Cheshire Quartzite (Lower Cambrian) - Mainly pure, white, glassy, tough quartzite

Dalton Formation (including Poughquag Quartzite and Lowerre Formation) (Lower Cambrian and perhaps partly older) - Gray, tan-weathering, medium grained, generally well layered gneiss or feldspathic quartzite, composed of quartz, microcline, plagioclase, muscovite, biotite, and generally tourmaline; some schistose micaceous layers have sillimanite, commonly as quartz-sillimanite nodules rimmed with muscovite. Layers of purer quartzite in many areas, especially near the top or where the formation is thin

Everett Schist (Cambrian?) - Grayish to greenish (some rusty-weathering), fine-to medium-grained, foliated but poorly layered schist or phyllite, composed or quartz, albite or oligoclase, muscovite, garnet, staurolite or chloritoid, and generally chlorite. Local layers are dark-gray to silvery schist or phyllite

Hoosac Schist (Cambrian?) - Light-to medium-gray, rusty-weathering, fine- to medium-grained schist and poorly layered schistose gneiss, composed of quartz, biotite, plagioclase, muscovite, and generally garnet and sillimanite or kyanite

Manhattan Schist (including Waramaug Formation and Canaan Mountain Schist) - (Cambrian?) - Dark-gray to silvery, rusty-weathering, generally coarse grained, foliated but poorly layered to massive gneiss or schistose gneiss, composed of quartz, oligoclase, microcline, biotite, and muscovite, and generally sillimanite and garnet. Amphibolite layers locally, especially near base where in places separately mapped as unit Cma

Amphibolite-bearing unit of Manhattan Schist (Cambrian?) - Like Manhattan Schist but with numerous lenses and layers of amphibolite

Lower slice of Canaan Mountain Schist (Cambrian?) - Dark-gray, rusty-weathering, coarse-grained, well-foliated and moderately well layered schist composed of quartz, plagioclase, biotite, muscovite, and generally garnet and sillimanite (or minor staurolite). Amphibolite layers rather rare

Upper slice of Canaan Mountain Schist (Cambrian?) - Dark-gray to silvery, generally rusty weathering, medium- to coarse-grained, well-foliated, massive to well-layered schist and schistose gneiss, composed of quartz, plagioclase, biotite, muscovite, and generally garnet and sillimanite; also layers of amphibolite

Basal member of upper slice of Canaan Mountain Schist (Cambrian?) - Gray, generally rusty- or tan-weathering gneiss, composed of quartz, plagioclase, microline, biotite, and muscovite, interlayered with feldspathic quartzite

Unit a [of Stockbridge Marble] (Lower Cambrian) - White to pale-gray, massive, smooth-weathering dolomite marble

Unit b [of Stockbridge Marble] (Upper and Middle? Cambrian) - White, pink, cream, and light-gray, generally well-bedded dolomitic marble interlayered with phyllite and schist and with siltstone, sandstone, or quartzite, commonly dolomitic

Unit c [of Stockbridge Marble] (Upper Cambrian) - Gray, generally massive dolomite marble, commonly contains quartz grains, locally beds of sandstone; may be calcitic near top

Waterbury Gneiss (Proterozoic Z or Cambrian or both) - Medium- to dark-gray, fine- to medium-grained, generally irregularly foliated and lenticular rather than regularly layered schist and schistose gneiss, composed of biotite, quartz, oligoclase, kyanite (or sillimanite), and garnet, also locally microcline, irregularly mixed with granitoid gneiss, composed of oligoclase or andesine, quartz, biotite, and commonly microcline and muscovite

Foliated quartz diorite (Devonian in part, probably Ordovician in part) - Mainly dark-gray, medium-grained, well-foliated gneiss (locally strongly sheared, especially near contacts), composed of plagioclase, quartz, biotite, and hornblende, locally also pyroxene (? denotes uncertainty about the designation of D (Devonian) for the age of the geologic formation).

Scotland Schist (Devonian or Silurian or both) - Gray to silvery, locally rusty, fine- to medium-grained schist, composed of quartz, muscovite, biotite, staurolite, and oligoclase, locally with kyanite or sillimanite; interlayered, especially below and to the west, with quartz-oligoclase-biotite schist and granofels and locally with quartzite

Quartzite unit [in Scotland Schist] (Devonian or Silurian or both) - Quartzite, generally micaceous, interlayered with mica schist

The Straits Schist (= Goshen Formation of Massachusetts) (Devonian or Silurian or both) - Silvery to gray, non-rusty, coarse- to very coarse grained, generally poorly layered schist, composed of quartz, muscovite, biotite, oligoclase, garnet, and commonly staurolite and kyanite or sillimanite; graphitic almost throughout

Southington Mountain Member [of The Straits Schist] (Devonian or Silurian or both) - Gray to silvery, non-rusty, medium-grained, well-layered alternating schist and granofels, composed of quartz, oligoclase, muscovite, biotite, and garnet, commonly with staurolite and kyanite (or sillimanite); schist commonly graphitic

