Includes light-gray to medium-light gray, aphanitic to fine-grained metarhyolite containing grayish-yellow potassium feldspar phenocrysts; and, medium-gray to medium-dark-gray tuffaceous metarhyolite containing potassium feldspar phenocrysts and quartz-filled amygdules. Metarhyolite occurs as dikes up to 50 m thick, cutting Grenville basement and the Swift Run Formation (Southworth, 1991); also as cream-colored rhyolitic metatuff which is interbedded with metabasalt (CZc) stratigraphically near the base of that unit in Loudoun and northwestern Fauquier Counties (Nickelsen, 1956; Gathright and Nystrom, 1974). Metarhyolite at two localities in Loudoun County has been dated at 564 ± 9 Ma and 572 ± 5 Ma (U-Pb zircon; Aleinikoff and others, 1991).
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.