At the first glance of the basaltic cliffs on
the opposite sides of the valley it was evident that the strata
once were united. What power, then, has removed along a whole line
of country a solid mass of very hard rock, which had an average
thickness of nearly three hundred feet, and a breadth varying from
rather less than two miles to four miles? The river, though it has
so little power in transporting even inconsiderable fragments, yet
in the lapse of ages might produce by its gradual erosion an
effect, of which it is difficult to judge the amount. But in this
case, independently of the insignificance of such an agency, good
reasons can be assigned for believing that this valley was formerly
occupied by an arm of the sea. It is needless in this work to
detail the arguments leading to this conclusion, derived from the
form and the nature of the step-formed terraces on both sides of
the valley, from the manner in which the bottom of the valley near
the Andes expands into a great estuary-like plain with
sand-hillocks on it, and from the occurrence of a few sea-shells
lying in the bed of the river. If I had space I could prove that
South America was formerly here cut off by a strait, joining the
Atlantic and Pacific oceans, like that of Magellan.
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