1834.] CORDILLERA. 1 77 



but they were scon distinct against 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 move- 

 ments of the neck, body, and tail 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 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. 



J.ju-il 29^. From some high laud we hailed with joy the white 

 summits of the Cordillera, as they wore seen occasionally peeping 

 through their dusky envelope of clouds. During the few suc- 

 ceeding 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 cir- 

 cumstances 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 



