1834.] BASALTIC LAVA. 171 



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 ono 

 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 26/i. 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 Ave 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 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 mountains of that great chain 

 for its source ; and worthy of such a source are streams that have 

 flowed over the gently inclined bed of the sea to a distance of one 

 hundred miles. At the first glance of the basaltic cliffs on the 

 opposite sides of the valley, it was evident that the strata once were 



