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Causes for these changes in direction may be : (1) a pre-Carbon-
iferous hill-and-valley topography, forming the noor of the
Basin ; (2) the deformation of a more level pre-Carbonif erous
land surface ; or, (3) a system of post-Carboniferous faults. If
the first supposition were true, the Carboniferous sediments
should often abut against the pre-Carboniferous, and there
should be little dependency between strikes of the strata and
strikes of the surface of unconformity separating the Carbon-
iferous and the pre-Carboniferous. But such is not the case.
There is a remarkably close parallelism between the attitude of
this surface of unconformity and the attitudes of the adjacent
Basin sediments. Indeed, it is just what would be expected if
the Bason floor had been originally comparatively flat and had
later shared in the diastrophism of the overlying strata. Fur-
ther evidence for deformation of the basement, according to
Shaler, is to be seen in a certain amount of schistose structure
in the eastern and western border-rocks, which decreases in
intensity away from the Basin.;]:

The JSasin floor has been deformed not only by bending, but
also by faulting. This is indicated locally by exceptional
straightness of tlie rim, by apparent displacement of beds or of
groups of beds, and by zones of fault brecciation. Whether all
of this fracturing is of the normal, or tension, type, or whether
some of it is of tne reversed, or compression, type, could not be
determined. Certainly many of the faults are normal.

Summarizing, we infer (1) that the original floor of the
Basin, which comes to the present topographic level in or near

*The strikes and dips have been more or less generalized for different
localities and have been plotted on the map (fig. 1).

f For these see Shaler, N. S., etc., op. oit., and Lahee, F. H., A stndy of
Metamorphism in the Carboniferoas Formation of the Narragansett Basin ;
thesis deposited in Gore Hall, Harvard University, in 1911.

t Shaler, N. S., op. cit., pp. 19-20.



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256 F. 11. Lahee — Metamorphiam and Geologioal StrvMui'e.

the pre-Carboniferou8 bonlere, had a relatively flat, or at most
a gently undulatiDg, surface ; and (2) that this surface under-
went deformation, both by folding and by faulting, in com-
pany with the superjacent strata.

The Basin Strata. — In drawing conclusions with respect
to the structural relations of the Carboniferous strata, consid-
erable latitude of interpretation is inevitable. Among other
reasons this is partly because plant remains, although common,
have not yet been proved to be of value in this region as indices
of stratigraj)hic horizon, and furthermore, because the entire
series of shales, sandstones, and conglomerates, of the Basin,
while characterized by numerous textural variations, is so sim-
ilar throughout that mineral composition and lithologic struc-
ture are almost worthless for correlation. In general, however,
a broad sequence has been made out, passing upward from
basal conglomerates and arkoses (Pondville arkose, Natick con-
glomerate, etc.), through a great thickness of conglomerates,
sandstones, and shales with some coal seams (Kingstown and
Aquidneck series), to an overlying coarse conglomerate (Digh-
ton and Purgatory conglomerates).

The deformation of these sediments has produced folds of
various sizes and shapes. We shall make a distinction between
moQor folds, which are sufficiently large and important to be
represented in an ordinary generalized vertical section, and
minor folds, which would usually be omitted from such a
section. By contortion we mean complex minor deformation
in which the strata are bent into closed or overturned folds, or
are otherwise severely compressed. Upon this arbitrary classi-
fication we shall base the succeeding description.

J/a;wyi?Z<5?m^.— Evidences for variations in the major folds
may be observed in such factors as the direction of strike, the
degree of dip, the direction and amount of dip of axial planes,
and the direction and amount of pitch, if there is a pitch.

Strikes — The greatest regularity of strikes (between 5° and
20° E. of N.) occure in the Kingstown formation, (1) in the
western coast belt — between the western border and the west-
ern coast of Narragansett Bay — on Boston Neck (B : 15)* and
northward to Barber's Height (B : 12), and (2) in northern
Conanicut Island (D : 10-13). That is to say, great uniformity
is found only in the southwestern portion of the Basin where
dips, as a rule, are rather steep. Such parallelism signifies that
the maximum component of force was here practically supreme.

Somewhat less uniformity of direction is shown between
Hamilton (Loc. 8, B:ll) and East Greenwich in the western
coast belt, and also on Prudence Island.

* The map (fig. 1) is coordinated by letters and fignres along the margins.
Some localities 81*6 numbered. These will be indicated as follows in the
text : Loc. 10, C : t3, i. e., Locality 10 in coordinate square, C : 18.



