EELATIVE AGES OF METALS. 
615 
In the first place, it is not true that veins in which tin 
abounds are the oldest lodes worked in Great 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 Devonian strata were deposited; in Cornwall, af¬ 
ter 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 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 cop¬ 
per and lead were produced, being of a date certainly poste¬ 
rior 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 can not 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 metalliferous veins. A few frag¬ 
ments also of tin found in Wicklow in the drift are supposed 
to have been derived from veins of the same age."^ 
Next, if we turn to Cornwall, we find there also the monu¬ 
ments of a very analogous sequence of events. First, the 
granite was formed; then, about the same period, veins of 
fine-grained granite, often tortuous (see Fig. 614, p. 561), pen¬ 
etrating both the outer crust of granite and the adjoining 
fossiliferous or primary rocks, including the coal-measures; 
thirdly, elvans, holding their course straight through granite, 
granitic veins, and fossiliferous slates; fourthly, veins of tin 
also containing copper, the first of those eight systems of fis¬ 
sures of different ages already alluded to, p. 607. 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 instances posterior to the oldest tin-bearing lodes, but 
the observations of Sir H. de la Beche during the survey led 
him to an opposite conclusion, and he has shown how the 
* Sir H. De la Beche, MS. Notes on Irish Survey. 
