Vol. 54.] WITWATERSRAITD AND OTHER DISTRTOTS IN S. TRANSVAAL. 



85 



The mine -workings on 

 the auriferous conglomerates 

 (Main Reef Series) have dis- 

 closed the presence of a great 

 number of dykes traversing 

 the formation. These dykes 

 are invariably of basic cha- 

 racter, and vary in width 

 from a few feet up to several 

 hundred. In depth, where 

 unaltered, they are close- 

 grained, dark-green coloured 

 rocks, but near the surface 

 they weather into a reddish 

 clay. 



I have had sections made 

 from specimens collected from 

 the deeper levels of the 

 mines (Ferreira, Meyer & 

 Charlton, Eobinson Deep, 

 Simmer & Jack, etc.) and 

 from some of the deep bore- 

 holes. They are all practi- 

 cally of the same description, 

 belonging to the epidiorite 

 group. To give examples, 

 a section of the Wemmer 

 dyke in the Ferreira Mine 

 shows fragmentary crystals 

 and shreds of strongly pleo- 

 chroic hornblende (brown 

 and green) in a felt-like 

 aggregate of pale green to 

 colourless hornblende, a little 

 plagioclase - felspar, apatite, 

 ilmenite, leucoxene, and cal- 

 cite. 



The margins of dykes are 

 invariably schistose, and the 

 smaller dykes are sometimes 

 completely transformed into 

 hornblendic and chloritic 

 schist. 



A section from the margin 

 of the big dyke in the Ferreira 

 Mine showed a mass of minute 

 interlacing needles of horn- 

 blende (uralite) with occa- 

 sional granules of felspar ; in 

 fact, a typical hornblende- 

 schist. This transformation 

 from massive dolerites of 

 purely igneous origin into 



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