ntry,
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
for strangers, and a small corral for the horses, all ma