Wepawaug Schist (Devonian or Silurian or both) - Medium- to dark-gray, medium- to fine-grained, well-layered schist or phylite and metasiltstone, composed of quartz, muscovite or sericite, plagioclase, biotite, and in appropriate metamorphic zones chlorite, garnet, staurolite, and kyanite. Schist or phyllite generally graphitic

Littleton Formation (Devonian) - Gray to silvery, generally non-rusty, medium-grained, massive to well-layered alternating schist and micaceous quartzite, composed of quartz, muscovite, biotite, garnet, and oligoclase, also staurolite, graphite, and ilmenite, and in certain areas kyanite or sillimanite in schist

Littleton Formation - see Dbl (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Mount Pisgah Member of Littleton Formation (Devonian) - Gray, medium-grained, well-layered (locally graded) granofels or micaceous quartzite with some schist, composed of quartz, oligoclase, biotite, garnet, and sillimanite

Canterbury Gneiss (may be equivalent to Ayer Granite of Massachusetts) (Devonian) - Light-gray, medium-grained, variably foliated, locally strongly lineated gneiss, composed of quartz, oligoclase, microcline, and biotite, locally also muscovite or epidote, and generally with megacrysts 1 to 2 cm long of either or both feldspars

'Eastford gneiss phase' [of Canterbury Gneiss] (Devonian) - Mainly light gray, medium-grained, foliated to strongly lineated gneiss, composed of quartz, microcline, oligoclase or albite, biotite, and muscovite

Erving Formation (Devonian) - Gray, medium-grained, well-foliated and generally well layered granofels and schist, composed of quartz, plagioclase, and biotite, also muscovite in schist, and accessory garnet and kyanite

Foliated granitic gneiss (Devonian?) - Light-gray, coarse-grained, strongly to weakly foliated gneiss, composed of phenocrysts of K-feldspar in a groundmass of plagioclase, quartz, K-feldspar, and biotite, with accessory sillimanite and garnet

Lebanon Gabbro (Devonian) - Dark, speckled, coarse-grained, massive but locally sheared gabbro, composed of hornblende, labradorite, and opaques. Some bodies contain biotite and quartz; some smaller ones are nearly pure hornblende with local augite

Dioritic phase [of Lebanon Gabbro] (Devonian) - White to black, streaked, medium-grained, foliated or sheared gneiss, composed of plagioclase, biotite, quartz, and generally hornblende

Lamprophyre (Devonian?) - Dark-gray to greenish, fine-grained, badly altered dike rock, composed of biotite, augite, K-feldspar, and accessory apatite and sphene, plus secondary minerals

Maromas Granite Gneiss (Devonian?) - Light-gray to buff, medium- to fine-grained granitic gneiss, composed of quartz and microcline with minor plagioclase and biotite. Central body is massive, but outlying strips are foliated and have accessory hornblende or garnet. Massive parts may be young anatectic intrusive rocks; foliated parts may include older felsic metavolcanic rocks belonging to unit Ochv. Pegmatite bodies are common in the vicinity

Hornblende norite (Devonian?) - Dark, coarse-grained, massive rock, composed of bytownite, hornblende, and hypesthene

Nonewaug Granite (Devonian) - White to pink, fine- to very coarse grained (commonly pegmatitic), massive to layered granite composed of albite, microcline, quartz, and muscovite, with minor biotite and garnet. Microcline commonly graphic; quartz and muscovite commonly in plumose aggregates

Buttress Dolerite (Middle? Jurassic) - Dark-gray to greenish-gray (weathers brown or gray), medium- to fine-grained, commonly porphyritic, generally massive with well-developed columnar jointing, grading from basalt near contacts to fine-grained gabbro in the interior, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine

Buttress Dolerite - see Jb (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

East Berlin Formation (Lower Jurassic) - Maroon siltstone, silty and sandy shale, and fine-grained silty sandstone, generally well laminated and commonly well indurated, alternating with dark fissile shale; dolomitic carbonate common in cement, concretions, and thin argillaceous laminae. Local arkose; grades eastward into coarse conglomerate close to eastern border fault

Hampden Basalt (Lower Jurassic) - Greenish-gray to black (weathers bright orange to brown), fine- to medium-grained, grading from basalt near contacts to fine-grained gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass

Holyoke Basalt (Lower Jurassic) - Greenish-gray to black (weathers bright orange to brown), fine- to coarse-grained, grading from basalt near contacts to gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass

Portland Arkose (Lower Jurassic) - Reddish-brown to maroon micaceous arkose and siltstone and red to black fissile silty shale. Grades eastward into coarse conglomerate (fanglomerate) 'Meriden Formation' of Krynine (1950) (Lower Jurassic)

Silicified rock and mylonite along Mesozoic faults (probably mainly Jurassic) - Close network of quartz veins and veinlets cutting each other and older rock, which is mostly replaced by very fine grained quartz. In places, incompletely replaced rock shows strongly mylonitic texture.

