94 DE. F. H. HATCH — GEOLOGY OF THE [Feb. 1 898, 



Y. The Volcanic Rocks. 



The period during which the beds constituting the Cape System 

 were deposited was undoubtedly one of great volcanic activity ; 

 an activity which was probably continued intermittently until the 

 Karoo Beds were deposited, since contemporaneous lava-flows are 

 quite a common feature in the Xaroo Beds of South Africa. The 

 Dwyka Conglomerate, which has given rise to so much discussion in 

 South African geology, lies at the base of the Karoo formation, 

 and although opinions are divided as to its mode of origin I should 

 be inclined to regard it as evidence of the enormous volcanic activity- 

 developed at that period. Specimens of this rock collected by the 

 late Prof. Green, ^ and examined microscopically by mj^self, have a 

 very marked resemblance to the aspect of a volcanic breccia or tuff, 

 consisting of distinctly angular fragments of quartz, felspar, augite, 

 olivine, epidote, magnetite, mica, besides chips of volcanic rocks 

 embedded in a minutely fragmental groundmass. 



"With regard to the Southern Transvaal, at least -J- of the whole 

 area described is made up of volcanic rock. I have already re- 

 ferred to the vast eruption of basic volcanic rock which took place 

 between the deposition of the Witwatersrand Beds and that of the 

 uncouformably overlying Black Reef, and known as the Kliprivers- 

 berg Amygdaloid. I have also described some of the types of the 

 contemporaneous flows of the Magaliesberg and Gatsrand Beds, and 

 mentioned the epidiorite-dykes which teem in the Witwatersrand 

 Beds. I have now to refer to a huge outpouring of lavas, some of 

 which are of a more acid type — rhyolites and andesites — whose age 

 I have not been able to determine. It is certain, however, that they 

 are younger than the Witwatersrand Beds. This volcanic area is met 

 with immediately west of Klerksdorp, extending thence in a northerly 

 direction towards Yentersdorp, a distance of 40 miles, and having in 

 the widest sections a breadth of 10 miles. One of the centres of this 

 eruption or series of eruptions appears to have been at Platberg, a 

 flat-topped hill 12 miles north of Klerksdorp. A part of the 

 rock occurring at this locality is of a true rhyolite or liparite-type ; 

 but rocks of a more basic (andesitic) character occur at the same 

 locality, and may be portions of flows emitted at a different 

 period. 



A section of the rhyolite examined under the microscope shows 

 it to consist of a cryptocrystalline or ' felsitic ' groundmass, in 

 which are embedded porphyritic crystals of quartz with character- 

 istic ' corroded ' edges and, less frequently, crystals of both ortho- 

 clase and plagioclase. This rock has also been described by Dahms,^ 

 under the name of quartz-porphyry, from the Makwassi Hills. 

 The andesite-type is distinctly vesicular, the vesicles being fiUed 

 with secondary minerals (chlorite, quartz, etc.). Under the micro- 

 scope it is seen to be composed of a mesh of felspar-microlites and 

 granules of augite, with few porphyritic constituents. 



^ Quart. Journ. Geol. Soc. toI. xliv (1888) p. 242. 

 2 Neues Jahrb. Beilage-Bd. vii (1891) p. 108. 



