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. [Illustration: 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.] In the rocks of Gebels Igli el Iswid, Mahali, and Hadarba, we have quartz-felsites which have been altered in quite a different way. The rocks are extremely hard, and almost flinty, breaking with a sub-conchoidal fracture. They are typically of a brown to nearly black colour, with white spots measuring a millimetre or two across. Under the microscope these white spots are seen in the rock from Gebel Igli el Iswid [10,372] to be chiefly porphyritic felspar and quartz crystals, sometimes corroded by the ground mass; while the black ground mass is largely composed of extremely fine micro-pegmatitic intergrowths of quartz and felspar, with some glassy matter, and here and there irregular small clouded and altered crystals of hornblende. The micro-pegmatitic material has possibly originated by devitrification of an originally glassy ground mass. Variations of the rock occur in which the porphyritic felspars are infrequent or even altogether absent, the rock passing gradually into a hornfels [10,371]. The slide from Gebel Mahali [10,402] exhibits clear porphyritic crystals of quartz and orthoclase, both minerals in more or less idiomorphic forms, embedded in, and occasionally corroded by, a semi-crystalline ground mass. The ground mass contains some calcite and iron oxides, probably arising from the decomposition of a hornblendic or micaceous mineral.