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Updated: May 19, 2025
The separation of the ingredients of a mass of lava, would, perhaps, sometimes take place within the body of a volcanic mountain, if lofty and of great dimensions, instead of within the underground focus; in which case, trachytic streams might be poured forth, almost contemporaneously, or at short recurrent intervals, from its summit, and basaltic streams from its base: this seems to have taken place at Teneriffe.
Trap Rocks. Name whence derived. Minerals most abundant in Volcanic Rocks. Similar Minerals in Meteorites. Theory of Isomorphism. Basaltic Rocks. Trachytic Rocks. Special Forms of Structure. The columnar and globular Forms. Trap Dikes and Veins. Alteration of Rocks by volcanic Dikes. Conversion of Chalk into Marble. Intrusion of Trap between Strata.
Proceeding still further south into the interior of the island, we here find a lofty plateau of an average elevation of 2,000 feet, interposed between the Tertiary beds of the Upper and Middle Waikato, and formed of trachytic and pitch-stone tuff, amongst which arise old extinct volcanic cones, such as those of Karioi, Pirongia, Kakepuku, Maunga Tautari, Aroha, and many others.
The talus now narrowed; the side-walls of dark trachytic blocks pinching it in. At this grisly hour they showed the quaintest figures towers and pinnacles, needles and tree-trunks, veiled nuns and monstrous beasts. Amongst them were huge bombs of obsidian, and masses with translucent, vitreous edges that cut like glass. Most of them contained crystals of felspar and pyroxene.
The probabilities appear to be that they are of Miocene age; and if so, the trachytic lavas, which in Antrim are older than those containing plants, may be referred to a still earlier epoch namely, that of the Eocene.
But a similar cause can hardly apply to the separation of the crystals of these lava-streams. The sinking of crystals through a viscid substance like molten rock, as is unequivocally shown to have been the case in the experiments of M. Dree, is worthy of further consideration, as throwing light on the separation of the trachytic and basaltic series of lavas. Mr.
Darwin attributes the lamination and fissile structure of volcanic rocks of the trachytic series, including some obsidians in Ascension, Mexico, and elsewhere, to their having moved when liquid in the direction of the laminae. The zones consist sometimes of layers of air-cells drawn out and lengthened in the supposed direction of the moving mass.
On a casual inspection, the whole series would be thought to be of the same plutonic or volcanic nature with the trachytic varieties and pitchstone; but this is far from being the case, as much of the porphyry is certainly of metamorphic origin.
The mountain Labo appears from its bell-shape and the strata exposed in the river bed to consist of trachytic hornblende. Half a league W.S.W., after wading through mud a foot deep, we reached the mountain Dallas where lead-glance and gold were formerly obtained by a mining company; and to the present day gold is obtained by a few natives in the usual mode.
Blanford says, with regard to this group, that there is a remarkable resemblance in its physical aspect to the scenery of the Deccan and the higher valleys of the Western Ghats of India, but the peculiarities of the landscape are exaggerated in Abyssinia. Many of the trachytic beds are brecciated and highly columnar; sedimentary beds are also interstratified with those of volcanic origin.
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