There are nearly 200 distinct copper ores, but the principal copper ores of commercial importance may be divided into seven classes, as follows:

Native copper, existing alone or practically so, as in the Lake Superior district, Bolivia, and elsewhere, and occurring in the oxidized zone of copper mines in almost every mineral field.

Oxide ores, of which Cuprite and Melaconite are good examples.

Carbonic ores, as Malachite and Azurite. These two ores are largely used as semi-precious gems, when pieces of sufficient hardness are found.

Sulphide ores, of which Chalcocite, Bornite and Chalcopyrite are the most important.

Sulphate of copper, as Chalcanthite, which occurs as an alteration product in many rich sulphide mines, and which is the source of the copper secured by precipitation from cupriferous mine water.

The Arsenides, of which Enargite is the most important.

The Chlorides, of which Atacimite is a good example.

In commercial importance, the sulphide group is easily in the lead, about three-quarters of the world's copper supply coming from ores of this class. Of this group Chalcocite alone produces nearly one-half of the world's supply of copper.

Next in importance commercially is native copper, which is mined extensively in the Lake Superior district.

Third in importance are the carbonates, Azurite and Malachite being the only ones found in large quantities. Malachite is the most important, as it is rich in copper and is easily smelted.

Most copper deposits carry both gold and silver, usually in small quantities, but frequently in amounts sufficient to add materially to the value of the mine. Lead and zinc are found very commonly in connection with copper ore, the three sulphide ores of copper, lead, and zinc being closely affiliated. Iron, while very rarely a commercial product of copper mines, is found in varying quantities in the great majority of copper mines.

Many mines have been opened for gold that really contained immensely greater values in copper at depth. There are frequent instances of gold mines turning into copper mines, the most recent example of importance of this change in metallic values being afforded by the Mount Morgan mine of Queensland, Australia, which for many years was one of the largest gold producers of the world, and which is now a copper mine of great value.

Examples of silver, lead, or zinc mines changing into copper mines at depth are numerous in Utah, Mexico, and elsewhere, the best examples being furnished by the mines in the Bingham district of Utah. The zinc ores of the mines at Leadville, Colorado, are replaced at depth by copper ores.

The Red Jacket shaft of the Calumet and Hecla mine in Michigan is the deepest copper shaft in the world, 4,920 feet.

The principal ores of copper with the approximate percentage of copper found in each is as follows:

Cuprite, a copper oxide which contains 89 per cent of copper. This is the ore that is richest in copper, frequently shading to crystals of native copper.

Chalcocite, a copper sulphide containing 80 per cent of copper. This is the richest commercial ore of copper, and it yields more than one-half of the world's entire copper supply. It is found in all copper districts.

Chalcopy rite, often called copper pyrites, or yellow copper ore, a copper and iron sulphide which contains 35 per cent of copper and 30 per cent of iron. This is the primary ore of copper, all other ores of copper being derived from it. It is found in all the copper fields of the world, and is second to-Chalcocite in importance as a commercial ore of copper.

Enargite, a copper sulphoarsenite bearing 48 per cent of copper. This is the most common and valuable ore at the largest copper producing district in the world, Butte, Montana.

Bornite, a copper and iron sulphide bearing 56 per cent copper, 16 per cent iron, 28 per cent sulphur, is another of the important commercial ores.

Azurite, sometimes called Chessytite or blue carbonate of copper, a copper carbonate with 55 per cent of copper. This is an ore of a most beautiful dark blue color, and is very largely used as a semi-precious jewel.

Malachite, a copper carbonate containing 58 per cent of copper, It is seldom found in large enough quantities to be of value as a commercial ore. It is dark green in color and the compact pieces are valued as semi-precious stones.

Adgadonite, a copper arsenide found in Chili and the Lake Superior district, bearing 85 per cent of copper.

Horsfordite, a copper antimonide with 76 per cent of copper, found in Asia Minor.

Covellite, a copper sulphide bearing 66 per cent of copper. This is a valuable commercial ore, found in Utah, Wyoming, and a few other localities.

Chrysocolla, a hydrous copper silicate with 36 per cent of copper. This ore is valuable as a commercial ore and also as a semi-precious stone for mounting in jewelry.

Stannite, a copper, tin and iron sulphide, with 30 per cent copper, 13 per cent iron, and 27 per cent of tin. It is found in Ireland and England.