GEOLOGY OF THE CORDILLERA. [CHAP. XT. 



are covered in the central parts, by a great thickness 

 of red sandstone, conglomerate, and calcareous clay-slate, asso- 

 ciated with, and passing into, prodigious beds of gypsum. Jn 

 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, crystallized 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 ot 

 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 Peu- 

 quenes 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 form- 

 ing ; but as the beds of the conglomerate have been thrown off 

 at an angle of 45 by the red Portillo granite (with the under- 

 lying 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 its great height to elevations of a still later date. Looking 

 to its earliest origin, the red granite seems to have been injected 

 on an ancient pre-existing line of white granite and mica-slate. 

 In most parts, perhaps in all parts, of the Cordillera, it may be 

 joncluded that each line has been formed by repeated upheavals 



