49 



The nature of the minerals and the accompanying matrix changes 

 with the changes of the rocks intersected by the fracture. In 

 South America the veins of fractures are also often found, — in 

 one part auriferous, in another argentiferous, again cupriferous, 

 according to the metalliferous character of the bounding rocks 

 (Pamplona, Ibague). 



The changes in the contents of fissures, when traversing sedi- 

 mentary rocks, are equally striking. The mining districts of 

 Aldstone Moor, Teesdale, Swaledale, &c, consist of sedimentary 

 beds of shales, grits and limestones^ traversed by fractures ; the 

 minerals in the fissures are chiefly found opposite the limestone 

 beds, which in the above district are the most metalliferous. 

 (Plate XV. fig. 3.) The ore is more abundant in the limestone 

 than in the gritstone, and in the shale ore seldom occurs. The 

 matrix of the vein as it passes through the gritstone is often 

 sulphate of barytes ; but when it enters the limestone it changes 

 to carbonate of barytes. When the rock on one side of a vein 

 is thrown up or down considerably, so as to bring a stratum of 

 limestone opposite a stratum of sandstone, or when the walls of 

 the vein are of two different kinds of stone, the vein is never so 

 productive in ore as when both sides of the vein are of the same 

 kind. The connexion of the opposite beds of limestone appears 

 essential to keep up the crystallising action, and consequently 

 the accumulations of the useful metals from side to side within 

 the fracture. When the strata are but slightly shifted, the com- 

 ponent parts, or the elements of each stratum, connect the oppo- 

 site walls obliquely : sometimes the shales cut through the veins 

 from side to side ; thus the transverse section of the contents of 

 the fracture exhibits the order of the sedimentary demarcations of 

 the bounding rocks 5 as shown in Plate XV. This important fact 

 alone is sufficient to invalidate the idea of veins having been 

 filled from above or below ; and proves very clearly that the veins 

 of fractures have been forced open and filled gradually by a late- 

 ral crystallization from the bounding rocks. 



In Derbyshire the beds of metalliferous limestone are inter- 

 stratified by hornblendic rocks, called toadstones. When a vein 

 of lead is worked through the first limestone down to the toad- 

 stone, it ceases to contain any ore, and often entirely disappears : 

 on sinking through the toadstone to the second limestone the 



