(a) SUB-ACID ROCKS.
Syenite and Syenite-porphyry.
Though hornblendic varieties of granite are fairly common, true syenite (i.e., orthoclase-hornblende rocks with little or no quartz) is amongst the rarest of rocks in South-Eastern Egypt. It occurs near Gebel Nazla (between Bir Um Gubur and Bir Masur), in the two remarkable conical hills called El Nahud, which rise from the plain near the head of Wadi Natash, and at Gebel Zergat Naam. In all these three localities the syenite appears to form intrusive bosses rising through the surrounding rocks.
The specimens from near Gebel Nazla [10,625] and from El Nahud [10,857] are very fine-grained reddish-brown rocks, which can be seen with a lens to be largely made up of red orthoclase crystals with dark specks of hornblende.
Fig. 15.—Syenite of Gebel Zergat Naam [11,515], × 10. o, orthoclase felspar, showing striations parallel to the basal planes; h, hornblende deep green in colour; g, interstitial quartz.
Fig. 16.—Syenite-porphyry, Gebel Zergat Naam [11,512], as seen between crossed nicols, × 10. f, porphyritic felspar (orthoclase); h, hornblende; g, microgranitic ground mass containing felspar, hornblende, and some quartz.
The syenite of Gebel Zergat Naam [11,515] rises as a great boss from among the surrounding dark schistose rocks. It is typically a pinkish-brown rock of rather fine grain (see [Plate XXIII]), in which can be seen shining crystals, three to four millimetres long, of pink orthoclase, and specks of dark hornblende, with here and there a little glassy-looking quartz. The sp. gr. is 2·62. Under the microscope the rock is found to be mainly composed of orthoclase, with a much smaller amount of hornblende and a little interstitial quartz. The felspar is fairly clear, in forms approximating to idiomorphic, and showing the characteristic simple twinning of orthoclase. The crystals are slightly clouded in streaks parallel to the basal planes, so that even in ordinary light a faint herring-bone structure is visible, the streaks on either side of the trace of the plane of composition being inclined to each other at a large angle. Between crossed nicols this herring-bone structure is very strongly marked, and is evidently due to a perthitic intergrowth (of albite?). The intergrown lamellæ are slightly irregular; though following generally the direction of the basal cleavage planes they are not perfectly straight, nor always continuous across the half of the crystal. The lamellæ extinguish and attain their maximum of brightness simultaneously with the respective halves of the crystal in which they occur, so that they become invisible in certain positions of the nicols; but as the nicols are turned they appear as well marked dark bands, clearly visible even under low powers. The hornblende is of exceptionally dark green colour, in irregular straggling masses, often considerably decomposed and clouded with iron oxide. Accessory minerals, other than the clear interstitial quartz, appear to be almost entirely absent in the slide examined.
| Ball.—Geography & Geology of South-Eastern Egypt. | PLATE XXIII. |