ROCK-FORMING MINERALS 21 
biotite, if cut at right angles to its vertical axis (or, in other words, if the 
slice be parallel to the cleavage-planes), appears deep brown or deep green 
to black, and shows little or no change of colour when rotated above the 
polariser. But when the section cuts across the cleavage-planes, which 
then appear as a series of parallel lines traversing the mica, as shown in 
Plate IV. 3, and the stage of the microscope is rotated, the change of 
colour is strongly pronounced. The polarisation colours are very 
brilliant in sections showing cleavage, and cut thin enough. Inclusions 
are frequently numerous, mostly of apatite and magnetite, and less 
commonly of zircon and rutile. 
Muscovite (potash mica) is sometimes ‘colourless, but usually pale- 
coloured or silvery ; occasionally, however, it assumes a pale shade of 
brown or green. It fuses on thin edges to a grey glass or white enamel, 
but is not attacked by acids, and as a rock-constituent is not so readily 
altered as biotite. As a primary rock-former its chief habitats are the 
crystalline schistose rocks (gneiss, mica-schist, phyllite), and the granites. 
It never occurs as an original constituent in any igneous rocks save 
granite, certain quartz-porphyries, and syenites. Being a mineral not 
readily decomposed, it frequently appears in the form of soft, worn- 
looking, non-elastic scales in sedimentary rocks of many kinds. Although 
muscovite has no great range as a primary constituent of crystalline 
eruptive rocks, it occurs in many as a secondary ingredient—the product 
of the alteration of silicates rich in alumina. Thus it often replaces such 
minerals as andalusite, felspar, nepheline, etc. Seen in thin sections 
under the microscope, muscovite is colourless or very faintly yellowish 
or light green. It shows no change of colour, or at most only a slight 
difference in the intensity of the colour, when rotated above the polariser. 
It polarises, however, more brilliantly than biotite. Inclusions are few. 
Although the micas, as rock-formers, occur most frequently in the 
form of scales, flakes, or plates of relatively small size, they now and 
again appear as large rough prisms, often tapering to a point—as, for 
example, in limestones which have been subject to metamorphic action. 
Very large individuals of muscovite also are met with in the pegmatitic 
veins (giant granite) associated with so many granitic masses. 
THE OLIVINE GROUP 
The minerals of this group are non-aluminous silicates, 
The only one of importance as a rock-former is Olivine 
(Peridote)—a silicate of magnesium and iron which crystal- 
lises in orthorhombic forms and shows an imperfect cleavage. 
It has a hardness of 6-5 to 7, and a specific gravity of 3 to 4. 
The proportion of iron varies—specimens containing very 
little being infusible, while those which are rich in iron are 
more or less readily fused. The mineral is slowly decomposed 
by cold hydrochloric acid with gelatinisation. It is usually 
