Bulletin No. 311 Series<| £' ^""P" »«&J. 1I5

\D, Petrography and Mineralogy, 35

DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

CHARLES D. WALCOTT, DIRECTOR

THE GREEN SCHISTS AND ASSOCIATED

GRANITES AND PORPHYRIES OF

RHODE ISLAND

BENJAMIN K. EMERSON

JOSEPH H. PERRY

WASHINGTON

GO VE R N M ENT PRINTING O F FICE 

1907

' ' CONTENTS.

	Page. 

Introduction............................................._..............'.. 7

Scope of paper................................... .................... 7

Previous work in the region............ I............................. 8

Table of formations..-..........-.......-..........-...:.........-... 8

Pre-Cambrian rocks...................................................... 9

Northbridge gneiss...........'........................................ 9

Cambrian rocks.......................................................... 10

Subdivisions and nomenclature......_-................-............... 10

Structure............................................................ 11

Grafton quartsdte..-.................--.....--..---..---.............. 12

General features ..............................._................... 12

Albion schist member............................................ 12

Marlboro formation................................................... .13

The green schists........................{\....................... 13

General characters............................................ 13

Varieties of the schist........................................ 14

Epidote-chlorite schist.................................... 14

Actinolite quartzite....................................... 15

Chlorite-actinolite schist........... ...................... 15

Hornblende schist.............. .......................... 15

Smithfield limestone member.................v.................... 16

General features.............................................. 16

Detailed descriptions of beds.................................. 18

Ores of copper, manganese, and iron....................... 18

Nonoccurrence of il vaite in Rhode Island.................. 19

Inclusion of limestone in granite porphyry................. 21

Harris bed at Lime Rock ................................. 22

Dexter quarry............................................ 24

Beds west of Providence and Pawtucket ................... 25

Cranston bed ............................................. 25

Attleboro bed............................................ 26

Beds in Woonsocket...................................... 26

Contact rocks on the Milford granite................................... 26

General features of the metamorphism ............................. 26

Contact rocks derived from the limestone .......................... 28

Trernolite rock and serpentine................................. 28

Tremolitic serpentine......................................... 28

Steatite...........................:........ ..................^ 28

Garnet rock.................................................. 29

Contact rocks derived from the green schists........................ 29

Contact effect of the Milford granite on inclosed Cambrian schists at

the Saylesville granite quarry .........:......................... 31

Contact rock from the Grafton quartzite ........................... . 33

Age relations......................................................... 33

3
CONTENTS.

Carboniferous rocks. i.....................................................

Scope of the discussion................................................

Carboniferous boundary ..............................................

Carboniferous conglomerate ................................. ^.........

Character and distribution ........................................

Outliers of Carboniferous conglomerate on the breccia of the East 

Greenwich group....................'..........................._...

Carboniferous shales and schists.......................................

Chloritoid (Masonite) in the metamorphosed Carboniferous conglomerate 

and phyllite ...;...................................................

Igneous rocks. ...................................^.......................

Igneous rocks of Cambrian age...............................:........

Diorite ..........................................................

Igneous rocks of post-Cambrian (pre-Carboniferous) age.......... ......

Milforcl granite...................................................

General description...........................................

The dark contact granite......................................

Dike rocks.......................................................

Aplite . ^.....................................................

Pyritous aplite...............................................

Basic eruptives...................................................

Gabbro ......................................................

Odinite......................................................

Igneous rocks of pre-Carboniferous or early Carboniferous age:.........

Subdivisions .....................................................

Quincy granitic group...... .......................................

Distribution and general character.............................

Petrographic descriptions.....................................

Granite porphyry............... J....................'....

Eiebeckite porphyry......................................

Hornblende granite.......................................

Microgranite. ........................'....................

Vein quartz at Diamond Hill..............................

"Warnsutta group" of Woodworth............................

East Greenwich group ....... ............:........................

Distribution and general features..............................

Petrographical descriptions....................................

Granite and granite porphyry .............................

Basic border of the granite porphyry ......................

Blue-quartz-microcline porphyry ..........................

Microgranite.............................................

Microgranite or graphic microgranite ......................

Dikes of microgranitic and micrographic texture............

Microclinite..............................................

Graphic microgranite breccia with granite-porphyry cement. 

Chemical relations of the Carboniferous igneous rocks...............

Besume showing relation of the porphyries to one another and to the Carboniferous sedimentary rocks .......................................

Igneous rocks of post-Carboniferous age. ...............................

Olivine-diabase porphyry .........................................

Porphyritic diabase........................-...---.---.-.-----.----

Index ...................................................................

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ILLUSTRATIONS.

Page. 

PLATE I. Map of the crystalline rocks in the vicinity of Providence and Narragansett Bay, Rhode Island................................ 10

II. Porphyritic green schist....... .^.................................. 30

FIG. 1. Calcite crystal from the Harris bed at Lime Rock.................... 23

2. Calcite crystal from the Harris bed at Lime Rock..................... 24

3. Calcite crystal of the square type from the Harris bed at Lime Rock.. 24

4. Map showing distribution of metamorphosed Carboniferous rocks.... 35

5. Crystals of feldspar embedded in quartz groundmass................ 61

6. Microcline Baveno twin from quartz porphyry south of Spencer Hill.. 61

5

THE GREEN SCHISTS AND ASSOCIATED GRANITES 

AND PORPHYRIES OF RHODE ISLAND.

Bv B. K. EMERSON and J. H. PERRY.

INTRODUCTION.

SCOPE OF PAPER.

A great mass of coarse, porphyritic granitoid gneiss occupies the 

towns of Sutton, Northbridge, and Douglas, in Massachusetts, and 

extends southward across western Rhode Island to Kingston, on the 

Sound, just west of the territory to be described in this bulletin. A 

series of quartzites and hornblendic rocks wraps round the northern 

part of this mass, resting on it unconformably with a very sinuous 

boundary. These two groups of rocks the first called the Northbridge gneiss, the second made up of the Grafton quartzite and the 

Marlboro formation may be assigned with considerable probability 

to the pre-Cambrian and Cambrian, respectively.

The continuity of the Cambrian rocks is greatly interrupted in 

Massachusetts by broad areas of intruded granite, and farther south, 

in the region here studied, only broad, isolated patches of the sedimentary rocks appear, like ice floes on a pond, in a confluent expanse 

of various eruptives, which differ in age and in lithologic character. 

This paper is devoted principally to a discussion of such of these 

interesting Cambrian remnants as lie within the limits of the State of 

Rhode Jsland and to a description of the associated eruptives, especially the remarkable series of very fresh porphyritic rocks that extend 

westward from the town of East Greenwich. The field work occupied three summers and covered the crystalline area along the western 

border of the Carboniferous rocks in the Providence and Narragansett 

Bay quadrangles. The work was undertaken for the purpose of publication in folios of the Geologic Atlas of the United States, but as the 

publication of the folios has been deferred and as the results of the 

survey are needed in the explanation of the structure of the territory 

adjacent on the north, in which work is now in progress, they are here 

presented in the form of a bulletin.

7
8 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND. 

PREVIOUS WORK IN THE REGION.

The rocks studied comprise the western border of the Carboniferous 

of the Narragansett basin in .Rhode Island and the more highly meta- . morphosed varieties of the Carboniferous rocks that extend over the 

older rocks. The results presented therefore supplement those contained in the monograph on the geology of the Narragansett basin 

by Shaler, Wooclworth, and Foerste/' in which these rocks were not 

discussed. The bibliography of the Cambrian and Carboniferous rocks 

of the basin given in that volume b includes nearly every publication 

that makes allusion to the region here described. The report on the 

geology of Rhode Island by Dr. C. T. Jackson c is the fullest account 

of this region that has been written. The area considered in this bulletin is represented in green and blue on Doctor Jackson's map, where 

it is labeled "hornblende rock and limestone" and referred to the Primary. Not the least interesting of the early writings on this region is 

the very clear description given by Prof. Ebenezer Ernmons/ in which 

he assigns the green schists to the Taconic that is, to the Cambrian 

(the age of the Stockbridge limestone) and finds several members of 

the "Taconic system" represented here. He gives an accurate section 

across the area from southwest to northeast through the limestone 

quarry. Prof. W. 0. Crosby, as the result of a brief visit to the 

region thirty-five years later, describes the rocks and assigns them 

doubtfully to the Huronian and Montalban/

TABLE OF FORMATIONS.

The rocks in the area investigated are tabulated below, the table 

beginning with the oldest:

Formations in Uie Narraganselt basin. 

EARLIEST ROCKS:

Pre-Cambrian.

'Northbridge gneiss. . 

SEDIMENTARY ROCKS: 

Cambrian.

Grafton qnartzite.

Albion schist member.

Marlboro formation, including green schists, amphibolite, mica schist, 

and, as a subordinate member, the Smithfield limestone, which 

changes into tremolite schist, steatite, and serpentine.

« Mon. U. S. Geol. Survey, vol. 33, 1899. 

Mdem, p. 212.

cJackson, Charles T., Report on the Geological and Agricultural Survey of the State of Rhode 

Island, 1839. Published in 1840.

rtEmmons, Ebenezer, Agriculture of -New York, vol. 1, 1846, pp. 91-93. 

eCrosby, W. O., Contributions to the Geology of Eastern Massachusetts, 1880, pp. 127-128.
PKE-CAMBRIAK KOCKS. V

SEDIMENTARY ROCKS Continued. 

Carboniferous.

Unaltered rocks. 

Shale.

Conglomerate. . 

'Metamorphosed rocks.

Biotite-spaugled phyllite. 

Chloritoid-spangled phyllite. 

Arenaceous chloritoid schist. 

Magnetitic mica schist. 

CongloiTieratic mica schist. 

IGNEOUS ROCKS: 

Cambrian.

Diorite. 

Post-Cambrian (pre-Carboni ferous).

Milforcl blue-quartz granite. 

Aplite. 

Gabbro. 

Odinite. 

Cumberlandite.

Early Carboniferous or late pre-Carboniferous. 

Quincy group.

Porphyritic granite. 

Granite porphyry. 

Riebeckite porphyry. 

Microgranite.

Hornblende granite. ^ 

East Greenwich group. 

Granite.

Granite porphyry. 

Blue-quarts porphyry.' 

Microgranite. 

Graphic microgranite.

Graphic microgranite and granite-porphyry breccia. 

Post-Carboniferous.

Olivine-diabase porphyry. 

Porphyritic diabase.

PBE-CAMBKIAN BOCKS.

NORTHBRIDGE GNEISS.

Northbridge gneiss is the name given elsewhere" to the great area 

of pre-Cambrian gneiss that extends southward from Southboro, Mass. 

A rock having all the characteristics of the North bridge gneiss and 

forming its probable southward extension occupies the southwest corner of the area mapped in Fl. I. It is a medium- to coarse-grained 

granitoid gneiss, of light color, containing smoky quartz in long pencils. The feldspar, the most abundant constituent of the rock, presents a notable contrast with the quartz in being regularly and evenly 

granular. The grains are just visible to the eye. Bladed biotite, 

magnetite, and a little muscovite appear. The rock crumbles into a

"Emerson, B. K., Geology of old Hampshire County: MOD. U. S. Geol. Survey, vol. 29, 1898, p. 18.
10 SCHISTS AND ASSOCIATED BOCKS OF RHODE ISLAND.

mass of pencils, as does the gneiss in Northbridge, Mass. It grows 

coarser and porphyritic toward the west. The distinction here, as in 

Massachusetts, between a coarser, less crushed, and porphyritic facies 

and a finer, more schistose, and more completely mashed and stretched 

facies is noted on the map. < »

The connection of the rock of this area with the Northbridge porphyritic granite-gneiss has not been definitely traced in the field, but 

is very probable, since Jackson a described the rock as of wide extent 

across Kingston and represented it on his map at many points in West 

Greenwich, Coventry, and- Foster, and we have traced it southward 

from Douglas, Mass., across Burrillville into Glocester, R. I., the 

town adjoining on the west the area here considered.

The porphyritic granite-gneiss that borders the Cambrian on the 

south and east of the Carboniferous rocks of the Narragansett basin, 

as, for example, around Conanicut, Newport, and Little Compton, 

would seem to be of the same age as the North bridge gneiss if the contention of Dale is correct, that it is an older member underlying the 

Cambrian. Pirsson, 6 Crosby and Barton/ and Foerste d are, however, 

certain that this granite-gneiss cuts the Cambrian and is thus the 

newer rock, and in this case it is probably of the same age as the Milford granite.

The least stretched varieties of the Northbridge gneiss and the most 

crushed facies of the Mifford granite are indistinguishable at some 

places in the field. The Northbridge gneiss is characterized by a 

coarse-meshed microcline, which is replaced in the Milford granite by 

a complex microperthite, the albitic constitutent of which is centrally 

dusted with epidote.

CAMBRIAN BOCKS. « 

SUBDIVISIONS AND NOMENCLATURE.

The Cambrian rocks, as they appear along the Blackstone Valley 

between Woonsocket and Pawtucket, are described in this text and 

shown on the geologic map (PI. I) under four subdivisions:

1. A central band of phyllite and fine-grained micaceous quartz 

schist the Albion schist member.

2. Two flanking bands of a granular massive quartzite theGrafton 

quartzite.

"Jackson, Charles T., Report on the Geological and Agricultural Survey of the State of Rhode 

Island, 1839. Published in 1840. 

SPirsson, L. V., On the geology and petrography of Conanicut Island, R. J.: Am. Jour. Sci., vol. 46, 

1893, pp. 363-378.

o Crosby, W. 0., and Barton, G. H., Oii the great dykes at Paradise, near Newport: Proc. Boston 

Soc. Nat. Hist. (1886), vol. 33, p. 325.

dSnaler, N. S.,Wood\vorth, J. B., and. Foerste, A. F., Geology of the Narragansett basin: Mon. U. S. 

Geol. Survey, 1899, vol. 33, p. 235.

e "Blackstone series" of Wood worth.
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POSTCAMBRIAN CAMBRIAN

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CAMBRIAN BOCKS, STRUCTURE. ' 11

3. Two broad exterior bands of green schists and amphibolite the 

Marlboro formation.

4. Thick beds of crystalline limestones with soapstone and serpentine in the sediments the Smithfield limestone.

This series of rocks in Massachusetts was several years ago traced 

southward by the senior author of this paper" from Westboro (where 

the quartzite and amphibolite appear in typical development) with a 

good degree of continuity to the area here .considered, and the names 

then proposed for them are here retained. Mr. Woodworth briefly 

touched upon the same rocks in connection with his description of 

the adjacent Carboniferous basin,* and proposed for them the name 

"Blackstone series." He gave the name "Cumberland quartzite" to 

the quartzose beds, and the name "Ashton schists" to the schistose 

beds. As we have found that the schistose beds are divisible, the 

name "Albion schist" is applied to the central quartz phyllite and 

mica schist upon whic"h the village of Albion stands, and the larger 

group of green schists, amphibolites, soapstones, and limestones are 

called the Marlboro formation, from their equivalent farther north.

STRUCTURE.

The whole series occurs in several elongate patches which are inclosed 

in a great granite batholith on all sides up to the point where they are 

in contact with the Carboniferous. All its members have a uniform 

 steep dip to the east. We can not tell whether the two broad bands 

of green schist that flank the central quartzite band are joined above 

the quartzite as a closed and overturned anticline, or beneath in synclinal arrangement. If our correlations are right, the green schists 

are probably the younger, as the hornblende schists of the Marlboro 

formation seem somewhat higher than the quartzites. At the south 

end of the isolated area of green schist north of Albion everything 

indicates that the green schist is the newer rock. The fact that the 

pebbles found in the green schists are of a quartzite like that of the 

central quartzite indicates that the green schists are the newer and 

that the structure is therefore anticlinal. The Providence area ma}r 

be taken as a more irregular anticline of the quartzite flanked by the 

green schists, and, like the other, included in the Milford granite.

The dips and strikes given on the map (PI. 1) show the probable 

posture of the beds in the middle of the Cumberland area. The 

isolated central area of the green schist on the Albion schist makes it 

probable that it is a passage bed between the Grafton quartzite and 

the Marlboro formation.

aMon. U. S. Geol. Survey, vol. 29,1898, p. 18. 

bMon. U. S. Geol. Survey, vol. 33,1899, p. 106.
12. SCHISTS AND ASSOCIATED .ROCKS OF RHODE ISLAND 

GRAFTON QUARTZITE.« 

GENERAL FEATURES.

The Grafton quartzite is general^ a line-grained, massive, saccharoidal quartzite of great purity, and resembles the Cheshire quartzite, 

of Cambrian age, in Berkshire County, Mass. It varies from white 

to pale flesh color, and some varieties have a greasy luster. It is 

entirely massive over large areas, as west of Manville, where it so 

exactly resembles the Cambrian quartzite of Berkshire that even the 

conchoidal fissures, which appear like gouge marks on the surface of 

the western rock, are here also present. At some places it shows thin 

lamination and cross bedding; at others it is greatly jointed or c thin 

foliated by crushing or reduced to rude columnar masses by complex 

strains, as in the long cut east of Manville, where bands of massive 

quartzite containing grains of blue quartz alternate with thin fissile 

crushed micaceous beds. It is most crushed and altered on granite 

contacts.

It appears in force in the south and west of Natick, where it is 

slightly biotitic and carries beds of biotite schist 3 feet thick. Here 

it breaks into small angular blocks, all of whose faces are coated with 

a film of secondary muscovite. In some places it is distinctly a 

conglomerate. ^

The quartzite in places contains purple fluor derived from the granite. Small red garnets (110) of perfect form occur in the quartzite 

and conglomerate, and, at a point one-half mile southwest of Ashton 

it contains grains of blue quartz.

ALBION SCHIST MEMBER.

Down the center of the Grafton quartzite runs a band of rock which, 

although originally in large part a quartz sandstone and still prevailingly quartzitic, is so far muscovitic that it may be called a mica schist 

or muscovite schist. The light-gray color dependent upon the absence 

of the ferruginous minerals (biotite, hornblende, chlorite) separates it 

from the green schist. As the Grafton quartzite is at most places 

slightly muscovitic, the distinction between it and the Albion quartzose mica schist is of subordinate importance; but after the line of 

boundary had been traced in the field, it was thought better to preserve 

the subdivision and represent it on the map. The finer grain of this 

rock and the presence in it of considerable beds of phyllite distinguish 

it from the other. The road on the side of the hill east of Cumberland 

village is cut in a dark-gray to drab phyllite, so little metamorphosed 

that we felt justified in searching it carefully for fossils. The soft 

beds alternate with beds of quartzite, ranging in thickness from 4 to

a "Cumberland quartzite" of Woodworth.
CAMBRIAN BOCKS, MABLUOBO FORMATION. 13

100 feet. The argillite shows in places the original bedding, which 

cuts the perfect slaty cleavage at a small angle. The rock resembles 

some phases of the Carboniferous slate of the Worcester basin, but is 

much more like the Cambrian slate of Weymouth.

MARLBORO FORMATION.

THE GREEN SCHISTS. 

GENERAL CHARACTERS.

The Marlboro formation, as developed in the region stretching from 

Woonsocket to Pawtucket, includes a great variety of green schists, 

ainphiboHtes, steatites, and limestones. In its least metamorphosed 

forms the green schist is a pale-green chlorite-sericite schist, soft and 

slaty; a green actinolite schist; a dark-gray, very fine-grained quartzschist, or a slaty and penciled arenaceous limestone. These rocks west 

of Pawtucket are so little metamorphosed as to suggest their identity 

with the Carboniferous schists in the large quarry in Pawtucket. Farther northwest, nearer the granite, a dark-green, fissile, aphanitic, 

chloritic ainphibolite, containing -in places small altered pyrite cubes, 

abounds. There were originally several thin beds of limestone in the 

formation, and at least one of considerable thickness. These are in 

part or whole altered to beds of tremolite, tremolitic steatite, or 

impure serpentine, and are discussed separately on pages 26-28.

The green schists are nearly everywhere severely crushed, aphanitic, 

and slaty, and are in places gashed with open fissures, and ragged and 

cavernous exteriorly, because of the solution of calcite, and" in the 

interior will nearly always effervesce with acid. Many specimens of 

the rocks are full of small magnetite octahedra.

In places, as, for instance, north of the Lime Rock quarry, beds of 

schist belonging to this formation contain traces of pebbles, some of 

them 1£ inches long. In rocks of this kind found north of Cumberland Hill the pebbles are much drawn out. These altered conglomerates extend from a point west of Sneech Pond 1 mile southward. 

Many of the pebbles are of epidotic quartzite.

Limestones and quartzites of undoubted sedimentary origin alternate with the green schists, amphibolites, and steatites at so many 

places and grade into them in so many ways that we have assigned to 

the whole series a sedimentary origin.

The large amount of iron present suggests the derivation of some 

part of the material from basic tuffs. We found but few beds in the 

green schist which, after study in the field or with the microscope, we 

were inclined to regard as directly derived from basic dikes or intruded 

sheets. The ferruginous sandstones and included tuffs and" impure 

limestone beds of the Triassic could be changed into a similar series,
14 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND.

and we have in a later section attempted to connect this formation 

with the similar ferruginous and calcareous Cambrian beds at North 

Attleboro. The more massive rocks, which most simulate eruptives, 

are the thick beds of tremolitic steatite and serpentine derived from 

limestone. A large slab of the conglomeratic green schist from a 

ledge on the top of the hill 100 rods north of the middle quarry at 

Lime Rock contains many well-rounded pebbles of a very fine-grained " 

white-bleaching actinolitic quartzite like the finest beds of the Albion 

quartzitic schist; also many pebbles of coarser and finer varieties of a 

diorite which appears in a considerable mass in the same hill. . This 

makes it certain that a part of the ferruginous material of the green 

schist is tuffaceous and related to these diorites. Before we made this 

discovery we had regarded the few occurrences of this diorite as 

eruptive. They occupy but a small area, and the hornblendic beds of 

the green schist proper are of a different and plainly metamorphic 

clastic type.

The aphanitic quartzite pebbles are unlike the coarse saccharoidal 

Grafton quartzite, but resemble the fine-grained calcareous quartzite 

of the green schist series itself, especially that form of it seen in inclosure* in the porphyry in Cumberland; that is, the actinolite quartzite is such a rock as would be produced if the calcite of the latter rock 

had gone to the formation of abundant actinolite needles.

The interesting relations of this tuffaceous conglomerate and the 

adjacent diorite are discussed on page 44.

