Cross sections and maps reveal the Concho arch as an imposing northwest trending structural feature, denuded in the Llano uplift area. The similarity of this arch to the Central Kansas uplift would be more obvious except for greater westward tilting of the Texas area. As in central Kansas, a comparatively thin Pennsylvanian section rests on truncated Ordovician and older beds along this broad axis. Uplift and erosion of the Ordovician evidently began during pre-Mississippian time. The Bend, and, to a greater degree, the Millsap Lake, Garner, and Mineral Wells beds show both pronounced thinning and some truncation over this broad regional feature.

Attention is also given to the evidence of progressive development of the Ouachita-Marathon mountains, the Electra and Muenster arches, the Bend flexure and other large structural features of this region.

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Because of the possible economic importance of the arched Ordovician beds, the identification of 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, a classification by "subseries" as in the northern Mid-Continent and West Texas regions has been used in this paper. Such procedure places emphasis on unconformities and faunal changes of wide regional importance. Former group names are retained but boundaries redefined where needed.

The probable influence of the regional conditions in this area upon oil and gas migration and accumulation is discussed, such as the overlap of structurally high reservoirs by apparent source beds; differential pressures resulting from wedge-shaped overburden; progressive development of structural trends and local folds; and the character and distribution of sedimentary material, especially the development of local sandstone or limestone reservoirs and stratigraphic traps.

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