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F, H. Lahee — Metaniorphism and Geological Sbmoiure, 257

Districts in which the strikes are conspicuously variable are
as follows : Northwest of Watchemoket Cove (D : 1), Ponham
Rock-Riverside area (D and E : 2), southeastern Cranston (A :4),
Warwick Neck, Bristol Neck, Hope Island, Potter's Cove on
Conanicut Island (Loc. 17, E : 13), eastern coast of Mackerel
Cove (Loc. 21, D : 14), Beaver Tail Peninsula (Loc. 23, D : 14),
Coaster's Harbor Island (Loc. 29, F : 13), Sheep Point district
(Loc. 42, F:15), area north and northeast of Warren Neck
(H and 1 : 3 and 4), Brayton Point (Loc. 49, J : 5), and the east-
ern coast belt.

Strikes of the western coast belt, then, are fairly regular,
although with some rather abrupt changes in direction. On
the contrary, strikes of the eastern border region are very irreg-
ular and, except along the coast north of Stone Bridge (Loc. 4y,
1 : 8), have no particular relation to the eastern cage of the
Basin. The lack of system here is probably due in part to
faulting.

Dips. — Without entering into detail, we may say that steep
dips predominate in the southern portion of the area, and low
dips in the northern. Many of the actual readings are recorded
on the map.

Pitch, — A definite northward pitch is indicated in the fol-
lowing places : Warwick Neck, Rumstick Neck, Hope Island,
southeast coast of Prudence Island, Gould Island, southern
Swansea, and High Hill Point (Loc. 45, 1 : 11-12). Southward
pitch was recorded near Silver Spring (Loc. 3, D-E : 2), half a
mile northeast of Riverside (E : 2), northeastern coast of Pru-
dence Island, north side of Butt's Hill (near Loc. 35, H : 9), one
mile west of Portsmouth Village (H : 9), Coddington Cove (Loc.
31, F : 12-13), Beacon Hill (Loc. 32, F : 13), probably north of
the Paradise tract (H : 14), and at Easton's Point.

Pitch is more commonly low than high. In general, it is
high where adjacent strikes and dips are variable {Swansea,
Coddington Cove, Warwick Neck), or where adjacent dips are
steep (Gould Island) ; and it is low in the broad folds (Pru-
dence Island, Easton's Point) ; but there are exceptions.

Axial planes,^ — Axial planes are vertical or dip either east-
ward or westward without regard to whether the fold is nearer
the eastern or western border of the Basin. That is to eav,
with reference to these border regions as comparatively rigid
beams through which the forces were applied against the sedi-
ments, both overthrusting and underthrusting were produced.

Continuity of the maior folds, — Because of these variations

in strike, dip, pitch, and symmetry, and especially because of

the wide water intervals between the land areas, the identity

of separate folds can rarely be discerned across many miles.

* See generalized sections, figs. 5 to 7.



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258 F, H. Lahee — Metamorphitm and Geological Structtire.

Almost without exception, however, the axes of the major folds
trend a little east of north, thus proving that the maximum
deforming forces acted along approximately east-west lines.

JRelative number of folds across the Basifi. — East- west sec-
tions across the Basin show a varying number of folds in differ-
ent latitudes. To illustrate this fact, sections along lines A-A,
B-B, and C-C (fig. 1) have been drawn, as nearly as possible
perpendicular to the strike (see figs. 5 to 7). By dividing the
number of major folds (both anticlines and synclines) inter-

Fias. 6, 8, 7.




SeaJ« in mil**.



FiQ. 5. Generalized vertical section along the line A-A in fig. 1.
Fio. 6. Generalized vertical section along the line B-B in fig. 1.
Fio. 7. Generalized vertical section along the line C-C in fig. 1.

sected by any line by the length of this line, we may estimate
the number of folds per unit of length of the given line, that
is, per unit of width of the Basin where the line is situated.
In each case land and water areas are traversed. Since the
determination of the folding is founded upon data obtained on
land, we have calculated the results not only upon the total
lengths of the lines, but also upon the sums of their land
portions. An important source oi error lies in the method of
interpreting the geological structure, and the opportunity for
mistake is greater in the north than in the soutli. To offset
such error we have chosen, in each case, that interpretation
which assumes the greatest reasonable number of folds. For
example, in the Cranston area, where the structure has been
explained by some as monoclinal and by others as consisting of



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F. R. Lahee — Metamorphiam and Geological Structure. 259

two synclines and an anticline, we have adopted the latter
hypothesis. Following are the results :

Line. Fold Length Nnmberof Sam of land Number of folds per
tra- of line folds per portions in ten miles jof

versed, in miles. ten miles. miles. land breadth.

