XV SILICIFIED TREES 355 



been spread out. The ocean which received such thick masses 

 must have been profoundly deep ; but again the subterranean 

 forces exerted themselves, and I now beheld the bed of that 

 ocean, forming a chain of mountains more than seven thousand 

 feet in height. Nor had those antagonist forces been dormant, 

 which are always at work wearing down the surface of the land : 

 the great piles of strata had been intersected by many wide 

 valleys, and the trees, now changed into silex, were exposed 

 projecting from the volcanic soil, now changed into rock, 

 whence formerly, in a green and budding state, they had raised 

 their lofty heads. Now, all is utterly irreclaimable and desert ; 

 even the lichen cannot adhere to the stony casts of former 

 trees. Vast, and scarcely comprehensible as such changes 

 must ever appear, yet they have all occurred within a period, 

 recent when compared with the history of the Cordillera ; and 

 the Cordillera itself is absolutely modern as compared with 

 many of the fossiliferous strata of Europe and America. 



April \st. — We crossed the Uspallata range, and at night 

 slept at the custom-house — the only inhabited spot on the 

 plain. Shortly before leaving the mountains, there was a 

 very extraordinary view ; red, purple, green, and quite white 

 sedimentary rocks, alternating with black lavas, were broken up 

 and thrown into all kinds of disorder by masses of porphyry of 

 every shade of colour, from dark brown to the brightest lilac. 

 It was the first view I ever saw, which really resembled those 

 pretty sections which geologists make of the inside of the 

 earth. 



The next day we crossed the plain, and followed the course 

 of the same great mountain stream which flows by Luxan. 

 Here it was a furious torrent, quite impassable, and appeared 

 larger than in the low country, as was the case with the rivulet 

 of Villa Vicencio. On the evening of the succeeding day we 

 reached the Rio de las Vacas, which is considered the worst 

 stream in the Cordillera to cross. As all these rivers have a 

 rapid and short course, and are formed by the melting of the 

 snow, the hour of the day makes a considerable difference in 

 their volume. In the evening the stream is muddy and full, 

 but about daybreak it becomes clearer and much less impetuous. 

 This we found to be the case with the Rio Vacas, and in the 

 morning we crossed it with little difficulty. 



