If yon no longer need this publication write to the Geological Survey In Washington for an 

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UNITED STATES DEPARTMENT OF THIS INTERIOR

GEOLOGICAL AND GEOPHYSICAL SURVEY OF

FLUORSPAR AREAS IN HARDIN COUNTY

ILLINOIS

Part 1. GEOLOGY OF THE CAVE IN ROCK DISTRICT

Part 2. AN EXPLORATORY STUDY OF FAU1 TS IN THE

CAVE IN ROCK AND ROSICLARE DISTRICTS

BY THE EARTH-RESISTIVITY METHOD

GEOLOGICAL SURVEY BULLETIN 942

UNITED STATES DEPARTMENT OF THE INTERIOR 

Harold L. Ickes, Secretary

GEOLOGICAL SURVEY 

W. £. Wrather, Director

Bulletin 942

GEOLOGICAL AND GEOPHYSICAL SUEVEY OF

FLUOKSPAE AEEAS IN HABDIN COUNTY

ILLINOIS

PART 1. GEOLOGY OF THE GATE IN BOCK DISTEICT

By L. W. CURRIER

WITH THE COLLABORATION OF O. E. WAGNER, JR.

PART 2. AN EXPLOEATOEY STUDY OF FAULTS IN THE

CAYE IN EOCK AND EOSICLAEE DISTEICTS

BY THE EAETH-EESISTIVITY METHOD

By M. KING HUBBERT

Prepared in cooperation with the

STATE OF ILLINOIS

DEPARTMENT OF REGISTRATION AND EDUCATION 

STATE GEOLOGICAL SURVEY DIVISION

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1944

For sale by the Superintendent of Documents, IT. S. Government Printing Office, Washington 25, D. 0.

Price $1.25

CONTENTS

Page

Foreword.________________________________________________________ IX

Part 1. Geology of the Cave in Rock district, by. L. W. Currier, with the

collaboration of O. E. Wagner, Jr._.________________________________ 1

Abstract_^-^ __________________________________________________ 1

Location and accessibility____________________________________ 2

Field work___-__-____-__-_________-_---_______-_^__------___ 3

Acknowledgments ____^_______-_-_____-_-____________-_---___-_ 4

Importance of Cave in Rock district._____----____-____-__-__-___ 4

Geologic features of the Illinois-Kentucky field__________________ 5

Stratigraphy and structure___--__---__--_____-___--_-__--__ 5

Sedimentary formations___-____-_______-__-_----_-_____ 5

Igneous rocks.________________________________________ 6

Faults__..._____________-_ _____'_________-___-_____ 8

Fluorspar deposits,________________________________________ 8

Geologic features of the Cave in Rock district_______-_-_-______ 10

Topography. ______________________________________________ 10

Stratigraphy._____________________________________________ 13

General statement___________________________________ 13

Descriptions of formations, by O. E. Wagner, Jr_____._____ 14

Mississippian series__-______-_-_________-__-_-_-___ 14

St. Louis limestone____________________________ 14

Ste. Genevieve limestone__------___-------- .__ 14

Chester group____---__--_____--___---_______ 17

Pennsylvanian series_______________________________ 21

Caseyville sandstone_--_-_-_-__-__-_------_-___ 21

Structural geology_______________________________._________ 21

General attitude of beds______________________________ 21

Folds._______________________________________________ 22

Joint systems and related fractures__-_____-______-__--__ 23

Faults______________________________________________ 24

Peters Creek major fault zone_______-_-______-----_- 24

Descriptions of faults, by O. E. Wagner, Jr___________ 25

Previous interpretations of faulting__________________ 28

Geophysical evidences of faulting.___________________ 29

Topographic evidences of faulting east of mapped area. 29

Fluorspar deposits of the district--..------------ ._-----_____ 30

General statement__________-________^_________________ 30

' Minerals of the deposits.-_-__-__--___--___--_---------_ 30

V Texture of the deposits_____________________________ 33

Paragenesis of the deposits__-----_-_-___-____-_-_..--_-_ 36

Fluorspar...______________________________________ 36

Quartz___-___________-______________-___-_-______ . 39

Sulfides.---___-_---_-______-______---__--___--_-- 39

Stratigraphic relations,______.-_______'___-__-__-________ 40

m
IV CONTENTS

Part 1. Geology of the Cave in Rock district Continued. Page 

Fluorspar deposits of the district Continued.

Structure of the deposits-..,____--______-_---_-________ 41

General form of the ore bodies._____________________ 41

Relation of ore bodies to minor folds___----_.________ 41

Roof veins._____________________________________ 42

Floor veins_________-_-____---___-_-__-___________ 43

Boundaries of mineral bodies.....___________________ 43

Partings_ _ ______________________________________ 44

Bedding veins in roof shale. -__-_-_-----_-_.________ 45

Fluorspar content of mineral bodies__-__.-_.___________ 46

Origin and geologic history of the deposits._______________ 47

Views of previous writers_______________________________ 51

Possible extension of the producing area._____-_-_-._-_________ 52

Suggestions for prospecting and exploration.__-_---___________ 54

Mines and prospects.__________-_____-__-_--_-----____..___- 55

Mining methods_________________--_---_--_--__,--_--_-____ 65

Milling methods_______________________________________ 65

Economic considerations__________________________________ 68

Uses and grades____-__-______-_--_---_-----------__-__ 68

Production __________________________ _____-___________ 69

Reserves. ________________________-_____-_------___-_-____ 70

Addendum___ __________________-__-__-_----------------_____ 71

Part 2. An exploratory study of faults in the Cave in Rock and Rosiclare

districts by the earth-resistivity method, by M. King Hubbert_______ 73

Abstract___ ______-_______________-__--_------ --.---_-_--__- 73

Introduction._________________________-__----_----___-________ 73

Classification of rocks on the basis of physical properties ___________ 74

Prospecting methods. _____________-___---__-_--_-_____________ 75

The earth-resistivity method.______________-_-__-_---_-_________ 77

Technique of earth-resistivity measurement._____--_--__-_________ 77

Variables in the Wenner method.-_______-__---____---_______ 78

The Gish-Rooney apparatus. __._-___-_-__-_--_--__-________ 79

Field methods used in Hardin County___________________________ 80

Field operations of resistivity party._________________________ 80

Method of control-________-_-___-_-_--_-----_----.___._. 81

Relation between apparent resistivity and geologic structure.-___-_- 82

Resistivity variation due to faults___________________________ 82

Effects due to interrelations between topography and strata_____ 86

Effects due to weathering and to alluvium^.___________________ 89

Fault interpretations as a function of distance between traverses. _ 90

Extent and scope of the present work_____--_____________________ 92

Acknowledgments ___________________--_----------------_-__-__ 93

Resistivity profiles of the Rosiclare district___-._-_-_-__-_________ 94

Discussion and interpretation. _-_______-_-------_-__-_______ 95

Eastern area (profiles l-16)______--_---------__-___-____ 95

Western area (profiles 17-27)_-_--------------_-----____ 107

Summary for the Rosiclare district_________-__-______________ 115

Resistivity profiles of the Cave in Rock district__-_--________.____ 119

Discussion and interpretation.______________________________ 120

Northeast-southwest profiles (1-12)____________________ 120

Northwest-southeast profiles(13-26)________.______:__ 127

Summary and conclusions for the Cave in Rock district-_______ 132
CONTECNTS V

Part 2. An exploratory study of faults in the Cave in Rock and Rosiclare Paw 

districts by the earth-resistivity method Continued.

Appendix._____________________________________________________ 134

Theory of the Wenner method of measuring earth resistivity. _ __ 134

Electrical resistance__________________________________ 134

Ohm's law_____________________ ______ 134

Resistance of an extended body.____________________ 134

Resistance of half space to current from single electrode.___ 136

Resistance of a single shell________________________ 137

Total resistance.__________________________________ 137

Two current electrodes.________________________________ 138

Equipotential surfaces and current flow lines_ _______ 139

Difference of potential between two points_________ 140

Inhomogeneous and anisotropic media_________________ 141

Apparatus used.,___________________________ ____________ 143

Index. ___________-_...-______._________________________ 149

ILLUSTRATIONS

PLATE 1. Topographic map of the Cave in Rock fluorspar district, Hardiu

County, 111.________________________________________ In pocket

2. Map showing drainage pattern of Cave in Rock district_______ 14

3. Geologic map of the Cave in Rock fluorspar district, Illinois. In pocket

4. Contour map of base of Rosiclare sandstone member in developed

area of Cave in Rock district_______-_____--_-_--__-___-____ 22

5. Map showing contours of top of mineralized zone and approximate

thickness of mineralized zone in several mines________________ 22

6. A, Photomicrograph of limestone showing microscopic quartz 

crystals; B, Photomicrograph of fine-grained siliceous band in ore 

specimens from Cave in Rock mine. ________________________ 30

7. Specimen of siliceous material from waste pile of Cave in Rock

mine______________-___-______-_____-___--_-____----___ 30

8. Sketches of parts of faces in mines on Spar Mountain_________ 30

9. Specimen of high-grade fluorspar ore from Spar Mountain showing

comb structure and vugs_____________-_--_____-_-__--_-___ 38

10. A, Specimen of high-grade "coon-tail" fluorspar from Spar 

Mountain; B, Specimen of low-grade material from Spar 

Mountain showing stratification_______________________. 38

11. Specimen of good ore from Spar Mountain._____________ _ 38

12. A, Photomicrograph of an impure fluorspar band showing unreplaced ferruginous carbonate grains and quartz; B, Photomicrograph showing contact between coarse-textured and fine-textured bands and interstitial impurities._____ _______*______.. 39

v 13. A, View in drift 3, Crystal mine; B, Exposure of fluorspar horizon

at-portal of Lead adit, Benzoji Fluorspar Co _______________ 46

14. Drift map of Crystal mine_--___---__-_-------------------_ 46

15. A, Face of a room in Crystal mine; B, Bedding veins in roof shale,

Victory No. 1 mine____.___________________________________ 46

16. Map of principal underground workings of Benzon Fluorspar Co. 62

17. A, Two levels of mineralization in Lead adit; B, Face of banded

fluorspar in Victory No. 1 mine__.-_-___--_----_-_----__--__ 62

18. Flowsheet, Victory Fluorspar Mining Co.'s mill______________ 62
VI CONTENTS

*

Page 

PLATE 19. Graph showing production.and utilization of fluorspar, 1916-40 70

20. Map of the Rosiclare district, Hardin County, 111., showing

location of earth-resistivity traverses and faults_______________ 94

21. Resistivity profiles for the Rosiclare district, Hardin County,

111.__________________________________________________ In pocket

22. Map of the Cave in Rock district, Hardin County, 111., showing

locations of earth-resistivity traverses and faults_!_________._ 118

23. Resistivity profiles for the Cave in Rock district, Hardin County,

111 ____________________________________________________ In pocket

24. The Gish-Rooney type of instrument used in field work_____ 142 

FIGURE 1. Key map showing location of Cave in Rock district and its geographic relation to Illinois-Kentucky fluorspar field____-_-_- 3

2. Condensed geologic map of Illinois-Kentucky fluorspar field.. 6

3. Generalized columnar section of formations exposed in the Cave

in Rock district_______,___---___---_-_-____--_._..__-. 7

4. Sketch of exposure at prospect on east side of Lead Hill showing

incipient mineralization along joints and bedding planes____ 86

5. Diagram illustrating "mushroom structure" as observed in adit 

No. 5 of Pittsburgh Fluorspar Products Co. and at abandoned 

workings of Basic Mineral Co., Lead Hill._______________ 44

6. Diagram illustrating formation of a flexure in flexible beds over

a minor fault in brittle beds__ ________________________ 50

7. Profiles across Lead adit workings showing relation of mineral

bodies to roof slopes..________________i____-_____-___- 59

8. Map of drifts in Victory No. 1 mine______________________ 61

9. Flow sheet, Crystal Fluorspar Co.'s concentrating mill______ 66

10. Flow sheet, Benzon Fluorspar Co.'s concentrating mill______ 67

11. Schematic diagram of the Wenner system for measuring earth

resistivity. ___________________________________________ 77

12. Three resistivity blocks associated with a vertical fault._______ 83

13. Variation of apparent resistivity across a vertical fault-______ 83

14. Variation of the apparent resistivity across a vertical highly

conductive sheet and across an insulating sheet..__________ 84

15. Apparent resistivity in a traverse across a vertical sheet of metal

with a slightly oily surface, immersed in water_____________ 85

16. Variation of apparent resistivity as a function of azimuth with

the station directly above the metal sheet of figure 15-____- 86

17. Variation of apparent resistivity along a traverse over a level

surface normal to strike of dipping strata _______ _ ________ 87

18. Variation of apparent resistivity produced by successive horizontal strata along a traverse across a ridge _______________ 87

19. Vaiiation of apparent resistivity produced by successive horizontal strata along a traverse across a valley ______________ 88

20. Variation of apparent resistivity along a traverse across horizontal strata with faulting and with topographic relief___l_. 88 f

21. Erratic variation of resistivity across karst topography _______ 89 /

22. Difficulty of correlating the faults shown by resistivity profiles.. 91

23. Electrical circuit for measuring resistance..-..______________ 134

24. Measurement of the resistance of a rectangular block to the flow

of current parallel to the Y axis________-___-____-______- 134

25. Radial flow of electric current into half space from a single hemispherical electrode (only quarter space shown)___________ 136

26. Resistive hemispherical shells about a hemispherical electrode

(only a quarter of a sphere shown)__-______^__..___-.____- 137
CONTENTS VII

Page

FIGTFKE 27. Determination of the potential at an arbitrary point_______ 138

28. Determination of the potentials at two points._____________ 140

29. Vertical section along the line of the electrodes showing the 

lines of flow of the electric current and equipotential surfaces 

in homogeneous and isotropic ground___________________ 144

30. Wiring diagram of the earth-resistivity instrument-_________ 145

31. Stamp and die for putting copper sleeves used for markers on

insulated wires_______________________.i______________ 146

FOREWORD

The manuscripts of parts 1 and 2 were transmitted to the Illinois 

Geological Survey in June 1937 and June 1938, respectively, for 

publication, according to a cooperative agreement between that 

bureau and the United States Department of the Interior, Geological 

Survey. Conditions since 1938 have interfered several times with 

plans made by the State bureau for publication of the report, so that 

the Federal Geological Survey offered to publish the report in order 

to avoid further delay. Accordingly, the manuscripts were returned 

to the Federal bureau in March, 1943.

Although the field work was done in 1934 and 1935 and the mine 

data and geologic data contained in the report are those that were 

obtained when the field work was done, it is believed that most of the 

information and discussions are as significant now as they were when 

the manuscripts were first prepared; the report is therefore published 

with little change, except for the revision of plate 19 to show production statistics up to 1940, the addition of data and references to the 

sections that deal with economic considerations and reserves, and an 

addendum in which significant mining and prospecting developments 

since 1937 are briefly reviewed. Publication of the report at the 

present time is particularly opportune in view of the current great 

need for additional supplies of fluorspar for purposes of national 

defense.

EC

GEOLOGICAL AND GEOPHYSICAL SUBVEY 01

ILUOESPAK AEEAS IN HARDIN COUNTY

ILLINOIS

PART 1. GEOLOGY OF THE CAVE IN EOCK DISTRICT

By L. W. CURRIER 

With the collaboration of O. E. Wagner, Jr.

ABSTRACT

The part of the Illinois fluorspar field known as the Cave in Rock district is 

characterized by horizontal deposits of fluorspar that have been formed by replacement of certain limestone beds in the Mississippian series of rocks. Unlike 

the vein deposits of the rest of the field, the Cave in Rock deposits occur at 

definite stratigraphic horizons. The principal ore horizon found so far is at the 

top of the Fredonia limestone member of the Ste. Genevieve limestone, just below 

the shaly base of the Rosiclare sandstone member. The Rosiclare sandstone is 

exposed along the east and south escarpments of Spar Mountain, a mesalike 

hill that rises about 200 feet above the adjacent lowland. Fluorspar deposits 

occur also at the same horizon at the top of Lead Hill, a small mesa near and 

southwest of Spar Mountain.

The present work seems to confirm the genetic theory previously published, 

namely that these deposits were formed by ascending solutions. These solutions 

probably followed minor fissures that connected below with larger fissures, which 

in turn probably connected with a major fault zone. It is believed that where 

such minor fissures extended upward only to the shale or other impervious cap 

rock, or were greatly reduced in size where they penetrated such beds, the solutions spread laterally along the contact and along the limestone beds beneath it 

and replaced the limestone. Differences in the texture and degree of purity of 

the limestone layers were among the factors that caused the layering of the ore. 

Stoichiometrical replacement (molecule for molecule) followed by further deposition is believed to have been the process that caused the crustification of the 

purer layers and the vug lines between them.

The bedding-replacement deposits of this district do not appear to be closely 

connected with major, regional faults, but rather with minor and local faults that 

are in turn probably related to the Peter's Creek major fault zone, that bounds 

the district on the northwest. The most favorable prospecting area, then, would 

be a belt generally parallel with the Peter's Creek fault but somewhat remote 

from it. Data obtained underground in all the accessible mines of the district 

seem to indicate that the fluorspar deposits are commonly localized on small local

1
2 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

folds as well as along minor fissures and that the long dimensions of the "ore" 

bodies are approximately parallel to the fold axes and the trends of the fissures; 

moreover, the structural axes and trends for the most part fall into two groups, 

one of which trends approximately N. 50° E. generally parallel to the Peter's 

Creek major fault and the other from N. 50° W. to N. 70° W., though a few 

structures fall outside these limits. The discovery of most of the minor structures, and particularly the folds, from surface geologic features alone would not 

be possible, however, because such features commonly do not extend far enough 

above the ore norizon to reach the surface, though several minor faults do appear 

at the surface.

Not only the northeast part of the mapped area, but also the unmapped area 

to the northeast of it should be considered favorable territory for prospecting. 

Particular attention should be given to evidences of minor structural deformation 

such as faults of even slight displacement, zones of closely spaced fissures, shattering and brecciation of formations, and reversed or unusually steep dips of the 

strata, in choosing sites for drilling. Such phenomena may lie along projections 

of the axes of similar phenomena known to exist in the mapped area and already 

developed mines, but, on the other hand, all zones of deformation will not necessarily be confined, to such projected lines. Geophysical methods of exploration as 

described in part 2, by M. K. Hubbert, may aid materially in locating areas for 

drilling.

The Rosiclare sandstone horizon would be found at increasingly greater depths 

to the northeast, owing to the gentle but prevailing regional northeast dip of the 

formations.

It is reasonable to assume that bedding-replacement deposits of the Cave in 

Rock type, probably occur in some other border areas of the Illinois-Kentucky 

field where lithologic sequences similiar to those of the Cave in Rock district occur 

and are somewhat remote from major faults. The presence of a shale or other 

compact and relatively impermeable bed overlying a thick limestone would be 

comparable to the lithologic sequences that appear to have influenced bedding 

replacement of limestone by fluorspar in the Cave in Rock district. The writer 

has found specimens of the banded type of fluorspar, such as characterize the Cave 

in Rock deposits, in dumps at the Hamp mines, on the north side of Hicks dome, 

and at the Renfro prospect, 4 miles east of Hicks. At both of these places the Rosiclare sandstone crops out. Charles Butts, of the Geological Survey, reported 

(unpublished manuscript) similiar specimens from the Rose prospect, near the 

center of Hicks dome, where the thick Chattanooga shale is exposed overlying 

Devonian limestone. Thus the lithologic sequences of beds and the geographic 

positions of these areas seem favorable for the existense of such deposits, but the 

structural details are not known,

LOCATION AND ACCESSIBILITY

The Cave in Rock district, as defined in this report, includes an area 

of approximately 3.3 square miles in the eastern part of Hardin County, 

111. It is about 4 miles northwest of Cave in Rock, a town on the Ohio 

River, which is the chief shipping point for fluorspar from this district. 

Cave in Rock is about 10 miles up the river from Rosiclare, a town 

which for many years has been the center of the fluorspar mining industry in Illinois, and which is the nearest railroad point to the Cave 

in Rock district. The district is reached by good highways from the 

north and west and a graded and graveled highway connects it with
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 3

the town of Cave hi Rock. There are no steep grades on the main 

roads between the producing area and the shipping points, Cave hi 

Rock, Rosiclare, and Elizabethtown. The location of the district and 

its geographic relation to the rest of the Illinois-Kentucky field is 

shown on the key map, figure 1.

In 1935, two operators were conveying the milled fluorspar product 

by motor trucks to loading bins on the Ohio River at Cave in Rock; a 

third operator was trucking similar material to the railroad at RosiNDIANA

r

CAVE IN ROCK DISTRICT

KENTUCKY

IL LINOIS - KENTUCKY 

FLUORSPAR-PRODUCING AREA

TENNESSEE

as____50____75 100 MILES

FIGURE 1. Key map showing location of Cave in Rock district and its geographic relation to IllinoisKentucky fluorspar fle,ld.

clare; and a fourth was trucking crude "ore" 13 miles to a mill 

reached by a railroad spur near Eichorn, about 5 miles northwest of 

Rosiclare.

FIELD WORK

In the summers of 1917 and 1919, the writer studied the fluorspar 

deposits of southern Illinois for the State Geological Survey, in connection with the geologic mapping of the area by Weller. 1 At that 

time the Cave in Rock district was but briefly examined, because^only 

since then has it become an important producing area. In 1920 and 

1921, the writer 2 studied the fluorspar deposits of the Kentucky part of 

the Illinois-Kentucky field.

« Weller, Stuart, and others, The geology of Hardin County, HI.: Illinois Qeol. Survey Bull. 41, 1920 

(chapters on fluorspar deposits and on igneous rocks, by L. W. Currier). 

2 Currier, L. W., Fluorspar deposits of Kentucky: Kentucky Oeol. Survey, ser. 6, vol. 13,1923.
4 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

The field work upon which this report is based was done in July 

1934 and July and August 1935. The first season's work was carried 

on by the Federal Geological Survey, under the auspices of the Public 

Works Administration, and in cooperation with the Illinois Geological 

 Survey. The second season's work was done in continuation of the 

first season's work as a cooperative project between the Federal 

Geological Survey and the Illinois Geological Survey."

O. E. Wagner, Jr., mapped the formations of the district, and prepared the sections of the report that describe the details of stratigraphy 

and the individual faults. C. G. Dickinson served as instrument man 

of the party and prepared the topographic base map of the district and 

the maps showing contours of the Rosiclare sandstone.

While the surface mapping and mine studies were being carried on, 

M. King Hubbert made an earth-resistivity survey of the Cave in 

Rock and Rosiclare areas for the purpose of ascertaining how clearly 

and accurately hidden faults might be discovered by the resistivity 

method. The results of Mr. Hubbert's studies are given in part 2.

ACKNOWLEDGMENTS

The writer is greatly indebted to the several operators and mine 

managers in the district who readily gave free access to the mines, 

furnished mine maps and other important data, and facilitated the 

study in various other ways. Acknowledgment is also made of helpful discussions and advice on the part of E. F. Burchard and other 

colleagues in the Federal Geological Survey.

IMPORTANCE OF CAVE IN ROCK DISTRICT

This special study of the Cave in Rock district was occasioned by the 

rapid rise in industrial importance of the district since 1919, and by 

significant geologic features uncovered by mining developments, 

mostly since 1926. The rapidly growing importance of the district 

is indicated by the statistics of production, which show an increase 

in the output of crude ore 3 from 9,930 short tons in 1919 to 32,295 

short tons in 1930. Since 1930 the production has fluctuated greatly. 

Of fluorspar shipped from the Illinois-Kentucky field in 1919, 1929 

(peak year), and 1934 (last year for which statistics were available), 

the Cave in Rock district furnished 5.5 percent, 11.7 percent, and 16.7 

percent, respectively.4 Economic features are discussed further on 

subsequent pages.

' Technically, the term "ore" is used only to denote a mine product from which a metal is obtained, but in 

this district the operators apply it to fluorspar bodies of commercial importance, and it is therefore used in 

this sense throughout this report.

« Statistics furnished by H. W. Davis, Div. Mineral Statistics, U. S, Bur. Mines. For total production 

since this report was prepared see page 71; in 1940 the Cave in Bock district produced 24.0 percent of the 

Dlinois-Kentucky production.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 5

The high quality of the Cave in Rock ores and the relatively low 

mining, and milling costs give the district a distinct advantage over 

much of the field.

The Cave in Rock district is on the margin of the extensive IllinoisKentucky fluorspar field, which is probably the largest and most 

productive area in the world for this mineral. As regards the major 

features relating to the geology and origin of its deposits, the Cave in 

Rock district is not materially different from the rest of the field; as 

regards details, however, not only of structure and origin, but also of 

economic development, the district has so many distinctive features 

that affect methods of prospecting and development as to deserve 

separate description.

GEOLOGIC FEATURES OF THE ILLINOIS-KENTUCKY

FIELD

A brief description of the principal geologic features of the entire 

Illinois-Kentucky field is presented at this point in order to make 

clear the genetic relationship between the bedding-replacement 

deposits of the Cave in Rock district and the vein deposits which 

characterize the rest of the field. Detailed descriptions of the geology 

of the field and discussions of the fluorspar deposits have appeared 

in earlier publications.5

STRATIGRAPHY AND STRUCTURE 

SEDIMENTARY FORMATIONS

The formations exposed hi the area comprise sedimentary rocks of 

Devonian and Carboniferous (chiefly Mississippian) age, and several 

small igneous bodies whose age is post-Carboniferous possibly late 

Cretaceous or early Tertiary. The rocks are cut by a great number 

of normal faults, giving marked distinction to this part of the Mississippi Valley province. The structural picture displayed by the map 

(fig. 2) is that of a broadly domed area that has collapsed into a mosaic 

of fault blocks and wedges. The longer diameter of the dome trends 

northwestward and its length was originally more than 30 miles. In 

the northwest portion of the field there is a subordinate structure 

known as Hicks dome, at the center of which are exposed a middle 

Devonian limestone and the Chattanooga shale, which is of upper 

Devonian (and lower Mississippian?) age. The limestone is the oldest 

rock exposed in the field.

5 Weller, Stuart, and others; The geology of Hardin County, 111.: 111. Qeol. Survey Bull. 41,1920. Weller, 

Stuart, Geology of the Qolconda quadrangle: Kentucky Geol. Survey, ser.6, vol.4, 1921; Geology of the 

Princeton quadrangle: Kentucky Geol. Survey, ser. 6, vol. 10,1923; Geology of the Cave in Rock quadrangle: 

Kentucky Geol. Survey, ser. 6, vol. 26, 1927. Currier, L. W., Fluorspar deposits of Kentucky: Kentucky 

Geol. Survey, ser. 6, vol. 13, 1923. Bastin, E. S., The fluorspar deposits of Hardin and Pope Counties, 111.: 

Illinois Geol. Survey Bull. 58, 1931.
6 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

The stratigraphic sequence is shown in figure 3. Its most important features so far as the fluorspar deposits are concerned are (1) the 

great thickness (about 1,400 feet) of limestone beds between the Bethel 

sandstone and the Chattanooga shale; (2) the presence of a thin 

sandstone member (the Kosiclare sandstone) near the top of this 

limestone sequence; and (3) the numerous relatively thin beds of

EXPLANATION

OUTCROP OF LOWER PART 

OF CHESTER GROUP 

(AS KEY HORIZON)

DIKES, PERIDOTITE 

AND LAMPROPHYRE

VOLCANIC BRECCIA

FAULTS

FIGURE 2. Condensed geologic map of Illinois-Kentucky field. Area within dotted line and marked M is; 

that covered by geologic map (pi. 3). D represents outcrop of Devonian strata. Compiled from maps by 

Stuart Weller, Charles Butts, and A. H. Sutton. Courtesy of J. M. Weller.

alternating sandstones, limestones, and shales of the Chester group, in 

which siliceous beds are dominant.

IGNEOUS HOCKS

Dikes and sills of mica peridotite and lamprophyre cut the sedimentary rocks. The many known occurrences are broadly distributed 

through the field but are mostly well within the domed area. All theknown dikes trend northwestward parallel to the major axis of thedome, and, as they do not occupy fault fissures and some are cut by 

fluorspar veins, it is believed that they were intruded before theperiod 

of general faulting.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT

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FORMATION

Caseyville sandstone

Kmkaid limestone 1

Degoma sandstone J

Palestine sandstone

Menard limestone

Tar Springs sandstone

Glen Dean limestone 

Hardinsburg sandstone

Golconda limestone

Cypress sandstone

Bethel sandstone

Renault formation

« Levias Is. mem. 

u Rosiclare ss. 

> member

-i- >- Fredonia ls.mem 

c including

^ "sub-Rosiclare 

. sandy zone"

St. Louis limestone

Salem and Warsaw 

limestones

Keokuk and Burlington 

limestones

Chattanooga shale

THICKNESS(FI)

400

150-200

60

80-120

15- 20 

15 

100-150

50-70 

30-100

150

80-100

40-50 

50-100

25-60

25-30 

15-40

150- 200 

350

250 

550

400

FIGURE 3 Generalized columnar section of formations exposed in the Cave in Eock district. 

551386 44 2
8 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

At two places, siliceous clastic material, which from its apparent 

mode of occurrence and composition may be of volcanic origin, 

occurs in the Illinois part of the field.

A few veins of fluorspar border and cross dikes, but in general there 

is no common coincidence in occurrence of veins and igneous rocks.

FAULTS

The formations of the Illinois-Kentucky field are cut by a great 

number of normal faults, which range in displacement up to 1,500 feet. 

The major faults and many of the minor faults have northeast-southwest trends, approximately at right angles to the longer axis of the 

dome, and some of these extend far into the border areas of the dome. 

To the southeast the major fault zones become progressively more 

easterly in trend and merge into a narrow belt of east-west faults that 

extends into central Kentucky. On the west, another belt of fractures extends almost continuously into the highly faulted area of 

Ste. Genevieve County, Mo.

FLUORSPAR DEPOSITS

The fluorspar deposits of the Illinois-Kentucky field have been classifiedas(l)vein deposits, (2) "blanket," "bedding," or "bedding-replacement" deposits, and (3) superficial deposits. The vein deposits have 

been described in several previous reports, but their main features - 

will be summarized briefly here, in order to show their genetic relation 

to the bedding-replacement deposits that characterize the Cave in 

Rock district. The terms "superficial deposits" and "gravel deposits," 

applied to local concentrations in the clay mantle at the apices of veins, 

are of economic rather than geologic significance. The "bedding" 

or "bedding-replacement" deposits, which are mined only in the Cave 

in Rock district, were formed by replacement of limestone strata beneath denser"cap rocks; they have been called "blanket deposits" by 

some geologists. The term "blanket deposit" has also been applied 

by some to the superficial deposits of this area, but although it is a 

broad term as applied to ore deposits its generally accepted significance is distinctly different from the term as applied to either the 

"bedding-replacement" or "gravel" deposits of this field,6 and 

the terms "bedding" and "bedding-replacement" deposit seem far 

more satisfactory for use in the Cave in Rock district. The term 

"blanket" is particularly confusable with the use as applied to deposits 

of the Joplin district, and "chalcocite blankets" as used by Lindgren.7 

The term "bedded deposit" has been rejected because it suggests 

that the deposits were laid down by sedimentary processes.

