What the thickness may be close to the Cordillera, I have
no means of knowing, but the platform there attains a height
of about three thousand feet above the level of the sea;
we must therefore look to the mountains of that great chain
for its source; and worthy of such a source are streams that
have flowed over the gently inclined bed of the sea to a
distance of one hundred miles. 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.
But it may yet be asked, how has the solid basalt been
moved? Geologists formerly would have brought into play