Geologic units in New Haven county, Connecticut

Mineral Resources On-Line Spatial Data

Pumpkin Ground Member [of Harrison (Prospect) Gneiss] (Middle? Ordovician) at surface, covers 1.0 % of this area

Pumpkin Ground (porphyritic) Member [of Harrison (Prospect) Gneiss] - 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. Pumpkin Ground and Beardsley Members of Harrison Gneiss, formerly considered conformable metavolcanic members, are here recognized as juxtaposed metaplutonic units and are renamed the Beardsley and Pumpkin Ground orthogneisses. Isotopic dating yields ages of 428+/-2 Ma (Early Silurian) for the Pumpkin Ground and 446+/-2 Ma (Late Ordovician) for the Beardsley, accepted by authors as crystallization ages. Pumpkin Ground intrudes the Trap Falls Formation. The Harrison Gneiss as described by Rodgers (1985) has no stratigraphic significance and cannot be correlated regionally (Sevigny and Hanson, 1993).

Lithology: gneiss; schist

Collins Hill Formation (Upper? and Middle Ordovician) at surface, covers 2 % of this area

Collins Hill Formation ( = Partridge Formation of New Hampshire) - 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 (coticule) layers.

Lithology: schist; gneiss; amphibolite; calc-silicate rock

New Haven Arkose (Upper Triassic; possibly Lower Jurassic at top) at surface, covers 27 % of this area

New Haven Arkose - 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.

Lithology: arkose; conglomerate; siltstone; sandstone

Upper part [of Maltby Lakes Metavolcanics] (Middle? Ordovician) at surface, covers 0.5 % of this area

Upper part [of Maltby Lakes Metavolcanics] - 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.

Lithology: greenstone; greenschist; metavolcanic rock

Buttress Dolerite (Middle? Jurassic) at surface, covers 0.5 % of this area

Buttress Dolerite - 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.

Lithology: diabase; basalt; gabbro

Monson Gneiss (Middle or Lower Ordovician?) at surface, covers 3 % of this area

Monson Gneiss (may be equivalent to part of Waterford Group) - 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.

Lithology: gneiss; amphibolite

The Straits Schist (Devonian or Silurian or both) at surface, covers 4 % of this area

The Straits Schist ( = Goshen Formation of Massachusetts) - Silvery to gray, non-rusty, coarse- to very coarse grained, generally poorly layered schist, composed of quartz, muscovite, biotite, oligoclase, garnet, and commonly with staurolite and kyanite or sillimanite; graphitic almost throughout. Distinctive sequence of metamorphic strata overlying the Collinsville Formation in the Bristol quad., is here correlated with the Straits Schist of Fritts (1963) in the Southington quad. Consists mainly of rusty-weathering, medium- to coarse-grained, commonly crenulated, garnet-bearing, graphitic, muscovite-rich, plagioclase-quartz schist with graded bedding. Unit is called The Straits Schist following the original usage of Rodgers and others (1959). In the southern part of the Bristol quad., unit includes the Southington Mountain Member (here reduced in rank) in its upper part. Age of The Straits is inferred to be Silurian or Devonian. Unit is tentatively correlated with the Goshen Formation based on lithologic similarities and a possible unconformity at the base (Simpson, 1990).

Lithology: schist

Buttress Dolerite (uncertain) (Middle? Jurassic) at surface, covers < 0.1 % of this area

Buttress Dolerite (uncertain)- 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.

Lithology: diabase; basalt; gabbro

Southington Mountain Member [of The Straits Schist] (Devonian or Silurian or both) at surface, covers 1 % of this area

Southington Mountain Member [of The Straits Schist] - 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. Metamorphosed strata named Southington Mountain Schist by Fritts (1963) is traceable into southern Bristol quad., and is here revised as Southington Mountain Member of The Straits Schist. Consists of alternating bands of quartz-feldspar granulite and of graphitic, muscovite-biotite schist. Unit is characterized by distinct, widespread, graded bedding and locally abundant staurolite. Unit may correlate with the Goshen Formation of the Heath quad, MA-VT, of Middle Silurian to Early Devonian age (Simpson, 1990).

Lithology: schist; granofels

Metavolcanic member [of Collins Hill Formation] (Upper? and Middle Ordovician) at surface, covers < 0.1 % of this area

Metavolcanic member [of Collins Hill Formation] - 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.

