Coal is a fuel substance of plant origin, composed largely of carbon with varying amounts of mineral matter. Coal belongs to a series of carbonaceous fuels that have varying compositions in the relative amounts of moisture, volatile matter, and fixed carbon. The most useful types of coal contain the largest amounts of carbon and the smallest amounts of moisture and volatile matter. The highest grade of coal is anthracite, or hard coal, which is nearly pure carbon and is used as a domestic fuel. Bituminous coal, or soft coal, with a lower carbon content, is used as an industrial fuel and in making coke. Lignite and peat are the lowest in carbon content. Large amounts of coal were formed in the Carboniferous period of geological time (approximately 280 to 345 million years ago). It is thought that great quantities of vegetable matter collected and underwent slow decomposition in swamps that were similar to present-day peat bogs and lagoons. The peat that formed was converted to lignite and coal by metamorphism. The pressure of accumulated layers of overlying sediment and rock forced out much of the volatile matter, leaving beds or seams of compact coal interstratified with shales, clays, or sandstones. Higher grades of coal were produced where the stress was greatest. Major U.S. coal fields are found in Appalachia, the Midwest, the Rocky Mountain region, and along the Gulf Coast. The chief coal-producing countries of Europe are Russia, Ukraine, Poland, Germany, Britain, France, and Belgium. Valuable coal deposits also exist in China, India, South Africa, and Australia.
