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THE GEOLCGY AED PETROGRAPHY OF LIT LE PRESQUE ISLE,

KARQUETTE COUHTY, KICKIGAN

By

flURIEL ELIZABETH TARA

A THESIS

Submitted to the School of Graduate Studies of Kichigan
State College of Agriculture and Applied Science
in partial fulfillment of the requirements

for the degree of

Department of Geology

1950

I )J {9'13

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4‘1)

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§./--‘»

THE GEOLOGY AND PEIBOGEAPHY OF LITTLE PBESQDE ISLE,

MARQUETTE COUNTY, MICHIGAN

CONTENTS

List of illustrations

.Acknowledgments

Abstract

Introduction
Location and extent of the area
Previous investigations
Objectives of the study
Field methods

Geography
Topography and drainage
Physiographic history
Vegetation

Geology
Stratigraphy
Structure

Relationship of Little Presque Isle to the
Marquette District

Sedimentary‘petrOIOgy
Potsdam sandstone

Igneous petrology
Diabase dikes

Laurentian granite

Sedimentary'petrography ,~,\ ,,
EEPIEL17£3

“.1

page
iii

vi

\oooooooc-uumu

10
ll
11
12
12
13
16

page

Procedure of sediment analyses 16
Results of sediment analyses 17
Data sheet, sediment analysis of Potsdam sandstone 18
Data sheet, sediment analysis of sand dune 20
Igneous petrography 22
Diabase dikes 22
Samples 8 and 9 23
Sample 1*5 24
Sample 51 25
Sample 36 25
Sample #6 26
Laurentian granite 26
Samples 1 and 5a 26
Samples #3 and 52 27
Samples 10 and 40 27
Conclusion 28
Summary’ 28
Possibilities for further study 28
Bibliography ‘ ' 30

ii

Plate

KAI-TU

10

11
12

13
14

15
16

LIST OF ILLUSTRATIONS

Marquette and vicinity showing location of Little
Presque Isle

. Little Presque Isle as mapped by.A. E. Seaman

Topography of Little Presque Isle
Surface geolOgy of Little Presque Isle

Figure 1. Sandstone cliff on east side of Little
Presque Isle

Figure 2. Shoreline of Little Presque Isle

Diagrammatic sketch of contact between the Laurentian

granite and the Potsdam sandstone

Figure 1. Little Presque Isle as seen from Sugar
Loaf Mountain

Figure 2. Chloritic pebble incorportaed in the base

of the sandstone
rigure l. Diabase dike
figure 2. Chevron banding in Potsdam sandstone
Figure 1. Conglomerate layer in sandstone
figure 2. Jointing in diabase dike
Figure l. Pegmatite dikes

Figure 2. Relationship of granite, gneiss and
pegmatite

Cumulative weight frequency curve for sandstone
Cumulative veight frequency curve for dune sand
Weight percent of heavy minerals

Vadell's method for computing roundness of quartz
grains

Photomicrographs, Samples 8 and 9

Photomicrographs, Sample 52

iii

page

pocket

pocket

12

12

13

13
16
16
16

16
23
2?

Plate

17 Photomicrographs, Samples 10 and to

iv

page

27

ACKNOWLEDGMENTS

The writer wishes to express sincere thanks and appreciation to
Dr. B. T. Sandefur of Michigan Statb College. who directed this study,
to Hrs. Jane 3. Smith and Drs. S. G. Bergquist, I. A. Kelly, and Justin
Zinn, also of Michigan State College, for their many helpful suggestions.
and to Ir. Frank Pardee and Hr._Bruce Kennedy of the Kichigan Geological
Survey who made available the original field work of A. I. Seaman.

The writer is indebted to Mr. Charles Milton of the Geochemical
and Geophysical Branch of the'U. 8. Geological Survey who made possible
the photomicrographs.

Love and deepest gratitude are expressed to Mr. and Hrs. O. R.
Tara, the writer's parents, and to Mary Jane Dockeray. geologist for
the Grand Rapids City Inseum, who were the writer's very capable and

willing field assistants.

ABSTRACT

Little Presque Isle lies in Lake Superior in sec. 1?, T. 49 N.,
.R. 25 V., of Harquette County. It's maximum length and width is
1,440 feet and 1,000 feet, respectively. The maximum altitude is
712 feet. _

The island is essentially a knob of Laurentian granite, which
was intruded into the Mona and.Iitchi schists of the Marquette District,
against which.unconformably lies the Potsdam sandstone. The east-central
part of the island is covered with drift.

The granite is out by IOIO forty or more diabase dikes, which range
in width from one-half inch to sixty feet, representing at least three
periods of intrusion. Further, this granite is cut by at least two
periods of pegmatite injection, which represent the final stages of the
granitic intrusion. The pegmatite is also cut by the diabase and is
therefore older.

Sediment analyses of the drift indicate that the deposit is an old
dune. Sediment analyses were of no value in determining the possible
origin of the sandstone.

Little Presque Isle is a part of the northern Basement Complex of

the Marquette District.

THE GEOLOGY AND PETROGRAPHY OF LITTLE PRESQUE ISLE,
MARQDETTE COUNTY, MICHIGAN

Muriel 1. Tara
INTRODUCTION

The area discussed in this report is commonly known by one of two
names, Little Presque Isle or Granite Point. Both are used synony-
mously here, and the latter is not to be confused with the Granite
Point which lies in sec. 27. T. 51 N., R. 26 l., of Marquette County.

ati and ent of th area -- Little Presque Isle lies in
Lake Superior, north of lat. 46°37'30“N., and west of long. 87°25'40'W.,
in sec. 17. T. 49 It, Re 25 V., (Plate 1). The island has a maximum
length of l,bho feet and a maximum width of 1,000 feet.

[reviqgg igzggtigatiogg - Although there have been many investi-
gations of the Iarquette District the writer found only one report
which was, in part, directly concerned.with Little Presque Isle. This

1/
was Seaman's original field notebook of the Marquette Range, which

 

‘ll Seanan,.a. 3., Marquette Range, Michigan Geological Survey, Field

Eotgbogk No, 28, pp, 52-53, 1888.

includes a map of Little Presque Isle (Plate 2) made by a pace and com-

 

pass traverse, and a few brief field notes concerning the rock types
found and their probable relationships to each other. To the writer's
knowledge none of Seaman's work on the Marquette Range was ever pub-
lished.

