332 ME. F. W. PEKTST OX THE VREDEFORT GRANITE. [Dec. I914, 



intruded tongues along lines of weakness in them, caused extensive 

 faulting, and. broken through thick beds of quartzite at right 

 angles to the strike, thereby completely isolating several large 

 and small masses of quartzite and slate. In the middle of it, 

 as indicated on the map, was found a small area of an amyg- 

 daloidal rock, containing many large amygdales, often tubular in 

 shape, chiefly filled with quartz. The nature of the junction 

 between this rock and the normal diabase is indefinite, and was 

 difficult to ascertain ; but the impression gained was that the 

 diabase suddenly became amygdaloidal. 



The case of the quartzite (Wl) in contact with the diabase near 

 the northern boundary of the farm ' Tweefontein ' is interesting : 

 it dips comparatively gently away from the granite, in contra- 

 distinction to the overtilted or nearly vertical dip which all the 

 sedimentary beds invariably have hereabouts. This is explained as 

 being due to a subsidiary granite intrusion into the diabase, the top 

 of which comes to light in the middle of the diabase to the west 

 of the quartzite-ridge ; this must have forced the sedimentary 

 beds outwards from the granite, thereby giving to its inner side a 



Fig. 1. — Quartzite-contact with diabase near the northern 

 boundary of the f arm ' Tweefontein? 



E. 



Sedimentary 

 ( Rocks 



curvature (fig. 1). When traced northwards the sedimentary beds 

 gradually assume a vertical, and then an overtilted position. 



This basic intrusion is a definite addition to the thickness of the 

 strata above the granite, since no evidence of absorption of the 

 sediments by it on any large scale was discovered. However, the 

 remarkable regularity of the boundaries which the slaty members of 

 the series exhibit towards the diabase, as opposed to the constantly 

 faulted outcrops of the associated quartzite-bands, is suggestive of 

 a certain amount of absorption by them. As a general rule, the 

 moderate-sized intercalations do not even produce metamorphism 

 visible to the naked eye in the slates adjacent to them. The 

 extreme extent and nature of the metamorphism which they effect 

 can be well studied in the masses of slate enclosed in the larger 

 intrusions remote from the granite, as, for instance, on the farm 

 'Tweefontein.' While still preserving its banding and cleavage, 

 the slate is changed to a pinkish-brown rock, somewhat phyllitic 

 in appearance, which, under the microscope, is seen to consist of 

 comparatively large flakes of sericite, roughly orientated, and 



