Outcrop mines, [60].
Output, factors limiting, [155].
giving least production cost, [154].
maximum, determination, [153].
Overhand stapes, [96], [98], [99].
Overproduction of base metal, [158].
Oxidation, [30].
Patchwork plant, mechanical inefficiency of, [158].
Pay areas, formation, [23].
Pillars, artificial, [121].