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.
