THE PORTAGE LAKE VOLCANICS 

(MIDDLE KEWEENAWAN) 

ON ISLE ROY ALE, 

MICHIGAN 

Prepared in cooperation with the 

National Park Service 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 754-C 

The Portage Lake Volcanics 

(Middle Keweenawan) on 

Isle Royale, Michigan 

By N. KING HUBER 

GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 754-C 

Prepared in cooperation with 

the National Park Service 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1973 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 73-600161 

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

Washington, D.C. 20402- Price $1.05 (paper cover) 

Stock Number 2401-02399 
CONTENTS 

Page 

Abstract -------------------------------------------------------------------------·-------------· C1 

Introduction ·····-·······-···---------------···················-··--········-··············· 1 

Historical background ·················--····--············--···--······-····- 1 

Scope of study ·····-·-·--··········--·-·············-····-·····················-···· 1 

Acknowledgments ··········--·····················--··-····--·········-···-·--· 2 

The Portage Lake Volcanics on the Keweenaw Peninsula__ 3 

The Portage Lake Volcanics on Isle Royale .......................... 4 

General description ··-··············-·············-------··-·············-···-· 4 

Volcanic rocks···--····--········-··-···················--------------------------- 5 

Classification and nomenclature ---------------- --------···-· 5 

Composition -----------------------------------------------··· ------------- 7 

Stratigraphic units ··-------------------------------------------------- 8 

Porphyrite flows -------------------------------------------------- 13 

Scoville Point, Middle Point, and 

Tobin Harbor Flows -------------------------------- 13 

Huginnin and Grace Island Flows ----------- 13 

Trap flows ----------------------------------------------·------------- 15 

Edwards Island and Long Island Flows __ 15 

Minong Flow ------------------------------------------------ 15 

Amygdaloid Island Flow --------------·------------- 18 

The Portage Lake Volcanics on Isle Royale--Continued 

Volcanic rocks-Continued 

Stratigraphic units-Continued 

Page 

Ophite flows ---------------------------------------------------------- C18 

Greenstone Flow-------------------------------------·--- 19 

Hill Point Flow -------------------------------------------- 19 

Washington Island Flow---------------------------- 20 

Sedimentary rocks -------------------------------------------------------------- 20 

Pyroclastic rocks --------------------------------------------------------------- 22 

Stratigraphic summary ---------------------------------------------------- 23 

Stratigraphic thickness -----------------------------------------------------· 23 

Correlations -------------------------------------------------------------------------------- 24 

Correlation between Isle Royale and the Keweenaw 

Peninsula ------------------------------------------------------------------------- 25 

Relation to other Keweenawan volcanic rocks -------------- 27 

Structure ------------------------------------------------------------------------------------- 28 

Origin of the Portage Lake Volcanics------------------------------------ 29 

References cited ----------------------------------·-··------------·---------·------------- 30 

ILLUSTRATIONS 

Page 

FIGURE 1. Sketch map showing generalized distribution of Keweenawan rocks in the Lake Superior region ------·------------------- C2 

2. Chart showing nomenclature for middle and upper Keweenawan rocks of northern Michigan and adjacent 

parts of Wisconsin--------------------------··--·----·--·----------··-----------------------------------------------------------·--·--··---------------------------------------- 3 

3. Longitudinal stratigraphic section of the Portage Lake Volcanics on the Keweenaw Peninsula------------··---------------- 4 

4. Photographs showing characteristic textures of volcanic rocks on Isle Royale ------------------------------------···--··--·-----------·---- 6 

5. Photograph showing coarse ophite with knobby weathered surface --------------------------·-·-·····-··----------------·-----···----------------- 7 

6. Chart showing nomenclature for named volcanic flows within the Portage Lake Volcanics on Isle Royale -·····---··· 7 

7. Plot showing tholeiitic affinities of analyzed specimens of rocks from the Portage Lake Volcanics on Isle Royale 8 

8. Plot illustrating the predominantly basaltic nature of analyzed specimens of rocks from the Portage Lake 

Volcanics on Isle Royale····················-·····-····-------···········-····-··········-·············-----------------·········-····--·---····--·-··-····-----·-····---- 9 

9. Shaded-relief map of Isle Royale National Park······-··-----·····-····-········-···-··--··-········----········-··------······-··-·--------·-·-··--····------- 12 

10. Schematic columnar section of t..~e Portage Lake Volcanics on Isle Royale-···-·····-···-·····-·-···--·········-·--···--············-------· 14 

11. Simplified map of bedrock geology of Isle Royale ············-------------------······---·······-··-··-------------··-···---------------·-·--··-----····------- 16 

12-16. Photographs showing: 

12. Porphyrite from Huginnin Flow ······--······---········-·····-·--····-········-·······-·····-··········-····--···-··--·-····--·-·-···-----··--····--·····--- 18 

13. Beach cobble of porphyrite from Grace Island Flow ·········-----·-····--··-------------·-··-···--·--····-···--------·--------·--····-········· 18 

14. Columnar jointing in Edwards Island Flow. ......... ----······················-··-·-·-·-·····---·····--····-·-···--···-··--······-·-·-····--·····-·--- 18 

15. Specimen of Amygdaloid Island Flow with characteristic agate amygdules --······-····-····---···--·--·······---·--·--·····-- 18 

16. Specimen of ophite from Washington Island Flow···········-···-----·--·-·····--·--········-··----·-··-············---··------······-··------- 20 

17. Columnar section of upper part of Portage Lake Volcanics showing distribution of sedimentary units in the 

Chippewa Harbor area··········--·-·····-·--·····----·--·-········-····-·········------------------·-----·--------····-·----········-·--··--······-----·--·-··-·--··--··--- 21 

18-21. Photographs showing: 

18. Scour-and-fill structure in pebbly sandstone near the west end of Chippewa Harbor --·-······-·····--·-···--·--·------- 21 

19. Breccia occurring above the Amygdaloid Island Flow ·--···--·--·--··---···········-··--·-·····----···-··-----------···-·····---·--·····--·-· 22 

20. Tuff-breccia overlying the Greenstone Flow. ........ -------------····--·------···-----···-·····--···-···-·········-···-·-···-·-·····-·--·-·--·-····--·· 22 

21. Agates typical of those occurring in the tuff-breccia overlying the Greenstone Flow---··--·--··--····--·--------··-···· 23 

22. Geologic section of Isle Royale in the vicinity of Lake Desor and Houghton Ridge ·-·--------·····--···-····-···-·--·-----------···--- 23 

23. Longitudinal stratigraphic section showing variations in thickness of the Portage Lake Volvanics on Isle Royale 24 

24. Stratigraphic diagram illustrating correlation of sedimentary horizons in the Portage Lake Volcanics between 

Isle Royale and the Keweenaw Peninsula ···-·····-----·--·----------···--······-····-·--··-·····-·············--···---·---··---······--··-···--·-··-······-· 27 

Ill 
IV CONTENTS 

TABLES 

Page 

TABLES 1. Analyses of volcanic rocks of the Portage Lake Volcanics, Isle Royale---------------------------------------------------------·---------------- ClO 

2. Correlation of clastic units in the Portage Lake Volcanics between the Keweenaw Peninsula and Isle Royale.... 26 

REPRO PROOF -tEf PACIFIC TYPOGRAPHIC SERVICE~ 655-0660 iliTI'iltroltroltro!~ltroltroltNilirnlihi 
GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

THE PORTAGE LAKE VOLCANICS (MIDDLE KEWEENAWAN) 

ON ISLE ROYALE, MICHIGAN 

By N. KING HUBER 

ABSTRACT 

The Portage Lake Volcanics is the upper part of the total 

volcanic sequence of Keweenawan (Precambrian) age in the 

Lake Superior region. On Isle Royale the formation consists of 

more than 10,000 feet of basaltic and andesitic lava flows with 

interbedded pyroclastic rocks and fluvially deposited sedimentary rocks. Certain stratigraphic units representing flows or 

groups of flows can be identified and traced in the field on the 

basis of their texture or other distinctive characteristics. Twelve 

such flow units have been distinguished and named on the 

island, and these key units provide stratigraphic and structural 

control within the volcanic sequence. 

Correlation between the Portage Lake Volcanics on Isle 

Royale and the type area some 50 miles southeast on the 

Keweenaw Peninsula on the opposite side of the Lake Superior 

syncline is excellent, and at least one individual lava flow is 

interpreted as being continuous across the basin. 

The Portage Lake Volcanics on Isle Royale does not appear 

to stratigraphically overlap the lower part of the Keweenawan 

volcanic sequence on the north side of the syncline, represented by the North Shore Volcanic Group in Minnesota, and 

there may be a stratigraphic break between them. The North 

Shore Volcanic Group is the source terrane for most of the 

clastic debris in the interbedded sedimentary rocks in the 

Portage Lake Volcanics on Isle Royale as well as in the overlying Copper Harbor Conglomerate. 

The flood basalts of the Portage Lake Volcanics were erupted 

from a major rift zone of continental proportions that trends in 

an arc through Lake Superior. Similar flood basalts underlie 

most or all of a belt nearly 100 miles wide and more than 1,000 

miles long. The depositional basin was subsiding as it was being 

filled, and during intermittent periods of volcanic quiescence, 

clastic debris was swept into the basin from its margins to form 

the sedimentary deposits interbedded with the lava flows. When 

volcanism ceased, subsidence continued, permitting the accumulation of a thick sedimentary sequence, the Copper Harbor 

Conglomerate and younger Keweenawan sedimentary rocks. 

INTRODUCTION 

HISTORICAL BACKGROUND 

It has long been proposed that part of the rock 

sequence on the Keweenaw Peninsula extends beneath 

the Lake Superior basin and is exposed again on Isle 

Royale (fig. 1). The basis for this interpretation is the 

similarity of the volcanic and sedimentary rock types 

and of the gross stratigraphic succession in both areas. 

As early as 1849, Charles T. Jackson (p. 474) stated 

that "this island has the same geological character as 

Keweenaw Point, and is of the same geological age." 

And in 1850, John W. Foster and Josiah D. Whitney 

(p. 81) noted that "in many respects, Isle Royale may 

be regarded as the counterpart of Keweenaw Point." 

Although the reconnaissance surveys of Jackson, 

Foster, and Whitney led to the first suggestions of correlation between the stratigraphic sections of Isle 

Royale and the Keweenaw Peninsula, not until the 

investigations of Alfred C. Lane (1898; 1911) was a 

firm foundation provided for such correlations by the 

recognition of parallel sequences of the varied rock 

types. Lane (1898, p. 101) concluded that "on the 

whole the coincidence is fairly satisfactory, and warrants us in saying that we have represented on Isle 

Royale practically the whole of the Copper Range 

[Portage Lake Volcanics] as it exists from the Central 

Mine to Portage Lake," on the Keweenaw Peninsula. 

Since Lane's study, no additional data regarding the 

Portage Lake Volcanics on Isle Royale have been 

published. 

SCOPE OF STUDY 

The present report, a part of a general study of the 

geology of Isle Royale National Park being carried out 

in cooperation with the National Park Service, focuses 

on the stratigraphy and structure of the Portage Lake 

Volcanics on Isle Royale and presents additional data 

on correlations with the Keweenaw Peninsula and on 

relations to other volcanic sequences of Keweenawan 

age. Only compositional and petrologic data necessary 

to characterize the rocks are given, as the rocks are 

similar in all respects to rocks of the Portage Lake Volcanics on the Keweenaw Peninsula and details can be 

obtained from extensive studies of those rocks on the 

peninsula. An earlier companion report describes the 

overlying Copper Harbor Conglomerate on Isle Royale 

Cl 
C2 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

93° 91° 89" 

49" 

ONTARIO 

MINNESOTA 

47" 

0 

WISCONSIN 

50 

87" 

MICHIGAN 

czz;v 

Michipicoten 

Island 

() 

85° 

100 150MILES 

45·L--------L-L---------------~----------------~------------------~----------------~~----~ 

ITIIIIJ 

Jacobsville Sandstone 

and Bayfield Group 

Freda Sandstone and 

Nonesuch Shale 

Copper Harbor 

Conglomerate 

z 

<( 

0:: 

Ill 

~ 

<( 

u 

w 

0:: 

D.. 

EXPLANATION 

Major intrusive 

rocks 

Volcanic 

sequence 

Sedimentary 

sequence 

z 

<( 

0:: 

Ill 

~ 

<( 

u 

w 

0:: 

D.. 

FIGURE !.-Generalized distribution of Keweenawan rocks in the Lake Superior region. Data from Green (1971), Halls (1966), 

Halls and West (1971), Irving (1883), White (1966a), and White and Wright (1960). 

(Wolff and Huber, 1973). A geologi-c map of Isle Royale 

National Park at a scale of 1:62,500, published separately (Huber, 1973b), provides details of the distribution and structure of the Portage Lake Volcanics 

not possible to show at the scale of maps in the present 

report, and it is recommended that that map be used 

in conjunction with this report. In fact, some of the 

place names mentioned in this report can be found only 

on that geologic map or on the Topographic Map of 

Isle Royale National Park (U.S. Geological Survey, 

1957). 

ACKNOWLEDGMENTS 

The generous support of the National Park Service 

staff made this study possible. Numerous discussions 

with Henry R. Cornwall and Walter S. White, drawing 

upon their extensive knowledge of the geology of the 

Keweenaw Peninsula, were very helpful. Field visits 

with John C. Green, Harold A. Hubbard, and WalterS. 
PORTAGE LAKE VOLCANICS C3 

"Wmte, in their respective areas of Keweenawan study, 

have been of help in understanding the regional aspects 

of Keweenawan geology. During summers on the island, 

I was ably assisted, successively, by Robert J. Larson, 

Harrison T. Southworth, Charles E. Bartberger, and 

David R. Chivington. 

THE PORTAGE LAKE VOLCANICS ON THE 

KEWEENAW PENINSULA 

The middle Keweenawan (Precambrian) Portage 

Lake Volcanics, in its type area on the Keweenaw 

Peninsula, is made up of a sequence of several hundred 

lava flows with rhyolite conglomerate separating some 

of the flows. Interbedded sandstone and pyroclastic 

rocks are less abundant. Since the first significant geologic account of the native-copper deposits in the 

Portage Lake Volcanics by Houghton (1841), the formation has been intensively studied by many workers. 

Although the latest comprehensive report is that of 

Butler and Burbank ( 1929), much of the more recent 

literature is covered in a review of the Keweenawan 

geology of the Lake Superior region by Halls ( 1966) 

and in papers on the geology of the Michigan copper 

district by White ( 1968, 1971). These reports are the 

source of most of the material in the following summary. 

The formation name Portage Lake Lava Series was 

introduced (White and others, 1953) as a comprehensive name to include rocks designated the Bohemian 

Range, Central Mine, Ashbed, and Eagle River 

Groups of earlier reports. (See Lane, 1911; Butler and 

Burbank, 1929, p. 17-18.) At the request of the Geologic Names Committee of the U.S. Geological Survey 

and in consultation with workers in the type area 

(W. S. White, written commun., 1972), the formation 

name Portage Lake Volcanics is used in this report in 

place of Portage Lake Lava Series in order to bring the 

formation name into conformity with the Code of 

Stratigraphic Nomenclature (American Commission 

on Stratigraphic Nomenclature, 1970, p. 7, 14). The 

formation is locally at least 15,000 feet thick on the 

Keweenaw Peninsula, but the base is nowhere exposed 

because the lower part of the sequence is bounded by a 

high-angle thrust fault, the Keweenaw fault. In westernmost Michigan, structural considerations suggest 

that the Portage Lake Volcanics unconformably overlies older Keweenawan volcanic rocks, which form the 

so-called South Trap Range (Hubbard, 1968). The 

Portage Lake Volcanics is conformably overlain by the 

Copper Harbor Conglomerate, which White (1972) 

proposed be reassigned from the upper to the middle 

Keweenawan. Locally the two formations are interfingered (fig. 2). The Copper Harbor Conglomerate is 

overlain by the upper Keweenawan Nonesuch Shale 

and Freda Sandstone of the Oronto Group. 

