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Bedrock Geology of 

Rhode Island

GEOLOGICAL SURVEY BULLETIN 1295

Prepared in cooperation with the State 

of Rhode Island Development Council

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Bedrock Geology of 

Rhode Island

By ALONZO W. QUINN

GEOLOGICAL SURVEY BULLETIN 1295

Prepared in cooperation with the State 

of Rhode Island Development Council

Lithology, structure, and metamorphism 

of Narragansett basin sedimentary rocks 

of Pennsylvanian age, younger Westerly 

Granite, and underlying igneous and 

metamorphic rocks of early Paleozoic 

or Precambrian age

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1971
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

Library of Congress catalog-card No. 76-610762

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.G. 20402
CONTENTS

Page

Abstract---__---___---_____-___---------------------.-_---------_ 1

Introduction. _______-_-__________---__---___--_____--___---'____.-_ 1

Earlier geological studies in Rhode Island____._.________.-__.-_-____. 5

Position of Rhode Island in the Appalachian Mountain system..________ 6

Stratigraphic units________________________________________________ 6

Age relationships-___-__-____---_-----__-__-________----_------ 6

Older metamorphic rocks.-__-_--____--_____-----------_-__----_ 8

Blackstone Series, Precambrian(?)___________________________ 8

General features...--..______-_____-____--____-___---._ 8

Mussey Brook Schi8t_-------_-__---_-__-----_--__-_--- 12

Marble. _--_-._-____--_---_-_-_---_-_-----_----_._--- 12

Quinnville Quartzite___-_-__-__._---__----___-_-__----- 12

Sneech Pond Schist___---_--_-____--__--_-_----_-_----_ 13

Hunting Hill Greenstone_______________________________ 13

Blackstone Series, undivided_--___--___----___---____--_ 13

Migmatite of Blackstone Series_________---__-__-___--_- 14

Older gneissic rocks of northwestern Rhode Island.____________ 14

Nipsachuck Gneiss------------------------------------ 15

Absalona Formation_---_-___-_____--__---____--___-_-_ 18

Woonasquatucket Formation___-__-_____-_____-____--_ 19

Light-colored gneiss.._--________-__--_-__----__-___---- 19

Metamorphic rocks of western and southwestern Rhode Island. _ 19

Metavolcanic rocks of western Rhode Island.___-_.___--._ 20

Cambrian(?) Plainfield Formation.. ------------------- 20

Ordovician(?) rocks.----.---------------.-------------- 21

Metamorphic rocks of southeastern Rhode Island_____-._--__ 21

Older plutonic rocks.___.-_____-__---__________-___--__---.---_ 23

West of the Narragansett basin.______-_-_________---_-_--_- 23

Metadiorite. ___-_.____-_-_____._________-_____.___._-_ 24

Ponaganset Gneiss:.__--_---_-_-_---_---__---__---_---- 24

Porphyritic granite gneiss.._._-______.__---____-------_- 25

Scituate Granite Gneis8-___-_--____--_-__-__--.-_------ 26

Hope Valley Alaskite Gneiss...-----.-._---_-----_------ 26

Ten Rod Granite Gneiss...._____---____---__-_-_--_--- 27

Fine-grained granite related to Scituate Granite Gneiss and

Ten Rod Granite Gneiss...-__-_--_-___-.....-.....-_ 27

Potter Hill Granite Gneiss...-._-__________.-_ 28

Quartz diorite, several types.--------------------.------ 28

Grant Mills Granodiorite.------------.-.--------------- 28

in
IV CONTENTS

Stratigraphic units Continued

Older plutonic rocks Continued

West of the Narragansett basin Continued Page

Esmond Granite-__-_-__-.-_-________________.__-___-__ 29

Fine-grained granite related to Esmond Granite__---__-.__ 30

Gabbro, probably of several ages-_--_-____---__--_______ 30

Cumberlandite ________________________________________ 31

Southeastern Rhode Island_____-___--_______.______________ 32

Metacom Granite Gneiss.-_____________________________ 32

Porphyritic granite at Newport and Conanicut Island_______ 32

Bulgarmarsh Granite-_________________________________ 33

Igneous rocks of Mississippian(?) or older age_____________________ 33

Quincy Granite and granite porphyry._______________________ 33

East Greenwich Group...----___--_--_-_.-_-_-_____________ 34

Spencer Hill Volcanics__-__-_--__-____--_-___-_-_-.-_ 35

Maskerchugg Granite.____--____-_________-______-_--__ 35

Granite porphyry associated with Cowesett Granite.______ 36

Cowesett Granite____________________________________ 36

Perthitic Cowesett Granite_____-____--____--__--_______ 36

Fine-grained granite related to Cowesett Granite __________ 37

Pennsylvanian rocks ___________________________________________ 37

Narragansett basin_______________________-____.__-____--__ 37

Pondville Conglomerate________________________________ 38

Wamsutta Forniation------_---_-__----__---_-_-----_-_ 39

Rhode Island Formation_______________________________ 39

Dighton Conglqmerate_______________--__-__________-_- 41

Felsite at Diamond Hill______.__._--___--_-_-______ 41

North Scituate and Woonsocket basins.______________________ 42

Bellingham Conglomerate____--__--__-___--__-----_---_ 42

Pennsylvanian or post-Pennsylvanian granitic rocks______-___--__ 42

Narragansett Pier Granite-_-__-_-__-__------_---_--------_- 42

Westerly Granite___-_________-____---__-__-_-__---_------- 43

Mafic dikes and sills._________-____________-_____---__--------_ 44

Vein quartz..--___-____---_______--_-_-_--_----_-------------- 45

Upper Cretaceous Raritan Formation(?) of Block Island-__________ 46

Structural geology--..----------.----------- _-____--__-----_---- 46

Older metamorphic rocks.._-______-__-____._--___--_-_---_----- 46

Older plutonic rocks._________--__----__--._--___-------------- 47

Igneous rocks of Mississippian(?) or older age_______--___.-------- 47

Narragansett basin._____-_____-___-___-____-__---------_-----_ 47

Pennsylvanian or post-Pennsylvanian granitic rocks________-_-_--- 48

Metamorphism. -_-_-___.________________---_-___-_---_------------ 49

Pre-Pennsylvanian metamorphism__--___-____-____-------------- 49

Pennsylvanian or post-Pennsylvanian metamorphism__________---- 50

Radiometric ages._____________-_----_----_---_----_--------------- 51

Geological history___-______-__-_---------_---------.-------------- 53
CONTENTS

Economic geology.

Carbonate rocks. __. 

Dimension stone 

Granite. ______

Other rocks. _..

Crushed stone..

Metal deposits._-..

Cumberland! te.

Meta-anthracite. _ _.

Quartz ____________

References cited. ___.__. 

Index----...._.--__...

Page 

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ILLUSTKATIONS

Page 

PLATE 1. Bedrock geologic map of Rhode Island.___________.-___ In pocket

FIGURE 1. Index map of Rhode Island____________

2. Generalized geologic map of Rhode Island.

TABLES

Page 

TABLE 1. Typical modes of Rhode Island rocks______-_-___--__--___. 15

2. Radiometric ages of rocks in Rhode Island and nearby

Massachusetts.___-_____._-_-__._.__-__---__--__---_-- 51

BEDROCK GEOLOGY OF 

RHODE ISLAND

By ALONZO W. QUINN

ABSTRACT

Probably the oldest rocks in Rhode Island are the Blackstone Series of Precambrian(?) age. These and several other groups of metamorphic rocks in the 

State include quartzite, quartz-mica schist, greenstone, amphibolite, marble, and 

several types of gneiss. All were folded and metamorphosed one or more times 

before the Pennsylvanian Period. Intrusive into these older metamorphic rocks 

are varied plutonic rocks that range from metadiorite through quartz diorite 

and granodiorite to granite, and from strongly gneissic syntectonic granitic rocks 

to almost massive granites. These plutonic rocks probably are of Paleozoic age, 

and they may belong to one series or to more than one. After solidifying at depth, 

they were exposed at the surface by erosion before the formation of the volcanic, 

hypabyssal, and plutonic rocks of the East Greenwich Group. This group and the 

Quincy Granite of northern Rhode Island were exposed by erosion before Pennsylvanian time when the Narragansett basin rocks were deposited.

The Narragansett basin contains several thousand feet of coarse clastic nonmarine rocks, which contain plant fossils and some meta-anthracite. These rocks 

were folded, faulted, and progressively metamorphosed in Pennsylvanian or 

post-Pennsylvanian time. They were also intruded by the Narragansett Pier 

Granite, which in turn was intruded by the Westerly Granite. Two masses of 

vein quartz and several mafic dikes have also been mapped. The Raritan Formation ( ?) of Late Cretaceous age is exposed on Block Island.

INTRODUCTION

Various aspects of the geology of Khode Island have been treated 

in many reports published over a long period of time. Since 1944, 

almost all the bedrock of the State has been mapped in a series of 

quadrangle maps on scales of 1:31,680 and 1:24,000. These were done 

under a cooperative project between the U.S. Geological Survey and 

the Khode Island Development Council and predecessor State agencies. 

Ehode Island probably is the first State to be geologically mapped on a 

scale of 1:31,680 (2 inches to 1 mile) or larger. On the basis of these 

maps, a geologic map of the whole State has been compiled on a scale 

of 1:125,000 (pi. 1).
2 BEDROCK GEOLOGY OF RHODE ISLAND

The present report, which is the descriptive text to accompany this 

map, is based mainly on the quadrangle reports; unpublished maps 

have also been made available by H. E. Dixon, U.S. Geological Survey, 

and G. E. Moore, Jr., Ohio State University and U.S. Geological 

Survey. Most of the information on Block Island has been provided 

by C. A. Kaye, U.S. Geological Survey. This help is gratefully 

acknowledged. I mapped several other areas, chiefly in quadrangles 

that are mostly in Massachusetts or Connecticut. Officers in the cooperating State agencies who worked most in this program were 

Mr. Clifton N. Lovenberg, Mr. Everett S. Woodmancy, and Mr. Lachlan F. Blair.

In the text, references are made to the names of specific quadrangles. 

These quadrangles are shown on the index map (fig. 1); those for 

which there are published reports or unpublished maps are listed below 

in alphabetical order. The publications dealing with these quadrangles 

are fully cited in the list of references at the end of this report.

Figure 2 is a generalized geology map of Ehode Island.

Quadrangle Reference

Ashaway ___________ Feininger, 1965a.

Blackstone___________ Quinn and Alien, 1950.

Bristol _______ Quinn and Springer, 1954.

Carolina____________. Moore, 1959.

Chepachet___________ Quinn, 1967.

Clayville ___________ G. E. Moore, Jr., unpub. data.

Coventry Center________. Moore, 1963.

Crompton____________. Quinn, 1963.

East Greenwich________ Quinn, 1952.

East Killingly_________. G. E. Moore, Jr., unpub. data.

East Providence________. A. W. Quinn, unpub. data.

Franklin____________ Quinn and Alien, 1950.

Georgiaville__________ Richmond, 1952; Richmond and Alien, 1951.

Hope Valley__________ Moore, 1958.

Kingston____________ Moore, 1964.

Narragansett Pier_______. Nichols, 1956.

Newport _________. G. E. Moore, Jr., unpub. data.

North Scituate_________ Quinn, 1951.

Oneco______________ Harwood and Goldsmith, 1971.

Pawtucket___________. Quinn and others, 1948,1949.

Providence-___________ Quinn, 1959.

Prudence Island________. G. E. Moore, Jr., unpub. data.

Quonochontaug _____ Moore, 1959.

Sakonnet Point ______. G. E. Moore, Jr., unpub. data.

Slocum_____________. Power, 1959.

Thompson.___________ H. R. Dixon, unpub. data.

Tiverton____________. Pollock, 1964.
INTRODUCTION

Quadrangle Reference

Voluntown___________. Feininger, 1965b.

Watch Hill___________ Moore, 1967.

Wickford____________. Williams, 1964.

*" -"~"~ <?" MASSACHUSETTS__ V ____ x < "

h + + '. ;-vl"±..I H

FIGURE 1. Index map of Rhode Island showing location of quadrangles.

418-781 0 71 2

BEDROCK GEOLOGY OF RHODE ISLAND 5

EARLIER GEOLOGICAL STUDIES IN RHODE ISLAND

Khode Island geology received some attention at a rather early date, 

as the State was an area of early scientific development. In addition, 

certain unusual geological features aroused the interest of early scientists and naturalists; among these features were the meta-anthracite, 

the Purgatory Conglomerate, and a considerable variety of minerals.

The earliest reference to Khode Island geology known 'to me is an 

1808 article on the meta-anthracite; the author is not known, but may 

have been William Meade. (Providence Franklin Soc., 1887, p. 2, 3). 

The "Bibliography of the Geology of Ehode Island," compiled in 

1944 and revised in 1950, lists all the published material known to the 

authors (Quinn and Swann, 1950). Many leading geologists of the 

last century made passing reference or significant contributions to 

Ehode Island geology. Among these were: W. O. Crosby, T. N. Dale, 

J. D. Dana, Amos Eaton, Ebenezer Emmons, A. F. Foerste, M. L. 

Fuller, C. H. Hitchcock, Edward Hitchcock, C. T. Jackson, J. F. 

Kemp, Leo Lesquereux, A. S. Packard, Jr., L. V. Pirsson, W. B. 

Eogers, S. H. Scudder, N. S. Shaler, C. U. Shepard, Benjamin Silliman, Gerard Troost, Warren Upham, Lardner Vanuxem, C. D. Walcott, J. D. Whitney, J. E. Wolff, and J. B. Woodworth.

The first geological study of the whole State was made by C. T. 

Jackson in 1840.

In 1887, the Providence Franklin Society published "Keport on the 

Geology of Ehode Island." This contains an annotated list of earlier 

publications, including newspaper articles; lists of rocks, minerals, 

and fossils; descriptions of a number of special geological features; 

but no geologic map.

The next survey of a large part of the State was "Geology of the 

Narragansett Basin" by Shaler, Woodworth, and Foerste (1899). This 

is an old report, but some of the descriptions of exposures at specific 

localities are still of considerable use.

Professor C. W. Brown of Brown University organized a Natural 

Eesources Survey of Ehode Island in 1909; he published preliminary 

reports in 1910 and 1911.

In 1917 B. K. Emerson published "Geology of Massachusetts and 

Ehode Island," which includes a geological map of the two States.

The present report is based in part on the above-mentioned early 

studies, but mostly on the series of quadrangle studies begun in 1944.
6 BEDROCK GEOLOGY OF RHODE ISLAND

POSITION OF RHODE ISLAND IN THE APPALACHIAN 

MOUNTAIN SYSTEM

Rhode Island and most of the rest of New England are commonly 

considered to be within the northern Appalachians. Actually, Rhode 

Island is almost in the middle of the mountain system that extends 

from southern Alabama through Newfoundland. This mountain system has gone through a long and exceedingly complex sequence of 

events, involving geosynclinal sedimentation and volcanism, folding, 

thrust faulting, metamorphism, plutonism, uplift, and erosion. In some 

parts of the system some of these events have been repeated several 

times, and in some places the compression features of orogeny have 

been broken by later tensional features.

In Rhode Island sedimentation and orogeny took place in early 

or middle Paleozoic time and probably also during Precambrian time. 

There were at least two episodes of plutonic activity in early or middle 

Paleozoic time. The Narragansett basin of eastern Rhode Island and 

adjacent Massachusetts contains the only large mass of known Pennsylvanian rocks in this part of the mountain system. It also has the 

only unequivocal geologic evidence of Pennsylvanian or later compressional folding and plutonic activity in this part of the Appalachians. 

Rocks and erogenic structures of equivalent ages are known in Nova 

Scotia (Belt, 1964, 1965), but they are not known elsewhere in New 

England. Other parts of New England may have been affected by this 

erogenic episode, but definite evidence of it is lacking. L. W. Lundgren 

(oral commun., 1966) has suggested that some of the geological features of southeastern Connecticut, as well as the radiometric ages there, 

may be correlated with this late Paleozoic activity so well demonstrated 

in Rhode Island.

STRATIGRAPHIC UNITS 

AGE RELATIONSHIPS

Age relationships of many of the rocks of Rhode Island, and of 

much of New England, are very uncertain because fossils are generally 

scarce, structural relations are complex, several episodes of metamorphism have affected parts of the area, lithologic characters are 

variable and gradational, and exposures are lacking in many critical 

places. A welcome exception to this uncertainty is the Narragansett 

basin (Quinn and Oliver, 1962) of eastern Rhode Island and adjacent 

Massachusetts. This is a complex synclinal mass containing several 

thousand feet of sedimentary rocks that contain plant fossils of Pennsylvanian age. This basin extends some 60 miles north through eastern 

Rhode Island and northeastward into Massachusetts; it is approximately 15 miles across.
STRATIGRAPHIC UNITS 7

Subject to the above uncertainties and excluding such minor rocks 

as mafic dikes and vein quartz, the rocks of Rhode Island are here 

divided into six groups.

1. Older metamorphic rocks. The ages of these rocks probably range 

from Precambrian to early Paleozoic. Important here are the 

rocks of the Blackstone Series of northern Rhode Island. Masses of 

somewhat similar rocks are scattered across the State to the southwest. Those near the southwest corner of the State have been correlated with Connecticut formations and assigned to the Cambrian 

and the Ordovician Systems (Feininger, 1965a,b). Other somewhat similar rocks east of the Narragansett basin are exposed in 

the Newport, Sakonnet Point, Tiverton, Fall River, and Bristol 

quadrangles. These are older than any other rocks in their vicinity, but there is no evidence concerning their relationships to the 

metamorphic rocks west of the Naragansett basin. Several gneissic rocks of northwestern Rhode Island have been assigned to 

this group, but relations of these are uncertain.

2. Older plutonic rocks. These are younger than the older metamorphic rocks and probably of early or middle Paleozoic age. This 

group includes several types of plutonic intrusive rocks, chiefly 

in the range of quartz diorite to granite. These rocks are areally 

the most extensive group. A few representatives of this group are 

the Scituate Granite Gneiss, the Hope Valley Alaskite Gneiss, 

the Esmond Granite, the Grant Mills Granodiorite, the Metacom 

Granite Gneiss, the Bulgarmarsh Granite, and the Ponaganset 

Gneiss. Present evidence is not sufficient to indicate whether these 

all belong to one plutonic or magmatic series or whether there 

are several such series. Again, rocks on the east side of the Narragansett basin cannot be certainly correlated with rocks on the 

west side.

3. Igneous rocks of Mississippian(?) or older age. This group includes the Quincy Granite and related rocks in northeastern 

Rhode Island and the East Greenwich Group of granitic, hypabyssal, and volcanic rocks.

4. Pennsylvanian rocks of the Narragansett basin and of smaller 

basins at North Scituate and at Wbonsocket. These are chiefly 

clastic sedimentary rocks of continental origin, more or less 

metamorphosed.

5. Pennsylvanian or post-Pennsylvanian granitic rocks. These include 

the Narragansett Pier Granite and Westerly Granite and associated pegmatites.

6. Cretaceous clay and sand exposed on Block Island. These constitute 

the youngest group of rocks.
8 BEDROCK GEOLOGY OF RHODE ISLAND 

OLDER METAMORPHIC ROCKS

The oldest rocks in Rhode Island are metamorphic rocks in several 

different parts of the State. Evidence is insufficient to determine 

whether these are closely related to each other or whether they include 

members of widely different and unrelated groups of rocks. The Blackstone Series in northern Rhode Island constitute a fairly definite 

group. Correlated with the Blackstone Series are several small bodies 

of metamorphic rocks to the west and southwest. Another group, in 

the extreme southwest, has been correlated with Cambrian (?) and 

Ordovician (?) rocks of Connecticut (Feininger, 1965a, b). The relation of these to the Blackstone Series has not been firmly established. 

In southeastern Rhode Island, on the east side of the Narragansetfc 

basin, are several types of old metamorphic rocks whose relations to the 

rocks on the west side of the basin are unknown. The older gneissic 

rocks of northwestern Rhode Island are included in this group although their relations are not well known.

BLACKSTONE SERIES, PRECAMBRIAN(P)

GENERAL FEATURES

The name Blackstone Series was originally used by Wood worth (in 

Shaler and others, 1899, p. 8,104-109), but it was dropped by Emerson 

(1917) and replaced with Marlboro Formation. In the Pawtucket 

quadrangle reports, the name Blackstone Series was revived (Quinn 

and others, 1948,1949). This series includes quartzite, chlorite-quartz 

schist, quartz-mica schist, marble, and mafic volcanic rocks exposed 

in the valley of the lower Blackstone River in the Pawtucket quadrangle. We divided the Blackstone Series into a lowermost Mussey 

Brook Schist, a thick middle formation of massive quartzite, the overlying Sneech Pond Schist, and the Hunting Hill Greenstone. We accepted Emerson's correlation of the middle thick massive quartzite 

with the Westboro Quartzite, but subsequent studies have convinced 

me that we should have rejected this correlation. There now seems to 

be no good basis for deciding which, if any, of the several quartzites 

in Massachusetts is equivalent to this thick quartzite of the Blackstone 

Series of Rhode Island. Therefore, I here propose that this quartzite 

be named the Quinnville Quartzite for exposures just north of the village of Quinnville that extend north to the west end of the Blackstone 

River bridge on George Washington Highway, State Route 116 in the 

Pawtucket quadrangle. The quartzite is also well exposed at the west 

end of the bridge of proposed State Route 295, approximately 0.8 mile 

farther northwest. No other modification of this stratigraphic unit is 

proposed.
OLDER METAMORPHIC ROCKS 9

At many places in Rhode Island the rocks of the Blackstone 

Series cannot be assigned to specific formations but are referred to 

as Blackstone Series, undivided. Only the Quinnville Quartzite and 

the Hunting Hill Greenstone are readily distinguished lithologically. 

