78 
SAND-DUNES 
CHAP. 
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 com¬ 
mencement 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 exceed¬ 
ingly 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 specula¬ 
tions 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, 
