i84 BASALTIC PEBBLES. [chap. ix. 



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 last two 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 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 beheath the 

 sea ; but the eruptions must have been on the grandest 

 scale. At the point where we first met this formation it 

 was one hundred and twenty 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 three hundred and twenty 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 united. What power, 

 then, has removed along a whole line of country a solid 

 mass of very hard rock, which had an average thickness 

 of nearly three hundred feet, and a breadth varying from 

 rather less than two miles to four miles? The river, 



