USGS Home
Contact USGS
Search USGS

Interactive Maps > Data Download > Geology > by state > Rhode Island Geology

Geologic units in Rhode Island (state in United States)

[Additional scientific data in this geographic area]

Hoppin Formation (Cambrian)

Hoppin Formation - Quartz arenite, argillite, and minor limestone; contains fossils of Lower Cambrian age.

Lithology: argillite; arenite; limestone

Pirate Cove Formation (Cambrian)

Pirate Cove Formation - Pink to buff-white, thinly laminated marble units (~ 3 m thick) containing diagnostic hyoliths, interlayered with magenta slate. The basal marble varies from pink to red to orange, whereas the overlying 15-m-thick phyllite varies from maroon and red to green, gray and silver. Intimately associated spatially with OCAcj, such that the two units are grouped together at the presented map scale.

Lithology: phyllite; slate; marble

cumberlandite (Late Proterozoic? to Devonian?)

cumberlandite - The designated state rock of Rhode Island. Black, medium-grained, porphyritic melatroctolite (phenocrysts of plagioclase), composed of magnetite, ilmenite, olivine, plagioclase, and accessory hercynite spinel; secondary chlorite and saussurite. Abundance of magnetite and ilmenite (up to 70% of rock) results in characteristic high specific gravity. The rock is massive, but locally exhibits a flow lamination caused by the planar orientation of plagioclase laths.

Lithology: troctolite

gabbro/diorite (Late Proterozoic? to Devonian?)

gabbro/diorite - Dark-gray to black, fine- to medium-grained, equigranular, faintly foliated rock, composed of plagioclase, clinopyroxene, hornblende, biotite, and accessory sphene, apatite, and opaque minerals. Includes quartz diorite, diorite, and gabbro.

Lithology: quartz diorite; diorite; gabbro

Scituate Igneous Suite - alkali-feldspar granite (Devonian)

Scituate Igneous Suite - alkali-feldspar granite - Gray to pink, medium- to coarse-grained, porphyritic to subporphyritic alkali-feldspar granite (phenocrysts of perthite). Contains perthite, quartz, and accessory biotite, hornblende, and lesser riebeckite, aegerine, zircon, allanite, sphene, fluorite, and opaque minerals. Plagioclase is sparse to absent. Secondary muscovite, chlorite, and calcite. Ovoid clots of mafic and accessory minerals are locally prominent. Rock is a hypersolvus, mildly peralkaline granite. Fabric ranges from massive to locally lineated and foliated. Includes rock mapped formerly in part as Scituate Granite Gneiss, Cowesett Granite, and perthitic Cowesett Granite.

Lithology: alkali-granite (alaskite)

Scituate Igneous Suite - diorite/gabbro (Devonian)

Scituate Igneous Suite - diorite/gabbro - Fine- to medium-grained, equigranular, massive to slightly foliated rock, composed of plagioclase, hornblende, clinopyroxene, and quartz, with accessory biotite, sphene, apatite, and opaque minerals; secondary chlorite, epidote, and saussurite. Varieties include quartz diorite, diorite, and gabbro.

Lithology: quartz diorite; diorite; gabbro

Scituate Igneous Suite - fine-grained granite (Devonian)

Scituate Igneous Suite - fine-grained granite - Gray to pink, fine-grained, equigranular to locally porphyritic granite (phenocrysts of perthite and plagioclase). Contains microcline, perthite, plagioclase, quartz, and accessory biotite, zircon, allanite, sphene, monazite, fluorite, and opaque minerals; minor garnet is present in some varieties. Partly massive, but locally lineated and foliated. Includes rock mapped formerly as fine-grained Scituate Granite Gneiss, and some rock mapped as Hope Valley Alaskite Gneiss.

Lithology: granite

Scituate Igneous Suite - granite (Devonian)

Scituate Igneous Suite - granite - Gray to pink, coarse-grained, porphyritic to subporphyritic subsolvus granite (phenocrysts of perthite, microcline, and plagioclase). Contains microcline, perthite, plagioclase, quartz, and accessory biotite, hornblende, allanite, zircon, apatite, sphene, fluorite, and opaque minerals; minor garnet is present in some varieties; some secondary muscovite, calcite, and epidote. Ovoid clots of mafic and accessory minerals are locally prominent. Some rock is massive, but most is commonly lineated and foliated, especially in the west and south near the Hope Valley shear zone. Includes most rock mapped formerly as Scituate Granite Gneiss, and some rock mapped as Hope Valley Alaskite Gneiss.

