Ch. 1: Gold and Silver in 1911

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	GOLD AND SILVER.	243

	Dredging supplied $10,311,589 in 1911, against $9,293,106 in 1910. Of this California alone produced $7,666,461 in 1911, against $7,550,254 in 1910. The total production of gold by dredging in California to the end of 1911 was $47,985,236. In 1911 Alaska produced by dredging $1,500,000, against $800,000 in 1910. The gold output by dredges and the number of boats in 1911 and 1910 for the United States is shown in the following table:



	The remainder of the placer production was chiefly from drift mining (which is especially important in Alaska in frozen ground at no great depth, and in California in ancient buried river channels,¹often at considerable depth), and from hydraulic and sluicing placers. In California, especially, hydraulic mining has been of much importance in the past and a special branch or the industry in itself, but restrictive laws relative to the debris and to disturbance of navigable streams have in recent years greatly confined mining activity of this kind. There is also a small annual output of placer gold from hand washing, and from the operation of so-called "dry-washing" machines in the arid regions of the Southwest. A larger production is probablv now being made from this last-named source in northwestern Mexico and Lower California. 'Figures for domestic production from "dry washing" are not yet available, but the output has been small to date.

	DRY PLACERS. In parts of the arid regions of the Southwest, notably Arizona, southern California, and adjoining areas in northern Mexico gold has long been known to occur in variable quantities, to some extent in veins 'but to much greater extent in weather-worn, concentrated form in dry, loose gravels near the surface and in lime-cemented gravel (caliche, argo masa) below. Natural concentration of the f old liberated by breaking down of vein croppings has been effected y the small annual rainfall of the country gathering temporarily in depressed areas and channels and leaving the heavier material in these depressions after it disappears. This rainfall, however, is of i Lindgreu, "Waldemar, The Tertiary gravels of the Sierra Nevada of California: Prof. Paper U. S. Geo!. Survey No. 73,1911.

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