VARIETIES OF THE SCHIST.

Epidote-cklorite schist. A typical green schist outcrops by the roadside a mile northwest of Lonsdale, near .the granite. It is a finegrained, irregularly fissile schist of gray-green color, with indistinct 

green veinlets of a shade .suggesting pistachio green. There are 

minute perfect crystals of pyrite (100) (111), superficially changed to 

limonite. The sheen of a micaceous mineral can be seen. The veins 

alone give slight effervescence with cold acid. The slide (362) shows 

with the lens porphyritic rounded spots, around which the constituents have a somewhat concentric arrangement. The field shows a 

dense mat of elongate green (chlorite) and pale-green (actinolite) 

blades with a few small, angular, colorless grains of quartz, more or 

less strained, the whole being dusted with a shining, granular, almost 

colorless mineral (epidote). The chlorite is uniaxial and positive (pennine), with pleochroistn ranging from yellow-green to emerald-green.

Some of the porphyritic spots, are relics of plagioclase phenocrysts, 

now almost wholly changed to groups of small, stout epidotes and. 

olive-green biotites. One of the plagioclase remnants gave extinction, 

13-33, with broad bands. These feldspars are Tare and are irregularly 

distributed. They may have been derived from diorite grains in the
CAMBRIAN BOOKS, MARLBORO FORMATION. 15

tuff. The granular mineral shows red of the second order in a slide 

of normal thickness, and so is epidote rather than zoisite.

Another rock from the same localit}^ is a similar green, mottled, very 

fine-grained schist or slate, with large chalcopyrite grains surrounded 

by a riui of just visible hornblende needles. The slide (370) is a mat of 

very fine-grained hornblende needles with blue absorption on c and 

chlorite scales in a clastic ground, with much epidote and many wisps 

of fasciculate tourmaline needles, with very strong absorption, salmoncolored parallel to c, blue-black parallel to a.

Actinolite quartzite. A fine-grained massive rock, varying from 

white to dark gray, occurs as pebbles in the tuifaceous conglomerate 

and in independent beds^ It is made up wholly of very fine grains of 

angular quartz and of needles of pale-green actinolite, the latter being 

in places abundant.

Chlonte-adinolite schist. Another rock collected at a place onethird of a mile from the granite and one-half mile west of Woodville, 

in West Providence, was the common, greenish-black, fissile rock, 

showing with the lens stout, minute actinolite needles and chlorite 

scales. The slide (437) showed abundant broad blades of chlorite and 

actinolite with ragged ends, which form an open network-in a ground 

made up of magnetite and epidote grains and minute actinolite needles.

Hornblende schist. Hornblende schist is especially abundant in the 

green schist around the Lime Rock limestone and east of the river in 

the land west of Sneech Pond about Cumberland Hill, which is marked 

hornblende rock on Jackson's map. There is at Smithfield a beautiful satiny, thin-fissile amphibolite, made up of brilliant black needles 

pointing in a common direction.

Doctor Jackson's remarks concerning the hornblende rock are 

extremely interesting and show that the doubts about its origin arose 

early (1839).

The rock * * * presents some of the most perplexing geological enigmas that 

have fallen under my notice. It is generally regarded as an unstratified rock of 

igneous origin, but not unfrequently presents the appearance of regular stratification. 

* * * It passes frequently by imperceptible shades into chloritic and argillaceous slates, so that it is very difficult to deifine its exact limits. * * * I am of 

opinion that it is decidedly a rock of igneous origin; * * * that it derives its 

occasionally stratiform structure from an admixture of argillaceous slate rock through 

which it was elevated * * * and * * * that the serpentine frequently is a 

product of the interfusion of the hornblende rock and the soapstone. * * * . It is 

highly probable that * * * the hornblende rock has charged the Smithfield 

limestone near its junction with magnesia.«

It will be noted that we assign a part of Doctor Jackson's hornblende rock to an igneous origin and part to a sedimentary and metamorphic origin. His last three propositions we should hardly indorse, 

though we made no study of the dolomite in the Smithfield limestone.

a Kept. Geol. and Agr. Survey of Rhode island, 1840, pp. 30-31.
16 SCHISTS AND ASSOCIATED EOCKS OF KHODE ISLAND.

SMITHFIELD LIMESTONE MEMBER.

3 

GENEKAL FEATURES.

The great " Harris " and "Dexter" beds of limestone in Lincoln 

(formerly part of Smithfield, from which the name Smithfield limestone is derived) have long been known because of their economic 

importance and the interesting minerals found in them. They have 

given name to the village of Lime Rock and still sustain a considerable industry, although they are less important than they were before 

the advent of the railroad.

Formerly many other beds were worked that are now abandoned, 

for large beds of this rock occur in many parts of the Marlboro formation, and those now found are only a part of the limestone which 

was present in the original rock. Mam^ beds of tremolite rock, soapstone, serpentine, amphibolite, and ores of iron and copper have 

replaced limestone. In color the rock is generally pure white, but is 

in some places banded in dark gray or tinged yellow with iron or pink 

with manganese, as if it contained rhodochrosite.

The Smithtield beds vary from massive saccharoidal marbles of line 

to above medium grain to rocks laminated through shearing with the 

development. of chlorite, asbestos, mountain leather, and talc upon 

the gliding planes. Professor Wood worth figures .the echeloned" fragments of a disjointed dike in the limestone in the southern bed in 

Lincoln, and calls attention to the fact that the limestone has under 

pressure flowed into all the interstices and now shows no trace of the 

separation."

These beds are plainly the contemporaneous members of a sedimentary series intercalated original!}7 with clayey, marty, and tuffaceous layers. ThejT attain in places a thickness of 150 feet or more, 

and their present appearance in isolated, long, elliptical masses is due 

to faulting, solution, and the metamorphism of much of the original 

limestone into other rocks. They are in part dolomitic and in part 

pure limestones.

They have been in many places replaced b}' ores of iron and copper, 

especially at Copper Mine Hill, north of Sneech Pond, in Cumberland.

Doctor Jackson says: &

The magnesium limes of Rhode Island have always ranked high in the estimation 

of masons, and they are prized for the quickness of their setting when converted into 

mortar, as also for the beautiful whiteness of the lime. Hence the Smithfield "hardjointer" rock, being a magnesium limestone, makes a variety of lime that commands 

a higher price in the market than any other. This kind of lime is, however, unfit 

for agriculture, and care should be taken to use only the soft rock for that purpose, 

since it is free from magnesia. . .

The dolomite of Rhode Island is that portion of the bed which lies 

in contact with or near the hornblende rock. In some places, however,

a Mon. U, S. Geol. Survey, vol. 33,1899, p. 108. & Jackson, idem, p. 36.
CAMBEIAN EOCKS, SMITHFIELD LIMESTONE. 17

the line of dolomitization appears to terminate abruptly at a distance 

of 6 or 10 feet from the hornblende rock. a

The figures given below show the annual value of the lime burned 

in Rhode Island, the industry being confined almost entirely to Providence County. 0 .'

Value of lime produced in Rhode Island, 1890-1905.

1890.......................... $27,625

1891 .......................... 25,000

1892 .......................... 30,000

1893................. ........ 24,800

1894.......................... 20,433

1895..........................

1896 .......................... 11, 589

1897 ........................... 11,555

1898........................... $10,215

1899.......................... 18,239

1900.......................... 16,715

1901.......................... 37,798

1902..........................

1903..... ..................... 34,432

1904 .......................... 31,871

1905 .......................... 42, 743

The following table of analyses is taken from Doctor Jackson's 

report.*

Analyses of limestones from Rhode Island.

No.

1

?

3 

4

5 

6

7 

8

9

in

n

1?!

13 

14

15 

16

17

IS

Locality, owner, etc.

Cumberland Hill, F.

Brown.

Johnston, Mr. Brown.

kins. 

North Providence ....

Newport Harbor, 

- Lime Islands. 

North Providence, 

lime quarry, 

Smithfleld, Harris

rock. 

Shore near Fort 

Adams. 

Smithfleld, Harris

quarries. 

Smithfield, Harris

rock. 

Smithfleld, Dexter

quarry. 

.....do ................

Smithfield, S. Arnold.

Smithfield, Harris 

quarry.

rock. 

.....do................

Smithfield, E. Angell.

Variety.

Jar. 

Greenstone (8.0 water)..

Stone, white with yellowish spots, crystalline.

lowish and green 

spots, crystalline. 

White, compact, subcrystalline. 

Compact, blue and buff 

colored. 

White and compact .....

Yellow, buff-colored, 

compact.

 White, granular, and

crumbly. 

Compact, white, insoluble matter in acicular 

crystals.

with talc and crystalline. 

First quality soft blue 

stone with blue arid 

white stripes, crystalline.

lar.

stratified and crystalline.

talline.

Carbonate 

of 

lime.

52.2

68.8

55.2 

97.2

56.1 

53.2 

68.6 

92.8

50.9 

92 4

94.8

64.6 

50.6

92.2 

95.8.

87.0

97.6

Insoluble 

matter.

6.0

23.2

46.6

.8

11.4 

7.0 

8.6

4.3 

6.0

1.0

. 1.6

2.0 

3.8

1.0 

1.4

12.4

1.0

Oxide 

of 

iron.

1.0

1.9

3.8

2.1 

.4

2.6

Tr. 

Tr.

Magnesia.

40.6

6.2

4.2 

.6

32.5 

37.9 

21.0 

3.4

42.7 

1.2

36.0

32.0 

J4.4

6.8 

2.8

Lime.

29.4

38.7

31.1 

64.6

31.6 

29.9 

38.6 

52.2

28.7 

52.0

34.0

53.4

36.4 

28.5

51.9 

68.0

49.0

54.9

Specific 

gravity.

2.728

2. 824

2. 822

2.660

2.853

2.681

2.715

"Twenty-first Ann. Kept. U. S. Geol. Survey, 1899, pt. 6 (continued), pp. 357-360; Mineral Resources 

U.S. for. 1901, p. C67. 

&Idem, p. 246.

Bull. 311 07 g
18 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND.

The following analysis of the limestone quarried by Mr. Herbert 

Harris at his quarry at Lime Rock, Providence County, was made by 

Prof. J. H. Appleton, of Brown University. 05

Analysis of limestone quarried, at Lime Rock, R. I.

Moisture................................................................ 0.040

Oxide of iron........................................................... .011

Alumina (A12 03 ) ........................................................ .-309

Siliceous matter (insoluble)............ ................................. 2. 748

Calcium carbonate (CaC03 ) ............................................. 88. 233

Magnesium carbonate (MgC03 ).......................................... 8. 797

100.138

DETAILED DESCRIPTIONS OF THE LIMESTONE BEDS.

ORES OF COPPER, MANGANESE, AND IRON.

On Doctor Jackson's map the symbol for copper is repeated several 

times in the area northwest of Sneech Pond. Reporting upon this 

area he says that 6 the limestone in the beds at Cumberland Hill, on 

the estate of Mr. F. Brown, runs IN. 25° W., dips 35° E., and is 6 to 

10 feet thick and dips beneath the granite. He states that it contains 

copper pyrites, tremolite, asbestos, and actinolite, and a number of 

curious minerals common to such limestones.

Continuing he writes:

Near Sneech Pond there is an ancient mine, sunk apparently for the purpose of 

extracting ores of copper, which are found there mixed with veins of granular magnetic iron ore. The shaft slopes to the northeast by east, following the dip. of the 

vein, and is 20 to 30 feet wide, but it has for a long time been filled with water, so 

 that its depth has not been ascertained.

Near the pond occurs a very thick bed of a remarkable ore of manganese, whiqh is 

peculiar in its composition, but most nearly resembles the knebelite of Beudant. 

The bed is no less than 40 feet thick, and when any method of making use of the 

mineral is discovered it may prove an important locality. One hundred grains of 

this mineral were taken for analysis, and the following results were obtained:

Silicic acid.............................................................. 26. 400

Protoxide iron .......................................................... 35. 912

Protoxide manganese.................................................... 32. 488

Carbonic acid. ........................................................... 5. 200

100. 000 

Another specimen yielded:

Silicic acid.............................................................. 29.400

Protoxide iron .......................................................... 37. 707

Protoxide manganese.................................................... 27. 692

Carbonic acid. .......................................................... 5. 200

99. 999 

Specific gravity mean of three specimens, 3.7881.

aTwentietzh Ann. Kept. U. S. Geol. Survey, pt. 6 (continued), p.442. &Idem, pp. 64-56.
CAMBRIAN ROCKS, SMITHFIELD LIMESTONE. 19

Associated with this mineral occur crystals of green quartz, or prase, and veins of 

quart/ penetrated by delicate green crystals of actinolite, forming a kind of ornamental stone for which the name Thetis Hair stone lias been proposed. Snlphnret 

of molybdena also occurs in the manganese ore, and the yenite formerly discovered 

near this place is said to have been formed in the accompanying quart/- veins. Ligneous actinolite abounds in the veins with the quart/ above mentioned.

Several excavations of considerable extent have been made in this vicinity, and 

were probably prompted by the'discovery of masses of yellow copper pyrites, which 

was doubtless mistaken for gold, as it generally is by persons unacquainted with mineralogy. It is worth remarking that there are no less than 50 different ancient mine 

holes in this hill, and it is estimated that more than half a million dollars must have 

been spent in these fruitless researches for the precious metals. The iron ore appears 

to have been generally neglected, although when wood was abundant in this vicinity 

it might have been profitably wrought for iron.

At one of these ancient mines it was easy to discover what minerals were sought 

for, since one of the casks in which the ore had been packed for the purpose of sending it to England had been left, and from its broken and partly decomposed staves 

we picked up an abundance of the yellow copper pyrites which is found accompanying the granular magnetic iron ore. I took a fair specimen of this ore and subjected it to separation by the magnet, and then reduced the copper ore which was 

left. It contains, in 100 grains of the picked ore:

Copper ................ r............................................... 36. 842

Iron................................................................... 31. 940

Sulphur ................................................................ 31.218

100.000

Professor Woodworth reports that angular brecciated fragments of 

the limestone lie in the ore-bearing mass at this locality, and assumes 

that the ores of iron and copper have replaced the limestone, as is 

doubtless the case. He mentions that there are eruptive rocks near, 

but finds no means of determining whether the ore has been deposited 

through the action of heated waters connected with these ernptives or 

through the downward percolation of acidulated surface waters. 

Fluorspar in small'quantity everywhere accompanies the eruptives, 

and we should expect to find it concentrated in considerable abundance 

in the ores if these were produced from solutions derived from the 

vicinity of these eruptives, and such we found to be the case.

Epidote occurs south of the road a mile east of Sneech Pond, in 

masses a foot thick, in the green schist doubtless derived from the 

alteration of small limestone beds.

NONOCCURRENCE OF ILVAITE IN, RHODE ISLAND.

The citation of yenite, above, depends upon a determination in 

1823 by Prof. C. U. Shepard^ of a mineral having apparently all the 

properties of }r enite. This mineral was found on Tower Hill, in Cumberland, in a matrix of quartz, epidote, and magnetite. Dr. Samuel 

Robinson cites the same mineral "1 mile east of Cumberland meetingo

house, associated with calcareous spar, actinolite, quartz, and prase."6

"Am. Jour. Sci., vol. 7,1824, p. 251. <> Am. Jour. Sci., vol. 8, 1824, p. 231.'
20 SCHISTS AND ASSOCIATED ROCKS OF ERODE ISLAND.

Professor Shepard describes it in his Mineralogy a in 1832 and 1835 as 

traversing quartz in seams and as associated with magnetic iron and 

hornblende. In his third edition, in 1852, he omits the locality 

entirely, and in his collection there were no specimens from Rhode 

Island labeled yenite, but there were black crystals labeled knebelite. 

Neither is there a specimen labeled yenite in the second collection he 

formed, which came to Amherst College after his death. The citation 

has remained in Dana's Mineralogy from the beginning, but it should 

be canceled.

Professor Dana originally reported from Cumberland the minerals 

manganese, epidote, actinolite, garnet, titaniferous iron, magnetite, 

red hematite, chalcopyrite. & In the sixth edition E. S. Dana (1892) 

copied this list, with the additions of bornite, malachite, azurite, 

calcite, apatite, feldspar, zoisite, mica, quartz crystals, and ilvaite. 

At Beaconpole Hill he reports crocidolite; at Sneech Pond, chalcopyrite, ilvaite, wad, molybdenite, magnetite, epidote, and chlorite.

It is probable that the knebelite crystals from the manganese locality 

near Sneech Pond gave occasion for the incorrect citation given under 

ilvaite in each edition of Dana's Mineralogy, viz:

Reported as formerly found at Cumberland, R. I., in slender black or brownish 

black crystals traversing quartz along with magnetite and hornblende.

It is remarkable also that knebelite is not entered in these lists from 

the report of Doctor Jackson, particularly as his citation is sustained 

by the two analyses quoted above, which agree well with other analyses of this mineral.

Mr. George F. Kunz writes as follows concerning gem stones found 

at Cumberland, R. 1.: c

The so-called Thetis hairstone described by Doctor Jackson, found at Cumberland, 

R. I., is really a quartz cat's-eye, and some very fair cat's-eyes have recently been 

cut from it by Mr. Edwin Passmore, one of them nearly two-thirds of an inch long, 

and quite equal to many from Hoff, Bavaria.

The beautiful specimens of limpid milky quartz, and also quartz crystals, the latter at times from three-fourths of an inch to 2 inches long, are found penetrated by 

crystals of black hornblende varying in size from acicular to those one-sixteenth 

inch in diameter and at times 6 inches long. They interlace and penetrate the 

quartz in every direction, making a very beautiful gem and ornamental stone. Fine 

pieces 6 inches square have been found. It occurs at the quarry at Calumet Hill, 

Cumberland, R. I., where the workmen, as a rule, knowing its value, secure the 

best specimens for disposal to the greatest advantage. Some hundreds of pounds of 

this material were sent abroad a few years ago to be cut up for jewelry at Idar and 

Oberstein. As, however, work has been suspended at the locality the mineral is 

likely to become somewhat uncommon.' Cut specimens sell at from 50 cents to $5, 

and specimens polished on one side at from 25 cents to $5. This locality is one of 

the best known for this association. 

aTreatise on Mineralogy, pt. 2, 1832, p. 288.

i> System of Mineralogy, 1883, p. 770.

cKunz, G. F., Precious stones: Mineral Resources U. S., 1883-4, pp. 755, 761.
CAMBRIAN ROCKS, SMITHFIELD LIMESTONE. 21

It is interesting to go back to what may be called the inineralogical 

period of geological science in America, initiated by the early volumes of the American Journal of Science, and read the first citation 

of-the molybdenite mentioned above (p. 20) as it appeared in that 

journal under the heading "Miscellaneous localities of minerals,"" in 

an item by Dr. Samuel Robinson, of Providence, R. L, viz:

Sulphuret of Molybdena with magnetic oxide of Iron and magnetic pyritous Iron $ a 

mile N. N. E. from C. M. H. [Cumberland meeting house], at a place called the 

"Mine Hole," on the west side of a Hill which overlooks "Sneerch's Pond," where 

a shaft was sunk for Copper 70 feet deep, 40 to 45 years ago.

INCLUSION OF LIMESTONE IN GRANITE PORPHYRY.

A remarkably interesting inclusion from the north end of the Cumberland granite-porphyry dike (slide 369) seems at first sight to be a 

* gray penciled slate or fine sandstone, but is a nearly pure, granular 

limestone with a few muscovite, biotite, quartz, and magnetite grains.

Many of the grains are idiomorphic, and are brought by pressure 

into a direction parallel to a single line, so as to give the pencil structure. The forces gave motion in only one direction, as in the "drawing" of a wire. There are parallel bands of coaly matter in rounded 

grains, but no magnetite.

A similar inclusion (slide 361) from the same place is a light-gray, 

fine-grained, coarse columnar or penciled micaceous limestone, very 

impure, but effervescing freely. It contains many grains of pyrite, 

and weathers rusty and cavernous, as is usual with a calcareous rock. 

It is full of dark grains, which may be partly anthracite, in foliation 

bands which are thickly crowded on the contact. Much magnetite is 

present. There are many trains of strained quartz grains. Some 

seem surrounded by secondary growth.

The small degree of metamorphism in the limestones found in the 

large Cumberland dike is noteworthy. It .would seem that they were 

involved in the porphyry when but little metamorphosed and that the 

porphyry acted on them at so low a temperature, and perhaps with so 

small an amount of water that it did not produce the most striking 

effects, especialty failing to utilize the lime and iron to make actinolite or garnet or the lime to make tremolite. It is, indeed, possible 

that its inclusion in the porphyry has protected it somewhat from the 

long-continued crushing and circulation of heated waters to which the 

rest of the schists were subjected, so that the inclusions represent a 

stage before the development of actinolite, tremolite, garnet, epidote, 

and serpentine. Other fragments of the same rock, without being 

more metamorphosed otherwise, were soaked so full of fluorite from 

the mineralizing of the porphyry that they have a rich purple color.

a Ain. Jour. Sci., 1st scr., vol. 8, 3824, p. 231.
22 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND.

HARRIS BKD AT LIME ROCK.

The Harris bed is the largest bed of limestone and has been most 

extensively worked. It is at least 150 feet thick, and the present outcrops are the remnants of a more continuous band. It is largely a 

wholly massive, highly crystalline marble of rather fine grain and of 

snowy whiteness. It would be a very beautiful building stone, or 

would serve even as a statuary marble, if it were not so much fissured. 

Its fractured and broken structure has given it the name "jointer 

rock," by which it has long been known. Doctor Jackson says:"

It is included immediately in greenstone or hornblende rock of a dark brownish 

green color, compact in structure and exceedingly hard. * * * The Harris lime 

rock is largely wrought, it being considered the best quarry, and is usually known 

by the name of the jointer ledge, a name derived from the joints in the rocks. * * *

That portion of the limestone which lies in contact with the hornblende rock is 

called the hard jointer, and is the variety of magnesian carbonate of lime called 

granular dolomite. About 10 feet from the hornblende rock the limestone graduates 

into pure granular carbonate of lime, occasionally colored with plumbaginous matter, 

oxides of manganese, and iron.

It is especially notable because of the dikes by which it is cut and 

the large number of interesting minerals which have been found in it.

