Geology and Magnetism. 527 



more metals be combined in the solution, and the current be very 

 feeble, only one of the metals will be formed in each crack at a 

 time : should the current be increased beyond that required for the 

 decomposition of one of the metallic salts, the others will be reduc- 

 ed proportionably and accordingly to their relative ease of decom- 

 position. The intensity of the current and the proportion of the 

 metallic salts may vary periodically, which may account for the 

 variety of crystals irregularly grouped together in the same vein 

 and at the very same spot. When the intensity of the currents is 

 very feeble the crystals are large, and when greater than sufficient 

 to reduce the metallic salts, hydrogen will evolve and the metals 

 will be precipitated in a massive powder. Hence, when minerals 

 are found in their metallic state and in large crystals, they indi- 

 cate feeble currents, and consequently unfavourable for the produc- 

 tion of large quantities in such veins. The most favourable indica- 

 tions for rich deposits are strong solutions of minerals, dammed by 

 fluccans or clayey veins, so that the excess may ooze out on the 

 surface, forming hydrous oxides and sulphates. The amount of 

 deposition in each fracture depends on the mechanical positions, as 

 illustrated in Plate XIV. The most favourable position of a frac- 

 ture for the accumulation of minerals is at right angles to the grain 

 of the district, and slightly dipping northward ; the unfavourable 

 fracture is, that dipping southward at a first angle under a heavy 

 hill. In a series of parallel fractures the bunches of mineral will be 

 found in a more or less meridional direction (Plates VII. and 

 VIII.), on the south somewhat deep, and on the north shallow 

 deposits. This rise of the metalliferous currents varies in South 

 America from 10° to 20° from the horizon. In Mexico the richer 

 bunches of minerals are found in the parts where the fissures inter- 

 sect the moist porpbyritic varieties of clay-slates. (Plate VII.) In 



battery, and it has been supposed that the small transverse^wwre^ produced by the 

 tension represent the phenomenon of cleavage. A very slight examination will 

 shew the distinction between them. Those who may feel disposed to imitate rea 

 polar laminae must furnish each pole with a piece of laminated rock ; without this 

 preparation cleavage planes cannot be produced by artificial means. A mass of 

 clay jammed between the walls in a vein of fracture will be cleaved across by the 

 natural magnetic currents in a very few years. It is this constant cleaving action 

 which is the cause of veins becoming obliterated. 