Lithology: granite

Scituate Igneous Suite - granodiorite (Devonian)

Scituate Igneous Suite - granodiorite - Dark-gray to black, fine- to medium-grained, massive to faintly foliated rock, containing plagioclase, microcline, quartz, and accessory hornblende, biotite, sphene, apatite, and opaque minerals. Includes some rock previously mapped as Cowesett Granite.

Lithology: granodiorite

Scituate Igneous Suite - monzonite/monzodiorite (Devonian)

Scituate Igneous Suite - monzonite/monzodiorite - Dark-gray to pink, fine- to medium-grained, equigranular rock, containing plagioclase, microcline, quartz, and accessory biotite, garnet, zircon, apatite, allanite, and opaque minerals; secondary muscovite and epidote. Mainly foliated. Varieties include quartz monzonite, monzonite, and monzodiorite. Includes some rock mapped formerly as Cowesett Granite.

Lithology: quartz monzonite; monzonite; monzodiorite

Scituate Igneous Suite - rhyolite (Devonian)

Scituate Igneous Suite - rhyolite - Pink to gray, fine-grained, locally porphyritic felsite (phenocrysts of quartz and Carlsbad-twinned orthoclase). Includes some hypabyssal intrusive rocks (feeder dikes and sills), and some probable lava flows. Contains accessory biotite, opaque minerals, zircon, monzonite, fluorite, sphene, apatite, and epidote. Quartz phenocrysts commonly are embayed and partly resorbed; matrix is fine-grained intergrowth of quartz and feldspar, probably formed from devitrified glass. Includes some rock mapped formerly as Spencer Hill Volcanics and Maskerchugg Granite.

Lithology: rhyolite

Scituate Igneous Suite - volcaniclastic rock (Devonian)

Scituate Igneous Suite - volcaniclastic rock - Dark-gray, pink, greenish, and purplish tuff and epiclastic rock (sandstone, breccia, conglomerate) containing abundant volcanic detritus. Tuff may include ignimbrite and ash-flow tuff. Fiamme common in some rock mapped formerly as Spencer Hill Volcanics.

Lithology: tuff; ignimbrite; ash-flow tuff; volcanic breccia (agglomerate)

monchiquite (Jurassic)

monchiquite - Dark-gray to greenish-black, fine-grained lamprophyre dikes, with flow-banding parallel to contacts; contains microphenocrysts of titaniferous augite, olivine, kaersutite, phlogopite, apatite, and opaque minerals enclosed in a matrix of analcite and calcite; ocellar texture locally developed. Locally abundant megacrysts (up to 5 cm) of pyroxene, and rounded xenoliths (up to 8 cm) composed of olivine, enstatite, chrome-diopside, and chrome-spinel.

Lithology: lamprophyre

Raritan Formation (Cretaceous)

Raritan Formation - Red, white, gray, and black clay and poorly lithified white sand. Black clay contains lignite fragments and pyrite and siderite nodules. Exposed only in a few wave-cut bluffs on Block Island, but inferred to have originally onlapped both the Esmond-Dedham and Hope Valley subterranes. This unit may occur as large glacial erratic blocks enclosed in till, and may not represent in situ bedrock.

Lithology: clay or mud; sand

alkali-feldspar granite of Cumberland (Devonian? or Mississippian?)

alkali-feldspar granite of Cumberland - Gray to blue-gray. medium- to coarse-grained, equigranular to porphyritic alkali-feldspar granite (phenocrysts of perthite). Massive to locally foliated. Contains perthite, quartz, abundant accessory riebeckite and aegerine, and lesser astrophyllite, anegmatite, zircon, allanite, fluorite, and opaque minerals. Plagioclase is notably absent. Hypersolvus, strongly peralkaline granite. Includes rock mapped formerly as Rhode Island Quincy Granite.

