1835.] SILICIFIED TREES. 333 
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 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 extraordi- 
nary view; red, purple, green, and quite white sedimentary 
rocks, alternating with black Javas, 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. = 
