Stamp mills were popular during the gold rush as a way to extract gold ore from surrounding rock and soil. Basically they would dump a load of rock in a hopper, it would get fed through the mill and crushed up, then run down a sluice box where the heavier gold ore would settle out from the rest of the material.

Pretty amazing that some of these structures are over 100 years old and still in such good shape...

More info here: http://en.wikipedia.org/wiki/Stamp_mill