Abstract Commercial production of natural gas in the Lima-Indiana district of Indiana and Ohio began in 1884–1886. Ten years later the largest area known to produce gas from rocks of Ordovician age was proved and outlined. The district extends in a curve for a distance of about 150 miles and covers an area of about 12,000 square miles. Only a few gas fields of small areal extent were found in Ohio; those in Indiana were numerous and large. All are now mostly exhausted. Most were coextensive with oil fields. In Indiana the gas field areas lie across the crest of a broad structural arch, in Ohio mostly east of the axis of a northward pitching anticline, according to published subsurface structure maps. Descriptions of local structures have not been published. Subsurface anticlines with 125 feet of structural closure, beds dipping at the rate of 40–120 or more feet per mile, and north-south axes are known in Indiana. In Ohio a fault with 100–200 feet of displacement and a general north-south trend is known. In Indiana at least, it appears that the commonly published structural form and attitude of the Cincinnati arch are not correct. The gas- and oil-bearing horizons occur in a dolomitic limestone zone of Ordovician age, mostly in the upper 50 feet of the “Trenton” formation. Porosity and percentage content of magnesium carbonate are considered to have been the controlling factors in trapping the occurrences. The writer believes that folding and genetic sites have been at least equally important as factors in determining the occurrences. Paleogeologic action, with earth movements of the arch active before Pennsylvanian time and possibly as early as Lowville (Middle Ordovician) time, is probably an important determinant.