Geology of West-Central Sinai. 81 



carry out a detailed topographical and geological survey of the area 

 in which the more important deposits occur. He has produced an 

 accurate map of an area of 380 square miles limited by the parallels 

 of 29° 15' and 28° 56' 20" N. lat. and by the meridians of 32° 9' 35" 

 and 33° 27' 30" E. long, on a scale of 1 : 50,000, on which the 

 distribution of the various geological formations, ranging from the 

 granite and gneiss of pre-Carboniferous age through the Carboniferous, 

 Cretaceous, and Tertiary to the Pleistocene and Recent deposits of 

 alluvium and blown sand, is laid down. The topographical and 

 geological details were recorded simultaneously, and as an illustration 

 of Dr. Ball's skill and neatness as a surveyor it may be mentioned 

 that the maps are based on direct photographic reproductions of the 

 field-sheets. In addition to the general map, the work is illustrated 

 by a larger scale map (1 : 25,000) of the environs of Um Bograa, 

 where the chief mines are situated, a plate of sections, numerous 

 photographs, and a large number of text-figures, including pen-and- 

 ink sketches of the most typical fossils drawn by the author from 

 actual specimens. 



After describing the general features of the district, which consist 

 of highly dissected tablelands and assemblages of rugged granitic 

 peaks, with occasional more open areas, the author gives an account 

 of the survey operations, of the methods adopted in laying out the 

 base-line, in determining its geographical position, and in connecting 

 up the triangulation with that of Egypt proper. Then follow 

 chapters dealing with the topographical features — the wadis, the 

 mountains and the plains — and with the geology. 



In the centre of the area lies a broad sandy plain at a height of 

 about 500 metres above sea-level, on which the base-line was laid 

 down. This is bounded on the north by the formidable escarpment 

 of Gebel el Tih, the edge of which is from 500 to 600 metres above 

 the plain and extends from east to west in a somewhat sinuous curve. 

 The base of the escarpment is formed of Nubian Sandstone, then 

 follow 200 metres of fossiliferous Cenomanian clays, marls and lime- 

 stones, which are capped by beds of Turonian limestone. The 

 escarpment forms the southern termination of a deeply dissected 

 plateau which stretches far to the north, and it is a remarkable fact 

 that although the valleys descend rapidly from the edge in that 

 direction, none appears to have been beheaded by the recession of the 

 escarpment. Many points on the edge have been accurately fixed. 



South of the central plain is a wild country with only relics of 

 plateau structure, in which many mountains rise to heights of 700 or 

 800 metres above the sea and one to over 1,000 metres. It consists 

 of the pre-Carboniferous complex of granite and metamorphic rocks, 

 on the planed-down surface of which rest strata of Carboniferous age. 

 The time available did not admit of any attempt to separate the rocks 

 of the complex, which is presumably Archaean. The Carboniferous 

 rocks consist of a lower sandstone (130), 1 a middle limestone with 

 fossils, similar to those occurring in Derbyshire and Yorkshire (40), and 

 an upper sandstone with Lepidodendron (130). To the west of the 

 central plain this upper sandstone underlies the Nubian Sandstone, 

 1 The figures in parentheses represent thicknesses in metres. 



DECADE VI. — VOL. IV. — NO. II. 6 



