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sediments at Tower Hill and elsewhere in that vicinity are
probably connected with the Sterling granite gneiss and
seem to indicate the post-Carboniferous age of the latter.

T Loughlin, G. F., this Journal, 29, 450-455, 1910.



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Hawkins — Notes on the Geology of Rhode Island. 443

It should be noted, however, that similar pegmatites also
cut the Sterling as freely as they do the sediments (op.
cit.). Such intrusions of both the granite and its border-
ing invaded rocks does not destroy the possibility that the
pegmatites may be a late phase following the intrusion
of the main portion of the Sterling batholith. The peg-
matites, however, seem quite free from gneissic shear
effects similar to those in the Sterling. Intrusion after
solidification of the Sterling batholith would naturally
appear in strongest development along its border,
especially if there were some faulting along that border,
as there may have been in the vicinity of Tower HilL
These pegmatite and granite dikes, as suggested by Pro-
fessor C. W. Brown, may well be more easily supposed to
be contemporaneous with the Westerly granite intrusion
(post-Sterling) than with the Sterling granite gneiss
itself ; then, too, a small dike of granite similar in type to
the Westerly granite, to which the writer's attention was
called by Professor Brown, intrudes the Carboniferous
sediments at Hamilton, just south of the point where the
trolley track intersects the highway. If actually con-
nected with the Westerly intrusive granite sills, this dike
would establish the post-Carboniferous age of the West-
erly granite. Additional and seemingly conclusive evi-
dence of the pre-Carboniferous age of the Sterling gran-
ite gneiss is furnished by the following field evidence : —
A few years ago the Natural Resources Survey of
Rhode Island (with which the writer was at that time
working in the field), in the course of work in the west-
ern part of the State, made careful records, under the
direction of the Superintendent, Professor Brown, of
dips and strikes of gneissic foliation of the various out-
crops of granite gneisses and associated rocks of the area.
In spite of scarcity of good outcrops, and the obscurity of
data caused by the extreme metamorphism, many good
readings were made and recorded. These the writer, in
the course of this present work, subsequently plotted, in
addition to other, personal, observations in the north-
western portion of the State. The Sterling granite
gneiss is a rock whose gneissic character is pronounced
and widespread, in Rhode Island as well as in eastern
Connecticut (a point not noted by Loughlin), giving it a
banded structure, which, whether due to flowage or
regional metamorphism, or both, is definitely connected



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444 Hawkins — Notes on the Geology of Rhode Island.

with a pitching geo-synclinal and -anticlinal structure of
Alpine proportions in southern and western Rhode Island
(see map, fig. 1). The largest and most clearly defined
fold in the granite, concerning whose anticlinal or syn-
clinal nature no conclusion can be reached, on account of
insufficient and ill-defined exposures, curves sharply
around the resistant mass of the Preston gabbro in the
adjacent part of Connecticut. The latter gabbro mass is
included in the granite gneiss ; hence the observed phe-
nomena might be attributed to actual flow of the granite
about the inclusion, or to later shear effects about the
resistant gabbro body during regional metamorphism.
The present writer favors the latter hypothesis because
the foliation in the granite gneiss seems to be connected
with the presence of feldspar phenocrysts and augen best
explained as secondary in origin due to such regional
deformation. If this latter supposition be true, then a
kind of deformation greatly different from and probably
vastly older than the folds of the Appalachians to the
westward is indicated. This deformation swings east-
ward toward the Narragansett Basin; it then turns
northward and extends in this direction through northern
Rhode Island and into Massachusetts, and northeastward
apparently to Boston Bay, forming a complicated series
of synclinal and anticlinal folds, one of the latter, after
deep erosion of the mountain mass in pre-Carboniferous
times, being filled with the Carboniferous sediments of
the Woonsocket Basin. It appears that the Carbon-
iferous sediments of the Narragansett Basin were sim-
ilarly laid down at this time in a more deeply and widely
eroded structural basin between the mass of deformed
granites just described and the very similarly deformed
granites of the eastern side of the Narragansett Basin.
Along the foliation caused by the deformation the West-
erly granite 8 found its way upward, the intrusion fol-
lowing an east-west line along the Connecticut and
Rhode Island shore and appearing probably in the small
north-south granite sill at Foster, and farther north, and
possibly in the vicinity of a small outcrop of a granite of
similar appearance on the Rhode Island-Massachusetts
line. The Westerly granite is massive and does not
share in the gneissic structure which is so typical of the

