190 THE CORDILLERA. [chap. ix. 



the blue sky. The head and neck were moved frequently, 

 and apparently with force ; and the extended wings seemed 

 to form the fulcrum on which the movements of the neck, 

 body, and tail acted. If the bird wished to descend, the 

 wings for the 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 soarings 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 move- 

 ment 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^th. — 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 eleven hundred 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 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, 



