THE GEOLOGY OF THE SAHARA. 383 



beds of the Hamada el Homra. Unless, then, the Nubian 

 sandstone is in part Cretaceous, the Cenomanian of the 

 Western Sahara is absent in the east, and has been over- 

 lapped by the higher beds. That the Cretaceous beds of 

 Egypt do overlap the beds below, is shown by the fact that 

 east of the Nile they rest for many miles directly upon the 

 crystalline rocks, whereas in the Libyan Desert they lie 

 conformably upon the Nubian sandstone. 



Tertiary. — It is only in the eastern part of the Sahara, 

 east of the Libyan Desert, that Eocene beds are developed. 

 They cover a wide area north of the Cretaceous belt of the 

 oases, and they consist chiefly of limestones. West of the 

 Nile they lie upon the Cretaceous beds ; but east of the 

 ruins of Thebes they rest for a short distance upon the 

 crystalline rocks. 



Towards the north and west the Eocene deposits pass 

 under Miocene beds. In the Oasis of Siwa these consist 

 of limestones, sandstones, and clays ; and the lower beds 

 are marine, while the upper are of freshwater origin. The 

 freshwater beds are probably the equivalents of the Jebel 

 Ah mar sandstone and the remarkable fossil forests of Cairo 



(35)- 



The Miocene beds extend along the southern shore of 



the Mediterranean, from the delta of the Nile westwards as 



far, no doubt, as the Gulf of Sidra. They occupy the Oasis 



of Siwa, and they probably form the whole of the Barka 



plateau (8, p. 131). They seem, like the other deposits of 



this region, to overlap the beds below ; and in Tripoli they 



rest directly upon the Cretaceous. 



The Pliocene of the Sahara is insignificant in extent, 



and structurally of little importance. Lenz, in his map of 



West Africa, has coloured a narrow strip of Cretaceous 



rocks along the West Coast from Morocco to Cape Blanco ; 



and most subsequent cartographers have accepted his 



authority. But it appears from the researches of Ouiroga 



at Rio d' Oro that the coast is formed, not of Cretaceous, 



but of Pliocene deposits. Limestones, sandstones, and 



marls occur, in which Ouiroga has found a very recent 



fauna, including many living species (25). 



27 



