Because of the possible economic importance of the arched Ellenburger group, the identification of its subdivisions by insoluble residues and general character of well cuttings has been attempted with encouraging results.

To facilitate correlation of the Pennsylvanian and lower Permian strata, classification by "series" is proposed, based on unconformities and faunal changes of wide regional importance. Rearrangement of certain formations and groups is also advocated. Former stratigraphic names are retained where possible even though redefined or changed in rank.

Cross sections and maps reveal the Concho arch as an imposing northwest trending structural feature, denuded in the Llano uplift area. Uplift and erosion of Ordovician beds along this axis evidently began during pre-Mississippian time. Beds from Barnett to Strawn in age show marked thinning and considerable truncation as this regional feature is approached.

Attention is also given to the evidence of intermittent growth of the Ouachita-Marathon Mountains, the Electra and Muenster arches. The Bend flexure and other large structural features of this region are described.

Regional conditions affecting oil and gas migration and accumulation are discussed. These include the progressive development of structural trends and local folds, overlap of structurally high reservoirs by apparent source beds, differential pressures resulting from wedge-shaped overburden, distribution of sedimentary material, and the development of local sandstone or limestone reservoirs and stratigraphic traps. A "compaction-hydraulic theory" of oil migration is favored as the main cause of movement of oil into and laterally within the reservoir toward areas of less overburden and pressure.