H Head grade The average grade of ore fed into a mill. Hoist The machine used for raising and lowering the cage or other conveyance in a shaft I Intrusive A body of igneous rock formed by the consolidation of magma intruded into other rocks, in contrast to lavas, which are extruded upon the surface. J Jaw crusher A machine in which rock is broken by the action of steel plates L Leaching A chemical process for the extraction of valuable minerals from ore; also, a natural process by which ground waters dissolve minerals, thus leaving the rock with a smaller proportion of some of the minerals than it contained originally. Lode A mineral deposit in solid rock. M Mill 1) A plant in which ore is treated and metals are recovered or prepared for smelting. 2) A revolving drum used for the grinding of ores in preparation for treatment. O Ore A mixture of ore minerals and gangue from which at least one of the metals can be extracted at a profit. Ore reserves The calculated tonnage and grade of mineralization which can be extracted profitably; classified as possible, probable and proven according to the level of confidence that can be placed in the data. P Plunge The vertical angle a linear geological feature makes with the horizontal plane. Porphyry Any igneous rock in which relatively large crystals, called phenocrysts, are set in a fine-grained groundmass. Probable reserves Valuable mineralization not sampled enough to accurately estimate the terms of tonnage and grade. Also called "indicated reserves." Proven reserves Reserves that have been sampled extensively by closely spaced diamond drill holes and developed by underground workings in sufficient detail to render an accurate estimation of grade and tonnage. Also called "measured reserves."