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The Slate Islands: a probable complex meteorite impact
structure in Lake Superior1
H.
C.
HALLS
Department of Geology, Erindale College, Universiry of Toronto, Mississauga, Ont., Canada
L5L
1C6
AND
R.
A.
F.
GRIEVE
Gravity and Geodynamics Division, Earth Physics Branch, Department of Energy, Mines and Resources, Otlawa, Ont.,
Canada
KIA
OY3
Received
2
March 1976
Revision accepted for publication 3 June 1976
Shock metamorphic effects in samples from the Slate Islands, Lake Superior (48"40' N, 87"OO'
W)
suggest that the islands are part of a meteorite impact structure. The islands form the central
uplift of a complex crater and are ringed by a submerged trough and annular ridge with a diameter
of 30 km. Precambrian bedrock units are locally brecciated and cut by allochthonous breccia
dikes. These dikes contain clasts of identifiable country rock and also fragments of a sedimentary
unit, possibly Upper Keweenawan in age, which is no longer present in outcrop. The orienta-
tions of shatter-cones present in the breccia host-rocks indicate the interior of the islands as the
approximate shock centre. Microscopic planar features, equivalent to those described from other
impact sites, occur in quartz and plagioclase and the level of shock deformation increases towards
the interior of the islands. The shock event postdates Keweenawan igneous activity (about
1.1
b.y. old) and, on the basis of the erosion level, may be early Paleozoic in age.
Les effets de mttamorphisme de choc dans des echantillons de Slate Islands, Lac Superieur
(48"40' N, 8T00'
0)
suggerent que les iles font partie d'une structure d'impact meteoritique. Les
iles forment le point eleve d'un cratere complexe et sont entourees par une fosse submergee et
'
une crkte en forme d'anneau d'un diametre de 30 km. Les roches du Precambrien contiennent par
endroits des zones de breche et sont recoupees par des dykes allochtones de breche. Ces dykes
contiennent des fragments identifiables des roches encaissantes et aussi des fragments d'une
unite sedimentaire, possiblement d'ige Keweenawien superieur qui n'existe plus
a
l'ttat
d'affleurement. Les orientations des c6nes de choc presents dans les breches encaissantes
indiquent I'inttrieur des iles comme point d'impact approximatif. Des petites structures, micro-
scopiques planes semblables
a
celles decrites pour d'autres sites d'impact, se rencontrent dans le
quartz et le plagioclase et le degre de deformation par choc augmente vers I'interieur des iles.
L'impact est posterieur la phase d'activite ignee du Keweenawien (environ
1.1
Ga) et en se
basant sur le niveau d'erosion, il a pu se produire au debut du Paltozoi'que.
[Traduit par le journal]
Can. J.
Earth
Sci.
13,
1301-1309(1976)
Introduction
In this paver we revort additional shatter-cone
A
A
slate ~~l~~d~
are
a coherent, roughly cir- observations and describe other characteristic
cular group
of
two
major and several smaller shock features that provide further evidence that
islands in Lake Superior, 10 km south of Terrace the Slate Islands may have foIXIed as a result of
Bay, Ontario (Fig. 1). The form of the islands
meteorite
impact.
and the surrounding lake bottom, and the report
of "shatter-cone-like" features and breccias by
Sage (1974) prompted the inclusion of the Slate
Islands in a list of possible impact structures in
Canada (Robertson and Grieve 1975). A similar
conclusion was also reached by Halls (1975)
from geological and paleomagnetic studies car-
ried out on the islands since 1973.
I
'Contribution from the Earth Physics Branch No.
606.
Form and General Geology of the Slate Islands
As a group, the Slate Islands have a diameter
of approximately 7 km and rise to elevations of
about 120 m above lake level. Hydrographic
data indicate a submerged trough with water
depths of 200-260 m at a radial distance of 7-8
km (Fig. 1). Between the islands and the main-
land to the north, the trough is partially ob-
scured, presumably due to glacial till or more
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1
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1976
FIG.
1.
Map of the Slate Islands showing surrounding lake bathymetry. The islands form a central
peak with a submerged peripheral trough (water depth
>
200
m) and annular ridge
(.=
100 m depth).