•For a new analysis of the Westerly granite, see Table ITT, analysis
no. 23.



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Hawkins — Notes on the Geology of Rhode Island. 445

Sterling and the Northbridge, and forms welded contacts
at times across the gneissic banding of the former. But
the field evidence shows clearly that the enormous pre-
Carbonif erous deformation, above described, affected the
Sterling, Northbridge, and Milford granite gneisses all
alike, together with the ancient quartzites and basic rocks
included in them. It is therefore indicated that all of
these granite gneisses are of pre-Carboniferous age.

Loughlin has sought to demonstrate the post-Car-
boniferous age of the Northbridge granite gneiss in
northeastern Connecticut, stating that it intrudes the Put-
nam gneiss, which he considers to be Carboniferous, 9
since it "has been traced northward into Massachusetts,
where it is represented by the Bolton gneiss. This rock
at Worcester, Mass., has been shown by Perry and
Emerson to lie conformably with quartzite and fos-
siliferous phyllite of known Carboniferous age, and is
regarded by them as Carbonif erous." But geological
conformity seems to have little value in this region as a
means of establishing identity of age. This is brought
forcibly to our attention by the discovery of Carbonifer-
ous sediments on the eastern slope of Woonsocket Hill,
in perfect apparent conformity with the finely exposed
white quartzite of Cambrian or possibly earlier age
which forms the backbone of the hill. Moreover, the
writer, as already stated, has found the Northbridge
granite gneiss in pre-Carboniferous relations along the
Woonsocket Basin, and has followed it from thence west-
ward through the northwestern part of Rhode Island into
Connecticut, throughout which area, though the outcrops
are widely scattered and the granite gneiss is modified in
different ways, it preserves its essential characteristics.
At a point a mile south of the hamlet of West Gloucester,
B. I., on the Putnam Pike, at the base of the southwestern
slope of a hill of Northbridge granite gneiss, the latter
may be seen as apparently well-defined sills in a quartz-
ite. This quartzite is the Plainfield quartz schist, 10 which
Bice and Gregory 11 regard as "only a prominent and

• Loughlin, G. F., Conn. State. Geol. and Nat. Hist. Surv., Bull. 13, 146-
148, 1910. This formation is mapped by Emerson (op. cit., 79) as gneiss
of "Age Undetermined. ' '

,0 Westboro ("Grafton") quartzite of Emerson, which he finds to lie
unconformably upon the Northbridije srranite gneiss.

u Rice, W. N., and Gregory, H. E., Conn. State Geol. & Nat. Hist. Surv.,
Bull. 6, 134, 1906.



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446 Hawkins — Notes on the Geology of Rhode Island.

