GEOLOGY OF THE CORDILLERA. 69 



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 sub- 

 marine lavas, alternating with angular and rounded 

 fragments of the same rocks, thrown out of the sub- 

 marine craters. These alternating masses are cov- 

 ered 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. Tn these upper beds shells are tolera- 

 bly 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, ci-ystallized, 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 Portil- 

 lo, is of a totally difterent formation : it consists 

 chiefly of grand, bare pinnacles of red potash-gran- 

 ite, which low down on the western flank are cov- 

 ered 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 thick- 

 ness, which have been upheaved by the red gran- 

 ite, and dip at an angle of 45° towards the Peu- 

 quenes line. I was astonished to find that this con- 

 glomerate was partly composed of pebbles, derived 

 from the rocks, with their fossil shells, of the Peu- 

 quenes range, and partly of red potash-granite, 

 like that of the Portillo. Hence we must con- 

 clude 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 throAvn 



