422 Notices of Memoirs — W. F. HiLine — Arid Lands. 



gives a rapid sketch of the geological history of Egypt as known to us 

 at present, the formation of the ancient core of pre-Cambrian or 

 Palaeozoic sediments, volcanic rocks with invasion by granitic magmas, 

 the brief Carboniferous marine advance, and later the much more 

 important Jurassic - Cretaceous transgression, which practically 

 affected almost the whole of Egypt, giving rise to the Nubian Sand- 

 stone and the important phosphate-bearing Cretaceous Series. The 

 Eocene strata which form the major portion of Central Egypt are 

 probably formed, at the base, of remade Cretaceous material, and only 

 in their upper portions show marked evidence that the underlying 

 sandstones and igneous rocks are undergoing erosion. 



The re-arranging of Cretaceous strata eroded during Eocene times 

 is regarded by the writer as explaining the great difficulty experienced 

 in drawing a lithological line of unconformity between the beds of 

 these respective periods, though the faunal differences indicate the 

 great break between them. 



Fringing the pre-Eocene and Eocene areas of Egypt are a series of 

 Miocene and more recent formations which are of great interest both 

 from tectonic and economic points of view. 



In considering the separate physiographic features it is pointed out: 



1. In the formation of the Oases it is necessary to consider the 

 denudation of the area by marine erosion while rising from the sea 

 and the effects of former more humid climatic conditions. Where the 

 ^Nubian Sandstone or other soft beds liave been exposed, as Beadnell 

 has pointed out, the Oases depression without outlet is produced by 

 "wearing through wind-blown sand. 



2. The Great Plains of the Libyan Desert are regions of low dip, 

 of meagre rainfall, and thus wind is the dominant factor. A sandy 

 region to the north supplies the sand necessary for erosion. The 

 character of the desert surface depends on the nature of the geological 

 strata present. The undulating gravel plateaux, or serir, the lime- 

 stone expanse, the ' melon ' country, and the fossil floors are various 

 forms in which the desert presents itself, the main feature being the 

 removal of all particles capable of being transported by wind. These 

 are deposited as sand-falls in the wind-shadows of the JS^ile Yalley 

 scarp or other depressions. The sand dunes which are locally 

 developed are in sharp contrast to the main desert, these probably 

 depending on three main factors — the existence of sandy deposits 

 determining their source of origin, the usual direction of the wind 

 their trend, and the relief of the ground their position. 



The Maaza Limestone Pegion is similar to the Libyan Desert, but 

 has a greater rainfall. It thus presents a fine example of the effects 

 when rain acts during short periods on rock-surfaces affected by 

 temperature variations. Deep ravines, remarkable water-holes, 

 caverns, natural bridges, and surface coloration films due to the 

 trickling down of ferruginous solutions over cliff-walls are among the 

 prevailing features in the southern part of this area. 



3. The present course of the Nile Valley appears to depend on 

 three factors : (1) the formation of the syncline, the axis of which it 

 partly follows ; (2) the erosion of the softer strata along their out- 

 crops determining the present nortli-south trend of the major courses 



