Basalt necks occur on the surface of Uvalde and Kinney Counties in southwest Texas. These basalt necks are the "serpentine plugs" that produce from the Taylor section in the subsurface of Zavala County. Many plugs are present in both the surface and subsurface.

Geology of the serpentine plugs indicates that most of the volcanic activity occurred as post-Austin subaqueous extrusions. Formations below the volcanic material show no structural deformation. Formations above the Cretaceous show evidence of the pile of volcanic material in few places. The most significant evidence of structure and faulting is within the Taylor section.

Strategically placed seismic lines will give obvious evidence of the existence of a serpentine plug.

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