The mines in the county of Cornwall consist chiefly of Tin and Copper, besides which there are some which yield Lead[87], Cobalt,[88] and Silver.[89] The ores are in veins which are provincially termed _Lodes_, the most important of which run in an east and west direction; during their course they vary considerably in width, from that of a barley-corn to 36 feet;[90] the average may be stated at from one to four feet. It is, however, by no means regular, the same lode will vary in size from six inches to two feet, in the space of a few fathoms. No instance has yet occurred of lodes having been cut out in depth; the deepest mine now at work is _Dolcoath_, which is about 235 fathoms from the surface to the lowest part.[91] _Crenver_ and _Oatfield_ have lately been stopped; they were 240 fathoms deep. The rocks through which the lodes descend are of different kinds, thus are Copper and Tin found in _granite_, as well as in _slate_.[92] The Tin in these veins[93] generally occurs in the state of an _oxide_; the only Copper ore of any consequence is _Copper Pyrites_, or Sulphuret of Copper; the _arseniates_, _carbonates_, &c. being too small in quantity to be of any importance in a mining point of view. _Iron_ and _Arsenical Pyrites_ are also very common attendants, and are both confounded under the name of _Mundic_. Besides the metalliferous veins which run easterly and westerly, we have already stated that there are others, not generally containing ore, which maintain a direction from North to South, and on that account are called _cross courses_, and often prove to the miner a great source of trouble and vexation; for they not only cut through the other veins, but frequently alter their position, or _heave_ them, as it is termed; and it is a very curious fact that most of the _Tin_ and _Copper_ lodes, thus _heaved_, are shifted in such a manner, as to be generally found by turning to the _right_ hand; _left_ handed heaves being comparatively rare. In _Huel Peever_ this vexatious phenomenon occurred, and it was not until after a search of forty years that the _lode_ was recovered.[94] The discovery of metalliferous veins is effected by various methods, the most usual one is by sinking pits to the solid rock, and then driving a trench north and south, so as to meet with every vein in the tract through which it passes; the process is a very ancient one, and is termed _Costeening_.[95] The operation, however, of opening a new mine from the surface, or from _Grass_,[96] as it is called, is not one of frequent occurrence.[97] The reworking of mines which have been formerly abandoned, on account of the produce being insufficient to pay the costs, from the fall of the standard price of ore, is quite sufficient to absorb all the speculative spirit of the country.