The heterogeneous strata of the Pennsylvanian system in the central and eastern states constitute a complex succession so different from the older Paleozoic systems that the interpretation of the Pennsylvanian is difficult and few geologists have attempted more than a generalization of its geological history. Because of variable lithology and the generally long or unknown range of invertebrate fossils, these standard means of correlation have proved, up to this time, to be of negligible service, and the fossil plants, which are probably no more accurate stratigraphic indices than are the invertebrates, have furnished the basis for practically all previous correlations between the Pennsylvanian strata of the different basins or between remote portions of the same basin.
In 1926 a broad stratigraphic study of the Pennsylvanian system in Illinois was begun by the Illinois State Geological Survey under the direction of J. M. Weller. As . . .