Formations of Bayfield Group, ~a. .!! ::J 

not differentiated here .... 0 >. .. 

~(!) z 

<( 

~ <( 

z 

UJ 

UJ 

~ 

Freda Sandstone UJ 

~ 

c. a:: ::J UJ 0 

<; 0.. 

0.. 

.s ::l 

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Nonesuch Shale 0 

z 

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Copper Harbor Conglomerate m 

:E (including mafic lava members) <( 

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Unnamed volcanic z formation <( 

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Portage Lake Volcanics 

FIGURE 2.-Nomenclature for middle and upper Keweenawan 

rocks of northern Michigan and adjacent parts of Wisconsin 

(from White, 1972). The unnamed volcanic formation occurs 

only in the Porcupine Mountains area (fig. 1). 

The lava flows are predominantly basalt or basaltic 

andesite, containing essential calcic plagioclase (generally labradorite), augite, and minor olivine. Compositional details of the volcanic rocks have been summarized most completely by Broderick (1935) and by 

Cornwall ( 1951a; 1951b). About 40 percent of the 

mafic flows have ophitic texture, coarsest in the 

thickest flows. The remaining flows are quite fine 

grained but show textural differences that depend 

mostly upon feldspar grain size and distribution. The 

uppermost 5-20 percent of most individual lava flows 

is conspicuously amygdaloidal and contains 5-50 percent vesicles filled with secondary minerals. The abundance of amygdules decreases downward toward the 

amygdule-free massive basalt of the middle and lower 

part of the flow. A thin amygdaloidal zone is generally 
C4 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

present at the base. In many of the flow tops, the 

vesicular crust was brecciated, probably during flow, 

and now consists of rubbly or fragmental material in 

which both amygdules within fragments and interstices 

between fragments are filled with secondary minerals. 

Many of the thicker flows have undergone magmatic 

differentiation, with the development of an internal 

zone having layers characterized by a pegmatitic texture; the Greenstone Flow is a prime example. Rhyolite 

flows or domes are present locally but are uncommon. 

The interbedded conglomerates are thoroughly lithified and normally consist of subangular to rounded 

pebbles, cobbles, or boulders of rhyolite and subordinate basalt in a sand matrix. Locally, granophyre pebbles are abundant. Individual beds of conglomerate or 

sandstone range from less than 1 inch to more than 100 

feet in thickness and tend to be lenticular. Some individual lava flows and most of the major interbedded 

Long 

89°W. 

I IV 

·~ 

Top of Portage Lake Volcanics 

-----=-- I 

Hancock Conglomerate 

CJ 

> 

Pewabic West Conglomerate 

I 

1 

'ii 

a.. 

conglomerate units can be traced for many miles along 

strike. They thus provide useful stratigraphic marker 

horizons, and many have been named (and (or) numbered) for ease of reference (fig. 3). 

Radiometric age determinations by the uranium-lead 

method suggest that Keweenawan igneous activity falls 

within the range of 1,120-1,140 million years ago (Silver and Green, 1972). The Portage Lake Volcanics thus 

clearly is within a newly defined division of the Precambrian, Precambrian Y (James, 1972), which has an 

age range of 800-1,600 million years. 

THE PORTAGE LAKE VOLCANICS 

ON ISLE ROYALE 

GENERAL DESCRIPTION 

The Portage Lake Volcanics on Isle Royale, as on 

the Keweenaw. Peninsula, consists of a thick sequence 

of basalt and basaltic andesite lava flows with inter-

...... 

...... ~ c: 

Qj 

(.) 

Long 

88°W . 

I 

THOUSANDS 

OF FEET 

No. 17 

15 

------~~A~I~Io~u~ez~C~o~n~g~lo~m~e~ra~t~e======~~~~~~~~~~~~~~~~~~~~~~~~~ No.15 

No. 14 

------------------------+-------- No.13 

-- _ Balticc 4-- ong/o Ppr0X;-- _ _ rnerate 

mate str -.--

I at, graph;--..._ /. ..--- ---..." 

c Posit JOn------...--\?>.\).\'\. '-' 

Of Keweena\111 " 

'\ 

\ 

\ 

0 10 20 30 MILES \ 

\ 

\ 

Kearsarge Flow 

Wolverine~ Sandstone 

\ / 

', ----... / " / I'- ' '- _,/ / 

No. 12 

No.9 

Nos. 8 

and 6 

10 

5 

0 

FIGURE 3.-Longitudinal stratigraphic section of the Portage Lake Volcanics on the Keweenaw Peninsula. Conglomerate beds 

are numbered according to local usage (from White, 1968). 
PORTAGE LAKE VOLCANICS C5 

bedded conglomerate, sandstone, and pyroclastic rocks. 

Exposures on the island indicate a minimum thickness 

of 10,000 feet, and as the base of the sequence is not 

exposed, the total thickness must be considerably 

greater. The formation is conformably overlain by the 

Copper Harbor Conglomerate (Wolff and Huber, 

1973). The entire sequence is tilted toward the southeast at dips ranging from 55° to less than 5°, with attitudes generally steeper on the north side of the island 

than on the south. Geophysical data suggest that a 

major high-angle thrust fault, the Isle Royale fault, lies 

between Isle Royale and the north shore of Lake Superior (Halls and West, 1971), thus increasing the structural similarity between Isle Royale and the Keweenaw 

Peninsula. 

Records from holes drilled during exploration for 

copper suggest that there may be more than 150 flows 

in the section on the island (Lane, 1898), but some 

units indicated as individual flows in Lane's section 

may actually be parts of composite flows or duplications caused by faulting. In the same drill logs Lane 

records 25 clastic units 1 foot or more thick; less than 

a third of these units are known from outcrop, and 

those occur primarily in the uppermost part of the 

section. 

VOLCANIC ROCKS 

CLASSIFICATION AND NOMENCLATURE 

The classification adopted here for the volcanic rocks 

of the Portage Lake Volcanics on Isle Royale is physically descriptive so as to be useful not only to the specialist but to the interested visitor to the National 

Park; it is based chiefly upon rock textures. Such a 

classification has not been widely used outside the Lake 

Superior region, but by 1898, when Lane's Isle Royale 

report was published, a textural classification was well 

established in the Michigan copper district and was 

used by Lane, who later reviewed the history of the 

development of this classification (1911, p. 51-66). As 

emphasized by Butler and Burbank (1929, p. 23-24), 

the one outstanding characteristic of the flows that is most 

useful in their classification is their texture, which serves as 

one of the major bases of correlation from section to section. 

Textural classification may be made in hand specimen, drill 

core, or outcrop. There may be some differences of opinion as 

to the appropriateness of the names used in this classification 

but there can be no doubt as to the usefulness of the distinctions. 

The following textural classification, based upon 

Lane (1898, 1911), is used in this report: 

Ophite.-Rock with an ophitic texture is produced 

by crystals of pyroxene that surround and enclose 

unoriented plagioclase laths, imparting a mottling (fig. 

4A). Weathering commonly accentuates the texture, 

both by increasing color contrast between the pyroxene 

crystals and the rock matrix and by producing a 

knobby surface (fig. 5). In freshly broken rock the 

texture can be distinguished by the flashing of 

pyroxene cleavage faces. The size of the pyroxene 

crystals varies with their distance from flow surfaces, 

becoming progressively larger toward flow interiors; 

crystals exceed 2 centimeters in diameter in the thickest ophitic flows on Isle Royale. 

Porphyrite.-Rock with a porphyritic texture is 

produced by well-defined plagioclase crystals scattered 

through a finer grained groundmass. The term is 

applied to two distinct varieties of such porphyritic 

rocks. One variety has small, blocky, millimeter-sized 

plagioclase crystals, more or less uniformly distributed 

through the groundmass (fig. 4B); where the plagioclase crystals tend to clot together, the term "glomeroporphyrite" has been used (fig. 4C). The other variety 

has larger, tabular crystals more randomly distributed 

and commonly occurring in clots (fig. 4D); the large 

crystals are often as long as 2 em. 

Pegmatite.-Rock with a pegmatitic texture is that 

in which the minerals, especially the plagioclase, are 

unusually coarse when compared with the other volcanic rock types (fig. 4E). This term, as applied to 

Keweenawan lava flows, is of relatively recent usage 

(Cornwall, 1951a); Lane used the term "dolerite," one 

that is confusing because it is widely used in a different 

sense. 

Trap.-Fine-grained massive rock called trap shows 

none of the distinctive textures indicated previously 

(fig. 4F). Lane tended to use "melaphyre" as a rock 

type and trap for a specific lava flow of that rock type, 

as for example, the Minong Trap. The term " trap" has 

also been used in the Michigan copper district to refer 

to the massive interior part of a flow, as distinguished 

from the amygdaloidal margins (amygdaloid). In this 

report "trap" will be used with a textural connotation 

and "is applied to any nonporphyritic aphanitic rock 

which is very dark gray or black, regardless of composition. It is thus equivalent to the term 'felsite' as used 

for lighter colored nonporphyritic aphanitic rocks" 

(Jackson,1970,p.300-301). 

The main advantages of a textural classification over 

a compositional classification for rocks of the Portage 

Lake Volcanics arise not only from the ease of application in the field but from the nature of the volcanic 

rocks themselves. Individual flows within the volcanic 

sequence generally have a characteristic texture 

throughout; consequently certain stratigraphic units 

representing flows or groups of flows can be identified 

and traced in the field on the basis of their texture and 

thus are excellent stratigraphic marker units. These 

features are especially important in areas of limited 

outcrop, as on the west end of Isle Royale or in areas 

that have been structurally disturbed. On Isle Royale, 
C6 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

A 

B 

1 3cm I __j c 

FIGURE 4.-Characteristic textures of volcanic rocks on Isle 

Royale. A, Typical ophite. Beach pebble from Mott Island. 

B, Fine-grained porphyrite. Scoville Point Flow from south 

side of North Government Island. C, Fine-grained porphyrite 

tending toward glomeroporphyrite. Tobin Harbor Flow from 

south side of Porter Island. D, Coarse porphyrite. Huginnin 

1

3cm 

1 E 

1

3cm 1 F 

Flow from shoreline just west of Huginnin Cove. E, Pegmatite. From the differentiated zone of the Greenstone Flow on 

the east end of Passage Island. F, Trap. Minong Flow from 

the west end of Isle Royale. A, B, D, and E show weathered 

surfaces. 
PORTAGE LAKE VOLCANICS C7 

FIGURE 5.-Coarse ophite with knobby weathered surface. 

South side of Raspberry Island. Penknife, 7 em long. 

12 such flow units have been distinguished within the 

Portage Lake Volcanics, and these key units provide 

stratigraphic and structural control for geologic mapping on the island (fig. 6). 

COMPOSITION 

It has been long recognized that there is a general 

relation between the texture and the composition of 

the volcanic rocks in the Portage Lake Volcanics on the 

Keweenaw Peninsula, although there is considerable 

overlap. Butler and Burbank ( 1929) noted that ophitic 

texture is best developed in the more mafic lavas and 

that porphyritic and glomeroporphyritic textures are 

characteristic of more felsic flows or those approaching 

andesite. Cornwall ( 1951a) stated that "the ophites 

are predominantly basalts, whereas the porphyrites, 

glomeroporphyrites, and melaphyres are predominantly 

basaltic andesites." 

The plagioclase in the ophites is generally labradorite; that in the porphyrites is labradorite or calcic 

andesine. The pyroxene is predominantly augite with 

small amounts of pigeonite in some ophitic flows. 

Ilmenite and magnetite are common accessory minerals. The major secondary minerals occurring both as 

alteration products and filling original voids include 

chlorite, sericite, prehnite, pumpellyite, chalcedony, 

laumontite, and calcite in addition to the opaque alteration products of the olivine. 

Primary textures of the volcanic rocks on Isle Royale 

are well preserved; there is no development of schistosity. Nevertheless, the rocks have been pervasively 

altered, for of the original minerals, only the pyroxene 

remains generally fresh. The degree of alteration appears to be much greater on Isle Royale than in similar 

rocks on the Keweenaw Peninsula, where commonly 

plagioclase and occasionally olivine are fresh enough 

for their original compositions to be determined. 

"' (.J 

c 

"' (.J 

0 

> 

., 

."'-

..J "' 

~ 

f! 

5 

C1. 

FIGURE G.-Nomenclature for named volcanic flows within the 

Portage Lake Volcanics on Isle Royale. Shaded areas represent the undivided remainder of the sequence, consisting 

predominantly of ophitic volcanic rocks with lesser amounts 

of sedimentary and pyroclastic rocks. 
CB GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

In the absence of quantitative mineralogical details 

from Isle Royale, chemical analyses of representative 

volcanic rocks are here used to characterize their composition (table 1). The extent to which alteration has 

affected the bulk chemistry of the rocks is unknown, 

but alteration is reflected to a certain extent by the 

presence of appreciable water, probably occurring 

chiefly in secondary minerals, and by the high o:ridation state of the iron. To partially compensate for oxidation, the ferrous-ferric ratio was adjusted to 2:1 prior 

to calculating the normative minerals in table 1. 

According to the chemical classification of Irvine and 

Baragar (1971) , which is based in part on normative 

mineralogy, the suite of rocks from Isle Royale is subalkaline, has tholeiitic affinities (fig. 7), and is predominantly basalt (fig. 8). The apparent variance of their 

classification with Cornwall's nomenclature results 

from his use of the classification system of Johannsen 

(1939), in which the dividing line between basalt and 

andesite is based solely on the modal anorthite content 

of the plagioclase (at 50 percent) and under which some 

of the basalts of this report would be called andesites. 

STRATIGRAPHIC UNITS 

The topography developed upon the rocks of the 

Portage Lake Volcanics on Isle Royale consists of a 

series of ridges and valleys alined in a northeast-southwest direction parallel to the regional strike of the formation (fig. 9). The southeast-facing slopes, controlled 

by the structural dip, are gentle compared with the 

northwest-facing antidip slopes and together provide a 

marked asymmetry to the ridge-and-valley cross profile. Because the sedimentary rocks in the series and 

the amygdaloidal zones at the base and top of flows are 

less resistant than the massive flow interiors, the interior parts of the thicker flows are the dominant ridgeforming units. The distinguishing texture of an 

individual flow is generally better developed and more 

readily apparent in the nonamygdaloidal or less-amygdaloidal flow interior. Fortunately, the part of a flow 

most useful for identification purposes is the part most 

likely to crop out. Contacts between flows, on the other 

hand, are seldom exposed, as they are generally concealed beneath alluvial materials or talus in the 

depressions between ridges. 

Because the volcanic rocks on Isle Royale are predominantly ophitic, the nonophitic or the few unusually thick coarsely ophitic flows are most useful as 

stratigraphic markers. These include finer grained porphyrites and traps which, in addition to their unique 

textural identity, are generally more resistant to erosion than the ophites and tend to stand out topographically, adding to their usefulness in geologic mapping. 

The 12 stratigraphic units within the Portage Lake 

Volcanics on Isle Royale that have been individually 

named include five porphyrites, four traps, and three 

ophites. All names are new except for the Huginnin and 

Minong Flows, formerly called Huginnin Porphyrite 

22 ,-----,-----,-----,------,-----,-----,-----,------.-----.-----. 

1-

z 

w 

20 

~ 18 w 

0.. 

II 

~ 16 w 

:5: 

~ 

.:.14 

0 

12 

EXPLANATION 

o Ophite 

X Porphyrite 

+ Trap 

• Pegmatite 

Calc-alkaline field 

• 71"6---- ............... .....__ 

Tholeiitic field x60A 

+49 

10~----~---- ---- -----L----~----~----~------L-----L---~ 

100 80 60 40 20 0 

NORMATIVE PLAGIOCLASE COMPOSITION IN PERCENT An 

FIGURE 7.-Tholeiitic affinities of analyzed specimens of rocks from the Portage Lake Volcanics on Isle Royale. Classification used is that of Irvine and Baragar (1971) . Numbers are keyed 

to analyses in table 1. 
PORTAGE LAKE VOLCANICS C9 

and Minong Trap, and the Greenstone Flow, originally 

named on the Keweenaw Peninsula. Six of these units 

can be traced the full length of the island, and two 

others about two-thirds the length. 

