The copper ore is cleansed and dressed by the same process as that adopted for tin, but as it generally rises in large masses, requires less washing. Owing to the expense of importing coal, the ore is disposed of after it is prepared for the smelting houses, and owing to the expense of importing coal, the _Smelting Houses_ at _Hayle_ have ceased working for a considerable time past. “Nothing,” says Dr. Maton, “were so deleterious as the fumes of arsenic constantly impregnating the air of these places, and so profuse is the perspiration occasioned by the heat of the furnaces, that those who have been employed at them a few months, became most emaciated figures, and in the course of a few years are generally laid in their graves.” The principal copper mines now working, are mostly in the neighbourhood of Redruth, of which the Gwennass, United, Poldice, Huel Unity, Cook’s Kitchen, and Dolcooth Mines, have yielded an abundant source of gain to their numerous adventurers. A very accurate and well executed geological map of the mining districts, by Mr. Richard Thomas, was published in the year 1819. _Lead_ is found in several parts of Cornwall, but not in any great abundance. The ores are very dissimilar, but the sort most frequently discovered is _galena_, or pure sulphuret of lead, which is found both crystallized and in masses. Its colour is most of a bluish grey, and the form of its crystals is generally the cube. The most common varieties are the cube, truncated at the angles and corners, and the octahedron of two four-sided pyramids, applied base to base. The principal mines are Huel Pool and Huel Rooe, near Helston. There are also a few others on the north coast, in the neighbourhood of Endellion and St. Minver, but of little consequence. The oxides of lead are valuable for painting and dying, and also for medicinal uses. Among the numerous mineral productions of Cornwall, _Gold and Silver_ ought not to be omitted; the former has been frequently found in extremely small granules, generally intermixed with the tin ore, in the stream works. The largest piece ever found, is mentioned by _Borlase_ to have weighed 15 pennyweights and 16 grains. The latter has been found at different periods in considerable quantities, particularly in a mine called _Huel Mexico_, some years ago, near St. Agnes; also in the Herland Copper Mine, in the parish of Gwinear.—A particular account of the discovery of silver in the Herland Mine, was furnished by the Rev. Malachy Hitchins, and printed in the transactions of the Royal Society for 1801. But it appears that after the mine was sunk to a considerable depth, the works were abandoned, the expenses of the mine having considerably exceeded the receipts. Within the last three years, a considerable quantity of silver has been discovered in a mine belonging to Sir Christopher Hawkins.