Fig. 6.—Biotite-granite near Gebel Abu Hegilig [10,390], × 17. q, quartz; f, felspar (orthoclase and oligoclase), somewhat turbid by decomposition; b, biotite; s, sphene.
Biotite-granite of a well-defined type [10,390] occurs somewhat largely in the mountains round the heads of Wadi Abu Ghusun and Wadi el Abiad. It is a whitish rock with dark-brown to black spots (see [Plate XXII]). The white portion of the rock consists of quartz and felspar; the dark spots are biotite. Generally the rock is of medium grain, the biotite being in nests two to four millimetres diameter scattered through the mass, giving it a speckled appearance. Sometimes the biotite is more uniformly scattered, giving the rock a grey aspect. At some places the rock shows signs of crushing, this being indicated in hand specimens by a tendency to laminar arrangement of the biotite patches. Under the microscope (see [Fig. 6]), the felspars, which are somewhat more abundantly present than the quartz, are seen to consist of orthoclase and oligoclase, with here and there crystals showing the characteristic cross hatching of microcline. The biotite is mostly in nests and clusters of ragged-looking crystals, but it also occurs in tiny flakes included in the felspars, sometimes showing a distinct arrangement along the cleavage planes of the latter. The biotite is usually fairly fresh; it is highly pleochroic, with colour ranging from pale yellow to deep olive-green in different positions over the nicol. Associated with the biotite are a few wisps of colourless mica, probably muscovite, and irregular granules of opaque iron oxides. Granules of epidote are to be seen in the altering felspars, and there are a few small irregular granules of a brownish highly refracting mineral, probably sphene.
A much more acid type of biotite granite [11,507] occurs in close proximity to serpentine in low hills near the wells of Abraq. In this the proportion of biotite is so small that the rock might almost be classed as an aplite. It is a fine-grained white rock, very fresh-looking, with dark spots; in hand specimens it almost exactly resembles the hornblende granite of Gebel Elba (see [Plate XXII]), but examination with a lens shows the dark spots to consist of biotite instead of hornblende. The microscopic slide shows quartz, orthoclase, and oligoclase with granitic structure, with here and there small straggling crystals of brown biotite; the brown biotite is sometimes altering to chlorite with change of colour from brown to green.
Muscovite-granite is rarely met with in South-Eastern Egypt. A specimen from Gebel Adar Qaqa[128] [12,138] is a fine-grained hard pinkish rock of sp. gr. 2·62, composed of quartz, orthoclase, oligoclase, muscovite, and garnet.
A very coarse-grained variety of muscovite-granite, in which the individual crystals measure several centimetres across, forms white hills to the east of Wadi Nugrus.
Hornblende-granite occurs in several forms, giving rise to rocks of different aspects in different parts of the district.
The most important and widely distributed form of hornblende-granite is a fine-grained and very acid rock of sp. gr. 2·59, which but for the manner in which it occurs might almost be spoken of as an aplite. It is a white rock, sometimes with a faintly pink or greenish cast, of medium to fine grain, with black specks of hornblende about a millimetre in diameter scattered through it (see [Plate XXII]). This rock forms Gebels Muelih and Elba; it also occurs in the low country round Gebel Hamrat Mukbud, and a gneissose variation of it forms the great boss of Um Rasein. Dykes of the same rock traverse the more basic rocks of Um Bisilla.
Fig. 7.—Hornblende-granite, Gebel Elba [12,118], × 17. q, quartz; f, felspar (mostly orthoclase), clouded by kaolinisation; h, hornblende; b, biotite, altering with separation of flakes of limonite.