Dr. W. F. Hume— Petrography of Egypt. 501 



represented. In general the latter consist of highly acid and horn- 

 blendic or mica-bearing varieties, as granitoid gneiss forming the main 

 rock in Uganda and the Southern Sudan. They extend from Kordofan 

 to the base of the Abyssinian hills (where the volcanic series overlies 

 this ancient formation), and once more reappear as prominent members 

 in the various cataract regions. The banded gneisses have also been 

 proved to be the fundamental member north of the Shabluka rapids 

 (Sixth Cataract), between Abu Hamed and Shirri Island in the 

 Fourth Cataract, forming the base-rock of the Third Cataract region, 

 of Semna, and of Ambugol, between Dongola and Wady Haifa, and 

 are again present in the outskirts of Aswan. Nor are they absent in 

 the great deserts east and west of the Nile, for the writer has met 

 with them near the Arbain Road far south of Kharga Oasis, and west 

 of the Nile between Ibrim and Dungul, while in the Eastern Desert 

 they form the bold peaks of Meeteq, of Sebahi, and the Abu Tiur, 

 familiar to all travellers down the Ked Sea, and are of the widest 

 extension both near the emerald mines of Sikait and in the desert 

 south-east of Aswan. This ancient series still requires most careful 

 and exhaustive study, and may in part at least represent an intrusive 

 magma anterior to the great post-Archaean sedimentary series, which 

 will be described later. Dawson has remarked on their great 

 resemblance to the Laurentian series of North America, and the 

 Cataract gneisses also appear to agree closely with the Bengal gneisses 

 of India (see Vredenburg, Summary Geol. India, p. 5). 



Although the main banding or foliation of these rocks appears to be 

 an original structure, or, if the result of earth-movements, of very 

 ancient date, a pseudo-foliation has in some cases been produced, due 

 to the intrusion of thin granitic veins between the foliated gneiss, 

 giving rise to a very coarsely banded variety, such as is present at 

 Nab Island, in the Third Cataract area, north of Dongola. 



The gneiss of the Dal Cataract region, south of Wady HaKa, can 

 also be distinctly traced back to a granite with large porphyritic 

 felspar crystals, but it is open to question whether this rock is of as 

 great an age as the more highly foliated members, rich in mica or 

 hornblende, which form the principal constituent of this division. 



(^) A second type of gneiss, that of the Arabian Desert, has a very 

 wide extension in the Eastern Desert and Sinai, but has no marked 

 foliation except in isolated instances. It agrees with the Bundelkhand 

 gneiss of India in having the general characters of a fairly coarse pink 

 granite, though many varieties have been observed. The evidence 

 available indicates that this series represents a granitic magma which 

 has played a most important part in the structure of the Eastern 

 Desert and Sinai. 



These great acid intrusions appear to have been of two dates, one 

 preceding the deposition of the pre-Carboniferous sedimentaries, as 

 these contain fragments of granitic rocks (both hornblendic and more 

 acid types) among the boulders and pebbles of their conglomerates. 

 A younger granite invasion has subsequently produced marked 

 contact changes, especially in Eastern Sinai. 



It is difficult at present to indicate definitely the origin of the 

 banded gneisses, though the writer after examination of these rocks 



