1833.] SAND-DUNES. 75 



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 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 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 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 bolt 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 