Schoolcraft observed the unconformity between the granite and

INDEX TO MARQUETTE AREA

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

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7
I ‘ ' a, MARQUETTE
e AREA
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WISCONSIN f LAKE
"‘3 MICHIGAN
MARQUETTE AND VICINITY
LOCATION OF
LITTLE PRESQUE ISLE
e7°3doo' 37.2540” erau'zo'
(V
LITTLE PRESQUE 1:“
° 6L8
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o
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Sneak-Loaf Partridge
24.: Islmd
Teen
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3 : Presque Isle
N N
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se‘ss'es' — -—ee'33'45“
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N MARQUETTE o ”'55
m
ersdoo' erzs'eo' erzfzo'

 

 

PLATE I

HURIEL E TARA

 

‘gf Schoolcraft, R. Re, narrative Journal of travels from Detroit

northwest through the great chain of.American Lakes to the sources

2f the Mississippi River in the year 1820, pp. 152-160I Albgpz, 1821.

the sandstone on Little Presque Isle and made the following comment:

“The granite rising out of the lake to a height of
200 feet, is connected to the shore by a neck of land
consisting of red and gray sandstone in horizontal
layers. The sandstone laps upon the granite and fits
into its irregular indentations in a manner that shows
it to have assumed that position subsequently to the
upheaving of the granite. Its horisontality is per-
fectly preserved, even to the point of contact, which
is laid bare to view.“

Roughton was one of the first to make a general statement with

 

‘1] Roughton, Douglas, #th.Annual Report of the State Geologist, House
of R resentat v no 2 1 Michi 1841i

respect to the ages of the Lake Superior rocks. as distinguished the

presence of three groups of rocks: primary, trap, and metamorphic.
His primary group is found on Little Presque Isle.

“The rocks constituting what may be considered as
the true primary group of this region, are chiefly
granite, syenite and syenitic granites. The members
of the group are first seen upon the south coast of
Lake Superior, constituting a rocky point known as
Little Presque Isle. ..... .'

Rominger in his studies of the larquette Iron Range made the

 

g] Rominger, Carl, Upper Peninsula, Pt. I, Marquette Iron Range. Vol.
lo ice 8 f hi 18 8- 880.

following reference to Granite Point.

'......An isolated knob of granite in a soft disin-
tegrating condition, situated at the end of a narrow
land tongue covered with sand and underlaid by Silurian

Bm-rOck.... eeee.

The writer investigated several other early reports of the

-2-

Marquette District but found no specific references to Little Presque
Isle. ~

Opjggtives of the stugy - The writer sincerely hepes that this
report of Little Presque Isle may be of some value to the Geology
Department of Michigan State College in connection with summer field
camp. The following were the major objectives of the study.

1. To prepare a topographic and a geologic map
of Little Presque Isle.

2. To determine the relative ages of the dikes
by field observations and, if possible, by
petrographic analyses.

3. To determine by petrographic study what changes,
if any, occurred in the granite as a result of
the intrusion of the diabase dikes.

4. To determine, if possible, by sediment analyses
the origin of the drift and of the sandstone.

5. To obtain a general over-all picture of the
geology of Little Presque Isle, and to note
the geologic relationship of Little Presque
Isle to the Marquette District.
11319_pgphgdg - The writer spent the first few days making a
reconnaissance of the area, locating the major geologic features.
Iield mapping was done with a telescopic alidade, plane table, and
stadia rod. The Seaman Stadia Arc was used for determining altitudes

and for converting slope distances into horizontal distances. A.scale

of 1:480 was chosen so that even the smaller dikes could be mapped.

GEOGRAPHY

Topoggaphz gpd draipggg - The tapography of Little Presque Isle.
though rugged, is relatively simple (Plate 3, pocket in back) and on

the basis of rock types is divided naturally into two units.

The first unit is a narrow sandstone neck (Plate h, pocket in back)

- 3 -

approximately 700 feet long, which extends to within about 200 feet of
the mainland and ranges in width from 100 to 200 feet. Except at the
southern tip of this neck wave action, sapping the sandstone, has formed
a vertical or undercut cliff. This cliff attains a maximum height of 5h
feet (Plate 5, Figure 1) along the east side of the island owing to pro-
tection from the granite along the northeast side of Little Presque Isle.
Below this high cliff the contact between the sandstone and the granite
is exposed: above it lies approximately 30 feet of unconsolidated sand
(Plate 6). On the western side of this narrow neck the sandstone cliff
has a maximum height of 14 feet.

The second unit, which forms the head of the island, is granite,
cut by numerous diabase and.pegmatite dikes. The major topographic
features of this unit are two granite ridges in the central part of
the island, trending approximately'l.65°w., and N.35°U., which attain
maximum altitudes of 687 feet and 712 feet, respectively. Two very
minor granite ridges, which trend east, are found in the northeast part
of Little Presque Isle. Along the shore the granite is completely ex-
posed and forms an uneven surface which in most places rises rather
abruptly for 20 to 50 feet or more above Lake Superior (Plate 5, Figure
2)

With one exception, that of a small rocky cove on the southwest
side of the island, there are no beaches, and in many places, especially
on the southern half of Little Presque Isle, it is very difficult to
reach the lake from the island.

There are no swamps or streams on the island. Msteoric water
simply disappears as run-off or seeps downward through the drift or
fractures in the bedrock.

Physioggaphig histopz - The following discussion of the physio-

graphic history of Little Presque Isle is concerned only with the

-4—

Plate 5

Figure l. Sandstone cliff on east side of Little Presque
Isle. Cliff is 5# feet high and is overlain by approximately
30 feet of unconsolidated sand. Rock at extreme right is
granite. The unconformity between the granite and the sand-
stone is exposed at the base of the cliff. Picture was taken
from the northeast side of the island.

figure 2. Shore of Little Presque Isle. Rock is granite.
The picture was taken from the northeast side of the island,
looking toward the north. Height of cliff is approximately
70 feet.

 

 

 

Figure l

 

Figure 2

Plate 5

 

 

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PLATE 6

postqdlgonquin, as the area was of insufficient altitude to have been
above the waters of Lake.dlgonquin. The physiographic history is of in-
terest in that as recently as 1888 the island was connected with the main-
land, forming what appeared to be a tombola (Plate 2). The disappearance
of this land connection would seem to be in exact reversal to the usual
sequence of shoreline development. Possible explanations for this dis-
appearance are: l) a lower lake level, 2) a submergence of the land, or

3) deposition on a narrow sandstone remnant (probably slightly submerged)
connecting the island with the mainland. Such deposition would be post-

Algonquin as the area was completely inundated by Lake Algonquin.

