Copper. This is one of the most useful of all metals. Several of its very important uses are as a conductor of electricity in the form of wire; in making alloys such as brass and bronze; in copperplate engraving; and in roofing and plumbing. Various minerals containing copper are found in many parts of the world, but only about six of them are really important as ores. These are native copper, chalcopyrite, chalcocite, azurite, malachite, and cuprite, most of which are described in the chapter on "Mineralogy." The number of places where they may be profitably mined as ore is distinctly limited. Fifteen or twenty countries produce more or less copper, but the United States is by far the greatest producer, with an output of nearly 2,000,000,000 pounds of copper in 1916, the output having fallen off some in 1918. This was two-thirds of the world's output and ten times as much as the nearest competitor. The other leading countries are Japan, Chile, Mexico, Spain, and Canada. In 1918 the four leading States in order were Arizona, Montana, Michigan, and Utah, with production ranging from nearly 765,000,000 pounds to about 230,000,000 pounds. In Arizona several great copper-mining districts lie in the southeastern one-fourth of the State. Almost invariably the ores are directly associated with limestone and an igneous rock (granite), both of late Paleozoic Age. The ores are almost always near the border between the two rocks, mostly as great irregular deposits within the limestone, and less commonly as veins within the granite. The original ores were carried in solution and deposited by hot liquids (or vapors) from the cooling granite. At lower levels the ores are mainly sulphides of copper (e.g., chalcopyrite and chalcocite), while at higher levels they are mostly carbonates (malachite and azurite) and oxides (e.g. cuprite). The difference is due to the fact that the ores nearer the surface have been subjected to weathering and altered from their original condition. The region around Butte, Mont., is next to the greatest copper producer. Nearly all the ores are sulphides of copper (mainly chalcocite) which occur with quartz in a great system of nearly parallel veins in granite of Tertiary Age. "It is supposed that in the copper veins the hot ore-bearing solutions ascended the fractures in the granite, replacing the rock by ore, and resulting in an intense alteration of the walls." (Ries.)