The numbers by which the principal coals of Illinois, Indiana, and western Kentucky are commonly known are a result of independent work by the geological surveys of these three states and for the most part do not correspond. Certain interstate correlations have been made during the last 75 or more years and much attention has been directed toward the correlation of coals within the individual states but no attempt has previously been made to effect a comprehensive correlation of the coals of the Eastern Interior basin as a whole.

Of the 50-odd seams present 13 are mined, 6 in Illinois, 5 in western Kentucky, and 8 in Indiana. The most important coal is Harrisburg No. 5 of Illinois (= Kentucky No. 9 = Indiana V). The Herrin coal No. 6 of Illinois (= Kentucky No. 11) is second in importance. Less important widespread seams are Rock Island coal No. 1 of Illinois (= Minshall of Indiana) with which is tentatively correlated Murphysboro coal "No. 2" of Illinois (= Mannington of Kentucky), Colchester coal No. 2 of Illinois, and Danville coal No. 7 of Illinois (= Indiana VII). Coals important only in restricted areas are lower and upper Brazil blocks of Indiana, Coals III, IV, and VI of Indiana, Grape Creek coal of Illinois, and coals No. 12 and 14 of Kentucky.