Lithology: alkali-granite (alaskite)

Conanicut Group - Dutch Island Harbor Formation (Cambro-Ordovician)

Conanicut Group - Dutch Island Harbor Formation - Dark gray phyllite, commonly with rhythmically bedded Bouma sequences and brown-weathering carbonate beds that contain concretions 10-30 cm long.

Lithology: phyllite

Conanicut Group - East Passage Formation (Cambro-Ordovician)

Conanicut Group - East Passage Formation - Red, orange-brown, and gray-green phyllite and thinly-bedded, nongraded sandstone and siltstone; rare limestone, and sparse volcanic rock including welded-tuff.

Lithology: phyllite; sandstone; siltstone; limestone; volcanic rock (aphanitic); welded tuff

Conanicut Group - Fort Burnside Formation (Cambro-Ordovician)

Conanicut Group - Fort Burnside Formation - Buff siltstone at base grading upward into black to gray phyllite; units commonly cyclically repeated. Intimately interstratified with OCAcj, such that the two units are grouped together at the presented map scale.

Lithology: siltstone; phyllite

Conanicut Group - Jamestown Formation (Cambro-Ordovician)

Conanicut Group - Jamestown Formation - Green to black phyllite interstratified with buff to pink siltstone; trilobite-bearing.

Lithology: phyllite; siltstone

Conanicut Group - undifferentiated rock (Cambro-Ordovician)

Conanicut Group - undifferentiated rock - Consists of associations of the above rock types.

Lithology: phyllite; siltstone; sandstone; limestone; volcanic rock (aphanitic); welded tuff

minette (Cambro-Ordovician? to Carboniferous?)

minette - Olive-gray to pale-gray, 1- to 4-m-wide dikes, mostly massive and porphyritic, but locally schistose, composed of biotite phenocrysts in a fine-grained groundmass of microcline, plagioclase, and quartz. Contains accessory apatite and pyrite, and secondary chlorite, sericite, calcite, and opaque minerals.

Lithology: lamprophyre

Other metasedimentary rocks - metaclastic rock, undivided (Pennsylvanian? or Late Proterozoic?)

Other metasedimentary rocks - metaclastic rock, undivided - Red to gray to green polymict conglomerate, sandstone, and shale. Variably metamorphosed. Includes rock mapped formerly as Bellingham Conglomerate.

Lithology: meta-conglomerate; metasedimentary rock

Narragansett Bay Group - Dighton Conglomerate (Pennsylvanian)

Narragansett Bay Group - Dighton Conglomerate - Gray conglomerate consisting predominantly of quartz clasts set in a sand-sized matrix. Minor lenses of litharenite and arkosic sandstone.

Lithology: conglomerate; arenite; sandstone

Narragansett Bay Group - Pondville Conglomerate (Pennsylvanian)

Narragansett Bay Group - Pondville Conglomerate - Gray to greenish conglomerate with sand-sized matrix, interbedded with quartz arenite and litharenite; typically lenticular and discontinuous. At type locality (Pondville Station, Massachusetts), unit consists of interbedded red and green slate, siltstone, arkose, and quartzite-pebble conglomerate.

Lithology: conglomerate; arenite; slate; siltstone; arkose

Narragansett Bay Group - Purgatory Conglomerate (Pennsylvanian)

Narragansett Bay Group - Purgatory Conglomerate - Buff to pale-gray conglomerate. Clasts consist entirely of quartzite; matrix primarily quartz, plus sparse amounts of magnetite. Cobbles and boulders are ubiquitously elongate, due to pressure-solution phenomena associated with deformation.

Lithology: conglomerate

Narragansett Bay Group - Rhode Island Formation (Pennsylvanian)

Narragansett Bay Group - Rhode Island Formation - In northern Rhode Island, consists of gray to black, fine- to coarse-grained quartz arenite, litharenite, shale, and conglomerate, with minor beds of anthracite and meta-anthracite. In southern Rhode Island, consists of meta-sandstone, meta-conglomerate, schist, carbonaceous schist, and graphite. Plant fossils are common.

