FORMATION OF VEINS. 



293 



frequently quartz and barytes than calcareous spar ; in calcareous 

 mountains, quartz is rarely the prevailing matrix. In the counties of 

 Durham and Northumberland, veins pass through siliceous sandstone, 

 argillaceous shale, and limestone. (See Plate VII. fig. 2.) The ore 

 is more abundant in the limestone than in the sandstone, and in the 

 shale, provincially called plate, ore, very rarely if ever, occurs. In 

 one mine at Welhope, the matrix of the vein, as it passes through 

 the sandstone, is cawk or the sulphate of barytes ; but when it en- 

 ters the limestone, it changes to carbonate of barytes in balls, having 

 a radiated diverging structure. It is still more deserving of notice, 

 that when the rock on one side of a vein is thrown up or down con- 

 siderably, so as to bring a stratum of limestone opposite a stratum of 

 sandstone, or when what are called the walls or cheeks of the vein 

 are of two different kinds of stone, (see Plate VII. fig. 5.) the vein 

 is never so productive in ore, as when both sides of the vein are of 

 the same kind. In the above figure, different strata are opposite to 

 each other, except where the strata are of great thickness : thus, 

 parts of the lower bed of limestone, a a, form the wall on each side 

 of the vein, and in such situations it is rich in ore ; but the upper 

 part of the bed, a, is brought opposite to a bed of sandstone, b, on 

 the left ; and in this part of the vein it will become poorer, and the 

 same will be the case when the vein passes through the upper stra- 

 ta ; in some it will contain no metallic ore. This fact alone seems 

 sufficient to invalidate ihe theory of Werner, that veins were filled 

 with metallic solutions, poured in from the upper part. Had this 

 been the case, the nature of the rock could have made no difference 

 in the quality or quantity of the ore. 



Werner, in his "Treatise of Veins," states one instance, as if it 

 were extraordinary, of the ore changing its quality, as the vein passed 

 through different rocks; and is inclined to admit that elective afBnity 

 for the rock may have contributed to the effect. The circumstance, 

 so far from being extraordinary, is of common occurrence, and known 

 to all working miners. The entire cessation of the ore in one part of 

 a rock, and its re-appearance below, are still more striking. 



In Derbyshire the beds of metalliferous lim^estone are separated by 

 beds of basaltic rock, called toadstone."^ When a vein of lead is 

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



* The fact of metallic veins being entirely cut ofl^ by the beds of toadstone, has 

 recently been doubted ; it is supposed that the vein is continued through the toad- 

 stone, though it contains no ore : but the fact of veins being cut otF by the seams 

 of clay, (called ivay boards,) if it could be established, would lead to the same con- 

 clusion as the separation of the vein by toadstone. My late visits to Derbyshire 

 have convinced me more fully, that Mr. Farey was too hasty in forming his opin- 

 ions, and that he did not always select his information from the best sources. 

 Neither the beds of clay nor toadstone may contain ore, and yet the vein may pass 

 through them, but, being unproductive, it is not noticed. In some instances, prob- 

 ably, the beds of toadstone were protruded between the beds of limestone, after the 

 formation of metallic veins, as Mr. Whitehurst originally maintained. 



