50 



ore is found again, but is cut off by a lower bed of toadstone, 

 under which it appears again in the third limestone. In some 

 situations, where the beds of limestone are divided by seams of 

 clay, they cut off the contents of the vein as effectually as the 

 toadstone. (Plate XV.) 



With regard to the metalliferous beds or channels of rocks, it 

 matters not of what variety they are, provided they be good con- 

 ductors and well charged with metallic solutions. The changes 

 in the contents of veins intersecting the more or less vertical 

 channels of the crystalline rocks are similar to those observed in 

 the sedimentary rocks. 



The mining districts of Gwennap, Redruth and Camborne 

 (Plate VII L), consist of crystalline channels of clay-slate, por- 

 phyry, 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 metallife- 

 rous character of the channels depends principally on a primary 

 porphyritic 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 frac- 

 tures 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 approaching to it, as 

 illustrated in Plate XV. : hence the miner's old rule in Corn- 

 wall, parallel lodes produce parallel bunches. This is an esta- 

 blished 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 be- 

 tween the channels can only receive such solvents as may pass 

 longitudinally through them : this is also the cause of the con- 

 tents of the two classes of veins exhibiting a different structure, 

 viz. the east and west from side to side, and the splits in longi- 

 tudinal plates. (Plate X.) 



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



