Stratigraphy of 

Pre-Keweenawan 

Rocks in Parts of 

Northern Michigan

GEOLOGICAL SURVEY PROFESSIONAL PAPER 314-C

Prepared in cooperation with the 

Geological Survey Division, Michigan 

Department of Conservation

Stratigraphy of 

Pre-Keweenawan 

Rocks in Parts of 

Northern Michigan

By HAROLD L. JAMES

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 314-C

Prepared in cooperation with the 

Geological Survey Division, Michigan 

Department of Conservation

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1958
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 20 cents (paper cover)
CONTENTS

Abstract...._______________________________________

Introduction. ____________________________________

Subdivision of Precambrian time________-_-__-_--___

Lower Precambrian rocks__________________.________

Older granite gneiss and included bodies of quartzite 

and schist_____________________________

Dickinson group____________________________

East Branch arkose_____-___-_--______-__-_-

Solberg schist.__________________________.__

Six-Mile Lake amphibolite.-____-_---______-_

Hardwood gneiss.______________________________

Quinnesec formation________________. ____________

Post-Dickinson gneissic granite_.____--______-____

Middle Precambrian rocks._________________________

Chocolay group______________________________

Menominee group__________________________ ___

Pace

27 Middle Precambrian rocks Continued

27 Baraga group.________________ 

28 Goodrich quartzite._ ______

31 Hemlock formation __________

Fence River formation._ 

31 Amasa formation__ ________

31 Michigamme slate. _________

31 Badwater greenstone _________

32 Paint River group______------___

32 Dunn Creek slate__________

32 Riverton iron-formation ______

32 Hiawatha graywacke.________

33 Stambaugh formation ________

33 Fortune Lakes slate.____.___

35 Middle Precambrian igneous rocks_____

35 References cited________-__--_- _.__

Index._____________________________

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ILLUSTRATION

FIGURE 2. Generalized geologic map of northern Michigan.

Page 

28

TABLES

TABLE 1. Lithologic sequence of Precambrian rocks in Iron and Dickinson Counties, Michigan____________ __ __ 30

2. Comparison of the stratigraphic position of the Animikie group of Minnesota with the Animikie series as defined

for part of Michigan.. __________ __ __ _ _______ 35

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

STRATIGRAPHY OF PRE-KEWEENAWAN ROCKS IN PARTS OF NORTHERN MICHIGAN

By HAROLD L. JAMES

ABSTRACT

The Precambrian rocks of northern Michigan are placed in 

three major categories Lower Precambrian, Middle Precambrian, and Upper Precambrian. This report summarizes stratigraphic nomenclature used by the U. S. Geological Survey for 

the Lower and Middle Precambrian rocks that occur in Iron and 

Dickinson Counties.

A sequence of Lower Precambrian rocks Archean of older 

reports is established on the basis of detailed mapping in 

central Dickinson County; it consists of metamorphosed sedimentary and volcanic rocks, herein named the Dickinson group. 

These strata rest unconformably on an older granite gneiss, 

and in turn have been invaded by granitic rocks of batholithic dimensions. The pre-Dickinson granitic gneiss contains 

inclusions of quartzite and schist, relics of a still older sedimentary sequence. The Dickinson group is divided into three 

formations East Branch arkose (oldest), Solberg schist with 

the interbedded iron-rich Skunk Creek member, and the SixMile Lake amphibolite. Separated by a covered interval from 

strata of the Dickinson group are rocks herein designated the 

Hardwood gneiss, which may be in part or wholly equivalent 

to the Dickinson group. The absolute age of the post-Dickinson 

granitic rocks is not known.

Middle Precambrian rocks consist chiefly of the Animikie 

series (Huronian of older reports) and post-Animikie, preKeweenawan igneous rocks. The term Huronian, used in the 

region for nearly 60 years, is not used because of the 

uncertainty of correlation with the type Huronian of Canada. 

The strata previously correlated with the Huronian are now 

referred to the Animikie, here expanded from group to series 

rank, on the basis of accepted correlations with the strata of 

the Mesabi district of Minnesota, which have been traced into 

the type Animikie of the Thunder Bay area of Canada.

The Animikie series rests with profound unconformity on 

the eroded surface of Lower Precambrian (post-Dickinson) 

granitic rocks in many places in northern Michigan. The 

Animikie is divided into four groups that are defined in this 

report. In order of age (oldest to youngest) these are (a) the 

Chocolay group, comprising the Fern Creek formation, Sturgeon quartzite, and Randville dolomite, and their stratigraphic 

equivalents; (b) the Menominee group, which in the type area 

consists of the Felch formation (Felch schist and equivalent 

rocks of older reports) and the Vulcan iron-formation, with 

the Traders, Brier, Curry, and Loretto members; (c) the 

Baraga group, comprising the Goodrich quartzite, Hemlock 

formation, Fence River formation and its probable equivalent 

the Amasa formation, Michigamme slate, and Badwater greenstone; and (d) the Paint River group, which comprises the 

Dunn Creek slate, Riverton iron-formation, Hiawatha graywacke, Stambaugh formation, and Fortune Lakes slate. The 

strata of the Paint River group, which occur chiefly in the 

Iron River-Crystal Falls district, previously have been considered part of the Michigamme slate.

The Animikie strata were deformed, metamorphosed, and intruded by dikes, stocks, and sills of gabbroic to granitic composition in a pre-Keweenawan erogenic interval. Recent determinations of A^/K40 and Rb'YSr87 indicate a probable age 

of at least 1,400 million years for the granitic rocks, which commonly have been referred to as "Killarney", on the basis of 

assumed equivalence in age with the Killarney granite of the 

north shore of Lake Huron. Recently reported ages of rocks 

from near the type area of the Killarney granite indicate that 

the assumed equivalence is not valid.

INTRODUCTION

Since 1943, the U. S. Geological Survey, in cooperation with the Geological Survey Division of the Michigan Department of Conservation, has been engaged in 

restudy of the iron-bearing districts of northern Michigan (fig. 2). In cooperation with the State of Wisconsin, this study has recently been extended into 

adjoining areas of Wisconsin. A number of reports on 

various aspects of the work have been published (Dutton, Park, and Balsley, 1945; Petti John, 1947; James, 

Clark, and Smith, 1947; James, 1948; Pettijohn, 1948; 

James and Wier, 1948; Dutton, 1949; Good and Pettijohn, 1949; Balsley, James, and Wier, 1949; Dutton, 

1950; Wier, 1950; James, 1951; James and Dutton, 

1951; Pettijohn, 1952; Wier, Balsley, and Pratt, 1953; 

James, 1954; James, 1955; Gair and Wier, 1956) and 

several more have ben placed on open file (Lamey, 1946; 

Lamey, 1947; Lamey, 1949; Wier and Kennedy, 1951; 

Pettijohn, 1951; Clark, 1953; Bayley, 1956). Eeports 

on the Lake Mary quadrangle (E. W. Bayley), Central 

Dickinson County (James, Lamey, Clark, and others), 

the Iron Eiver-Crystal Falls district (James, Pettijohn, and Dutton), and Southern Dickinson County 

(Bayley, Lamey, and Dutton) are in various stages of 

preparation for publication. Most of the early reports

27
28 SHOETEK CONTRIBUTIONS TO GENERAL GEOLOGY

Upper Precambritm (Keweenawan) rocks

^^^"Tk" A/-*Di£rRICT<

FIGURE 2. Geologic sketch map of northern Michigan.

and all those placed on open file were preliminary in 

nature; stratigraphic names were used in conformity 

with existing terminology or were described only in 

terms of lithology, without formal designation. The 

reports now in preparation, however, will contain many 

new stratigraphic names and revisions of terminology 

of the pre-Keweenawan rocks. The present paper is 

written so that the problem can be reviewed in its entirety rather than piecemeal; in part it will summarize 

that which will be presented in more detail in later 

reports on individual areas.

The conclusions summarized in this brief report are 

based on more than 200 man-months of field work. In 

preparing the report I have drawn heavily on the 

published and unpublished work of my colleagues: 

R. W. Bayley, L. D. Clark, C. E. Button, Jacob Freedman, J. E. Gair, C. A. Lamey, F. J. Petti John, W. C.

Prinz, James Trow, and K. L. Wier. Each of these 

men criticized preliminary drafts of the manuscript 

and their suggestions have been incorporated. Complete unanimity of view on all points is not to be 

expected, but we are in agreement on all major aspects 

of the stratigraphy. The final responsibility for the 

statements made herein is mine, but the report is a joint 

effort and I hope it will be so regarded.

SUBDIVISION OF PRECAMBRIAN TIME

Unlike the situation with respect to post-Precambrian time, there exists no widely accepted reference 

framework of eras and periods to which Precambrian 

lithologic units and geologic events can be related. The 

principal reasons, of course, are obvious: lack of diagnostic fossils and the greater degree of deformation 

and metamorphism. Even within the Canadian shield
PRE-KEWEENAWAN ROCKS IN PARTS OP NORTHERN MICHIGAN 29

no satisfactory subdivision has been achieved, although 

the literature is strewn with the debris of many unsuccessful attempts. Trust in particular lithologic aspects (as in the original definitions of the Archean and 

Algonkian) and a shield-wide unconformity (the 

Eparchaean interval) has been shown to have been 

misplaced; it now seems safe to say that virtually all 

the familiar time and time-stratigraphic terms Archean, Algonkian, Laurentian, Keewatin, Algoman, 

Huronian, and Killarney have become so beclouded by 

misuse they are of dubious value except in the type 

areas (Gill, 1955).

The monograph studies of the Lake Superior region, 

summarized by Van Hise and Leith (1911), employed 

a standard twofold classification of Precambrian time 

into two major units the Archean and the Algonkian. 

