J))\ W. F. Hume — Petrography of Egypt. 507 



separate this coarse metamorpliic rock from a highly complicated 

 grouping of schists and serpentines. 



In the Cataract districts are schists of very complex structure and 

 origin, judging from the preliminary studies already made. One of 

 these from the neighbourhood of Amara (between Wady Haifa and 

 Dongola), which was examined by the writer with the kind co-operation 

 of Dr. J. W. Evans, proved to be a labradorite-wollastonite-zoisite 

 rock. In close connection with this series were some calcareous- 

 looking bands which did not respond to the action of dilute acid, but 

 in microscopic section proved to be dolomites whose origin must for the 

 time being remain doubtful, though it is probable that they are 

 derived from the alteration of early Palaeozoic or even pre-Cambrian 

 limestones. Another line of change is indicated by the presence of 

 true marble bands intercalated between the schistose and gneissose 

 members. 



Taking the distribution of the metamorphic and sedimentary rocks 

 as a "whole, the writer is inclined to the view that sedimentation and 

 intrusion have succeeded each other at least twice in the pre- 

 Carboniferous periods in Egypt, foliation being most marked in the 

 archaic epochs represented by the Cataract gneisses. Up to the 

 present time no fossil records have been obtained which would supply 

 a date for the ancient sedimentary strata, though there is always 

 a possibility that such materials may yet be forthcoming. Such 

 evidence as is available shows conclusively that all these ancient strata 

 are pre-Carbonif erous, the first f ossiliferous sandstones containing a late 

 Carboniferous fauna lying unconformably upon them. 



Y. — Carboniferous and Post-Cakboniferous Yolcanic Activity. 



There have been at least two periods of volcanic activity in Egypt 

 since the I^ubian Sandstone was first laid down. The earliest 

 occurrences of this nature at present known are the basic eruptions 

 recorded in Western Sinai by Mr. Barron, the most interesting rock- 

 type being a uralite-diabase from the summit of Sawasia. 



During the Survey operations of 1905 a series of volcanic rocks of 

 varied composition (rhyolites, andesites, and basalts) were noted in 

 la*t. 25° N., about midway between the Nile and the Red Sea, and 

 immediately underlying the Nubian Sandstone, while in the same 

 neighbourhood were conspicuous summits (Jebel Sufra,^ the Nahuds, 

 etc.) which are either volcanic rocks or lava-flows still showing 

 a distinct columnar structure. Mr. Ferrar studied the Nahud necks 

 and found them to be trachytic in nature, while Charteris Stewart 

 brought andesitic rocks from the summit of Sufra. At El E,anga, on 

 the Eed Sea, in about lat. 24° 50', an andesite (once a lava-flow) is now 

 intercalated between the beds of sandstone, thus fixing the age of these 

 lavas as not being greater than that of the Nubian Sandstone itself. 



In the Northern Sudan south of Wady Haifa is a series of basic rocks 

 of basaltic character, which is evidently younger than the associated 

 Nubian Sandstone, the latter having undergone marked change to 



' Mr. "Wells had previously brought the writer an instructive photograph of Jebel 

 Sufra, and had remarked on the marked evidence of volcanic action at that locality. 