Shuttle Meadow Formation (Lower Jurassic) - Maroon to dark-gray, silty shale, siltstone, and fine-grained silty sandstone, generally well and thinly laminated. In the southern part of the State includes a layer, up to 5 m thick, of blue, commonly sandy, fine-grained limestone or dolomitic limestone, grading laterally into calcareous siltstone. Coarser and more arkosic to east and south, grading into conglomerate near the eastern border fault

Talcott Basalt (Lower Jurassic) - Greenish-gray to black (weathers bright orange to brown), fine- to medium-grained, grading from basalt near contacts to fine-grained gabbro in the interior, composed of pyroxene and plagioclase with accessory opaques and locally olivine or devitrified glass. Pillows in may places; volcanic breccia with fragmentary pillows in others

West Rock Dolerite (Lower Jurassic) - Dark-gray to greenish-gray (weathers bright orange to brown), medium- to fine-grained, grading from basalt near contacts to fine-grained gabbro in the interior, generally massive with well-developed columnar jointing, composed of plagioclase and pyroxene with accessory opaques and locally devitrified glass, quartz, or olivine

West Rock Dolerite - see Jwr (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Rowe Schist (Lower Ordovician or Cambrian or both) - Light-gray to silvery, fine- to medium-grained, generally poorly layered schist composed of quartz, muscovite, biotite, oligoclase, and generally garnet, staurolite, and kyanite or sillimanite. Layers of granofels common; also some layers of amphibolite, quartz-spessartine rock (coticule), and calc-silicate rock

 Rowe Schist and Amphibolite unit in Rowe Schist undivided -  see OCr and OCra

Rowe Schist - see OCr (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Stockbridge Marble (including Inwood Marble (Lower Ordovician and Cambrian) - White to gray, massive to layered marble, generally dolomitic but containing calcite marble in upper part, locally interlayered with schist or phyllite and with calcareous siltstone or sandstone

Allingtown Metavolcanics (Middle? Ordovician) - Green, fine-grained, massive greenstone, composed of epidote, actinolite, albite, and chlorite, commonly with abundant megacrysts of saussurite, interlayered with minor green phyllite, generally containing quartz and sericite. Dark amphibole in western outcrops

 Allingtown Metavolcanics and Maltby Lakes Metavolcanics undivided -  see Oa and Oma

Brookfield (dioritic and granodioritic) Gneiss (including Newtown Gneiss of Crowley, 1968) (Middle? Ordovician) - Dark and light, commonly speckled or banded, medium- to coarse-grained, massive to poorly foliated gneiss, composed of plagioclase, biotite, and hornblende, generally with quartz and K-feldspar, the latter commonly as megacrysts 1 to 3 cm across (also plagioclase megacrysts in darker rocks), locally associated with amphibolite or hornblende schist

Brimfield Schist (includes Hamilton Reservoir Formation) (Upper? and Middle Ordovician) - Gray, rusty-weathering, medium- to coarse-grained, interlayered schist and gneiss, composed of oligoclase, quartz, K-feldspar, and biotite, and commonly garnet, sillimanite, graphite, and pyrrhotite. K-feldspar partly as augen 1 to 3 cm across. Minor layers and lenses of hornblende and pyroxene-bearing gneiss, amphibolite, and calc-silicate rock

Brimfield Schist - see Obr (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Gneiss (metavolcanic) member [of Brimfield Schist] (Upper? and Middle Ordovician) - Medium-gray, medium-grained, layered gneiss and schist, composed of oligoclase, quartz, and biotite; some gneiss and most schist layers contain garnet and sillimanite; some gneiss layers contain garnet, hornblende, or pyroxene or grade into amphibolite or calc-silicate rock. Probably includes metavolcanic rocks

Bristol Gneiss (Middle? Ordovician) - Light, medium-grained, massive to well-layered gneiss, composed of plagioclase, quartz, and biotite, also muscovite and garnet in many layers, interlayered in places with dark amphibolite

Collinsville Formation (Middle Ordovician) - Mixture of rock types as described for the two members; in many areas felsic and mafic striped metavolcanic rocks predominate

Hornblende gneiss member [of Collinsville Formation] (Middle Ordovician) - Dark, fine- to medium-grained, well-layered amphibolite and hornblende gneiss, composed or hornblende and plagioclase, commonly with biotite, garnet, or epidote, interlayered with light-gray felsic gneiss and pink quartz-spessartine rock (coticule). Grades into Bristol Gneiss

