But it supported many different kinds
of plants, and the grass, though brown and withered, was
more abundant, as the thorny bushes were less so.
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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