The other two formations both include interbedded mica schist, quartzmica schist, chlorite-quartz schist, thin-bedded quartzite, and marble. 

The assignment of rocks to the Mussey Brook Schist or to the Sneech 

Pond Schist is mostly on the basis of whether they are above or below 

the Quinnville Quartzite.

Distribution. The largest area of Blackstone Series rocks is along 

the valley of the Blackstone River in the Pawtucket quadrangle. Other 

areas of these rocks are a short distance northwest in the Georgiaville 

and Blackstone quadrangles and further northwest in the Chepachet 

and Uxbridge quadrangles. To the south in the Providence and North 

Scituate quadrangles is an area of rocks very similar to those of the 

Blackstone River valley. The Quinnville Quartzite is well developed 

there. Further south in the East Greenwich and Crompton quadrangles 

is an area of Blackstone Series rocks in which the Quinnville Quartzite 

is typical but in which the Sneech Pond Schist has more thin-bedded 

amphibolite than is usual. Blackstone Series rocks in two areas in 

the Clayville quadrangle probably were similar to the Sneech Pond 

Schist and the Mussey Brook Schist, but they have been much modified 

by migmatization. A considerable number of other areas to the south, 

southwest, and west across Rhode Island have metamorphic rocks similar to those of the Blackstone Series, but correlation is by no means 

certain. Feininger (1965, a, b) has correlated somewhat similar rocks 

of the Ashaway and Voluntown quadrangles with Cambrian(?) and 

Ordovician (?) formations in Connecticut. G. E. Moore, Jr. (oral 

commun., 1966) states that those metamorphic rocks east of the nearly 

north-south fault in the Ashaway quadrangle are more like rocks of 

the Blackstone Series than they are like the Cambrian (?) and Ordovician (?) rocks of Connecticut.

Age and correlation. The age and correlation of the Blackstone 

Series constitute perhaps the most vexing problem in the geology of 

Rhode Island. Direct evidence is not definite. Indirect evidence involves correlations on lithologic characters of variable rocks in patches 

separated by younger plutonic rocks.

Within the East Greenwich area, the geologic relations indicate a 

considerable interval of geologic time between the deposition of the 

Blackstone Series and the deposition of the Pennsylvanian sediments 

of the Narragansett basin. First was the deposition of probably 15,000 

to 20,000 feet of sandstone, shale, limestone, and mafic volcanic rocks 

of the Blackstone Series. These rocks were then folded and meta-
10 BEDROCK GEOLOGY OF RHODE ISLAND

morphosed and later were intruded at depth by the Esmond Granite 

of the older plutonic series. This event was followed by a period of 

erosion long enough to expose the Esmond Granite. The Spencer 

Hill Volcanics were then formed, and some of the conglomeratic 

beds in these volcanic rocks contain cobbles of the Esmond Granite. 

After a considerable, but unknown, thickness of Spencer Hill Volcanics accumulated, there was a second plutonic intrusion, this time 

the Oowesett Granite and several related rocks. A second erosion 

interval followed, during which the Cowesett Granite was exposed 

before the deposition of the Pennsylvanian sediments of the Narragansett basin. These geologic events must have required a considerable 

length of time, so that the Blackstorie Series probably was deposited 

in early Paleozoic or in Precambrian time.

Similar relations are exposed in northeastern Khode Island, mostly 

in the Pawtucket quadrangle, and in nearby Massachusetts. There, 

the evidence indicates: (1) deposition of perhaps 10,000 feet of Blackstone Series, (2) deformation and metamorphism of the Backstone 

Series, (3) intrusion of Esmond Granite, Grant Mills Granodiorite, 

and related plutonic rocks, (4) intrusion of the Devonian (?) or older 

Quincy Granite and granite porphyry, (5) erosion which exposed 

the Quincy Granite and granite porphyry, and (6) deposition of the 

Pennsylvanian Pondville Conglomerate, exposed as "giant conglomerate" on the south border of the Blue Hills of Massachusetts (Chute, 

1964). Here, also, the time involved for the sequence of events seems 

stone Series, (2) deformation and metamorphism of the Blackstone 

Series.

Indirect evidence suggesting a Precambrian, rather than a Paleozoic, age is found in the Hoppin Hill area in North Attleborough, 

Mass. Shale and limestone in that area are so little metamorphosed 

that they contain fossils, which have been identified as of Early Cambrian age (Shaw, 1950; Dowse, 1950). The rocks of the Blackstone 

Series are more metamorphosed than are these Cambrian rocks; 

therefore they were probably metamorphosed before the deposition 

of the Cambrian rocks. This argument is not strong, however, because 

the nearest Blackstone Series rocks 'are 4 miles distant from Hoppin 

Hill and the intervening area is covered by Pennsylvanian rocks and 

glacial drift.

Another indirect line of evidence on the age of the Blackstone Series 

has to do with the Northbridge Granite Gneiss of adjacent Massachusetts. Moorbath and staff (1962) made a rubidium-strontium 

investigation of the Northbridge and, on the basis of six samples, obtained an age of 535±15 m.y. (million years) Middle or Late Cam-
OLDER METAMORPHIC ROCKS 11

brian. This date was changed to 558±11 m.y. (Fairbairn and others, 

1965, p. 5). The Northbridge Granite Gneiss includes a considerable 

variety of gneissic rocks that extend into similar rocks of the Ponaganset Gneiss of Rhode Island. The Ponaganset Gneiss of the Chepachet quadrangle is intrusive into and younger than the Blackstone 

Series. Therefore, if the Ponaganset and the Northbridge are correlative, as seems probable, the Blackstone must be early Cambrian or 

Precambrian. However, a complication in this correlation comes from 

the fact that Emerson (1917, p. 155) wrote of the Northbridge, "It 

is considered Archean because the Algonkian (?) quartzite overlaps 

it normally and the Milford granite cuts both rocks." The Algonkian ( ?) quartzite referred to by Emerson was the Westboro Quartzite. 

Emerson did not specify the locality of the supposed unconformity, or normal overlap; he may have inferred it from the general map 

pattern. In disagreement are exposures at a recent roadcut on Route 

140,0.73 mile south-southeast of Grafton, Mass., at the position where 

Emerson's map has a contact between the Westboro Quartzite and 

the Northbridge Granite Gneiss. These exposures show that the 

gneiss is younger than quartzite and schist layers of the Westboro. 

This relation is the same as that between the Ponaganset and the 

Blackstone Series in Rhode Island.

Altogether, it seems to me that no definite age assignment can now 

be made for the Blackstone Series but that the known evidence favors 

a Precambrian age.

Thicknesses. At no place is it possible to measure or to estimate 

the total thickness of the Blackstone Series, because later intrusive 

rocks have removed some of the lower part of the Mussey Brook 

Schist and some of the upper part of the Sneech Pond Schist. The 

full thickness of the Quinnville Quartzite may be present along the 

Blackstone River in the Pawtucket quadrangle, but even in the most 

favorable places the possibility remains that folding or faulting has 

repeated or cut out some of the original layers of these formations.

The greatest remaining thickness of the Mussey Brook Schist is 

in the Providence quadrangle, where it is approximately 8,500 feet. 

The thickness of the Quinnville Quartzite along the Blackstone River 

is about 3,300 feet, but this quartzite seems to be at least 5,000 feet 

thick along the border between the Crompton and the East Greenwich quadrangles. A rather uncertain estimate of the thickness of the 

Sneech Pond Schist in the Pawtucket quadrangle is 8,500 feet. The 

Hunting Hill Greenstone is so irregular that an estimate of thickness 

would have little value. The total thickness of the Blackstone Series 

seems to be at least 15,000 feet, and it may be more than 20,000 feet.

418-781 O 71 8
12 BEDROCK GEOLOGY OF RHODE ISLAND

MUSSEY BROOK SCHIST

The type locality of the Mussey Brook Schist is in the Pawtucket 

quadrangle, where Mussey Brook leaves Handy Pond. This formation also occurs in the Georgiaville, the Providence, and the North 

Scituate quadrangles. It includes a considerable variety of rocks, 

chiefly greenish to gray fine-grained and thin-bedded chlorite-quartz 

schist and thin-bedded quartzite; similar rocks in the Georgiaville 

quadrangle have been altered by feldspathization. There are also 

thin beds of light-gray mica schist, dark-gray amphibole schist, greenstone, and white marble, and a few small lenses of serpentine and 

steatite.

Several of the thin beds of marble and lenses of steatite were worked 

in a small way during colonial times, and steatite was worked by 

Indians in prehistoric time in the Providence quadrangle at a point 

just north of Hartford Avenue and 400 feet west of the Providence 

City line.

Quartzite. In the Providence quadrangle and extending into the 

North Scituate quadrangle is a layer of quartzite in the lower part 

of the Mussey Brook Schist. This layer, which is approximately 1,500 

feet thick, is largely gray to light-gray, medium-grained to finegrained, massive to thick-bedded quartzite, but it also includes beds of 

quartz-mica schist.

MARBLE

Layers of marble are sparsely interbedded through the Mussey 

Brook Schist and the Sneech Pond Schist. Most of these beds are too 

small to map, but two within the Pawtucket quadrangle are shown 

on the State map. One of these is at the now active quarry near Lime 

Rock (the old Harris quarry), and the other is at the abandoned quarry 

about a mile east (the old Dexter quarry). Several other beds in the 

Pawtucket quadrangle and in the Providence quadrangle were once 

quarried in a small way. The rock is white to light-gray, fine- to medium-grained marble and much of it is dolomitic. Common metamorphic minerals are tremolite, forsterite, diopside, talc, the bowenite 

variety of serpentine, and other lime-silicate minerals.

QUINNVILLE QUARTZITE

The Quinnville Quartzite is extensively exposed along the Blackstone River in the Pawtucket quadrangle and along the Woonasquatucket River in the Providence quadrangle. No explanation has been 

found for the courses of these two rivers in what appears to be 

resistant quartzite. The Quinnville is also found in the East Greenwich 

and Crompton quadrangles. It is largely light-gray or light-bluish,
OLDER METAMORPHIC ROCKS 13

clean, massive, medium-grained quartzite, but in many places it contains thin beds of light-gray to greenish quartz-mica schist.

SNEECH POND SCHIST

The Sneech Pond Schist, named for exposures around Sneech Pond 

in the Pawtucket quadrangle, overlies the Quinnville Quartzite in the 

Pawtucket quadrangle, in the Providence quadrangle, and in an area 

along the border of the East Greenwich and Crompton quadrangles. 

This formation includes a great variety of metamorphosed sedimentary and volcanic rocks. It consists chiefly of gray to light-greenish, 

fine-grained quartz-mica schist, with many interbeds of quartzite, 

greenstone, quartz-chlorite schist, amphibole schist, feldspathic mica 

schist, marble, and serpentine.

HUNTING HILL GREENSTONE

The Hunting Hill Greenstone was named for exposures on Hunting 

Hill in the Pawtucket quadrangle. The formation is also exposed elsewhere in the Pawtucket quadrangle and in the Providence quadrangle ; many bodies of this rock are too small to be shown even on a 

1:24,000 map. The rock is largely dark-green, fine-grained greenstone 

that resulted from low-grade metamorphism of basaltic rock, although 

some may have had an andesite composition. Chief minerals are 

amphibole, plagioclase, and epidote, with minor chlorite, biotite, and 

quartz. Knots and veins of epidote are common. Some of the rock 

shows the texture of a pyroclastic rock, some seems to have formed 

as lava flows, some shows pillow structure, and some formed as 

shallow intrusives. It occurs at various stratigraphic positions 

throughout the Blackstone Series. This occurrence implies that 

volcanic and shallow intrusive activity took place during the time of 

deposition of several thousand feet of sandstone, shale, and limestone 

of the Blackstone Series.

A few small bodies of serpentine and of steatite occur in the Blackstone Series, mostly in association with the Hunting Hill Greenstone.

BLACKSTONE SERIES, UNDIVIDED

At several places in Rhode Island are bodies of metamorphic rock 

which are correlated tentatively with the Blackstone Series, but which 

cannot be assigned to specific formations within the series. Most of 

these bodies are surrounded by later intrusive granitic rocks. The 

lithologic types of these metamorphic rocks encompass the range of 

the Blackstone Series in the Pawtucket quadrangle quartzite, mica 

schist, quartz-mica schist, chlorite schist, amphibole schist, and marble.
14 BEDROCK GEOLOGY OF RHODE ISLAND

Most of them were metamorphosed at a higher intensity than that 

which prevailed in the Pawtucket quadrangle. In the Ohepachet 

quadrangle, for example, are massive quartzite beds like the Quinnville Quartzite of the Pawtucket quadrangle, but mica schist beds 

contain staurolite and garnet, and mafic layers are amphibolite; schists 

in the Kingston quadrangle contain sillimanite. Most of the schists 

surrounded by granitic rocks are feldspathic, and much of the rock 

is gneissic. The variety of lithologic types is fully as great as might 

be expected from the great variety of rocks of the Blackstone Series 

that were subjected to different intensities of metamorphism and 

different degrees of metasomatism. Some of the common rocks that 

resulted are: quartzite, biotite schist, muscovite schist, garnet-sillimanite-muscovite schist, hornblende schist, biotite gneiss, muscovite 

gneiss, augen gneiss, and porphyroblastic gneiss.

MIGMATITE OF BLACKSTONE SERIES

In the Clayville quadrangle are two areas of migmatite formed 

from the Blackstone Series. Apparently before migmatization, the 

rocks were chiefly biotite schist and quartzite. These were then intimately invaded by stringers, sills, and dikes of granitic, aplitic, and 

pegmatitic material, and also by quartz veins, all probably derived 

from the adjoining Ponaganset Gneiss. Some contacts are sharp and 

some are gradational. The main minerals are feldspars, quartz, biotite, 

and muscovite. Other minerals, less abundant but in notable amount, 

include kyanite, staurolite, epidote, garnet, actinolite, scapolite, tourmaline, and apatite.

OLDER GNEISSIC ROCKS OF NORTHWESTERN RHODE ISLAND

In northwestern Ehode Island are several gneissic rocks whose 

relations to other rocks are not clearly revealed. These rocks are exposed rather extensively in the Georgiaville, North Scituate, Chepachet, and Clayville quadrangles. Formations included are the Nipsachuk Gneiss, the Absalona Formation, the Woonasquatucket Formation, and an unnamed light-colored gneiss. Richmond (1952; Richmond 

and Alien, 1951) named the first three of the above for exposures in 

the Georgiaville quadrangle.

These gneissic formations are older than the Scituate Granite Gneiss 

of the older plutonic group, as is shown by intrusive relations on Snake 

Hill in the Chepachet quadrangle. Richmond indicated that the gneisses are older than the Blackstone Series. The map pattern gives some 

support to his belief that the Blackstone Series lies unconformably 

over the gneisses. However, glacial drift covers the areas where the 

supposed unconformity might be exposed. On the basis of present
OLDER METAMORPHIC ROCKS 15

knowledge, the gneisses may be older than, equivalent to, or younger 

than the Blackstone Series.

Their lithologic characters indicate that these gneisses probably 

formed as thick accumulations of graywacke, feldspathic sands, tuffs, 

ash flows, or lava flows, mostly of felsic composition. Richmond interpreted the structure of these rocks as a complicated overturned anticline, with the oldest rocks exposed in the middle. On this basis the 

total thickness of the gneisses would be at least 5,000 feet and might be 

10,000 feet or more.

NIPSACHUCK GNEISS

The Nipsachuck Gneiss is exposed mostly in the Georgiaville quadrangle, but it extends westward into the Chepachet quadrangle. According to Richmond (1952; Richmond and Alien, 1951), it is exposed 

in the core of an anticline that is overturned toward the west. The rock 

is gray to light-tan fine-grained to medium-grained gneiss, with prominent lineation of biotite streaks and prominent foliation. Indistinct 

compositional layering appears to be bedding. The main constitutents 

are microperthite, albite-oligoclase, quartz, biotite, and muscovite. A 

typical mode is given in table 1. The composition, texture, and structure suggest that this rock was formed by the metamorphism of graywacke, f eldspathic sandstone, or felsic volcanic rock.

TABLE 1. Typical modes of Rhode Island rocks

Quartz Potassium Plagio- Biotite Musco- Acces- References 

feldspar clase vite series

Westerly Granite

27.8

27.5

27.5

23.1

24.3

23.2

27.1

28.7

32.9

32.0

35.4

27.1

27.8

27.7

33.9

25.1

32.7

34.4

31.4

40.8

41.3

42.1

33.6

43.6

3.7

3.9

3.2

6.3

5.5 ...

4.9 ...

1.8

1.6

1.9

1.4

1.3

1.4

2.5

.3

1.0 Chayes, 1950; (11) .12

.8 L. W. Lundgren, unpub. data; (15) .1 2

1. 2 Fairbairn and others, 1961; (16). > 2

1.3 Chayes, 1950; (5). is

1.1 Quinn, 1943; (6) .is

1. 2 Do.8

. 7 Do."

Narragansett Pier Granite

29.2

28.6

23.6 

31.0 

35.2

22.5 

27.0 

34.0

31.3 

25.8

27.9

33.8

32.6 

28.4 

24.1

49.0 

30.0 

36.0

29.7 

33.4

34.7

34.3

37.0 

34.7 

28.4

23.5 

35.0 

25.0 ...

34.1 

38.6

5.9 ...

2.6 ...

5.2 

3.5 

7.1

3.5 

5.5

2.8 

.1

1.0 

1.3 

5.1

.5 

.5 

3.0

.6 

2.1 ...

2.4

.7

.6 

1.1 

1.0

.5 

1.0 

1.0

1.5

Quonochontaug quad. 

Moore, 1964. Kingston quad. 

A. W. Quinn, unpub. data Ashaway quad. 

Quinn and others, 1957, p. 550. Narragansett 

Pier quad. 

Feininger, 1964. Ashaway quad. 

Do. 

Do.

Do. 

Do.

See footnotes at end of table.
16 BEDROCK GEOLOGY OF RHODE ISLAND

TABLE 1. Typical modes of Rhode Island rocks Continued

Quartz Potassium 

feldspar

Plagio- Biotite Musco- Acces- References 

clase vite series

Esmond Granite

31.8 

41.1 

36.9

27.1 

35.3

43.1 

31.6 

43.0

45.9 

35.9

18.5 

22.7 

16.4

24.8 

27.0

quad. 

2.3 0.7 1.6 A. W. Quinn, unpub. data. North Scituate 

quad.

quad. 

1. 3 .8 . 1 A. W. Quinn, unpub. data. Crompton quad 

.6 .8 . 4 A. W. Quinn, unpub. data. Chepachet quad

Metacom Granite Gneiss

31.8 

21.4 

37.4

38.0 

68.7 

24.8

4.2 

7.4 

29.2

16. 2 4. 0 5. 8 A. W. Quinn, unpub. data. Bristol quad. 

7.2 3.6 2.6 Do. 

4.2 3.2 1.2 Do.

Woonasquatueket Formation

70.9 

22.7

Quartz

19.4 

72.0

Potassium 

feldspar

1.0 

1.3

0. 3 8. 0 0. 7 A. W. Quinn, unpub. data. North Scituate 

quad. 

1.3 1.2 1.5 Do.

Plagio- Biotite Musco- Horn- Acces- References 

clase vite blende sories

Sdtuate Granite Gneiss

24.8

22.6 

31.6

19.3 

39.6 

34.7 

35.3 

32.2 

39.0

Quartz

45.4

49.3 

31.6

45.1 

33.7 

38.1 

42.1 

35.4 

50.6

Potassium 

feldspar

27.2

24.9 

30.2

21.4 

22.5 

22.4 

16.7 

24.0 

4.7

Plagioclase

1.7 0.2 .......... 0.7 Quinn and Moore, 1968, p. 272.

North Scituate quad. 

2.0 1.3 .................... Do.

Coventry Center quad. 11.6 1.9 .......... .7 Do.

2.9 .................... 1.4 Do.

5.6 .......... .4 1.0 Quinn and Moore, 1968, p. 272.

AccesBiotite sories References

Cowesett Granite

21.8 

42.6

40.4 

46.9 

47.7 

37.0

49.2 

52.6

40.6 

49.8 ..

43.8 

42.2

23.8 

.7 

15.4

3.1

18.9

3.3 1.9 Qiunn and others, 1957, p. 550. East Greenwich 

quad. 

3.7 .4 Quinn and Moore, 1968, p. 272. East Greenwich 

quad. 

2. 9 .7 Do. 

3.0 .3 A. W. Quinn, unpub. data. Crompton quad. 

6.0 .4 Do. 

.7 1. 6 Do.

Grant Mills Granodiorite

35.3 

44.0

14.2 

16.4

45.7 

34.6

43 0. 5 A. W. Quinn, unpub. data. Pawtucket quad. 

5.0 ............ Do.