 Fay, A. H., Glossary of mining terms: TJ. S. Bur. Mines Bull. 95,1920.

i Lindgren, Waldemar, Mineral deposits, 3d ed., pp. 712 et seq., New York, McQraw-Hill Book Co., 1928.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 9

The fluorspar veins were deposited in fault fissures, partly by direct 

precipitation of the fluorspar and partly by replacement of limestone wall rock and an earlier vein filling of calcite. The coarse vein 

calcite was derived probably in part from brecciated limestone wall 

rocks by recrystallization through the agency of ascending thermal 

waters, and in part by transportation from beds at lower horizons. 

In general, developments show that the larger fluorspar bodies are 

found where at least one wall is within the thick limestone formations 

below the Bethel sandstone. The thinner beds of limestone alternating with noncalcareous formations in the Chester group, above 

the Ste. Genevieve limestone, were apparently much less favorable 

sites of deposition for large or continuous ore bodies.

The veins commonly pinch or disappear between walls of shale, 

or shale and sandstone. This lithologic selectivity, together with 

the marked degree to which calcite is replaced by fluorspar, bespeaks 

the strong influence of calcareous material in the veins and wall rocks 

on the fixation of fluorine from the original solutions.

Sulfides, chiefly galena and sphalerite, are prominent in some veins 

and absent from others, and were deposited later than most of the 

fluorspar, after a period of fracturing that reopened old fissures or 

developed new minor fissures within the fluorspar bodies or even 

entirely new fissures not related to earlier faults. The sulfides partly 

replaced the calcite and fluorspar of earlier deposition although 

they were doubtless derived from the same ultimate source as the 

fluorspar. As the sulfides most commonly were deposited in fractures 

in the fluorspar and calcite veins, sulfide deposits not associated with 

fluorspar are comparatively uncommon, though there are many fluorspar veins devoid of sulfides.

Quartz was deposited in some veins, particularly along secondary 

fissures, so that some of the fluorspar bodies are siliceous. Some recrystallization of sandstone wall rocks took place to form quartzite, that 

by virtue of superior hardness and resistance to erosion forms quartzite "reefs," some of which mark the courses of fault zones.

'At the Daisy mine of the Rosiclare Lead and Fluorspar Co., vein 

material has been followed to a depth of 800 feet, the greatest depth 

yet reached in the Illinois-Kentucky field. How much deeper the 

vein continues is not known, nor are there sufficient data to indicate 

an average or consistent rate of decrease in fluorspar content. Also 

there is no apparent stratigraphic limitation of the veins. At a depth of 

about 400 feet one shaft in Kentucky penetrates the Warsaw limestone; 

the same formation is reached in one wall of the Rosiclare vein at 

500 to 600 feet. It is probable that the thermal gradient as well as the 

character of wall rocks determined the depth of the zone in which 

fluorspar was precipitated. The thermal gradient was doubtless 

relatively low, a condition that would be favorable for the formation
10 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

of fluorspar bodies through a thicker zone, than in cases where the 

gradient was relatively high, as in many of the western ore deposits. 

The calcareous nature of the wall rocks apparently was a very influential factor, providing the most favorable chemical environment for 

the fixation of fluorine as fluorspar. As there is known to be a total 

thickness of about 1,200 feet of limestone below the Ste. Genevieve 

formation in the fluorspar field, it seems reasonable to assume that 

fluorspar bodies may exist at much greater depths than has been demonstrated so far, and that in some places, they may extend well into 

or even through the Osage group.

This view may not seem to be supported by the apparently restricted 

stratigraphic range of the Cave in Rock bedding-replacement deposits, 

although their fluorine is believed to have had the same source as that 

in the veins. For reasons discussed more fully below, the localization 

of the Cave in Rock deposits is believed to have been dependent upon 

the presence of dense strata within beds of permeable strata, and 

minor fracturing. Extensive deposits of this type at greater depths 

than those at which such deposits have been found in the Cave in 

Rock district are not indicated by any available data, though such 

deposits might conceivably be found at any stratigraphic horizon 

within the temperature range of fluorspar deposition where structural 

and lithologic conditions were favorable. Fluorspar has been found 

in the St. Louis limestone, which underlies the Ste. Genevieve, about 

a quarter of a mile northeast of Tower Rock school, where it appears 

to occur along the bedding of the rock, but the exposures at this 

place were not sufficient to determine whether or not this fluorspar 

represents a bedding-replacement deposit of the type that characterizes the Cave in Rock district.

GEOLOGIC FEATURES OF THE CAVE IN HOCK DISTRICT

TOPOGRAPHY

The Illinois portion of the fluorspar field lies in an area of maturely 

dissected plateau land, that some geologists regard as a spur of the 

Ozark plateau.8 Many of the divides are narrow, nearly flat-topped ridges 

and the intervening valleys are likewise narrow and V-shaped. (See 

pi. 1.) The summits of the divides for the most part reach a general 

altitude that, according to Salisbury,9 marks an imperfect peneplain 

developed by early erosion of the uplifted Ozark Plateau. Most of the 

present ridge tops and mesalike areas are remnants of this plain, the 

uplift of which quickened the streams and caused a deepening of the 

valleys, with a consequent development of bluff like valley walls in 

places where the bedrock formations were most resistant. Some of 

the exceptionally straight and persistent valleys, bluffs, and ridges

 Weller, Stuart, and others, The geology of Hardin County, 111.: Illinois Qeol. Survey Bull. 41,1920, p. 19. 

9 Idem, p. 47.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 11

mark the positions of faults, whereby bedrock formations of different 

degrees of resistance were brought into juxtaposition previous to the 

uplift and dissection of the peneplain. Subsequent erosion caused 

the more resistant beds in one wall of each of the faults to form the 

present bluffs and ridges.

Several areas bordering the Ohio River, o'n the other hand, are 

either flood plains of this stream or areas underlain by thick and 

soluble limestone formations. The northern and eastern parts of 

Cave in Rock township in particular comprise a broad limestone area 

whose altitude (about 400 to 460 feet above sea level) is roughly 150 

to 200 feet lower than the summits of the adjacent area. In this lowland, underlain by the St. Louis and Fredonia limestones, much solution of the limestones by underground waters has taken place, so that 

many sinkholes have been developed, and a thick mantle of insoluble 

residual clay soil has accumulated. Such an area is particularly 

difficult ground for prospecting, even though the lithology and structure of the rocks may be favorable, for the ordinary indications of 

faults and veins or other mineral bodies are generally completely 

obscured.

On approaching Cave hi Rock district from the south, one is faced 

by a steep bluff that rises about 200 feet above the general level of a 

lowland hi which there are numerous sinkholes. The escarpment 

continues northwestward for a mile without being interrupted by 

prominent valleys. Midway in this stretch there is a broad indentation as if a block were offset by cross faulting; there is no certain 

evidence of a fault here, however. At the northeast end of this mile 

a pronounced steep-walled valley having a N. 40° W. trend cuts 

through the escarpment. The regularity of the drainage pattern of 

this valley and its tributaries suggests adjustment to geologic structures, particularly faults, and surface geologic evidence together with 

geophysical data indicate that some minor faults are present. Plate 2 

displays the drainage pattern of the district, and the geologic map (pi. 

3) shows where available data suggest the presence of faults.

West of the plant of the Benzon Fluorspar Co. (see pi. 1), the escarpment broadens, and from a point about a quarter of a mile west of 

the plant it trends approximately N. 40° W. for a distance of threefourths of a mile to Peters Creek. There may be some'structural 

significance in the fact that this direction of trend is practically parallel to a minor fault a quarter of a mile west of the Benzon plant, and 

is also parallel to the previously mentioned valley at the northeast end 

of the mapped area.

Near the middle of the northwestward-trending escarpment a 

broad low saddle separates the escarpment from a hill to the southwest. 

This saddle is nearly bisected by two very minor creek valleys in
12 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

alinement, heading toward each other, and draining to the northwest 

and southeast, respectively. The axes of these valleys trend about N. 

30° W., approximately parallel to the trends previously mentioned. 

Some prospect holes on, the-saddle, between the valleys, have shown 

fluorspar, but there are no rock exposures, so that the existence of a 

fault trace along these valleys could not be proved.

An eighth of a mile southwest, the broad low saddle connects with 

the north end of Lead Hill, a short, narrow, flat-topped, north-south 

ridge about half a mile in length. Lead Hill is a buttelike feature in 

which the underlying strata are nearly horizontal, and which has been 

isolated from Spar Mountain by erosion. The crest of Lead Hill is at 

an altitude of 660 feet above sea level, or 160 to 170 feet above the level 

of the surrounding plain. Several fluorspar prospects and small mines 

are situated around the south end and along the east side of the ridge.

The top of the Spar Mountain mesa is at an altitude of slightly more 

than 670 feet above sea level at a point on the edge of the escarpment 

a few hundred feet west of No. 2 shaft of Victory Fluorspar Co. (see 

map, pi. 1). The general altitude of the top of the escarpment along 

the south side of Spar Mountain is about 640 to 650 feet. The top 

surface of the mesa slopes gently northward from this locality nearly 

to the northwest limit of the mapped area where the general altitude 

is 550 feet along a narrow northwestward-trending belt. On the 

northwest side of this relatively low belt, the surface rises comparatively steeply to an altitude of slightly more than 700 feet, forming 

another escarpmentlike feature trending approximately N. 50 E. to 

the limits of the mapped area. This escarpment continues the line of 

Peters Creek, north of Lead Hill, and marks the zone of faulting 

known as the Peters Creek fault zone, the only major fault zone 

known to exist in the district. The similarity in trend of this escarpment to the trends of several minor valleys on Spar Mountain (pi. 2) 

and to the general trend of the escarpment that forms the southeast 

side of the mesa suggests that the positions of these valleys may have 

been controlled by faults, but this interpretation is not supported by 

any available stratigraphic evidence.

To the northwest of Peters Creek fault escarpment the general 

surface merges into the characteristic mature topography of Hard in 

County. *

The lowland comprising the southern and eastern parts of the 

mapped area is part of the large area of sinkholes that, as already 

mentioned, covers a large proportion of Cave in Rock township. In 

the mapped area, this lowland is directly underlain by the Fredonia 

limestone, the top of which is the chief horizon for fluorspar deposits. 

This horizon is exposed close to the top of Lead Hill at its south end 

and midway up the Spar Mountain escarpment, but as fluorspar has 

also been prospected at a lower stratigraphic horizon, on a knoll
PABT 1. GEOLOGY OF CAVE IN ROCK DISTRICT 13

in the flats east of the escarpment, a part of the lowland has been 

included in the map of the district.

The lowland has a gently rolling surface and contains several 

broad, shallow depressions, or sinkholes, which are drained through 

underground channels. The largest sinkhole is in the southeastern 

part of the mapped area and covers nearly half a square mile of this 

area besides nearly a square mile of adjacent territory to the east and 

south. The rim of this sink is approximately at an altitude of 420 

feet. In times of continued and heavy rains the sink has filled with 

water nearly to the 400-foot level, forming a broad shallow lake almost 

a mile across; generally, however, the submerged area is much less. 

Drainage of this sink is very slow at times and a considerable body of 

water may stand in it for several months. Many small sinks are scattered over the rest of the mapped lowland, and at times contain ponds 

that are drained veiy slowly. At times the sinkhole ponds extend 

over the roads leading from the main highway to the mines so that, 

in order to maintain access, it is necessary to elevate sections of the 

roads by filling.

There are no physiographic features in the lowland area that suggest influence of geologic structures upon the topography.

STRATIGRAPHY 

GENERAL STATEMENT

All bedrock formations exposed in the Cave in Rock district are of 

sedimentary origin; they consist of limestones, sandstones, and* shales 

belonging to the Mississippian and Pennsylvanian series of the Carboniferous system. Igneou^ rocks, though known to be present in 

other parts of Hardin County, have not yet been discovered in this 

district.

The formations are nearly horizontal, with a slight general regional 

dip to the north and northeast, except where they have been disturbed 

by local faulting that has produced slight to moderate dips in other 

directions.

The lowest unit exposed in the area is the Fredonia limestone member of the Ste. Genevieve limestone, which constitutes the bedrock of 

the lowlands and the lower parts of the slopes in the southwestern and 

southern parts of the area. Successively higher formations of the 

entire Chester group are exposed toward the north, the sequence 

being broken in places by faults. One formation, the Palestine sandstone, is not exposed, because of faulting, but all other formations 

of the Chester group are exposed at one or more places in the district. 

The highest formation exposed is the Caseyville sandstone, of lower 

Pennsylvania age, which caps the escarpment of a down-faulted 

block of the Peters Creek fault zone in the northwest part of the dis-
14 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

trict. The distribution of outcrop areas of the numerous formations 

is shown by the geologic map, plate 3, and a generalized columnar 

section of the formations is shown in figure 3.

DESCRIPTIONS QF FORMATIONS

BY O. E. WAGNER, JR.

SUSSISSIPPIAN SERIES

/

ST. LOUIS LIMESTONE

Although it is not exposed within the limits of the mapped area, 

the St. Louis limestone crops out not far to the south, along the Ohio 

River, extending continuously from Hosick Creek, about 2 miles east 

of Elizabethtown, to a point about half a mile east of Cave in Rock. 

The belt of outcrop is about 2 miles wide at its widest point. From 

this belt the formation dips northward under younger rocks of Mississippian age, and in the Cave in Rock mining district it lies beneath 

the surface, being probably about 100 to 125 feet below the base of 

the escarpment at the Benson Fluorspar Co. mill.

The St. Louip limestone is commonly a hard, dense rock, mostly 

dark blue or almost black in color on fresh surfaces and light bluishr 

gray and smooth when exposed to weathering. Some beds of crystalline limestone occur in the formation; oolitic beds are almost entirely 

absent. Chert is quite common and is usually dark colored where fresh 

and lighter where exposed to weathering; it occurs as lenticular and 

irregular masses in planes parallel to the bedding of the limestone. 

The thickness of the St. Louis limestone in Hardin County, 111., is 

estimated by Weller to be about 350 feet. The exact position of its 

contact with the overlying Fredonia limestpne is very difficult to determine but it is usually placed below the lowest oolitic beds in the 

section.

Fluorspar has been found in the St. Louis limestone at several small 

prospect pits located in the SW^SEtf sec. 17, T. 12 S., R. 9 E., 

about a quarter of a mile northeast of Tower Rock school. It appears 

to occur as deposits along the bedding of the rock.

A little mining, chiefly of lead carbonate (cerussite) has been done 

also in the SWKNWJi sec. 16, T. 12 S., R. 9 E., in what appears, from 

the chert on the mine dumps., to be St. Louis limestone. Slickensided 

rock on the mine dumps indicates a fault, but no exposure of the fault 

was seen.

STE. QENEVIEVE LIMESTONE

The Ste. Genevieve limestone formation consists of three members, 

the Fredonia limestone (oldest), the Rosiclare sandstone, and the 

Levias limestone (youngest).

Fredonia limestone member. The Fredonia limestone is the lowermost and thickest member of the Ste. Genevieve limestone. It con-
GEOLOGICAL SURVEY BULLETIN 912 PLATE 2

MAP SHOWING DRAINAGE PATTERN OF CAVE IN ROCK DISTRICT.

PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 15

sists essentially of even-bedded limestone which is variable in lithologic 

character, but which, especially in the upper 75 feet, is markedly 

oolitic. The oolitic beds are white on weathered surfaces and in places 

show cross bedding, especially at and near the top of the member. 

Interbedded with the oolitic beds are beds of gray limestone, of which 

some are crystalline and fossiliferous and some are dense and almost 

as fine-grained as lithographic limestone, breaking with a conchoidal 

fracture. The crystalline beds weather light gray, either with smooth 

surfaces or with surfaces that are studded with fossils, prominent among 

which are the elliptical stem plates of the crinoid Platycrinus penicillus. Some of the dense beds weather with smooth buff-colored 

surfaces.

The Fredonia limestone underlies the extensive sinkhole plain in 

the southern part of the district, but outcrops are scarce there and 

are usually found only in some of the gullies and in the deeper sinks. 

This limestone is well exposed arorind the base of Lead Hill and along 

the Spar Mountain escarpment from the Crystal mine southwestward 

and westward to the easternmost branch of the Peters Creek fault 

zone; outcrops are generally quite good in these areas.

From 50 to 65 feet below the top of the Fredonia limestone member 

there is a markedly cross-bedded, sandy limestone, which is known 

locally as the "sub-Rosiclare sandstone" and which ranges from a few 

inches up to 11 feet in thickness. It can be traced around the foot of 

the Spar Mountain escarpment from the old lower workings of the 

Cave in Rock mine eastward and northeastward to the low hill south 

of the Crystal mine. To the west, on Lead Hill, it appears to have 

thinned out or is not of mappable thickness. In outcrop this rock is 

extremely porous because of the leaching of the calcareous material by 

weathering processes.

The full thickness of the Fredonia limestone is not revealed in the 

district and the base has not been reached by either shafts or drill 

holes. Its total thickness in Hardin County, 111., is about 180 to 

200 feet 10 .

Rosiclare sandstone member. The Rosiclare sandstone member of 

the Ste. Genevieve limestone overlies the Fredonia limestone member, 

with which it appears to be conformable. It is fine grained, and porous 

and yellowish brown where weathered; its beds are usually thin, 

averaging about 2 inches in thickness, but in places, as on the north 

end of Lead Hill, they are thicker and resemble sandstone beds of 

higher formations; generally, however, the Rosiclare sandstone can 

readily be distinguished from sandstone of the Chester group. The 

unweathered Rosiclare sandstone is a very sandy limestone or calcareous sandstone which is gray in color and quite compact. The

w-Weller, Stuart, and others, The geology of Hardin County, HI.: Illinois Geol. Survey Bull. 41, pp. 113- 

114,1920.
16 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

leaching out of the limebyweatheringproducesthesandstoneofporous 

texture. In thickness the Rosiclare sandstone member varies from 

15 to 40 feet, but the average thickness is about 25 feet.

At the base of the Rosiclare sandstone, and resting upon the 

Fredonia limestone, is a green sandy shale which varies considerably 

in thickness within short distances. On the north end of Lead Hill 

this shale is apparently absent, but an eighth of a mile northeast, in 

an old shaft on the Martin property, 8 feet of the shale was penetrated. 

Ordinarily its thickness ranges from 6 inches to a foot or slightly more.

The Rosiclare sandstone is exposed around Lead Hill and along 

Spar Mountain escarpment from the east branch of the Peters Creek 

fault zone (fault No. 3, pi. 3) eastward to the edge of the district. It 

is exposed at the top of the south end of Lead Hill, but because of its 

gentle northerly dip its outcrops are 70 to 80 feet lower at the north 

end of the hill. From the escarpment of Spar Mountain it dips northward and underlies younger formations. Its contacts, both with the 

underlying Fredonia limestone and with the overlying Levias limestone, can readily be traced. Its outcrop declines gradually northeastward along the escarpment so that in the eastern part of the mapped 

area, in the latitude of the Crystal mine, it passes below the surface. 

Northwestward along the southwest side of Spar Mountain it gradually leaves the escarpment and passes into a sinkhole area, where 

it is abruptly terminated by the Peters Creek fault. In the area 

between Lead Hill and Spar Mountain it appears at the top of the 

westernmost of two low knolls and around the sides of the easternmost. A large number of contacts with the Fredonia limestone can 

be seen in the many mine adits, open pits, and cuts in the district.

Levias limestone member, Overlying the Rosiclare sandstone member is the Levias limestone member of the Ste. Genevieve limestone. 

This limestone is almost indistinguishable, except for its stratigraphic 

position, from the Fredonia limestone. It consists of interbedded 

oolitic limestone and gray, crystalline to lithographic limestone, with 

thin partings of shale.. It is thin-bedded at the base, and thickerbedded toward the top. The oolitic beds are not as white in this area 

as the oolitic beds of the Fredonia limestone.

Good exposures of the Levias limestone member can be seen on 

the escarpment in the vicinity of the old Cleveland mine, in a gully 

north of the logwa'sher on the Martin property, and on Lead Hill. In 

these places the Levias limestone member consists of fairly thick beds 

of limestone with a total thickness of 25 to 30 feet. Elsewhere the 

outcrops are partly covered, so that the contacts can be judged only 

from the topography, but nearly everywhere it is better exposed than 

limestone and shale beds of the overlying Renault formation.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 17

The name "Levias limestone" was proposed in 1932 by J. M. Weller 

and A. H. Sutton, to supplant the term "Lower Ohara," the term 

that appears in earlier reports on the Illinois-Kentucky field.

CHESTER GROUP

Renault formation. -The Renault formation consists of interbedded 

light gray to bluish-gray and brown limestone and gray, green, and 

red shale. The limestone beds are more or less politic, fossiliferous 

and argillaceous, and range in texture from very fine to coarse. They 

weather with smooth surfaces, which may be gray, but lighter than 

the fresh rock, or yellow to light brown. Ordinarily the Renault 

formation is covered with talus derived from the overlying Bethel 

sandstone, and in most of its outcrops only the limestone is exposed; 

fragments of a gray calcareous shale may be seen, however, on the 

dumps of several prospect pits. Nowhere was an exposure found in 

which a section of an appreciable thickness could be measured.

The Renault formation extends along the Spar Mountain escarpment, where its distribution is generally similar to that of the Levias 

limestone member. It also occurs on the top of Lead Hill and on the 

higher of the two knolls between Lead Hill and Spar Mountain. It 

is exposed in the bottom of a large sinkhole just north of the mill ponds 

of the Victory Fluorspar Co., on the sloping top of Spar Mountain.

As shown by records of diamond-drill borings, the interval between 

the Bethel sandstone and the Rosiclare sandstone ranges from 55 to 

80 feet. As this interval is occupied by the Levias member and the 

Renault formation, the latter probably ranges from 25 to 60 feet in 

thickness.

Bethel sandstone. A large part of the Cave in Rock district is 

directly underlain by the Bethel sandstone. This resistant formation 

constitutes the upper part of the Spar Mountain escarpment, and much 

of the sloping upland plain north of the escarpment consists of dip 

slopes of the sandstone.

Wherever seen along the top of the escarpment the Bethel sandstone is a massive irregularly cross-bedded sandstone that forms a 

steep bluff, skirted with talus which obscures the contact of the 

sandstone with the underlying Renault formation. The sandstone 

is fine-grained, compact, and yellowish-brown. Freshly broken pieces 

show characteristic large, uniformly distributed brown spots. Where 

the sandstone is exposed in the gullies and valleys along the dip slopes 

its massive character is not evident. The upper beds of the Bethel 

sandstone, near its contact with the Paint Creek formation, are thin, 

somewhat shaly, and lighter in color than the lower beds.

Paint Creek shale. The Paint Creek shale is exposed at only a few 

places in the district and the mapping of this formation was done 

mainly on the basis of topographic expression. The best exposures are
18 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

in the northeastern part of the district, along the northward-flowing 

creek about a quarter of a mile north of the Crystal mine, and in the 

creek valley an eighth of a mile farther northeast. A shale which is 

believed to be the Paint Creek shale is also revealed in an old shale on 

the middle fault of the Peters Creek fault zone.

The best measure of the thickness of the shale was obtained in the 

exposure north-northeast of the Crystal mine where the full thickness 

is 12 feet; in places, however, the shale may be as much as 20 feet thick. 

Wherever seen it is dark gray and thinly laminated, and contains 

interbedded thin layers of sandstone.

Cypress sandstone. The Cypress sandstone, like the Bethel sandstone, is for the most part massive, irregularly cross-bedded, yellowishbrown to white, fine-grained, and compact. The upper part is thinbedded and contains at least one conspicuous bed of light gray shale.

This sandstone forms the capping of the Spar Mountain mesa an 

eighth of a mile north of the Crystal mine, and also occurs in a strip 

that crosses the northern part of the area. This strip is narrow near 

the Peters Creek fault zone, where the dip is relatively steep, and 

broadens out eastward where the dips are not so steep. Two of tire 

blocks of the faulted zone on the west side of the district, have been 

mapped as Cypress sandstone, but the identification is not made with 

absolute certainty. \

No complete section of the Cypress sandstone is exposed in the area, 

but from data obtained in adjoining areas it is estimated to be 100 to 

125 feet. There is a marked unconformity at its base.

Golconda limestone. The Golconda limestone is made up of three 

intergrading parts, the upper part consisting of coarse-grained, oolitic 

limestone, the middle part of shale, and the lower part of argillaceous 

limestone and shale. "It is the most poorly exposed formation in the 

district. It has been mapped partly from scattered exposures in the 

gullies along the base of the Peters Creek fault escarpment in the 

northern part of the district, but its distribution has to some extent 

been inferred from the topography. The scattered exposures in the 

Cave in Rock district are mostly of dark gray coarsely crystalline 

limestone, and do not reveal the shale which, to judge from better 

exposures in other parts of Hardin County, probably makes up the 

greater part of the formation.

The thickness of the formation in the district could not be determined, but in other places where it is better exposed the formation is 

from 100 to 150 feet thick.

Hardinsburg sandstone. The Hardinsburg sandstone is moderately 

fine-grained, compact, and white to gray where it is not weathered; 

weathered surfaces are darker, commonly dark brown. It contains 

some beds of sandy shale, especially near the base, which grade into
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 19

massive beds of sandstone resembling those of the Cypress and Bethel 

sandstones.

Exposures of the Hardinsburg sandstone are meager, and in no 

place can a complete section of it be seen. It is exposed in short, 

narrow bands at several places along the Peters Creek fault zone. In 

the extreme northern part of the district, a band of this sandstone 

diverges from the fault zone and extends eastward, through a broader 

area of outcrops, beyond the mapped area. Along the Peters Creek 

fault escarpment the talus from the Caseyville sandstone covers most 

of the Hardinsburg sandstone. A good exposure, showing regular beds 

of white to gray sandstone, occurs at the northern end of the mapped 

area, along an old road west of the present Forest Service road.

No exact determination of its thickness was possible, but the formation is probably about 100 feet thick.

Glen Dean limestone. Like the Golconda, the Glen Dean limestone 

is made up of both shale and limestone beds, but limestone predominates. Its only exposures in the mapped area are in the small block 

northwest of the Peters Creek fault zone, along^ the valley of Peters 

Creek. 'Outcrops of the gray, crystalline limestones just below the 

contact with the overlying Tar Springs sandstone, can be seen along 

the wagon road on the west side of this block. On the tributary to 

Peters Creek just east of the house in the southwest corner of the 

block there is an outcrop of crystalline, gray, fossiliferous limestone; 

the fossils include the bryozoan Prismopora serrulata, which is characteristic of the Glen Dean limestone. Blocks of a brecciated, darkgray, crystalline limestone, with calcite veins, that is believed to be the 

Glen Dean were seen on the dump of an old prospect shaft near the 

north end of the middle fault of the Peters Creek fault zone.

At no place in the district is the full thickness of the Glen Dean 

limestone exposed. In other parts of Hardin County it attains a 

thickness of from 50 to 70 feet, according to Weller. 11

Tar Springs sandstone. The Tar Springs sandstone, like the Glen 

Dean limestone, is well exposed northwest of the Peters Creek fault 

zone. It is at the surface in a large part of this block, and is probably 

present also in the block bounded by faults 1, 2, and 6. (See geologic 

map, pi. 1.)

The Tar Springs sandstone is a moderately fine-grained, yellowishbrown sandstone which occurs as uneven beds, most of which are thin. 

In this district the formation is characterized by carbonaceous fragmentary plant remains, which are especially abundant in the shaly 

beds that occur near the base and middle of the formation.

The thickness of the Tar Springs sandstone appears to be about 

100 feet or slightly less.

11 Weller, Stuart, and others, The geology of Hardin County, HI.: Illinois Qeol. Survey Bull. 41, p. 195, 

1920.
20 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

Vienna limestone. Overlying the Tar Springs sandstone in the block 

between faults 1 and 5 (see pi. 3) is a limestone whose lower part is 

thick-bedded, dark gray, and coarsely crystalline. This limestone 

weathers with dark-brown surfaces that appear much like the surfac& 

of a weathered sandstone. The upper beds of the limestone are dense 

and almost black, and they break with a sharp, chertlike fracture. 

Their weathered surfaces are dark greenish-brown, and pitted. The 

limestone is about 15 feet thick and probably corresponds to the 

Vienna limestone of the standard Chester section.

Waltersburg sandstone. Overlying the Vienna limestone, and occurring in the same fault block is a thin-bedded, fine grained, shaly sandstone, which from its position in the section is thought to be the 

Waltersburg sandstone. It is from 15 to 20 feet thick.

Menard limestone. Above the Waltersburg sandstone are beds of 

dense light-gray fossiliferous limestone that contain a large form of 

Spirifer increbescens. These beds weather with smooth brown surfaces. 

They dip steeply and contain numerous "gash" veins of calcite. 

Although they are not lithologically similar to the typical Meuard 

limestone they have" been mapped as Menard because of their 

containing large Spirifers that appear to be characteristic of the 

Menard limestone elsewhere.

Palestine sandstone. No surface exposures of the Palestine sandstone were observed in the district, but the rock probably underlies 

the younger formations west of fault No. 4.

Clore limestone, Degonia sandstone, Kinkaid limestone. The three 

uppermost formations of the Chester group occur along the escarpment in the block west of fault No. 4 and north of fault No. 5 (pi. 

3). They are poorly exposed because of the covering of talus derived 

from the Caseyville sandstone, which crops out in the upper part of 

the escarpment. Because of the poor exposures as well as the thinness 

of both the Clore limestone and Degonia sandstone the three formations have been mapped as a unit.

The Clore, which consists of thin beds of limestone and shale, is probably covered in this area at least no beds were seen which could 

definitely be correlated with it..

The Degonia sandstone consists of thin-bedded, fine-grained, 

yellowish-brown, shaly sandstone. Only two exposures of it were 

found, both in gullies along the escarpment, and its thickness could 

not be measured.

The Kinkaid consists largely of limestone with inteVbedded shales. 

The limestones are dense and dark gray, and weather with lightercolored surfaces. In this respect they somewhat resemble the typical 

Menard limestone. Exposures of the Kinkaid limestone can be seen 

along the wagon road on the southward-facing escarpment at the
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 21

western border of the mapped area, just north of the east end of 

fault No. 5.

The thickness of the Kinkaid limestone is variable because of the 

unconformity between it and the overlying Caseyville sandstone.

PENNSYIVANIAN SERIES 

CASEYVILLE SANDSTONE

Resting unconformably upon the Kinkaid limestone there is a 

series of thick, massive beds of sandstone and conglomerate, with 

thinner shale beds, representing the Caseyville sandstone of the Pottsville group. These beds form the high cliffs along the escarpment in 

the northern part of the district. Much of the part of the formation 

that occurs in the area mapped consists of conglomerate beds.

STRUCTURAL GEOLOGY

The larger structural features of the Illinois-Kentucky field, and 

the relation of the Cave in Rock district to this field, are described on 

pages 5-10.

GENERAL, ATTITUDE OF BEDS

The bedrock formations of the Cave in Rock district are nearly 

horizontal, except where local faulting has produced sharp flexures, 

known as "drags," in the wall blocks of the faults. Because the district lie^s on the north-northeast side of a broad domal uplift (see page 

5) there is a general slight dip in that direction. In the southern part 

of Spar Mountain the average dip amounts to about 200 feet to the 

mile, or slightly more than 2°, and the average strike is about N. 80° W. 

In the north-central and northeastern parts of the district the average 

dip, apart from local relatively steep dips that may be the result of 

faulting, ranges from about 2° (185 feet to the mile) to about 3K° 

(320 feet to the mile), and the average strike is about N. 70° W. On 

Lead Hill the average strike is about N. 85° E., and the average dip is 

a little less than 2°.

