342 
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 0 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 0 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 
