146 G. T. PRIOR. 



Beaclij Natal, contains large glomero-porphyritic masses of 

 clear, glassy felspar, broken up and invaded by the variolitic 

 base, as seen in PI. Ill, fig. 2. Most of the felspar shows no 

 twin-striations, lint one section in which albite twinning was 

 observed gave symmetrical extinction about the twin-lamella? 

 as high as 30°. A few phenocrysts of enstatite are also 

 present in the slide. Of still denser variolitic character are 

 pale grey specimens of so-called " felsite " from the Coal 

 Measures, Urahlali, Natal. One of these (215) shows a few. 

 porphyritic labradorites, micro-pegmatitically intergrown with 

 quartz (see PI. IV, fig. 1), and one or two small, rounded 

 augites, in a dense base consisting of variolitic felspathic 

 material, through which runs a complicated network of lines 

 made up of a doubly refracting mineral enclosing thickly 

 distributed grains of magnetite. These interlacing lines 

 are distinct from the long radiating felspar-needles of the 

 variolitic felspathic base ; in some specimens they have high 

 extinction angles and consist probably of augite; in others, 

 however, tliey give nearly straight extinction, and in these 

 cases the augite has doubtless been altered to hornblende. 

 The structure is strikingly similar to that presented by many 

 iron-slags. Precisely similar specimens come from Intam- 

 banana Creek (388), and from "a hill north of Umhlatuzi 

 crossing above middle drift," Zululand (428), where the rock 

 is intrusive in Ecca shales. In the rock from Intambanana 

 Creek the interlacing curved lines of doubly refracting 

 material with included grains of magnetite present a most 

 fantastic appearance (see PI. IV, fig. 2), and consist for the 

 most part of strings of globulites. In another specimen (385) 

 from the same creek, the fine needles of augite, giving high 

 extinction-angles and enclosing magnetite grains, are shorter 

 and suggest by their arrangement a more or less well-defined 

 flow structure ; still another specimen (386) from the same 

 locality is a dense brown tachylyte showing incipient spheru- 

 lites. These are all probably junction specimens of the 

 coarser-grained sub-ophitic dolerite (387) from the same 

 locality, while specimen 428 from above the middle drift. 



