VALVE. The quantity of metal Beef'sSary to make mining profitable depends largely on the character of the ore. Lake Superior copper contain as little as 0.tt5 per cent. of native copper; and many copper sulphide ores running as low as 2 or 3 per cent. metallic copper are SlieeessfIllly worked. 3Iany low-grade lead and zinc ores are profitably worked because their gold and silver contents more than pay the cost of metallurgical treatment. Odd ores alone, run ning as low as $2 to $3 per ton. can be success fully worked under favorable conditions. In nearly every ease the metallic contents of the ore is increased by mechanical concentration or by roasting fin the ease of sulphides), or both, before the or is smelted.
CLAssIFIc.vrtox. Numerous attempts have been made to develop a suitable classifiention of ore deposits, and the schemes suggested have been based either on the form, mineral contents, or mode of origin of the ore body. The first is perhaps the most practical from the miners' standpoint. The second is undesirable because several kinds of ore may often be found in the same ore body. The third is probably the most seientific. and is of value to the mining geologist and mining engineer. as it serves as a guide toward judging the possible extant or irregularity of the ore masses under consideration. The classi fication proposed by W. 11. Weed, which is given • below, includes the most recent discoveries and ideas on the origin of ore deposits. This elassi tication is based entirely on the origin of the ore mass.
• Igneous ore deposits constituting the first diVi 5i011 arc those in which the metallic minerals ha% e crystallized directly from the igneous magma during cooling. inch deposits may occur in dikes. ur on the periphery of igneous masses; the collecting or gathering of the ore particles being the result sometimes of mutual attraction ur again of convection currents set up dining cooling. In pnetunatolitic deposits it is con,,ia crud that the conditions bear out the statement that the igneous mass during its intrusion alio cooling may give off metallic minerals, sili ceous tsanpounds, and gases. Thus at San Jor•6, Tamaulipas, :Ilexico, a great laccolitie mass of andesitic rock has penetrated a Cretaceous HMV Stolle, the latter being a nearly pure carbonate of lime, and yet in the contact zones are found garnet. magnetite, pyrite, and chalcopyrite, all texeept the garnet ) containing matter foreign to the limestone. All of these most therefore have been emissions front the igneous magma. The gas-aqueous deposits include those which have been deposited from a mixture of water and steam, probablv under pressure and at a high temperature. They may either till true fissures or porous deposits. or replace the wall rock lining 11:1/TOW fissure, The last class recognized is the result of meteoric circulation, the waters carrying the ore part k-les to points of eoneentra lion; this may occur either underground or on the surface.