86 CHARLES DARWIN 



passed by the toldos of the Indians. These are round like 

 ovens, and covered with hides ; by the mouth of each, a taper- 

 ing chuzo was stuck in the ground. The toldos were divided 

 into separate groups, which belong 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, dif- 

 fering 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 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 commencement 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 coun- 

 try 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 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 



