268	MINERAL RESOURCES, 1912.
	The first item consists of all classes of ores, tailings, and slag pro­ducing gold and silver in the United States in 1912. The grand total shows that this tonnage increased from 30,287,175 short tons in 1910 to 32,928,522 tons in 1911 and to 38,594,806 tons in 1912. This takes no account of placer gravel but represents product of mines producing ore only. The most important increases in 1912 were in Alaska, Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, and Utah, and decreased outputs are noted from California and South Dakota. In reference to the second item, many gold and silver mills employ concentrating apparatus, and the concentrates resulting are com­bined with those from straight concentrating mills in the column of the table "concentrates produced." The gold and silver recovery under this item is mainly by amalgamation and cyanidation, as is shown in detail in the last table of this report. The total quantity of