According to the Pictorial History of California (Frost, 1850), in which this engraving appeared under the heading "The Arrastra, or Primitive Quartz Mill":

Gold which had been ground up by the action of the rivers was deposited in the gravel and sand bars. These bars contained "rich dirt" which was gathered and panned or washed out by one of several methods. When, however, the gold was found in its natural condition in veins of quartz rock, it became necessary to grind the rock to a fine powder in order to separate the gold. The Mexicans used a machine as shown here to grind the rich mineral bearing quartz. The gold was then readily obtained by washing.