224 



PATAGONIA. 



April, 1834. 



the momentum gained by the rapid descent seemed to utge 

 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 sur- 

 face 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 that fluid (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 con- 

 dor, 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, wheel- 

 ing 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 occa- 

 sionally 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, and its 

 character was much altered. The well-rounded pebbles of 

 porphyry were in this part mingled with many immense 

 angular fragments of basalt and of the rocks above men- 

 tioned. The first of these erratic blocks which I noticed^ 

 was sixty-seven miles distant from the nearest mountain; 

 another which had been transported to rather a less dis- 

 tance, measured 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 plains here were not quite so level as those nearer the 

 coast, but yetj they betrayed little signs of any violent action. 

 Under these circumstances, it would be difficult, as it appears 

 to me, to explain this phenomenon on any theory, excepting 

 through that of transport by ice while the country was under 

 water. But this is a subject to which I shall again recur. 



