RliPOilX ON MINERAL VEINS. 23 



cause, though it seems to offer objections to some received 

 theories, it may, when better understood, assist in developing 

 the truth. 



This is tlie relation that the contents of a vein bear to the 

 nature of the roclc in which tlie fissure is situated. 



Thus in the older rocks, we see the same vein intersecting 

 clay-slate and granite : it is itself continuous, and there is no 

 doubt of its identity ; and yet the contents of the part inclosed 

 by the one rock shall differ very much from what is found in 

 the other. In Cornwall, a vein that has been productive of 

 copper ore in the clay-slate, passing into the granite becomes 

 richer, or, what is more remarkable, furnishes ores of the same 

 metal differently mineralized. If we pursue it further into the 

 granite, the produce of metal frequently is found to diminish. 



Veins in some cases cut through the elvan courses, as well as 

 the clay-slate inclosing these porphyries : the ores are rich and 

 abundant in the latter; in other instances they fail altogether. 



Less striking differences in the structure of the rock seem to 

 affect the contents of the veins ; and appearances as to the tex- 

 ture and formation of the strata are often regarded by miners 

 with more anxiety than the indications presented by the vein 

 itself; and a change of ground is relied upon with an assu- 

 rance, derived from experience, as a more certain basis to au- 

 gur upon, for better or for worse, than almost any other which 

 the difficult art of mining has to offer. 



Numberless facts might be collected and adduced to show 

 that this is not mere speculation ; but it will nowhere appear 

 more clearly than if we examine the various beds of limestone 

 grit, &c., in the great lead mines in the North of England. 



Here we shall find a series of stratified rocks, and that por- 

 tion of the series which has been most productive of lead ores, 

 occupying a thickness of nearly 280 yards. It is divided into 

 55 distinct beds, which are accurately described in Mr. West- 

 garth Forster's section, each having its name known to the 

 miners of the country. Nine of these beds are of limestone, 

 about 18 are of gritstone or siliceous sandstone, and the re- 

 mainder are plate or black shale, with thin beds of imperfect 

 coal. 



Now the lead veins pass through all these beds, and have 

 been worked more or less into all of them; and it has thus been 

 proved, that though the fissure is common to all, yet lead ore is 

 only found abundantly in particular beds, and those very much 

 the same, if we examine the immense number of mines which 

 are working in this district. 



Where the veins pass through the shale, little or no ore is to 



