side appeared fertile, and it is supposed that they are well adapted to
the growth of corn. Turning northward from the river, we soon entered
on a country, differing from the plains south of the river. The land
still continued dry and sterile: 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. The belt of sand-dunes
is about eight miles wide; at some former period, it probably formed
the margin of a grand estuary, where the Colorado now flows. In this
district, where absolute proofs of the recent elevation of the land
occur, such speculations can hardly be neglected by any one, although
merely considering the physical geography of the country. Having
crossed the sandy tract, we arrived in the evening at one of the
post-houses; and, as the fresh horses were grazing at a distance we
determined to pass the night there.
The house was situated at the base of a ridge between one and two
hundred feet high--a most remarkable feature in this country. This
posta was commanded by a negro lieutenant, born in Africa: to his
credit be it said, there was not a ranche between the Colorado and
Buenos Ayres in nearly such neat order as his. He had a little room
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