great pile of strata, have been dislocated, baked, crystallized and
almost blended together, through the agency of mountain masses of a
peculiar white soda-granitic rock.
The other main line, namely, that of the Portillo, is of a totally
different formation: it consists chiefly of grand bare pinnacles of a
red potash-granite, which low down on the western flank are covered by
a sandstone, converted by the former heat into a quartz-rock. On the
quartz, there rest beds of a conglomerate several thousand feet in
thickness, which have been upheaved by the red granite, and dip at an
angle of 45 degs. towards the Peuquenes line. I was astonished to find
that this conglomerate was partly composed of pebbles, derived from the
rocks, with their fossil shells, of the Peuquenes range; and partly of
red potash-granite, like that of the Portillo. Hence we must conclude,
that both the Peuquenes and Portillo ranges were partially upheaved and
exposed to wear and tear, when the conglomerate was forming; but as the
beds of the conglomerate have been thrown off at an angle of 45 degs.
by the red Portillo granite (with the underlying sandstone baked by
it), we may feel sure, that the greater part of the injection and
upheaval of the already partially formed Portillo line, took place
after the accumulation of the conglomerate, and long after the
elevation of the Peuquenes ridge. 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
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