200 
Transactions of the Boyal Society of South Africa. 
The Quartz- or Granite Porphyry. 
The fine-grained granite porphyry does not differ in mineral composition 
from the medium-grained porphyritic granite. The chief differences are, 
firstly, that biotite occurs as an additional phenocryst ; secondly, that the 
groundmass is microcrystalline ; and, thirdly, that the granite porphyry is 
characterised by containing, throughout a large portion of its mass, the 
rudely spherical tourmalinised patches previously mentioned. The quartz 
phenocrysts show a distinct tendency to idiomorphism ; the boundaries of 
individuals, however, are not straight linear ones, but irregular, indicating 
corrosion by the groundmass. Penetration of the groundmass along cracks 
is also shown. Mineral inclusions are not common ; occasionally one finds 
biotite. Eows of extremely minute fluid and gaseous inclusions occur. The 
extinction is undulatory. In the majority of cases the mica phenocrysts 
have become almost opaque through decomposition ; in some cases the 
opacity is almost certainly due to the inclusion of rutile needles and the sub- 
sequent formation of material resembling leucoxene. One supposed pheno- 
cryst, when examined in thin section, proved to be an aggregate of biotite 
and quartz. Apatite is occasionally found included in the biotite ; zircon 
was not identified. 
4. The Modifications of the Gtkanite. 
(1) The Tourmalinised Patches. — These occur not only in the normal 
granite porphyry, but occur also in some of the other modifications described 
below. Microscopic examination shows that, whilst some are composed of 
quartz, tourmaline, felspar (much decomposed ; generally microcline) and 
muscovite, in others the felspar is either residual or has entirely disappeared ; 
some, again, contain only quartz and tourmaline. 
(2) The Andalusite-hearing Varieties. — The varieties described hereunder 
are exposed along the contact zone on the sides of the ravine in Morgen Ster 
— particularly that portion which has a N.W.-S.E. trend. They are charac- 
terised by containing spherical tourmalinised patches similar to those 
described as occurring in the granite porphyry. One common variety is a 
rock resembling the latter in texture but differing from it in containing 
muscovite and andalusite. Varieties occur — according as the proportion of 
muscovite increases (both the felspar and the andalusite become converted 
into muscovite) and that of the felspar decreases, between this and one 
composed of quartz and muscovite and, in patches, tourmaline. To the last- 
named variety the term greissen could well be applied. 
Mention must be made here of a specimen (labelled 73, S. end of Har- 
monie) collected by Dr. Eogers. The bulk of it is composed of a lavender- 
grey mineral with a prismatic habit and a fairly good cleavage. Muscovite 
and patches of black tourmaline (enclosing prisms of the grey material) also 
occur. Under the microscope the lavender- grey mineral is seen to be anda- 
