19Q Dr. Mac Culloch on Mineral Veins. 



highly productive of veins and metals, which, in another, is 

 deficient and barren. That they arc most abundant in the pri- 

 mary or ancient rocks, is, however, certain. They are also 

 more common in the stratified substances, namely, in gneiss, 

 micaceous schist, and argillaceous schist, than in granite, or in 

 the older porphyries. In the secondary or recent strata, they 

 occur chiefly in the lowest, as in that which has been called in 

 England the mountain limestone, and are scarcely found in the 

 upper strata, or above coal. In the same manner, they are 

 rare in the later trap rocks : but, if Racquet's observations are 

 correct, they occur at Nagyag, either in these, or, as he thinks, 

 in ancient volcanic rocks. 



In the primary rocks, they are sometimes found at the junc- 

 tions of granite with the strata, as happens in Cornwall and at 

 Stroatian. But it is fruitless to attempt to derive any prac- 

 tical advantages from any thing yet known on this subject ; 

 unless as the experience acquired in particular districts may be 

 a guide for these. The limitation of tin to Cornwall and a few 

 other spots, and its exclusion from countries formed of the 

 same materials, — the barrenness of gneiss in Scotland, com- 

 pared with its fertility in Saxony, may be added to a thousand 

 other instances, to prove that we must be content to possess 

 mines wherever they are found, without wasting our hopes and 

 our means in vain endeavours after them, where we have no 

 evidence of their existence. That much false philosophy should 

 have been adopted on the subject of mines, is a natural conse- 

 quence of that perversion of judgment which so often attends 

 the pursuit of Avealth, and of that subversion of the reasoning 

 powers which is produced by examples of its sudden acqui- 

 sition. 



The contents of mineral veins are various ; and although the 

 metallic substances form the most valuable part of them, they 

 bear a very small proportion to the rest. No general rules re- 

 specting these contents can be given, as they vary in almost 

 every country, in every vein, and, often, in every part of a vein. 

 It is common, however, to find that the sides next to the in- 

 cluding rocks are formed of earthy matters of very ordinary 