clearly marked variation of the Putnam formation ;"
which invades Rhode Island from the west in a semicircu-
lar area not more than a mile long and half a mile wide.
"All igneous rocks of the district occur as intrusions in
the Putnam gneiss, and this formation is therefore older
than the various dikes, sheets, pegmatite veins, and igne-
ous masses found associated with it" (op. cit.). "The
abundance of sheets of Sterling granite gneiss intruded
into the Putnam formation" is especially emphasized;
the same relationships obtain at Preston, Connecticut,
farther south. The Putnam gneiss and associated quartz-
ites of the Plainfield series are hence older than the Ster-
ling and Northbridge members which here compose the
great granite batholith by which they are surrounded;
and for this reason they must be pre-Carboniferous in
age ; in fact the writer believes that this whole sedimen-
tary series, including quartzite and gneiss, belongs among
the rocks of vastly more ancient origin (which may be of
Cambrian age or may possibly belong to a still older
series like the GrenvUle, 12 but whose age, in the
absence of fossils or other conclusive evidence, remains
unknown), represented by the quartzites, limestones,
£reen schists, and gabbros (the Blackstone series), pres-
ently to be described. Through intense metamorphism
the basic rocks and the Carboniferous sediments have
locally been altered so as to approach each other in
appearance and mineral composition; by intrusion the
Carboniferous shales of Wakefield, R. L, have been
changed into hornblende schists, and by shearing in the
Woonsocket Basin they have become mica schists ; fun-
damental changes of this kind are such as to render
correlation difficult by lithologic characteristics alone.
Thus, for instance, there is little evidence to show
whether the dark inclusions in the Westerly granite are
Carboniferous in age or much older; (the writer, on
account of facts observed in the field, favors the
latter opinion).

A part of the southern portion of Conanicut Island is
underlain by a granite which bears a strong resemblance
to the Sterling granite gneiss to the west 13 ; phenocrysts
derived from it are found in the adjacent Carboniferous

13 All classified together as " Algonkian ! ' f by Emerson (idem).
"Loughlin, G. F., U. S. Geol. Surv., Bull. 492, 134, 1912.



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Hawkins — Notes on the Geology of Rhode Island. 447

arkose, 14 thus proving it to be "pre-Carboniferous." 16
Similar granites are found at Mount Hope, in Bristol,
and at Common Fence Point, on the northern end of
Aquidneck Island.

At three points along the southern Rhode Island coast
there are exposures of granite of a different and perhaps
later type. A very coarse reddish biotite granite with
white feldspars underlies Sakonnet Point. It shows
spots of dioritic appearance, inclusions, or due to differ-
entiation. It cuts chlorite schist and is cut by aplite and
minette. It becomes more gneissic eastward. Contact
relations with surrounding formations are not exposed.
A granite of closely similar appearance is extensively
exposed at Newport. 16 Again relations are concealed.
A similar state of affairs exists at the headland of Quo-
nochontaug, which is formed of a coarse red biotite
granite, cut by heavy pinkish pegmatite dikes.

Emerson and Perry 17 favor "the early Carboniferous
age" of the East Greenwich granite group ; yet "there is
indication of a blending of the (granite porphyry)
breccia upward with the ordinary Carboniferous con-
glomerate." This is taken to "suggest the idea that
they are the result of an eruption of tuffaceous material
rather than the result of slow erosion on the surface of
the laccolith," the fragments seeming "to have been car-
ried along and to have been cemented by a small quantity
of the granite porphyry." However, "the recent dis-
cussions of Barrell, Mansfield, and others on the conti-
nental transportation of unaltered feldspathic material
in semiarid regions suggests another explanation of
the fresh granite pebbles in the conglomerate" (op.
cit.). "The conclusive evidence seems to be lacking"
(Foerste) 18 ; but "the succession of events becomes
simpler if we assume that the porphyries and micro-
granites of the series formed the surface of a rather
thinly covered batholith, which was just exposed by
erosion in early Carboniferous time." 17 The present
writer agrees with the latter statements. The contact
of the East Greenwich granite group with the adjacent

"Shaler, Woodworth, and Foerste, op. cit., 233.

M Emerson and. Perry, op. cit., 46.

1€ Shaler, Woodworth, and Foerste, op. cit., 316.

" Op. cit, 69.

u Shaler, Woodworth, and Foerste, op. cit.



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448 Hawkins — Notes on the Geology of Rhode Island.

