No volcanic rocks of acid type have been with certainty identified in South-Eastern Egypt. It is, however, possible that some of the rocks above described are in reality of volcanic origin, though the manner of their occurrence is more suggestive of intruded masses and sheets. It is also quite likely that some of the hornfels found associated with the schists, as for instance at Gebel Um Semiuki, near Abu Hamamid, are altered forms of glassy acid lavas; but their vast age and the intense metamorphism to which they and the surrounding rocks have been subjected render it impossible to be certain of the manner of their origin. INTERMEDIATE IGNEOUS ROCKS. (_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. [Illustration: 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.] [Illustration: 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.]