a lump of petrified plant matter, frequently spheroid, found in coal seams of the Upper Carboniferous Period (from 325,000,000 to 280,000,000 years ago). Coal balls are important sources of fossil information relating to the forests preceding the Coal Age. As a result of a variety of conditions, small pockets of plant debris in Carboniferous swamps, infiltrated by mineral salts, became petrified rather than changed into coal. These petrifactions, ranging from a few grams to several hundred kilograms, have been uncovered in the central United States, in England, in a broad area from Belgium to the Ukraine, and in Spain.