THE GEOLOGY OF THE SAHARA. 385 



Again, at the north-west end of the Tarso range, 

 Nachtigal describes a well-formed crater; and this clearly 

 points to a recent period of vulcanicity (20, vol. i., pp. 



389-39i)- 



In none of these cases, however, is there any really 



satisfactory evidence of the age of the rocks ; and it is only 



among the more recent deposits, the Cretaceous and 



Tertiary, that we are able to fix a definite limit for the date 



of the eruptions. Near the northern border of the Tripoli 



plateau several cones of volcanic rock rise through the 



Cretaceous beds ; and these must clearly be of late 



Cretaceous, or more likely, of Tertiary age. They consist 



chiefly of phonolite, and to a certain extent, of olivine 



basalt (16). 



But the most extensive volcanic masses in this area are 

 probably the Jebel-el-Soda and the Jebel Haruj, or the 

 " Black Mountains". Hornemann long ago mentioned the 

 presence of basalt in these mountains ; but there can be 

 little doubt that he has sometimes been misled by the 

 peculiar black weathering of the surface which is so 

 characteristic of the Sahara — a mode of weathering which 

 gives a very deceptive volcanic look to the sandstones and 

 other rocks which cover so wide an area. In this case, 

 however, Hornemann was probably correct, for Duveyrier 

 brought home specimens which were determined by 

 Descloiseaux to be amygdaloidal basalt (30, p.~ jj). 



It is probable, however, that the eruptive rocks do not 

 form the whole of this mountain mass. Beurmann states 

 that he has nowhere found any traces of basalt, and he can 

 scarcely believe in its presence (38). Vogel, too, describes 

 the range as consisting of limestone and sandstone (39) ; 

 and Rohlfs, who accepts Duveyrier's view, also speaks 

 of the presence of sedimentary rocks (19, p. 125). We can 

 only conclude, then, that these mountains form a continuation 

 of the Cretaceous plateau, penetrated here and there by 

 eruptive rocks of later date. 



Volcanic rocks are also supposed to occur in the 

 Ahaggar Mountains (30, p. 54), and on the Tasili plateau. 



Lastly, in the Egyptian area, basalt occurs in the 



