THE GEOLOGY OF THE SAHARA. 375 



regions are tolerably complete ; but in the central parts of 

 the Sahara we have little besides the notes of Barth (18) 

 and Rohlfs (19), who were not geologists. It is to be 

 regretted that so careful an observer as Nachtigal (20) 

 should have told us so little of the character of the rocks 

 over which he passed. 



With regard to maps the Sahara is highly favoured, for so 

 inhospitable a country. The sketches of Suess and Rolland 

 are accompanied by geological maps, and Zittel is respon- 

 sible for the map of Africa in Berghaus' Geological Atlas 

 (21). Rolland's map, with some slight alterations and 

 improvements, is reproduced by Schirmer (4, p. 9). 



All these general maps are based mainly upon Lenz' 

 geological map of West Africa (22), Rolland's of the 

 Algerian Sahara (15), and Zittel's of the Libyan Desert (8). 



Speaking broadly, the Sahara is geologically the 

 southern half of an enormous basin. The outer rim. 

 which is not perfect, is formed by granite, gneiss, and other 

 crystalline rocks, presumed to be of Archaean age. Such 

 rocks occur along the west coast, south of the Wadi Draa, 

 and throughout the Tiris country. In Senegambia they 

 are replaced by slates and quartzites, doubtfully referred to 

 the Silurian. But gneiss reappears in the Kong Mountains, 

 and there is reason to believe that Archaean rocks extend 

 nearly continuously across the Soudan as far as Khartum. 

 Here again, as in the west, the rim is broken by a strip of 

 later rocks ; but granite and gneiss reappear further north, 

 and form the eastern border of the continent. 



Within this rim there is -a vast area of Palaeozoic rocks, 

 while the centre of the basin is occupied by Cretaceous and 

 Tertiary deposits. Jurassic beds are unknown except in 

 the extreme north, on the borders of Tunis (23); and of the 

 Trias there is probably no trace. 1 



Granitic and gneissic rocks. — On Lenz' map of West 

 Africa, a broad belt of Devonian and Carboniferous rocks 



1 Berghaus' map shows Trias and Dyas at various points on Barth's 

 route from Kuka to Timbuktu, and a large area in Kordofan. Of 

 the first the evidence is extremely unsatisfactory ; the second will be 

 discussed in a future article. 



