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You may have come to it, as we did, from the Petrified Forests, where you saw the silicified trunks of thousands of gigantic trees or tree ferns, that grew millions of years ago, most of them uncovered, but many of them protruding from banks of clay and gravel, and in their interiors rich in all the colors of the rainbow, and you wonder if you may not now be gazing upon some petrified antediluvian city of temples and holy places exhumed by mysterious hands and opened up to the vulgar gaze of to-day.
The beds of conglomerate just referred to, and the extraordinarily numerous silicified trunks of fir-trees at Los Hornos, perhaps at Coquimbo and at two distant points in the valley of Copiapo, indicate that land existed at this period in the neighbourhood.
If the two fragments of the same wood are, one of them silicified and the other simply carbonized and preserved in sandstone, the diminution in volume will have occurred in all directions in the latter of the two. Figs. 9 and 11, which represent a portion of the fibrous region of Calamodendron wood, may give an idea of the shrinkage that has taken place therein.
After these considerations, I cannot doubt but that the jasper of Ascension may be viewed as a volcanic rock silicified, in precisely the same sense as this term is applied to wood, when silicified; we are equally ignorant of the means by which every atom of wood, whilst in a perfect state, is removed and replaced by atoms of silica, as we are of the means by which the constituent parts of a volcanic rock could be thus acted on.
These concretions are remarkable from the great number of large silicified bones, apparently of cetaceous animals, which they contain; and likewise of a shark's teeth, closely resembling those of the Carcharias megalodon. Shells of the following species, of which the gigantic Oyster and Perna are the most conspicuous, are numerously embedded in the concretions:
Kent collected for me some silicified wood and shells out of the concretions in the sandstone from Tome, situated a short distance north of Lirguen. They consist of: Natica australis, d'Orbigny, "Voyage, Part Pal." 2. Mactra Araucana, d'Orbigny, "Voyage, Part Pal." 3. Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal." 4.
A great many silicified trees are met with lying on the ground all over this part of the country; some are broken off horizontally, and stand upright; others are lying prone, and broken across into a number of pieces. One was 4 feet 8 inches in diameter, and the wood must have been soft like that of the baobab, for there were only six concentric rings to the inch.
Looking at the geology of the district, the plateaux on each side of the Rovuma are masses of grey sandstone, capped with masses of ferruginous conglomerate; apparently an aqueous deposit. When we ascend the Rovuma about sixty miles, a great many pieces and blocks of silicified wood appear on the surface of the soil at the bottom of the slope up the plateaux.
Robert Brown showed me in an agate, formed within a cavity in a piece of silicified wood, some little specks, which were only just visible to the naked eye: these specks, when placed by him under a lens of high power, presented a beautiful appearance: they were perfectly circular, and consisted of the finest fibres of a brown colour, radiating with great exactness from a common centre.
The fossil wood of the drift is sometimes beautifully silicified, and occasionally the trunks of trees are replaced by iron pyrites, but gold seems not to have been found as in the pyrites of similarly petrified trees in the drift of Australia. The formation of recent metalliferous veins is now going on, according to Mr. Phillips, in various parts of the Pacific coast.
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