382 SCIENCE PROGRESS. 



the lower of Rolland's terraces. On the top of the plateau 

 the same explorer obtained Inoceramus impressus, Cardita, 

 Area, Exogyra Overwegi, and Ostrea larva ; the first and 

 last of which are Senonian forms, while Ex. Overwegi is 

 almost peculiar to the Sahara. 



Other travellers also have found Senonian fossils upon 

 this plateau. The collections made by Vatonne on his 

 journey to Ghadames have been examined by Coquand 

 (15, p. 545). They include Ostrea vesicular? s, 0. larva, 

 O. ostracina, O. Overwegi, and several other Senonian 

 forms, together with a few Cenomanian species. Rohlfs 

 also, on the way to Ghadames, found Ostrea armata, O. 

 larva, Exogyra Overwegi, and Ex. Matheroniana (34); while 

 south of that town, Duveyrier discovered the Cenomanian 

 form, Exogyra columba, near the southern escarpment of the 

 plateau (30, pp. 49, 50). 



It is tolerably clear then that the Tripoli plateau, the 

 Hamada el Homra, is formed of Senonian beds, resting 

 upon Cenomanian, and perhaps Turonian ; and this agrees 

 with Rolland's observations in the Algerian Sahara. 



Farther east, the " Black Mountains " (Jebel el Soda 

 and Jebel Haruj) rise up amidst the Cretaceous rocks. 

 They have been the subject of much discussion ; but are 

 probably in part volcanic, and will be described in speaking 

 of the eruptive rocks. 



On the eastern side of the Libyan Desert the Cretaceous 

 and later beds have been very fully described by Zittel (8), 

 who accompanied one of Rohlfs' numerous expeditions. 

 But I shall confine myself here to a very brief and general 

 account of the character and distribution of these deposits ; 

 the details will come more fittingly in a description of 

 Egypt and the Valley of the Nile. 



The Nubian sandstone of Upper Egypt is succeeded 

 conformably by a series of Cretaceous limestones and marls, 

 which occupy the floors of the Kharga, Dakhla, and 

 Farafra Oases, and stretch away westwards beneath the 

 sands of the desert. The whole series belongs to the 

 Upper Cretaceous ; and the lower beds, which contain 

 Exogyra Overwegi in abundance, correspond with the upper 



