Dr. W. F. Hume — Petrography of Egypt. 505 



also been obtained from Wadi Barud (Eastern Desert of Egypt north 

 of Qosseir) and at several localities in East Sinai. 



5. Intermediate types. — Ball has recorded the presence of a boss 

 of highly-altered syenitic porphyry between Aswan and Shellal, 

 characterized by the presence of abundant lath-shaped orthoclase 

 felspars.^ This type is, however, rare in the Eastern Desert and 

 Sinai, rocks which from hand- specimens had been regarded as such 

 being mainly very fine-grained granites with quartz in fairly large 

 proportions. 



Well-marked dykes of diorite have been observed in "West Sinai, 

 and further study will probably show these to be of considerable 

 importance elsewhere. 



Very noticeable in every part of the igneous region from Sinai to 

 the Eourth Cataract are a series of chocolate-brown or light-yellow 

 rocks (often with satiny lustre) which are referable to the porphyrites. 

 Ball has described some interesting examples from near Aswan, 

 including a variety rich in enstatite from near Sehel Island, and 

 another from East Sinai, near Dahab, was closely related to a Bostonite, 

 but was more coarsely crystalline (see Hume, ** Eastern Sinai," 

 p. 163). 



6. Basic types. — These are extremely numerous, and often as 

 much as thirty metres wide, showing cannon-ball weathering. The 

 dense black types are usually dolerites, the easily decomposed varieties 

 diabases. 



All these intrusive and crystalline rocks are pre-Carboniferous in 

 age, and were formed anterior to the deposition of the Nubian Sandstone. 



The researches of the Survey have brought into view the importance 

 of a further group of rocks, which, though geographically closely 

 connected with the intrusive granites and gneisses, are of totally 

 different origin and character. 



Ill, — The Pre-Carboniferous Volcanic and Sedimentary Series. 



At a very early stage in the work of the Survey it was found that 

 volcanic rocks closely connected with sedimentarj^ formations played 

 a most important part in the history of the Eastern Desert and Sinai. 

 Rhyolites of typical character were obtained from near Qosseir, in the 

 Eerani Hills of East Sinai, and were also well developed in the 

 Shabluka (Sixth Cataract region) north of Khartoum. To this 

 series also belongs the well-known manganese-bearing andesites and 

 porphyrites of Jebel Dokhan (the Imperial Porphyry), the andesites 

 of Jebel Katherina and many other summits in Sinai and the South- 

 Eastern Desert, and the dolerites or basalts developed on a large 

 scale between Q,ena and Qosseir. Serpentines also are of the widest 

 distribution, and near Sikait are exceptionally developed, while 

 Dr. Ball has now reported them to attain an unusual importance 

 in the region he has recently studied in the South- Eastern Desert near 

 the frontier-line of the Sudan and Egypt, he having proved that the 

 mountains of Abu Dahar, Uerf, Korabkansi, etc., are entirely composed 

 of this type of rock. 



' The identification is based on the predominance of simple twinnin*. 



