(b) SUB-BASIC ROCKS.
Diorite.
Though not forming such conspicuous features as the granites, rocks of dioritic composition are very widely distributed over the country, and are specially abundant in the districts to the south of Ras Benas. They are almost always sharply marked-off from the granites, being generally closely connected with the more basic igneous rocks and schists. The coarser-grained varieties usually form parts of irregular intrusive masses, shading off gradually into the more basic forms of diabase, gabbro, and hornblende rock; the finer-grained types occur as dykes and irregular bands traversing other igneous rocks and schists, and are themselves frequently so crushed as to resemble schists. Owing to their intimate association with other dark rocks, it is no easy matter to trace the limits of the diorites in the field. The same difficulty is found when they are microscopically examined, for one finds in their mineralogical composition every gradation from true diorites, through the intermediate stage of augite-diorite to diabase and gabbro, while many of the harder dark schists and hornblende gneisses turn out to be merely highly crushed diorites, so that classification must be more or less arbitrary.
Diorite typically forms low hill country of dark aspect. The rock weathers as a rule far more easily than granite, and in some cases the debris of rounded grains set free by disintegration cover the surface and render the climbing of the hills somewhat dangerous, the effect being like one would imagine to result from walking over slopes covered with hard peas.
Fig. 18.—Diorite of Gebel Allawi [10,313], × 10. pl, plagioclase felspar, clouded by decomposition products; h, hornblende, olive-brown in colour; ha, hornblende altering to pale-green chlorite; m, magnetite.
Diorite in the narrowest sense of the term (plagioclase-hornblende rock) is not by any means abundant. Curiously enough, it is generally found in the neighbourhood of old gold mines, as for instance in the Kurdeman and Allawi districts, and it was employed by the ancient miners for their crushing pans. The rock from Gebel Allawi [10,313] is a medium-grained one composed of black and milk-white minerals in about equal proportion (see [Plate XXIII]). Its sp. gr. is 2·95. Under the microscope the milk-white material is seen to be plagioclastic felspar, generally much decomposed, while the dark grains are of hornblende, pale green to olive-brown in thin section, with somewhat feeble pleochroism. Like the felspar, the hornblende is considerably altered; in places it has lost nearly all its colour and is converted into pale green chlorite. The accessory minerals are iron-oxides and a little sphene. Rocks of somewhat finer grain [10,358] occur in the Rod el Ligaia. In a slide from the last-named locality, granules of ilmenite are surrounded by sphene, suggesting the formation of sphene by the alteration of ilmenite. A variety of diorite [10,403] occurs in the Wadi Huluz in which the hornblende is nearly colourless, showing only a trace of green colour in thin section, and is accompanied by a small amount of augite.
Fig. 19.—Diorite, Wadi Baaneit [12,151], × 10. h, hornblende; pl, plagioclase; b, accessory biotite; q, accessory quartz.
A form of diorite which occurs in and about the Wadi Baaneit [12,151] is very similar to the rock just described, but differs from it in the more irregular manner in which the two principal mineral constituents are distributed. The hornblende is in patches varying from mere specks to eight millimetres in diameter; the felspars have a sugary appearance in the hand specimen. The sp. gr. is 2·81.