Under the microscope the grains of calcite in some specimens are 

of coarse, even grain, and are rarely twinned. In other bands the 

calcite is crushed into a mass of elongate grains or finely powdered and 

many of the grains are twinned. The rock in some slides shows 

grains of colorless pyroxene and large plates of colorless hornblende 

broken up into separate fields by a growth of brightly polarizing talc, 

which itself merges into an aggregate of pale bluish plates of the colorless serpentine called bowenite. In other slides the rock contains 

much brown granular essonite. At its contact with the schist the 

limestone is tinted with malachite. Veins of a velvet-black, very 

greasy quartz run through the rock, and fine quartz crystals are found 

in the fissures.

The talc at the Harris quarry in Lime Rock is white, thin, and fissile, and some layers still eifervesce with cold acid. It is derived from 

the tremolite, and inherits from it a matted fibrous structure. Tremolite, pure white asbestos, mountain leather, -the pale-green serpentinous mineral " bowenite^" and essonite are found at the eastern Lime 

Rock quarry. The last two are formed at the contact on the small dikes 

which cut the limestone.

Veins of compact epidote 3 inches thick occur in the white marble 

in Smithfield, and a similar rock made up of irregular alternating layers, about an inch thick, of dark-brown garnet and green epidote occur 

at Lime Rock. Similar epidote veins contain fragments of the black

a Idem, pp. 57,68.
CAMBRIAN EOCKS, SMITHFTELD LIMESTONE. 23

massive hornblende rock, which become so abundant in places as to 

form a breccia.

A pure white pyroxene occurs in the Lime Rock quarry, in coarse, 

radiate, columnar aggregates, the individuals of which are in some 

specimens developed in perfect crystals of the Nordmark type 100, 

010, 001, 110, with parting parallel to 001. They are in prisms a half 

inch square, and resemble, even in the perfect basal parting, the 

canaanite from the Stockbridge marble.

The order of crystallization of the calcite in fissures in the limestone 

at Lime Rock is interesting. At first drusy surfaces of minute crystals having the form of the unit rhombohedron without modification 

were formed. These were calcite, as their effervescence with dilute acid 

shows. Upon this surface are perched simple forms 1/2 R and a 

very acuminate rhombohedron simulating a hexagonal prism. Upon 

this come scalenohedral forms of the greatest 

complexity. Some of these are flattened to 

resemble the half of one of the Egremont 

butterfly twins, or form square prisms with 

two opposite faces striated, and the two remaining faces polished. The corners of this 

apparently four-sided prism are replaced by 

the faces of an acute four-sided pyramid, 

producing an apparently quadratic form.

Next an infiltration of limonite has covered 

the surface of the crystals with a tarnish of 

brilliant peacock colors. There has then followed a new growth of limpid calcite, which 

has placed upon the apex of the amber 

scalenohedra a capping having the form ocP 

and 1. (See fig. 2.) Finally, rare needles 

of scolecite have grown in the drusy cavities. These are amber 

colored, transparent, and of the size of a fine hair, and have been 

bent in growing across the cavity. Only four of these delicate needles 

were found. One was found to be optically negative. Its greatest 

elasticity was parallel to its length, and a positive bisectrix emerges 

from the middle of one of its sides. It was decomposed by acid 

without effervescence. On revolution on the long axis the angle of 

extinction proved to be 7°.

Professor C. Palache has been so kind as to exami ne the calcite crystals 

and to send the following description and accompanying figures:

Fig. 1 shows the more important forms and the habit of most of the crystals. 

The dominant form is y, R 5, 3 2 5 1, whose obtuse angles are truncated by the 

positive rhombohedron M, 4 R, 4 4 0 1. The termination consists of r, R, 1 0 11, 

with narrow planes of scalenohedrons of the principal zone, of which n, R 5/3, 4153, 

and E. 1/2R 5/3, 4156, are the commonest, e, 1/2R, 0112, is also sometimes 

present in the zone. M and r generally meet in a series of steplike oscillations not 

shown in the drawings.

FIG. 1. Calcite crystal from the 

Harris bed at Liine Rock.
24 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND.

Other steep scalenohedrons of the principal zone are also often present, sometimes 

in addition toy, sometimes taking its place. On one measured crystal the forms of 

this group found were as follows:

Rll/8, 19.3.22.16 

n, R5/3,_4153 

R 9/5, 7295 

R4, 5382

 R 16/3, 19^13.32.6 

R6, 7.5. 12. 2__. 

R 19/3, 11. 8.19._3 

R 20/3, 23.17.40. 6

Fig. 2 shows an interesting parallel growth which was observed on one crystal. 

On a crystal of the ordinary habit is placed in parallel position a second consisting 

simply of the combination of prism of second order, a, co 

P2, 1120, and negative rhombohedron, e, 1/2R, 0112.

Another type of habit is 

illustrated in fig. 3. The 

forms are dominated by the 

same scalenohedron prominent in the other types, y, 

3251, + R5. No other scalenohedron is present but 

both first and second order 

prisms are found and a series 

of negative rhombohedrons; 

of these the only one accurately determined by measurement was e, 0112, 1/2R. 

The others form a more or 

less rounded surface between e and the prism m, 

but the edges of the scalenohedron truncated by 

planes of a rhombohedron

on several crystals, indicating the form X, 0772, 7/2R, shown in the drawing, and one measurement from .a 

sharp face gave a value corresponding to L, 0887, 8/7R which is also indicated in 

the figure.

m 1010 oo P

a 1120 oo P2

e 0112 -1/2R

L 0887 -8/7R 

0772 7/2R

y 3251 +R5

DEXTER QUARRY.

The rock at the Dexter quarry, at Smithfield, is called locally the 

Dexter rock or the South rock. This outcrop is next in importance, 

economically, to the bed at Lime Rock, and is still worked in an extensive quarry for lirne. It ranges in width from 12 to 15 rods and 

extends several times this distance on the strike. It is cut off abruptly 

on the south by the green schists but may extend far north in the 

depressed area beneath the till.

FIG. 2. Calcite crystal from 

the Harris bed at Lime Rock. 

A later colorless addition is 

made to the golden yellow 

scalenohedron.

FIG. 3. Calcite crystal of the 

square type from the Harris 

bed at Lime Rock.
CAMBRIAN BOOKS, SMITHFIELD LIMESTONE. 25

It is greatly jointed, pure white to gray banded, and carries fewer 

minerals than the former bed, quartz and calcite crystals and talc 

pseudomorph after tremolite being the only common ones found.

Octahedrite is reported as occurring in small crystals at " Dexter's 

lime rock at Smithfield, R. I." a I had not been able to verify this 

citation, nor to find any trace of the mineral at the locality, nor any 

specimen of the mineral from this place in any collection until 

recently Professor Palache wrote me that he had found in the Cambridge Museum a very pretty specimen, in a collection from the 

Dexter quarry made many years ago by Prof. M. E. Wadsworth.

BEDS WEST OF PROVIDENCE AND PAWTUCKET.

Doctor Jackson mentioned several small beds of limestone on the 

Aza Arnold farm. 6 This is the bed a mile west of Lonsdale, which- is 

20 to 30 feet wide and 300 feet long, and has now been about all 

quarried out. Unlike the other beds, this is isolated in the granite, 

but lies near the border of the green schist. It is bordered on the 

east side by a bed of tremolite rock several feet thick, and on the 

west by a similar thick bed of compact talc which is fibrous and pseudomorphic after tremolite. Both these beds have been derived from 

the limestone, and the transition can be plainly seen. The bed as a 

whole can be traced southeast with the strike for a mile to near Saylesville by a series of tremolitic serpentine outcrops.

Doctor Jackson mentions c also an extensive bed of limestone on 

the farm of Nathan Brown, 4 miles from Providence, which was 

worked for lime before the discovery of the Smithfield lime rocks. 

The bed is about 6 rods wide, and is included between hornblende 

schist and a compact altered graywacke that is nearly as crystalline as 

granite. Chlorite slate rocks in beds 10 feet thick lie southwest of 

the limestone, and include beds of talcose rock or soapstone. Green, 

acicular, and fasciculated actinolite and crystals of magnetic iron ore 

of an octahedral form are found here in chlorite and green talc. 

Granular and compact limestone also occurs on the Olney farm, in 

hornblende rock associated with beds of chlorite slate, soapstone, and 

serpentine. Coarsely granular layers of a bright-pink color occur 

west of the road on the west slope of Neutaconkanut Hill. They alternate with green chloritic limestone. The rock may contain some 

rhodochrosite. Associated with it are heavy beds made up of brown 

garnet and green epidote.

CRANSTON BED.

An ore bed is marked on Doctor Jackson's map at a place in Cranston, and this was the site of an iron mine. Professor Woodworth 

says of this bed: " The iron of the old mine in Cranston in the ' duga Kunz, G. F., Mineral Resources U. S., 1883-84, p. 772. bldem, p. -57. cldem, pp. 80, 81.
26 SCHISTS AND ASSOCIATED ROCKS OF KHODE ISLAND.

way' is probably a ferruginous replacement of the limestone." In the 

bed of a small brook above this mine there is a vertical bed of limestone 10 feet thick.

ATTLEBOKO BED.

In the Massachusetts State collection, under VII, Devonian or Old 

Ked Sandstone rocks, there is a specimen marked " 88 Petalite in limestone, Attleborough." This is a white chalcedony in a white crystallized limestone that effervesces with cold acid. This is wholly unlike 

the compact limestone interstratified with red shale from the ordinary 

Cambrian at Attleboro, and is exactly like the crystalline limestone of 

the green schist. It may be that it is a contact product of the intrusives of the region around Attleboro, which lies outside the area of our 

investigation. This may be taken to slightly increase the probability 

of the derivation of the green schists from strata similar to the Cambrian rocks at Attleboro, as maintained on page 33.

BEDS IN WOONSOCKET.

The green schist with limestone extends in scattered fragments out 

over the granite to the north and west. Thick beds of coarse pink 

marble occur in pyritous quartzose mica schist in Woonsocket.

A 3-inch layer of very coarse calcite (each grain 1£ inches square) 

occurs in a coarse biotite schist carrying coarse prisms of sillimanite 

in North Snrithfield. These are just beyond the limits of the area 

mapped in PI. I.

CONTACT ROCKS ON THE MILFORD GRANITE AND OTHER

IGNEOUS ROCKS. '

GENERAL FEATURES OF THE METAMORPHISM.

The normal members of the Cambrian formation are a fine-grained 

generally aphanitic chlorite slate, a fine-grained hornblende schist, a 

white crystalline limestone, and a slightly muscovitic saccharoidal 

quartzite. These represent the normal minimum of metamorphism 

which may be ascribed to the mashing to which the pre-Cambrian and 

Cambrian rocks were subjected, probably in Ordovician time and 

before the advent of the Milford granite.

In a broad band adjacent to the Milford granite these normal rocks are 

at some points greatly altered; the slates have become coarse chloritic 

or biotitic amphibolites, the Limestones have changed to tremolite 

rock which may show transitions to steatite or impure steatiteserpentine masses, and the quartzites have become dark biotite schists. 

We have associated with these contact rocks some similar beds from 

the interior of the series for reasons given below.

The whole Cumberland regipn and the smaller area to the south are 

in effect large inclusions in the great batholith of Milford granite.
CAMBRIAN METAMORPHIC ROCKS. 27

This is the justification of the term '" batholithic structure" applied 

°by Wood worth. The large Lonsdale limestone bed described above 

is inclosed wholly in this granite. The great beds of tremolite which 

have been formed from this limestone at the contact (since the limestone is wholly inclosed in the granite) can best be' explained as formed 

by the union of silica derived from the granite with the bases of the 

limestone.

Southeastward for a mile along the strike, and at the same time 

along the border, there extends a thick bed of massive impure serpentine, or steatite-serpentine mixture, whose niassiveness suggests an 

igneous origin, but whose fibrous texture indicates its derivation, 

through a tremolitic stage, from the limestone bed, as does its position 

in the strike of the limestone. The same kind of rock occurs at 

several other places on the border, as indicated on the map. These 

occurrences we ascribe to the contact influence of the Milford granite. 

Therefore, when we find the same impure serpentine and steatite rock 

associated at many places with the coarse garnet rock and forming a 

continuation of the great limestone beds in the midst of the green 

schists, we may fairly conclude that the granite beneath has produced 

the change, as at the border. Indeed, a small amount of granite 

appears at places where the serpentinous change is best developed, as 

at a point just north of the Lime Rock quarry. Where the tremolite 

border is lacking on the limestone beds, as it generally is in the interior of the limestone region, the granite was too distant to effect the 

change.

Again at some places the least altered beds of hornblende schist and 

chlorite schist contain pebbles and grade into quartzites and are interstratified with limestones; at other places the most highly crystalline 

beds, in which the coarse hornblende schists are porphyritic with large 

hornblende crystals and contain plagioclase, are also interstratified 

with limestone beds or their derivatives tremolitic and garnetepidote rocks and ore beds and every gradation can be observed 

between the two kinds of rock. Thus some of the beds, such as the 

diorites described below among the igneous rocks, still show trace of 

igneous origin and are undoubted!}1 igneous, while other tremolitic 

and hornblendic beds are of different type and under the microscope 

and in the field give evidence of sedimentary origin. In this section 

we have considered only those contact effects which are exomorphic 

to the granite. A broad band of the latter is general^ much darker 

thairusual, a probable result of differentiation, though this is not so 

marked as in Massachusetts, where a broad hornblendic band is usually 

present.
28 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND. 

CONTACT BOCKS DERIVED PROM THE LIMESTONE.

Tremolite rock and serpentine. An interesting outcrop of a partly 

serpentinized tremolite rock occurs in Lime Rock, near the point where 

the road turns off to the quarry. It is a massive greenish-gray rock, 

having the appearance of serpentine, with abundant black porphyritic 

crystals that show the cleavage of hornblende.

The black-looking crystals are colorless hornblende, their black 

appearance being due to an abundance of magnetite arranged along 

.the cleavage planes. The ground between these black crystals is made 

up of a matted network of tremolite needles. In this rock the grains 

of iron oxide are in some places so arranged that they mark out a 

former hornblende cleavage over large areas, showing that in these 

areas there were formerly large tremolite crystals that have by a kind 

of paramorphism been changed into the tremolite needles. Large 

patches are changed to a green serpentine. The great amount of the 

iron in the limestone has doubtless been brought in from the adjacent 

basic tuffs, and the ore beds mentioned above represent a more complete replacement of the limestone.

Several slides cut from the large dikelike mass 50 rods long by 20 

rods wide on the top of the serpentine hill, just north of the Lime 

Rock quarry, are of the same type as the rock east of the Lime Rock 

road, and the two outcrops have been connected on the map. The 

rock is massive and of dark-gray color, the black hornblende crystals 

showing more or less distinctly. The serpentinous change in this rock 

is farther advanced than in the rock described above, and at many 

places only the dense masses of magnetite grains, with angular boundaries, indicate the former presence of the large crystals of hornblende, 

since this and the finer fibrous tremolite have changed to a mat of 

serpentine blades.

Tremolitic serpentine. A rock from the granite contact, threefourths of a mile southwest of the Lime Rock quarry, in Lincoln, is a 

dark-gray, 'harsh-feeling serpentine, full of large tremolite blades and 

rhombs of dolomite. '

The slide (356) shows a black-dusted homogeneous serpentine made 

up of slightly polarizing flakes and showing traces of the rectangular 

cleavage of a former mineral, perhaps a pyroxene. Each of the 

untwinned rhombs of dolomite is surrounded by a band of dolomite 

differently orientated.

Steatite. Just west of Manville, at the fork in the road, is a bed of 

a soft, dark-gray, coarse-grained steatite full of large rhombohedra of 

ankerite, many of them one-half inch in diameter, which range in color 

from wine-yellow to black. Another similar bed occurs a half mile 

west of Manton. These beds seem also to have been derived from a 

magnesian limestone, but have passed through an actinolitic stage.
CAMBKIAN METAMORPHIO ROOKS. 29

Prof. C. W. Brown called our attention to the Indian "Ollah" quarry 

at Merino, where half-finished pots still remain attached to the ledge. 

Garnet rock. In the hill north of the Lime Rock limestone quarry, 

at the serpentine locality in the green schist, there is a thick short 

band 4 inches wide of brown-red garnet, and on each side of this 

there is a band 2 inches wide of a fine, granular, pistachio-green mass 

mixed with granular limestone. The rock contains much fine crystalline magnetite. The garnet is without strain or inclusions. The 

green mixture is fine, fibrous actinolite filled with fine granular epidote. Here and there are white, fine, granular veins of quartz. The 

rock passes into an epidote-garnet quartzite. A garnetiferous 

quartzite appears also as a border bed to the limestone at the Dexter 

quarry, and in considerable quantity in connection with several other 

limestone beds.

CONTACT ROCKS DERIVED FROM THE GREEN SCHISTS.

On Neutaconkanut Hill, west of Providence, and at points farther 

south, we were compelled to map separately a broad contact border 

because on the one side small offshoots of the granite intricately penetrate the schists and on the other the schists become coarsely crystalline. Generally the line of contact of the granite and the schists is 

remarkably straight, no lobes of the granite penetrating the schists. 

Here, however, the granite is intimately mixed with the schists in thin 

layers, which are at many places intruded between layers of the schist, 

and although the rocks here, are all much .more coarsely crystalline 

than usual the schist, limestone, and quartzite can still be distinguished 

from one another and from the granite.

Farther north, in Massachusetts, the Milford granite is at many 

places bordered by a black hornblende band, much injected by small 

graniter veins, and part of this band may be of the same origin as that 

just described, but the calcareous and ferruginous green schists are so 

well fitted to change to similar hornblende schists that it is not easy to 

separate the two.

The common rock at the border on Neutaconkanut Hill is dark por- 

 phyritic, being closely dotted with perfect, stout, black hornblende crystals from one-eighth to one-fourth inch long, in a light groundmass 

made up of fine white grains of quartz and feldspar. The microscope 

shows it to be a biotite-actinolite-epidote schist. It is without calcite 

or magnetite, but is very cavernous at the surface, from the removal 

of some constituent. The large, perfectly bounded hornblende crystals are partly or wholly replaced by aggregates of biotite scales, 

which, where the replacement is partial, are in some cases grouped in 

the center of the crystal and in others are arranged in an annular 

band at or near its border.
30 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND.

The contact rocks are well shown along the road in North Providence, 2 miles south of Olney Pond. The noncalcareous chloritesericite schist can be traced into a greenish-gray massive rock containing a green mica in a fine-granular quartz-feldspar ground. The 

intervening calciferous green schist passes into coarse-grained dark 

green, pyroxenic amphibolite, or a light-colored gneissoid rock mottled by black, square, hornblende crystals or similar square biotite 

aggregates, as at Neutaconkanut Hill. We m&y assume that the 

first influence of the granite here was thermal, producing the coarser 

crystallization of the hornblende ; the second chemical, consisting of 

a transfer of alkaline solutions from the granite to the schist, whereby 

the enlarged hornblendes have been in whole or part changed into 

groups of biotite scales.

A very peculiar rock occurs by the north-south road at the northwest foot of Neutaconkanut Hill. (See PI. II.} It is the common 

greenish-black contact form of the calcareous green schist, massive, 

aphanitic, and homogeneous in appearance when freshly broken, but 

developing on weathering the two structures shown in the figure 

distant, straight, parallel ridges, which seem to represent a concealed 

foliation, and stout, prismatic projections, which generally comprise 

regular crystals that have more or less rounded edges and give in cross 

section the angles of hornblende. These crystals represent a mineral 

that has grown in the green schist during its first transformation, with 

only a partial expulsion of the original material of the soft schist 

that is, they have grown in the schist somewhat as chiastolite grows in 

clay slates.

An examination of these crystals in thin section (466) bears out this 

conclusion. The ciystal cross sections are markedly different from 

the surrounding ground, but their composition is so variable that they 

differ from one another as one kind of rock differs from another kind. 

The magnetite octahedra, single and grouped, are equally abundant in 

the crystal sections and in the ground outside, showing that the magnetite was crystalized before them both. The ground is a mica schist 

in which half the surface is of olive biotite scales in a colorless ground. 

The crystals are formed of a green substance, not resolvable by the 

lens, which is shot through by colorless rods in a sort of rnicrographic 

arrangement. There are also some biotite scales somewhat larger 

than in the ground. With the microscope the groundmass is seen to 

be made up of the small angular and rounded quartz grains of a veiy 

fine-grained sandstone (exactly like the calcareous sandstone described 

on p. 14) in which the biotite blades and magnetite are disseminated.

The large hornblende crystals preserve one original structure. This 

is a poikilitic intergrowth of plagioclase, still wholly fresh, which is 

regularly spread throughout the crystal nearly to its border, or appears 

in broad patches. It is in long notched rods and plates more or less
U. S. GEOLOGICAL SURVEY BULLETIN NO, 311 PL. II

PORPHYRITIC GREEN SCHIST.

The ground is a dark mica schist. The distinctly crystalline projections are made up of matted 

actinolite, pseudomorphous after pyroxene or hornblende (See oage 32.)
CAMBEIAN METAMOEPHIC BOOKS. 31

rounded, with a parallel arrangement in places, as if controlled by the

host. The plates are only rarely twinned and then extinguish at 23°.

The normal growth of the large hornblendes continued slightly after

the intergrowth had ceased and then sometimes a few blades shot out

o

from the ends of the large crystals, becoming themselves rather large, 

branching, and fasciculate, as in garben-schiefer. Groups of the same 

fasciculate actinolite occur independently. The whole of each large 

hornblende crystal was then changed into an exceedingly fine-grained 

felt of actinolite needles of several sizes, mostly so fine and overlapping that they present with low power an almost amorphous mass in 

which the larger blades shine out.

The rock was a veiy fine-grained calcareous and ferruginous sandstone, in which first a part of the iron crystallized in the black octahedra and a foliation was produced during the folding of the rock. 

Then followed the crystallization of the stout hornblende crystals with 

at first an iutergrowth of a lime feldspar, producing a structure not 

uncommon in limestone contacts. We may infer that these crystals 

were not formed during the folding of the rock, since they cut across 

and disturb the remains of the foliation. As this required only the 

constituents of the original rock, we may assume that the heat of the 

adjacent granite produced the change. Then followed the development of the abundant biotite in the ground outside the crystals, and 

this also may be brought into connection with the intrusion of the 

great granite masses, which involved the sending out into the green 

schist of alkaline solutions that contributed the potash that joined 

with the magnesium and iron to make the abundant biotite of 

the ground. This regularly disseminated biotite is absent from the 

hornblende crystals, because they were formed at an earlier period. 