Collins Hill Formation ( = Partridge Formation of New Hampshire) (Upper? and Middle Ordovician) - Gray, rusty-weathering, medium- to coarse-grained, poorly layered schist, composed of quartz, oligoclase, muscovite, biotite, and garnet, and commonly staurolite, kyanite, or sillimanite, generally graphitic, interlayered with fine-grained two-mica gneiss, especially to the west, and with calc-silicate and amphibolite layers, also rare quartz-spessartine (colicule) layers

Metavolcanic member [of Collins Hill Formation] (Upper? and Middle Ordovician) - Ranges from mafic to felsic, from dark layered amphibolite and hornblende schist, locally with garnet or epidote, to light-gray (in places purplish), laminated gneiss, composed of quartz, oligoclase, and biotite, in which some layers contain garnet (generally manganiferous) and hornblende or cummingtonite

Cobble Mountain Formation (Middle Ordovician) - Gray to silvery (not rusty), medium- to coarse-grained, generally layered schist and granofels, composed of quartz, oligoclase, muscovite, biotite, and garnet, and locally kyanite and staurolite or sillimanite. Some amphibolite layers

Sweetheart Mountain Member [of Collinsville Formation] (Middle Ordovician) - Gray and silvery (not rusty), medium- to coarse-grained, poorly layered schist, composed of quartz, oligoclase, biotite, muscovite, and garnet, and in places kyanite or sillimanite. Amphibolite layers common; also layers of quartz-spessartine rock (coticule)

Ordovician(?) granitic gneiss (including local terms Ansonia, Mine Hill, 'Tyler Lake', 'Siscowit') (Middle Ordovician?) - White, light-gray, buff, or pink, generally foliated granitic gneiss, composed of sodic plagioclase, quartz, microcline, muscovite, and biotite, and locally garnet or sillimanite. Commonly contains numerous inclusions or layers of mica schist and gneiss

Ordovician? granitic gneiss - see Og (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Golden Hill Schist (may be equivalent to part of Trap Falls Formation) (Lower? Ordovician) - Gray to silvery, medium- to coarse-grained, generally layered schist and granofels, composed of quartz, muscovite, biotite, plagioclase, and garnet

Golden Hill Schist - see Ogh (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Glastonbury Gneiss (Middle? Ordovician) - Gray, medium- to coarse-grained, massive to well-foliated granitoid gneiss composed of oligoclase, quartz, microcline, and biotite, (as patches), also epidote and hornblende in many areas, commonly associated with layers of amphibolite; elsewhere minor muscovite and garnet

Harrison Gneiss (including Prospect Gneiss) (Middle? Ordovician) - Interlayered dark- and light-gray, medium-grained, well-foliated gneiss, composed of andesine, quartz, hornblende, and biotite (also locally K-feldspar as megacrysts 1 to 5 cm long). Thought to be metavolcanic equivalent of unit Ob

Harrison Gneiss - see Oh (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Beardsley (hornblendic) Member [of Harrison (Prospect) Gneiss] (Middle? Ordovician) - Gray to dark-gray, medium-grained, lineated gneiss

Hawley Formation (carbonaceous schist facies) (Middle Ordovician) - Gray, rusty-weathering, fine- to medium-grained, generally layered schist and granofels, composed of quartz, oligoclase, and biotite; some muscovite and graphite, rare garnet and kyanite or sillimanite. Layers of quartz-spessartine rock (coticule) common

Nodular member [of Harrison Gneiss] (Middle? Ordovician) - Harrison Gneiss containing prominent quartz-sillimanite nodules

Pumpkin Ground (porphyritic) Member [of Harrison (Prospect) Gneiss] (Middle? Ordovician) - Medium- to light-gray, medium- to coarse-grained, well-layered and foliated gneiss, composed of oligoclase, microcline, quartz, and biotite; some layers have numerous microcline megacrysts 1 to 5 cm across; others have hornblende. Minor layers of garnetiferous schist and gneiss

Litchfield Norite (Middle? Ordovician) - Dark, medium- to coarse-grained, mostly massive mafic rock (olivine norite, quartz norite, hypersthene pyroxenite), composed of labradorite, hypersthene, augite, and olivine in varying proportions, also hornblende and biotite (and minor quartz in quartz norite). Associated with small mineral deposits of pyrrhotite, pentlandite, and chalcopyrite

Middletown Formation ( = Ammonoosuc Volcanics of New Hampshire) (Middle Ordovician) - Heterogeneously interlayered dark- to light-gray, generally medium grained gneiss and granofels, ranging from quartz-biotite gneiss through felsic amphibolite gneiss to amphibolite and characteristically containing anthophyllite or cummingtonite with or without hornblende. Also layers of calc-silicate rock and of biotite gneiss with quartz-sillimanite nodules

Middletown Formation -  see Om (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Maltby Lakes Metavolcanics (Middle? Ordovician) - Green to gray-green, fine-grained, massive to well-foliated and layered greenstone, greenschist, and schist; also dark amphibolite to west and southwest

