Dykes of granite-porphyry also occur traversing the granite of Gebel Kahfa [11,537 B] and in the basic rocks of Gebel Um Bisilla [11,518]. At both these places the rocks are far less fresh than that above described. In the hand specimen they are of a greyish-white or greenish-white colour, strongly resembling fine-grained aplites in appearance. With the lens, crystals of quartz and felspar can be made out, and specks of hornblende and chlorite. The microscopic slides show the characteristic structure of granite porphyry, but the felspars are all clouded by decomposition products, and the hornblende, which is rather sparingly present in the ground mass, is mostly changed to chlorite and epidote. The specimen from Gebel Kahfa contains a small amount of muscovite.
Quartz-felsite.
Though covering only small areas, quartz-felsite is of very wide distribution in South-Eastern Egypt. It typically forms dykes and intrusive sheets traversing the plutonic and metamorphic rocks, but likewise occurs occasionally in larger masses. Felsite dykes are specially abundant in the neighbourhood of Gebels Muelih, Zergat Naam, and Um Reit. Larger masses occur in the Wadi Huluz, and form the summits of Gebels Nigrub el Foqani and Hamata.
Felsites are frequently met with in a highly altered condition. The commonest alteration, especially in dykes, is kaolinisation of the felspars and a clouding of the whole rock with finely disseminated iron oxides. In some dykes traversing the granite of Um Reit, a bleaching action has reduced the felsite to the appearance of a limestone. In other cases the rock has been devitrified and indurated to a high degree; this is well seen in Gebel Igli el Iswid (latitude 25°) and at Gebel Hadarba (latitude 22°), where extensive hill-tracts consist of felsitic rocks of almost flinty hardness.
The quartz-felsites are in general among the youngest of the igneous rocks of the district, since they commonly form dykes and intrusions, not only in the schists, but also in the granites and other eruptive rocks.
Fig. 10.—Quartz-felsite, Wadi Huluz [10,394], × 10. q, quartz; f, felspar (orthoclase and oligoclase); g, ground mass, showing flow structure round the porphyritic quartz and felspar.
The quartz-felsite of Wadi Huluz [10,394] occurs in considerable masses about a kilometre below the water holes of Um Gerifat. In the hand specimen, it is of granitoid appearance, with opalescent quartz crystals two millimetres or more in diameter plentifully scattered in a light greyish ground mass. The sp. gr. is 2·71. Under the microscope the crystals of quartz are seen to have rounded forms and to be accompanied by other porphyritic crystals of orthoclase and oligoclase, often approximating to idiomorphic shape (see [Fig. 10]). The porphyritic constituents are embedded in a cryptocrystalline ground mass, in which augite and minute grains of quartz, felspar, and biotite can be made out, and there are some strings and granules of epidote and fairly large specks of iron oxides. The ground mass shows a fluidal arrangement, the little biotite flakes in it being often arranged in lines which sweep round the porphyritic crystals. The rock has undergone considerable alteration, the felspar crystals being clouded and full of tiny micaceous flakes of high double refraction; the epidote is also doubtless due to the alteration of augite and other minerals in the ground mass.
The quartz-felsite which forms the upper part of Gebel Hamata [10,906] is a dark brown coarse-textured rock, with remarkably glassy porphyritic quartz in granules about two millimetres diameter scattered plentifully through it. It is a very hard rock which rings under the hammer and weathers into rusty brown blocks. The sp. gr. is 2·71. Under the microscope the quartz crystals are seen to be much cracked. There are also porphyritic crystals of orthoclase and oligoclase, mostly of irregular shape, and a few crystals, of still more irregular form, of dark green hornblende. The cryptocrystalline ground mass consists chiefly of quartz and felspar, through which are scattered tiny granules of dark green hornblende. The ground mass shows a matted texture between crossed nicols; there is no trace of fluidal movement. Like the foregoing, this rock is somewhat altered, the hornblende in particular being very much attacked; the felspars are fairly fresh, but the crystals, like those of the quartz, are often cracked, and thus show the rock to have undergone considerable crushing.