The heat of this intrusion may have caused the paramorphism of the 

bisilicate to a uralitic pseudomorph.

CONTACT EFFECT OF THE MILFORD GRANITE ON INCLOSED CAMBRIAN 

SCHISTS AT THE SAYLESVILLE GRANITE QUARRY.

The phenomena described below were at first believed to indicate 

the post-Carboniferous age of the Milford granite and to suggest that 

the sericite schists of the Carboniferous border may be due to contact 

metamorphism caused by that granite. A special study, therefore, 

was made of the Saylesville quarries northwest of Pawtucket, in Lincoln, because of the interesting inclusions of schist in the granite at 

that place.

The inclosing rock is a gray biotite granite, coarse, often subporphyritic, containing in some places blue quartz, and much jointed. 

At the time the quarry was visited, in 1900, large masses of pale 

greenish schist rose from the floor of the quarry with a width of 3 to
32 SCHISTS AND ASSOCIATED EOCKS OF EHODE ISLAND.

4 rods, a strike N. 35° W., and dip of 50° N.E. Pieces taken from 

the centers of the masses are a soft, greasy sericite slate of leek-green 

color, closely like the original sericite schist of the Taunus. It is 

found that a portion of the Albion schist farther west is closely like 

the least altered portion of the included masses.

At the border the rock is granitoid and the quartz and feldspar of 

the granite are diffused through the dark material of the schist. For 

several feet beyond the border of the granite and schist the granite 

has lost its normal subporphyritic texture and seems to have been shattered by the influence of the moisture of the inclusion and recemented. 

Outside this belt on every side the granite shows its normal and original 

texture, with coarse fresh feldspar and distinct biotite films.

A slide (371) was cut from this granite at its contact with the large 

inclosure. The rock is thoroughly shattered, narrow granulated bands 

running through several grains and recementing them. The quartz is 

generally a mosaic, but at some places the rock contains blue quartz 

in perfect crystals, which ar-e penetrated by lobes of the groundmass. 

It contains also squarish broken feldspars, some of which are orthoclase, judging from their refraction, but the commonest feldspar is 

albite. Pale red garnets surrounded by a kelyphitic rim of pale amber 

pyroxene appear, and also a colorless, limpid, isotropic mineral with 

purple border, doubtless fluorite. The slide shows also apatite and 

brilliant zircons.

Under the microscope the inclosed slate in its central portion is seen 

to be composed of matted parallel filaments of sericite with wavy 

boundaries dusted full of the coaly and clayey material of the unaltered 

slate and differing entirely from the nearest green schist, which is 

made up of sharply outlined crystals of actinolite, biotite, and epidote. 

The transition of this slate to microflaserig mica schist is clearly 

traceable. Near the center of the slate are a few lenticular groups of 

interlocked and greatly strained quartz grains, and these increase in 

number and size outward, away from the central portion, until a clear 

flaserig texture is produced and the micaceous films undulate between 

the augen of strained and interlocking quartz grains as if they had 

been spread apart by the growing quartz. These grains are often full 

of rutile needles, like true granite quartz. There is thus far no new 

mineral except the blue quartz added to the original schist, which 

consists of white and brown micas, chlorite, and a granular mineral 

which seems to be epidote; but in one section taken only a foot from 

the border feldspar, both orthoclase and plagioclase, is introduced in 

rather large crystals, forming a perfect paragneiss. In specimens 

taken just adjacent to the granite the grains of quartz and feldspar are 

larger, more abundant, and more irregular in size and shape, and are 

included in a small amount of the dark schistose material, which may 

explain the granite-quartz grains mentioned above,
CAMBRIAN ROCKS, AGE RELATIONS. 33

The sharp border of the great blocks of schist against the granite 

and the uncrushed condition of the granite away from the border 

show that the two.rocks have riot been subjected to a common mashing since the granite cooled. The complete mashing of the schists 

elsewhere in the region has produced no blue quartz. The blue quartz 

is the characteristic of the inclosing granite. The granite has thus 

penetrated the schist for some distance; the blue quartz has been carried in solution still farther in, but is lacking in the central part. 

This occurrence is analogous to that of a granite stock which sends out 

into a country rock pegmatitic apophyses that grade outward into 

quartz veins.

CONTACT HOCK FEOM THE GKAFTON QUAKTZITE.

An interesting contact rock on the Milford granite occurs in the 

quartzite west of Manton. It is a black, very fine-grained mica schist 

that shows in the slide (437) colorless spots in a fine-grained biotite 

schist, thus having a porphyritic aspect. With polarized light these 

.spots prove to be a colorless uniaxial mica, within which all the grains 

of the original sand still retain their places. Here also potash was 

introduced in large quantities, forming biotite.

AGE RELATIONS.

Prof essor Wood worth proposed the name "Blackstone series" for 

the complex of quartzites and green schists along the bed of Blackstone River." The limestone and the hornblende rock of this series 

were assigned to the Primary by Dr. C. T. Jackson, in 1840, in his 

report upon the geology of Rhode Island. Prof. Ebenezer Emmons 

referred them, in 1846, to the Taconic & that is to the age of the 

Stockbridge limestone in the Cambrian or at the base of the Silurian 

an assignment which seems probable.

Professor Shaler, in his paper upon the Cambrian of Attleboro, 6' 

devotes three pages to a discussion of the character and age of the 

" Blackstone series." He regards it as of sedimentary origin, 

considers it pre-Cambrian, possibly Huronian, and estimates its 

probable thickness at more than 5,000 feet. He states that pebbles of 

the "Blackstone series" are found in the Cambrian, behaving then 

assigned all the red beds in Attleboro to the Cambrian. Later, 

Foerste, Woodwbrth, and Shaler <? give a much less area to the Cambrian, and do not cite pebbles of the "Blackstone " in the true Cambrian. 

The strike of the "Blackstone series" is northwest-southeast; that of 

the Cambrian at Attleboro northeast-southwest. Professor Woodwor th

aMon. U. S. Geol. Survey, vol. 33,1899, p. 106.

b Agriculture of New York, vol. 1,1846, pp. 90-93; also American Geology, vol. 1,1855, p. 22. 

cShaler, N. S., On the geology of the Cambrian district of Bristol County, Mass.: Bull. Mus. Comp. 

Zool., vol. 16, No. 2,1888, pp. 15-18.

. U. S. Geol. Survey, vol. 33,1899.

Bull. 311 07 3
34 SCHISTS AND ASSOCIATED KOCKS OF RHODE ISLAND.

bases the idea of a difference in age between this series and the Cambrian rocks on the fact that the Blackstone beds are greatly metamorphosed, the fossiliferous Cambrian very little, and refers the series to 

the Algonkian period. He says:"

The determination of the pre-Cambrian age of the group of limestones, schists, 

slates, and quartzites in the Blackstone River area rests upon the relation which it 

bears to the Lower Cambrian strata in North Attleboro. The Olenellus fauna occurs 

in little-altered, red, calcareous shales and slates at this latter place in close proximity 

to granite (hornblendic granitite). Four miles west of this inlier of the Carboniferous area occur the sediments involved in the complex already described. These 

strata are highly altered sediments, now hornblendic and chloritic schists, mainly of 

a green color, altered sandstone or quartzites, and crystalline limestones. The presumption that these rocks are pre-Carnbrian rests, at present, therefore, on the difference in metamorphism between them and the Lower Cambrian rocks in the same 

field. The criterion appealed to in this case is embodied in the statement that where 

two sets of rock coexist in the same dynamic field, that group which has undergone 

one dynamic movement more than the other is the older. If this view is maintained, this series of rocks falls into the Algonkian. Evidence of unconformity with 

the Lower Cambrian is necessary to make this conclusion positive. The relation of 

the granitic intrusives to the pre-Cambrian on the one hand and to the Cambrian on 

the other is simply to show that the granitite is younger than the former, and that 

the sedimentary rocks are of different ages.

The reasons which have led us to believe that the "Blackstone 

series" is Cambrian and that it is the equivalent of the "Attleboro 

series" are as follows: The highly ferruginous and highly calcareous 

green schists must have been derived from rocks exactly like the red 

calcareous shales of the "Attleboro series" and the quartzites from 

rocks closety like the sandstones of the Braintree Cambrian. Evidence that there is a considerable bed of highly crystalline limestone 

in the Attleboro area is given above (p. 26).

When these rocks are traced farther northwest ifc is found that they 

wrap round a series of coarse, porphyritic, granitoid gneisses like the 

pre-Cambrian of the Berkshire Hills, and they are themselves indistinguishable from the Cambrian quartzites of Berkshire. Although 

the two are only 2 or 3 miles apart, the Attleboro rocks lie along a 

zone of least metamorphism, while the Blackstone rocks lie along a 

zone of maximum metamorphism. To make this point clear there is 

reproduced in fig. 4 a map drawn by Professor Woodworth, which 

shows a zone of maximum disturbance of the Carboniferous rocks 

extending along the west side of the basin and a zone.of lesser change 

extending along to the east of the former. To Professor Woodworth's 

map have been added the areas of the " Blackstone series," marked d, 

which it will be seen lie in the continuation of this zone of maximum 

metamorphism, and the area of the Attleboro fossiliferous Cambrian, 

marked e, which lies in the zone of least change. The Carboniferous rocks in Attleboro contain annelid markings, impressions of

«Mon. U. S. Geol. Survey, vol. 33, 1899, p. 105.
CAMBBIAN BOCKS, AGE EELAT10NS. 35

raindrops, etc., and are as little altered as the Cambrian there, while 

farther west, adjacent to the "Blackstone series," Carboniferous conglomerates are metamorphosed into the semblance of granites and into 

coarse, crumpled mica schist, while the shales are changed into very 

coarse ottrelite garnet schists, fully as much metamorphosed as the 

green schists. Professor Woodworth emphasizes this contrast and 

the suddenness of the transition. He says: a

The geologist who should pass from the nearly vertical metainorphic strata of 

the East Side in the city of Providence, R. I., to the slightly folded and unaltered 

shale beds of East Providence would, from a comparison of the rocks alone, be 

led to infer that there was in this field a set of very ancient tilted rocks flanked on 

the east by strata of much less antiquity. So short is the space between the two rock 

phases at this point, being the width 

of the Seekonk River only, that one 

is led to believe that an intermediate 

zone of considerable width has been 

concealed by a fault.

We may be sure that the 

Cambrian of the Attleboro 

area extended some miles farther west of its present limits 

at the time of its deposition. 

Indeed, a small area of Cambrian rocks is shown b}7 Professor Woodworth6, with associated hornblende granite, only 

a mile from the green schist, 

on the north border of the 

Diamond Hill felsite. The 

geologic history of the region 

will present to our minds a. 

much simpler picture if we 

assume that these rocks are 

represented in an altered state 

by the wholly similar "Blackstone series " and, farther 

west and northwest, b}^ the Grafton quartzite and the Marlboroamphibolite, which wrap round the older Northbridge granitoid gneiss. The 

alternative hypothesis would involve an additional incursion of the se"a 

into the region in pre-Caiiibrian time, to account for the deposition 

of the green-schist series. The same hornblendic granite appears in 

the Cambrian near the fossil beds and in the "Blackstone series" 

near Albion.

FIG. 4. Map showing distribution of metamorphosed 

Carboniferous rocks, a, Narragansett Bay area of 

maximum metainorphisrn; b, Winneconnet area; c, 

Morrill's area, in Norfolk County basin; rf, Cambrian; 

e, Attleboro fossiliferous Cambrian.area.

aMon. U. S. Geol. Survey, vol. 33, 1899, p. 119. Mdern, PI. XVII:
36 SCHISTS AND ASSOCIATED EOCKS OF EHODE ISLAND.

Green schist underlies Carboniferous rocks at the south end of 

Conanicut Island a, and there is a large area in Little Compton, made 

up mostly of quartzite with some green schist, that is referred by 

Foerste to the Cambrian, with some reserve. Similar rocks occur 

about Newport Neck and Newport Harbor. To justify the probable 

correlation of these rocks at the mouth of Narragansett Bay with the 

Cambrian beds at North Attleboro, Mr. Foerste cites the very fact noted 

above, viz, the presence of layers of limestone in the Newport beds in 

considerable abundance, as in the Attleboro beds, and their absence 

from the Carboniferous. 6

In relation to the quartzites and green schists'of the "Blackstone 

series" from Natick to Manville, he says: c

The question as to the' geological position of these quartzites is very important, but 

is at present without a solution. On lithological grounds alone they might be considered of Cambrian age, but there is little real basis for such a determination. Perhaps the best reason so far known for considering these quartzites as of Cambrian 

age is the abundant occurrence of Cambrian quartzite pebbles in the Carboniferous 

conglomerates of the Narragansett basin.

But while the quartzite pebbles occur in all horizons in the Carboniferous those containing Cambrian fossils appear only in the upper 

beds, and these quartzites differ from any known outcrop of quartzite 

surrounding the basin. Moreover, while the fossiliferous quartzite 

pebbles are quite abundant in the Carboniferous, the quartzite beds 

around the basin are nonfossiliferous.

The "amphibolite aggregate" of Hitchcock (Mass. Col., VII, 102) 

from Middletown, near Newport, is made up of dark biotite changing 

to chlorite shot through with hornblende and very like the rock along 

the Blackstone, and the zoisite, which is cited as occurring with it (VII, 

124), is in a sheet an inch thick, having a columnar structure. The 

rocks of Little Compton and Conanicut also bear close resemblance to 

those of the "Blackstone series."

The limestone of the Attleboro locality, of the Newport locality, 

and of the " Blackstone series" are alike in amount and distribution 

and in thickness of beds. A comparison of the analyses of the Newport and "Blackstone" limestones on page 17 will show that they are 

both prevailingly dolomitic.

In the harbor of Newport there are two islands of yellow, compact 

limestone, which are highly magnesian and manganesian.^ The larger 

of these islands is 90 feet wide and 210 feet long; the smaller is half 

as large. At Fort Adams, in Newport Harbor, there are limestone 

beds 45 and 15 feet wide. 6 It" is true that these limestones are less 

crystalline than those of the "Blackstone series," as the flinty slates 

associated with them around Newport are less metamorphosed than

a Foerste, A. F., Mon. U. S. Geol. Survey, vol. 33, 1899, p. 234. rf Jackson, idem, p. 34. 

b Foerste, A. F., idem, pp. 381-383. eldem, pp. 91, 92. 

o Idem, pp. 383-385, ' 
CARBONIFEROUS ROCKS. . 37

the schists associated with the Blackstone rocks. In degree of change 

they are in fact intermediate between the two extremes, and they lie 

geographically along the band of transition between these two extremes. The crystalline limestone from Attleboro, cited on page 26, 

shows that the rock there may be in some places as ciystalline as that 

from the Blackstone region.

A comparison of the altered limestones of the "Blackstone series" 

with the Cambro-Silurian limestone of the liousatonic Valley and the 

pre-Cambrian limestone of the Berkshire Hills is interesting in this 

connection.

The abundant tremolite, actinolite, canaanite, garnet, epidote, and 

iron ores of the " Blackstone" are closely like the derivative minerals of 

the western valley limestone (Cambrian), while most of the characteristic minerals of the older limestone are wanting, or rare, viz, phlogopite, chondrodite, scapolite, spinel, titanite, dark pyroxene, pyrrhotite.

CARBONIFEROUS ROCKS.

SCOPE OF THE DISCUSSION.

The pre-Carbonif erous rocks in the Providence and Narragansett 

Bay quadrangles will be next considered, the discussion covering the 

ground not occupied by Messrs. Shaler, Woodworth, and Foerste in 

their monograph on the geology of-the Narragansett basin." It was 

not their purpose to report upon the isolated patches of Carboniferous 

conglomerate that rest upon the crystalline rocks in the area that lies 

west of the line which they had set as the western boundary of the 

basin, nor did they occupy themselves especially with some of the 

problems connected with the metamorphic changes of the Carbonif-. 

erous rocks in immediate contact with the granite along the western 

border of the basin. Some of these problems are therefore here discussed, but no attempt is made to present a general description of the 

Carboniferous rocks.

CARBONIFEROUS BOUNDARY.

The high bluffs of crystalline rock extending northward from the 

southern edge of the area mapped are taken as the approximate 

boundary of the Carboniferous, although in the low ground at their 

base the conglomerates are not exposed for 6 miles to the east. From 

a point west of Wickford Junction, a half mile west of the edge of 

the Narragansett Bay quadrangle, the boundary of the conglomerate 

could be closely traced northward by bowlders. The character of the 

boundary along this line could not be determined.

The easternmost ledges in bluffs east of Davisville show traces of 

much-altered conglomerate. From this point to Natick the bluffs

aMon. U. S. Geol. Survey,-vol. 33,1899.
38 SCHISTS AND ASSOCIATED BOCKS OF RHODE ISLAND.

everywhere show ledges of the older rocks, and the boundary of the 

Carboniferous is concealed beneath the Quaternary deposits.

From Natick northward for 2i miles into Cranston the boundary is 

well exposed in the bluffs. Beyond this point it is partially exposed 

on to Knightsville. It is a normal boundary on the different Cambrian formations and on the Milford granite. In the area just north 

of this the conglomerates are exposed at only a few places, and are 

not represented in that section on Professor Woodworth's map. We 

found traces of the basal beds as far north as Geneva, west of Providence. North of this point argillites adjoin the crystallines, except 

for a short distance southeast of Hunting Hill, and we have followed 

Professor Woodworth in o assuming a fault along this portion of the 

boundary.

Just at the northwest corner of the area mapped begins another 

large area of the Carboniferous conglomerate in Woonsocket, the 

white quartzite pebbles in which are in some places mashed to rods 

and plates 12 to 14 inches long.

CARBONIFEROUS CONGLOMERATE. 

CHARACTER AND DISTRIBUTION.

In the bare bluff above the church in the village of Natick the Carboniferous conglomerate rests in normal contact on massive jointed 

Cambrian quartzite. It is a very coarse conglomerate; one block is 3 

feet in diameter, and many of the blocks are angular. A little farther 

north a fine greenish quartz grouridrnass envelops the large cobbles. 

This purely quartzose facies is confined to the basal portion of the conglomerate and to places where it is in immediate contact with the 

Cambrian quartzite.

Eastward across Natick, away from the contact, the pebbles become 

smaller. Many of them appear to be of a finer and whiter quartzite, 

but prove under the microscope to be a microgranite exactly like that 

which forms the border of the conglomerate for a long way south in 

East Greenwich. Pebbles of granite also occur, and these also, when 

studied in thin section, prove to be of the type of the East Greenwich 

granite found farther south, and quite unlike the Milford granite, 

found farther east and north.

A fine-grained interstitial material developed between the pebbles 

is metamorphosed into a shining white finely crumpled muscovite or 

sericite schist, in places full of small magnetite octahedra and spangled 

with biotite. Some specimens contain small garnets and chloritoid.

The friable microgranite pebbles crush easily, and, mingled probably with fine material of the same character, form this sericite schist. 

Many quartzite and granite pebbles remain intact and the microgranite 

pebbles wrap round them. Some of the pebbles are penetrated by
CARBONIFEROUS CONGLOMERATE. 39

large secondaiy magnetite octahedra for an inch in from the surface. 

These octahedra also fill the paste and were doubtless carried into the 

pebbles under heavy pressure by the same solutions that brought the 

biotite, and both seem to have come from the microgranite, the only 

magnetite-bearing rock that is common in the conglomerate.

Between the church and the schoolhouse in Natick there is a completely crushed zone of sericite schist. Farther on, through and east 

of the village, are abundant outcrops of the conglomerate with sericiteschist paste as described above. In Natick the transition from the 

basal, locally derived, quartz conglomerate to the microgranitic conglomerate derived from the East Greenwich rocks to the south is so 

sudden that we have suggested that the eruptive material was furnished by an explosive rupturing of the dome of the East Greenwich 

mass rather than by erosion. (See p. 69.)

The interesting relations of the conglomerate to the adjacent .rocks 

in Natick and at a point a mile north of that place are beautifully 

shown in abundant outcrops.

One who follows the boundary three-fourths of a mile northward 

from Natick Church to the point where it crosses the east-west road 

will come to a point where the conglomerate rests on the Milford 

granite. This boundary can also be followed by abundant outcrops 

northeastward over a wooded hill to a place where it crosses the road 

and the Cranston line at the same time. The conglomerate contains 

abundant large cobbles, embedded in a paste of sericite schist. The 

majorit}^ of these are quartzite which has drifted northward from the 

Natick ledges. There are also many pebbles of microgranite and a 

f CAY of granite, which might lead one to assume that they were derived 

from the adjacent Milford granite, and to infer the granite to be the 

older. But the quartzite pebbles are angular and larger, the microgranite and the granite pebbles are rounded and smaller, which would 

harmonize with the idea that they were far traveled and had been 

derived from the East Greenwich series which formed the shore to 

the southeast beyond the quartzite. The Milford granite is a coarse 

biotite gray-quartz microperthite rock. The granite in the pebbles 

is a blue-quartz plagioclase granite containing garnets and showing a 

marked micrographic structure, characters that are found in the East 

Greenwich pre-Carboniferous granite, but not in the Milford granite. 

The material is thus all far traveled.

Seven rods east of the bare rocky southern apex of the hill along 

which the boundary runs is a contact of the Milford granite and the 

conglomerate which might be interpreted as an intrusive contact, 

since 1 it runs east-west (the prevailing direction of the boundaiy being 

north-south) and cuts across the laminse of the sericite schist. The 

alteration of the rock here is probably due to crushing. The extreme
40 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND.

metamorphism of the conglomerate, combined with the small amount 

of mashing and jointing, is very characteristic.

One-half mile east of Natick, near the base of the conglomerate, all 

the pebbles retain their shape and the paste is a coarse sericite schist 

full of biotite, garnet, and chloritoid.

On the Cranston line black amphibolite and green schist pebbles 

appear in the conglomerate. Here the adjacent rock is the green 

schist, but the far-transported material still predominates. If we follow the boundary 3 miles farther north, to the great hill a mile and a 

half southwest of Knightsville, we come on an area where the granite 

and the conglomerate are so mashed together that the whole east slope 

of the hill may be looked upon as the contact surface between the two. 

At the south end and just west of the road, in front of a new" country 

house, the large ledge is on one side completely granitoid in aspect; 

on the other large rounded cobbles 6 to 8 inches long can be clearly 

distinguished.

