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. Daily it is forced home on the mind of the
geologist that nothing, not even the wind that blows, is so
unstable as the level of the crust of this earth.
I will make only one other geological remark: although the Portillo
chain is here higher than the Peuquenes, the waters, draining the
intermediate valleys, have burst through it. The same fact, on a
grander scale, has been remarked in the eastern and loftiest line
of the Bolivian Cordillera, through which the rivers pass:
analogous facts have also been observed in other quarters of the
world. On the supposition of the subsequent and gradual elevation
of the Portillo line, this can be understood; for a chain of islets
would at first appear, and, as these were lifted up, the tides
would be always wearing deeper and broader channels between them.
At the present day, even in the most retired Sounds on the coast of
Tierra del Fuego, the currents in the transverse breaks which
connect the longitudinal channels are very strong, so that in one
transverse channel even a small vessel under sail was whirled round
and round.
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