The term ore implies an economic conception and means a source of supply of a metal which can be profitably worked, hence the proportion of the metallic constituent which must be present for profitable working depends very largely upon the price of the metal. Iron ore, ready for the blast-furnace, must have at least 35 % of the metal, while a 3 % ore of copper may be employed. The table of the elements which chiefly make up the accessible parts of the earth's crust (see p. 6) shows that the only commercially important metals which are among the first eight elements are aluminium and iron, while the other metals form but an excessively small proportion of the crust. It has been estimated that lead, tin, and zinc form some hundred-thousandths of a per cent each, copper is in the hundred-thousandths or millionths of a per cent, silver a tenth or a hundredth as much as copper, and gold one tenth as much as silver. (Vogt.) Infinitesimal as these proportions seem, the metals are very widely disseminated in the rocks, and the processes of ore deposition are therefore, above all, processes of concentration, by which the scattered particles of the metallic compounds are brought together in relatively large quantity.

The variety of ore deposits, regarded from the standpoint either of their contents, their mode of formation, or the rocks in which they are found, is excessively great, and no classification of them is satisfactory. All that can be attempted here is a description of some of the commoner and more typical kinds of ore deposits, with a brief discussion of the problems concerning their origin, problems which are still very far from definitive solution.

Stratified Ore Deposits are usually of iron, or less commonly manganese, and occur in beds interstratified with other rocks.-The ores themselves may be found in continuous sheets, thick beds, or scattered nodules, and were evidently deposited from solution in water, like the bog and lake ores which are now in process of formation. Very frequently bedded ores of iron are found among highly metamorphic rocks, especially the crystalline schists. Placers are river gravels which contain grains, or nuggets of heavy metals, such as gold, platinum, or tin oxide (stream tin). They are due to the concentration of the metallic particles, originally scattered through veins or rocks by erosion and stream transportation, and owing to their high specific gravity the metallic particles are thrown down where gravel is deposited. The stratified ore deposits thus offer no particular difficulty of explanation.