THE GEOLOGY OF THE SAHARA. 381 



fossils by Schweinfurth in the Wadi Araba 1 (33), has led 

 most subsequent writers to refer a part of the series to the 

 Pakeozoic. Rolland indeed has gone so far as to include 

 the whole of the Nubian sandstone with the Palaeozoic 

 upon his map ; but this is due no doubt simply to the 

 impossibility, in the present state of our knowledge, of 

 drawing a line between the Upper and Lower divisions. 



Cretaceous. — Turning now to the Cretaceous deposits, 

 we find that in the Algerian Sahara they form the plateaux 

 of the Mzab and Tedmaid (near In Salah) ; and in Tripoli 

 the Hamada el Homra. The southern and western 

 boundaries have already been traced, and the Cretaceous 

 rocks, except where covered by later deposits, extend 

 from these boundaries to the Atlas and Mediterranean. 

 Between the Mzab and the Hamada el Homra lies a broad 

 area of Quaternary deposits, through the middle of which 

 runs the depression called the Wadi Igharghar — a de- 

 pression which figures very prominently in certain accounts 

 cf North Africa, but which is not everywhere very clearly 

 defined. 



Rolland (15) finds that the Cretaceous plateau of the 

 Algerian Sahara rises in two steps or terraces ; the lower 

 terrace being formed of Middle Cretaceous and the upper 

 terrace of Upper Cretaceous beds. Each consists of a 

 limestone, resting upon a series of gypseous marls ; and the 

 lower stage is often very fossiliferous. Among the fossils 

 are Ostrea flabellata, Plicatula aziressensis, both of them 

 essentially Cenomanian forms ; and a variety of other 

 species, nearly all of which are common in the Cenomanian 

 of the Atlas. 



In the Algerian Sahara no fossils have been found in 

 the upper stage, but in Tripoli they are fairly abundant. 



On the north edge of the plateau that lies south of 

 the town of Tripoli, Overweg (16, 17) discovered R?idistes, 

 Exogyra simiata, and Ex. conica. These are Cenomanian, 

 and the beds in which they occur clearly correspond with 



1 This is the Wadi Araba which opens into the Gulf of Suez, about 

 2 9 N. Lat., not the Wadi Araba of Syria. 



