Porphyritic albite-felsite sills and dikes are exposed in west-central Virginia in Highland County. Two biotite separates have K-Ar ages of 46.9 ± 1.1 and 47.2 ± 1.3 m.y. Seven whole-rock and the two biotite separates were analyzed for Rb, Sr, and Sr isotopic composition. These results indicate that the felsite is 37 ± 3 m.y. old; because of the very low percentage of radiogenic Sr87 in the whole-rock and biotite samples the Rb-Sr age is considered to be less reliable than the K-Ar age.
These results establish a location for Eocene igneous activity in the south-central Appalachian area. The period of time during which igneous activity occurred along the 38th-parallel lineament has been extended considerably. Highland County, Virginia, may have been at least one of the sources of volcanic ash found in sediments from the Coastal Plain of North America. The initial Sr87/Sr86 ratio of 0.704 indicates that the felsite magma was derived from the lower crust or upper mantle.