Lithology: amphibolite; schist; gneiss

Waterford Group (Proterozoic Z?) at surface, covers 1 % of this area

Waterford Group (may be equivalent in part to Monson Gneiss) - 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. Usage of Waterford Group follows Goldsmith (1980; 1985). Described as a sequence of metavolcanic and metaplutonic plagioclase gneisses and amphibolites that unconformably overlies the Plainfield Formation in the Hope Valley terrane. (Hope Valley and Esmond-Dedham terranes compose the Avalon superterrane of this report.) Thickness is variable; ranges to 3,100 m. Subdivided (ascending) into Mamacoke Formation with its upper Cohanzie Member (first used?), New London Gneiss, and Rope Ferry Gneiss. Age is Late Proterozoic based on U-Pb analyses of zircon and sphene in the Rope Ferry Gneiss (620+/-3 Ma, Wintsch and Aleinikoff, 1987) (Skehan and Rast, 1990).

Lithology: gneiss; amphibolite

Potter Hill Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers < 0.1 % of this area

Potter Hill Granite Gneiss plus Narragansett Pier Granite - Potter Hill Granite Gneiss - 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. 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.

Lithology: granitic gneiss; granite; pegmatite

Talcott Basalt (Lower Jurassic) at surface, covers 0.9 % of this area

Talcott Basalt - 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 many places; volcanic breccia with fragmentary pillows in others.

Lithology: basalt; gabbro; volcanic breccia (agglomerate)

Shuttle Meadow Formation (Lower Jurassic) at surface, covers 2 % of this area

Shuttle Meadow Formation - 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.

Lithology: shale; siltstone; sandstone; limestone. conglomerate

Lamprophyre (Devonian?) at surface, covers < 0.1 % of this area

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

Lithology: lamprophyre

Silicified rock and mylonite along Mesozoic faults (probably mainly Jurassic) at surface, covers < 0.1 % of this area

Silicified rock and mylonite along Mesozoic faults - 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.

Lithology: mylonite

Oronoque Schist (Lower? Ordovician) at surface, covers 2 % of this area

Oronoque Schist - 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.

Lithology: schist; granofels; amphibolite; greenstone

Waterford Group plus Stony Creek Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers 1 % of this area

Waterford Group plus Stony Creek Granite Gneiss plus Narragansett Pier Granite - Waterford Group (may be equivalent in part to Monson Gneiss) - 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. Stony Creek Granite Gneiss - Red to pink, unevenly medium to very coarse grained, variably foliated granite or granite gneiss, composed of oligoclase, K-feldspar, and quartz with minor biotite and magnetite, sporatic garnet (in foliated varieties), and local muscovite. Commonly contains granite and pegmatite of Narragansett Pier type (and probably age). In much of area both granites occur as innumerable veins penetrating other units or as larger bodies full of inclusions of those units, which can be mapped through the bodies of granite. 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.

Lithology: gneiss; granitic gneiss; granite; pegmatite; amphibolite

Allingtown Metavolcanics (Middle? Ordovician) at surface, covers 0.6 % of this area

Allingtown Metavolcanics - 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.

Lithology: greenstone; phyllite

Light House Gneiss (Proterozoic Z?) at surface, covers 0.6 % of this area

Light House Gneiss - 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.

Lithology: granitic gneiss

Taine Mountain and Collinsville Formations undivided (Middle to Lower? Ordovician) at surface, covers 2 % of this area

Taine Mountain and Collinsville Formations undivided - see Ot and Oc.

Lithology: gneiss; amphibolite;; granofels; felsic metavolcanic rock; mafic metavolcanic rock

Taine Mountain Member (Lower? Ordovician) at surface, covers 4 % of this area

Taine Mountain Member - 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. The Taine Mountain Formation of Stanley (1964) is here adopted in the Collinsville and Bristol quads, CT. Includes 3 pinstriped units: the Wildcat, Scranton Mountain, and Whigville Members, also adopted from Stanley (1964). Underlies Bristol Gneiss (revised). Correlates with the Savoy Schist of Emerson (1898), Missisquoi Schist or Group of Richardson (1919, 1924), in MA; and the Moretown Formation of Cady (1956) in MA and VT. Age is Middle Ordovician (Simpson, 1990).

Lithology: granofels

East Berlin Formation (Lower Jurassic) at surface, covers 2 % of this area

East Berlin Formation - 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. The East Berlin Formation of the Hartford basin contains eight facies: trough cross-bedded sandstones, horizontally stratified sandstones, interbedded sandstones and mudrocks, ripple cross-laminated siltstones, black shales, stratified mudrocks, disrupted shales, and disrupted mudstones. These facies are interpreted as a continental depositional system and are divided into two assemblages. Sandflat/alluvial plain facies assemblage (sandstones and siltstones) is composed of sheet-flood deposits. The lacustrine assemblage (shales and mudrocks) represents a saline lake-playa system (Gierlowski-Kordesch and Rust, 1994).

Lithology: siltstone; shale; sandstone; dolostone (dolomite); arkose; conglomerate

Carringtons Pond Member [of Trap Falls Formation] (Middle or Lower Ordovician) at surface, covers 0.2 % of this area

Carringtons Pond Member [of Trap Falls Formation] - 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.