Emphasis was placed on the complex, crystalline nature 

of the Archean system, and upon the profound unconformity that separated the Archean from the Algonikian. Subsequent work in the Canadian shield was to 

show that in many places the Archean contains thick 

sequences of only weakly metamorphosed sedimentary 

rocks and that the "Eparchaean" unconformity is not 

readily defined. The increased uncertainty as to the 

precise correlation of rocks in separated areas finally 

led Leith (1934), in his scholarly review of the Precambrian, to propose that the terms Archean and Algonkian be used only in a relative sense essentially to 

indicate the nature of the lithology. Leith, Lund, and 

Leith (1935) adopted the scheme whereby only one 

term Precambrian is used in a strict time sense, 

and Archean-type and Algonkian-type were used to 

characterize Precambrian rocks but without definite 

time significance. This method of designation has not 

been followed by other workers in the region, chiefly 

because unless the terms have time sense they are of 

no value; with less confusion one may refer to a terrane as consisting simply of an older sequence and a 

younger sequence of rocks. Furthermore, many truly 

older ("Archean") rocks are not "Archean-type", 

whereas some younger rocks elsewhere are.

In 1934, a special committee of Section IV of the 

Eoyal Society of Canada (Alcock, 1934) recommended 

that the terms Archean and Proterozoic be used in 

Canada for Early and Late Precambrian, respectively, 

which were the terms then in current usage by the Geological Survey of Canada. The base of the Bruce 

series (lower Huronian) was proposed as the dividing 

line between the two major divisions. The Geological 

Survey of Canada follows the recommended terminology, but with recognition of the difficulty of application in some areas and recognition of the uncertainty 

as to the exact position of the dividing line between 

the Archean and the Proterozoic (J. M. Harrison,

1957, written communication). Although the two-fold 

subdivision would seem quite applicable to the rocks 

in northern Michigan, there remain the same questions 

of actual time significance that plagued Leith and led 

him to abandon Archean and Algonkian. The rocks 

of many, if not most, areas of the Canadian shield can 

be placed in two main groups separated by a profound 

unconformity. But is this unconformity of the same 

age in each area ? May it not be that the rocks beneath 

the unconformity in one area are actually timeequivalent to rocks above a comparable unconformity 

in another area? What of the interval represented by 

the unconformity itself? And in some districts profound unconformities separate the rocks into three or 

more groups. How are the rocks then to be classified 

in a twofold time sense ?

Only one term Precambrian is now used in a strict 

time sense in reports of the U. S. Geological Survey; 

the terms Archean and Algonkian were abandoned more 

than 20 years ago. For a region in which at least 2,000 

million years of geologic history is recorded in the rocks 

exposed at the surface and for which the geologic record locally has been worked out in as great detail as in 

many well-studied younger terranes, the situation obviously is unsatisfactory. Time was an element equally 

important prior to Olenellm as it was later, and the 

analysis of geologic events demands a greater subdivision than that permitted by the single term Precambrian. Yet it seems equally evident that until gross 

subdivisions can be placed in an actual time scale by 

age determinations, they have little general value and 

in fact may lead to many incorrect conclusions regarding regional problems. Fortunately, many laboratories 

are perfecting methods of geochronology; the number 

of absolute age determinations is increasing rapidly, 

and methods such as potassium-argon permit determination of the age of rocks other than those of intrusive 

origin. Within a decade, perhaps, it may be possible to 

define major subdivisions of time in terms of adequately 

mapped and geologically understood rock sequences in 

the Canadian shield.

Until an adequate basis for time division is available, 

informal subdivisions such as Lower Precambrian, 

Middle Precambrian, and Upper Precambrian are used 

by the U. S. Geological Survey. Strictly speaking, these 

are meant to apply only to rock units within a given 

area, not a subdivision of time (hence Lower and 

Upper, rather than Early and Late), and the designations apply only to the particular area under discussion. 

Inevitably, however, because of the overriding need for 

subdivisions of time, the terms are used informally in 

a time sense and, regardless of qualifications held by the 

originators, they will be so interpreted by others (see, 

for example, Wilson, 1956). Furthermore, unless the
30 SHORTER CONTRIBUTIONS' TO GENERAL GEOLOGY

implication of time is heeded, workers within the same 

region or even in adjoining areas presumably could, 

on the basis of local geology, place a recognized correlative unit in different categories. Indeed this has 

already happened: Grout, Gruner, Schwartz, and Thiel 

(1951) place the Biwabik and Gunflint formations in 

the Late Precambrian an eminently justifiable decision considering the area whereas in Michigan the 

agreed correlatives are referred to the Middle Precambrian (James, 1955), which is equally justifiable on the 

basis of the geology of that area. Until enough absolute 

age determinations are available properly to define gross 

subdivisions of time for the region, this obviously unsatisfactory situation seemingly must be endured; all 

that can be done to minimize the confusion is to define 

usages as clearly as possible and to apply the same terms 

to time-equivalent units, so far as this is possible with 

the means at hand.

TABLE 1. Lithologic sequence of Precambrian rocks in Iron and Dickinson Counties, Mich. 

[New stratigraphic names are Indicated by asterisks]

Precambrian

Upper Precambrian

Middle Precambrian

Lower Precambrian

Keweenawan series Diabase dikes and sills (probable age about 1,100 million years) -Intrusive contact 

Granitic intrusive rocks (probable age at least 1,400 million years)

 Intrusive contact-

-Intrusive contactMetadiabase and metagabbro

Animikle series

Paint River group*

Baraga group*

Menominee group*

Ohocolay group*

Fortune Lakes slate*

Stambaugh formation*

Hiawatha graywacke*

Riverton iron-formation*

Dunn Creek slate*, with Wauseca pyritic member*

Badwater greenstone*

Michlgamme slate

Fence River formation Amasa formation

Hemlock formation, with Mansfield iron-bearing slate member and Bird ironbearing slate member*

Goodrich quartzlte

-UnconformityVulcan iron-formation

Loretto slate member

Ourry iron-bearing member

Brier slate member

Traders iron-bearing member

Felch formation

Randville dolomite

-UnconformitySturgeon quartzlte

Saunders formation

Fern Greek formation UnconformltyGneisslc granite and other crystalline rocks

-Intrusive or replacement contact??-

Dickinson group*

Six-Mile Lake amphibolite*

Solberg schist*, with Skunk 

Creek member*

Granite gneiss

East Branch arkose* UnconformityHardwood gneiss* 

position uncertain

Quartzlte and schist (small bodies included in granite gneiss)

Quinnesec formation 

(position uncertain)

Margeson Greek gneiss 

(position uncertain)

For the rocks of northern Michigan, the gross subdivisions that will be followed are given in comparison 

with previously used terminology.

Van Hise and Leith 1911

Proterozoic

Algonkian

Archean

Leith, Lund and Leith 1935

Precambrian Algonkian-type

Archean-type

Grout et al. (Minnesota) 1951

Late Precambrian

Middle Precambrian

Early Precambrian

Present Geological Survey usage for 

northern Michigan

Precambrian

Upper Precambrian

Middle Precambrian

Lower Precambrian
PRE-KEWEENAWANI ROCKS IN PAETS OF NORTHERN MICHIGAN 31

Most of the recent work of the Geological Survey in 

Michigan has been confined to Iron and Dickinson 

Counties (fig. 2), an area which includes the Iron 

River-Crystal Falls, Menominee, Felch Mountain, and 

Calumet districts. The stratigraphy of the Precambrian rocks of this area, including terms that will be 

defined later in the present report, is summarized in 

table 1.

LOWER PRECAMBRIAN ROCKS

The subdivisions of Lower Precambrian rocks, given 

in table 1, have been defined on the basis of detailed 

mapping in central Dickinson County, chiefly by Lorin 

D. Clark, C. A. Lamey, F. J. Pettijohn, Jacob Freedman, and James Trow. Preliminary descriptions of 

some parts of this area have been made available 

(Clark, 1953; Lamey, 1946, 1947, 1949; Pettijohn, 

1951) and a general report on the entire area is in preparation (James, H. L., Lamey, C. L., Clark, L. D., and 

others).

OLDER GRANITE GNEISS AND INCLUDED BODIES 

OF QUARTZITE AND SCHIST

A strongly deformed granite gneiss is exposed in the 

northern part of T. 42 N., Ks. 28, 29, and 30 W. and is 

overlain unconformably by the East Branch arkose 

(table 1). The rock is described by Clark (1953, p. 10) 

as follows:

Although the composition and texture of the gneiss range 

widely, a coarse brick-red to very light-red rock of porphyritic 

aspect is most common. The largest phenocrysts are 1% inches 

long and are microcline or albite. A marked cataclastic texture 

with strong secondary foliation is characteristic of the red 

porphyritic granite. Many of the large feldspar grains are 

fractured and rounded. They are separated by laminae of 

micaceous or chloritic minerals in which are embedded microcline fragments and thin, tabular quartz grains.

Less common varieties of granitic gneiss exposed in T. 

42 N., R. 28 W. include fine-grained granite and quartzrich granite.

The gneiss is especially well exposed in the north part 

of sec. 7, T. 42 N., E. 28 W. and in adjoining parts of 

adjacent sections. Foliation trends easterly and is 

nearly vertical.

Inclusions of schist and of quartzite measurable in 

tens of feet in width and length are sparsely distributed 

in the granite gneiss. They are the only known 

remnants of the terrane in which the granite was 

emplaced.

DICKINSON GROUP

The rocks here named in the Dickinson group occur 

in an easterly trending belt about four miles wide in the 

central part of T. 42 N., Rs. 28, 29 and 30 W. This area 

is in the central part of Dickinson County, for which the

449221 58 2

group is named. Magnetic green schist believed correlative with the Dickinson group occurs in the Kiernan 

,and Lake Mary quadrangles to the northwest (Gair and 

Wier, 1956, p. 27; R. W. Bayley, 1956, p. 13). Detailed 

mapping by Lorin D. Clark and C. A. Lamey has shown 

that the group can be divided into three formations in 

central Dickinson County. These formations East 

Branch arkose (oldest), Solberg schist, and Six-Mile 

Lake amphibolite are defined below. The rocks are 

steeply dipping or vertical and the sequence is believed 

to be conformable.

