The minerals which are in constant use by nearly all people and that are found abundantly in the United States, are gold, silver, copper, lead, zinc, and the elements used in manufacturing building materials.
Gold is valuable chiefly because it has been made the standard of money value of the world. Africa produces one-third of the world's supply, next come the United States and Australia, producing almost equal amounts, Russia and Canada each produce a limited amount, and various other countries together produce about one-sixteenth of the whole. (In the statements of the gold supply of the United States the territory of Alaska is included.)
Gold is not found alone but contained in quartz rock or sand. The method of taking gold from the rock is first by blasting, and afterward grinding the rock in a stamp mill, which reduces it to powder, after which the gold is separated by refining processes. The gold which occurs in the sand, gravel, or clay soil, is washed out. When done on a small scale this is called "panning." The larger operations of this kind are called "placer" and "dredge" mining. There is also a considerable amount of gold obtained as a by-product from copper mining.
Generally speaking, quartz mines are in the mountains and placer mines in the river valleys. Placer mining by powerful water pressure, called hydraulic mining, destroys the banks, and also fills up the river beds with masses of rock and gravel. Some of the large rivers of California have been made unfit for steamboat traffic, and serious damage has been done to the harbor of San Francisco. For this reason hydraulic placer mining has been stopped by law. This has greatly lessened the gold production of California.
In 1907, the United States produced $94,000,000 worth of gold. Of this, Colorado produced more than any other state. Next in their order come Alaska, California and Nevada. Each produced from $15,000,000 to $20,000,000 worth. Together they furnished nearly four-fifths of the entire supply. The remaining one-fifth comes from Utah, South Dakota, Montana, Arizona, Idaho, and Oregon, with very small amounts from the southeastern states, the two Carolinas and Georgia, New Mexico, Washington, and Wyoming. South Dakota has the most profitable single gold mine in the United States. It has produced nearly $60,000,000 in gold, and is now turning out about $5,000,000 worth a year.
The United States has many unworked gold mines, "gold reserves" they are called, whose value can not in any way be exactly estimated. The value of the placer mines can be better judged than that of the lode or quartz mines. The placer mines are chiefly in Alaska and California. These mines may yield gold to the amount of a billion dollars. There are lesser, but important resources of placer gold in Montana, Idaho, and Oregon.
The placer gold mined in 1907 was valued at $24,000,000, and it is thought that about this quantity can be supplied for a long time.
The amount of gold yielded in the reduction of copper ores was about $5,500,000. It is probable that this amount will be gradually increased, and can be relied on to last many years. From the lead ores a little over $2,000,000 worth of gold was taken. This will probably slowly decrease for the next ten or twenty years. From gold and silver-bearing quartz mines $55,000,000 was taken.
No calculation can be made as to the amount of gold contained in quartz mines. New discoveries are always probable and many new mines are opened up each year, but their value can only be estimated as the work in them progresses.
Just how long they will last nobody knows, but it would seem that their decline is far off. The government report says, "Unless very important new discoveries are made it is thought unlikely that the production of gold in the United States will rise much above $110,000,000; nor is it likely that it will sink below $60,000,000 within a long period of years."