110 VEINS THEIR NATURE AND ORIGIN. 



rocky framework. In this way the fissure becomes a vein ; 

 and as each set of veins has a fixed direction, and is 

 charged with its own peculiar vein-stuff and metal, the 

 ascertaining of these directions and peculiarities is at once 

 of the highest scientific and economic importance. As the 

 original fissures are produced by volcanic convulsions, and 

 the subsequent fillings-in by slow and gradual precipitation 

 from solution, veins will occur most abundantly, of course, 

 in districts that have been longest subjected to those 

 agents ; and such tracts are necessarily those occupied by 

 primary and transition strata. In these districts, bleak 

 and barren and inhospitable, the mining industry of the 

 world is chiefly situated, their subterranean wealth compen- 

 sating, and often more than compensating, for their want 

 of agricultural fertility and amenity. As the metals are in- 

 dispensable to mechanical, manufacturing, and commercial 

 progress, so they are generally regarded as powerful auxili- 

 aries of civilisation ; hence the importance of all that at- 

 taches, scientifically and industrially, to their history, their 

 modes of occurrence, the means of obtaining them, and the 

 processes of reducing them from their ores and associated 

 vein-stuffs. 



