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Transactions of the Royal Society of South Africa. 
all of these sheared granite, resembling — on account of the complete pulveri- 
sation of the felspars — a metamorphosed sediment rather than a metamor- 
phosed igneous rock, has been thrown out. From the two upper ones fresh 
granite showing no signs whatever, in the hand specimen, of mechanical 
deformation, has been thrown out in addition. 
Only one specimen of an inclusion of a fragment of the invaded series 
was found. This contains abundant, almost black mica, and is noticeably 
foliated. It resembles in general appearance many of such inclusions which 
occur so abundantly in the granite of the Cape Peninsula. 
Detailed Description of the Medium-grained Porphyritic Granite. 
Macroscopic Characters : 
Constituent Minerals : Grreyish- white felspar and blue- grey quartz 
abundant ; somewhat bronzed biotite only sparingly present. 
Structure : Porphyritic ; phenocrysts of greyish-white felspar (many 
showing Carlsbad twinning) and blue-grey quartz. About 250 
yards to the north of the quarries on the road to Caledon this 
character is accentuated by the felspars being unusually large 
(some are If in. long and J in. broad), and having proved more 
resistant to the action of the weather than the groundmass. The 
surface is studded with them and also with large quartz pheno- 
crysts. 
Specific gravity: 2'63 (the average of several determinations). 
Microscopic Characters : 
The Felspar Phenocrysts : 
A microscopic examination of such portions of the phenocrysts as 
occur in the rock-sections confirmed the assumption — based on the 
initial inspection of the hand- specimens — that they were essentially 
composed of a felspar characterised by simple — Carlsbad — twin- 
ning. In addition, they are traversed by veiulets of a mineral 
which, though largely altered to a micaceous aggregate, shows 
obscure traces of lamellar twinning parallel to the 010 cleavage 
of the phenocrysts. The behaviour of some of the phenocrysts 
between crossed nicols led one to suspect the presence of a lamellar 
twin structure in them, and it was deemed necessary, for a com- 
plete specific determination of the mineral, to prepare specially 
thin sections from cleavage flakes. This was done. Sections 
prepared from basal cleavage flakes showed, between crossed 
nicols, a structure suggestive of anorthoclase ; areas of fine 
lamellar twinning — " cross-hatching" — pass without any sharp line 
of demarcation, into others free from twinning. 
The average of several determinations of the extinction of the 
homogeneous portions with regard to the 010 cleavage was 15° 
