THE VOYAGE OF THE BEAGLE 339 



ridge, on the Mendoza side, which is 14,305 feet. The lower 

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

 to the westward of it, are composed of a vast pile, many thou- 

 sand feet in thickness, of porphyries which have flowed as 

 submarine lavas, alternating with angular and rounded frag- 

 ments 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 cal- 

 careous clay-slate, associated with, and passing into, pro- 

 digious beds of gypsum. In these upper beds shells are toler- 

 ably 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 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 peb- 

 bles, 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 Peu- 

 quenes and Portillo ranges were partially upheaved and ex- 

 posed 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 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 accumula- 

 tion 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 



