a group of rolling hills projects westward for a mile or more beyond the Reynosa escarpment. Such rounded hills (not mesas) are also present west of the Schott- A viators field, four miles south of Mirando City, but are rarely pres ent at other points along the Reynosa scarp. The topography and the scanty geologic evidence obtainable from exposures thus suggest small anticlines with north-south axes at the Ojuelos and Schott- A viators fields in southeastern Webb County.
Gentle opposing tilts of the siliceous sandstone cappings of the mesas which partially surround the Mirando Valley oil field in northeastern Zapata County indicate the presence there of a small dome or nearly equidimensional anticline.
The large reentrant in the Reynosa scarp near Randado, in northwestern Jim Hogg County, with an extensive east ward projection of older rocks into the reentrant, suggests the presence of an anticline in this region. An oil field has been discovered in northwestern Jim Hogg County since the writers last visit, which furnishes confirmatory evidence of such a structure.
Faulting may have played a minor role in the causation of petroleum and natural gas accumulation in the Webb- Zapata and Jim Hogg county fields but the surface evidence points to small anticlines or, as Sellards 84 suggests, anti clinal noses as the important agencies in the accumulation of petroleum and natural gas.
The widening of the Gueydan outcrop and the broad reentrant of Gueydan in the vicinity of Moglia, in east central Webb County, suggests the existence of a north-south structural terrace or of gentle anticlinal and synclinal fold ing in this region. Important additional evidence of fold ing here is afforded by the predominance of north dips of 1-5° at the northern margin of this reentrant of Gueydan. The Carolina-Texas gas wells west of Moglia may owe their existence to one of these inferred structural features. Some doubt is cast on the validity of folding in this region by the
84 Sellards, E. H., "Notes on the Oil and Gas Fields of Webb and Zapata Counties," Univ. Texas Bull. No. 2230, pp. 5-29, 1922.