Lithology: schist; gneiss; amphibolite

Plainfield Formation plus Stony Creek Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers 0.8 % of this area

Plainfield Formation - 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-grey 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. Stony Creek Granite Gneiss - Red to pink, unevenly medium to very coarse grained, variably foliated granite or granite gneiss, composed of oligoclase, K-feldspar, and quartz with minor biotite and magnetite, sporatic garnet (in foliated varieties), and local muscovite. Commonly contains granite and pegmatite of Narragansett Pier type (and probably age). In much of area both granites occur as innumerable veins penetrating other units or as larger bodies full of inclusions of those units, which can be mapped through the bodies of granite. 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.

Lithology: quartzite; mica schist; gneiss; granitic gneiss; granite; amphibolite; calc-silicate rock; pegmatite

Quartzite unit [in Plainfield Formation] plus Stony Creek Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers 0.2 % of this area

Quartzite unit [in Plainfield Formation] plus Stony Creek Granite Gneiss plus Narragansett Pier Granite - Quartzite unit [in Plainfield Formation] - light-gray, glassy, generally thin bedded quartzite, also feldspathic and micaceous quartzite containing quartz-sillimanite nodules. Stony Creek Granite Gneiss - Red to pink, unevenly medium to very coarse grained, variably foliated granite or granite gneiss, composed of oligoclase, K-feldspar, and quartz with minor biotite and magnetite, sporatic garnet (in foliated varieties), and local muscovite. Commonly contains granite and pegmatite of Narragansett Pier type (and probably age). In much of area both granites occur as innumerable veins penetrating other units or as larger bodies full of inclusions of those units, which can be mapped through the bodies of granite. 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.

Lithology: quartzite; granitic gneiss; granite; pegmatite

Maltby Lakes Metavolcanics (Middle? Ordovician) at surface, covers 0.3 % of this area

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

Lithology: greenstone; greenschist; schist; amphibolite

Waterbury Gneiss (Proterozoic Z or Cambrian or both) at surface, covers 7 % of this area

Waterbury Gneiss - 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.

Lithology: schist; gneiss

Stony Creek Granite Gneiss plus Narragansett Pier Granite (Proterozoic Z? and Permian) at surface, covers 0.3 % of this area

Stony Creek Granite Gneiss plus Narragansett Pier Granite - Stony Creek Granite Gneiss - Red to pink, unevenly medium to very coarse grained, variably foliated granite or granite gneiss, composed of oligoclase, K-feldspar, and quartz with minor biotite and magnetite, sporatic garnet (in foliated varieties), and local muscovite. Commonly contains granite and pegmatite of Narragansett Pier type (and probably age). In much of area both granites occur as innumerable veins penetrating other units or as larger bodies full of inclusions of those units, which can be mapped through the bodies of granite. 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.

Lithology: granitic gneiss; granite; pegmatite

Trap Falls Formation (Middle or Lower Ordovician) at surface, covers 1 % of this area

Trap Falls Formation (may be equivalent in part to Golden Hill Schist) - 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.

Lithology: schist; gneiss; granulite; amphibolite

Golden Hill Schist (Lower? Ordovician) at surface, covers 1 % of this area

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

Lithology: schist; granofels

Portland Arkose (Lower Jurassic) at surface, covers 1 % of this area

Portland Arkose - Reddish-brown to maroon micaceous arkose and siltstone and red to black fissile silty shale. Grades eastward into coarse conglomerate (fanglomerate).

Lithology: arkose; siltstone; shale; conglomerate

Allingtown Metavolcanics plus Maltby Lakes Metavolcanics (Middle? Ordovician) at surface, covers 0.2 % of this area

Allingtown Metavolcanics plus Maltby Lakes Metavolcanics - Allingtown Metavolcanics - 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. Maltby Lakes Metavolcanics - Green to gray-green, fine-grained, massive to well-foliated and layered greenstone, greenschist, and schist; also dark amphibolite to west and southwest.

Lithology: greenstone; schist; greenschist; phyllite; amphibolite

Rowe Schist (Lower Ordovician or Cambrian or both) at surface, covers 2 % of this area

Rowe Schist - 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.

Lithology: schist; granofels; amphibolite; calc-silicate rock

Lower part [of Maltby Lakes Metavolcanics] (Middle? Ordovician) at surface, covers 0.8 % of this area

Lower part [of Maltby Lakes Metavolcanics] - 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 metavolcanics and metasedimentary rocks.

Lithology: greenschist; greenstone; schist; phyllite; metavolcanic rock; metasedimentary rock

Ratlum Mountain Schist (Lower? Ordovician) at surface, covers 0.3 % of this area

Ratlum Mountain Schist - 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.

