Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/142

132 occurred; and I was informed by Capt. William Davey, of Redruth, one of the most skilful and intelligent practical miners of the present day, and who was the principal manager of Huel Gotland mine, that for some fathoms both in length and depth one wall of one of its veins was of granite and the other of schist; another instance is mentioned in the annexed notice of the accompanying section of Tin Croft mine.

The alterations in the country from schist to granite and back again to schist, are very frequent in some of the mining districts of Cornwall, so that it is impossible in a word to say in which some of the mines are situated, but I suspect this to be principally the case with those mines that are at the foot, or in the immediate neighbourhood of granite hills. I have not noticed any instances of the junction of these two substances in which the granite has not shewn a tendency to decomposition: it is sometimes separated from the schist by a slight ferruginous seam.

It will of course be understood that great variations in the texture and hardness both of granite and schist are observable. Of the former there seems every possible difference in hardness between the decomposed granite of Tol Cam mine, which, as well as that of several others, it was impossible to keep apart without lining the adit or the level with boards close to each other, both above and on each side, and the compact and fine grained granite, of which quartz forms a principal ingredient, and which in hardness is almost equal to porphyry. Of the great difference in the hardness of schist, I shall notice one instance in Huel Allred; in the sinking of two shafts in that mine, not much exceeding the distance of 50 fathoms from each other, the pay to the miner was in one instance £55 per fathom, but in the other only at £5.

These frequent variations in the country through which the miner