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