250 VEINS AND INCLUSIONS IN STELLENBOSCII GRANITE. 



felspars on either wall. A noticeable feature, which is brought 

 out quite clearly in photographs Nos. 3 and 4, is that the tourma- 

 linisation has affected chiefly the porphyritic felspars of the 

 granite, so that these are often entirely replaced by tourmaline 

 through a thickness of one or two millimetres on either side of 

 the fissure. These pseudomorphs retain the form of the felspar 

 crystals perfectly. 



In connection with these veins, in which the tourmaline is 

 clearly secondary, I wish to mention here a peculiar brecciated 

 rock of which numerous blocks are to be found on the south 

 slopes of Papagaaisberg, from the level of the Plateau nearly to 

 the summit. This rock has a black base in which there are set 

 abundant fragments of white quartzite, the whole having the 

 appearance of a fault-breccia. Under the microscope the quartz- 

 ite shows a typical mosaic of interlocking quartz grains, through 

 which tiny crystals of tourmaline, with pleochroism from brown 

 to colourless, are scattered. The dark base of the rock seems to 

 be made up entirely of brown tourmaline and decomposition pro- 

 ducts, with also occasional fragments of deep blue tourmaline. 

 Some crystals are half brown, half blue. This rock is not ex- 

 posed in place anywhere on the hill, but the abundance of 

 weathered blocks of it suggests that a very extensive brecciated 

 tourmaline vein passes right through the hill. Indications of 

 cassiterite or other useful ores are, unfortunately, absent. The 

 source of the quartzite fragments is not obvious. There are no 

 such pure quartz rocks in the Malmesbury of this neighbourhood, 

 and it would be contrary to all experience to suppose that the vein 

 penetrated the former covering of Table Mountain Sandstone. 

 The mosaic structure of the quartzite suggests that the latter was 

 recrystallised at a high temperature. It is unfortunate that the 

 relations of this interesting vein to the granite and to the Malmes- 

 bury cannot be definitely ascertained. 



Micaceous xenoliths, type (a). These very fine-grained 

 micaceous xenoliths contain spongy brown and green biotite as 

 their chief constituent. Both quartz and cordierite are present, 

 but owing to the difficulty of distinguishing between these two 

 minerals it is not possible to determine their relative abundance. 

 Occasional fragments of larger size than the average afforded in 

 some cases the positive uniaxial interference figure of quartz, in 

 others the negative biaxial figure of cordierite ; while the faint 

 bluish colour and pleochroism of the latter mineral were also 

 discernible with the naked eye in some specimens. A few turbid 

 crystals of albite or oligoclase are present, and little needles of 

 sillimanite are rather plentiful as inclusions in the other minerals. 

 From these characters it is clear that the xenoliths in question 

 are altered fragments of Malmesbury slate — a fact which in the 

 field is so obvious as scarcely to require demonstration. 



Type (b). In the coarser-grained xenoliths quartz is reduced 

 in quantity and microcline is, after biotite, the most important 

 constituent. Cordierite was not certainly identified, but is not 

 necessarily absent on that account. Sillimanite is absent from 



