THE GEOLOGY OF THE SAHARA. 373 



In the centre of the region rise several mountain masses 

 of considerable altitude. The most important is the Tarso 

 range in Tibesti, the culminating peak of which is estimated 

 by Nachtigal to attain a height of about 7900 feet. Another 

 great mountain mass is the Ahaggar, in the country of the 

 Tuareg. Both of these are supposed to consist of Archaean 

 rocks : the sedimentary deposits form plateaux rather than 

 mountains. 



Even the sandy wastes themselves lie at a considerable 

 elevation ; and the only parts of Northern Africa which are 

 sunk beneath the level of the sea are the Tunisian Shotts, 

 west of the Gulf of Cabes ; and perhaps a strip of country 

 between the Gulf of Sidra and the Oasis of Siwa, or Jupiter 

 Ammon. The Shotts have been carefully surveyed by 

 the French Government (1) ; but the elevation of the oases 

 west of Siwa has been determined only by barometrical 

 observations, and the results of two expeditions differ by 

 more than 100 metres. 



So greatly do the plateaux exceed the lower lying- 

 districts in extent, and so considerable is the elevation even 

 of the deserts, that Chavanne estimates the average height 

 of the Sahara at 460 m. (2) above the sea-level, and accord- 

 ing to him the region is constituted as follows (3, p. 

 789) :- 



Hamadas and Serirs, 1 3,600,000 sq. kilos; steppes, 1,500,000 sq. kilos. 

 Rocks and mountains, 2,000,000 ,, sand, 350,000 ,, 



Oases and cultivated ground, 200,000 sq. kilos. 



For fuller information on the physical features of the 

 Sahara, I cannot do better than refer to the admirable 

 description in Reclus' GSographie Universe I /e (3), and to 

 the monograph of H. Schirmer (4). 



Although many travellers have penetrated into the 

 difficult and dangerous regions of the Sahara, few of 

 them have had any pretensions to a knowledge of geology. 



1 The Hamadas and Serirs are stony plateaux destitute of vegetation 

 and almost of soil. 



