3 1 8 Pampean Formation. paet n. 



with the tosca-rock appearing only in four or five spots : 

 this sierra, precisely like that of Tapalguen, is bordered 

 by horizontal, often cliff-bounded, little hills of tosca- 

 rock, higher than the surrounding plain. Here, also, 

 a new appearance was presented in some extensive and 

 level banks of alluvium or detritus of the neighbouring 

 metamorphic rocks ; but I neglected to observe whether 

 it was stratified or not. Between Guitru-gueyu and 

 the Sierra Vent ana. I crossed a diy plain of tosca-rock 

 higher than the country hitherto passed over, and with 

 small pieces of denuded table-land of the same forma- 

 tion, standing still higher. 



The marly or calcareous beds not only come up 

 nearly horizontally to the northern and southern foot 

 of the great quartzose mountains of the Sierra Yentana, 

 but interfold between the parallel ranges. The super- 

 ficial beds (for I nowhere obtained sections more than 

 twenty feet deep) retain, even close to the mountains, 

 their usual character : the uppermost layer, however, 

 in one place included pebbles of quartz, and rested on 

 a mass of detritus of the same rock. At the very foot 

 of the mountains, there were some few piles of quartz 

 and tosca-rock detritus, including land-shells ; but at 

 the distance of only half a mile from these lofty, jagged, 

 and battered mountains, I could not, to my great sur- 

 prise, find on the boundless surface of the calcareous 

 plain even a single pebble. Quartz-pebbles, however, 

 of considerable size have at some period been trans- 

 ported to a distance of between forty and fifty miles to 

 the shores of Bahia Blanca. 1 



The highest peak of the S. Yentana is, by Captain 

 FitzEoy's measurement, 3,3-40 feet, and the calcareous 

 plain at its foot (from observations taken by some 



1 Schniidtnieyer (' Travels in Chile.' p. 150) states that he first 

 roticed on the Pampas, very small bits of red granite, when fifty 

 miles distant from the southern extremity of the mountains of 

 Cordova, which project on the plain, like a reef into the sea. 



