Fig. 11.—Quartz-felsite from dyke at Gebel Kolaiqo [12,145], × 10. q, quartz (a group of idiomorphic crystals); f, felspar (mostly orthoclase); g, microgranitic ground mass of quartz and felspar, with wisps of hornblende.
One of the quartz-felsite dykes traversing the schists at Gebel Kolaiqo [12,145] is a very similar rock to that of Gebel Hamata, except that here the ground mass, instead of being dark brown in the hand specimen, is of a reddish brown colour (see [Plate XXII]), and the porphyritic crystals are partly pink felspar and partly glassy quartz. The sp. gr. is 2·62. Under the microscope the quartz and felspar crystals are seen to be less crushed than in the Hamata rock, while the ground mass, instead of being of a matted structure, is microgranitic, with tiny elongated grains of green hornblende scattered through it (see [Fig. 11]).
Another quartz-felsite dyke at Gebel Kolaiqo [12,135] has a rather different composition, and in the hand specimen has more the appearance of a felspar porphyry, porphyritic white felspars being scattered with quartz grains through a dark grey to black ground mass (see [Plate XXII]). Its quartz crystals frequently approximate to idiomorphic forms; they are much corroded by the ground mass. The felspar is chiefly oligoclase, in more or less idiomorphic crystals, a little clouded by decomposition products. The ferro-magnesian mineral here is chiefly brown biotite, in crystals of smaller size than those of the quartz and felspar, scattered porphyritically through the ground mass. One of the quartz crystals includes a crystal of biotite. The ground mass is crystalline, and appears to consist of quartz and felspar with a little biotite and some glassy matter.
A peculiar type of quartz-felsite [10,377] occurs associated with serpentine and ophicalcites in the upper part of Gebel Ghadir. The rock, which has a sp. gr. of 2·66, is nearly white, with a faint greyish tinge and scattered reddish-brown specks. At first the rock was taken for a granulite, which it very much resembles in appearance, and the reddish-brown specks for garnet. But examination with a lens shows the specks to be ferric oxide, and here and there little crystals of glassy quartz can be seen. Under the microscope the rock is found to consist mainly of an extremely fine-grained semi-granulitic colourless ground mass in which are sparsely scattered clear crystals of quartz, sometimes showing corrosion, and straggling irregular patches of opaque iron oxide, often mixed with nearly colourless wisps of mica. A few of the clear crystals are in four-sided forms, and may possibly be felspar; but they are perfectly free from alteration or twinning. The iron oxide is doubtless the product of alteration of a ferro-magnesian mineral, probably biotite. The minerals of the ground mass are difficult of determination owing to the minuteness of the grains and the absence of any colour, but apparently consist of quartz, felspar, a colourless hornblende, and sericite.
The quartz-felsite dykes [10,356] which cut through the granite and schists of Gebel Muelih appear as grooves in the granite, and as back-bone ridges in the surrounding schists. They are pinkish-brown close-textured rocks, with a dark marbling in places. The microscopic slides show quartz and felspar in a confused ground mass, with little flakes of a white micaceous mineral (sericite?) aggregated round the porphyritic crystals and distributed through the felspars and the ground mass.
The highly altered quartz-felsite [12,158] which forms Gebel Butitelib, near Gebel Niqrub el Tahtani, appears to have formed a boss or neck in the surrounding schists. The rock is a very fine-grained grey to purplish one, in which no crystals can usually be made out with the unaided eye. The sp. gr. is very low, being only 2·32. Under the microscope one sees small highly corroded crystals of quartz scattered through a clouded and glassy ground mass. No traces of felspars can be seen, the crystals, if they existed, having been decomposed and become undistinguishable from the ground mass; nor can any ferro-magnesian minerals be made out, though there are tiny granules and wisps of iron oxides which in places show an arrangement suggestive of their having been derived from the decomposition of such minerals.
The specimen [12,121] brought back from the summit of Gebel Shendib, by the guide who was sent to erect the triangulation beacon on it, is a compact brown rock with reddish patches, which under the microscope appears to be an altered and brecciated felsite.[130] The slide shows quartz and altered felspars in a spherulitic ground mass, the whole of the mineral being very much clouded by finely disseminated ferric oxide.
The felsite dykes [11,538] which seam the granite of Gebel Um Reit are a still more highly altered rock. The dykes are about two metres wide, of a white colour, and resemble limestone in appearance; being soft, they have weathered more rapidly than the granite around them, leaving vertical-sided chasms. But for the manner of its occurrence, the rock would have been easily mistaken in the field for a sedimentary one. The microscope, however, confirms its eruptive origin. The slides cut from the rock show a confused and clouded semi-crystalline aggregate, containing clearer small areas of quartz. Even the quartz crystals are full of specks, and only the faintest traces can be seen of the felspars, which are so decomposed as to be hardly distinguishable from the ground mass.
Fig. 12.—Altered quartz-felsite, Gebel Igli el Iswid [10,372], as seen between crossed nicols, × 40. f, felspar (mostly orthoclase in the position of extinction); h, a crystal of hornblende; g, hemi-crystalline ground mass; m, micropegmatitic intergrowth of quartz and felspar, separating the porphyritic crystals.