DETAILED DESCRIPTIONS OF ROCKS. 5' 



Considering the chain of hills, extending from Baorli to Thob, as 

 a whole, it is worthy of notice that, speaking generally, the rocks to 

 the north-east of Nagona dip towards the north-east while those to 

 the south-west are inclined in the opposite direction. Moreover, we 

 have seen that about the centre of the chain, in the large block of hills 

 surrounding Nagona, there is some e\idence of the presence of one 

 of the vents through which the lavas were poured out. It seems at 

 least possible therefore that this chain of hills represents the remnants 

 of an ancient volcanic cone. The inclination of the lava flows on 

 either side of the central area, and the form of the scarps, which points 

 to a large amount of denudation having taken place, is not inconsist- 

 ent with the supposition of the former existence of such a cone ; but 

 the enormous extent to which the surrounding rocks are covered up 

 by sand, leaving only the merest fragments of what must have been 

 thick flows of lava, extending over large areas, now visible, renders 

 it impossible to say with certainty whether the lavas were erupted 

 from a single vent, forming a cone of the Vesuvian type, from a number 

 of fissures, or from smaller vents of the Puy type scattered over the 

 surface of the country. 



Beyond the southern end of the chain of hills just described, no 

 rocks occur in the plain until the river Luni is reached at Jasol, about 

 20 miles to the south-west of Thob. Here on the south bank of the 

 river are one or two isolated hills of rhyolite, and here the first 

 instance occurs of rocks of a different age being associated with the 

 rhyolites. On the southern side of the large hill close to Jasol, which 

 consists of a dark purplish porphyritic rhyolite, two broad vertical 

 dykes of basic rock (olivine dolerite) are intruded into the lavas. The 

 dykes are 50 and 30 feet broad respectively, parallel to each other and 

 striking W. 30 N. Branches are thrown off into the surrounding 

 rocks, and at one point a dyke about 15 feet broad connects the two, 

 running diagonally from one to the other. The material of the dykes 

 is dark green in colour, consisting mainly of plagioclase felspar, ser- 

 pentinised olivine and hornblende. It decomposes much more rapidly 



( 5» ) 



