An agreement of the State geological surveys of Iowa, Kansas, Missouri, Nebraska, and Oklahoma on all major and many minor features of Pennsylvanian classification affecting territory northeastward from northern Oklahoma is an important step toward simplification of nomenclature and elimination of confusion.

The stratigraphic succession of Pennsylvanian rocks in the northern Mid-Continent area is well suited to serve as an important standard of reference in studying Pennsylvanian sections and geological history in other areas. The classification of these rocks in the several states, developed mainly during the last 50 years, has had many features in common but also has exhibited noteworthy discrepancies. Establishment of a classification that ignores State boundaries has general interest to geologists.

The State surveys agree in recognizing Pennsylvanian rocks as a geological system and in defining major time-rock subdivisions within the system, which in upward order are called Desmoinesian, Missourian, and Virgilian. Older units, such as Springeran, Morrowan, and Atokan, are not excluded from recognition where they may be differentiated. Groups are defined as follows, in upward order: Cherokee, Marmaton, Pleasanton, Kansas City, Lansing, Pedee, Douglas, Shawnee, Wabaunsee, except that in northern Oklahoma and part of southern Kansas, rocks of Missourian age are recognized as belonging to the Skiatook and Ochelata groups. Bourbon, previously used in Kansas and Nebraska, is abandoned in favor of Pleasanton. The Kansas City group is defined as extending from the base of the Hertha lim stone to the base of the Plattsburg limestone; it contains three sub-groups called Bronson, Linn (new), and Zarah (new). The Wabaunsee group, also, may be divided into subgroups named Sacfox, Nemaha, and Richardson.

Not considered by the inter-state conference is use of lithologic classificatory terms for units

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classed as formations which are made up of two or more kinds of rock. Some surveys are disposed to designate such units by the term "formation," rather than to call them "limestone," if their dominant constituent is limestone, or "shale," if they are mainly shale but contain other kinds of sedimentary rock also.