Notices of Memoirs — Dr. Hume — Fetrography of Egypt. 465 



V. — Notes on the Petrography of Egtpt.^ By "W. F. Hume, D.Sc. 



1. The ancient core of the North-East African Continent consists of 

 the Cataract and Sudan Banded Gneisses, which may represent a very 

 ancient igneous magma. They are usually much veined by granitic 

 dykes. 



2. In certain places in the Arabian Desert, Cataracts, etc., these 

 underlie highly metamorphosed Schists (the Mica-Schists of Sikait, the 

 Calcareous Schists of tJm Garaiart and Haimar and of the Amara 

 Cataracts, also the Dolomites of the latter region), which are sharply 

 separated from the Banded Gneisses, and are possibly the oldest 

 sedimentary representatives in Egypt. 



3. The greater part of the mountainous regions of the Eastern Desert 

 and Sinai is occupied by two types of rock, («) a schistose constituent 

 overlying or surrounded by (h) an acid member. The first-named (a), 

 the Dokhan Volcanic Bocks and Schists, are partly volcanic in origin 

 and partly sedimentary, the former being represented by lavas of 

 various types, while the latter are clearly altered sedimentary strata 

 (grits, conglomerates, etc.). No fossils have yet been found, but they 

 have their nearest analogues in the latest Pre-Cambrian and Cambrian 

 series. Here are included some of the most interesting rocks of 

 Egypt, such as the Imperial Porphyry and the Breccia Yerde Antico. 



(h) The igneous member intruded into these ancient sediments, etc., 

 includes a great diversity of igneous rocks, varying from highly basic 

 to acid types. Contact phenomena of complex nature occur at the 

 junctions of a and b. 



4. Bed Granite and Dyke Bocks, whose parallelism and extent of 

 distribution present one of the most conspicuous features of the Eastern 

 Desert of Egypt, mark the final eruptive action before Carboniferous 

 times. 



5. Three periods of volcanic activity have been subsequently noted. 

 («) In Western Sinai in late Carboniferous times. 



[h) An undated series of eruptions interbedded with the base of 



the Nubian Sandstone or intrusive into it with marked 



contact alterations. 

 {c) The Basic intrusions near Cairo and the Fayum, etc., which 



are intimately associated with the Oligocene Continental 



Period in Egypt. 



YI. — The Tourmaline Bocks of Cwm Dwythwc, near Llanberis 

 (North Wales). ^ By W. G. Fearnsides, M.A., F.G.S. 



SOME years ago, when examining sands from the neighbourhood of 

 Caernarfon, I found that both the river sands of the Seiont and 

 the beach sands of the Menai contain tourmaline. In order to trace 

 the mineral to its source I have since examined the heavy mineral 

 residues of the sands of nearly all the tributaries of the Seiont, and 

 find that all those which flow across the Cambrian Slate Belt contain 

 either needles or broken grains of brown tourmaline. The sand from 



1 Eead before the British Association, Section C (Geology), Dublin, Sept. 1908. 



DECADE V.^VOL. V. — NO. X. 30 