 

37H Leverett, Frank, and Taylor, I. 3.. The Pleistocene of Indiana and
Michigan and the History of the Great Lakes, U. S. Geological Survey
o o a h #1 l
The first explanation may be discounted as hydrographie records
show that over the period from 1860 to 1926 the mean water level of
Lake Superior was 602.82 feet. The minimum and maximum levels recorded
during this period m. 600.82 feet (1926) and 603.10 feet (1916),

respectively; The mean level of Lake Superior in 1888 was 602.28 feet.

 

,6] Horton, H. 1.. and Grunsky, O. 1., Hydrology of the Great Lakes,

2;, 1;], Appggdix II, pp, 400.405, l227.
During the period from 1927 to 1949 the mean lake level was 602.41 feet

 

and the minimum and maximum levels recorded were 601.90 feet (192?) and
602.88 feet (1943), respectively. In June and July of 1950, during
which time the field work for this report was done, the lake level was

603.35 feet and 603.60 feet, respectively.

 

 

 

If U. S. Army, Corps of Engineers, U. S. Lake Survey Office, 360

119de $513,, pgtzoit, Eighigap.

The submergence of the land as a possible explanation is also in-

 

valid as the shore of Lake Superior has been emerging slightly since the
retreat of the glacier. This is proven by the old Algonquin and Hipie-

sing shorelines, now lying above the present shore of Lake Superior.

 

8/ op. cit., Leverett, Frank, and Taylor, 1'. 3.. PP. 399-463. 1915.

 

In regard to the third explanation early investigations of the area

are interesting. Schoolcraft described this land connection as “red

 

2/ op. cit., p. 157-160, 1821.

 

197
and gray sandstone in horizontal layers“, and Hominger as a “narrow

 

lg] op. cit., p. 11, 1878-1880.

 

land tongue covered with sand and underlaid by Silurian sand-rock“.

Seaman mapped it as sandstone and sand beach (Plate 2). The reports

 

11/ op. cit., p. 52, 1888.

of these early geologists substantiate the theory of deposition on a
narrow sandstone remnant. However, they are incorrect in part for there

is no gray sandstone in the area, and the sandstone is Upper Cambrian,

 

127 Allen. R. A., and Barrett, L. 19., Contributions to the pro-Cambrian
geology of Northern Hichigan and Wisconsin, Michigan Geological

S l S 1+ 1;

 

not Silurian.

Inasmuch as the lake was only from one to five feet deep between
the island and the mainland it was also possible to examine the lake
floor. This was found to be sandstone and.kept completely devoid of
sediments by the action of currents. Also, the mainland, which pro-
Jects toward the island as a long narrow tongue (Plate 7, Iigure l),
is sandstone. Unlike the island, the sandstone on the mainland is cov-
ered by a deposit of sand. The writer believes, on the basis of this
field evidence and on the reports of the early geologists, that the
former land connection was a narrow remnant of sandstone, probably
slightly submerged, which formed a barrier causing deposition of sand
during the late Algonquin, or early Hipissing. Once the underlying sand»
stone had been completely severed the unconsolidated material above would
have been rapidly carried away and deposited in deeper water.

The erosion of the sandstone and the removal of the sand would have
been greatly aided by: l) the weak, crumbling nature of the sandstone.

2) the frequency of the Lake Superior storms, thus increasing the cutting
power of the waves, and 3) the deflection of the waves around the north-
east end of the island, resulting in wave attack from the northeast as
well as from.the northwest, which is the dominant wind direction.

Also of physiographic interest is the deposit of unconsolidated sand
lying above the highest sandstone cliff (Plate 6). The writer believes
this feature to be an old dune because of its high degree of sorting and
its position above the sandstone. Sediment analyses showed the uncon-
solidated sand to be 2.67 times as well sorted as the sandstone (note
results of sediment analyses, page 17). The island attains a maximum
altitude of 712 feet, and therefore would have been too low to develop

the highest.dlgonquin beach, which at larquette is surveyed at 940

Plate 7

Figure 1. Little Presque Isle as seen from Sugar Loaf
Mountain, approximately 2 miles distant. The long point of the
mainland which Jute toward the island is sandstone, overlain
by a deposit of sand.

Figure 2. Chloritic pebble incorporated in the sandstone.
Just above the unconformity. The pebble, which is approximately
four inches in diameter, may be derived from the diabase.
Photographed at a distance of four feet.

 

 

 

Figure l

 

Plate 7

11/
feet. The island was high enough to be above the Hipissing waters.

 

13/ op. cit., Leverett, Prank, and Taylor, I. 3., p. 431, 1915.

 

The ei:7ation of the lipissing beach at Marquette is surveyed at 628
14
feet or approximately 26 feet above Lake Superior. In other words,

 

14/ idem.. p. 460, 1915.

 

since Hipissing time there has been approximately 26 feet of uplift in the
region of larquette. The dune lies 54 feet above Lake Superior, or 28 feet
above the Hipissing beach. Therefore, because the island was inundated by
Lake.dlgonquin, and the dune lies 28 feet above the Bipissing shore, the
sand was probably deposited sometime between 1ate.dlgonquin and early
lipissing. The formation of this dune may be contemporaneous with the
deposition of the sand which formed the land connection to the mainland.
Iggltatign - lxcept for the east, north, and west edges of the is-
land which are practically devoid of vegetation, a mixed pine and hardwood
(oak, maple, beech) growth covers Little Presque Isle. The southern part
of the island, which is underlain by sandstone, has a much denser growth

of underbrush and small shrubs than that portion underlain by granite.
GEOLOGY

fitggtigggnhy - The stratigraphy of Little Presque Isle includes
the following units.

Pleistocene - drift

Upper Cambrian Potsdam sandstone

1 - Diabase dikes

Laurentian Granite with numerouw pegmatite dikes

-8-

The Laurentian granite was in:§7ded into the old.Mona and Kitchi
l
schists of the Marquette District, however, this relationship is not

 

,15/ Van Hise. C. H., Hayley, I. S.. and Smyth, H. L.. The Marquette

Iron-bearing District of Michigan,‘U. S. Geological Survey Mono-

a h 28 14 l l 9?.

in evidence on Little Presque Isle. The granite on the island has been
cut by intrusions of diabase of at least three different ages. It is not
possible to date exactly the time of the diabase intrusions from the evi-
dence available on Little Presque Isle. It is qudte certain that they
were intruded prior to the deposition of the Potsdam.sandstone, for the
sandstone is not cut by the dikes. lurthermore, chloritic pebbles, which
possibly were derived from the diabase, are incorporated in the base of
the sandstone (Plate 7, Figure 2). However, there is also the possibility
that these pebbles are from the Mona schist. The relative ages of the
dikes were determined by noting which dikes out existing ones and which in
turn were cum by others. The writer, by this method, determined three dif-
ferent periods of diabase intrusion.

