152 G. T. PRIOR. 



of quartz appears to be certain from the fact that the refrac- 

 tion is practically the same as Canada-balsam, and that some 

 of the triangular sections show in convergent light a well- 

 defined positive uniaxial figure. In another rock (15) 

 " cropping out in middle of plain to the south-east of Dundee, 

 Natal," the structure is of the more usual type, with rounded 

 grains of quartz surrounded by spherulitic and granophyric 

 material. 



Perhaps the most interesting point brought out by the 

 examination of the dolerites is their general resemblance to 

 those of the Ferrar Glacier in South Victoria Land. In both 

 regions, dykes and sills of a somewhat similar dolerite, of 

 medium acidity and of Pacific type, occur over a large area, 

 intrusive in sandstone and shale formations, which, it seems 

 not unreasonable to suggest, may possibly be of about the 

 same ag-e. In both the actual Continental areas over which 

 these ancient dolerite intrusions are spread, no volcanic 

 rocks of recent date are found, but in each case in neigh- 

 bouring islands (Koss Island, etc., in South Victoria Land, 

 Madagascar in South Africa) in later geological times volcanic 

 eruptions have taken place, of which the products are in both 

 regions of very similar character, viz. soda-rich phonolitic 

 rocks of decidedly Atlantic type. 



Ehyolites of the Lebombo Range. 



Most of the specimens from the Lebombo Range are of 

 acid glassy rocks. Though on the Avhole best classed as 

 rhyolites, some of them, containing less silica and more lime- 

 soda felspar than typical rhyolites, approach to trachytes of the 

 Toscanite type of Washington.^ Closest to typical rhyolites 

 are a number of specimens of glassy rocks from near 

 " Crossly's Store " at the southern end of the Lebombo Range. 

 In some of these the original glassy base is altered and 

 devitrified ; in most, however, the glass and also the enclosed 

 felspars and augites are quite fresh and unaltered. 

 ^ See ' Jouvn. of GeoL, Chicago,' v (1897), p. 37. 