In the northern part of this area, one-half mile west of Wayland 

station, the conglomerate becomes a shining muscovite schist with 

finely corrugated foliation surfaces, while the pebbles are in part present in broad, flat disks, and here and there a perfect cobble of granite 

4 or 5 inches across remains quite intact.

The same blending continues farther westward between the Milford 

granite and the sericite schist, and distinct traces of pebbles rarely 

appear in the latter. A slide of the Milford granite from one side of 

a ledge near the summit showed uncrushed microcline with biotite in 

small distant bands. This granite appears in great force farther west. 

Another slide from a spot a few feet farther along the same ledge was 

a sericite schist with distant distinct biotites and garnets and strained 

blue quartz (all characteristics of the altered conglomerate and wanting in the adjacent granite), but with a single filament of microcline 

granite showing the intimacy of the blending.

OUTLIERS OF CARBONIFEROUS CONGLOMERATE ON THE BRECCIA OF 

THE EAST GREENWICH GROUP. a

One who follows the road along the south slope of Spencer Hill in 

Warwick westward will keep on the breccia for a considerable distance. Turning then northward along a road he will come, in about 

40 rods, to a small schoolhouse. Just north of this school is a very 

interesting ledge that is broadly exposed on either side of the road. 

Areally it seems to be a part of the breccia, which is in place on every 

side, but it is plainly a thin film of the conglomerate, about 3 rods 

square, resting on the breccia, and as the abundant interstitial material 

is a fairly pure muscovite schist we are compelled by all our previous 

experience to assume that it is a Carboniferous conglomerate. It

a See p. 64 for description of the breccia.
CARBONIFEROUS CONGLOMERATE. 41

contains red garnets, many of which are found in the altered paste of 

the Carboniferous conglomerate, and a few Cambrian quartzite pebbles 

(the larger angular and 2 inches across), as well as clastic grains of 

various feldspars,- micrecline orthoclase, oligoclase, and a few of blue 

quartz, of types exactly like those derived from the blue-quartz 

porphyry and the porphyritic inicrogranite. It contains also pebbles 

of the blue-quartz porphyry and of the granite, and one specimen 

exhibits a small block of the breccia with a half dozen fragments, 

each an inch long, of the graphic microgranite embedded in a paste 

of the normal blue-quartz porphyry. Adjacent to this block is a 

rounded pebble of the Cambrian quartzite, which seems at first sight 

to be also included in the porphyry, but proves, on careful study, to be 

adjacent to the latter and not included. Most of these grains and pebbles are almost wholly unchanged, although they have been subjected to 

influences which in places changed the paste into a clear, coarse mica 

schist. Here and there a feldspar grain is largely changed to coarse 

muscovite. The grains are also slightly fissured, and, as will be seen 

below, have probably been 'penetrated secondarily by magnetite. 

Rounded grains of quartzite exactly like those in the Cambrian quartzite are surrounded by attached magnetite grains. This magnetite is 

thus certainly secondary. Where it is spread regularly through the 

mica-schist paste it is also almost certainly secondary, and as it penetrates the feldspar grains in whole or in part very generally but not 

always, and as these feldspars are otherwise exactly like the porphyry 

feldspars which do not contain much magnetite, it is probably secondary here also.

This conglomerate contains also rather large pebbles of a peculiar 

rock which is so exceptional in character that it has been described 

elsewhere (p. 64) under the name microclinite as a distinct variety 

of the rocks of the East Greenwich group. Part of the microcline 

phenocrysts in this rock have been penetrated by the magnetite that 

is regularly distributed through it, and part of them have not. The 

secondary penetration of the magnetite into the Carboniferous conglomerate at the Cranston locality has already been described, and may 

be considered a second evidence of the Carboniferous age of the rock* 

here discussed.

This film of arkose conglomerate, which contains all types of rocks 

of the eruptive series and a few quartzite pebbles, probably passed 

upward into a true Carboniferous quartz conglomerate. It rests on a 

brecciated tine-grained, graphic granite that is without porphyry 

cement. These associated rocks, as will be explained below (see 

p. 69), possibty had their origin in an explosive eruption of the mass 

of porphyry and graphic microgranite, which furnished the large 

amount of unaltered igneous material found here and at localities 

farther east, across Natick and Cranston.
42 SCHISTS AND ASSOCIATED ROCKS OF EHODE ISLAND.

This determination of. the age of the East Greenwich group makes 

it agree in its time of eruption with the " Wamsutta series" and with 

the "Quincy series" as fixed by Wood worth. The alternative 

hypothesis, which is very plausible, would be that the breccia was a 

rather deep-seated formation a reibungs breccia that it was uncovered by erosion, and that the material which it has contributed to the 

Carboniferous conglomerate was transported a long time after. This 

would make the intrusion pre-Carboniferous. (See p. 67.) « A similar outcrop of the Carboniferous conglomerate rests on the 

breccia just west of the spring locality of the breccia.

CARBONIFEROUS SHALES AND SCHISTS.

The prevailing rock among the Carboniferous shales and schists is 

a dark roofing slate, which has reached but a low degree of metamorphism in the area extending from Providence northward, but in 

that extending. from Providence southward consists of much more 

metamorphosed beds, mainly highly muscovitic schists, here and there 

spangled with biotite. The rock is at many places just such a spangled 

mica schist as occurs in the calciferous mica schist (Goshen schist) in 

the Devonian rocks of the Berkshire Hills and of Bernardston. It is 

the same light, shining, crumpled muscovite schist, with small black 

biotite crystals set at all angles to the bedding. There are many muchstrained quartz grains, which seem to be derived from the granite and 

which contain sheets of fluid pores with moving bubbles and rutile 

needles. The biotite plates blend with the surrounding rock and are 

full of dark aureoles.

CHLORITOID (MASONITE) IN THE METAMORPHOSED CARBONIFEROUS CONGLOMERATE AND PHYLLITE.

Chloritoid is very abundant in bowlders, but we found it in place 

only in the brook cutting on the south line of East Greenwich, where 

it occurs in a shining mica schist, and at a point 40' rods east of the 

eastern railroad station in Natick, in a fine-grained Carboniferous conglomerate containing uncrushed pebbles in a paste of sericite schist 

with garnet and biotite. The plates of chloritoid are only one-eighth 

inch across.

A specimen of a dark-gra}7 silvery schist from Cranston, very similar to the above, is also full of small, thin, black scales resembling 

biotite, but the rock is more mashed and the scales are of brittle 

chloritoid, so opaque that only with difficulty could light be seen 

through the thinnest plates.

A peculiar variety of this masonite occurs as a bowlder in Warwick, 

1 mile south of Cowesett and 5 miles south of Natick. This is a coarsegrained massive rock, made up almost entirely of thick, squarish plates 

of masoiiite one-fourth inch across, set close together. It contains
CHLORITOID IN CARBONIFEROUS CONGLOMERATE. 43

scattered red garnets of about the same size, and passes into a coarse 

garnet rock. Under the microscope c = yellowish, 1) indigo blue, 

a = greenish yellow. This bowlder and the very similar ones found 

in Natick, Avhich contain larger crystals of masonite, 'certainly come 

from the Carboniferous contact area north or northwest of Natick. 

Some of the great plates of masonite are 1 inch thick and 2 inches 

across.

The mother rock is the same finely corrugated silvery and finely 

arenaceous muscovite schist'that forms the matrix of the Carboniferous 

conglomerate, and is doubtless of Carboniferous age. The typical 

masonite is associated with large garnets, and many of the small black 

plates that are scattered in the 'matrix are biotite like that of the 

spangled biotite phyllite described above.

Doctor Jackson makes the following report concerning the masonite 

at this locality : ft

JSearly opposite the brick cotton factory in Natick, upon the roadside, there are 

three large blocks of a peculiar rock composed of a new mineral associated with mica 

and garnets. These blocks of stone are erratic and now rest on a totally different 

rock formation.

I have been informed by Professor Hitchcock that the same kind of rock is met 

in the town of Ward, in AVorcester County, Mass. It is certain that no rock of the 

kind exists in place in Rhode Island, and it may appear surprising that these blocks 

should have been removed so far from their native locality. I measured one of 

them and found it to be 15 feet long, 10 feet wide, and 4 feet thick. * * *

My attention was first called to these rocks by Mr. Owen Mason, in Providence, 

and I examined them for-the purpose of ascertaining the nature of a singular mineral 

of which they are chiefly composed. This mineral I have analyzed and find it to 

constitute an entirely new^species, to which I propose giving the name masonite, in 

honor of one to whom the geology and mineralogy of the State is so much indebted. 

Masonite consists of the following ingredients:

Ratio. 

Water .......................... 4. 000 containing oxygen.......... 3. 555

Silicic acid ...................... 33. 200 containing oxygen.......... 1.7.247 =3

Alumina........................ 29. 000 containing oxygen.......... 13.543 =2

M agnesia .................. 1.... 0. 240 containing oxygen.......... 0. 092

Protoxide of iron.............. 25. 934 containing oxygen.......... 5.904 =1

Oxide of manganese............. 6. 000 containing oxygen.......... 1. 814

98.374

Al» 

Its mineralogical formula is Fe: Si 3+A9 .

Mn

It is a silicate of alumina and protoxide of iron plus silicate of manganese, plus 

water. Its specific gravity is 3.450. It occurs in tabular crystals which cleave ve"ry 

easily with brilliant planes perpendicular to their axes and with great difficulty in 

other directions. Its primary appears to be a right rhombic prism. It scratches 

glass and yields to the knife with difficulty. It is with difficulty fusible to a darkgreen enamel.

a Idem, p. 87.
44 SCHISTS AND ASSOCIATED EOCKS OF EHODE ISLAND.

This analysis was made at a time when "silicic acid" (that is, silica) 

was supposed to be a trioxid (SiO3 ) instead of a dioxid (SiO2 ). The 

mineralogic formula deduced from the analysis given above is therefore subject to' correction.

IGNEOUS ROCKS. >

IGNEOUS ROCKS OF CAMBRIAN AGE. 

DIORITE.

The diorite of the pebbles in the conglomerate green schist on the hill 

a hundred rods north of the middle Lime Rock quarry varies in texture from medium to fine grain and shows in one type, on a continuous 

dull-white ground, black hornblende grains and lobate forms in somewhat regular arrangement. The hornblende increases in amount until 

the roles of the two minerals are reversed and the white lobate forms 

stand out on a black ground. The pebbles vary greatly in coarseness. The adjacent diorite ledge, which borders the great dikelike 

bed of tremolitic serpentine, comprises all these types of diorite, and 

in addition other varieties. A diminution of the amount of feldspar 

produces a black, normal, fine-grained, wholly massive diorite, in 

which the lens still detects traces of the white constituent, and finally, 

where it is in contact with the serpentine, a very harsh-fracturing, 

black, almost aphanitic rock. In another portion of the mass is a 

coarse black hornblendite, massive and fresh-looking, with the stout 

phenocrysts often automorphic and a half inch long. Several slides 

(416, 417, 420, and 422) showed with a lens a gabbroid texture and 

little or no trace of crushing.

The microscope showed that a common feature of this rock was the 

abundant occurrence of grains of black ore surrounded by large borders of leucoxene, or even broad, cleaved crystals of titanite, exhibiting no trace of crushing. The white fields have the shape of stout 

feldspar blades and show in places traces of triclinic striation, but are 

thoroughly saussuritized and filled with actinolite needles. The dark 

fields are partly large squarish hornblendes, partly groups of smaller 

hornblendes. The hornblende shows moderate pleochroism in yellow, 

green, and bluish green, which may be secondan^, but gives no trace 

of any earlier mineral. One doubtful remnant of pyroxene was noted.

Pebbles of the diorite show a texture that is very similar to that of 

fragments taken fresh from the ledge, so that the ledge has evidently 

suffered no considerable metamorphic change since the fragments were 

broken off in Cambrian time; in other words, it was then, as now, a 

hornblende-plagioclase rock a diorite.

Another 20-foot dike which seems to be of the same type occurs in 

the quartzite one-fourth mile northwest of Albion, on the railroad.
PRE-CARBONIFEROUS IGNEOUS BOOKS. 45

Still farther south, one-half mile west of Woodville, by the roadside, is a similar fine-grained massive rock, in which the ey& just 

detects abundant squarish hornblende grains and rare plagioclase 

prisms. In the slide the hornblende grains are like those in other 

slides of this diorite, but here and there distinct augite centers appear, 

and some of the saussuritized feldspars show distinct banding with 

extinctions of 13° to 16° on either side the twinning plane.

Another rock of the same type is found in what appears to be a dike 

at the northern foot of Copper Mine Hill, on the northern border of 

the green schist area at the contact of the schist and the granite. It 

is a fine-grained, traplike rock containing much magnetite in feathery 

crystal groups, but showing no trace of leucoxene. The large square 

anhedra of green hornblende show low pleochroism. The rock, as a 

whole, is distinctly gabbrolike in structure, and these hornblendes 

occupy about the position of the original bisilicate, while the intervening stout feldspar blades retain their boundaries intact, although 

internally they are largely saussuritized, but show rarely broad twinning 

bands which extinguish at 10° on either side6

IGNEOUS ROCKS OF POST-CAMBRIAN (PRE-CARBONIFEROUS) AGE.

MILFORD GRANITE.

General description. A great granite area (batholith) of a constant 

type that extends across Massachusetts and Rhode Island in the western part of the area here studied has been named by the writers from 

the well-known quarries in Milford, Mass.

This is a compact, massive rock, somewhat above medium grain, 

and of light color. The light flesh color of the feldspar and the blue 

of the quartz give it in some places a slight pinkish tint, and it is now 

much used as a structural stone under the name "pink granite." It 

differs from the Quincy granite, found farther east, in lacking hornblende and also in the fact that it is unaccompanied by the many 

varieties of rock that accompany that granite (porphyries, felsites, and 

breccias), and it differs from the rocks of many other groups that 

cross Massachusetts farther west by the lack of coarse porphyritic 

varieties.

Its two especially characteristic constituents constantly present are 

blue quartz and a microcline microperthite in which the albite is alwa}'s 

dusted with minute crystals of muscovite and epidote, especially 

centrally, while the microcline is free from these minerals. These 

perthitic bands of albite also generally extend out beyond the surface 

of the microcline and cover it with a more or less continuous veneer. 

The rock also shows a fine micrographic structure in contact with 

quartz.
46 SCHISTS AND ASSOCIATED ROCKS OF EHODE ISLAND.

The feldspars project idionaorphically into large fields of quartz, 

which seem to have been single grains but are now somewhat cracked. 

Most of this quartz is blue, and this color appears also in the contact 

zones and even in the secondary quartz that is found in fragments of 

schist which are inclosed in the granite and which have been greatly 

altered by it. The fractured grains of quartz show with polarized 

light the strongest undulatory extinction, which indicates a state of 

strain that has probably produced the blue color.

The biotite is in small amount, and is here and there associated with 

epidote grains. In specimens of granite taken at the quarries at 

Graniteville it is evenly distributed or gathered in small blotches, as 

in the Milford type, and the rock in these quarries can hardly be distinguished from the Milford granite. In the northwest portion of the 

area, near Woonsocket, the biotite occurs in distant, interrupted, 

rudely parallel films, as in the rock at the Fayville quarries, north of 

Milford. In granite from a point south of Woonsocket the central 

portions of the spots and bands of albite in the microcline are crowded 

with muscovite crystals more strikingly than in the rock at Fayville. 

The albite is in coarser bands in and around the microcline and is 

filled with much larger scales of muscovite.

The granite from the country north of Spencer Hill, in Warwick, is 

a rather coarse subporphyritic rock, without blue quartz. The large, 

perfect, microcline niicroperthite crystals are characteristic, and in 

one specimen of this rock a soda orthoclase was observed. It contains 

rarely garnet, and the distinct feldspars and large quartz grains give 

it a porphyritic aspect.

It will long be difficult to separate the pre-Cambrian porphyritic 

granite-gneiss (the Northbridge gneiss) from the post-Cambrian and 

pre-Carboniferous coarse porphyritic granites, like those on the south 

end of Conanicut, and the coarse, subporphyritic granite of the Milford 

type.

The dark contact granite. In a zone that is at some places one-half 

mile wide, surrounding the green schists, the rock is generally present 

in contact facies of considerable variety. The first sign of approach 

to the green schist is in the increase of biotite, causing a darker color. 

Thus the rock becomes more or less foliated and jointed, and near the 

schist it is distinctly and at times decidedly hornblendic. In the hills 

west of Providence the included fragments of the schists become so 

abundant that the}7 occupy more of the surface than the granite, and 

farther west is a band in which the green schist is cut in every direction by great numbers of small granite veins. For this reason a broad 

area between granite and green schist could be mapped only as a contact zone between the two.

The dark granite near the contact north of Woodville is full of 

beautiful titanite crystals 1 to 2 millimeters long, of the regular 

"envelope" shape, and of dark aureoles around zircon.
PRE-OARBONIFEKOUS DIKE BOOKS. 47

Where the Milford granite is in contact with limestone, as west of 

Lonsville, the dark border is wanting. Here, however, the rock is 

very fine grained near the limestone. From a point 1 mile west of 

Copper Mine Hill its contact with the riebeckite porphyry can be 

well observed for a long distance. Both rocks are normal up to the 

boundary and neither sends out branches into the' other. This-more 

basic granite is apparently in part due to an endoniorphic contact 

change within the limits of the eruptive mass of the Milford granite. 

This change appears to have been an enrichment with iron which may 

be explained as in part a differentiation of the granitic magma and a 

transference of the iron to the border, and as in part an absorption of 

so much of the ferruginous green schist that the rock has become 

more basic and darker. This interpretation is supported by the fact 

that granite of this type is not found at the contact of the Milford 

rock with limestone, quartzite, and porphyry. Along the boundary 

line of the granite and. schist many good-sized fragments of muchaltered green schist are inclosed in the granite.

DIKE ROCKS.

Aplite. A very interesting dike of aplite, only one-half inch across, 

cuts the 2^-foot odinite dike in the eastern Lime Rock quariy, west of 

Lime Rock, and is continued beyond through the limestone. It is a 

bluish-white, fine-grained rock, only slightly different from the marble 

in appearance, as it contains many calcite grains. It is made up wholly 

of rounded grains of feldspar, part of which is plagioclase, sometimes 

twinned in broad bands, and having low extinction angles. It contains much microcline, and perhaps orthoclase. Mica and ore are 

absent, and radiated wisps of tremolite needles are scattered through 

the rock. Quartz is present only in the abundant micrographic portion. Scattered through the rock, in irregular grains, many of them 

of large size, is a very large quantity of calcite which has been taken 

up from the limestone, dissolved and recrystallized in the magma, 

since it is intercrystallized with the plagioclase and included in it, and 

is of different size and shape from the grains of calcite of the marble.

At the border of the aplite arid the amphibolite, wisps of hornblende 

of much larger size than that of the average of the amphibolite project 

into the aplite.

There is also a 4-inch aplite dike in the limestone at the eastern 

Lime Rock quarry, but under the microscope it shows a texture that 

is very different from that of the rock already described. The halfinch dike is a very fine and even-grained microgranite with many small 

grains of microcline, curiously abundant micrographic intergrowth, no 

mica and no alteration. The 4-inch dike is coarser grained, with 

microgranitic texture, except for large grains of plagioclase (albite to
48 SCHISTS AND ASSOCIATED BOOKS OF EHODE ISLAND.

oligoclase), with centers made opaque by the great mass of white mica 

scales developed in them, and a few large grains of microcline.

Pyritous aplite. Another interesting,dike of the same type as the 

last runs across the path that goes up from the south end of the same 

Lime Rock quarry. It is 8 inches wide, and is composed of a clear, 

rather dark-gray, aphanitic rock of harsh feel, broken up into halfinch plates crusted on all sides by cubes of pyrite, which also appears 

elsewhere in the rock, though in less quantity. It effervesces very 

actively with acid. When this rock is examined with a power of 85 

the field is seen to be covered with many opaque white spots, which 

may be leucoxene or kaolin. With polarized light much of the surface is seen to be covered with irregular-lobed areas of calcite, which 

may have been derived from the limestone, but if so have crystallized 

in places in peculiar lobate shapes. Except for this calcite, the field 

is strewn about equally with minute low-polarizing plates, which seem 

to rest in an almost apolar ground, and which take up about half the 

surface. With a power of 230 the apolar places are seen to be occupied by overlapping plates of the same colorless low-polarizing material. This seems all to be a fine-grained mosaic of orthoclase, and in 

the larger plates are seen here and there more brightly polarizing 

plates of muscovite, and the opaque white areas which cluster round 

the larger orthoclase grains seem with the higher power to be distinctly kaolin derived from its decomposition. The rock- is thus an 

aplite, and its bluish-gray color is caused by its very exceptional content of pyrite. All these small dikes are interesting as possible members of the distant East Greenwich group described below, and as 

similar micrographic and microgranitic types occur in the Quincy 

group they suggest a connection between the two. Its abundant content of coarse microcline, however, seems to ally this rock more closely 

with the Milford granite.

BASIC ERUPTIVES.

Gdbbro. A great range of gabbro and syenite hills and isolated 

bosses, which have not been described in detail, extends southwestward 

along the south side of the Norfolk County basin from Canton Junction a to Wrentham. The remarkable mass of cumberlandite in Cumberland Hill lies in the extension of this range. This hill stands just on 

the northern border of the area here mapped and probably enters it 

beneath the drift. It has been fully described by Wood worth. b It is 

largely coarse massive menaccanite with a few porphyritic plagioclase 

phenocrysts. Where bronzite and plagioclase appear abundantly in 

the menaccanite mass it becomes a gabbro.

aWoodworth, J. B., Geology of the Narragansett basin: Mon. U. S. Geol. Survey, vol. 33,1899, p. 118, 

b Bull. Mus. Comp. Zool., vol. 1, p. 183; Proc. Boston Soc. Nat. Hist., vol. 21, p. 195. .
PKE-CARBONIFEKOUS BASIC EBUPTIVES. 49

In the prolongation of the range to the southwest an ideal gabbro 

of the same type, of coarse grain and wholty massive, occurs in a large 

bowlder on the hill a half mile northeast of Albion. This was probab\y brought by the ice from some point 5° or 10° west of north but 

near at hand, since in the sheet farther south, which we have studied 

in detail, this t}^pe is not found. It is a dull, dark rock, with spots of 

1 a faint shade of brown or green, large squarish areas of a brownish 

pyroxenic mineral, and large, shining, striated plagioclase cleavages 

and gi:ains of menaccanite.

