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.
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.