DOLEKITES AND EHYOLITES 0¥ NATAL AND ZULULAND. 151 



quartz-mosaic. The oligoclases are much altered with develop- 

 ment of a fibrous hornblendic mineral and patches of micropeg- 

 matite similar to those in the hybrid rock. The " fine-grained 

 granite vein " in the hybrid rock is similar^ but of finer grain, 

 and consists of similarly altered oligoclases in a mosaic of 

 quartz, orthoclase, and microcline, with altered biotite only in 

 small amount. 



Much altered dolerites (amphibolites, and typical epidiorites 

 with uralitic hornblende and clear plagioclastic felspar) occur 

 as dykes in the granite-gneiss between the eighteenth and 

 twentieth mile-pegs on the Melmoth and Eshowe Road, 

 Zululand. 



Most of the pebbles in the Dwyka Conglomerate at Park 

 Rynie, Natal, are of dolerites which have been much altered 

 with development of epidote, uralitic hornblende and 

 leucoxene ; they generally show coarse interstitial micro- 

 pegmatitic material like so many of the dolerites intrusive in 

 the Karoo formation. 



Associated with the dolerites in Cape Colony, as is well 

 known,^ are granophyric rocks said to be differentiation 

 products of the magma which supplied the dolerites, so many 

 of which contain granophyric patches. In the present 

 collection there are a few specimens of these rocks. One of 

 them-(197), labelled "decomposed dyke,'' comes from Umhlali 

 Beach, Natal. It is a white rock showing no ferro-magnesian 

 constituents. Under the microscope it is seen to consist of 

 small felspar prisms and skeletal crystals of quartz surrounded 

 by' spherulitic material. In thin slices (PI. VI, fig. 2) under 

 the microscope central square sections of felspar, giving 

 straight extinction, are seen surrounded by a clear zone of 

 quartz with diagonal extinction, which in its turn is surrounded 

 by spherulitic material. The skeletal crystals of quartz give 

 sections of various shapes, triangular, nearly rectangular 

 (with diagonal extinction), or elongated along a central axis 

 with lateral notched projections (see PI. VI, fig. 2). They 

 appear to be mostly bipyramidal crystals. That they consist 

 ' See Rogers, 1. c, p. 275, and pi. xix, p. 277. 