A. 9 19 4-73 16 6-62

B. 13 16 8-12 8-75 14-85

C. 16 13-5 11-86 9 17-78

This table indicates that the number of principal folds per
unit of Basin width, and, therefore, the degree of compression,
regularly increases southward.

Minor folding. — Contortion of the strata was recorded at
the localities listed below :



Number of


Location on




locality*


map


Type of rock


1.


Di 1


Shale, sandstone, little conglomeratef


4.


D-E : 2-3


Chiefly sandstone and shale


6.


D:6


Shale


6.


D:7


Shale and sandstone


7.


C :7


Shale and fine sandstone


10.


C : 13-14


Shale and sandstone


11.


C :15


Shale and sandstone


12.


B : 16


Shale and sandstone (inclusions)
Shale and sandstone (inclusions)


13.


B : 16


14.


C-D : 13


Chiefly shale and sandstone


15.


D : 10-11


Chiefly shale


16.


E : 11


Chiefly shale


18.


E : 13


Shale


19.


D : 13


Shale


20.


C-D : 14


Shale and sandstone


21.


D : 14


Shale


22.


D :14


Shale


25.


D : 16


Shale


26.


D : 15


Shale


27.


E : 12


Chiefly shale


28.


E-F: 13


Shale and arkose


29.


F : 13


Shale, sandstone, and conglomerate


80.


F : 12-13


Shale and sandstone


33.


H:9


Chiefly shale


36.


H:ll


Chiefly shale


37.


H:12


Chiefly shale


38.


H:14


Shale and arkose


40.


G:14


Shale, sandstone, and conerlomerate


41.


F-G: 15


Shale


44.


1 : 12


Chiefly finer rocks


46.


J : 10


Chiefly sandstone and conglomerate


48.


I :8


Shale, sandstone, and conglomerate


49.


J : 5


Shale and fine sandstone


60.


G:6


Shale and sandstone



* These numbers are plotted on the map.

f For convenience the rocks are here spoken of as shaleSi sandstones, or
conglomerates, whether or not they have been metamorphosed.



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Google



F. II. Lahee — Metamorphism and OeologicaX Structure.

Localities 4, 16, 27, and 33 are near the axial regions of
major folds (all pinched anticlines) ; 10, 14, 15, 20, and 48 are
on the limbs of major folds ; and the othei-s cannot be surely
placed in this respect. Evidently, then, there is no hard and
fast rule for the location of contortion in the larger folds.

If the map be divided into four equal rectangles (by lines in
F and in 8 of the coordinate squares, tig. 1), three of tlie locali-
ties of contortion will be included in the northeastern area ;
five in the northwestern ; eleven in the southeastern ; and fif-
teen in the southwestern. That is, more contortion is found
southward and westward in the Basin.*

In five of the localities the contortion affects shale, sandstone,
and conglomerate, or sandstone and conglomerate ; in twenty-
nine it affects shale and sandstone — usually fine — or shale alone.
According to this, contortion is limited chiefly to the elastics of
finer texture (see p. 254).

Areal distribution ojf^ variations in the major and minor
folding. — Taking into consideration both dip and strike of
major and minor folding, we could show that, if we should
pass across certain regions in the Basin, the complexity of the
deformation would increase. Thus, there is evidence for an
increase in the complexity of folding,

(1) westward, in East Providence (D-E : 1-2) ;

(2) eastward and westward from the middle of the western
coast belt, north of East Greenwich ;

(3) southward, in Warwick Neck ;

(4) westward, in the western coast belt, between East Green-
wich and Wickford ;

(6) southward, in the western coast belt, south of East
Greenwich ;

(6) eastward, in Prudence Island ;

(7) westward, from Prudence Island to Hope Island ;

(8) eastward, in northern Conanicut Island ;

(9) westward, from southern Conanicut Island to the western
coast belt ;

(10) southward, on Aquidneck Island ;

(11) eastward, in northern Aquidneck Island ;

(12) eastward, from Aquidneck Island to the eastern coast
belt;

(13) westward, from middle Aquidneck Island to eastern
Prudence Island, Coddington Point, and Coaster's Harbor
Island; and

(14) southward, in Swansea.

Obviously there is not uniform increase in complexity from

* This relation is not dne essentiaUy to a greater number of ontcrops in
the sonthern district.



