514 



NA TURE 



[September 19, 1907 



its conslruclion would involve a very great expendi- 

 ture, and occupy a long time; and it is very unfor- 

 tunate that the fact of Lake Tsana, near the head- 

 waters of the Blue Nile, being in Abyssinia is con- 

 sidered as precluding its utilisation as a storage reser- 

 voir for tlie irrigation of the Sudan and Egypt, with 

 its great natural advantages of extensive area and 

 commanding position. Several sections of reservoir 

 dams are given in a chapter on dams and reser- 

 voirs, which, being drawn to various scales, are not 

 readily compared, though forming ar. interesting 

 series ; but the foundations of the new Croton dam 

 have had to be carried down more than forty feet 

 deeper than shown on the section ; and the Salt River 

 dam, constructed for the irrigation of an arid district 

 in Arizona, only about seventeen feet less in maximum 

 height than the Croton dam, has the crest of its waste 

 weirs raised the unprecedented height of 225 feet above 

 the deepest part of the river-'bed at the dam. 



Interesting descriptions are given of the diversion 

 weirs across rivers in India to raise their water-level 

 for supplying inundation canals, and of the Zifta, 

 Delta, and Assiout barrages for regulating the supply 

 of irrigation water from the Kile, w-orked by Stoney's 

 sluice-gates sliding on free rollers. The important 

 function, however, performed by the Assiout barrage 

 in raising the water-level of the river in August, K)o2, 

 during so low a flood of the Nile that the water could 

 not flow into the great canals, until, by the prompt 

 action of Mr. Webb in closing the gates, a sufficient 

 head was obtained, for which the barrage had not been 

 designed, should not be overlooked, as it preserved 

 a large tract from a loss in non-irrigated crops of more 

 than 600,000/. Moreover, the Esna barrage, in cour.se 

 of construction across the Nile between .Assuan and 

 Assiout, has been designed of sufficient strength to 

 perform a similar duty when needed in perfect safety. 

 Standard books, indeed, have been previously issued 

 dealing with irrigation works on their grandest scale, 

 in " The Irrigation Works of India," by Mr. Buckley, 

 and " Egyptian Irrigation," by Sir William Willcocks ; 

 but the book under review will be very valuable for 

 all persons interested in irrigation, bv dealing in a 

 single volume with and contrasting the principal 

 works and systems of irrigation in these two great 

 countries, and thus presenting a very comprehensive 

 view of that most important subject of irrigation for 

 the development of arid regions. 



GEOLOGICAL EXPLORATIONS IN SINAI. 

 The Topography and Geology of the Pciiiiistda of 

 Sinai {South-eastern Portion). By W. F. Hume. 

 Pp. 280+plates. (Cairo : National Printing De- 

 partment, IQ06.) 

 ' I 'HE interesting region described in this memoir 

 -•- by the superintendent of the Geological Survey 

 of Egypt extends from near Dahab along the western 

 borders of the Gulf of Akaba to the promontory 

 of Ras Mohammed. It lies east of Mount Sinai 

 proper, or Gebel Musa, which is not, however, the 

 highest of the Sinai mountains. 



Attractive as the region is from an historical point of 

 view, and interesting as it is to the naturalist, it offers 

 NO. 1977, VOL. 76I 



few temptations for the ordinary tourist ; and this not j 

 merely because it is an arid country. The Arab in- 

 habitants are honest and obliging, but theiH 

 dwellings consist simply of cloth stretched on a few 

 poles, and placed under the protection of a rocky 

 ridge. Keen as sportsmen, they use flint-lock guns, 

 often of great length, serviceable for small game, as 

 well as for the leopard, hysena and ibex, which are the 

 more abundant ol the larger mammals. The country 

 itself has been spoken of as " one of the most moun- 

 tainous and intricate regions on the face of the earth," 

 and Dr. Hume admits that much of it is a veritable 

 wilderness with a bewildering complex of topo- 

 graphical structure. To the mountaineer it will hardly 

 appeal, as there are no inaccessible peaks, but to the 

 botanist and zoologist, who will find chapters specially 

 devoted to them, and to the geologist, there are many 

 attractions, not the least of which may be the absence 

 of any extensive literature on their subjects. 



The main portions of the area are composed of 

 igneous and metamorphic rocks of ancient date — pre- 

 Carboniferous at any rate. They rise in a mountain 

 system trending north-east and south-west with a 

 transverse chain that parts the region into two districts. 

 The rocks include gneiss and sundry schists, granites, 

 andesites, felsites, &c. The earliest sedimentary and 

 volcanic rocks were penetrated by masses of granite, 

 and some of the later intrusions have been arranged in 

 strikinglv symmetrical lines. The northern portion of 

 this region is largely plateau, an old plain of marine 

 denudation that appears only recently to have lost its 

 capping of Nubian Sandstone. It has an average level 

 of 1220 metres, except where cut by narrow gorges. 

 In the southern portion, which comprises a multitude 

 of ranges and peaks, the valleys are more deeply ex- 

 cavated, and in consequence the mountains, though 

 lower, appear relatively higher than those in the north. 



Still further north there is an area composed of 

 barren Nubian Sandstone with overlying fossiliferous 

 limestones of Cenomanian age, where the succession 

 is greatly disturbed by trough-faults that have led to 

 the production of rift valleys. 



The structure, in a broad sense, is comparatively 

 simple, as Dr. Hume observes, the main features 

 having been produced by upheaval and dislocation 

 rather than by erosion. Thus the principal mountain 

 chain is due to a fault with a westerly downthrow of 

 more than 5000 metres. Nevertheless, the influence of 

 the rocks on the configuration of the land is well 

 marked. The summits of many peaks are formed by 

 felsite dykes ; elsewhere parallel dykes of felsite and 

 dolerite form the remarkable " dyke country," while 

 the granite, which wears away in shells, presents 

 curious rounded knobs and pillars. The coastal plain 

 exhibits many features of interest in the presence of 

 Miocene strata, and also of raised coral-reefs of Pleis- 

 tocene age. Moreover, there are terraces of roughly 

 stratified gravel in the principal valleys, as well as 

 countless loose boulders. These were probably of tor- 

 rential origin, distributed during the Glacial period, 

 when, as Dr. Hume remarks, a small amount of nive 

 might have accumulated on the Sinai mountains. Of 

 still later date are some curious calcareous sandstones 