§5;3553;1,- With one exception the dikes appear to be vertical.
The exception, a dike approximately two feet wide extending almost com-
pletely across the north end of Little Presque Isle, has a dip of approxr
imately 54° to the south. This dip is readily apparent along the eastern
end of the dike (Plate 8, Figure 1). This dike is also unique in that
twice its strike changes nearly 900 which would suggest intrusion along
Joint planes (Plate 4, pocket in back). Other portions of the granite.
however. do not show Jointing parallel to these directions.

faulting is found in several dikes, the best example being the major

dike which extends across the northwest side of Little Presque Isle. A

Plate 8

Figure l. Diabase dike, approximately two feet wide, in~
truded into granite. Twice the strike of this dike changes nearly
90°, which would suggest intrueisn along Joint planes in the
granite. The dike dips about 54 to the south.

Chevron banding in the sandstone, Jumt above the

Figure 2.
PhotOgraphed

unconformity on the east side of Little Presque Isle.
from about four feet.

 

Plate 8

second, and earlier period of faulting is believed to have caused the
displacement of the two dikes from which samples 45 and 51 were taken
(Plate 4. pocket in back). These dikes crop out along the northeast
(Sample 45) and northwest (Sample 51) sides of Little Presque Isle.
They trend nearly east-west, are approximately 20 feet wide, contain
numeroue granite stringers, and have no Jointing perpendicular to the
dike walls. The dikes not only differ markedly from the other dikes in
their physical characteristics but also contain a different plagioclase
(note sections on petrology, page 12, and petrography, page 22). It was
on the basis of this petrologic and.petrographic evidence that the writer
inferred.an earlier period of faulting. The writer was unable to find a
trace of this fault in the field.

lati as of ittl s t th istri t - In
considering the general geology of the Marquette District there are three
series which must be dealt with. the Basement Complex (further subdivided
into the northern and the Southern), the Lower Marquette Series (now divided

into Lower and Middle Huronian ). and the Upper Marquette Series (Upper

 

16] Leith, C. K., Land. He J.. and.Leith, Andrew. Pre-Cambrian rocks of

the Lake Superior Region, U. 8. Geological Survey Professional Paper

lfifi, Sizatiggaphig gharts 1925.

Huronian ).

 

 

12] idem.. 1925.

 

Polding of these series took place in a very complicated manner. In
general, the directions of folding are north-south, and east-west. The
largest, although the least conspicuous fold, is a north-south anticline.

which goes through Marquette. It has the effect of giving the east-west

-10-

folds a westward pitch. "However, the major part of the district was
affected by a more powerful north-south.pressure and therefore the east-
west folds, even though of smaller amplitude and wave length, are more
conspicuous. As a result of this north-south pressure the Marquette
Series was bent together in a synclinorium, flanked on either side by
the rocks of the Basement Complex. ‘

Little Presque Isle is believed to be a part of this Hortherm.Base—
ment Complex, which is comprised mainly of gneissoid granites, syenites,
greenstone-tuffs, conglomerates, schists, diabase, vein granites and

epidiorites.

 

ld/ op. cit., Van Bias. 0. 3., Bayley, w. s.. and Sewth, H. 1... pp.

142-129, 554-522, 1897.

 

SIDIMENTLHI PETROLOGY

ts s ds - The Potsdam sandstone is here characteristically
very weak, crumbly and easily eroded. It is a non—fossiliferous, coarse.
rather loosely cemented red rock which shows considerable leaching, not
only along fractures, but in small spots scattered over the surface,
lending to the entire outcrop a sort of widely spaced.polka dot pattern.
The sandstone is essentially horizontal, even where it comes into contact
with the granite. Just above the unconformity. chloritic pebbles (Plate 7.
Figure 2), possibly derived from the diabase dikes, and granite pebbles
are incorporated in the sandstone. Also, near the unconformity. is a small
area of banded sandstone. The banding is unique in that the bands are
concentrically arranged and have sharp angles, forming a pattern not un-
like that of chevron folding (Plate 6. and Plate 8, Figure 2). The band-
ing appears to be the result of some minor local deformation rather then

the result of deposition. Unfortunately it was impossible to see the bands

.11..

ing in the third dimension and thus possibly obtain a clue to its origin.
Scattered throughout the sandstone are fine conglomerate lenses (Plate 9.
Figure l) which would indicate that the sandstone had been derived from
some near by land mass and that there must have been minor fluctuations
of sea level during deposition. The sandstone is not believed to have
been derived from the granite with which it is in contact as the pebbles
of the conglomerate lenses are mainly quarts. Had the sandstone been
derived from the granite the conglomerate lenses would.probably be arko-

sic.
IGNEOUS PETROLOGI

Qiabase diggg - Some forty to forty-five diabase dikes, ranging in
width from one-half inch to sixty feet, are exposed on Little Presque Isle.
The weathered surface of most of the dikes is characteristically a dark,
greenish to brownish.gray. The dikes are relatively fresh and the wider
dikes show a considerable range in texture, becoming progressively coarser
toward the center of the dike. Jointing perpendicular to the dike walls
is well developed in the larger dikes, and consequently as blocks of the
dike are weathered out a sort of step-like topography is formed.(P1ate 9,
Pigure 2).

There are two notable exceptions to these general characteristics
of the diabase dikes. The first exception is exposed along the south-
west dide of the island. Here two dikes, approximately eight inches
wide (trending southeast) may be traced for eight or ten feet from the
edge of the lake. These dikes are very rotten and so completely weathered
that they are porous, and their original mineralogical composition is un-
determinable. Owing to the very advanced degree of weathering these

dikes would possibly suggest still another and much earlier period of

-12-

 

Plate 9

Figure 1. One of the conglomerate lenses in the sandstone.
This particular lens is about three inches wide, although these
lenses may range from one to more than four inches in width, and
may extend from six to more than thirty feet. PhotOgraphed at a
distance of four feet.

Figure 2. Diabase dike approximately forty-five feet wide.
is the dike weathers the Jointing, perpendicular to the dike walls,
causes a step-like topography,

 

Figure 1

 

Figure 2

Plate 9

few feet to the west of these dikes is a third, very much weathered,
rather schistose dike (not shown on map) which has been.pushed apart
along the planes of schistosity by pressure owing to the growth of quartz
crystals. Some crystals are three-quarters of an inch long.

