Most of the gold of South Africa comes from Witwatersrand district where the native metal occurs in a unique manner in beds or layers of conglomerate associated with other strata, all the rocks being considerably folded and somewhat faulted. Some of the mines are more than a mile deep (vertically), the deepest in the world. The gold either accumulated in placer form with gravel which later consolidated into conglomerate, or it was introduced into spaces between the pebbles subsequently by ore-bearing solutions. Silver. For many years the United States and Mexico have been the world's greatest silver producers, sometimes one and sometimes the other leading, with Canada third, and Australasia fourth. In 1918 the United States produced nearly 68,000,000 ounces of silver and Mexico over 62,000,000 ounces. In the United States in 1918 the four leading States were Montana, Utah, Idaho and Nevada with outputs ranging from over 10,000,000 to over 15,000,000 ounces each. 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 Coeur 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.