The pegmatitic granite is usually of a far more pronounced red colour than the normal granite. This is doubtless in part due to the greater abundance of orthoclase, but it also arises in some cases from the orthoclase having itself a deeper colour. The rocks about the lower parts of Wadi Gemal and Wadi Kreiga have a strong brick-red aspect. Their proximity to the coast suggests that there may possibly be some connexion between the intensity of their coloration and their position near the sea; this idea is supported in some measure by the strong red colouration of the dioritic rocks of Ras Benas, which occupy a similar position, but the actual manner of production of the oxide of iron which gives the colour is not clear.
Fig. 5.—Red pegmatitic granite, from low hills near the mouth of Wadi Gemal [12,106], × 10. q, quartz; f, felspar (orthoclase) turbid and full of finely disseminated red oxide of iron.
In the hand specimen (see [Plate XXII]), the red pegmatitic granite [10,389] is a coarse-grained aggregate of red orthoclase and quartz. The quartz is generally of grey aspect in the mass, running in strings and networks through the felspar. The rock usually breaks easily into irregular pieces under the hammer, owing to the facility with which the large felspar crystals can be cleaved, but the very red variety from near the mouth of Wadi Gemal [12,106] is extremely hard and tough, and strikes fire very easily under the hammer. Under the microscope (see [Fig. 5]) the red colour of the orthoclase often persists even in the thinnest sections, being evidently due to finely disseminated iron oxide which clouds the felspars. The quartz shows sharply defined angular forms, and appears to have got somewhat the start of the felspar in crystallisation. Owing to its cloudy nature, the felspar looks like a ground mass, and between crossed nicols shows a peculiar patchy appearance, so that a first glance at the slide suggests a quartz porphyry; but the whole of the patches forming a crystal extinguish at once, and moreover the characteristic twinning of the orthoclase can be seen in some of the crystals. A little oligoclase is also present in addition to the orthoclase. Crushing of the rock is frequently shown by cracked and brecciated crystals, more especially in the hard form of the rock from Wadi Gemal.
Dykes of pegmatitic granite are very frequent in the neighbourhood of Gebel Hamrat Mukbud; they are typically rather paler in colour than the larger masses, and occasionally show graphic structure.
In ascending Gebel Migif, the gneiss slopes of the mountain were found to be strewn here and there with crystals of green microcline [10,366], similar to the well-known Pike’s Peak mineral. The crystals, which are of imperfect idiomorphic form, measuring about one to two centimetres in length, sometimes show cross-striations on certain faces. Though the rock was not traced in situ, the crystals are doubtless derived from pegmatite dykes traversing the gneiss.
Aplite, or fine-grained binary granite consisting essentially of quartz and felspar, with very little or no mica or hornblende, differs from pegmatite not only in its greater fineness of grain, but in its structure, which is essentially granitic rather than pegmatitic, the two main minerals being both in allotriomorphic grains. In colour, different aplites vary with that of their felspars from red to white, and most of them have tiny dark spots in them, sometimes only visible with a lens, due to the presence of small quantities of hornblende or biotite. As already mentioned on [p. 268,] most of the granites of South-Eastern Egypt incline to the acid or aplitic type, and one is frequently in doubt whether to classify an acid granite, such as that of Gebel Elba for instance, as an aplite with a little hornblende, or as a hornblende granite. Under aplites will be here considered only those granites in which the proportion of ferro-magnesian minerals is extremely small, not forming more than about one or two per cent of the rock. When the definition of an aplite is thus limited, aplites are comparatively scarce in the area.
Gebel Abu Hireiq consists of a red granitic rock; I did not visit the range, but the guide sent to erect the triangulation beacon brought back as the typical rock a pink aplite [12,134] of rather fine grain, consisting essentially of quartz and pink felspar, with sparsely scattered specks of hornblende.
The dykes [11,536] which seam the granite and diorite near where the Wadi el Kreim joins Wadi Garara may be classed as siliceous altered aplites. They are greyish-white to purplish-pink rocks of rather fine grain, with a few blackish specks, the latter visible only with a lens. The microscopic slide shows granitic quartz and red-stained decomposed matter, with a rather plentiful sprinkling of grains and powder of iron oxide; the decomposed matter is almost certainly the result of alteration of felspar and hornblende, the latter in very small proportion.
The hill called Marwot Rod el Ligaia, near the head of Wadi Muelih, is a boss of pink aplite, and is of interest as lying in close connexion with certain quartz veins which are believed to be of igneous origin (see [p. 266]).