548 SIR DAVID BREWSTER ON THE OPTICAL PHENOMENA OF 
The other crystals of tourmaline which I have discovered in mica have a very 
different character: They have been formed subsequently to the crystallisation 
of the mica, and exist only between its laminze. I have not been able to discover 
any cavities in mica containing fluids or gases, but I have found thousands from 
which the fluids and gases have escaped,—the one crystallisiny into hexagonal 
plates of tourmaline, and the other separating the laminz, or running between 
them, and carrying along with it minute portions of crystallisable matter. 
The hexagonal crystals thus formed have their faces perpendicular to the axis 
of double refraction, which is the axis of the prism; and what is peculiarly in- 
teresting, the fluid from which they were formed has insinuated itself between 
several of the laminze, and the different plates of tourmaline which they formed 
have, of course, the sides of the hexagon incoincident. Sometimes these crystals 
extend to different distances from the centre of the original cavity, and are occa- 
sionally formed round it in a circular group. See Plate XV., Fig. 1. ; 
The centre of the cavity from which these crystals have been projected is oc- 
cupied by a spherical group of granular or capillary crystals, which is generally 
very opaque, though such groups sometimes exhibit, in particular spots, double 
refraction, and a speck of light is occasionally seen through the centre of the 
group. In some cases I have observed these very thin hexagonal plates without 
this opaque centre; and they have probably been formed by a portion of the fluid 
projected to a distance between faces of easy cleavage. The black spherical 
eroup already mentioned has its outward surface bristled with points, which are 
the extremities of the crystals radiating from its centre ; and in one fine specimen 
to be farther described, it is surrounded with a ring of less opacity than the nu- 
cleus, and analogous to what is common in circular crystals. See Fig. 1. 
The thin plates thus formed between the laminze, whether hexagonal or pris- 
matic, are always of a faint brownish yellow, which at an increased thickness be- 
comes green; and so exceedingly thin are these plates, especially those farthest 
from the nucleus, that with a power of 400, it is often very difficult to see their 
terminal lines. 
In order to convey an idea of these phenomena, I have given a drawing in 
Fig. 1 of a very interesting one, where the prismatic crystal nearest the black 
central group is a bright green in all azimuths with polarised light, surrounded 
with three or four larger prismatic yellowish plates, growing fainter both in tint 
and outline as they recede. In some cases the crystals are brown, and in others 
beautifully dichroitic, being bright green and pink in the different azimuths of 
polarised light. 
As considerable forces must have been in operation during the production of 
these phenomena, we may expect to see the effects of them upon the surrounding 
mica. We accordingly observe the polarisation produced by pressure round 
almost all of these crystalline groups. Rents and other marks of violence are dis- 

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