378 SCIENCE PROGRESS. 



has yet penetrated ; but it is probable that the granite 

 which Barth and von Bary crossed, is simply the eastern 

 part of a mass of crystalline rock of which the Ahaggar is 

 the centre. It is true that Ismail-Boii-Derba is said to 

 have found pebbles of lava in the Wadi Igharghar, and 

 these pebbles appeared to have come from Ahaggar (30, 

 p. 54). If this be so, there are probably lava-flows in these 

 mountains just as in the hilly granitic region of Air. 



Barth's and von Bary's observations, in conjunction 

 with those of Rohlfs, give us the eastern boundary of the 

 Ahaggar massif ; and the northern limit is determined by 

 the discovery of Devonian fossils in the Tasili plateau. But 

 the southern boundary is less certain. Rolland assumes 

 that the Tasili-wan-Ahaggar, south of the great Ahaggar, 

 is a Palaeozoic plateau. This cannot be said to be 

 certain ; but it seems likely that the granitic rocks do not 

 extend far to the south of the mountain mass itself. 



East of Ahaggar and Air there is sandstone, but it is 

 uncertain how far this extends to the south. According to 

 Rohlfs, granite occurs in the Geissiger Mountains (19, vol. 

 i., p. 272) ; but sandstone and limestone still predominate. 

 It is not till we proceed to the east that we meet with any 

 more granitic rocks. At the north-west end of the Tibesti 

 Mountains, Nachtigal has noticed the presence of granite 

 (20) ; and from this and the character of the hills, Suess infers 

 that the whole range is Archaean. 1 If this be correct, it 

 probably forms only a narrow strip, for sandstone hills occur 

 on the south side of the ranee i n the district of Borku. 



This narrow strip is probably connected on the east 

 with the extensive mass of Archaean rocks in Kordofan and 

 Sennaar ; 2 but the intermediate country requires examina- 

 tion. Near Khartum these rocks are succeeded by the 

 Nubian sandstone ; but farther north there is another large 



1 From Nachtigal's description I cannot but think that the main mass 

 of this range is Palaeozoic rather than Archaean (20, vol. i., pp. 389-391). 



- I have for the present followed Suess and Rolland in including the 

 Kordofan and Sennaar rocks in the Archaean. Zittel, in Berghaus' map,, 

 has coloured the region as Trias and Dyas, with x^rchaean masses rising 

 through it. 



