36 
INTERMEDIATE TYPES 
The intermediate types form large areas of the main intrusion. One 
peculiar variety consists, in the hand specimen, of brown or pink feldspar 
crystals, up to one-half inch in length, set in a fine-grained, holocrystalhne, 
greenish grey groundmass, the green colour being due to tiny needles of 
pyroxenes. Under the microscope large phenocrysts of orthoclase and 
microcline can be discerned. They are considerably kaolinized. Several 
large crystals of quartz also occur and they contain fine-grained feldspar 
irregularly or concentrically arranged as already described. The ground- 
mass is fine-grained, holocrystalline, and consists of albite, some orthoclase, 
and a small amount of quarts. 
Scattered throughout the section and largely confined to the ground- 
mass are numerous prisms of pyroxene, up to 2 millimetres in length. 
This pyroxene has yellow to green pleochroism, negative elongation, 
and an extinction angle X A C, of 5 degrees to 10 degrees, thus being 
proved to be nearly pure aegirine. Also occurring in the ground- 
mass, are numerous prisms, of about the same average size as those of 
pyroxene, of a colourless mineral which in a few sections exhibits very 
good amphibole cleavages, inclined at 124 degrees and 56 degrees. The 
optical properties of this mineral follow. It is biaxial negative. The optic 
angle is 0° -j- to about 60 degrees. It has positive elongation. The dis- 
persion y<v, is extreme. The indices of refraction are: 
a = 1 - 623,0 - 1 - 630,7 = 1 - 635 . 
The birefringence is about 0 *012. The plane of the optic axes coin- 
cides in general direction with the elongation or the prismatic cleavage 
traces. The extinction Z A C is approximately 40 degrees, but it was 
extremely difficult to obtain the exact extinction angle on account of the 
great dispersion, an anomalous berlin blue colour being the lowest obtainable. 
The mineral is commonly twinned parallel to 100. In thin section the 
mineral is absolutely colourless. 
From the well-developed cleavage the mineral may safely be assumed 
to be an amphibole of some variety that, as far as the author can ascertain, 
has not heretofore been described microscopically. Separation by heavy 
liquids, with a view to obtaining sufficient material for analysis, was at- 
tempted, but a sufficiently clean product could not be obtained. Even 
the purer grains of the amphibole were found to contain many impurities. 
The writer's conclusion as to the nature of this mineral is that it is a NaAl 
or NaCa MgAl amphibole. 
The edges of all the large feldspar and quartz phenocrysts are extremely 
irregular and have apparently been considerably resorbed by the ground- 
mass before it solidified, A few, very coarse grains of apatite constitute 
the only other primary minerals in the slide. Titanite and limonitic 
material are the chief alteration products. 
The original feldspar phenocrysts are cut and replaced by numerous 
stringers of the fine-grained groundmass and in many cases numerous 
fine laths of albite appear isolated in the interior of the crystals. These 
laths are particularly abundant in the centres of the phenocrysts, in some 
cases uniting to form large areas of clear plagioclase, suggesting that the 
interior of the phenocrysts is slightly different in composition from the 
exterior, and is thus more susceptible to replacement by albite. 
