388 ON MINERAL VEINS. 



become more productive, under others less, and that, 

 after the intersection of a more recent vein, the me- 

 tallic produce of the antient one disappears. The 

 value of such remarks is not very intelligible, and the 

 same proposition is often both true and false at the 

 same time. Like many other conclusions of a si- 

 milar nature, their chief value consists in warning us 

 not to rely on observations guided by no principles. 



There is one observation, however, respecting the 

 variation of 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 

 sufficiently extensive. It is said that in all countries 

 where veins traverse strata of different natures, 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 the facts are neither very numerous nor very 

 definite: it remains to be proved whether they arc 

 not swallowed up by a mass of exceptions. It is said, 

 that in a vein in Cornwall, passing through schist and 

 granite, the copper is found in the former and the tin 

 in the latter portion ; that similar veins are poor in 

 the schist and rich in the granite; that veins are most 

 productive at the junction of the schist and granite 

 not only in that district but in Silesia and elsewhere. 

 There is not one example of this nature to which 

 there are not exceptions many times exceeding them, 

 for which the reports of the same observers may be 

 consulted. 



But it is fruitless to record all the observations 

 which have been made on these subjects ; since the 

 conclusion would be to draw, as might equally be 

 done without them, no conclusions. Whether, as to 

 the influence of strata over the contents of veins, anv 



