Gneisses.
The most conspicuous occurrence of gneiss in South-Eastern Egypt is the great mass which extends from Gebel Mudergeg and Gebel Migif in the north, down the range of Gebel Hafafit, and along the greater part of the Wadi Gemal. With this mass is closely connected another stretch of gneiss to the south of Gebel Abu Khrug; the hills which rise from the sandy plain round the head of Wadi Natash are mostly gneiss, so that the gneiss is probably continuous across the plain except where it is penetrated by the syenitic cones called El Nahud. The total area covered by gneiss in this locality alone is some two thousand square kilometres, and includes some remarkable mountain masses, such as Gebels Migif and Hafafit.
Other gneissose areas of less extent occur round Bir Abu Beid and to the south-west of Gebel Hamrat el Feg; while still smaller areas are found in the hills to the north-west of Berenice, in the remarkable triple-headed mountain called Gebel Um Rasein, and to the north of Bir Um Bishtit.
Gneiss typically forms bold scenery of precipitous hills, the colour of which varies with the composition of the rock, from pink to grey. At Gebel Migif one ascends the mountain most easily by the northern slopes, which are formed by the foliation-planes of the rock, while to the south these are cut across by sheer precipices; the same is the case at Gebel Um Rasein.
Fig. 52.—Granite-gneiss, Gebel Um Rasein [12,108], viewed between crossed nicols, × 30. q, quartz; ol, oligoclase; or, orthoclase; h, hornblende; m, “mortar” of crushed quartz and felspar between the larger crystals. The black areas are quartz in the position of extinction.
Of gneisses evidently produced by the deformation of granite, we may take the rock of Gebel Um Rasein [12,108][134] as a type. The bulk of the mountain is formed of a nearly white acid rock, of sp. gr. 2·67, with dark spots, resembling the granite of Gebel Elba in appearance, except that here the dark minerals have a marked arrangement along definite planes. (Compare the coloured figures of the two rocks on Plates [XXII] and [XXV]). In places the rock contains thin bands of almost pure biotite, and these are often thrown into sharp contortions by the strong compression which the mass has undergone. The strike of the foliation-planes in Gebel Um Rasein itself is about south-south-east, with a dip of about 20° to the east-north-east, but the strike swings round to about south at the south end of the mass, and the inclination of the foliation-planes become much steeper. The gneiss shows very marked pitting due to weathering, the holes ranging from a few centimetres to a metre or more in diameter and depth. A microscopic section of the rock shows it to be essentially an acid hornblende-granite similar to that of Gebel Elba, but crushed so that there is a “mortar” of broken up quartz and felspar between the larger crystals, and the larger where unbroken frequently show undulose extinction as the result of strain.
Fig. 53.—Diorite-gneiss, Wadi Nugrus [10,386], × 17. h, hornblende, with small enclosures of felspar and sphene; pl, plagioclase felspar; b, biotite; q, quartz; ap, apatite; sp, sphene.
In the Wadis Nugrus and Gemal there is a great deal of strongly foliated grey gneiss [10,386-7], derived from the intense crushing of diorites. A figure from a typical hand specimen of this gneiss, looking at the edges of the foliation-planes, is given on [Plate XXV.] The sp. gr. of the rock is 2·88. The microscopic slides show plagioclase and green hornblende in about equal proportions, with some quartz and small amounts of biotite, sphene, iron oxides, and apatite. The hornblende is very fresh-looking, in strongly pleochroic (yellowish to deep blue-green) straggling crystals, aggregated together along the foliation-planes; the crystals, though variously oriented, show a tendency to lie with their cleavages in the direction of foliation. A very striking feature of the slides is the abundance of small grains of enclosed felspar in the hornblende, giving the green crystals a perforated appearance in ordinary light. The felspars, which are likewise fairly fresh, are much smashed up, forming with the quartz a mosaic between the irregular bands of hornblende; the grains often show undulose extinction. Biotite, of a warm brown colour where not bleached, is sparingly mixed with the hornblende. The sphene is in small rounded grains, mixed with, and sometimes containing, granules of ilmenite; the grains show a tendency to form little aggregates, sometimes in the hornblende, and sometimes in the felspathic portions of the slide. One or two stumpy grains of apatite are enclosed in the felspars, but this mineral is extremely scarce in the slides examined, and does not show its usual long prismatic habit.
| Ball.—Geography & Geology of South-Eastern Egypt. | PLATE XXV. |
METAMORPHIC ROCKS.
GRANITE-GNEISS.
Gebel Um Rasein.
RED GNEISS.
Gebel Abu Beid.
DIORITE-GNEISS.
Wadi Nugrus.
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.