Geology and Magnetism, 525 



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 sand- 

 stone, 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 shift- 

 ed, the component parts, or the elements of each stratum, connect 

 the opposite 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, 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 lateral 

 crystallization from the bounding rocks. 



In Derbyshire the beds of metalliferous limestone are interstrati- 

 fied by hornblendic rocks, called toadstones. When a vein of lead 

 is worked through the first limestone down to the toadstone, it 

 ceases to contain any ore, and often entirely disappears : on sinking 

 through the toadstone to the second limestone, the ore is found 

 again, but is cut off by a lower bed of toadstone, under which it 

 appears again in the third limestone. In some situations, where 

 the beds of limestone are divided by seams of clay, they cut off the 

 contents of the veins as effectually as the toadstone. (Plate XV.) 



With regard to the metalliferous beds or channels of rocks, it 

 matters not of what variety they are, provided they be good con- 

 ductors and well charged with metallic solutions. The changes in 

 the contents of veins intersecting the more or less vertical channels 



* For this and similar references to plates, see note, p. 518. 



3 Y 



