ON THE MICROSCOPICAL STRUCTURE OF ROCKS. 
11 
to recognise a twin crystal under ordinary light, on account of 
the way in which it may be cut ; but with polarised light it can 
always be seen with great certainty in the case of all double-re- 
fracting minerals, owing to the reversal of the optic axes, which 
causes the appearance of different colours in the different laminae 
of the crystal. It may also be observed, that whenever the 
optic axis of one of the twins coincides with that of the Nichol 
prism that twin will appear dark, whilst the other will be 
coloured, and this effect will, of course, be reversed with the 
reversal of the Nichol. This phenomenon of twin formation is 
very commonly met with in microscopic sections of many 
minerals. 
Mica . — Examining our minerals rather in the order of their 
importance than of their strict classification, we will look next 
at some of the various species of mica. The ordinary mica 
found in granite is potash mica, sometimes called biaxial mica, 
or muscovite. 
Muscovite forms table-shaped crystals, and its sections 
coincide with the chief plane of cleavage, and show an irregular 
polygonal shape. Owing to the laminated structure of mica, 
its sections show a chaos of fine lines and of Newton’s rings on 
the surface, which mark the boundaries of its torn leaves, some 
of which may be only ^5X0 o"o P ar ^ °f an i nc h thick. Its 
leaves are sometimes differently arranged as regards their optic 
axes — “ differently oriented,” the Germans would say — and thus 
when polarised such a mica will never be dark in a horizontal 
position, as its planes of vibration can never in all the leaflets 
coincide with those of the polariser and analyser. Mica, as a 
rule, does not enclose many other mineral substances; occa- 
sionally apatite needles are found in it, pretty crystals of 
haematite, and also magnetite, whilst between its leaves may 
sometimes be seen dendritic markings. In the mica of granite 
fluid cavities are not uncommon. It is usually very transparent, 
and, owing to this, shows brighter colours than biotite, although 
it is difficult under the microscope to distinguish between the 
two. Mica is dichroic, but usually manifests only clearer or 
darker tints of the same colour. 
Biotite. — Biotite, or uniaxial or magnesia mica, has hexagonal 
crystals, but is generally seen in thin, irregular, polygonal, or 
roundish leaflets, or elongated tables. In the former case it has 
a smooth, shining surface, in the latter is laminated. As I have 
just observed, its microscopic characteristics are but slight. The 
little leaflets, which are perpendicular to the small axis, remain 
dark between crossed prisms. Those cut parallel to the axis 
only become so when the direction of their thin stripes is 
parallel to the chief diagonal of. the Nichol prism. The powerful 
dichroism* of biotite is one of its most characteristic marks 
