THE GEOLOGY OF THE SAHARA. 



IN the eyes of a geologist, Northern Africa, with all 

 its political complications, falls into two, and only two, 

 great divisions. They are of very unequal extent, and the 

 boundary between them lies along the southern flanks of 

 the Atlas Mountains. In the northern division, in Tunis, 

 Algeria, and Morocco, the rocks have been thrown into 

 folds which run somewhat north of east ; and these folds 

 have given rise to mountain chains approximately parallel 

 to the Sierras of Spain. This part of Africa, indeed, as even 

 so ancient a writer as Sallust has remarked, is physically a 

 part of Europe. South of the Atlas, on the other hand, 

 even the Devonian and Carboniferous beds are nearly 

 horizontal, and they form broad plateaux instead of narrow 

 mountain chains. It is this division, the Sahara, with 

 which the present article is concerned. 



Sahara is a word of doubtful origin, and in its applica- 

 tion it is somewhat vaQfiie. In its widest sense it includes 

 the whole of that portion of Africa which extends from the 

 Atlas southwards to the fertile districts of the Soudan ; and 

 from the Atlantic eastwards to the borders of the Red Sea. 

 Even the shores of the Mediterranean, from the Gulf of 

 Cabes to the Suez Canal, belong to it ; and Egypt and the 

 Nubian Desert lie within its limits. But to confine the 

 article within reasonable bounds, the valley of the Nile 

 and the regions to the east must be left for future con- 

 sideration. 



In its physical features the Sahara of Northern Africa 

 is very different from the Sahara of our older text-books. 

 It is not the broad, low-lying waste of sand which they 

 depict ; but the greater part is occupied by a succession 

 of elevated rocky plateaux, and the sandy deserts form a 

 comparatively small part of the whole. The largest of the 

 sandy wastes is the Libyan Desert, between Fezzan and the 

 Nile ; while in the west the most extensive are those of 

 Igidi and El Juf. 