Because of this general northerly dip, the chief fluorspar horizon 

the top of the Fredonia limestone will be found at progressively 

lower altitudes north and northeast from the present productive area. 

At a point in the extreme northern part of the mapped area where the 

surface altitude is 600 feet, for example, the limestone would apparently be reached by drill holes or shafts at depths of 500 to 550 feet. Obviously the initial cost of mine development at such a point would be 

considerably greater than where the deposits are now being exploited. 

Nevertheless, deposits as rich as some that have been developed at 

shallower depths in the district could be mined profitably at depths 

appreciably greater than 500 feet.
22 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

FOLDS

There are no prominent or pronounced folds in the Cave in Kock 

district. The dips of the strata are the result partly of regional 

doming and partly of very minor, local folding. The broad doming of 

the Illinois-Kentucky field (see page 5) developed peripheral dips 

that range up to 20°, though rarely more than 15°, and northerly or 

northeasterly dips of this order of magnitude should not necessarily be 

interpreted as local structures. Such dips would be the normal result 

of the general doming, for the Cave in Eock district lies on the northeast flank of the dome. Northeasterly dips in excess of 20°, however, 

may commonly have some local structural significance, provided there 

is confirmatory evidence of local deformation. Dips in other directions 

than north or northeast, even though relatively low, may be highly 

significant, possibly as indicating local minor folds, but more probably 

as being related to local faulting. Such features ar,e considered below 

in connection with minor faults.

Small minor folds and gentle warpings, discernible in the mine 

openings, appear to follow definite directions, as if related to definite 

systems of fractures, and the degree of mineralization appears to be 

related in many places to such minor flexures. A general structural 

pattern that seems significant has emerged from the detailed mapping 

and contouring of the shale bed at the base of the Rosiclare sandstone. 

The details of these minor folds, and their apparent relations to 

fluorspar concentration, are described and discussed*on pages 41-42. In 

general the axes of the minor folds may be grouped into two sets, one 

with axes striking N. 50° W. to N. 65° W., and the other with axes 

striking N. 40° E. to N. 50° E. Thus the first set, which is the more 

prominent of the two, is nearly normal to the Peters Creek fault 

zone and the second set is nearly parallel to this fault zone. These 

directions are also generally parallel to the trends of the minor fissures 

in the mines.

As minor folds or flexures, with associated fissures, seem clearly to 

have localized ore deposition at least in some places they should be 

important guides in prospecting for new ore bodies. But to discover 

them from surface indications is practically impossible because of the 

scarcity of good rock exposures in the mapped area, except along the 

Spar Mountain escarpment. For the most part only observations 

near the contact of the Fredonia limestone and Rosiclare sandstone 

would be of particular value, for in such places the stratigraphy is 

especially varied through a narrow range, and there is, moreover, 

reason to believe that many of the folds and minor fissures weaken or 

die out entirely in overlying formations. The Fredonia-Rosiclare contact lies deeply buried in most places north of the escarpment. Drilling to determine altitudes of this horizon and thus to contour it would 

ordinarily prove impracticable because of the generally small sizes of
GEOLOGICAL SURVEY BULLETIN 942 PLATE 4

480

\ \\\ x A\V \ V \X \ X \

M\X vVa 4U-- ^

"- CLEVELAND

-"--LEAD0 

'WEST'MORRISON

32*007

OXFORD^-. 

PITS -;

(6OO

EXPLANATION

 -..... - OUTCROP LINE - -- - MINE ROOFS

- -INTERPOLATED FROM DRILL RECORDS AND 

OTHER SOURCES 

CONTOUR INTERVAL, 5 FEET

Jtr 

'

500 1000 1500 2000 250O FEET

BY C.G. DICKENSON, 1935

CONTOUR MAP OF BASE OF ROSICLARE SANDSTONE MEMBER IN DEVELOPED AREA OF CAVE IN ROCK FLUORSPAR DISTRICT.

Interpreted from mine data, drill records, and surface exposures.

GEOLOGICAL SURVEY BULLETIN 942 PLATE 5

THICKNESS OF 

MINERALIZED ZONE

5'T07'||]0 T03'

CONTOUR ON TOP OF 

MINERALIZED ZONE AT 

BASE OF ROSICLARE

SANDSTONE

so too 200 FEET

C.COMF'OSITE Ol

ADJACENT OPENINGS

MAP SHOWING CONTOURS OF TOP OF MINERALIZED ZONE AND APPROXIMATE THICKNESS OF MINERALIZED ZONE IN SEVERAL MINES.

For profiles see figure 7.

PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 23

the folds. A very close spacing of drill holes as well as very accurate 

measurements of depths to the key horizon would be necessary, and 

the procedure would be very costly.

Plate 4 presents an interpretation of the structure of the Rosiclare 

sandstone throughout the developed area, and plate 5 gives a more 

detailed contouring of the roof of the ore bodies in several of th'e mines.

JOINT SYSTEMS AND RELATED FRACTURES

Joints of the type commonly found in rock formations have not 

greatly influenced the localization of fluorspar in this area, except 

where, by their fortuitous occurrence in mineralized zones, they may 

have offered additional space and facility for the spread of solutions. 

In general, such fractures are discontinuous both horizontally and 

vertically, and many of them are restricted to certain beds or formations that are relatively brittle. Being everywhere present they have 

in many places, helped to prepare a terrain for the reception of mineralizing solutions, and it is easy to ascribe to them a greater importance 

than they deserve. However, where joint systems conform in orientation to regional axes or planes of deformation by folding and faulting, 

and display locally a closer spacing than elsewhere, they may deserve 

special attention from the prospector. Such prominent development 

of a joint system may, indeed, betoken a more direct connection with 

fissure zones of deeper origin, or with local folds. Exposures in Cave 

in Rock district are generally inadequate for displaying the connection 

of joints with important mineral bodies. In a few places, however, 

prominent joint systems are exposed, and in at least two localities in 

the undeveloped part of the district they are not only in belts of possible minor faulting, but their strikes of N. 50° E. and N. 60° E., 

respectively, coincide approximately with one of the major structural 

trends of the region. These localities are on the larger tributaries of 

the prominent creek valley in the northeastern part of the mapped 

area.

Minor fissures of little or no displacement are commonly found in 

the mines, and in such relations with mineral bodies and local structures as to indicate a close genetic connection with them. These 

fissures, filled with fluorspar, appear as veins in the roofs of drifts and 

in several of the workings. They have been regarded as "leaders" 

in underground exploration and development. Their structural significance and relations to ore bodies are discussed in some detail in a 

succeeding section (page 45). In general, these fissures are narrow 

cracks, usually not more than an inch wide, and may be remarkably 

persistent. They probably are tension cracks formed by local warping, 

and in part at least trend parallel to the axes of the minor folds with 

which they may be connected. Many of these fissures probably belong

551386 44 3
24 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

to a system trending from N. 45° E. to N. 60° E., average about N. 50°E. 

These trends correspond to those of the minor folds whose axes 

range from N. 40° E. to N. 50° E. (See page 22.) Several fissures not 

belonging to this set were measured, and their trends are N.47° W. to 

N. 85° W. There is generally no appreciable displacement along 

these "roof cracks," but some of them may be directly over very minor 

fault fissures that penetrate through the floor to the ore horizon.

FAULTS

PETERS CREEK MAJOR FAULT ZONE

The Peters Creek fault zone has been described by Weller 12 and 

appears on his geologic map of Hardin County. That portion of the 

zone that borders the Cave in Rock district has been mapped on a 

larger scale and in greater detail, and appears on the geologic map 

accompanying this report (pi. 3).

The Peters Creek fault zone is one of the major structural features 

of the Illinois fluorspar field, and has been traced for about 11 miles, 

from the Ohio River nearly to the Saline River. The general strike of 

the fault zone is N. 45° to 50° E. Through the northeast third of the 

zone it consists of a single major fault. At a point in the north-central 

part of Cave in Rock district, the single fault passes southwestward 

into a fault zone comprising three major faults and at least three 

minor cross faults. The larger scale of the base map used for this 

report has permitted a closer identification and mapping of details 

than was possible at the time of Mr. Weller's field work. This mapping and the resistivity profiles (pi. 21) show that the northern part 

of the area is deformed by minor faults, which probably are connected 

with the Peters Creek fault zone.

Regarding the faults of the Peters Creek zone, Weller says "the 

downthrow is on the north side of each of these faults and the accumulated effect north of Peters Creek is a downthrow of 1,000 feet or 

more, nearly the full thickness of the Chester group." In the portion 

of the fault zone bordering the Cave in Rock district the aggregate 

displacement is about 500 feet. The stratigraphic details are shown 

on the geologic map (pi. 3), and detailed descriptions of the individual 

faults of the zone, as they occur in the mapped areas are given below.

Some prospecting has been done along the Peters Creek fault zone, 

particularly at the F. E. Martin and the Eureka prospects, about \% 

miles northeast of the mapped area, where shallow shafts have revealed 

a small vein containing fluorspar, calcite, a little sphalerite and galena, 

and a very little chalcopyrite. These prospects are on the part of the 

fault zone that apparently consists of a single break. Prospects on the 

zone within the mapped area have given no evidence of important

" Weller, Stuart, and others, The geology of Hardin County, 111.: Illinois Geol. Survey Bull. 41,1920.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 25

mineralization, but along this part of the zone, except for a short strip 

north and northwest of Lead Hill, the wall rocks at the surface are 

composed almost entirely of sandstone and shale belonging to the 

Chester group and the Caseyville conglomerate. Such formations are 

notably less favorable to prospecting, in general, than the lower, thick 

limestones. North and northwest of Lead Hill the southeast wall 

consists of Fredonia limestone at the surface, but the low, flat sink-- 

hole character of the area and the thick residual clay mantle are 

greatly obscuring features. Also, for a short distance along the course 

of the easternmost fault of the zone the Peters Creek valley has an 

exceptionally thick cover of alluvium, in which there would be no 

evidence of a subjacent vein. Farther southwest the fault zone enters 

an extensive area of Fredonia limestone, in which the stratigraphy 

and lithology should be favorable for prospecting, but in which the 

surface mantle is thick and obscuring. In this area prospecting is apt 

to be expensive.

DESCRIPTIONS OF FAUITS 

By O. E. WAGNER, JR.

For convenience of treatment the individual faults shown on the 

geologic map, plate 3, have been numbered Fl, F2, etc., and the 

following brief descriptions are correspondingly designated. Faults 1, 

2, 3, and 4 comprise the principal faults of the Peters Creek fault zone 

within the mapped area, and faults 5 to .17 inclusive are minor faults.

Fault 1, An excellent exposure of the northwesternmost fault of the 

Peters Creek fault zone, called No. 1, can be seen in the west bank of 

Peters Creek near the southwest corner of the district, at a bend where 

the creek is cutting this bank. At one place the fault plane is exposed 

and the sandstone to the east has been brecciated and altered to 

quartzite, a rock which is more resistant to erosion than the other 

rocks of the area and forms prominent bluffs and ridges nearby, known 

locally as "reefs." The strike of the fault here is N. 55° E. and its dip 

is steeply to the northwest. The footwall, on the southeast side, 

consists of Cypress sandstone, and the hanging wall is probably of 

Hardinsburg sandstone, so that the displacement is about 200 to 300 

feet.

About 500. feet upstream from this exposure a steeply dipping 

quartzite with quartz veinlets is exposed. The bedding strikes N. 50b 

E. and dips to the northwest. About 3,000 feet farther upstream 

another good exposure of this fault is to be seen on a northwestwardflowing creek which crosses the fault plane. The outcrop shows a gray 

shale that strikes N. 10° E. and dips 80° NW. The shale dips under 

Tar Springs sandstone and is thought to belong to one of the shale beds 

within that formation. The rock at the surface on the southeast or 

hanging-wall side of the fault is probably Tar Springs sandstone,
26 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

supposedly brought up on a cross fault designated on plate 3 as fault 

6 which has taken up most of the displacement.

Fault 2. Fault 2, southeast of fault 1, can be traced with reasonable 

certainty southwestward from an old prospect shaft, located on the 

actual fault plane near its northeast end, to a point just southwest of a 

cross fault, No. 7. Between these points its course is marked by out- 

 crops of quartzite, by slickensides, and by exposures of steeply dipping 

rocks; farther to the southwest its course is inferred from the character 

of the topography and from the areal distribution of the strata. The 

displacement northeast of fault 7 appears to be from 300 to 400 feet, 

but is reduced at fault 7 to about 100 feet.

Fault 3. Fairly good exposures of brecciated quartzite with quartz 

veins and of brecciated limestone with calcite veins can be seen along 

fault 3 near its northeast end, where its displacement is estimated to 

be about 200 feet, but to the southwest the displacement becomes 

smaller and breccia less common. Along most of the length of the 

fault the displacement is from 50 to 100 feet. Southwest of fault 7 the 

position of fault 3 is mapped on the basis of topography and stratigraphy, except on the ridge at the southwest end, where there are good 

exposures of quartzite.

Fault 4- Faults 1,2, and 3 converge within a short distance and the 

Peters Creek fault zone is represented to the northeast by what is 

called fault 4. This may be a zone of closely spaced faults, rather than 

a single fault. The displacement on fault 4 is estimated to be about 

600 feet at its southwest end, but it gradually diminished northeastward. No exposure of the actual fault plane has been found because 

the talus along the escarpment forms a cover that obscures the actual 

position and character of the fault trace or traces, but the course of the 

fault is marked by the presence of abundant quartzitic and brecciated 

material.

Fault 5. Fault 5 is across fault trending westward from about the 

middle of the Peters Creek zone to a major fault of northeasterly strike 

outside the mapped area. It is one of the most difficult to trace beause of the similarity of the limestones in the two walls, but steeply 

dipping beds and exposures of brecciated limestones veined with calcite are found along its course.

Faults 6 and 7. Two cross faults, numbered 6 and 7, are inferred 

from the stratigraphy and dips of the nearby rocks, but their locations 

and strikes have been only roughly determined.

Fault 8. Evidences of fracturing observed at a shallow prospect 

shaft on the saddle northeast" of Lead Hill are believed to indicate a 

minor break which is mapped as fault 8. The evidence for appreciable 

displacement and the exact position of the fault are uncertain.

Fault 9. Near the old shaft in the valley northeast of Lead Hill 

there are indications of a fault within the Fredonia limestone. This
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 27

fault, indicated as fault 9, is of undetermined, but probably small, 

displacement, and has a strike of N. 60° E. The limestone near 

the fault has been replaced by quartz to form a solid mass of gray 

quartzite. Limestone on the dump pile beside the shaft contains 

both fluorspar and galena.

Fault 10. In the lower working of the old Cave in Rock mine there 

is a zone of brecciated limestone containing fluorspar and a relatively 

large quantity of galena. Some of the limestone has been replaced by 

quartz to form quartzite. A fault striking about N. 20° W. is believed 

to pass through this zone, and is mapped as fault 10.

Fault 11. Fault 11, which has a strike of N. 55° E., is encountered 

in an adit penetrating the Levias limestone. It is revealed by the displacement of a bed of shale within this limestone, the shale being 3 

feet lower on the west side of the adit than on the east. Fissure veins 

of fluorspar in the limestone nearby all strike parallel to the fault.

Several of the prospects near this fault show bedding-replacement deposits of fluorspar in the Levias limestone and in limestone of 

the Renault formation.

Fault 12. The evidence for fault 12 consists in a slight displacement 

of the base of the Rosiclare sandstone, together with the discovery of 

a fragment of slickensided sandstone in the gully through which the 

fault is thought to pass. The displacement, as indicated by the difference of altitude of the base of the Rosiclare sandstone on each side 

of the gully, is about 10 feet. The downthrow is on the east, but the 

exact strike of the fault could not be determined. %

Fault 13. A fault with horizontal displacement and a strike of 

N. 70° W. is said to be crossed by the old adit just east of the plant of 

the Benzon Fluorspar Co. The limestone east of the adit has prominent fissures, 1 to 3 inches wide, filled with fluorspar, and strikes N. 

70° W. and N. 40° E.

Fault 14- In the workings of the Victory Fluorspar Co.'s mine No. 

1 there is a postmineralization fault of horizontal displacement. The 

fault is nearly vertical and strikes N. 70° E.

Fault 15. The workings of the Crystal mine reveal a postmineralization fault of small displacement, striking N. 55° E. A vein of barite, 

as much,as 6 inches in width, extends along this fault.

Fault 16. Fault 16 is considered to be one of the most important 

minor faults in the area. It is best exposed in the creek a quarter of 

a mile north of the Crystal mine. Here the Paint Creek shale on the 

foot-wall side is dragged along the fault plane so that it dips steeply 

to the southeast and the Bethel sandstone on the hanging-wall side 

has been distorted and altered to quartzite. In the quartzite along 

the fault plane sphalerite, chalcopyrite, and greenockite were found.

Fault 17. The mapping of fault 17 is based on rather doubtful 

evidence. Where the fault supposedly crosses the creek bed at this
28 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

locality, Cypress sandstone is exposed. The sandstone beds are considerably distorted and there are prominent joints with a strike of 

N. 50° E.

PREVIOUS INTERPRETATIONS OF FAULTING

In previous publications, as noted below, two faults have been inferred for which the present investigation has yielded no decisive 

evidence.

Bain's report 13 records a fault of east-by-south trend at the foot of 

the south escarpment of Spar Mountain. The topographic pattern 

here is somewhat suggestive of a fault zone, but there is no stratigraphic evidence of its existence. The vertical displacement along a 

fault here could not be more than a few feet, probably 20 to 30 feet at 

the most, to judge from the altitudes of the Rosiclare sandstone a 

. good horizon marker as exposed in Lead Hill and in the southern 

face of Spar Mountain.

Schwerin 14 describes a fault, which he calls the McWade fault, as 

extending eastward from the Peters Creek fault zone across the north 

end of Lead Hill, through the old Cave inRockmme, thence northeastward to the top of Spar Mountain, thence northward through the prominent sinkhole to the Peters Creek fault zone in the northern part of the 

mapped area. At several places along this course there are undoubted 

evidences of minor faulting, but^ that these points lie on a single and 

continuous fault was not demonstrated by the present work. Certain 

of the underground evidence cited by Schwerin, however, was not 

accessible at the time of the investigation. That more minor faults 

exist in the area than are indicated by field data at hand seems very 

probable, but such faults could be disclosed only by underground development. Theoretically, however, if faulting has occurred along the 

course indicated by Schwerin, it is probably represented by intersecting breaks, rather than by a single broadly curved fault, and probably 

one of these breaks follows a course similar to that suggested earlier 

by Bain. The apparently unbroken continuity of the Rosiclare sandstone in the narrow southward-extending promontory of Spar Mountain, where the Oxford open pits are situated, argues against the presence of an east-west fault through the promontory. If there is 

such a fault, certainly the displacement either vertical or horizontal 

must be very slight. To prove that a "McWade fault" exists, and 

crosses Spar Mountain, and to trace its course, would require extensive 

underground exploration, or accurate stratigraphic measurement of 

numerous drill holes. Such a fault, if it exists, might have considerable 

economic importance.

is Bain, H. F., Fluorspar deposits of southern Illinois: IT. 8. Geol. Survey bull. 255, p. 54,1905. 

i* Schwerin, Martin, An unusual fluorspar deposit: Bng. and Min. Jour., vol. 126, No. 9 (Sept. 1,1928), p. 

335.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 29 

GEOPHYSICAL EVIDENCES OF FAULTING

As a part of the cooperative project comprising a comprehensive 

study of the Cave in Rock district, M. King Hubbert of the State 

Geological Survey made 30 geophysical traverses across the district, 

excepting Lead Hill and most of the sinkhole area. The traverses were 

run both parallel and perpendicular to the Peters Creek fault zone and 

were as nearly equally spaced as topography would permit. The details, 

results, and interpretations of this work are presented in part 2 of this 

volume. It should be pointed out here, however, that several resistivity breaks found in different parts of the area where surface evidence of 

faults is wanting, particularly in the north-central and northeastern 

parts, indicate the probability that these parts of the area are also 

probably deformed by minor faulting.

TOPOGRAPHIC EVIDENCES OF FAUITING EAST OF MAPPED AREA

The topographic form of the long, straight escarpment extending 

southeasterly from the prominent creek valley on the eastern edge 

of the mapped area suggests that it may have been formed by faulting, 

but it was not possible to extend the mapping and detailed field studies 

to include this area. The trend of the escarpment is such that it could 

have been determined by normal processes of erosion, because it is 

essentially parallel with the strikes of the beds that are exposed in the 

escarpment, but the shape and extent of the adjacent area of Fredonia 

limestone, southwest of the escarpment, suggests also a downfaulted 

block. Faulting along this escarpment might be of considerable importance in prospecting. However, fluorspar deposits are not known 

to exist along it although a small amount of prospecting has been done 

at a few points. The Rosiclare sandstone is exposed along the escarpment, and it has the usual general northeast dip, but there is no obvious 

discrepancy in altitudes between its outcrop here and the exposure of 

the same horizon along the east side of Spar Mountain. Moreover, the 

sandstone belts along both escarpments join in the creek valley to the 

north without apparent displacement, hence any faulting at the base 

of the southeasterly trending escarpment must have very small displacement unless the sinkhole area between them is part of a downfaulted block. Again, the presence of "sub-Rosiclare sandstone" as 

the capping of a knoll in the lowlands, within the mapped area, 

precludes the possibility of much displacement of the western part of 

such a block.

It is perhaps pertinent to call attention at this point to the presence 

of a bedding deposit in the sinkhole area, about three-fourths of a 

mile northeast of the village of Cave in Rock, and about the same 

distance from the southeastern part of the escarpment. This deposit 

(prospected in 1917) is at a much lower horizon than the Spar Mountain 

deposits, being near the base of the Fredonia limestone, but it appears
30 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

to be similar in structure and texture, and its presence here demonstrates 

the fact that mineralizing solutions have risen along minor fractures 

far outside of the mapped area.

FLUORSPAR DEPOSITS OF THE DISTRICT

GENERAL STATEMENT

The economic fluorspar deposits of the Cave in Rock district are 

elongated tabular or lenticular bodies that clearly have replaced certain beds of the thick Ste. Genevieve limestone. They are here called 

"bedding-replacement deposits," or "bedding deposits," for reasons 

discussed on page 8. Several distinctive features indicate that the 

stratigraphic, lithologic, and structural conditions controlling the formation of these deposits differ from those that governed the deposition of the vein deposits elsewhere in the field, although the sources 

of the mineralizing solutions and the time of formation were the same.

The distinguishing features of these deposits are (1) they occur at 

certain stratigraphic horizons of which one in particular the top of 

the Fredonia limestone has so far proved to be of particular importance; (2) they replace the host limestone along its bedding planes 

and preserve the coarser textural features of stratification and cross 

bedding; (3) they were deposited in readily replaceable horizons 

beneath dense, unreplaceable beds that form a well defined cap rock;

(4) their localization was favored by local very minor folds or flexures;

(5) they were deposited by solutions that penetrated laterally from 

minor fissures of little or no displacement, which, in turn, were greatly 

constricted in passing upward into the dense shale cap rock or that 

failed entirely to penetrate the cap rock; (6) their mode of replacement resulted in the development of characteristically layered ores, 

generally called "coon-tail spar" the layers marking stratification 

planes and cross-bedding laminae; (7) they are not connected with 

veins of commercial size.

The mode of occurrence and the form of the bedding-replacement 

deposits permit comparatively simple methods of mining and milling, 

and together with the high quality of much of the ore give them a 

distinct advantage over many of the vein deposits.

MINERALS OF THE DEPOSITS

The mineral composition of the bedding-replacement deposits is 

comparatively simple and resembles that of the vein deposits except 

that sulfides are much less abundant and more localized 15 in the bedding-replacement deposits than in the veins. The minerals present 

in the deposit are fluorite (fluorspar), calcite,.quartz, barite, galena, 

sphalerite, chalcopyrite, marcasite, smithsonite, and cerussite, all 

of which were introduced after formation of the host rocks, and a

15 See addendum, page 71.
GEOLOGICAL SURVEY BULLETIN 942 PLATE 6

A. PHOTOMICROGRAPH OF LIMESTONE SHOWING MICROSCOPIC QUARTZ CRYSTALS.

B. PHOTOMICROGRAPH OF FINE-GRAINED SILICEOUS BAND IN ORE SPECIMEN FROM

CAVE IN ROCK MINE.
GEOLOGICAL SURVEY BULLETIN 942 PLATE 7

SPECIMEN OF SILICEOUS MATERIAL FROM WASTE PILE OF CAVE IN ROCK MINE.
GEOLOGICAL SURVEY BULLETIN 942 PLATE 8

LIMESTONE BEDDING 

(PARTINGS)

APPROXIMATE SCALE APPROXIMATE SCALE

SHALE ROOF

= ' FINELY BANDED

SPAR 

HIGH GRADE

APPROXIMATE SCALE

SKETCHES OF PARTS OF FACES IN MINES ON SPAR MOUNTAIN. 

Show alternating bands of pure (shaded) and impure (unshaded) fluorspar, attitude of bands, and bedding and cross-bedding structure of fluorspar ore.

PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 31

ferriferous carbonate (probably ankerite) that was probably an original constituent of some of the limestone layers and which is observable 

in the more impure layers of the ores, where it has apparently escaped 

complete replacement.

Fluorite. The chief mineral of the deposits is fluorite (commercial 

name fluorspar). This mineral is much more coarsely crystalline in 

the purer layers of ore than in the less pure, siliceous or ferriferous 

layers. In large vugs and cavities, the majority of which appear to 

occur chiefly at the tops of ore bodies, just below the roof rock, exceptionally large crystals are common; some of these crystals measure 

more than 6 inches across the crystal faces. Fluorite generally crystallizes as simple cubes, ancl this is the prevailing form in these deposits 

also, but occasionally druses or large aggregates are found that display 

well developed dodecahedral faces modifying the edges of the dominant cube forms.

The fluorite ranges in color from colorless or white to various shades 

of amber, deep purple, violet, lavender, and blue. Large crystals 

attached to the walls of cavities commonly have an amber to colorless 

core and a purple shell, the two being sharply distinct. A similar color 

zoning is found also in the "combs" of coarser grains that make up 

the purer layers of the "coon-tail" spar. Amber and purple hues are 

more common than lavender and violet, and the blue and greenishblue shades are exceptional. The strong hydrocarbon odor obtained 

by breaking the purple variety suggests that its color is due to organic 

material, and the fact that purple fluorite bleaches out to pale shades 

when exposed to sunlight, as in dump piles, lends some support to 

this hypothesis. The blue and violet hues may, however, be due to 

the presence of metallic ions in the mineral, as some crystals of 

nearly colorless fluorite have been found in which halos of these colors 

surround inclusions of chalcopyrite. Small inclusions of pyrite and 

marcasite, on the other hand, have no colored halos. Comb structure 

is common in the pure layers of the ores (see below), and the relatively 

coarse-grained fluorite of a pure layer is marked by deeply colored 

material along the center of the layer; this colored portion is crystallographically continuous with the less strongly colored material 

commonly amber or colorless above and below.

Calcite ("calc"; " calcspar"). Calcite is. not particularly prominent 

throughout the bedding-replacement deposits, although it is an abundant mineral of the vein deposits and predominates in some parts of 

the veins. In part it occurs as small grains in the less pure bands of 

the ores, where replacement has been incomplete. To a large extent 

however,the carbonate grains of these layers are a ferriferous variety 

 perhaps ankerite instead of the pure calcium carbonate that forms 

the mineral calcite. Coarsely crystalline white calcite is locally abundant where it has crystallized in cavities and brecciated zones, as it has
32 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

done near fissures, particularly just beneath the roof rock. Some of 

these crystals are several inches in diameter and the masses are similar 

in appearance to the typical coarse calcite of the vein deposits.

A later generation of calcite is represented by well-formed crystals 

that were deposited upon crystals and drusy surfaces of fluorite in 

open spaces. Doubly terminated complex crystals of the mineral 

may be found in this situation. The crystals range from minute 

sizes up to several inches in length, and are mostly transparent or 

translucent, and colorless to buff-colored.

Barite (barytes; "heavy spar"} Barite is present at a few places, 

but is to be considered a very minor constituent of the deposits, and 

consequently is not of economic importance. It appears in vugs, 

cavities, and locally disturbed zones, generally in the upper parts of 

the deposits, and is practically entirely absent from the more massive, 

and compact, "banded" portions of the ore deposits. Apparently it 

is a mineral of relatively late development.

An excellent description of the mineral and its paragenetic relationships has been given by Bastin, 16 who regards it as a late secondary 

mineral replacing fluorite, calcite, and limestone.

Quartz. This mineral is generally present in the deposits, but in 

most of the developed fluorspar bodies it occurs so sparingly that 

simple milling will easily give a product that satisfies the commercial 

requirements. On the other hand, at a few places, the deposits 

obviously contain so much quartz that a satisfactory product cannot 

be obtained by the milling methods in use.

Minute prismatic crystals of quartz are disseminated in the oolitic 

limestone occurring in the ore bodies chiefly in the impure layers. 

Plate 6, A, is a photomicrograph showing quartz crystals in material 

from the bedding-replacement deposits. To -some extent these 

crystals probably represent silica transported in fluoriferous solutions, 

but some of it, especially in the impure layers, may well have recrystallized during the mineralizing epoch. Quite similar quartz crystals 

appear in the wall rocks of the vein deposits at some places, as in the 

"jasperoid"of the Mary Belle mine in Kentucky. 17

A third mode of occurrence is illustrated by specimens obtained 

from some dump material at the old Cave in Rock mine and from 

certain prospects on the east side of Lead Hill. These specimens 

show alternate layers of pure crystalline fluorspar and quartz. In 

one the 'quartz layers are composed of well-formed crystals arranged 

in comb structure; in other specimens (seepi.7) layers of crystalline 

purple fluorspar alternate with fine-grained layers of quartz. Under

i« Bastin, E. S., The fluorspar deposits of Hardin and Pope Counties, m.: Illinois Oeol. Survey Bull. 

68, pp. 60-62, 1931.

v Currier, L. W., Fluorspar deposits of Kentucky: Kentucky Geol. Survey, ser. 6, vol. 13, pp. 18, 70, 

92, 95, 1923.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 33

the microscope the quartz layers are seen to be clearly made up of 

quartz that has replaced the matrix, partly as perfect crystals, partly 

as elongated "feathery" crystals, and partly as fine-grained "matted" 

aggregates of nearly chalcedonic texture (see pi. 6, B). Some of the 

grains have outlines that suggest original clastic texture modified 

slightly by recrystallization. Many of the grains and crystals attributed to replacement show included patches and bands of minute 

calcite grains, and many of these bands are parallel with the crystal 

faces of the quartz grains that include them.

A fourth mode of occurrence of quartz is that of drusy surfaces 

of crystals upon the faces of fluorspar crystals. This quartz apparently 

belongs to a relatively late epoch of deposition.

Galena. The primary sulfide of lead, galena, is not abundant 

in most of the bedding-replacement deposits, though it is prominent in 

many of the vein deposits elsewhere in the field. Where found it is well 

crystallized, and appears within and upon the layers of relatively 

coarsely crystalline fluorspar, where it is intergrown with the fluorspar 

or else occurs as crystals upon fluorspar crystals. It has been found 

particularly at the old Cave in Rock mine, and at the C. M. Miller 

mine on the west side of Lead Hill. It was deposited relatively late 

in the mineralizing epoch, and hence was both contemporaneous with 

fluorspar and later.