Lower part [of Maltby Lakes Metavolcanics] (Middle? Ordovician) - Gray-green to green, fine-grained, generally well foliated greenschist, greenstone, and schist or phyllite, composed of albite and chlorite, plus quartz and sericite or epidote and actinolite, mixed metavolcanic and metasemidentary rocks

Upper part [of Maltby Lakes Metavolcanic] (Middle? Ordovician) - Green to gray-green, fine-grained, layered and foliated to massive greenstone and greenschist, composed of epidote, albite, actinolite, and chlorite, and locally minor quartz, sericite, garnet, pyrite, or calcite. Mainly metavolcanic

Lower member [of Middletown Formation] (Middle Ordovician) - Dark- to medium-gray amphibolite and hornblende gneiss, commonly with garnet, diopside, or epidote, interlayered with light-gray gneiss composed of oligoclase, quartz, biotite, and generally one or more amphiboles, also garnet

Massive mafic rock [in Middletown Formation] (Middle Ordovician) - Dark, coarse-grained, massive amphibolite and metagabbro, composed of hornblende and plagioclase; in places with quartz and epidote, in others with patches of actinolite or anthophyllite, chlorite, and epidote or garnet. May be intrusive

Monson Gneiss (may be equivalent to part of Waterford Group) (Middle or Lower Ordovician?) - Interlayered light to dark, mostly medium to coarse-grained gneiss and amphibolite; gneiss composed of plagioclase, quartz and biotite, with hornblende in some layers and microcline in others; traces of garnet, epidote, and magnetite

Upper member [of Middletown Formation] (Middle Ordovician) - Light-gray, generally rusty weathering, well-layered gneiss and granofels, composed of oligoclase, quartz, biotite, and amphibole (cummingtonite, anthophyllite, gedrite, or hornblende, or several of these), also garnet and chlorite. Many layers of amphibolite and biotite gneiss throughout

Oronoque Schist (Lower? Ordovician) - Gray to silver, medium- to fine-grained, well-layered to laminated schist and granofels, composed of quartz, oligoclase or albite, muscovite or sericite, biotite or chlorite, and in western belt local garnet, staurolite, and kyanite. Small lenses of amphibolite or greenstone

Preston Gabbro (Middle Ordovician or older) - Dark, medium- to coarse-grained, mainly massive gabbro, composed of labradorite, augite, and opaques, generally with hornblende, locally hypersthene or olivine or both

 Preston Gabbro and Quinebaug Formation undivided -  see Op and Oq

Dioritic phase [of Preston Gabbro] (Middle Ordovician or older) - Medium- to dark-gray, streaked or speckled, medium-grained diorite and quartz diorite, gneissic where sheared near contact, composed of plagioclase, hornblende, and biotite, and locally quartz and relic pyroxene

Quinebaug Formation (Middle Ordovician or older) - Medium- to dark-gray, commonly greenish, medium-grained, well-layered gneiss, composed of hornblende, andesine, biotite, and epidote, commonly with quartz or garnet, interlayered with amphibolite

Black Hill Member [of Quinebaug Formation] (Middle Ordovician or older) - Gray, medium- to fine-grained, well-layered schist and granofels, composed of oligoclase, quartz, and biotite, commonly with hornblende or muscovite, and locally with calcite, garnet, or epidote

Felsic gneiss member [of Quinebaug Formation] (Middle Ordovician or older) - Light- to medium-gray, fine- to medium-grained gneiss, composed of plagioclase, quartz, biotite, and muscovite, commonly with K-feldspar

Ratlum Mountain Schist (Lower? Ordovician) - Gray, medium-grained, interlayered schist and granofels, composed of quartz, oligoclase, muscovite (in the schist), biotite, and garnet, also staurolite and kyanite in the schist. Numerous layers and lenses of amphibolite; also some of quartz-spessartine (coticule) and calc-silicate rock

 Ratlum Mountain Schist and Amphibolite unit in Ratlum Mountain Schist undivided -  see Or and Ora

Ratlum Mountain Schist - see Or (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Amphibolite unit [in Ratlum Mountain Schist] (Lower? Ordovician) - Black or mottled, generally massive amphibolite and hornblende gneiss, composed of hornblende and andesine, commonly with minor quartz and magnetite, and locally with garnet, biotite, and epidote

Units e and d [of Strockbridge Marble] (Lower Ordovician) - White to gray massive calcite marble, commonly mottled with dolomite and locally interlayered with dolomite marble and calcareous siltstone and sandstone

Units g and f [of Strockbridge Marble] (Lower Ordovician) - White to gray massive calcite marble with layers and laminae of dolomitic marble

Taine Mountain Formation (Lower? Ordovician) - Gray, medium-grained, generally fairly well layered to well-laminated ('pin-stripe') gneissic or schistose granofels, composed of quartz, oligoclase, biotite, muscovite, and garnet, and locally staurolite and kyanite or sillimanite

