So That The Portillo, The Loftiest Line
In This Part Of The Cordillera, Is Not So Old As The Less Lofty
Line Of The Peuquenes.
Evidence derived from an inclined stream
of lava at the eastern base of the Portillo, might be adduced
to show, that it owes part of its great height to elevations of
a still later date.
Looking to its earliest origin, the red
granite seems to have been injected on an ancient pre-existing
line of white granite and mica-slate. In most parts, perhaps in
all parts, of the Cordillera, it may be concluded that each line
has been formed by repeated upheavals and injections; and
that the several parallel lines are of different ages. Only
thus can we gain time, at all sufficient to explain the truly
astonishing amount of denudation, which these great, though
comparatively with most other ranges recent, mountains have
suffered.
Finally, the shells in the Peuquenes or oldest ridge, prove,
as before remarked, that it has been upraised 14,000 feet
since a Secondary period, which in Europe we are accustomed
to consider as far from ancient; but since these shells
lived in a moderately deep sea, it can be shown that the area
now occupied by the Cordillera, must have subsided several
thousand feet - in northern Chile as much as 6000 feet - so
as to have allowed that amount of submarine strata to have
been heaped on the bed on which the shells lived. The proof
is the same with that by which it was shown, that at a much
later period, since the tertiary shells of Patagonia lived,
there must have been there a subsidence of several hundred
feet, as well as an ensuing elevation.
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