Sie PORTILLO PASS. [chap. xv. 



piles of debris. At the lower end or mouths of the valleys^ 

 they are continuously united to those land-locked plains (also 

 formed of shingle) at the foot of the main Cordillera, which 

 I have described in a former chapter as characteristic of the 

 scenery of Chile, and which were undoubtedly deposited when 

 the sea penetrated Chile, as it now does the more southern 

 coasts. No one fact in the geology of South America, interested 

 me more than these terraces of ru,dely-stratified shingle. They 

 precisely resemble in composition, the matter which the torrents iu 

 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 

 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 slowly rose. If this be so, and I cannot doubt it, the grand 

 and broken chain of the Cordillera, instead of having been sud- 

 denly 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 tlie 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 Maypu made, as 

 it rushed over the great rounded fragments, was like that of the 

 sea. Amidst 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. The sound spoke 

 eloquently to the geologist; the thousands and thousands of 

 stones, which, striking against each other, made the one dull 

 uniform sound, were all hurrying in one direction. It w^as like 

 thinking on time, where the minute that now glides past is irre- 



