THE MINERALS OF SOME 

 GRANITES. 



SOUTH AFRICAN 



Bv F. P. MicxxKLi., F.Cj.S., Cuhatok of ihI'. Rhodksia 



MlSiaM, BULAWAYC). 



Plutonic rocks of acid com]K)sition are ver\ extensively develop- 

 ed in Africa south of the Equator. These rocks pre.sent many 

 features of interest ami. especiallv under the microscope, many 

 minerals may he recognised besides the usual quartz, felspar and 

 ferromagnesiaii constituents. Thus the granite of Capelown itself is 

 remarkal)l\ rich in accessories. Tourmaline, in narlicular is verv 



UIXKNU'OSINC LdKUUMn I !■;, CAI'K TOWN (.KANnK 



abundant in places. In thin sections, it is of -a yellowish brown colour 

 frequently bordered and zoned with [)ale blue, while some cr\stals 

 shew alternate l)ands of vellow and brown. The l)asic patches, which 

 are no doubt derived from the fu.sion and sul).sequent recrystallisation 

 of fragments of the adjacent slate are es])ecially interesting. Some 

 are largely made up of andalusite in good crwstals or scmiewhat round- 

 ed grains. Cordierite is often seen and is sometimes quite fresh and 

 almost indistinguishable from quartz, while in other cases it is entirely 