The thickness of the units is difficult to estimate, in 

part because contacts are so sparsely exposed. Some 

thickness data can be obtained from Lane's (1898) 

descriptive log of a series of holes drilled across the 

island near its west end in 1891 and 1892, although 

correlation problems introduce some uncertainties. 

Several of the stratigraphic units used in this report 

appear to correlate with groups of flows in Lane's log 

rather than with individual flows. It is possible that 

Lane, in basing his conclusions on the presence of 

EXPLANATION 

o Ophite 

60 x Porphyrite 

+ Trap 

50 

X 

w 

0 

z 

-40 

a: 

0 

...J 

0 

(.) 

w 

> 

i= 

<( 30 ~ 

a: 

0 

z 

20 

10 

• Pegmatite 

66 00 70A 

74+ o11A 0 

15 

+40 

22Bx_ X36 

amygdaloidal rock in the core samples, overestimated 

the number of individual flow tops present or that there 

is duplication by faulting. Lane was aware of the problem, noting in numerous places in the log that the limits 

of some flows were very uncertain; in one place he 

noted that the samples from a core box had been mixed 

up. It is equally possible that some of the units reported here do indeed consist of multiple flows that are 

not individually discernible in the field. Each unit has 

its distinctive rock type, however, and must represent 

a single volcanic episode that produced rock differing 

in some respect from that immediately underlying or 

overlying the unit. The units thus are rock stratigraphic, and all may not necessarily be single flows. 

Basalt 

Andesite 

OBLO--------~~------~----------~--------L---------~--------~2~0--------~1~0--------~0 70 60 50 40 30 

NORMATIVE PLAGIOCLASE COMPOSITION IN PERCENT An 

FIGURE B.-Predominantly basaltic nature of analyzed specimens of rocks from the Portage Lake Volc_anics on Isle Royale. 

Classification used is that of Irvine and Baragar (1971). Numbers are keyed to analyses m table 1. 
ClO GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

TABLE 1.-Analyses of volcanic rocks of 

[Chemical analyses made by a single-solution procedure (Shapiro, 1967). Analysts: P. L. D. Elmore, Lowell Artis, S. D. Botts, Gillison Chloe, J. L. Glenn, series 1, 0.7, 0.5, 0.3, 0.2, 0.15, and 0.1, which represent approximate midpoints of interval data on a geometric scale. The assigned interval for semiquanAg, As, Au, Bi, Cd, Ge, Hf, Hg, In, Li, Mo, Pd, Pt, Re, Sb, Sn, Ta, Te, Th, Tl, U, W, Zn] 

Rock type ............................................................................................................................................... . Ophite 

Hill Green- Stratigraphic unit .................................................................................................................................. Undivided Point Undivided stone Flow Flow 

Field No ................................................................................................................................................. . IR-llA IR-66 IR-15 IR-70A 

Chemical analyses (weight percent) 

Si02 ------------------------------------------------------------------------------------------------------------------------ 44.7 47.1 47.4 47.6 

Al20a -----------------------------------------------------------------------------------------·---------------------------- 16.3 16.1 16.1 16.2 

Fe20a ---------------------------------------------------------------------------------------------------------------------- 5.6 5.3 10.2 3.9 

FeO ------------------------------------------------------------------------------------------------------------------------ 6.5 6.3 3.4 6.6 

MgO --------·---------------------------------------------------------------------------------·-·-····--··-···········-··· 7.5 7.0 5.9 7.3 

CaO -····-··-·······--·-··--------------------------------------------------------------···---------------·----------------- 8.8 10.2 9.0 11.1 

Na20 -------··----------------------------------------------------------------··--···-----------------------···--------··-· 2.4 2.4 2.3 2.5 

K20 ------------------------------------------------------------------------------------------------------···--··---------·- .52 .35 .45 .18 

H2o- ---------------------------------------------------------------------------------------------------------------------· 1.8 .81 .71 1.3 

H20+ ---------------------------------------------------------------------------------------------------------------------- 3.6 2.4 2.0 1.7 

Ti02 .. ------------------------------------------.. -------------------------------------------··-----------------····--·-···· 1.9 1.5 2.2 1.3 

P205 ------------------------------------------------------------------·---------------------------------------------·-··--·· .29 .20 .34 .14 

MnO -----····-·-··---------------------------------------------·····--·-------·---------------------------··--····--.. ··-· .16 .18 .10 .16 

C02 -----------------------------------------···-------------------------------------------------------------------·-------· <.05 <.05 <.05 <.05 ----------------------------------------------- Sum ·------------------------·--······----·····-··-----------------------------------··---------------··----·--· 100 

Semiquantitative spectrographic analyses 

B -----------------------------------------------------------------------------------------······--·--···-········--------···· Ba --------------------------·-·---------------------------------------------------------------··-·--------·-··-----------··· Be-----------------------------------------------------······--------------······----···-·------------··--··-·-------------· 

Ce .. ----------------------------------------------------------------------------------------------------·--------------------· Co----------··-····-----------------········---------------------····-···········---------·-------------------------···-···-

Cr ........................................................................................................................... . 

Cu --------·---------------------·-------·-·---------------------------------.. ·--·-----·-··---·----------.. -------.. ----··-· Ga----------------------------------.. ·-------------------------------.. ·-------.............. -----------------··--------··· La------------------------------------------------------------------------·-------------------------------------------------

Nb ----------------------------------------------------··----------------------·---·-··----------------------·-···-----··-·--

Ni --------------··----------------------------·-----------·----------------------------------------------------------------· Ph-···---------------------------------------------------------------------------------------------------------------------- Sc ___________________________________________________________________________________________________________________________ _ 

Sr ------------------------··--------------------------------------------------------------·-"···--·----------.. ·-----------.. v ------------------------------·-··-·"···---------.. -----------------------·--·------------------·------------------------·- y ............................. ____________________ , _________________________ , ______ , ________ , ___________________________ _ 

Yb ---··--------·-------------------------····--·······----------------------·------------··--·--·--·---------------------- Zr --····-·--·---------------·---------·-·--------·--·-------------··-····----··-----------------·--··---·-----··-------------· Nd ...... --------------------------------------------------------------............ __________________________ .. ____ .. _______ _ 

0.015 

.005 

.01 

.02 

.0015 

.002 

.02 

.003 

.07 

.03 

.005 

.0002 

.01 

CIPW norms (weight percent) 

Q --------------··---------------------------·------......................................................................... . 

or-------------------------------------------------···------------------------------------··--·-----------------.. -----------· ab __________________________________________________________________________________________________________________________ __ 

an ... ·------------------------··--------------------------------····---------------------------------------------------------- di --------------------------------···---···---------------·-------------------------------------------------------------------

hy 

wo --------------·---------------------------------------------------------------···-.. ·-----------------.. ·--------· 

en·----------------------------------------------------------------------------------------------------------.. ·------ fs -------------------------------------------------------------------------------------------------------------.. -----

Sum ______________________________________________________________________ , ____________________ , _____________ _ 

Normative plagioclase (percent an) -------------------.. ------------------------------------------- Normative color index ---------------------------------------------------------··--------------------------· 

3.3 

21.5 

34.0 

8.2 

(4.2) 

(2.7) 

(1.2) 

16.2 

(11.1) 

(5.0) 

6.1 

3.8 

6.2 

(4.1) 

(2.1) 

.7 

100.0 

61.3 

41.2 

100 

0.015 

........................... 

.005 

.02 

.02 

.0015 

.0015 

.02 

.005 

.05 

.02 

.003 

.0003 

.007 

-------------· 2.1 

21.1 

33.3 

14.4 

(7.4) 

(4.7) 

(2.3) 

19.4 

(13.0) 

(6.5) 

-------------· 5.7 

3.0 

.5 

(.3) 

<.2) 

. 5 

100.0 

61.3 

43.5 

IR-llA. South shore of Raspberry Island. IR-66. North side of Hill Point. IR-15. South shore of West Caribou Island. 

IR-23. North shore of Edwards Island. IR-74. South shore of Merritt Island. IR-22B. South shore of Edwards Island. 

100 

0.015 

.. ............................ 

............................. 

.005 

.007 

.03 

.0015 ............................. 

.002 

.02 ............................ 

.003 

.03 

.03 

.005 

.0004 

.015 ............................... 

3.4 

2.7 

20.1 

33.3 

8.5 

(4.4) 

(2.6) 

(1.6) 

20.2 

(12.6) 

(7.6) 

.............................. 

6.5 

4.3 

---------·----

.............................. 

......................... 

.8 

100.0 

62.5 

40.5 

IR-70A. North side of Blake Point near The Palisa<'.'S. IR-36. North shore of Tobin Harbor near Fire Island. 

100 

............................ 

0.015 

........................... 

............................ 

.007 

.03 

.01 

.0015 .. .......................... 

.0015 

.02 .............................. 

.005 

.03 

.03 

.005 

.0005 

.015 .. ....................... 

.......................... 

1.1 

21.8 

33.5 

18.1 

(9.3) 

(6.0) 

(2.7) 

15.1 

(10.4) 

(4.6) 

............................. 

5.2 

2.5 

2.4 

(1.6) 

(.8) 

.3 

........................... 

100.0 

60.6 

43.6 
PORTAGE LAKE VOLCANICS Cll 

the Portage Lake Volcanics, Isle Royale 

James Kelsey, and Hezekiah Smith (rept. 68 WRC 26). Results of semiquantitative spectrographic analyses are reported to the nearest number in the titative results will include the quantitative value about 30 percent of the lime. Analyst: Chris Heropoulos (rept. 68 MS-38 ). Looked for but not found: 

Columnar Fine-grained Coarsegrained Trap Pegmatite trap porphyrite porphyrite 

Edwards Green- Scoville Tobin Amygda- Green .. Island stone Point Harbor Huginnin Minong laid Undivided stone Flow Flow Flow Flow Flow Flow Island Flow Flow 

IR-23 IR-74 IR-22B IR-36 IR-60A IR-40 IR-.49 IR-32 IR-71B 

Chemical analyses (weight percent)-Continued 

47.1 48.5 49.2 48.5 52.7 52.5 56.6 49.6 50.0 

17.7 16.2 17.2 16.9 13.0 14.9 12.7 12.1 14.7 

3.1 4.9 6.4 5.2 4.6 4.1 9.8 10.4 5.9 

5.2 5.4 4.2 6.2 8.2 7.4 3.5 3.4 5.4 

8.2 7.3 4.8 4.6 4.8 5.5 2.7 3.7 5.5 

10.9 9.8 9.5 8.4 5.5 7.2 3.0 12.3 8.1 

1.8 2.2 3.6 3.6 3.8 3.2 3.2 2.6 3.7 .13 .14 .96 1.3 .28 1.3 2.9 .12 1.2 

2.2 2.2 .34 .54 .91 .45 .74 .06 .53 

1.9 1.6 1.9 2.3 3.2 1.8 1.9 1.7 2.5 

.93 1.4 1.5 2.1 2.3 1.7 2.0 3.3 2.1 .08 .15 .16 .26 .39 .29 .71 .43 .20 .15 .16 .16 .16 .15 .15 .18 .18 .13 

.31 <.05 .08 <.05 <.05 <.05 < .05 <.05 <.05 

100 100 100 100 100 100 100 100 100 

Semiquantitative spectrographic analyses-Continued 

---------·---- -------------- 0.003 0.001 0.001 0.001 0.002 0.002 

0.005 0.01 .02 .03 .015 0.03 .07 .002 .05 

----········-- --·----------- -------------- -------------- ·····-·······- .0003 --·········--· .............. .01 -------------- .015 -------------- .005 .005 .005 .005 .005 .005 .003 .003 .005 

.03 .02 .02 .015 .003 .01 .007 .02 

.01 .015 .007 .007 .03 .02 .02 .07 .015 

.001 .0015 .0015 .0015 .0015 .0015 .002 .0015 .0015 

-------------- --····-------- ------·------- .007 .005 .01 .003 -------------- ·-···--------- .0015 .0015 .0015 .003 .0015 .005 .002 .002 

.03 .03 .007 .007 .007 .015 .0003 .005 .005 .............. .003 .001 .001 .001 -------------- .005 .003 .005 .005 .005 .003 .003 .005 .005 .03 .05 .05 .07 .015 .05 .015 .05 .05 

.02 .02 .03 .03 .03 .02 .007 .05 .05 

.002 .005 .003 .007 .007 .005 .01 .007 .007 

.0002 .0005 .0003 .0005 .0007 .0007 .001 .0007 .0007 

.005 .01 .01 .02 .02 .02 .05 .02 .015 

···········--- -------------- .01 .............. 

CIPW norms (weight percent)-Continued 

1.2 3.3 9.7 4.8 14.8 8.2 -----········· .8 .9 5.8 7.9 1.7 7.8 17.7 .7 7.3 

15.9 19.4 31.3 31.4 33.6 27.6 28.0 22.6 32.4 

41.7 35.3 28.7 26.9 18.4 22.9 10.6 21.6 20.6 

9.9 11.5 14.8 11.7 6.1 9.4 31.3 15.9 

(5.1) (6.0) (7.6) (6.0) (3.1) (4.8) ---·······-··· (15.9) ~8 2) (3.6) (3.9) (4.4) (3.4) (1.7) (2.9) (8.5) 5.0) 

(1.2) (1.6) (2.8) (2.3) (1.3) (1.7) (6.9) (2.7) 

23.5 21.3 5.7 6.4 18.5 17.9 16.4 1.6 12.7 

(17.8) (15.0) (3.5) (3.9) (10.8) (11.1) (7.0) (.9) (8.3) 

(5.7) (6.3) (2.2) (2.5) (7.7) (6.8) (9.4) (.7) (4.4) 

.6 --····----···- 4.2 5.1 5.1 5.6 6.4 5.6 6.4 6.5 5.5 

1.8 2.8 2.9 4.1 4.6 3.3 3.9 6.4 4.1 

·-··-·-··-···· 5.2 5.4 .9 

---·-·····-··· (3.1) (3.2) ··-·····------ -------------- 6~ ------------·· (2.2) (2.3) ----------···- (.3 .2 .4 .4 .6 1.0 .7 1.7 1.0 .5 

.7 .2 -------------- -------------- ---·····------

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 

72.5 64.5 47.9 47.0 35.4 45.4 27.5 48.9 38.9 

40.4 41.1 34.2 33.8 36.6 36.9 28.9 47.9 39.7 

IR 60A. Small point on west side of Lane Cove. IR 32. Shoreline near Mine Point, south of Conglomerate Bay. lR- 40. East end of most northwesterly point on main island, IR- 71B. On end of Blake Point near lighthouse. 1 mile west of Locke Point. IR-49. South shore at east end of Amygdaloid Island. 
/ N 

-c 

) 

0 1 2 3 4 5 6 7 8 9 10 MILES 

I I I I I I I I I I I I I I I I I I I I I o 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 KILOMETERS 

FIGURE 9.-Shaded relief map of Isle Royale National Park Prepared by Alexius J. Burgess (U.S. Geol. Survey, 1957). 

/' 

0 

1--' 

tv 

Q 

trj 

0 

t"' 

0 

Q 

>-<: 

0 

'"1j 

...... 

rn 

t"' 

trj 

::0 

0 

>-<: 

> t"' 

trj 

z 

> >-3 

...... 

0 

z 

> t"' 

'ij 

> ::0 

.r 

:= 

...... 

a 

:r: 

..... 

Q 

> z 
PORTAGE LAKE VOLCANICS C13 

In the discussion that follows, the stratigraphic units 

are grouped by rock type and not in any specific stratigraphic sequence. In order to provide a stratigraphic 

framework within which to view the individual units, 

the sequence is shown in a schematic columnar section 

(fig. 10). The distribution of some of the units is shown 

on the bedrock map (fig. 11). 

