5 1 6 Section of the Uspallata Range, pabt n. 



porphyries, apparently belonging to the rocks of the 

 Uspallata chain. This plain corresponds geographically 

 with the valley of Tenuyan between the Portillo and 

 Peuquenes ranges ; but in that valley the shingle, 

 which likewise has been derived both from the eastern 

 and western ranges, has been cemented into a hard con- 

 glomerate, and has been throughout tilted at a con- 

 siderable inclination ; the gravel there apparently attains 

 a much greater thickness, and is probably of higher 

 antiquity. 



The Uspallata Range. — The road by the Villa 

 Vicencio Pass does not strike directly across the range, 

 but runs for some leagues northward along its western 

 base : and I must briefly describe the rocks here seen, 

 before continuing with the coloured east and west section. 

 At the mouth of the valley of Canota, and at several 

 points northwards, there is an extensive formation of a 

 glossy and harsh, and of a feldspathic clay-slate, includ- 

 ing strata of grauwacke, and having a tortuous, nearly 

 vertical cleavage, traversed by numerous metalliferous 

 veins and others of quartz. The clay-slate is in many 

 parts capped by a thick mass of fragments of the same 

 rock, firmly recemented ; and both together have been 

 injected and broken up by very numerous hillocks, 

 ranging north and south, of lilac, white, dark and 

 salmon-coloured porphyries : one steep, now denuded, 

 hillock of porphyry had its face as distinctly impressed 

 with the angles of a fragmentary mass of the slate, with 

 some of the points still remaining embedded, as sealing- 

 wax could be by a seal. At the mouth of this same 

 valley of Canota, in a fine escarpment having the strata 

 dipping from 50° to 60° to the XE., 1 the clay-slate 



1 Nearly opposite to this escarpment, there is another correspond- 

 ing one, with the strata dipping not to the exactly opposite point, or 

 SW., but to S3 W. : consequently the two escarpments trend towards 