See footnotes at end of table.
OLDER METAMORPHIC ROCKS

TABLE 1. Typical modes of Rhode Island rocks Continued

17

Quartz Potassium 

feldspar

Plagtoclase

Blotite Horn- Acces- References 

blende series

Quartz dlorlte

20.6 

28.5 

24.6 ..

25.2

1.9 

7.3

2.2

52.1 

61.3 

59.5

54.0

1.8 

2.5 .

15.6 ..

8.8 .,

22. 8 0. 8 A. W. Quinn, unpub. data. Pawtucket quad . 

._-....- .4 Do.

.... __ .4 Quinn and Moore, 1968, p. 272. Pawtucket

quad. 

. _ _ 9.8 A. W. Quinn, unpub. data. Providence quad.

Hope Valley Alaskite Gneiss

36.1

34.1 

31.3 

43.0

34.5 

28.5 

24.2

28.3

31.9 

34.4 

34.2

18.4 

32.2 

39.9

32.7 

31.9 ..

31.3 

19.6

42.1 

35.5 

33.1

1.8 .

2.5 .

1.4 .

4.3 .

2.5

1.9 .

......... 1.0 Quinn and Moore, 1968, p. 272. Hope Valley

quad. 

......... 1.7 Do.

......... 1.5 Quinn and Moore, 1968, p. 272. CarolinaQuonochontaug quad. 

......... .8 Do.

0. 9 .4 Moore, 1963. Coventry Center quad. 

......... 1. 0 Moore, 1964. Kingston quad.

Ten Rod Granite Gneiss

30.4

31.4 

39.0 

31.8 

34.2 

31.9 

31.4

Quartz

28.6

28.8 

23.5 

31.8 

30.6 

22.1 

25.1

Potassium 

feldspar

35.0

31.5 

32.1 

30.9 

27.6 

38.9 

36.5

Plagioclase

5.0 .

4.2 

4.5 .

4.1 .

6.8

6.1 .

6.2

Blotite

quad. 

2. 8 1. 3 Do. 

......... .9 Do.

. ____ .8 Moore, 1963. Coventry Center quad.

......... .8 Do.

Horn- Epidote Musco- Acces- References 

blende vlte sories

Ponaganset Gneiss

38.1

37.0 

32.7 

29.0 

39.3

35.3 

35.8 

36.5 ..

Quartz

51.6

30.8 

27.3 

31.2 

22.5

16.4 

4.2

Potassium 

feldspar

9.0

25.6 

24.2 

29.4 

31.9

34.8 

39.5 . 

39.4

Plagloclase

0.8

6.1 ..

13.4

7.7 ..

5.0 ..

10.0 

14.7 ..

12.7

Blotite

.. ____ ... __ 0.3 0.2 Quinn, 1967. Chepachet

quad. 

.................... ......... .5 Do.

0.8 0.7 .......... .9 Do.

................... .2 2.6 Do.

data. Chepachet quad. 

1. 2 .6 .3 1. 4 Quinn, 1967. Chepachet 

quad.

Center quad. 

7. 3 2. 7 .......... 1. 4 Quinn . 1967. Chepachet

quad.

Riebeck- Aeglrite Acces- References 

ite sories

Quincy Granite

37.0 

39.7 

75.3 

35.4

55.5 ...

51.4 ...

14.6 ...

53.1 0.9 ..

2.4 3.8 1.2 A. W. Quinn, unpub. data. Paw 

tucket quad. 

6.6 2.4 .......... Quinn and Moore, 1968, p. 272.

Pawtucket quad.

tucket quad. 

........ 7.4 3.2 A. W. Quinn, unpub. data. Franklin quad.

See footnotes at end of table.
18 BEDROCK GEOLOGY OF RHODE ISLAND

TABLE 1. Typical modes of Rhode Island rocks Continued

Quartz Potassium Plaglo- Chlorlte Reference 

feldspar clase

Porphyrltic granite of Conanlcnt Island

35 32 27 6 D. R. Nichols, unpub. data. Narragansett Pier quad.

Quartz Plagio- Horn- Biotite Acces- References 

clase blende sorles

1.1

1.3 

13.1

9.7 

1.9

38.0 

12.5

30.5 

54.3 

46.5 

52.1

59.4 

86.0 ..

56.1 

8.1 

34.7 

45.4

0.1

10.9 

21.7 

7.1 

.6 ...

Metadlorlte

1. 4 Qulnn, 1967. Chepachet quad. 

1.5 Do.

1.2 Do. 

2.8 Do. 

2. 0 O. E. Moore, Jr., unpub. data. Clayvllle quad.

Plaglo- Blotlte Olivlne Pyrox- Magnetite, Apatite Alteration Reference 

clase ene llmenite products

Gabbro

37.1 4.2 3.0 16.4 7.4 7.8 24.1 Moore, 1963. Coventry Center

quad.

Quartz Potassium Plaglo- Blotlte Carbonate Acces- Reference 

feldspar clase minerals sorles

Absalona Formation

37.8 41.6 4.3 12.4 2.1 1.8 A. W. Qulnn, unpub. data. North Scituate

quad.

Quartz Potassium Plaglo- Biotite Musco- Carbonate Acces- Reference 

feldspar clase vite minerals sories

Nlpaachuek Gneiss

50.6 31.2 0.8 14.6 1.0 1.5 0.3 A. W. Qulnn, unpub. data.

Chepachet quad.

i Number In parentheses indicates number of thin sections studied. 

8 Rocks from Smith quarries at Westerly. 

1 Rocks from quarries near Bradford.

ABSALONA FORMATION

The Absalona Formation, which appears to overlie the Nipsachuck 

Gneiss, is a dark-gray biotite schist or gneiss with prominent porphyroblasts, an inch or more across, of pink or light-gray microcline. 

Microcline, microperthite, albite, quartz, and biotite are the main constituents, but muscovite, epidote, and hornblende are common. A typi-
OLDER METAMORPHIC ROCKS 19

cal mode is given in table 1. Round grains of quartz are also common. 

In a few places some of these grains are bluish quartz and some are 

colorless quartz. This probably indicates a sedimentary origin, quartz 

grains having been derived from different sources. Foliation and lineation are well developed.

In the Georgiaville quadrangle, in addition to the dominant porphyroblastic biotite gneiss, G. M. Richmond mapped several layers of 

quartz-biotite schist, ampibolite, and feldspathic quartzite.

WOONASQUATUCKET FORMATION

The Woonasquatucket Formation is exposed in the Georgiaville and 

the North Scituate quadrangles. According to Richmond (1952; Richmond and Alien, 1951), it is younger than the Absalona Formation. 

The formation is chiefly light-gray, medium-grained, indistinctly 

foliated and layered gneiss and schist. Large round grains of bluish 

quartz and phenocrysts of microperthite are prominent. The main 

constitutents are microperthite, albite, quartz, muscovite, and biotite. 

Modes are given in table 1. Much of the formation appears to have 

originated as crystal tuff or as lava flows, but Richmond described 

conglomeratic layers and more schistose rock in the Georgiaville 

quadrangle.

LIGHT-COLORED GNEISS

In the Chepachet and the Clayville quadrangles are two areas of 

light-colored gneiss. One of these is enclosed by the Absalona Formation of the Chepachet quadrangle. The other is west of the Absalona 

Formation and extends southward into the Clayville quadrangle. The 

rock is light-gray to gray, fine-grained to medium-grained distinctly 

foliated gneiss. The main constitutents are microcline, oligoclase, 

quartz, biotite, and muscovite; hornblende porphyroblasts are prominent locally. At a few localities are layers of quartzite and feldspathic 

quartzite. The original unmetamorphosed rock probably was feldspathic and quartzose sediment or f elsic volcanic debris.

METAMORPHIC ROCKS OF WESTERN AND SOUTHWESTERN 

RHODE ISLAND

Among the metamorphic rocks of western and southwestern Rhode 

Island are metavolcanic rocks in west-central Rhode Island and 

several formations in the extreme southwest that are continuous with 

Cambrian(?) and Ordovician(?) rocks of Connecticut (Feininger, 

1965a,b).

418-781 0 71-
20 BEDROCK GEOLOGY OF RHODE ISLAND

METAVOLCANIC ROCKS OF WESTERN RHODE ISLAND

An irregular mass of metavolcanic rocks and some metasedimentary 

rocks is exposed in the Coventry Center, Oneco, East Killingly, and 

Clayville quadrangles. These rocks are older than the enclosing plutonic rocks, but there is not sufficient evidence for any more specific age 

assignment. Much of the rock is light-gray to dark-gray, fine-grained, 

interlayered feldspathic gneiss, schist quartzite, amphibolite, and limesilicate rock. The main constituents are feldspars, muscovite, biotite, 

quartz, and amphibole. Staurolite and sillimanite are present locally.

CAMBRIAN (?) PLAINFIELD FORMATION

Medium-grained feldspar-quartz-biotite gneiss. In the Ashaway 

quadrangle are several metamorphic rocks that have been correlated 

with the Plainfield Formation of Cambrian(?) age of Connecticut 

(Feninger, 1965a). Medium-grained feldspar-quartz-biotite gneiss is 

one of the most voluminous of these rocks. Approximately 1 mile east 

of Ashaway is a synclinal structure with this gneiss as the lowermost 

formation. Structural complications and lack of outcrops in critical 

places prevent the estimation of the full thickness of the gneiss, but it 

appears to be greater than 2,000 feet. The rock is gray, mediumgrained, feldspar-quartz-biotite gneiss and schistose gneiss. Compositional layering is well-developed, with feldspathic quartzite, biotite 

quartzite, and dark hornblende gneiss.

Fine-grained quartzite. Fine-grained quartzite is exposed at several places in the Ashaway quadrangle. One layer of this quartzite is 

within the synclinal structure east of Ashaway, where it overlies the 

gneissic rocks assigned to the Plainfield Formation. This quartzite 

seems to underlie the same gneissic rocks, so that the quartzite probably 

forms different beds at different stratigraphic positions. The upper 

layer in the syncline appears to be approximately 700 feet thick. The 

rock is predominantly pale-tan to gray, fine-grained to mediumgrained, vitreous, massive to lenticularly bedded clean quartzite. This 

unit also contains layers of mica schist and calc-silicate rock.

Medium- to coarse-grained calc-silicate rock. Within the mediumgrained feldspar-quartz-biotite gneiss of the Ashaway quadrangle are 

several lenses of medium- to coarse-grained calc-silicate rock. The 

longest of these extends almost east through Hopkinton for 2 miles, 

with an outcrop width of approximately 300 feet. The rock is gray 

to dark-gray and black, massive to weakly foliated calc-silicate quartzite and calc-silicate granofels. It is composed chiefly of quartz, 

andesine, actinolite-tremolite, epidote, and diopside. It includes brightgreen epidote-rich gneiss southeast of Hopkinton.
OLDER METJIMORPHIC ROCKS 21

Plainfield Formation, undivided. Near the northwest corner of 

Rhode Island is a small area where material in the drift and somewhat 

uncertain outcrops suggest that the bedrock must be Plainfield Formation. Most of the rock seems to be thin-bedded quartzite and quartzmica schist.

ORDOVICIAN(?) ROCKS

Within the Ashaway and the Watch Hill quadrangles are three 

formations that have been assigned tentatively to the Ordovician System (Feininger, 1965 a, b) : layered felsic gneiss, pink facies of the 

layered felsic gneiss, and amphibolite.

Layered felsic gneiss. In the southern part of the Ashaway quadrangle and extending into the Watch Hill quadrangle is a rather 

extensive area of layered felsic gneiss. Included here is mostly finegrained to medium-grained, light- to medium-gray, strongly layered 

felsic gneiss. Most of the rock is composed of microcline, oligoclase, 

quartz, and biotite, but it also contains layers of biotite schist and 

amphibolite. The rock is intersected by many small irregular stringers 

of pink granite and pegmatite.

Pink facies of layered felsic gneiss. Within the Watch Hill quadrangle is an area of rock like the layered felsic gneiss, except that it 

has more microcline and less biotite and is pink. The pink facies is 

overlain and underlain by the gray felsic gneiss.

Amphibolite. In the Ashaway quadrangle are several areas of 

amphibolite. One of these is within the syncline east of Ashaway; the 

amphibolite there lies above the fine-grained quartzite of the 

Plainfield Formation. Other amphibolite bodies occupy different 

stratigraphic positions, however. This rock is predominantly mediumgrained to coarse-grained, layered and foliated amphibolite whose 

main constituents are hornblende, plagioclase, quartz, and biotite. 

Locally, it is porphyroblastic. Included also are layers of dark biotite 

schist, feldspathic granofels, and quartzite.

METAMORPHIC ROCKS OF SOUTHEASTERN RHODE ISLAND

In the vicinity of Newport are two groups of older metamorphic 

rocks: (1) slate and quartzite and (2) volcanic tuff, conglomerate, 

and quartzite. On the east shore of the Sakonnet River are two other 

groups of older metamorphic rocks: (1) mica-chlorite schist of Sakonnet and (2) chlorite-biotite schist of Tiverton. A fifth type of older 

metamorphic rock in southeastern Rhode Island is the mica schist of 

Bristol. The relation of these rocks to other rocks of the older metamorphic group is unknown.
22 BEDROCK GEOLOGY OF RHODE ISLAND

Slate and quartzite of Newport vicinity. Slate and quartzite are 

well exposed on the shores of the western part of Newport Neck. Rocks 

included here are greenish-gray, very light gray, and purple, finegrained to coarse-grained, thin-bedded slate and quartzite in alternating thin beds. Graded bedding and crossbedding are well developed 

at many outcrops. There are some thin beds of conglomerate and lenses 

of fine-grained marble. Discordance between bedding and cleavage is 

well exposed. Age relations of these rocks are not revealed, but general 

lithologic and structural characters suggest that they grade upward 

into the volcanic tuff, conglomerate, and quartzite of Newport Neck.

Volcanic tuff, conglomerate, and quartzite of Newport mcinity. 

The shores of Newport Neck also expose a considerable mass of volcanic tuff, conglomerate, and quartzite. Other exposures of these rocks 

are on the southeastern part of Conanicut Island, Cliff Walk of Newport, the outer shore of Sachuest Point east of Newport, and Gould 

Island near the north end of Sakonnet River. The formation is mostly 

dark-gray, dense, flinty to fine-grained felsic metavolcanic rocks that 

weather light gray to greenish gray. Originally, the rocks were probably tuff, lapilli tuff, volcanic sandstone, and volcanic siltstone. Some 

of the more massive layers may have been flows. Near the base are conglomerate beds as much as 50 feet thick, which contain pebbles of 

quartzite, quartz, granite, and volcanic rocks. There are also a few 

lenses of white to gray marble near the base. These rocks are separated 

from the slate and quartzite of Newport Neck by a fault. Exposures 

on Cliff Walk show that these rocks were intruded by the porphyritic 

granite of Newport and Conanicut Island. The porphyritic granite is 

overlain unconformably by the Pondville Conglomerate on the east 

shore of Mackerel Cove of Conanicut Island.

Mica-chlorite schist of Sakonnet. A rather large area of micachlorite schist is exposed along the shore north of Sakonnet Point 

in the Tiverton and Sakonnet Point quadrangles. This rock was 

intruded by the Bulgarmarsh Granite, which is Devonian (?) or 

older. Relations to other metamorphic rocks on the east side of the 

Narragansett basin are not known. The rock is mostly light-gray to 

greenish, fine-grained, thin-bedded mica-chlorite schist and is composed chiefly of muscovite, biotite, chlorite, and quartz. There are also 

many thin beds of marble and quartzite.

Chlorite-Hotite schist of Tiverton. Within the Tiverton quadrangle are two masses of chlorite-biotite schist that 'are almost surrounded 

by the Bulgarmarsh Granite. The granite is intrusive into the schist. 

The rock is mostly green to gray, fine-grained, poorly foliated chloritebiotite schist and is composed mostly of chlorite, biotite, and quartz. 

There are also some beds of amphibolite and of quartzite.
OLDER PLUTONIC ROCKS 23

Mica schist of Bristol. Pre-Pennsylvanian basement is exposed in 

an area on Bristol Neck in the Bristol quadrangle. Within this area is 

schist that is older than the Metacom Granite Gneiss, which apparently 

formed in part at least by the granitization of the schist. Inclusions 

or relicts of schist are common in the gneiss. Both the gneiss and the 

schist are overlain unconformably by the Pennsylvanian rocks. The 

rock is pink to light-gray, thin-bedded, strongly lineated schist composed chiefly of muscovite, quartz, and biotite.

OLDER PLUTONIC ROCKS

WEST OF THE NARRAGANSETT BASIN

A major part of Rhode Island is underlain by plutonic granitic 

rocks, and a major event in the geological history of this area was 

the formation of these rocks. They range in structure from distinctly 

foliated gneisses to almost massive granites and in composition from 

diorite or quartz diorite to alaskite. These rocks extend northward into 

Massachusetts and a short distance westward into Connecticut; they 

apparently constitute a complex batholith. Evidence in Rhode Island 

suggests that these rocks, perhaps with the exception of cumberlandite 

and some gabbro, resulted from one episode, perhaps somewhat prolonged, of plutonic activity. However, so few reliable geological ages 

are known that the possibility of more than one plutonic episode still 

remains. The earliest of the plutonic rocks are so strongly foliated that 

they are interpreted as having been syntectonic. Some of the later 

plutonic rocks are almost massive. The more mafic rocks are generally 

more foliated, but there are exceptions to this; some granite gneiss in 

northwestern Rhode Island is more foliated than some quartz diorite 

in northeastern Rhode Island.

The Scituate Granite Gneiss, the Hope Valley Alaskite Gneiss, and 

the Ten Rod Granite Gneiss were formerly all included in the Sterling 

Granite Gneiss and were assigned a late Carboniferous or postCarboniferous age (Loughlin, 1910; Loughlin and Hechinger, 1914). 

However, later mapping demonstrated that these several types of rocks 

can be distinguished. Furthermore, field relations indicate that these 

rocks are older than the Pennsylvanian rocks of the Narragansett 

basin (Quinn, 1951, 1952). Granitic rock that Loughlin observed to 

be intrusive into the Pennsylvanian rocks is the Narragansett Pier 

Granite. Recently, Goldsmith (1966) revived the name "Sterling," as 

the Sterling Plutonic Group in the New London area of Connecticut.
24 BEDROCK GEOLOGY OF RHODE ISLAND

METADIORITE

Several bodies of metadiorite and schistose diorite are exposed at 

widely separated localities in the Chepachet, Thompson, Clayville, 

Providence, and Crompton quadrangles. Age relations are not known 

for all the metadiorites, but some of these are younger than the Blackstone Series, and some are older than granitic rocks of the older Plutonic series. The rocks are rather variable, even within small exposures. 

They vary from gray to dark gray and greenish, fine grained to coarse 

grained, and schistose and streaky to almost massive. The main constituents are hornblende, plagioclase, quartz, biotite, epidote, and 

chlorite. Eetrograde alteration of these minerals is common. Typical 

modes are given in table 1. Angular and sharp inclusions of quartzite 

and schist, probably of the Blackstone Series, are exposed at several 

places. In the Crompton quadrangle the metadiorite seems to form 

dikes extending into the Blackstone Series. Most of these rocks probably formed as diorite intrusives, but some may have resulted from 

the metamorphism of sedimentary or volcanic rocks.

PONAGANSET GNEISS

The earliest of the granitic older plutonic rocks probably is the 

Ponaganset Gneiss of northwestern Rhode Island, which underlies 

a large area, chiefly in the Chepachet and Clayville quadrangles and 

also in the Uxbridge, Oxford, Thompson, East Killingly, Oneco, and 

Voluntown quadrangles. It is probably continuous into and equivalent 

to the Northbridge Granite Gneiss of Massachusetts, although, as 

stated earlier, a definite correlation involves confusion concerning 

ages. Detailed study of a considerable area in Massachusetts will be 

needed in order to determine how far the Ponaganset Gneiss extends 

into Massachusetts and what relationships it has to the metamorphic 

rocks and the granitic rocks there.

The Ponaganset Gneiss in Rhode Island includes a considerable 

variety of gneissic and granitic rock. The different types are arranged 

chiefly in north-trending lenses and layers, mostly too small to show 

even on a scale of 1:24,000. The rock is mostly gray to light gray, but 

some is pinkish, and some lenses are dark gray. The texture is mostly 

medium grained to coarse grained, but some rock is fine grained and 

some is strikingly porphyroblastic. All types are distinctly gneissic 

or foliated, and most are lineated. In most places the gneissic structure 

is steep and trends northward, and the lineation plunges northward. 

In some places a second foliation trends westward and dips gently 

northward. The chief minerals are potassium feldspar, plagioclase, 

quartz, and biotite. Minor minerals include hornblende and muscovite.
OLDER PLUTONIC ROCKS 25

Most of the potassium feldspar is microcline with fine twinning, and 

some is perthitic. Composition of the plagioclase varies from Anu to 

An35 . The compositions of the rocks correspond to quartz diorite, 

grandodiorite, quartz monzonite, and granite. Typical modes are given 

in table 1.