The topographic expression is similar to that of an eroded compkx meteorite crater.
A
and
B
are loca-
tions referred to in text. A
:
Archean diorite intrusion in Lawrence Bay;
B
:
breccia locality on mainland,
1 km north of McKellar Harbour.
recent sedimentation, although local water
depths of 180 m are still recorded.
The trough is surrounded by a discontinuous
annular ridge with relatively shallow water
depths of 60-120 m at a radial distance of 15-16
km. Beyond the ridge, water depths increase to
about 200 m, the average for this part of Lake
Superior. The form of the topography-a central
peak formed by the islands, peripheral trough,
and annular ridge-is that of a complex hyper-
velocity impact crater (Dence 1972) and is similar
to that of several other large impact structures in
Canada (Robertson and Grieve 1975).
The islands are geologically complex. The
principal rock units are Archean felsic to mafic
meta-volcanics and pyroclastics with dioritic
and felsic intrusives (Sage 1974). On the western
shore of Patterson Island (Fig.
I),
these early
units are unconformably overlain by approxi-
mately 20 m of Aphebian black argillite and
taconite of the Animikie Group. The Animikie
in turn appears to be overlain with little dis-
cordance by about 120 m of reversely magnet-
ized, Lower Keweenawan basalt flows and minor
sandstone, which are correlative with the Osler
Volcanic Group exposed on the mainland to the
west (Halls 1975). On the Slate Islands, the vol-
canic~ have a general dip of 30-40"
W
into the
lake, although dips of up to 70' occur locally
toward the base of the sequence. Numerous east-
trending diabase dikes, presumably feeders for
the Keweenawan flows, cut across the islands,
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HALLS
AND
GRIEVE
1303
but appear to be thicker and more abundant
toward the south.
In places, the bedrock units are highly frac-
tured and locally form autochthonous breccias.
The most striking breccia development is, how-
ever, in the form of cross-cutting polymict alloch-
thonous breccia dikes, which range in width
from a few cm to 50-60 m. They are well dis-
played in shoreline outcrops, particularly around
Patterson Island, and are stratigraphically the
youngest rocks exposed on the islands.
I
Shock Deformation Features: General Aspects
Twenty-two circular structures exhibiting a
class of deformation known as shock metamor-
phism have previously been identified in Canada
(Robertson and Grieve 1975; Caty
et al.
1976).
Shock deformation features are distinct from
those formed in normal tectonic or volcanic en-
vironments and in static loading experiments
(Carter 1971). They have been reproduced in
nuclear explosions and in numerous dynamic
loading experiments, and apparently require
transient pressures of 2-60 GPa (1 GPa
=
10
kbar), with accompanying strain rates of the
order of 1x per microsecond for their formation
(Borg 1972; Stoffler 1972). Shock-metamorphic
effects have not been observed in rocks known
to be genetically linked to explosive volcanism,
but are displayed in many meteorites and lunar
samples, and in the ejecta of young terrestrial
hypervelocity impact sites that contain meteorite
fragments or Fe-Ni spherules. This evidence has
led to the general acceptance of shock-metamor-
phic features in association with a circular struc-
ture as indicating the site of a hypervelocity col-
lision of a large cosmic body with the earth's
surface. Reviews of shock metamorphism and
its implications are given by French and Short
(1968), Horz (1971), and Stoffler (1972, 1974).
On the Slate Islands, various shock metamorphic
features have been recognized and are described
below.
Megascopic Deformation Features
Shatter-Cones
Shatter-cones have been produced in nuclear
explosions (Bunch and Quaide 1968) and in
laboratory shock experiments (Shoemaker
et al.
1961). They are formed by the interaction of a
shock wave with inhomogeneities in the rock at
pressures in excess of the Hugoniot elastic limit
(Johnson and Talbot 1964), which in the case of
non-porous silicate rocks is of the order of 4.0-
9.0 GPa (Ahrens and Gregson 1964). Accord-
ingly, shatter-cones, which have been identified
at approximately 30 terrestrial impact structures,
are generally regarded as an indicator of rela-
tively low-grade shock metamorphism (Dietz
1968; Short and Bunch 1968; Robertson and
Mason 1975).
