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. [Illustration: 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.] Under the microscope the rock from Gebel Elba [12,118] is seen to consist of quartz, orthoclase, a little oligoclase, hornblende, and small quantities of biotite (_see_ Fig. 7). The orthoclase is somewhat clouded in strips parallel to the clino-axis of the crystals. The hornblende is in irregular grains of sharply marked curved outlines; it is of a very deep bottle-green colour. The accessory biotite is brown, occurring as small wisps associated with the hornblende. The rock of Gebel Muelih [10,355] is similar to that of Gebel Elba, but the hornblende here is of an extremely pale greenish brown colour, only slightly pleochroic, with well-marked vertical cleavage and a very small extinction angle, so that it might be mistaken for biotite, which latter mineral appears to be absent. [Illustration: Fig. 8.—Hornblende-granite, Gebel Hamata [10,405], × 10. _q_, quartz (the rock contains a larger percentage of quartz than appears in the drawn portion of the slide); _f_, felspar (orthoclase), clouded; _h_, hornblende, altered.] Another type of hornblende-granite, differing from that last described in being of coarser grain and containing much more abundant hornblende, and thus having much more the appearance of an ordinary grey granite, occurs in the lower part of Gebel Hamata and the surrounding mountains. This rock [10,405] is slightly heavier than the foregoing type, having a sp. gr. of 2·66. Under the microscope (_see_ Fig. 8) the hornblende is seen to be of a dark green colour, very much altered and clouded by separated iron oxides and epidote. A third type of hornblende-granite is formed by the variation of the normal pink granite already referred to, in which the place of biotite is largely taken by hornblende. The mass of Gebel Mishbih appears[129] to consist of a rock of this type, which passes by insensible gradations into a syenite. =Granite-porphyry.=