TRANSACTIONS OF SECTION C. 369 
re-made 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 Hocene times is regarded 
by the writer as explaining the great difficulty experienced in drawing a litho- 
logical 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 :— 
(A) 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 
have been exposed, as Beadnell has pointed out, the Oases depression without 
outlet is produced by wearing through wind-blown sand. 
(B) 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 limestone 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 es wind. These 
are deposited as sand-falls in the wind-shadows of the Nile Valley 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 Region 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. 
(C) The present course of the Nile Valley appears to depend on three factors : 
(a) The formation of the syncline, the axis of which it partly follows; (b) the 
erosion of the softer strata along their outcrops determining the present north- 
south trend of the major courses of the river; and (c) the possible effect of the 
rotation of the earth (Van Baer’s law), the stream tending to hug its eastern 
bank. Attention is called to the region of exceptional erosion where heavy 
masses of Eocene limestone rest on and have slipped over the subjacent soft 
Cretaceous marls and slates. These slips must have been connected with greater 
rainfall and earth-movement as widespread terraces extend in front of the main 
cliff and rise to some 110 métres above the present river-level. The triple 
terracing of the Nile is briefly considered. 
(D) The Mountain Region of the Eastern Desert is essentially an anticlinal 
area, where tension is in excess of compression. The differential movements 
are considerable, minor folds play a conspicuous part, and great fractures deter- 
mine earth-features of considerable magnitude. The result is that the masses 
of granite and metamorphic rocks hidden beneath the surface in Central Egypt 
are here exposed by denudation, forming the Red Sea Hills and Sinai 
mountains. 
The different geological formations give rise to very varied surface features. 
Attention is called to the importance of rain as a sculpturing agent. The soft 
Nubian sandstone is easily eroded both by wind-borne sand and by water, giving 
rise to conspicuous depressions. In the granitic areas temperature variation 
breaks up the solid rock, huge domes are produced by flaking off of concentric 
shells. Dykes give rise to marked differences in surface outline, the harder 
quartz-porphyries determining the form and general trend of many of the 
mountain summits, while the softer diabases, being easily eroded, give rise to 
gullies seaming the precipitous sides of the granitic hills. The general character 
of the country where schists and volcanic rocks are present is also described. 
1914. BB 