The slide shows a ground of coarse-grained labradorite, with maximum extinction in the albite twinning of 23°, and with the twinning 

bands rigidly parallel. The second original mineral is bronzite, which 

is abundantly dusted with black grains. It was formerly present in 

large squarish fields, which are now wholly or mostly changed into a 

coarse radiate fibrous mass of a bluish-green hornblende, in which 

a=ochre. yellow, J=pure green, c=deep blue; c<.l)~>a. Outside this 

there is around many of the fields a rim of an oil-green, scarcely 

polarizing mineral full of black dust and calcite grains, which has the 

aspect of a serpentine. Outside this, against the plagioclase, is a 

second rim, perhaps a reaction rim, of white, brightly shining plates 

like muscovite or paragonite, which have eaten into the plagioclase. 

A few large garnet grains occur.

The large menaccanite plates are cracked and are surrounded by a 

rim of biotite in large plates full of rounded scales of menaccanite 

like those common in the bronzite. They are gathered centrally and 

arranged in curved, beaded bands. There is no leucoxene or rust; no 

strain or crushing of constituents, and thus no dynamic metamorphism. The change went on without motion of part on part, and without oxygen; but water, silica, and alkali were needed to change the 

labradorite into the shining white paragonite, the bronzite into the 

blue hornblende, and this into the chloritic or serperitinous mineral, 

and to develop biotite from the menaccanite.

Odinite. It is noteworthy that beside the olivine-diabase dike 

described below from the Lime Rock marble quarry, which is remarkable for its freshness and for the sharp-cut completeness of all its 

crystals, there should run another dike, of about the same thickness 

(2i feet) and with the same strike (N. 50C W.) and dip (60° NE.), of a 

dark, aphanitic trap which should prove under the microscope to be so 

unlike the other. It can be recognized in the quarry by the fact that 

it is cut by a half-inch aplite dike, and by its lighter gray color and 

more aphanitic and even texture, the minute porphyritic needles of 

plagioclase and glassy grains of olivine of the diabase being absent. 

Under the microscope it is found to be made up wholly, or almost 

wholly, of a confused network of hornblende needles, all very minute, 

Bull. 311 07 4
50 SCHISTS AND ASSOCIATED EOCKS OF RHODE ISLAND.

so that it requires high power of the microscope to study it. The 

whole field has much more the aspect of a common, fine-grained hornblende schist of metamorphic origin than of an unchanged intrusive 

dike. The hornblende needles are ragged-edged and fibrous, so that a 

surface rarely shows a common polarizing color, and a basal section is 

made up of a bundle of diamond-shaped rods. This hornblende has 

very faint pleochroism; «=pale yellow, J=yellowish-green, c=pale 

bluish, 1) showing the strongest absorption.

This rock contains some large crystals of hornblende of the same 

type as the others. With the finer hornblende is intermixed over 

irregular and isolated patches a biotite that occurs here and there in 

parallel or radiate forms. This biotite is salmon-colored parallel to 

axis c, and nearly colorless in other directions. It is at some places 

changed into a pale-green chlorite. The rock contains a few menaccanite grains, most of which are surrounded by leucoxene, and many of 

the larger green hornblendes are centrally dusted with menaccanite 

grains and plates. Small areas of feldspar appear in the interstices 

between the hornblende needles. These are indeterminate because 

they are without cleavage or twinning, but have almost the same refractive index as the Canada balsam of the slide, or a little higher, and so 

are probably near oligoclase. This is probably the same dike that is 

described as amphibolite and figured from the southernmost limestone 

locality at Lincoln by Prof. J. B. Woodworth, a because its parts have 

been disconnected by faulting in the limestone, while all trace of these 

faults has disappeared from the adjacent limestone. He notes hornblende, chlorite, muscovite, titanite, magnetite, apatite, and quartz as 

constituents of this rock, and considers it-, probably, an altered diabase. It bears considerable resemblance to the diorite in the serpentine hill directly north of the quarry, and may possibly be an apophysis of this rock. It is massive and unstrained, and has not suffered 

any dynamic change except the slight displacement which it and the 

limestone experienced together. Its walls against the limestone are 

intact. It does not effervesce with acid, and it seems to me possible 

that the only change since consolidation may be the chloritization of 

a part of the biotite and the growth of the leucoxene around the 

menaccanite. The central crowding of the hornblendes with black ore 

grains may be an original structure.

If the rock has suffered only the amount of change indicated above, 

it may be called an odinite and be associated with the gabbros of the 

region. It was intruded in the limestone after these had been metamorphosed, and is cut by the aplite which is here associated with the 

Milford granite.

a Geology of the Narragansett basin: Mon. U. S. Geol. Survey, vol. 33,1899, p. 108.
IGNEOUS BOOKS. 51

IGNEOUS ROCKS OF PRE-CARBONIFEROUS OR EARLY CARBONIFEROUS AGE.

SUBDIVISIONS.

The newer acid eruptive rocks, which occupy a large portion of the 

area mapped, may be conveniently divided into two large groups:

(1) The hornblende granites and granite porphyries in Cumberland 

on the northeast, which may be considered a southern prolongation of 

the granite and porphyry series that extends southward from Ipswich 

across the Boston basin and includes, in the middle of its extent, the 

large quarries at Quincy, from which these rocks may be called the 

Quincy group.

(2) The East Greenwich group of biotite-granite porphyries and 

graphic microgranites and the brecciated phases of the latter. It is 

a question whether this should be regarded as a southward prolongation of the Quincy or as an independent group. The rocks of the 

two groups show only partial resemblances.

QUINCY GRANITIC GROUP. 

DISTRIBUTION AND GENERAL CHARACTER.

The rocks of the Quincy granitic group occupy the whole area 

between the green schist and the Carboniferous rocks along the northeastern border of. the green schist from Diamond Hill southward to 

Berkele}7 , forming a southern lobe of the broad Quincy band in the 

Boston basin.

The commonest rock in the central parts, notably in all the large 

central lobes 2 miles south of Diamond Hill, is a coarse, light porphyritic granite with a few squarish feldspar phenocrysts two-thirds 

of an inch long almost a granite porphyn^ with here and there limpid 

quartz phenocrysts inclosing lobes of a microgranitic base.

A second type is a granite porphyry in which all the constituents 

are of dark-gray color, and the porphyritic character, although perfectly developed, comes/out most distinctly only on weathered surfaces 

or under the microscope. At one extreme it is a rather fine-grained rock 

with abundant small square feldspars and dark-blue rounded but distinct primary quartz phenocrysts; at the other extreme it is a very 

peculiar rock, made up almost entirely of square feldspars (2 to 4 mm. 

across), the quartz appearing only in the microgranitic groundmass. 

An identical rock appears in the great Cumberland dike 1 mile west, 

and a similar dike occurs 1 mile west of Lonsdale, 2 miles south of the 

southernmost area of the granite porphyry, this occurrence serving to 

lessen the gap that separates this rock from those of the East Greenwich group.

A third and most interesting type is a riebeckite porphyiy and 

granite in which the large rounded phenocrysts of quartz and smaller
52 SCHISTS AND ASSOCIATED BOOKS OF RHODE ISLAND.

squares of feldspar are inclosed in a microgranitic base that is at some 

places of a purplish tint, from the content of fluorspar grains and blue 

riebeckite needles. This forms a compact area along the north edge 

of the area mapped in PL I, and extends for an unknown distance 

northward.

A fourth t}^pe, forming the border rock near the green schist, is a 

bluish-white niicrogranite. This appears only along the south side 

of the southern separate area between Hunting Hill and Berkeley. 

In the western portion of this southern area is a fifth type a dark, 

hornblendic granite and a dark chloritic facies of the same rock o

appears in a small patch at the border of the granite west of Diamond 

Hill.

The boundaries of the subdivisions as-shown on the map (PL I) are 

only approximately exact, as these subdivisions were fixed after .the 

field work was completed, partly by study of the material collected, 

and so no sharp, definitely marked boundary lines are drawn. Full 

notes and sufficient material were taken in the course of the field 

work, but much of the area is covered b}^ till, and the determination 

of the nature of the underlying rock at some places is therefore difficult and doubtful.

PETROGRAPHIC DESCRIPTIONS.

Granite porphyry. The granite porphyry occurring along the east 

branch of Sneech Brook in Cumberland (slide 359) and that of the 

Cumberland dike is a dull-black rock, with dark square feldspars and 

quartz c^stals, generally smoky, but in a section (slide 371) from the 

north end of the dike the quartz was bipyramidal and of blue color. 

It was limpid, without microlitic inclusions, and with deep lobes of 

the, light-gray microgranitic ground. The feldspar was untwinned 

soda orthoclase, and showed extinction of 12° on M (010) and the 

vertical emergence of a positive bisectrix. Some of the feldspars 

inclose great numbers of large, stout, sharply rectangular rods of an 

acid plagioclase. These rods are singly twinned with small angle, have 

the suture parallel to the side of the square, and show a higher index 

of refraction and birefringence than their host. The feldspar also 

in places incloses many lobes of the groundmass. The microgranitic 

ground contains quartz, feldspar, magnetite, biotite, muscovite, and 

calcite.

The rock shows distinctly a fluidal structure, and some specimens 

carry fluorite. It incloses fragments of a calcareous sandstone so full 

of fluorite as to impart to it a deep purple tinge. Where it lies in 

contact with the green schist this is only slightly indulated, but is 

soaked full of purple fluorite for one-half inch from the boundary. 

At the north end of the Cumberland dike the rock contains an inclusion of the coarse white granite of this series, which was also full of
IGNEOUS ROCKS, QU1NCY GRANITIC GROUP. 53

purple fluorite. A slide (384) cut from the rock adjacent to an inclusion of calcareous sandstone shows fluidal structure, adjacent phenocrysts being connected by bands of a much coarser groundmass, and so 

united into somewhat parallel trains. In this coarser groundmass ma'ny 

of the grains are fresh untwinned and unstained calcite that include 

and are included by the other constituents so that they appear to be 

an original constituent. Calcite is much rarer in the intervening finegrained ground, but at some places trains of groups of calcite grains 

run through this also, each group being surrounded by a dense halo 

of black ore grains. These calcite grains are wholly unlike the calcite 

of the inclusions. In one slide an exceptionally large calcite grain is 

partly inclosed by a large phenocryst of soda orthoclase. The calcite 

must therefore have crystallized out early in the magma before the 

feldspar phenocrysts had been completely formed or the black ore 

had solidified. The latter is almost wanting in the phenocrysts, and 

was formed by a reaction of the carbonate and the iron-bearing solutions in the magma. M.ost of the grades that have the same shade as 

the large grains are also calcite.

There is another dike of the granite porphyry in the green schist by 

the schoolhouse 1£ miles west of Lonsdale, near the Moshassuck River, 

in Lincoln. It is 20 feet wide, with sharp boundaries. Under the 

microscope the rock (slide 402) shows the extreme of alteration without 

the slightest disturbance of its original structure. The feldspars are 

wholly idiomorphic and are set so closely together that there is but 

little interstitial ^material. These^ feldspars are in places changed to 

a congeries of epidote crystals without losing the original sharpness of 

boundary.

The groundmass shows micrographic structure where it borders the 

feldspars, from which the quartz rods radiate, and this structure passes 

into a microgranitic arrangement at the center of the interspaces. 

The abundant biotite is largely chloritized. This rock differs from 

that of the Cumberland dike in its greater quantity of feldspar and in 

the absence of large quartz phenocrysts of first consolidation.

Riebeckite porphyry. The riebeckite porphyry is a fine-grained 

granite porphyry made up largely of small, square feldspars, larger 

rounded or bipyramidal quartz grains, and black amphibole (riebeckite) 

blades, these three constituents being embedded in a microgranitic 

ground which looks like a fine, white quartzite.

The feldspars are Manebach twins of orthoclase microperthite, 

with broad anastomosing bands of albite in places in peglike arrangement; extinction, 16|° on (010). In places these bands coalesce at 

the surface to form broad continuous layers of albite. Some of 

them are dusted full of long needles of riebeckite. Others have the 

moire surface and the optical character of soda orthoclase. The large 

quartz grains, some of which are bluish, are nearly all idiomorphic
54 SCHISTS AND ASSOCIATED BOCKS OF EHODE ISLAND.

_> 

and have all the characteristics of porphyry quartz. They are limpid

and show slight strain and sharp straight lines of cleavage and cavities with moving bubbles. . They are penetrated by deep lobes of the 

fine, granular quartz-feldspar ground, Avhich as a microgranitic 

groundmass also incloses the other constituents.

Riebeckite is present not only in the many microlites that penetrate 

the feldspar and are absent from the quartz, but in blades that inclose 

large angular grains of orthoclase. The mineral has the very strongest absorption: a = deep blue, 1) = dark greenish blue, c 3r ellowbrown; a>l>c. It has perfect prismatic and basal cleavage.

Purple fluorite is an interesting original constituent of the porphyiy. 

Jt is found in rather large grains, especially in .the vicinit}' of the 

riebeckite, and its presence may have determined the formation of the 

riebeckite rather than another form of hornblende or augite. ft The best 

locality to study the rock is on the hill a mile southeast of Copper Mine 

Hill, in Cumberland.

Along its southern border, on the north slope of Copper Mine Hill, 

the rock retains its texture, except that all the iron has gone into 

magnetite. On page 66 is given an analysis of the riebeckite porphyiy, and for comparison one of the riebeckite granite of the Hardwick quarry at Quincy, Mass.

White's description and figure of the rock of the Weymouth Fore 

River in the Quincy basin, 6 north of the area here studied, agree 

closety with that of the rock here discussed.

The "crocidolite" announced in 18T9 C and described in 1887 tf by 

A. H. Chester and F. I. Cairns from "Beacon Pole Hill," 6 would 

seem to be the same mineral that we have here determined optically 

to be riebeckite. It occurs in seams in a granite ledge.

It is usually disseminated in fine particles through feldspar but often occurs in large 

masses up to the si/e of a butternut. Unbroken surfaces sometimes present a 

botryoidal appearance, and the nodules, Avhen broken, show a radiated.structure. 

* * * Its color is usually a dark bluish gray, the radiated nodules, however, being 

darker, almost an indigo blue. It is associated with dolomite, glassy quart/, and 

rarely with light purple fluorite./

a This suggestion has been independently developed as the result of an extended study of the riebeckite rocks by Mr. Murgucci in a paper read before the Geological Society in Philadelphia in 

December, 1904. (See Am. Jour. Sci., 4th ser., vol. 20, 1905, p. 133.)

b White, T. G., Contribution to petrography of the Boston basin: Proc. Boston Soc. Nat. Hist., vol. 

28,1899. p. 132, PI. IV, fig. 12.

cDana, Man. Min. and Lith., 3d ed., p. 252.

('Am. Jour. Sci., 3d ser., vol. 34, p. 108.

eThis is the hill on the north edge of the area mapped northeast of Copper Mine Hill, which is 

apparently the Tower Hill of Shepard, cited above, and the Cumberland Hill of Chester and Cairns.

/Chester, A. H., and Cairns, P. I., Crocidolite from Cumberland, R. I.: Am. Jour. Sci., 3d ser., vol. 

34,1887, p. 108.
IGNEOUS ROCKS, QDINOY GRANITIC GROUP.

Analyses of "crocidolite."

55

SiO, ...........................

Fe2 08 . ........................

FeO ..........................

MgO. .........................

I.

53.13

15.93

01 f)Pi

0.22

H.

51.03

17.88

21 19

0.09

NaoO...... ...................

H,0. :........................

Total.......'............

I.

C. 20

3.95

100. 74

II.

0.41

3. (i'l

100. 24

Hornblende granite. A hornblende granite occurs abundantly in the 

southern area between Berkeley and Hunting Hill. It is dark, coarse, 

massive, and fresh looking, varying from a rock resembling a gabbr.o, 

in which the large feldspar and biotite phenocrysts and grains stand 

out on a greenish-black ground, to a more granitic facies, in which, on 

the dark mottled surface, the large black hornblende phenocrysts are 

relieved by a white border and many of them luster-mottled by large 

apatite crystals.

Slides of rock of both these types (366, 361) have under a lens an 

identical and very characteristic appearance. The magnetite, biotite, 

and hornblende, all of which occur in small amount, are massed in 

groups and more or less altered. The whole field seems to be made 

up of closety packed feldspar crystals of uniform size and shape, 

having perfect crystal faces except against the hornblende-biotite 

groups, and the small amount of limpid quartz is arranged in narrow 

areas between these ciystals. They are uniformly Carlsbad twins of 

soda orthoclaee, which, except around a narrow border band, are 

dusted full of perfect crystals and crystal groups of almost colorless 

hornblende, epidote, and garnets, the latter in model-like dodecahedra.

The quartz is free from inclusions, granulation, or strain, and some 

crystals are of bipyramidal shape. The rock has thus the texture of 

a porphyry without a groundmass. The quartz in the center of the 

intersertal fields shows, where it adjoins the feldspar, a micrographic 

and microgranitic texture.

A similar rock on the west side of Diamond Hill has a more mashed 

and chloritized aspect and is probably adjacent to a fault. The slide 

(357) shows the feldspars to be a dense mass of overlapping blades of 

hornblende and epidote. The quartz is slightly strained and granulated 

and the biotite is chloritized. The rock contains the same abundance 

of large apatites and the same peculiar model-like garnets that were 

noted by Woodworth in the corresponding hornblende granite from 

the Cambrian area on Hoppin Hill, in Attleboro.

Microgranite. The mlcrogranite is a light pearl-gray rock of very 

fine grain, with a slight banding, due probably to flow structure. It 

is a mixture of granular quartz and microcline, which in some specimens of the rock interlock so completely as to produce almost a micrographic structure. The gray color is due to small grains of magnetite 

and biotite. (Slide 353.) It resembles very closely the microgranite
56 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND.

of East .Greenwich. (See p. 62.) It can be studied along the road 

southwest of Hunting Hill.

Vein quartz at Diamond Hill. The enormous mass of vein quartz 

at Diamond Hill, only the southern half of which is shown on the map, 

lies on the border between the Carboniferous sediments and the granite 

porphyries, in the continuation of this mass to the north, beyond 

the limits of the area mapped in PI. I, Professor Woodworth has 

mapped a large area of felsite, which he assigns to the same age as 

that of the felsites around Attleboro. According to his determination a 

the quartz was deposited by hot springs connected with the volcanic 

activity of which the felsites are the product, and its formation was 

therefore contemporaneous with that of the earlier Carboniferous beds 

in this area. This seems to be the only possible explanation of the 

great veinstone body. The mass is made up of many layers of quartz, 

much of which occurs in radiating crystals, suggesting diamond, and 

from these the hill gets its name. The mass comprises also much 

white compact and chalcedonic quartz.

Phosphate of lime and red hematite occur in beautiful botryoidal 

and stalactitic groups in the quartz on the southern slope of Diamond 

Hill. 6

Mr. George F. Kunz writes as follows concerning the varieties of 

quartz found at this locality:

The highly modified crystals from Diamond Hill and Cumberland Hill, Rhode 

Island, also the fine ones from White Plains and Stony Point, Alexander County, 

and from Catawba and Burke counties, N. C., are worthy of mention, and lately 

formed the subject of a crystallographic memoir by Prof. Gerhard vom Rath, c

Jasper agate is found inconsiderable quantity at Diamond Hill, Cumberland, R. I., 

in all shades of white, yellow, red, and green; these colors are also all intermixed in 

one specimen, usually mottled, and at times beautifully banded in irregular seams 

of white, creamy brown, greenish, and brecciated. It is found in large quantities, 

and although fully 1,000 pounds is taken away every year by .visitors and collectors 

not over $100 worth is sold or polished per annum. d

"WAMSUTTA GROUP" OF WOODWORTH. .

The group of granite porphyries, felsites, and fine graphic granites 

described bjr Professor Woodworth, e the border of which makes a great 

curve in the " Wamsutta group " of Carboniferous beds from North 

Attleboro by Lanesville to Arnolds Mill is most interesting. The 

curve may perhaps be continued to include the larger granite-porphyry 

area at Diamond Hill. It conforms with the Carboniferous beds in 

their folding around the central Cambrian island. The rocks of this 

group are accompanied 'by a series of basic dikes. The larger f elsitic 

mass has some of the characteristics of a tilted laccolith.

«Mon. U. S. Geol. Survey, vol. 33, 1899, p. 155.

fcJackson, loc. cit., p. 52.

cMineral Resources U. S. for 1883 and 1884, p. 749.

f'Op. cit., p. 762.

«Mon. U. S. Geol. Survey, vol. 33,1899, p. 153, and map (PI. XVII).
IGNEOUS ROCKS, " WAMSUTTA GROUP." 57

There is frequent flow structure, with crumpled layers, and beneath 

the massive flow of felsite is a bed wherein the same felsite is the 

matrix of an agglomerate of rounded pebbles of felsite, granite 

porphyry, quartzite, and hornblende granite. Professor Woodworth 

states that "the peculiar features of the Wamsutta series * * * 

point to a volcano or volcanoes existing in this field in Carboniferous 

time," and compares the rocks with the flows and intrusives of the 

quartz-porph}^ry series in the Boston basin. He speaks of "Wamsutta volcanoes," and connects the intrusive granite porphyries 

described above with the effusive felsites and granite porphyries of 

the " Wamsutta series," and finds the same story repeated in the Blue 

Hills region on the north side of the Norfolk County basin, and still 

farther north in the larger felsite area about Boston." He mentions 

and maps an area of granite porphyry, about 2 miles long, north of 

the quartz veinstone mass of Diamond Hill. 6 This is possibly the 

northern apex of the area shown on PI. I. Another somewhat 

smaller area lies farther east, within the limits of the Carboniferous, 

one-half mile northwest of Arnolds Mill. The rock from this area 

contains rare microscopic garnet and fluid inclusions with moving 

bubbles. Professor Woodworth associates these two stocks of granite 

porphyry with the adjacent felsites, which are clearly interbedded 

with the Carboniferous, and therefore deduces the probable Carboniferous age of the whole series of the granite porphyries extending from 

this region to the vicinity of Boston, including the rocks in Cumberland. This is doubtless the most probable relation, but no certain 

evidence was found along'the border between the granite porplryry 

and. the Carboniferous to indicate that the granite porphyry was 

intruded into the Carboniferous, nor does Professor Woodworth 

adduce such evidence.

