51 



different crystals in the same fracture, place a mass of clay- slate 

 between the poles of a battery, immersed in a metallic solution ; 

 it will be seen that the currents pass only in the direction of the 

 cleavage : if the slate be broken across, so as to represent veins 

 of fractures, crystals will be observed to grow in each fracture 

 transversely, i. e. in the direction of the cleavage planes*. If 

 two or 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 reduced proportionably and accordingly to their 

 relative ease of decomposition. 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 to- 

 gether 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, hydro- 

 gen 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 indicate feeble currents, and conse- 

 quently unfavourable for the production of large quantities in 

 such veins. The most favourable indications 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 position, as illustrated in 

 Plate XIV. The most favourable position of a fracture for the 

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

 district, and slightly dipping northward ; the unfavourable frac- 



* We have already insisted that cleavage planes are formed in the direction 

 of the currents from pole to pole. Experiments have been made with the view 

 of imitating these crystalline planes, by placing a mass of clay between the 

 poles of a battery, and it has been supposed that the small transverse fissures 

 produced by the tension represent the phenomenon of cleavage. A very 

 slight examination will show the distinction between them. Those who may 

 feel disposed to imitate real 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, as represented in Plate XI. 



E 2 