"pre-Cambrian" Northbridge gneiss is mapped by
Emerson and Perry as an intrusive one, the East Green-
wich granites being the younger. Relations with the
Milford granite gneiss are not discussed by them. The
field relations are obscure, but dikes of aplitic and gran-
itic material which cut the granite gneisses in the vicinity
of Riverpoint and Arctic might perhaps be referred to
the East Greenwich group.

Felsites are not prominent in this State, and yet small
occurrences are present in at least two places. In the
adjacent area of South Attleboro, Massachusetts, they
are extensively developed, being associated with Carbon-
iferous sediments and diabase. At Diamond Hill, Cum-
berland, R. I., a considerable mass of dense felsite has
been largely replaced by vein quartz, 19 but fragments of
it are still plainly visible. There is also present on the
crest of this quartz mass a small area of unknown extent
underlain by a coarse biotite granite similar in appear-
ance to that exposed in the quarries a mile farther west ;
relations of granite and vein quartz are not exposed.
Again on the northwest slope of Bald Hill, in Scituate, in
local drift, are found narrow red felsite dikes cutting a
granite gneiss. This latter occurrence extends the zone
of rhyolite occurrences in Rhode Island southward a
little farther toward the far-away field of similar rocks
at South Mountain, Pennsylvania.

A very well-defined zone extending from Wakefield
through Newport to Sakonnet Point is occupied by pre-
Carboniferous green schist, quartzite, limestone, and
granite, cut by prominent intrusions of pegmatite,
granite, diabase, gabbro, and minette, and bordered on
the north and possibly also on the south by sediments of
Carboniferous age. The writer suggests that this zone
of ancient rocks may represent an elevated block or
horst, separated by fault zones of nearly east-west strike
(probably pre-Carboniferous in age) from adjacent
blocks which have fallen away on the north and south.
Within this zone of disturbance, at various times, the
numerous intrusions have found their way upward. It
is in line with the Westerly granites and pegmatites, also
parallel to the coast line.

19 Warren, C. H., and Powers, S., Bull. Geol. Soc. Am., 25, 472, 1914.



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Hawkins — Notes on the Geology of Rhode Island. 449

Basic Igneous Bocks.

Of basic igneous rocks there are three distinct types in
the region herein studied, viz: Green Schist, Diabase,
and Gabbro.

(1). The Green Schist.

This occurs at several localities within the area under
consideration, namely, in the Blackstone Valley, north-
west of Providence, and in Johnston and Cranston, west
of Providence, where it is associated with white quartz-
ite, as already mentioned; in similar relations at
Premisy Hill, a small eminence 3 miles west-southwest
of 'Woonsocket; and in several scattered exposures
between West Greenville and Primrose. In the Black-
stone Valley it has been mapped by Emerson and
Perry (op. cit.) and assigned to the Cambrian (Marl-
boro formation). Other writers of earlier (Shaler, et
al., op. cit.) and later date (Warren and Powers, op. cit.)
would place it in the pre-Cambrian. (Compare also
Emerson, op. cit.) On account of lack of evidence,
determinations of the age of the green schist-quartzite
series have been made from lithologic similarity of these
rocks to others not visibly connected with them but of
known age (the Cheshire and Westboro ("Grafton")
quartzite; compare views of Emerson and Perry (op.
cit.) with those of Warren and Powers (op. cit.)).
("Algonkian?" of Emerson). There are several local-
ities in the southern part of the State where the occur-
rence of green schists or slaty rocks has repeatedly
attracted attention. They have been enumerated by
Foerste (op. cit.) as follows:

At Church's Cove, west of Tiverton Four Corners, and
southward to Little Compton, a series of greenish shales
is exposed, associated with rusty limestones intersected
by numerous quartz veins. Green schists and slates on
Sachuest Point are described, also those on Conanicut
Island south of Jamestown, in the vicinity of the
Dumplings, and those at Newport. At least two of these
localities show the greenish rocks only in the most uncer-
tain relations with the known Carboniferous sediments
and the other adjacent rocks, so that little is known of
their age, except that they are probably for the most part
pre-Carboniferous. The altered dioritic " dikes" of
" Paradise," near Newport, and a hornblende and

Am. Jour. Sci.— Fourth Sehibs, Vol. XLVI, No. 272.— August, 1918.
16



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450 Hawkins — Notes on the Geology of Rhode Island.

actinolite schist along Sin and Flesh Brook, southeast of
Tiverton, are apparently of similar age.