The felsite of Diamond Hill is unlike anj^thing in the Quincy and 

East Greenwich groups. A sample taken from the northeast slope of 

the hill is a light pearl-gra}7 rock, breaking in plates, and wholty 

aphanitic and horn'stonelike. It fuses easily to a white enamel. The 

microscope shows a few small, angular kaolin spots, which are traces 

of former feldspar crystals, and minute quartz veins, in a uniform 

microfelsitic base of exceedingly minute plates and short needles, 

which polarize brightly and lie apparently in a colorless glass. .There 

is no iron oxide. This rock is precisely like the felsite in slides taken 

from rock at Natick, Mass., in the middle of the Boston basin.

The granite mass in Cumberland forms the southern end of an 

extensive belt of igneous rocks, the main body of which lies north of 

the area mapped .in PI. I. The felsites (aporhyolites) of the Boston 

basin approach this area at the northeast base of Diamond Hill and 

swing round the Cambrian area in the " Wamsutta" region to the east.

"Idem, pp. 155-166. bMou. U. S. Geol. Survey, Vol. 33,1899, pp. 117, 165.
58 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND.

Distinct surface flows are therefore not found in this southern extension of the Quincy rocks. The content of riebeckite and soda orthoclase connects the series with the Quincy granite. Eleolite found in 

the Essex basin is lacking here, but a nepheline tephrite occurs at a 

point not much farther west, at Fairmount Farms, in Woonsocket. 

(Slide 25920 of the Tenth Census rocks.)

Microgranite also is common in both groups. The hornblende 

granite described above, which might almost be called a granite 

porph}Ty, is much like the granite porphyry of the "Wamsutta" region. 

Both contain beautiful microscopic garnet. There is also a close 

resemblance between the porphyry and the microgranite of the Cumberland area and the corresponding rocks of the East Greenwich 

group described below, though hornblende is lacking in the latter and 

the limited dark border beds are biotitic.

EAST GREENWICH GROUP.

DISTRIBUTION AND GENERAL FEATURES.

The East Greenwich group of eruptives became interesting in the 

course of this survey because of the disco very'that it carries the 

porphyries and fine graphic granites of the Boston basin much farther 

south than they have been known before.

A dozen broad bands of acid eruptive rocks cross the State of Massachusetts from north to south, either as continuous areas or in rows 

of great batholiths. Each band has many distinguishing characteristics. Porphyries and basic rocks are rare or for the most part wholly 

lacking in all of these bands that lie west of the broad easternmost 

band which extends southward from Ipswich across the Boston basin. 

In this belt porphyries and a great range of basic rocks accompany 

the granites. Professor Woodworth has connected the porphyries of 

the adjacent "Wamsuttagroup" with the porphyries and felsites of the 

Boston basin, and the porphyries of the two groups described in the 

present report show many points of resemblance to each other, to 

those of the "Wamsutta group," and to those of other members of the 

same band farther north, and, although isolated in surface occurrence 

from the rocks of the Boston basin, they seem to represent a southward prolongation of those rocks.

Much of the country occupied by the East Greenwich group, especially its central and most interesting portion, is covered by till, and 

the contact relations can be observed at only a few places. This 

group includes granite porphyry, some of it canning. rather small 

phenocrysts, microgranite, a massive and banded fine-grained micrographic rock, and a breccia formed of a great mass of angular fragments of all sizes of the latter rock, either alone or inclosed in the 

granite porphyry. A black biotite granite occurs in small amount on
IGNEOUS BOOKS, EAST GREENWICH GROUP. 59

the border. A distinguishing common characteristic is the microgranitic and micrographic groundmass. The former is even-granular, 

like a fine sandstone. Its grains become in places more and more 

lobate and interlocking until a continuous micrographic texture is 

produced. The microgranite appears in purity over large areas; the 

micrographic rock appears mostly as fragments in the breccia. Where 

quartz and microcliue crystals appear in abundance in a much finer 

microgranitic groundmass porphyry is produced, and appears in continuous masses and as the paste of the graphic microgranite breccia. 

Thus where phenocrysts become predominant and the microgranitic 

groundmass is interstitial or absent we have granite porphyry and 

granite, an intermediate type occurring where the microcline wholly 

predominates. At the border these peculiarities are masked by the 

abundance of biotite in the black contact granite..

PETKOGRAPHIC DESCRIPTIONS.

Granite and granite porphyry. The granite of the East Greenwich 

series is always in aspect allied to granite porphyiy. The feldspar 

and quartz are more or less idiomorphic and are inclosed in a small 

amount of granitic ground of rather fine grain. Where the small 

flesh-colored feldspars grow more distant and distinct and the rounded 

blue-quartz grains more individualized the rock becomes a granite 

porphyry, which is the prevailing form of rock in the areas mapped 

as granite and granite porphyry.

Another variety is found only in large bowlders at the quarry in 

the village of East Greenwich. Large flesh-colored feldspars an inch 

long and large blue-quartz grains appear in a rather coarse microgranitic ground, and great oval and parallel blotches of biotite cover 

rude foliation faces.

At the extreme south end of the granite porphyry a specimen was 

obtained which showed its contact with the light-gray microgranite, 

and the contact was a rather sharp one, though more like a pronounced schlieren contact than an eruptive contact. This seems to 

indicate that the granite porphyry was consolidated a little later than 

the other because it is free from biotite and is distinctly coarser for a 

quarter of an inch from the contact.

J3asic border of the granite porphyry. Along the northern border 

of the granite porphyry of the East Greenwich group on Bald Hill in 

Warwick, separating it from the Cambrian quartzite, stretches a band, 

about 40 rods wide, of heavy black subporphyritic rock, which is in 

places spotted white with feldspar crystals and groups of crystals. It 

is of medium grain, and its dark color is due to its abundant content of 

biotite and magnetite. It is well exposed along the southern slope of 

Bald Hill and crosses the road just east of the hill, where it is slightly 

gneissoid, It grades at many points into the granite porphyry, the
60 SCHISTS AND ASSOCIATED BOCKS OF RHODE ISLAND.

 gradation showing that the two are parts of one mass. At some places 

the boundary of the two rocks is intrusive, the granite porphyry 

being the newer rock. At some localities the granite porphyry is 

unchanged up to the contact, though in other places it becomes finer 

grained in a belt several inches wide.

The black granite is cut by many small dikes of graphic microgranite 

or of microgranite containing small crystals of pj'rite, and by one 

small dike of granite porphyry.

Rarely a fragment of the Cambrian quartzite is found inclosed in 

the granite. It is clear that this pure quartzite can not have supplied 

the iron to the border granite, which must be regarded as a basic 

. border differentiate of the granite porphyry. The same brown garnets are found in both. As shown in several slides, the feldspars 

have the same idiomorphic character, and the graphic structure is 

abundantly developed in both.

A slide (652) shows a vein of graphic microgranite crossing at the 

top and a portion of the black granite below. A large idiomorphic 

feldspar in the center'of the field extinguished as a single individual, 

but is dusted so full of minute crystals of an epidotic mineral and of 

micrographic quartz as to produce the effect of aggregate polarization. 

Dark spots are biotite aggregates. The slide shows with the lens so 

many well-defined feldspar crystals that it can be called a granite porphyry, though its porphyritic character is disguised by the abundant biotite. The vein of micrographic texture is very interesting. It is about 

2 millimeters wide; has a central suture. It is white, and thus stands 

out clearty against the black granite. In thin section the micrographic structure of the black granite is seen to be continued into the 

white vein, in which it grows inward from both sides of the vein with 

a coarser texture than in the granite and meets along the central 

suture. This minute vein, which is free from the abundant epidote 

dust as well as the dark minerals that fill all the rest of the rock, is 

analogous to the limpid perthitic aftergrowth of many of the epidotefilled nuclei of the feldspars. The same slide contains a wider dike of 

microgranite.

The black granite is thus of the same type as the other members of 

the series, but is slightly older and is cut .by veins or small dikes of 

them all.

Blue-quartz-microcline porphyry. The porphyry from the important locality one-half mile southeast of the top of Spencer Hill is a 

fresh rock with a bluish to chocolate-colored base, j ust resolvable with 

the lens into a granular mass. It contains pale flesh-colored feldspars 

two or more millimeters long; large, clear, blue-quartz phenocrysts, 

and black biotite scales just visible with the lens. The feldspar groups 

show small miarolitic cavities. An analysis of this rock is given on 

page 66.
IGNEOUS BOCKS, EAST GREENWICH GROUP. 61

Under the microscope the quartz crystals are the most prominent 

and abundant (see fig. 5.) They have often perfect forms, with prism 

and double pyramid; they are free from inclosures but are penetrated 

by lobes of the groundmass. Here and there two or three of these 

crystals are grouped together. Many of them are often fissured with 

perfect cleavage and polarized with fine 

broad undulation, indicating incipient 

strain. The large-lobed blebs of quartz 

in the feldspars and the quartz grains of 

the ground are full of magnetite grains.

The feldspar is generally microcline. It 

showed with heavy solution the same specific gravity as the Ceylon moonstone. 

The microcline twinning is generally flamy 

and irregular, as shown in fig. 6. The 

composites are also twinned after the Carlsbad or Baverio law. Fig. 6 shows an upper 

crystal twinned to right and left with- crystals on the Baveno law. 

There are also large included plates of bytownite in the large microcline, showing extinction 30 on M and emergence of an axis.

Biotite is regularly but distantly disseminated in the ground in long 

blades, many of them full of rounded quartz grains. Muscovite is

FIG. 5. Crystals of feldspar embedded in quartz groundmass. 

F, Feldspar; M, magnetite; Q, 

quartz.

X65

FIG. 6. Microcline Baveno twin cut at right angles to a. Negative bisectrix nearly central. 

From quartz porphyry south of Spencer Hill.

abundant, in irregular plates surrounding the feldspar. Magnetite 

is more abundant, but occurs in smaller grains than the'biotite. It is 

often in regular octahedra and arranged in long nuidal trains. The 

ground is microgranitic, and at some places shows fluidal structure. 

In this quartz-feldspar ground a few of the feldspars show micropeg-
62 SCHISTS AND ASSOCIATED EOCKS OF RHODE ISLAND.

matitic structure, vermiform quartz rods penetrate the feldspar and 

themselves contain blebs of feldspar orientated with the host. A fine 

dust of magnetite in octahedra or in vermiform shapes is scattered 

through the ground. Zircons cause dark borders in the biotite.

Outside the locality named above, southeast of Spencer Hill, the 

porphyry is generally of lighter color, and a slight fissilit}r is in places 

produced by the aggregation of the biotite into flattened concretionary 

groups. That portion of the porphyry which forms the cement of 

the extrusive graphic granite-porphyry breccia is the latest member 

of the series. In a block from the southern border of the porphyry, 

at its contact with the microgranite, the microgranite penetrates the 

porphyry in delicate Jobes, as seen in thin section, and has a distinct 

differentiation border, which is black from the concentration of magnetite grains. This is apparently a slightly later dike of the microgranite, resembling those penetrating the black border granite.

Microgranite. The microgranite is best exposed in the quarry in 

the northwest corner of East Greenwich village. It is a fine-grained 

aplitic rock, often splitting in slaty slabs of a fresh bluish-white to 

dark-gray color, in which the biotite scales are just visible. Macroscopically it is like the graphic microgranite, but is generally darker; 

microscopically it has a very regular automorphic granular texture. 

An anatysis is given on page 66.

In some parts of its mass small quartz and feldspar phenocrysts 

form in the microgranite; in others the quartz becomes more abundant, occurring in spherical grains nearly the size of a pea the unit 

form with rounded faces. This forms a transition to the blue-quartz 

porphyry. These transitional forms show under the microscope a 

limpid wavy polarizing quartz, as in the porpl^ry. Seen with low 

powers the feldspar seems to be untwinned, but with high powers 

shows the finest laminae. The feldspar is generally microcline. One 

section on M gave central positive bisectrix and extinction 11°, and is 

therefore soda orthoclase. It was full of long plates parallel to a steep 

positive orthodome of about 76°. Purple fluor occurs in the microgranite where it has been slightty sheared and has become banded with 

biotite.

The microgranite in the East Greenwich quarry has been partially 

crushed into flat fragments, which have been recemented by thin, black 

films of biotite, one of the many indications of the migration of the 

biotite. At some places the granite-porphyry cement of the breccia 

passes into this biotite facies by the disappearance of the quartz and 

feldspar phenocrysts, especially in thin layers, and the rock then generally becomes black near the contact from the great concentration of 

the magnetite in bands that may be an inch or more wide. The magnetite found in the conglomerate is probably derived from this rock, 

as the other constituents are mostly free from iron.
IGNEOUS BOOKS, EAST GREENWICH GROUP. 63

Microgranite or graphic microgranite. The ruicrographic facies 

of the central granite mass is present in rather large amount, but 

mostly as a constituent of the breccia. It makes up some part of the 

rock mapped as microgranite, but can not well be separated without 

extensive microscopic work. Even at the East Greenwich quarry 

some slides are half micrographic in texture. It is a fine, evengrained rock, distinctly granular with the lens, bluish white, weathering pale fawn color. Under the microscope it is seen to be made up 

of rather large, rounded, or interlocking grains of orthoclase or microcline with exceedingly fine twinning structure, each grain filled with 

vermiform quartz rods in perfect micrographic structure, which are 

in places so abundant that they almost entirely mask the feldspar. 

Small, long plates of biotite.are scattered evenly and distantly through 

the rock, which swarms with small octahedra of magnetite.

Many fragments of the graphic microgranite in the breccia, some 

over a foot across, show very distinct nuidal structure in the form of 

thin wavy bands of lighter and darker gray, and one of these fragments 

itself contains fragments, about an inch long, of micrographic texture, 

around which the fluidal bands bend. This banding is caused' by the 

concentration of magnetite and biotite in zones and by the alternating 

narrow layers of micrographic and microgranitic rock.

An analysis of the graphic microgranite taken from a large fragment in the graphic microgranite breccia at a spring near Spencer 

Hill, at a locality described belo\v, is given on page 66. It differs from 

the analysis of the microgranite mainly in showing the presence of 

the Fe2O3 .

Dikes of microgranitic and micrographic texture. Besides the large 

stocks of microgranite and graphic microgranite described in the preceding sections and designated on the map, there are many veins or 

small dikes of the same rock. A minute graphic microgranite dike in 

the dark granite seen in one slide is described on page 60. The same 

slide contains a vein or minute dike of microgranite. This constant 

association shows that the two types are closely allied. These dikes 

easily escape notice in the light-colored rocks, but are very abundant 

in the dark border granite and confirm the idea that it is the older 

rock. A dike of unusually coarse graphic microgranite, 2£ inches 

wide, was found on the border of this dark granite, where one wall 

was the normal granite porphyr}^ and the other the dark fine-grained 

microgranite. One slide shows the microgranite in contact with the 

graphic microgranite which occupies the lower three-fourths of the 

slide. A large dark square at the bottom is a perfect crystal of soda 

orthoclase cut on M, and the nucleal ciystal is full of microlites of a 

shining epidotic mineral, and is greatly enlarged outwardly by limpid 

and less twinned material in which the graphic structure first appears 

and is very marked. (j
64 SCHISTS AND ASSOCIATED ROCKS OF "RHODE ISLAND.

A part of the same slide shows two dark adjoining miarolitic cavities, into which perfect crystal ends project, as.well as forms with 

rounded crystal faces, to which the twin structure conforms, also projecting into the cavities. Both microcline and soda orthoclase appear.

Microclinite. The Carboniferous conglomerate on the west slope of 

Spencer Hill contains grains of blue quartz and feldspar from the 

porphyry, grains of microgranite, and pebbles of Cambrian quartzite, 

as well as small pebbles of a very peculiar rock.. This rock, here 

called microclinite, is made up wholly of large, squarish grains of 

microcline, all of about the same size and shape, which are filled full 

of magnetite in rounded grains and in. beaded and branching growths 

that simulate very closely the micrographic texture of some of the 

granites. Muscovite in minute scales is present in the microcline in 

large amount, and assumes the same distribution as the black ore. 

Here and there is a bleb of quartz or a scale of biotite. There is no 

intel'stitial material between the interlocking grains of microcline, so 

the rock may be considered a form of microcline poryhyry, made up 

wholly of the feldspathic constituent, with secondary magnetite as 

the only important accessory.

The branching magnetite and muscovite growth penetrates the grain 

for a certain distance, and then halts abruptly in the middle of a row 

of microcline grains, as if the iron had been brought in from without, 

as in the Carboniferous conglomerate in Cranston. The magnetite 

grains are more abundant and larger in the mica-schist paste, but are 

at some places concentrated in a broad black band that passes through 

the 'microlinite.

Graphic microgranite breccia with granite-porphyry cement. The 

microgranite is in places broken into thin, splinter}7 fragments, which 

are cemented together by films of black biotite. This kind of fracturing occurs locally in the quarry in the northwest part of East 

Greenwich village, but only in a narrow layer, and the small fragments 

are only slightly moved upon one another.

The large area at Spencer Hill, in Warwick, which is represented 

on the map as about a mile square, is of much greater interest. The 

simplest form of this breccia occurs on the western slope of Spencer 

Hill, in the Kent quadrangle, just across the western boundary of 

the Narragansett Bay quadrangle, at a point south of a schoolhouse 

in latitude 41° 40' 30", longitude 71° 29' 30", and in the field to the 

east. It is a beautiful, fine-grained, mottled, pearl-gray rock, the 

angular fragments of graphic microgranite being cemented by thin 

films of magnetite and biotite. Here and there regular and partly 

rounded Cambrian quartzite fragments, 1 to 3 inches long, occur in 

the breccia, indicating that some part of the breccia must have formed 

near the surface, and that if the superincumbent beds had been pre-
IGNEOUS BOOKS, EAST GREENWICH GROUP. 65

served the rock would here show a transition upward into the Carboniferous conglomerate.

The most interesting form of the breccia occurs at a spring about 10 

rods south of the road running south of Spencer Hill, in a depression 

just east of a house, latitude 41° 40' 15", longitude 71° 29' 15". In 

clearing the fields here large masses of the rock have been blasted to 

pieces, which afford samples of fresh rock for study. The breccia is 

made up of pearl-gray fragments of almost aphanitic graphic microgranite, ranging in size from fine dust to pieces a foot across. These 

fragments are mostly massive, but some large pieces show perfect 

fluidal structure and themselves inclose small fragments of massive 

graphic microgranite. These small fragments are cemented by a 

small quantity of a blue-quartz porphyry, which itself shows fluidal 

structure between the included pieces, marked by trains of magnetite 

grains that in some places pass through the large corroded biotite 

scales. This porphyiy at some points at the border contains fluorite.

Small shining scales of biotite are abundant in the central portion 

of some of the micrographic fragments, but absent from the outer 

half inch. It may perhaps be assumed that the brecciatiou took place 

so soon after solidification that the porphyry filling the fissures 

reheated the graphic microgranite so that a resorption of the few 

scattered biotite grains took place. - It may also be assumed that the 

cementing of the fragments with biotite or biotite and magnetite 

occurred at the same time in areas where the porphyry could not 

penetrate, but where, by pneuuiatolitic processes, superheated steam 

and other vapors carried the cementing minerals to their present position, and the same pneurnatol.itic process may. have resorbed the 

biotite in the micrographic fragments.

The breccia.also contains small pockets of granitic debris and large 

rounded cobbles of granite, from an inch to a foot in length. This 

granite is a medium-grained two-lnica rock. The quartz and feldspar 

have a certain individuality; the feldspar is' in small, perfect, opaque 

Carlsbad twins, and both quartz and feldspar are included in a small 

amount of fine ground, a characteristic that connects this rock with 

the granite porphyries described above.

CHEMICAL RELATIONS OF THE CARBONIFEROUS IGNEOUS ROCKS.

'The analyses given below show the chemical composition of the igneous rocks of the two groups. Analyses of riebeckite-bearing granite 

from Quihcy, Mass., and of the Milford granite are presented in 

columns V arid VI for comparison. 

Bull. 311 07 5
66 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND.

Analyses of igneous rocks.

SiOo..... .................................

A1203 .... .................................

Fe203 .... .................................

FeO.......... ............................

MnO .....................................

MgO. ....................................

CaO. .....................................

Na,O .....................................

KoO.... ..................................

S. ..............................:.........

BaO......................................

SrO ......................................

LijjO. .....................................

U.20 + .................. ..................

H20-110° ...............................

I.

76.81

10.57

0.00

3.74

0.13

0.05

0.32

3.42

5.30

0.00

0.25

0.00

100. 59

II.

74.52

10.07

3.74

2.81

0.20

0.01

0.86

3.88

3.46

0.00

0.86

0.07

100. 48

III.

77 V\

10.89

1.98

2.82

O i<*

0.09

0.70

4.24

3.38

0.00

0.26

. 0.05

101.89

IV.

71 m

11.23

2.53

3.66

0 1Q

0.02

0.75

5.56

3.66

0.05

0.06

0.01

0.00

0.55

0.00

101.28

V.

7<? Q3

12.29

2.91

1.55

0.04

0.31

4.66

4.63

0. 41

0. 18

100.91

VI.

77.08

12 54

0.00

0.95

0.01

0.75

3 64

4.99

99.96

I. Microgranite. East Greenwich, R. I. From the quarry at the northwest 

corner of the village.

II. Graphic microgranite. East Greenwich, R. I. From the spring locality south 

of Spencer Hill, described on page 60.

III. Blue-quartz porphyry. East Greenwich, R. I. From a point a half mile 

southeast of the apex of Spencer Hill.

IV. Riebeckite porphyry. From top of hill 1 mile northeast of Sneech Pond, 

Cumberland River.

V. Riebeckite-bearing granite from Hardwick quarry, Quincy, Mass. Presented 

for comparison.

VI. Milford granite. (For comparison, see p. 45.)

Analysis I to IV, by J. H. Perry; analysis V, by Mr. H. S. Washington (Am. 

Jour. ScL, 4th ser., vof. 6, p. 181, 1898); analysis VI, by L. P. Kinnicutt.

In the quantitative classification the three types are related as follows:

Petrographic relations of igneous rocks analyzed.

Rock.

Mierogranite. . ............

Blue-quartz porphyry. ....

Riebeckite porphvry . .....

Class.

