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 the 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.
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