Hence, Also,
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
beachheads 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 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.
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