Lithology: arenite; shale; conglomerate; coal; metasedimentary rock; meta-conglomerate; schist

Narragansett Bay Group - Sachuest Arkose (Pennsylvanian)

Narragansett Bay Group - Sachuest Arkose - Gray, smoky-quartz granule-conglomerate, sandstone, and pebble to cobble conglomerate, interbedded with black carbonaceous phyllite. Includes some rock mapped formerly as Pondville Conglomerate.

Lithology: conglomerate; sandstone; phyllite

Narragansett Bay Group - Wamsutta Formation (Pennsylvanian)

Narragansett Bay Group - Wamsutta Formation - Red sandstone, shale, and conglomerate, locally containing abundant volcanic detritus as clasts and matrix. Plant fossil localities occur in adjacent Massachusetts. Minor, but significant amounts of interstratified bimodal-composition volcanic lava flows consisting of alkalic basalt (locally pillowed) and rhyolite are present in adjacent Massachusetts.

Lithology: conglomerate; shale; graywacke; sandstone

Narragansett Pier Plutonic Suite - fine-grained granite (Permian)

Narragansett Pier Plutonic Suite - fine-grained granite - Gray to pink, fine-grained equigranular, massive to faintly foliated granite composed of microcline, oligoclase, quartz, biotite, magnetite, zircon, monazite, allanite, and apatite; secondary chlorite, muscovite, calcite. Occurs mainly as elongate (up to 2.5 km) dikes, up to 80 m wide. Mapped formerly as Westerly Granite.

Lithology: granite

Narragansett Pier Plutonic Suite - granite (Permian)

Narragansett Pier Plutonic Suite - granite - Dark-pink to pale-gray, medium-grained equigranular granite, with lesser granodiorite and quartz monzonite. Composed of microcline, oligoclase, quartz, and accessory biotite, magnetite, ilmenite, apatite, sphene, zircon, monazite, apatite, and allanite; muscovite and garnet locally present; secondary chlorite and calcite. Mainly massive, but locally exhibits faint flow foliation. Cut locally by abundant pegmatite, aplite, and composite aplite-pegmatite of mineralogy similar to the host granite. Unit is a 5 km by 40 km batholith that underlies the southern coast of RI. Intrusion during the latter part of Alleghanian deformation of the Narragansett Basin is indicated by its Permian age, rapid cooling, and field relations. The age of the Narragansett Pier is well constrained by several factors. Brown and others (1978) report a Pennsylvanian fossil contained in an inclusion in the pluton. Radiometric ages of 276 Ma by Kocis and others (1978), 272+/-4 Ma by Hermes and others (1981), and 273+/-2 Ma by Zartman and Hermes (1987) are consistently Early Permian. Structural studies by Mosher (1983) and Reck and Mosther (1988) indicate that intrusion began during the third period of deformation in the Narragansett Basin. Argon release patterns (Dallmeyer, 1982) indicate that the granite and intruded sediments had cooled below argon retention temperatures of hornblende by the Late Permian and of biotite by the Early Triassic. Intruded by the Westerly Granite, which has been radiometrically dated at 276+/-7 Ma. The two granites are probably genetically related based on similarities in age, mineralogy, and geochemistry (Hozik, 1992).

Lithology: granite; granodiorite; quartz monzonite; pegmatite; aplite

Narragansett Pier Plutonic Suite - leucocratic granite (Permian)