Sphalerite (zinc blende). Only a few crystals of sphalerite were 

noted in the deposits that were studied18 though the mineral is quite 

common hi the vein deposits of the Illinois-Kentucky field and in 

some of the veins it constitutes recoverable ore bodies. Vein relations 

indicate that, like galena, it was deposited relatively late in the 

mineralizing epoch.

Chalcopyrite and marcasite. Chalcopyrite and marcasite occur in 

very small quantity as minute crystal inclusions in fluorspar. In the 

large crystals of fluorspar that show them best these sulfides are 

restricted to the outer zones of the host mineral, as if their deposition had begun later than that of the fluorspar, and had continued 

while fluorspar was still being deposited. The relation of colored halos 

to chalcopyrite inclusions has been mentioned (p. 31).

Cerussite and smiihsonite. The carbonates of lead (cerussite) and 

of zinc (smithsonite) have been observed in some of the deposits. 

The variety of smithsonite that is known, from its color and appearance, as "turkey-fat" has been found at Lead Hill as a coating on 

fluorspar crystals.

TEXTURE OF THE DEPOSITS

The fluorspar deposits of the Cave in Rock district show a well developed and characteristic layering, which gives a markedly banded

« Local concentrations of sphalerite in the Cave in Rock district have been reported since this report was 

written; see addendum, page 71.
34 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

appearance to exposed surfaces. From a study of the ores, the unreplaced limestone of the ore horizon, and the structures of the beds, it 

is believed that the ore layers reflect original stratification and cross 

bedding of the host rocks. The reasons for this belief are discussed 

below.

The typical ore consists of alternate layers of pure, coarsely crystalline fluorspar, and relatively impure fine-grained material that is, 

however, dominantly fluorspar. The layers range in thickness from a 

small fraction of an inch to several inches, mostly from% inch to linch. 

In places the layers are remarkably continuous, and uniform in thickness; in others they are short and bifurcated, and are also truncated 

by roof or floor beds or by other groups of bands. The pattern of the 

layers clearly simulates the cross-bedded structure of the host rock 

(see pi. 8).

A noteworthy feature of the coarser fluorspar layers is their prevalent crustification and comb structure formed by crystals that 

project toward the central zones of the layers, which are generally 

marked by vugs (see pi. 9). This structure shows that crystals of 

fluorspar grew from the outer borders of the pure layers toward their 

centers, but not quite filling the space originally occupied by rock 

material. Such crustification is common in veins formed, by deposition 

in open fissures; it is inconceivable, however, that closely spaced and 

extensive horizontal open fissures could have existed here and the 

comb structure is therefore attributed to progressive replacement. 

The zone of vugs may be centrally situated within the layer or it may 

be closer to one side than to the other (see pis. 10, A, 11), but it is 

nearly always recognizable from the presence of open spaces and also 

from a deeper coloration of the fluorspar bordering the vugs (see pi. 9).

In some of the ore the pure layers have few vugs and are colorless, 

but show a line of contact between the crystals from the two walls 

of each layer (see pi. 11). Some of the ore breaks readily along the 

vug lines, exposing surfaces of well-formed crystals (see pi. 9). A 

specimen from the east side of Lead Hill showed druses of quartz crystals in comb structure between each two layers of purple fluorite 

crystals, with the vug zones between the quartz layers; in these the 

fluorite layers were fine-grained and somewhat impure. Another 

specimen showed layers of similarly crustified quartz crystals alternating with layers of clear, coarsely-crystalline, purple fluorite.

The less pure layers in the ores consist of relatively fine grains of 

fluorite, carbonates, and in some layers quartz. Such layers show no 

comb structures or crustification. They are generally gray or brown 

and somewhat resemble original rock material, not only in color, but 

also in texture. Even the stratification caused by variations in the 

original texture and composition of the rock is clearly apparent in 

some of these layers. In several specimens collected at the Cave in
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 35

Rock mine such layers contained fossil shells and oolites that had been 

partly replaced by fluorite.

Plate 10, B, shows differences in texture and composition, as well as 

the lack of sharpness in the contact between the coarse and finegrained layers. Most of the impure layers contain unreplaced carbonates. Some of this is calcite but much of it consists of small 

rhombs of a ferriferous carbonate ankeritic or sideritic calcite as 

indicated by the indices of refraction and by the strong yellow-brown 

color of the weathered material. Under the microscope these grains 

commonly show outer zones that are stained brown with limonite, 

and some limonite extends into the grains along cleavage cracks (see 

pi. 12, B}. A difference in sharpness between the two contacts of an impure layer with adjacent pure layers (see pis. 10, A, 11) reflects original 

variations of texture and composition in the original limestone.

A study of several thin sections shows that most of the minut& 

secondary quartz crystals found in the Cave in Rock ores occur in the 

relatively impure layers, and a condition that also reflects differences 

in composition of the original rock layers.

The range in fluorspar content of the less pure, fine-grained layers is 

variable within wide limits; some of these layers contain as much as 

90 percent of CaF2 but in general their fluorspar content is much less and 

even in the highest grade ores probably averages less than 80 percent. 

These layers are not separated in milling the ores, however, and for 

much of the richer ore taken out the requirements for metallurgical spar 

of 85 percent of CaF2 and not over 5 percent of silica were met with 

very little milling1 of the ore. Toward the limits of the ore bodies, on 

the other hand, the average fluorspar content decreased so that marginal ores requiring carefully controlled milling were mined in places. 

It has also been the practice to mix such marginal ores with richer 

material from other parts of the ore bodies in order to obtain a product 

of standard specifications. The lateral boundaries of the ore bodies 

are thus economic in large part rather than structural, but the upper 

and lower boundaries are more apt to be stratigraphic and structural 

boundaries.

Specimens collected from the dumps at the lower adit of the Cave 

in Rock mine, and at the Miller mine on the west side of Lead Hill, 

consist of alternate layers of coarsely crystalline fluorspar, showing the 

usual crustification, and fine-grained, slightly porous quartzite. Thin 

sections of the quartzite reveal a texture that indicates replacement 

of impure limestone or calcareous sandstone by quartz. In general the 

grains of this quartz are interlocking, elongated,"feathery"edged, or 

sharply euhedral, rather than rounded like sand grains (see pi. 6, B). 

The faces of the fluorspar crystals of the other layers are commonly 

coated with druses of quartz crystals.
36 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

PARAX3ENESIS OP THE DEPOSITS

The order of deposition of the several minerals in the bedding deposits appears to be generally consistent with the interpretations applied 

to the vein deposits, as presented in the writer's earlier report on the 

Illinois fluorspar field.19 The assemblage of minerals is the same as 

in the veins, but prominent amounts of course vein calcite are lacking 

in .the bedding-replacement deposits, and consequently the replacement relation between fluorite and calcite, so obvious in the veins, 

is not clearly displayed. The paragenetic relations of quartz, also,

FIGURE 4. Sketch of exposure at prospect on east side of Lead Hill showing incipient mineralization along 

joints and bedding planes. Stippling indicates fluorspar, light lines indicate bedding and cross bedding 

of limestone, and heavy lines indicate joints.

are more complex in the bedding-replacement deposits than in the 

veins.

FLUORSPAR

The fluorspar of the Cave in Rock deposits clearly replaces carbonate grains, oolites, and the fine-grained calcareous matrix of the limestone country rock. Pseudomorphs of oolites and fossil fragments 

may be observed. In many places it is clearly seen that the mineral 

follows stratification planes and layers of the host limestone, extending as replacement layers or streaks from small cross-cutting fractures. 

(See fig. 4.) In general, the writer believes that the characteristic 

layering of the fluorspar bodies is due primarily to replacement of

' Currier, L. W., Fluorspar, lead, and zinc, in Weller, Stuart, and others, The geology of Hardin County, 

111.: Illinois Geol. Survey Bull. 41, pp. 257-260.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 37

bedding and cross-bedding layers. This matter is discussed further 

in the subsequent section dealing with origin of the deposits.

In some weathered exposures of the unmineralized Fredonia limestone at the ore horizon, layers of relatively pure calcitic material 

alternate with layers of less pure, ferriferous carbonate that are made 

prominent by oxidation of the iron content, so that the rock surface 

shows a series of alternating gray and brown bands. In places of light 

or moderate mineralization irregular layers of clear, pure, 'crystalline 

fluorite alternate with impure ferriferous layers in which brown carbonate rhombs are abundantly disseminated (see pis. 10, B, 12, A). 

Contacts between the layers are in part quite sharp, and in part gradational. It is inferred that the iron-bearing carbonate material was 

less readily acted upon by fluorine to form calcium fluoride than was 

the pure calcitic material. The fluorspar of the nonferriferous layers 

is coarsely crystalline and clear, and the microscope reveals an inconsequential amount of other material. In many of the relatively finergrained layers, however, the fluorspar is in the matrix, between and 

surrounding rhombs and grains of ferriferous carbonate (see pis. 10, 

B, 12). From this mode of occurrence it is inferred that the alternation 

of coarsely crystalline pure fluorspar with impure layers of much finer 

texture reflects differences of composition and texture in the orginal 

layers of the rock.

Schwerin 20 has ascribed the crystal-lined cavities and vugs to reduction of volume attending the formation of CaF2 from CaCO3 . The 

stoichiometrical volume change required by this reaction, however, 

obviously would form a larger amount of open space than is now apparent; therefore it seems necessary to assume that additional CaF2 was 

precipitated from solution. Such a process would explain, also, the 

lack of structural distortion of the replaced beds.

Two alternative explanations for the formation of the comb structure by replacement are suggested.

According to one hypothesis active fluoriferous solutions from a 

feeding fissure proceeded along the purer limestone beds, progressively 

replacing the limestone. Hydrofluoric acid would easily and quickly 

react with the calcium carbonate of the limestone to form calcium 

fluoride, according to the reaction:

H2F2 +CaC03->CaF2 +H2O+CO2

The ratio between the amount of CaCO3 dissolved and the-CaF2 

formed would be completely stoichiometrical, and as the specific volume of CaF2 is only about two-thirds the specific volume of CaCO3 

the fluorspar thus formed by the above reaction would have been of

20 Schwerin, Martin, An unusual fluorspar deposit: Eng. and Min. Jour., vol. 126, No. 9 (Sept. 1,1928), 

pp. 336, 339.
38 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

insufficient volume to fill completely the space that was occupied by 

the replaced calcite. In any solution cavity thus formed, additional 

fluorspar, existing as CaF2 in the mineralizing solution and picked up 

by these solutions because of reactions along the paths of travel, could 

be precipitated almost immediately, so that at no time would the open 

space be extensive. This process would result in the building up of 

crystals from the walls of the progressively forming cavity toward 

its center. Fundamentally this becomes a comb structure similar to 

such structures formed, without replacement, in open fissures by simple deposition from solutions. The fluorspar added to that formed by 

replacement might indeed appear to belong to a second generation, 

whereas, in reality, it belongs to a single and continuous genetic epoch 

of mineralization, although, as pointed out by Bastin,21 an inconsequential amount of fluorspar may be interpreted as having been deposited subsequently to that of the main body.

A second hypothesis for the development of comb structure by 

replacement along the bedding of the limestone depends upon the 

relative permeability of the different beds of the limestone. As already 

remarked, weathered exposures of the limestone display layers of 

different composition. The siliceous and ferriferous layers appear 

to have been less readily*replaced by CaF2 than the purer calcite layers, 

but, in some places at least, they are seen to be more porous, so that 

such beds might have afforded more opportunity for the entrance 

and travel of fluoriferous solutions, whether liquid or gaseous. Possibly 

with these more porous, though impure, layers as more readily permeable channels of access, they became more quickly impregnated with 

hydrofluoric acid solutions which then attacked and replaced the 

adjacent denser and more calcitic layers, working into such layers 

from both sides toward the middle. If this occurred, the "comb" 

crystals of fluorspar in part represent true metasomes.

The alternation of layers of fluorspar with layers of quartz crystals, 

or with layers of silicified limestone, as found at the old Cave in Rock 

mine and places on Lead Hill, requires a special explanation. Close 

examination of the "sub-Rosiclare sandstone" reveals that it is made 

up of two kinds of alternate layers, one kind being dominantly calcitic 

material and the other being dominantly clastic quartz grains. Both 

represent sedimentary layers as originally deposited. All of this rock 

is cross-bedded. Fluoriferous solutions might be expected to develop 

fluorite in the calcareous beds according to the reaction:

(1) H2F2 +CaCO3+CaF2 -fH2O+CO2 ;

n Bastin, E. 8., The fluorspar deposits of Hardin and Pope Counties, El.: Illinois Geol. Survey Bull. 

58, p. 67, 1931.
GEOLOGICAL SURVEY BULLETIN 942 PLATE 9

SPECIMEN OF HIGH-GRADE FLUORSPAR ORE FROM SPAR MOUNTAIN 

SHOWING COMB STRUCTURE AND VUGS.
GEOLOGICAL SURVEY BULLETIN 942 PLATE 10

A. SPECIMEN OF HIGH-GRADE "COON-TAIL" FLUORSPAR FROM SPAR 

MOUNTAIN.

B. SPECIMEN OF LOW-GRADE MATERIAL FROM SPAR MOUNTAIN SHOWING 

STRATIFICATION.
GEOLOGICAL SURVEY BULLETIN 942 PLATE 1]

SPECIMEN OF GOOD ORE FROM SPAR MOUNTAIN.

Shows wide colorless bands of pure fluorspar and narrow bands of impure fluorspar that contain unreplaced

rhombs of carbonate.
GEOLOGICAL SURVEY BULLETIN 942 PLATE 12

A. PHOTOMICROGRAPH OF AN IMPURE FLUORSPAR BAND SHOWING UNREPLACED 

FERRUGINOUS CARBONATE GRAINS AND QUARTZ.

B. PHOTOMICROGRAPH SHOWING CONTACT BETWEEN COARSE-TEXTURED AND FINETEXTURED BANDS AND INTERSTITIAL IMPURITIES.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 39

in the siliceous beds, on the other hand, the following reactions might 

prevail:

(2) 2H2F2 +SiO2-»SiF4+2H2O, 

(3) SiF4 H-2CaCO3->2CaF2 +SiO2 +2CO2 .

Thus the silica of the original sand grains might first be dissolved by 

hydrofluoric acid and then reprecipitated, under the influence of an 

excess of CaCO3 , as quartz crystals. A similar origin may be ascribed 

to the druses of minute quartz crystals in the fluorite layers. In 

similar fashion there might well result a silicification of the calcareous 

layers by a matted aggregate of minute "feathery" quartz crystals, 

as described on page 33. Plate 9 illustrates such layers of quartz 

crystals alternating with layers of fluorite.

In Schwerin's paper is a quotation from a private report by C. W. 

Greenland attributing reorganization of the silica to the action of 

fluoriferous solutions. This change could have been effected by the 

reactions given in equations 2 and 3 above. These reactions, however, 

indicate that the deposition of the fluorspar and of the quartz were at 

least in part contemporaneous, and not separated by a tune interval as

suggested by Greenland.

QUARTZ

The modes of occurrence of quartz are described on page 32, 

but the paragenetic relations call for some further comment. That 

some of the quartz was deposited subsequently to the fluorspar is 

indicated by the druses of quartz crystals deposited upon fluorspar in 

some of the layers, vugs, and larger cavities. Microscopic replacement 

crystals of quartz also occur in the less pure, fine-grained layers of 

some of the ore bodies, though rarely in sufficient amount to be detrimental except, perhaps, in border zones. The writer has observed no 

evidence, however, that all the quartz was deposited later than the 

fluorspar; it appears, rather, that the two minerals were in part 

contemporaneous, but that deposition of quartz continued after that 

of the fluorspar. This overlapping of the minerals in paragenetic 

sequence is in accord with the time relations noted for the vein deposits of the entire Illinois-Kentucky field, as described in earlier reports. 22

SUIFIDES

The deposition of the sulfide minerals galena, sphalerite, chalcopyrite 

and marcasite appears to have begun during the later stages of fluorite 

deposition and to have continued beyond it. This is also in accord

92 Currier, L. W., Fluorspar, lead, and zinc, in Weller, Stuart, and others, the geology of Hardin County 

111.: Illinois Geol. Survey Bull. 41,1920. Currier, L. W., Fluorspar deposits of Kentucky: Kentucky Qeol. 

Survey ser. 6, vol. 13, p. 59,1923. Bastin. E. S., The fluotspar deposits of Hardin and Pope Counties, 111.: 

Illinois Geol. Survey Bull. 58, p. 26, 1931.

551386 44 4
40 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

with the sequence of mineralization found in the vein deposits.23 

The formation of marcasite rather than pyrite was probably due to 

the high acidity and epithermal character of the depositing solutions.

STRATIGRAPHIC RELATIONS

Fluorspar deposits have been found at several stratigraphic horizons 

in the mapped area, most of them within the upper part of the Fredonia 

member of the Ste. Genevieve limestone. The thickest, richest, and 

most extensive fluorspar bodies so far discovered are found in the 

Fredonia limestone member immediately below its contact with the 

Rosiclare sandstone member, whose basal shale bed constitutes the 

roof of the fluorspar bodies in the principal mines. It has become 

axiomatic in the field that the most important fluorspar deposits are 

to be sought at this horizon. According to the theory of origin outlined on pages 47-51, however, bedding-replacement deposits might be 

found at other horizons in the limestones, depending upon whether or 

not the lithologic sequences and structures at such horizons were 

favorable.

At several places in the district minor bedding-replacement deposits 

have, in fact, been found below dense limestone beds, silicified limestone beds, and compact sandstone or quartzite beds, all within about 

60 feet below the base of the Rosiclare sandstone. At no locality yet 

discovered, however, is there evidence that the conditions at these 

horizons were as favorable for localization of deposits as beneath the 

basal shale of the Rosiclare sandstone; these lower deposits are apparently smaller, thinner, and less continuous than those of the main 

ore-bearing horizon. Other than the Rosiclare sandstone, the unit 

known locally as the "sub-Rosiclare sandstone" has thus far shown 

most promise. The old Cave in Rock mine is at this horizon, and 

recently the Benzon Fluorspar Co. has'found fluorspar deposits at the 

same horizon in the lowlands east of the escarpment. These deposits 

have not yet been opened to an appreciable extent; consequently their 

probable economic importance cannot be judged at this time.

As already mentioned, a small bedding-replacement deposit beneath 

a dense limestone bed near the base of the Fredonia limestone occurs 

three-fourths of a mile northeast of the village of Cave in Rock. A 

prospect called the Miller mine was opened here in 1917 but was soon 

abandoned.

Although, as just pointed out, bedding-replacement deposits may 

exist at many horizons in the Ste. Genevieve limestone, and even in 

limestones below it, the minor structures that seem to have controlled 

the deposition of bedding-replacement deposits would not be determinable at the surface over deep-lying deposits, which would thus be 

discoverable only by costly drilling.

w See references given in footnote 22.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 41

STRUCTURE OF THE DEPOSITS

GENERAL FORM OF THE ORE BODIES

The ore bodies are roughly tabular elongated masses tending to 

lenticular shapes, and generally lie with the long axes parallel to local 

joints and fold axes. The limits of individual ore bodies are economic 

rather than sharply defined, in many places, and a single fluorspar 

deposit generally shows considerable variation in thickness. Variations in thickness and forms are illustrated by plate 5, which was 

drawn from data obtained underground in four mines of the district. 

The method of studying these features consisted in constructing a 

contour map of the roof of the ore bodies that is, the base of the 

Rosiclare sandstone member and measuring the thickness of the 

mineralized zone at many places.

RELATION OF ORE BODIES TO MINOR FOLDS

The contours shown in plate 5 indicate the shape of the roof of the 

mineralized zone in the mines that were accessible for study. Variations in altitude of the shale bed appear to be due to structural deformation rather than to deposition upon an old erosion surface, because 

the bedding laminae of the shale are parallel to the plotted surface. 

The troughs and rises indicated by the contours have definite trends 

for the most part, their axes being parallel to other principal deformational features of the region, chiefly the joint systems and faults. An 

^interpretation of the structure of the same horizon for the entire 

developed area and for some additional ground is presented in plate 4. 

The data for this structural map was obtained partly from actual 

exposures of the Rosiclare sandstone along the escarpment, partly from 

records of scattered drill holes in areas adjacent to the mines, and 

partly from the mine maps upon which plate 5 was based. Therefore, 

the contours shown in plate 4 are necessarily generalized, because they 

were compiled from three sets of data of different degrees of accuracy.

It is evident from plate 5 that most of the thickest mineral bodies 

are elongated parallel, or nearly so, to the axes of the folds. The heaviest mineralization has occurred, apparently, on the narrow, sharp, 

pitching folds although it is not entirely confined to such positions. 

Most of the richer ore bodies seem, to be on the flanks of folds, and 

nearer the synclinal troughs than the anticlinal crests. Mine develop-1 

ment, therefore, should generally follow down the pitch of the roof 

to reach richer areas, if drifting has b een carried along a structural "high/' 

or anticlinal crest. In two places thick ore bodies occupy the crests 

of anticlines but both of these folds are particularly sharp. In general, 

broad crests or troughs, as indicated by wide spacing of contours, are 

apt to be more thinly mineralized than areas of sharper folding. 

These relations of ore bodies to roof contours are clearly seen in many 

places underground where development has been carried down the 

dip of the roof, notably in part of the Crystal mine (see pis. 5, 13, A).
42 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

Although this correlation of ore deposits with small local folds may 

be of considerable importance to operators during the development 

of a mine, it will probably be of little service in prospecting or exploration, because its applicability hi such operations depends upon the 

acquirement of a considerable amount of accurate data from many 

closely spaced drill holes, and the expense of getting enough of this 

information to be serviceable would generally be too great. Lack 

of rock exposures renders surface data entirely inadquate for determining the positions of such minor structures.

ROOF VEINS

Veins of fluorspar cut the roof rock in many places in the mines. All 

these veins are very narrow, generally less than an inch wide, but they 

are impressively persistent, and some of them may be followed continuously for long distances in the workings. For the most part they 

belong to two sets, one having a course N. 45° to 60° E., the other and 

less common set trending N. 60° to 80° W. The approximate coincidence of their trends with the regional axes of major and minor deformation leads to the belief that the fissures in which these veins were 

deposited were genetically connected with the faulting and folding 

of the country rock and that, in large part, they represent tension 

joints in the places of sharpest bending and twisting of the strata.

Inasmuch as these veins are closely associated with and intersect 

the ore bodies in some places, it is customary in some of the mines to 

follow them during development as "leaders" to ore bodies. The 

roof veins are not, however, necessarily above the thickest parts of the 

bedding-replacement bodies, but in many places they occupy lateral 

positions along the folds. Some of them even cross the folds at approximately right angles, but, though the fissures that these veins fill 

were probably also tension cracks and were formed contemporaneously 

with the longitudinal roof fissures, they differed from the longitudinal fissures somewhat in mode of origin. Roof fissures of both 

types seem to mark sites of deformation rather than intense mineralization; they probably never acted as the main channels of access for 

mineralizing solutions, hence their lack of close and consistent association with major ore bodies.

At the Crystal mine several roof fissures were followed parallel to the 

long axis of the ore body (see pi. 14). In this mine the ore body is 

situated on the limb of a pitching syncline, of northeast trend, along 

which the prominent roof fissure veins are spaced rather evenly, 

though the spacing is somewhat closer where dips are steeper. Plate 

13, A, shows the pitch of the roof and the related variation in thickness of the ore body at one place in drift No. 3.

At a face in the No. 1 adit of the Pittsburgh Fluorspar Products Co., 

on the east side and near the top of Lead Hill, two diverging ore bodies
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 43

were found to follow distinct sags in the roof. Several veinlets in the 

roof follow the centers of the sags, and it was below these that ore 

bodies were thickest.

In the Lead adit of-the Benzon Fluorspar Co., a similar relation of 

thick mineral zone to roof sags and veinlets was seen at two points. It 

was reported that roof seams in this mine were the best guides in development.

In a pillar near the shaft in the mine of the Crystal Fluorspar Co., 

two narrow roof veins were observed above wide veins that extended 

along a fault fissure of a few inches displacement in the limestone 

below the roof. Here the fluorspar body spreads out appreciably 

just below the cap rock, and the vein continues downward in the floor.

FLO OB VEINS

At several places veins were found in the ore horizon filling fissures 

that probably served as channels of access through which the mineralizing solutions rose. These veins appear in the floors of drifts and extend upward toward the roof, widening rather abruptly upward, 

and passing laterally into the characteristic horizontally layered mineral bodies, in cross section the structure roughly simulating a mushroom (see fig. 5). The structure was noted in 1917 at the old Miller 

mine on the west side of .Lead Hill.2i Here a thin bedding-replacement 

deposit was seen to continue downward into the floor as a vein. At 

the No. 5 adit of the Pittsburgh Fluorspar Products Co., in the south 

end of Lead Hill, about 40 feet below the Rosiclare sandstone, a dense 

limestone bed forms a cap rock for a thin bedding-replacement deposit 

of the usual type. At the drift face a vein of coarsely crystalline fluorspar extended downward into the floor, directly beneath a slight 

sag in the floor rock. At the floor level the vein was narrower, 

having pinched to about a foot wide, and had been followed 

for some distance along the floor. In the roof directly above the vein 

at the face a 3-inch roof vein was seen. This "center seam" had been 

followed in drifting from the outcrop.

BOUNDARIES OF MINERAL BODIES

The contacts of the bedding-replacement bodies with the roof rocks 

are sharply defined. In some places a relatively thick layer of pure, 

transparent fluorspar is found at the contact, suggesting that the 

upper layers of the replaced limestone beds at these points were particularly pure. In most places, however, the topmost layers of fluorspar are thin. They commonly follow the roof contact withoutdeviation, but inclined layers just beneath believed to represent 

cross bedding in the original limestone terminated against or

» Currier, L. W., op. cit. (111. Qeol. Survey Bull. 41), p. 255.
44 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

coalesce with the top layers. This feature is represented in plate 8. 

Bedding veins of clear, crystalline fluorspar are found in pla.ces in the 

roof shale above the contact, but these are doubtless fissure fillings 

rather than replacement bodies; an explanation for the origin of 

these flat veins is given below.

At the contact also may be found thin, lenticular cavities, that 

rarely exceed 3 or 4 feet in maximum dimension. They are commonly 

lined with crystals of fluorspar, some of which are as much as 6 

inches in diameter. It would appear that local solution greatly exceeded deposition at these places during the process of replacement. 

Such cavities, however, may have resulted only in part from the

FIGURE 5. Diagram illustrating "mushroom structure" as observed in adit No. 5 of Pittsburgh Fluorspar 

Products Co. and at abandoned workings of Basic Mineral Co., Lead Hill. A, Coarse vein fluorspar; 

B, banded fluorspar; C, roof vein; D,' dense cap rock.

reduction in volume that has been attributed to the difference in 

specific volumes of fluorspar and calcite as pointed out by Schwerin26 

and elaborated upon in this report; to some degree the cavities may 

also represent local slumping or settling into solution cavities in the 

limestone below.

Laterally the bedding-replacement layers converge and coalesce, 

and ultimately pinch out in the limestone. In many places a massive, 

compact limestone floor of very dense texture limits the mineral bodies 

at the base in places and also the surface of the limestone rises toward 

the roof along the upper side of the ore body serving to reduce and 

pinch out the replaced horizon (see pi. 13).

PARTINGS

Within the mineral bodies, unmineralized beds from a fraction of 

an inch to several feet in thickness are often encountered. Some of 

these layers are thin shale beds, and some are limestone beds that,

M Schwerin, Martin, an unusual fluorspar deposit: Eng. and Min. Jour., vol. 126, No. 9 (Sept. 1, 1928), 

p. 309.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 45

being more argillaceous or very dense textured, were not readily replaced. At an adit portal of the Green mine a layer of dense-textured 

ferriferous limestone about 2 inches thick was seen as a parting between beds of banded fluorspar. At several places in the Victory mine 

beds of dense or partly crystalline brown limestone, of variable thickness, occur as partings between beds of typical "coon-tail" fluorspar. 

Plate 15, A, illustrates a similar limestone ted, pirtly fractured and 

slightly mineralized by veinlets and streaks of fluorspar.

BEDDING VEINS IN ROOF SHALE

At many places in the several mines flat-lying thin veins of 

coarsely crystalline fluorspar without comb structure occur in the 

shale roof adjacent to thick mineral bodies (see pi. 15, B), Such peins 

bifurcate and coalesce, but for the greater part follow the stratification planes of the shale. It is believed that these veins were formed 

by deposition in more or less open spaces between shale layers, rather 

than by progressive replacement, as the boundaries are sharp and 

enclose no fragments of shale that appear to be replacement remnants.

In the Crystal mine, at a point near the adit portal, a mixture of 

coarsely crystalline fluorspar and shale fragments is exposed at the 

roof of the fluorspar body. The promiscuous arrangement of the shale 

fragments in the fluorspar and the steep pitch of the roof at this place 

indicate dynamic deformation with attendant'fracturing of the roof 

material, probably at the beginning of the epoch of mineralization.

Two processes probably operated in producing the phenomena just 

described. Progressive replacement of limestone by fluorspar, as outlined in previous pages and for the reasons stated therein, resulted in 

a net volume reduction. In part this reduction was probably offset 

by deposition of CaF2 , existing as such in the solution and migrating 

from other points; in part, however, the net result was the progressive 

development of open spaces in the limestone horizon just below the 

roof. During the temporary existence of such spaces, the collapse and 

spreading apart of overlying shale beds progressively afforded interbedding space for direct precipitation of fluorspar. As open space 

existed temporarily at these points and below the shale, the crystallization of the fluorspar between shale laminae assisted in spreading the 

layers farther apart, until the voids were entirely filled or the 

supply of solutions failed. In places some spalling of shale fragments 

into local cavities may well have taken place during the time of imn.- 

eral deposition.

Open spaces in the shale roof may have been brought about by 

some spreading of the layers on the steeper parts of the folds by the 

slippage and displacement of the beds as a result of the folding. Such a 

process would tend to weaken the shale along bedding planes, and to
46 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

brecciate the layers without much rotation of the fragments. Open 

spaces may also have been produced by the actual spreading apart of 

more competent layers above and below the thin stratum of shale at 

the base of the Rosiclare sandstone.

FLUORSPAR CONTENT OF MINERAL BODIES

From the structural and textural characteristics of fluorspar deposits as described on foregoing pages it is clear that the calcium fluoride 

content of mined materi il must vary widely. The variations in thickness of the layered ores, the presence of partings, the gradual merging 

of border zones into unmineralized limestone, the differing relative 

thicknesses of pure layers and impure layers and probably several 

other factors would all tend to the same result. Information given 

by the several operators indicates that, according to mining practice, 

mill heads average from 50 percent to 80 percent calcium fluoride. 

That the minimum of this range is so low is due to the fact that some 

of the deposits have disintegrated at the surface and have become 

mixed with residual soil. Such deposits are mined by steam shovel 

and the crude ore is sent to the mill for washing and concentration. 

Their silica is mostly in the form of admixed clay and sand, which are 

readily washed out in the first stages of milling. In material from 

underground workings the average CaF2 content appears to be about 

70 percent, but the maximum is much higher than that of the surficial 

ore bodies and the average SiO2 content is accordingly much lower. 