On the Slate Islands, shatter-cones are present
in the autochthonous bedrock units and in clasts,
but not the matrix, of the allochthonous brec-
cias. Cone segments up to 30 cm in axial length
are common in the Keweenawan volcanics (Figs.
2a,b) and have a preferred eastward apical direc-
tion (Sage 1974). Similar-sized cones are ob-
served occasionally in the interiors of Keweena-
wan diabase dikes and also in an Archean diorite
intrusion exposed along Lawrence Bay on the
north side of Patterson Island (location A in Fig.
1). Smaller shatter-cones (less than 10 cm in
length) occur abundantly in Animikie argillite.
They show finer detail than in the coarser-
grained basalts and diorites, and have well devel-
oped parasitic cones, with splitting of individual
striations to form distinct horsetails (Fig. 2c).
Similar cones are found in comparatively fine
grained rocks elsewhere on the islands: in
Archean tuffaceom(?) units along the east coast
of Patterson Island, in Archean greenstones on
the south side of Mortimer Island, and in chilled
margins of the Keweenawan dikes.
In many of the more strongly cleaved and
fractured Archean rocks, cones are relatively
rare and occur as apparently isolated fractures
with curved striations (Fig. 2d), or as a her-
ringbone pattern formed on roughly planar inter-
faces by juxtaposed cone segments with common
orientation. These features result from pre-im-
pact structures within the rocks, which behaved
as open interfaces during passage of the shock
wave. Shatter-coning was initiated along these
interfaces and the formation of symmetrical,
fully developed cones, retarded. As a result,
along the south and east shores of Patterson
Island, where the Archean rocks
are
strongly
ma-
foliated and thus not conducive to cone
h-..
tion, Keweenawan diabase dikes often provided
the only source of well developed partial cones.
A full analysis of the shatter-cone orientations
such as that described by Manton (1965) for the
Vredefort Dome is yet to be completed. How-
ever, the azimuth of the 'direction of point' was
measured for a number of partial cone segments
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CAN.
J.
EARTH
SCI.
VOL.
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1976
at three localities where they are exposed on ap-
proximately horizontal surfaces. These cones
exhibit horsetail striations and occurred either as
joint-controlled corrugated sheets or formed
surfaces that were not obviously part of a pre-
existing joint pattern. These data, together with
the determination of Sage (1974), show that the
shatter-cones point radially inwards and have a
reasonably coherent intersection (Fig.
3).
The
geological complexity of the Slate Islands largely
pre-empts any attempt to restore the rocks to
their possible pre-impact attitude. However, the
upward sequence Archean, Animikie, Keweena-
wan observed on the west side of Patterson
Island, and the uniform sub-vertical attitude of
diabase dikes throughout the islands, suggest
that no overturning of the rocks as a result of
impact has occurred. Therefore, since shatter-
cone axes are parallel to the local path of the
shock wave (Johnson and Talbot 1964) the
azimuth data indicate the middle of Patterson
Island as lying on the vertical projection of the
shock wave origin.
Breccias
Both the allochthonous and autochthonous
breccias are clastic down to the limit of optical
resolution. Clastic breccias, although extremely
common at impact sites (Short and Bunch 1968),
can be produced by a number of geological
processes and they are not, in themselves, defin-
itive of meteorite impact. However, as described
later, quartz grains in the Slate Island breccias
have microscopic planar dislocation lamellae
characteristic of shock metamorphism, which
suggest that the brecciation is related to a shock
event.
The allochthonous breccias were formed by
injection; they have sharp contacts with the
country rock and many have an obvious dike-
like form (Fig. 2e).
In
places, they form an in-
tersecting network and constitute the matrix of a
country rock megabreccia. They have a highly
variable clast population and their matrix ranges
in colour from brick-red to pale green or buff.
