4 o8 DISTRIBUTION AND DIRECTION OF VEINS. 



dislocated by slips, and divided by fissures at the period of their eleva~ 

 tion. It is not the absolute height of the ground, but the circumstance 

 of its having been much exposed to subterranean convulsion, infiltra- 

 tion, and denudation, that determines the prevalence of mineral veins. 

 The rich mines of Cornwall are in comparatively low situations, but 

 they are all in the vicinity of elevated rocks which have been heated. 



There appears to be no limit either of height above or depth below 

 the sea, which defines the productiveness of veins, though in some 

 countries like America, higher situations are most favourable. 



It is sometimes found that the contents of a vein vary with the 

 depth, without any particular geological conditions ; as, for instance, 

 in Cornwall copper is prevalent in the mines at greater depths than 

 tin ; and in the slate tract of Cumberland veins which bear lead near 

 the surface yield copper at a depth. In other cases there appears a 

 peculiar determination of the metallic ingredients to particular situa- 

 tions. The mines about Ecton yield copper; those of Derbyshire 

 and the Pennine chain generally yield lead, but toward the eastern 

 and western limits of the district, copper becomes less uncommon. 



The length of a vein or fissure is perhaps hardly in any case cer- 

 tainly known ; because, when it ceases to be worth working, it is for 

 all the ordinary purposes of mining said to be dying out, or cut out, 

 or ended. The richest veins are productive for limited lengths, but 

 the fissures which they fill may be, and often are, extended far beyond 

 the spaces occupied by metallic impregnations. Some of them are 

 known to extend, and to be productive for many miles in the Harz, in 

 Cornwall, and in the North of England. The width is various in 

 different veins, but generally nearly constant in the same vein. A 

 width of twenty feet is very unusual. Most veins are less than six 

 feet wide. 



Directions of Mineral Veins. The most general direction of the 

 great dykes and faults in the North of England may perhaps be 

 defined to be nearly east and west. Eut this is much more certainly 

 true with respect to the mineral veins of the limestone districts of 

 Weardale, Allendale, Alston Moor, and all the mining districts of 

 Yorkshire ; and it is equally recognised in the Primary tracts of 

 Cumberland, Westmoreland, and Lancashire. This is so general a 

 fact, that the east-and-west veins are called right-running veins, while 

 the few which range more nearly north and south are called cross 

 courses. These latter are seldom rich in metal. They often cut 

 through and shift the right-running veins laterally, as both of them 

 shift the strata vertically. There is often to be observed a sort of 

 compensation in the dislocating effects of veins. In Weardale most 

 of the veins throw down to the south, while the parallel courses in 

 Allendale and Alston Moor throw down to the north. The lead veins 

 of Flintshire and Cardiganshire have the same east-and-west direction, 

 and so have those of Mexico. 



The lodes and veins of Cornwall are most generally east-and-west 

 veins, or nearly so ; and these are the oldest veins in that district, 

 being traversed by oblique veins and by cross course elvans and 



