Breccias.
A very curious rock [10,385] which from a distance looks like a giant diorite, occurs as a band in gneiss on the west side of the lower part of Wadi Nugrus, is doubtless a fluidal breccia. It contains ovoid masses of white aplite, sometimes measuring five centimetres in diameter, embedded in a dark fine-grained dioritic matrix with a marked tendency to schistose structure. Most likely the rock has been formed by the catching up of the fragments of a crushed aplite in an intrusive diorite, and then the whole mass has been subjected to the same pressure which foliated the surrounding gneisses.
More normal breccias are found at various points. One which occurs between schists and hornfels near the summit of Gebel Abu Hamamid [10,398] is made up of fragments of various altered volcanic rocks, with large black flint-like lumps of hornfels, all cemented into a very hard rock which breaks across the fragments composing it. This breccia was doubtless formed by the same movements which produced the schists of the summit of the mountain; these latter (see [p. 341]) are themselves almost as much fine breccias as schists.
To the west of Gebel Zergat Naam the stones in the wadis are sometimes cemented into hard breccias by calcareous matter, doubtless deposited by drainage waters which have dissolved out the lime from felspathic rocks.
In the hill called Ti Keferiai, a little below the triangulation point which marks the summit, a highly altered fine-grained dioritic rock, containing much epidote, has been crushed into a coarse breccia [12,123] cemented by rose-coloured quartz.
A remarkable green breccia is found in Gebel Hamata, where it appears to form a large mass in the mountain-side to the east of the main peak. This rock [10,407], which has a sp. gr. of 2·92, is darker in colour and somewhat softer than the ornamental “breccia verde antico” of the Wadi Hammamat district further north,[135] but it also is a very beautiful rock. In the hand specimen, it consists of black angular fragments, up to two centimetres in diameter, embedded in a dark green ground mass, the whole of very fine grain and barely scratchable with a knife. Under the microscope, the black fragments seem to be of basaltic nature, while the green matrix is probably a highly crushed and brecciated diorite-porphyrite; the whole of the slide is clouded by decomposition products.
Fault-breccias, produced by differential movement of the two sides of faults, occur in the neighbourhood of the Wadi Saalek, where the sandstones and schists are much faulted (see [p. 359]). The breccias here [11,539] are narrow bands which stand up like dykes; they are very calcareous and highly ferruginous, with occasional green stains, perhaps due to traces of copper.