96 RIO COLORADO. 



miles, the face of the country is everywhere com- 

 posed, of shingle : the pebbles are chiefly of por- 

 phyry, 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 coun- 

 try 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 be- 

 tween 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 C olorado 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 geogi'aphy 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 Af- 

 rica : to his credit be it said, there was not a rancho 

 between the Colorado and Buenos Ayres in nearly 

 such neat order as his. He had a little room for 



