A few metals are occasionally, though rarely, found disseminated through the substance of Rocks. Thus Tin is sometimes found disseminated through Granite, and Copper through the cupriferous slate at the base of the Hartz, at Mansfeld, &c.
The most numerous and rich of the metallic veins in Cornwall, and in many other mining districts, are found near the junction of the Granite with the incumbent Slates. These vary in width from less than an inch to thirty feet and upwards; but the prevailing width, both of Tin and Copper Veins in that county, is from one to three feet; and in these narrower veins, the Ore is less intermixed with other substances, and more advantageously wrought.[1]
Several hypotheses have been proposed to explain the
manner in which these chasms in solid rocks have become
filled with metallic ores, and with earthy minerals, often of
a different nature from the rocks containing them. Werner
supposed that veins were supplied by matter descending
into them from above, in a state of aqueous solution; whilst
Hutton, and his followers, imagined that their contents were
and Tertiary strata which have been rendered crystalline by its heat, and are traversed by dikes from the granitic mass, are now inclined at high angles, and form regular, and complicated anticlinal lines. These same sedimentary strata, and also lavas are there traversed by very numerous true metallic veins of iron, copper, arsenic, silver, and gold, and these can be traced to the underlying granite. (Lond. and Edin. Phil. Mag. N. S. Vol. 8. p. 158.)
- ↑ An excellent illustration of the manner in which metallic veins are disposed in the Rocks which form their matrix, may be found in Mr. R. Thomas's geological Report, accompanied by a Map and Sections of the mining district near Redruth. This map comprehends the most interesting spot of all the mining districts in Cornwall, and exhibits in a small compass the most important phenomena of metallic veins, slides, and cross courses, all of them penetrating to an unknown depth, and continuing uninterruptedly through Rocks of various ages. In Pl. 67, Fig. 3, I have selected from this work a section, which exhibits an unusually dense accumulation of veins producing Tin, Copper, and Lead.
Much highly valuable information on these subjects may shortly be expected from the Geological Survey of Cornwall, now in progress by Hr. De la Beche, under the appointment of the Board of Ordnance.