Red-matrix breccias owe their colour to exten-
sive development of hematite and are well ex-
posed along the western shore of Patterson
Island. Individual dike-like bodies contain angu-
lar lithic clasts, ranging up to 5 m in size, of
Archean, Animikie, and Keweenawan rock types
(Fig.
2f).
Many of the smaller, millimetre-sized
clasts are partially recrystallized and planar
shock features in quartz are partially healed.
Several clasts that lack planar features have ir-
regular contorted outlines with devitrification
textures. or are converted to sheet silicate. These
clasts may represent diaplectic mineral or rock
glasses produced during the shock event, which
have subsequently altered and/or devitrified.
Pale green to buff breccias are well developed
on the southern and eastern shores of Patterson
Island. Besides fragments of Keweenawan igne-
ous rocks and assorted Archean lithologies,
breccias on the south shore carry numerous
clasts of a pale green, buff, or purple, well bedded
siltstone and brown or purple arkosic sand-
stone. These clasts are lithologically unlike ex-
posed Animikie and Keweenawan interflow
sedimentary units, but resemble the Upper
Keweenawan Freda Formation. The nearest
exposures of this formation are in northern
Michigan, about 150-200 km to the southwest.
There is, however, good geological and geophys-
ical evidence that Upper Keweenawan sedimen-
tary rocks underlie most of Lake Superior,
including that part in the vicinity of the Slate
Islands (Halls 1966, 1972; Hinze
et
al.
1966;
Halls and West 1971; Dell 1975). If these rocks
once covered the area presently occupied by the
islands, the existence of Freda fragments at a
lower erosional level than the parent formation
could indicate that the breccia was injected in a
downward direction.
A small breccia outcrop containing meta-vol-
canic, quartzite and arkosic sandstone clasts oc-
curs on the mainland east of Jackfish Bay (loca-
tion B in
Fig.
l).
Examination of several samples
FIG.
2.
Megascopic deformation features found on the Slate Islands. (a) Shatter-cone in Keweena-
wan basalt flow.
(b)
Typical shatter-coned outcrop of Keweenawan basalt, showing horsetail features.
(c) Well developed shatter-coned surface in clast of Animikie argillite from allochthonous breccia.
Width of field of view is
5
cm. (d) Shatter-coned surface of Archean felsic metavolcanic. Cones are
poorly developed and appear only as curved striations. Width of field of view is
10
cm. (e) Dike-like,
allochthonous, red-matrix breccia with few clasts, cutting Archean country rock. (f) Allochthonous
red-matrix breccia with numerous angular country rock fragments. Largest clast is approximately
2
m
in maximum dimension.
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HALLS AND
GRIEVE
1305
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CAN.
J.
EARTH SCI.
VOL.
13,
1976
O
I2
Jim
FIG.
3.
'Direction of point' of shatter cones in the
field. Their intersection approximates present location
of the shock centre. Data from the western shore of Pat-
terson Island taken from Sage
(1974).
Remaining data are
averages and ranges of at least
10
separate cone measure-
ments per site. Solid dots show other shatter cone local-
ities where adequate directional data could not be ob-
tained, owing to poor preservation and infrequency of
shatter cone occurrence.
failed to yield evidence of shock deformation.
The outcrop is 30 km from the Slate Islands and
is beyond the limits of the complex crater de-
fined on bathymetric data. Thus, its relationship,
if any, with the Slate Islands event remains an
open question.
Microscopic Deformation Features
Quartz
Planar features, multiple sets of microscopic
lamellae, form in quartz at low to moderate shock
pressures of 7.5-30 GPa and are widely recog-
nized as a microscopic shock metamorphic de-
formation effect (Carter 1968; Robertson et al.
1968
;
Engelhardt and Bertsch 1969; Stoffler
1972). Planar features, or shock lamellae, ident-
ical in form and Orientation to those described
from other impact sites were recognized in all but
two quartz-bearing samples collected from Slate
Islands breccias and host rocks. In general the
planar features are 'decorated' with minute in-
clusions. At their weakest development, on the
western shore of Patterson Island, they occur
only in some grains, and no grain has more than
one set (Fig. 4a). Inland, virtually all quartz
grains have lamellae, with some having as many
as
6
or
7
distinct sets (Fig. 4b).