Lithology: schist; granofels; amphibolite; calc-silicate rock

Holyoke Basalt (Lower Jurassic) at surface, covers 2 % of this area

Holyoke Basalt - 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.

Lithology: basalt; gabbro

Basal member [of The Straits Schist] (Silurian) at surface, covers 0.1 % of this area

Basal member [of The Straits Schist] ( = Russell Mountain Formation of Massachusetts) - 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.

Lithology: schist; amphibolite; marble; calc-silicate rock; quartzite

Beardsley Member [of Harrison (Prospect) Gneiss] (Middle? Ordovician) at surface, covers 3 % of this area

Beardsley (hornblendic) Member [of Harrison (Prospect) Gneiss] - Medium- to dark-gray, medium-grained, well-layered and lineated gneiss, composed of plagioclase, quartz, microcline, hornblende, biotite, and epidote. Microcline may occur as megacrysts 1 to 3 cm across. Minor layers of garnetiferous schist and rarely of calc-silicate rock or marble. Pumpkin Ground and Beardsley Members of Harrison Gneiss, formerly considered conformable metavolcanic members, are here recognized as juxtaposed metaplutonic units and are renamed the Beardsley and Pumpkin Ground orthogneisses. Isotopic dating yields crystallization ages of 428+/-2 Ma (Early Silurian) for the Pumpkin Ground and 446+/-2 Ma (Late Ordovician) for the Beardsley. Age of the Beardsley based on analysis of seven zircon and two sphene fractions. The Harrison Gneiss as described by Rodgers (1985) has no stratigraphic significance and cannot be correlated regionally (Sevigny and Hanson, 1993).

Lithology: gneiss; schist; calc-silicate rock; marble

Waterford Group and Branford Gneiss (Proterozoic Z?) at surface, covers 0.9 % of this area

Lithology: gneiss; granitic gneiss; amphibolite

West Rock Dolerite (Lower Jurassic) at surface, covers 1 % of this area

West Rock Dolerite - 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.

Lithology: diabase; basalt; gabbro

Middletown Formation (Middle Ordovician) at surface, covers 4 % of this area

Middletown Formation ( = Ammonoosuc Volcanics of New Hampshire) - Heterogeneously interlayered dark- to light-gray, generally medium grained gneiss and granofels, ranging from quartz-biotite gneiss through felsic amphibole 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.

Lithology: gneiss; granofels; amphibolite; mafic gneiss; biotite gneiss; calc-silicate rock

West Rock Dolerite (uncertain) (Lower Jurassic) at surface, covers < 0.1 % of this area

West Rock Dolerite (uncertain) - 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.

Lithology: basalt; gabbro

Hampden Basalt (Lower Jurassic) at surface, covers 0.2 % of this area

Hampden Basalt - 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.

Lithology: basalt; gabbro

Schist and granulite member [of Trap Falls Formation] (Middle or Lower Ordovician) at surface, covers 0.4 % of this area

Schist and granulite member [of Trap Falls Formation] - 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.

Lithology: schist; granofels

Wepawaug Schist (Devonian or Silurian or both) at surface, covers 6 % of this area

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

Lithology: schist; phyllite; metasedimentary rock

Ordovician? granitic gneiss (Middle Ordovician?) at surface, covers 0.5 % of this area

Ordovician? granitic gneiss (including local terms Ansonia, Mine Hill, "Tyler Lake," "Siscowit") - 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.

Lithology: granitic gneiss; schist

Collinsville Formation (Middle Ordovician) at surface, covers 4 % of this area

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

Lithology: gneiss; amphibolite; felsic metavolcanic rock; mafic metavolcanic rock; granulite

Ultramafic rock (Ordovician or older) at surface, covers < 0.1 % of this area

Ultramafic rock - Ultramafic rock, originally composed of olivine and pyroxene, now generally altered to tremolite, talc, chlorite, or serpentine.

Lithology: ultramafic intrusive rock; serpentinite

New Haven Arkose plus Buttress Dolerite (Upper Triassic; possibly Lower Jurassic at top plus Middle? Jurassic) at surface, covers < 0.1 % of this area

New Haven Arkose plus Buttress Dolerite - New Haven Arkose - 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. 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.

Lithology: arkose; diabase; conglomerate; siltstone; sandstone

Syenite (Permian) at surface, covers < 0.1 % of this area

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

Lithology: syenite

Brookfield Gneiss (Middle? Ordovician) at surface, covers < 0.1 % of this area

Brookfield (dioritic and granodioritic) Gneiss (including Newtown Gneiss of Crowley, 1968) - 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.

Lithology: orthogneiss; amphibolite; mica schist

Basal member [of Taine Mountain Formation] (Lower? Ordovician) at surface, covers 3 % of this area

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

Lithology: granofels

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