248 VEINS AND INCLUSIONS IN STELLEN BOSCH GRANITE. 



The next exposure to be described is met close to the main 

 road, about half-a-mile north-east of Vlottenberg station. This 

 is in some respects the best exposure of all, because, although of 

 small superficial area, it has been more deeply quarried than 

 the other exposures. True segregation-veins are not seen here, 

 but tourmalinised fissure-veins intersect the granite abundantly. 

 Inclusions are of three types : — 



(a), (b) Micaceous xenoliths similar to (a) and (b) as 

 described above. 



(/) Tourmaline-bearing xenoliths. 



These inclusions are scattered indiscriminately through the rock, 

 but they show a striking point of difference, which is apparent 

 even at a little distance. It consists in this, that the tourmaline- 

 bearing xenoliths are surrounded in every case by a white collar 

 of large felspar crystals, this feature being absent from the mica- 

 ceous xenoliths. Photograph No. 2 shows such a xenolith in place 

 within the granite, and brings out the collar quite distinctly. The 

 average size of the xenoliths of both kinds approaches that of a 

 cricket ball. 



Microscopic Characters of the Rocks. 



The granite has precisely similar characters over the whole 

 area, and does not differ externally from the Lion's Head granite. 

 It contains very numerous large phenocrysts of microcline in the 

 form of Carlsbad twins. These range from two to four inches 

 in diameter. The majority of the smaller felspars in the base 

 of the rock belong to microcline, with the exception of a quite 

 small proportion in which the twinning and extinction are those 

 of albite. Quartz is very abundant, together with the usual 

 brown biotite and minute crystals of apatite and zircon. Tour- 

 maline does not appear to be a usual primary constituent of the 

 granite. 



The segregation veins are chiefly reddish-grey aplites of fairlv 

 uniform millimetre grain. The veins range from a fraction of 

 an inch up to about eighteeen inches in width, and can be traced 

 for considerable distances through the granite. In places they 

 pass, either marginally or centrally, into thin pegmatites. Under 

 the microscope the aplite is seen to be composed almost entirely 

 of uniform anhedral grains of microcline and quartz, with a few 

 flakes of biotite and, as a novelty, one or two crystals of musco- 

 vite. Tourmaline has not been observed in the aplites. The 

 pegmatite veins are seldom more than an inch in thickness ; they 

 show well-developed graphic intergrowths of quartz and micro- 

 cline, and here and there contain radiating bundles of tourmaline 

 needles. An intermediate stage between aplite and pegmatite is 

 apparent where felspar phenocrysts are developed in the aplitic 

 base, producing a rock which must be described as granite- 

 porphyry. Some writers would restrict the name granite-por- 

 phyry to aschistic dyke-rocks, that is, such as differ only in 

 structure, but not at all in composition, from the parent rock. 



