WIDTH OF METALLIC VEINS. 411 



A few metals are occasionally, though rarely, found dis- 

 seminated 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 

 Gornw^all, 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 on to three feet; and 

 in these narrower veins, the Ore is less intermixed with 

 other substances, and more advantageously wrought.* 



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 cr3'stallinc by its heat, and 

 are traversed by dikes from the granitic mass, are now inclined at high 

 angles, and form regular, and conaplicated 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 dis- 

 posed 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 im- 

 portant 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 PI. 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 ex- 

 pected from the Geological Survey of Cornwall, now in progress by Mr. De 

 la Beche, under the appointment of the Board of Ordnance. 