EAST BRANCH ARKOSE

The strata here named the East Branch arkose are 

well exposed in broad glacially polished outcrops in 

sees. 17 and 18, T. 42 N., R. 28 W., along the East 

Branch of the Sturgeon River (Clark, 1953). Many 

other excellent exposures occur for about two miles 

both east and west of this area. The strata comprise 

arkose, arkosic conglomerate, and interbedded metamorphosed basalt flows and basic tuffs. The sedimentary rocks are characterized by coarseness of grain, 

poor sorting, and beds of conglomerate. The pebbles 

of the conglomerate are deformed into elliptical bodies 

in the plane of foliation. About 75 percent of the 

fragments more than an inch long are quartzite, but 

pebbles similar or identical to the underlying granite 

gneiss are abundant. The arkosic beds have been reconstituted to a gneissic-appearing rock but crossbedding is well preserved; the beds are steeply dipping 

or vertical, and the top direction is consistently to the 

south.

The actual contact with the older granitic gneiss has 

not been observed, though in some places the two rock 

units are exposed but a few feet apart. The younger 

age of the arkose is shown by the presence of granitic 

gneiss pebbles in the conglomerate and by the absence 

of any metamorphic effects that can be related to the 

contact. The strata previously have been referred to 

the Sturgeon quartzite of the Huronian series by 

earlier investigators (W. S. Bayley, 1899, p. 471; Van 

Hise and Leith, 1911, p. 301), who assumed it to underlie Randville dolomite that is exposed a short distance 

to the north. The pre-Animikie age for the arkose is 

shown by the following facts:

1. In much of the area, the two formations are separated by a wedge of granite gneiss.

2. The top directions in the arkose are consistently to 

the south, away from the dolomite that was assumed to overlie it conformably.

3. The arkose shows strong internal deformation, in 

contrast to the absence of such deformation in 

the adjacent dolomite. In the SW*4 sec. 3, T.
32 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

42 1ST., R. 28 W., for example, fine-grained dolomite with undef ormed algal structures occurs only 

a few hundred feet north of broad outcrops of 

arkosic conglomerate in which the arkose is 

gneissic in appearance and texture and in which 

the pebbles of the conglomerate are flattened 

discs.

The East Branch arkose, after .making due allowance 

for structural complications, is believed to be at least 

1,000 feet thick in the type area.

SOLBEKG SCHIST

The unit here defined as the Solberg schist is named 

for Solberg Lake, in sees. 17 and 20, T. 421ST., R. 29 W., 

east of which the rock is exposed in scattered outcrops 

for about 10 miles in a belt 1 to 2% miles wide. The 

more northerly (stratigraphically lower) parts of the 

unit are dark fine-grained hornblende and biotite 

schists. Some exposures show a banding, parallel or 

nearly parallel to the steeply dipping foliation that 

may represent original layering. The more southerly 

exposures are of massive gray quartz-mica schist and 

micaceous quartzite.

Interlayered with the Solberg schist is a bed of ironformation, here named the Skunk Creek member. It is 

named for Skunk Creek, a tributary of the East Branch 

of the Sturgeon River, which crosses the iron-rich 

member near the SE cor. sec. 17, T. 42 1ST., R. 28 W. 

The rock is exposed only in a small outcrop in the ISTE^ 

sec. 23, T. 42 1ST., R. 29 W., but the magnetic anomaly 

caused by the rock has been traced for more than 20 

miles and the bed has been drilled in several places by 

the M. A. Hanna Company. The drill core shows the 

rock to consist principally of three interlayered rock 

types: (a) biotite-hornblende schist with magnetite 

layers; (b) a distinctive thin-bedded rock consisting of 

alternating layers of metachert and magnetite; and (c) 

various mixtures of hornblende, biotite, grunerite, garnet, and epidote. The Skunk Creek member is about 

100 feet thick.

The aggregate thickness of the Solberg schist is difficult to determine accurately, but it is about 3,000 feet. 

Some interbedding with the East Branch arkose is indicated in exposures near the south edge of sec. 18, T. 

42 1ST., R. 28 W. The Skunk Creek member is about 

1,000 feet below the top of the formation.

SIX-MILE LAKE AMPHIBOLITE

The Six-Mile Lake amphibolite is named for Six-Mile 

Lake, in sec. 22, T. 42 1ST., R. 29 W. The rock is well 

exposed in low knobs and ridges immediately south of 

the lake and for several miles to the east and west. It 

is a massive dark fine- to medium-grained aggregate of

hornblende and plagioclase, with strong preferred 

planar and linear orientation of the minerals. The 

amphibolite is basaltic in composition and represents 

a metamorphosed sequence of flows and possible tuffs. 

The amphibolite grades to the south (across the strike) 

into a mile-wide zone of banded gray gneiss that is the 

product of reaction between the amphibolite and postDickinson gneissic granite, a granite that forms the 

basement to the Animikie series in parts of the area.

The thickness of amphibolite is about 3,000 feet, but 

a considerably greater original amount can be inferred 

from the widespread occurrence of amphibolite as relict 

bodies in the gneiss the granite adjoining to the south.

HARDWOOD GNEISS

The rock here named the Hardwood gneiss is exposed principally in sections 5 and 8, T. 411ST., R. 27 W., 

Dickinson County, a short distance west of the community of Hardwood for which the unit is named. Excellent exposures occur along and adjacent to highway 

M69, within an area mapped in detail by Jacob Freedman, 1951 (written communication).

The gneiss contains some highly distinctive rocks, 

many of which are quite unlike any others in northern 

Michigan known to me. Much of the rock is strongly 

layered in units a fraction of an inch to a few feet in 

thickness. The principal rock is dark, medium-grained 

gneiss composed of hornblende, plagioclase, and pyroxene, which is interlayered with dark fine-grained 

gneiss, beds of light-colored gneiss with light and dark 

laminae, garnet-quartz-mica schist, and light-colored 

rock that resembles quartzite but which probably is 

volcanic in origin. The layering in general dips easterly at angles less than 40° but many exposures show 

complex crumpling.

The gneiss, which represents original volcanic and 

tuffaceous rocks of basic to intermediate composition 

together with some interbedded rock of sedimentary 

origin and intrusive metagabbro sills, may well be 

equivalent to or part of the Dickinson group, but exposures are inadequate to establish relationships. The 

known thickness, including metagabbro sills, is not 

great possibly no more than several hundred feet.

QTJINNESEC FORMATION

The term Quinnesec formation applies to greenstone, amphibolite, and schist that form a belt in the 

southernmost part of Dickinson County and adjacent 

parts of Wisconsin. These rocks, under the name 

Quinnesec schists, were referred to the Archean by Van 

Hise and Bay ley (1900), but later (Van Hise and 

Leith, 1911) were assigned a late Huronian age. Leith, 

Lund, and Leith (1935) changed the designation to
PRE-KEWEENAWAN1 ROCKS IN PARTS OF NORTHERN MICHIGAN 33

Quinnesec greenstone and doubtfully assigned the unit 

to the middle Huronian.

The type area of the Quinnesec formation has been 

mapped in detail recently by W. C. Prinz (1957, 

written communication), who finds that rocks placed 

in the Quinnesec formation by previous workers comprise metavolcanic rocks and younger metagabbro sills. 

The term Quinnesec formation is retained for the metavolcanic rocks, which are clearly older than the granitic rocks that form a mass of batholithic dimensions to 

the south. Relationships of the Quinnesec formation 

and of the granitic rocks to known Animikie strata 

are not clear; the metavolcanic and granitic rocks are 

believed to be pre-Animikie, and are so shown in table 

1, but until absolute age measurements are made this 

conclusion is only tentative. The younger metagabbro 

sills are intrusive into the Quinnesec formation and are 

younger than the granitic rocks to the south. They are 

tentatively correlated with the post-Animikie metadiabase and metagabbro.

POST-DICKINSON GNEISSIC GRANITE

Large areas of northern Michigan are underlain by 

masses of pre-Animikie gneissic granite and related 

rocks which are surrounded by or contain infolded 

Animikie strata. In central Dickinson County it has 

been established that the large mass of crystalline rocks 

between the Felch and Calumet troughs of Animikie 

strata is of post-Dickinson age. The basement rocks to 

the Animikie series in many other parts of the region 

probably are equivalent to this post-Dickinson granite, 

rather than to the pre-Dickinson granite, but proof is 

not available. The pre-Animikie Margeson Creek 

gneiss of northeastern Iron County (Gair and Wier, 

1956, p. 18-26) is in this category; it could be either 

post-Dickinson or pre-Dickinson in age.

The term "Algoman," commonly used in the Lake 

Superior region in reference to upper pre-Animikie 

granitic rocks, may be applicable to the post-Dickinson 

granite gneiss, but in the absence of actual determinations of age this assignment should be considered 

tentative or doubtful.

MIDDLE PRECAMBRIAN ROCKS

Middle Precambrian rocks are represented by a thick 

sequence of metasedimentary and metavolcanic rocks 

and by scattered bodies of metagabbro and of granitic 

rocks. The metasedimentary and metavolcanic sequence 

has been referred to the Huronian series for over 60 

years. This designation is now changed in favor of 

Animikie series.

The term Huronian was applied by Logan and Hunt 

(1855) to a sequence of sedimentary rocks that lies

beneath the recognized Silurian strata on the north 

shore of Lake Huron. The beds, which consist of conglomerate, quartzite, limestone, and tillite, have an 

aggregate thickness on the order of 20,000 feet, and 

were described in some detail by Logan (1863). Restudy of the area by Collins (1925) showed that the 

strata were divided by an unconformity into two units, 

the lower of which is now referred to as the Bruce series 

and the upper as the Cobalt series.

Correlation between the Marquette district of Michigan and the type area of the Huronian was early proposed by Van Hise (1891). The Lower Marquette 

series of the Marquette range consisting principally 

of the rocks now called the Mesnard quartzite, the Kona 

dolomite, the Ajibik quartzite, the Siamo slate, and the 

Negaunee iron-formation was correlated with the 

Lower Huronian (Bruce series of the type area) ; and 

the Upper .Marquette series of the Marquette range 

consisting chiefly of the rocks now called the Goodrich 

quartzite and Michigamme slate was correlated with 

the Upper Huronian (Cobalt series of the type area). 

