338 
PORTILLO PASS 
CHAP. 
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 
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 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 
