Hornblende-schists and Crushed Volcanic Rocks.
Under the heading of hornblende-schists and crushed volcanic rocks are classified a great variety of rocks, ranging in colour from green to grey and almost black, in fissility from the finest lamellation to almost massive forms in which the schistosity is barely evident in the hand specimen, but all definable as fine-grained dark-coloured schistose rocks consisting largely either of hornblende or of its alteration products. Their origin is not always clear, but many of them appear on microscopic examination to be altered fine-grained syenites, diorites, and lavas, and it is probable that practically the whole are altered forms of various intermediate and basic igneous rocks. The reason for including the hornblende-schists and crushed volcanic rocks in one great group is the difficulty of separating the different classes in the field. They pass one into the other, and many rocks which in the field would be put down as hornblendic turn out on detailed examination to contain little or none of that mineral, its place being taken by decomposition products.
Schists of the types above defined occur probably more abundantly than any other class of metamorphic rocks in the South-Eastern Desert. They enter largely into the composition of some of the main mountain masses, such as those of Gebels Abu Hamamid and Abu Gurdi, besides covering great expanses of lower hill country. In the mountain ranges they rise in steep-sided ridges with knife edge summits, a good idea of which is given by the photograph of Gebel Abu Hamamid on [Plate VII.] In the lower hill country we have typically a dreary confused looking waste of thousands of dark hills and ridges separated by small winding wadis; sometimes there is a semblance of system in the distribution of the hills, owing to the occurrence of parallel dykes harder than the schists which they penetrate, forming thus back-bones for long lines of schist-ridges.
Fig. 54.—Schist produced by crushing of syenite, near Gebel el Anbat (Wadi Hodein). [11,532 B], × 30. f, crushed and clouded felspar, probably orthoclase; lc, mixture of calcite and limonite, produced by alteration of hornblende, no trace of which remains.
Schist derived from Syenite.—Among the schists occurring near Gebel el Anbat, near the Wadi Hodein, is one which appears to be a crushed and altered syenite. It is a speckled grey and reddish-brown rock with a dull and rather granular fracture, of sp. gr. 2·92. The slide [11,532 B] shows a rather coarse granitic mixture of clouded felspar and heavily iron-stained calcite. The felspar is considerably altered, showing no twinning, but between crossed nicols it extinguishes in irregular bands and is seen to be much crushed. The ferruginous and calcareous matter is almost certainly altered hornblende; it forms irregular masses, in which the iron oxide is mostly arranged in parallel lines as though along the cleavage planes of the parent mineral, while a clouded calcite fills up the spaces between the lines. It would seem that in addition to pressure, carbonic acid has been the main agent of metamorphism here, the ferro-magnesian silicate being attacked and its silica entirely removed, while the aluminous silicate of the felspar has remained to some extent unchanged.
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.
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.