?42 GEOLOGY OF THE CORDILLERA chap. 



of the Peuquenes ridge, and of the several great lines to the 

 westward of it, are composed of a vast pile, many thousand 

 feet in thickness, of porphyries which have flowed as submarine 

 lavas, alternating with angular and rounded fragments of the 

 same rocks, thrown out of the submarine craters. These 

 alternating masses are covered in the central parts by a 

 great thickness of red sandstone, conglomerate, and calcareous 

 clay-slate, associated with, and passing into, prodigious beds 

 of gypsum. In these upper beds shells are tolerably frequent ; 

 and they belong to about the period of the lower chalk of 

 Europe. It is an old story, but not the less wonderful, to hear 

 of shells which were once crawling on the bottom of the sea, 

 now standing nearly 14,000 feet above its level. The lower 

 beds in this great pile of strata have been dislocated, baked, 

 crystallised and almost blended together, through the agency of 

 mountain masses of a peculiar white soda-granitic rock. 



The other main line, namely that of the Portillo, is of a 

 totally different formation ; it consists chiefly of grand bare 

 pinnacles of a red potash -granite, which low down on the 

 western flank are covered by a sandstone, converted by the 

 former heat into a quartz-rock. On the quartz there rest 

 beds of a conglomerate several thousand feet in thickness, 

 which have been upheaved by the red granite, and dip at an 

 angle of 45° towards the Peuquenes line. I was astonished to 

 find that this conglomerate was partly composed of pebbles, 

 derived from the rocks, with their fossil shells, of the Peuquenes 

 range ; and partly of red potash-granite, like that of the Portillo. 

 Hence we must conclude that both the Peuquenes and Portillo 

 ranges were partially upheaved and exposed to wear and tear, 

 when the conglomerate was forming ; but as the beds of the 

 conglomerate have been thrown off at an angle of 45° by the 

 red Portillo granite (with the underlying sandstone baked by 

 it), we may feel sure that the greater part of the injection and 

 upheaval of the already partially formed Portillo line took 

 place after the accumulation of the conglomerate, and long 

 after the elevation of the Peuquenes ridge. So that the 

 Portillo, the loftiest line in this part of the Cordillera, is not 

 so old as the less lofty line of the Peuquenes. Evidence 

 derived from an inclined stream of lava at the eastern base 

 of the Portillo might be adduced to show that it owes part of 



