The yield reached its lowest point in 1899, but subsequently increased through the application of improved machinery, while the tailings of the old diggings were treated by the cyanide process with profitable results.
This interruption, due to the practical prohibition of the industry by the United States courts, on the ground that it was injuring, through the deposit of tailings, agricultural lands and navigable streams, was lessened, though not entirely removed, by compromises and regulations which permit, under certain restrictions, the renewed exploitation of the ancient river-beds by the hydraulic method.
There are belt conveyers for discharging the gravel and tailings at the end of the vessel remote from the buckets.
The process was introduced in 1858 by Deetken at Grass Valley, California, where the waste minerals, principally pyrites from tailings, had been worked for a considerable time by amalgamation.
The process is rarely applied to ores direct; free-milling ores are generally amalgamated, and the tailings and slimes, after concentration, operated upon.