500 C. B. Horicood Sf A. Wade— The Old Granites of Africa. 



frequently contain small rounded pebbles, sometimes almost in sufficient 

 quantity to constitute a 'banket', and occasional pebbles are also 

 sometimes found at slightly higher horizons. The pebbles are of 

 vein-quartz and quartzite ; granitic fragments could not be detected 

 with certainty. 



" Apart from the presence of these pebbles, the basal portion of the 

 quartzite is usually fine-grained and uniform in texture, though along 

 a certain portion of the section it was seen to become coarse and gritty. 

 These features recall those so often found at the base of the Black 

 Eeef quartzites, Avhere they rest upon similar granite, and tend to the 

 conclusion that the base of the "VVitwatersraud quartzite here constitutes 

 a true basement bed, which has been deposited upon the granite." 



On the other hand, he points out that the intensely foliated character 

 of the granite along the junction is similar to the foliated marginal 

 facies which is intrusive in the Swaziland System, and that its petro- 

 graphical features tend to strengthen the resemblance. Also the 

 planes of foliation usually, although not always, strike in a direction 

 parallel to the strike of the overlying quartzites. Further, he says : 

 " the porphyry-like facies of the granite along the line of contact is 

 certainly suggestive of intrusive behaviour, but one cannot accept 

 this fact alone as good evidence of intrusion without more definite 

 and confirmatoiy indications in the field ; whereas, as already pointed 

 out, the field evidence leads distinctly to the conclusion that the 

 quartzites were deposited upon the granite. Further, the large 

 quartz grains in the coarse grit, sometimes found at the base of 

 the quartzite, are seen under the microscope to consist of aggregates 

 of quartz showing strain shadows, and resembling the quartz blebs 

 seen in the foliated granite beneath, a fact which strongly suggests 

 their source of supply, and further favours the conclusion of the later 

 age of the quartzites." Personally, I think the foliation and porphyiy- 

 like facies alluded to suggest close proximity to the contact with 

 beds belonging to the Swaziland System, the junction being hidden 

 by the overlying and unconformable quartzites. 



He also describes two small outliers of the ICoodies Series, consisting 

 of shales and quartzites, with associated basic schists and serpentines, 

 and also quartz-felsites, some 3 miles west of Mulder's Drift, in 

 which the granite is intrusive ; and shows that the basic schists to the 

 south and south-west of the granite mass form a sort of peripheral 

 basic zone. 



He calls attention to the foliated character of the granite along its 

 contact with the older basic schists and quartz-felsites of the Swaziland 

 System, also to the variations of the granite itself. Thus we have 

 biotite - granites, hornblende - granites, and quartz - diorites ; to the 

 development either of biotite or hornblende, the one at the expense of 

 the other ; to the local development of foliated quartz-diorite along 

 its margin, due to the magmatic segregation of the more basic minerals 

 towards the cooler portions; to dyke-like intrusions of diabase; and 

 to the occurrence of lines of crush in the granite, forming conspicuous 

 ridge-like features, having a general north and south trend, along 

 which foliated and crushed granite of an acid type is found well 

 developed. 



