An area comprising Illinois, Indiana, Ohio, northern Kentucky, and the western parts of Pennsylvania and West Virginia is considered as a distinct structural province. Stratigraphic evidence is cited to substantiate the premise that major positive structural features are supported by crystalline cores of pre-Cambrian mountain ranges, and that the dominant crustal movement during the entire Paleozoic era was subsidence of the intervening areas.

Importance of the weight of basins of accumulation and the results of their drag on the positive features are considered. Directions and probable extent of resultant lines of weakness in the basement complex are postulated and types of structures formed in sediments over them in Michigan, Illinois, and on the western flank of the Pittsburgh-Huntington basin are discussed. These features are shown to be primarily the results of differential subsidence, certain of the forces involved being the reverse of compressional.

The Appalachian orogenesis is considered the culminating result of an era of subsidence. Lateral effects of orogenic stress are described and evidence reviewed supporting an opinion that the province discussed was shielded from direct compression by the formation of the Pittsfburgh-Huntington basin. Deformation through central Kentucky and southern Illinois is explained as the result of a westward expression of orogenic stress through greater thickness of sedimentary rock.

An experiment is described by which the general pattern of basal lines of weakness in the area were duplicated at a small scale.

The paper is intended to lay the groundwork for a new approach to the study of geologic structure in this province.