526 Geology mid Magnetism, 



of the crystalline rocks are similar to those observed in the sedi- 

 mentary rocks. 



The mining districts of Gwenap, Redruth and Cambrone (Plate 

 VIII.), consists of crystalline channels of clay-slate, porphyry, 

 greenstone, granite &c. When the lodes intersect the pale-blue 

 massive clay-slate, they are generally productive in copper, and tin 

 in the chloritic variety. When the channels dip towards the east 

 or the west, the bunches of ore dip in the same direction. If the 

 channels of ground be very dry and of a close crystalline grain, they 

 seldom produce minerals. The metalliferous character of the 

 channels depends principally on a primary prophyritic and moist 

 base ; this kind of rock appears to be the richest soil, as it were, 

 for the production of minerals. These crystalline channels of rocks 

 being more or less in the meridian, as we have already explained in 

 a previous chapter, and the fractures called lodes intersecting them 

 from east to west (Plates VII. and VIII.), each fracture will contain 

 similar deposits of mineral in the same meridian, or in a line ap- 

 proaching to it, as illustrated in Plate XV. : hence the miner's old 

 rule in Cornwall, parallel lodes produce parallel bunches. This is 

 an established fact in all mining districts ; but it must be remembered 

 that the rule cannot hold good in split veins. The east and west 

 cracks being across the meridional grain, are exposed to the whole 

 crystallising action of the series, whereas split veins between the 

 channels can only receive such solvents as may pass longitudinally 

 through them : this is also the cause of the contents of the two 

 classes of veins exhibiting a different structure ; viz. the east and 

 west from side to side, and the splits in longitudinal plates. (Plate X.) 



In order to exhibit the mode of filling, and the formation of 

 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 trans- 

 versely, i. e. in the direction of the cleavage planes*. If two or 



* 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 



