534 The Uspallata Range. part il 



deposition of the Peuquenes strata, and which certainly 

 accompanied the deposition of the equivalent beds near 

 the Puente del Inca, in this line of section. 



The Uspallata chain corresponds in geographical 

 position, though on a small scale, with the Portillo line ; 

 and its clay-slate formation is probably the equivalent 

 of the mica-schist of the Portillo, there metamorphosed 

 by the old white granites and syenites. The coloured 

 beds under the conglomerate in the valley of Tenuyan, 

 of which traces are seen on the crest of the Portillo, 

 and even the conglomerate itself, may perhaps be 

 synchronous with the tufaceous beds and submarine 

 lavas of the Uspallata range ; an open sea and volcanic 

 action in the latter case, and a confined channel between 

 two bordering chains of islets in the former case, having 

 been sufficient to account for the mineralogical dissimi- 

 larity of the two series. From this correspondence 

 between the Uspallata and Portillo ranges, perhaps in 

 age and certainly in geographical position, one is 

 tempted to consider the one range as the prolongation 

 of the other ; but their axes are formed of totally diffe- 

 rent intrusive rocks ; and we have traced the apparent 

 continuation of the red granite of the Portillo in the 

 red porphyries diverging into the main Cordillera. 

 Whether the axis of the Uspallata range was injected 

 before, or, as perhaps is more probable, after that of 

 the Portillo line, I will not pretend to decide ; but it 

 is well to remember that the highly inclined lava- 

 streams on the eastern flank of the Portillo line, prove 

 that its angular upheavement was not a single and 

 sudden event ; and therefore that the anticlinal eleva- 

 tion of the Uspallata range may have been contempo- 

 raneous with some of the later angular movements by 

 which the gigantic Portillo range gained its present 

 height above the adjoining plain. 



