218 J. C. MOULDEN. 
(6.) Plagioclase is present in much smaller proportion than the 
foregoing, and is exactly similar, save that between crossed nicols 
the twin lamellz are broad and distinct. All of the felspars con- 
tain numerous inclusions of zircon, apatite, and small indetermin- 
ate colourless needles having a definite orientation. 
Quartz is very abundant, and is of the ordinary granitic type. 
In some cases small grains form a “mosaic” area. It contains 
numerous fluid inclusions, in many of the smaller of which, 
beautiful examples of spontaneously moving bubbles are shown. 
Cordierite is likewise abundant, mostly in irregular grains, 
though sometimes in rudely prismatic forms. The grains vary in 
size greatly. It is colourless and transparent in good thin sections 
and shows no pleochroism. It resembles quartz closely in having 
irregular cracks, a low refractive index, and a low index of double 
refraction, hence the polarization colours are low, and vary from 
the grey and white tones to the yellow of the first order on the 
Newtonian colour scale. Alteration has set in around the edges. 
of the grains and along the cracks, giving rise to an illdefined 
granular clouded material of greyish-white colour, which has a 
fairly high index of refraction, and, in consequence, often imparts 
to the unaltered cordierite a false appearance of relief. Inclusions 
of zircon are very common, and show pleochroic haloes when the 
section is cut in a direction other than parallel to the basal plane 
of the cordierite. Biotite also occurs as an inclusion, and occurs 
too, as a wreath, in conjunction with chlorite, around the crystal 
edges in such a manner as to suggest that it arises from the alter- 
ation of the mineral. I have observed, between crossed nicols, 
what seems to be a complicated twin structure of lamell and 
interpenetrations, which I am rather at a loss to understand so far. 
Biotite is present in varying amount in the form of wisps and 
plates, some of which include small zircons. 
Zircon, in the form of grains and small colourless prisms, is VeTY 
abundant as an accessory constituent, and nearly every erystal of 
cordierite contains one or more inclusions of this mineral. 
