140 LElectro-magnetic properties of the mines of Cornwall. 
in a pure state, or to the proportion of them in combination. Silver 
and mercury, for example, are combined with, comparatively, very 
small quantities of sulphur ; and zinc, which seems to hold an o 
site place to silver in the electrical scale, is also found in combination 
with a much less proportion of sulphur than is contained in copper 
pyrites, though the latter is one of the best mineral conductors of 
electricity. There are many other analogous examples, which 
prove that no conclusion can be drawn, a priori, from the nature or 
chemical arrangements of minerals, as to their relative electrical 
properties. — 
_ Much time and attention have been bestowed by geologists on the 
consideration of the origin and comparative ages of veins, and but 
little on the purposes for which they are designed. 
The author thinks it will prove a vain attempt to reconcile a mul 
titude of facts observable in mines with any known natural causes. 
ist. The very oblique descent of a large proportion of the veins 
into the earth, in some cases in very hard rock, and in others m 
ground so soft, that it would immediately fall in, however small the 
excayation, without being completely supported by timber. Were 
it possible to conceive fissures to exist under such circumstances, it 
is not reasonable to suppose that they would not take the direction in 
which the resistance would be least, that is, either the vertical, or the 
line of the cleavage of the rocks. 
2d. Veins are often divided into branches, which unite again at 4 
considerable depth, including between them vast portions of rock, 
perfectly insulated by the ore or vem stones from the general mass} 
these, it is evident, could not have existed as fissures for a moment. 
3d. Veins are continually subject to changes in their horizontal di- 
rection and underlie; their size also often varies exceedingly, one 
part being many times wider than another, without any reference t0 
their relative position or depth under the surface. 
4th. Although a portion of their vein stones are usually quite dis- 
tinct in their characters from the rocks they traverse, they are ge" 
erally, in part, of the same nature, and vary with the containing rocks, 
whether granite, elvan, killas, &c., and they are cammonly too regu 
larly arranged in the veins, and are found inclosing insulated portions 
of the ore, &c. in their very substance, to admit of the idea of thelr 
haying been originally mere broken fragments of the inclosing rocks. 
At Dolcoath mine, there is an instance of one ore vein intersect 
ing another at different depths, and being itself intersected, and even 
shifted by the same vein at a greater depth. 