Eelationships along the contact between the Ponaganset and the 

Blackstone in the eastern part of the Chepachet quadrangle show 

clearly that the Ponaganset Gneiss is younger than the Blackstone 

Series. Some of this evidence indicates that the Ponaganset came in as 

liquid or partly liquid material that pushed schist and quartzite of 

the Blackstone Series aside and rotated blocks within this liquid. In 

other places there is clear indication of considerable granitization of 

the Blackstone Series. Evidence of these two processes, liquid intrusion 

and granitization, is present not only along the margin, but also well 

within the area of the Ponaganset Gneiss. An abandoned quarry just 

south of Pascoag in the Chepachet quadrangle shows clear examples of 

relicts. G. E. Moore, Jr., has observed rotated inclusions within the 

Clayville quadrangle ( written commun., 1963).

This evidence is interpreted to mean that a mass of sedimentary rock, 

probably graywacke or other feldspathic sediments, was heated until 

some became liquid or partly liquid so that it intruded higher rocks. 

Some water-rich solutions also invaded the higher rocks. This all occurred under considerable stress, and these rocks are syntectonic.

At Escoheag Hill in the Voluntown quadrangle is a dark-gray 

coarse-grained and markedly porphyritic foliated quartz diorite 

gneiss. Textural layers are prominent. Tan microcline phenocrysts as 

much as 3 inches long give the rock an unusual appearance; many of 

these are discordant to the foliation. Chief minerals are oligoclaseandesine, quartz, microcline, and biotite. Magnetite is common and 

hornblende is present locally. Feininger (1965b) described this as 

magmatic in origin. Harwood and Goldsmith (1971) have called this 

unit the porphyritic facies of the Ponaganset Gneiss in the Oneco 

quadrangle.

PORPHYRITIC GRANITE GNEISS

In the Voluntown quadrangle are a few areas underlain by porphyritic granite gneiss of rather heterogeneous character. The rock ranges 

from light gray to pink, most is medium grained but some is fine 

grained, and foliation is well developed at most places. Platy microcline phenocrysts are mostly parallel to, but some are discordant to, the 

foliation. Layering is prominent at many outcrops and is emphasized 

by sills of Hope Valley Alaskite Gneiss in some areas. The porphyritic 

granite gneiss is composed of almost equal proportions of microcline, 

oligoclase, and quartz; biotite is the main mafic constituent.
26 BEDROCK GEOLOGY OF RHODE ISLAND

This gneiss was intruded by both the Scituate Granite Gneiss and 

the Hope Valley Alaskite Gneiss. It is probably most closely related 

to the Ponaganset Gneiss. Feininger (1965b) described it as magmatic 

and syntectonic.

SCITUATE GRANITE GNEISS

The Scituate Granite Gneiss is one of the most widely exposed formations in Rhode Island. The largest area of exposure is in the North 

Scituate, Clayville, Crompton, Coventry Center, Slocum, and Hope 

Valley quadrangles, but smaller areas of this rock are exposed in most 

of the other quadrangles west of the Narragansett basin.

The Scituate Granite Gneiss is grouped with other members of the 

older plutonic rocks for reasons discussed earlier in this report. Especially clear is the evidence that it is closely related to the Hope Valley 

Alaskite Gneiss and the Ten Rod Granite Gneiss. The Scituate Granite 

Gneiss is gradational into the Hope Valley Alaskite Gneiss in the Hope 

Valley quadrangle. The Ten Rod Granite Gneiss is intrusive into the 

Scituate in the Hope Valley quadrangle. The Scituate Granite Gneiss 

probably is younger than the Ponaganset Gneiss, as is suggested by the 

greater development of foliation in the Ponaganset. Evidence in the 

North Scituate quadrangle seems to show that the Scituate Granite 

Gneiss is older than the Esmond Granite.

The Scituate Granite Gneiss is pink, tan, or gray, medium grained to 

coarse grained, and commonly has a lineation marked by splotches of 

biotite. Locally, it is also foliated. The main constituents are microcline and microperthite, albite-oligoclase, smoky quartz, and biotite; 

commonly, hornblende and magnetite are megascopically visible. The 

splotches of biotite commonly also contain unusual concentrations of 

such accessory minerals as sphene (keilhauite; Young, 1938), magnetite, zircon, epidote, and apatite. Typical modes are given in table 1.

At most outcrops the origin of the Scituate Granite Gneiss is not 

shown, but at several places in the North Scituate and the Hope Valley 

quadrangles there is evidence of its magmatic intrusion into the Blackstone Series and other rocks. There is no evidence that any large part 

of this rock was formed by granitization.

HOPE VALLEY ALASKITE GNEISS

. The Hope Valley Alaskite Gneiss was named by Moore (1958) for 

extensive exposures in the Hope Valley quadrangle. It is also exposed 

widely in the Carolina, and other quadrangles in the southern part of 

the State and in the Thompson and Oxford quadrangles near the southwest corner of the State. Moore (1958) discovered that the Hope Valley 

Alaskite Gneiss is gradational into and of the same age as the Scituate 

Granite Gneiss.
OLDER PLUTONIC ROCKS 27

The Hope Valley Alaskite Gneiss is mostly pink or light tan, but 

also gray, medium-grained, lineated alaskite gneiss. Lineation consisting of rod-shaped aggregates of smoky and gray quartz is a conspicuous character of the rock, as is the scarcity of biotite. Magnetite grains 

are common. The main constituents are microcline, albite-oligoclase, 

quartz, and magnetite. Typical modes are given in table 1.

Fine-grained Hope Valley Alaskite Gneiss. A few small areas are 

underlain by rock that is like the Hope Valley Alaskite Gneiss, except 

that the grain size is smaller. Some of those are shown on the map.

TEN ROD GRANITE GNEISS

The Ten Rod Granite Gneiss was named for exposures on Ten Rod 

Road in the Hope Valley quadrangle (Moore, 1958). One rather large 

area of this rock is within the Slocum, Kingston, Wickford, and Narragansett Pier quadrangles. Smaller areas of this rock have been 

mapped in the Coventry Center, Hope Valley, and Carolina quadrangles.

The Ten Rod Granite Gneiss is pinkish-gray to medium-gray, 

medium-grained to fine-grained, porphyritic or porphyroblastic granite gneiss. The chief minerals are microcline, albite-oligoclase, quartz, 

and biotite. It generally has a well-developed foliation and some lineation. Large crystals of pinkish to tan microline give this rock a distinctive appearance. Typical modes are given in table 1.

Moore (1958) described evidence indicating that the Ten Rod Granite Gneiss is younger than the Scituate Granite Gneiss 'and the Hope 

Valley Alaskite Gneiss and that it has a magmatic origin. However, 

Nichols (1956) in the Narragansett Pier quadrangle and Power (1959) 

in the Slocum quadrangle believed that this "augen gneiss" resulted 

from granitization of masses of older rocks such as the Blackstone 

Series.

Nonporphyritic fades of Ten Rod Granite Gneiss. In the southwest corner of the Wickford quadrangle is a nonporphyritic facies of 

the Ted Rod Granite Gneiss, which Williams (1964) mapped separately. It is gradational into Ten Rod Granite Gneiss that has the usual 

porphyritic or porphyroblastic texture. The nonporphyritic facies 

differs also by containing muscovite.

FINE-GRAINED GRANITE RELATED TO SCITUATE GRANITE GNEISS AND TEN ROD

GRANITE GNEISS

Throughout the areas of Scituate Granite Gneiss and Ten Rod 

Granite Gneiss are numerous granitic instrusives, some of which are 

large enough to be shown on the map. These commonly appear to have 

approximately the same composition as the surrounding coarse gneiss,

413-731 O 71 -5
28 BEDROCK GEOLOGY OF RHODE ISLAND

but they are finer grained and they commonly show less foliation and 

lineation. In some places, foliation is parallel to the margins of the 

finer grained rock. In the North Scituate and Crompton quadrangles, 

the finer grained rock seems to have been formed as late-stage intrusives into the coarser rock.

POTTER KILL, GRANITE GNEISS

The Potter Hill Granite Gneiss was named for a village in the Ashaway quadrangle (Feininger, 1965a). Feininger mapped a considerable 

area of this rock in Khode Island and nearby Connecticut. The rock is 

light orange to pink, fine grained to medium grained, equigranular to 

locally porphyritic, and very strongly foliated. Most outcrops are 

weathered, deeply stained, somewhat crumbly, and generally slabby. 

The rock is composed chiefly of microcline, oligoclase, quartz, biotite, 

magnetite, and minor muscovite. Feininger (1965a) described it as 

largely magmatic, but locally containing many inclusions of gneiss of 

the Plainfield Formation. He used the foliation to delineate the shapes 

of the several Potter Hill intrusives. This rock grades into the Hope 

Valley Alaskite Gneiss.

QUARTZ DIORITE, SEVERAL TYPES

There are several bodies of quartz diorite in the Providence, Pawtucket, and Franklin quadrangles. The rock in an individual body is 

somewhat variable and streaky, and separate bodies differ somewhat 

for each other. Most of them are probably approximately the same age, 

but within the Pawtucket quadrangle there is evidence of at least two 

different ages. Generally the quartz diorite seems to be older than the 

associated more granitic rocks.

Most of the quartz diorite is dull gray, medium grained to coarse 

grained, and variably foliated. Some is porphyritic. The chief minerals 

are oligoclase or andesine, quartz, biotite, locally occurring hornblende, 

and minor microcline. Much of the plagioclase has been altered to more 

sodic composition, with resulting formation of epidote and muscovite. 

Typical modes are given in table 1. Most of the quartz diorite seems to 

be of magmatic origin, but replacement of mafic metamorphic rocks 

is also indicated for some quartz diorite in the Pawtucket quadrangle.

GRANT MILLS GRANODIORITE

Grant Mills Granodiorite, which was named for a former settlement 

in the Franklin quadrangle (Warren and Powers, 1914), is exposed 

in several moderate-sized areas in the Franklin, Pawtucket, and 

Providence quadrangles.
OLDER PLUTONIC ROCKS 29

This granodiorite is gray to greenish gray, strikingly porphyritic, 

with a, medium- to coarse-grained matrix, and massive to foliated 

and streaky. It consists chiefly of albite-oligoclase, microcline microperthite, quartz, and biotite. The phenocrysts are microcline microperthite. Much of the plagioclase has been changed to albite, epidote, 

and muscovite, and some of the biotite has been altered to chlorite. 

Typical modes are given in table 1.

Evidence of magmatic intrusive origin has been described in reports on the Pawtucket and the Providence quadrangles. Intrusive 

relations show that the granodiorite is younger than some of the quartz 

diorite in the Pawtucket quadrangle. Also in the Pawtucket quadrangle, the Grant Mills Granodiorite grades into the Esmond Granite, 

which presumably means that these two rocks are of the same age.

ESMOND GRANITE

The Esmond Granite was named (Quinn and others, 1948) for the 

Esmond district just beyond the southwest corner of the Pawtucket 

quadrangle. Formerly there was a rather large granite-quarrying 

industry there. The Esmond Granite is also exposed in the Franklin, 

Blackstone, Chepachet, Georgiaville, North Scituate, Providence, East 

Greenwich, and Crompton quadrangles. This granite was correlated 

with the Milford Granite of Massachusetts by Emerson (191Y, p. 165, 

166), but the lithologic resemblance is not very strong, and intervening areas have not been studied in much detail.

Much of the Esmond Granite is light pink to light gray, light tan, 

and greenish, medium to coarse grained, and massive. However, some 

is distinctly foliated. The greenish color characteristic of much of this 

rock is due to the presence of much fine-grained epidote. The rock consists chiefly of microperthite, albite, quartz, and biotite. Much of the 

plagicoclase has been altered to albite, epidote, and muscovite, and 

some of the biotite has been altered to chlorite. Typical modes are 

given in table 1.

At several places sharp angular and rotated inclusions of Blackstone Series and other rocks within the Esmond Granite give evidence 

of a magmatic origin for the granite; such evidence has been described 

in the Providence, Pawtucket, Chepachet, and North Scituate quadrangle reports. There is also some evidence of granitization, especially 

in the Georgiaville quadrangle. Intrusive relations show that the 

Esmond Granite is younger than the Blackstone Series. Near the north 

margin of the North Scituate quadrangle it appears to be intrusive 

into and younger than the Scituate Granite Gneiss. As indicated above, 

it appears gradational into the Grant Mills Granodiorite in the Pawtucket quadrangle. Intrusive relations in the Pawtucket quadrangle 

show that the Esmond is older than the Quincy Granite.
30 BEDROCK GEOLOGY OF RHODE ISLAND

FINE-GRAINED GRANITE RELATED TO ESMOND GRANITE

Fine-grained granite related to the Esmond Granite is exposed in 

several areas too small to show on this State map and in one larger 

area shown in the Pawtucket and Georgiaville quadrangles. Except 

for the fine grain, this rock is very similar to the Esmond Granite. In 

the Pawtucket quadrangle there seems to be some gradation into the 

Esmond Granite, but in the Georgiaville quadrangle some dikes of the 

fine-grained rock intrude the Esmond Granite.

The rock is light gray to gray and pink, fine grained, and massive 

to faintly foliated. It consists of microperthite, albite, quartz, biotite, 

and muscovite. Plagioclase and biotite are altered in the same way as 

in the Esmond Granite.

GABBRO, PROBABLY OF SEVERAL AGES

At three places in Rhode Island are gabbroic rocks of sufficient extent to show on the State map. There rocks are widely separated and 

there is little evidence concerning their ages and their relationships 

to each other.

The northernmost gabbro body is near the Massachusetts State line 

in the Pawtucket and Franklin quadrangles, just west of the cumberlandite at Iron Mine Hill. In addition to the Pawtucket reports (Quinn 

and others, 1948, 1949), a report on Woonsocket (Quinn and Alien, 

1950) describes this gabbro briefly and gives references to earlier reports. The rock is dark gray, medium grained to coarse grained, and 

massive. It is composed chiefly of labradorite, altered ferromagnesian 

minerals, and magnetite. Within Rhode Island the only evidence concerning the age of this rock is that inclusions of it are contained in the 

cumberlandite. Warren and Powers (1914, p. 450) reported that at 

Sheldonville, Mass., a similar gabbro was intruded by biotite granite 

that probably is related to the Esmond Granite.

Two masses of gabbroic rock are near the southern margin of the 

Georgiaville quadrangle. This rock was coarse grained and porphyritic, 

but it has been much altered, and the grain size has been reduced. It had 

platy phenocrysts of plagioclase, but these have been altered. It now 

consists chiefly of chlorite, albite, epidote, and magnetite, with small 

amounts of amphibole and quartz. Apparently it is younger than the 

Woonasquatucket Formation and the Absalona Formation and older 

than the Pennsylvanian Bellingham Conglomerate of the North Scituate basin.

A third area of gabbro is chiefly in the Coventry Center quadrangle 

(Moore, 1963), although it extends short distances into the Clayville, 

East Killingly, and Oneco quadrangles. The rock is uniformly coarse 

grained, dark gray to grayish purple, with platy crystals of grayish-
OLDER PLUTONIC ROCKS 31

purple andesine-labradorite, and lesser amounts of augite, olivine, biotite, magnetite, and apatite. A typical mode is given in table 1. The 

rock has a well-developed foliation which appears to be a primary 

flow structure. This rock is intrusive into the surrounding metamorphic 

rocks. It shows no effects of the regional metamorphism that has affected even the Pennsylvanian rocks in Rhode Island, so that this is 

probably a Pennsylvanian or younger rock. In this respect it appears 

to differ from the other gabbroic rocks of Rhode Island.

CUMBERLANDITE

Cumberlandite, the titaniferous magnetite rock at Iron Mine Hill 

in the Franklin quadrangle, has been known for 250 years or more. The 

peculiar character of the rock and the possibility of its economic use 

as an iron ore have attracted the attention of many people. As a 

result, many articles have been written about the rock, the most useful 

being those by Johnson (1908), Wadsworth (1881, 1882), and Singewald(1913).

The rock is black to brown with scattered white or greenish platy 

crystals (originally of labradorite) as much as half an inch long. The 

matrix is fine grained to medium grained, and in places the platy 

crystals are alined, probably as a result of flow. The rock is very heavy 

and is also weakly magnetic. The fresh rock consists of magnetiteilmenite intergrowth, olivine, labradorite, and spinel. Fresh rock is 

rare, however; in most cumberlandite, the olivine and labradorite have 

been replaced by sepentine, actinolite, and other alteration products. 

This rock is peculiar to Rhode Island, although a somewhat similar 

rock has been described at Taberg, Sweden (Singewald, 1913, Iron 

Minehill, p. 40-46).

The original hill has been described as 1,200 feet long and 600 feet 

wide, but perhaps half of it has been quarried away. No contact with 

other rocks is exposed, but the cumberlandite contains a few inclusions 

of the gabbro that crops out to the west. Schist of the Blackstone Series 

is exposed a few hundred feet to the northeast, but the relationship is 

not known. Hurley and Goodman (1943) obtained an age of 1,500 m.y. 

for the cumberlandite, by using the helium method on magnetite, but 

this method is of doubtful value. The mineral composition and the 

presence of inclusions of gabbro suggest an igneous origin of the 

cumberlandite.

The cumberlandite meets the requirements of an ideal glacial boulder 

train: there is only one source, the source is small but large enough 

to supply an adequate amount of material, the rock is readily recognized, and it is resistant to weathering. As a consequence, this boulder 

train was described long ago (Shaler, 1893). The train extends a little
32 BEDROCK GEOLOGY OF RHODE ISLAND

east of south, mostly on the east and central parts of Narragansett Bay 

to Newport and beyond. Cumberlandite boulders have been discovered 

on Block Island and on Martha's Vineyard.

SOUTHEASTERN RHODE ISLAND

Several older plutonic rocks are exposed in southeastern Ehode 

Island. There is no strong lithologic resemblance or other evidence for 

correlating these rocks with pre-Pennsylvanian rocks on the west side 

of the Narragansett basin, nor with Dedham Granodiorite (Emerson, 

1917).

METACOM GRANITE GNEISS

The Metacom Granite Gneiss was named for exposures on Metacom 

Avenue in the Bristol quadrangle (Quinn and Springer, 1954). Other 

exposures are on the northeast part of Aquidneck Island and on the 

mainland to the east, in the Tiverton and Fall River quadrangles.

The gneiss is gray to pink, medium grained, locally porphyritic, 

strongly lineated with oval patches of biotite, and foliated locally. 

It is composed chiefly of microcline, microperthite, albite, quartz, 

biotite, and muscovite. Typical modes are given in table 1. Small aplite 

dikes and quartz veins are common. Oriented inclusions of mica schist, 

and gradation of the gneiss into the adjacent mica schist of Bristol 

are evidence that the gneiss originated by granitization of the schist.

The mica schist and the gneiss, together with the Bulgarmarsh 

Granite, constitute the basement on which the Pennsylvanian sediments 

of that part of the Narragansett basin were deposited.

PORPHYRITIC GRANITE AT NEWPORT AND CONANICUT ISLAND

Porphyritic granite is exposed on Newport Neck in the Newport 

quadrangle and on Conanicut Island in the Newport and Narragansett 

Pier quadrangles.

The rock is gray, pink, and greenish, coarse grained, contains pink 

phenocrysts of microcline, and is generally massive. It consists chiefly 

of microcline, microperthite, orthoclase, albite-oligoclase, quartz, biotite, hornblende, and chlorite. Locally, the mineral grains are much 

strained and crushed; in these places, alteration has produced chlorite, 

epidote, and sericite. A typical mode is given in table 1.

On the Cliff Walk on Newport Neck and on Conanicut Island the 

granite is intrusive into and younger than the volcanic tuff, conglomerate, and quartzite. On the east shore of Mackerel Cove in the Narragansett Pier quadrangle is an exposure showing that Pennsylvanian 

conglomerate lies unconformably on the granite. No similar porphyritic granite is known on the west side of the Narragansett basin.
IGNEOUS ROCKS OF MISSISSIPPIAN (?) OR OLDER AGE 33 

BULGARMARSH GRANITE

The Bulgarmarsh Granite was named for exposures on Bulgarmarsh 

Eoad in the Tiverton quadrangle (Pollock, 1964). It is widely exposed 

in the Tiverton, Fall River, and Sakonnet Point quadrangles and in 

adjacent Massachusetts.

The Bulgarmarsh Granite is pink to gray, coarse grained, and foliated to massive. It is composed chiefly of microcline, microperthite, 

albite, quartz, biotite, chlorite, and muscovite. Plagioclase and biotite 

are somewhat altered.

The granite intruded the chlorite-biotite schist of Tiverton in a manner indicating a magmatic origin of the granite. It is overlain unconf ormably by Pennsylvanian conglomerate of the Narragansett basin in 

the Fall River and Tiverton quadrangles.

IGNEOUS ROCKS OF MISSISSIPPIAN (?) OR OLDER AGE

Igneous rocks of considerable variety that appear to be younger than 

the older plutonic rocks of Rhode Island and older than the Pennsylvanian rocks of the Narragansett basin occur in the Pawtucket and 

Franklin quadrangles of northeastern Rhode Island and in the East 

Greenwich and Crompton quadrangles of central Rhode Island. The 

northeast group has been correlated with the Quincy Granite of Massachusetts, which had been assigned a Carboniferous (Emerson, 1917, 

p. 188) or a Mississippian(?) age (Quinn and others, 1948, p. 16), 

but is now considered to be Devonian(?) in age (Chute, 1969). Correlation of the Quincy Granite and the granite porphyry with the East 

Greenwich Group seems probable, although evidence for this is not 

strong.

