48 



dience to the polar force. Those who may wish to know whe- 

 ther this is a known fact in England may consult the numerous 

 experiments of Mr. Fox, who has proved the existence of the 

 subterranean currents and mineral salts in Cornwall by repeated 

 experiments. Indeed, to doubt this is equal to doubting the 

 meridional action of the magnetic needle ; the latter is the effect 

 of the same power. 



We have already observed that the meridional channels of 

 rocks contain different kinds of minerals, the fractures intersect- 

 ing which form what are called lodes or mineral veins ; we shall 

 now consider the nature of their contents. 



The Filling of Veins. 



Agreeably to the preceding observations, we should naturally 

 conclude, that if fissures be formed in a rock of any given che- 

 mical composition, the pores of such a rock being filled with 

 solvents, the fissures traversing it would contain the predomi- 

 nating mineral substance : hence we should find in limestone, 

 veins filled with carbonate of lime ; in siliceous rocks, veins 

 of quartz; in hornblendic granite or slate, veins of hornblende, 

 &c. : consequently those veins which may intersect a series 

 of rocks varying in their chemical composition, would be filled 

 with a corresponding variety of minerals. Should the laminae 

 or pores of any given channel of rock be more strongly satu- 

 rated with mineral salts than another, the traversing crack, or a 

 series of cracks, would be found to contain rich deposits oppo- 

 site to, and within the limits of such a channel. This variation in 

 the contents of the bounding rocks produces a corresponding 

 variation in the fissure : hence the cause why the minerals are 

 formed in isolated masses (called " bunches 5 '), a well-known fact 

 in every mining district. (Plate XV.) The reality of the depend- 

 ence of the masses of metallic ores in a continuous vein upon 

 the qualities of the bounding rocks, is very perfectly demon- 

 strated by facts long known in England. 



Some veins of fracture change so much in their horizontal 

 direction as to be considered in one part a tin lode and in an- 

 other a copper lode. This is particularly the case with Chase- 

 water lode, which at Wheal Daniel is called a tin lode, at Chase- 

 water mine a tin and copper lode, and at Treskerby a copper lode. 