Because of the exceptionally high grade of some parts of the solid ore 

bodies it has been possible to mine exceptionally low grade material 

from the border zones or from thinner and poorer ore bodies, so that 

by mixing the two, mill heads would be obtained that would yield a 

product of standard metallurgical grade that is, containing 85 per 

cent of CaF2 and 5 per cent (or less) of SiO2 . Indeed, parts of some ore 

bodies have yielded ore that contained 90 per cent or more of CaF2 

and less than 5 per cent of SiO2 . Selective mining and marketing of 

high-grade ore has not been encouraged by market prices for the 

higher grades of fluorspar, because by mixing the ores it has been 

possible to prolong the life of the mine, 'to conserve the resources, and 

to obtain a greater financial return in the long run.

The differences in fluorspar content between the two kinds of alternating layers of the typical bedding-replacement ores are not great, 

and are, indeed, far less than might be expected from the contrasting 

appearances of the layers. The finer-grained, less pure layers probably contain, for the most part, from 70 to 85 per cent of CaF2 , except 

in the leaner, marginal parts of ore bodies, and one analysis of tlie 

"sandy-looking" layers was reported to show 95.09 per cent of CaF2, 

4.12 per cent of SiO2 , and 0.86 per csnt of CaCO3 .26 It is therefore 

not necessary to separate the layers in milling.

19 A. H. Cronk, personal communication.
GEOLOGICAL SURVEY BULLETIN 942 PLATE 13

A. VIEW IN DRIFT 3, CRYSTAL MINE.

B. EXPOSURE OF FLUORSPAR HORIZON AT PORTAL OF LEAD ADIT, 

BENZON FLUORSPAR CO.

GEOLOGICAL SURVEY BULLETIN 942 PLATE 14

AIRSHAFT

0 5 10 2O 3O 40 SO CO 70 BO 9O IOO FEET

SHAFT

PORTAL

DRIFT MAP OF CRYSTAL MINE. 

Position of roof fissures indicated by broken lines. Courtesy of Crystal Fluorspar Co.

GEOLOGICAL SUKVEY BULLETIN 942 PLATE 15

A. FACE OF A ROOM IN CRYSTAL MINE.

B. BEDDING VEINS IN ROOF SHALE, VICTORY NO. 1 MINE*

PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 47 

ORIGIN AND GEOLOGIC HISTORY OF THE DEPOSITS

In origin and geologic history, the bedding-replacement deposits 

of the Cave in Rock district are, in their general aspects, similar to 

the vein deposits that characterize the rest of the Illinois-Kentucky 

field. The source and character of fluoriferous solutions and 

the time of deposition are believed to be the same for both types of 

deposits. Only in regard to the details of structural control 

does the genetic history of the bedding-replacement deposits differ 

from that of the veins. Theories of origin for the entire field have 

been stated in previous publications, already cited. A brief summary 

is presented here, however, so that the genetic relations that the 

writer believes to exist between the bedding-replacement deposits 

and the veins may be clearly understood.

Three general factors are foremost in any problem relating to the 

origin of mineral deposits formed by circulating underground waters. 

These are (1) the source of the solutions; (2) the channels of access 

from the points of origin of the solutions to the sites of mineral deposition; and (3) the conditions that determined the sites of deposition 

of the mineral bodies. As regards the sites of deposition, the problem 

also involves the geologic processes by which the host rock was prepared for mineral reception.

The source of mineralizing solutions for the entire fluorspar field 

is believed to have been a deep-lying mass of molten rock material, 

or magma, that existed at considerable depth, doubtless several 

thousands of feet below the present surface.

The geologic structure of the Illinois-Kentucky fluorspar field has 

been described as a large oval dome that collapsed to form a mosaic 

of fault blocks and wedges. The doming of the strata may have been 

'caused by the upward pressure of a large body of magma intruded 

at considerable depth, or it may have been caused by crustal forces 

that warped and uplifted the strata and thus permitted magma to 

invade the space, nearer the surface, in which pressure had thus been 

diminished by displacement of the strata. The presence of an underlying magma body that was intruded by either of these processes is 

indicated by the widespread occurrence of dikes in the domed area, 

most of the exposures of which are found in the broad central part of 

the field. As these dikes are generally not found in fault fissures, and 

some of them are offset by major faults, they must have been intruded 

before the dome collapsed. The striking symmetry of the Hicks dome, 

in the northwestern part of the field, suggests that the dome was 

caused by intrusion of a laccolith an igneous body shaped like a 

mushroom, and forced between layers of stratified rocks but this 

local intrusion could not have caused the doming of the field as a 

whole.
48 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

The general fault pattern of the field is believed to be more suggestive of regional warping by crustal forces than of deformation by 

simple uplift directly due to igneous intrusion. The main faults appear 

to be grouped roughly into three broad belts transverse to the long 

diameter of the field. In general, the trend of these belts is northeasterly, but from northwest to southeast the trends of the faults become 

progressively more easterly, and in the extreme southeastern part 

of the field the faults trend practically east-west. According to the 

geologic map of Kentucky narrow faulted belts extend from the 

fluorspar field for relatively long distances into the central and eastern 

parts of the State.

Bucher,27 indeed, has suggested that an incipient arcuate geosyncline follows the west side of the Mississippi embayment, and that 

its axis runs generally parallel with the Mississippi Valley as far north 

as southern Illinois, whence it swings to the northeast, following the 

Ohio River Valley in a general way and conforming broadly in trend 

with the Appalachian geosyncline. It is interesting to note, hi this 

connection, that the sharp bend of the axis of this postulated geosynclinal furrow lies within the broadly domed fluorspar field.

It seems likely, therefore, that the region was broadly warped and 

domed by widespread crustal forces, that invasion by magma accompanied or followed this deformation, and that the igneous activity 

was followed by collapse and settling of the uplifted and warped area, 

giving rise to the major faults after solidification of the igneous material. The Cretaceous beds of western Kentucky apparently were 

faulted, so that a part, at least, of the faulting must have been of 

post-Cretaceous age.

Late hydrothermal emanations from the deeper part of the magma 

basin ascended along major fault fissures, which must have cut throughthe congealed upper part of the deep-seated magma as well as through 

the overlying strata. These emanations were especially rich in fluorine, so that hydrofluoric acid, formed by hydrolysis, was relatively 

abundant in the ascending solutions. Wherever calcium carbonate 

(as limestone or as vein calcite) was encountered along the paths of 

circulation, chemical reactions resulted in the formation of fluorspar. 

So long as the upward flow of the solutions was relatively unimpeded, 

by virtue of the more or less open character of the fissures, a considerable excess of hydrofluoric acid probably remained in the solutions 

and was carried upward until it gradually became used up by reaction 

with calcium carbonate encountered along the fissures. It is a natural 

result of such action that some of the fault fissures contain fluorspar 

through a large vertical range. For long distances and in many places 

along faults, also, smooth, compact, slickensided walls gave little 

chance for replacement of the wall rock, even where it was composed

» Bucher, W. H., The deformation of the earlh's crust, pp. 354-357, Princeton Univ. Press, 1933.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 49

of limestone, except at intersections with other fissures. But even 

where wall rocks could not be replaced vein fluorspar was probably 

deposited directly from solution wherever the progressively decreasing 

pressure and temperature of the solution made it unable to carry the 

calcium fluoride obtained at lower horizons. Wherever crystalline 

calcite was encountered as fissure filling, as, for example, between 

walls of the thick Ste. Genevieve limestone and lower formations, 

fluorspar was deposited both by reaction of the hydrofluoric acid of 

the solutions with calcite, and through volume-for-volume replacement 

of calcite by the calcium fluoride carried in the solutions.

The formation of'the characteristic bedding-replacement deposits 

of the Cave in Rock district seems to have been due to a combination 

of certain structural and stratigraphic conditions peculiar to that 

district. The forms of the principal deposits and their relation to the 

shale at the base of the Rosiclare sandstone member have led to the 

belief that this lithologic unit effectively shut off or at least impeded 

the rise of solutions along minor fissures, retarding their flow and 

causing them to spread laterally along the shale. Other dense beds 

below the shale, and probably some above, acted similarly, but apparently to a lesser degree. Reactions of the solutions with the more 

penetrable limestone beds directly below comparatively dense cap rocks 

thus developed replacement bodies, which preserved the beddiag 

and cross-bedding laminae of the replaced limestone.

The channels of access for these solutions were prominent joint 

fissures and minor fault fissures, connected with larger fissures, which 

in turn were connected ultimately with a major fault zone, probably 

the Peters Creek zone. The writer's conception of the entire aggregation of the feeding fissures is that of more or less diverging sets, along 

the uppermost and outermost members of which there was little or. 

no displacement, in marked contrast with the fissures in which veins 

were deposited elsewhere in the fluorspar field. The minor fissures of 

the Cave in Rock district, according to this theory, either extend 

upward just to the cap rock, or, in passing through the basal shale of 

the Rosiclare sandstone (or other comparatively "tight" horizon), 

are sharply narrowed, owing to the incompetency of the shale to 

develop or preserve extensive and relatively wide fissures. In the shale 

such cracks would tend to be closed by flowage and folding as well as 

by slippage along the weakly coherent beds, whereas in the relatively 

competent limestone beds fissures could remain partly open, the walls 

being supported in places, of course, by points of contact along the 

fracture.

Such a system of minor fissures, dying out upward at horizons of 

favorable differences in lithologic competency, might well be more 

numerous in the marginal part of a domed area than in the more 

central part, where faulting is prominent. In the marginal belt the
50 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

regional strain features become progressively weaker toward the 

border and finally disappear. Thus a combination of geographic 

position with stratigraphy, structure, and the proximity of a major 

fault zone made the Cave in Kock district peculiarly favorable to 

the formation of bedding-replacement deposits. Probably some other 

marginal areas of the Illinois-Kentucky field possess a similar 

combination of favorable features, and may ultimately yield similar 

deposits. The detection of such areas is certain to be difficult, in 

general, because of the lack of pronounced displacements of the strata, 

the general obscurity of minor folds and faults that appear to be 

necessarily associated with such deposits, and the fact that the deposFIGTJEE 6. Diagram illustrating formation of a flexure in flexible beds over a minor fault in brittle beds,

its themselves are parallel to the bedding of the strata and hence 

would be less commonly exposed at the surface than would veins.

The sites of deposition were thus near the minor terminal fissures 

of a complex system, developed during the doming and faulting of 

the Illinois-Kentucky field. Minor folding of the strata assisted in 

preparing the ground for mineralization, by the forming of local 

minor tension fissures along positions of greatest curvature or steepest 

pitch, both along the limbs of minor folds and in directions across 

the folds. In one or two places, as illustrated by figure 6, there is a 

suggestion that steep pitches of the roof may follow floor fissures of 

small displacement, as if these minor faults may be represented in the 

roof rock by flexures. There is no tangible evidence, however, that the 

roof flexures were generally formed in this way; indeed, the pronounced narrow pitching anticlines and synclines rather oppose this, 

interpretation in some places, as does also the slight sag in the roof 

observed in the adit at the south end of Lead Hill (see fig. 5).
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 51

Erosion after the deposition of the fluorspar deposits removed a 

considerable but indeterminate thickness of the cover rocks, exposing 

the mineral horizon along the sides of Spar Mountain and of Lead 

Hill. Doubtless the mineralized beds once extended farther to the 

south and east over part of the adjoining lowland.

VIEWS OF PREVIOUS WRITERS

Bain 28 in 1905 briefly described some bedding deposits in the Cave 

in Kock district and attributed their origin to replacement of limestone 

strata along the bedding. He explained the comb structure of the 

layers as an example of "pseudocrustification," with the central 

lines of the pure layers representing stylolitic contacts between 

individual layers of the limestone.

The writer's first study of the fluorspar field, made in 1917, included 

only a brief examination in the Cave in Kock area. At that time a 

small cut was being operated at the Cave in Eock mine. The workings 

at Lead Hill were inactive, but part of the Miller mine (Basic Mineral 

Co.) on the west side of the hill was accessible. The general form and 

lithologic relations of the deposits were recognized, the "mushroom 

structure" as described on page 43 was reported,29 and the relation of 

the fluorspar to minor fissures was suggested, but it was not possible 

to determine the structural and textural details as outlined in this 

report.

Bastin 30 studied the district in 1926. He agrees with the theory of 

metasomatic origin of the deposits, and also attributes them to mineralizing solutions that followed small fractures and penetrated limestone beds, particularly at the contact between the basal shale of the 

Rosiclare sandstone member and the underlying Fredonia limestone. 

He considers that the banding of the deposits is "nearly parallel to the 

bedding planes of the inclosing rock." 31 He believes, however, 

that the fluorspar layers do not follow stratification planes exactly, 

but make various angles with them, that these divergences "demonstrate that the banding is not an inheritance from either bedding or 

cross bedding of the sediments," and that one of the outstanding features of the deposits is "the common development of a peculiar type 

of banded ore which implies rhythmic deposition by replacement." 31 

Thus, according to him, the bands or layers were formed by diffusion 

through the limestone bed, outward from cross-cutting fissures and 

downward from the shale-limestone contact.

Evidence presented through the present report is believed to show 

that the layers of the Cave in Rock ores represent original stratifica28 Bain, H. F., Fluorspar deposits of southern Illinois: U. S. Geol. Survey Bull. 255, pp.54-58,'1905.

n Currier, L. W., in Weller, Stuart, and others, The geology of Hardin County, 111.: Illinois Geol. Survey 

Bull. 41, pp. 254-255, 1920>

soBastin, E. S., The fluorspar deposits of Hardin and [Pope counties, 111.: Illinois Geol. Survey 

Bull. 58,1931; 16th Internal. Geol. Cong. Guidebook 2, pp. 39-42,1933.

3»Op. cit. (Guidebook 2), p. 40.
52 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

tion and cross bedding of the replaced limestone, and that the differences in composition between successive layers in the ore deposits reflect 

differences in texture and purity of original laminae of the limestone. 

The uniform spacing of the layers through considerable thicknesses, 

the comb structure displayed by many of the pure layers, the fact that 

even the less pure finer-textured layers of most of the core consist predominantly of fluorite, and the fact that the layers hi some ore bodies 

consist alternately of nearly pure quartz and fluorspar, all favor the 

theory of simple bedding replacement rather than rhythmic deposition. 

Except for this difference in mode of penetration and replacement 

by fluoriferous solutions, however, both agree that the fluorite 

bodies were formed by replacement of permeable limeatone below 

dense beds of shale or limestone, and that the sites of deposition were 

determined by structural features.

POSSIBLE EXTENSION OF THE PRODUCING AREA

The geologic history, mode of origin, and structural relations of the 

bedding-replacement deposits, as outlined and interpreted in the foregoing pages, suggest that such deposits may be present in places 

through a considerable area to the northeast of the area of present development. The occurrence of such bodies elsewhere will, of course, 

be determined by the presence of certain favorable conditions, as 

outlined above. Bedrock exposures are too sparse and inadequate 

for complete determination of the potentialities of the unexplored 

area northeast of the mines, but the formations can be mapped with 

confidence, and the presence of some minor faults can be inferred from 

the field data. In addition, the geophysical work done by Mr. Hubbert 

has indicated definite breaks in the resistivity profiles that can rationally be construed as due to structural disturbances. For these reasons 

the area to the northeast should be considered worthy of careful prospecting. This can be done satisfactorily only by drilling, for the Kosiclare sandstone is not exposed and, as pointed out in a previous section, 

it will be found at progressively greater depths toward the northeast. 

Sinking of prospect shafts would therefore be too costly and it usually 

is not an economical method of primary exploration. The increase in" 

depth to the favorable horizon will also tend to set economic limits to 

the development of the deposits. The limits of the geologic map, 

however, are not related to any known or inferred limits of the area 

worth prospecting; they merely outline a convenient unit for this 

investigation.

The formations of the lowland area south and southeast of the 

escarpment are entirely below the Kosiclare sandstone horizon, 

which has here been removed by erosion. The " sub-Rosiclare sandstone" appears in a small part of this area. Any bedding deposits present
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 53

in the lowland must be related to the " sub-Rosiclare" or still lower horizons in the Fredonia limestone. According to the theory of origin outlined above, bedding deposits may be present in places at these lower 

horizons, providing the structural and lithologic conditions are favorable. The area underlain by the Fredonia limestone is very extensive, 

but a thick mantle of residual clay conceals details in this rock. It 

is not possible, therefore, to indicate any particularly favorable 

ground for exploration in the broad lowlands between the Spar Mountain escarpment and the Ohio River; prospecting by drilling would be 

entirely of the expensive "wildcatting" variety. Incidentally, much 

of the low area in question is underlain directly by the St. Louis limestone, a formation in which no certain signs of bedding deposits have 

yet been found, and which appears to be of more uniform lithologic 

composition than the formations above..

The long escarpment that extends southeastward from the broad 

creek valley in the northeast part of the mapped area is favorable 

prospecting ground so far as the stratigraphy is concerned, for the 

Rosiclare sandstone is exposed along nearly its entire length; but as 

geologic investigations made in connection with this report were not 

carried into that area, no statement can be made regarding its minor 

structural features. Except at its northwest end, however, this escarpment is remote from any known major or minor fault zone.

Since one of the chief features that characterize the present producing 

district is its marginal position with respect to the domed area of the 

fluorspar field, it might be expected that bedding-replacement deposits 

may be found in other marginal areas of the field. Indeed, at the 

Renfro prospect, in sec. 23, T. 11 S., R. 8 E., about 5 to 6 miles northwest of Spar Mountain, the dump shows material of the characteristic 

bedding-replacement type. Here the Rosiclare sandstone and a ,thin 

shale bed are present, and there is a mapped fault nearby.32 The 

material at this prospect would appear to be significant and to warrant 

further investigation of the prospect b'ecause of the known geologic 

conditions. The extent to which shale forms the base of the Rosiclare 

sandstone in this peripheral belt might be one of the vital factors.

Specimens of "banded" fluorspar showing the typical layered 

structure of the Cave in Rock ores have been obtained also at the 

dumps of the Hamp mine, 3 miles north of the center of the Hicks 

dome, where the Rosiclare sandstone is exposed, but no statement 

can be made regarding the extent of the deposits at this locality. Vein 

deposits have been worked here intermittently, but so far as could be 

determined no bedding-replacement deposits of appreciable size 

have been found during the operations. The following note is repeated 

from the writer's earlier report: 33

32 Weller, Stuart, and others, The geology of Hardin County, 111.: Illinois Geol. Survey Bull. 41, pi. 1,1920.

33 Weller, Stuart, and others, op. cit., p. 303.
54 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

At a small cut west of the shafts, banded fluorspar of the "bedding" type of 

deposit was seen. Unlike the fluorspar near Cave in Rock the mineral of these 

bands was very deeply colored and nearly opaque. When freshly broken, the 

mineral from this deposit gave a very strong odor of petroleum.

Similar material has been found at the Rose property, and according 

to Charles Butts,34 formerly of the United States Geological Survey,

a vein of high-grade fluorspar occurs in the Devonian limestone, and probably in 

the Hamilton part of it, on Goose Creek, about one mile southeast of Hicks, 

where it has been prospected and worked on the Rose property. The vein, 

which lies parallel to the bedding, is apparently a replacement of the limestone 

layers. The enclosing beds are chert, which also is a replacement of the limestone 

layers.

Extensive beddings-replacement deposits should hardly be expected 

to exist in areas of pronounced faulting well within the domed area 

where vein deposits are common, such as in the Rosiclare district, 

because the originally more open fissures of major fault zones would 

not be apt to favor the penetration of minor terminal fissures by 

large volumes of mineralizing solutions, although some minor replacement along the bedding planes of limestone walls could be expected 

in places. s

SUGGESTIONS FOB PROSPECTING AND EXPLORATION

Bedding-replacement deposits should be sought particularly in marginal areas of minor fissuring, fairly near but not along prominent fault 

zones, where the Rosiclare sandstone is present and has a shaly base, 

or where other beds that are similarly compact are underlain by penetrable limestone. So far as indicated by present knowledge of the field 

the. most favorable ground is on the top of Spar Mountain and in the 

northeastern part of the mapped area and still farther to the northeast. 

The territory for some distance east of the mapped area seems less 

encouraging because so far very little fluorspar has been discovered 

along the escarpment that trends to the southeast; however, the 

anomalous strikes and dips along the eastern boundary of section 35, as 

shown on Weller's map 35 indicate structural disturbance in that locality. Those parts of the mapped area in which minor faulting or warping of the beds is indicated by geologic data, or suggested by resistivity 

anomalies, should be most favored, but other parts, in which faulting 

or warping is not evident on the surface, should not be omitted, 

for most of the minor structural features are generally obscured on 

the surface and hence could not be recognized by ordinary geologic 

methods.

Prospecting for bedding-replacement deposits must generally be 

carried on by drilling,.as shafts are economical only where the expected 

deposits lie at very shallow depths and where there is some surface

« Unpublished manuscript.

«»Weller, Stuart, and others, op. cit., pi. 1.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 55

indication of favorable structure. Core drilling is unquestionably the 

most satisfactory drilling procedure, as the stratigraphy and structural 

features of mineralized beds can best be detected by this method.

During the development of a mine, structural features should be 

closely observed and progressively mapped, so that gradual changes 

in the dip of the beds can be constantly noted. It seems clear that for 

.the most part the axes of developed ore bodies tend to follow the structural trends that are regionally characteristic. In places these trends 

are indicated not only by the roof fissures and the floor veins but also 

by the axes of minor folds. It also appears that in many places underground development can go from lean to richer portions by following 

the pitch of the roof which is essentially the dip of the beds from 

the crests of folds or "rolls." This direction would not necessarily be 

that of the main axis of the individual mineral bocly, which would most 

commonly be expected to trend in the direction of structural axes, such 

 as roof fissures, floor veins, and folds axes, and hence would ordinarily 

be more or less nearly at right angles to the direction of dip of the beds. 

In the area represented in plate 5, for example, starting in the lean 

part of an ore body perhaps at the top or bottom of a fold and 

following the dip of the beds, as shown approximately by the contours, 

would in most places lead to richer zones. The developed areas shown 

by plate 5 represent a very small proportion of the district upon which 

to establish a rule, but data do suggest that a structural pattern of 

control exists and 'that close attention should be paid to structural 

elements during exploration and development to the end that this 

apparent rule may be substantiated or a more exact one established.

MINES AND PROSPECTS

Mine development in the Cave in Eock district presents a constantly 

changing picture. Because of the comparatively shallow depth to 

which the known deposits are confined and because they have, for the 

most part, been entered through adits, it has been possible to develop 

them from numerous points of attack and rather quickly to abandon 

individual openings that have been worked out or that have proved 

unproductive. It has been customary, when an ore body has been 

opened, to follow it only so long as it yields profitable material. For 

these reasons the scenes of activity have been constantly shifting 

from mine to mine or within a mine, the development of new openings 

being soon followed by their temporary or permanent abandonment. 

The writer feels that no particular service would be rendered by describing in detail all the accessible mine workings when it would be 

impossible to give a similarly detailed account of the inaccessible workings that would be essential to a balanced picture. Therefore, only 

brief and general descriptions of the present workings will be given,

551386 44 5 .
56 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

together with still briefer statements concerning the locations and 

general relationships of former openings. Brief descriptions and historical facts relating to these earlier workings have appeared in earlier 

publications.36 The locations of all mines and prospects listed hero 

are indicated on the geologic map, plate 3.

Crystal Fluorspar Co. The mine operated by the Crystal Fluorspar 

Co. (post office address, Elizabethtown, 111.), is the most northeasterly mine of the district. It was opened in 1930 through an inclined 

adit at the base of the eastern slope of Spar Mountain. The underground workings comprise two main drifts, 40 to 70 feet apart, that 

extend in a general N. 35° or 40° E. direction, and are connected by 

several crosscuts. Several short drifts also penetrate for short 

distances into the bordering areas in the northwest and southeast, and 

small irregular rooms or galleries are developed in the area between 

the main drifts. The length of the developed area (see drift map, pi. 

14), in the summer of 1934 was about 400 feet, and the average width 

was about 60 feet, but since that time the southeastern drift has been 

considerably extended to the northeast. A shallow service shaft 

connects the southwest end of this drift with the surface. A prospect 

shaft, the Lackey shaft, has been sunk at a point about 550 feet N. 70° 

E. from the adit portal.

Notable general features seen in the Crystal mine are the steep 

southeasterly and easterly dips of the shale roof along the northwest 

(drift 3), the marked thickening of the ore body, from zero at places in 

the northwest wall, to more than 8 feet down the pitch of the roof (see 

pis. 5, 13, A), and the numerous mineralized roof seams, or "leaders," 

trending N. 55° to 60° E., parallel to the long axis of the ore body. 

Along the southeast drift, the roof rises appreciably, but its slope is 

less than that of the northwest drift, so that the thickest parts of the 

ore body appear to lie in the center and northwest limb of the synclinal 

trough.

At a point in the southwest crosscut about midway between the 

main drifts, a roof fissure showing no appreciable displacement is 

exposed above a sharp drop in the floor of the crosscut on the south 

side, in which the exposed thickness of the mineral body is 9 feet; it 

was reported that fluorspar was followed 5 feet deeper on this side. 

No floor vein was seen here, but the floor was not carried to the bottom 

of the mineral zone. There is some suggestion that a displacement 

of the floor is reflected in the marked warping of the roof and development of a tension crack. In a pillar near the shaft, there appears to 

be a fault of very small displacement, filled with coarsely crystalline

36 Bain, H. F., The fluorspar deposits of southern Illinois: U. S. Geol. Survey Bull. 255, 1905. Currier, 

L. W., Economic geology, in Weller, Stuart, and others, The geology of Hardin County 111.: Illinois 

Geol. Survey Bull. 41, pt. 5,1920. Bastin, E. S., The fluorspar deposits of Hardin and Pope Counties, El.: 

Illinois Geol. Survey Bull. 58,1931.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 57

vein fluorspar, leading to a narrow mineralized roof fissure. Shale 

fragments, oriented as if by drag and separated by streaks of fluorspar, were seen in the pillar. Some postmineralization faulting 

appears to have cut the ore body on the east side of the southeast 

drift (drift 62).

At a few places, coarsely crystalline calcite appeared in the limestone at the contact with fluorspar, and replacement of the calcite 

by purple fluorspar was noted.

Benzon Fluorspar Co. The property of the Benzon Fluorspar Co. 

covers an extensive area along the east and south slopes of the Spar 

Mountain escarpment and in the adjoining lowland. The northeastern limit of the property is about a quarter of a mile southwest 

of the Crystal mine. The property extends southwesterly and westerly, nearly to the Cave in Rock mine. From the northeastern limit 

to the western limit the chief workings are known successively as 

the Green and Defender, Cleveland, 32 cut, Lead adit, West Morrison, 

and Oxford pits. Other pits and prospect shafts are interspersed 

between these. The Cleveland mine was not accessible at the time 

of the investigation. In all these workings the principal development 

has been at the base of the Rosiclare sandstone, but a few cuts have 

been made in beds down to and including the "sub-Rosiclare sandstone" horizon. Plate 16 is a map of the principal underground 

workings.

Ore from open cuts consists largely of residual material in clay, the 

result of weathering and decomposition of bedding-replacement ore 

bodies. This material is excavated by steam shovel and sent to the 

mill for washing. Most of the production during the summer season 

has come from the open pits, whereas winter operations are restricted 

to the underground workings. The marketable products are transported by truck to storage bins on the Ohio River at Cave in Rock. 

The local company office is at Cave in Rock, Illinois.

Green and Defender mines. The Green and Defender mines are 

about a quarter of a mile southwest of the Crystal mine. Originally 

opened through the Defender shaft, these two mines are now operated 

through adits. The ore bodies are in the Fredonia limestone horizon 

at the base of the Rosiclare sandstone. The base of the sandstone 

here consists, as usual, of shale which is the general roof rock of the 

deposits. At a few spots the mineralized beds exceed 7 feet in thickness, but for the most part the thickness is between 3 and 5 feet. 

Replacement of the limestone is relatively less complete, and roof 

fissures appear to be less common, at some of the other mines. The 

workings of these two mines cover an area approximately 625 feet 

long and 500 feet wide, but only about half of this area has been mined 

out. A barren or very low-grade strip crosses this development area 

in an east-west direction, and is flanked on both sides by belts of
58 FLUORSPAR AREAS IN -HARDIN COUNTY, ILL.

higher-grade material, in which all the mining has been carried on. 

Areas of very low grade or unmineralized ground apparently border 

the -workings on both the north and the south sides, but the widths 

of these areas have not been determined.

At the adit portal near the east property line the mineralized zone 

is 3 or 4 feet thick and shows incomplete replacement of cross-bedded 

limestone. About 20 to 25 feet below this, in the loading cut, weathered exposures of entirely unmineralized limestone clearly display 

cross bedding.

Cleveland mine and 82 cut. These openings of the Cleveland mine

and 32 cut embrace several adits, drifts, and open pit workings ex-

. tending northeastward for about a thousand feet, the northeast end

being approximately 700 feet southwest of the Green-Defender adit.

The mine was not accessible for study. The chief fluorspar bodies

were just beneath the Rosiclare sandstone, but sporadic mineraliza-

. tion was found at a lower level.

The 32 cut connects with the southw'est end of the underground workings of the Cleveland mine. At this cut an extensive exposure of 

mineralized cross-bedded sandstone was seen. Typical banded fluorspar was also seen between a sandstone roof and an uneven floor of 

dense, massive limestone. Comparison of the beds along both sides 

of the 32 cut and in a small cut on the south side of the roadway 

suggests a minor fault trending approximately N. 45° E. and aprominent 

roof fissure of this trend is said to extend throughout the length of 

the Cleveland workings with which the 32 cut is connected.

The Cleveland mine was opened prior to 1903, by the ClevelandIllinois Fluorspar Co. Bain,37 who examined the mine in 1903, states 

that the ore body was from 18 inches to 6 feet thick, and was found to 

diminish in thickness northeastward where the workings extended 

into the hill. He describes the ore as consisting of "interbanded" 

limestone and fluorite with minor amounts of calcite and galena.

Lead adit. The lead adit is a short distance west of the Cleveland 

mine. From its portal, about a hundred feet north of the mill, the 

workings extend for approximately 1,000 feet in a west-northwest 

direction. The widest part of the workings is about 350 feet. At the 

northwest end the drift has been carried about 500 feet southwestward, following a roof fissure, but no stoping has been done along this 

part of the drift. Two sets of roof fissures are present in this mine, 

one set trending approximately N. 65° W-, and the other N. 45° to 

60° E. The mined area covers about 112,000 to 113,000 square feet 

including pillars. In places mining has been carried to depths of 15 

to 20 feet below the main level. Plate 17, A, is a photograph of a 

lower level in this mine.

w Bain, H. F., Fluorspar deposits of southern Illinois: U. S. Geol. Survey Bull. 255, p. 54,1905.
PAET 1. GEOLOGY OF CAVE IN ROCK DISTRICT 59

A series of cross sections of this mine, showing roof profiles and 

thickness of mineralized zones, is given in figure 7, and a contour map 

of the roof is shown on plate 5. The average thickness of ore is from 

3 to 5 feet, but considerably greater thicknesses were seen in several

^: .,

G' H

H *

FIGURE 7. Profiles across Lead adit workings showing relation of mineral bodies to roof slopes. Sections 

are along lines indicated on plate 5. Vertical scale 5 times horizontal scale. Heavy vertical lines indicate measured thicknesses of mineralized zone at points on or close to section line; hachured vertical lines 

indicate measurements intermediate between sections; solid lines indicate top of mineralized zone (base 

of Rosiclare sandstone member); dashed lines indicate interpreted base of mineralized zone; S, points 

of streaky or slight mineralization; N, points of practically no mineralization.

places. The thicker bodies occur beneath steep dips of the roof, and 

seem to be near or along the axes of pitching synclinal troughs.

