MINERAL DEPOSITS. This term is generally understood as a collection of metalliferous ores occurring in g,eological formations where they- have been deposited by the processes of nature, and have, with some exceptions, undergone more or less alteration, either in composition or position, by subsequent changes. Sometimes the ore is a native metal, but is more frequently a mixture of compounds of different metals. A single metal may be the principal one, the associated metals forming a mixture which is called a gangue, the principal ore and the gangue constituting the deposit. In gen eral, ores inay be classified as follows: compact, when the structure is close and fine grained; granular, when composed of visible particles; micaceous or finely laminated. when existing in the form of minute scales, as, for example, micaceous specular iron ore; disseminated, when scottered throughout the gangue in laminw or coarse grains; porphyritic, when distributed in distinct crystals; banded, when the principal ore, or the gangue, or both, are arranged in parallel layers, or bands. Sometimes the bands are arranged concentrically, when the deposit is said to be concentric-banded; brecciated, when the deposit contains fragments of other rock or of older ore, these fragments often forming nuclei around which the ore or the gangue has formed further deposits or crys tals; and it is called drusy when there are many cavities lined with custals. Mineral deposits may also be divided into superficial, stratified, and unstratificd deposits. Super ficial deposits are those in which the materials lie in a more or less unconsolidated or loose condition where they have been washed from cliffs and mountain slopes whose rocks contained metals, ores, or gems. The surface gold deposits of California. Aus tralia, and the Ural are examples, as also the platinum beds of Oregon and Siberia, and the stream tin of Cornwall, Australia, and Durango, rnd the diamond, sapphire, and ruby "mines" of Brazil, South Africa, and the Indies. The ease with which such deposits are worked renders them as a rule the most profitable when first discovered. Stratified deposits have their examples in the coal beds, and many beds of iron, such SE the clay iron-stone, of tbe coal measures; and the schistose copper beds in the triassic sandstones of New Mexico. Unstratified deposits have their examples in those meta
morphosed rocks which have been much disturbed by geologic forces, as the iron ores of Missouri, lake Superior, and the Alleghanies. These deposits were formerly supposed to be of eruptive origin, but it is now understood that they are principally stratitied deposits which have been subjected to great disturbance and to metamorphism. The vast deposits of metallic copper in the lake Superior region were once supposed to have been formed from subterranean fusion, but it has been pretty clearly demonstrated that the metal was deposited from solution under the influence of galvanic or magnetic action. There are, however, eruptive rocks which contain minerals in dissenunated condition, such as the volcanic rocks containing the amygdaloid copper of lake Superior, and the volcanic rocks in Japan, from which large quantities of copper are obtained. Among the unstratified deposits are what are called contact deposits. These occur at the junction, or surface of contact, of two different formations, as where sedimentary have been displaced by igneous rocks. Concretions and sheets of ore are thus found at the junction of trap and sandstone. Deposits occur also as impregnations, where the metalliferous mineral is diffused through a mass of rock in irregular streaks of more or less richness, as is generally the case with deposits of quicksilver. FaWands is a name given to deposits where the ore is diffused through certain layers which become more softened or rotten, or fah2, than the other strata. Examples of fahlbanas are more fre quently met with in mines in Scandinavia than elsewhere. Stockwork is a kind of deposit where the rock is penetrated in every direction, so that the ore must be taken out with the mass of the metalliferous rock. Some of the great iron ore deposits of the world, as the magnetic stock at Tagilsk in the Ural and the hematite of the Missouri iron mountain are examples, as also the copper mines of lake Superior, and the silver mines of Norway, Saxony, and Nevada.