PORPHYRITE FLOWS 

SCOVILLE POINT, MIDDLE POINT, AND TOBIN HARBOR FLOWS 

The rock in the Scoville Point, Middle Point, and 

Tobin Harbor Flows is similar and somewhat unusual; 

it is not known to crop out elsewhere in the stratigraphic section on Isle Royale. Lane (1898, p. 170), 

recognizing their distinctive character, informally referred to them together as "the Tobin porphyrites" for 

occurrences in the Tobin Harbor area. 

The Scoville Point Flow is well exposed on the end 

of Scoville Point, its type locality, at the east end of 

Rock Harbor. It also is exposed on Edwards and North 

Government Islands, along part of the north shore of 

Rock Harbor, along the north shore of Lake Richie, 

and at scattered localities over the length of the island. 

On the western part of the island, Red Oak Ridge, 

which there rivals Greenstone Ridge in height, is preserved because it is capped and armored by the resistant Scoville Point Flow. The rock characteristically 

has fine equant millimeter-sized plagioclase crystals 

distributed uniformly througi1 a fine-grained matrix; 

the texture can best be seen on weathered surfaces (fig. 

4B). The unit is at least 100 feet thick at Scoville Point 

and locally may be as much as 200 feet thick. 

The Middle Point Flow is nowhere well exposed. The 

most accessible outcrop is a small one, considered its 

type locality, on the south shore of Grace Harbor about 

half a mile northeast of Middle Point. The flow can be 

traced northeastward through scattered outcrops for a 

distance of 12 miles; it appears to be absent for about 

9 miles further along strike, and then is exposed over 

an extent of 2 miles north of Siskiwit Lake. It may not 

occur on the eastern third of the island. The texture of 

the Middle Point Flow is similar to that of the Scoville 

Point Flow except that the plagioclase crystals are less 

uniform in size and distribution and have a tendency 

to clot together as in a glomeroporphyrite (fig. 4C). 

The thickness of the flow is probably somewhat more 

than 50 feet. 

The Tobin Harbor Flow is well exposed on the south 

arm of Porter Island, along the north side of Tobin 

Harbor, its type locality, and westward through scattered outcrops for the length of the island. Good outcrops occur on the Mount Franklin Trail just north of 

Tobin Creek. North of Siskiwit Lake and south of Lake 

Desor, the Tobin Harbor Flow makes up much of the 

south slope of Greenstone Ridge. The rock is identical 

in appearance with the Middle Point Flow, tending 

toward a glomeroporphyrite (fig. 4C). The flow is probably at least 50 feet thick in the Tobin Harbor area and 

may be somewhat more than 100 feet thick in the 

central part of the island. 

HUGINNIN AND GRACE ISLAND FLOWS 

The rock of the Huginnin and Grace Island Flows 

is the most distinctive volcanic rock on the island, with 

its large euhedral tabular plagioclase crystals floating 

randomly in a fine-grained groundmass (fig. 12). Even 

as beach pebbles or cobbles, this rock is easily recognized (fig. 13). In referring to the Huginnin Flow, Lane 

( 1898, p. 89) stated that there was no other flow like it 

in the whole series; except for the Grace Island Flow, 

which he apparently overlooked, this appears to be true. 

The Huginnin Flow was originally named the Huginnin Porphyrite by Lane for outcrops that occur in the 

bed of Huginnin Creek just a short distance from 

where the creek flows into Huginnin Cove near the 

northwest corner of the island. Although the outcrops 

in the creek are rather poor, excellent wave-washed 

outcrops occur on the shoreline about a quarter of a 

mile southwest of Huginnin Cove. At that locality, the 

upper contact of the flow is exposed and the plagioclase 

phenocrysts can be seen to extend undiminished in 

abundance and size through the amygdaloidal zone to 

the top of the flow. On the western third of the island, 

the Huginnin Flow occurs beneath a rather thick cliffforming ophitic flow and as a consequence is generally 

buried under talus. However, it can be traced by scattered outcrops from just west of Huginnin Cove to 

Little Todd Harbor. From Todc;l Harbor eastward it 

can be traced for the length of the island, the most 

accessible outcrops being at the east end of Todd Harbor, at the mouth of McCargoe Cove, and on a small 

peninsula on the west side of Lane Cove. Lane's drill 

log gives a thickness of 67 feet for the Huginnin Porphyrite at the northwest corner of the island. 

The rock of the Grace Island Flow is indistinguishable from that of the Huginnin Flow. The best and 

most easily accessible exposures occur on the offshore 

islands at the west end of Isle Royale proper; on a point 

south of the campground on Grace Island, its type 

locality, on Booth Island, and on both ends of Washington Island. On the west end of Washington Island, 

the base of the flow is exposed, and the plagioclase 

phenocrysts can be seen to extend through the basal 

amygdaloidal zone to the bottom of the flow. Outcrops· 

of the Grace Island Flow are not only poorbutscarce on 

the main island, and the unit can be traced by means of 

very scattered outcrops for about 6 miles eastward from 
C14 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

~ C) 0 

0 o Copper 0 0 

0 Harbor 0 

Named lava flows: ~ 

Conglomerate 0 

0 0 Unnamed rock sequences: ~ 0 0 0 0 

lr1-.lr! _L 

~-:~:. ~~.L~o Numerous thin ophitic flows with as 

11 T many as seven interbedded sandstone l_Q •0 • D• •O·o • o • o and conglomerate units (see fig. 17 for h'" ;..>,.4. 

o·O ·•·cO-o·o details) ,....,.... 

Scoville Point Flow (porphyrite) (psp) ,,,,,,,,,, 

/ ,,, 1\ 71, 

1¥ .lJ l I \_l l ~ Several thin ophitic flows \ \ (\ \\ \ 

Edwards Island Flow (trap) (pei) 

·a. 0 0 

0 •• 0 0 Conglomerate-known from drill records 0 • 

Middle Point Flow (porphyrite) (pmp) ....... ,,-,/ ...... , -" 1_' 1 ' 1 I I 1 

ti" ::t \JJti-r- Several thin ophitic flows 

\ \ \ 

Long Island Flow (trap) (pli) 

.. Sandstone-known from drill records 

Tobin Harbor Flow (porphyrite) (pth) I' / \ / '' I 

I \ I '\ '' 

Washington Island Flow (ophite) (pwi) X Xx-'< ~ ~: )( 

<:I. <I 4~ "' Tuff-breccia [). 6. t, <I ~ 

)( )( X X X 

Greenstone Flow (ophite) (pg) X )( 

y.. )( X 

X X X X )< X 

0 0 0 Conglomerate-known from drill records • 0 0 

Grace Island Flow (porphyrite) (pgi) / 'I ,., ' / / 

' I \ \ I \ I I 

n-n.lll..ll Tnt Sequence of thin to thick (more than ~.o·o·o·O·o 100 ft) flows chiefly ophitic, with one 

"\ or more sedimentary units suggested 

TI L-ttt by drill data 

.\ 

4 Ll 4 4 4 <3 <I V' Tuff-breccia 

Minong Flow (trap) (pm) 

1/ Sequence of thin to thick flows, chiefly ) IJl 0 0 0 0 0 0 0 ophitic, with one or more sedimentary ~ units suggested by drill data 

..lllll .l J.tiiiiiT71 I I 

Huginnin Flow (porphyrite) (ph) \/ .... I I -, I -

" / ' I '' LU. J I LLl.J, One or more ophitic flows present locally J1 ( 

Hill Point Flow (ophite) (php) x x X X X >( X >< X X 

0. 0 •..2. 0 .o .o . o. Sequence of thin to moderately thick 

flows, chiefly ophitic. Several sedi-

•0• .o.o.o.o•O mentary units and a felsite indicated 

by dri II records 

<lt,..t. <J6.<::1 <ILl Breccia 

Amygdaloid Island Flow (trap) (pai) 

..I 

~ 

_l J...lj _l Lava flows, chiefly ophitic 

FIGURE 10.-Schematic columnar section of the Portage Lake Volcanics on Isle Royale. Not drawn to scale. 
PORTAGE LAKE VOLCANICS C15 

Grace Harbor. That the unit is more extensive is suggested by a single recognized occurrence of the distinctive porphyrite in the same stratigraphic position 

just beneath the Greenstone Flow on Greenstone Ridge 

in the central part of the island just north of the 

Greenstone Ridge Trail about 1 mile east of the east 

end of Hatchet Lake. On Washington Island and for 

some distance eastward, the Greenstone Flow is relatively\ thin, but at about the point where the Grace 

Island Flow becomes difficult to trace, the Greenstone 

Flow thickens eastward and becomes more and more 

a cliff former, burying its own base, as well as the 

immediately underlying section, beneath massive piles 

of talus. The Grace Island Flow is probably about 50 

feet thick on Washington Island; elsewhere its thickness is unknown. 

TRAP FLOWS 

EDWARDS ISLAND AND LONG ISLAND FLOWS 

The Edwards Island and Long Island Flows, chemically the most basaltic of the rocks analyzed (fig. 8), 

have important secondary attributes that help in field 

recognition. With the exception of the upper "chill" 

zone of the Greenstone Flow, they are the only flows in 

the stratgraphic section to exhibit well-developed 

columnar jointing. Lane noted what he referred to as 

"basaltic" jointing and conchoidal fracture and referred 

to the rock as "columnar trap." Where the columnar 

jointing is not well developed, the rock usually exhibits 

polygonal rather than rectangular jointing, and this, 

together with fine grain size and conchoidal fracture, 

aids in field identification. 

The type locality for the Edwards Island Flow is the 

good exposure on the north side of Edwards Island. It 

is also exposed on adjacent islands and on the point just 

north of Scoville Point on the south side of Tobin 

Harbor; columnar jointing is also well developed at 

these localities (fig. 14). From the Tobin Harbor area, 

the unit can be traced westward through scattered outcrops for nearly 30 miles to just northwest of Hay Bay. 

West of Hay Bay the amount of surficial cover on the 

island increases markedly, and although the Edwards 

Island Flow may continue farther west, it is concealed. 

Other accessible areas where the columnar jointing can 

be observed are the place where the trail from Rock 

Harbor to Mount Franklin crosses the flow and the 

south slope of Ransom Hill just west of the trail from 

Daisy Farm Campground to Mount Ojibway. In thickness the Edwards Island Flow probably averages about 

50 feet. 

The Long Island Flow is exposed on a chain of small 

narrow islands in Tobin Harbor, including Long Island, 

its type locality. Columnar jointing is not noticeable in 

the Tobin Harbor area, but polygonal jointing is well 

developed on Long and Third Islands. Although the 

Long Island Flow can be traced as far west as the 

Edwards Island Flow, to just northwest of Hay Bay, 

outcrops are generally very poor, and at only one relatively inaccessible locality was well-developed columnar jointing obs~rved. About 1% miles west of Lake 

Richie along the north fork of the creek draining into 

the west end of the lake, columns form a cliff about 25 

feet high. The Long Island Flow commonly contains 

sparse small bluish agate amygdules that help distinguish isolated outcrops of this unit from those of 

the Edwards Island Flow, in which agates have not 

been observed. Because the islands where the flow is 

best exposed project only a few feet above water level, 

a stratigraphic thickness of no more than 25 feet is 

visible; however, the flow is probably less than 50 feet 

thick. 

MINONG FLOW 

The Minong Flow was originally named the "Minong 

Trap" by Lane (1898, p. 16) for its occurrence at the 

Minong mine. The rock is dark and fine grained and 

has a pronounced conchoidal fracture (fig. 4F). Lane 

noted a tendency toward "basaltic" (columnar) jointing, but no well-developed columns were observed in 

the present study. Because it is thick and resistant, the 

Minong Flow is an excellent ridge former and can easily 

be traced the length of the island. In fact, Minong 

Ridge is generally characterized by the precipitous 

antidip slopes on its north side. The rock is further 

characterized by the presence of sparse bluish-white 

agate amygdules. The Minong Ridge Fire Man way 

(trail) follows the Minong Flow for much of its length 

and thus makes numerous outcrops of the flow readily 

accessible. Excellent wave-washed outcrops are found 

on some of the small promontories jutting into Lake 

Superior, especially north of Locke Point, in Five 

Finger Bay, and at the far west end of Isle Royale. At 

the west-end locality, the base of the unit is exposed 

and the rock is uniformly dense down to the contact, 

with almost no amygdaloidal zone present. The thickness of the Minong Flow is 77 feet in Lane's drill log. 

Lane (1898, p. 82) also described and named a 

"Minong Porphyrite," which he designated more felsic 

than the rock he described as "Tobin porphyrite" and 

which occurs immediately above his "Minong Trap." 

In fact, he considered the porphyrite and trap possibly 

to be magmatic differentiates of a single flow. In 

describing the porphyrite from drill samples, Lane 

noted "the extreme fineness of grain, the scoriaceous, 

porous and brecciated appearance, peculiar in that the 

pores are fine and irregular." Lane's Minong Porphy-
C16 

48° 10' 

GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

EXPLANATION 

• : 

. 

. 

. 

. 

. : 

Copper Harbor Conglomerate 

D 

Portage Lake Volcanics 

Subunits shown on map: 

M:f~~:gJ~::~~;·j Scoville Point Flow (psp) 

- Greenstone Flow (pg) 

Minong Flow (pm) 

ll!l!l!ll!lllllll!l!lllll!l!llllllJ Hill Point Flow (php) 

1----------1 Sandstone and conglomerate 

Cumberland Point 

20 

Strike and dip of sedimentary strata or lava flow 

Symbols plotted over water to avoid crowding apply to 

adjacent shoreline 

Fault 

Approximate location of hold drilled in exploration for 

copper . 

A A' 

Location of geologic cross section (fig. 22) 

FIGURE H.-Simplified bedrock geology of Isle Royale. Scale too small to show all stratigraphic units described 
PORTAGE LAKE VOLCANICS C17 

50' 40' 88" 30' 

0 5 10 M ILES 

0 5 10 KILOMETERS 

in text. For additional details, see larger scale geologic map (1: 62,500) published separately (Huber, 1973b). 
C18 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

FIGURE 12.-Porphyrite from Huginnin Flow. Loose slab on 

beach a t Huginnin Cove. Pencil, approximately 15 em long. 

FIGURE 13.- Beach cobble of porphyrite from Grace Island 

Flow with a rather unusual distribution of plagioclase phenocrysts. Dark spots are amygdules of pumpellyite. From beach 

on Grace Island; long dimension, 15 em. 

rite, now believed to be of pyroclastic origin, is discussed in a later section. 

AMYGDALOID ISLAND FLOW 

The Amygdaloid Island Flow, an andesite, is the 

most felsic volcanic rock of those analyzed (fig. 8; table 

1). It is exposed only on Amygdaloid Island near the 

base of the exposed stratigraphic section of the Portag-e 

Lake Volcanics. Like the other traps, it is a resistant 

rock and forms the highest ridge running the length of 

Amygdaloid Island. Some of the best and most accessible exposures are those on the promontory south of 

Crystal Cove at the east end of the island, its type 

locality, and at the natural bridge in the central part of 

the island-a wave-cut arch formed at a time when a 

higher lake level existed in the Superior basin (Huber, 

1973a). One of the most distinctive aspects of the rock 

is the presence of rather abundant agates with a characteristic flesh-pink cast and commonly with centers 

of massive or drusy quartz (fig. 15). The Amygdaloid 

Island Flow is probably less than 50 feet thick. 

OPHITE FLOWS 

The volcanic rocks in the Portage Lake Volcanics on 

Isle Royale are predominantly ophitic. The stratigraphic marker units described to this point are nonophitic and thus are useful because of their conspicuous 

3cm 

I I 

FIGURE 15.-Specimen of Amygdaloid Island Flow with characFIGURE 14.-Columnar jointing in Edwards Island Flow on teristic agate amygdules (sawed surface) . From northeast 

north side of Edwards Island. end of Amygdaloid Island. 
PORTAGE LAKE VOLCANICS C19 

contrast within the generally ophitic sequence. Three 

ophitic flows on Isle Royale, however, are useful marker 

horizons because they also stand out from the mass of 

ophitic flows: the Greenstone, Hill Point, and Washington Island Flows. 

