Volcanic rocks have been reported from a number of exploratory wells located throughout central and northwestern Mississippi. K-Ar dates have been determined for two samples obtained from a cored igneous section in southern Humphreys County, Mississippi. A biotite analcimite taken from a depth of 1,287 m (4,224 ft) yielded an age of 91.3 ± 3.4 m.y. A phonolite from a depth of 1,576 m (5,172 ft) yielded an age of 78.3 ± 2.9 m.y. These K-Ar dates are in agreement with crosscutting relationships present in the local stratigraphy. In the vicinity of the well site, the volcanic section is overlain by transgressive chalk of the Upper Cretaceous Selma Group.