196 CHARLES DARWIN 



of the Strait of Magellan at the rate of eight knots an hour, 

 yet we must confess that it makes the head almost giddy to 

 reflect on the number of years, century after century, which 

 the tides, unaided by a heavy surf, must have required to 

 have corroded so vast an area and thickness of solid basaltic 

 lava. Nevertheless, we must believe that the strata under- 

 mined by the waters of this ancient strait, were broken up 

 into huge fragments, and these lying scattered on the beach, 

 were reduced first to smaller blocks, then to pebbles and 

 lastly to the most impalpable mud, which the tides drifted 

 far into the Eastern or Western Ocean. 



With the change in the geological structure of the plains 

 the character of the landscape likewise altered. While ram- 

 bling up some of the narrow and rocky defiles, I could almost 

 have fancied myself transported back again to the barren 

 valleys of the island of St. Jago. Among the basaltic cliffs, 

 I found some plants which I had seen nowhere else, but 

 others I recognised as being wanderers from Tierra del 

 Fuego. These porous rocks serve as a reservoir for the 

 scanty rain-water; and consequently on the line where the 

 igneous and sedimentary formations unite, some small 

 springs (most rare occurrences in Patagonia) burst forth; 

 and they could be distinguished at a distance by the circum- 

 scribed patches of bright green herbage. 



April 2fik. The bed of the river became rather narrower, 

 and hence the stream more rapid. It here ran at the rate 

 of six knots an hour. From this cause, and from the many 

 great angular fragments, tracking the boats became both 

 dangerous and laborious. 



This day I shot a condor. It measured from tip to tip 

 of the wings, eight and a half feet, and from beak to tail, 

 four feet. This bird is known to have a wide geographical 

 range, being found on the west coast of South America, 

 from the Strait of Magellan along the Cordillera as far as 

 eight degrees north of the equator. The steep cliff near the 

 mouth of the Rio Negro is its northern limit on the Pata- 

 gonian coast; and they have there wandered about four 

 hundred miles from the great central line of their habita- 

 tion in the Andes. Further south, among the bold preci- 



