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



columnar structure where in direct contact with the intrusive material. 

 One of the most conspicuous occurrences is on a low hill at the 

 east end of the Kaibar Cataract, and other striking examples are seen 

 at Delgo, at Jebel Alarambia, near Kerma, at Jebel Alibersi, and at 

 El Lagia, in the El Kab Oasis near Dongola. The contact alterations 

 are distinctly marked not only by the formation of the columnar 

 sandstones, but by highly crystallized quartzites. A.s the intrusive 

 rock has the altered sandstone both above and below it in the 

 Alarambia district, its age is probably of the same period as those 

 previously mentioned. 



VI. — Early Tertiary Volcanic Occurrknces. 



The basaltic rocks in the north of Egypt and Sinai appear to belong 

 to a later period of eruption, being closely connected with the 

 Oligocene Continental Period. In the Fayum and near Cairo they 

 overlie limestones of Upper Eocene age, and are overlaid by the 

 Miocene and Oligocene strata where the latter are present, in 

 the former case giving rise to the well-known precipitous shelf 

 above the Birket-el-Qurun Lake, while at Abu Zabel, north of Cairo, 

 they are of considerable economic importance. The rock is classed as 

 a basalt on account of its ophitic structure, but in its typical develop- 

 ment olivine is markedly absent. A rock of the same type has also 

 a wide extension both to west and east, having been recorded from 

 Baharia Oasis, from Gara Soda and Gebel Gebail in the Nile Valley, 

 and from Western Sinai. These resemble the Cairo basalts in all 

 essential particulars. 



In addition to these occurrences in the north of Egypt, some 

 interesting intrusions of andesitic and trachytic masses into Eocene 

 limestones have been recorded by various members of the Survey. 

 This is notably the case in an area east of Assiut examined by 

 Beadnell, and from Jebel Abuhad, near Qena, examined by Barron 

 and the writer. 



Summary. 



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 Um 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 are occupied by two types of rocks, a schistose 

 constituent overlying or being surrounded by the acid member. 

 (ff) The first-named, the JDokhan volcanic roclcs and schists, are partly 

 volcanic in origin and partly sedimentary, the former being repre- 

 sented by lavas of various types, while the latter are clearly altered 

 sedimentary strata (grits, conglomerates, etc.). No fossils have yet 



