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CHAP. XIX. 



On Mineral Veins. 



IN a practical view, this is one of the most important 

 subjects in geology, yet, though long studied hy 

 miners and geologists hoth, nothing is less understood. 

 No general practical rules have been established, as to 

 the districts or rocks in which they should occur, the 

 courses they may hold, the forms and accidents they 

 may display, or the substances they may contain. 

 And as little have we been able to form any theory 

 respecting their origin and the mode of their pro- 

 duction. 



Mineral veins may exist without metallic sub- 

 stances, and many minerals are occasionally found in 

 repositories which cannot be called veins. Metallic 

 minerals thus occur in the compound rocks, so as to 

 form parts of their composition. Thus oxydulous 

 iron is found in granite, gneiss, sandstone, and trap, 

 molybdena in gneiss, and iron pyrites in micaceous 

 schist, slate, shale, and limestone. They sometimes 

 also occur independently; neither forming part of 

 the composition of rocks, nor included in distinct re- 

 positories. In this way, pyrites is found in many 

 situations, copper in the trap rocks, and oxydulous 

 iron in volcanic ones. Some of these, also, are occa- 

 sionally accumulated in such quantities in particular 

 spots as to be wrought for use ; and thus Cobalt and 

 Copper occur in sandstone. Iron, in the form of iron 

 stone and bog ore, forms beds; the first among the 

 coal strata, and the latter in alluvial soils. Thus also, 

 tin and gold are found among these, and the latter in 

 great abundance; but, in the two last cases, the origin 



