sedimentary. In the latter, the ores of iron and manganese are the most abundant, and often found in beds of large extent. Some ores, as well as native metals, are also found in alluvial deposits; gold, platinum, etc., in those known as placers. Placer products, sometimes called placer ores, have been derived from the degradation and wearing away of older rocks, the minerals having been washed out and redeposited by the agency of water.
OREADS, in Greek mythology, nymphs of the mountains.
ORE DEPOSITS. Any considerable accumulation of metal bearing matter in the crust of the earth. A metalliferous rock is spoken of as an ore when it contains metallic minerals (or in some cases non-metallic minerals, such as sulphur) in such quantities and mixtures as to make possible their profitable extraction. The metal contained in an ore is sometime found in its elemental form, as in the case of gold or platinum, but more frequently as a sulphur compound, or as an oxide carbonate or silicate. Often several different forms of one metallic element are found in the same deposit and several metals are often found together. Quartz, feldspar, hornblende, fluorite are the common gangue materials, and are sometimes so evenly mixed with the minerals that the ore must be crushed and metal picked out by magnets or separated by specific gravity devices. In other cases the gangue is in such a form that it can be avoided in mining. Some deposits were formed at the same time as the rocks by which they are contained, but in the greater number of cases, the mineral was deposited after the rocks were formed. The greater number of ore deposits are in igneous rocks and not infrequently near hot springs. It is believed that ground water, frequently hot, and sometimes under high pressure because of its depth and in vapor form carried many of the metallic elements in solution until a decrease in temperature or pressure caused the precipitation of the metallic salts they had been carrying in solution.
When metal bearing salts are deposited along a crevice or fissure in tubular form, the deposit is called a vein; and when the veins are parallel and closely spaced, they are known as a lode. A large pocket rich in ore is known as a bonanza. When the metal is found in the open mixed with gravel, as is frequently the case with gold and platinum, they are called placer deposits. The form of the deposit, the mineral contents or the origin of the ore body are the three most widely used methods of classifying ore bodies. The former is used by miners and the latter by geologists and mining engineers. The five main divisions under this method of classification are:
- Igneous, those which were formed with the rocks;
- Pneumatolytic Deposits, made by gases above the critical point;
- Fumarole Deposits, made by lava;
- Gas Aqueous Deposits, made by ground water at high temperature;
- Deposits by ordinary ground or surface water.
Ore deposits are found over a wide range of territory. They are found where there has been igneous activity, or where they have resulted from the work of meteoric waters.
Copper is found in the United States in Michigan, Arizona, Utah and Montana; zinc in New Jersey, and both lead and zinc are found in Missouri, Illinois, Iowa and Wisconsin; iron ore is found around Lake Superior and in parts of the Appalachian region, and gold and silver are found in Alaska, the Black Hills, and in the Cordilleran region.
OREGON, a State in the Pacific Division of the North American Union; bounded by Washington, Idaho, Nevada, California, and the Pacific Ocean; admitted to the Union, Feb. 14, 1859; capital, Salem; counties, 36; area, 96,699 square miles; pop. (1890) 313,767; (1900) 413,536; (1910) 672,765; (1920) 783,389.
Topography.—The surface of the State is mountainous, three ranges dividing it from N. to S.; the Coast Range from 10 to 30 miles from the ocean; the Cascade Mountains, from 110 to 150 miles inland; and the Blue Mountains in the E. The Coast Range has an extreme altitude of 4,000 feet, and is covered with dense forests. The Cascade Mountains, a continuation of the Sierra Nevadas, have an extreme height of 7,000 feet, with several peaks rising 2,000 to 5,000 feet higher. Mount Hood reaches an altitude of 11,500 feet, McLoughlin, 11,000 feet; and Jefferson, 10,500 feet. The Cascades are heavily timbered to the snow line. Four transverse ranges connect the Coast Range with the Cascades; the Calpooia, Umpqua, Rouge River and Siskiyou Mountains. The Willamette river valley, lying between the Coast Range and Cascade Mountains, and the Columbia river and California spur, is 150 miles long, from 30 to 70 miles wide, and is extremely fertile. Eastern Oregon, embracing two-thirds of the State, is a high table-land, with little rain-
