The Central Eureka's Cyanide plant in the 1930's. Amalgamating gold with mercury only worked on the free particles of gold. Cyanide leaching was developed to chemically separate the gold from the other elements.
Central Eureka head frame
Excerpt Amador Ledger - Nov 22, 1907
The Central Eureka has been working a few men since most of them were laid off about three weeks ago, taking out a pillar of rock, which is in the rich streak that paid so well
in years gone by, and is run through the mill at night.
During a two-year period (1938-1939), 85,517 tons of ore averaging 0.62 oz. gold per ton were mined and milled, as compared to the production of approximately 210,000 tons of 0.315-oz.
Information, photographs courtesy of the Amador County Archives, The Historical Marker Database, and the Chronicling America Database