GREENSTONE FLOW 

Lane ( 1898, p. 98) referred to the Greenstone Flow 

as "the Greenstone, the 'backbone' [of the island] and 

biggest ophite of all." Application of this formal name 

to the flow on Isle Royale resulted from Lane's correlation of this flow with the type Greenstone of the 

Keweenaw Peninsula, the thickest ophitic flow there. 

The name "Greenstone" eventually evolved into 

"Greenstone Flow" on the peninsula, and as the correlation with Isle Royale is still considered valid, the 

name Greenstone Flow is also used on the island. The 

Greenstone Flow on Isle Royale, as on the peninsula, 

has undergone magmatic differentiation, and this feature primarily, in addition to thickness, led to the 

correlation. Extensive descriptions and analytical data 

for the Greenstone Flow on the Keweenaw Peninsula 

were presented by Broderick (1935) and Cornwall 

(1951a, b). 

As late as 1911, some workers considered the "Greenstone," both on the Keweenaw Peninsula and on Isle 

Royale, to be an intrusive sill because of its thickness 

and differentiated character (VanHise and Leith, 1911, 

p. 381, 389). Lane (1911, p. 939-940), however, argued 

convincingly for its origin as a lava flow, an interpretation confirmed by subsequent work on the peninsula 

based on thousands of feet of diamond-drill samples 

(for example, Broderick and others, 1946, p. 683-684). 

The Greenstone Flow is indeed the backbone of Isle 

Royale, for it forms the most prominent ridge running 

the length of the island, Greenstone Ridge. The Greenstone Ridge Trail, which follows the ridge for nearly its 

entire length, provides access to numerous scattered 

outcrops, but only at the far east end of the island near 

Blake Point is a reasonably complete cross section of 

the flow exposed. There the flow can be seen to consist 

of four units with the following approximate thicknesses: A lower ophitic zone (100 ft), a central pegmatitic zone (75ft), an upper ophitic zone (175ft), 

and an uppermost columnar-jointed trap (50 ft), for a 

total thickness of about 400 feet. The Greenstone Flow 

is thickest in the cental part of Isle Royale, where it is 

estimated to be nearly 800 feet thick. It thins westward 

to less than 100 feet on Washington Island. 

The lower ophitic zone, which makes up the cliffs of 

The Palisades near Blake Point at the east end of the 

island, shows an increase in grain size of the ophitic 

augite crystals from a few millimeters near the base 

(base not exposed) to as much as 1 ~ em in the upper 

part of the zone. The contact between the lower ophitic 

zone and the pegmatitic zone is not exposed but is 

considered to be fairly abrupt, as it can be projected 

with good continuity between outcrops of the two rock 

types for a distance of more than 30 miles and at places 

can be located within a stratigraphic interval of 25 feet. 

The pegmatitic zone is extremely variable in texture 

and grain size but characteristically has an interlocking mat of elongate plagioclase crystals, which are as 

much as 2 em long (fig. 4E). The contact of the pegmatitic zone with the upper ophitic zone is gradational 

over a few tens of feet, with irregular patches of the 

two rock types intermingling. The upper ophitic zone 

is fairly coarse at its base, with augite crystals as much 

as 1 em in diameter, but grades upward rather rapidly 

into finer ophite. · 

The columnar-jointed trap is exposed on the chain of 

small islands south of Merritt Lane, on Red Rock Point, 

and at a few additional isolated localities on the main 

island. The trap was not seen in actual contact with 

the upper ophite, for the contact between them apparently is a zone of low resistance to erosion and everywhere occupies a physiographic depression. The trap 

is considered to be part of the Greenstone Flow, rather 

than a separate overlying flow, by analogy with the 

type Greenstone Flow on the Keweenaw Peninsula, 

where a columnar-jointed "melaphyre" is typically 

found near the top of the flow (Cornwall, 1951a; 1954a, 

b, c). Compositionally the trap on Isle Royale plots 

closer to the ophite of the Greenstone Flow than to 

other columnar-jointed trap in the section (fig. 8) on 

the basis of one analysis of each that is available. 

The lower ophite forms the commonly precipitous, 

antidip slopes on the north side of Greenstone Ridge, 

whereas resistant pegmatite commonly armors the 

gentler, south dip slope. The contact between the lower 

ophite and the pegmatite usually is near the crest of 

the ridge; because it is a relatively sharp contact, it 

provides a very useful internal marker horizon, especially for the recognition of offset along faults that cross 

the ridge. The pegmatitic zone can be traced in outcrop 

from Blake Point as far west as Mount Desor, west of 

which outcrops are sparse. Six miles west of Mount 

Desor, where the flow is perhaps 265 feet thick, the 

pegmatite appears to be poorly developed, according to 

Lane's drill log. At the west end of Isle Royale and on 

Washington Island, the pegmatitic zone appears to be 

absent. The pegmatitic zone is likewise missing on the 

Keweenaw Peninsula, at places where the Greenstone 

Flow is thin (Davidson and others, 1955). 

HILL POINT FLOW 

The Hill Point Flow is a typical ophite useful as a 

stratigraphic marker only because of its thickness and 
C20 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

exceptionally coarse grain size; it contains poikilitic 

augite crystals as much as 2 em in diameter. The flow 

is 158 feet thick near the west end of the island, according to Lane's drill log. It crops out in imposing cliffs 

along the north shore of the island from Huginnin Cove 

to McCargoe Cove and then along the south side of 

Pickerel Cove eastward to Hill Point, the type locality. 

A few thin seams of pegmatite can be seen near McCargoe Cove, but they are not extensive enough to help in 

identifying the flow elsewhere. 

WASHINGTON ISLAND FLOW 

The rock of the Washington Island Flow is difficult 

to describe but distinctive when seen; perhaps Lane's 

application of the term "glomeroporphyritic ophite" 

suggests the problem. The rock is clearly an ophite, as 

indicated by the poikilitic augite crystals, but the 

ophitic texture is somewhat obscured by the fact that 

the plagioclase crystals are somewhat larger than in 

the typical ophites and have about the same distribution and size within the augite crystals as in the groundmass. Other distinctive features are occasional larger 

porphyritic plagioclase crystals, about one-half centimeter in average size, and a pervasive greenish speckled 

appearance caused by the rather uniform distribution 

of chlorite (fig. 16). In thin section the rock exhibits a 

relict diktytaxitic texture, chlorite and other secondary 

minerals filling what appear to have been interstitial 

voids, as well as replacing earlier minerals, such as 

olivine. 

1

3cm 

1 

FIGURE 16.-Specimen from Washington Island Flow. Ophitic 

texture is obscure, but characteristic dark chlorite splotches 

are visible. Compare with figure 4A. 

The Washington Island Flow is well exposed at its 

type locality on the south side of Washington Island, 

where it caps the highest ridge. On the main island it 

can be traced eastward from Grace Harbor for about 

5 miles, beyond which it appears to pinch out as the 

Greenstone Flow increases in thickness. A single outcrop of similar rock in the same stratigraphic position 

above the Greenstone Flow occurs near the east end of 

the main island on a small point on Porter Island that 

partially separates a narrow bay from an interior cove. 

The unit is more than 200 feet thick on Washington 

Island but thins to about 80 feet 5 miles to the east; it 

can be no more than a few tens of feet thick on Porter 

Island. 

SEDIMENTARY ROCKS 

Epiclastic sedimentary rocks crop out only in the 

upper part of the Portage Lake Volcanics, although 

they are known from drill records to occur lower in the 

section. Those that crop out are all above the uppermost of the named volcanic units, the Scoville Point 

Flow, and are best exposed in the Chippewa Harbor 

area, where seven units of pebble conglomerate and 

sandstone are interbedded with a series of generally 

thin ophitic lava flows (fig. 17). These sedimentary 

units can be traced eastward to a point where most of 

them disappear beneath the waters of Moskey Basin 

and Rock Harbor; two are exposed discontinuously on 

the chain of small islands on the south side of Rock 

Harbor. All the units can be traced westward from 

Chippewa Harbor as far as Siskiwit Lake, but farther 

west outcrops are sparse and correlation of individual 

units is uncertain. The most accessible outcrops are 

those in the Chippewa Harbor area, on the south side 

of Siskiwit Lake, the north side of Conglomerate Bay, · 

the south side of Moskey Basin, and the south side of 

Mott Island. Conglomerate can been seen on the dumps 

at the Island Mine, northwest of Siskiwit Bay. 

The sedimentary units vary in thickness and in grain 

size along strike, but on the whole the units seem surprisingly uniform in thickness, considering their probable lateral extent, more than 40 miles. For example, 

the uppermost unit is predominantly pebble conglomerate for its entire length and appears to thicken eastward very slightly. It is about 30 feet thick west of 

Siskiwit Bay and at Hay Bay and Chippewa Harbor 

and increases to about 40 feet on Mott Island. The 

lower-most sedimentary unit, the Island Mine Conglomerate Bed (Lane, 1898, p. 99), is a pebble conglomerate about 20 feet thick south of Washington Harbor 

and at the Island Mine and the west end of Siskiwit 

Lake; it changes to a sandstone also about 20 feet thick 

at Chippewa Harbor, and it is a sandstone at Moskey 

Basin, probably slightly thinner. 
PORTAGE LAKE VOLCANICS C21 

8: 

_,1 / -:: t> 

r. o o u o 

"- 0~ 

-' ..... ,--;.' / ::::_l 

-; --;/ \ /-/-t-; 

-. ;-!. \ I\..__ 

,-,!,/~' 

~:..,,,/,\ 

1/'~- '\ ,_z ,~.:_t-; 

lo.s>_o oo 

~ .... ,,; ..._\ 

\1/'-'~ ,-,.,..,-:~-

/\I\-/ ........_\ 

~~~,-- ~-;; 

~~~~~~/ 

-' _ I/ I 

'\ I ,-/ 'I/ /.,- - -1 

-;:._,-/,_:_ 

- - / \ 

=~~~~~~; 

~:~~%~:::: 

Projected position of 

Copper Harbor 

Conglomerate . 

200-250 ft 

25-30 ft 

130·140 ft 

160-170 ft 

26-30 ft 

60-70 ft 

25-30 ft 

100·110 ft 

10-15 ft 

60-70 ft 

25-30 ft 

170-180 ft 

16-20 ft 

200ft 

Scoville Point Flow 

EXPLANATION 

Conglomerate 

and 

sandstone 

FIGURE 17.-Columnar section of upper part of Portage Lake 

Volcanics showing distribution of sedimentary units in the 

Chippewa Harbor area. All the ophitic flow zones consist of 

multiple flows. 

The pebbles in the conglomerate are essentially all 

of volcanic rock, with mafic varieties about twice as 

abundant as felsic ones. In this respect the conglomerate differs somewhat from the Copper Harbor Conglomerate, higher in the section, where felsic varieties 

predominate (Wolff and Huber, 1973). The sandstones 

are very feldspathic, plagioclase being the most abundant among granular constituents. Fine-grained hematite is ubiquitous, giving all the sedimentary rocks a 

reddish cast and generally obscuring other fine-grained 

material in the matrix. 

The conglomerates and coarser sandstones commonly exhibit a scour-and-fill structure (fig. 18), suggesting an origin in a braided stream system. Locally 

the sedimentary units are fine-grained even-bedded 

sandstone throughout and probably represent floodplain deposits. The presence of desiccation cracks and 

raindrop impressions also point toward a fluvial depositional environment. Paleocurrent data are too scarce 

to treat statistically, but foreset crossbeds and pebble 

imbrication suggest that the direction of sediment 

transport was down the present structural dip, or generally southeast toward the axis of the Lake Superior 

syncline. This conclusion is compatible with paleocurrent data for the Copper Harbor Conglomerate (Wolff 

and Huber, 1973). 

A sandstone unit below the Long Island Flow, a conglomerate immediately beneath the Greenstone Flow, 

and several conglomerates below the Hill Point Flow 

are indicated in Lane's drill log and described as being 

similar to those mentioned; none of these are known 

from outcrop. 

Another type of rock, with at least a sedimentary 

component, is one known in the Michigan copper district 

as a "scoriaceous conglomerate" or "amygdaloid conglomerate," or even "scoriaceous amygdaloid" (Lane, 

FIGURE 18.-Scour-and-fill structure in pebbly sandstone near 

the west end of Chippewa Harbor. Handle of penknife approximately 7 em long. 
C22 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

1911, p. 69). This type consists of irregular-shaped 

fragments of highly amygdaloidal rock or scoria in a 

matrix of fine sand or silt and might more properly be 

called a breccia (fig. 19). The percentage of fragments 

diminishes stratigraphically upward. The rock was 

formed when fine sedimentary material swept over the 

fragmental top of a lava flow and incorporated in the 

resulting deposit various-sized fragments from the 

underlying flow top. Lane described several units of 

this type in his Isle Royale drill log, but only one is well 

exposed at the surface. It occurs immediately above the 

Amygdaloid Island Flow and can best be seen in a milelong exposure along the south shoreline of Amygdaloid 

Island near the east end of the island. 

PYROCLASTIC ROCKS 

Lane recognized that some of the so-called sedimentary rocks in the Portage Lake Volcanics on Isle Royale 

are of pyroclastic origin or contain appreciable pyroclastic debris. He referred to these rocks as ash beds 

and tuffs and described fragments having "the conchoidal forms of glass ash" (Lane, 1898, p. 171-172). 

Unfortunately, the pyroclastic rocks, like the epiclastic 

rocks, rarely crop out and are therefore of little use in 

the field as stratigraphic markers. A tuff-breccia immediately overlying the Greenstone Flow is the single 

exception; it is exposed sporadically for the length of 

the island and indeed helps to identify the Greenstone 

Flow on Washington Island, where that flow is thin 

FIGURE 19.-Breccia occurring above the Amygdaloid Island 

Flow. Specimen from south shore at east end of Amygdaloid 

Island. 

and lacks much of its distinctive character, looking 

much like any other ophite. Other pyroclastic rocks 

that occur just above and below the Minong Flow rarely 

crop out. 

The basal part of the tuff-breccia overlying the 

Greenstone Flow contains much rubble that appears to 

have been obtained from the Greenstone Flow (fig. 20). 

Both fragments and matrix within the main body of the 

tuff are very fine grained, suggesting that much of the 

rock was originally glassy. Relict shard structures and 

collapsed pumice fragments are sometimes visible in 

thin section, and many rock fragments are scoriaceous. 

Other rock fragments exhibit pilotaxitic texture. Corroded quartz and plagioclase crystals are common, and 

staining techniques reveal appreciable K-feldspar in 

the fine-grained matrix, indicating a rather felsic composition. From the east end of the island to Mount 

Desor, the tuff is also characterized by numerous agates 

with a pink or red cast (fig. 21). Unlike the ovoid 

agates typically occurring in many of the volcanic :flows, 

these agates tend to have a very irregular shape similar 

to the "thunder eggs" occurring in tuffs of the Columbia River Plateau in the Western United States. Such 

agates have been interpreted by Ross (1941) as 

chalcedony-filled spherulitic cavities in welded tuff, and 

their presence in the tuff on Isle Royale, together with 

other small ellipsoidal vesicles now filled with secondary 

minerals, suggests that the pyroclastic material was hot 

when deposited. The tuff where exposed on Washington 

Island contains quartz and plagioclase crystals, relict 

shards and pumice, and scoria fragments, but no 

agates, amygdules, or other evidence of deuteric 

thermal activity. Perhaps it was far enough away from 

the pyroclastic source to have been deposited at a lower 

temperature than the tuff to the east. The most accesFIGURE 20.-Tuff-breccia overlying the Greenstone Flow. Fragmental character is clearly evident in this outcrop, which is 

close to the base of the pyroclastic unit on the north shore of 

Tobin Harbor near Newman Island. Knife, 7 em long. 
PORTAGE LAKE VOLCANICS C23 

FIGURE 21.-Agate typical of those occurring in the tuff-breccia 

overlying the Greenstone Flow. Knife, 7 em long. 

sible outcrops of the tuff-breccia are on the north side 

of Tobin Harbor near Newman Island. Its maximum 

thickness is probably between 25 and 50 feet. 

