A brown to black combustible rock that originated by accumulation and subsequent physical and chemical alteration of plant material over long periods of time, and that on a moisture-free basis contains no more than 50% mineral matter. The plant debris accumulated in various wet environments, commonly called http://wiki.answers.com/topic/peat swamps, where dead plants were largely protected from decay by a high water table and oxygen-deficient water. The accumulating http://wiki.answers.com/topic/spongy, water-saturated, plant-derived organic material known as peat is the precursor of coal. Over time, many changes of the original vegetable matter are brought about by bacteria, fungi, and chemical agents. The process progressively transforms peat into lignite or brown coal, subbituminous coal, http://wiki.answers.com/topic/bituminous-coal, and http://wiki.answers.com/topic/anthracite. This progression is known as the http://wiki.answers.com/topic/coalification series. The pressure exerted by the weight of the overlying http://wiki.answers.com/topic/sediment and the heat that increases with depth, as well as the length of exposure to them, determine the degree of coalification reached. See also http://wiki.answers.com/topic/fossil-fuel; http://wiki.answers.com/topic/kerogen; http://wiki.answers.com/topic/lignite-1; http://wiki.answers.com/topic/peat.