192 Dr. Mac Culloch on Mineral Veins. 



are subject to such important alterations, and that chance, in 

 •the ordinary acceptation of the term, baffles ull the calculations 

 of the proprietor. Yet rules are still to be found in every min- 

 ing country. These, too, are, unquestionably, of occasional 

 value in practice ; but they are always local, and if they may 

 sometimes serve valuable purposes in practice, they offer no 

 facts on which a philosophical geologist can possibly reason. 



The intersections of veins are sometimes observed to produce 

 variations in the nature and disposition of their metallic con- 

 tents ; but these, like most other rules, are of a local nature. 

 It is also said that masses of ore are found at the intersections 

 of more recent veins, and that intersecting veins of different 

 periods, necessarily differ in the nature of the metals which 

 they afford. It is asserted, further, that in Cornv/all, " if two 

 metalliferous veins cross fron. .opposite sides of the line pcr- 

 pendici'lar to their intersection, they become less productive at 

 and after the junction ; but that, if they cross i'rom the same 

 side of it, the reverse etl'ect takes place." It is further there 

 remarked, that, "after the intersection of a more recent vein, 

 the metallic produce of the ancient vein disappears." If any 

 remarks of this nature have a value, it is not very intelligible. 

 The same proposition is both true and false at the same time ; 

 since it is evident, that where the miner may have chanced to 

 work in an opposite direction, the very reverse effect must take 

 place. Like too many other conclusions, of a similar nature, 

 their chief value consists in warning us not to lely on observa- 

 tions made at hazard, and guided by no principles. 



There is one circumstance, hov.ever, respecting the variation 

 qf the contents of metalliferous veins, which is of importance 

 towards a rational theory of them ; if, indeed, it should prove 

 to be really founded on facts sufHciently extensive. 

 . It is said to be a general remark, that, in all countries where 

 veins traverse strata of different nature?, their metallic contents 

 vary with some relation to these ; and that, in the same vein, 

 the vicinity of some strata renders the vein more productive 

 than that of others. But tlie facts adduced to prove the truth 

 0^ this observation are neither very numerous nor very definite : 