The thinning of banded fluorspar between the shale roof and 

the massive floor limestone is well shown in the exposures at the adit 

portal (see pi. 13, B}.

West Morrison adit and open cuts, and Oxford pits. The West 

Morrison adit is about 850 feet slightly south of west from the Lead adit
60 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

portal. The adit and open cuts extend southwesterly from this point 

about 850 feet to the Oxford pits. At the adit very little development 

has been done; from 3 to 4ieet of fluorspar containing an appreciable 

amount of unreplaced carbonate appears beneath a shale roof in which 

there is a roof fissure trending N. 45° E. The pits are in a residual 

"gravel" deposit, with blocks of fluorspar resting in thick surface 

clay mantle, derived from original bedding-replacement ore at the 

Rosiclare sandstone horizon. To the southwest a mineral zone 20 

to 25 feet below this horizon has yielded some ore.

The Oxford pits, essentially a continuation of the West Morrison 

pits, obtain material from a disintegrated zone extending through a 

range of 30 to 40 feet, between the Rosiclare sandstone and "subRosiclare sandstone." At the south Oxford pit the contact between 

the "sub-Rosiclare sandstone" and the overlying dense limestone 

pitches to the northeast, about 10 feet in 100, and at the northeast it 

dips below pinnacles of the dense limestone, around which "gravel 

tipar" occurs.

The highly calcareous nature of the markedly cross-bedded "subRosiclare sandstone" is well displayed in fresh cuts at this locality. The 

layers of this rock vary from calcareous sandstone to sandy limestone, 

with the sandstone facies predominating.

Prospect shafts east of Green-Defender mine. The Benzon Fluorspar 

Co. has recently prospected on the crest and sides of a low knoll, 

1,500 to 2,000 feet east of the Green-Defender workings, in the lowlands (East Green prospect on pi. 1). The "sub-Rosiclare sandstone" 

appears near the top of this knoll and has been penetrated by several 

prospect shafts and drill holes. A considerable thickness of fluorspar, 

largely in a clay and sand matrix, and resting on an irregular limestone 

surface, has been reported.

Victory Fluorspar Mining Co. The properties of the Victory Fluorspar Mining Co. (post office address, Elizabethtown, 111.) are situated 

north of the property of the Benzon Fluorspar Co., on the nearly 

level top of Spar Mountain, just north of the escarpment.

The workings are all underground and comprise approximately 

4,000 feet of drifts and cross cuts, besides many large irregular rooms. 

The company operates two shafts, the older of which, No. 1, is near 

the east end of the property, and the other, No. 2, about 1,100 feet 

west by south of No. 1. The workings from the two shafts did not 

connect in 1935. Up to 1937, most of the drifting and mining has been 

done in No. 1 mine, but No. 2 mine was being developed with a view 

to connecting it soon with the other. The drift map of No. 1 mine as 

it was in 1934 is shown is figure 8. The company has built a new 

mill of 150-ton capacity per 8-hour day at the No. 2 shaft; a flow 

sheet of this mill is given in plate 18.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 61

The shafts reach the ore horizon at the base of the Rosiclare sandstone, at depths of about 145 and 130 feet, respectively. For the 

most part underground development has followed the ore bodies, but 

in addition several long drifts have been driven with a view to developing a retreating method of mining. The area opened up by the 

No. 1 mine was about 500 feet square in 1934.

The thickest mineral bodies occupy the sharper folds, particularly 

where the pitch of the roof is steepest, but two broad areas of this 

mineralized ground underlie parts of the roof that are low and flat. 

The average thickness of the developed ore bodies is probably

FIGURE 8. Map of drifts in Victory No. 1 mine. Courtesy of Victory Fluorspar Mining Co.

between 5 and 6 feet, with a maximum thickness of approximately 

12 feet. Plate 17, B, is a photograph of an ore face. So far, only the 

Rosiclare sandstone horizon has been explored. Roof fissures are 

present but have,not been used as "leaders" in this mine.

No. 1 shaft has been sunk within a comparatively large area of 

barren or low-grade material, but about 150 feet north of the shaft, 

the roof pitches steeply northward and the fluorspar body thickens 

somewhat abruptly. About 200 feet north of the shaft from 3 to 5 feet 

of fluorspar was found, and the mineral zone thickens to 7 feet a short 

distance beyond this point, as the roof rises sharply on the limb of a 

narrow, pitching anticline whose axis trends about N. 70° W. Excellent ore was seen for the greater part of 200 feet. Drifts running 

westward to northwestward from the shaft followed this ore along a
62 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

stretch where it was comparatively thin, being under a level part of 

the roof, but reached a thick body at a distance of approximately 

350 feet from the shaft, where the roof rises markedly to the southeast 

and the contours indicate local warping. From 50 to 75 feet north of 

this point, however, the roof is low and exceptionally level through a 

width of 100 feet, and here the mineralized zone is thin. At a point 

100 feet southwest of the shaft and for 200 feet west-southwest, the 

roof slope was again seen to rise abruptly and two small but thick (5 to 

7 feet) ore bodies were found. It thus appears helpful to watch the 

dip of the roof closely as a guide in development.

At one place on an east-west drift along the northern border of 

the workings a small fault trending N. 70°E. and dipping 83° to the 

north by west, was observed. The fault is of postmineral origin, as 

is shown by brecciation and pulverizing of the fluorspar body it crosses; 

striations on the slickensided surface indicate a nearly horizontal 

displacement.

At several places, particularly in No. 2 mine, flat cavities at the top 

of the ore body are lined with large crystals of fluorspar, some cubes of 

which show faces several inches across. In some places thin seams of 

clear crystalline purple fluorspar penetrate the shale roof parallel to 

the bedding. (See pi. 15, B.) Barite was seen at several places, but 

it is not abundant or conspicuous in the mine.

, At several places in No. 1 mine, beds of dense, brown limestone, an 

inch to a foot or two thick were seen to separate beds of typical "coontail" fluorspar. In general such limestones appear to have been replaced only slightly or not at all, while adjoining beds of more porous 

texture and variable crystallinity were replaced by bodies of high 

grade ore. It is common for several thick layers of well-layered 

fluorspar, showing cross-bedding structure, to be separated by thin 

partings of dense brown limestone or shale which range in thickness 

from a fraction of an inch to several inches but are usually not more 

than 3 inches thick.

Cave in Rock mine. The abandoned Cave in Rock mine is situated 

at the base of the southwest slope of Spar Mountain, about 1,200 feet 

northwest of the Oxford Pits of the Benzon Fluorspar Co., in the 

NW.Ji NE.% sec. 4, T. 125, R. 9 E. Adits have been driven into the 

slope at two levels, one at the base of the Rosiclare sandstone, and the 

other at the horizon of the "sub-Rosiclare sandstone," which at this 

locality is about 40 feet lower. The workings were not accessible, 

but a brief description of them has been given by Bastin.38

A fault of north by west trend £fault No. 10, pi. 1) appears to be 

intersected by the lower workings, and probably also by the upper 

workings. In the lower workings the cap rock appears to be a dense

88 Bastin, E. S., The fluorspar deposits of Hardin and Pope counties, 111.: 111. Geol. Survey Bull. 58, pp. 

92-93,1931.
GEOLOGICAL SUBVEY BULLETIN 942 PLATE 16

EXPLANATION

JI PORTAL

C* PILLAR

.;-;;' UNDERCUT OR LOWER LEVEL

, BENCH ?

 PROMINENT ROOF FISSURE

(BROKEN WHERE INFERRED)

INCOMPLETELY MAPPED

O_____ICO ZOO__ 3OO 4OO 50O FEET

WEST MORRISON ADIT 

AND PITS

GREEN AND DEFENDER

MAP OF PRINCIPAL UNDERGROUND WORKINGS OF BENZON FLUORSPAR CO. 

Courtesy of J. W. H. Blee.

GEOLOGICAL SURVEY BULLETIN 942 PLATE 17

A. TWO LEVELS OF MINERALIZATION IN LEAD ADIT.

B. FACE OF BANDED FLUORSPAR IN VICTORY NO. 1 MINE.

GEOLOGICAL SURVEY

5"Grizzly

200-ton ore bin

20 tons per hour 

20" belt

f

Water

Scrubbing 

trommel 4'xl4'

Leahy 

vibrating 

screens

5 cells 30"y3tf'

K/ /

5 cells 30"x36" 5 cells

30*36"

\A

/ ^^-O Hutch & bed

Dewztering drag

Overflow

- 6 mesh

Waste 

bin

McLanahan 

_+ stone 

DO roll jaw 

-If] I8"xl8"

OO Rolls Overflow

Slimes

Classifier

Overflow

To pond

Log washer

Truck to river

bin 

2000 tons capacity

FLOW SHEET, \1CTORY FLUORSPAR MINING CO.'S MILL. 

Courtesy of Victory Fluorspar Mining Co.

PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 63

siliceous bed, either a completely silicified limestone, or the completely 

quartzitized "sub-Rosiclare sandstone." The ore body was apparently 

1 to 4 feet thick, but in much of the dump material fluorite layers alternate with layers of crystalline quartz. Considerable galena is said to 

have been found here in pockets. So far as could be judged from surface observations, this mine appears to be partly at least, in a local 

zone of silicification, probably closely connected with a fault fissure. 

The silicification of the cap rock and of the mineral body appears to be 

somewhat similar to that found at the Miller mine (now abandoned) 

on the west side of Lead Hill, and to certain prospects on the east 

side of the hill.

Pittsburgh Fluorspar Products Co, The Pittsburgh Fluorspar 

Products Co., of Pittsburgh, Pa., operates property on the south end 

of Lead Hill. The workings consist of several adits and short, irregular 

drifts on the east and south sides of the hill, near the top. The fluorspar 

is mined chiefly at the base of the Rosiclare sandstone, but some exploratory adits have been driven at levels about 40 feet lower. The ore 

is of the usual "banded" variety. Although the ore bodies are 

comparatively thin, some very high-grade material has been obtained. 

In a typical specimen of ore from one of the upper adits layers of pure, 

coarsely crystalline fluorspar 2 inches thick and showing comb structure alternate with fine-grained impure bands, half an inch thick. 

The ore from these deposits lacks the conspicuous quartz layers that 

are in much of the material found farther north on Lead Hill.

At the No. 1 adit, where the Rosiclare sandstone forms the roof rock, 

the immediate cap rock consists of about 2 feet of shale. Near the portal the underlying limestone is well bedded and breaks in plates about 

1 to 3 inches thick, with thin shaly partings. From \% to 2 feet of high 

grade fluorspar was seen in this opening, and the pure layers greatly 

exceed the impure layers in thickness (see pi. 11). At the face two 

diverging "runs" of ore followed distinct sags in the roof, and several 

veinlets in the roof followed the axis of the sag above the thickest part 

of the mineral body.

At No. 2 adit, about 100 feet south of No. 1, the shale cap rock 

attains a thickness of 4 or 5 feet and pitches to the north. The fluorspar body thickens down the dip of the roof. Bedding veinlets occur 

in the lower part of the shale cap rock.

At No. 5 adit on the south side of the hill the cap rock is a dense limestone bed that lies 40 feet below the base of the Rosiclare sandstone. 

The adit follows a central roof vein in a sag, and trends N. 20° E. This 

vein has a maximum thickness of 3 inches and was followed from the 

surface exposure. At the face of the adit coarse-grained vein material 

was seen to extend from the floor to the roof. The vein widens toward 

the roof, and bedding-replacement fluorspar extends from the vein
64 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

laterally below the capping, but this bedding material averaged 

only a few inches in thickness.

At the portal of a new adit on the southeast side of the hill, about 41 

feet below the Rosiclare sandstone there obviously has been replacement along bedding planes of the limestone. The mineralization at 

this point is slight to incipient and definitely shows penetration along 

bedded and cross-bedded limestone, in part extending from a mineralized minor joint plane. A layer of thin-bedded limestone 4 to 12 inches 

thick is more strongly mineralized than more massive beds above and 

below. Both fresh and weathered limestone show considerable variation in texture, and thickness of individual layers.

C. M. Miller mine (abandoned). The abandoned C. M. Miller mine, 

operated last by the Basic Mineral Co. of Pittsburgh, Pa., is situated on 

the west side of Lead Hill, about 500 feet west by north of the No. 1 

adit of the Pittsburgh Fluorspar Products Co.'s workings. The mine 

was inaccessible at the time of this investigation, but the workings 

have been briefly described in earlier publications. 39 Present exposures 

at this locality show considerable silicification of limestone. Bastin 

reported as follows:

Several small branching drifts and small pits develop a replacement deposit, 

similar in general type to the deposits at Spar Mountain, in Fredonia limestone 

not far below the Rosiclare sandstone.

Within a zone about six feet thick occur two or three flat-lying replacement zones 

of fluorspar ore as much as \/z l feet thick and parallel to the bedding. * * *

In some vugs crystals of white quartz as much as one-half inch across appear to 

have formed contemporaneously with the fluorite, and in others it encrusts fluorite 

crystals and is clearly younger. Galena is of spotty distribution but may form 

masses three to four inches across.

In 1917 the present writer made the following observation in workings that are now caved:

Near the entrance to the northern end of the two tunnels running into the hill, 

the horizontal mass of fluorspar is seen to pass downward, funnel-shaped, into 

a narrow vein, which evidently represents the channel followed by the ore- bearing 

solutions.

Miscellaneous prospects. Several other prospect shafts and adits 

are distributed over the west and east sides of Lead Hill and in the 

topographic saddle between Lead Hill and Spar Mountain (see pi. 3), 

but little was learned from them, for in 1935 none of them had been 

developed to the producing stage and all were inaccessible.

At a prospect (the Chester Martin?) part way up on the east side 

of Lead Hill a bedding-replacement deposit has been exposed below 

a roof of massive, blocky, dense limestone. A persistent layer of 

chert 2 to 3 inches thick forms the immediate cap rock of the deposit,

3» Bastin, E. S., op. eit., p. 94. Currier, L. W., in Weller, Stuart, and others, The geology of Hardin 

County, 111.: Illinois Geol. Survey Bull. 41, pp. 300-301, 1920.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT' 65

which is about 4 feet thick. The alternate layers are siliceous and 

consist in large part of quartz crystals. Some of the limestone below 

the chert shows prominent silicification.

MINING METHODS

The methods of mining the bedding-replacement deposits are simple 

as compared with the vein deposits and entail no particular difficulties. Where the deposits are exposed along a slope, they are entered 

through horizontal adits, from which drifts, crosscuts, and irregular 

rooms are developed. Generally these openings follow the mineral 

bodies, avoiding the lean or barren material so far as possible, and 

leaving pillars for support as needed. After a mine has been worked 

out it is planned to retreat to the portal, removing such pillars as 

contain ore. The procedure thus resembles the retreating method 

utilized in coal mining. Because of the general practice of mining 

to the economic limits of ore bodies, and first following the richer 

runs, and because of the structural characteristics of the ore bodies, 

the rooms and drifts are not developed according to a definite pattern 

or with regular spacing.

The practice, in some mines, of following roof veins as "leaders" 

has already been mentioned. There appears to be a geologic foundation for this practice, and probably further attention should be paid 

to the relation of ore bodies to roof contours, especially with regard to 

the advisability of developing exploratory crosscuts toward the structural low points in the roof flexures.

Where the ore horizon is not exposed at the surface, vertical shafts 

are sunk, from which drifts and rooms are developed. In these workings also a retreating method is generally planned, but economy 

forces the operator to develop a larger area hi shaft mining than in 

adit mining before the pillars are removed.

Very little timbering is required in underground operations as 

practiced hi this district. The amount of underground water encountered, also, is very small and presents no particular difficulties. 

In deeper mining further northeast, however, greater quantities of 

water will doubtless be met, increasing the costs of operation.

Deposits of "gravel" fluorspar in surface clays are mined by opencut methods, and a steam shovel is serviceable in places.

MILLING METHODS

Milling fluorspar ores low in silica from the bedding-replacement 

deposits is a comparatively simple process. The constituents of the 

ores are chiefly fluorspar, limestone, and clay, though very subordinate 

amounts of shale and sandstone also find their way into the mill heads, 

and these rocks contain most of the silica that is in the crude ores.
66 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

Sulfides have so far been too rare to affect the design of flow sheets. 

As the minerals of the ores are mostly coarse-grained and relatively 

free, simple methods of crushing and classifying serve to separate the 

mineral components sufficiently for economical concentration.

By careful selection in the mines, much material has been obtained 

that required simple washing to remove adhering clay, but the usual 

practice, regarded as more economical, is to send all the mine product

Run of mine ore

Blake jaw crusher 

set at 2"opening

Doable steel log washer Minus 65 mesh overflow 

 »-E|evato_r

Minus '4-

Finished bin 

more than 85 % CaF2 

less than 5% Si02

 24"Symons __ crusher- j__ Picking belt 

Waste only_picked

Minus 325 mesh to waste-" Dewatering drag

Flotation stock minus 65 plus 325 mesh

FIGURE 9. Flow sheet, Crystal Fluorspar Co.'s concentrating mill. Courtesy of Crystal Fluorspar Co.

to the mill for coarse crushing and washing, and then to remove the 

waste from the oversize product of the washing trommel by hand 

picking from a picking belt. The cleaned and picked material is then 

crushed and, together with the trommel undersize product, sized and 

sent to concentrating machines. Details of operations at the three 

mills of the district are shown in figures 9 and 10 and plate 18, the data 

for which were kindly furnished by the respective operators.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 67

Surface Ores Mine Ores

3"Grizzlv 3" Grizzly

"1 _ (OVERSIZE) f~~

Feed bin i

Overflow Log washer 

to waste^* 1 1 

Elevator

Cr

R 

Waj

t 

UJ

N 

</)

a.

LJ 

Q

i J

Sizing trommel

1 (OVE

\ 

aw crusher 

^ (UNDERSIZE)

ude ore bin

otary feeder 

>hing trommel

"^111 

Picking belt

1 -Waste

aw crusher

* * 

Elevator u

,. Rolls

*^ .

Hydraulic classifier

:RSIZE) _ . 1

, (FINES) "^ 1

4 Ha'-z Jigs

T CH

(C

Dewaterer 

Tailings 'conveyor 

Tailings bin 

Waste

Wi If lev tables

UTCH) ^

, f TAILS') ,

ONCENTRATES)

Dewalerer

Concentrates conveyor 

Concentrates bin

Market

V

FIGURE 10. Flow sheet, Benzon Fluorspar Co.'s concentrating mill. Courtesy of Benzon Fluorspar Co.
68 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

ECONOMIC CONSIDERATIONS 40 

USES AND GRADES

The uses and market grades of fluorspar have been'described in 

earlier publications 41 and need to be only briefly reviewed Jiere for 

the purpose of showing the general adaptability of the Cave in Rock 

ores to the chief industrial requirements.'

The principal use of the fluorspar is as a metallurgical flux in the 

production of steel by the basic open-hearth process. For this purpose 

admixed calcite is not objectionable, and a moderate amount of silica 

is allowable; silica, however, reduces the available effective units of 

calcium fluoride in the flux, so that a standard maximum content is 

generally specified. Sulfur, even in very small amounts, is objectionable, and, if sulfide minerals and barite are present hi the ore they 

must be largerly eliminated by milling. Metallurgical fluorspar is 

generally required to be in "gravel" form, and the standard gravel spar 

is composed of fragments less than 1 inch in diameter; "fines" are 

objectionable because of excessive losses in the flues brought about by 

the furnace drafts. On the other hand, fluorspar that is too coarse 

is not readily assimilated. For these reasons a standard size range is 

specified. Metallurgical fluorspar is used also in the manufacture of 

"electric-steel" and ferroalloys, and in foundry work, but only a small 

proportion of the metallurgical fluorspar produced is used for these 

purposes. Most of the Cave in Rock ores mined up to the present time 

have been milled easily to produce standard fluxing spar, for their 

fluorspar content has been high, their calcium carbonate and silica 

contents correspondingly low, and their sulfur content negligible; 

moreover, fine crushing has not been necessary.

Where a large amount of sphalerite is present, as hi deposits recently 

discovered and opened (1937 and 1941, see Addendum.) p. 71) in 

the northeastern part of the area, a special milling process is required 

to separate the fluorspar and the zinc sulfide; a flotation method is 

used for this purpose and it requires fine grinding of the ores, but it 

permits adequate separation of the minerals to give a high-grade 

fluorspar concentrate that may be of "acid" grade, as well as a zinc 

concentrate. For use as a flux in the steel industry the fine-grained 

fluorspar product of the flotation process requires sintering and pelletization.

M Although no attempt has been made to bring this report as a whole up to date (May 1943), some data 

regarding market grades of fluorspar production and recent references have been added to this and the 

succeeding section on reserves.

« Bastin, E. S., The fluorspar deposits of Hardin and Pope Counties, 111.: 111. Geol. Survey Bull. 58, pp. 

101-104, 1931. Ladoo, R. B., Fluorspar, its mining, milling, and utilization: U. S. Bur. Mines Bull. 244, 

1927. U. S. Bur. Mines, Mineial Resources U. S., 1926, pt. 2, pp. 30-34; Minerals Yearbook for years 1935 to 

1940, and 1941 (Review of 1940), chapters on fluorspar and cryolite. Hatmaker, Paul, and Davis, H. W., 

The fluorspar industry of the United States, with special reference to the Illinois-Kentucky district: Illinois 

Geol. Survey Bull. 59, 1938.
PART 1. GEOLOGY OF CAVE IN ROCK DISTRICT 69

For use by the chemical and alumium industries fluorspar of particularly high grade is required. Both silica and sulfur-bearing minerals 

are very objectionable, and even calcium- carbonate is undesirable 

because of its frothing action. Standard specifications for "acid 

spar," the grade used for these purposes, have required at least 

98 percent of CaF2 and not over 1 percent of SiO2 . Though the richer 

parts of some of the ore bodies in the Cave in Rock district have 

often yielded very high grade ore, suitable for the production of 

"acid spar," some of the ores, particularly from the margins of ore 

bodies, have required mixing with high grade ore in order to meet the 

requirements for standard metallurgical fluorspar, and consequently 

most of the fluorspar marketed from this district has been of metallurgical grade.

The ceramic industry uses fluorspar in the manufacture of certain 

glasses and enamels, and for these purposes ground fluorspar that 

contains at least 95 percent of CaF2 and not over 3 percent of SiO2 

is considered ^standard.

The chief commercial grades of fluorspar as of 1940 are summarized 

in the following table, quoted from U. S. Bureau of Mines, Minerals 

Yearbook, Review of 1940:

Grade

Metallurgical _.

Acid.. .........

Form

Washed gravel, less than 1 inch and not more than 

15 percent fines. 

Glass and enamel -

Lump, gravel, and 3

CaF2 

(minimum 

percent)

85

95 

98

SiOj 

(maximum 

percent)

5

3 

1

FezOs 

(maximum 

percent)

0.12

Current market prices have been based upon these grades, but the 

specifications as given above have not been strictly adhered to, 

particularly with respect to metallurgical spar, for some consumers 

have accepted as fluxing spar material that contained as low as 80 percent of CaF2 and as high as 6 percent of SiO2 , the price being adjusted 

accordingly.

PRODUCTION

Prior to 1919 production from the bedding-replacement deposits of 

the Cave in Rock district was intermittent and inconsiderable in comparison with the total production of the Illinois-Kentucky field. Since 

that year, as a result of more consistent development of old properties, and exploration and development of new ones, the district has 

gained rapidly in importance as a producer of fluorspar, and it is likely 

to make a considerable further production in the future.

The purity of much of the Cave .in Rock ores and the low cost of 

mining and milling them as compared with many of the vein deposits 

gave a distinct economic advantage to the district in its earlier days,
70 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

and it seems fairly probable that in the future it will be possible to 

extend operations over a much larger area, thus materially increasing 

the reserves, and that the district will, for some time, becorrfe increasingly productive. The production of fluorspar in this district by 

years since 1919, as compared with the total production in Illinois 

and the United States, is shown graphically on plate 19. 42 The data 

upon which these curves are based represent shipments rather than 

actual production, so that for any single year only approximate production is indicated; the difference, however, from actual production 

figures is probably small. Over a period of 2 years or more the accordance will doubtless be close, and the curves will therefore give a fair 

idea of the relative industrial importance of the district.

RESERVES

Owing to lack of extensive development and of sufficient exploration 

in advance of mining, it is impossible to compute the reserves of the 

district in the usual manner, but the structural and genetic relations 

of the deposits, and the geologic features of the district seem sufficiently clear to justify a rough estimate based on reasonable assumptions and on the extrapolation of certain calculated factors. The four 

primary factors that enter into the estimation of fluorspar reserves in 

the Cave in Rock district are (1) area of geologically favorable territory, (2) percentage of that area that is mineralized, (3) average concentration of fluorspar in mineralized ground, and (4) average percentage recovery of marketable fluorspar from mineral bodies of the 

bedding-replacement type.

These factors cannot be evaluated closely. In particular, the extent 

of the mineralized ground and the average concentration of fluorspar 

in that ground are determinable only from data to be obtained 

through extensive and well-distributed underground exploration. An 

estimate of possible reserves in the mapped area was made in 1936.45 

It seemed probable, when the known major geologic features were 

viewed according to the theory of origin (presented in this report) that 

between 900 and 1,000 acres of the mapped area were favorable to the 

deposition of fluorspar ore. Less than a third of this area, however, had 

yielded specific data for evaluating the factors involved in the calculation of possible reserves. The factors thus derived were applied to the 

remaining two-thirds or more of the area, and a reserve of 500,000 to 

700,000 tons of marketable fluorspar was thus calculated for the area 

mapped. It was pointed out, however, and is reaffirmed here, that 

this estimate was based largely upon geologic interpretation, and that 

its chief value would probably consist in the stimulus it might give to 

prospecting and exploring in the undeveloped parts of the district.

« This set of curves has been brought up to 1940 since preparation of this report.

<3 Currier, L. W., Geologic factors in the interpretation of fluorspar reserves in the Illinois-Kentucky field: 

.8, Geol. Survey Bull. 886-B, p. 13, 1937.
GEOLOGICAL SURVEY 

275

250

BULLETIN 942 PLATE 19

55

5O

1916 19,7 |9|8 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940

FLUORSPAR

&) (CURVE PER TON S

5o 

POUND

3 Ui

LJ 

>

cr

D

y- 7t 75 

1- 

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ul 

U

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/

s

/

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I

1 

1

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\ \ \

^

, ..

$-£**

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<§U.^'

ENTAGE

t.AGE_ V

 RSPAR

DOMES

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UJJLIZA

nc FLU

R TON_

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2£S£AR__

£LUXING

CTOR-. _P

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-^ORS

OUNDS L

EEU_JIiS

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-*- 

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 ^'

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w o> DOLLARS IN

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5 £o

AVERAGE VALUEPER TON

(CURVE

GRAPH SHOWING PRODUCTION AND UTILIZATION OF FLUORSPAR, 1916-40.

ADDENDUM

Since the field work that forms the basis of this report was completed, 

in 1935, several significant developments have taken place in the 

Cave in Rock district. It has not been possible to do further field 

work in order to bring the body of the report up to date, but it is 

desirable to add a brief summary of the significant data recently 

acquired from various sources.

According to available data 44 about 300,000 tons of fluorspar 

have been shipped from the Cave in Rock district from the beginning 

of 1936, when the above reserve estimate was made, to the end of 

1942. Most of this production has come from properties that were 

being operated in 1936, but a considerable part has .come from a deposit 

discovered in the northeastern part of the mapped area since the 

field study for this report was made. Another large deposit recently 

discovered and explored northeast of the mapped area has greatly 

extended the district and is expected to greatly increase the potential 

reserves.

In 1937, shortly after the completion of this report, the Mahoning 

Mining Co. acquired options on a tract of land in the northeastern 

part of the area as mapped for this report. The property was prospected by drilling to a depth of 300 to 400 feet. A shaft 300 feet deep 

was sunk at a point in the NW% NE% sec. 34, about 3,500 feet north of 

the Crystal mine adit, and near faults 16 and 17 (see pi. 3) in an area 

where both surface geologic data and resistivity data indicate structural features favorable to mineralization. It is reported that mineralized beds have been found on this property in the Renault formation, 

in the top of the Fredonia limestone, and in the "sub-Rosiclare sandstone," about 60 feet below the base of the Rosiclare sandstone member; 

the "sub-Rosiclare" is said to contain the main ore body. The ore 

bodies here are composed of alternating layers of rather coarsely 

crystalline fluorspar and sphalerite. Both minerals are recovered by 

flotation at the company's mill in Rosiclare. The ore bodies appear 

to be closely associated with two minor faults about 100 to 120 feet 

apart. The company has also sunk three shafts to other bedding 

replacement deposits in the northwest and northeast quarters of section 35, east of the W. L. Davis shaft; at two of these places the mineralized horizons are said to be in the upper parts of the Renault 

formation and the Fredonia limestone.

Another recent development of considerable significance is that of 

the Minerva Oil Co., which has drilled extensively in an area consisting

« U. B. Bur. Mines, Minerals Yearbook for years 1937 to 1940, inclusive. H. W. Davis, personal communication, May 8, 1942. /

71 

651386 44 6
72 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

of 3,000, acres that lie northeast of the W. L. Davis mine. According 

to a summary report of the Bureau of Mines 45 more than a hundred 

holes have been drilled here, and 311,000 tons of ore containing 5.47 

percent of zinc and 37.1 percent of fluorspar and 219,000 tons of ore 

containing 3.33 percent of zinc, 20.53 percent of fluorspar, and 0.56 

percent of barite are indicated. The ore is said to occur as beddingreplacement deposits in the Renault formation, the Levias limestone, 

and the top of the Fredonia limestone. A shaft is being sunk to a 

depth of about 640 feet, where it will presumably reach into or through 

the Levias limestone. The shaft is located approximately at the center 

of the southeast quarter of section 24, nearly three miles northeast 

of the Crystal mine adit.

In 1936 the workings of the Crystal mine were at the top of the 

Fredonia limestone, and the shale base of the Rosiclare sandstone 

formed the cap rock of the bedding-replacement deposits that constituted the ore body. Since that year, not only have the original workings been greatly extended to the northeast the length in this direction being now 1,750 feet but crosscuts to the northwest have reached 

a second ore body parallel with the first; moreover, ore is reported to 

be obtained at several levels in the upper 45 feet of the Fredonia limestone, and at the " sub-Rosiclare" horizon. A small amount of ore 

is reported to occur also in the Renault formation.

According to the data cited above, the recently discovered deposits 

occur at the same stratigraphic horizons as those previously studied 

in the area mapped; that is, in the uppermost beds of the Fredonia 

limestone member, in the "sub-Rosiclare sandstone," and hi the 

limestone beds of the Renault formation. In addition the Levias 

limestone member is said to be mineralized in places. Details of structural control in the newly developed areas are not known, but it is 

clear that in part, at least, the deposits are closely associated with 

minor faults, and that these faults and the longer dimensions of the 

mineral bodies trend approximately parallel with the Peters Creek 

fault zone the principal axis of regional deformation although 

quite remote from this zone.

The discovery of bedding-replacement deposits in the southeastern 

quarter of section 24, nearly 3 miles northeast of the crystal adit and 

nearly a mile and a quarter from the Peters Creek fault zone, has 

definitely extended the area to be recommended for prospecting 

considerably beyond the limits of the mapped area. Such deposits, 

and consequently favorable structural conditions are thus known to 

exist through an area of at least five square miles, all of which is worthy 

of serious consideration. A detailed geologic and geophysical investigation is recommended.