The tuff-breccia above the Minong Flow is similar 

to that above the Greenstone Flow, with abundant 

spherulitic structures, some containing agate. This unit 

is probably the same as Lane's Minong Breccia (or 

Minong Conglomerate) together with his Minong 

Porphyrite ( 1898, p. 82, 87). Maximum thickness probably does not exceed 25 feet. The only known exposures 

of this unit occur in a belt about 1 mile long at the 

far west end of the island and on the peninsula east of 

Five Finger Bay at the east end of the island, although 

it was reported from the workings of the Minong Mine. 

Below the Minong Flow a single exposure on a shoreline cliff at the west end of the island shows an ash 

deposit only a few inches thick in its now-compacted 

state. It appears to be an air-fall deposit; shard structures are beautifully preserved, and no rock fragments 

were noted in the few samples examined. The upper 

part of the ash deposit was later fused by the heat of 

the overlying Minong Flow, with the downward development of spherulitic structures and the creation of a 

compaction foliation. A rhyolitic composition is indicated by the refractive index of fused glass beads of 

the tuff. 

SEA 

LEVEL 

-1200' 

-2400' 

Northwest 

A 

0 

STRATIGRAPIDC SUMMARY 

The individually named flows of the Portage Lake 

Volcanics on Isle Royale have been described in some 

detail, along with the other assorted rock types that 

occur in the sequence. The stratigraphy of the largely 

volcanic pile has been summarized in a schematic 

columnar section presented earlier as figure 10, a composite from various parts of the island; not all units 

shown are everywhere present. Most of the clastic 

interbeds are known only from drill core, and such 

data are limited to the western part of the island. The 

columnar section is not drawn to scale, but some sense 

of scale can be obtained from the geologic section 

(fig.22). 

STRATIGRAPHIC THICKNESS 

Some general data have been presented regarding 

the thickness of individual stratigraphic units, but for 

most units data on thickness variations along strike 

are scanty. Because of the hogback asymmetry of 

ridges formed by individual flows and the tendency of 

the upper amygdaloidal parts of flows to be eroded, the 

base rather than the top of most units can be more 

precisely located even when both contacts are concealed. By utilizing the bases of certain key flows as 

marker horizons, thickness variations for larger segments of the Portage Lake Volcanics can be calculated 

for a number of sections across Isle Royale with considerable reliability; the results are presented in a 

longitudinal stratigraphic section with the base of the 

Greenstone Flow as a datum plane (fig. 23). Some 

additional data on the section below the Hill Point 

Flow, in the vicinity of Amygdaloid Island, have been 

obtained by tracing on aerial photographs certain topographic lineaments presumed to be stratigraphic horizons. 

The longitudinal section brings out two significant 

features. First, the Greenstone Flow is thickest near 

the central part of Isle Royale and thins toward both 

the west and east ends of the island. Secondly, the 

entire exposed section of the series on Isle Royale also 

thins toward both ends from a maximum in the center. 

The Copper Harbor Conglomerate has a similar geCopper Harbor Conglomerate ~ 

psc 

Houghton 

Ridge 

"-!..:::::: ::: ....... 

Southeast 

A' 

Lake 

Superior 

FIGURE 22.-Geologic section of Isle Royale in the vicinity of Lake Desor and Houghton Ridge. Letter symbols represent units 

shown in figure 10, with the addition of psc, sandstone and conglomerate; pp, pyroclastic rock; pu, Portage Lake Volcanics, 

undivided. 
C24 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

. 

