Featherstonhaugh\^ Geological Report. 



73 



then contracted into a very small space, and afterwards dilated 

 again into pockets, resembling a number of blown bladders 

 connected by a long wire. Another curious phenomenon is, 

 that their contents are modified on entering a different rock ; 

 they usually have a case or sheath differing from the mineral 

 matter of the rocks they are found in. The walls of sulphu- 

 ret of lead, in passing through sandstone, are often formed 

 of sulphate of barytes, but, on entering the limestone above, 

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

 diverging structure. Sulphuret of zinc is sometimes abundant 

 in the upper part of veins, that lower down become rich in 

 copper ; and in the mine of Cook's kitchen, Cornwall, after 

 first working through tin, and then through copper, to the 

 depth of eleveii hundred feet, tin is again found, and v./orked 

 to the depth of near thirteen hundred feet. The same vein, 

 at Dalcoath mine, is sometimes contracted to six inches, and 

 sometimes dilated to forty feet. Although it appears demon- 

 strable that the contents of many metallic veins have been 

 injected from below, it appears probable that the contents of 

 others have been modified by chemico-electric action, and that 

 metals may be formed by processes analogous to those upon 

 which the formation of saline crystals depends. Mines which 

 have been closed, on account of inundation, for two centuries, 

 have, on being re-opened, exhibited the curious spectacle of 

 native silver coating the wooden supports which had been left 

 there. The walls of veins and the atmosphere may stand in 

 the relation of galvanic plates to each other, and decompose 

 and reeompound gaseous bodies. 



Many of the appearances which have been alluded to are 

 of constant occurrence in the United States. In the aurifer- 

 ous region of this country the course of the principal produc- 

 tive veins is from northeast to southwest. 



Where metalliferous veins abound, red earth is usually found 

 in great abundance. It seems to have a constant connexion 

 with them. It is very much the case in the United States, is 



