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Updated: May 19, 2025
The Professor goes on to say that "at intervals along the skirts of the Carpathians, and in more central detached situations, volcanoes seem to have been in active operation, vomiting forth masses of trachytic and basaltic lava, which were sometimes mingled with the deposits forming under the waters of the lakes.
Amongst the group are trachytic domes such as the Mountain of Tobreonu in the Society Islands, rising to a height probably not inferior to that of Etna, with extremely steep sides, and holding a lake on its summit. The linear arrangement of some of the volcanic islands of the Pacific is illustrated by those of the Tonga, or Friendly, Group, lying to the north of New Zealand.
According to H. von Dechen, there are sheets of basalt older than the greater mass of the brown coal formation, and others newer than the trachyte; while dykes of basalt traversing the trachytic lavas are not uncommon.
Crossing some cultivated fields, a bold eminence of trachytic tufa, covered with scanty grass and tufts of brushwood, rises between you and the sea, forming part of a range of low hills, which evidently mark the ancient coast-line.
Although in plutonic rocks two distinct species, corresponding to the trachytic and basaltic series, do not exist, I much suspect that a certain amount of separation of their constituent parts has often taken place. I suspect this from having observed how frequently dikes of greenstone and basalt intersect widely extended formations of granite and the allied metamorphic rocks.
It can by no means be inferred that trachyte predominated at one period of the earth's history and basalt at another, for we know that trachytic lavas have been formed at many successive periods, and are still emitted from many active craters; but it seems that in each region, where a long series of eruptions have occurred, the lavas containing feldspar more rich in silica have been first emitted, and the escape of the more augitic kinds has followed.
AN unfathomable gulf twenty-five miles long, and twenty miles broad was produced, but long before historic times, by the falling in of caverns among the trachytic lavas of the center of the island. And these waters falling from the surrounding heights have taken possession of this vast basin. The gulf has become a lake, but it is also an abyss, and no lead-line has yet sounded its depths.
The greater number of the hills, however, are composed of a quite white, friable stone, appearing like a trachytic tuff. Obsidian, hornstone, and several kinds of laminated feldspathic rocks, are associated with the trachyte. There is no distinct stratification; nor could I distinguish a crateriform structure in any of the hills of this series.
There is every variety of composition in lavas; some are trachytic, as in the Peak of Teneriffe; a great number are basaltic, as in Vesuvius and Auvergne; others are andesitic, as those of Chili; some of the most modern in Vesuvius consist of green augite, and many of those of Etna of augite and labrador-feldspar.
In the clear winter-air we could distinctly trace the bold contour of the upper heights tipped by the central haystack, El Nublo, a giant trachytic monolith. We passed Confital Bay, whose 'comfits' are galettes of stone, and gave a wide berth to the Isleta and its Sphinx's head.
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