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Show 406 PASSAGE OF CORDILLERA. March, 1835. the newer horizontal beds on the shores of the Pacific. From this resemblance I expected to find silicified wood, which is generally characteristic of those formations. I was gratified in a very extraordinary manner. In the central part of the range, at an elevation probably of seven thousand feet, on a bare slope, I observed some snow-white projecting columns. These were petrified trees, eleven being silicified, and from thirty to forty converted into coarsely-crystallized white calcareous spar. They were abruptly broken off; the upright stumps projecting a few feet abo,ve the ground. The trunks measured from three to five feet each in circumference. They stood a little way apart from each other, but the whole formed one distinct group. Mr. Robert Brown has been kind enough to examine the wood : he says it is coniferous, and that it partakes of the character of the Araucarian tribe (to which the common South Chilian pine belongs), but with some curious points of affinity with the yew. The volcanic sandstone in which they were embedded, and from the lower part of which they must have sprung, had accumulated in successive thin layers around their trunks ; and the stone yet retained the impression of the bark. It required little g~ological practice to interpret the marvellous story, which this scene at once unfolded; though I confess I was at first so much astonished that I could scarcely believe the plainest evidence of it. I saw the spot where a cluster of fine trees had once waved their branches on the shores of the Atlantic, when that ocean (now driven back 700 miles) approached the base of the Andes. I saw that they had sprung from a volcanic soil which had been raised above the level of the sea, and that this dry land, with its upI ·ight trees, had subsequently been let down to the depths of the ocean. There it was covered by sedimentary matter, and this again by enormous streams of submarine lava-one such mass alone attaining the thickness of a thousand feet ; and these deluges of melted stone and aqueous deposits had been five times spread out alternately. The ocean which received such masses must have been deep; but again the subterra- 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 thousand 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 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 ~tate ~hey 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 difference in their volume. In the evening the stream is muddy and full, but about daybreak it becomes both clearer and much less impetuous. |