550 WIDTH OF METALLIC VEINS. 
A few metals are occasionally, though rarely, 
found disseminated through the substance of 
Rocks. Thus Tin is sometimes found disse- 
minated 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 intermixt with other substances, 
and more advantageously wrought.^ 
Several hypotheses have been proposed to 
* An excellent illustration of the manner in which metallic 
veins are disposed in the Rocks which form their matrix, may 
he 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 con- 
tinuing uninterruptedly through Rocks of various ages. In PL 
67, Fig. 3, I have selected from this work a section, which ex- 
hibits 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 Mr. De la Beche, under the appointment of 
the Board of Ordnance. 
