Petrography of Rocks. 283 



The feldspar is very variable in character ; it includes orthoclase 

 and a little microcline, and a good deal of plagioclase; the latter 

 shows indices of refraction in different crystals, both higher and lower 

 than that of quartz, so that the range of composition is apparently 

 wide. 



The mica is of a greenish-brown colour, and strongly pleochroic. 

 It is in parts more or less completely converted into the usual pale 

 green chloritic decomposition product. Some slides show a little 

 colourless muscovite. There is a small amount of magnetite, and 

 crystals of a greenish-yellow pleochroic epidote appear to be original 

 constituents. 



Some specimens show a certain amount of parallel orientation of 

 the minerals, especially as regards the mica, and it appears that the 

 general structure is slightly gneissose. Whether this structure is 

 primary or superinduced by pressure it is impossible to say, but there 

 is not much evidence, of foliation in the specimens examined, except 

 in one case (2 24d) from the deepest level in the Kimberley Mine, 2520 

 feet, where there has evidently been some kind of shearing or thrust- 

 ing movement, so that the decomposed feldspar has been broken up 

 and crushed, while the quartz grains are almost unaltered ; however, 

 a few of the latter also are shattered. The flakes of mica have been 

 squeezed between the hard quartz grains, and some show good 

 examples of strain-slip-cleavage. (Ausweichungsclivage). This is a 

 purely local phenomenon, and has not affected the mass of the rock. 



Associated with the normal granite there are also some examples 

 of pegmatites, of much coarser texture. The best specimen (ti4b) 

 is composed chieflv of large crystals of a white feldspar, up to 2 or 

 3 inches across, with clear, glassy quartz, and dark mica in nests and 

 patches ; there is no visible muscovite, as in so many pegmatites. 

 The feldspar is somewhat milky in colour, and shows very well- 

 developed twinning on the albite law, with very numerous striations 

 on the c cleavages. The extinction in small fragments is nearly 

 straight, so that the feldspar must be identified as oligoclase. 



Some specimens differ from the foregoing in being chiefly com- 

 posed of pink feldspar instead of white, and small fragments of this 

 under the microscope show the characteristic cross-twinning of micro- 

 cline. Some quartz is present, but no mica. Thus there are two 

 somewhat different types of pegmatite characterised by oligoclase 

 and microcline respectively. However, transition types between these 

 extremes can be recognised, and the whole of them probablv belong 

 to one phase of intrusion. 



3. The Bultfontein, Dutoitspan, and Wesselton Mines. 



At the present time these mines have reached a much smaller 

 depth than is the case at the Kimberley and De Beers Mines. The 

 deepest of this group is Dutoitspan, but even this only reaches about 

 750 feet from the surface. Sections of Bultfontein and Dutoitspan 

 are given bv Gardner Williams, * but as regards Wesselton we hrive 



* Science in South Africa, p. 323. 



