340 



CHARLES DARWIN 



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 gran- 

 ite 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 concluded that each line 

 has been formed by repeated upheavals and injections; and 

 that the several parallel lines are of different ages. Only 

 thus can we gain time, at all sufficient to explain the truly 

 astonishing amount of denudation, which these great, though 

 comparatively with most other ranges recent, mountains have 

 suffered. 



Finally, the shells in the Peuquenes or oldest ridge, prove, 

 as before remarked, that it has been upraised 14,000 feet 

 since a Secondary period, which in Europe we are accus- 

 tomed to consider as far from ancient; but since these shells 

 lived in a moderately deep sea, it can be shown that the area 

 now occupied by the Cordillera, must have subsided several 

 thousand feet — in northern Chile as much as 6000 feet — so 

 as to have allowed that amount of submarine strata to have 

 been heaped on the bed on which the shells lived. The proof 

 is the same with that by which it was shown, that at a much 

 later period, since the tertiary shells of Patagonia lived, 

 there must have been there a subsidence of several hundred 

 feet, as well as an ensuing elevation. Daily it is forced home 

 on the mind of the geologist, that nothing, not even the wind 

 that blows, is so unstable as the level of the crust of this 

 earth. 



I will make only one other geological remark : although 

 the Portillo chain is here higher than the Peuquenes, the 

 waters draining the intermediate valleys, have burst through 

 it. The same fact, on a grander scale, has been remarked in 

 the eastern and loftiest line of the Bolivian Cordillera, 

 through which the rivers pass: analogous facts have also* 

 been observed in other quarters of the world. On the sup- 

 position of the subsequent and gradual elevation of the Por- 

 tillo line, this can be understood ; for a chain of islets would 

 at first appear, and, as these were lifted up, the tides would be 

 always wearing deeper and broader channels between them. 



