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.