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Updated: June 9, 2025
The basement beds of tuff and gravel contain, besides pebbles of flint and chalk, others of sanidine trachyte, showing that highly acid lavas had been extruded and consolidated before the first eruption of the plateau-basalts; another point of analogy between the volcanic phenomenon of Antrim and the Inner Hebrides.
A gutter leads from the mouth, showing signs of water-wear, and the blocks of trachyte are so loaded with glossy white felspar that I attempted to dust them before sitting down. Local tradition connects this ice-cave with the famous burial-cavern near Ycod, on the northern coast; this would give a tunnel 8 miles long and 11,040 feet high.
These veins are extraordinarily numerous, intersecting in the most complicated manner both coloured varieties of the earthy trachyte: they are best seen on the flanks of the "Crater of the old volcano."
The ground rose appreciably as it moved away from the sand flats by the waves, and we soon arrived at some long, winding gradients, genuinely steep paths that allowed us to climb little by little; but we had to tread cautiously in the midst of pudding stones that weren't cemented together, and our feet kept skidding on glassy trachyte, made of feldspar and quartz crystals.
The Magdala group is unconformable to that of Ashangi in some places. A still more recent group of volcanic rocks appears to occur in the neighbourhood of Senafé, consisting of amorphous masses of trachyte, often so fine-grained and compact as to pass into claystone and to resemble sandstone. At Akub Teriki the rocks appear to be in the immediate vicinity of an ancient vent of eruption.
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.
Boussingault, in company with Colonel Hall, accomplished the ascent as far as the foot of the mass of columnar "trachyte," the upper surface of which, covered by a dome of snow, forms the summit of the mountain.
The range consists of a succession of conical hills sometimes flat-topped as in the case of Petersberg; and at the Drachenfels, near the centre of the range it presents to the river a bold front of precipitous cliffs of trachyte porphyry.
This stone had reappeared in large quantities, the moment we crossed the divide; the pale grey of the Jebel Ziglab and its neighbours was evidently owing to its presence; and from this point it will be found extending southwards and seawards as far as El-Hejaz. He brought with him a hard white stone much resembling trachyte, and fragments of fine green jasper.
Trachyte, which consists chiefly of feldspar, with some hornblende and oxide of iron, has a specific gravity of about 2.45; whilst basalt, composed chiefly of augite and feldspar, often with much iron and olivine, has a gravity of about 3.0. This order of eruption has been observed by Beudant, Scrope, and by other authors; three instances, also, have been given in this volume.
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