The occurrence and distribution of much oil and gas in the Mid-Continent region have been influenced by the structural deformation of the belt of greatly deformed rocks of the Ouachita Mountain facies. They have also been influenced by the effect of the deformation of these rocks on the adjacent regions. Also, the structural history and character of the rocks on either side of the Ouachita belt are such that the belt separates areas yielding Cretaceous and Tertiary oil and gas on its gulfward side from areas of Paleozoic oil and gas northwest of it.

The belt of deformed rocks occupies a wide Paleozoic geosyncline, with arcuate bends, that is largely concealed beneath the Cretaceous and younger rocks of the Gulf Coastal Plain. The rocks of the geosynclinal belt are of Paleozoic age and their only exposures are in the Ouachita Mountains of Oklahoma and Arkansas and in the Solitario and Marathon regions of Texas. The Ouachita Mountains form the northernmost culmination of a great arc in the course of the geosyncline. Deep well data appear to indicate that the arc reaches from central Texas past the Ouachita Mountains and thence to southern Alabama. There is a significant parallelism between the course of this arc and the belts of outcrop of the Cretaceous and younger rocks in the Gulf region. The numerous masses of alkali-rich igneo s rocks of Upper Cretaceous and early Tertiary age in and near the Gulf region are confined to the geosynclinal belt. They include many occurrences of serpentine which yields commercial oil along and near the Mexia fault zone in central Texas. This fault zone, which extends from Texas into Arkansas and Louisiana, is interpreted by the writer as marking the boundary that separates pre-Cambrian crystalline rocks of the Paleozoic land Llanoria from the Paleozoic rocks of the geosynclinal belt.