XV GEOLOGY OF THE CORDILLERA 343 



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 

 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 truh' 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 1 4,000 feet since 

 a Secondary period, which in Europe we are accustomed 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 supposition of the sub- 

 sequent and gradual elevation of the Portillo 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. At the present day, even 

 in the most retired Sounds on the coast of Tierra del Fuego, 

 the currents in the transverse breaks which connect the longi- 

 tudinal channels are very strong, so that in one transverse channel 

 even a small vessel under sail was whirled round and round. 