Narragansett Pier Plutonic Suite - leucocratic granite - White to gray leucocratic granite composed of microcline, oligoclase, and quartz. Up to 10% accessory muscovite and garnet, with lesser apatite, zircon, and monazite. Biotite and opaque minerals notably sparse or absent. Mainly massive, but locally exhibits flow foliation caused by alternating layers of pegmatite, aplite, and medium-grained equigranular granite; pegmatitic and aplitic material of mineralogy similar to the host granite. The granite contains widespread xenolithic inclusions and screens of metasedimentary rock, some of which are PAnbr; bedding and fabric in the screens generally is oriented parallel to that in the host metasedimentary country rock. Unit is a 5 km by 40 km batholith that underlies the southern coast of RI. Intrusion during the latter part of Alleghanian deformation of the Narragansett Basin is indicated by its Permian age, rapid cooling, and field relations. The age of the Narragansett Pier is well constrained by several factors. Brown and others (1978) report a Pennsylvanian fossil contained in an inclusion in the pluton. Radiometric ages of 276 Ma by Kocis and others (1978), 272+/-4 Ma by Hermes and others (1981), and 273+/-2 Ma by Zartman and Hermes (1987) are consistently Early Permian. Structural studies by Mosher (1983) and Reck and Mosther (1988) indicate that intrusion began during the third period of deformation in the Narragansett Basin. Argon release patterns (Dallmeyer, 1982) indicate that the granite and intruded sediments had cooled below argon retention temperatures of hornblende by the Late Permian and of biotite by the Early Triassic. Intruded by the Westerly Granite, which has been radiometrically dated at 276+/-7 Ma. The two granites are probably genetically related based on similarities in age, mineralogy, and geochemistry (Hozik, 1992).

Lithology: granite; pegmatite; aplite; metasedimentary rock

Narragansett Pier Plutonic Suite - porphyritic granite (Permian)

Narragansett Pier Plutonic Suite - porphyritic granite - Similar to Png, except contains phenocrysts of microcline and plagioclase up to 1.5 cm in a medium-grained groundmass. Unit is a 5 km by 40 km batholith that underlies the southern coast of RI. Intrusion during the latter part of Alleghanian deformation of the Narragansett Basin is indicated by its Permian age, rapid cooling, and field relations. The age of the Narragansett Pier is well constrained by several factors. Brown and others (1978) report a Pennsylvanian fossil contained in an inclusion in the pluton. Radiometric ages of 276 Ma by Kocis and others (1978), 272+/-4 Ma by Hermes and others (1981), and 273+/-2 Ma by Zartman and Hermes (1987) are consistently Early Permian. Structural studies by Mosher (1983) and Reck and Mosther (1988) indicate that intrusion began during the third period of deformation in the Narragansett Basin. Argon release patterns (Dallmeyer, 1982) indicate that the granite and intruded sediments had cooled below argon retention temperatures of hornblende by the Late Permian and of biotite by the Early Triassic. Intruded by the Westerly Granite, which has been radiometrically dated at 276+/-7 Ma. The two granites are probably genetically related based on similarities in age, mineralogy, and geochemistry (Hozik, 1992).

Lithology: granite

diabase (Triassic?)

diabase - Greenish-gray to gray-black, fine-grained, commonly porphyritic, generally massive dike rock composed of plagioclase, augite, opaque minerals, olivine pseudomorphed by serpentine and chlorite minerals, and sparse quartz.

Lithology: diabase

vein quartz (Triassic?)

vein quartz - En echelon and crosscutting veins (millimeter to meter widths) of massive quartz; crystal terminations common in vugs and open cavities. Contains local concentrations of cryptocrystalline jasper and quartz, and irregular distributions of hematite and goethite, some in botryoidal masses. Locally present are abundant rectangular crystals of barite (up to 2 cm), or hollow molds after dissolved barite. Age uncertain, but rocks lack deformation features present in nearby Pennsylvanian rocks of the Narragansett Basin; emplacement most likely postdates Alleghanian deformation

Lithology: plutonic rock (phaneritic)

Blackstone Group - greenstone, amphibolite, serpentinite (Late Proterozoic? or older?)

Blackstone Group - greenstone, amphibolite, serpentinite - Dark-green, massive to foliated metamorphosed gabbroic and basaltic rock interlayered with epiclastic rock; local pillow-structures preserved. Consists primarily of epidote, actinolite, chlorite, and plagioclase. Includes rock mapped formerly as Hunting Hill Greenstone.

Lithology: greenstone; amphibolite; serpentinite

Blackstone Group - quartzite (Late Proterozoic? or older?)

Blackstone Group - quartzite - Gray, medium-grained, massive to thinly bedded quartzite; commonly tectonically interleaved with other units of the Blackstone Group. Interpreted as olistostromal deposits. Includes rock mapped formerly as Quinnville Quartzite.

Lithology: quartzite

Blackstone Group - epidote and biotite schist (Late Proterozoic? or older?)

