Illinois coals range from high volatile C to high volatile A bituminous rank. The rank increases gradually from the northwestern to the southeastern part of the Illinois Basin. In the northern and central parts of the Illinois Basin, the iso-rank lines (lines of equal seam moisture and of equal calorific value calculated on the moist, mineral-matter-free basis) for the Herrin (No. 6) coal member of the Carbondale Formation (Pennsylvanian) exhibit an increase in rank with depth and parallel the structural contour lines fairly closely. In southern Illinois, however, the rank continues to increase southeast-ward although the coals become shallower and finally crop out on the southern edge of the Illinois Basin. The continued southward increase in rank was probably caused either by southwardly increasing depth of burial of the coals during coalification and the tectonic closure of the basin after the coalification process had essentially terminated or by increased heat flow above a possible plutonic intrusion in that part of Illinois.The increase in rank with depth (expressed in percentage of moisture content or in calorific value) is in the same range as that reported in other coal basins in which the coal is of comparable rank.

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