death.
April 24th.--Like the navigators of old when approaching an unknown
land, we examined and watched for the most trivial sign of a change.
The drifted trunk of a tree, or a boulder of primitive rock, was hailed
with joy, as if we had seen a forest growing on the flanks of the
Cordillera. The top, however, of a heavy bank of clouds, which
remained almost constantly in one position, was the most promising
sign, and eventually turned out a true harbinger. At first the clouds
were mistaken for the mountains themselves, instead of the masses of
vapour condensed by their icy summits.
April 26th.--We this day met with a marked change in the geological
structure of the plains. From the first starting I had carefully
examined the gravel in the river, and for the two last days had noticed
the presence of a few small pebbles of a very cellular basalt. 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
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