MINERALS IN VEINS. 



45 



cept among volcanic rocks. Jt is composed of silex, 

 lime, and magnesia, and is usually dark brown, 

 p ^ black, or green, but sometimes light 

 coloured. It resembles hornblende. 

 .^-^J^\. but is much harder, and strikes fire 

 f Y'S--— with steel. It crystallizes in short 

 j four-sided or six-sided prisms, termi- 



\. I nated by two faces, as in the adjoiniii^ 



nJ figures. 



Atlgite is found in considerable abun- 

 ^ig- 8 . dance in volcanic rocks, and in some 

 \ of the trap rocks, which are generally 

 /l^ ^ \ supposed to be of volcanic origin. It 

 I lis met with also among primitive rocks. 



II J It often exists in a granular or mass- 



\J 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 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 metalhc 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 



