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.
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.
Fig. 9.—Granite-porphyry, Kreishim Hill [12,150], viewed between crossed nicols, × 10. q, quartz (the dark triangular area to the left of figure is also a quartz crystal in the position of extinction); f, felspar (oligoclase); g, micro-granitic ground mass.
Under this heading are comprised rocks of granitic composition in which porphyritic crystals of quartz, felspar, and mica are surrounded by a fine-grained ground mass of entirely granitic (holocrystalline) character. They differ from porphyritic granites in the much finer grain of the general body of the rock, and from the quartz felsites in the entire absence of glassy matter from the ground mass. Rocks of this type are scarce in South-Eastern Egypt. An example [12,150] occurs at the hill called Kreishim, sixteen kilometres west of Halaib. Here the rock, which appears to form a small boss, consists of rounded white porphyritic crystals set in a finely crystalline grey ground mass. The sp. gr. is 2·69. Under the microscope ([Fig. 9]) the porphyritic crystals are seen to consist of quartz and oligoclase, the latter in approximately idiomorphic crystals, often beautifully zoned; the ground mass, of micro-granitic structure, is made up of smaller allotriomorphic crystals of quartz, felspar, and biotite.