ON MINERAL VEINS, 387 



of the vein, and are crystallized wherever cavities are 

 present. The metallic minerals are variously dis- 

 posed ; sometimes lining similar cavities, in their cry- 

 stalline forms, at others, collected into deposits in 

 different parts of the vein ; and, at others again, more 

 generally diffused among the mass of materials. In 

 some instances, only one metal is found in a vein, 

 in others, two or more; and these are sometimes 

 distinctly separated, at others intinately mixed, so 

 as to he a source of much trouble to the miner. 

 Occasionally, the minerals, whether metallic or 

 earthy, are arranged in layers parallel to the sides of 

 the vein ; and, in some of these instances, there is 

 further, a perfect correspondence on the opposite 

 sides. Such also is the capricious disposition of the 

 metals, that they sometimes disappear altogether, after 

 having abounded through a large space; so that it 

 becomes necessary to abandon a mine which had once 

 proved profitable. It is from these perpetual varia- 

 tions in the contents of mineral veins, that the cha- 

 racters of particular mines are subject to such impor- 

 tant alterations, and that chance, in the ordinary ac- 

 ceptation of the term, baffles all the calculations of 

 the miner. Yet rules are still to be found in every 

 mining country, of occasional value in practice, but 

 always local; while they offer no facts on which a 

 philosophical geologist can safely reason. 



The intersections of veins sometimes produce varia- 

 tions in the nature and disposition of their metallic 

 contents; 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, neces- 

 sarily differ in the nature of the metals which they 

 afford; that under peculiar modes of crossing, they 



c t 2 



