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.
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