INTRODUCTION AND ACKNOWLEDGMENTS The lithologically complex Pennsylvanian successions in the eastern United States are traced and classified with difficulty, but the great economic value of the coal, clay, shale, and other mineral resources, and the abundant and well-preserved plant and invertebrate fossils contained therein have stimulated detailed studies. The writer’s interest in problems of the Pennsylvanian was developed in 1925 and 1927 during detailed mapping of the Alexis and Havana quadrangles in northwestern and west-central Illinois for the Illinois State Geological Survey. Several very thin lithologic units were recognizable in both these areas, which are about 60 miles apart. In 1929 the correlations were confirmed by tracing many of the thin but diagnostic key beds through the intervening area. Reconnaissance studies of the outcropping rocks of the Illinois coal field have been continued for several years by the writer and J. M. Weller, S. E. Ekblaw, H. B. Willman, W. A. Newton, and others until at present outcrops in all parts of the field have been correlated into a general composite section of more than 300 lithologic units. During the summer of 1931, the writer and J. M. Weller spent a few weeks in southeastern Iowa and eastern Missouri, in the Pennsylvanian areas of the Western Interior coal field. Some writers had stated that the existing coal basins had been largely separated during the Pennsylvanian (Culver, 1927, p. 10–13, Savage, 1930, p. 130–131). It was therefore surprising to find that many thin lithologic units, widely distributed . . .