128	MINERAL RESOURCES.
	mined exclusively for silver form a very small class and less than 100,000 tons of them were mined. Under copper ores are classed those which contain 2-1/2 per cent or more of copper, as well as those which contain less than this amount, provided their principal value is in copper. The Lake Superior ores contain, for instance, only from 1 to 2 per cent of copper, and from the Bingham porphyry ores less than 2 per cent is also recovered. The lead ores are those which con­tain over 4-1/2 per cent of lead, irrespective of their tenor in precious metals. Special divisions of mixed ores have been established. A comparatively small amount of zinc is mined as zinc ores, and these contain at least 25 per cent of that metal. Most of the argentiferous zinc ores of the Rocky Mountain States are mined as lead-zinc ores and are subsequently concentrated. Ore production in short tons and average extraction value of gold and silver per ton in 1907, by States and Territories. a