THE GEOLOGY OF THE SAHARA. 377 



may be, the number of points at which granite occurs 

 justifies us in concluding that a large part of the region 

 enclosed in the great bend of the Niger is composed of 

 granite or gneiss. 



It does not appear, however, that these rocks extend 

 much farther to the north. It is true that Barth notes 

 granitic boulders in the course of the Niger some 150 miles 

 east of Timbuktu ; but near the point where the river 

 bends from east to south, stratified rocks occur on the north 

 side of the river, and south of Gogo there are steep sand- 

 stone cliffs. At Say itself sandstone also occurs. 



East of the Niger, between Say and Sokoto, and again 

 nearly as far as Katsena, there are no records of rock, and 

 the country is for the most part a plain. But a little west 

 of Katsena, Barth again mentions granitic rock ; and 

 granite also occurs at intervals as far as Mashena. 



We have now traced the granitic rim of the Sahara 

 basin from the Tiris country to the plains of Lake Tsad, 

 where it is lost under recent deposits. But there are several 

 masses of granite which appear to be separated from the 

 main belt, and which rise up in the midst of the sedimentary 

 deposits within the basin. Lenz records granite in El 

 Eglab on the southern border of the Igidi Desert (12, vol. 

 ii., p. 60) ; and according to Barth, a considerable part of 

 the Air country is made of granite. In Air it is associated 

 with flows of basalt, and both on the north and the south 

 it is overlaid by the sandstone which occupies such wide 

 areas in the Central Sahara (18, vol. i.). 



But a much more extensive Archaean mass than either 

 of these, is that which forms the Ahaggar. Both Barth and 

 von Bary passed to the east of these mountains on their 

 way from Ghat to Air ; and both of them found the 

 country, from the Wadi Egeri southwards, to consist mainly 

 of granite (18, vol. i., map; 29, 12th Jan., 1877). Here 

 and there the granite is overlaid by plateaux of sandstone, 

 which seem to be tongues projecting west from the more 

 extensive plateau crossed by Nachtigal and Rohlfs on 

 their way to Kuka. 



Into the centre of the Ahaggar no European traveller 