The other exception is found in a dike on the northwest side of
Little Presque Isle, and also on the northeast side of the island, in a
dike of similar character (Plate #, pocket in back: Samples #5 and 51).
These two dikes, though similar to each other are radically different
from the other dikes exposed in the area. Jointing is not apparent, but
rather the dikes have a lineation parallel to the strike and have eroded
along the contact forming a chasm 20 or 30 feet deep, with a slab of dike
some ten feet wide remaining in the center. The writer believes these
dikes were faulted prior to the second period of diabase intrusion (note
section on structure, page 9). lo trace of faulting is found in the field,
and therefore this conclusion is based completely on the physical character-
istics of the dikes.

e i i -- The weathered surface of the granite ranges

 

12] op. cit., Van Rise. 6. 3., Bayley. w. 8.. and sum. H. I... pp. 1&9-
190:;l892.
from a light dirty gray to a rather deep orange red.where washing of the
rock by the waves has kept it free of lichens. The fresh surface of the
granite is dark gray with considerable red feldspar, some iron stain, and
in some places chlorite and epidote. The granite is cut by numerous peg-
matite dikes (Plate 10, rigure l) which range in width from tiny stringers
to three or more inches. These dikes extend in all directions, without
any pattern.

The term gneissoid granite would.perhaps he s better term inasmuch as

-13-

Plate 10

Figure 1. Pegmatite dikes, approximately one inch and two
and one-half inches wide. cutting the granite. The relationship
of these two dikes is evidence of at least two stages of pegb
natitic inJection.

figure 2. Relationship between the granite (light gray),
gneiss (dark gray), and the pegmatite. lots that the pegmatite
cuts both the granite and the darker gneiss. A.fault in the
pegmatite is shown in the lower center of the photo. Photo-
graphed fron a distance of approximately ten feet.

 

 

 

 

Plate 10

parts of the granite are very definitely banded. In a few places within
the granite a dark almost schistose rock is exposed, which upon mega-
scopic examination appears quite basic. However, microscopic examination
shows it to have essentially the same composition as the granite (note
igneous petrography; page 22), and it would seem to be merely a gneissic
phase of the granite. In some cases this rock is in sharp contact with
the granite and in other instances the two seem to be blended together.
In either case both are cut by the pegmatite (Plate 10, Figure 2).

No discussion of the petrology of the granite is complete without
considering the problem of origin. In attempting to answer the question
of origin the writer realises that the problem is one which would involve
investigation of all the granites within the Harquette District and per-
haps in the surroundingtarea. Such an investigation was not within the
scape of this study and therefore any ideas presented.here concerning the
origin of the granite are based solely upon such evidence as was found on
Little Presque Isle, and.upon the work of earlier geologists.

Iield evidence of granitisation, such as: l) inclusions within the
granite preserving the strike, dip and lineation of beds outside the
granite, 2) large phenocrysts of feldspar within the granite and in the
surrounding country rock, or 3) rock beyond 337 edges of the granite more

basic than the regular type of country rock, was not found in the area.

 

g9] Cooke, H. 0.. Back to Laganz A.discussion on granitisation:
Transactions of the Royal Society of Canada, Fol. ILII, Series III,
S ti 2 -#O 1 45.
Whether or not such evidence would be found if the entire flarquette Dis-
trict were studied the writer does not know. It does seem to be generally

accepted.by previous investigators of the larquette District (Van Rise.

-14-

Bayley, Leith, and others) that the Laurentian granite is an intrusive
body. It seems to the writer that the presence of the numerous peg-
matite dikes would further substantiate this theory of magmatic origin
as it is generally recOgnized thit the pegmatite represents the last

stages of a granitic intrusion.

 

g;] Johannsen, A. J.. A descriptive petrOgraphy of the igneous rocks,

Vol. II, The quarts-bearing rocks, p. 77. university of Chicago

zrggs, 3rd igprgssion, 19h9.
The writer believes a possible explanation for the gneissic char-

 

acter of parts of the granite may be litdpa::;it injection of the
22
granitic magma into the old schists. Grout made the following state-

 

,g;/ Grout, I. 1., Petrography and petrology, McGrawbHill Book Company,

Igg.. fig! Iggk, p, hho, 1932.
ment regarding lit-par-lit injection.

 

ICycle recorded in many batholithic injections in-
cludes a peripheral schistosity of the walls, an early
injection and breaking or stoping of the schist, a late
injection of pegmatite and an alternation of squeezing
and injection or filter pressing, which finally makes
those phases earliest to solidify.I

21/
Rice defines litapar-lit injection as

 

.gz] Rice, C. M.. Dictionary of geological terms, p. 225, Edwards Bros.

Ig§,. Ag; Agbggl gighiggg, 12&§.

“a term used to designate the intimate penetration
of bedded schistose or other fissile formations by in-
numerable narrow :h7ets and tongues of granitic magma.I
2a
According to Duly litdpar-lit injection may also be applied to

 

 

g5] Daly, BM.L., Igneous rocks and the depths of the earth, pp. 209, 212,

McGrawaHill Book Company,Inc., Dew York, 1933.

-15..

parallel sheets or sills of notable thickness.

SEDIMENTARY PETROGRAPHY

Egogedure oz sediment analyses - The sediment analyses of the sand-

stone and of the sand dune were carried out in the following manner.

1.

7.

The samples were split with a Jones Sample Splitter
and 100 grams of each sample were weighed out.

The carbonates were removed by leaving the samples
in dilute hydrochloric acid for fifteen heurs.
The samples were then washed, dried and weighed.

A.sieve analysis was made by shaking in a Ro-Tap
Automatic shaking machine for eight minutes.
Tyler sieves, ranging from .701mm to .088mm (35
to 200 mesh) inclusive were used.

The material from each sieve was collected, weighed,
and the weight percent and cumulative percent deter-
mined (Plates 11 and 12).

The heavy minerals were separated from the light with
bromoform. The principle here involved is one of

specific gravities. Bromoform has a specific gravity
of 2.87 and therefore the quarts (Sp. Gr. 2.65) and
fcldspal‘ (Sp. Gr. 2.55 to 2.75) may be floated off.

The heavies and lights were washed with alcohol, col-
lected, dried, and the lights weighed, thus giving
the weight of heavies by subtraction (Plate 13).

The heavies retained on the 200 mesh screen were
mounted in resin (piperine, n : 1.68) and in canada

balsam. .L heavy mineral count was made for each
sample (note data sheets, pages 18-21).

