226 PETROGRAPHY OF THE LAVAS 



they are present vary. The felspar of the earlier genera- 

 tion is most frequently labradorite, that of the second 

 is usually andesine ; the augite is commonly colourless ; 

 olivine is often present, but seldom in quantity, and is 

 more or less changed to greenish serpentine ; magnetite 

 is always, and apatite often, present, and occasionally 

 enstatite or a green augite. Brown glass containing 

 microlites is found in many of the lavas. 



Many of the vesicular lavas are basalts having a 

 varying amount of glass and microcrystalline base, in 

 which are embedded more or less well-formed crystals 

 of olivine, augite and felspar ; the enstatite-andesites of 

 the Belmore volcano are highly glassy and show fluxion 

 and perlitic structures. The doleritic lavas may possess 

 a small amount of residual glass ; the felspar is either 

 in fair-sized porphyritic crystals between which small 

 augites and felspars lie, or in smaller laths partially or 

 wholly enclosed by masses of augite. The last-men- 

 tioned type of rock is very similar in character to the 

 dolerite of the intrusive sheets and dykes of the Karroo, 

 but the ophitic structure is rarely so well developed in 

 the Drakensberg lavas as in the latter. There are no 

 coarsely porphyritic lavas. 



The specific gravity of the doleritic varieties ranges 

 from about 2'8 to 2'95, and is less than that of the 

 Karroo dolerites, which range from 2 '95 to 3*0. 



The amygdaloidal lavas have their steam-holes as a 

 rule filled in with secondary minerals such as calcite, 

 chalcedony (agate) or zeolites, amongst which heulandite, 

 stilbite, thomsonite and scolecite have been recognised. 

 A layer of green chlorite or delessite sometimes lines 