*> U. 8. Bur. Mines, War minerals report No. 118, Zinc-fluorspar property of the Minerva Oil Co., Cave 

in Rock, Hardin Connty, 111., October, 1942.
Part 2. AN EXPLORATORY STUDY OF FAULTS IN THE

CAYE IN ROCK AND ROSICLARE DISTRICTS

BY THE EARTH-RESISTIVITY METHOD

By M. KING HUBBERT

ABSTRACT

This report is a presentation of the results of an electrical-resistivity survey 

conducted in the fluorspar-bearing areas of Hardin County, IU., principally during 

the field seasons of 1934 and 1935. Gish-Rooney apparatus employing the Wenner 

4-electrode method of measurement, was used. Traverses were run in directions 

usually transverse to the strike of the dominant systems of faults. In all but two 

short profiles, the electrode spacing was 100 feet with the measurements made at 

100-foot intervals. The line of electrodes was parallel to the line of traverse.

Most of the work was done in two districts, one near the town of Rosiclare and 

the other in the mining district northwest of Cave in Rock. In many cases the 

presence of faults was clearly indicated. A fault was mapped for a distance of 

about 2 miles in the alluvium-filled valley of Wallace Branch, about 3.5 miles west 

of Rosiclare. Some of the branches of the Peters Creek fault in the Cave in Rock 

district were clearly shown. In many other instances anomalies appeared, 

the causes of which were not evident, but they may be related to structural 

disturbances.

' One curious anomaly was discovered at station 83.5 on profile 13 in the Rosiclare 

district. It suggests an unusual subsurface condition, which warrants further 

consideration.

In the Cave in Rock district a region of marked disturbance was indicated in 

the northeast part of the resistivity grid, suggesting the possibility of complex 

fracturing of small displacement favorable to the infiltration of mineralizing 

solutions. This locality is indicated as being one of the most hopeful prospects 

of the territory surveyed.

Although the significance of many of the observed anomalies is not clear, it is 

possible that their existence may provide useful clues for the guidance of direct 

prospecting. " With this in view, not only have the author's own interpretations 

been presented, but the direct measurements themselves have been plotted in 

graphical form.

INTRODUCTION

One of the principal fluorspar-producing regions of the world is 

located in southern Illinois and western Kentucky. Except for a 

small area in Pope County, the .Illinois portion of this Illinois-Kentucky field is hi Hardin County, and the Rosiclare and Cave in Rock 

districts, dealt with in this report, are in that county.

73
74 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

Hardin County is bounded on the south and east by the Ohio River 

(see fig. 1). Its topography is rather broken, with elevations ranging 

from 350 to more than 700 feet above sea level. The geologic formations exposed in the county are principally sedimentary, though 

there are several small intrusions of igneous rocks hi the area. The 

outcropping sedimentary rocks range in age from Devonian to Carboniferous. They are partly covered with wind-blown loess over the 

uplands and stream alluvium in the valleys. The rocks are described 

in part 1 of this report, and the stratigraphic sequence is shown in 

figure 3.

CLASSIFICATION OF ROCKS ON THE BASIS OF PHYSICAL

PROPERTIES

For the purposes of the present investigation the stratigraphic 

column of the area falls into three principal subdivisions, based upon 

the physical properties of the rocks. The lowest of these divisions extends up to the top of the Fredonia limestone. It is characterized by 

hard, massive, electrically-resistive rocks limestone, chert, and shale. 

The middle division extends from the top of the Fredonia limestone 

to the bottom of the Pottsville group. Owing to successive repetition 

of sandstones, shales, and limestones, its physical properities are 

variable. The upper division extends upward from the base of the 

Pottsville group. The sandstones of the Pottsville are massive and 

are highly resistive to the flow of electricity. They form the highlands 

of the county and frequently crop out as bluffs.

Most of the work for the present report has been done in areas 

containing outcrops of formations of the middle division and of the 

upper part of the lower division.

Structurally, Hardin County is chiefly characterized by an intricate 

system of normal faults, which cut the sediments into a mosaic of 

slightly tilted fault blocks in which the dip of the beds commonly ranges 

from 0° to 15°. Most of the major faults strike northeastward, and 

the blocks between them are cut by shorter transverse faults into 

many small irregular segments.

Isolated occurrences of fluorspar are found in all parts of the county, 

but the greater part of the mining production has come from two 

rather small areas. One of these, called the Rosiclare district, is in a 

faulted belt that extends 3 miles in a direction about N. 30° E. from 

the Ohio River through the west edge of the town of Rosiclare. The 

second is called the Cave in Rock district, though it lies about 4 miles 

northwest of the town of Cave in Rock.

The deposits are mostly of two kinds, vein deposits and beddingreplacement deposits. The vein deposits are characteristic of the 

Rosiclare district, and the bedding-replacement deposits characterize 

the Cave in Rock district. In both kinds of deposits the chief gangue
PAET 2. STUDY BY EARTH-RESISTIVITY METHOD 75

mineral is calcite. The vein depqsits consist of veins of fluorspar extending along normal faults whose dips range from 45° to 90°. The 

veins vary in thickness, their maximum thickness being about 15 feet, 

and some of them extend as deep as the mines have been worked, 

which is more than 700 feet. The bedding-replacement deposits consist of nearly horizontal sheets roughly parallel to the bedding of the 

sedimentary rocks. All those that have been worked occur in the 

Cave in Rock district at and near the top of the Fredonia limestone. 

Detailed descriptions of the bedding-replacement deposits and of their 

geologic setting are given in part 1 of this report.

PROSPECTING METHODS

The methods of prospecting for additional fluorspar deposits have 

heretofore been more or less direct; the fluorspar has been detected by 

actual observation of the outcropping vein or of the residual gravel in 

the top soil in the vicinity of a deposit, or it has been sought by drilling and by digging test pits. This direct prospecting has always been 

conducted by private enterprise.

Related to the direct methods is that of geologic mapping, whereby 

the rocks cropping out on the surface "have been identified and the 

principal faults located, thus permitting a choice of favorable locations 

for direct prospecting. Hardin County and a part of Pope County 

have been mapped geologically by the Illinois State Geological Survey 

and the Federal Geological Survey. 1

In the search for more fluorspar there remains the possibility of 

applying some of the geophysical techniques currently employed in the 

search for petroleum and for certain ore minerals. Geophysical 

methods of prospecting may be divided into two classes: those by 

means of which the presence of the mineral sought is detected directly, 

and those which are useful indirectly because they detect structures 

commonly found in association with the minerals sought. Salt domes 

in the Gulf Coast region, for example, are directly detected by the use 

of seismographs and torsion balances; but in locating salt domes, oil 

and sulfur are commonly the minerals actually sought, since the salt 

domes frequently, though not in variably, are accompanied by oil and 

sulfur, which are therefore often found indirectly by prospecting for 

salt domes.

All geophysical prospecting consists in measuring physical properties (on top of the ground or in mines or drill holes) that are in some 

manner influenced by the presence or absence of the mineral or struc*- 

tures sought. This requires that the mineral or structure shall have 

some physical property that is susceptible of measurement. Such 

properties include density, elasticity, thermal conductivity, radioactivity, magnetic susceptibility, and electrical conductivity or

i Weller, Stuart, and others, The Geology of Hardin County HI.: Illinois Oeol. Survey Bull. 41,1920.
76 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

resistivity. In respect to whatever property is to be used for its detection, the mineral or structure sought must contrast sharply with the 

surrounding rocks.

Fluorspar differs sharply from many minerals in being virtually 

nonmagnetic and nonradioactive and in having high electrical resistivity, but in these respects it closely resembles the calcite and the 

dense crystalline limestone with which it is most commonly associated. Its density, however, is 3.18, whereas that of calcite is only 2.7, 

and if the deposits of fluorspar were larger, they doubtless could be 

located by gravitational measurements, but their small size, together 

with the unevenness of the local topography, makes it impracticable 

to detect them by this means.

Direct methods of geophysical prospecting', then, do not appear to 

be applicable to fluorspar; but there remain the indirect methods, 

especially the detection of associated structures, of which the most 

common are faults. If the faults in the region can be accurately 

located, some of the fault fissures may be found to contain, or to lead 

up to, hitherto unknown fluorspar deposits.

Such a method is afforded by electrical-resistivity measurements, 

for there is a wide range of difference in electrical resistivity among 

the common sedimentary rocks. In rocks composed of the nonmetallic minerals, as Sundberg 2 has shown, this difference is due 

principally to the conductivity of the water filling the pores. When 

such rocks are free from water, they are almost perfect nonconductors 

of electricity, and if two rocks differing in porosity are both saturated 

with water, the more porous rock will be the better electrical conductor, provided the water in both is equally ionized. If two samples of 

similar rock having equal porosity are both saturated with water but 

the concentration of ionized salts is different in the two samples, the 

sample containing the lesser concentration of ions will be the more 

resistive.

It has been observed in the field,3 and confirmed by laboratory tests, 

that water-saturated gravel deposits are more resistive than watersaturated sand, and the latter more resistive than clay.

As the county has already been mapped geologically, it might at 

first thought appear futile to devote more time and effort to locating 

faults. This would be true were it nt)t for the fact that the geologic 

mapping is based largely on surface outcrops of rocks and that such 

outcrops are discontinuous and local. Not only are the rocks concealed 

in large part by residual soil, but the larger valleys contain extensive 

deposits of alluvium; over large parts of the county, moreover, there 

lies a blanket, ranging up to several feet in thickness, of the wind2 Sundberg, Karl, Effect of impregnating water on electrical conductivity of soils and rocks: Am. lust. 

Min. Met. Eng. Trans., Geophysical prospecting, pp. 367-391,1932.

»Hubbert, M. K., Results of earth-resistivity survey on various geologic structures in Illinois: Am. Inst. 

Min. Met. Enf. Trans., vol. 110, pp. 9-39,1934.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 77

Iblown deposit known as loess. Underground mining operations, 

according to A. H. Cronk,4 superintendent of the Rosiclare Lead and 

Fluorspar Mining Co., have disclosed two or three times as many 

faults as were detected by geologic mapping on the surface.

The evidence indicates, therefore, that there still are undiscovered 

faults in the area and that faults inferred from widely separated 

exposures will in many instances bear more precise location.

THE EARTH-RESISTIVITY METHOD

Where rocks have been faulted, formations of different lithology 

frequently occur on opposite sides of the fault. Where such differences 

occur at or near the surface of the ground, they should afford a means 

of locating the fault, provided a method can be found that is sensitive 

to lithological differences.

Since there are wide variations of porosity and water content as well 

as different degrees of ionization in the contained water in ordinary 

sedimentary rocks, rocks differing in composition and texture should 

differ in electrical resistivity. Differences of resistivity, measured in 

the field, may therefore be of service in locating geologic boundaries 

that cannot be traced by ordinary geologic methods.

TECHNIQUE OF EARTH-RESISTIVITY MEASUREMENT

In 1915 Wenner,5 of the United States Bureau of Standards, described how the resistivity of the ground could be measured. The

MILLIAMMETER

*~*

I

B-BATTERY 

Hllll lllll IIIFIGTJSE 11. Schematic diagram of the Wenner system for measuring earth resistivity.

method is essentially as follows: Four metal stakes, which are to serve 

as electrodes, are driven into the ground at equal intervals along a 

straight line. The arrangement is shown schematically in figure 11. 

Through the outer stakes Ci and C2 and the ground an electric current 

of strength I is made to flow. This causes a potential difference E to 

be set up between the inner pair of stakes, PI and P2 . The quantities 

measured are the current I, the potential difference E^and the electrode 

spacing a. Wenner showed that for the case in which the ground is

* Personal communication.

" Wenner, Frank, A method of measuring earth resistivity: Bur Standards Sci. Paper 258, July 15,1914. 

Reprinted in Bur. Standards Bull., vol. 12, pp. 469-478,1916.
78 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

flat, and homogeneous and isotropic with respect to electrical conductivity, the specific electrical resistivity p is given by the equation:

where p is in ohm-centimeters when a is in centimeters, Ein volts, and 

/ in amperes. (Were a measured in meters or feet, p would be in ohmmeters or ohm-feet, respectively).

VARIABLES IN THE WENNER METHOD

In actual field practice the condition that the surface be flat is usually approximated. The remaining two conditions for which the Wenner equation is valid a ground homogeneous and isotropic to distances great as compared with a are seldom realized. When under 

these circumstances the Wenner equation is applied to the measured 

values of a, E, and /, the value of p obtained no longer represents a 

true or even average resistivity of the ground, but an apparent resistivity whose manner of variation reflects departures of the ground 

from the ideal conditions postulated.

There are yet two more variables involved in the application of the 

Wenner method of resistivity measurements. These are the location 

of the station and the orientation or azimuth of the line of electrodes. 

The location of the station may conveniently be defined as the location 

of the center point between PI and P2 .

On a ground that is flat, homogeneous, and isotropic (such as a large 

and deep body of water) the value of p would be the same for all station locations and azimuths and for all values of a. If, on the contrary, the ground is mhomogeneous and anisotropic, the value of p differs in general for different station locations and azimuths and for 

different values of a. Thus the variation of the apparent resistivity 

p as a function of position, azimuth, and electrode spacing serves as an 

indicator of the inhomogeneity of the ground.

Three principal techniques of measurement are immediately suggested. They consist, respectively, in varying one of the three quantities position, azimuth, and electrode spacing while keeping the 

other two constant. The results of applying these techniques are 

briefly as follows :

I. Variation of azimuth:

This technique consists in taking a series of readings in different 

azimuths about a single station, with constant electrode spacing. The 

results are plotted graphically by means of polar coordinates with the 

station as the origin. For each azimuth a pan- of equal and oppositely 

directed radius vectors whose lengths are proportional to the value 

of p for that azimuth are drawn. The ensemble of these vectors will,
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 79

in general, give a dumbbell-shaped figure symmetrical about the 

station, with an axis of greatest and another of least apparent specific 

resistivity. (See fig. 16.) This procedure is useful in determining 

the strike of structural features.

When the resistivity is independent of the azimuth, the dumbbellshaped figure will degenerate into a circle.

II. Variation of electrode spacing:

The second technique which suggests itself is to vary the electrode 

spacing while keeping the azimuth and the station position fixed. In 

this case the variation of p with the electrode spacing a reflects inhomogeneities with depth. It is useful where the ground is homogeneous in horizontal layers and permits the approximate determination 

of depths of distinctive layers.

III. Variation of station location:

The third technique consists in making measurements at successive 

stations while keeping the electrode spacing and azimuth constant. 

Measurements are made along a line of traverse and the value of p 

plotted as ordinates against distances as abscissas in a resistivity profile. The variations of resistivity in such a profile are due to lateral 

inhomogeneities of the ground.

In a resistivity profile, the effective depth to which the readings are 

influenced is controlled by the magnitude of the electrode spacing, a. 

Although, theoretically, inhomogeneities at any depth influence the 

readings, the variations in the apparent resistivity are due predominantly to inhomogeneities occurring at depths not greater than the 

electrode spacing a.

The azimuth remains an arbitrary element in resistivity traverses. 

Two principal cases suggest themselves (1) when the line of electrodes is parallel to the line of traverse, (2) when the line of electrodes 

is perpendicular to the line of traverse. The latter arrangement has 

many theoretical advantages, but it renders field work slow and difficult; the former lends itself much more readily to rapid field work.

THE GISH-ROONEY APPARATUS

Gish and Rooney 6 were among the first to build apparatus by which 

to apply Wenner's method to measurements of earth resistivity. They 

developed an apparatus that differed somewhat from the one suggested by Wenner but that employed the same basic theory.

The Gish-Rooney apparatus, a form of which was employed in the 

work in Hardin County, consists of a radio B-battery to supply the 

current, a milliammeter to measure the current /, a potentiometer to 

measure the voltage E, and a double commutator. The purpose of the 

commutator is to alternate t}ie current entering the ground and then

 Gish, O. H., and Rooney, W. J., Measurement of resistivity of large masses of undisturbed earth: Terrestrial magnetism and atmospheric electricity, vol. 30, pp. 161-188,1925.
80 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

to rectify the alternating potential difference E occurring across 

the potential stakes. Direct-current instruments may thus be used 

while an alternating current is maintained in the ground. An alternating current is used because it eliminates the effect of stray ground 

currents and balances out effects due to the large potential differences 

that occur spontaneously between the potential stakes because of electrolytic reaction with the ground.

The use of a potentiometer for measuring E has the additional advantage that it allows no current to flow in the potential circuit while 

readings are being made, so that the contact resistances of the potential stakes do not appreciably affect the readings.

The commutators used by Gish and Rooney gave an alternating 

current frequency of about 25 cycles per second. The frequency produced by the present apparatus was about 10 to 12 cycles per second.

It has been found that, in spite of the commutation, enough stray 

voltage frequently gets through to the potentiometer to make its readLags extremely unsteady, and h^nce inaccurate. This unsteadiness can 

be totally eliminated by placing a condenser in series between the 

commutator and the potentiometer. A condenser of 25 microfarads 

was used in the present work, but subsequent tests have shown that a 

very much smaller condenser, possibly as small as 1 microfarad, would 

do equally well. The optional use of a condenser was suggested by 

Gish and Rooney.

FIELD METHODS USED IN HARDIN COUNTY

The method employed in Hardin County was that of Wenner, 

described above, and the apparatus used was one of the Gish and 

Rooney type, copied from a design by F. C. Farnham of the Missouri 

Bureau of Geology and Mines.

The measurements were made along traverses in which the location 

of the station was varied but the azimuth and electrode spacing were 

kept constant. The line of electrodes was kept parallel to the line of 

traverse. The electrode spacing a was 100 feet except on two traverses 

on which it was 200 feet. Readings were taken at intervals equal to 

the electrode spacing 100 feet in all but the two traverses mentioned 

above.

In the Rosiclare area, most of the traverses ran east and west, transverse to the strike of the dominant system of faults (N. 30° E.). In 

the Cave in Rock area, two sets of mutually perpendicular traverses, 

extending in general northwest and northeast, were run. These were 

respectively about parallel and perpendicular to the Peters Creek 

fault zone, the dominant structure of that area.

FIELD OPERATIONS OF REJSISTIVITY PARTY

The crew of the resistivity party consisted of an instrument man, 

a front lineman, and a rear lineman. Each of these men followed a
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 81

 definite routine. The instrument man took a reading, which he either 

recorded himself or called out to. the rear lineman, serving as note 

taker. The rear lineman then pulled up the last stake and brought it 

forward to the front lineman. The instrument man disconnected the 

two front lines from the instrument and moved forward a distance 

of one electrode interval, dragging the rear lines. The front lineman 

dragged forward the two front lines, set the front stake, and connected 

the lines. The instrument man reconnected the front lines; the rear 

lineman connected the two rear lines to the stakes and another reading 

was taken. This cycle was repeated until the traverse was covered. 

The speed at which traversing can be done depends upon the nature 

of the country to be crossed. Along a road it is possible to make 100 

stations, at intervals of 100 feet, in a day. In wooded or brushy 

country the rate is very much slower, 50 stations, or a mile of traverse, 

being a good day's work. If the lines of traverse are to be kept straight 

in wooded or brushy country, especially in country infested with 

blackberry brambles, it is almost necessary for the front lineman to 

be equipped with some form of machete (a corn knife does very well) 

and time can be saved by providing machetes for the instrument man 

and the rear lineman also, in order that all hands may join in clearing 

the line ahead when particularly brushy stretches are encountered.

METHOD OF CONTROL

The ordinary Geological Survey topographic map, on the relatively 

small scale of about one inch to the mile, is too generalized and not 

sufficiently accurate in small details to be of service, except as a 

general guide map, in work of this sort. The following method of 

control was therefore adopted:

The traverses, except when a winding road was followed, were run 

as nearly as possible in straight lines controlled by a Brunton compass. 

The leads to the ground electrodes were marked and used as tapes for 

measuring distances along the traverse,-which could thus be determined within ± 1 percent. At the beginning and end of each traverse, 

and wherever a traverse crossed a main road, a flag was set that was 

marked with the station number for that point a flag set 30 feet 

ahead of station 28, for example, would be marked station 28.3.

In addition to the nearly straight cross-country traverses, planetable traverses were run along the principal roads, on which the positions of the flags, fences, and principal land lines were plotted. The 

traverses run in the Cave in Rock area were located on a large-scale 

topographic map, which was made primarily as a base for the geologic 

work described in part 1.

Between the points on the resistivity traverse that were located by 

plane-table, control was effected by a graphic system of note taking. 

In the left-hand margin of the notebook page a vertical line was drawn
82 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

to represent the line of traverse. A point on this line represented 

graphically the location of the station, and the locations of houses, 

fences, roads, creeks, ridge crests, and various geologic features with 

reference to the station were sketched in.

When the readings were computed and a resistivity profile plotted, 

the resulting map of the traverse, properly oriented, was plotted to 

the same scale below the graph of the profile (pis. 21 and 23). With 

this map in hand it is possible, by following the line of traverse, to 

locate any station within a few feet by measurement from some nearby 

feature whose position has been recorded.

RELATION BETWEEN APPARENT RESISTIVITY AND 

GEOLOGIC STRUCTURE

Since any subsurface inhomogeneity, within the depth range of the 

instrument, of the rocks along the line of traverse may cause variation 

in the observed apparent resistivity, it is well before attempting to 

interpret the actual resistivity curves obtained in the field to consider, 

at least qualitatively, the variations that are likely to be caused by 

the various geologic conditions that are most likely to be encountered. 

Faults are what is chiefly looked for, but the other features by which 

resistivity is affected must be taken account of in order that their 

influence may not be wrongly ascribed to faults.

RESISTIVITY VARIATION DUE TO FAULTS

The present discussion concerns only the faults that have dips of 

more than 45°, for no faults of lower dip occur in Hardin County. 

The resistive properties along these steep faults may be divided into 

three more or less distinct parts the resistivity of the two blocks on 

the one side and on the other side of the fault zone and the resistivity 

of the fault zone itself.

An idealized condition is shown in figure 12 , where the resistivity of 

the block to the left is p^ that of the fault zone p2 , and that of the 

block to the right p3 . Various combinations of pi, p2, and p3 are 

possible. For example, we may have all three unequal:

or we may have what amounts only to two blocks with a sharp 

bounding interface, when:

or the two fault blocks may be of equal resistivity while the fault 

plane itself is either more resistive or more conductive than the blocks:
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 83

In nature, a fault zone rarely consists, as_ these equations assume, 

of a simple sheet whose resistivity is uniform; it may consist of laminae 

that differ greatly hi resistivity. This might well be the case for a 

fault containing both resistive vein material, such as fluorspar or calcite,

FIGURE 12. Three resistivity blocks associated with a vertical fault.

and highly conductive fault gouge. Such a fault zone would be highly 

anisotropic, showing high resistivity to a transverse flow of current 

but low resistivity to a longitudinal flow. There is field evidence that 

zones of this type are not uncommon.

VALUE OF 5

FIGURE 13. Variation of apparent resistivity across a vertical fault. After Tagg.

Some enlightening theoretical and experimental work pertaining to 

these various combinations and permutations has been done by several

men.

Tagg 7 (fig. 13) has computed the resistivity curves for the case 

of a simple fault where

' Tagg, G. F., The earth resistivity method of geophysical prospecting: Mining Mag., vol. 43, pp. 150-158, 

Sept. 1930.
84 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

with the line of electrodes at right angles to the strike of the fault 

plane. The readings are indicated continuously rather than at discrete intervals, and for all ratios of pi/p2 - It is worth note that 

the curve is not smooth; it becomes angular wherever an electrode 

crosses the fault plane.

Howell 8 (fig. 14) has computed similar curves for the cases where 

Pi=Pa and the fault zone is either infinitely conductive or infinitely

1.5

1.0

POSITION OF CONDUCTING SHEET RELATIVE TO ELECTRODES

(a)

POSITION OF INSULATING SHEET RELATIVE TO ELECTRODES

0

c, p'

(b)

FIGURE 14. Variation of the apparent resistivity across (a) a vertical highly conductive sheet and (6) 

across an insulating sheet. After Howell.

resistive. In these cases also the curves are angular at the points 

where an electrode crosses the fault. When the resistance is infinite, 

the curves are even discontinuous at these points. The outstanding 

characteristic of the curves, however, is their W shape. When the 

fault zone is infinitely resistive, the central peak of the W is higher

»Howell, L. G., Discussion of paper by M. King Hubbert: Am. 1st. Mini-Met. Eng. Trans., vol. 110, 

Geophysical prospecting, pp. 34-37, 1934.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 85

than the resistivity of the surrounding region; when it is infinitely 

conductive, this peak is equal to the resistivity of the two sides.

Others 9 have made tank experiments on conductive and resistive 

sheets. For thin sheets a W shape, similar to that given by Howell, is 

characteristic. A thick conductive sheet, however, produces, a resistivity trough. As the sheet is made thinner, an arch begins to form in 

the bottom of the trough, and as the thickness of the sheet approaches 

zero this arch rises to the level of the resistivity of the surrounding 

medium. If the conductive sheet is then replaced by a resistive one, 

the peak becomes still higher, and it continues to rise as the resistive 

sheet is thickened.

The anisotropic condition caused by alternation of conductive and 

resistive sheets was inadvertently produced by the author 10 (fig. 15).

30,000

20,000

1 0,000

V

6 12 18 24 30 36 42 48 

DISTANCE, INCHES

54 60 66 72

7

WATER

j &&&£$£&£ SAND Z/Xg:*::**.

tUJ

I

J

_l '

£ WATER J

UI ^

^- ? -T--^»^y/;^^J

FIGURE 15. Apparent resistivity in a traverse across a vertical sheet of metal with a slightly oily surface.

immersed in water.

A piece of sheet metal with a greasy surface was immersed in a tank 

of witter and a profile transverse to the sheet was taken (fig. 15). From 

what has been said about it is evident that the high-peaked W indicates a resistive sheet. Next, the station was taken directly above 

the sheet and readings made hi different azimuths (fig. 16). The 

radial vectors representing the resistivity in various directions, when 

plotted in polar coordinates, generate a symmetrical dumbbell-shaped 

figure, which shows a resistivity higher than that of the water in a 

direction normal to the sheet and a resistivity lower than that of the

«Zuschlag, Theodore, Discussion of paper by M. King Hubbert: Am. Inst. Mm. Met. Eng. Trans., vol. 

110, Geophysical prospecting, p. 30, 1934.

1° Hubbert, M. K., Results of resisitivity survey on various geologic structures in Illinois: Am. Inst. Min. 

Met. Eng. Trans., vol. 110, Geophysical prospecting, figs. 2, 3,1934. See also discussion by L. G. Howell, 

op. cit., pp. 34-37.
86 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

water in the direction parallel to the sheet. This indicates that the 

sheet is more resistive than the water to a transverse current, and 

more conductive than the water to a longitudinal current. Such a figure as this would give the strike of the sheet were it not already known.

In the limiting case where pi=p2=pz there obviously will be no 

variation but this case would virtually never occur in nature.

The foregoing discussion provides us with a clue to what may be 

expected when a real fault is crossed perpendicularly by a resistivity 

traverse. It is not to be forgotten that in actual field work readings 

are not taken continuously, but at discrete points separated by the 

electrode interval a. The readings obtained should be points on a 

curve resulting from some combination of the separate effects described

FIGURE 16. Variation of apparent resistivity as a function of azimuth with the station directly abovethe

metal sheet of figure 15.

above. If the fault zone is narrow as compared with the electrode interval, its effect, if it has any, should be visible in the readings for not 

less than three consecutive stations, since the line of electrodes spans 

the fault of three consecutive stations. The resistivity curve may 

merely oscillate, or it may level off to a higher resistivity on one side 

and a lower resistivity on the other, depending upon the resistivities of 

the blocks on the opposite sides of the fault.

If the width of the fault zone is w, the resistivity of the zone should 

affect the readings on a traverse perpendicular to the zone for a distance of not less than iu+3a. On a traverse oblique to the fault zone, 

the effect of its resistivity will persist for a longer distance.

EFFECTS DUE TO INTERRELATIONS BETWEEN TOPOGRAPHY AND

STRATA

Different stratified rocks, such as sandstone, shale, and limestone, 

differ in resistivity, so that contacts between them would be marked 

by flexures in transverse resistivity profiles. Only a small proportion 

of such contacts are fault contacts. When the contact between two
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 87

sedimentary rocks reaches the surface, it is most commonly because 

the strata are folded and tilted, or because the surface is uneven, or 

because the two conditions co-exist. In one class of ideally simple

35

FIGURE 17. Variation of apparent resistivity along a traverse over a level surface normal to strike of

dipping strata.

cases, tilted and folded but unfaulted strata are truncated by a plane 

topographic surface; in other cases, more often approached in nature, 

horizontal strata are dissected by an uneven eroded surface. The most

FIGURE 18. Variation of apparent resistivity produced by successive horizontal strata along a traverse

across a ridge.

general case, however, and the commonest, is that of folded strata cut 

by an uneven eroded surface.

In those parts of Har'din County in which the present work has been 

done, the rocks are but slightly flexed. The dips of the strata in the 

individual fault blocks vary from zero up to a maximum of 10° or 15°.

551386 44 7
88 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

The cases that concern us, therefore, are those involving strata with 

gentle homoclinal dips; and these strata crop out on a topography that 

is not rugged on the whole but that includes some steep slopes as well

V/SANDSTONE'.Trx

SHALE 

i i i

" i II

^

-1 i " LIMESTONE -1-

^*S?:. SANDSTONE'. 

^s*^~= SHALE 

r~\ \ 1 1

-1 I " 1 ' 1 ' I" 

1 !__._._..._. 1 1

FIGURE 19. Variation of apparent resistivity produced by successive horizontal strata .along a traverse

across a valley.

as fairly extensive areas of very low relief. Some effects of structures 

and topographic features resembling those found in this region are 

illustrated in figures 17-20. Figure 17 gives an idea of the type of

^*& 

1 "l ' 1

-LIMESTONE-

=^

II i , 

 ' i-LIMESTONEi ' 

 '.' SANDStbNE>\_ ^"'T 1

 SHALE 

- T-LIMESTONEFIGURE 20. Variation of apparent resistivity along a traverse across horizontal strata with faulting and

with topographic relief.

resistivity variation that may be expected from a traverse across the 

strike of uniformly dipping strata. The sharpness of the transition will 

vary with the steepness of the dip, being gradual for gentle dips and-
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 89

.more abrupt for steep dips. The variations are asymmetrical. Figures 

18 and 19 depict traverses across a valley and a ridge respectively, 

both underlain by horizontal strata. The resistivity curve across either 

a valley or a ridge may itself be either troughlike or ridgelike, depending upon whether the more resistive rocks crop out on the sides or 

in the middle. But any resistivity curve crossing a symmetrical valley 

or ridge underlain by horizontal unfaulted strata should be more or 

less symmetrical about the axis of the valley or ridge, and any marked 

asymmetry in a resistivity curve that cannot be accounted for either 

by dipping strata or by topography is quite probably due to the presence of a fault (fig. 20).

EFFECTS DUE TO WEATHERING AND TO ALLUVIUM

Other effects are due to the fact that the surface of the ground is 

blanketed over large areas either by residual soil or alluvium. These

FIGTOE 21. Erratic variation of resistivity across karst topography.

surficial deposits, with the exception of loose dry sand, are almost 

always highly conductive, and where their thickness is large in 

comparison with the electrode interval used they may lead to false 

interpretation of the profiles.

