DOLERITES AND EHYOLITES OF NATAL AND ZULULAND. 143 



following account is therefore devoted mainly to a description 

 of the more interesting structural variations presented by these 

 rocks. 



Mineral Composition. — The rocks consist mainly of 

 labradorite, brown augite, which is nearly colourless in thin 

 slices, and (generally in comparatively small amount) iron 

 ores (magnetite and ilmenite). Accompanying the monoclinic 

 pyroxene, in many specimens, is enstatite, a constituent which 

 does not appear to have been noted in South African 

 dolerites previously described. Biotite and original horn- 

 blende, mentioned as often present in the dolerites of Cape 

 Colony,^ were not found in these rocks of Natal and Zululand. 



Olivine is present in large amount only in a few specimens 

 of coarse-grained ophitic dolerites, such as the rocks from 

 '' dyke in the Metamorphic Series," half way up Bosnian's 

 Folly, north of the Umhlatuzi River in Zululand (442),- from 

 the top of a hill near Ingogo battle-field (20), and from In- 

 hlazanMt., thirty miles west of Pietermaritzburg, Natal (245). 

 These show numerous irregular olivines, mostly enclosed in 

 large ophitic plates of augite. In a dolerite intrusive in coal- 

 measure sandstone at Ugata Hill, Hlabisa, Zululand (424), the 

 olivine, like the augite, occurs in irregular plates, interpene- 

 trated by and enclosing felspar laths (see PI. Ill, fig. 1). In 

 other specimens (e.g. dolerites from Majuba Hill, Natal; 

 Makowe Hills and Umlalaas, Zululand) the olivine occurs in 

 small grains only sparingly distributed. By far the greater 

 number of dolerites in the collection are without olivine. 



Many of the specimens (e.g. a coarse-grained, gabbro-like 

 rock, intrusive in sandstones and shales at the mouth of the 

 Umhlali River, Natal (160), and altered dolerites from the 

 Makowe Hills in Zululand) show in thin sections patches of 

 more acid material, with quartz in spherulitic and micro- 

 pegmatitic intergrowth with felspar, such as occur so commonly 

 in the dolerites of Cape Colony. In this and many other 

 respects (mode of occurrence, mineral composition, etc.) these 



^ See Rogers, I.e., p. 273. 



- The numbers in brackets refer to the list of specimens. 



