Drag-stone mill for pulverizing ores to isolate silver by the patio process , apparently used in pre-Columbian America.
The silver ore was crushed and ground by mule power in the arrastras (shallow circular pits paved with stone). Large blocks of stone attached by beams to a central rotating post were dragged around the arrastra, reducing the ore to a fine mud. Further steps resulted in the isolating of the silver.