778 RELATIVE AGE OF METALS. [Oh. XXXVIII. 



(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 Devonian strata were deposited ; in Cornwall, after the car- 

 boniferous 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 Devonian 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 the Silurian, and 

 anterior to the Devonian ; for they do not enter the latter, and, what 

 is still more decisive, streaks or layers of derivative copper have been 

 found near Wexford in the Devonian, not far from points where mines 

 of copper are worked in the Silurian strata.* 



Although the precise age of such copper lodes cannot be defined, 

 we may safely affirm that they were either filled at the close of the 

 Silurian or commencement of the Devonian period. Besides copper, 

 lead, and silver, there is some gold in these ancient or primary metal- 

 liferous veins. A few fragments also of tin found in Wicklow in the 

 drift are supposed to have been derived from veins of the same age.f 



Next, if we turn to Cornwall, we find there also the monuments of 

 a very analogous sequence of events. First the granite was formed ; 

 then, about the same period, veins of fine-grained granite, often tor- 

 tuous (see fig. 744, p. 713), penetrating both the outer crust of gran- 

 ite and the adjoining fossiliferous or primary rocks, including the coal- 

 measures ; thirdly, elvans, holding their course straight through gran- 

 ite, granitic veins, and fossiliferous slates ; fourthly, veins of tin also 

 containing copper, the first of those eight systems of fissures of differ- 

 ent ages already alluded to, p. 769. Here, then, the tin lodes are 

 newer than the elvans. It has indeed been stated by some Cornish 

 miners that the elvans are in some few instances posterior to the old- 

 est tin-bearing lodes, but the observation of Sir H. De la Beche during 

 the survey led him to an opposite conclusion, and he has shown how 

 the cases referred to in corroboration can be otherwise interpreted.]; 

 We may, therefore, assert that the most ancient Cornish lodes are 

 younger than the coal-measures of that part of England, and it follows 

 that they are of a much later date than the Irish copper and lead of 

 Wexford and some adjoining counties. How much later it is not so 

 easy to declare, although probably they are not newer than the begin- 

 ning of the Permian period, as no tin lodes have been discovered in 



* I am indebted to Sir H. De la Beche for this information. See also maps and 

 sections of Irish Survey. 



f Sir H. De la Beche, MS. notes on Irish Survey. 

 X Report on Geology of Cornwall, p. 310. 



