180 S. CRUZ, PATAGONIA. [CHAP. ix. 



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 moun- 

 tains themselves, instead of the masses of vapour condensed by 

 their icy summits. 



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

 geological structure of the plains. From the first starting 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 dis- 

 tance 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 the surrounding boulder-formation, were equally numerous. 

 None of the fragments of any considerable 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 example 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 thick- 

 ness ; 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. What the thickness may 

 be close to the Cordillera, I have no means of knowing, but the 

 platform there attains a height of about three thousand feet 

 above the level of the sea : we must therefore look to the moun- 

 tains of that great chain for its source ; and worthy of such a source 

 are streams, that have flowed over the gently inclined bed of thr 



