194 CHARLES DARWIN 



dislocated and bones broken, showed how they had met their 

 death. 



April 24th. Like the navigators of old when approaching 

 an unknown land, we examined and watched for the most 

 trivial sign of a change. The drifted trunk of a tree, or a 

 boulder of primitive rock, was hailed with joy, as if we had 

 seen a forest growing on the flanks of the Cordillera. The 

 top, however, of a heavy bank of clouds, which remained 

 almost constantly in one position, was the most promising 

 sign, and eventually turned out a true harbinger. At first the 

 clouds were mistaken for the mountains themselves, instead 

 of the masses of vapour condensed by their icy summits. 



April 26th. We this day met with a marked change ii 

 the geological structure of the plains. From the first start- 

 ing I had carefully examined the gravel in the river, and 

 for the two last days had noticed the presence of a few small 

 pebbles of a very cellular basalt. These gradually increased 

 in number and in size, but none were as large as a man's 

 head. This morning, however, pebbles of the same rock, 

 but more compact, suddenly became abundant, and in the 

 course of half an hour we saw, at the distance of five or 

 six miles, the angular edge of a great basaltic platform. 

 When we arrived at its base we found the stream bubbling 

 among the fallen blocks. For the next twenty-eight miles 

 the river-course was encumbered with these basaltic masses. 

 Above that limit immense fragments of primitive rocks, 

 derived from its surrounding boulder- formation, were 

 equally numerous. None of the fragments of any consider- 

 able size had been washed more than three or four miles 

 down the river below their parent-source: considering the 

 singular rapidity of the great body of water in the Santa 

 Cruz, and that no still reaches occur in any part, this ex- 

 ample is a most striking one, of the inefficiency of rivers in 

 transporting even moderately-sized fragments. 



The basalt is only lava, which has flowed beneath the sea ; 

 but the eruptions must have been on the grandest scale. At 

 the point where we first met this formation it was 120 feet 

 in thickness; following up the river course, the surface 

 imperceptibly rose and the mass became thicker, so that at 

 forty miles above the first station it was 320 feet thick. 



