Fig. 38.—Olivine-diabase, from a dyke at the junction of Wadis Huluz and Gemal [10,393], × 17. o, olivine; pl, plagioclase.
Turning now to the occurrence of diabase in dykes, a large dyke in the gneiss at the junction of Wadis Huluz and Gemal [10,393] consists of a very hard and heavy, strongly magnetic, greyish black rock of rather fine grain, in which a dark-brown platey mineral is mixed with grey and white matter. The sp. gr. is 2·95. Under the microscope the constituents are seen to be plagioclase, olivine, augite and magnetite, with small amounts of biotite and apatite. The plagioclase appears to have formed in two generations, for while the bulk of it is in small lath-shaped crystals (frequently with radial grouping), there is a very large porphyritic zoned crystal in the slide. The olivine is in large crystals, usually rounded, but occasionally tending to hexagonal outline with the usual strongly marked black irregular cracks. A little serpentinisation has gone on at the edges and along cracks of a few of the crystals, but, as a rule, the olivine is very fresh; it is never intergrown with felspars. The augite is slightly pleochroic, of a purple to brown tint, in irregular forms, partly in moderate sized crystals and partly in tiny grains in the ground mass. The crystals are much cracked. Ophitic structures are not conspicuous. Brown biotite is very sparingly present in small flakes. Magnetite is liberally scattered in small grains in the augite of the ground mass. Apatite occurs in minute prisms included in the felspars.
Fig. 39.—Diabase, from a dyke in Wadi Kreiga [12,110], × 17. pl, plagioclase felspar; a, augite; h, hornblende; l, limonite strings.
The diabase dykes which penetrate the granite in Wadi Kreiga [12,110] differ from the rock last described in their freedom from olivine and in showing marked ophitic structure. They are dense brown to black rocks of very fine grain, with porphyritic felspar crystals here and there. The sp. gr. is 2·98. Microscopic study shows them to be holocrystalline rocks, composed of an ophitic mixture of rod-shaped plagioclase with altering augite and hornblende. Both the ferro-magnesian minerals are very much clouded, and contain plentiful strings of iron oxide. The hornblende is dark green, often forming celephytic shells round the augite, and is probably largely an alteration from augite. There is very little of the nature of a ground mass, the augite and hornblende practically filling all the spaces between the felspars.
Mica-diabase.
Fig. 40.—Mica-diabase, Gebel Um Khariga [10,373] × 17. pl, plagioclase felspar; b, biotite, with separated opaque flakes of magnetite, often in geometric forms; a, clouded mineral, probably altered augite, with which the felspars are ophitically intergrown.
The rock which forms the top of Gebel Um Khariga [10,373] appears to be an altered mica-diabase. It is highly magnetic; the compass was found to point 20° out of its normal position at the station on the hill, while hand specimens broken off the rock showed strong polarity, some parts attracting and other parts repelling the needle; a fragment of the size of a pea deflected the compass needle several degrees when placed near it. It is a dark brown rock, of sp. gr. 2·83, very rotten, which in the mass looks like an altered dolerite. Microscopic study of a slide reveals the presence of altered plagioclase, in rather large lath-shaped crystals; biotite, largely altered to opaque iron oxide, the flakes of which show marked geometric forms; scattered grains of magnetite, and some secondary calcite. The brown clouded mineral polarises as a confused fine-grained and fibrous aggregate in low colours; it is probably altered augite, with which the plagioclases were ophitically intergrown, but is in too highly altered a state for certain identification. The strongly magnetic character of the rock would appear to indicate that the iron oxides produced by the alteration of the biotite are, like the primary grains, in the form of magnetite.