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F, H, Lahee — Metamorphism and Geological Stnccture. 261

the middle of the Basin to the walls. Such an increase does
occur, however, within three or four miles of the borders,
a phenomenon which might be explained by the greater
proximity of these rocks to the pre-Carboniferous mass which
transmitted, probably in part, the deforming thrust.

Apparently there are alternating, nearly north-south belts of
greater or less intensity, belts which cannot usually be traced
continuously. These belts are as follows : (1) along the west-
em border; (2) Warwick Neck, northern Narragansett Bay,
and Providence Eiver in the latitude of East Providence,
and the city of Providence; (3) eastern part of northern
Conanicut Island ; (4) Gould Island, (boaster's Harbor Island,
Coddington Point, and eastern Prudence Island; and, (5)
Sakonnet River, and eastern coast belt.

These facts refer to variations along east-west lines. As for
north-south directions, only southward intensification of the
deformation was noted, and this was in Swansea and, in gen-
eral, from the latitude of Prudence Island.

Conclusions, — From the preceding statements certain infer-
ences may be drawn.

(1.) The high dips of Carboniferous strata resting uncon-
formably upon the pre-Carboniferous border rocks, the frequent
parallelism of strikes of the Carboniferous sediments with the
trend of the border, and the diversity of overturn and under-
turn relations at the borders, as exhibited by the axial planes,
indicate that the forces which operated through the pre-Car-
boniferous, whatever their original character, must have been
multiple in value and in direction at nearly all places where
they encountered the Carboniferous.

(2.) Within the Basin strata these forces acted in all direc-
tions, but with much greater intensity along east-west lines
than along north-south lines.

(3.) In different parts of the Basin the deformation effected
by these forces varies in complexity according {a) to variations
in the direction and potency of the forces themselves ; (J) to the
texture (and therefore rigidity) of the rock affected ; and {c)
to vertical position in a given fold.

(4.) Variations in the deformation, due to variations in the
forces, are so distributed that {a) there is a marked increase in
the complexity of folding and in the amount of compression
from north to south ; (h) there are approximately north-south
alternating belts of more or less intense deformation ; and {c)
within a few miles of the border there is sometimes observable
an increase in intensity.

(5.) Variations in the deformation (only minor folding), due
to differences of texture, are important, but local. Finer rocks
are more highly contorted than coarser ones.



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262 F. H. Lahee — Meiamorphism and Geological Stmcture.

(6.) YariationB in the deformation, dne to vertical position in
the folds, are of little importance and are commonly local.
The folding appears to be of the parallel type except in rare
instaDces of very minnte crnmpling.

(7.) The factors upon which variations in the folding are
dependent, mentioned in order of lessening importance, are :

(a) relative position of outcrop along north-soath lines ;
(h) rock texture ;

(c) distance of outcrop from walls of Basin ;

(d) relative vertical position of outcrop in fold.

Cambridge, Mass., Jan. 6, 1912.

(To be con tinned.)



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Warren — IVmenite Rooks near St Urhain^ Quebec. 263



Abt. XXV. — The llmenite RocTm nea/r St. Urhain^ Quebeo ;
A New Occurrence of RuUle and SapphiHne; by Ohablbs
H. Wareen.

Introductory. — One of the notable occurrences of ilmenite,
mentioned in treatises on mineralogy, is that near Bay St.
Paul, a town located on the north snore of the St. Lawrence
Eiver, about sixty miles east of Quebec. This occurrence is
more accurately located as being just west of the little village
of St. Urbain in the parish of that name, which is located about
ten miles north of Bay St. Paul on the River Gouffre. In the
Geology of Canada, 1863, Dr. T. S. Hunt gives a brief descrip-
tion of this occurrence. He states that the ilmenite bodies are
" intercalated in the stratification " of the anorthosite rock in
which they occur. One bed, 90 feet thick, was traced for a
distance of 300 feet, and was reported continuous for over a
mile. Writing further, he states, '* it contains in many parts
orange-red transparent grains of pure titanic acid." The
density is given as from 4*56 to 4*66. A chemical analysis
gave : TiO, 48-60 ; Fe,0„ 10-42 ; FeO, 37-06 ; MgO, 3-6. Total,
99-68. Just what type of ore this analysis represents is not
stated, but assuming it to be a fairly correct analysis, the molec-
ular ratio derived from it indicates that it was made on rutile-
free material. The presence of the rutile appears to have
been practically forgotten, at least no other mention of it
occurs in the literature so far as the writer is aware. Its pres-
ence in the ilmenite was again noted in the summer of 1909
by Dr. W. R. Whitney of Schenectady, N. Y., while on a visit
to the locality, and it was through tne latter's interest in the
deposit that the writer had an opportunity of visiting the
locality in the spring of 1910. A representative collection of
material was made at that time with the expectation of later
using it for a thorough study of this unusual rock. A more
careful examination of the material in the laboratory sfiowed
it to be more unusual in character than was at first supposed,
but, unfortunately, it was also found that the material had
suffered so much from alteration that it has been thought best
to defer any exhaustive chemical study in the hope that fresher
material may eventually be obtained, when it is also hoped
that further details regarding the extent of the rutile-bearing
portions may be also available.

