486 



On Mineral and Metallic Veins. 



they are attached, and the atmosphere, which standing in the 

 relation of galvanic plates, decompose and re-compound the 

 gaseous bodies which surround them. When glass is interposed 

 between the wall and the atmosphere, the production of salt 

 soon ceases. We cannot say that salt is suspended in the at- 

 mosphere, for dry frosty weather is favourable to the quick 

 production of nitre. When a wall is coated with paint, crystalli- 

 zation still forms upon the paint. 



Amidst the curious phenomena which metalliferous and other 

 veins present, is the fact that their contents- are modified on en- 

 tering a different rock. The walls of veins change also with 

 the change of beds. At Welhope, the walls are sulphate of 

 barytes in passing through the sandstone, but on entering the 

 limestone, they change to carbonate of barytes in balls, with a 

 radiated diverging structure. It has been observed also that 

 when mineral beds of a different character are so shifted that 

 their faces are opposed to each other, that part of the veins is 

 impoverished. This could hardly be, under the Werne- 

 rian theory, and may be more plausibly attributed to electric 

 action. Veins usually have a sheath or case difFeiing from the 

 rocks they intersect. This mineral matter is sometimes mixed 

 up with the metal contained in the vein ; the sheath or case is 

 called the walls of the vein, or gangue, or matrix. It is gener- 

 ally of a slaty structure, and in cases of igneous origin, may 

 have been produced by cooling ; the metal concentrating by 

 affinity, and the slaty mineral remaining at the sides. At Cas- 

 tleton in Derbyshire, the vein of fluor spar, has a wall of cawk, 

 or sulphate of barytes ; the vein dilates into cavities, and again 

 contracts into a small space, containing nothing but the cawk, 

 which serves as a clue to the miner to conduct him to another 

 repository of the fluor spar. Blende, a sulphuret of zinc, is oc- 

 casionally abundant in Cornwall, in the upper part of veins that, 

 lower down, become rich in copper. Tin also is found near the 

 surface, with rich copper lying below. But in the mine of 

 Cook's kitchen, after working first through tin, and then through 

 copper to the depth of eleven hundred feet, tin is again found, 

 and is still worked there to the depth of near thirteen hundred 

 feet. The same vein at Dalcoath mine is sometimes contracted 

 to six inches, and sometimes spread out to forty feet. 



Trusting that we have said enough to draw the attention of 



