1'24> DE. A. L. DU TOIT OX THE [vol. lxxv r 



has made its way along the bedding-planes, and has sent little 

 sheets in between and across the beds. Just at this point the rock 

 is distinctly gneissose, but generally it is granitoidal, dark red, 

 and medium-grained, with irregular bands and patches grading 

 into pegmatite. From this lobe arise two vertical dykes of red 

 biotite-granite, which can be traced up and across the ridge into 

 the Umzimkulu valley, the southernmost being intersected by the 

 adit at the limekiln on Umdwendwe, where it is somewhat gneissic 

 in places. It is probably the dyke mentioned by Dr. F. H. Hatch. 

 Along it the marble has been silieified, and large blocks o£ the 

 siliceous product strew the slopes. 



On F.A. No. 1 and the northern part of West Slopes the marble 

 is split up by several parallel sheets of gneissic granite with a 

 south-westerly dip, and one of these descends from the plateau 

 into the bed of the river at the sharp bend and ascends the 

 opposite slope on Westlands. Another sheet-like body crossing 

 the bedding-planes and dipping westwards descends from the 

 eastern beacon of Ndongini, crosses the Umzimkulu, and ascends 

 East Slopes, swelling out just behind Umdwendwe Hill. This 

 must be the granite observed by Draper, though it does not form 

 the actual summit, as stated by him. 



It is in the main a red porphyritic gneissose variety, and in 

 places the red felspars measure as much as 4 inches across. In 

 the bend of the river, where it cuts obliquely through the white 

 marble cliff, are several smaller dykes. One of these is a dark- 

 grey aplitic variety, and includes a couple of ' calcitized ' marble 

 xenoliths with cleavage-surfaces exceeding an inch in width. 



So far as can be judged, all the acid intrusions — there are a few 

 of fine-grained Karroo dolerite also — belong to and form part of 

 the gneiss of the region. The bigger sheets are generally red, 

 and exhibit conspicuous foliation in places ; while the small dykes, 

 of which there are a fair number, are granitic in structure and 

 usually paler in tint, although there are some narrow red dykes 

 as well. In the case of the very smallest injections they are 

 generally composed almost wholly of quartz and grey or white 

 felspar ; some of them, including even ' nipped-off ' portions, are 

 distinctly foliated. 



IV. Reactiox-Phexomexa aloxg Dykes. 



The envelopment of the mass of dolomitic limestones by the 

 orthogneiss has naturally caused the destruction of the original 

 texture, and a transformation, as Hatch & Rastall have recorded, 

 into white crystalline marble too coarse in grain to be of any use 

 for statuary purposes, or, in the case of the impurer kinds, into 

 varieties containing silicates. 



The phenomena along granite-dykes, however, deserve some 

 detailed consideration, because of the proof which they furnish of 

 the actual interchange of material across the active surfaces of 

 contact. 



