This machine made possible the recovery of gold from the rock of the ore. The first part would crush the ore to gravel sized pieces. This part, the stamp, would pound the gravel into dust. Attached to that round rod at the top was a heavy hammer (stamp) that would be repeatedly dropped on the ore. The dust would then be washed down a sluice box or over metal plates coated with mercury which attracts the gold.
The mercury paste would be heated in a retort like the one in the foreground. The mercury would boil off, condense, and be captured for reuse. The gold would be left in the retort. The formula for such a mill was: 1 ton of ore processed in 24 hours yields 1 ounce (28.3 grams) of gold. This assumes ore of a high enough grade. At the time, gold was worth $30 per ounce.
Joshua Tree National Park
Part of the Great American West series.