70 RIO COLORADO. [CHAP. iv. 



lu the morning we started for Bahia Blanca, which we reached 

 in two days. Leaving the regular encampment, we passed by the 

 toldos of the Indians. These are round like ovens, and covered 

 with hides ; by the month of each, a tapering chuzo was stuck in 

 the ground. The toldos were divided into separate groups, which 

 belonged to the different caciques' tribes, and the groups were again 

 divided into smaller ones, according to the relationship of the 

 owners. For several miles we travelled along the valley of the 

 Colorado. The alluvial plains on the side appeared fertile, and it 

 is supposed that they are well adapted to the growth of corn. 

 Turning northward from the river, we soon entered on a country, 

 differing from the plains south of the river. The land still con- 

 tinued dry and sterile : but it supported many different kinds of 

 plants, and the grass, though brown and withered, was more abun- 

 dant, 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 de- 

 posit, 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 

 cast 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 Piio 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 laud occur, 

 such speculations can hardly bo neglected by any one, although 



