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. =PLATE XXIII.= [Illustration: Ball.—Geography & Geology of South-Eastern Egypt. =INTERMEDIATE IGNEOUS ROCKS.= =PINK SYENITE.= Gebel Zergat Naam. =TRACHYTE.= Gebel Kahfa. =DIORITE.= Gebel Allawi. =DIORITE WITH PRISMATIC HORNBLENDE.= Wadi Muqur. =BANDED ANDESITE.= Gebel Sufra. =VEINED KERSANTITE.= Gebel Fereyid. NATURAL SIZE.] In places, the rock of Gebel Zergat Naam takes on a finer grain and a greyer aspect in the mass. A slide cut from this variety [11,512] shows essentially the same composition, but the felspars are here porphyritic in a crystalline ground mass of felspar and hornblende. The rock thus passes into syenite-porphyry. The porphyritic felspar crystals show the same lamellar structure as those in the more coarsely crystalline rock. =Trachyte.= [Illustration: Fig. 17.—Trachyte, from a dyke at Gebel Kahfa [11,537 A] × 10. A clear crystal of orthoclase felspar is seen in the centre of the field, surrounded by a finely crystalline ground mass of hornblende and felspar.]