Coys Hill Porphyritic Granite Gneiss (MADchgr;0)

State	Massachusetts
Name	Coys Hill Porphyritic Granite Gneiss
Geologic age	Lower Devonian
Original map label	Dchgr
Comments	Part of Merrimack Belt (Silurian, Devonian and Pennsylvanian Rocks) Secondary unit description per MA003 as reported in USGS Geologic Names lexicon (ref. MA024): Coys Hill Granite named by Emerson (1898) of Early Devonian age is here renamed Coys Hill Porphyritic Granite Gneiss to more accurately reflect its character in MA. Characterized by variously deformed tabular microcline and locally sodic plagioclase phenocrysts 3 to 10 cm long in a finer matrix of quartz, plagioclase, and biotite with or without muscovite, garnet, and sillimanite (Field, 1975). May be continuous with Cardigan pluton of Kinsman Quartz Monzonite of Billings (1956) (P.J. Thompson, 1983, unpub. data). (Authors note that Kinsman should also be called a granite under current terminology.) Lenses of hornblende-pyroxene gneiss within the Coys Hill unit are mapped separately (Robinson and Luttrell, 1985).
Primary rock type	granitic gneiss
Secondary rock type
Other rock types
Lithologic constituents	Major Metamorphic > Gneisscoarse-grained porphyritic microcline granite gneiss, commonly containing garnet and sillimanite with or without muscovite
Map references	Unpublished Digital Geologic Map of Massachusetts received from Rudi Hon at Boston College in 1998.
Unit references	Zen, E-An (ed.), Goldsmith, R. (comp.), Ratcliffe, N.M. (comp.), Robinson, P. (comp.), Stanley, R.S. (comp.), Hatch, N.L., Jr., Shride, A.F., Weed, E.G.A., Wones, D.R., 1983, Bedrock Geologic Map of Massachusetts: U.S. Geological Survey, Reston, VA, scale 1: 250,000. Robinson, Peter and Luttrell, G.W., 1985, Revision of some stratigraphic names in central Massachusetts, IN Stratigraphic notes, 1984: U.S. Geological Survey Bulletin, 1605-A, p. A71-A78. [http://pubs.er.usgs.gov/publication/b1605A] USGS Geologic Names Lexicon (GEOLEX) [http://ngmdb.usgs.gov/Geolex/]
Geographic coverage	Hampden - Hampshire - Worcester