Fig. 55.—Hornblende-schist (probably a metamorphosed fine-grained diorite), Wadi Muelih [10,357], × 40. h, pale green hornblende; f, mass of decomposed felspar, containing abundance of finely acicular hornblende and some little patches of quartz mosaic; m, magnetite.
Schists derived from Diorites.—Hornblendic schists which appear to have been produced by the metamorphism of fine-grained diorites occur largely in the low hill country round the upper parts of the Wadi Muelih, and in the mountain mass of Gebel Abu Gurdi. A specimen of the less altered rock from the Wadi Muelih [10,357] is a fine-grained hard grey basaltic-looking rock, of sp. gr. 3·04, in which the schistosity in not very evident, though it is well seen in the mass. The microscopic slide shows pale green hornblende in ragged fibrous forms in a matted-looking clouded ground mass made up of finely acicular green hornblende and plagioclase, with a little quartz and orthoclase and a few grains of magnetite. Hornblende fibres are often enclosed in the decomposing felspars, which rarely show definite outlines and appear much shattered. The quartz is probably of secondary formation, occurring as little patches of mosaic.
The schist which forms the summit of Gebel Abu Gurdi [10,416] is probably also a crushed and altered diorite; it is a rather fine-grained hard greenish-grey rock, which with a lens can be seen to be a mixture of greenish-white felspathic material and dark hornblende; the felspar is mostly dull, while the hornblende, on the other hand, is frequently in shining crystals. The sp. gr. of the rock is 3·02. The microscopic slide shows the hornblende to be of a very pale green colour, in large irregular crystals, often including felspars. The felspar appears to be plagioclase; it is typically in smaller crystals than the hornblende, with a strongly marked tendency to idiomorphism, highly cracked and almost entirely changed to kaolin. Some rather large straggling crystals of a dark brown clouded and semi-opaque highly refracting mineral, white by reflected light, are perhaps altered sphene. Round the larger crystals is more finely crystalline matter, much clouded, apparently composed of altered felspar and hornblende.
The crushed dioritic dykes already mentioned as cutting the quartz-schist to the south-west of Gebel Abu Gurdi are possibly offshoots from the same magma which formed the main mass of the mountain. Specimen [10,415], taken from one of these dykes, is a fine-grained grey rock, of sp. gr. 2·93, with an even more decided schistosity than that of the main mountain. The microscopic slide shows clear hornblende of a green colour, strongly pleochroic (greenish-yellow to blue-green) in irregular grains which are frequently aggregated into nests and strings running in the direction of foliation of the rock; the remaining material is a mosaic of clear quartz, clouding kaolin, and sericite, presumably representing altered felspar.
Fig. 56.—Schist composed of fragments of various volcanic rocks, summit of Gebel Abu Hamamid [10,397], × 30.
Schists formed by crushing of Volcanic Rocks.—Schists derived from the crushing of volcanic rocks are very abundant in the mountains round Gebel Abu Hamamid and in the Wadi Beida. Typical specimens from the summit of Gebel Abu Hamamid [10,397] are hard green to grey rocks, of sp. gr. 2·7, breaking with a rough dull fracture. With a lens, spots and strings of dull white matter, with ill-defined outlines, are seen in a green to nearly black ground mass. The microscopic slides show the rock to be a breccia rather than a simple crushed rock, for in the same slide very various structures can be seen. Some portions, evidently andesitic, consist of perfectly idiomorphic lath-shaped felspars scattered with a little decomposed hornblende in a glassy brown ground mass. Others, more abundant, seem to be altered quartz-diorite-porphyrite; in these parts, porphyritic quartz and orthoclase crystals, in forms strongly inclined to idiomorphism, clouded and strained, are scattered in a cryptocrystalline ground mass containing a good deal of green chlorite and epidote. In other parts of the slide, again, the chief porphyritic constituent is formed by large green grains, which have evidently once been biotite or hornblende, but which now consist of chlorite. In yet other parts of the rock we have fragments of devitrified glassy lava. The different parts, which are not always well outlined, are separated by schistose bands composed mainly of strings of chlorite and epidote. Whether the rock is a tuff, or a crushed conglomerate of igneous boulders and pebbles, or due to complicated crushing of a series of contiguous igneous rocks in situ, is not quite clear. In some places, especially about the Wadi el Sheikh, the schists look like crushed conglomerates, but these may possibly be rocks crushed in situ rather than accumulations of rolled fragments transported by streams.
The hornblende-schists which surround Gebel Zergat Naam are of a peculiar type of which the origin is not evident, but are possibly altered andesites. The typical rock [11,526] is a hard grey basaltic one of sp. gr. 2·95, with a rusty-brown skin on exposed surfaces. The microscopic slide shows little plates and brushes of nearly colourless hornblende, liberally scattered in a clouded ground mass consisting mainly of hornblende fibres in radiating and dendritic groups, with a small amount of kaolinic matter and a few grains of magnetite. The slide contains no distinct quartz, but possibly a small amount of this mineral may be present in very minute grains with the kaolinic matter.
To the metamorphism of andesitic lavas, too, are somewhat doubtfully ascribed the grey and green schists of the Wadi Beida, which have specific gravity of about 2·75. The slides from the grey variety of the schists [12,116 and 12,159] show a very fine-grained clouded rock, apparently a mosaic of quartz, kaolin, chlorite and sericite, with larger scattered irregular plates of dark green chlorite. In a pale green variety from the head of the wadi [12,111], there is less chlorite and a considerable amount of calcite.