164 
MELLO: MICROSCOPICAL STRUCTURE OF ROCKS. 
bands being interrupted and crossed by others, giving a grating-like 
appearance. The bands or striae run at right angles to each other. 
Twin crystals of the Carlsbad type are common in orthoclase. 
Enclosures of other minerals are abundant in this felspar, although 
from its general opacity it is not easy to recognise their species. 
Turning to the next group, the Plagioclastic Felspars are easily 
distinguished when the polariscope is used, by their splendid banded 
appearance, parallel bands of colour are seen, which are due to the 
polysynthetic or twinned crystallization, which is highly characteristic 
of this mineral species, although this structure must not be regarded 
as essential, since we may not unfrequently find plagioclastic felspars, 
anorthite, oligoclase, and even labradorite, which exhibit no trace of 
lamellar twinning, a structure which is considered by several eminent 
petrographers to result from pressure or strain, and not to be 
invariably an original character. When we examine the varieties of 
mica which occur in rock formations, we find that two of them arc 
most common, Muscovite, or potash mica, and Biotite, or magnesia 
mica. The former, which is optically biaxial, forms tabular hexagonal 
crystals. Under the microscope it is transparent and shows bright 
colours, and its sections generally exhibit a chaos of fine lines or 
of Newton's rings on the surface, marking the boundaries of its torn 
leaves. This mica is but slightly dichroic, manifesting only clearer 
or darker tints of the same colour. Biotite or uniaxial mica, has also 
hexagonal crystals, it generally occurs in thin irregular or polygonal 
leaflets or in elongated tables, its colour is black or dark gi'een or brown. 
The leaflets present a smooth glistening surface, but the tables which 
are cut across the edges of the leaflets are laminated. The leaflets 
which are cut at right angles to the short axis are dark between crossed 
Nichols. Those cut parallel to the axis only become so when the 
direction of their striae is parallel to the chief diagonal of the prism. 
Biotite is strongly dichroic, and this is its most characteristic micro- 
scopical feature. 
Chlorite may often be seen in igneous rocks, in the form of 
little green leaflets and scale-like aggregations. Its leaflets are 
frequently arranged in vermicular or radiating forms caused by a 
partial overlapping. It is also found in concentric layers. Some- 
