A tonstein that occurs within the Middle Pennsylvanian Fire Clay coal zone is a widespread marker bed that has been used for correlation purposes since the early 1900?s. Seiders (1965) first demonstrated that this tonstein is an altered volcanic air-fall ash deposit. Lyons and others (1992) have correlated the tonstein across Kentucky, West Virginia, Virginia, and Tennessee on the basis of mineralogical and geochemical ?fingerprinting?. The correlation of the tonstein, herein often referred to as the Fire Clay tonstein over this four-state area, and the abundance of thickness data on this tonstein permit construction of an isopach map that depicts its distribution. In general, the tonstein thins from Tennessee, where it is up to 12 inches (30 cm) thick, northeastward to central West Virginia, where it is less than 4 in. (10 cm) thick, and its preserved area is about 14,300 sq. mi. (37,000 sq. km). The preserved volume is 0.90 cu. mi or 3.7 cu. km. The compaction ratio of ash to rock is 6:1, so the original ash volume deposited in this area was 5.4 cubic miles (22.2 cubic kilometers). The trends in thickness suggest the source of the ash was a volcano in the Yucatan tectonic plate, which, had it not moved relative to the North American Plate since the Pennsylvanian, would be located in the present Gulf of Mexico, south of Louisiana. The thickness and extent of the tonstein appears to have been controlled partly by winds and partly by the surface on which it landed. Its absence in some areas indicates that the ash was not preserved, probably because it floated downstream as pumice.