acted. If the bird wished to descend, the wings were for a moment
collapsed; and when again expanded with an altered inclination, the
momentum gained by the rapid descent seemed to urge the bird upwards
with the even and steady movement of a paper kite. In the case of any
bird soaring, its motion must be 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 movements 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 29th.--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 slate rocks, and of
granite. The plain bordering the valley has 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 in
betrayed no signs of any great violence. Under these circumstances it
is, I believe, quite impossible to explain the transportal 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,
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