Subsequently, A. E. Seaman (see plate 19 in Van Hise 

and Leith, 1911) discovered an unconformity within 

Van Hise's Lower Marquette series, at the base of the 

Ajibik quartzite, and as a result the strata in the Marquette range were divided into three unconformable 

units rather than two. An international committee reviewed the problem and accepted a proposal for a threefold division of the Huronian (Van Hise, Adams, Bell, 

and Leith, 1905), in which the Huronian was divided 

into three units, only two of which the lower and the 

middle are represented in the type area. The Animikie group of the Thunder Bay area of Ontario was 

to form the third unit (upper), which was correlated 

with the Goodrich and Michigamme sequence of the 

Marquette range on the basis of lithologic similarity. 

The latter sequence, therefore, while retaining the designation of upper Huronian, actually received an entirely 

different correlation from that originally proposed; instead of being correlative with the Cobalt series of the 

type Huronian, it was made post-Cobalt in age. This 

scheme was followed in Monograph 52, the final report 

of the U. S. Geological Survey series of monographs on 

the region (Van Hise and Leith, 1911). Although 

many geologists raised objections see, for example, articles by Collins, Lawson, Coleman, and others in the 

proceedings of the 12th International Geological Congress (1914) the revised classification received and 

still receives fairly wide acceptance. A later report by 

the (Canadian) National Committee on Stratigraphic 

Nomenclature, however, reinstated Upper Huronian as 

a designation for the Cobalt series of Canada (Alcock, 

1934, p. 118).
34 iSHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Marsden (1955) has suggested that the MesnardKona sequence of the Marquette district is "Middle 

Huronian", but otherwise few major objections have 

been raised to the fundamental matter of correlation of 

the Huronian of the type area with the strata of the 

Marquette district about 200 miles to the west. Yet no 

iron-formation such as the Negaunee is present in the 

Huronian of the type area, nor, except by indirect correlation, can relationship be established between the 

Upper Huronian of the Marquette district, the Animikie group of the north shore of Lake Superior, and 

the Huronian of the type area. The correlation is based 

almost entirely upon supposed lithologic similarity between the Bruce series (lower Huronian of the type 

area) and the Mesnard quartzite and Kona dolomite sequence of the Marquette range, and upon similarities in 

structural and igneous history.

Actually, the lithologic similarity between the Bruce 

series of Canada and the Mesnard-Kona sequence of 

Michigan is not close. If trust is to be placed in lithologic similarities a questionable procedure at best 

other correlations can be made that are of at least 

equal validity. One of the most distinctive rocks in 

the Huronian sequence of the type area is the Gowganda 

tillite, which forms the base of the Cobalt series of 

Canada. The tillite is overlain by the thick massive 

vitreous white Lorrain quartzite. In Dickinson County, 

Michigan, tillite of virtually identical appearance forms 

part of the Fern Creek formation (Pettijohn, 1943), 

and the Fern Creek formation is overlain by the Sturgeon quartzite, which is a thick vitreous quartzite comparable in thickness and lithology to the Lorrain 

quartzite that overlies the Gowganda tillite. If this 

correlation were to be accepted, it would imply that the 

entire "Huronian" sequence of Michigan is post-Bruce 

in age, but other equally plausible correlations could 

be made.

A second view is that the Michigan strata are preHuronian in age. The argument in favor of this view 

is as follows:

1. In the type Huronian area, the two principal 

quartzite formations the Lorrain and the Mississagi contain abundant pebbles and fragments of 

chert and jasper, material which is relatively 

scarce in the surrounding pre-Huronian terrane. 

Crossbedding in these two units indicates northerly and westerly sources for the clastic material 

(Pettijohn, 1957, p. 471). The abundance of jasper and chert suggests that major chert-bearing 

units were undergoing erosion during Huronian 

time; the iron-formation strata of Michigan are 

possible sources.

 2. The probable age of the granitic rocks that cut the 

Middle Precambrian sequence of Michigan is more 

than 1,400 million year. (See p. 39-40.) On the 

north side of Lake Huron, the Wavy Lake granite 

(Lake Huron sheet, Map 155-A, Canadian Geological Survey) appears to be essentially a part 

of the Killarney granite of post-Huronian age. 

Its age has been determined as 1,025 million years 

by the potassium-argon method and 1,075 million 

years by the rubidium-strontium method 

(Wetherill, Davis, and Aldrich, 1957, pp. 39-40). 

East of this area the Cutler batholith is known 

to cut the Sudbury series of probable preHuronian age, but though the granite is adjacent 

to the basal Mississagi quartzite of the Huronian 

series, no direct evidence of crosscutting has been 

found. Age determinations on rocks from the 

batholith are given by Wetherill, Davis, and 

Aldrich (1957, p. 40) as 1,325, 1,760, and 1,750 

million years by the rubidium-strontium method, 

and 1,380, 1,420, and 1,350 million years by the 

potassium-argon method. The Cutler batholith, 

therefore, which may well be overlain unconformably by the basal formation of the Huronian, 

appears to be of comparable age to the granitic 

rocks that cut the supposed Huronian series in 

Michigan.

The arguments for retention of the term Huronian 

for the Michigan strata are essentially negative namely that the correlation has not actually been disproved 

and that the term is deeply entrenched in the literature. 

Because there are equally plausible alternatives to the 

correlation, it is our view that continued designation 

of the Michigan strata as Huronian is not justified.

Application of the term Animikie to the Michigan 

strata is open to far fewer major objections. The term 

was used originally in the Thunder Bay area on the 

northwest side of Lake Superior (Hunt, 1873) to strata 

that underlie the Keweenawan series. On the basis of 

physical continuity and bed-by-bed correlation, it has 

been accepted as a group name - by Grout, Gruner, 

Schwartz, and Thiel (1951), for strata in Minnesota 

that previously had been referred to the Huronian. 

These authors use Animikie rather than Huronian because (p. 1042) "* * * the Minnesota outcrops are well 

correlated with the original Animikie locality and not 

well correlated with the type locality of the Huronian."

The strata assigned to the Animikie group by Grout 

and his associates can be correlated with the Michigan 

strata with reasonable assurance, chiefly because of the 

presence of major units of iron-formation. The Gun-
PRE-KEWEEOSTAWAN ROCKS EST PARTS OF NORTHERN MICHIGAN 35

flint of the type Animikie area and of adjacent parts of 

Minnesota, the Biwabik of the Mesabi district, the 

Iron wood of the Gogebic district (Wisconsin and Michigan), the Negaunee of the Marquette district (Michigan), and the Vulcan of the Menominee district 

(Michigan) are accepted as at least approximately 

equivalent in age (Leith, Lund, and Leith, 1935). In 

general, therefore, the extension of the term Animikie 

to the Michigan strata rests on far firmer grounds than 

the extension of the term Huronian.

The Animikie is given series rank in application to 

the Michigan strata because it probably includes some 

rocks that are older and some that are younger than 

those in the type area, and because it is divided into 

four groups. Using the generalized sequence represented in Iron and Dickinson County for comparison, 

the relationship between the Animikie group of Minnesota and the Animikie series of Michigan is given in 

table 2.

TABLE 2. Comparison of the stratigraphic position of the Animikie group of Minnesota with the Animikie series as defined for 

part of Michigan.

Minnesota (Mesabi 

district)

Animikie group

* «> o

Virginia slate

Biwabik ironformation 

Pokegama 

quartzite i? ?

Michigan (Iron and Dickinson Counties)

 S "&..

0>Ift

<D

=a .§ 

3

Paint River 

group

Baraga group

Menominee 

group

Chocolay 

group

Fortune Lakes slate 

Stambaugh formation 

Hiawatha graywacke 

Riverton iron-formation 

Dunn Creek slate

Badwater greenstone 

Michigamme slate 

Fence River and Amasa formations 

Hemlock formation 

Goodrich quartzite

Vulcan iron-formation 

Felch formation

Randville dolomitelSaunders 

Sturgeon quartzite ) formation 

Fern Creek formation

CHOCOLAY GROUP

The Chocolay group, as here defined, is equivalent to 

the Lower Huronian of earlier reports. It is named for 

Mount Chocolay and for Chocolay Junction on the 

Duluth, South Shore, and Atlantic Kailroad, about 3 

miles southeast of Marquette, Michigan. At Mount 

Chocolay and the adjacent area to the west (Van Hise 

and Bayley, 1897, Atlas Sheet 39) the Mesnard quartzite and Kona dolomite are widely exposed. These two 

formations are accepted as direct correlatives of the 

Sturgeon quartzite and Randville dolomite of Iron and 

Dickinson Counties. Lithologically the sequences are 

almost identical and the correlation has been a basic 

tenet of Lake Superior geology for 60 years.

In Dickinson County the basal beds of the Chocolay 

group comprise conglomerate and arkose, with beds of 

tillite (Pettijohn, 1943). Pettijohn has designated

these strata the Fern Creek formation, which is described in more detail by Trow (1948).

In parts of Iron and Dickinson Counties the only unit 

of the Chocolay group is the Randville dolomite (in 

southern Iron County, its partial correlative the Saunders formation). Whether the Sturgeon is absent because of nondeposition or whether the dolomite in these 

areas is time-equivalent to both Sturgeon and Randville 

is uncertain.

MENOMINEE GROUP

The Menominee group is named for the Menominee 

district of southern Dickinson County, Michigan. It 

is to be noted that the term Menominee series was used 

in Monograph 46 (Bayley, 1904) and discarded seven 

years later in Monograph 52 in favor of Huronian 

(Van Hise and Leith, 1911). The present definition 

covers only the middle part of the Menominee series as 

originally defined.

In the type locality this group consists of two formations the basal Felch formation, and the Vulcan ironformation. The latter formation is made up of four 

members which are, in ascending order, the Traders 

iron-bearing member, the Brier slate member, the 

Curry iron-bearing member, and the Loretto slate member. The Loretto was defined originally by Alien 

(1919) as a post-Curry unit preserved only locally; in 

most places it was removed by erosion prior to disposition of the overlying Goodrich quartzite or equivalent 

rocks. The existence of the Loretto is confirmed by 

more recent studies (R. W. Bayley, 1957 written communication).

