564 REPORT—1904. 
extensively worked in large quarries for building material, and on that account I 
was able to collect a number of suitable specimens for investigation. 
The granite itself is of much interest on account of the presence of several 
constituents which rarely occur in this kind of rock. It exhibits on the whole 
a uniform medium-grained structure, but is in some parts porphyritically de- 
veloped, and contains large well-defined crystals of white orthoclase; it is also 
traversed by pegmatitic veins, which contain much tourmaline and apatite, while 
mica disappears. 
When examined under the microscope, thin sections of this granite are 
observed to be composed of the following constituents: Quartz, orthoclase, oligo- 
clase, albite, white and brown mica, tourmaline, apatite, zircon, magnetite, 
fluorite, andalusite, cordierite, and chlorite—the last-named as a decomposition 
product. Further constituents, occurring only in cavities in the pegmatitic veins, 
are lepidolite and gilbertite. Quartz occurs in the usual form of grains or 
granular aggregates, and contains numerous liquid inclusions, some of them show- 
ing a crystal of salt. 
The orthoclase is of a pure white colour; its phenocrysts are of considerable 
size, being sometimes about 4 inches long and 2} inches wide. They are Carlsbad 
twins, and occasioually two such crystals form penetration twins of cruciform 
type, similar to the pseudomorphs of cassiterite after orthoclase, which are well 
known from Cornwall, the twinning plane being probably a face of the pyramid P 
(i11). Under the microscope the ortioclase is seen to be more or less decomposed, 
passing into a kaolin-like substance. It contains a great number of inclusions: 
numerous flakes of black mica can be observed macruscopically, which generally 
accumulate in the centre of the crystals or are arranged in layers parallel to their 
faces. White mica also is very abundant, and occurs in micropoikilitie inter- 
growth with the orthoclase, forming sponge-like aggregates; there can be no 
question about this mica being a primary constituent and contemporaneous with 
the orthoclase in which it is embedded. Microperthitic intergrowth of orthoclase 
and albite is very common; and it may be mentioned here that albite frequently 
envelops the orthoclase crystals, which occur well crystallised in cavities of the rock. 
There is also a fair amount of plagioclase present ; its colour differs somewhat 
from that of orthoclase, being yellowish white. Under the microscope it exhibits, 
as a rule, a better state of preservation ; the sections are perfectly transparent, and 
contain but few inclusions and alteration products. ‘Twin lamellation is well 
marked, and the optical properties are those of oligoclase. 
Brown and white mica both occur abundantly. The former contains numerous 
small crystals of zircon, which are always surrounded by pleochroic halos, whereas 
the white mica is almost free from inclusions, Parallel intergrowth of the two is 
very common ; the brown mica is often seen to pass into green chlorite; the white 
mica is often twinned according to the common law; the composition plane being 
sometimes the twin plane itself, in which case a twinned flake shows different 
optical orientation in different parts of the basal plane, and the interference-figures 
appear disturbed. 
It has been suggested that in tourmaline-bearing granite the tourmaline may 
have been produced through the action of vapours containing boric acid on mica, 
and that in this way it replaces the mica. It seems as though our granite con- 
firms this view. Tourmaline is a never-failing constituent of the rock throughout, 
and although it does not occur very abundantly, yet one or more grains of it can 
be detected in every thin section. The pegmatitic veins of the granite, however, 
contain a great quantity of tourmaline, and it occurs in large crystals and erystal- 
line aggregates of radiating structure; the prisms reach a length of 10-12 em. ; 
biotite and muscovite, on the other hand, are very rare. Of great interest is 
the parallel intergrowth of biotite and tourmaline; it sometimes appears as if 
the tourmaline intrudes into the mica, and the pleochroic halos surrounding the 
inclusions in the latter are sometimes half situated in the mica and half in 
tourmaline. This intergrowth has not been observed with tourmaline and the 
white mica. The colour of the tourmaline in thin section is brown, or seldom 
blue, both colours also appearing on the same crystal; zonal structure is very 
conspicuous in most crystals, 
.“ 