QUINCY GRANITE AND GRANITE PORPHYRY

The Quincy Granite is gray to light gray, medium grained, and 

mostly massive but has flow structure along the contacts. Microperthite, quartz in small grains, riebeckite, and aegirite are the main 

constituents. Patches and veins of bright-purple fluorite are common. 

Typical modes are given in table 1.

The granite porphyry is gray to dark gray, fine grained with round 

phenocrysts of quartz and squarish phenocrysts of microperthite, and 

generally has flow structure. Bright-purple fluorite is common, as are 

inclusions of schist.

The Quincy Granite and granite porphyry form several hills in the 

northern part of the Pawtucket quadrangle and also in the Franklin 

quadrangle to the north. The intrusive bodies are moderate sized to 

small, and some of them include both granite and granite porphyry.
34 BEDROCK GEOLOGY OF RHODE ISLAND

The granite of the quarry area of Quincy, Mass., has such a distinctive composition (perthitic feldspar, granular quartz, riebeckite, and 

aegirite) that the similar rocks of northeastern Khode Island are correlated with it with some assurance. The association of the granite with 

granite porphyry in both places is further evidence. In the Blue Hills 

area of Massachusetts, a pre-Pennsylvanian age seems well established 

by the presence of boulders of the Blue Hill Granite Porphyry in the 

"giant conglomerate" of the Pennsylvanian Norfolk basin, at Routes 

28 and 128 (Chute, 1964).

Chute (1966, p. B28) has described evidence that the Blue Hill 

Granite Porphyry of the Norwood, Mass., quadrangle is not closely 

related in age to the Quincy Granite, but Chute (1969) considers Blue 

Hill and Quincy to be Devonian (?) and the Blue Hill to be younger 

than Quincy. (See "Igneous rocks of Mississippian( ?) or older age.") 

The evidence in Rhode Island is that the granite porphyry in Rhode 

Island is closely related in age to the Quincy Granite of the Pawtucket 

quadrangle. The Quincy Granite and associated rocks seems to be 

somewhat younger than rocks of the Boston vicinity that have been 

assigned to the Devonian System (Emerson, 1917, p. 188). However, 

the Devonian age of the older rocks is not well established, and, therefore, this limit to the age of the Quincy Granite is not well established 

by geological relations. In the Pawtucket quadrangle, the rock mapped 

as Quincy Granite is intrusive into the Esmond Granite. The 

rubidium-strontium whole-rock age of 3-05±14 m.y. for the Quincy 

Granite as mapped in Massachusetts (Bottino, 1963) is not quite in 

agreement with a Mississippian age. More recently, R. E. Zartman and 

others (U.S. Geological Survey, 1967, p. A166-A167) have obtained 

radiometric ages that suggest an Ordovician age for the same Quincy 

Granite (nine samples, Rb-Sr whole-rock age, 340-400 m.y.; four 

samples, K-Ar hornblende age, 430-457 m.y.). Zartman favors the 

greater hornblende age.

EAST GREENWICH GROUP

The name, East Greenwich Group (called "East Greenwich Granite 

Group" on their map), was originated by Emerson and Perry (1907, 

p. 58-70). It has also been discussed in the East Greenwich Quadrangle 

report (Quinn, 1952). The rocks of this unit appear to constitute a 

closely related group of extrusive, shallow intrusive, and plutonic 

rocks. Some of the volcanic rocks contain boulders that appear to be 

Esmond Granite. These volcanic rocks were intruded by the Cowesett 

Granite. The whole group appears to be overlain unconformably by 

the Pennsylvanian rocks of the Narragansett basin. This evidence 

leaves some uncertainty about the age of the East Greenwich Group,
IGNEOUS ROCKS OF MISSISSIPPIAN (?) OR OLDER AGE 35

but a Mississippian or earlier age is plausible. The perthitic character 

of the granitic rocks and the association of volcanic, hypabyssal, and 

plutonic rocks gives some support to a correlation of the East Greenwich Group with the Quincy Granite and associated rocks. Riebeckite 

and aegirite are absent from or rare in the group in the East Greenwich 

quadrangle but are sparingly present in the Crompton quadrangle. The 

table of radiometric ages (table 2) lists lead-alpha ages on zircon from 

the Cowesett Granite ranging from 257 to 275 m.y. These are in disagreement with the pre-Pennsylvanian age, which seems well 

established.

SPENCER HILL VOLCANICS

The Spencer Hill Volcanics were named in the East Greenwich 

quadrangle report for exposures on Spencer Hill near the west margin 

of the quadrangle. These rocks extend eastward a considerable distance 

in the East Greenwich quadrangle and westward a short distance into 

the Crompton quadrangle. They include rhyolitic rocks that appear 

to be flows and a great amount of volcanic tuff, breccia, and conglomerate. They are gray, dark gray, pink, greenish, and purplish. Rhyolites are mostly fine grained and some are porphyritic. Flow structure 

is well developed. The composition is mostly felsic, but some appears 

to be as mafic as andesite. Chief minerals are microcline, microperthite, plagioclase, quartz, biotite, and muscovite. The pyroclastic 

rocks contain poorly sorted pebbles and other fragments. Most fragments are 2 or 3 inches in diameter, but some are as much as 2 feet 

across. The fragments include a considerable variety of rock types, 

such as rhyolite, quartzite, schist, and granite. The trend of flow 

structure and irregular bedding is mostly east, and the dips are moderate to steep northerly. New roadcuts in 1966 on U.S. Route 95 made 

good exposures of the Spencer Hill Volcanics.

/ MASKERCHUGG GRANITE

East and southeast of the Spencer Hill Volcanics is an area of 

Maskerchugg Granite, named in the East Greenwich quadrangle 

report for exposures along Maskerchugg River. This granite appears 

to be intrusive into the Spencer Hill Volcanics and to have been intruded by Cowesett Granite and by granite porphyry. It is overlain 

unconf ormably by the Pennsylvanian rocks of the Narragansett basin.

The rock is gray to light tan, fine grained with a few small phenocrysts, and streaky with a well-developed flow structure. The main 

constituents are microcline, microperthite, micropegmatite, quartz, 

and biotite. Phenocrysts include microperthite, micropegmatite, and 

quartz.
36 BEDROCK GEOLOGY OF RHODE ISLAND

GRANITE PORPHYRY ASSOCIATED WITH COWESETT GRANITE

A small round area of granite porphyry transects the contact of 

the Spencer Hill Volcanics with the Maskerchugg Granite in the East 

Greenwich quadrangle. The granite porphyry appears to be intrusive 

into both the Spencer Hill Volcanics and the Maskerchugg Granite, 

but exposures are sparse and relations are uncertain.

The granite porphyry is gray, bluish gray, to pink, fine to medium 

grained, porphyritic, and massive. The main constituents are microcline, microcline-microperthite, oligoclase, quartz, and biotite. The 

phenocrysts are chiefly pink microcline, microcline-microperthite, gray 

to bluish-gray quartz, and biotite.

COWESETT GRANITE

The most extensive member of the East Greenwich Group is the 

Cowesett Granite, named for Cowesett Road in the East Greenwich 

quadrangle; it extends also into the Crompton, Slocum, and Wickford quadrangles. It is intrusive into rocks of the Blackstone Series, 

the Spencer Hill Volcanics, and the Maskerchugg Granite. It appears 

to be overlain unconformably by Pennsylvanian rocks of the Narragansett basin. Contacts with quartzite of the Blackstone Series are 

sharp, and some blocks of the quartzite were rotated in the Cowesett 

Granite magma.

The Cowesett Granite is light gray to pink, medium grained to 

coarse grained, subporphyritic, and mostly massive but has flow structure along contacts. The main constituents are microcline, microperthite, -albite, quartz, and biotite. Typical modes are given in table 1.

PERTHITIC COWESETT GRANITE

Along the west side of the Cowesett Granite area of the Crompton 

quadrangle are two areas of perthitic Cowesett Granite. It appears 

to be gradational into the Cowesett Granite.

The granite is gray where fresh, but tan or brown where weathered. 

It is coarse grained to medium grained, subporphyritic, and has 

oval clusters of biotite and some flow structure. The main constituents 

are glassy feldspar, quartz, and biotite. The feldspar is chiefly microperthite, containing only very minor plagioclase in separate grains. 

Eiebeckite is present in a few places in this rock, and purple fluorite 

is common. The presence of riebeckite and purple fluorite add some 

further support to correlation of the Cowesett Granite with the Quincy 

Granite.
PENNSYLVANIA ROCKS 37 

FINE-GRAINED GRANITE RELATED TO COWESETT GRANITE

In the East Greenwich quadrangle near the northern margin of 

the Cowesett Granite area is a dikelike intrusive mass of dark finegrained granite, intrusive into the Cowesett Granite.

The rock is gray to dark gray, fine grained and porphyritic, and 

mostly massive but has flow structure along the margins. It consists 

chiefly of microperthite, micropegmatite, oligoclase, quartz, and 

biotite.

PENNSYLVANIAN ROCKS

The Narragansett basin which contains a large mass of Pennsylvanian rocks, is the most prominent geologic feature in eastern Rhode 

Island and adjacent Massachusetts. Two smaller masses of probably 

Pennsylvanian rocks, contained in the Woonsocket and North Scituate 

basins are exposed a few miles west of the Narragansett basin. These 

Pennsylvanian rocks are in marked contrast to the adjacent older 

rocks; they lie unconformably on the older rocks, trend in different 

directions, include many layers of fossiliferous rocks and some unmetamorphosed rocks, and are generally lithologically dissimilar.

The most recent general description of the Pennsylvanian rocks in 

this area is by Quinn and Oliver (1962). A massive report was published in 1899, by Shaler, Woodworth, and Foerste. The following 

published quadrangle reports (see "Introduction") treat at least small 

areas of Pennsylvanian rocks: Georgiaville, Pawtucket, North Scituate, Providence, Crompton, East Greenwich, Kingston, Bristol, 

Tiverton, and Narragansett Pier.

NARRAGANSETT BASIN

The Narragansett basin occupies the area of Narragansett Bay, 

trending northward in eastern Rhode Island and extending northeastward into Massachusetts. Actually, more of the Narragansett 

basin is in Massachusetts than in Rhode Island, but exposures are 

better in Rhode Island, and much of what is known about these rocks 

has been learned there. This basin is a complex synclinal mass of 

clastic sedimentary rocks. It is almost 55 miles long and. about 15 

to 25 miles wide. The western margin of the basin is in the western 

part of Providence, R.I., and the east margin runs through Fall 

River, Mass. The northeast extremity is near Hanover, Mass. Exposures of older rocks on Conanicut Island and on Newport Neck suggest that the southern end of the basin is near the mouth of Narragansett Bay. The drowned-valley system now occupied by the bay 

has been eroded in the less resistant rocks of the Narragansett basin.
38 BEDROCK GEOLOGY OF RHODE ISLAND

The rocks of the Narragansett basin are predominantly clastic 

nonmarine sedimentary rocks of graywacke and arkosic composition 

that range from coarse conglomerate to shale. Sharp and irregular 

variations, both vertical and lateral, in lithologic character, together 

with probably complex structure, combine to make the study of these 

rocks difficult. Total thickness has been estimated at approximately 

12,000 feet (Shaler and others, 1899, p. 134), but the evidence for this 

is weak. The greatest measured stratigraphic thickness, apparently 

without duplication, is in a tunnel in the Providence quadrangle where 

a thickness of 2,260 feet was measured. This section appears to be 

only a small part of the whole. Scattered exposures reveal many folds 

and some faults, but, in the absence of stratigraphic units that can 

be traced more than a few hundred feet, the character and amount 

of the folding and faulting are not known in satisfactory detail.

The Pennsylvanian age of these rocks has been determined on the 

basis of numerous plant fossils that have been found at scattered 

localities within the basin. No recent study of the fossils has been 

made, but an Allegheny age seems to be established (Knox, 1944). A 

few insect and other animal fossils have also been found (Lesquereux, 

1899 ;Scudder, 1893).

The coarseness of the sediments, the presence of much unweathered 

feldspar and rock fragments, and the character of the bedding suggest 

that these sediments were deposited in a lowland surrounded by an 

upland area of considerable relief. The presence of coal beds indicates 

that there were also fairly extensive swampy areas somewhat removed 

from the sources of sediments (Mutch, 1968).

On both the west and east margins are exposures showing an unconformable relationship, some of the Pennsylvanian conglomerate 

having been derived from the nearby underlying older rocks. Such a 

relationship is exposed in the Providence, Crompton, East Greenwich, 

Narragansett Pier, and Tiverton quadrangles. For the most part, however, the Pennsylvanian sediment appears to have been derived from 

a more distant source, probably from the northeast (Towe, 1959).

Although the lithologic character of the Pennsylvanian rocks of the 

basin varies in an irregular way, it has been possible to divide these 

rocks into several somewhat vaguely defined mappable units the 

Pondville Conglomerate, the Wamsutta Formation, the Khode Island 

Formation, conglomerate in the Rhode Island Formation, the Dighton 

Conglomerate, and a f elsite at Diamond Hill.

PONDVILLE CONGLOMERATE

The Pondville Conglomerate was named for Pondville Station in 

Massachusetts (Shaler and others, 1899, p. 134-139). This name is 

applied to conglomeratic layers lying directly on the older rocks at the
PENNSYLVANIA ROCKS 39

base of the Pennsylvanian rocks. This unit is discontinuous, being absent at many places where sandstone or shale of the Rhode Island Formation or of the Wamsutta Formation lies directly on the older rocks. 

Its thickness is approximately 160 feet near Natick, Mass., and may 

be almost 500 feet in the Tiverton quadrangle.

In most places the Pondville Conglomerate is gray to greenish coarse 

conglomerate with abundant sandy matrix, and there is much irregular 

interbedding with sandstone and lithic gray wacke. Pebbles and cobbles 

are commonly 6 inches or more in diameter, and some are as much 

as 2 feet across. Most of the pebbles are quartzite, but in a few places 

there are pebbles of granite and schist derived from nearby exposed 

sources. In the Tiverton quadrangle and to the south the basal Pennsylvanian beds are light-gray to dark-gray granule conglomerate, containing pebbles of smoky quartz as much as 5 millimeters in diameter.

WAMSUTTA FORMATION

The Wamsutta Formation was named for a proposed but not adopted 

name for Attleboro, Mass. (Shaler and others, 1899, p. 141). It is a large 

mass of red rocks that lie mostly in North Attleborough, and Attleboro, 

Mass.; the unit extends into Ehode Island only in the Pawtucket and 

Franklin quadrangles.

The formation includes a considerable thickness of interbedded red 

coarse-grained conglomerate, lithic graywacke, sandstone, and shale. 

Crossbedding and interfingering of layers are characteristic. The 

conglomerate layers contain many pebbles of felsite. Some of the conglomerate in the Pawtucket quadrangle contains boulders as much as 4 

feet in diameter. A very few lenses of limestone, one flow of red 

rhyolite, and several sheets of basalt are exposed in adjacent Massachusetts but not in Rhode Island.

The Wamsutta Formation is partly equivalent to the Pondville 

Conglomerate, for locally it lies directly on the pre-Pennsylvanian 

rocks. It is also partly equivalent to the Rhode Island Formation, for 

red layers of the Wamsutta Formation interfinger with gray and black 

layers of the Rhode Island Formation to the south. A few plant fossils 

have been discovered in the Wamsutta Formation (Knox, 1944).

RHODE ISLAND FORMATION

The most extensive and thickest (perhaps 10,000 feet) of the Pennsylvanian formations in Rhode Island is the Rhode Island Formation, 

originally called Rhode Island Coal Measures (Shaler and others, 

1899, p. 134, 159). By far the largest part of the Narragansett basin 

is underlain by it. Included here are fine to coarse conglomerate, sandstone, lithic graywacke, graywacke, arkose, and shale, and a small
40 BEDROCK GEOLOGY OF RHODE ISLAND

amount of meta-anthracite. Most of the rock is gray, dark gray, and 

greenish, but some is black especially the shale and the meta-anthracite. 

These are interbedded in a most irregular way; crossbedding is common. Field examination seems to show that the sediments were derived 

from the northeast and were carried to the south and southwest because 

there seems to be more shale to the south and southwest and more conglomerate to the north and northeast. Furthermore, microscopic examination seems to show that quartz is more abundant (the sediment 

more mature) to the south and southwest (Towe, 1959). This conclusion is given further support by directions of crossbedding, alinement 

of plant stems, and other sedimentary features.

In the north, the rocks of the Rhode Island Formation are strong 

and well indurated but not metamorphosed. To the south, however, 

they are progressively metamorphosed, as is shown by the successive 

appearance of metamorphic minerals of higher grade, by such structural features as metamorphic cleavage and schistosity, and by the 

elongation of pebbles (Quinn and Glass, 1958). These more intensely 

metamorphosed rocks include quartz-mica schist, feldspathic quartzite, 

garnet-staurolite schist, and some quartz-mica-sillimanite schist.

Coal in the Rhode Island Formation has long been known (Ashley, 

1915). Most of it is meta-anthracite, but some is anthracite (Quinn 

and Glass, 1958). Numerous beds of meta-anthracite have been mined 

or otherwise exposed at many places in the Narragansett basin. Most 

of these beds have not been traced very far, and most are thin, but the 

bed at the Cranston mine was as much as 35 feet thick in places. Some 

of this thickness may have been caused by deformation. The metaanthracite is dark gray to black. Some has the blocky fracture of lower 

rank coal, but much of it looks like sheared and slickensided graphite. 

The ash content is high. Vein quartz and fibrous quartz, which is a replacement of aphrosiderite (Richards, 1925), commonly are associated 

with the coaly layers, as is pyrite.

Conglomerate of the Rhode Island Formation. Conglomerate layers occur throughout the Rhode Island Formation, but only a few of 

them are thick and extensive enough to show on the State map. These 

layers are shown in the Bristol, Wickford, Prudence Island, Tiverton, 

and Newport quadrangles.

These conglomerate layers are gray to greenish, mostly very coarse, 

with stones as much as a foot or a few feet long, and interbedded with 

sandstone and graywacke. These stones are predominantly quartzite. 

In the vicinity of Newport, stones of porphyritic felsite are common, 

and in the same area some of the quartzite stones contain Cambrian
PENNSYLVANIA ROCKS 41

and Ordovician brachiopods (Walcott, 1898). Increasingly to the 

south, the pebbles and cobbles have been elongated as a result of tectonic forces. Purgatory Chasm, approximately 2 miles east of Newport, has long been known for its "stretched pebbles."

These conglomerate layers are somewhat more resistant to erosion 

than are the surrounding rocks, so that they stand higher 

topographically.

DIGHTON CONGLOMERATE

The Dighton Conglomerate was named for Dighton, Mass. (Shaler 

and others, 1899, p. 184^187).

Within the town of Dighton, and also west of Taunton and west of 

Attleboro, Mass., are bodies of conglomerate that appear to have synclinal structures. According to this interpretation, these rocks are the 

youngest in the Narragansett basin. These have been correlated with 

each other and have been placed in one formation, the Dighton Conglomerate. The structure of intervening areas is too little known to 

make these correlations or age assignments firm. The maximum thickness has been estimated at 2,000 feet (Shaler and others, 1899, p. 184). 

Only the southwest end of the conglomerate of Dighton extends into 

Rhode Island.

This formation includes gray conglomerate and lithic graywacke 

and sandstone, irregularly interbedded. Most of the conglomerate is 

coarse, with quartzite clasts in a sandy matrix.

The presence of this coarse conglomerate at the top of the section 

and at places now several miles from the margins of the basin implies 

a very vigorous transportation over considerable distances.

FELSITE AT DIAMOND HILL

At Diamond Hill in the Pawtucket and Franklin quadrangles is a 

small mass of f elsite that is much altered and cut through by the vein 

quartz of Diamond Hill. A rhyolite flow and some basalt layers are 

enclosed by the Wamsutta Formation in Attleboro, Mass., to the southeast. The Diamond Hill felsite appears to be intrusive but probably 

was almost contemporaneous with the extrusions in Massachusetts.

The felsite at Diamond Hill is gray, greenish gray, and reddish 

purple, fine grained, and porphyritic with phenocrysts of quartz and 

altered feldspars. In places it shows flow structure, but much of it is 

massive. The composition of the phenocrysts suggests that the fresh 

rock was nearly dacite.
42 BEDROCK GEOLOGY OF RHODE ISLAND

NORTH SCITUATE AND WOONSOCKET BASINS 

BELLINGHAM CONGLOMERATE

The probable Pennsylvanian rocks in the Woonsocket and the North 

Scituate basins have been correlated with rocks at Bellingham, Mass. 

(Mansfield, 1906, p. 99). These rocks are exposed in the Blackstone, 

Franklin, Georgiaville, and North Scituate quadrangles. No fossils 

have been found in these two basins, but a correlation with the Narragansett basin is strongly supported by the general lithologic and 

structural similarity and by strong discordance of the basins to structures in the older rocks.

Rocks included are gray to green conglomerate, sandstone, lithic 

graywacke, and phyllite, all irregularly interbedded. Along State 

Route 146 southwest of Woonsocket the pebbles are greatly elongated, 

and the rocks have a distinct schistosity. Further south, especially in 

the North Scituate basin, conglomerate is less abundant, sand-size 

grains are predominant, and pebbles are less elongated.

