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.