The lights retained on the 100 mesh screen were
mounted in resin (piperine, n 3 1.68). 100 quarts
grains from each sand were projected onto a table
and traced off for the purpose of making roundness
and sphericity measurments. Roundness was computed
by Wadell's methodigjf whereby roundness equals the
ratio of the sum of the roundness values of N cor-
ners to the number of corners (Plate 1“). Spher-
icity was computed by Riley's method,2§/ which is
the square root of the ratio of the maximum in-
scribed circle and the minimum circumscribed circle.

- 16 -

¢UMULATEDQ FREQUENCY-

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Ladell's Kethod for Computing Roundness of Quartz Grains

 

 

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_ Total degree of roundness

r : Radius of curvature of one corner
R = Radius of maximum inscribed circle
N = Number of corners

S:(r/R) : Sum of the roundness values of N corners

Plate 14

 

‘25]T’Vadell, Hakon, Volume, shape and roundness of quartz particles:
Journal of Geology, Vol. 43, No. 3, p. 269, April-May 1935.
‘29] Riley, l..A.. Projection sphericity: Journal of Sedimentary
Pet 10 To No. 2 4 A at 1 .
9. Kurtosie, sorting coefficient, quartile de-
viation, and skewness were computed from the
cumulative curve according to methods out-

lined by Krumbfiin and.PettijohnggZ/ (note
data sheets, pages 18-21).

 

gz/ Krum‘o'ein, I. 0.. and Pettijohn, I. J., Manual of sedimentary
petrography, pp. 228-267, Appletoanentury-Crofts, Inc., now York,

1938.

 

s ts f sediment ana s -.A summary of the data obtained from
the sediment analyses may be noted on the data sheets which follow (pages
18-21).

A comparison of the data obtained for the two sands shows a greater
degree of roundness for the dune sand, but a lesser degree of shpericity.
Both sands are well sorted, as indicated not only by the quartile de-
viation, but also by the sorting coefficient. The values obtained for
the quartile deviation and for the sorting coefficient show a greater de-
gree of sorting for the dune sand. This is well illustrated by comparing
the values of the logs of the sorting coefficient. These were .132 and
.083 for the sandstone and the dune sand, respectively. In other words
the spread of the sand grains in the sandstone is 2.67 times as great as
in the dune sand, or the dune sand is 2.67 times as well sorted as the
sandstone. This greater degree of sorting for the dune sand is not born
out by the kurtosis (measure of the peakedness of the curve) but this
may be accounted for by the rapid flatening of the cumulative curve be-

tween the 90th and 100th percent lines. Skewness (measure of the symmetry

- 17 -

Data Sheet

Sediment Analysis of Potsdam Sandstone

Amount of carbonate negligible

Sieve analysis

 

 

Screen Mesh Weight weight Cumulative
opening retained percent percent
in grams
.701 35 8.038 8.0# 8.04
.#95 #8 27.883 27.88 35.92
.351 65 34.616 3b.62 70.5“
.246 100 16.208 16.20 86.7h
.175 150 6.227 6.23 92.97
.124 200 3.087 3.09 96.06
.088 over 200 3.382 3,32 99.45
99.h48 99.45
Sieve loss

.___l252_
100.000 100.00

Percent heavies

 

 

Screen opening percent
.701 .05
.e95 .29
.351 .78:
.246 1.60
.175 1.33
.124 .55

Note: The heavy minerals were predominantely hematite or were so
coated with hematite that identification was practically
impossible. A few grains of tapes, sircon, garnet. tour-
maline and hornblende were recognized.

-18-

Sediment Analysis of Potsdam Sandstone (cont'd)

Median .450 Sorting VQ37Q1 1.36
Q1 .320 Leg sorting logQB-logQ,1 .132
2
Q3 .570 Skewness Q3Q1/Md .901 _
P10 .210 .-Kurtosis QB-Q1/2(P904P10) .265
P90 .680 Roundness z (r/R)/N .356
Quartile Q3-Q1/2 .125 Sphericity \/i7Dc .795
deviation .

Note: Md median
01 rirst quartile which is that diameter that has 25$ of
the distribution smaller than itself and 75% larger
and is therefore the 75% line on the cumulative curve.
Q3 Third quartile which is that diameter that has 25% of
the distribution larger than itself and 75% smaller
and is therefore the 25% line on the cumulative curve.
P10 That diameter that has 10% of the distribution smaller
than itself and 90% larger and is therefore the 90%
line on the cumulative curve.
That diameter that has 10% of the distribution larger
than itself and 90$smaller and is therefore the 10‘
line on the cumulative curve.
i Diameter of maximum inscribed circle possible in quarts
grain.
Dc Diameter of minimum circumscribed circle possible for
quarts grain.
lots: for values for the equation for the determination of

roundness see Plate 14.

-19..

Data Sheet

Sediment Analysis of Sand Dune

Amount of carbonate negligible

Sieve analysis

 

 

 

 

Screen Mesh Height Weight cumulative
opening retained percent percent
in grams
.701 35 2.911 2.91 2.91
.495 48 45.040 45.04 47.95
.351 65 44.068 44.07 92.02
.2u6 100 6.390 6.39 98.41
.175 150 1.110 1.11 99.52
.120 200 0.136 0.13 99.65
.088 over 200 0,022 0.08 99.73
99.733 99.73
Sieve loss 26 2
100.000 100.00
Percent heavies
Screen opening percent
.701 .12
.495 .79
.351 1.09
.246 1.16
.175 .52
.124 .08

- 20 -

Sediment.1nalysis of Sand Dune (cont'd)

Heavy mineral count

lagnetite
Zircon
Angite
Hematite
Chlorite
Hornblende
Topaz

Others

67.16%
10.14
7.46
n.62

' 3.43
1.63
1.04

4.52

(including leucoxene, apatite, tourmaline, corundum,
biotite, epidote. muscovite, fluorite, sillimanite,

and titanite)

Note: A total of 670 grains was counted.

ledian .485
Q1 .395
Q3 .580
210 .360
P90 .660

Sorting V037 Q1

Log sorting logQ3-1ogQ1
Skewness Qa-Ql/Mdz
Kurtosis Qa-Q1/2(P904P10)
Roundness E (r/B)/l

Quartile 03‘01/2 .092 Sphericity' Vi7ne

deviation

Note: For the values for the above equations see page 19.

- 21 -

1.21
.083
.974
.308

O 785

of the curve) shows greater symmetry for the sandstone, and for both sands
indicates that the degree of sorting lies on the coarser side.

