Or- 

 bigny lays some stress on the supposed absence of this 

 mineral in the Pampean formation. 



Southward of the Salado the country is low and 

 swampy, with tosca-rock appearing at long intervals at 

 the surface. On the banks, however, of the Tapalguen 

 (sixty miles south of the Salado) there is a large extent 

 of tosca-rock, some highly compact and even semi- 

 crystalline, overlying pale Pampean mud with the usual 

 concretions. Thirty miles further south, the small 

 quartz-ridge of Tapalguen is fringed on its northern 

 and southern flank, by little, narrow, flat-topped hills 

 of tosca-rock, which stand higher than the surrounding 

 plain. Between this ridge and the Sierra of Guitru- 

 gueyu, a distance of sixty miles, the country is swampy, 