Taine Mountain and Collinsville Formation undivided - see Ot and Oc

Tatnic Hill Formation (Upper? and Middle Ordovician) - Medium- to dark-gray, medium-grained gneiss or schist composed of quartz, andesine, biotite, garnet, and sillimanite, locally kyanite, muscovite, or K-feldspar, interlayered with locally mappable units and thinner layers of rusty-weathering graphitic pyrrhotitic two-mica schist, amphibolite, and calc-silicate rock

Fly Pond (calc-silicate) Member [of Tatnic Hill Formation] (Upper? and Middle Ordovician) - Light-gray, medium-grained, layered to massive calc-silicate gneiss, composed of andesine, quartz, hornblende or actinolite, epidote, and commonly diopside, biotite, and scapolite; some layers are calcitic

Yantic Member [of Tatnic Hill Formation] (Upper? and Middle Ordovician) - Medium- to dark-gray, fine- to medium-grained schist, composed of quartz, oligoclase, biotite, and muscovite, some layers with garnet, staurolite, and kyanite or garnet and sillimanite, local epidote or K-feldspar; some layers of rusty-weathering graphitic, pyrrhotitic, two-mica schist

Basal member [of Taine Mountain Formation] around Waterbury dome (Lower? Ordovician) - Differs from rest of Taine Mountain Formation in being especially well layered and generally less micaceous and schistose

Trap Falls Formation (may be equivalent in part to Golden Hill Schist) (Middle or Lower Ordovician) - Gray to silvery, partly rusty weathering, medium-grained, generally well layered schist, composed of quartz, sodic plagioclase, biotite, muscovite, and garnet, locally with sillimanite or kyanite, interlayered with two-mica gneiss and granulite and with amphibolite

 Trap Falls Formation and Ordovician? granitic gneiss undivided -  see Otf and Og

Carringtons Pond Member [of Trap Falls Formation] (Middle or Lower Ordovician) - Interlayered medium- to dark-gray, rusty-weathering, medium-grained schist and light-gray, fine- to medium-grained gneiss, composed of quartz, sodic plagioclase, biotite, muscovite, and garnet; schist locally contains sillimanite or kyanite; gneiss locally contains K-feldspar; amphibolite layers common

Schist and granulite member [of Trap Falls Formation] (Middle or Lower Ordovician) - Interlayered gray to silvery, medium- to coarse-grained schist and fine-grained granofels, composed of quartz, sodic plagioclase, biotite, and muscovite; garnet common in schist

Shelton (white gneiss) Member [of Trap Falls Formation] (Middle or Lower Ordovician) - White, light-gray, or buff, fine- to medium-grained, generally well foliated granitic gneiss, composed of sodic plagioclase, quartz, microcline, muscovite, and garnet (in tiny almost ubiquitous grains), also commonly minor biotite; generally interlayered with mica schist, biotite gneiss, and calc-silicate rock. Thought to be metavolcanic equivalent of unit Og

Scranton Mountain Member [of Taine Mountain Formation] (Lower? Ordovician) - Gray, rusty-weathering, medium-grained schist, composed of quartz, muscovite, biotite, plagioclase, garnet, and generally kyanite

Wildcat Member [of Taine Mountain Formation] (Lower? Ordovician) - Gray, medium-grained, generally fairly well layered to well-laminated ('pin-stripe') gneissic or schistose granofels, composed of quartz, oligoclase, biotite, muscovite, and garnet, and locally starolite and kyanite or sillimanite

Whigville Member [of Taine Mountain Formation] (Lower? Ordovician) - Gray, medium-grained, generally fairly well layered to well-laminated ('pin-stripe') gneissic or schistose granofels, composed of quartz, oligoclase, biotite, muscovite, and garnet, and locally staurolite and kyanite or sillimanite

Walloomsac Schist (Middle Ordovician) - Black to dark-or silvery-gray, rarely layered schist or phyllite, composed of quartz, albite, and commonly garnet and staurolite or sillimanite (locally strongly retrograded to chlorite and muscovite). Locally feldspathic or calcareous near the base

Basal marble member [of Walloomsac Schist] (Middle Ordovician) - Dark-gray to white, massive to layered schistose or phyllitic calcite-phlogopite marble

Narragansett Pier Granite (Permian) - Pink to red, medium- to coarse-grained (commonly pegmatitic), generally massive (not gneissic) granite, composed of microcline, oligoclase, quartz, and biotite and accessory muscovite and magnetite. Considerable associated pegmatite

Mafic phase [of Narragansett Pier Granite] (Permian) - Gray to reddish, medium-grained, generally massive granite, like Pn but with more biotite and locally hornblende

Porphyry (dacite or rhyolite) (Permian) - Light-colored, very fine grained, massive porphyry with phenocrysts of quartz, feldspar, and biotite; muscovite and accessory fluorite in ground mass

Pinewood Adamellite (Permian) - Light-gray, medium-grained, massive adamellite, composed of microcline, albite, quartz, and muscovite with accessory fluorite. High radioactivity

