These Gradually Increased
In Number And In Size, But None Were As Large As A Man's
Head.
This morning, however, pebbles of the same rock,
but more compact, suddenly became abundant, and in the
course of
Half an hour we saw, at the distance of five of
six miles, the angular edge of a great basaltic platform.
When we arrived at its base we found the stream bubbling
among the fallen blocks. For the next twenty-eight miles
the river-course was encumbered with these basaltic masses.
Above that limit immense fragments of primitive rocks,
derived from its surrounding boulder-formation, were
equally numerous. None of the fragments of any considerable
size had been washed more than three or four miles
down the river below their parent-source: considering the
singular rapidity of the great body of water in the Santa
Cruz, and that no still reaches occur in any part, this example
is a most striking one, of the inefficiency of rivers in
transporting even moderately-sized fragments.
The basalt is only lava, which has flowed beneath the sea;
but the eruptions must have been on the grandest scale. At
the point where we first met this formation it was 120 feet
in thickness; following up the river course, the surface
imperceptibly rose and the mass became thicker, so that at
forty miles above the first station it was 320 feet thick.
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.
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