-" 0 :l ~ 1 -~ 

~ ~ 

LL 

I ~ t;/ ---------------- ~--:cr---~~~--

-------..!!~1 ~//l/ 

---------:- ~ ~~ ~ i--;-: ~!j /;;:;: / / :_: - 10 MILES 

---------.c - INDEX MAP SHOWING LOCATION OF DRILL HOLES 

~~~I ~ ~ ~ AND INDIVIDUAL CROSS SECTIONS USED IN 

I 

// l/ ~ ~ ~:'-----------C_Oc_N __ ST __ R_U_C_T_I_N_G __ G-=Ic---T_U_:D_:I_:N_:A_:L=--=-S=-EC~T.:_: I O::_:N__:___ 

/ ~ 174 -~ f\p.\E. :g J: ~ 

• C20P!~~~ -; ... - t::?~gpli ...J aNGLO~:..: -c 1 1ii E ~ ~ .-' 4:-~0~ G r~cgt.~ ~ .j!gl : E-'~ -------;; ~~----~r--

~~- '-Cgl. Cgl.- --'--1---- ---~i-----a:Cgl. r :/i!i 

Cal. Cgl. ~ s~t ~ j-:l~t--------+-= =~C"""-1.--+--

----- ~ Longlsland - ~ =-------:: 

-

l'aulj Fault ____- r~------_.:_:~----~ 

1 Base of Greenstone Flow 

:£ lautlt (used as datum) 

:g 

______.f• ~ 

GREENSTONE 

Base of Edwards Island 

_ _____ r-__ FLOW I r----~" 

----------~ ----~,~ 1 -~--- ~--------~----r----- ---- ------~------4----------~~~-- ~ .... ----_ Base of Minong Flow ---

0 ------

FEET 

2000 

~ SCALE 

100:~ 

0 5 MILES 

. 

z 

FIGURE 23.-Longitudinal stratigraphic section showing variations in thickness of the Portage Lake Volcanics on Isle Royale. 

Data on the Copper Harbor Conglomerate from Wolff and Huber (1973). 

ometry at the west end of the island. (See Wolff and 

Huber, 1973, for details.) These features may have 

some bearing on the topographic expression of Isle 

Royale as a ridge in the Lake Superior basin, as one 

would expect the entire section to be more resistant to 

erosion and stand topographically higher where the 

individual flows are thickest. 

CORRELATIONS 

The Keweenawan volcanic rocks on Isle Royale have 

long been thought to be correlative with those on the 

Keweenaw Peninsula, the Portage Lake Volcanics. But 

all the Keweenawan volcanic rocks exposed around the 

Lake Superior basin have at one time or another been 

thought to be more or less correlative. Only on Isle 

Royale and the Keweenaw Peninsula, however, is the 

upper part of the total Keweenawan volcanic sequence 

both present and in contact with overlying younger 

formations. At other places around the basin (fig. 1), 

the basal part of the volcanic sequence is present, 

including the volcanic rocks of the so-called South Trap 

Range in westernmost Michigan and Wisconsin, the 

North Shore Volcanic Group of Green (1971) in Minnesota, and the Osler Group in Ontario. The correlation problem for Isle Royale is thus twofold: (1) to 

provide a detailed correlation of the volcanic sequence 

on Isle Royale with the highly similar sequence on the 

Keweenaw Peninsula and (2) to determine possible 

overlap between the section on Isle Royale and the 
PORTAGE LAKE VOLCANICS C25 

lower sequences, particularly the North Shore Volcanic 

Group. 

CORRELATION BETWEEN ISLE ROYALE 

AND THE KEWEENAW PENINSULA 

One of Alfred C. Lane's major contributions to 

Keweenawan geology was his recognition of the correlation of specific rock units on Isle Royale with their 

counterparts on the peninsula. In fact, he was not only 

the first but the only geologist to personally make 

detailed studies on both Isle Royale and the peninsula. 

The key to the correlation is the Greenstone Flow, 

for as Lane (1911, p. 22) stated, "the backbone of Isle 

Royale and that of Keweenaw Point is the same lava 

flow." In each place the Greenstone is the thickest, 

most persistent flow and is further distinguished by its 

magmatic differentiation. Henry Cornwall, who studied 

the Greenstone Flow on the peninsula in great detail 

and briefly examined the flow on Isle Royale, confirmed 

their similarities and pointed out that although a few 

other flows on the peninsula show evidence of differentiation, they differ from the Greenstone Flow in other 

respects (oral commun., 1971). 

Even more convincing than mere similarities between 

the "backbone" of Isle Royale and that of the Keweenaw Peninsula, however, are the similarities in the 

entire rock sequence in the two areas, including the 

position of the Greenstone Flow within the sequence. 

It was Lane's recognition of these broader similarities 

that strengthened his arguments for correlation. 

Very thick flows like the Greenstone Flow are the 

exception, however, for while it does specifically correlate across Lake Superior, to suggest that all the 

flows in the Portage Lake Volcanics are continuous 

across the lake would be unrealistic. But different rock 

types were produced at different times during the volcanic cycle, and both Isle Royale and the Keweenaw 

Peninsula show parallel sequences of similar types, the 

part of the sequence above the Greenstone Flow providing the clearest correspondence. 

The flows above the type Greenstone Flow on the 

Keweenaw Peninsula were previously divided into two 

groups, the Ash bed Group of Irving ( 1883) and the 

overlying Eagle River Group of Lane and Seaman 

(1907), both of which Lane thought he recognized on 

Isle Royale. On the island the Ashbed Group would 

include the rocks from the top of the Greenstone Flow 

to the top of the Scoville Point Flow, and the Eagle 

River Group, those from the top of the Scoville Point 

Flow to the top of the Portage Lake Volcanics. 

Although these formal group names are no longer generally used, they were applied to sections on the peninsula almost directly across from Isle Royale and are 

still useful for purposes of discussion. 

Correlation criteria for the Ashbed Group are as 

follows: 

1. At the base of the Ashbed Group just above the 

Greenstone Flow on the peninsula is the Mesnard 

Epidote of Lane and Seaman ( 1907, p. 680), 

described as a "volcanic ash" by Lane (1911, p. 37, 

688). The tuff-breccia immediately above the Greenstone Flow on the island is in the same stratigraphic 

position. 

2. The Washington Island Flow is unique in appearance on the island and has its counterpart in the 

Pewabic Flow in the same relative position on the 

peninsula (Lane, 1911, p. 689; Cornwall, oral commun., 1971). 

3. The Ashbed Group "is characterized by lavas which 

contain more soda than the regular ophites and 

which are relatively feldspathic and fine grained 

[porphyrites and glomeroporphyrites]" (Lane, 1911, 

p. 35). These flows occur in three distinct zones in 

the Ashbed Group on the Keweenaw Peninsula (for 

example, see Davidson and others, 1955), and the 

middle zone contains the type Ashbed Flow. The 

three zones are represented on Isle Royale by the 

Tobin Harbor, Middle Point, and Scoville Point 

Flows. In fact, at least part of the Middle Point Flow 

of this report was specifically correlated with the 

Ashbed Flow by Lane (1911, p. 670). 

4. On the Keweenaw Peninsula "columnar basalt occurs 

only in two flows, the Greenstone and a 'melaphyre' 

just above the Ashbed Flow" (Cornwall, 1954b). As 

noted earlier, on the island the Greenstone Flow is 

interpreted as having a columnar-jointed zone, and 

the columnar-jointed Edwards Island Flow appears 

to be in the same stratigraphic position as the "melaphyre." The other columnar-jointed flow on the 

island, the Long Island Flow, which lies between the 

Tobin Harbor and Middle Point Flows, is in the 

same stratigraphic position as some fine-grained 

melaphyres on the peninsula. Columnar jointing can 

be seen in only a few of the outcrops of the Long 

Island Flow, however, and it is possible that similar 

but concealed jointing occurs in the probable equivalent rocks on the peninsula, where they are largely 

known only from drill core. 

The uppermost group, the Eagle River Group of Lane 

and Seaman (1907), has been described on the Keweenaw Peninsula as "characterized by waning volcanic 

activity. It contains numerous sandstones and conglomerates, and on the whole they are more rounded 

in this group than in others. The individual lava flows 

are generally not thick nor coarse grained" (Lane, 1911, 

p. 37). The rocks are also generally ophitic. This 

description also accurately fits the upper part of the 

Portage Lake Volcanics on Isle Royale, that part above 

the Scoville Point Flow (fig. 17). 
C26 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

Below the Greenstone Flow it is more difficult to 

make specific correlations of volcanic units. On both 

sides of the Lake Superior syncline, flows of the lower 

part of the Portage Lake Volcanics are chiefly ophitic, 

including a number with above-average thickness. The 

thick Hill Point Flow on the island could well be correlative with a thick ophitic flow such as the Scales 

Creek Flow (fig. 3) on the peninsula, as suggested 

indirectly by Lane (1911, p. 739), but such a correlation is only one of several options. Equivalents of the 

other stratigraphic markers on the island have not been 

recognized on the peninsula. 

Several bodies of felsite occur in the lower part of 

the Portage Lake Volcanics near the east end of the 

Keweenaw Peninsula, at and below the horizon of the 

Bohemia Conglomerate (fig. 3). This would be in the 

upper part of the Bohemian Range Group of the older 

terminology. The body at Bare Hill, for example, is a 

fine-grained reddish-brown rhyolite that contains scattered feldspar phenocrysts as much as 2 millimeters 

long and is considered to be an extrusive dome (Cornwall, 1954c). A felsite reported to be lithologically 

similar to that of Bare Hill was intersected at the very 

bottom of a hole drilled at the north shoreline of Isle 

Royale (Lane, 1911, p. 755); projection of the felsite 

would reach the surface of Lake Superior about 1% 

miles offshore. This felsite, together with some thick 

felsite-pebble conglomerates above it, led Lane to suggest that its correlative stratigraphic position on the 

peninsula might lie within the upper part of the Bohemian Range Group at the approximate horizon of the 

Bohemia Conglomerate (Lane, 1911, p. 749). 

Correlation of the Greenstone Flow on opposite sides 

of Lake Superior, or at least correlation of its relative 

position in the stratigraphic sequence, is also supported 

by paleomagnetic evidence. Books (1972) determined 

paleomagnetic field orientations for a cross section of 

the Portage Lake Volcanics near Calumet on the 

Keweenaw Peninsula. He found that the field directions 

are fairly uniform throughout the sequence, except for 

an anomalous variation that occurs just below the 

Greenstone Flow. He found an identical field variation 

just below the Greenstone Flow on Isle Royale. 

The clastic deposits within the Portage Lake Volcanics are chiefly stream deposits laid down on top of 

what were essentially flat-lying lava flows within the 

volcanic sequence. Although these conglomerates and 

sandstones are lenticular rather than continuous 

sheets, their horizons can be carried across many of 

the gaps in their continuity. In places where the beds 

are missing, their horizons generally can be recognized 

in drill core by detrital material in the underlying 

amygdaloidal flow top, and as a result, the sedimentary 

units provide excellent horizon markers within the 

Portage Lake Volcanics where subsurface data are 

available (White, 1968). 

If the period of volcanic quiescence during which the 

sedimentary rocks were deposited was basin wide, then 

coeval, if not connected, sedimentary deposits might 

exist on opposite sides of the basin, even though differences in source areas might create dissimilarities in the 

deposits. On this assumption, Lane attempted to correlate individual sedimentary units on Isle Royale with 

the well-established sequence on the peninsula, utilizing both the nature and relative position of the sedimentary units and the nature of the intervening lava 

flows. In view of the general lack of outcrop of the sedimentary rocks, the attempt itself would not have been 

possible without the data from the series of exploratory 

holes drilled across the island. 

If Lane's correlations are accepted (table 2), which 

is not unreasonable, they can be used to compare relaTABLE 2.-Correlation of clastic units in the Portage Lake 

Volcanics between the Keweenaw Peninsula and Isle Royale 

Keweenaw Peninsula 

(for strat. section, see fig. 8) 

Hancock Conglomerate 

(No.17). 

Pewabic West Conglomerate 

(No. 16). 

Mesnard Epidote of Lane 

and Seaman (1907). 

Allouez Conglomerate 

(No. 15). 

Houghton Conglomerate 

(No. 14). 

Calumet and Hecla 

Conglomerate (No. 13). 

Isle R071ale1 

Conglomerate between Edwards 

Island and Middle Point 

Flows. 

Sandstone between Long Island 

and Tobin Harbor Flows. 

Tuff-breccia above Greenstone 

Flow. 

Conglomerate at base of Greenstone Flow. 

Conglomerate 200 ft below base 

of Greenstone Flow. 

Very thin sandstone bed 875 ft 

below base of Greenstone 

Flow. 

Kingston Conglomerate Tuff-breccia above Minong 

(No. 12) (=Kearsarge Flow. 

Conglomerate of Lane). 

Wolverine Sandstone (No.9) __ Very thin sandstone bed 350ft 

below base of Minong Flow. 

Bohemia Conglomerate 

(No.8). 

Series of conglomerates beginning about 450 ft below Hill 

Point Flow. 

lQn Isle Royale only the two indicated tuff-breccias are known from outcrop. 

Correlation of clastic units between Keweenaw Peninsula and the Isle Royale drill records is by Lane (1898); correlation of named lava flows on Isle Royale 

with Lane's drill log is by Huber (this report). 

tive thicknesses of groups of flows on Isle Royale and 

the Keweenaw Peninsula, as in figure 24. For Isle 

Royale, one stratigraphic section (A) was obtained 

directly from Lane's drill log. By using key volcanic 

units as control, another section (B) was obtained from 

extrapolating the relative position of the sedimentary 

units in the drill core to the stratigraphic section near 

Wright Island (fig. 23). The stratigraphic diagram dis-
PORTAGE LAKE VOLCANICS C27 

regards structural considerations and the probability 

that the formation is thickest in the central part of the 

Lake Superior basin and thins in both directions 

toward Isle Royale and the peninsula; thus the 

"planes" in the illustration are very artificial and are 

used only to tie the individual stratigraphic sections 

together. 

The thickness of the upper part of the sequence is 

somewhat similar on both sides of Lake Superior, but 

the lower part becomes increasingly thicker on the 

peninsula than on Isle Royale. This difference in the 

lower part would suggest that the measured stratigraphic sections on the peninsula were perhaps closer 

to the axis of the subsiding depositional basin than the 

sections on Isle Royale (fig. 1). Such an interpretation 

is supported by geophysical evidence (for example, 

White, 1966b; Wold and Ostenso, 1966). 

The thickest equivalent sections on Isle Royale and 

the peninsula occur where the distance between the 

two belts of the Portage Lake Volcanics on opposite 

sides of the basin is the least-near the central part of 

the island and near the Cliff mine on the peninsula 

(B-D in fig. 24). Local thickness of the section probably bears some relation to distance from the margin 

of the original depositional basin. This relation is indicated for at least the west end of Isle Royale by the 

fact that there the Portage Lake Volcanics thins toward 

the west-the direction from which the clastic debris in 

the overlying thick wedge of Copper Harbor Conglomerate was derived, as shown by various criteria indicating direction of sedimentary transport (W ol:ff and 

Huber, 1973). As this fluvially deposited conglomerate 

contains boulders as much as 2 feet in diameter, its 

source area could not have been far. 

RELATION TO OTHER KEWEENAWAN VOLCANIC ROCKS 

The Portage Lake Volcanics on Isle Royale represents, as noted, the uppermost part of the Keweenawan 

volcanic sequence on the north side of Lake Superior, 

and the North Shore Volcanic Group of Green (1971) 

and the Osler Group represent the lowermost part. In 

Minnesota the North Shore Volcanic Group overlies 

lower Keweenawan sedimentary rocks (Puckwunge 

Formation of Winchell, 1897). In Ontario the Osler 

Group has not been studied in detail, but as it overlies 

lower Keweenawan sedimentary rocks (Sibley Group), 

it is probably at least in part correlative with the North 

Shore Volcanic Group. Correlation is also supported by 

the fact that a reversely polarized paleomagnetic field 

with similar orientation exists in the lower parts of the 

Osler Group and the North Shore Volcanic Group as 

well as in the volcanic rocks of the South Trap Range 

(Books, 1968, 1972; Halls, 1972; Palmer, 1970). AII 

I 

I 

I 

I 

I 

I 

-------- ~~~ 

Sources of data: 

} 

Greenstone Flow 

No. 15-AIIouez Cgl. 

No. 14-Houghton Cgl. 

No. 13-Calumet and Hecla Cgl. 

No. 12-Kingston (Kearsarge) Cgl. 

No. 9-Wolverine Cgl. 

No. 8-Bohemia Cgl. 

A. Drill log (Lane, 1898, p. 57-1051. 

B. Extrapolation from figure 23, Wright Island section. 

See text for details. 

C. Section near Calumet (White and others, 1953; Cornwall 

and Wright, 1956). 

D. Section near Cliff mine (Cornwall, 1954a; Davidson and 

others, 1955). 

FIGURE 24.-Stratigraphic diagram illustrating correlation of 

sedimentary horizons in the Portage Lake Volcanics between 

Isle Royale and the Keweenaw Peninsula. Numbers and 

names are those used on the peninsula. (See fig. 3.) 

though all these volcanic rocks have traditionally been 

assigned to the middle Keweenawan, along with the 

Portage Lake Volcanics, it has been suggested that perhaps the lowermost ones, those reversed ones below 

the horizon of the paleomagnetic change to normal 

polarity, should be designated as lower Keweenawan 

(for example, Books, 1968; Green and Books, 1972). 

The exposed section of the Osler Group may be as much 

as 10,000 feet thick (Tanton, 1931); that of the North 
C28 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

Shore Volcanic Group is approximately 23,000 feet 

thick (Green, 1971). 

The Portage Lake Volcanics and the North Shore 

Volcanic Group do not appear to overlap stratigraphically. Overlap cannot be completely ruled out, however, as ophitic basalts do occur both at the top of the 

exposed North Shore Volcanic Group and at the base 

of the exposed Portage Lake Volcanics. Further, we 

have no knowledge of the composition or thickness of 

the unexposed portions of each sequence beneath Lake 

Superior. The major difference between the two stratigraphic sections is that felsic volcanic rocks are nearly 

absent from the Portage Lake Volcanics, whereas they 

are relatively abundant in the North Shore Volcanic 

Group (Green, 1971). Andesites also may be more 

abundant in the North Shore Volcanic Group. 

Other evidence suggests a stratigraphic break of 

some sort between the two volcanic sequences, 

although interpretation is far from unequivocal. The 

low degree of metamorphism and lack of schistosity of 

the volcanic clasts in the sedimentary rocks interbedded with the lava flows in the Portage Lake Volcanics and in the Copper Harbor Conglomerate on Isle 

Royale are typical of Keweenawan volcanic rocks and 

unlike any pre-Keweenawan rocks in the Lake Superior 

region. The North Shore Volcanic Group is the most 

likely source terrane for this clastic debris. This conclusion requires that at least part of the North Shore 

Volcanic Group, including both felsic and mafic volcanic rocks, be exposed and subjected to active erosion 

at the margin of the depositional basin, while volcanic 

activity continued in the central part of the basin during the formation of the Portage Lake Volcanics. An 

unconformity between the North Shore Volcanic Group 

and the Portage Lake Volcanics, as suggested by Wolff 

and Huber ( 1973), would account for such erosion of 

the lower sequence. Or it is possible that part of the 

North Shore Volcanic Group was tectonically uplifted 

at the margin of the basin and that consequently the 

Portage Lake Volcanics was deposited in progressively 

smaller areas in the central part of the basin as uplift 

proceeded. It is also possible that the North Shore Volcanic Group and the Portage Lake Volcanics formed in 

separate tectonic basins isolated from each other, 

rather than in a single large basin, as has been suggested for some of the Keweenawan volcanic sequences 

on the south side of the Lake Superior basin (White, 

1966a; White and others, 1971). Finally, it is possible 

that there is a major stratigraphic break within the 

North Shore Volcanic Group at the approximate horizon of the paleomagnetic reversal. In such a case, the 

greater part of the North Shore Volcanic Group, with 

normal polarity, could be continuous upward into the 

Portage Lake Volcanics, with the necessary clastic 

debris for middle Keweenawan sedimentary rocks 

being derived from only the lower part of the North 

Shore Volcanic Group, where the required rock types 

are apparently present (Green, 1971; Green and Books, 

1972). 

Each of these structural settings would permit erosion of at least part of the North Shore Volcanic Group 

to provide clastic debris for younger Keweenawan sedimentary rocks. It is hoped that future work will provide evidence for narrowing the choice. 

STRUCTURE 

The layered bedrock sequence on Isle Royale is tilted 

toward the southeast. Dips of the strata range from 

less than 10° to 55°; they are generally steeper on the 

north side of the island than on the south side and 

average less than 20°. The increase in dip toward the 

north side is probably related in part to increasing 

proximity to the Isle Royale fault in that direction (fig. 

1). The existence of a high-angle thrust fault along the 

north side of Isle Royale, similar to the well-documented Keweenaw fault on the Keweenaw Peninsula, 

was postulated on physiographic grounds by Irving 

and Chamberlain (1855). Considerable geophysical 

evidence has since been marshalled in support of its 

existence (Wold and Ostenso, 1966; Halls and West, 

1971), and Halls and West estimated that the southeast side has been upthrown at least 1-2 kilometers. 

The structural setting of Isle Royale is thus analogous 

to that on the Keweenaw Peninsula, where dips 

increase markedly toward the Keweenaw fault; dips 

