In Montana most of the silver is in the native form, more especially in the upper portions of the great veins rich in copper and zinc ores near Butte. These ores and their origin are described above under the captions “Copper” and “Zinc.”
The two greatest silver districts of Nevada are Tonopah and Comstock Lode where silver and gold minerals are associated as ores in Tertiary igneous rocks, the ores having been deposited in veins by hot ore-bearing solutions from the igneous rocks.
In Idaho the Cœur d’Alene district produces most of the silver, the ore there being a silver-bearing lead ore (galena). The nature and origin of these deposits are described above under the caption “Lead.”
In Utah the silver is also obtained from silver-bearing galena especially in the Tintic, Cottonwood Canyon, and Bingham Canyon districts where the ores occur mainly as irregular deposits and in fissure veins in Paleozoic strata (chiefly limestone) directly associated with igneous rocks, hot ore-bearing solutions from the igneous rocks having furnished the ores.
Tin. Production of tin in the United States has never amounted to much, a little mining having been carried on from time to time in South Carolina, Black Hills of South Dakota, and southern California. About one-half of the world’s supply of tin (121,000 long tons 1918) comes from the Malay Peninsula and two small islands near by. The only other great producer is Bolivia, though a number of other countries produce from 1,000 to 9,000 tons each.
The only important ore of tin is the mineral cassiterite (oxide of tin) described above in the chapter on “Mineralogy.” In the Malay region the ore all occurs in placer deposits and is, therefore, of secondary origin, the source of the ore not being known. In Bolivia the tin ore occurs in veins in and close to granite, the ore having been carried by very hot vapors or liquids which were derived from the still highly heated granite.
Tin is used chiefly in the making of tin plate, bronze, pewter, gun metal, and bell metal.
Aluminum. The mineral called bauxite (a hydrous oxide of aluminum) is the great ore from which aluminum is obtained by an electrical process. Bauxite is noncrystalline, relatively light in weight, white to yellowish in color, and in the form of rounded grains, or earthy or claylike masses. The United States and France are the only two great producers of bauxite, most of which is treated for metallic aluminum. In 1918 the United States produced more than 100,000 tons of aluminum. In the United States the principal deposits are in Georgia, Alabama, and Arkansas. Bauxite is probably always a secondary mineral formed by decomposition of igneous rocks rich in certain aluminum silicate minerals. In some cases, as in the Georgia-Alabama region, the bauxite appears to have been formed and concentrated in deposits by hot solutions from uncooled igneous rocks.
Aluminum is most used in the manufacture of wire for electric current transmission. It is also mixed with certain other metals like copper, zinc, magnesium, and tungsten to form special types of alloys, some of which possess remarkable tensile strength up to nearly 50,000 pounds per square inch. Aluminum is used in powdered form to generate very high temperatures in certain welding processes. It is also made into many kinds of utensils and instruments.
Mercury. This metal, commonly known as “quicksilver,” is of special interest because it is the only one which exists in liquid form at ordinary temperatures. The metal occurs in only small quantities in nature, most of it by far being obtained from the red mineral cinnabar described in the chapter on “Mineralogy.” In order of importance the greatest quicksilver producing countries in 1916 were Italy, United States, Austria, and Spain. In the United States, California is by far the leading State, while Texas and Nevada are the only other important producers.