The data obtained from the sand analyses was of value in deter-
mining the origin of the sand dune, however, it gave no clue as to the
origin of the sandstone. The theory of dune origin attributed to the
unconsolidated sand above the sandstone was based on the position of the
sand, 54 feet above Lake Superior and 28 feet above the old Hipissing

beach, and on the high degree of sorting. According to Sherser the

 

‘28] Sherser, I. H., Criteria for the recognition of the various types
of sand grains: Bull. of the Geological Society of America, Vol. 21,

:92, 6253362. lgv‘empgr 1910.
degree of sorting for the various types of sands is highest for the

 

aeolian deposits, With regard to the sandstone such an analysis might
have value as a method of correlation between formations, as certain
heavy minerals could conceivably be found in one sandstone and not in
another, or sphericity and roundness measurements might shov trends for
different formations if a sufficient number of grains were measured to

give valid results.

IGNEOUS PETBOGRAPHY

Diabasg dike; - The term diabase is here used to include all the
basic dikes found in the area. The term is textural and is discussed

by Johannsen as follows:

 

.22] Johannsen, A. J., A descriptive petrOgraphy of the igneous rocks,
Vol. I, Introduction, textures, classifications and glossary,

ni ersi hi 0 Pr s rd 1 ession 0 4

- 22 -

I'Diabasic (Rosenbusch) - Generally used as synony-
mous with ophitic, since this texture is common in dia-
bases. Por Kemp the difference between ophitic and dia-
basic 'lies in the fact that in the former the augite is
in excess and the feldspar is involved in it. In the
latter, the feldspar is in excess, and the augite fills
the interstices between its lath-shaped crystals“.

I'Diabase (Brongniart) -- Originally proposed as a
substitute for Grdnstein, the word has come.to mean.a
pre-Tertiary basalt in Germany, a decomposed basalt in
England, and a dike-rock with ophitic texture in the
Uhited States and Canada“.

Inasmuch as the term diabase is a textural term.and fails to give
any clue as to the mineralogy of the dikes they are discussed separately

‘39]

and classified according to Johannsen's system. The percentages are

 

'30] op. cit.. pp 141-161, 1948.

 

estimated.
Samples 8 and 9
Gabbro, 2-3-12.

MegascOpic description -- Weathered surface dark, dirty gray
brown owing to growth of lichens. Fresh surface dark,
grayish green. Both samples are from the same dike
(Plate 4, pocket in back), however, one is taken from
the center (8), and the other from the edge (9) of the
dike. A.marked difference in texture in the two samples
is readily apparent, becoming progressively coarser
toward the center owing to the slower rate of cooling
and the increased length of time for crysta1 growth.

MicroscOpic description -— Texture is diabasic, however,
again a notable difference in texture is readily
apparent between the two samples (Plate 15, Figures
1 and 2). The labradorite occurs as lathe and the
augite as grains.

Mineralogy
Essentials Accessories Secondary
Labradorite 50% Magnetite Calcite
Augite 38 Iaolinite
Olivine 5 Sericite
Picotite 7 Chlorite

Myrmekite 7!?

- 23 -

Plate 15

Figure 1. Sample 8, ordinary light, magnification 28.61.
Olivine (light gray) in center surrounded by labradorite. The
lath shape of the labradorite is not well illustrated under
ordinary light. Note the larger grain size as compared to
Figure 2, a photomicrograph of a thin section from the same
dike, but taken from the edge.

Pigure 2. Sample 9, ordinary light, magnification 28.61.
Laths are labradorite and the larger grains are augite. Reso-
tasis is plagioclase.

 

Figure l

 

Plate 15

Samples 8 and 9 (cont'd)

Remarks - Myrmekite is a secondary intergrowth of quartz
and plagioclase. It would therefore be possible to
have quarts in this form.in the same rock with olivine.
However, according to Drescher‘ll/ myrmekite is formed
only along the contact of a potash feldspar and a
plagioclase. For this reason the writer questions the
identification of the myrmekite owing to the apparent
lack of a potash feldspar. However, if very minor amounts
of potash feldspar were present it would no doubt be
sufficient to account for the very minor amount of myrmekite
found. The sequence of crystallization appears to be
olivine. picotite, larbradorite, and pyroxene. According
to Vogt13g/ .'The sequence of crystallization of plagioclase
and pyroxene depends upon the relative quantity of the two
minerals for the crystallization commences with the solidi-
fication of the mineral present in excess of a certain
limit.“

 

 

‘31/ Drescher, van, 3. K.. Die Feldspat-quarts - Beaktionsquefdge der
Granite and Gneise und ihre genetische Bedeutung, Mineralogie und
Petrographie in linzeldarstellungen, pp. 21-22, Berlin, 1948.

‘12/ Vogt, J. H. 1., The physical chemistry of the crystallization and

magmatic differentiation of igneous rocks: Journal of Geology,
191, zglg, pp, 438-440I 1221.

 

Sample 45

XegascOpic description .. Weathered surface a dark, dirty grayish
green. Presh.surface dark, grayish green. Pyrite crystals
visible in hand specimen.

Microscopic description - The alteration of the minerals has
been so complete that the determination of the feldspar
and the pyroxene is difficult with any degree of accuracy.
The feldspar is thought to be andesine and the pyroxene
augite. Ohlorite is pale green and slightly pleochroic.
Some chlorite shows pseudohexagonal crystals under
ordinary light.

Mineralogy
Essentials Accessories Secondary

Andesine 50% Pyrite Leucoxene

Augite 4O Calcite

Diotite 6 Kaolinite

Magnetite 4 Chlorite
Chalcedony
Dralite

-21.}-

Sample 51
Diorite, 2-2-12.

Megascopic description - Weathered surface dark, dirty gray
green. Fresh surface dark, grayish green.

Microscopic description -- Considerable alteration of the
minerals and therefore the determination of the feldspar
and the pyroxene is difficult with any degree of accuracy.
Pyroxene has been altered to a fibrous amphibole(uralite).
Biotite is the brown variety and is considerably altered
to chlorite. Chlorite is pale green and slightly plea-
chroic. Some chlorite shows pseudohexagonal crystals under
ordinary light.

Mineralogy
Essentials Accessories Secondary
Andesine 1 50% Calcite
Microcline 2 Leucoxene
Augite ? 36 Chalcedony
Biotite 7 Chlorite
magnetite 5 Uralite

Kaolinite

Remarks - Owing to the marked similarity between samples 45
and 51 the writer believes these two dikes may be cor-
related and were faulted prior to the second period of
diabase intrusion (Plate 4, pocket in back). Although
the two samples differ in minor respects the plagioclase
in each case is thought to be andesine and therefore the
dikes would be diorite. All other major dikes on the
island are gabbro.

