chap. xrv. Section by the Portillo Pass. 481 



strip on the steep southern side of the valley, at the 

 height of 400 or 500 feet above the river-bed ; judging 

 from an apparently corresponding line of cliff on the 

 northern side, the valley must once have been filled up 

 to this height by a field of lava. On the summit of a 

 lofty mountain some leagues higher up this same valley 

 of the Cachapual, I found columnar pitchstone porphy- 

 ritic with feldspar ; I do not suppose this rock to be 

 of volcanic origin, and only mention it here, from its 

 being intersected by masses and dikes of a vesicular 

 rock, approaching in character to trachyte ; in no other 

 part of Chile did I observe vesicular or amygdaloidal 

 dikes, though these are so common in ordinary volcanic 

 districts. 



Passage of the Andes by the Portillo or Peuquenes 



Pass. 



Although I crossed the Cordillera only once by this 

 pass, and only once by that of the Cumbre or Uspallata 

 (presently to be described), riding slowly and halting 

 occasionally to ascend the mountains, there are many 

 circumstances favourable to obtaining a more faithful 

 sketch of their structure than would at first be thought 

 possible from so short an examination. The mountains 

 are steep and absolutely bare of vegetation ; the atmo- 

 sphere is resplendently clear ; the stratification distinct ; 

 and the rocks brightly and variously coloured : some of 

 the natural sections might be truly compared for dis- 

 tinctness to those coloured ones in geological works. 

 Considering how little is known of the structure of this 

 gigantic range, to which I particularly attended, most 

 travellers having collected only specimens of the rocks, 

 I think my sketch-sections, though necessarily imper- 

 fect, possess some interest. The section given in Plate I. 



