86 BR. F. H. HATCH GEOLOGY OF THE [Feb. 1 898, 



typical crystalline schist is, of course, due to dynamic causes which 

 are connected with the reversed faulting, and other earth-move- 

 ments that have affected these beds, to which I shall have 

 occasion to refer later on. 



3. The Black Reef xst) Xlipriveesbeeg Amygdaloid. 



The Black Eeef formation consists of a thin band of quartzites, 

 not more than 50 feet thick at the most, but inclined at a very 

 slight angle, and therefore in places spreading out over a compara- 

 tively large area. These quartzites have at their base a thin, narrow 

 seam of pebbles, which is often highly mineralized with sulphides and 

 oxides of iron. In places also it carries gold. The pebbles consist 

 of quartz, as in the Witwatersrand conglomerates, but a distin- 

 guishing feature is the presence of pink and dark- coloured cherty 

 and jaspery pebbles. The matrix is also much more pyritic, and 

 the pyrites is sometimes present in the form of nodules. It is this 

 seam which, on account of its dark colour, gives the name to the 

 formation. The name is universally adopted in the Transvaal ; 

 although Molengraaff ^ uses the designation ' Boschrand Series,' from 

 the name of a small ridge formed near Klerksdorp. 



The Black Beef overlies unconformably the Witwatersrand Beds, 

 and although along the Band proper it may appear to conform to 

 the older series, there are several places where it can be seen over- 

 lapping them : for instance, at Bandfontein, Hill's Waterfall, and 

 Elandsvlei, west of Bandfontein ; also at Middelvlei, Bietfontein, 

 Blaauwbank, and Elandsfontein, near Wonderfontein. The sinuous 

 course pursued by the Black Beef will be well seen on the accom- 

 panying map. An interesting little inlier of the Black Beef is 

 worthy of note at Xatdoombosch, close to Prederikstad. 



Between the deposition of the Witwatersrand Beds and the 

 Black Beef formation there was a considerable eruption of basic 

 volcanic rock now forming the Klip Biver Hills, and known as the 

 Klipriversberg Amygdaloid, on account of the number of infilled 

 vesicles which distinguish it. These vesicular cavities are filled with 

 the usual secondary minerals, chiefly chlorite, quartz, and calcite. 

 A section of the rock shows a felt-like aggregate of felspar-needles 

 and augite-granules, in which are embedded small porphyritic 

 crystals of augite, and less frequently large crystals of plagioclase- 

 felspar. 



The flows are of immense thickness, amounting in the aggregate 

 to at least 5000 feet. They were produced by the volcanic out- 

 pourings which took place after the deposition of the Witwatersrand 

 sediments ; and this fact may help to explain the unconformity 

 which exists between those beds and the Black Beef. 



4. The Dolomite Formation. 



The Dolomite is one of the most interesting and widespread 

 members of the Cape System in South Africa. It was first 



^ Neues Jahrb. Beilage-Bd- ix (1895) p. 216. 