Blackstone Group - epidote and biotite schist - Green to gray, fine-grained, massive to thinly-bedded mica schist, quartzite, and marble. Schist consists of quartz plus chlorite, muscovite, and/or biotite. Includes rock mapped formerly as Sneech Pond Schist, Mussey Brook Schist, and marble.

Lithology: mica schist; quartzite; marble

Blackstone Group - undifferentiated rock (Late Proterozoic? or older?)

Blackstone Group - undifferentiated rock - Consists of associations of the above rock types. Locally cut by granitic rocks of the Esmond Igneous Suite, resulting in migmatitic appearance. Includes rock mapped formerly as undifferentiated Blackstone Series and migmatite.

Lithology: migmatite

Esmond Igneous Suite - augen granite gneiss (Late Proterozoic)

Esmond Igneous Suite - augen granite gneiss - Pale- to dark-gray, medium- to coarse-grained, granite gneiss characterized by large feldspar porphyroclasts (> 1 cm). Composition ranges from tonalite to granodiorite to quartz monzonite. Composed of sodic plagioclase, quartz, microcline/orthoclase, biotite, opaque minerals; hornblende and epidote common in more mafic types; accessory apatite, sphene, allanite, and zircon; secondary chlorite. Typically massive, but with strong penetrative foliation and lineation defined by major minerals. Commonly contains structurally conformable layers of amphibolite. Includes most rock mapped formerly as Ponaganset Granite Gneiss and Ten Rod Granite Gneiss.

Lithology: augen gneiss; amphibolite

Esmond Igneous Suite - fine-grained granite (Late Proterozoic)

Esmond Igneous Suite - fine-grained granite - Gray to pale-pink, fine-grained, equigranular rock, containing microcline, perthite, plagioclase, quartz, and accessory biotite, epidote, sphene, zircon, monazite, apatite, and opaque minerals. Secondary muscovite, chlorite, and calcite. Mainly massive. Includes rock mapped formerly as fine-grained Esmond Granite.

Lithology: granite

Esmond Igneous Suite - felsic volcaniclastic rock (Late Proterozoic)

Esmond Igneous Suite - felsic volcaniclastic rock - Light-colored to dark-gray ash flow tuff, ignimbrite, and felsite; may include some shallow-level intrusive rock. Phenocrysts of quartz, orthoclase, and plagioclase in a fine-grained matrix of quartz, feldspar, biotite, and opaque minerals. Some varieties contain fiamme and lithic fragments. Includes rock mapped formerly as Pennsylvanian felsite at Diamond Hill.

Lithology: ash-flow tuff; ignimbrite; felsic volcanic rock

Esmond Igneous Suite - granite (Late Proterozoic)

Esmond Igneous Suite - granite - Gray, tan, greenish, or pale-pink, medium- to coarse-grained, mainly equigranular rock. Contains microcline, perthite, plagioclase, quartz, and accessory biotite, epidote, zircon, allanite, monazite, apatite, sphene, and opaque minerals; secondary muscovite, chlorite, and calcite. Mainly massive, but locally foliated and lineated. Includes rock mapped formerly as Esmond Granite.

Lithology: granite

Esmond Igneous Suite - granodiorite (Late Proterozoic)

Esmond Igneous Suite - granodiorite - Gray, tan, greenish, or pale-pink, medium- to coarse-grained, mainly porphyritic rock (phenocrysts of microcline). Contains microcline, perthite, plagioclase, quartz, and accessory biotite, epidote, zircon, allanite, monazite, apatite, sphene, and opaque minerals; secondary muscovite, chlorite, and calcite. Mainly massive. Includes rock mapped formerly as Grant Mills Granodiorite.

Lithology: granodiorite

Esmond Igneous Suite - granite gneiss (Late Proterozoic)

Esmond Igneous Suite - granite gneiss - Pale-gray to pale-pink, fine- to medium-grained granite gneiss, rarely with porphyroclasts of microcline/orthoclase. Composition ranges from quartz monzonite to granite. Composed of sodic plagioclase, quartz, microcline, biotite, opaque minerals; minor muscovite common, garnet more rare; accessory apatite and zircon; sphene and hornblende present in some rocks; secondary chlorite. Typically massive, but with strong penetrative foliation and lineation defined by major minerals. Commonly associated with, and in part gradational into, augen gneiss. Includes rock mapped formerly in part as Ponaganset Granite Gneiss, Scituate Granite Gneiss, Ten Rod Granite Gneiss, and Hope Valley Alaskite Gneiss.

