64 PORTILLO PASS. 



more southern coasts. No one fact in the geology 

 of South America interested me more than these 

 terraces of rudely-stratified shingle. They pre- 

 cisely resemble in composition the matter which 

 the torrents in each valley would deposit if they 

 were checked in their course by any cause, such 

 as entering a lake or arm of the sea ; but the tor- 

 rents, 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 impossi- 

 ble here to give the reasons, but I am convinced 

 that the shingle terraces were accumulated, during 

 the gradual elevation of the Cordillera, by the tor- 

 rents delivering, at successive levels, their detri- 

 tus on the beach-heads of long, narrow ai'ms of the 

 sea, first high up the valleys, then lower and lower 

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

 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 up- 

 heaved 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 I'ivefs which flow in these valleys ought 

 rather to be called mountain-torrents. Their in- 

 clination is very great, and their water the colour 

 of mud. The roar which the Maypu made, as it 

 rushed over the great rounded fragments, was like 

 that of the sea. Ainidst the din of rushing waters, 

 the noise from the stones, as they rattled one over 

 another, was most distinctly audible even from a 

 distance. This rattling noise, night and day, may 

 be heard along^ the whole course of the torrent. 