PENNSYLVANIAN OR POST-PENNSYLVANIAN GRANITIC ROCKS

At the mouth of Narragansett Bay and extending westward along 

the shore at least as far as Westerly are granitic rocks that intrude 

the Pennsylvanian rocks of the Narragansett basin. This is one of the 

very few places where geological relations demonstrate that there are 

Pennsylvanian or later granitic intrusive rocks in New England. The 

Narragansett Pier Granite is readily distinguished from the Westerly 

Granite, and the Westerly Granite is definitely younger, but the two 

probably are closely related and of almost the same age.

NARRAGANSETT PIER GRANITE

The Narragansett Pier Granite was named for extensive exposures 

on the shore just south of Narragansett (then called Narragansett 

Pier) in the Narragansett Pier quadrangle. It is the same as the "red 

Westerly Granite" of the Westerly quarry area. The rock is shown on 

the map as a continuous mass, but much of this area is covered by 

end moraine and outwash plain, and exposures there are sparse. It 

is exposed in the Narragansett Pier, Kingston, Carolina, Quonochontaug, Watch Hill, and Ashaway quadrangles.

The rock is mostly pink or tan, tout also light gray, equigranular, 

medium grained, locally porphyritic, and massive to weakly foliated, 

the foliation apparently being flow structure. It is quartz monzonite 

to granodiorite and consists chiefly of microcline, oligoclase, quartz, 

biotite, and minor muscovite. Typical modes are given in table 1. It has 

a rather uniform composition: microcline, 30-35 percent; oligoclase,
PENNSYLVANIAN OR POST-PENNSYLVANIAN GRANITIC ROCKS 43

30-35 percent; quartz, 25-30 percent; and biotite, 3-5 percent. At 

Quonochontaug, Kemp (1894) described several boulders of orbicular 

granite. No bedrock source of these boulders is known. Some of the 

matrix of these boulders is very similar to the Narragansett Pier 

Granite, however, so it seems probable that the orbicular rock is a 

facies of the Narrangansett Pier Granite and that the source is under 

the drift a short distance north of Quonochontaug.

Granitic sills and dikes that intrude Pennsylvanian sedimentary 

rocks of the Narragansett basin are exposed at the south end of Tower 

Hill in the Narragansett Pier quadrangle. The granite is light gray, 

but otherwise is very similar to the Narragansett Pier Granite. Exposures are not complete, but this rock seems to extend into the type 

area of the Narragansett Pier Granite. Therefore, this is interpreted as 

the Narrangansett Pier Granite, and the granite is assigned a late 

Pennsylvanian or post-Pennsylvanian age.

Wave-cut cliffs expose large masses of pegmatite and granite that 

intrude the Pennsylvanian sedimentary rocks in the Narragansett Pier 

quadrangle, southward from Wesquage Beach for a mile or more. 

Pegmatite is predominant at the north, and granite is more abundant 

to the south. The pegmatite is mostly simple in composition; feldspars, 

perthite, graphic granite, quartz, muscovite, and biotite constitute most 

of the rock. Small garnet crystals are common; there are a few crystals 

of beryl as much as 6 inches across and a few crystals of apatite.

WESTERLY GRANITE

The Westerly Granite is exposed chiefly in the Westerly area in 

the Ashaway, Watch Hill, Carolina, and Quonochontaug quadrangles. 

Most of this rock is in east-trending dikes that dip south, but there 

are also small irregular intrusive masses. The dike just south of 

Bradford is 1.6 miles long and approximately 65 feet thick, and it 

dips about 28° S. Other dikes are somewhat shorter and most are 

thinner. Dale (1908, p. 190), who had opportunity to see more quarries 

in operation then than now, stated that the dikes of Westerly Granite 

range in thickness from 50 to 150 feet.

The Westerly Granite is mostly gray to light gray, but workers in 

the stone industry recognized several color varieties, such as white 

statuary, blue, blue white, old blue, pink, light pink, pink gray, and 

pink statuary. The texture is mostly equigranular and fine. The structure is mostly massive, but a faint foliation can be detected in places. 

The rock is quartz monzonite to granodiorite in composition, and the 

main constituents are oligoclase, microcline, quartz, biotite, and minor 

muscovite. Modes are given in table 1.
44 BEDROCK GEOLOGY OF RHODE ISLAND

The fine grain and uniform composition of this rock have long been 

known in the stone industry, and it has had extensive use for statuary 

and monuments. More recently, the fine grain and uniform composition 

have resulted in the selection of this rock as a standard for chemical, 

mineralogical, and other analyses and measurements. As a result, many 

analyses of different kinds have been published; new ones appear from 

time to time and in many .different publications. Probably this rock 

is more thoroughly known than is any other rock in the world. Sample 

G-l (Westerly Granite) was taken from the Smith quarries in Westerly (Fairbairn and others, 1951). Later, as the first sample was nearing exhaustion, a second sample was taken from the quarries just south 

of Bradford (Chayes and Suzuki, 1963). The rock within any one dike 

has unusual uniformity, although small differences can be detected. 

Differences between different dikes are somewhat larger, but even 

these differences are small compared with differences in most other 

granitic rocks. "A rather detailed study of small samples of the granites 

of Westerly and Bradford, Khode Island, indicates that these granites 

are remarkably homogeneous in composition" (Chayes, 1950, p. 403).

Geological relations show only that the Westerly Granite is intrusive 

into and younger than the Narragansett Pier Granite, which is younger 

than the Pennsylvanian rocks of the Narragansett basin. The two 

granites probably are closely related and of almost the same age.

MAFIC DIKES AND SILLS

Mafic dikes and sills are widely scattered in Ehode Island. Approximately 40 have been discovered in the following quadrangles: Blackstone (Quinn and Alien, 1950), Franklin, Georgiaville, Pawtucket, 

North Scituate, Crompton, Ashaway, Voluntown, Narragansett Pier, 

Newport, Sakonnet Point, and Watch Hill. Only a few dikes are 

shown on the State map, so that the quadrangle reports should be 

consulted for locations and descriptions. After the report was printed, 

one dike in the Georgiaville quadrangle was uncovered in a new excavation on State Koute 146, approximately 900 feet southeast of 

Pound Hill Road. Most of the dikes are small, only a few inches or 

a few feet across, but one in the Blackstone quadrangle is 40-60 feet 

wide and almost a mile long.

The dikes axe dark gray to black and fine grained. Some have the 

composition of ordinary basalt, diabase, and olivine diabase, and others are of less common types. The dikes on Beavertail in the Narragansett Pier quadrangle have been called minette, although they are 

somewhat altered. Two dikes (possibly two exposures of one dike) 

in the Watch Hill and Ashaway quadrangles appear to be lampro-
VEIN QUARTZ 45

phyre, possibly monchiquite. Several of the older dikes are so altered 

that their original compositions are not known.

These dikes are of more than one age. In the Pawtucket quadrangle, 

some were altered by the regional metamorphism that affected the 

Blackstone Series. Others there are fresh and unaltered, presumably 

because they were intruded after the metamorphism. The large dike 

in the Blackstone quadrange intersects sedimentary rocks of probable Pennsylvanian age, and the minettes on Beavertail in the Narragansett Pier quadrangle also intersect Pennsylvanian rocks. The dike 

in the Blackstone quadrangle is ordinary diabase, which makes plausible a correlation with Triassic diabase of the Connecticut Valley.

It seems probable that each of the several episodes of plutonic and 

tectonic activity was accompanied by some dike intrusion and that 

the compositions of the intrusive rocks varied. The Connecticut Valley Triassic igneous and tectonic activity was also probably represented 

by dikes in Rhode Island.

VEIN QUARTZ

Two masses of vein quartz are large enough to be shown on the 

State map at Diamond Hill in the Pawtucket and Franklin quadrangles and on Bristol Neck in the Fall River quadrangle.

The prominent western face of Diamond Hill is part of a mass of 

vein quartz more than 1,000 feet wide and a mile long. This occurrence 

is probably along a fault between Pennsylvanian rocks of the Narragansett basin on the east and pre-Pennsylvanian rocks on the west. 

Some of the quartz appears to have replaced the felsite of Diamond 

Hill, but much of it was open-space filling. The many intersecting 

veins indicate that the quartz was deposited during repeated fracturing along this zone. Open cavities with quartz-crystal terminations are 

common. Considerable hematite and goethite were deposited, some 

in the form of botryoidal masses, some as thin coatings, and some as 

specular hematite flakes. Cryptocrystalline quartz and jasper with 

both massive and layered structure form a minor part of the deposit. 

"Negative crystal" cavities have been found in a few places.

A considerable mass of vein quartz crops out on the eastern part 

of Bristol Neck and on the east slope of Mount Hope (in the Fall River 

quadrangle, but described in the Bristol quadrangle report). This 

mass is probably along a fault between the Metacom Granite Gneiss 

on the west and Pennsylvanian sedimentary rocks on the east. The 

quartz is mostly milky and massive, only a few cavities containing 

quartz crystals. In a few places, especially near the south end of the 

vein on the shore, some of the quartz of the cavities is amethystine.
46 BEDROCK GEOLOGY OF RHODE ISLAND

The amethyst was once much sought, and very little can now be found. 

A very few pieces containing fluorite have been found in this vein. Unlike Diamond Hill, there is little staining by hematite and goethite.

UPPER CRETACEOUS RARITAN FORMATION (?) OF BLOCK ISLAND

In a few places the bluffs of Block Island expose unconsolidated 

Cretaceous rocks, including red, white, gray, and black clay and 

white sand. The black clay contains fragments of lignite and nodules 

of pyrite and siderite. These rocks have been correlated tentatively 

with the Raritan Formation (C. A. Kaye, written commun., 1965). 

All these layers have been involved in complex folding, presumably 

by glacial push.

No consolidated bedrock is exposed on Block Island. It apparently 

is 1,000 feet or more below sea level (Tuttle and others, 1961; Hansen 

and Schiner, 1964, p. 7). Most of the island is composed of glacial 

drift, which is exposed prominently on several wave-cut bluffs. The 

drift includes a considerable thickness and a wide variety of till, gravel, 

sand, and clay, probably representing more than one glaciation.

STRUCTURAL GEOLOGY 

OLDER METAMORPHIC ROCKS

The masses of Blackstone Series and other older metamorphic rocks 

now exposed are only small remnants of the original masses. Very large 

parts of these original masses of the metamorphic rocks must have 

been removed or displaced by the intrusions of the plutonic rocks that 

now occupy a large part of Rhode Island. Other parts of the metamorphic rocks were covered by the Pennsylvanian rocks of the Narragansett basin.

The large body of Blackstone. Series in the Blackstone River valley between Pawtucket and Woonsocket has a broadly simple structure northwest strikes of bedding and schistosity and moderate northeast dips. The patch of Blackstone Series just west of Providence and 

that west of Natick are similar, except that west of Natick the strikes 

are more to the west. Some of the smaller structures of these rocks 

are more complex. The trends in these rocks are not in line with the 

general north and northeast trends of Appalachian structure in the 

nearby parts of New England. Schistosity and bedding generally are 

parallel in the Blackstone Series.

The Cambrian (?) and Ordovician (?) rocks in the southwest part 

of the State are fragmentary, but mapping in Connecticut may tie 

them into major structures to the west and northwest.
STRUCTURAL GEOLOGY 47

The structures of the older gneissic rocks (Nipsachuck, Absalona, 

Woonasquatucket, and light-colored gneisses), chiefly in the Georgiaville quadrangle, seem even less related to regional structure. These 

structures may be more complex continuations of the structures in the 

Blackstone Series, or they may be at least in part remnants of more 

ancient structures, as was suggested in the Georgiaville quadrangle 

reports.

The small bodies of metamorphic rocks on the east and southeast 

side of the Narragansett basin are even more fragmentary than those 

on the west side. However, this fragmentary character leaves the 

possibility that these metamorphic masses are isolated pieces of a oncesystematic major structure.

OLDER PLUTONIC ROCKS

Most of Khode Island west of the Narragansett basin is underlain 

by granitic plutonic rocks. These may make up one magmatic series, 

in which case the whole body of rocks constitutes a complex batholith. 

In a broad way this is a discordant intrusion, cutting sharply across 

the Blackstone Series and other metamorphic rocks. The plutonic rocks 

range from strongly gneissic for the early syntectonic members to 

almost massive for the latest. In the northwest part of the State, in the 

Chepachet quadrangle, a north-trending foliation and lineation are 

strongly developed. In that quadrangle there is also a later westerly 

foliation dipping moderately northward. A similar later foliation is 

present more sparingly in the more southern areas of the older plutonic 

rocks.

IGNEOUS ROCKS OF MISSISSIPPIAN(?) OR OLDER AGE

The Quincy Granite and granite porphyry in the Pawtucket quadrangle were intruded as sills and irregular partly concordant bodies. 

One intrusive body seems to be along a fault.

The Spencer Hill Volcanics in the East Greenwich and Crompton 

quadrangles strike eastward and dip northward.

NARRAGANSETT BASIN

The Narragansett basin is strikingly discordant to the adjacent older 

rocks. Contact lines of the older rocks and such smaller structural features as foliation are cut sharply by the margins of the Narrangansett 

basin. The two smaller basins, at North Scituate. and at Woonsocket, 

also are sharply discordant to the older structures, which is part of the 

evidence for their Pennsylvanian age. This unconformity indicates 

clearly that the older rocks were deformed and then eroded to a considerable depth before the Pennsylvanian sediments were deposited.
48 BEDROCK GEOLOGY OF RHODE ISLAND

The structure within the Narragansett basin is complex, but the 

scarcity of outcrops and the irregular variability of the Pennsylvanian 

sedimentary rocks in the basin have prevented discovery of much of 

the details of structure. The major structure of the basin is synclinal; 

this syncline trends north through eastern Rhode Island and bends 

northeastward into Massachusetts. Three masses of Dighton Conglomerate in Massachusetts have been intercepted as three smaller synclines 

within the basin. At several localities in Rhode Island and Massachusetts certain beds can be traced for a few hundred feet. These and 

observations on some single outcrops show clearly that there are many 

folds and faults. The exposures of pre-Pennsylvanian rocks surrounded 

by Pennsylvanian rocks on Bristol Neck and on the northeast end of 

Aquidneck Island are due to folds and faults.

To the south, especially on Beavertail in the Narragansett Pier 

quadrangle, a pronounced slaty cleavage has been developed. The 

cleavage is axial-plane cleavage, and it is, therefore, generally discordant to the beds. Another consequence of the deformation of the Pennsylvanian rocks is the distortion of clasts in conglomerate. Clasts in 

conglomerate in the northern parts of the State have approximately 

their original shapes, but they are progressively distorted to the south. 

At Purgatory Chasm, a little more than 2 miles east of Newport, the 

clasts are triaxial in shape, and the ratio of dimensions is 4.1:1.5:1 

(Agron, 1963, p. 22). The two longer dimensions are in the axial planes 

of folds. In the Woonsocket basin, clast dimension ratios are more 

extreme, 13.5: 2.5 :1 (Hall, 1963, p. 54).

The west margin of the Narragansett basin at Diamond Hill is along 

a fault. A similar relationship is indicated by the vein quartz at Mount 

Hope, described in the Bristol quadrangle report. There is some evidence that the east margins of the Woonsocket and North Scituate 

basins may be faults in the Georgiaville and North Scituate quadrangles. Probably there are many other faults not revealed by existing 

exposures.

How far the Narragansett basin extends southward out to sea is 

unknown. The presence of pre-Pennsylvanian rocks on Newport Neck 

and on the southeast part of Conanicut Island (Newport, Prudence 

Island, and Narragansett Pier quadrangles) gives some evidence that 

the basin may come to an end near the mouth of Narragansett Bay.

PENNSYLVANIAN OR POST-PENNSYLVANIAN GRANITIC ROCKS

The Narragansett Pier Granite is intrusive into the Pennsylvanian 

rocks 'at the southwest extent of the Narragansett basin. This granite 

appears to form a body extending westward some 20 miles or more 

along the south shore. Such a west-trending structure is not in line
METAMORPHISM 49

with most recognized structures in nearby parts of New England. In 

support of the reality of this west-trending structure are the dikes of 

Westerly Granite, most of which strike west and dip moderately south.

METAMORPHISM

Rhode Island was affected by two or more episodes of regional metamorphism in pre-Pennsylvanian time and by one in Pennsylvanian or 

post-Pennsylvanian time. There was also some minor contact metamorphism, caused by the intrusion of plutonic rocks.

PRE-PENNSYLVANIAN METAMORPHISM

The Blackstone Series and others of the older rocks were involved 

in regional metamorphism. Evidence is not clear whether this all 

occurred at one time or at several times. Because of the scattered and 

patchy distribution of these rocks, the drawing of isograds seems not 

to be feasible. However, some variations may be detected. The large 

mass of Blackstone Series rocks in the Blackstone River valley (Pawtucket and Georgiaville quadrangles) shows low intensity of metarnorphism, about the quartz-albite-epidote-almandine subf acies of the 

greenschist facies. Approximately the same intensity prevails in the 

Providence and North Scituate quadrangles. To the northwest (Chepachet quadrangle) and south and southwest (Crompton, Hope Valley, 

and Coventry Center quadrangles) the intensity rises to the lower part 

of the almandine-amphibolite facies. Further south and southwest 

(Kingston, Carolina, Ashaway, and Voluntown quadrangles) the intensity has risen to the sillimanite-almandine-muscovite subf acies of 

the almandine-amphibolite facies. The metamorphism of rocks east and 

southeast of the Narragansett basin is mostly of low intensity, about the 

quartz-albite-epidote-biotite subfacies of the greenschist facies.

In a few places, contact metamorphism has affected these older 

rocks. The marble at Lime Rock in the Pawtucket quadrangle has been 

intruded by quartz diorite of the older plutonic rocks. Away from the 

contact, the marble contains tremolite as a result of regional metamorphism. At the contact, forsterite and diopside were caused by the 

higher temperature. Some of these minerals have been altered down 

to serpentine, bowenite variety. Several small mineral deposits in the 

vicinity of Sneech Pond, also in the Pawtucket quadrangle, are contact 

metamorphic deposits (Quinn and Young, 1937).

Regional metamorphism of the Blackstone Series and other older 

metamorphic rocks occurred before the intrusion of the older plutonic 

rocks, as is shown by rotated schist inclusions in the plutonic rocks. 

The plutonic rocks also show effects of metamorphism, which indicates 

a second episode. In the west and southwest areas, Moore (1958,1959,
50 BEDROCK GEOLOGY OF RHODE ISLAND

1963,1964) discovered granulation, a metamorphic foliation, and late 

porphyroblasts in the Scituate Granite Gneiss, the Hope Valley 

Alaskite Gneiss, and the Ten Rod Granite Gneiss. The second foliation 

of the Ponaganset Gneiss in the Chepachet quadrangle is also evidence 

of a later metamorphism.

PENNSYLVANIAN OR POST-PENNSYLVANIAN METAMORPHISM

The Pennsylvanian rocks of the Narragansett basin provide a good 

illustration of progressive regional metamorphism, as was described 

by Lahee (1912, 1914) and later by Quinn and Glass (1958). These 

three papers described gradual and systematic change from nonmetamorphosed rocks in the north to more intensely metamorphosed rocks 

to the south. Subsequent work has made it feasible to draw isograds 

that show an increase of metamorphic intensity to the south and an 

increase toward the margins, particularly the west margin, of the basin 

where the rocks must have been under the greatest thickness of cover. 

This area of most intense metamorphism is also near the intrusive 

bodies of Narragansett Pier Granite.

Well-indurated but very weakly metamorphosed rocks occur at the 

northeast corner of the State. Going southward, the change of intensity of metamorphism is shown by the appearance of biotite, then 

garnet, and then staurolite. The pattern and rate of increase of metamorphism suggest that the rocks at the south end of Tower Hill in the 

Narragansett Pier quadrangle are at the sillimanite grade. No sillimanite has been discovered there, but the original rocks probably did 

not have the right composition to yield sillimanite. To the west, in the 

Kingston quadrangle, are sillimanite-bearing rocks that have been 

assigned to the Pennsylvanian. Along with these mineralogical indications of increasing metamorphism are textural changes. The mineral 

grains become more and more interlocking. More striking is the increasing deformation of pebbles, cobbles, and boulders in conglomerate 

described above. Still another effect of this progressive metamorphism 

is the increase in rank of coal. Quinn and Glass (1958) showed that 

the rank of coal increases from high anthracite near the ProvidencePa wtucket line to meta-anthracite at the Cranston mine, near the south 

margin of the Providence quadrangle.

A remarkable fact is that plant fossils in graphitic shale interbedded 

with garnet-staurolite schist layers have been found at shore exposures 

on Conanicut Island 1,500 feet south of the east end of the Jamestown 

Bridge.

The probable Pennsylvanian rocks of the basins at North Scituate 

and Woonsocket have been metamorphosed to the intensity that they
RADIOMETRIC AGES 51

contain much biotite and scattered garnet crystals. Apparently they 

are at about the quartz-albite-epidote-almandine subf acies of the greenschist f acies.

A puzzling feature of both metamorphism and structure of the prePennsylvanian rocks in Rhode Island is that they show so little of the 

effects of the Pennsylvanian or post-Pennsylvanian deformation and 

metamorphism.

RADIOMETRIC AGES

Radiometric ages of rocks in Rhode Island and nearby Massachusetts 

are assembled in table 2. The ages must be considered with respect to 

methods of determination, but even so, they do not all agree with ages 

indicated by geological evidence.

