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PORTILLO PASS 



their course by any cause, such as entering a lake or arm of the 

 sea ; but the torrents, instead of depositing matter, are now 

 steadily at work wearing away both the solid rock and these 

 alluvial deposits, along the whole line of every main valley and 

 side valley. It is impossible here to give the reasons, but I am 

 convinced that the shingle terraces were accumulated, during the 

 gradual elevation of the Cordillera, by the torrents delivering, 

 at successive levels, their detritus on the beach-heads of long 

 narrow arms of the sea, first high up the valleys, then lower 

 and lower down as the land slpwly rose. If this be so, and I 



SOUTH AMERICAN BIT. 



cannot doubt it, the grand and broken chain of the Cordillera, 

 instead of having been suddenly thrown up, as was till lately the 

 universal, and still is the common opinion of geologists, has been 

 slowly upheaved in mass, in the same gradual manner as the 

 coasts of the Atlantic and Pacific have risen within the recent 

 period. A multitude of facts in the structure of the Cordillera 

 on this view receive a simple explanation. 



The rivers which flow in these valleys ought rather to be 

 called mountain -torrents. Their inclination is very great, and 

 their water the colour of mud. The roar which the Ma}pu made, 

 as it rushed over the great rounded fragment.s, was like that of 

 the sea. Amidst the din of rushing waters, the noise from the 



