IV.—Notes on the Petrography of Egypt

Abstract

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. (a) The first-named, the Dokhan volcanic rocks 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 lithological 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 Verde Antico. (b) 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. Red granite and dyke rocks, 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—(a) In Western Sinai in late Carboniferous times.(b) 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 Oligocena Continental Period in Egypt.