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Updated: June 28, 2025
Phonolite, which I may observe is often, if not always, an injected rock, also, often has a fissile structure; this is generally due to the parallel position of the embedded crystals of feldspar, but sometimes, as at Fernando Noronha, seems to be nearly independent of their presence.
If this theory be correct, the volcanoes must have gone on dying out from west to east, from north to south, till only Kilauea remains, and its energies appear to be declining. The central mountain of this island is built of a heavy ferruginous basalt, but the shore ridges contain less iron, are more porous, and vary in their structure from a compact phonolite, to a ponderous basalt.
Almost at its foot, and in the midst of the mandelstein tract we have just been describing, a porphyritic phonolite, a mass of compact feldspar of a greenish grey, or mountain-green, containing long crystals of vitreous feldspar, appears exposed.
The formation of conical bosses of rock on lines of fissure, the walls of which are in most cases parallel, may probably be attributed to inequalities in the tension, causing small transverse fissures, and at these points of intersection the edges of the strata would naturally yield, and be easily turned upwards. Jago, however, the cones of phonolite, though tapering, have a regular form.
We ascended several hills in order to obtain general views, and found that the level country, over which we had travelled during the last two days, was of less extent than I had anticipated. To the north-east by east, ranges rise with the characteristic outlines of the basalt and phonolite, in peaks and long stretched flat-topped hills, with undulations openly timbered extending at their base.
It is surrounded by pyroxenic amygdaloid; it would no doubt be seen below, issuing immediately from gneiss-granite, like the phonolite of Biliner Stein, in Bohemia, which contains fragments of gneiss embedded in its mass.
From the highest accessible point of this hill, I could distinguish in different parts of the group several other conical hills, apparently of the same nature. At St. Helena there are similar, great, conical, protuberant masses of phonolite, nearly one thousand feet in height, which have been formed by the injection of fluid feldspathic lava into yielding strata.
Those geologists who have doubted whether granite could have been formed by igneous liquefaction, because minerals of different degrees of fusibility impress each other with their forms, could not have been aware of the fact of crystallised hornblende penetrating phonolite, a rock undoubtedly of igneous origin.
The coarse granite of Harney Peak indicating that, as the central point of the earliest upheaval, and the three porphyries known as rhyolite, trachyte, and phonolite, showing the uplifts of later periods to have had their centers a little more to the north, but the entire area is said to be only about sixty miles long and twenty-five miles in width.
The most remarkable feature is a conical hill, about one thousand feet high, the upper part of which is exceedingly steep, and on one side overhangs its base. The rock is phonolite, and is divided into irregular columns. On viewing one of these isolated masses, at first one is inclined to believe that it has been suddenly pushed up in a semi-fluid state. At St.
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