Coal mining is an important industry in Illinois, and the state contains the largest known reserves of bituminous coal in the United States. Minable coal beds underlie about two-thirds of the state. As many as 20 different coal beds have been mined in Illinois, the most important being the Herrin (No. 6) and the Springfield (No. 5) or Harrisburg (No. 5). The coal in most mining areas averages 5 to 7 feet thick and in places attains a thickness of 15 feet. In underground mines the coal is approached by vertical or inclined shafts. In open cut, or strip, mines all of the overlying material (overburden) is removed, leaving the coal exposed. Coal as much as 100 feet deep is now being strip mined in Illinois. Illinois coal is now used mainly for generating electric power, for industrial purposes, and for heating. In industry, coal is used extensively for power, heating, burning cement, firing clay products such as brick, tile, pottery, porcelain, and china, and making coke. Certain Illinois coals when mixed with coal from the eastern part of the United States, produce metallurgical coke for making steel. Gases, oils, and tars derived in processing coal for coke have been used for making many chemical products, including dyes, perfumes, explosives, medicines, insecticides, plastics, and road tar. PETROLEUM (15) PETROLEUM (crude oil) is classed as a mineral resource although it is a liquid hydrocarbon and not technically a mineral or rock. It is, however, found in the pores and fractures of rocks. The color of crude oil ranges from yellow through green and brown to black. [Illustration: uncaptioned] Arch (anticline) Gas Porous rock Nonporous rock Oil saturated rock Water Break (fault) Nonporous rock Oil saturated porous rock Water saturated porous rock Trap formed by change in character of rock Water Nonporous rock Porous rock Petroleum had its origin in the plants and animals buried in ancient sediments. The organic matter changed slowly into the complex mixture of hydrogen and carbon compounds that is petroleum. Because gas is lighter than oil, and oil in turn lighter than water, gas and oil move upward in a porous rock containing all three. Gas moves to the highest position with oil next below and water in the lowest part of the rock. Oil pools exist where geologic barriers have stopped the movement of gas and oil. Arches (upward folds or anticlines), breaks (faults), and lateral changes from porous to nonporous rock are geologic features that serve to localize oil pools within the reservoir rock.