The type locality of the Felch schist, here changed 

to the Felch formation, is in the Felch Mountain district of central Dickinson County, where the designated 

strata occur between the Randville dolomite and the 

Vulcan iron-formation. In the Menominee district the 

term Felch formation is applied to the distinctive sericitic slate-quartzite sequence which similarly lies above 

the Randville dolomite and below the Vulcan iron-formation, and which has been previously referred to as 

"Traders quartzite" (Bayley, W. S., 1904), and "Footwall slate" (Dutton, C. E., and Lamey, C. A., 1939). 

The Felch formation, according to Leith, Lund, and 

Leith (1935), is probably equivalent to the Palms 

quartzite of the Gogebic district, and to the Ajibik 

quartzite and Siamo slate of the Marquette district.

BABAGA GROUP

The Baraga group, here named for Baraga County 

in which the rocks of this group are .widely exposed, 

comprises most of the strata referred to as Upper 

Huronian of previous reports, namely, (a) Goodrich
36 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

quartzite (basal formation), and (b) Michigamme 

slate, the lower part of which in the Marquette district 

includes the Bijiki iron-formation member, the Clarksburg volcanics member, and the Greenwood iron-formation. In Iron and Dickinson Counties six formations are represented Goodrich quartzite (lowermost 

unit), Hemlock formation, Fence River formation, and 

its probable correlative, the Amasa formation Michigamme slate, and Badwater greenstone (uppermost 

unit).

GOODRICH QTJARTZITE

Within the area of this report, the Goodrich has been 

recognized only in the Kiernan quadrangle of northeastern Iron County (Gair and Wier, 1956, p. 34-39), 

but local beds of conglomerate overlying the Vulcan 

iron-formation in the Menominee district (Alien, 

1920) might well be assigned to this unit, as they 

occupy a position at the base of the Michigamme slate 

entirely comparable to that of the Goodrich quartzite 

of the type area in the Marquette district.

HEMLOCK FORMATION

The Hemlock, named by Clements and Smyth 

(1899a, pt. 3, 19th Ann. Kept., p. 45-63) for extensive 

exposures of volcanic rocks along the Hemlock River 

in northeastern Iron County, has been considered to 

be Middle Huronian in most previous reports. This 

assignment was made on the basis of relationships in 

the Michigamme Mountain area of the Kiernan quadrangle, where it was believed that the volcanic rocks 

were stratigraphically between the Randville dolomite 

and a Middle Huronian iron-formation (Clements and 

Smyth, 1899b, p. 453-456). Gair and Wier (1956, 

p. 41-57) have since shown, however, that the volcanic 

rocks that correlate with the Hemlock actually overlie 

the iron-rich strata (the latter redefined as a highly 

ferruginous facies of the Goodrich quartzite rather 

than Negaunee iron-formation) so that the previous 

assignment is no longer considered valid.

The Hemlock, though several miles thick in its type 

area on the west flank of the Amasa oval, is not a persistent unit. On the east flank of the oval, the thickness is 3,000 feet or less, and the formation is missing 

entirely in most of Dickinson County. These variations in thickness are original, not the result of later 

erosion.

The Hemlock formation consists largely of metamorphosed basaltic rocks, but interbedded sedimentary 

strata occur in many places. Two of these sedimentary 

units are sufficiently different and continuous to warrant distinction as members. The Mansfield iron-bearing slate member (Van Hise and Leith, 1911, p. 291- 

296), named for the Mansfield mine in eastern Iron 

County, consists in part of chert-siderite iron-formation

and in part of slate, with a maximum thickness of about 

500 feet (R. W. Bayley, 1956, p. 68). The member occurs within the lower third of the Hemlock formation. 

The second member, here designated the Bird ironbearing member, lies about 1,300 feet below the top of 

the Hemlock formation in eastern Iron County. The 

member attains a maximum thickness of 300 feet in sec. 

13, T. 43' N., R. 32 W. at the Bird Exploration, for which 

it is named. At this locality the rock exposed in trenches 

and test pits consists of oolitic hematite-chert, highly 

ferruginous quartzite and schist, and a distinctive breccia made up of angular chert and jasper fragments in 

a matrix of hard blue hematite. The member is overlain and underlain by greenstone.

FENCE RIVER FORMATION

The Fence River formation was named by Gair and 

Wier (1956, p. 57). In previous reports this unit was 

assigned to the middle Huronian Negaunee iron-formation (Leith, Lund, and Leith, 1935). It is exposed only 

in sec. 21, T. 45 N., R. 31 W. but it is magnetic and has 

been traced for many miles on the east flank of the 

Amasa oval by magnetic surveys. The Amasa formation, which flanks the west side of the oval, probably is 

correlative with the Fence River, but the connection 

has not been completely traced.

The Fence River formation is not known to be present 

much south of the Amasa oval. Its absence (in part at 

least) is believed due to post-Fence River, pre-Michigamme erosion.

AMASA FORMATION

The belt of iron-rich strata that overlies the Hemlock 

formation and flanks the Amasa oval on the west side 

was referred to the Vulcan iron-bearing member of the 

Michigamme slate by Van Hise and Leith (1911, p. 298- 

299), and was believed by them to be areally continuous 

with the iron-formation at the Dunn and Mastadon 

mines south of Crystal Falls. Stratigraphically the 

strata lie between the Hemlock formation and the Michigamme slate, and it is now known that the formation 

correlates with neither the Vulcan (Negaunee) of the 

type area nor the strata near Crystal Falls (Gair and 

Wier, 1956, p. 58 j Pettijohn, 1947). It occupies the 

same stratigraphic position as the Fence River formation, but direct equivalence has not yet been proved.

The iron-rich strata were named the Amasa formation 

by Royce (1936, p. 86), for the town of Amasa which 

is about centrally located on the belt. Royce states that 

the Amasa formation "* * * is separated by a conglomerate from the overlying Upper Huronian slates [Michigamme slate of this report]; intimately associated with 

contemporaneous volcanics, called the Hemlock volcanics, at its base." Ore has been mined from the for-
PRE-KEWEENAWANi ROCKS IN PARTS OF NORTHERN MICHIGAN 37

mation at the Hemlock, Michigan, Warner, Porter, Hoilister, McDonald, Armenia, Lee Peck, Cayia, and Hope 

mines all, except the Warner, now abandoned and inaccessible. The formation contains a wide variety of 

rocks pyritic slate, ferruginous slate and quartzite, 

chert, and cherty hematitic iron-formation that were 

in part clastic and in part chemical sediments. The aggregate thickness of the ferruginous strata reaches a 

maximum of about 1,500 feet in sees. 13, 14, 23, 24, and 

26,T.43N.,R.32W.

MICHIGAMME SLATE

The Michigamme slate is the most extensive formation of the Animikie series in northern Michigan. It is 

the only formation that is physically continuous between most of the districts. Keliable estimates of thickness are not easily made, but present evidence indicates 

a maximum thickness of about 5,000 feet in Iron and 

Dickinson Counties; locally it may be much less, and in 

some places the formation is absent. Most of the rock 

is monotonously interbedded slate and graywacke or 

their more highly metamorphosed equivalents, but ironrich beds of apparently local extent occur in the uppermost part of the formation.

BADWATEB GREENSTONE

Large areas of southern Iron County, northern 

Florence County (in Wisconsin, immediately south of 

Iron County, Mich.), and southwestern Dickinson 

County are underlain by greenstone. The formation 

is hlere named the Badwater greenstone for the extensive 

exposures in sees. 1, 2, and 11, T. 40 N., R. 31 W., in the 

vicinity of Badwater Lake, southern Dickinson County. 

The greenstone in the adjacent area of southeastern 

Iron County, Dickinson County, and Florence County 

(Wisconsin) has been referred to as the MastadonSpread Eagle-Lake Antoine belt (Leith, Lund, and 

Leith, 1935, p. 4). The greenstone in the eastern part 

of this belt was assigned to the Archean by Bayley 

(1904), but in Monograph 52 (Van Hise and Leith, 1911, 

p. 345) the statement is made:

This area of schists at the time the Menoruinee monograph 

was written, was supposed to be equivalent in age to the Quinnesec schist of Menominee River, then regarded as Archean. 

Later work by G. W. Corey and C. F. Brown (unpublished field 

notes, 1905) has shown that they are really intrusive and extrusive in the upper Huronian or in part contemporaneous flows.

Leith, Lund, and Leith (1935) doubtfully assigned the 

greenstone to the Middle Huronian.

Detailed mapping in the northern part of the Menominee district by C. A. Lamey, and in the Commonwealth quadrangle of southern Iron County and adjoining areas of Wisconsin south of the Brule River 

by C. E. Dutton, K. L. Wier, and the .writer has

demonstrated that the greenstone is stratigraphically 

above the Michigamme slate. The conclusion as to relationship is based on many top directions established 

by graded bedding in the Michigamme and ellipsoidal 

structures in the greenstone. This relationship is of 

critical importance in that it establishes the position 

not only of the greenstone but of the sequence of rocks 

that overlies it in the Florence district of Wisconsin 

and the Iron River-Crystal Falls district of Michigan. 

The maximum thickness attained by the Badwater 

greenstone is several miles (in the area bounding the 

Iron River-Crystal Falls district on the north), but 

such thicknesses are not maintained far along the 

strike. As the beds now exposed are about vertical, the 

map patterns are cross-sections of the original volcanic 

accumulations. They reveal great mound-like piles in 

which thicknesses of several miles diminish to the vanishing point within as little as 15 miles along the 

strike. The presence of ellipsoidal structures at many 

localities indicate that the greenstone is largely submarine in origin.

PAINT RIVER GROUP

The definition of the stratigraphic relationship between the Badwater greenstone and the Michigamme 

slate permits or demands redefinition of the stratigraphic position of the strata in the Iron RiverCrystal Falls district. In previous reports these 

strata, which include productive iron-formation, generally were considered part of the Michigamme slate 

(Leith, Lund, and Leith, 1935). The Iron RiverCrystal Falls district is a deep tightly folded major 

synclinal structure incompletely bounded by Badwater 

greenstone. In the area immediately east of Crystal 

Falls, greenstone is absent and the strata of the district proper rest directly on the Michigamme slate, 

which doubtless is the reason they were previously considered an extension of that unit. Recognition of the 

stratigraphic position of the Badwater greenstone 

forms the basis for definition of a new group, here 

named the Paint River group for the Paint River, 

which drains the northern and eastern part of the district. The five formations that comprise the group 

are the youngest strata of the Animikie series in Iron 

and Dickinson Counties, and probably are the youngest 

Animikie strata preserved in northern Michigan. Some 

of their characteristics have been described in previous 

papers (James, 1951, 1954).

