MINERALS IN VEINS. 



45 



cept among volcanic rocks. It is composed of silex, 

 lime, and magnesia, and is usually dark brown, 

 F 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 44 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 limited 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 

 most 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 



