1835.1 SILICIFIED TREES. 533 



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 

 tires, now changed into silex, were exposed projecting from the 

 volcanic soil, now changed into rock, whence formerly, in a 

 orreen 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 extraordi- 

 nary 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 dav we crossed the plain, and followed the course of 

 the same great mountain stream whicn 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, 



