1833,] SAND-DUNES. 75 
river, we soon entered on a country, differi ing 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 dis- 
appeared, and the plains were left without.a thicket to cover 
their nakedness. This change in the vegetation marks the com- 
mencement of the grand caleareo-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 charac- 
teristic 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 advan- 
tage 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 
