MINERALOGY OF BLACK LAKE AREA. 
67 
2 mm. thick, upon a fine granular to compact matrix, which also forms 
the nucleus of each little sphere where the structure is botryoidal. Fre- 
quently these small spheres of colerainite are themselves coated by a 
dolomitic film, so delicate that it can only be detected under the micros- 
cope with a very high power. The analysis on page 68 as well as the 
general blowpipe and other characters, indicate that this matrix is very 
nearly related to, if not actually identical with, the crystals themselves. 
The entire vein is composed almost exclusively of this compact material, 
there being only a little associated brown platy mica (biaxial, negative, 
with small axial angle) , an occasional small crystal of garnet and zircon, 
and some pale green actinolite in short radiated fibres. 
Individual crystals of colerainite are essentially colourless and trans- 
parent with a vitreous lustre; actually, the lustre is more generally 
pearly, due to the curvature and superposition of the plates, and possibly 
also to a basal cleavage. In bulk, the pure mineral appears white, with 
a glistening or dull lustre, depending on the size of the crystals; other 
colours exhibited are faint pink and pale brown, due to isomorphous 
replacement or staining. The compact matrix is generally white with 
a dull matte ’ lustre; adjacent to the crystals there is usually a narrow 
zone which is translucent and nearly colourless, and there is thus a tend- 
ency for the material to be banded, although this feature is not very 
pronounced. 
The hardness is 2§ to 3; the specific gravity of the crystals is 2*51 
and of the matrix about 2 • 44. 
Examined under the microscope between crossed nicols, crystal 
flakes are isotropic, and in convergent light they yield a uniaxial figure, 
the optic axis emerging normal to the flake. The birefringence is positive 
and weak, with mean refractive index about 1 * 56. The matrix is very 
finely crystalline and has similar optical characters. 
Microphotographs of a thin section of the rock are reproduced in 
Plates XI and XII. Examined in ordinary light, the rock is seen to 
have a drusy character, and to be composed almost entirely of a colourless 
mineral of fairly low refractive index (colerainite) through which are 
scattered dusty patches and streaks of a white, opaque, clay-like 
substance, which appear black in the photograph. One or two minute 
flakes of a pale brown mica are also present in the section examined. 
The structure is best observed between crossed nicols. The drusy 
cavities are seen to be everywhere bordered by aggregates of colerainite 
crystals. These are almost without exception wedge-shaped, each little 
wedge having its base facing toward the open druse; a number of wedges 
being arranged side by side in this manner produce spherical, or in 
section circular, groups of individuals. The simultaneous extinction, 
in such groups, of four component wedges inclined at 90 degrees to one 
