On the boundaries of this granite, and in the adjacent slate, are numerous tin and copper-mines, the most interesting of which are Delcoath and Cock’s Kitchen; the latter extends into the parish of Illogan, but is a continuation of the Camborne lodes. Delcoath has been for many years the deepest mine in Cornwall. It stands at the surface, about fifty fathoms (300 feet) above the level of the sea, and the deepest shaft is about a hundred and eighty fathoms (1080 feet) below the sea level, making on the whole a depth from the surface of nearly 1400 feet. These mines exhibit the curious geological phenomena of alternating granite and slate; that is, in sinking a perpendicular shaft, the miner passes repeatedly out of one of these rocks into the other. Various theoretical opinions have been entertained on this subject: some geologists supposing that the layers are no more than irregular protuberances from the main mass of the rock; whilst others consider them as large veins dipping towards and communicating with the granite at a great depth. But it is ascertained that these granite layers are sometimes detached or insulated masses, whilst at other times they form large veins or courses, which have regular bearings to a considerable distance, and are then called elvans; to form, however, a correct idea of the features of these phenomena, we must become acquainted, not with the appearance only, but with the nature and composition of these two rocks. Granite and slate are usually considered, from their exterior character, as very dissimilar, whereas in this situation their real composition is nearly alike. The granite immediately in contact with the slate, consists of compact felspar, containing particles or crystals of felspar, quartz, and mica, in variable proportions, but the whole generally increasing towards the centre of the mass. So that the granite is changed into a felspar rock or porphyry rock, scarcely ever resembling a well characterised granite; while the slate in contact has received the various names of greywacke, greenstone, clayslate, and killas; but it appears to be a rock _sui generis_, consisting almost entirely of compact felspar, coloured purple or blue by its intimate union with a dark-coloured micaceous mineral, sometimes seen distinct on the surface of the slate, and from which it appears to derive its lamellar structure. The bases of these two rocks are therefore the same, and at the point of contact it is often difficult to draw a line between them; for the slate passes into white compact felspar by the gradual disappearance of its colouring mineral; and this granite rock, by the more and more additions of felspar, quartz, and mica, reassumes its usual character.