These latter in a
short space entirely disappeared, and the plains were left without
a thicket to cover their nakedness. This change in the vegetation
marks the commencement of the grand calcareo-argillaceous deposit,
which forms the wide extent of the Pampas, and covers the granitic
rocks of Banda Oriental. From the Strait of Magellan to the
Colorado, a distance of about eight hundred miles, the face of the
country is everywhere composed of shingle: the pebbles are chiefly
of porphyry, and probably owe their origin to the rocks of the
Cordillera. North of the Colorado this bed thins out, and the
pebbles become exceedingly small, and here the characteristic
vegetation of Patagonia ceases.
Having ridden about twenty-five miles, we came to a broad belt of
sand-dunes, which stretches, as far as the eye can reach, to the
east and west. The sand-hillocks resting on the clay, allow small
pools of water to collect, and thus afford in this dry country an
invaluable supply of fresh water. The great advantage arising from
depressions and elevations of the soil, is not often brought home
to the mind. The two miserable springs in the long passage between
the Rio Negro and Colorado were caused by trifling inequalities in
the plain, without them not a drop of water would have been found.
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