DTJNN CREEK SLATE

The basal formation of the Paint River group is a 

sequence of siltstones and slates here named the Dunn 

Creek slate. The name derives from Dunn Creek, in 

eastern Iron County south of Crystal Falls, in the
38 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

vicinity of which are abundant exposures of the unit. 

The rocks in this area have been described by Pettijohn (1947). Much of the Dunn Creek is graywacke 

and slate that are physically indistinguishable from 

rocks of the Michigamme slate, but in the type area 

of eastern Iron County the formation is thicker and 

more complex; it contains one or more layers of slaty 

sideritic iron-formation. The probable maximum 

thickness is about 800 feet; the probable minimum, 

about 400 feet.

The uppermost part of the Dunn Creek, immediately 

underlying the productive iron-formation, is a black 

pyritic graphitic slate, probably 30 feet or less in true 

thickness but commonly much greater or less because of 

squeezing and shearing. The rock typically contains 

35 to 40 percent pyrite, extremely fine grained, and 5 

to 10 percent free carbon (James, 1951). This distinctive rock is exposed at only one place, a road cut south 

of the town of Alpha (Pettijohn, 1947; 1948), but is 

known from test pits, mine workings, and drill holes 

throughout the district. It is here named the Wauseca 

pyritic member, for the Wauseca mine at Iron River, in 

which the unit is exposed in thousands of feet of mine 

workings. The member itself is subdivided into two 

or three parts in the detailed geologic mapping of the 

mines. One of these subunits is of particular interest. 

It is a chaotic breccia that ranges from a foot or so to 

about 50 feet in thickness and is composed of small (less 

than 1 inch diameter) fragments of pyritic slate in a 

matrix of the same composition. The uniform composition of the rock, the complete lack of extraneous material, and its occurrence over distances measurable in 

miles strongly suggest that the rock is the product of 

a widespread submarine slump. If so, the unit is precisely time equivalent over the entire district.

RIVERTON IRON-FORMATION

The productive iron-formation of the Iron RiverCrystal Falls district the host rock for the ore bodies 

 is here named the Riverton iron-formation. The 

name is taken from the Riverton mine at Iron River, 

the first site of mining in the district about 75 years ago. 

The formation is visible in the walls of the caved workings of the old mine and the original discovery of iron in 

the district about 100 years ago was in one of the few 

natural exposures of the formation in the Iron River 

area, within a hundred feet of the old mine. Small exposures are widespread in the eastern part of the district (Pettijohn, 1947; Good and Pettijohn, 1949; Pettijohn, 1952), but much of the knowledge concerning the 

formation is derived from the hundreds of miles of mine 

workings and the thousands of diamond-drill holes that 

have intersected the rock.

The formation consists dominantly of interbedded 

chert and siderite. Its thickness ranges from less than 

100 feet in the western part of the district to about 600 

feet in the eastern part. Locally the upper part of the 

formation is absent because of erosion prior to deposition of the overlying Hiawatha graywacke, but most of 

the variation in thickness is to be ascribed to the nature 

of sedimentation.

HIAWATHA GRAYWACKE

Overlying the Riverton iron-formation, locally with 

minor unconformity, is a unit of clastic rock, here named 

the Hiawatha graywacke. The name is taken from the 

Hiawatha mine at Iron River, near which the rocks are 

exposed in scattered outcrops and in which the formation is crossed by extensive mine workings. As the name 

implies, graywacke is the dominant rock, but it is interbedded with considerable amounts of slate, particularly 

in the western part of the district. The lower part of 

the formation quite commonly is a breccia that consists 

of scattered angular chert fragments in a graywacke 

matrix (see photographs in James, 1951, pi. 2). This 

rock is locally well exposed along the course of the Paint 

River at the north edge of the town of Crystal Falls 

(Pettijohn, 1952).

The formation ranges in thickness from 50 feet or less 

in the eastern part of the district to 400 feet or more in 

the western part the reverse of the thickness aspect 

of the underlying Riverton iron-formation. The formation also becomes more arkosic in composition toward 

the west.

STAMBAtJGH FORMATION

The strata here named the Stambaugh formation are 

iron-rich rocks that range from chlorite mudstone and 

slate to a laminated cherty siderite-magnetite rock. The 

distinctive feature of the formation is that it is magnetic (the only formation that is consistently so over 

most of the district) and as a result can be traced by 

magnetic methods. Tedious though the magnetic surveying is, it has been of inestimable value in working 

out the map patterns in this structurally complex area 

of poorly exposed rocks. The distribution of the magnetic slate, as it is called locally, is better known than 

that of any other unit in the district.

The formation is named for the town of Stambaugh, 

in the western part of the district. The hill on which 

the town is built is in large part underlain by the formation (here compressed into a tight complex syncline) 

and numerous small exposures can be seen on the 

grounds of the Stambaugh High School. Better exposures are present in the eastern part of the district 

(Pettijohn, 1947; 1952), but like most units of the Paint 

River group, a large part of the information has come 

from mining explorations and developments.
PRE-KEWEENAWAN ROCKS IN PARTS OF NORTHERN MICHIGAN 39

FORTUNE LAKES SLATE

The strata underlying the central part of the Iron 

Eiver-Crystal Falls synclinal basin are very poorly 

known, but they underlie a larger area than any other 

unit of the Paint River group. The main areas of exposure are between Crystal Falls and Fortune Lakes, 

for which the unit here is named, and in the area between Alpha and the Brule River. The exposed rocks 

consist mostly of slate and minor graywacke, with 

characteristic thin beds (10 feet or less) of coarse massive graywacke about 300 feet above the base. Sideritic 

slate is present in some areas between Crystal Falls and 

Alpha, and in one exposure near Fortune Lakes oxidized 

chert-siderite rock (iron-formation) is present. The 

character of the bulk of the formation is not known, 

however, because of lack of exposure.

The aggregate thickness of the unit probably is at 

least 4,000 feet. This estimate is based on structural 

inferences rather than any measured thickness.

MIDDLE PRECAMBRIAN IGNEOUS ROCKS

The Middle Precambrian igneous rocks of northern 

Michigan fall into two principal groups metadiabase 

and metagabbro of Animikie or post-Animikie age, 

and granite and allied rocks that are younger than the 

metadiabase and metagabbro. Other Animikie or postAnimikie igneous rocks appear to be exceedingly 

scarce.

The metadiabase and metagabbro were metamorphosed during the post-Animikie, pre-Keweenawan interval (James, 1955). Several large bodies of metagabbro have been mapped; one, the West Kiernan sill 

(Gair and Wier, 1956; R. W. Bayley, 1956) of the Lake 

Mary and Kiernan quadrangles of eastern Iron County, 

is a differentiated sheet, now vertical but originally 

horizontal, that is about a mile thick.

The granitic rocks are believed relatively minor in 

surface expression but are widely distributed. Narrow 

dikes of granite and pegmatite are common in the 

Felch area (where they cut Animikie strata as well as 

older rocks) and a complex mass, perhaps several miles 

across, occurs in the southern part of the Lake Mary 

quadrangle of eastern Iron County (R. W. Bayley, 

1956).

The existence of post-Animikie granite in northern 

Michigan has been known almost since geologic work 

was begun more than a hundred years ago, but considerable debate has arisen as to its extent. Lamey 

(1931, 1933, 1934, 1937) contended that some of the 

large areas of granitic rocks regarded by others as 

pre-Huronian (pre-Animikie of this report) are in 

fact post-Huronian, and introduced the term Republic 

granite for the rock between Republic and the Marquette district (1933). In subsequent papers he extended this term and concept to include the large areas 

of granitic rock in Dickinson County (Lamey, 1934; 

1937). Dickey (1936) took issue with this conclusion 

and reaffirmed a pre-Huronian age. Later Dickey 

(1938), on the basis of the intrusive relationships of 

granite to quartzite in the Ford River area of northern 

Dickinson County, introduced the possibility of a postLower Huronian granite (post-Chocolay of this report) for the Republic granite. At that time, however, Dickey was not aware of the existence of the preAnimikie sedimentary sequence in the Norway Lake 

area (Clark, 1953) a few miles south of the area studied 

by him. The Ford River area has been examined by 

the writer and his colleagues. The quartzite referred 

to by Dickey almost certainly is pre-Animikie rather 

than Sturgeon, and the pre-Aiiimikie age for the granitic rock was established by finding an exposed unconformable contact of Randville dolomite on the 

granite in the NE^SE^, sec. 15, T. 43 N, R. 29 W.

Petti John's conclusions (1943) on the age relations 

of the main bodies of granitic rock in southern and 

central Dickinson County have been borne out by the 

detailed mapping of the area by the U. S. Geological 

Survey, in which Dr. Lamey has participated. These 

conclusions are in essential agreement with the earlier 

studies summarized by Van Hise and Leith (1911), 

but the detailed mapping has also shown that postAnimikie granite is more abundant than previously 

recognized.

The Republic area has not been mapped in detail 

during the present study, but a study of the metamorphism of northern Michigan (James, 1955) indicates 

that in the type area of the Republic granite described 

by Lamey the main mass of the granite is not postHuronian (post-Animikie) but rather is pre-Animikie, 

although some post-Animikie granite is present. With 

Lamey's concurrence, it is recommended that the term 

Republic granite be abandoned because of the practical 

difficulty in distinguishing older from younger granite.

Measurements of absolute age have been made on 

several samples of the post-Animikie granite of Michigan by the lead-alpha (zircon) method, the potassiumargon method, and the strontium-rubidium method. 

Determinations by the lead-alpha method on zircon 

concentrated from a post-Animikie granite body near 

the town of Crystal Falls by H. W. Jaffe were earlier 

reported to yield an age of 820 million years (James, 

1955, p. 1458). Potassium-argon determinations on 

post-Animikie granite from the Felch area were reported by Wasserburg, Hay den, and Jensen (1956, p. 

