§ 32. METALLIFEROUS VEINS. 867 



the richest depositories of the metals occur in certain 

 cavities termed metalliferous veins ; which are separations 

 in the continuity of rocks, of a determinate width, but ex- 

 tending indefinitely in length and depth, and more or less 

 filled with metallic and mineral substances of a different 

 nature from that of the masses they traverse. These natural 

 stores of hidden treasures are not confined to any epoch or 

 formation, nor to any tracts of country ; they are, however, 

 most abundant in rocks that have been exposed to intense 

 igneous action, hence their prevalence in the plutonic : but 

 veins of iron, copper, arsenic, silver, and gold, also occur 

 in tertiary strata (ante, p. 284). 



Some veins are evidently fissures of mechanical origin, 

 having been opened by elevatory forces ; in many instances 

 they have been filled from beneath by the sublimation of 

 metalliferous matter by heat; and in others from the surface, 

 by infiltration, or by various materials deposited by streams, 

 which have flowed into them. But in general, the veins are 

 connected by a gradual mineral transition with the contiguous 

 rock, and appear to have resulted from an electro-chemical 

 separation, or segregation, of certain mineral and metallic 

 particles from the enveloping mass, while it was in a soft or 

 fluid state, and their determination to particular centres. 

 The nature of these veins receives illustration from the 

 nests of spar and other minerals common in masses of trap, 

 and in which there appears to have been no possibility of 

 the introduction of any foreign substance from without. 



From the observations of M. Fournet in the mines of 

 Auvergne, it seems probable that in many instances sulphu- 

 rets of iron, copper, lead, zinc, sulphate of barytes, and other 

 minerals, have been introduced at different periods, by 

 electro-chemical action, accompanied by new fractures and 

 dislocations of the rocks, and the widening of previous fis- 

 sures.* The observations and experiments of Mr. Fox add 



* Mr. Lyell's Anniversary Address. 

 VOL. II. 3 L 