The Enclosing Anortlvosite. — Like so many other occur-
rences of ilmenite the containing rock of the St. Urbain deposits
is an anorthosite. The extent of the anorthosite area in the
present instance is not known, but it appears to be a large one,
and may be, as was believed by Hunt, continuous with the



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264 Warren — llmenite Rocks near St. Cfrhain^ Quebec;

anorthosite located near Quebec, just north of the Chateau
Richer, and known by that name. It is probably distinct from
the great mass of anorthosite lying to the north and northeast
about the upper waters of the Saguenay.

The anorthosite calls for no special description. It may be
noted, however, that it is rather poor in femic constituents,
and that such as occur are largely or wholly altered to chloritic
or serpentinous products. The disseminated grains of ilmenite
are always highly xenomorphic. Locally the rock shows some
crushing, and throughout, the feldspars, which are chiefly ande-
sine, show some evidence of strains.

The Ilmenite Masses i?i General. — The contacts with the
ilmenite bodies are, as a rule, quite sharp, although there is in
places some gradation. Along the contacts there is commonly
a narrow band of a dark brown mica developed. Small spheri-
cal or irregular masses of ilmenite occur at many places in the
vicinity of St. Urbain, also narrow dike-like streaks. The
larger bodies appear in general to have the form of elongated
masses, sometimes dike-like in their general outlines. The
elongation follows an indistinct gneissoid structure in the
anorthosite, which here has an east-westerly direction. The
dip is usually highly inclined, although some of the ore bodies
bend over and lie almost horizontally, conforming, doubtless,
to local flextures in the enclosing rock.

Large Deposit of Rutile-free Ilmenite. — One of the two
most important exposures of the ilmenite rock is found about
one-half way up the hillside, which rises rather steeply directly
west of the village of St. Urbain and forms the western rim of
the broad valley of the River Gouffre. The ilmenite has been
partially uncovered, and probably a few thousand tons were
mined many years ago for iron. In fact, the ruins of an old
smelter may still be seen just below the deposit. Tlie total
exposure at the old workings is perhaps 100 feet wide by 200
feet long (east and west.) (rood outcrops occur at intervals
for some distance to the west along the bed of a small stream
which runs down the hillside at this point. On the south side
of workings a dike-like body of very massive ilmenite is
exposed and may be followed up the hill for some 200 feet.
This has an average width of about 10 feet and near its upper
end bifurcates, one branch bending off to the northwest. A few
feet north of this, another dike, possibly ramifying below with
the first, bends off to the north with a flattening dip until it
becomes almost horizontal. Just north of this again comes
another mass, which has been uncovered over an area measur-
ing some 70 feet on a side, with a depth, as exposed, of some
20 feet. This is cut near its northernmost exposure by a nar-
row streak of anorthosite rock with a nearly vertical dip and
an east-west trend.



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New Occurrence of Rutile and Sajpphirine. 265

Megascopically, the ilmenite rock from this occurrence con-
sists essentially of a dense black, medium to rather coarsely
granular ilmenite, through which are scattered small grains oi
feldspar, or its decomposition products, occasional grains of a
dark green spinel, and plates of dark brown mica. Long
exix>8ure to surface weathering develops a brown limonite coat,
but, as a whole, the ilmenite rock is very resistant to weather-
ing processes. The ore, studied in thin sections and on pol-
ished surfaces, shows that the ilmenite, as well as the other
constituents, lack altogether any crystallographic outlines.
The ilmenite grains range in size from individuals 3-4'"" in
cross section to ones 10-12"", the average being perhaps 6-7"".
The feldspar is the same variety as that in the anorthosite and
forms rounded grains. It is often largely replaced by second-
ary products. Lying along the border of many of the feldspar
grains, next to the ilmenite, biotite is developed. This may
lie parallel to the margin or may project out into the feldspar,
sometimes replacing a good portion of it. Its occurrence is
such as to suggest that it may be of later origin than the feld-



Online LibraryJohn Elihu HallThe American journal of science → online text (page 26 of 61)