Notices of Memoirs — W. F. Hume — Arid Lands. 423 



of the river; and (3) the possible effect of the rotation of the earth 

 (Van Baer's law), the stream tending to hug ils eastern bank. 

 Attention is called to the region of exceptional erosion where heavy 

 masses of Eocene limestone rest on and have slipped over the sub- 

 jacent 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 metres 

 above the present river-level. The triple terracing of the Nile is 

 briefly considered. 



4. 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 con- 

 spicuous part, and gi'eat fractures determine earth-features of 

 considerable magnitude. The result is tliat the masses of granite and 

 metamorphic rocks hidden beneath the surface in Central Egypt are 

 here exposed by denudation, forming the lied Sea Hills and Sinai 

 Mountains. 



The difterent 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 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 manj^ 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. 



5. In the Gulf of Suez area another factor has come into play. 

 Here sea-arms project far inland between land-surfaces subject to 

 desert conditions, and their waters become centres of far-reaching 

 chemical activity. Thus coral-reefs are changed to dolomites, sea- 

 sliells of carbonate of lime to gy[)sum, hydro-carbons are in qiuintities 

 of economic importance, and mineralized areas of lead and zinc ores, 

 of manganese oxide, of iron pyrites, and of sulphur are present in the 

 young Tertiary beds which fill these Red Sea depressions. From 

 Suez to beyond Hiilaib, that is, throughout the length of Egypt, 

 gypsum forms a conspicuous fringe between the ancient hills and the 

 sea, generally dipping <;ently seaward on the borders of the lied Sea 

 itself. Further north, in the Gulf of Suez area, the conditions are 

 more complicated. Dyapir, or piercing folds such as have been 

 described by Professor Mrazec in Roumania, are of common occurrence, 

 and there is remarkable interplay between the hard and soft members 

 of the folded series. 



The surface structure of an arid land is not only the direct reflex 

 of its geological structure, but also of former climatic change. Many 

 factors in Egypt point to great rainfall in the past, such as gravels of 

 igneous material in the Nile Valley far from their source of origin, 

 masses of travertine in the Oases, the varying terraces of the Nile 

 Valley itself, the evidence of expansive lakes at Kom Umbo, etc. 



