Hornblende Schists of doubtful origin.—In the more highly foliated and harder varieties of hornblende schist, which are typically of a darker colour than most of those already described, we have rocks in which the process of re-crystallization has been so complete that no trace of the original rock remains. These rocks, which are true hornblende-schists in the narrowest sense of the term, are less abundant than the foregoing types, and generally occur as comparatively narrow bands associated with gneisses and mica and talc schists. All the minerals in them being of secondary origin, they are usually in a quite fresh state, and fractured surfaces, examined with a lens, exhibit a mass of glistening small crystals of hornblende, quartz, and felspar. The density is usually about 2·9. Microscopic slides show elongated crystals of clear green hornblende, strongly pleochroic (pale yellowish-green to deep blue-green) arranged along the planes of schistosity, separated by clear granules of quartz and a little felspar, with scattered magnetite grains.
Fig. 57.—Hornblende-schist, near Gebel Eqrun [12,117], × 30. h, hornblende; q, quartz; m, magnetite. A little felspar is present mixed with the quartz, from which it is distinguishable only in polarised light.
Near Gebel Eqrun are found hornblende-schists which exhibit a curious banding in planes at right angles to the main foliation, in the form of darker lenticular stripes a few millimetres wide and about the same distance apart. A slide [12,117] cut from this variety shows little trace of the banding, the lighter spaces between the dark bands merely showing a clouding of the quartz and felspar by tiny granules of epidote. The stripes are most probably the consequence of a secondary compression in a direction perpendicular to the original one rather than relics of a banded structure in the parent rock.
Actinolite-schists.—Very beautiful bright green schists, in which the hornblende is in the fibrous to silky form called actinolite, occur in small quantity associated with mica and talc schists at Sikait [10,380] and elsewhere. In these rocks the actinolite fibres, which often reach two centimetres in length, are generally aggregated into bundles, with radiating structure. In the microscopic slide the rock presents even a more beautiful appearance than in the mass, the long fibres of actinolite polarising in the most brilliant tints; associated with the actinolite, there is nearly always more or less chlorite and talc.
Hornfels.
Associated with the schists of Gebel Abu Hamamid and the neighbouring mountains there are great masses of very hard horny-looking rock [10,401] of green to grey colour, breaking with a sub-conchoidal fracture, and of such close texture as to appear homogeneous even with a strong lens. The pyramidal peak called Gebel Um Semiuki, which rises to 1,282 metres above sea, three kilometres to the north-east of Gebel Abu Hamamid, is almost entirely composed of rocks of this type; in the mountain faces the rock looks red, but this is only due to a film covering weathered surfaces, the interior being of a green to grey colour. The rock, which has a sp. gr. of 2·71, is frequently beautifully banded, light and dark layers alternating with each other, and often contains tiny cubes of pyrites [10,399]. The microscopic slide from Gebel Um Semiuki shows a very fine-grained clouded compact rock, apparently consisting of glassy matter with minute granules of quartz and altered felspar, together with a little sericite, the latter especially along certain bands. The slide from Gebel Abu Hamamid is similar, but here the granules of quartz and felspar are a little larger, though they are still too small to be seen with a lens in the hand specimen; the appearance is that of a quartz felsite on a small scale. It has already been mentioned ([p. 281]) that the quartz felsites of Gebels Igli and Hadarba pass gradually into hornfels, and when we remember that the schists of the Abu Hamamid district are mostly crushed volcanic rocks, it becomes almost certain that the hornfels associated with them is a crushed and devitrified glassy lava of acid composition.
A yellowish horny rock with grey streaks [10,379], which occurs near Gebel Sabahia, is conspicuous in the field owing to its weathered surfaces being covered with a bright red ferruginous skin, resembling cinnabar in colour. The sp. gr. is 2·52. Examination with a lens shows the grey streaks to be filled with myriads of brilliant yellow specks of pyrites. The microscopic slide shows these to be aggregates of little cubes, while the bulk of the rock is a schistose felsitic mass of quartz and felspar, with scattered larger felspar crystals, much broken up and bent. In this rock too we have therefore a rolled up and altered felsite.
Mica-schists.
Mica-schists, composed mainly of golden-brown lustrous laminæ of biotite with more or less quartz, occur near the base of Gebel Zabara and at Gebel Sikait [10,626], as well as in small quantity at one or two other points. They are always associated with gneiss, and appear to form irregular bands, alternating and mixed with talc and other schists. The laminæ of mica can seldom be separated in any large size, breaking up at a touch into small scales; they are often highly contorted.