Clay-schists.
Clay-schists cover extensive areas in the South-Eastern Desert. They occur abundantly in the high hills to the east of Gebel Nugrus and form a large portion of Gebel Zabara. They abound also to the south of Wadi Antar about the longitude of Gebel Um Goraf, in the hills on the east side of Gebel Abu Dahr, in the Wadi Hodein round about where Wadi Um Tenedba joins it, in the hills flanking the sandstone plateau of Gebels Dif and Anfeib, at Gebels Kolaiqo and Eqrun, on the western flanks of the serpentine mass of Gebel Meneiga, and in the low hills on the plain between Gebels Gerf and Korabkansi.
Clay-schists are typically of a grey colour, though greenish, purple, and reddish varieties occur. They are soft rocks, easily scratched with a knife. In some places, as for instance at Gebel Zabara, they are practically slates, cleaving into slabs often of considerable size, while in other localities, as for instance near Gebel Abu Dahr, they are crushed into matchwood-like splinters, which can be used as slate-pencils. They are usually associated with other metamorphic rocks such as crushed diorites and hornblende and mica-schists. The direction of foliation of the slaty and slabby forms is often persistent—at Gebel Zabara, for instance, a general south-easterly dip of about 28° is maintained over a considerable area; but there are numerous local variations, and the number of observations thus far made is too small for any general conclusions to be drawn as to the regional distribution of dip. On the whole, the clay-schists are remarkably free from knots or other irregularities, though knotted varieties, doubtless the result of contact alteration, have been observed on the south side of Wadi Seiga and at a few other places.
The clay-schists have probably for the most part been produced by the metamorphism of ancient argillaceous sediments; in some places, as for instance in the Wadi Beida, a passage can be observed into what appears to be a crushed conglomerate, while in other places they pass into quartzites which doubtless represent altered intercalated sandstones. The relation between the direction of the foliation-planes and that of the original bedding is unknown; it would appear likely that in most cases the traces of bedding have been entirely obliterated by pressure. Nor have any fossils been found in the rocks, though a sharp look-out was kept in the more likely-looking localities, so that the age of the beds is unknown. At present I am inclined to regard them as most likely of Archæan age, antedating the great granitic intrusions; but further evidence, either in the finding of fossils or from a more careful study of the field relations of these rocks to the others, is required before one can be certain that they are of so great an antiquity.