Syenite (Permian) - Medium-gray, massive syenite, composed of microcline, amphibole (arfvedsonite), and biotite with accessory apatite and sphene

Westerly Granite (Permian) - Light-gray, pink-weathering, fine-grained, massive, aplitic granite, composed of oligoclase or albite, quartz, and K-feldspar, with minor biotite and accessory muscovite, magnetite, allanite, and sphene

Mylonite along Paleozoic faults (Upper or Middle Paleozoic) - Mylonite, blastomylonite, and blastomylonitic gneiss, composed of intensely granulated quartz, plagioclase, biotite, and epidote, in places with hornblende or microcline and commonly with secondary minerals. In places has later been silicified (compare unit Jsi)

Lower member [of Bigelow Brook Formation] (Silurian or Ordovician or both) - Chiefly gray, medium-grained, well-layered granofels, composed of quartz, oligoclase, and biotite, commonly with garnet and sillimanite, interlayered with thinly fissile sillimanitic, graphitic, pyrrhotitic biotite schist and with calc-silicate rock

Middle member [of Bigelow Brook Formation] (Silurian and perhaps Ordovician) - Greenish-gray, medium-grained calc-silicate rock, composed of plagioclase, quartz, and diopside (locally hornblende and scapolite), interbedded with schist and granofels composed of plagioclase, quartz, biotite, and commonly garnet and sillimanite

Upper member [of Bigelow Brook Formation] (Silurian and perhaps Ordovician) - Chiefly gray, rusty-weathering, medium-grained, generally well layered and locally fissile schist, composed of plagioclase, quartz, biotite, garnet, and sillimanite, locally with K-feldspar or cordierite, fissile layers commonly with graphite and pyrrhotite, interlayered with quartzose granofels with less biotite but with calc-silicate minerals

Hebron Gneiss (Silurian and Ordovician) - Interlayered dark-gray, medium- to coarse-grained schist, composed of andesine, quartz, biotite, and local K-feldspar, and greenish-gray, fine- to medium-grained calc-silicate rock, composed of labradorite, quartz, biotite, actinolite, hornblende, and diopside, and locally scapolite. Local lenses of graphitic two-mica schist

Southbridge Formation (Silurian or Ordovician or both) - Dark- to light-gray, locally rusty, fine- to medium-grained interlayered granofels and schist, composed of quartz, plagioclase, and biotite, with muscovite in schist and amphibole, calc-silicate minerals, or K-feldspar in certain layers; also locally mappable units and thinner layers of calc-silicate rock, amphibolite, and sillimanite-garnet and sillimanite-graphite-pyrrhotite schist

Southbridge Formation -  see SOs (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Porphyritic member [of Southbridge Formation] (Silurian or Ordovician or both) - Light-to medium-gray, fine-grained porphyritic massive to layered gneiss, composed of quartz, oligoclase, microcline, and biotite, with megacrysts 1 to 2 cm long of microcline

Clough Quartzite (Silurian) - White, medium-grained, glassy to granular, well-layered quartzite and muscovitic quartzite, locally with garnet; conglomeratic (commonly with tourmaline) in lower part

Fitch Formation (Silurian) - Gray, fine- to medium-grained calc-silicate rock, composed of quartz, biotite, calcite, actinolite, diopside, microcline, and locally garnet, scapolite, or epidote, interlayered with two-mica schist

Basal member [of The Straits Schist] ( = Russell Mountain Formation of Massachusetts) (Silurian) - Distinguished by presence of layers of amphibolite, marble, calc-silicate rock, and quartzite within more uniform schist like that on either side. Minor, unevenly distributed mineralization in W, Bi, Cu, Ni, and other metals

New Haven Arkose (Upper Triassic; possibly Lower Jurassic at top) - Red, pink, and gray coarse-grained, locally conglomeratic, poorly sorted and indurated arkose, interbedded with brick-red micaceous, locally shaly siltstone and fine-grained feldspathic clayey sandstone

 New Haven Arkose and Buttress Dolerite undivided -  see TRnh and Jb

Gneiss of Highlands massifs, (including Fordham Gneiss) (Proterozoic Y; may contain some older rocks) - Mixture of rock types described below, where not separately mapped

Augen gneiss (including local term 'Danbury Gneiss', equivalent in part to Tyringham Gneiss of Massachusetts) (Proterozoic Y) - Medium-gray to spotted, fine- to medium-grained, porphyritic, foliated and lineated granitic gneiss, composed of microcline (largely as megacrysts or augen up to 10 cm long), quartz, albite, or oligoclase, biotite and minor hornblende

Hornblende gneiss and amphibolite (Proterozoic Y) - Dark-gray to mottled, fine- to medium-grained, massive to foliated amphibolite and gneiss, composed of hornblende and plagioclase, also commonly biotite and minor quartz; commonly interlayered with banded felsic gneiss. Locally contains calc-silicate rock or diopsidic calcite marble