In the extreme northern part of the State, at Round
Top, a fissile, closely plicated muscovite schist is asso-
ciated with quartzite. The schist carries chlorite, and
also garnets of small size and rarely black tourmaline.
It is probably also closely related to the green schist
series, being mapped as part of the Westboro quartzite
by Emerson.

The green schists are quite generally gabbroid in com-
position (see analyses, Table II), and in appearance and
behavior, though badly sheared, suggest "the derivation
of some part of the material from basic tuffs' 9 (Emer-
son and Perry, op. cit.), the tuffs being interbedded with
the quartzites. Interlayering of the two kinds of rock at
the contacts also affords for these authors additional evi-
dence of sedimentary origin for the schist. There are
a few beds of the green schist in the quartzite, however,
which seem to actually represent intrusive sills in lit-
par-lit injection. One of these appears as a breccia of
quartzite fragments in green schist matrix at Violet Hill,
Manton (underneath the perched granite bowlder on the
hill just north of the Manton Avenue quarry) ; and thin,
ramifying, sill-like layers of schist in a quartzite at a
small exposure southeast of Oak Valley, in North Smith-
field. The appearance of large, isolated masses of
quartzite, too large and isolated to represent conglom-
eratic bowlders, seems also to point toward actual
intrusion.

The schists in their coarser phases exhibit a mass of
long green actinolite blades intermingled sometimes with
areas of granular or saccharoidal feldspar. An extreme
case of the production of secondary feldspars of this kind
is in the rock exposed along the highway at West Green-
ville, where it is filled with closely set pseudo-phenocrysts
of a saccharoidal feldspar near albite in composition,
lath-shaped, some attaining a size of 1 cm. X 3 cm., and
showing roughly parallel arrangement simulating flow
structure. About two miles north of this latter place
the green schist grades into a true gabbro. In contrast
to this, the gabbro at Woonsocket (Huntington Avenue)
contains inclusions of green schist surrounded by reac-
tion rims.



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Hawkins — Notes on the Geology of Rhode Island. ±51



SiO, .
A1A
Fe a O a
PeO .
MgO
CaO .
Na,0
K*0 .
H,0 .
CO, .
MnO
S ...
TiO, .



Sp. Gr.





Table II.








f Basic Igneous Bocks from Bhode Island.




(A.)


Green Schists.






11


12


13


14


15


6658


4360


5087


46-39


4162


1304


26-90


19-47


18-32


1316


708


3-20


429


644


671


1-40


8-60


788


682


4-48


101


251


285


4-66


8-88


276


10-83


8 99


12-58


1771


526


175


221


198


142


344


2-52


2-57


131


108


16


•64


73


•73


72


Tr


"*07


75


43
Tr


476







10


12
25


14


10073


10062


10071


10003


10068


2844












11.

12.
13.

14.
15.



Man ton, B. I. (Unaffected by granitic intrusion). A. C. Hawkins.
Man ton, B. I. (Largely affected by intrusion). A. F. Buddington.
Neutaconkanut Hill, Thornton, B. I. (Slightly affected). A. 0. Haw-
kins.

Berkeley, B. I. (Unaffected by granitic intrusion). A. G. Hawkins.
Berkeley, B. I. (Somewhat affected by intrusion). A. 0. Hawkins.
Local drift; porphyritic augites.



SiO, ..
A1A
FeA
FeO ..
MgO ,
CaO ..
Na,0 .
K,0 ..
H s O+
H t O—
Ti0 2 .
CO, ..
PA .
S ....
MnO .



