SILICIFIED TREES. 319 



rauean forces exerted themselves, and 1 now beheld the bed of that 

 ocean, forming a chain of mountains more than seven thousand 

 feet in height. Nor had those antagonistic 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 list. 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, Avhich 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 Kio Vacas, and in the morning we crossed it with 

 little difficulty. 



The scenery thus far was very uninteresting, compared with that 

 of the Portillo pass. Little can be seen beyond the bare walls of 

 the one grand flat-bottomed valley, which the road follows up to 



