524* Geology and Magnetism. 



The Filling of Veins. 



Agreeably to the preceding observations, we should naturally 

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

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

 fissures traversing it would contain the predominating mineral 

 substance : hence we should find in limestone, veins filled with car- 

 bonate 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 mi- 

 nerals. Should the laminae or pores of any given channel of rock 

 be more strongly saturated with mineral salts than another, the 

 traversing crack, or a series of cracks, would be found to contain 

 rich deposits opposite 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"), a well- 

 known fact in every mining district. The reality of the dependence 

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

 of the bounding rocks, is very perfectly demonstrated by facts 

 long known in England. 



Some veins of fracture change so much in their horizontal direc- 

 tion as to be considered in one part a tin lode and in another 

 a copper lode. This is particularly the case with Chasewater lode, 

 which at Wheal Daniel is called a tin lode, at Chasewater mine 

 a tin and copper lode, and at Treskerby a copper lode. The na- 

 ture of the minerals and the accompanying matrix changes with the 

 changes of the rocks intersected by the fracture. In South Ameri- 

 ca the veins of fractures are also often found, — in one part aurifer- 

 ous, in another argentiferous, again cupriferous, according to the 

 metalliferous character of the bounding rocks. (Pamplona, Ibague.) 



The changes in the contents of fissures, when traversing sedi- 

 mentary rocks, are equally striking. The mining districts of 

 Aldstone Moor, Teesdale, &c, consist of sedimentary beds of swale- 

 dale grits and limestones, traversed by fractures ; the minerals in 

 the fissures are chiefly found opposite the limestone beds, which in 



