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 degrees 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 degrees 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.
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