628 RELATIVE AGE OF METALS. [Ch. XXXVIIl, 



the theory ; and it must be owned that the direction of veins in different 

 mining districts varies so entirely that it seems to depend on lines of 

 fracture, rather than on the laws of voltaic electricity. Nevertheless, as 

 different kinds of rock would be often in different electrical conditions, 

 we ma}' readily believe that electricity must often govern the arrange- 

 ment of metallic precipitates in a rent. 



''I have observed," says Mr. K. Fox, 'Uhat when chloride of tin in 

 solution is placed in the voltaic circuit, part of the tin is deposited in a 

 metallic state at the negative pole, and part at the positive one, in the 

 state of a peroxide, such as it occurs in our Cornish mines. This experi- 

 ment may serve to explain why tin is found contiguous to, and inter- 

 mixed with, copper ore, and likewise separated from it, in other parts 

 of the same lode."^' 



Relative age of the different metals. — After duly r\3flecting on the 

 facts above described, we cannot doubt that mineral veins, like eruptions 

 of granite or trap, are referable to many distinct periods of the earth's 

 history, although it may be more difficult to determine the precise age 

 of veins ) because they have often remained open for ages, and because, 

 as we have seen, the same fissure, after having been once filled, has 

 frequently been re-opened or enlarged. But besides this diversity of 

 age, it has been supposed by some geologists that certain metals have 

 been produced exclusively in earlier, others in more modern times, — 

 that tin, for example, is of higher antiquity than copper, copper than 

 lead or silver, and all of them more ancient than gold. I shall first 

 point out that the facts once relied upon in support of some of these 

 views are contradicted by later experience, and then consider how far 

 any chronological order of arrangement can be recognised in the position 

 of the precious and other metals in the earth's crust. In the first place, 

 it is not true that veins in which tin abounds are the oldest lodes worked 

 in Grent Britain. The government survey of Ireland has demonstrated^ 

 that in Wexford veins of copper and lead (the latter as usual being 

 argentiferous) are much older than the tin of Cornwall. In each of the 

 two countries a very similar series of geological changes has occurred at 

 two distinct epochs, — in Wexford, before the Devoniam strata were 

 deposited; in Cornwall, after the carboniferous epoch. To begin with 

 the Irish mining district : We have granite in Wexford, traversed by 

 granite veins, which veins also intrude themselves into the Silurian 

 strata, the same Silurian rocks as well as the veins having been denuded 

 before the Devoniam beds were superimposed. Next we find, in the 

 same county, that elvans, or straight dikes of porphyritic granite, have 

 cut through the granite and the veins before mentioned, but have not 

 penetrated the Devonian rocks. Subsequently to these elvans, veins 

 of copper and lead were produced, being of a date certainly posterior 

 to fhe Silurian, and anterior to the Devonian; for they do not enter 

 the latter, and, what is still more decisive, streaks or layers o*f 



* R. W. Fox on Mineral Veins, p. 38. 