The initial development and abundance of
planar features with specific crystallographic
orientations characterize different levels of shock
deformation. Orientations of the planar features
were measured on the universal stage, and fol-
lowing the classification of Robertson et al.
(1968), four levels of shock deformation were
recognized in quartz grains in samples from the
Slate Islands. In terms of increasing shock de-
formation, they are: type A, with one set of
planar features parallel to c {0001)
;
type B, with
sets parallel to
o
(1013) and usually c; type C,
with sets parallel to
o,
(22411, and usually
c;
and Type D, with sets parallel to
o,
n{lOi2), and
usually (2231). Additional orientations, for
example {1011), were observed and occurred in
quartz with C or D type deformation. Figure
5
shows frequency plots of the angle between the
c-axis and the pole to plane of the shock lamellae
typical of the various deformation levels re-
corded in quartz from two Slate Island samples.
Orientations and the number of sets of planar
features per grain have been measured in 20
grains in each of 14 country rock samples, and
an average recorded shock pressure for each
sample has been estimated (see Grieve and
Robertson 1976 for details). Recorded shock
pressures range from about 6 to 15 GPa (60-1 50
kbar), and generally increase toward the shock
centre deduced from the shatter-cone orienta-
tions in Fig. 3.
Plagioclase
As with quartz, low to moderate shock pres-
sures produce multiple sets of deformation
lamellae in plagioclase, parallel to low-index
crystallographic planes (Stoffler 1967; Robertson
et
al. 1968; Dworak 1969). The lamellae form at
pressures above approximately 15 GPa (Stoffler
1972, fig. 16). Although their orientation does
not seem to characterize different shock levels,
lamellae formed at higher pressures (>20 GPa)
are twin-like and have measurable widths. On
the Slate Islands, shock lamellae in plagioclase
were only observed in samples with quartz planar
features indicative of the higher (C-D) levels.
They were weak, had no measurable width, and
were present in only some grains (Fig.
4c).
Maskelynite, a dense diaplectic plagioclase
glass, the transformation to which is complete at
shock pressures above 30 GPa (Stoffler and
Hornemann 1972; Gibbons
et
al. 1975), was
identified in a diabase clast from a buff breccia
on the southern shore of Patterson Island. Other
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HALLS
AND
GRIEVE
1307
FIG.
4.
Microscopic deformation features found on the Slate Islands. (a) Type A quartz from western
shore of Patterson Island, with few basal planar features. (b) Type
D
quartz,
1.5
km inland from type
A, with numerous planar features belonging to several sets. (c) Shock lamellae in altered plagioclase
phenocryst from an Archean felsic metavolcanic. (d) Intense microtwinning in calcite from an amyg-
dule in a Keweenawan basalt flow. Quartz from approximately the same location shows only type A
or no obvious shock deformation.
clasts, believed to have been shock-deformed to
maskelynite, are either devitrified or altered to
microcrystalline sheet silicates.
Calcite
A unique style of shock deformation has yet to
be identified in calcite. However, twinning along
e
(0112) and
r
{lOTl), common in statically
deformed calcite, is intensely developed under
shock-deformation conditions (Roddy 1968;
Robertson and Mason 1975). In the Slate Islands
samples, universal stage measurements revealed
up to 9 sets of polysynthetic twins, with predomi-
nant
e
and
r
orientations, within single calcite
grains (Fig. 4d). Microtwinning is common along
{0221), and several unindexed orientations at
high angles to the basal plane were also observed.
Intense calcite microtwinning is present in rocks
which lack obvious shock features in quartz, and
it is, therefore, regarded as a lower-grade de-
formational feature than planar features in tecto-
silicates. This is in keeping with the lower
Hugoniot elastic limit of calcite with respect to
quartz and feldspar (Ahrens and Gregson 1964).
Summary and Conclusions
The occurrence of shatter-cones, characteristic
mineral deformation features, and maskelynite
indicates that rocks on the Slate Islands have
been subjected to a shock event. Shatter-cone
orientation data further suggest that the shock-
wave origin is centrally located within the Slate
Islands, an observation that is consistent with
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1308
CAN.