Table II.

Analyses of Basic Igneous Bocks from Bhode Island.

(B.) Gabbros and Belated Types.



22-35

526

1405

28-84

1610

117

•44

10

"-42
1011
02
•02
38
43



Sp. Gr. .. 3-92
* PA by A. C. H.



2


3


4


5


6


7


8


9


10


46-48


50-48


4957


4436


4433


48-43


51-86


4611


39-30


15 69


21-26


1240


1815


20-58


1576


1572


14-53


1558


710


205


358


5 46


129


192


....


220


2-65


8-66


6 86


704


1162


11-39


768


1487


451


4-62


274


215


5-76


412


479


6-34


550


5-73


400


11-42


7-23


1586


1019


740


1019


720


7-82


13-49


214


303


213


265


2-94


2-22




129


1-27


•70


2-07


229


191


•37


1-48


• • • •


3-84


403


•20


•83


•48


20


•08


•56


• • . •


2-90


2-28


03


•40


•02


12


12


04




....


•37


3-80


200


Tr


105


500


3-00


2-75


•84


15


-28


95


26


64


•68


234


....


732


721


11


98


•16


Tr


•17


Tr


•73


•22*


514


80


•08


•77


Tr


60


26


•06


1-37


•39


Tr


Tr


Tr


Tr


Tr


Tr


115


Tr


Tr


.00-15


100-53


100-32


10047


9994


100-26


10040


9965
2904


10025


3036


2928


2970


3112


3-203


2962


....


2-961



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452 Hawkins — Notes on the Geology of Rhode Island.

Norms of the Basic Igneous Bocks Analysed {Table II).





1


2


3


4


5


6


7


Q






579











Or


56


424


1258


1374


10-29


230


9-97


Ab


. 367


19-45


25-82


18-32


20-35


24-90


794


An


. 5-56


3439


3081


1801


29-50


32-93


33-25


Di




20-91






1624




668


Hy






i3-28












Ens














1764


01


. 45-70


809




1976


i323


1550


914


Mt


. 20-65




265


5-24


738


186


314


n


. 1900


775


380




171


9-50


6-26


Pr


, . 1 15






255








Ap






202











Ak




4-50





2i-76









Cor






325










Gar













li-50





Spinel . . .


355














Calcite . . .




•67




62


130


iii


5-98



Classification of the Basic Igneous Bocks Analyzed {Table II).

1. Peridotite, rhodose, V.3.1.2. Iron Mine Hill, Cumberland.* C. H.
Warren.

2. Gabbro, auvergnose, III.5.4.3. Ironstone Beservoir, Mass.t A. C.
Hawkins.

3. Gabbro (Hybrid), shoshonose, II.5.3.3. Woonsocket, R. I. A. C.
Hawkins.

4. Gabbro, oronose-auvergnose, III.5.4.2. Pascoag, R. I. A. C- Hawkins.

5. Gabbro (Aetinolite Schist), auvergnose, III.5.4.3. West Greenville,
R. I. A. C. Hawkins.

6. Gabbro, near auvergnose, III.5.4.5. Moosup Valley, R. I. A. C.
Hawkins.

7. Olivine Diabase, oronose-auvergnose, III.5.4.2. Snake Den, R. I. A.
C. Hawkins.

8. Diabase, South Attleboro, Mass. Chemist, Am. Steel and Wire Co.

9. Minette, Conanicut Island, R. I. J. P. Iddings.
10. Minette, Sakonnet Point, R. I. A. C. Hawkins.

The last four rocks were probably not fresh.
* Compare Emerson (op. cit., 183 and 185).
t Compare Emerson (idem, 170).

(2). The Diabases.

The development of diabases is strongest toward the
northern border of the State; western Rhode Island is
free from them. They show a tendency to follow prom-
inent joint-planes which mark lines of structural weak-
ness, arranged in several zones with a north-south or
northeast-southwest strike. One of these appears to be



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