382 ON MINERAL VEINS. 



that the materials of veins were ejected from the 

 depths of the earth, then indeed they may he indefi- 

 nite, or at least, incapahle of definition, in their down- 

 ward progress. But this is speculative matter. 



The ahsolute antiquity of veins cannot he con- 

 jectured; but there are two modes of judging of their 

 ages, within certain limits. It is evident that they 

 are all posterior to the induration of the strata, as they 

 imply fracture of these : and should veins be found in 

 the primary strata, not also existing in the secondary, 

 they must be of an earlier origin than the deposition 

 of the latter. The veins of Cornwall are, probably, 

 anterior to the formation of the English secondary 

 rocks, because they do not occur in the secondary 

 districts; but to render the proof of this complete, we 

 ought to produce secondary strata unbroken above 

 these veins, or discover them after removing these. 

 Thus also, if the lead veins of Derbyshire are really 

 confined to the secondary strata, they are of more 

 recent date than the last changes in the primary rocks 

 which caused the fissures. A difference of age in 

 veins is, however, proved where two co-exist, and 

 where the one intersects the other. This circumstance 

 is not uncommon on an extensive scale. In Cornwall, 

 most, if not all of the easterly veins are intersected by 

 the northerly; the former being metalliferous, and the 

 latter wanting in metals. These intersections are 

 attended by circumstances as interesting to geology 

 as they are important in the art of mining, in which 

 they are often the source of much labour and expense. 



As the first class of veins is frequently attended by 

 dislocations of strata, so is the second; and, with the 

 consequence of dislocating the former set. Thus the 

 effect of a second vein is to produce a shift in the first, 

 often attended by peculiar circumstances in the state 



