Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/137

Rh iron pyrites, which occasions its being abandoned; in others are found the ores of tin or copper, sometimes both, intermingled with some, or even most of the foregoing substances; but if they occur together, the copper for the most part prevails to such an amount, as that all traces of tin are lost in depth. In some veins however they continue to be found, even to a considerable depth, though not often much intermingled but in separate bunches; this as has already been noted, was the case in Cook's Kitchen Mine; the neighbouring mine, Tin Croft, also furnished a somewhat similar instance, though not I believe in the same vein; but these must be considered as somewhat rare occurrences.

Tin is commonly found much nearer the surface than copper. When either is very near, especially if abundance, it is in the estimation of the miner an indication that it will not continue to a very considerable depth, of which Cornwall furnishes numerous instances. There is a tract of country situate about midway between Truro and Redruth on the north of the high road, in most of the mines of which, and amongst them the great mine called North Downs, the ore for the most part occurred near the surface, and was almost uniformly found to extend but to a small comparative depth.

Although there are veins in various parts of the country which have yielded copper in its pure state in considerable abundance, in masses interspersed through them, and but little intermingled with other substances, or even with the ores of copper, yet the most common state in which this metal is found is that called yellow copper ore. It is more continuous and lasting than any other. There are veins which yield only the grey sulphuret, but this rarely or never continues, being found only in bunches. It is sometimes mingled with the yellow ore. As a proof of the great difference in the value of the ores of veins, it may be noticed that there are many which