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. [Illustration: 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.