Sample 36
Gabbro, 2-3-12.
MegascOpic description -- Weathered surface a dark, gray green,
almost black. Fresh surface dark, greenish black. Texture

fine to medium.

Microscopic description - Texture is diabasic. Plagioclase
occurs as lathe, the augite as grains.

Mineralogy
Essentials Accessories Secondary
Labradorite 55% Kaolinite
Augite 3O Leucoxene
Biotite 6 Chlorite (var.
Magnetite 9 penninite)

Chalcedony
Myrmekite ll?

- 25 -

Sample 46
Gabbl‘o , 2-3-12 e

Megascopic description - Weathered surface dark, dirty gray.
Iresh surface dark, green black. Texture is fine to medium.

Microscopic description - Texture is diabasic. Plagioclase
occurs as lathe, the augite as grains. Ilmenite has well
developed skeleton crystals.

Mineralogy
Essentials Accessoriée Secondary

Labradorite 50% Ilmenite Deucoxene

Augite 32 Chlorite (var.

Biotite 6 penninite)

Magnetite 8 Kaolinite

Olivine 4 Chalcsdony
uralite (secondary

hornblende)

Myrmekite 1??

Remarks - Owing to a somewhat similar mineral composition, and
the same general trend, width, and megascopic characteristics
it is thought possible that the dikes from which samples
36 and 46 were taken may be correlated.

Leggentian ggagitg -.A discussion of the petrography of the granite
must include two phases, one a granitic and the other a gneissic.
Samples.l and 54

Granite, 2-2-6.

Note - Although these two samples were taken from different parts
of the island.their mineralogy is the same and therefore they
are discussed together.

MegascOpic description - Weathered surface is a rather dark, pinkish
gray. The fresh surface is a dark gray with considerable red
feldspar.

Microscopic description - The texture is hypautomorphic granular.
In general the biotite is brown, very strongly pleochroic,
and the birds-eye-structure is well developed. Some magnetite
is completely altered to hematite and leucoxene. One or two
crystals of ilmenite are found.

Mineralogy
Essentials Accessories Secondary
Quartz 25$ Zircon Chlorite

Samples 1 and 5h (cont‘d)

Mineralogy
Essentials .Accessories Secondary
Microcline 8i Apatite Leucoxene
Orthoclase 35 Magnetite Kaolinite
Oligoclase 25 Ilmenite Calcite
Biotite b Hematite

Muscovite
Samples 43 and 52
Granite, 2-2-6.

Note - Although these two samples were taken from different parts
of the island their mineralogy is essentially the same and
therefore they are discussed.together.

Hegascopic description - Weathered surface dark, dirty gray. fresh
surface dark gray. Schistose and somewhat crumbly. Biotite
is readily apparent in hand specimen.

Microscopic description - The gneissic phases of the granite differ
from the normal granite mainly in the amount of mafites, which
include some blue-green hornblende, and very minor amounts of
augite which has been altered to uralite (Plate 16, Figure 1).
Xafites are aligned parallel with the length of the crystals.
One or two well deve10ped skeleton crystals of ilmenite are
found (Plate 16. Figure 2).

Mineralogy

Essentials Accessories Secondary
Quart: 18% _—Pyrite Oralite
Hicrocline 7 Rutile Kaolinite
Orthoclase 30 Titanite Sericite
Oligoclase 15 Magnetite
Biotite 8 Ilmenite
Hornblende l7
Augite 5

Samples 10 and no

Note - These two samples were taken near the contact of the granite
' and diabase (Sample 10) and at the contact (Sample #0). The
purpose of these thin sections was to see what changes occurred
in the granite as a result of the intrusion of the diabase.
The only major change seems to be the addition of calcite and
chalcedony (Plate 17, Figures 1 and 2).

Plate 16

Figure 1. Sample 52, ordinary light, magnification 28.61.
Alteration of augite to uralite (fibrous amphibole) shown on
left side of photomicrograph. Note the parallel orientaion of
the mafites (dark gray) and quarts (clear light gIEY). Opaques
are pyrite.

figure 2. Sample 52, ordinary light, magnification 28.61.
Skeleton crystal of ilmenite on left side of photomicrOgraph.
The clear light gray mineral is quartz, cloudy light gray feld-
spar, and the darker gray minerals biotite and augite. late the
parallel orientation of the minerals.

 

Figure 2
Plate 16

Plate 17

Figure 1. Sample 10, crossed nicols, magnification 28.61.
Photomicrograph of granite near contact with diabase. Fins
grained minerals in center of photo are calcite and chalcedony
which apparently occur in the granite as a result of the in-
trusion of the diabase.

Figure 2. Sample 40, ordinary light, magnification 28.61.
Photomicrograph of granite in contact with diabase (dark grey
on right side of photo). Husbase is considerably altered to
chlorite and kaolinite. Small opaque grains are leucoxene.
Clear light gray minerals are quartz and secondary calcite.

 

Plate 17

CONCLUSION

Summggy - The geology of Little Presque Isle may be briefly
summarized as follows: 1) the intrusion of the Laurentian granite into

the older Mona and Kitchi schists, 2) at least two injections of

 

lljf—vop. cit., van Hise, C. 3., Bayley, W. 5., and Smyth, H. L., pp.

149-190; 1892;._
pegmatite, probably representing the final stages of granitic intrusion,
3) the intrusion of the oldest diabase, b) faulting ?, 5) the second in-
trusion of diabase, 6) faultinso 7) a third period of diabase intrusion,
8) uplift, diastrophism.and a long erosional interval, 9) deposition of
the Potsdam sandstone upon an old erosion surface, 10) uplift and erosion,
ll) deposition of the Pleistocene drift, and 12) erosion of the land con-
nection between Little Presque Isle and the mainland.

In regard to the original objectives of the study the writer con-
cludes that the sediment analyses was of value in the determination
of the origin of the Pleistocene drift, but of no welue in the deter-
mination of the possible origin of the sandstone. The petrographic
study of the rocks did aid in the correlation of the dikes and did show
some changes in the granite, which were apparently the result of the
intrusion of the diabase.

figssibilitigs f9; fggther study - The following possibilities for
further study in the Marquette District and surrounding area occurred to
the writer while working on this report: 1) a detailed examination of
the granite to determine if some of the granite now attributed to a mag»
matic origin might not be granitised sediments, 2) an attempt to cor-
relate the sandstones along the Lake Superior shore by a sediment

analysis, 3) a possible study in petrofabrics on some of the rocks of the

-28-

Marquette District, and 4) more detailed studies of the island and.points

along the Superior shore in the vicinity of Marquette.

-29-

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