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Updated: May 19, 2025
Its double cone forms the limit of a trachytic belt which stands out distinctly in the mountain system of the island. From our starting point we could see the two peaks boldly projected against the dark grey sky; I could see an enormous cap of snow coming low down upon the giant's brow.
The volcanic products of Mezen have been erupted from one central orifice of vast size, and consist mainly of extensive sheets of "clinkstone," a variety of trachytic lava, which have taken courses mainly towards the north-west and south-east.
Whatever may be thought of the explanation here advanced of the laminated structure of the rocks of the trachytic series, I venture to call the attention of geologists to the simple fact, that in a body of rock at Ascension, undoubtedly of volcanic origin, layers often of extreme tenuity, quite straight, and parallel to each other, have been produced; some composed of distinct crystals of quartz and diopside, mingled with amorphous augitic specks and granular feldspar, others entirely composed of these black augitic specks, with granules of oxide of iron, and lastly, others formed of crystalline feldspar, in a more or less perfect state of purity, together with numerous crystals of feldspar, placed lengthways.
These rocks, therefore, although at first appearing like baked clay or some altered sedimentary deposit, contain all the essential ingredients of trachyte; from which they differ only in not being harsh, and in not containing crystals of glassy feldspar. As is so often the case with trachytic formation, no stratification is here apparent.
It was a strange story which Charles Turold heard by that grey Cornish sea a story touched with the glitter of adventurous fortune in the sombre setting of a trachytic island, where wine-dark breakers beat monotonously on a black beach of volcanic sand strewn with driftwood, kelp, dead shells, and the squirming forms of blindworms tossed up from the bowels of a dead sea.
To show how exceedingly firm this recent conglomerate is, I may mention, that I endeavoured with a heavy geological hammer to knock out a thick bolt of iron, which was embedded a little above low-water mark, but was quite unable to succeed. FERNANDO NORONHA. Precipitous hill of phonolite. TERCEIRA. Trachytic rocks: their singular decomposition by steam of high temperature.
The ores of Tolfa, Milo and Nipoligo; those of Montione, in which silica does not accompany the alumina; the siliceous breccia of Mont Dore, which contains sulphur in its cavities; the alumiferous rocks of Parad and Beregh in Hungary, which belong also to trachytic and pumice conglomerates, may no doubt be traced to the penetration of sulphurous acid vapours.
The cones situated in the higher parts of the forest zone are chiefly clothed with lofty pines; while those at a lower elevation are adorned with chestnuts, oaks, and beech trees. These cones have from time to time been buried amidst fresh lava-streams descending from the great crater, and thus often become obliterated. Ideal Section through Etna. Older lavas, chiefly trachytic; b.
These lavas consisting of basalt and dolerite, with their varieties, and extruded from vents and fissures, spread themselves in all directions over the pre-existing lake deposits or the older sheets of augitic lava, and probably entirely buried the trachytic hills. These later sheets solidified into more solid masses than those of the second stage.
The soft white stone above mentioned is remarkable from its singular resemblance, when viewed in mass, to a sedimentary tuff: it was long before I could persuade myself that such was not its origin; and other geologists have been perplexed by closely similar formations in trachytic regions.
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