Cuttings and cores from test wells in 13 Mississippi counties were examined for igneous material. Samples appearing to be igneous were studied in 41 thin sections. Rocks were classified as intrusive dikes, extrusive volcanics, and basement granite. Alteration of rock types is moderate to severe.

Intrusive dike rocks and extrusive volcanic rocks are present at depths from 3,562 ft (1,085 m) to 10,010 ft (3,043 m). The basement granite observed is present at 11,010 ft (3,347 m) in Lafayette County in north Mississippi and 18,826 ft (5,738 m) in Jackson County in the coastal area.

Basement granite of Precambrian age represents the oldest stratigraphic interval recognized. Volcanic extrusive rocks and plutonic intrusive rocks have been interpreted to be present in sediments of Paleozoic through Cretaceous ages. Available age dates of the volcanic material indicate Jurassic to Late Cretaceous activity.

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