504 Dr. W. F. Hume —Petrographij of Egypt. 



Second Cataract, forming the sides of the Bab-el-Kebir rapid, whose 

 presence is due to the wearing away of a porphyrite dyke intrusive 

 in the massive basic rock. 



Norites, etc., have been observed locally in the Eastern Desert, but 

 their general relations are still obscure, and massive beds of dolerite 

 form an important constituent between Qena and Uosseir. 



The rarity of felspathoid rocks in Egypt renders occurrences of this 

 nature of special interest. In 1896 Mr. Barron brought back a rock 

 from the Wadi Zeidun which in section showed hexagonal and 

 rectangular sections strongly recalling nepheline in appearance, though 

 internal strains had rendered the hexagonal sections nonisotropic. 

 The position of the specimen was not precisely fixed, but last year 

 the writer brought back a variety from Jebel Hadarba which attracted 

 M. Couyat's attention. Having made a section from the specimen, 

 he has privately informed the writer that we are here dealing Avith 

 a representative of the nepheline-syenites, so that the central region 

 of the Arabian Desert between lats. 25° and 26° ^N". will probably 

 yield felspathoid-bearing varieties over an extended area. 



II {d). — A much younger group of acid intrusions is of the greatest 

 geographical importance, it being the most notable mountain-former 

 in Eastern Egypt and Sinai. This is the red granite (mainly composed 

 of quartz and felspar, with occasional nests and films of muscovite 

 mica), which is widely distributed in its range. Some of the finest 

 mountain masses are composed of this rock (Ilm Shoraer and Ed Deir 

 in Sinai, El Shaib and the Gattar range, etc., in the Eastern Desert), 

 while in the Cataract regions it gives rise to the rugged hills of 

 Akasha, south of Semna, and the boulder-strewn region of Shirri, 

 in the Fourth Cataract. Possibly in part contemporaneous with 

 it are — 



II {e). — The dykes of varied composition (granites, felsites, 

 dolerites, etc.) which seam the greater part of the Egyptian igneous 

 and metamorphic rocks, and themselves have complicated inter- 

 relationships, as mentioned in the Egyptian Geological Survey 

 Memoirs. 



1. Acid types. — Of these the most conspicuous and common are the 

 close and often coarse intergrowth of quartz and orthoclase felspar, 

 frequently known under the name oi pegmatite granite. It is probable 

 that every region in Egypt provides examples of this type. Ball 

 (" Aswan Cataract," p. 84) states that it is the most frequent rock at the 

 Cataract, though actual graphic structure is uncommon. It is also 

 well developed in the Eastern Desert north of Qosseir and near the 

 Sikait emerald mines, while a beautiful graphic form was obtained by 

 Stewart at Gebel Kibdab in the south, near the Sudan frontier. 



2. In the second rank are a group of microgranites (notably in West 

 Sinai) occasionally containing porphyritic crystals of orthoclase and 

 plagioclase, while locally a beautiful granophyric structure is developed. 



3. The quartz porphyries^ or felsites with microcrystalline base, are 

 not so prominent as those of micrograuitic type, but typical examples 

 were obtained by Barron at Ain-el-Akhdar, in West Sinai, and from 

 Jebel Zeit, in the Eastern Desert. 



4. FeUitic types, showing well-marked spherulitic structures, have 