exceeding 30° are found only within 4 or 5 miles of the 

Keweenaw fault (White, 1968). A slight flexure in the 

strata between McCargoe Cove and Pickerel Cove on 

the northeast end of Isle Royale also may be related to 

movement on the Isle Royale fault. 

The only major distortion superimposed upon the 

homoclinal structure of Isle Royale is in the area 

between Siskiwit Lake and Rock Harbor, where the 

upper part of the Portage Lake Volcanics is warped 

around a node of apparent uplift and fracturing. The 

style of deformation there is different on opposite sides 

of a fault that extends through Chippewa Harbor and 

Lake Richie. On the southwest side of that fault, the 

stratigraphic units from the Edwards Island Flow 

upward, though cut by numerous minor faults, all swing 

with relative continuity around a point centered near 

the south end of Lake Richie. On the northeast side, 

only the stratigraphically highest sedimentary units in 

the Portage Lake Volcanics maintain continuity. The 

volcanic sequence between those units and the Scoville 

Point Flow is broken by a series of closely spaced 

normal faults trending parallel to the strike direction 

of the flows and consistently downthrown on the north, 
PORTAGE LAKE VOLCANICS C29 

resulting in extreme duplication of that part of the 

stratigraphic section. 

Lane (1898, p. 210), in discussing this area, noted 

that "the strata appear as though wrapped over some 

large mass below" and that "this suggests either that 

there is an intrusion below or that we are on the flanks 

of an old volcanic focus." In support of this suggestion, 

he cited the presence of thin "apparently intrusive 

sills," which in the present study are interpreted as 

pegmatitic differentiates of extrusive volcanic rocks; no 

intrusive rocks are now thought to be exposed in the 

area. Nevertheless, the area seems to have been domed 

upward, and it is possible that a buried intrusive was 

the cause. 

Lane's map of this deformed area is somewhat misleading and can be interpreted as indicating an appreciable nontectonic thickening of the stratigraphic section in the Lake Richie-Chippewa Harbor region, an 

interpretation made by Halls and West (1971, p. 622), 

among others. Lane was aware of the limitations of his 

map, however, when he stated that "I can hardly hope 

to have connected all the isolated outcrops exactly" 

(Lane, 1898, p. 209). Geologic mapping during the 

present study indicates that what appears to be stratigraphic thickening on Lane's map is due partly to 

decrease in dip of the strata in the central part of the 

upwarped zone and partly to extensive duplication of 

section by a complex pattern of normal faults. Consequently, the outcrop pattern is strongly influenced by 

topography, and the map pattern of the thin sedimentary units gives the impression of a more complex 

structure than actually exists. It is only fair to state 

that most of the refinements of the present mapping 

were made possible solely through the use of resources 

unavailable to Lane, in particular modern aerial photographs and modern mosquito repellent that permitted 

the field use of those photographs. 

Topographic lineaments and smaller scale fractures 

related to a regional fracture system are superimposed 

upon the homoclinal structure. One prominent joint 

set of this system trends northward at the west end of 

Isle Royale and changes progressively eastward to 

trend about 30° E. of N. at McCargoe Cove. Minor 

faulting has occurred along some of the lineaments 

related to this set, but displacements are generally 

small and probably rarely exceed 100 feet. All displacement appears to be near vertical, with the east 

side upthrown relative to the west side. Some fractures 

of a second set, trending east-west, commonly are faults 

showing somewhat greater displacements but still no 

more than a few hundred feet. Displacements along 

this set also are vertical, with the south side upthrown 

relative to the north side. The systematic orientation 

and nature of displacement of these fracture sets attest 

to the regional nature of their pattern. The fractures 

were probably formed in response to warping of the 

bedrock strata on the flank of the Lake Superior syncline, probably in part caused by thrust movement on 

the Isle Royale fault. 

ORIGIN OF THE PORTAGE LAKE VOLCANICS 

The Portage Lake Volcanics is the upper part of the 

total Keweenawan volcanic sequence, one of the world's 

major accumulations of plateau or flood basalt flows. 

This sequence, together with the associated Keweenawan continental red clastic rocks, is probably more 

than 40,000 feet thick locally. These materials fill a 

trough or series of elongate basins nearly 100 miles 

wide and more than 1,000 miles long, coincident in part 

with the midcontinent and mid-Michigan high gravity 

and magnetic anomalies (Bayley and Muehlberger, 

1968). The structure has been interpreted as a major 

rift system of continental proportions. (See, for example, Hinze and others, 1971; King and Zietz, 1971; 

Lyons, 1970; Pettijohn, 1970.) 

The source of the lavas appears to have been along 

the axis of the trough or rift system, as directional flow 

data on the Keweenaw Peninsula indicate flow from 

the Lake Superior basin toward the peninsula, and the 

Portage Lake Volcanics has been interpreted as wedging out completely within 15-20 miles south of the 

present line of outcrop of the sequence on the peninsula 

(White, 1960). On the north shore of Lake Superior in 

Ontario, Tanton (1931, p. 64) noted that in a number 

of localities flowage wrinkles on the tops of lava flows 

in the Osler Group indicated flowage toward the north 

or northeast prior to consolidation. He concluded that 

"all observations indicate that the lava came from a 

southerly source and it seems probable that the lava 

was poured out along fissures in the site of Lake Superior." Although the Osler Group is probably stratigraphically below the Portage Lake Volcanics on Isle 

Royale, the two sequences appear to have many similarities, and no reason now indicates that the lavas 

came from different sources. Definitive directional flow 

data are lacking on Isle Royale, but stratigraphic considerations previously mentioned suggest that the Isle 

Royale vicinity was not far from the northwest margin 

of the depositional basin at the time the Portage Lake 

Volcanics was erupted. Assuming this location, there 

is almost no alternative but that great volumes of lava 

were erupted from fissures near the axis of the trough 

and spread laterally and rapidly toward both margins 

of the trough, finally ponding and cooling in place to 

form vast sheets covering literally thousands of square 

miles. 
C30 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

White ( 1960, p. 372-373) summed up the mechanism 

of basalt flooding as follows: 

The principal consequence of the great volume of lava floods 

is that on a relatively flat surface a flow may come approximately to rest while its interior, at least, is still molten, giving 

the top time to approach a hydrostatic level. The near-level 

surface of such a flow provides the floor for a succeeding flow, 

making the upper and lower surfaces of successive flows nearly 

parallel over large areas. As envisaged here, the formation of a 

basalt flood has its closest observed counterpart in the initial 

burst of lava from vents on shields. The great volume of lava 

and the gentle slope keep the flow thick, molten, and exceedingly wide, and do not permit the top to become anchored or 

tunnels to form. Motion stops primarily because the lava is 

ponded, either against an uphill slope or behind the dam of its 

own frozen margin. After eruption ceased, there would, of 

course, be some movement of lava from central to marginal 

parts of the flow, and some local advances of the front itself as 

long as the leveling process continued, but these movements 

would be minor compared with those of the initial flooding. The 

crystallization of a large part of the interiors of flows would 

thus take place under essentially static conditions. 

White arrived at his interpretation largely on the 

basis of field evidence, speculating that the great 

volume of lava alone, rather than an abnormally high 

fluidity, favored the formation of flood basalts instead 

of the construction of shield volcanoes of the Hawaiian 

type, which are made up of rock types that do not differ 

greatly. 

Shaw and Swanson ( 1970a, b) recently calculated 

some eruption and flow rates for flood basalts on the 

basis of certain physical parameters for the Yakima 

Basalt flows of the Columbia River Plateau and measured viscosity data for Hawaiian basalt. They ( 1970a) 

concluded the following: 

Fluid dynamical calculations suggest average flow velocities 

in the turbulent regime of 10 to 20 km per hour for flows of 

about 10 m or more in thickness assuming average effective 

viscosities of approximately 500 poises. Such velocities indicate 

that heat loss during flow was mainly by radiation, and that 

lavas 200 km from their source cooled an average of about 50°C 

during flow. Turbulent flow implies homogeneity of flow units 

and little development of surface crust during movement, and 

the small temperature depression during flow implies that 

quenched lavas would have low crystal contents if they were 

initially at or above their liquidus. Both implications are consistent with field studies. 

They further concluded that with only minor and altogether realistic changes in their assumptions, chiefly 

somewhat larger source fissures, surface flow rates 

approaching 50 km per hr can be calculated. The field 

evidence suggesting rapid eruption and flowage, with 

ponding and cooling in place, is not inconsistent with 

theoretical flow mechanics. 

Data from the Keweenaw Peninsula and Isle Royale 

indicate that the clastic debris in the sedimentary rocks 

interbedded with the lava flows of the Portage Lake 

Volcanics and in the Copper Harbor Conglomerate and 

other Keweenawan formations above the lava sequence 

was transported into the basin from marginal source 

terranes (White, 1952, 1960; White and Wright, 1960; 

Hamblin and Horner, 1961; Wolff and Huber, 1973). 

The evidence for basinward flow of streams, contrasted 

with evidence that the lavas flowed toward the margins 

of the basin, shows that there were at times reversals 

of the prevailing slope over large areas, and the picture 

of a basin subsiding as it was filled seems established 

beyond any reasonable doubt. According to White's 

analysis (1960), the lava was horizontal or sloped 

gently toward the margins of the basin as long as filling 

kept pace with downwarping. When extrusion was 

interrupted, however, downwarping produced basinward slopes over small to large areas, permitting sedimentary debris to be swept into the basin during those 

periods of volcanic quiescence. Finally, with the gradual cessation of volcanic activity, continued subsidence 

permitted the accumulation of the Copper Harbor 

Conglomerate and younger deposits to form a thick 

sedimentary sequence in the basin. 

The gross synclinal form of the Keweenawan basin 

resulted from subsidence coincident with filling of the 

basin rather than later folding. However, attitudes of 

strata near the margins of the basin, as on the 

Keweenaw Peninsula and Isle Royale, were subsequently steepened by movement on major thrust faults, 

the Keweenaw fault and the Isle Royale fault, thereby 

accentuating the synclinal structure. 

REFERENCES CITED 

American Commission on Stratigraphic Nomenclature, 1970, 

Code of stratigraphic nomenclature: Tulsa, Okla., Am. 

Assoc. Petroleum Geologists, 19 p. 

Bayley, R. W., and Muehlberger, W. R., compilers, 1968, Basement rock map of the United States, exclusive of Alaska 

and Hawaii: U.S. Geol. Survey, 2 sheets, scale 1:2,500,000. 

Books, K. G., 1968, Magnetization of the lowermost Keweenawan lava flows in the Lake Superior area, in Geological 

Survey research 1968: U.S. Geol. Survey Prof. Paper 

600-D, p. D248-D254. 

---1972, Paleomagnetism of some Lake Superior Keweenawan rocks: U.S. Geol. Survey Prof. Paper 760,42 p. 

Broderick, T. M., 1935, Differentiation in lavas of the Michigan 

Keweenawan: Geol. Soc. America Bull., v. 46, p. 503-558. 

Broderick, T. M., Hohl, C. D., and Eidemiller, H. N., 1946, 

Recent contributions to the geology of the Michigan copper district: Econ. Geology, v. 41, no. 7, p. 675-725. 

Bulter, B.S., and Burbank, W. S., 1929, The copper deposits of 

Michigan: U.S. Geol. Survey Prof. Paper 144, 238 p. 

Cornwall, H. R., 1951a, Differentiation in lavas of the Keweenawan Series and the origin of the copper deposits of Michigan: Geol. Soc. America Bull., v. 62, p. 159-202. 

---1951b, Differentiation in magmas of the Keweenawan 

Series: Jour. Geology, v. 59, no. 2, p. 151-172. 

---1954a, Bedrock geology of the Phoenix quadrangle, 

Michigan: U.S. Geol. Survey Geol. Quad. Map GQ-34, 

scale 1: 24,000. 
PORTAGE LAKE VOLCANICS C31 

---1954b, Bedrock geology of the Delaware quadrangle, 

Michigan: U.S. Geol. Survey Geol. Quad. Map GQ-51, 

scale 1:24,000. 

---1954c, Bedrock geology of the Lake Medora quadrangle, 

Michigan: U.S. Geol. Survey Geol. Quad. Map GQ-52, 

scale 1:24,000. 

Cornwall, H. R., and Wright, J. C., 1956, Geologic map of the 

Laurium quadrangle, Michigan: U.S. Geol. Survey Mineral lnv. Field Studies Map MF-47, scale 1:24,000. 

Davidson, E. S., Espenshade, G. H., White, W. S., and Wright, 

J. C., 1955, Bedrock geology of the Mohawk quadrangle, 

Michigan: U.S. Geol. Survey Geol. Quad. Map GQ-54, 

scale 1:24,000. 

Foster, J. W., and Whitney, J. D., 1850, Report on the geology 

and topography of a portion of the Lake Superior Land 

District in the State of Michigan; Part 1, Copper Lands: 

U.S. 31st Cong., 1st sess, House Ex. Doc. 69, 224 p. 

Green, J. C., 1971, The North Shore Volcanic Group, in Inst. 

Lake Superior Geology, 17th Ann., 1971, Abs. and Field 

Guides: Duluth, Minn., p. 74-96. 

Green, J. C., and Books, K. G., 1972, Paleomagnetic evidence 

for the extent of lower Keweenawan lavas in Minnesota 

[abs.], in Inst. Lake Superior Geology, 18th Ann., 1972, 

Abs. and Field Guides, Paper 8: Houghton, Mich. 

Halls, H. C., 1966, A review of the Keweenawan geology of the 

Lake Superior region, in The earth beneath the continents 

-a volume of geophysical studies in honor of Merle A. 

Tuve: Am. Geophys. Union Geophys. Mon. 10 (Natl. Acad. 

Sci., Natl. Research Council Pub. 1467), p. 3-27. 

---1972, Magnetic studies in northern Lake Superior: 

Canadian Jour. Earth Sci., v. 9, p. 1349-1367. 

Halls, H. C., and West, G. F., 1971, A seismic refraction survey 

in Lake Superior: Canadian Jour. Earth Sci., v. 8, p. 610-

630. 

Hamblin, W. K., and Horner, W. J., 1961, Sources of the 

Keweenawan conglomerates of northern Michigan: Jour. 

Geology, v. 69, p. 204-211. 

Hinze, W. J., Davidson, D. M., and Roy, R. F., 1971, Continental rifts [abs.], in Inst. Lake Superior Geology, 17th Ann., 

1971, Abs. and Field Guides: Duluth, Minn., p. 29. 

Houghton, Douglas, 1841, [Fourth] annual report of the State 

Geologist: Michigan House of Representatives [Doc.] no. 

27,184 p. 

Hubbard, H. A., 1968, Stratigraphic relationships of some 

Keweenawan rocks of Michigan and Wisconsin [abs.], in 

Inst. Lake Superior Geology, 14th Ann., 1968, Superior, 

Wise., p. 35-36. 

Huber, N. K., 1973a, Glacial and postglacial geologic history. of 

Isle Royale National Park: U.S. Geol. Survey Prof. Paper 

754-A, 15 p. 

---1973b, Geologic map of Isle Royale National Park: U.S. 

Geol. Survey Misc. Geol. Inv. Map. 1-796, scale 1:62,500. 

Irvine, T. N., and Baragar, W. R. A., 1971, A guide to the 

chemical classification of the common volcanic rocks: 

Canadian Jour. Earth Sci., v. 8, p. 523-548. 

Irving, R. D., 1883, The copper-bearing rocks of Lake Superior: 

U.S. Geol. Survey Mon. 5, 464 p. 

Irving, R. D., and Chamberlin, T. C., 1885, Observations on the 

junction between the Eastern Sandstone and the Keweenaw 

Series on Keweenaw Point, Lake Superior: U.S. Geol. 

Survey Bull. 23, 124 p. 

Jackson, C. T., 1849, Geological and mineralogical reports: U.S. 

31st Cong., 1st sess., House Ex. Doc. 5, p. 371-935. 

Jackson, K. C., 1970, Textbook of lithology: New York, McGraw-Hill Book Co., 552 p. 

James, H. L., 1972, Subdivision of Precambrian: an interim 

scheme to be used by U.S. Geological Survey: Am. Assoc. 

Petroleum Geologists Bull., v. 56, no. 6, p. 1128-1133. 

Johannsen, Albert, 1939, A descriptive petrography of the 

igneous rocks, v. 1: Chicago, Univ. Chicago Press, 318 p. 

King, E. R., and Zietz, Isidore, 1971, Aeromagnetic study of 

the Midcontinent Gravity High of central United States: 

Geol. Soc. American Bull., v. 82, no. 8, p. 2187-2208. 

Lane, A. C., 1898, Geological report on Isle Royale, Michigan: 

Mich. Geol. Survey, v. 6, pt. 1, 281 p. 

---1911, The Keweenaw series of Michigan: Michigan Geol. 

and Biol. Survey. Pub. 6, Geol. Ser. 4, 2 vols., 983 p. 

Lane, A. C., and Seaman, A. E., 1907, Notes on the geological 

section of Michigan; Part 1, the pre-Ordovician: Jour. 

Geol., v. 15, no. 7, p. 680-695. 

Lyons, P. L., 1970, Continental and ocean geophysics, in Johnson, Helgi, and Smith, B. L., eds., The megatectonics of 

continents and oceans: New Brunswick, N. J., Rutgers 

Univ. Press, p. 147-166. 

Palmer, H. C., 1970, Paleomagnetism and correlation of some 

Middle Keweenawan rocks, Lake Superior: Canadian 

Jour. Earth Sci., v. 7, no. 6, p. 1410-1436. 

Pettijohn, F. J., 1970, The Canadian Shield-A status report, 

1970, in Baer, A. J., ed., Basins and geosynclines of the 

Canadian Shield-symposium, Ottawa, 1970: Canada 

Geol. Survey Paper 70-40, p. 239-255. 

Ross, C. S., 1941, Origin and geometric form of chalcedonyfilled spherulites from Oregon: Am. Mineralogist, v. 26, no. 

12, p. 727-732. 

Shapiro, Leonard, 1967, Rapid analysis of rocks and minerals 

by a single-solution method, in Geological Survey research 

1967: U.S. Geol. Survey Prof. Paper 575-B, p. 187-191. 

Shaw, H. R., and Swanson, D. A., 1970a, Speculations on the 

fluid mechanical history of Yakima Basalt flows [abs.], in 

Columbia River Basalt Symposium: 2d, Cheney, Wash. 

1969, Proc., p. 330. 

---1970b, Eruption and flow rates of flood basalts, in Columbia River Basalt Symposium: 2d, Cheney, Wash. 1969, 

Proc., p. 271-299. 

Silver, L. T., and Green, J. C., 1972, Time constants for 

Keweenawan igneous activity [abs.]: Geol. Soc. America 

Abstracts with Programs, v. 4, no. 7, p. 665-666. 

Tanton, T. L., 1931, Fort William and Port Arthur, and 

Thunder Cape map areas, Thunder Bay District, Ontario: 

Canada Geol. Survey Mem. 167, 222 p. 

U.S. Geological Survey, 1957, Topographic map of Isle Royale 

National Park, Michigan (culture revised 1970): scale 

1:62,500, contour and shaded-relief editions. 

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

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

White, W. S., 1952, Imbrication and initial dip in a Keweenawan conglomerate bed: Jour. Sed. Petrology, v. 22, no. 4, 

p. 189-199. 

---1960, The Keweenawan lavas of Lake Superior, an 

example of flood basalts: Am. Jour. Sci., v. 258-A, Bradley 

vol., p. 367-374. 

---1966a, Tectonics of the Keweenawan Basin, western 

Lake Superior region: U.S. Geol. Survey Prof. Paper 

524-E, 23 p. 

---1966b, Geologic evidence for crustal structure in the 

western Lake Superior basin, in The earth beneath the 

continents-a volume of geophysical studies in honor of 

Merle A. Tuve: Am. Geophys. Union Geophys. Mon. 10 

(Natl. Acad. Sci., Natl. Research Council Pub. 1467), p. 

28-41. 
C32 GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN 

---1968, The native copper deposits of northern Michigan, 

in Ridge, J. D., ed., Ore deposits of the United States, 

1933-1967, v. 1: New York, Am. lnst. Mining, Metall., and 

Petroleum Engineers, p. 303-326. 

---1971, Geologic setting of the Michigan copper district, in 

Soc. Econ. Geologists, Guidebook for field conference, 

Michigan copper district, 1971: p. 3-17. 

---1972, The base -of the upper Keweenawan, Michigan 

and Wisconsin: U.S. Geol. Survey, Bull. 1354-F, 23 p. 

White, W. S., Brooks, E. R., Hubbard, H. A., Johnson, R. F., 

and Wilband, J. T., 1971, Keweenawan stratigraphy of 

westernmost Michigan, in Inst. Lake Superior Geology, 

17th Ann., 1971, Abs. and Field Guides: Duluth, Minn., 

P. 71-72. 

White, W. S., Cornwall, H. R., and Swanson, R. W., 1953, 

Bedrock geology of the Ahmeek quadrangle, Michigan: 

U.S. Geol. Survey Geol. Quad. Map GQ-27, scale 1:24,000. 

White, W. S., and Wright, J. C., 1960, Lithofacies of the Copper Harbor Conglomerate, northern Michigan, in Geological Survey research 1960: U.S. Geol. Survey Prof. Paper 

400-B, p. B5-B8. 

Winchell, N. H., 1897, Some new features in the geology of 

northeastern Minnesota: Am. Geologist, v. 20, p. 41-51. 

Wold, R. J., and Ostenso, N. A., 1966, Aeromagnetic, gravity, 

and sub bottom profiling studies in Western Lake Superior, 

in The earth beneath the continents-a volume of geophysical studies in honor of Merle A. Tuve: Am. Geophys. 

Union Geophys. Mon. 10 (Natl. Acad. Sci., Natl. Research 

Council Pub. 1467), p. 66-94. 

Wolff, R. G., and Huber, N. K., 1973, The Copper Harbor Conglomerate (middle Keweenawan) on Isle Royale, Michigan, and its regional implications: U.S. Geol. Survey Prof. 

Paper 754-B, 15 p. 
Geology of Isle Royale 

National Park, Michigan 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 754 

This volume was published 

as separate chapters A-C 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 
CONTENTS 

[Letters designate the separately published chapters] 

(A) Glacial and postglacial geologic history of Isle Royale National Park, Michigan, by 

N. King Huber. 

(B) The Copper Harbor Conglomerate (middle Keweenawan) on Isle Royale, Michigan, 

and its regional implications, by Roger G. Wolff and N. King Huber. 

(C) The Portage Lake Volcanics (middle Keweenawan) on Isle Royale, Michigan, by 

N. King Huber. 

• *u.s. GOVERNMENT PRINTING OFFICE: 1973 Q-543-576/9