Reviews — Geology of 8.E. Egypt. 267 



scale of 1 : 750,000 and in a geologically coloured map on the same 

 scale. These two taken together give an excellent picture of a region 

 of which hitherto little has been known. As a result of this state of 

 things a large part of this volume is taken up by detailed descriptions 

 of the drainage systems, the hills and mountains, and the water 

 supplies, which are mainly topographical but which contain a certain 

 amount of geological information. A most valuable chapter treats of 

 the special methods adopted in the survey to meet the conditions which 

 prevailed, and tliis will repay careful study by all who may have to 

 prepare their maps in hilly regions which are not largely timbered. 

 Triangulation was extensively employed, and a very free use of vertical 

 angular measurements furnished a very reliable representation of the 

 relief of the region. 



Sedimentary deposits are scantily represented by the coral reefs and 

 raised beaches along the coast, the gypsum beds which occur on part 

 of the coast, and the Nubian Sandstone which covers a considerable 

 area in the western portion of the region. Besides these, alluvial 

 deposits and accumulations of rock waste in the valleys and on the 

 plains at the foot of the hills cover wide tracts, but igneous and 

 metamorphic types of rocks form the surface over by far the larger 

 part of this desert. 



The igneous rocks fall broadly into two main divisions, one of an 

 acid type represented by a granite rich in felspar, and the other of 

 a basic type having gabbro for its representative. A few ultra-acid 

 and ultra-basic rocks occur, while an intermediate division is also 

 represented, but rocks of the granite and gabbro types predominate. 

 The former occur largely as forming the bigher mountains and 

 dominant peaks, while the basic rocks form the lower hill-country. 



Full descriptions are given of typical specimens from various 

 localities, together with the special characters which microscopical 

 examination brings to light. Volcanic rocks are comparatively scarce, 

 and nothing like the wide occurrence which Dr. Hume has recorded 

 in the north-eastern portion of this desert has been found by Dr. Ball 

 in the south-eastern. Basic rocks predominate somewhat in the 

 southern portion of this region, and rocks of the ultra-basic group, 

 usually altered to serpentine, cover as much as 200 square miles of 

 the desert. 



Gneisses and schists cover very large areas, and the former constitutes 

 some of the boldest scenery of the i)recipitous hills of the central 

 portion of the range. , Many ancient mining sites occur in this area, 

 where the quartz veins have been worked for gold in the past, but in 

 many cases they do not pay sufficiently to be worked under modern 

 conditions. Besides the maps already mentioned, several plans on 

 larger scales usefully supplement the text, while photographs of several 

 mourttain-summits and types of country illustrate the weathering 

 of the rocks. The whole question of erosion and the development of 

 the surface relief, for which Dr. Ball must have ample material, 

 promises to be most interesting, and will doubtless be dealt with in 

 due course. As it is, the present volume contains a very valuable 

 account of a region offering many problems, for whose solution Dr. Ball 

 has furnished important material. 



