The Granite Area of the Schapenherg, Somerset West. 
199 
that of several determinations of the sum of the extinction angles 
of two adjacent strife was 29°. Again, in sections prepared from 
010 cleavage flakes, the average of several readings of the extinc- 
tion angle with regard to the 001 cleavage was 5° 30'. 
A comparison of the above values with the corresponding 
values recorded for microcline induces me to regard these pheno- 
crysts as microcline perthitically intergrown with albite. 
There are occcasional inclusions of quartz and decomposed 
albite. 
The Quartz Phenocrysts : 
These show only a slight tendency to idiomorphism. Mineral 
inclusions are not common ; occasionally biotite is included. Rows 
of extremely minute fluid and gaseous inclusions occur. The 
extinction is undulatory. 
The Groundmass : 
This is composed chiefly of microcline and quartz ; soda-lime 
felspar and, sparingly, biotite and muscovite, occur. The muscovite 
appears to be almost wholly a secondary mineral formed from the 
microcline. Microcline occurs in abundance and shows a tendency 
to idiomorphism. Occasionally the marginal portions of indi- 
viduals are an intergrowth of microcline and muscovite. The 
quartz contains numerous minute inclusions and occasionally inclu- 
sions of apatite, zircon, and biotite. The extinction is undulatory. 
The biotite, which is much altered, occurs as small, irregularly- 
shaped individuals. The soda-lime felspar is an acid variety — 
albite to albite oligoclase. The accessories apatite and zircon are 
only rarely met with. 
Mention must be made of a mineral which occurs both as a 
phenocryst and in the groundmass. It has a pale-green colour, a 
form similar to that of a felspar, possesses at least one good cleav- 
age, and is easily scratched with a knife-blade. In thin section' it is 
seen to be a much decomposed mineral ; the decomposition product 
is essentially an aggregate of minute flakes of muscovite. It con- 
tains in addition small needle-like crystals of a colourless mineral 
(sillimanite ?). Between crossed nicols faint zoning is to be 
observed in some individuals ; others exhibit faint traces of lamellar 
twinning. A section parallel to the cleavage showed simple twin- 
ning combined with lamellar twinning. For a long time I was in 
doubt as to whether this mineral was pinite pseudomorphous after 
cordierite, or a much decomposed soda-lime felspar. Now, however, 
I incline to the latter view and rather regard it as a phenocryst 
which existed previous to the growth of the microcline ]iheno- 
crysts. 
