MINERALS IN VEINS. 
45 
cept among volcanic rocks. It is composed of silex, 
lime, and magnesia, and is usually dark brown, 
Fig. 7. 
black, or green, but sometimes light 
coloured. It resembles hornblende, 
but is much harder, and strikes fire 
with steel. It crystallizes in short 
four-sided or six-sided prisms, termi- 
nated by two faces, as in the adjoining 
figures. 
Augite is found in considerable abun- 
dance in volcanic rocks, and in some 
of the trap rocks, which are generally 
supposed to be of volcanic origin. It 
is met with also among primitive rocks. 
It often exists in a granular or mass- 
ive form ; when granular, it is called 
coccolite, from " coccos,^^ a grain. 
Minerals found in veins or beds. — The metallic and 
other minerals which are occasionally interspersed 
throughout the mass of earths, or more generally 
forming veins or beds of hmited extent, are far more 
numerous, and present a wide range of study to the 
mineralogist ; but the limits of the present work 
forbid our entering upon this branch of natural sci- 
ence in detail. It is, however, necessary to notice 
such as are most common, and which it is absolute- 
ly necessary for the geologist to be able to recog- 
nise whenever he may see them. These are the 
ores of iron, lead, gold, silver, tin, copper, and zinc ; 
mercury, manganese, titanium, bismuth, antimony, 
and cobalt, also occur. Many of these are com- 
paratively rare. 
These are all metallic minerals, and may be dis- 
tinguished from the earthy by their possessing a 
brilliant metallic lustre and great specific gravity. 
Iron. — Of all metallic minerals, iron is by far the 
jnost abundant and most generally distributed ; there 
is, indeed, scarcely a rock or soil in which it does 
not occur. Its ores are numerous, but the most 
