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