THE VOYAGE OF THE BEAGLE 



33S 



they are easily cultivated by irrigation. They may be traced 

 up to a height of between 7000 and 9000 feet, where they 

 become hidden by the irregular 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 rudely-stratified shingle. They precisely 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 

 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 val- 

 leys, 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 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 

 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 rat- 

 tling noise, night and day, may be heard along the whole 

 course of the torrent. The sound spoke eloquently to the 



