20 STRUCTURAL AND FIELD GEOLOGY 
In thin slices under the microscope it is generally easy to distinguish 
between hornblende and augite. The faces of the unit prism in horn- 
blende are inclined to each other at an angle of 124° 30’, while in augite 
the corresponding angles are 87° 6’, or very nearly a right angle. The 
cleavage-planes being in the direction of these faces, it is obvious that 
those of augite must intersect at nearly 90°, while the angles between the 
two directions of cleavage in hornblende, in transverse sections of the 
crystal, will be 124° 30’ and 55° 30 (see Plates III., IV.). The cleavage 
in hornblende is usually more marked than in augite. Again, hornblende 
is very distinctly pleochroic, while in augite the change of colour is 
usually feeble, and often altogether wanting. 
THE MICA GROUP 
The micas, as rock-formers, mostly occur as thin plates 
and scales, the surfaces of which show a pearly to submetallic 
lustre. Usually these plates are irregular in shape, but now 
and again they are six-sided. The micas, however, are really 
monoclinic with pseudo-hexagonal symmetry. The cleavage 
is perfect, all micas being readily split up into exceedingly 
thin, transparent, and elastic leaflets. They are all rather 
soft (2-5 to 4 in the scale), and the specific gravity ranges 
from 2-7 to 3. They are essentially silicates of aluminium 
and potassium (or sodium), some kinds containing magnesium 
andiron. Only two micas are important rock-formers, namely, 
the brown to black Biotite or ferromagnesian mica, and the 
silver-white Muscovite or potash mica. They are essential 
_ constituents of many schistose rocks and of granite, and are 
met with in a large number of eruptive rocks of all ages. 
Soft, non-elastic scales of mica are also of common occurrence 
in many derivative rocks, particularly in fissile sandstones. 
Biotite (ferromagnesian mica) is usually dark brown to black, but 
green and red varieties are known. It is decomposed by strong sul- 
phuric acid; and in nature alters readily to chlorite, with separation of 
iron-oxide. Not infrequently, however, biotite becomes pale through loss 
of iron, and then assumes a golden yellow to silver-grey colour, thus 
sometimes closely resembling muscovite. It is a primary or original 
constituent of granites, rhyolites, some syenites and diorites, trachytes, 
etc. In effusive rocks the scales often show blackened borders, which, as 
in the case of basaltic hornblende, appear to be due to the corrosive 
action of the igneous magma. Biotite occurs also in certain schistose 
rocks. Being a less durable mineral than muscovite, it is not so often met 
with in sedimentary rocks. In thin rock-sections under the microscope, 
