16 
POPULAR SCIENCE REVIEW. 
will occur in short colourless rectangles, the apatite will form 
long colourless needles, both minerals having hexagonal 
sections. 
Sodalite . — Sodalite is a representative of a group of isomor- 
phic regular minerals found in volcanic rocks. Microscopic 
sections show dodecahedral forms, and they are often very 
translucent, though frequently also full of various microlitic 
enclosures and large gas cavities, and glass cavities containing 
bubbles ; occasionally fluid cavities may be also found in soda- 
lite from Vesuvius. 
Nosean . — Nosean or noseite is a brown or grey variety of 
hauyne, of which lapis lazuli is the well-known blue form. 
Nosean is occasionally found in volcanic rocks, and may be 
readily known in the microscope by its quadrangular crystals, 
which are filled with dark granular specks, occupying the centre 
or else symmetrically following the planes of the crystal. 
Epidote . — Epidote is found in some granitic, dioritic, and 
other rocks, in granular crystallised forms, and it is frequently 
seen in small elongated crystals, clouded with a rather dirty 
green colour; in some specimens it seems to have a radiate 
fibrous structure, and it shows a play of colours when polarised, 
green, yellow, and brown tints prevailing. Epidote is associated 
with quartz in some rocks, and when mixed with more or less 
of that mineral, and sometimes with garnet, it forms the rarer 
mineral, epidosite. 
Serpentine. — Serpentine, which is often found in enormous 
masses, in the microscope shows a ribbon-like structure of deep- 
coloured greenish or bluish green, nearly opaque mineral 
matter, and in thin sections a kind of laminated or net-work 
appearance is produced ; on the outer edge there may be often 
observed brush-like aggregations of needles pointing towards 
the interior of the serpentine. In the polariscope serpentine 
shows double refraction. 
Iron . — Iron is constantly present in igneous rocks of every 
age, usually in the form of crystals and grains of magnetite, 
occasionally as titaniferous iron, and as specular iron and hae- 
matite. Curious clusters of opaque magnetite crystals are 
frequent. Translucent crystalline plates of haematite are often 
found in felspar, and it is owing to the light reflected from 
these that the variety of oligoclase called sunstone owes its 
beautiful opalescent appearance. 
Calcite . — We may consider now one or two minerals which 
usually occur as secondary formations, filling up fissures and 
cavities in the igneous rocks. One of the most important of 
these is calcite, which in its varied forms is one of the most 
abundant of minerals, occurring not merely as an occasional 
after product in volcanic rocks, but still more frequently in 
