
TOURMALINE, &c., WITHIN MICA AND OTHER MINERALS. 553 
one specimen, in particular, the included crystals form a larger mass than the 
garnet, which is merely a cement for holding them together. These crystals have 
various crystalline forms, while some are amorphous, though regularly crystallised 
in their interior. All these crystals are doubly refracting, and give the colours of 
polarised light from their small size. 
_ In another specimen, many of the crystals, in the form of hexagons and 
rhombic plates, are opaque, and exhibit by polarised light the remarkable pheno- 
menon, which I had never before seen, of having luminous edges, so that when 
the rest of the crystal and all the field of view is dark, we observe hexagons and 
rhombs, and other geometrical figures, depicted in lines of red light. It is not 
easy to ascertain the cause of this singular appearance, because we cannot see the 
form of the crystals where the light exists; but I have no doubt that the lumi- 
nous lines consist of light depolarised by reflexion from the sides of the hexagonal 
and rhombic plates, because the illuminating pencil is much larger than the crys- 
tals, and the crystals much smaller than the pupil of the eye, so that light 
must be reflected from the prismatic faces of the hexagons and rhombic plates, 
if they have sufficiently broad faces, and that light so reflected must enter the 
pupil of the eye. 
In this specimen and in others there are many spherical cavities, surrounded 
with sectors of polarised light, and also several amorphous masses of matter, 
round which there is also polarised light,- indicating, as all the phenomena of the 
crystals do, that the matter of the garnet must have been in a soft state, and com- 
pressed by some force emanating from these cavities. 
In another specimen of garnet, a large fissure in its interior is occupied with 
granular matter, which must have issued either from a burst cavity containing a 
fluid or a gas, or both; but what is very interesting, and what I have never ob- 
served in any other mineral, the matter has, in several places, formed circular 
crystals of singular beauty, some being very simple, and others very composite. 
St LEONARD’s COLLEGE, St ANDREWS, 
December 11, 1852. 
