Talc-schists.
Talc-schists, though not widely distributed, are abundant in certain localities. They form the main rock in many of the hills in the district called Um el Huetat, to the west of Gebel Atut in latitude 25°, where there are numerous old mines. In this district there are immense masses of talc-schist of remarkable purity. The rock [10,364] is of a grey to green or brown colour, distinctly schistose, cleaving easily with a soapy feel, and easily scratched by the finger nail; more massive forms occur which can be easily carved into pipes, and these are frequently smoked by the Bedouin. I descended one of the old shafts at Um el Huetat, and found no evidence of other minerals having been worked than the talc itself. A somewhat more earthy variety of talc-schist [10,396] was met with near the Rod el Ligaia. Talc-schists also occur at Gebel Sikait [10,383] and Gebel Zabara, where, as at Um el Huetat, they alternate with mica and hornblende schists. But at Sikait and Zabara the talc-schists are decidedly subordinate to the mica-schist in which the beryls are found.
To the class of talc-schists is also probably best referred a fissile and very rotten purple-brown schist, containing cubes of limonite, which was found near the triangulation station on Gebel Hamida, and again near the old mines of Romit. The microscopic slide of the Romit specimen [12,139] shows the bulk of the rock to consist of talc and chlorite, with blotchy stains and floculent-looking masses of brown iron oxide and occasional clear irregular granules of quartz. In this are plentifully embedded perfect little cubes of limonite. The limonite cubes [12,129 and 12,136] weather out easily from the rotten matrix, and can be gathered from the ground at the foot of the exposed surfaces.
Calcareous Schists and Marble.
Some very curious calcareous schists are found in Gebel el Anbat and its neighbourhood, near the Wadi Hodein. In Gebel el Anbat itself a mass of these rocks rises to a height of over 250 metres above the wadi level. They are earthy-looking rocks of varying colour, chiefly brown and reddish, often purplish on the rough weathered surfaces, and sometimes have a talcose feel. A typical specimen [11,532 A] has a sp. gr. of 2·92, and the slide cut from it shows it to be almost entirely composed of grains of calcite, with here and there larger irregular granules of quartz and of what looks like altered felspar, and a liberal sprinkling of iron oxide. It is difficult to assign an origin to this rock, but a variation of it [11,532 B] has been already described ([p. 339]) as probably an altered syenite, and the main rock may therefore represent the extreme form of alteration of an igneous rock rich in lime felspars.
A hard close-grained reddish-brown rock of sp. gr. 2·91, with dark streaks [12,107], which forms a high ridge, swathed in blown sand, rising to 350 metres above sea-level near the Wadi Kreiga, eleven and a half kilometres to the south-east of Gebel Beida, resembles a quartzite in appearance, but turns out on careful examination to be a calcareous schist. The microscopic section shows a very fine-grained mosaic of calcite with a little quartz, and scattered grains and strings of iron oxide. The origin of this rock is uncertain; it may be a metamorphosed limestone.
The summit of a high hill rising to 686 metres above sea, on the east side of the Wadi Um Khariga in latitude 24° 56′ 30″, is a gozzany mass resembling the outcrop of a mineral vein traversing the schists. On a fresh fracture, the interior of the rock [10,369] is seen to consist mainly of dark crystalline calcite, with some cubical crystals of pyrites, numerous rusty looking spots and patches of limonite, and veinlets of white calcite. The microscopic slide shows a mixture of calcite with kaolinic and serpentinous matter, with a very pronounced schistose structure, containing “eyes” of mixed calcite and iron oxides; the rock is therefore in reality a ferruginous calcareous schist. The mass is too highly metamorphosed for more than a guess as to its origin; but the slide contains some granules resembling picotite and one or two small patches of what looks like altering felspar, and the suggestion is that the schist is a metamorphosed basic igneous vein.
White crystalline marble has been found only at one point, namely, about three kilometres south of the jagged peak called Qash Amir, west of Gebel Elba, where it forms a small patch in crystalline rocks.
Graphite-schists.
A considerable mass of graphite-schist occurs associated with diorite rocks near some ancient gold workings at Gebel Allawi. The extent of the graphite-schist has not been mapped, but it occurs on the north slope of the mountain, and as seen from the summit it appears to run out westward for miles as a broad black band. The rock [10,378], which has a sp. gr. of 2·70, consists of a mixture of graphite with calcareous and earthy matter. The parts richest in graphite are readily cut with a knife, and mark paper easily. Analysis shows that the graphite, though sufficiently abundant to appear in the hand specimen the dominant mineral of the rock, is nevertheless present in surprisingly small proportion; a sample tested by Mr. Pollard was found to contain only a trifle over one per cent. of carbon.