12 
POPULAR SCIENCE REVIEW. 
when the polariser alone is used, and if the direction of the 
cleavage is perpendicular to its shorter diagonal, the colours 
may be greenish yellow to brown ; if the direction of the two is 
parallel, we get dark brown to black owing to the greater 
absorption. Biotite is distinguished from other dichroic mine- 
rals, such as epidote and hornblende, by the absence of the 
marked pleochroism which these present, and from tourmaline 
by its fine striation. 
Talc . — Talc, which occurs both in large quantities in talc 
slate, and also in little curved scales and crystals as a consti- 
tuent of some granular rocks, like mica shows both by trans- 
mitted as well as by reflected light similar irregular lines and 
brilliant coloured rings, owing, as in that mineral, to the partial 
separation of its leaflets. It may be known from mica and also 
from chlorite in possessing no perceptible dichroism. 
Chlorite . — Chlorite forms little green leaflets, and scale-like 
aggregations in various rocks. Its scales or leaflets are fre- 
quently arranged in vermicular or radiating forms, caused by 
their partially overlapping each other ; it is also found in con- 
centric layers, like the coats of an onion. In many specimens 
the leaflets seem to consist of twisted or irregularly intergrown 
fibres. Sometimes leaflets of chlorite look like large green glass 
cavities, and present a beautiful appearance in sections of the 
chloritic schists. Now and then hexagonal scales will be seen, 
which remain dark between the crossed prisms. The polarisa- 
tion colours of chlorite are often very feeble ; blue and brown 
tints prevail. The mineral shows decided dichroism, although 
of varied strength in different specimens. 
Hornblende . — We will next turn to hornblende, a widely 
dispersed and very important constituent of many rocks. It is 
found distinctly crystallised, frequently in six-sided forms, but 
it is more often found in crystalline masses, which might be 
called crystalloids, and in sheaf-shaped aggregates : these are 
especially common in diorite. It may be at once distinguished 
from augite, which mineral it resembles somewhat in its sec- 
tions, although its prism angle is larger, by its fibrous 
structure ; this serves also to distinguish it from augite, when as 
uralite its crystalline form is the same. Twin crystals also are 
not so common in hornblende as they are in augite, but one of 
the most striking characteristic features of hornblende is its 
strong dichroism, the “orientation” of which, to use the 
German term, distinguishes it from mica, which it rather re- 
sembles in form in some specimens. Besides its dichroism, its 
striking pleochroism might well serve to distinguish hornblende 
from biotite, augite, or diallage. Epidote might possibly be 
mistaken for it, but the dichroism of this mineral is not so 
strong. 