Layered gneiss (Proterozoic Y) - Gray, medium-grained, well-foliated and generally well layered, light and dark, but locally wispy gneiss, composed of quartz and plagioclase, with microcline locally in the light layers and abundant biotite and common hornblende in the dark layers; garnet or epidote locally. Layers and lenses of calc-silicate rock and amphibolite in some areas

Pink granitic gneiss (Proterozoic Y) - Light-pink to gray, medium- to coarse-grained, foliated but generally massive or poorly layered granitic gneiss, composed of quartz, microcline, oligoclase, and either biotite or muscovite or both, also locally amphibole or epidote

Rusty mica schist and gneiss (equivalent in part to Washington Gneiss of Massachusetts) (Proterozoic Y; may contain some older rocks) - Dark-gray, rusty-weathering, well-foliated and well- to poorly layered schist and gneiss composed of quartz, plagioclase, biotite, muscovite, sillimanite, and locally garnet; some layers of feldspathic quartzite and garnetiferous amphibolite

Light House Gneiss (Proterozoic Z?) - Light-pink or gray to red, medium-grained, generally well foliated granitic gneiss, composed of K-feldspar, oligoclase, quartz, biotite, and magnetite, with local muscovite but no garnet

Plainfield Formation (Proterozoic Z?) - Interlayered light-gray, thin-bedded quartzite, in places with feldspar, mica, graphite, or pyrite, light- to medium-gray gneiss composed of quartz, oligoclase, and biotite (rarely microcline), medium- to dark-gray schist composed of quartz, oligoclase, biotite, sillimanite, and garnet, dark-gray or green gneiss composed of plagioclase, quartz, biotite, and hornblende (commonly with diopside), amphibolite, diopside-bearing quartzite, and calc-silicate rock. In places contains quartz-sillimanite nodules

 Plainfield Formation, Stony Creek Granite Gneiss and Narragansett Pier Granite undivided -  see Zp, Zsc and Pn

Plainfield Formation, Potter Hill Granite Gneiss and Narragansett Pier Granite undivided -  see Zp, Zsph and Pn (? denotes uncertain identification of the formation, with the most closely associated formation symbol used for the map unit).

Quartzite unit [in Plainfield Formation] (Proterozoic Z?) - Light-gray, glassy, generally thin bedded quartzite, also feldspathic and micaceous quartzite containing quartz-sillimanite nodules

 Quartzite unit in Plainfield Formation, Stony Creek Granite Gneiss and Narragansett Pier Granite undivided -  see Zpq, Zsc and Pn

 Stony Creek Granite Gneiss and Narragansett Pier Granite undivided -  see Zsc and Pn

Hope Valley Alaskite Gneiss (Proterozoic Z?) - Light-pink to gray, medium- to coarse-grained, locally porphyritic, variably lineated and foliated alaskitic gneiss, composed of microcline, quartz, albite or oligoclase, and minor magnetite, and locally biotite and muscovite. Lineation formed by rods of quartz. Locally contains quartz-sillimanite nodules

Ponaganset Gneiss (Proterozoic Z?) - Dark-gray, coarse-grained, porphyritic, well-foliated gneiss, composed of oligoclase, quartz, microcline (mostly as megacrysts up to 8 cm long), biotite, magnetite, and generally hornblende; also garnet and muscovite where hornblende is absent

Potter Hill Granite Gneiss (Proterozoic Z?) - Light-pink to gray, tan-weathering, fine- to medium-grained, rarely porphyritic, well-foliated (not lineated) granitic gneiss, composed of microcline, quartz, oligoclase (or albite), biotite, and magnetite, minor muscovite, and local garnet

 Potter Hill Granite Gneiss and Narragansett Pier Granite undivided -  see Zsph and Pn

Porphyritic phase [of Potter Hill Granite Gneiss] (Proterozoic Z?) - Light- to medium-gray, fine- to medium-grained, porphyritic, well-foliated (note lineated) granitic gneiss, composed of microcline (much of it as megacrysts up to 4 cm long), quartz, oligoclase, biotite, and magnetite

'Scituate' Granite Gneiss (Proterozoic Z?) (probably not equivalent to type Scituate in Rhode Island, which is probably Devonian) - Light-pink to gray, medium- to coarse-grained, generally porphyritic, well-lineated and locally foliated granitic gneiss, composed of microcline, quartz, albite or orthoclase, biotite, hornblende, and magnetite. Megacrysts of microcline up to 3 cm long; lineation formed by splotches of biotite or by rods of quartz

Waterford Group (may be equivalent in part to Monson Gneiss) (Proterozoic Z?) - Interlayered part (but layers locally distinct) of Waterford Group, light to dark, generally medium grained gneiss, composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others. Some layers of amphibolite

 Waterford Group and Branford Gneiss undivided -  see Zw and Zb