I. Persalane..

I. Persalane..

I. Persalane..

II. Dosalane..

Order.

13. Columbare.........

Rang.

1. Liparase . . .

Subrang.

3. Liparose.

4. Pantellerose.

It is probable that the analysis of the microgranite is slightly incorrect, as all the slides cut from this rock show a little magnetite. They 

come near the line between orders 3 and 4, but within order 3.

The slight increase in the calcium just throws the graphic microgranite into the second rang. The considerable increment of sodium 

the rock receives in passing from microgranite to porphyry is indicated by the subrangs. The above table shows the general order of 

the appearance of the rocks, the blue-quartz porphyry being the later. 

We may assume that the similar blue-quartz porphyry in the Cumberland area, which contains soda orthoclase and a little riebeckite, may 

carry even more sodium.
RESUME. 67

All these types come in the category of alkali granite, as does also 

the Milford granite. Of the four series present in the Essex County 

basin only the granito-dioritic and the related series of dike rocks 

occur in the region here considered. The foyaitic series and its dike 

rocks we absent, although the riebeckite porphyry has a foyaitic 

aspect."

RESUME SHOWING- RELATION OF THE PORPHYRIES TO ONE 

ANOTHER AND TO THE. CARBONIFEROUS SEDIMENTARY ROCKS.

We may imagine the complex: of intrusive rocks here described to 

have intruded into the Cambrian schists^ in an early stage of the Carboniferous period, as a laccolithic mass whicht before solidifying, was 

slightly differentiated along its northern side intoa.more basic border, 

represented by the black gneiss of Bald Hill, which occurs along the 

only portion of the boundary that is exposed.

A broad, continuous band of the granite adjoins this narrow border 

on the south. The rock of this band seems to have solidified mainly 

a little later than that of the border, which it seems to underlie, and 

to have formed the nucleus of the stock. Still farther south is the 

extensive microgranite and graphic microgranite complex, which 

seems also to be a little older than the granite.porphyry and to have 

mantled over it around the southern side, as the dark granite did 

around the northern. Indeed, *a little of the microgranite is found on 

the north side where the dark gneiss joins the granite porphyr}', and 

this occurrence increases the probability that the two former rocks 

are portions of a common mantle over the granite.

Only a part of the stock is exposed and of this the outcrops at critical points are so poor that no very clear evidence could be gathered 

concerning the exact relations of the three rock types, either as to 

time or superposition. They seem to blend by sharp transitions into 

one another and to be three very similar facies of a common stock, 

since they have a microgranitic groundmass and many other peculiarities in common and are closely allied chemically.

In the interesting central area the graphic microgranite is probabty 

a variant of the granular microgranite formed somewhat below the 

present surface, and since the chemical constitution of the .two is so 

nearly the same their difference of texture was caused by some small 

but unknown difference of physical conditions. This graphic microgranite has been brecciated and has been penetrated by a portion of 

the granite porphyry, and a certain stratiform structure has been 

given to the whole mass of the resulting breccia a structure showing 

northerly dips, as represented upon the map.

The breccia contains more graphic microgranite in its southern por-

<i Washington, H. S., The petrogruphical province of Essex County, Mass.: Jour. Geol., vol. 7, 1899, 

p. 469.
68 SCHISTS AND ASSOCIATED ROCKS OF RHODE ISLAND.

tion and more granite porphyry in its northern portion where it 

adjoins the granitoid rock. The breccia preserves the record of several stages: (1) The formation of a fine massive graphic microgranite, 

(2). a partial breaking up of the rock, by which it was involved in a 

second microgranite or micrographic granite, which shows flow structure by distinct banding, and (3) the blending of great angular pieces 

of these two very fine-grained rocks and of granite with many small 

and very small fragments of the same rocks which seem (4) to have 

been carried along and to have been cemented by a small quantity of 

the granite porplryry. There is (5) indication of a blending of the 

breccia upward with the ordinary Carboniferous conglomerate. The 

fragments have been transported very slightly or not at all and still 

fit together and at places where the porphyry could not penetrate 

are cemented'by films of biotite and magnetite brought in by heated 

solutions. This interstitial porphyiy is much more irregular in 

grain than the normal porphyiy outside, and is somewhat crushed 

and filled with trains of magnetite grains. There is no scoriaceous 

structure which would indicate a surface flow, and the rock was 

probabty a somewhat deep-seated deposit of the intruding porphyry. 

The great quantity of angular and fresh fragments of the microgranite and granite porphyry that occur in the adjacent Carboniferous conglomerate suggest the idea that they are. the result of 

an eruption of tutfaceous material rather than a result of slow erosion 

of the surface of the laccolith. The identity, under the microscope, of material taken from the fresh rock in the microgranite 

ledges and from small fragments in the Carboniferous conglomerate 

serves to confirm this suggestion, as does the fact that the later rnetamorphism of this conglomerate did not form aluminous silicates 

(chiastolite, fibrolite, andalusite), the common products of metamorphism of argillaceous Carboniferous rocks farther north, but formed 

sericite, biotite, and chloritoid; that is, if the igneous material of the 

breccia had been obtained by erosion it would have been largely altered 

and kaolinized and would have been largely metamorphosed into the 

aluminous silicates mentioned above, butas it consisted of fresh fragments of the microgranite it altered into the alkaline silicates named. 

The presence of chloritoid would be due to the abundant presence of 

iron, indicated by the abundance of secondary magnetite.

Under the postulates of piezocrystallization the presence of these 

minerals may be taken to indicate a somewhat deep-seated crystallization, though without invalidating the former conclusion. The microgranite mass and the dark border bed seem to have been first solidified 

against superincumbent rocks, which favored a slight basic differentiation on the north and the development of a fine-grained facies on 

the south. The granite solidified a little later, and lower. Later 

microgranite dikes penetrated the granite and the dark border granite.
RESUME. 69

An explosive eruption seems to have blown off the capping of microgranite and graphic microgranite, furnishing the great mass of microgranite and porphjiltic granite fragments which find place in the 

adjacent Carboniferous conglomerate, and permitting the blending of 

the two seen at the schoolhouse west of Spencer Hill. That portion 

of the magma which ran in this conduit consolidated as a porphyry, 

because its quartz and feldspar phenocrysts had already solidified 

below, and on the sudden transference of the mass to a higher level 

the remainder solidified as an exceptionally fine-grained groundmass. 

The one part of this porphyry cemented shattered fragments .of the 

graphic microgranite; the other filled the remainder of the chimney 

forming the central mass of porphyry.

The existence of the Wamsutta volcanoes of this age in the adjacent 

territory studied by Mr. -Wood worth and the conclusion of Mr. 

Washington that the Essex County complex is laccolithic and not deep 

magmatic favor the conclusion that the consolidation of the mass may 

have been so superficial that a last explosion may have broken through 

the roof and supplied the fragmental material for the conglomerate.

The next step would be the further spreading of the Carboniferous 

conglomerates over the area, and the wide distribution of fragments of 

the granites and microgranites as bowlders in these conglomerates. 

Then came a metamorphism of the latter, changing their paste into a 

coarse mucovite schist, developing therein garnet, biotite, and chloritoid, and carrying iron in considerable quantity into the rock, where 

it is developed as magnetite in the paste and penetrated the pebbles of 

the microgranite and the feldspar of the microclinite, producing elaborate dendritic growths of the black oxide. In this connection we 

may recall the fact that scales of biotite are scattered through the 

inner portions of the fragments of graphic microgranite and are absent 

from their borders, where they come in contact with the quartzporphyry, the condition indicating that they were resorbed by the 

caustic effect of the inclosing rock.

Continued stud}7 of the problem, however, has weakened each of the 

lines of evidence relied upon to prove the early Carboniferous age of 

.the East Greenwich group:

(1) Professor Jaggar has collected much evidence to show that the 

Quinc^y hornblende granite differs in age from the biotite granite of 

the Boston basin, with which the East Greenwich group is more nearly 

allied.

(2) The recent discussions by Barrell,® Mansfield, 6 and others on the 

continental transport of unaltered feldspathic material in semiarid 

regions suggest another explanation of the fresh granite pebbles in the 

conglomerate.

o Barrell, J., Studies for students: .lour. Geol.. vol. 14, pp. 810-356, 430-457, 524-508, 1906. 

I'O. R. Mansfield, Roxbury conglomerate: Bull. Mus. Com p. Geol., vol. 49, 1906, p. 91.
70 SCHISTS AND ASSOCIATED EOCKS OF EHODE ISLAND.

(3) A further study of the schoolhouse locality mentioned above has 

shown that, while the transition from the breccia to the conglomerate 

seems complete, a break may be concealed by the considerable metamorphic change.

(4) The succession of events becomes simpler if we assume that the 

porphyries and microgranites of the series formed the surface of a 

rather thinly covered batholith, which was just exposed by erosion in 

early Carboniferous time.

Gould Island, east of Couanicut, would seem, from Mr. Foerste's 

description, to be made up of a graphic granite breccia like the one 

here described."

IGNEOUS ROCKS OF POST-CARBONIFEROUS AGE.

Olivine-diabase porphyry. In a quarry by the roadside, 1 mile 

southwest of Olney Pond, near the south line of Lincoln, is-a dike 1£ 

to 2 feet wide, running N. 50° E. and dipping 60° NE. It indurates 

the granite, and is more compact at its border. It is a fine-drained, 

fresh, black trap, minutely porphyritic, with feldspar needles and 

olivine crystals^ and contains quartz grains,' probably derived from 

the granite. The large feldspars are labradorite, with extinction 

angles of 23i° on (010). The smaller feldspars are anorthite with 

maximum extinction 34° in the albite zone. The olivine phenocrysts 

are half serpentinized; otherwise the rock is very fresh. Granular pyroxene and feathery magnetite make the ground in the coarser 

center of the dike; an aphanitic ground of glass full of black oxide 

makes the ground on the border and incloses porphyritic labradorite 

and olivine exactly like those in the center of the dike, showing that 

these had floated there after first consolidation elsewhere.

In the deep limestone quarry north of the road and nearest Lirne 

Rock are two dikes, 2 and 2^ feet thick, which run N. 50° W., and 

dip 60° NE. The trap dikes are echeloned by several slight displacements, while the limestone that incloses them, as noted by 

Woodworth, shows no different texture here to indicate the motion that has taken place. Many contact pieces obtained directly 

underneath the limekilns show no visible influence of the trap on 

the limestone, and in the slide the only endomorphic effect was a 

darkening of the ground of the trap for one^eighth inch. These 

dikes branch from a single one at the head of the quarry. The trap 

is the same jet-black rock as the trap at Lincoln described above, not 

quite so coarse at the center, but quite the same at the border. The 

large feldspars are labradorite, beautifully twinned on the albite and 

pericline laws. The olivine shows peculiar alteration. The olivine 

network does not appear. The few fissures and the surface are black 

with secondary magnetite. The whole mass is changed into a fibrous

"Geology of the Narragansett basin: Mon. U. S. Geol. Survey, vol. 33, 1899, p. 272.
POST-CARBONIFEROUS IGNEOUS ROOKS. 7l

mass of talc needles radiating from many centers. At some places a 

later agent has changed the central portion of the olivines into an 

oil-green serpentine. At others the large feldspars, though generally 

quite fresh, are changed within sharply defined fields, or within segments of a single crystal, into a fibrous mass of a white micaceous 

mineral.

A similar dike, or vein, 1£ inches wide, has just been exposed 

beneath the buildings at the quarry. It is fused with the limestone, 

and portions taken from its black pitchstonelike mass, at places 

where it is penetrated by the pure white limestone, form beautiful 

cabinet specimens. This dike has produced no metamorphic effect on 

the limestone.

Doctor Jackson" maps another dike east of the river opposite the 

Dexter quarry.

Porphyritic diabase. One mile southeast of Spencer Hill, in Warwick, a rather coarse and exceptionally fresh diabase was found, which 

is of exactly the same type as the Holyoke trap. It has a porphyritic 

aspect, due to the presence of small groups of stout plagioclase plates 

of first consolidation, which are near bytownite in composition. 

The augite is amethystine, often twinned and mostly xenomorphici 

The rock contains rather large blebs of brightly polarizing devitrified 

glass, wherein an original perlitic structure is now marked by lines of 

black ore grains.

Basic eruptive rocks are rare in the region, especially in the Carboniferous area. Professor Wood worth maps a series of altered diabase dikes that extend from North Attleboro by Lanesville to Arnolds 

Mill, in the " Wamsutta group" of the Carboniferous. ''

Mr. Foerste mentions two minette dikes that cut the Carboniferous 

shales in the southern half of Conanicut Island. 0 The authors found 

dikes of coarse gabbroid diabase in Woonsocket.

nReport on the Geological and Agricultural Survey of the State of Rhode Island, 1840. 

feMon. U. S. Geol. Survey, vol. 33,1899, p. 152. 

fldem, p. 232.

INDEX.

Page. 

Actinolito quartzite, occurrence anil charac- '

tor of............................ 15

Agate, occurrence of........................ 56

Albion, rocks near.......................... 44,49

Albion schist member, occurrence and character of...................... 10,12-13

Aplitc, occurrence and character of......... 47-48

pyrite in................................ 48

Ashton schists, correlation of.............. 11

Attleboro bed, occurrence and'character of. 20 

Attleboro series, age of..................... 34

extent of................................ 35

Bald Hill, rocks of......................... 67

Barton, G. H., and Crosby, \V. 0., on geology of region................... 10

Basic eruptives, occurrence and character

of......'."..................... 48-51,71

Blackstone series, ago of.................... 34-35

correlation of........................... 11

minerals of............................. 37

occurrence of ............................ 33-30

Blue-quartz-microclinc porphyry, analysis

of............................... . 06

occurrence and description of........... CO-02

Breccia, conglomerate in................... 40-42

occurrence and diameter of...... <i4-85,(>7-68

Cairns, F. I., and Chester. A. II., on crocidolite.............................. 54

Calcite, figures showing..............:..... 23,24

Cambrian igneous rocks, occurrence and

character of.................... 44-45

Cambrian rocks, age relations of........... 33-37

occurrence and character of.......... 8,10-37

Carboniferous conglomerate, occurrence and

character of.................... 38-42

Carboniferous igneous rocks, chemical relations of......................... 65-07

occurrence and character of............ 51-70

Carboniferous rocks, metamorphosed, distribution of..................... 37-38

distribution of, map showing............ 35

occurrence and character of.......... 9,37-44

Carboniferous schist, occurrence and character of......................... 42

Carboniferous sedimentary rocks, boundary

of............................... 37-38

relations of porphyries and............ 07-70

Carboniferous shale, occurrence, and character of........................... 42

Cat's-eyeS) occurrence of.................... 20

Cement, granite porphyry, occurrence and

character of.................... 64-05

Chester, A.M., and Cairns, F. I., oncrocidolitc.............................. 54

Chlorite-actinolite schist, occurrence and

character of.................... 15

Chloritoid, occurrence of................... 42-44

Conanicut Island, dikes in.................. 71

Conglomerate, Chloritoid in................ 42-44

occurrence and character of... 38-44,04-05,67

Page. 

Copper, occurrence of.................... 10,18-19

Copper Mine Hill, rocks near............ 45,47,54

Cowesctt, rocks near...... i................ 42

Cranston, rocks near.................... 25-26,42

Cranston bed, occurrence and character of.. 25-26 

Crocidolite, occurrence and character of.... 54

Crosby, W. O., on geology of region....'.... 8

Crosby, W. O., and Barton, 0. 11., on geology of region................... 10

Cumberland dike, character of............. 21,52

Cumberland Hill, limestone at............. 18

limestone at, analysis of............... 17

rocks near............................ 13,15,48

Cumberland quartzite, correlation of...... 11

Cujnberlanclite, occurrence of............... 48

Dale, T. N.. on geology of region........... 10

Dana, J. D., on Rhode Island minerals..... 20

Davisville, rocks near...................... 37

Dexter limestone, analyses of.............. 17

occurrence and character of......... 10,24-25

Diabase, occurrence and character of...... 70-71

Diamond Hill, rocks near............ 51,55,50,57

Dikes, occurrence and character of:........ 44-45,

47-48,52-53,63-04 

Diorite, occurrence and character of........ 44-45

East Greenwich, rocks near................ 7,02

East Greenwich group, age of.............. 69

breccia of............................... 04-65

conglomerate in.................... 40-42

occurrence and character of............ 58-65

Emmons, Ebenezer, on geology of region... 8,33 

Epidote-chlorite schist,occurrence and character of......................... 14-15

Eruptive rocks, occurrence and character of. 7,

48-50,71 

Fairmount Farms, rocks at.-............... 58

Felsite, occurrence of....................... 50,57

Field work, extent of....................... 7

Foerste, A. F., on geology of region. 10,33,30,70,71 

Formations, table of....................... 8

Gabbro, occurrence and character of....... 48-49

Garnet rock, derivation of.................. 29

Geneva, rocks near......................... 38

Gneiss, occurrence of....................... 7

Gould Island, rocks of...................... 70

Grafton quartzite, occurrence and character

of.......................... 7,10,12-13

Granite, analyses of........................ CO

intrusions of............................ 7

occurrence and character of...... 45-47,51-71

Granite porphyry, inclusion in............. 21

occurrence and character of....... 52-53,59-00

Granitevilie, rocks at....................... 46

Graphic microgranite, analyses of.......... 00

occurrence and character ot............. 03-65

Green schists, nietarnorplusm of........... 29-31

occurrence and character of. .. :...... 13-14,30

varieties of............................. 14-15

Harris limestone, analyses of............... 17

occurrence and character of.......... 16,22-24

73
74 INDEX.

Page.

Hornblende granite, occurrence and character of......................... 55

Hornblende schist, occurrence and character

of............................... 15

Hunting Hill, rocks near................... 38

Igneous rocks, analyses of.................. 66

chemical relations of..................... 65-67

occurrence and character of............. 44-71

Ilvaite, nonoccurrence of................... 19-21

Iron, occurrence of...................... KM 4,1C

Jackson, C. T., on geology of region........ 8,

10,15,18-19,22,25,33,43,71 

Jagger, T. A., on Quincy granite........... 67

Knebelite, occurrence of.................... 20

Knightsville, rocks near......'.............. 38,40

Kunz, G. F., on Rhode Island minerals..... 20,56

Lime industry, condition of................ 16-17

Lime Islands, limestone of, analysis of..... 17

Lime Rock, limestone at................... 22-24

analysis of.............................. 18

rocks near.................. 13,15,16,44,47,70

Limestones, analyses of.................... 17-18

inclusion of, in granite porphyry....... 21

metamorphism of..................... 26-29

occurrence and character of.......... 16-26,36

Lonsdale, rocks near................. 14,25,47,53

Magnetite, figure showing.................. 61

occurrence of........................... 61

Manganese, occurrence of................... 18

Manton, metamorphism near.............. 33

Manville, rocks near........................ 12

Marlboro formation, occurrence and character of.................... 7,11,13-26

. Masonite, analysis of....................... 43

occurrence of........................... 42-44

Massachusetts, gneiss in.................... 7

Metamorpbic rocks of region............... 26-33

Metamorphism, occurrence of........... 26-33,69

Microcline porphyry, figure showing........ 61

occurrence and description of........... 60-62

Microclinite, occurrence and character of... 64 

Microgranite, analyses of................... 66

occurrence and character of.... 55-56,62-64,67

Milf ord, rocks in............................ 45

Milford granite, analysis Of................. 66

metamorphism by...................... 26-33

occurrence and character of............. 45-47

Minette dikes, occurrence of................ 71

Molybdenite, occurrence of................. 21

Moshasshuk River, rocks near.............. 53

Narragansett basin, rocks of............... 8-9

Narragansett quadrangle, work in......... 7

Natick, rocks near................. 12,37-40,42-43

Neutaconkanut Hill, rocks on.............. 25,29

North Providence, limestone at, analysis of. 17 

Northbridge gneiss, occurrence and character of........................ 7,9-10

Odinite, occurrence and character of........ 49-50

Olivine-diabase porphyry, occurrence and

character of.................... 70-71

Olney Pond, rocks near.................... 70

Page.

Palache, Charles, on Rhode Island minerals......................... 23-24,25

Phyllite, masonite in....................... 42-44

Pirsson, L. V., ongeology of region......... 10

Porphyritic diabase, occurrence and character of......................... 71

Porphyry, occurrence and character of..... 52-55

59-61,64,71 

relations of............................. 67-70

Providence, rocks near..................... 25,46

Providence quadrangle, work in............ 7

Quartz, feldspar in, figure showing......... 61

See also Vein quartz; Blue-quartz 

Quartzites, metamorphism of.............. 26,33

See also Actinolite quartzite. 

Quincy, rocks near......................... 54

Quincy granitic group, correlation of ...... 42

occurrence and character of............ 51-58

Rhode Island, lime industry in............. 16-17

limestone of, analyses of................ 17-18

rocks in................................. 7

Riebeckite porphyry, analysis of ........... 66

occurrence and character of............ 53-55

Robinson, Samuel, on Rhode Island minerals. ........................... 19

Saylesville, metamorphism at.............. 31-33

Schists, metamorphism of.................. 31-33

occurrence and character of......... 13-15,42

varieties of............................. 14-15

See also Green schists; Hornblende 

. schist; Epidote-chlorite schist; 

.Chlorite-actinolite schist. 

Serpentine, derivation of................... 28

Shaler, N. S., on geology of region........... 33

Shephard, C. U., on Rhode Island minerals. ......................:.... 19-20

Slates, metamorphism of................... 26

Smithfield, limestone at................. 22,24-25

limestone at, analyses of................ 17

Smithfield limestone, occurrence and character of...................... 1],16-26

Sneech Brook, rocks near................... 52

Sneech Pond, copper ore near.............. 18

rocks near.............................. 13,15

South rock. See Dexter limestone.

Spencer Hill, rocks near.............. 62,64,65,71

Steatite, derivation 'of..................... 28-29

Structure, description of.................... 11

Tremolite rock, derivation of............... 28

Vein quartz, occurrence and character of.... 56

Wamsutta group, correlation of............ 42,58

occurrence and character of............ 56-58

Washington, H. S., on Essex County complex. ............................ 69

White, T. G., on geology of region.......... 54

Wdodville, rocks near...................... 15,45

Woodworth, J. B., on geology of region.. 11,16,

19,25,33-35,38,48,50,

55,56,57,58,69,70,71

Woonsocket, rocks in....................... 26

Yenite, occurrence of....................... 20