158-159), as follows:
40 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Locality

Pegmatite quarry in sec. 

19, T. 42 N., R. 29 W., 

Dickinson County. 

As above..___ ___ 

As above.. 

Groveland mine, sec. 31, 

T. 42 N., R. 29 W., 

Dickinson County.

Material 

Muscovite..

Microcline_.

Microcline. .

Total granite

(mica free).

Branching 

ratio

0.126

.085

.085

.085

Apparent

age

(million 

years)

1,610

1,490

1,450

1,410

The value of the branching ratio (0.085) used by 

Wasserburg and his associates to compute the ages of 

the microcline and the granite is an empirical value 

assumed to correct for loss of argon. The value of 0.126 

used for the mica determination is a maximum value 

and the age so computed is a minimum. Unpublished 

determinations made by L. T. Aldrich, G. W. Wetherill, 

and G. L. Davis by the strontium-rubidium method on 

micas from several samples of metamorphosed Animikie 

strata and from post-Animikie, pre-Keweenawan granitic rock yield values of about 1,400 million years, and 

feldspar from the same pegmatite reported on by 

Wasserburg, Hayden, and Jensen yields a value near 

1,750 million years (L. T. Aldrich, oral communication, 

1957).

Eecent determinations by the potassium-argon method on rocks from the Lake Superior region have been 

reported by Goldich, Nier, and Baadsgaard (oral presentation and program abstract for the annual meeting 

of the American Geophysical Union, at Washington, 

D. C., April 29-May 2,1957). Mica from granite near 

Crystal Falls, Mich., yielded an age of 1,700 million 

years, as computed using the branching ratio of 0.118 

that now appears most suitable for mica. Baadsgaard, 

Nier, and Goldich (1957) also report determinations on 

microcline from Dickinson County, Mich., probably 

from the same localities from which the samples run by 

Wasserburg, Hayden, and Jensen (1956) were obtained. 

The values of A 40/K 40 are closely comparable, but the 

age calculation using a branching ratio of 0.118 (as 

compared with 0.085 by Waserburg, Hayden, and Jensen) yields values of 1,210, 1,200, 1,220, and 1,210 

million years. These values, considering the probable 

argon leakage, are likely to be significantly lower than 

the true ages.

The net results to date of work on absolute ages 

suggest that the age of the post-Animikie, pre-Keweenawan epoch of diastrophism, metamorphism, and granite intrusion is more than 1,400 million years. For 

comparison, the age of the Keweenawan or post-Keweenawan intrusive rocks (Duluth gabbro and related 

rocks) is about 1,100 million years (Goldich, Nier, and 

Baadsgaard, 1957). The term Killarney commonly has 

been applied to the post-Animikie, pre-Keweenawan

granitic rocks in Michigan, but the validity of this 

designation is uncertain. Wetherill, Davis, and Aidrich (1957) report that the apparent age of the Wavy 

Lake granite of Ontario, which is near the type area 

of Killarney, is between 1,000 and 1,100 million years. 

If this is actually the age of the Killarney granite 

proper, the post-Animikie, pre-Keweenawan rocks of 

Michigan are distinctly older, and the term Killarney 

is not applicable.

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(Sec. iv), v. 28, p. 113-121.

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in Michigan: Am. Inst. Mining Metall. Engineers Bull. 135, 

p. 2579-2594; Am. Inst. Mining Metall. Engineers Trans., 

v. 63, p. 188-212 [1920].

Baadsgaard, H., Nier, A. O. C., and Goldich, S. S., 1957, Investigations in A^/K 40 dating: Am. Geophys. Union, program 

abstracts for annual meeting, Washington, D. 0., April 29- 

May 2,1957, p. 40.

Balsley, J. R., James, H. L., and Wier, K. L., 1949, Aeromagnetic 

survey of parts of Baraga, Iron, and Houghton counties, 

Michigan, with preliminary geologic interpretation: U. S. 

Geol. Survey, Geophys. Inv. Prelim. Kept.

Bay ley, E. W., 1956, Preliminary report of geology of Lake Mary 

quadrangle, Iron County, Michigan: U. S. Geol. Survey openfile rept.

Bayley, W. S., 1899, The Sturgeon River tongue: Chap. 6, vn 

Clements and Smyth, U. S. Geol. Survey Mon. 36.

 1904, The Menominee iron-bearing district of Michigan:

U. S. Geol. Survey Mon. 46, 513 p. 

Canada Geological Survey, 1933, Lake Huron Sheet, Map 155A

(3d ed.) : Pub. 1553. 

Clark, L. D., 1953, Precambrian geology of the Norway Lake

area: U. S. Geol. Survey open-file rept. 

Clements, J. M., and Smyth, H. L., Jr., 1899a, The Crystal Falls

iron-bearing district of Michigan: U. S. Geol. Survey, 19th

Ann. Rept., pt. 3, p. 45-63. 

Clements, J. M., and Smyth, H. L., Jr., 1899b, The Crystal Falls

iron-bearing district of Michigan: U. S. Geol. Survey Mon.

36, 512 p. 

Collins, W. H., 1925, North shore of Lake Huron: Canada Geol.

Survey Mem. 143.

Dickey, R. M., 1936, The granitic sequence in the southern complex of Upper Michigan: Jour. Geology, v. 44, p. 317-340.

 1938, The Ford River granite of the southern complex

of Michigan: Jour. Geology, v. 46, p. 321-335. 

Button, C. E., 1949, Geology of the central part of the Iron River

district, Michigan: U. S. Geol. Survey Circ. 43.

 1950, Progress of geologic work in Iron and Dickinson 

Counties, Michigan: U. S. Geol. Survey Circ. 84.

Button, C. E., and Lamey, C. A., 1939, Geology of the Menominee 

range, Bickinson County, Michigan: Michigan Dept. Conserv., Geol. Survey Biv. Progress Rept. 5,10 p.

Button, C. E., Park, C. F., and Balsley, J. R., 1945, General 

character arid succession of tentative divisions in the stratigraphy of the Mineral Hills district: U. S. Geol. Survey 

Prelim, rept.

Gair, J. E., and Wier, K. L., 1956, Geology of the Kiernan quadrangle, Michigan: U. S. Geol. Survey Bull. 1044, 88 p.
PRE-KEWEENAWAN ROCKS IN PARTS OF NORTHERN MICHIGAN 41

Gill, J. B., 1955, Precambrian nomenclature in Canada: Royal

Soc. Canada Trans.3dser. (Sec. iv) v.49,p.25-30. 

Goldich, S. S., Nier, A. O. C., and Baadsgaard, H., 1957, A^/K40

dating of rocks of the Lake Superior region: Am. Geophys.

Union, program abstracts for annual meeting, Washington,

D. 0., April 29-May 2,1957, p. 41. 

Good, S. B., and Pettijohn, F. J., 1949, Magnetic survey and

geology of the Stager area: U. S. Geol. Survey Circ. 55. 

Grout, F. F., Gruner, J. W., Schwartz, G. M., and Thiel, G. A.,

1951, Pre-Cambrian stratigraphy of Minnesota: Geol. Soc.

America Bull., v. 62, p. 1017-1078. 

Hunt, T. S., 1873, The geonostical history of the metals: Am.

Inst. Mining Metall. Engineers., Trans., v. 1, p. 331-395. 

James, H. L., 1948, Field comparison of some magnetic instruments, with analysis of Superdip performance: Am. Inst.

Mining Metall. Engineers Tech. Pub. 2293; Trans., v. 178,

p. 490-500.

 1951, Iron formation and associated rocks in the Iron 

River district, Michigan: Geol. Soc. America Bull, v. 62, 

p. 251-266.

 1954, Sedimentary facies of iron-formation: Econ. 

Geology, v. 49, p. 235-293.

1955, Zones of regional metamorphism in the Precambrian of northern Michigan: Geol. Soc. America Bull., v.

66, p. 1455-1488. 

James, H. L., and Dutton, C. B., 1951, Geology of the northern

part of the Iron River district: U. S. Geol. Survey Circ. 120. 

James, H. L., Smith, L. E., and Clark, L. D., 1947, Magnetic

survey and geology of the Ice Lake-Chicagon Creek area:

U. S. Geol. Survey Prelim. Rept. 3-213. 

James, H. L., and Wier, K. L., 1948, Magnetic survey and

geology of the eastern and southeastern parts of the Iron

River district: U. S. Geol. Survey Circ. 26.

Lamey, C. A., 1931, Granite intrusions in the Huronian formations of northern Michigan: Jour. Geology, v. 39,

p. 288-295.

 1933, The intrusive relations of the Republic granite: 

Jour. Geology, v. 41, p. 487-500.

 1934, Some metamorphic effects of the Republic granite: 

Jour. Geology, v. 42, p. 248-263.

 1937, Republic granite or basement complex: Jour. 

Geology, v. 45, p. 485-510.

 1946, Geology of an area near Randville: U. S. Geol. 

Survey open-file rept.

 1947, Geology of an area near Randville: U. S. Geol. 

Survey open-file rept.

 1949, Geology of a part of the Felch Mountain district:

U. S. Geol. Survey open-file report.

Leith, C. K., 1934, The pre-Cambrian: Geol. Soc. America 

Proc. 1933, p. 151-180.

Leith, C. K., Lund, R. J., and Leith, A., 1935, Pre-Cambrian 

rocks of the Lake Superior region * * *: U. S. Geol. Survey Prof. Paper 184, 34 p.

Logan, W. B., 1863, Report of progress of the Geological Survey of Canada from its commencement to 1863: Geol. Survey of Canada.

Logan, W. B., and Hunt, T. S., 1855, Bsquisse geologique du 

Canada * * * a 1'Exposition Univ. de Paris [Geol. Survey of Canada].

Marsden, R. W., 1955, Precambrian correlations in the Lake 

Superior region in Michigan, Wisconsin, and Minnesota: 

Geol. Assoc. Canada Proc., v. 7, pt. 2, p. 107-116.

Pettijohn, F. J., 1943, Basal Huronian conglomerates of Menominee and Calumet districts, Michigan: Jour. Geology, 

v. 51, p. 387-397.

