ORES. Any mineral or combination of minerals containing as much metal as to be profitably extracted, is reckoned by miners an ore. The proportion necessary for this purpose is, of course, very various, according to the value of the particular metal and the facility or difficulty of reducing the ore. A rock containing only 1 per cent, of iron is never called an ore; one containing the same proportion of gold is a very rich ore. Metals rarely exist in ores in a pure or native state; they are almost always chemically combined with oxygen, sulphur, or other elements.
Ores present themselves in a multiplicity of forms and positions in the solid erust of the earth. Sometimes they are sprinkled through the whole mass of the rocks in which they occur, as is often the case with gold, till ore, and magnetic iron ore. Sometimes they are deposited in regular parallel beds between the strata of other rocks, as in the case of many iron-stones and of cupreous schist. At other times they occur in irregular lumps or concretions; or they fill up the fissures of other rocks, forming veins, particu larly silver, copper, and lead ores; or lastly, they are found in detiitus, gravel, sand, and other alluvial deposits. This last form is evidently the result of disturbance and
transport from some of the other positions above specified. And as the metallic parts of the mineral masses or rocks so disturbed and transported are the heaviest, and are insoluble in water, they are more concentrated in these deposits than in their original position. and can therefore be extracted with greater advantage. Such deposits are called ica,shings, from the metal being separated from the other debris by the process of wash ing. Gold and platinum are mostly got in this way in the Ural and Altai mountains, and gold in Guiana, California, and Australia. Tin ore is also found in alluvial deposits in Cornwall and India. The reduction of ores is treated of under _METALLURGY and the names of the several metals.