Lithology: granitic gneiss; augen gneiss

Esmond Igneous Suite - mafic/intermediate rock (Late Proterozoic)

Esmond Igneous Suite - mafic/intermediate rock - Dark-gray, purple, or black, medium- to coarse-grained rock that may contain plagioclase, quartz, clinopyroxene, hornblende, olivine, and accessory biotite, epidote, sphene, zircon, apatite, and opaque minerals; secondary chlorite, sericite, and saussurite. Massive to variably foliated. Composition includes tonalite, quartz diorite, diorite, and gabbro. Includes rock mapped formerly as quartz diorite.

Lithology: tonalite; quartz diorite; diorite; gabbro

Harmony Group - Absalona Formation (Late Proterozoic? or older?)

Harmony Group - Absalona Formation - Gray, medium- to coarse-grained biotite granite gneiss characterized by alkali feldspar porphyroblasts. Consists of biotite, hornblende, quartz, microcline, microperthite, albite, epidote, garnet, and chlorite. Subordinate amounts of quartz-biotite schist, amphibolite, and quartzite occur as layers and isolated bodies.

Lithology: granitic gneiss; schist; amphibolite; quartzite

Harmony Group - Nipsachuck Formation (Late Proterozoic? or older?)

Harmony Group - Nipsachuck Formation - Gray to tan, fine- to medium-grained granite gneiss, characterized by biotite streaks. Consists of biotite, muscovite, quartz, microperthite, and sodic plagioclase.

Lithology: granitic gneiss

Harmony Group - Woonasquatucket Formation (Late Proterozoic? or older?)

Harmony Group - Woonasquatucket Formation - Gray, medium-grained felsic gneiss, composed of microcline-microperthite, quartz, albite, muscovite, and biotite. Commonly phyllonitic.

Lithology: felsic gneiss

mica schist (Late Proterozoic? or older?)

mica schist - Gray to green, fine-grained, thinly bedded schist consisting of muscovite, biotite, chlorite, and quartz. Locally contains think beds of quartzite, marble, and amphibolite. Includes rock mapped formerly as mica schist of Bristol, chlorite-biotite schist of Tiverton, and mica-chlorite schist of Sakonnet.

Lithology: mica schist; quartzite; marble; amphibolite

Newport Group - Fort Adams Formation (Late Proterozoic? or older?)

Newport Group - Fort Adams Formation - Large clasts of dolostone and quartz arenite (olistoliths) enclosed within a matrix of tuff, siltstone, slate, and conglomerate; interpreted as an olistostrome.

Lithology: olistostrome

Newport Group - Newport Neck Formation (Late Proterozoic? or older?)

Newport Group - Newport Neck Formation - Sequences of gray, green, and maroon graded rocks, ranging from fine-grained feldspathic granule-conglomerate to maroon slate.

Lithology: conglomerate; slate

Newport Group - Price Neck Formation (Late Proterozoic? or older?)

Newport Group - Price Neck Formation - Fine-grained graded beds of feldspathic siltstone and sandstone, interstratified with carbonate conglomerate, and ash-flow and lapilli tuff; some units may be lahar deposits.

Lithology: siltstone; sandstone; conglomerate; ash-flow tuff; tuff; lahar

Plainfield Formation (Late Proterozoic? or older?)

Plainfield Formation - Pale-tan to gray, fine-grained quartzite, and pale- to medium-gray quartz-mica schist. Quartzite is massive to layered (bedded?). Includes Plainfield Formation and some rock mapped formerly as Blackstone Series.

