Vol. 67.] WORCESTER, ROBERTSON, AND ASHTON (CAPE COLONl). 717 



presumably curved joints, but presenting a strong deceptive resem- 

 blance to the curved ; boiler-plate slabs ' so characteristic of igneous 

 rocks in glaciated regions. The rock is much weathered, and it is 

 somewhat difficult to obtain really fresh material. 



Sections were cut from specimens collected from different parts 

 of the mass, but all are very similar, the chief difference consisting 

 in the presence or absence of phenocrysts of felspar, which are never 

 very abundant and sometimes wanting. Another difference lies in 

 the amount of foliation: the central portion, especially in Wolve 

 Kloof, is free from foliation, whereas near the margin a gneissose 

 structure is often conspicuous. In certain parts aplitic veins are 

 fairly abundant, but otherwise the rock shows little variation. 



The granite is a rock of moderately coarse grain, of a prevailing 

 bluish-grey colour when fresh, with scattered phenocrysts of white 

 felspar measuring up to 1 inch in length. The principal minerals 

 are quartz, felspar, and mica, with accessory iron-ores and sphene. 

 The quartz, which was the last mineral to crystallize, forms large 

 irregular crystals, much cracked and shattered, and exhibiting 

 conspicuous strain-shadows. Several varieties of alkali-felspar can 

 be recognized, including orthoclase, microcline, microcline-perthite, 

 and microperthite. These are usually quite clear and fresh, whereas 

 the plagioclase, which occurs in subordinate amount, is distinctly 

 turbid, and for the most part converted into an aggregate of white 

 mica and zoisite. The plagioclase shows very small extinction-angles, 

 corresponding to oligoclase. The soda-molecule of the plagioclase has 

 evidently given rise to soda-mica, while the lime-molecule has formed 

 zoisite. The principal coloured mineral is an olive-brown biotite, 

 which is usually much chloritized and then green in colour. It is 

 full of inclusions, some of which are epidote, while others consist 

 of black opaque iron-ore surrounded by a ring of sphene. This 

 latter fact suggests that the iron-ore may be ilmenite, which has 

 given rise to sphene as a reaction-product. A few large original 

 sphenes are also present. Some of the bigger flakes of muscovite 

 which occur in places may possibly be of primary origin, but the 

 greater part of this mineral is undoubtedly derived from plagio- 

 clase-felspar. Some specimens of the granite, especially the more 

 porphyritic examples, show a strong tendency to micrographic inter- 

 growths of the quartz and felspars of the ground-mass [9470-71]. 



The aplite forms narrow veins and strings penetrating the granite 

 in all directions, but its distribution is somewhat restricted. It 

 consists of quartz and a great variety of felspars, all of alkaline 

 composition, with only the merest trace of biotite. Sphene and 

 small garnets are occasionally present. In this rock the quartz has 

 commonly crystallized before the felspar. No notable difference of 

 composition can be detected between the normal granite and the 

 aplite ; the variation is chiefly a matter of texture, though the aplite 

 appears to be of somewhat more acid character [9468-69]. 



Since the inner parts of the granite mass, where cut through 

 by the Nels River in Wolve Kloof, show no foliation, while the 

 outer portions are much crushed and shattered, it is clear that the 



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