384 SCIENCE PROGRESS. 



Pliocene beds are also found in Egypt, but these need 

 not be considered here. 



Quaternary. — Quaternary deposits cover wide areas in 

 the Sahara, and were at one time supposed to be of marine 

 origin. It was even held that the great sand dunes of the 

 deserts must have been formed under the sea ; but at this 

 date it is scarcely necessary to discuss such a view. There 

 are many other points of interest in connection with the 

 dunes ; but space does not permit me to touch upon them, 

 and I can only refer to the work of Walther on denudation 

 in the deserts (36). 



The advocates for a Quaternary Sahara sea argued 

 chiefly from the discovery of marine shells at several points 

 in the middle of the Sahara. But Tournouer has shown 

 that to call in the aid of a great ocean in order to explain 

 the presence of one or two shells is a needless expenditure 

 of energy (37). Indeed it would be difficult for the ocean 

 to have deposited them without leaving more extended 

 evidence of its existence ; and in some cases, at least, the 

 shells seem to have been brought by man, like the money 

 cowries in use at the present day. 



Eruptive rocks. — It is probable, to say the least, that 

 much of the granitic rock observed by travellers is of 

 eruptive origin, and some of it may well belong to a much 

 later period than the Archaean. But the evidence is 

 generally incomplete. 



In the Tiris country, Quiroga (25) describes granite and 

 elaeolite- syenite, which he believes to be eruptive and dis- 

 tinct from the Archaean. Eruptive rocks of basic character 

 also occur in this region. Both Douls (26) in the north, and 

 Panet (27) in the south, speak of basalts ; and Quiroga also 

 has described the occurrence of diabase and melaphyre. 



Von Bary, in the diary of his journey from Ghat to Air, 

 notices basalt in connection with the Granite ; and accord- 

 ing to him, Tegindshir in Air is an extinct volcano (29, 

 p. 362, etc.). Barth, also, in his account of the same 

 region, describes Teleshera as consisting of sandstone, 

 capped by trachyte ; and further south he records a terrace 

 of basalt (18, vol. i., p. 501). 



