Trachyte, the volcanic representative of syenite, occurs in dykes [11,537 A] seaming the granite of Gebel Kahfa. It is a light grey rock, of very fine grain, breaking with a rough surface, containing pores and small stumpy white pearly-looking porphyritic crystals of orthoclase (_see_ Plate XXIII). The sp. gr. is 2·56. The microscope shows the ground mass surrounding the porphyritic idiomorphic orthoclase crystals to be a holocrystalline mixture of hornblende and felspar. The felspars of the ground mass are usually in more elongated forms than the porphyritic crystals, and appear to be partly plagioclase. The hornblende, in small and very irregular crystals of dark olive-green colour, often clouded and dirty looking, is scattered plentifully among the feslpars of the ground mass. Accessory primary minerals appear to be entirely absent. A rock which occurs in a hill at the head of Wadi Amba-ut [10,375] appears to be essentially similar to the above, but the porphyritic orthoclases are more numerous and the ground mass contains small quantities of augite and magnetite. This rock is in a highly altered condition, the felspars being full of kaolin and epidote, while the hornblende, which is here of a paler colour than in the trachyte of Gebel Kahfa, is highly chloritised. (_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.