45. The Geology of Egypt
[8]. The north-east corner of Africa, lying between the Red Sea on the east and the sand merged portion of the Libyan Desert on the west, and stretching from the Mediterranean to the 22nd parallel of north latitude, both in its topographical and geological characters is distinctly tripartite, as follows:—
(1) A rugged broken undulating sandstone desert, forming the southern part of the country;
(2) Elevated plateaux, for the most part of limestone, stretching from lat. 25° N. (approximately) to the Mediterranean;
(3) The mountainous igneous range of the Red Sea Hills, with peaks over 1800 metres (6000 feet) in height.
[8] In writing this note at the request of Sir William Willcocks I have made free use of all sources, of information, but am chiefly indebted to the publications of Schweinfurth and the late Professor Zittel, Capt. Lyons, and my past and present colleagues on the Geological Survey of Egypt.
As a whole one of the most waterless and desolate areas in the world, the country is traversed from south to north by a narrow highly cultivated and thickly populated strip of alluvial land, formed and watered by the Nile. In the southern sandstone country the river occupies only a shallow valley, but to the north flows over the floor of a deep gorge cut down from the surrounding limestone plateaux. On either side of the river are alluvial plains of varying extent, composed of the finest loam, a fertile soil for the most part formed by the disintegration of the volcanic rocks of the Abyssinian highlands, annually denuded by rains and brought down by the Atbara and Blue Nile floods and deposited in the lower courses of the river. Unlike most countries therefore, the soil of Egypt has no connection with the underlying rocks, being entirely of extraneous origin and owing its existence absolutely to the peculiar conditions of rainfall in Abyssinia and the direction of drainage from the watersheds of that country.
46. Igneous rocks.
The most ancient rocks in Egypt are found in the central igneous ranges of the Red Sea Hills and in the crystalline floor underlying the sandstones in the southern part of the country.
In Nubia the crystalline rocks consist largely of granite and gneiss, with associated diorites and schists, traversed by basaltic and felsitic dykes. Cataracts have been formed at those points where the river crosses the hard igneous belts, which may be regarded as the summits of the higher ridges of an old eroded continental land surface.