J. EARTH
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VOL.
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1976
0
3
C
4
+
4
C
+
SL-75-28:
20
grains
37
sets
L
10
20 30 40 50
60
70 80
90
c
-
axis
Al
planes ,degrees
63
sets
c
-
axis
A1
planes ,degrees
FIG.
5.
Frequency plot of planar feature orientations
in
20
quartz grains from two Slate Islands samples. Sam-
ple
SL-75-28
exhibits basal
{OW1
}
features (type A), and
o
{10i3}
features (type
B).
Sample
SL-75-24
has more
sets of planar features and has a generaIly higher recorded
shock level with development of
o
features,
{2241}
fea-
tures (type
C),
and
n
{10i2}
features (type
D);
other
orientations were recorded, for example
{I071
},
but no
grains have basal
(0001
} features.
the spatial variation of shock deformation esti-
mated from the development of planar features
in quartz.
The shock effects, particularly the microscopic
deformation features in quartz and feldspar, are
equivalent to those described from terrestrial
meteorite craters, nuclear explosion events, and
hypervelocity impact experiments. The estimated
recorded shock pressures range from 4 to greater
than
15
GPa (40 to
>
150 kbar) and are con-
siderably higher than peak values of 0.15-0.3
GPa (1.5-3 kbar) calculated for pressures im-
mediately prior to the most energetic volcanic
explosions (Gorshkov 1959). It is concluded,
therefore, that the shock event is due to meteorite
impact and the Slate Islands represent the central
uplift of a complex hypervelocity crater. The
chaotic geology of the islands, with intense brec-
cia development, together with an associated,
rapidly acquired remanent magnetization (Halls
1975, 1976), is further evidence of a sudden cata-
strophic event, although these features are not in
themselves definitive of meteorite impact.
The age of the shock event is presently un-
known. However, radiometric age-dating of the
most highly shocked samples is currently under-
way. The event must be younger than about 1.1
b.y. because the breccia dikes cut Keweenawan
igneous rocks. The paleomagnetic pole position
given by the shock-induced remanence
(24"
S,
166"
E)
is chronologically not diagnostic (Halls
1975), as it occurs in a region where Grenville and
Lower Paleozoic poles also lie (Irving
et
al.
1974;
Beales
et
al.
1974). However, the distribution of
shock effects with respect to present topography
is similar to that observed at the Charlevoix
impact structure, Quebec (Robertson 1968), and
suggests a similar level of erosion. The Charlevoix
impact event occurred 360
f
25
m.y. ago
(Rondot 1971), and it may be that the Slate
Islands event has a comparable age.
Acknowledgements
The financial support of the National Research
Council of Canada to H. C. Halls is gratefully
acknowledged. The assistance of
P.
B.
Robertson
to
R.
A.
F.
Grieve in the field, in the measure-
ment of planar features, and in general discus-
sion of the structure is greatly appreciated.
AHRENS,
T.
J.
and GREGSON,
V.
G., JR.
1964.
Shock com-
pression of crustal rocks: data for quartz, calcite and
plagioclase rocks.
J.
Geophys. Res.
69,
pp.
4839-4874.
BEALES, F.
W.,
CARRACEDO, J. C., and STRANGWAY,
D.
W.
1974.
Paleornagnetism and the origin of Mississippi
Valley-type ore deposits. Can.
J.
Earth Sci.
11,
pp.
21 1-223.
BORG,
I.
Y.
1972.
Some shock effects ingranodiorite to
270
kilobars at the Piledriver site.
In:
Flow and fracture of
rocks.
(H.
C. Heard,
I.
Y.
Borg, N.
L.
Carter, and C. B.
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HALLS
A
LND
GRIEVE
1309
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293-3 13.
BUNCH, T. E. and QUAIDE, W. L. 1968. Shatter cones in
the Danny Boy nuclear crater. In Shock metamorphism
and natural materials (B. M. French and N. M. Short,
Eds
.)
Mono Book Corp., Baltimore, MD, pp. 285-286.
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