Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/126

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The East and West, or metalliferous veins, are generally from one to three feet in width, but vary to 30 feet. In the old workings of Relistian mine for tin, there are chasms, both open to day and underground, full 30 feet wide. A vein from one to three feet in width is preferred, because, though many instances of the contrary have occurred, it is found that the product is generally as good as in wider veins, on account of there being, for the most part, less admixture of foreign substances with the ore.

A vein sometimes varies in width in the same mine very materially, and is often found to increase in that respect in going down. One of the lodes in Huel Alfred varies from 9 to 24 feet: there is a peculiarity in its direction which is noticed in speaking of that description of irregular veins called Contres. As opposed however to the width of that vein, those of Herland and Drannack and Prince George mines, which are separated from Huel Alfred only by a brook and a cross vein, may be cited. There, instead of continuous veins of a somewhat irregular width, they are remarkably small, most of them varying only from 2 to 6 inches in the widest part, which was about the middle, and going away East and West in mere strings. A tin vein in Whealan Coates mine, not 3 inches wide, was very rich, and found to be worth working.

If in working on the course of a vein, or in sinking through the load, the country is found to assume a greater hardness in a very considerable degree, the vein generally becomes narrower.