Lithology: quartzite; mica schist

Sterling Igneous Suite - alaskite gneiss (Late Proterozoic)

Sterling Igneous Suite - alaskite gneiss - Pale pink, orange, or gray, fine- to medium-grained granite gneiss, rarely with porphyroclasts of microcline/orthoclase. Composition is granite with generally less than 3% dark minerals. Composed of sodic plagioclase, quartz, microcline/orthoclase, minor biotite, and opaque minerals; minor muscovite (in part secondary), and rare garnet and sphene in some rocks; accessory apatite and zircon; secondary chlorite. Varies from massive to layered. Strongly foliated and locally well lineated. Includes most rock mapped formerly as Hope Valley Alaskite Gneiss.

Lithology: granitic gneiss

Granites of southeastern Rhode Island - granite (Late Proterozoic)

Granites of southeastern Rhode Island - granite - Pink to gray, coarse-grained, equigranular rock composed of microcline, perthite, plagioclase, quartz, and accessory biotite, sphene, zircon, and opaque minerals; secondary chlorite and muscovite. Generally massive, but locally foliated and lineated. Includes some rock mapped formerly as Bulgarmarsh Granite and Metacom Granite Gneiss.

Lithology: granite

Granites of southeastern Rhode Island - porphyritic granite (Late Proterozoic)

Granites of southeastern Rhode Island - porphyritic granite - Gray, pink, or greenish, generally massive, coarse-grained, porphyritic (microcline phenocrysts) granite. Contains microcline, perthite, plagioclase, quartz, and accessory biotite, hornblende, apatite, sphene, and opaque minerals; secondary chlorite, epidote, and sericite. Includes some rock mapped formerly as Bulgarmarsh Granite and Newport Granite Porphyry.

Lithology: granite

Sterling Igneous Suite - granite gneiss (Late Proterozoic)

Sterling Igneous Suite - granite gneiss - Pale pink to gray, medium-grained granite gneiss, commonly with small porphyroclasts of microcline/orthoclase. Similar to alaskite gneiss, but with more than 3% dark minerals. Composition is granite with generally less than 3% dark minerals. Compositions range from quartz monzonite to granite. Composed of sodic plagioclase, quartz, microcline, biotite, opaque minerals; minor muscovite common, garnet more rare; accessory apatite and zircon; sphene present in some rocks; secondary chlorite. Typically is compositionally homogeneous, with strong foliation and locally well-developed lineation defined by major minerals. Includes some rock mapped formerly as Ten Rod Granite Gneiss, Hope Valley Alaskite Gneiss, Potter Hill Granite Gneiss, and Scituate Granite Gneiss.

Lithology: granitic gneiss

Sterling Igneous Suite - mafic/intermediate gneiss (Late Proterozoic)

Sterling Igneous Suite - mafic/intermediate gneiss - Gray medium-grained biotite gneiss, commonly with porphyroclasts of microcline/orthoclase, and amphibolite gneiss. Compositions range from quartz diorite to granodiorite. Composed of plagioclase, quartz, microcline/orthoclase, biotite, opaque minerals (commonly as conspicuous magnetite octahedra); accessory apatite, zircon, sphene; secondary chlorite. Varies from massive to layered, with strong penetrative foliation and lineation defined by major minerals. Includes some rock mapped formerly as Blackstone Series in southwestern and westernmost Rhode Island.

Lithology: biotite gneiss; gneiss

Waterford Group - Mamacoke Formation (Late Proterozoic? or older?)

Waterford Group - Mamacoke Formation - Light- to dark gray, medium-grained amphibolitic gneiss composed of plagioclase, microcline, quartz, biotite, hornblende, and garnet; locally contains quartz-sillimanite nodules, and thin layers of quartzite and calc-silicate rock.

Lithology: gneiss; quartzite; calc-silicate rock

Waterford Group - Rope Ferry Gneiss (Late Proterozoic? or older?)

Waterford Group - Rope Ferry Gneiss - Light- to dark gray, fine- to medium-grained tonalite gneiss composed of plagioclase, quartz biotite, and hornblende, commonly interlayered with granite gneiss composed of microcline, plagioclase, quartz, biotite, and hornblende; local layers of amphibolite.

Lithology: mafic gneiss; granitic gneiss; amphibolite

unknown (unknown)

unknown

Lithology: indeterminate