GREEN BRECCIA.
Gebel Hamata.
EMERALD AND QUARTZ IN MICA-SCHIST.
Gebel Sikait.
NATURAL SIZE.
As an example of gneisses in which the foliation is so intense that all original structures have been obliterated by recrystallization, we may take the red gneiss of the Abu Beid Hills [10,659], of which a coloured natural size representation is given on [Plate XXV.] In this rock, which has a sp. gr. of 2·65, the main constituent is a pink felspar, mixed with a little quartz and arranged in fine lenticles separated by greenish laminæ which are doubtless altering biotite. This rock bears a strong resemblance to the well-known red gneiss of the Erzgebirge, which, from its chemical composition, is usually believed to be a metamorphosed granite poor in mica.
Grey gneisses, equally intensely foliated with the Abu Beid rock, occur in the range of Gebel Hafafit; these contain a much larger proportion of biotite, and remind one of the grey gneisses of the Freiberg district. But as no analyses have yet been made of the Egyptian rocks, the absolute similarity to those of the Erzgebirge is not quite certain.
Quartzites and Quartz-schists.
About ten kilometres south of Gebel Abu Gurdi is a tract of low hills and ridges composed mainly of quartz-schists. The limits of these rocks have not been mapped, but they appear to cover a somewhat elliptical area with its long axis, measuring some eight or ten kilometres, pointing east and west. Where first encountered, at a point about twelve kilometres east of Gebel Selaia, on the march from that hill towards the pass into the head of Wadi Lahami, the rocks strongly resemble rather friable white to brown fine-grained sandstones, with bedding planes dipping about 35° to the north. The rock [10,413] consists almost entirely of white or slightly iron-stained quartz grains, with, in places, scattered minute dull black rods which are apparently hornblende. Traced further east, the rocks have more and more the character of quartz schists, with harder bands of quartzite, and the planes of separation, which gradually become nearly horizontal, have more the appearance of foliation-planes. The harder quartzite bands [10,414] are usually brownish to greenish-white in colour, but sometimes take on a purple tint, and show micaceous and chloritic scales on cleaved surfaces; in some places the rock contains “eyes” of minette-like composition. To the south the bedding or cleavage planes seem to reverse their dip, this being now to the south as though there were an anticlinical fold in the mass. There is considerable mixture of other schists with the rocks, and dykes of diorite and pegmatite, as well as veins of quartz, are frequent. A dioritic dyke which cuts through the rocks at a point nine kilometres south-south-west of Gebel Abu Gurdi, and which is itself crushed almost into a hornblende-schist, has hardened and altered the quartzose rock on either side of it into a close-grained horny looking rock [10,415] apparently containing a great deal of finely granular andalusite.
These quartz-schists may have arisen from the metamorphism either of sandstones, in which case the parts to the west of the tract are the least altered, or of fine-grained aplitic rocks, on which view the sandstone-like aspect of the western portion of the tract is due to weathering; the rock is certainly more highly weathered where it most resembles an unaltered sandstone, and the presence of hornblende and the “eyes” of micaceous matter are suggestive of igneous rock. From a consideration of all the evidence I was able to gather in a rapid march over the sea, however, I incline to the view that the rocks are altered sandstone older than the Nubian series, and probably of about the same age as the ancient clays which gave rise to the clay-schists of Zabara and elsewhere. The rocks are completely detached from the nearest undoubted Nubian sandstone beds (which lie some forty kilometres to the south-west, beyond Gebel Zergat Naam), and they are mixed with schists and veined and altered to a degree which is nowhere approached by beds known to be of the Nubian series. Still, it is just possible that the quartz-schists represent the remains of Nubian sandstone strata, for the nearest beds of that series show-considerable folding and faulting near Gebel Zergat Naam, and we should expect still greater disturbance and alteration here near the main watershed and close to a principal axis of mountain elevation.
A quartz-schist which is certainly derived from the metamorphism of aplitic dykes or sheets, occurs as great inclined bands cutting across the Wadi Shenshef close to some old ruins. The main rock of the district, in which the bands occur, is a medium-grained diorite. The quartz-schist is a greyish-white fissile rock, which splits easily into great slabs; it has supplied an excellent building material for the houses of the ruined town or encampment, being used not only for the walls but also for lintels over doors and windows.