IX TRACES OF INDIANS 197 



sufficiently rapid, so that the action of the incHned surface of its 

 body on the atmosphere may counterbalance its gravity. The 

 force to keep up the momentum of a body moving in a 

 horizontal plane in the air (in which there is so little friction) 

 cannot be great, and this force is all that is wanted. The 

 movement of the neck and body of the condor, we must 

 suppose, is sufficient for this. However this may be, it is truly 

 wonderful and beautiful to see so great a bird, hour after hour, 

 without any apparent exertion, wheeling and gliding over 

 mountain and river. 



April 2gih. — From some high land we hailed with joy the 

 white summits of the Cordillera, as they were seen occasionally 

 peeping through their dusky envelope of clouds. During the 

 few succeeding days we continued to get on slowly, for we 

 found the river-course very tortuous, and strewed with immense 

 fragments of various ancient slaty rocks, and of granite. The 

 plain bordering the valley had here attained an elevation of 

 about 1 100 feet above the river, and its character was much 

 altered. The well-rounded pebbles of porphyry were mingled 

 with many immense angular fragments of basalt and of primary 

 rocks. The first of these erratic boulders which I noticed was 

 sixty-seven miles distant from the nearest mountain ; another 

 which I measured was five yards square, and projected five feet 

 above the gravel. Its edges were so angular, and its size so 

 great, that I at first mistook it for a rock in situ, and took out 

 my compass to observe the direction of its cleavage. The plain 

 here was not quite so level as that nearer the coast, but yet it 

 betrayed no signs of any great violence. Under these circum- 

 stances it is, I believe, quite impossible to explain the trans- 

 portal of these gigantic masses of rock so many miles from 

 their parent- source, on any theory except by that of floating 

 icebergs. 



During the two last days we met with signs of horses, and 

 with several small articles which had belonged to the Indians — 

 such as parts of a mantle and a bunch of ostrich feathers — but 

 they appeared to have been lying long on the ground. 

 Between the place where the Indians had so lately crossed the 

 river and this neighbourhood, though so many miles apart, the 

 country appears to be quite unfrequented. At first, considerins; 



