Basalt (Mole Hill, Rockingham County, Brent, 1960), labradorite, augite, olivine, magnetite, serpentine, chlorite, dolomite, epidote(?). Radiometric age: 47± 1 Ma (Eocene, Wampler and Dooley, 1975). Dikes in Highland and adjacent counties are of three types: Mafic rocks, felsic rocks, and breccia. Mafic rocks are dark gray to black and range from aphanitic to porphyritic in texture. Phenocrysts include plagioclase, clinopyroxene and, occasionally, olivine and biotite. Amygdules may be present, and xenoliths of country rock are common. Felsic rocks are mineralogically variable and texturally complex.They are light to medium-gray when fresh and are generally porphyritic with phenocrysts of plagioclase, biotite, hornblende, orthopyroxene, and orthoclase. Breccia (or "diatreme") contain abundant xenoliths of older igneous rock, sedimentary county rock, and xenocrysts of various minerals. Radiometric dates for some of the intrusive rocks are Eocene in age (Fuller and Bottino, 1969). Field relationships of other bodies suggest Mesozoic emplacement. Alkalic intrusive rocks, including nepheline-syenite, teschenite-syenite, and teschenite-picrite occur in Augusta County (Rader, 1967). Diabase dikes occur in several counties and are shown in red on the map.
Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000.
Virginia Division of Mineral Resources, 2003, Digital Representation of the 1993 Geologic Map of Virginia, Publication 174, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, scale 1:500,000.