April, 1835. 



SILICIFIED TREES. 



407 



nean forces exerted their power, and I now beheld the bed of 

 that sea forming a chain of mountains more than seven thou- 

 sand feet in altitude. Nor had those antagonist forces been 

 dormant, which are always at work to wear down the surface 

 of the land to one level: the great piles of strata had been in- 

 tersected 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 that Cordillera itself is modern as compared 

 with some other of the fossiliferous strata of South America. 



April 1st. — 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, 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 appearing larger than in the low country ; as was the 

 case with the rivulet of Villa Vincencio. 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 

 due to the snow melted by the sun^s heat, the hour of the 

 day makes a considerable diff"erence in their volume. In 

 the evening the stream is muddy and full, but about day- 

 break it becomes both clearer and much less impetuous. 



