Deep wells that have been drilled in the part of the Gulf Coastal Plain south of the Ouachita Mountains have passed through gently inclined Cretaceous strata and entered Paleozoic rocks. The Paleozoic floor, which is relatively smooth, has a steep southerly dip; therefore, it has not been reached by wells at great distances from the mountains. It has, however, been reached at many places in a wide southwestward trending belt that extends from Oklahoma into Texas. The wells that are described in this paper include a few in southwestern Arkansas, a few in southeastern Oklahoma, and several in Grayson, Fannin, Lamar, and Red River counties, Texas. The Paleozoic rocks that have been penetrated by these wells are similar in character and age to the Paleozoic rocks exposed in t e Ouachita Mountains.

Some of the wells in the counties adjacent to Red River in Texas have entered pre-Carboniferous rocks. Areas of such rocks are perhaps southwestward extensions of the exposure of rocks of this age in the Ouachita Mountains in Oklahoma. The other wells within the area described entered rocks that are assigned to the Carboniferous or simply to the Paleozoic. There seems to be no evidence that any of the wells entered pre-Cambrian rocks.

The western boundary of the Ouachita Mountain facies of rocks seems to pass in a southerly direction through the western part of Grayson County, Texas. West of this boundary are Paleozoic rocks that are similar to those exposed in the Arbuckle and the Wichita Mountains and Criner Hills. This boundary, the writers believe, is marked by a southwestward continuation of the Choctaw fault from Oklahoma.

The axis of the Ouachita geosyncline--the geosyncline in which the rocks of the Ouachita Mountain facies were deposited--has an east-west trend in Arkansas, but it swings toward the southwest in Oklahoma and crosses Red River into Texas. The part of the geosyncline extending from Arkansas across Oklahoma into Texas thus displays an arcuate form.