IX 
TRACES OF INDIANS 
197 
sufficiently rapid, so that the action of the inclined 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 2 gth .—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 1100 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, considering 