The first published ages were made by the lead-alpha method (Q/uinn 

and others, 1957). Most of these ages seemed reasonable according to 

the time scale accepted at the time of the determinations (older plutonic 

rocks, Devonian; Quincy Granite, Mississippian; Narragansett Pier 

and Westerly Granites, Late or post-Pennsylvanian). However, subsequent lengthening of the geologic time scale (Kulp, 1961; Faul, 1960) 

places most of these dates in the Pennsylvanian Period or later. This 

placement probably is wrong; it seems very doubtful that the older 

plutonic rocks were intruded at depth during Pennsylvanian time and 

then exposed by erosion before the formation of the volcanic and 

plutonic rocks of the East Greenwich Group and that those in turn 

were exposed by a second period of erosion, all before lower or middle 

Allegheny time, which is the age indicated by the fossils of the Wamsutta Formation (Knox, 1944).

TABLE 2. Radiometric ages of rocks in Rhode Island and nearby Massachusetts

Sample Method 1 Material Age (m.y.) 2 Keference 

Narragansett Pier (npg) and Westerly (wg) Granites

Q55-4 (wg)_-_-__- Pb-a_.- Monazite_._ 220 Quinn and others,

1957.

S47 (wg)__.______ Pb-a __ Zircon____._. 243 Do.

B-3042(wg)_.-._ Rb-Sr--_ Biotite.-.-__- 259±10 Bottino, 1963.

G-l standard (wg). Rb-Sr..-_ Whole rock... 299±40 Do.

B-3814(wg)-____._ K-Ar____ Biotite_-_-_-- 240±12 Hurley and others,

1960.

Q55-3(npg)_--____ Pb-a_.. Zircon____._. 208 Quinn and others,

1957.

S46(npg)_-----_-. Pb-a -------do_-----_ 225 Do.

Q55-l(npg)._-_.-- Pb-a -------do-----.- 235 Do.

53S-49(npg)______ Pb-a.__-------do.--._ 274 Do.

See footnotes at end of table.
52 BEDROCK GEOLOGY OF RHODE ISLAND

TABLE 2. Radiometric ages of rocks in Rhode Island and nearby 

Massachusetts Continued

Sample Method Material Age (m.y.) 2 Reference

Narragansett basin and North Scituate basin rocks and metamorphic minerals

Wamsutta rhyolite Rb-Sr__ Whole rock.__ 211±23 Bottino, 1963.

R-3917

R-3818--.------- K-Ar____.____do__-----

R-3817. _____-_-- K-Ar____ Impure biotite

B-3817A-.._----. K-Ar____ Biotite_-.-_--

B-3817A.________ Rb-Sr_______do-_-.___

Rb-Sr... ______do_-_--_- 244± 12

253 ± 13 Hurley and others,

	1960.

230± 12 Do.

260± 13 Do.

275± 15 Do.

250±12 Do.

244± 15 Do.

237±11 Do.

230± 11 Do.

200

Do. 

Harakal, 1964.

Cowesett Granite (eg) and Quincy Granite

Q48-3(eg)_________ Pb-a.____ Zircon.______ 272 Quinnand others,

1957.

272

257 

QG-1------------ Pb-a__________do___-__- 275 Do.

273 

Quincy Granite__ Rb-Sr___ Whole rock.__ 305± 14 Bottino, 1963.

Older plutonic rocks Scituate Granite Gneiss (sg), Hope Valley Alaskite Gneiss (hva). Ten Rod Granite 

Gneiss (trg), and Northbridge Granite Gneiss (ngn)

Q53-38(trg)___-_-_ Pb-a-_._- Zircon___._._ 289 Quinn and others,

	1957. 

Q53-39(hva)_--_-_ Pb-a--_._____do------- 303 Do.

Q50-17(sg)------- Pb-a-----.___do------ 299 Do.

Q53-37(sg)------- Pb-«-_---.___do------ 337 Do.

	303 

l(sg)__--_-__-. K-Ar____ Biotite____. 245 Harakal, 1964.

2(sg) _---_----- K-Ar_________do-_.---- 200 Do.

3(sg)-__.-------- K-Ar_________do._----_ 225 Do.

5(sg) -.---_..._ K-Ar_______-_do------- 200 Do.

23(sg)- -------- K-Ar_________do------- 230 Do.

23(sg)__-.__---_ K-Ar____ Muscovite..-- 250 Do.

25(sg)__-.___ K-Ar____ Biotite____ 230 Do.

4849(ngn)____._ Rb-Sr__ Whole rock___ 538 Moorbath and staff,

	1962. 

4850(ngn)________ Rb-Sr__---....do.----_- 550 Do.

4851(ngn)____-_-_ Rb-Sr.___...-_do------ 517 Do.

4852(ngn)-------- Rb-Sr._.______do---_--- 529 Do.

4853(ngn)___-__._ Rb-Sr___._.--_do--_--_- 458 Do.

4144(ngn)________ Rb-Sr ___--_do_------ 535 Do.

(ngn)__-__----___ isochron..._--_________- 535±15

1 Published or recalculated ages based on the following decay constants:

Rb-Sr................................................................. ......._..X=1.47X10-»yr-i

K-Ar....................................................................... --.X0=4,72X10-'°yr-»

X«=0.585Xl(HOyi-»

2 Part of time scale according to Kulp (1961): Million years 

Jurassic-...-------......-------.-...-.-........-.----.----..--------.---..---..--.----------- 181

Triassic-.------------.-.---..---...---------.------.---..-----------.-----.------.--.-------- 230

Permian..................----....................-----........--..----.-.------..-.--.-.-..-- 280

Pennsylvanian--._....-_....-_--.__.__-..-..- ...............-.....-.-.-.-.-.......--.-.-- 310

Mississippian.......-.....-....-........--........--....-....---.......--.---..---.-.--------- 345

s This is listed as being from or associated with granite intrusive into Pennsylvanian rocks and appears 

to be a feldspathic sandstone.
GEOLOGICAL HISTORY 53

Early rubidium-strontium whole-rock determinations (table 2) are 

compatible with geologic evidence. The Northbridge Granite Gneiss, 

which is correlated with the Ponaganset Gneiss, is believed to be of 

early Paleozoic age. In 1968, however, Day obtained an age of 638±42 

m.y. for the Hope Valley Alaskite Gneiss, the Ten Kod Granite Gneiss, 

and probably the Scituate Granite Gneiss of Rhode Island. This additional evidence supports a Precambrian age for the Blackstone Series. 

However, the rubidium-strontium method gave a Triassic age for the 

rhyolite flow of the Wamsutta Formation, but the Allegheny age 

referred to previously was of rocks that almost certainly overlie and 

are younger than this rhyolite. The rubidium-strontium whole-rock 

determination of the Westerly Granite (G-l) gives an Early Pennsylvanian age. The Westerly Granite is intrusive into Narragansett 

Pier Granite, which is intrusive into the Pennsylvanian rocks of the 

Narragansett basin. Therefore, the geological evidence gives a Late 

Pennsylvanian or a post-Pennsylvanian age for the Westerly Granite. 

The three rubidium-strontium age determinations (table 2, B-3042, 

B-3817A, and B-3815) on individual minerals give approximately the 

same age for the metamorphism of the Pennsylvanian rocks and the 

crystallization of the Westerly Granite; this similarity seems geologically probable.

Potassium-argon determinations appear to give ages younger than 

the time of formation of the rocks on which they were made, because 

the age given is probably the time at which both the igneous minerals 

and the metamorphic minerals cooled to a temperature at which the 

radiogenic argon was retained within these minerals. This time may 

have been considerably later than the crystallization of the rock.

In addition to the ages shown in table 2 is an age of 1,500 m.y. for 

cumberlandite by the helium method on magnetite (Hurley and Goodman, 1943). This method now appears to be unreliable.

More recent age determinations on the Quincy Granite in Massachusetts have been discussed earlier (p. 34).

GEOLOGICAL HISTORY

The earliest geologic event shown by the rocks of Rhode Island is 

the deposition of the Blackstone Series, probably in Precambrian 

time. A large part of this series is clean quartzite, quartz-mica schist, 

and marble. This series originated as sediments deposited in a body of 

water, probably marine. Intel-layered with these sediments were lava 

flows and pyroclastic rocks. The basin of sedimentation was receiving 

sediment from the erosion of a landmass, and it was also receiving 

extrusive material from nearby volcanoes. This time of sedimentation
54 BEDROCK GEOLOGY OF RHODE ISLAND

and volcanic activity must have been very long, as the thickness of 

the Blackstone Series may have been 15,000 to 20,000 feet.

The other metamorphic rocks both to the west and to the east and 

southeast of the Narragansett Bay show a somewhat similar history, 

although they are not exactly the same lithologically and the present 

remains are not so thick. Whether they were contemporaneous with 

the Blackstone Series is not known.

Presumably after the deposition of the Blackstone Series came the 

deposition of materials forming the Cambrian (?) and Ordovician (?) 

rocks in the southwest part of Rhode Island. These rocks, too, resulted from the deposition of a large thickness of sediments and the 

extrusion of volcanic material.

All these sediments and volcanic rocks were buried to a considerable 

depth and were involved in tectonic activity that included regional 

metamorphism. It is not known whether all these rocks were involved 

in one episode of tectonism and metamorphism or whether different 

groups were affected by different tectonic-metamorphic events. The 

intensity of the metamorphism seems to have been less in the northeastern part of Rhode Island.

In the waning stages of the metamorphism, the intrusion of the older 

plutonic rocks began. Large masses of liquid or partly liquid granitic 

material were squeezed upward into the metamorphic rocks. This material may have been derived from melting or partial melting of the 

just-accumulated geosynclinal sediments and volcanic rocks, or it may 

have been derived from the underlying sialic crust. The intrusion 

continued beyond the tectonic climax, so that the syntectonic intrusive 

rocks that have pronounced foliation and lineation were followed by 

posttectonic rocks that are more nearly massive. During the development of this intrusive complex there was also some change from more 

mafic to less mafic rocks, but this change is not very well defined. The 

principal rock units included in this large intrusive sequence are the 

Ponaganset Gneiss, the Scituate Granite Gneiss and associates, and 

probably the Grant Mills Granodiorite and the Esmond Granite and 

associated intrusives. Again there is no good evidence concerning relations with rocks on the east side of Narragansett Bay. These intrusions probably were emplaced in middle or early Paleozoic, or, if Day's 

(1968) rubidium-strontium ages are correct, in Precambrian time.

These older plutonic rocks, having coarse textures, must have solidified at depth. The next recorded event in the geological history of the 

State is that the plutonic rocks were uncovered during a long period 

of erosion. This is shown by the fact that eroded boulders of the Esmond Granite are in the Spencer Hill Volcanics of the East Greenwich 

quadrangle.
GEOLOGICAL HISTORY 55

The East Greenwich Group, which is tentatively correlated with the 

Quincy Granite and granite porphyry, reveals an episode of volcanic 

activity associated with and followed by small hypabyssal intrusions 

and larger plutonic intrusions. This activity may have been in the Mississippian Period, but recent radiometric ages indicate an earlier time 

of formation.

After the East Greenwich Group was formed, erosion again attacked 

the area and in time exposed the Cowesett Granite and the Quincy 

Granite before the beginning of sedimentation in the Pennsylvanian 

Period.

The sedimentation of the Pennsylvanian Period appears to have 

been entirely nonmarine and to have been in an area of high relief. 

Erosion was rapid and weathering was largely physical, so that feldspars were not completely destroyed in the weathering. Vigorous 

streams attacked the high surrounding areas and strewed the clastic 

sediments in an intermontane lowland. Some parts of the lowland were 

far enough away from the sediment-laden streams for quiet swamps 

to escape sedimentation for a time. During this time, plants grew in 

profusion, indicating a mild climate, and large masses of vegetation 

accumulated. These masses later became the beds of meta-anthracite. 

The clastic sedimentation continued long enough for several thousand 

feet of sediments to accumulate. Late in this time some of the streams 

must have rejuvenated, which resulted in the deposition of the coarse 

Dighton Conglomerate.

The Pennsylvanian sedimentation came to an end when the area was 

again uplifted and caught in intense tectonic action. The Narragansett 

basin sedimentary rocks were tightly folded, faulted, and metamorphosed. Near the south end of the present Narragansett Bay, the Narragansett Pier Granite was intruded into the Pennsylvanian rocks 

and also into older granitic rocks along the position of the present 

south shore. The Westerly Granite was intruded along a few southdipping fractures.

A few mafic dikes were intruded after the Westerly Granite. Possibly some of the dikes and faults are Triassic.

The Cretaceous rocks of Block Island are a very small part of a 

considerable thickness of marine sediments deposited along the east 

coast of the United States in Cretaceous and Tertiary time. These 

sediments constitute the Coastal Plain, which is submerged in the 

north except for such scattered areas as Long Island, Block Island, 

Martha's Vineyard, and Nantucket.

Otherwise, the succeeding time has been a period of erosion, probably a period during which stream-cut valleys were drowned to form 

Narragansett Bay, and a period of glaciation.
56 BEDROCK GEOLOGY OF RHODE ISLAND

Thus, the evidence in Ehode Island is in general agreement with 

other evidence pointing to great complexity in the formation of a 

mountain range and specifically to the recurrence of various types of 

structural and igneous activity for a long time during the development 

of the Appalachians. In Rhode Island, at least one and perhaps several 

episodes of deformation and metamorphism followed the deposition of 

materials of the Blackstone Series and other older metamorphic rocks, 

in Precambrian or early Paleozoic time. Then, at least one and perhaps 

more than one episode of plutonic activity formed the older plutonic 

rocks, in Paleozoic or possibly in Precambrian time. This period was 

followed by erosion and the volcanism, plutonic activity, and deformation that formed the Quincy Granite and the East Greenwich 

Group in Mississippian (?) or earlier time. Another period of erosion 

exposed these rocks, and then the materials of the Pennsylvanian rocks 

were deposited in great thickness. Finally, these rocks were deformed, 

metamorphosed, and cut by intrusive rocks in Pennsylvanian or postPennsylvanian time. This last activity, the "Appalachian Revolution," 

is better demonstrated in Rhode Island than in any other New England 

State. The greatest possible simplification of these events leaves a long 

and complex orogenic sequence in the development of this part of the 

Appalachian Mountain system.

ECONOMIC GEOLOGY

CARBONATE ROCKS

Carbonate rock, limestone, has been of economic use in Rhode Island 

since a very early date. Bishop (1861, v. 1, p. 218) stated, "The earliest 

mention of its [lime's] manufacture that we have seen, was in Rhode 

Island, where in January 1662, a Mr. Hacklet, of Providence, applied 

to the town for liberty to burn lime, and to take stone and wood from 

the commons for that purpose, which was granted him for a limited 

time." Evidence concerning the location of this early quarry is conflicting. Published data point to a small quarry in Manton just northwest of Providence (Quinn, 1954). On the contrary, Mr. Ed Conklin, 

of the Conklin Limestone Co., maintains that early deeds indicate that 

this earliest quarry was at Lime Rock, at the site of the present operating quarry.

Marble beds within the Blackstone Series are the carbonate rocks 

quarried here. The two largest beds are in the Pawtucket quadrangle, 

the "Harris" quarries (including the present operating Conklin 

quarry) at Lime Rock about 7 miles northwest of the center of Providence and the "Dexter" quarry about a mile east of the "Harris" 

quarry. The layer of marble at the "Harris" quarry is at least half a
ECONOMIC GEOLOGY 57

mile long and somewhat more than 400 feet thick. Several other marble 

beds are known in the Pawtucket and Providence quadrangles, but 

most of them are too small to be of value now and some of them have 

been almost worked out. Earlier use of these rocks was for mortar, 

plaster, and so forth, but the present use is largely agricultural. Much 

of this marble is dolomitic, so that it is not suitable for portland 

cement.

DIMENSION STONE

GRANITE

Granite has been one of the most valuable geological materials in 

Rhode Island. Many small quarries are scattered about the State and 

there are also a considerable number of large quarries. Most are now 

abandoned.

The Quincy Granite of the northern part of the Pawtucket quadrangle and the southern part of the Franklin quadrangle was once 

worked rather extensively. The quarries have been abandoned for 

many years.

The Esmond Granite was once quarried extensively in many quarries 

in the Esmond-Graniteville area near the corner of the Georgiaville, 

Pawtucket, North Scituate, and Providence quadrangles. A notable 

fact about this area is that it supplied the 12 large 3- by 22-foot monolithic columns for the Arcade building in Providence. These are reported to have been the largest granite monoliths in the world in 1828 

when the Arcade was built. These quarries are also now abandoned.

The Ponaganset Gneiss, the Scituate Granite Gneiss, the Hope 

Valley Alaskite Gneiss, the Ten Rod Granite Gneiss, and the porphyritic granite of Newport all have been quarried at several places; 

most of the quadrangles west of the Narragansett basin have at least a 

few abandoned quarries.

Probably the most important granite quarry area in the State is 

the Westerly area, which still has some quarry activity (Dale, 1908, 

1923; Macomber, 1958). Some of these quarries are in the Watch Hill 

and Ashaway quadrangles near Westerly, and some in the Carolina 

quadrangle near Bradford. These quarries worked either the Narragansett Pier Granite or the Westerly Granite; both rocks were taken 

from a few quarries. The dike form added to the difficulty of working 

the Westerly Granite. The quarry south of Bradford was extended 

back a considerable distance underneath the overlying Hope Valley 

Alaskite Gneiss. The fine grain and uniform composition make the 

Westerly Granite especially suited for statuary and monuments. The 

Narragansett Pier Granite ("red Westerly granite") is used more in
58 BEDROCK GEOLOGY OF RHODE ISLAND

buildings. In recent years, the waste blocks of these quarries have had 

large use in building breakwaters along the nearby shores. Only one 

quarry now is in use at Westerly and one at Bradford.

OTHER ROCKS

Only minor use for dimension stone has been made of such rocks as 

marble and Pennsylvanian sandstone. Very minor use has been made 

of soapstone in lenses of the Blackstone Series. The sandstone at 

Woonsocket was used for making "ten thousand dozen" whetstones in 

1840 (Jackson, 1840, p. 71).

CRUSHED STONE

Crushed stone has been quarried in large amounts at several places 

and in such different rocks as greenstone of the Blackstone Series, 

Metacom Granite Gneiss, several other granites and gneisses, sandstone 

and conglomerate of the Narragansett basin, and cumberlandite.

METAL DEPOSITS

Rhode Island, like other New England States, has several old 

prospect pits and the remains of small mines. There is generally very 

little information about when the deposits were worked, the extent 

of the mining, or even what was sought. Below is a list of seven of 

these deposits, the first six of which have been referred to in the 

quadrangle reports:

Durf ee Hill gold mine, Chepachet quadrangle

Furnace Hill iron deposits, North Scituate quadrangle

Snake Den gold mine, North Scituate quadrangle

Bald Hill gold mine, East Greenwich quadrangle

Gold mine southeast of Victory Highway and Plainfield Pike.

Coventry Center quadrangle 

Copper Mine Hill deposits, Pawtucket quadrangle 

Foster ("Homestrike") gold mine, half a mile northeast of

Cucumber Hill, East Killingly quadrangle

In the Pawtucket quadrangle, mostly east and northeast of Sneech 

Pond, are several prospects and small abandoned mine holes. Jackson 

(1840, p. 55) wrote, "no less than fifty different ancient mine holes 

in this hill." The area was once known as Copper Mine Hill. These 

are high-temperature hydrothermal deposits, containing tremolite, 

diopside, garnet, quartz, magnetite, scheelite, molybdenite, pyrite, 

chalcopyrite, galena, sphalerite, covellite, chalcocite, and other minerals. The mining was probably for copper, although some of the work 

may have been for gold. Nearby are also a few small deposits of manganese silicates and oxides.
ECONOMIC GEOLOGY 59 

CUMBERLANDITE

The cumberlandite of Iron Mine Hill in the Franklin quadrangle 

has been used for iron ore and for crushed rock. It has also Been 

considered as a possible source of titanium. It is reported to have 

been mixed in 1703 with iron ore from Cranston (Furnace Hill Brook 

limonite) and used to make cannon for the siege of Louisburg in 1745 

(Johnson, 1908). The iron content is low; Warren (1908, p. 24) gives 

the following analysis: SiO2 , 22.35 percent; A1 3O3 , 5.26 percent; 

Fe2 03 , 14.05 percent; V2 03 , 0.18 percent; Cr2 03 , trace; FeO, 28.84 

percent; MgO, 16.10 percent; CaO, 1.17 percent; Na^.0, 0.44 percent; 

K2 0, 0.10 percent; H2 0, 0.42 percent; C02 , 0.02 percent; Ti02 , 10.11 

percent; P2 05 , 0.02 percent; S, 0.38 percent; MnO, 0.43 percent; Zn, 

0.71 percent; Cu, 0.08 percent; Co and Ni, 0.08 percent; Pb, trace. 

The average of 10 analyses gives Fe, 33.49 percent &nd Ti02 , 9.75 

percent. The presence of titanium makes smelting difficult. The intergrowth of magnetite and ilmenite is so fine (Singewald, 1913) that 

separation is difficult.

Johnson (1908, p. 6) estimated the surface dimensions of this rock 

as 1,200 by 600 feet. With this area there would be 8 million tons of 

rock for 100 feet of depth, and the rock probably extends down a 

considerable distance. The area is probably larger beneath the cover 

of drift, and the body of rock is probably large enough to justify 

large-scale mining if an economic method of separating the iron and 

titanium can be found.