 1947, Geology of the Crystal Falls-Alpha iron-bearing

district, Iron County, Michigan: U. S. Geol. Survey Prelim.

Rept. 3-181. 

Pettijohn, F. J., 1948, Magnetic and geological data of parts of

Crystal Falls iron-bearing district: Michigan Dept. Conserv., Geol. Survey Div.

 1951, Geology and magnetic anomalies of T. 42 N., R. 30 

W., Dickinson County, Michigan: U. S. Geol. Survey openfile rept.

 1952, Geology of the northern Crystal Falls area; U. S. 

Geol. Survey Circ. 153.

 1957, Paleocurrents of Lake Superior Precambrian

quartzites: Geol. Soc. America Bull. v. 68, p. 469-480.

Royce, Stephen, 1936, Geology of the Lake "Superior iron 

deposits: Lake Superior Mining Institute, Proc. v. 29, p. 86.

Trow, J. W., 1948, The Sturgeon quartzite of the Menominee 

district, Michigan: Unpublished Ph. D. thesis at the University of Chicago, 60 p.

Van Hise, C. R., 1891, An attempt to harmonize some apparently 

conflicting views of Lake Superior stratigraphy: Am. Jour. 

Sci., v. 41, p. 117-137.

Van Hise, O. R., Adams, F. D., Bell, J. M., and Leith, C. K., 1905, 

Report of the special committee for the Lake Superior 

region: Jour. Geology, v. 13, p. 89-104.

Van Hise, C. R., and Bayley, W. S., 1897, The Marquette ironbearing district of Michigan: U. S. Geol. Survey Mon. 28, 

608 p.

Van Hise, C. R., and Bayley, W. S., 1900, Description of the 

Menominee special quadrangle, Michigan: U. "8. Geol. Survey Atlas, folio 62.

Van Hise, C. R., and Leith, C. K., 1911, The geology of the Lake 

Superior region: U. S. Geol. Survey Mon. 52, 641 p.

Wasserburg, G. J., Hayden, R. J., and Jensen, K. J., 1956, 

A^-K40 dating of igneous rocks and sediments: Geochim, 

et Cosmochim. Acta, v. 10, p. 153-165.

Wetherill, G. W., Davis, G. L., and Aldrich, L. T., 1957, Age 

measurements on rocks north of Lake Huron: Am. Geophys. Union, program abstracts for annual meeting, Washington, D. C., April 29-May 2, 1957, p. 39-40.

Wier, K. L., 1950, Comparisons of some aeromagnetic with 

ground magnetic profiles: Am. Geophys. Union Trans., v. 

31, p. 191-195.

Wier, K. L., Balsley, J. R., and Pratt, W. P., 1953, Aeromagnetic 

survey of Dickinson County, Michigan, with preliminary 

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Map GP-115.

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data of the Sholdeis-Doane and Red Rock explorations, Iron 

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v. 67, p. 1743.

INDEX

Page

Acknowledgments ___._____________________ 28 

Algoman_ _____________________________________ 29,33 

Algonkian _________________________ 29 

Amasa formation, general description______________________ 36-37 

mines ____..... ...__... 36-37

name . __________________________________ 36 

rocks in ________________________ 37 

stratigraphy________._________________________ 36 

thickness..__________________________________ 37 

Animikie . .__ .....__.......___......._ 32,33,34-35

Archaean ________________.________ 29 

Archean.. .... . . ..... ......... 29,32,37

Badwater greenstone _ _________. ______ 36,37 

name ___..____________________ 37 

origin . ___________________________ 37 

outcrop ______......___________________________ 37

stratigraphy__________________________________ 37

thickness.._._____.____________________________ 37

Baraga group._____________________________________ 35-37

name . __......._..__...._. 35

rocks in . ___________________________ 35-36 

Bird iron-bearing member, general description_____________.___ 36 

outcrop ___________________________________ 36 

stratigraphy____________________________.____ 36 

thickness........ _________________________ 36

Brace series ........ ..._______________________... 29,33,34

Canadian shield_____. ___________________________ 28-29

Chaotic breccia__________________________________ 38

composition of________________________________ 38

thickness... __._____...________________ 38

Chocolay group__............_________________________ 35

name. __________________________________ 35

rocks in_______________________________________ 35

Clark, quoted___________________________________ 31

Cobalt series. .. _..... ________ __..... 33,34

Dickinson gneissic granite, post_____ . _______________ 32,33

Dickinson group ___________________________ 31-32

dip... ...........__.__...._...._........____.. ........ 31

name.- ___________________________________ 31 

Dunn Creek slate, general description _______ _____. 37-38 

name __ __ _______________ 37 

outcrop __.__ ___________________________ 38 

rocks in..___________________________________ 37,38 

thickness___________________________________ 38

East Branch arkose, general description__.__________________ 31-32

dip . ..-......-.. ....-..-.-...-........- . .......- 31

evidence of pre-Animikie age. _... . __________^____ 31-32

outcrop -_ _______....._______________ 31

rocks in . . __._.. .___.... 31

thickness___________________________________ 32

Eparchaean interval______________________________ 29

Felch formation. ... ........ . ...________.___.__ 35

rocks in ______.... ______.___________________ 35

stratigraphy.______.____________._____________ 35 

See Menominee group.

Fence River formation______________________________ 36

name. _____ ______....________________ 36

outcrop..._____....__________________________ 36

Fern Creek formation. ______________________________ 34,35

Fortune Lakes slate, general description..-___________________ 39

name - _____._______________.____________ 39

outcrop_______________________________________ 39

rocks in_. __________________________________ 39

thickness__....______...____________________.. 39

Page

Qeochronology.. 29 

Gneissic granite_____ ____ - __ - 32,33 

Goodrich quartzite 35-56 

outcrop 36 

Granite, age of. _ -__ -..... .... - - 34,39-40 

Granite gneiss, general description 31

Hardwood gneiss, general description 32 

composition. 32 

dip 32

name_____ 32

outcrop__... 32 

thickness____-_...._____~ _ - 32

Hemlock formation, general description . -.. 36 

name __ 36 

rocks in. _ __ . 36 

thickness____ __ - 36

Hiawatha graywacke, general description. 38 

name.-__.___________________________-___ 38

outcrop____ .. 38 

rocks in.._ . . 38 

thickness_.. - - 38

Huronian....-..___... _ - ... 29,33,35 

correlation with the strata of the Marquette district. . 33-34 

rocks - 33 

subdivisions 33 

thickness___... - - 33 

Lower, rocks in.. .. . . 33,34 

Upper, rocks in__ __ _ 33,34 

See Frecambrian rocks, Middle.

Iron River-Crystal Falls district, stratigraphy...- . . 37 

structure____ 37

Keewatin . - 29 

Keweenawan rocks, pre 28 

Killarney . 29,40

Konadolomite. _ - - . .. 33,34,35

Lake Huron, north shore, unconformity 33 

Lake Superior region 29,35 

Laurentian_____.. . 29 

Lead-alpha _.... ... 39

Location of area covered by report 31 

Lorrain formation, appearance 34

Magnetic rocks___ . - - - 31,32,36,38

Mansfield iron-bearing slate member, general description 36

name_____ - - 36

rocks in.__ 36

thickness_ - 36

See Hemlock formation.

Marquette district, correlation with type area Huronian 33-34

See Frecambrian rocks, Middle

Marquette series, Lower, rocks in... 33

unconformity 33

Upper, rocks in 33

Menominee group 35

formations in. - 35

name. __ - 35

Mesnard quartzite 34,35

Metadiabase.. .... ... __ 33,39

Metagabbro ....................................... ............. 33,39

Metasedimentary rocks 33

Metavolcanic rocks 33

Michigamme slate 36,37

rocks in______ ... . 37

thickness____.... .___.. . . -. . 37

Mississagi formation, composition . 34

43
44 INDEX

Page 

Paint River group________________________________ 37-39

name . .... . _.._..__._....._ 37

See Iron River-Crystal Palls district.

Potassium-argon. _ _ ...____________._ 29,34 

Precambrian, divisions of.__ ..___ ________________ 29 

Precambrian igneous rocks, Middle.____-_________________ 39-40

groups. . ___ 39 

Precambrian rocks, Middle___________________________ 33-35

rocks in . . ..____......____.. 33

See Huronlan, Marquette district.

Precambrian rocks, Lower_ _______.______________ 31-33 

Proterozoic__ _______________________________ 29

Quartzite. _ _ .. __.. 31,34

Quinnesec formation . ..__....._______ 32-33

age . __ __ _____________ 33 

rocks in. ...._ __.__....._______________ 32

Randville dolomite___________._______._._______.__ 31,35

References cited . .._ ___. _. _ 40-41

Riverton iron-formation, general description. ..___ ________ 38

composition _ __ ______________ 38

name. _____ ________________________ 38

outcrop._ ____ _________________________ 38

thickness ______.________________________ 38

Royal Society of Canada, nomenclature of... - ____________... 29

Rubidium-strontium____________________.__________ 34

Six-Mile Lake amphibolite, general description. ..____..._______ 31,32 

composition . _ _____..______._________ 32 

name. _____._________________________ 32 

outcrop_.._________________________________ 32 

thickness . . . .... . -....-.-.__...._.. 32

Skunk Creek member, general description__ . 

composition 

magnetic anomaly 

name. 

outcrop_ - 

thickness__ , 

Solberg schist, general description_ 

composition.. 

name___-- - ----

outcrop. ---------

thickness -.-- 

See Skunk Creek member.

Stambaugh formation, general description . 

name . 

outcrop ,- 

rocks in.._ . 

Strontium-rubidium . 

Sturgeon quartzite... . 31

U. S. Geological Survey, Precambrian subdivisionsPage 

32 

32

31

32

31,32

32

32

32

38

40

34,35

29

Van Hise and Leith, quoted 37 

Vulcan iron-formation 35,36

members__ 35

See Menominee group.

Wauseca pyritic member, general description. 38

composition 38

name- 38

outcrop_ 38

subdivisions of... . . - 38

thickness.._________ . . . 38 

See Dunn Creek slate.

U. S. GOVERNMENT PRIUT1NS OFFICEt 1918