META-ANTHRACITE

Ashley (1915) recounted the history of mining Rhode Island coal. 

The earliest use recorded by him was by British soldiers in Newport 

in pre-Revolutionary time. One of the few extended and successful 

uses was in Portsmouth from 1860 to 1883 for smelting copper ore from 

Cuba and South America. Some of the slag can still be seen on the 

shore. Other mines were opened in Valley Falls, Cumberland, Bristol, 

Providence, and Cranston, and in Mansfield, Mass. The material from 

the Cranston mine, operated until 1959, was last used for foundry 

graphite but was earlier used for fuel. It is even reported that 250 to 

300 tons a week of Rhode Island coal was shipped to Pittsburgh, Pa., 

in 1887 (Ashley, 1915, p. 11).

The coal beds are fairly common in parts of the Rhode Island 

Formation, and some are as much as 35 feet thick. Two main difficulties in the use of this material are (1) that the ash content is high in 

most of the coal, and (2) that metamorphism has changed much of the 

coal to the meta-anthracite rank, in which the volatile combustible
60 BEDROCK GEOLOGY OF RHODE ISLAND

content is too low for good use (Quinn and Glass, 1958; Toenges and 

others, 1948). These characteristics of the meta-anthracite, together 

with the general trend away from some uses of coal, make a situation 

very unfavorable to the use of Rhode Island coal.

QUARTZ

At Diamond Hill in the Pawtucket and Franklin quadrangles is 

a mass of vein quartz approximately 1,000 feet wide and a mile long. 

This might constitute a source of silica, but the iron content is much 

too high for many uses of silica. Hematite, goethite, and limonite are 

abundant throughout the mass of veins as stains, coatings, and botryoidal masses in vugs.

The large quartz vein on Mount Hope (described by Quinn and 

Springer, 1954) has some iron, but less than that at Diamond Hill. 

I do not know of any systematic sampling or economic study of this 

mass of quartz.

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62 BEDROCK GEOLOGY OF RHODE ISLAND

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64 BEDROCK GEOLOGY OF RHODE ISLAND

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Univ. Bull. 19, p. 219, 1881; Boston Soc. Nat. History Proc., v. 21, p. 194-197, 

1882.

Walcott, C. D., 1898, Note on the brachiopod fauna of the quartzite pebbles of the 

Carboniferous conglomerates of the Narragansett basin, R.I.: Am. Jour. 

Sci., 4th ser., v. 6, p. 327-328.

Warren, C. H., 1908, Contributions to the geology of Rhode Island; the petrography and mineralogy of Iron Mine Hill, Cumberland: Am. Jour. Sci., 

4th ser., v. 25, p. 12-38.

Warren, C. H., and Powers, Sidney, 1914, Geology of the Diamond Hill-Cumberland district in Rhode Island-Massachusetts: Geol. Soc. America Bull., v. 

25, p. 435-476.

Williams, R. B., 1964, Bedrock geology of the Wickford quadrangle, Rhodfe 

Island: U.S. Geol. Survey Bull. 1158-C, 15 p.

Young, J. A., Jr., 1938, Keilhauite, a guide mineral to the Sterling Granite Gneiss 

of Rhode Island: Am. Mineralogist, v. 23, no. 3, p. 149-152.
INDEX

[Italic page numbers Indicate major references]

Page 

Absalona Formation....................... 14,/8,30

Acknowledgments............................ 2

Age, Allegheny........................._... 51,53

Blackstone Series..................._... 9

Cretaceous.....-...-..--.--.--.--.-...-.- 55

Devonian.......----..--.--.--..-.-...... 51

early Paleozoic......... . ...... 54,56

Early Pennsylvanlan..................... 53

Late Pennsylvanlan...................... 51,53

middle Paleozoic--..---.-....-..-.-...... 54

Mlssisslpplan....--._-..--..-............. 47

Paleozoic......--.---.--------...-----.-.- 53

Pennsylvanlan-..-. ................. 47,56

post-Pennsylvanlan...................... 51,53

Precambrlan............................. 53

radlometrlc_......_............._... SI

rubidium-strontium.... . 54

Tertiary........--------.....-...---.....- 55

Trlasslc._._._...-.._-.--..---.-.-....._.- 53,55

Age relationships..........._..._...__. 6

Algonklan quartzlte.......................... 11

Allegheny age..__._ .................. 51,53

Amethyst..........-.-....-.....--..-.-.....- 46

Anthracite............-..--..-.-.....-..--- 40,50

Appalachian Mountain system............... 56

Appalachian Revolution........._......... 56

Appalachian structure_..................... 46

Aquldneck Island............................ 32,48

Ashaway quadrangle.......... ... .. 2,

9,20,21,28,42,43,44,45,49,57 

Attleboro, Mass........................... 39,41

Bald Hill gold mine.......................... 58

BeavertaU................................. 44,45,48

Belllngham, Mass....-....-.....-..---.-- 42

Belllngham Conglomerate.-..------... 30,#

Bibliography_._..._.............._... 60

Blackstone quadrangle............. 2,9,29,42,44,45

Blackstone River___.___....._... 8,9,11,12

Blackstone River valley...................... 46,49

Blackstone Series......... 8,36,45,46,47,49,53,56,58

undivided.-..----........----.--. IS

Block Island............................ 7,32,46,55

Blue Hill Granite Porphyry................ 34

Blue Hills.................................... 10

Blue Hills area_............................ 34

Bradford.........-..--.. ........... 43,44,57

Bristol...........--..- .............. 21,23,59

Bristol Neck...............- ......... 23,45,48

Bristol quadrangle............ 2,7,23,32,37,40,45,48

Bulgarmarsh Granite_................ 7,22,32,SS

Bulgarmarsh Road................._....... 33

Page 

Cambrian brachlopods..- .................. 40

Cambrian rocks.............................. 46,54

Carbonate rocks..... ..................... 68

Carolina quadrangle_........ 2,26,27,42,43,49,57

Chepachet quadrangle_.__........____ 2,

9,11,14,15,24,25,29,47,49,50,58 

Clayville quadrangle......... 2,9,14,20,24,25,26,30

Cliff Walk, Newport.....-..-.... ........ 22,32

Coal.....-....-..--..-......-..-..-... 40,59

Coastal Plain....-..-..-..-...-.....--.... 55

Conanlcut Island.................... 22,32,37,48,50

Conklln Limestone Co........._ ....._ 56

Conklln quarry__.......................... 56

Connecticut Valley........................... 45

Copper Mine Hill............................. 58

deposits......___........._......... 68

Correlation, Blackstone Series..... ....._ 9

Coventry Center quadrangle... 2,20,26,27,30,49,68 

Cowesett Granite...... ....__.. 10,34,35,88,55

perthitic_...........____.......... 30

Cowesett Road.............___...__..... 36

Cranston mine.....__.................. 40,50,59

Cretaceous rocks........._...__.......... 46,55

Cretaceous time........... .............. 65

Crompton quadrangle..-..................... 2,

9,11, 12, 13, 24, 26, 28, 29, 33, 35, 36, 37,

38, 44, 47, 49 

Crushed stcne........._____......___ 68,59

Cumberland_.___.............._.__ 59

Cumberlandlte............................... S/,59

D 

Dedham Granodlorite....... . . .. 32

Deposits, Copper Mine Hill.................. 68

Furnace Hill, Iron...._______...... 58

Devonian age__............................ 61

Dexter quarry..____............__..... 12,56

Diabase, Trlasslo............................. 45

Diamond Hill.......................... 38,46,48,60

felsite..................................... 41

Dighton, Mass_............................ 41

Dighton Conglomerate....-..-..-... 38,^,48,65

Dikes, mafic.................................. 4$,55

Dimension stone_ _. ________. ___ 57 

Distribution, Blackstone Series_.__..... 9

Durfee Hill gold mine........................ 58

E 

Early Paleozoic time....... . ........ 64,56

Early Pennsylvanlan age... ....... 53

East Greenwich Granite Group_......._ 34

65
66 INDEX

Page

East Greenwich Group.. __.. __.. 7, 34, 51,55,56 

East Greenwich quadrangle.._.....____ 2,

9,11,12,13. 29,33,35,36,37,38,47,54,58 

East Kllllngly quadrangle............ 2,20,24,30,58

East Providence quadrangle.................. 2

Economic geology............._....._.... 56

Esccheag Hill................................ 25

Esmond Granite............... 7,10,26, g9, 34,54,57

Esmond-Granitevllle area___........__.. 57

Fall River, Mass............................. 37

Fall River quadrangle........_....__ 7,32,33,45

Felsite, Diamond Hill........................ 41

Foster goldmine.....__................... 58

Franklin quadrangle.... .....--........- 2,

28, 29, 30, 31, 33, 39, 41, 42, 44, 45, 57, 60 

Furnace Hill Brook limonlte............. _. 59

Furnace Hill Iron deposits.................... 58

G 

Gabbro..... ......................._... SO

Geological history............_........._ 6,53

Geological studies, earlier.........._....... 5

Geology ......................_.._...... pi. 1

Georglaville quadrangle..__._____ 2,

9,12,14,15,19, 29,30,37,42,44,47,48,49,57

Glaciation...........................___.. 55

Gneiss, fine-grained...._................... 27

light-colored.. .. _................... 19

porphyritic granite. ...._........... 15

Gneissic rocks, older, northwestern Rhode 

Island--..-..--........ ............ 14

Gold mines:

Bald Hill........-... ................ 58

DurfeeHill... ........................ 58

Foster.................................... 58

Homestrike.............................. 58

Snake Den._....... .................. 58

Southeast of Victory Highway and Plainfield Pike............................ 58

Gould Island................................. 22

Granite.. .............................. 56

fine-grained.....-..-..-.-.-....- 27,30,37

porphyritic. .... _....._....... 32,57

Granite gneiss, porphyritic.....__.......... 25

Granite porphyry............__...__.... 33,36

Granitic rocks, Pennsylvanian..........._. 4?

post-Pennsylvanlan...................... J$

Grant Mills Granodiorite ..7,10,28,54

Handy Pond...-..-.-.-..-....--.-.- 12

Harris quarry..................... ........ 12,56

Heliummethod ... -............. ........ 53

Homestrike gold mine.... ._........_... 58

Hope Valley Alasklte Gneiss.. 7, 23, 25, S6, 50, 53, 57 

Hope Valley quadrangle................. 2,26,27,49

Hopkinton................................... 20

Hoppin Hill area............................. 10

Hunting Hill................................. 13

Hunting Hill Greenstone........- .. 8,11, 18

Page 

[gneous rocks, Mississlppian age.............. 47

Iron deposits. Furnace Hill................... 58

Iron Mine Hill. ........................ 30, 31, 59

Iron ore..................................... 59

Jamestown Bridge. 50

Kingston quadrangle... ._ 2,14, 27, 37, 42, 49,

Lamprophyre.. .. ...................... 45

Late Pennsylvanian age..................... 51, 53

Lime Rock............................... 12, 49, 56

Limonite Furnace Hill Brook......____.. 59

Long Island .- .. .. __ 55

M 

Mackerel Cove..... ...................... 22, 32

Mafic dikes.................................. 44, 55

Mafic sills......-..-.-.-..-......- .. 44

Mansfield Mass-...-.--...-.........-... 59

Manton__.................................. 56

Marble.......-.. ........................ 12, 56

Martha's Vineyard....................... 32, 55

Maskerchugg Granite........................ 35, 36

Maskerchugg River..__.____.......... 35

Meta-anthracite........................ 5,40, 50, 69

Metacom Avenue_.......................... 32

Metacom Granite Gneiss.-- .. .... 7, 23, 32, 45, 58

Metadlorite................-.-.....-.- . S4

Metal deposits..- -------------------------- 68

Metamorphic rocks, older. 8, 46

southeastern Rhode Island............... SI

southwestern Rhode Island__.......... 19

western Rhode Island............. .... 19

Metamorphism............................... $?

Metavolcanic rocks, Western Rhode Island... 80 

Middle Paleozoic time........................ 64

Migmatite, Blackstone Series.__......._.. 14

Milford Granite...... .................... 11, 29

Mines, gold:

Bald HiU................................. 58

DurfeeHill ........................... 58

Foster................................. 58

Homestrike__..........._........... 58

Snake Den....-.-.-.----.------- 58

Southeast of Victory Highway and Plainfield Pike........................ 58

Minette....... ...... ......------ 44

Mississippian age_.__............. 47, 51

Mississippian Period..... .........-------- 55

Mlssissippianrocks...-.-. . --- - --- 47 

Mississippian time.... ._.._...._ .. ---- 56

Monchiquite.... 45

Mount Hope.....-.. -----...--.---- 45,48,60

Mussey Brook -. .. -. 12

Mussey Brook Schist........-.--- --. 8,11,/jS

N 

Nantucket....................... 55

Narragansett.---................-..-.--.-- 42
INDEX 67

Page

Narragansett basin........................... 6,

7, 8, 9,10, 32, 33, 34, 35, 36, 57, 39, 40, 41, 

42, 43, 44, 46, 47, 50, 53, 55, 57 

Narragansett Bay................... 32,37,48,54,55

Narragansett Pier Granite_................ 7,23,

27, 42, 44, 48 50, 51, 53, 55, 57 

Narragansett Pier quadrangle................ 2,32,

37, 38,42,43,44,45,48,50 

Natick, Mass................................. 39,46

New London area............................ 23

Newport...................... 21,22,32,40,41,48,59

Newport Neck......................... 22,32,37,48

Newport quadrangle............... 2,7,32,40,44,48

Nipsachuk Gneiss......................... 14,^5,18

Norfolk basin................................. 34

North Attleborough, Mass.................... 10,39

North Scituate basin........... 7,30,37,42,47,48,50

North Scituate quadrangle................... 2,9,

12,26,28,29,37,42,44,48,49, 57, 58 

North bridge Granite Gneiss............ 10,11,24,53

Norwood, Mass_...._..................... 34

0 

Oneco quadrangle.................... 2,20,24,25,30

Ordovician brachiopods...................... 41

Ordovicianrocks............................. 46,54

Oxford quadrangle............................ 24,26

Paleozoic age._---.._-__-_-.._.--..-__...... 53

Pascoag...................................... 25

Pawtucket................................... 46

Pawtucket quadrangle....................... 2,8,

9,10,1112,13,14, 28, 29, 30, 33, 34, 37, 39, 

41, 44, 45, 47, 49, 56, 57, 58, 60 

Pennsylvanian age. .......................... 47

Pennsylvanian metamorphism............... 50

Pennsylvanian Period........................ 51,55

Pennsylvanian rocks.......... 37,42,46,48,53,55,56

Pennsylvanian sedimentary rocks............ 48

Pennsylvanian time.......................... 56

Plainfleld Formation......................... 20,28

undivided................................ 21

Plutonic rocks, older......................... 23,47

Plutonic series_............................ 24

Ponaganset Gneiss....... 7,11,14,24,26,50,53,54,57

Pondville Conglomerate................... 10,22,38

Pondville Station............................. 38

Porphyritic granite........................... 32

Newport................................. 57

Porphyritic granite gneiss.................... 25

Portsmouth.................................. 59

Post-Pennsylvanianage...................... 51,53

Post-Pennsylvanian metamorphism.......... 60

Potassium-argon determinations. _.......... 53

Potter Hill Granite Gneiss................... 28

Pound Hill Eoad............................ 44

Precambrian age............................. 53

Precambrian time......................... 53,54,56

Pre-Pennsy Ivanian metamorphism........... 49

Progressive regional metamorphism.......... 50

Providence............................. 37,46,56,59

Providence quadrangle....................... 2,

9,11,12,13,24,28,29,37,38,49,50,57

Page 

Prudence Island quadrangle................ 2,40,48

Purgatory Chasm............................ 41,48

Purgatory Conglomerate..................... 5

Q 

Quadrangles:

Ashaway.........2,9,20,21,28,42,43,44,45,49,57

Blackstone........................ 2,9,42,44,45

Bristol................... 2,7,23,32,37,40,45,48

Carolina................... 2,26,27,42,43,49,57

Chepachet.............................. 2,

9,11,13,14,15,24,25,29,47. 50,58 

Clayville................. 2,9,14,20,24,25,26,30

Coventry Center........... 2,20,26,27,30,49,58

C.'ompton....__--...-...--..---.--._..-.- 2,

9,11.12.13,24,26, 28, 29, 33, 35, 36, 37, 38, 

44,47,49 

East Greenwich.......................... 2,

9,11,12,13,29,33,35,36,37,38,47,54,58 

East Killingly.................... 2,20,24,30,58

East Providence......................... 2

Fall River.......................... 7,32,33,45

Franklin................................. 2,

28,29,30,31,33,39,41,42,44,45, 57,60 

Georgiaville.............................. 2,

9,12,15,19,29,30,37,42,44,47,48,49,57 

Hope Valley......................... 2,26,27,49

Kingston................... 2,14,27,37,42,49,50

Narragansett P.er__....__._ . 2,

32,37,38,42,43,44,45,48,50 

Newport......................... 2,32,40,44,48

North Scituate........................... 2,

9,12,26,28,29,37,42,44,48,49,57,58 

Oneco............................ 2,20,24,25,30

Oxford................................... 24,26

Pawtucket............................... 2,

8,9,10,11,12,13,14,28, 29, 30,33,34,37,39, 

41,44,45,47,49,56,57,58,60 

Providence............................... 2,

9,11,12,13,24,28,29,37,38,49,50,57 

Prudence Island........................ 2,40,48

Quonochontaug........................ 2,42,43

Sakonnet Point...................... 2,7,22,33

Slocum.............................. 2,26,27,36

Thompson............................. 2,24,26

Tiverton...-..-...-...-.-.-.--..---.---.. 2,

7,22,32,33,37,38,39,40

Uxbridge................................. 9,24

Voluntown..................... 2,9,24,25,44,49

Watch Bill................. 2,21,42,43,44,45,57

Wickford............................ 2,27,36,40

Quarries_........__._.__.......... 44,56,57

Quartz....................................... 60

Quartz diorite................................ 28

Quincy, Mass................................. 34

Quincy Granite....... 7,10,29,33,36,47,51,53,55,57

Quinville Quartzite................... 8,11,12,13,14

Quonochontaug_............................ 43

Quonoch;. ntaug quadrangle................. 2,42,43

R 

Radiometric age.............................. 51

Raritan Formation...._.___.___..... AS
68 INDEX

Page 

Red Westerly granite........ .......__ 57

Rhode I land Appalachian Mountain system. 6 

Rhode I land Coal Measures...... ......... 39

Rhode I land Formation..................... 38,59

Rock groups._ . ..... ......_... 7

Rubidium-strontium age..................... 54

Rubidium-strontium whole-rock determinations. .__- -_............_-._....... 63

8 

Sachuest Point._______...._....__. 22

Sakonnet..................................... 22

Sakonnet Point......__...____.___ 22

Sakonnet Point quadrangle............ 2,7,22,33,44

Sakonnet River._........___..____ 21,22

Scltuate Granite Gneiss___..__.__... 7,

14,23, m, 27, 29,50,53,54,57

Sheldonvllle Mass.......................... 30

Sills, mafic................................... 44

Slocum quadrangle.-..-..........-.- 2,26,27,36

Smith quarries..... ____....__........ 44

Snake Den gold mine_..__.......___.. 58

Snake HiU.................................... 14

Sneech Pond.......................... 13,49,58

Sneech Pond Schist._.................... 8,11,/S

Soapstone........___.__________ 58

Spencer Hill.................................. 35

Spencer HiU Volcanlcs............... 10, SB, 36,47,54

Steatite..................

Sterling Granite Gneiss.. 

Sterling Plutonic Group. 

Stratigraphic units......

Stretched pebbles.......

Structural geology_. _

T

Page

Taberg, Sweden__..._....._._....... 31

Taunton, Mass. ..... .. . ... 41

Ten Rod Granite Gneiss.....__ 23,26,27,50,53,57

Ten Rod Road..........-..--......-..-- 27

Tertiary time__............................ 55

Thickness, Blackstone Series._.___.. 11 

Thompson quadrangle__......... __ 2,24,26

Titanium..._............................... 59

Tiverton....... . ... ...- 21,22

Tiverton quadrangle...... 2,7,22,32,33,37,38,39,40

TowerHill 43,50

Triassic age......... ........ .... .... 53,55

Triassic diabase..___........_.. . ... 45

U V 

Uxbridge quadrangle.................... . 9,24

Valley Falls.. 59

Vein quartz..-.... ......................... Jfi

Voluntown quadrangle_.......... 2,9,24,25,44,49

W 

Wamsutta Formation................ 38,59,41,51,53

Watch Hill quadrangle..... .... 2,21,42,43,44,45

Wesquage Beach_...._..........._ .. 43

Westboro Quartzlte.............. ........ 11

Westerly...... . 42,44

Westerly area_.............................. 43,57

Westerly Granite............ 7,42, 43, 49,51,53,55,57

Westerly quarry area..............-.-......-. 42

Wickford quadrangle..................... 2,27,36,40

Woonasquatucket Formation __.......... 14, 19, 30

Woonasquatucket River__..___.__..... 12

Woonsocket__...._._......_____. 30,46,58

Woonsocket basin................. 7,37, 48, 47,48,5o

US. GOVERNMENT PRINTING OFFICE: 1971 O 413-731