The Fredonia limestone in Hardin County is particularly hazardous 

in this respect. Upon weathering, its lime carbonate is dissolved, 

and there remains a stiff, red clay, which is highly conductive, whereas 

the limestone itself is among the most resistive rocks in the area. The 

limestone does not dissolve uniformly; its surface, beneath the residual 

clay, is marked with crevices and deep pockets of various depths,
90 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

with pinnacles of undissolved limestone between. A further complication is produced by the fact that the Fredonia forms sinkholes and 

caverns, which may contain open air spaces of infinite resistivity 

(fig. 21). Resistivity curves crossing such a limestone may be expected 

to oscillate erratically between the widest extremes, and so in fact 

they do.

FAULT INTERPRETATIONS AS A FUNCTION OF DISTANCE BETWEEN

TRAVERSES

Let us suppose that a series of parallel resistivity traverses, spaced 

at a uniform distance d, have been run across an area transverse to 

the strike of the dominant faults. Assuming that all the faults so 

crossed show up on the resistivity curves (which is improbable) we 

have then the problem of correlating the faults found on one traverse 

with those on the next.

The complexity and difficulty of this problem are much increased 

if the faults are numerous, irregular in strike, and short. If, for 

example, there is only one fault in the area and this fault shows up 

on three or more adjacent profiles, its location is uniquely determined. 

If, on the contrary, the rocks are broken into polygonal blocks bounded 

on all sides by faults, and if many of these faults are so short that they 

do not extend from one traverse to the next, there are an infinite 

number of possible interpretations of the resistivity data, all but one 

of which are wrong. In other words, the probability of finding one, 

and only one, fault pattern that completely satisfies all the resistivity 

data is zero.

When, therefore, an area of complicated block faulting exists and 

the distances between, adjacent traverses are too large to enable one 

to determine the pattern, complete solution of the problem would 

require closer spacing of the traverses. A 100-foot spacing of the 

lines forming the grid work is not at all too close for the precise locating 

of features for mining purposes, particularly for the purpose of controlling exploration by drilling or other methods. Stations on a regular 

grid can be supplemented by stations taken on lines of different 

azimuth drawn through single points, in order to determine the strike; 

and depth tests may be made where needed.

The uncertainty of correlation, between traverses that are too 

widely spaced in a hypothetical area of complex fault pattern is shown 

in figure 22. Figure 22, a, shows what is assumed to be the actual 

fault pattern. It is assumed, also, that there are only two kinds of 

rock at the surface, one of high and the other of low resistivity. A 

fault will then be indicated on a resistivity profile across the area only 

if that profile crosses the fault where it brings rocks of high and low 

resistivity together at or near the surface. Two traverses across the 

area are shown. In figure 22, b and c, the results are shown of two
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 91

L|H H/L

(b)

H|L L|H

H|L

(c)

FIQUEE 22. Difficulty of correlating the faults shown by resistivity profiles A-A' and J3-J3': (o) The 

true fault pattern; (6) and(c) attempts to correlate the resistivity anomalies.
92 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

attempts to correlate the resistivity data on these and to construct 

the fault pattern from them; both results, though consistent with the 

data, fail to give the fault pattern of figure 22, a, which by hypothesis 

is the correct one.

EXTENT AND SCOPE OF THE PRESENT WORK

Preliminary test traverses, together with considerations such as 

the foregoing, early made it evident that isolated or too widely spaced 

traverses in Hardin County would be comparatively useless. Work 

in minute detail, on the other hand, such as mining companies might 

find it profitable to do on their own properties, were much beyond 

the facilities of the present undertaking; a season's work on that 

scale would probably have covered less than one square mile. Hardin 

County as a whole, on the other hand, was too large a unit to be 

covered in the time available. A compromise plan was therefore 

adopted, whereby the work was confined to two of the more likely 

fluorspar-bearing areas, across which traverses were run as close 

together, as time would allow. The closest spacing achieved was about 

one-eighth of a mile. Although this spacing was adequate for locating 

the longer faults, it was not close enough for certain areas where the 

fault pattern appears to be complex; in a number of cases, correlation of 

major faults across several traverses seems certain, but any attempted 

correlation in the more complicated areas would probably be wrong. 

On the whole, the resistivity work that has'been done must be regarded 

as reconnaissance. Enough has been done, however, to indicate a 

number of things about each of the districts that were studied, and 

to give a fair idea of what may be expected from further work of this 

kind.

The two districts selected for study are the Rosiclare and the Cave 

in Rock. The Rosiclare district is bounded on the east by a northsouth line about one mile east of the town of Rosiclare, on the north 

by the center line of T. 12 S., on the west by Big Grand Pierre 

Creek, and on the south by the Ohio River. The Cave in Rock 

district is contained within sees. 27, 33, and 34, T. 11 S., R., 9 E., 

and sees. 3, 4, and 5, T. 12 S., R. 9 E.

In the summer of 1931 A. H. Bell, of the State Geological Survey, 

and the writer made an inspection trip to Hardin County for the purpose of ascertaining what, if any, kind of geophysical method could be 

applied. During this time Mr. Robert Weaver of the Franklin Fluorspar Co. was kind enough to demonstrate to us a set of Gish-Rooney 

apparatus, which he had obtained from the Colorado School of Mines. 

Later in the same summer P. S. McClure and the writer returned to 

Hardin County with a Megger ground tester, with which tests were 

made on selected known faults in various parts of the county. The 

results as a whole seemed positive enough to Warrant further work.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 93

In the summer of 1932, therefore, the writer, with Robert P. Stevens 

and Ben Richards as field assistants, returned to Hardin County with a 

set of Gish-Rooney apparatus, and did six weeks' work in the Rosiclare 

district. Profiles were constructed from the data that were obtained, and possible faults were indicated. These profiles were then 

taken into the field by J. Marvin Weller, who checked the indicated 

faults by geologic evidence. Many of the inferences from the preliminary traverses were thus confirmed 11 and it was accordingly decided 

that more detailed work was justified.

In 1934 the work was resumed as a Public Works Administration 

project, under the direction of the Federal Geological Survey and 

with the cooperative sponsorship of tne Illinois State Geological 

Survey. This time a larger party, comprising both a geologic and 

a resistivity crew was available. Its members were: L. W. Currier, 

geologist; M. King Hubbert, geophysicist; C. G. Dickinson, geologist and plane-table man; D. S. Hughes, physicist; O. E. Wagner, Jr., 

stratigrapher; and R. P. Stevens, lineman and computer.

Six weeks were allotted to Illinois and six to Kentucky. During 

this season resistivity work was confined to the Rosiclare district 

except for a few test lines in the Cave in Rock district; geologic work 

was carried on in both districts.

The task was concluded in 1935, still under the joint auspices of the 

Federal Geological Survey and the Illinois State Geological Survey, 

The party was the same as that of the preceding summer except that 

Hughes and Stevens were replaced by Gilbert H. Cady, Jr., and A. L. 

Skrobisch. During this season both geologic and resistivity work 

were confined to the Cave in Rock district, except for a few final 

traverses run in the Rosiclare district. .

ACKNOWLEDGMENTS

Acknowledgments are due to many of those engaged in the fluorspar-mining industry of Hardin County for faithful cooperation, without wh^ch much of the present work would have been impossible. 

Among the many to whom we are especially indebted are: A. H. 

Cronk, Superintendent of Rosiclare Lead and Fluorspar Mining Co.; 

E. C. Reeder, late Superintendent of Hill Side Fluorspar Mines Co.; 

J. W. H. Blee, Superintendent of Benzon Fluorspar Co.; Martin 

Schwerin, President of Victory Fluorspar Mining Co.; Miles Haman, 

Engineer, Crystal Fluorspar Mining Co.; Robert Weaver, Engineer, 

Franklin Fluorspar Mining Co., and many other members of the 

staff; Arthur J. Lay, Crystal Fluorspar Mining Co.; and Dodson 

Gibson, Engineer, Crystal Fluorspar Mining Co.

" Hubbert, M. E., and Weller, J. M., Location of faults in Hardin County by the earth-resistivity method: 

Am. Inst. Min. Met. Eng. Trans., vol. 110, pp. 40-48,1934.
94 FLUORSPAR AREAS IN HARDIN COUNTY, ILL. 

RESISTIVITY PROFILES OF THE ROSICLARE DISTRICT

The Rosiclare district as defined for the purposes of this report 

extends about 1 mile east and about 3 miles north of the town of 

Kosiclare; it is bounded on the west by Big Grand Pierre Creek and 

south by the Ohio River. As will be seen by reference to plate 20, 

this area was by no means fully covered by resistivity traverses, but 

the above boundaries enclose all the traverses run in this part of 

Hardin and Pope Counties. Altogether 224,000 feet of traverses, with 

readings taken at every 100 feet, were run in the Rosiclare district. 

This work represents 2,240 separate measurements of resistivity.

The locations of the various traverses in the Rosiclare district are 

shown in plate 20, and the profiles are plotted in plate 21. In general 

the traverses largely followed straight east-west lines, tKough some 

of them (notably profiles 3, 10, and 16) were run along roads that 

extend only approximately east and west.

Although a few of the profiles extended entirely across the district, 

most of them were confined to one or the other of two parts of it, one 

to the east and the other to the west of the township line between 

R. 7 E. and R. 8 E. The work in the eastern area was mainly focused 

upon the fault system associated with the active mining district 

around Rosiclare; that in the western area was mainly devoted to a 

detailed study of the alluvium-filled valley of Wallace Branch.

Between these two areas is a strip r6ughly a mile wide, extending 

along the north-south township line, in which little work was done, 

because the country rock is mostly Caseyville conglomerate in the 

southern part of the strip and Fredonia limestone in the northern 

part, and faults in either of these formations are not likely to be 

revealed by resistivity profiles. Similar considerations apply to other 

regions left blank on the map. Because of the limited time, the work 

was confined to the more promising parts of the district.

In plate 21, also, an endeavor was made to correlate, between successive resistivity profiles, the anomalies or irregularities that appear 

to indicate intersections with faults. Where a similar anomaly 

appears on many successive profiles along a nearly straight or gently 

curving line, there can be little doubt that it represents a single fault. 

, In some cases, however, correlation from one profile to another is 

highly conjectural, and there may even be doubt as to whether or 

not certain individual anomalies are due to faults. In the vicinity 

of Stone school, for example, the anomalies can be correlated in many 

different ways, and here a study of the areal geology has revealed a 

complex pattern of faults.

The geophysical data are not complete enough to justify revision 

of the areal geology map of the Rosiclare district. Although areal 

mapping was done in this district, the results did not differ enough
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 95

from the earlier work of Stuart Weller to justify a revision of his maps. 

It is true that mining operations have disclosed more faults than were 

detected by geologic mapping. Resistivity methods, however, can 

at best only determine the location of faults; they cannot of themselves identify the wall rocks. They could not, therefore, fill in all 

the missing details of a map made by ordinary methods, even if they 

succeeded in locating every fault in the mapped area; the identity of 

the formations flanking a concealed fault located by geophysical 

methods would have to be guessed at. Fortunately, however, it is 

the location of the faults, rather than, the identity of the bordering 

formations, that is of primary importance in prospecting for vein 

deposits of fluorspar. It therefore seems worth while to try to locate 

faults that had not been located by geologic mapping. 

 Since the data in many cases can be variously interpreted, many 

particular interpretations may be wrong. This report therefore presents the original resistivity data, in order that they may be reinterpreted, when it seems desirable, by or on behalf of persons who are 

seeking fluorspar. The interpretations are subjective, representing 

the author's opinions; the data are objective and based on instrumental observation.

DISCUSSION AND INTERPRETATION 

EASTERN AREA

The traverses made in the eastern area of the Rosiclare district 

are plotted, from south to north, on profiles 1 to 16, plate 21. All 

except profiles 1 and 2 fqrm a sequence of approximately east-west 

profiles with station intervals of 100 feet and electrode spacing of 

100 feet.

Profiles 1 and 2 were taken with stations at 200-foot intervals and 

electrode spacing of 200 feet. They lie apart from the others and are 

discussed separately on pages 106 and 107.

PROFILE 3

The noteworthy resistivity anomalies in profile 3, reading from 

west to east, are as follows: A small rise between stations 77 and 68 

coincides with a topographic rise that is flanked on either side by 

alluvium. The higher resistivity is probably due to the nearness of 

bedrock to the surface.

Between stations 55 and 51 there is an abrupt rise in resistivity, 

with the higher level approximately four times the lower. Station 

53 is at the crest of a small ridge. If the rocks were the same on 

both sides of the ridge, the resistivity anomaly should be symmetrical 

with respect to the ridge, crest, but in fact the anomaly is highly 

asymmetrical and suggests a fault near station 53.5.
96 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

r The high-resistivity block encountered at 53.5 ends near station 40, 

on the crest of another small ridge. This fact is interpreted as being 

due to a fault at or near station 40.

From stations 33 to 29 there is an abrupt rise of resistivity, east 

of which the curve remains high to station 20, but from there to station 

18 it drops sharply. This region of high resistivity coincides in general 

with a rather broad ridge, of which the top consists of Hardinsburg 

sandstone and the slopes of Golconda limestone. These two changes 

in resistivity thus appear to be due to the transition from the Golconda 

to the Hardinsburg and back again in ascending and descending 

the ridge.

From stations 7 to 4 the curve shows some minor oscillations and 

then rises abruptly. Station 5, near which the curve rises to a high 

peak, is at a prospect shaft on the Blue Diggings fault.

PROFILE 4

Between stations 8 and 9 on profile 4 there is a sharp rise that coincides with the western border of a dike or volcanic neck. How far 

the dike extends in the line of traverse is somewhat uncertain, but it 

is exposed at station 12, and the decline of the resistivity curve at 

station 15 suggests that it may not extend any farther east.

The low resistivity from stations 17 to 28 represents an alluviumfilled valley. Between stations 30 and 31 the curve rises somewhat 

sharply. This rise might be regarded as merely marking the boundary 

of the alluvium, but its abruptness and sharpness, and its correlation 

with similar features in the adjacent profiles, indicate that it has some 

other cause. Correlation across several profiles indicates a fault at 

about station 30, apparently the same as the one at station 53.5 on 

profile 3.

Station 41.5 is at the crest of a ridge of Palestine sandstone, and at 

station 44 there is an outcrop of limestone. As the ridge crest is 

approached from the west the resistivity profile descends rather 

sharply an effect that might be due either to faulting or to the fact 

that the sandstone dips .westward and as the crest is passed the influence of the Menard limestone and shale becomes evident.

Geologic data show that there is a fault between stations 40 and 47, 

for the sandstone on the next ridge to the east is the Hardinsburg 

sandstone, and at station 45, moreover, there is a small valley which, 

as shown by the areal map, has Golconda limestone on the east side 

and Menard limestone on the west. A fault is accordingly placed at 

station 45.

Near station 49 the curve rises again, apparently marking the transition from the Golconda limestone to the Hardinsburg sandstone in 

normal stratigraphic sequence.
GEOLOGICAL SURVEY BVLLET1N ffl FLATE 20

Sfi->&

L / / ' '

n^- * 41 ^ NO. 25 /' .//___ /__ ,J^ //}/ /lx ~

/5/ NO. 9 I / 

\-^ ' V

i\ \\ /

WtT^ *

// JrP^T?!^ '/--f.g'- j/ J y

LINE OF TRAVERSE AND SELECTED STATION 

NO. 3 TRAVERSE NUMBER

 FAULT, EXPOSED IN MINE WORKINGS

 -^ PROBABLE FAULT INDICATED BY RESISTIVITY

 POSSIBLE FAULT INDICATED BY RESISTIVITY

 ROAD BUILDING

 -- SECONDARY B MINE SHAFT 

ROAD OPEN CUT

 TRAIL DIKE 

RAILROAD ^g) POND 

FENCE jr-^ WATER COURSE

1000 ' 2000____3000 4000 5000FEET

1937

MAP OF THE ROSICLARE DISTRICT, HARDIN COUNTY, ILL., SHOWING LOCATION OF KARTH-RESISTIVITY TRAVERSES AND FAULTS.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 97

At station 67 the curve descends abruptly, the descent coinciding 

with an eastward-facing scarp of Hardinsburg sandstone, probably 

at its eastern contact with the Golconda limestone.

At station 74 the curve rises, and at station 75 there is a prospect 

pit on what is evidently the northward extension of the Blue Diggings 

fault.

From stations 76 to 81 the traverse had to be interrupted, because 

of a small pond. Mine data show that the Daisy fault crosses at about 

station 80. The drop between stations 85 and S6 is due to the Eosiclare 

fault, which mine maps show to be at station 86.

The rise at station 89 has not been correlated with any known feature. It may represent a hitherto unknown branch of the Rosiclare 

fault system, or, merely a variation in thickness of the overburden 

on the Fredonia limestone.

The large rise in the curve which begins at station 115 would strongly 

suggest a fault were it not for the fact that the rock to the east is 

certainly Fredonia limestone. At station 116 there is a sinkhole. At 

other places where the Fredonia likewise occurs on both sides of such 

an anomaly, the anomaly appears to mark a change in thickness of 

the overburden. The anomaly at station 115 may have such a cause, 

but it may represent a fault with the Fredonia limestone on both sides.

PROFILE 5

The gradual descent of the curve from stations 1 to 9 is apparently 

due to the transition from the^Kinkaid limestone and Degonia sandstone on the west to alluvium on the east.

The rise near station 23 correlates with that near station 30 on 

profile 4, to the south.

Between stations 38 and 39 the curve drops sharply. This drop 

occurs just east of the crest of a ridge of Palestine sandstone, and is 

believed to be due to a fault.

About station 46 an oscillation occurs, with regions of low resistivity 

on either side. At about this point the geologic map indicates a fault 

between the Menard limestone and the Golconda linlestone-^-both 

formations of low resistivity.

The region of high resistivity between stations 53 and 65 is apparently due to a hill capped by Hardinsburg sandstone.

The rise between stations 68 and 69 occurs near the strike extension of both the Blue Diggings and the Argo faults, and it appears to 

be due to a-fault of this system. The further oscillation extending to 

station 74 may be due to a rather wide fault zone or to several closely 

spaced faults.

Near station 76 the traverse crosses the Daisy fault, but here the 

rocks on the two sides are so nearly equal in resistivity that there is
98 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

not the slightest indication of the fault on the resistivity profile. 

There are a number of instances of this sort.

The peak at station 82 represents the Rosiclare fault. The peaks 

of the same order of magnitude as stations 85 and 90 may represent 

something of importance, but there is no particular reason for supposing that they do.

From stations 93 to 99 there is a stretch of high resistivity, followed 

by one of low, and east of station 107 the resistivity again becomes 

high. At stations 94 £nd 98 the Fredonia limestone crops out. The 

entire area east of the Rosiclare fault has been mapped geologically 

as Fredonia limestone, and all of the above noted anomalies may be 

due merely to the vagaries of the Fredonia (see p. 89). In the low area 

from stations 99 to 106 no outcrops were observed but some residual 

sandstone was found. This fact suggests the possibility of a graben 

in this area but the adjacent profiles offer little supporting evidence.

PROFILE 6

The oscillation at stations 91 and 92 is unaccounted for. It occurs 

on the dip slope of the Palestine* sandstone, but it does not correlate 

with anything on the profiles to the north and south. It could be 

due to a small shear zone.

The drop in resistivity at station 79 occurs near the crest of a ridge. 

It correlates with a similar feature at station 38 on profile 5, and with 

other profiles to the north, and a fault is accordingly believed to extend 

along this scarp.

The high resistivity between stations 59 and 56 appears to be due 

to the northward extension of the Hardinsburg sandstone.

There is a small oscillation in the curve between stations 50 and 46, 

about on the strike line of the Blue Diggings and Argo faultteystems.

At stations 42 and 35.5 tlj.e traverse crosses the Daisy ana Hillside 

faults, respectively, but again the curve gives no indication of their 

presence.

The curve rises rather sharply near station 25, in a way that suggests 

the possibility of a fault. A diamond-drill hole, pitching 45° eastward, 

about 200 feet south of the line of traverse encountered nothing but 

Fredonia limestone below the surface zone of weathering. The same 

results were obtained in another hole near station 30. In the east wall 

of the vein in the Hillside mine, at about the point crossed by the line 

of traverse, Fredonia limestone is the uppermost formation. -It is 

weathered, however, and is covered with an overburden" of residual 

clay to a depth of nearly 100 feet, and this clay evidently accounts for 

the low resistivity immediately east of the fault.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 99

PROFILE 7

Between stations 77 and 66 of profile 7 there is little change in resistivity. Between stations 66 and 65, however, there is an abrupt 

rise, which occurs in gently rolling topography underlain by Caseyville 

conglomerate. This rise correlates with similar features on profiles 8. 

and 10 to the north, and it may represent a north-south fault.

The drop in resistivity at station 61 is an isolated occurrence. Here 

the topography is somewhat rough, and the low resistivity may be due 

to that fact.

The decline of the curve from stations 56 to 50 corresponds with the 

transition from the Caseyville conglomerate to the alluvium of Three 

Mile Creek.

Station 40 is on the top of a small ridge which is mapped geologically 

as Kinkaid limestone. Both east and west of this ridge are alluviumfilled valleys. The bedrock in this locality has a regional westward 

dip of about 500 feet per mile. Across this ridge the resistivity curve 

is markedly asymmetrical, dropping very sharply between stations 40 

and 39, within 100 feet of the ridge crest. This drop may be due merely 

to the line's crossing formations that have a homoclinal dip, but its 

abruptness suggests faulting.

The low resistivity between stations 37 and 20 corresponds to an 

area of alluvium. The rather sudden rise near station 21 occurs at 

about the eastern boundary of the alluvium and may be due solely to 

the alluvium, but as it correlates with similar features to the south and 

to the north it may be due to a fault.

A sudden drop between stations 3 and 4, together with a similar drop 

on the same ridge of Palestine sandstone noted in the profiles farther 

south, appears to be due to a fault along the strike of the ridge.

The eastern part of profile 7 was run in 1932. The western part was 

run in 1934, with an improved instrument that gives somewhat more 

reliable readings than those obtained in 1932. In 1932 a commutator 

correction factor was applied, which made the results systematically 

higher than those of 1934, when its use was discontinued. These 

facts account for the offset in the curve at station 1, which is common 

to the eastern and western parts.

As certain instrumental difficulties had not yet been eliminated in 

1932, when the eastern part of the traverse was run, it is uncertain 

whether some of the smaller oscillations are due to geologic causes or 

to instrumental errors. The sharp dip between stations 12 and 15, 

however, indicates an anomaly somewhat larger than the adjacent ones 

and at this place a fault between the Menard limestone and the 

Golconda formation is shown on the geologic map.

The high-resistivity peak occurring at station 24 is not so clearly 

accounted for. The geologic map shows nothing that explains it, and
100 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

it correlates with, nothing to the south. In the next profile to the north 

however, about N. 30° E. from the peak in question, there occurs a 

similar and more pronounced peak, and it seems probable that both 

peaks are on a fault zone.

The sharp oscillation between stations 38 and 42 corresponds in 

position with the Daisy and Hillside faults. It is evidently the result 

of that fault system, and, though it coincides with none of the faults 

that are shown on the mine maps, it might be worth investigation.

The low stretch on part of the curve between stations 42 and 48 

marks the position of the Hillside fault and of a swampy area covered 

with waste from the Hillside mine. The sharp rise at station 48 is near 

an outcrop of Fredonia limestone and is evidently due to the eastward 

thinning out of the overburden upon the limestone.

PROFILE 8

Profile 8, like profile 7, was run at two separate times and consists of 

two number sequences, one extending westward and the other eastward from station 1.

Beginning at the west end of the profile the oscillations between 

stations 84 and 73 are probably due to faulty readings because of poor 

ground contacts. As readings between stations 73 and 65 were made 

with fairly good ground contacts, the rise at station 69 is apparently 

real and possibly represents a fault.

The large oscillations between stations 65 and 57 are also erratic, 

owing to the fact that they were taken on a slope of conglomerate 

that forms a bed in the Caseyville sandstone, on which the contact 

resistance between the ground and the stakes was so great that not 

enough current could be made to flow to give reliable readings.

The small peak on. the curve at station 43 may not be significant. 

It appears, however, to correlate with the anomaly at station 40 on 

profile 7, and both anomalies are interpreted as being on a single fault.

The dip in the profile between stations 37 and 17 represents alluvium. 

The rather sudden rise at station 14 is interpreted as being due to the 

same fault that crosses profile 7 at station 19.

The abrupt drop in resistivity near station 2 in the eastern part of 

profile 8 correlates with similar drops on the profiles to the north and 

to the south, and is apparently due to a long northeastward-striking 

fault.

The small abrupt rise between stations 13 and 14 occurs about 

where the traverse crosses a small valley. The terrain is nearly flat. 

The asymmetry of the resistivity curve with reference to this valley suggests that the rocks on the two sides of the valley are different, and it is 

inferred that the valley follows a fault.

The large oscillations between stations 29 and 34 are somewhat 

west of the main Rosiclare fault, which crosses near station 37. It is
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 101

about Jiere that the Blue Diggings, Daisy, and Kosiclare faults all 

converge. The resistivity curve suggests a fault near station 29 and 

another near station 34. The rise in the curve at station 40 seems to 

correlate with anomalies on the next two profiles to the north, and is 

believed, like them, to represent a fault.

Some further light is thrown on this area by the data from a line 

of diamond-drill holes, furnished to the writer by Mr. A. H. Cronk, 

Superintendent of the Rosiclare Lead and Fluorspar Mining Co. 

The line of drill holes runs nearly east-west, only a few hundred feet 

north of the line of traverse. The holes all pitch eastward or westward, 

so that successive holes overlap horizontally.

These holes indicate faulting and fluorspar veins near station 35. * 

Another small vein was encountered at considerable depth near 

station 29 or 30. A drill hole near station 38 or 39 is in gray limestone 

to a vertical depth of about 200 feet, where it enters cherty limestone. 

The remaining four holes encounter the cherty limestone at a depth of 

about 140 feet. The discrepancy of 60 feet occurs close to station 40.

PROFILE 9

Near the west end of profile 9 the curve rises sharply from stations 

82 to 80. The supposed fault passing through station 14 on profile 

8 would pass very near station 82 or just to the west of it.

The large anomaly between stations 75 and 70 is interpreted as 

indicating a fault block bounded by one fault near station 75 and 

another near station 70. Station 70 is at the crest of a ridge of sandstone that dips gently westward, and station 75 is OQ the west slope of 

this ridge. The anomaly just noted correlates with a similar one both 

to the south and to the north. It is ascribed to a fault, of which there 

also is geologic evidence: Palestine sandstone is exposed on the ridge 

and a prospect pit east of the ridge exposed Menard limestone.

The rise in the curve at station 52 probably represents another 

fault. Geologic mapping shows Hardinsburg sandstone to the east 

and Menard limestone to the west, with a fault between them somewhere in this vicinity.

The Kosiclare fault is crossed at station 36, but with little effect 

on the resistivity curve. At station 32, however, there is a small 

anomaly, which appears to represent the southward extension of a 

fault that is indicated by the next two traverses to the north and by 

the anomaly at station 40 on profile 8.

The meaning of the very sharp rise in resistivity beginning near 

station 23 is uncertain. It strongly suggests a fault with Fredonia 

limestone to the east, yet there were similar features in some of the 

profiles farther south where no fault was indicated by mining or drilling 

data. This rise is regarded, however, as possibly due to a fault.
102 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

PROFILE 10

In going eastward from station 100, at the west end of profile 10', 

a very sharp rise in resistivity is encountered between stations 89 and 

84. This occurs on a hill composed of Caseyville conglomerate. There 

is no geologic evidence of faulting here but the occurrence of similar 

anomalies near the same north-south line on profiles 7 and 8 as well 

as 10 is regarded as indicating a possible fault.

The sharp oscillations in the curve between stations 84 and 77 are 

due to poor ground contacts on the steep slope of Caseyville con- 

. glomerate.

There is an anomalous region of high resistivity between stations 

30 and 15, which correlates with similar features on the profiles both 

to the south and to the north. It is believed that there is a fault near 

station 30 and another near station 16. Correlation with the profiles 

to the south and the north suggests the possibility that the oscillations near station 23 may represent a cross-fault striking about northsouth.

The geologic map of Hardin County by Stuart Weller shows a 

fault near station 1 or 2. Wagner placed the same fault more nearly 

at station 2. At neither place, however, is there much variation 

in the resistivity curve. ,

The sharp rise in the curve between stations 16 and 17 coincides exactly with the location of the Rosiclare fault as known 

from mining data.

The sharp rise in the curve between stations 28 and 29 is difficult to account for except by faulting, though its correlation with 

the profiles to the north is somewhat uncertain.

The oscillations to the east of station 29 may possibly be explained by faulting, but they are regarded as more probably due to 

sinkholes in the Fredonia limestone.

PROFILE 11

The anomalous region of high resistivity between stations 77 and 

62 corresponds to the similar region on profile 10 between stations 

30 and 15. Faults are indicated near stations 77 and 66, but the location of the one at station 66 is somewhat uncertain. The anomaly 

actually extends to station 62, but the sharpest break occurs near 

station 66, and station 66 is in a line with the corresponding points in 

the adjacent profiles.

The sharp breaks near stations 37.5, 33.5, 30, 25, and 16 may all 

indicate faults, though their correlation with the neighboring profiles, 

especially those to the north, is rather uncertain.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 103

The break at 33.5 is near an old shaft on the main Rosiclare fault. 

The block between stations 30 and 25 has sandstone at the surface, 

underlain by limestone, and the block contains sinkholes with runs 

of sandstone. Both Weller and Wagner map this as Bethel sandstone, 

underlain by Renault limestone and Shetlerville shale.

The break at station, 16 is of uncertain significance. East of it are 

outcrops of Fredonia limestone; west of it is a hill containing.loose 

sands tone., Weller shows a narrow fault block of Cypress sandstone 

in this area, but Wagner mapped the same rock as Rosiclare sandstone. The sharpness of the break at station 16 seems to favor 

faulting.

PROFILE 12

Between stations 79 and 60 profile 12 crosses an anomalous area 

of high resistivity that is also recorded in the profiles to the south. 

Faults are tentatively placed near stations 74 and 62, though their 

exact locations are uncertain.

The geologic map shows a fault near station 47.5 between the 

Menard and the Glen Dean limestones, but these are both of low, 

resistivity and the fault is not indicated on the resistivity profile. 

There is possibly a fault between stations 35 and 30, and it is placed at 

station 32, which is on top of a hill and on moderately level ground. 

The rise beginning near station 36 can be accounted for as due to 

transition from limestone and shale to sandstone on a steep slope.

The drop in resistivity near station 28 may indicate either faulting 

or transition from sandstone to limestone and shale on the east slope 

of the hill; the latter interpretation seems the more probable.

At station 24 is the Eureka No. 2 shaft, which is close to the Rosiclare fault. The footwall block, to the east, is limestone.

The drop in resistivity near stations 18 to 16, followed by a very 

large increase near station 13, may be significant, but it is apparently 

due to the peculiarities of the Fredonia limestone.

PROFILE 13

The rather large anomaly between stations 89 and 83 on profile 13 

was quite unexpected and is entirely unaccounted for by the known 

areal geology. It occurs in open fields of gentle topographic relief, 

in which the country rock is mapped as Degonia sandstone. Between 

stations'89 and 84 the resistivity rises from 5,000 to 125,000 ohmcentimeters, and then suddenly drops at station 83 to 25,000 ohmcentimeters.

After the profile was completed a station was occupied at 83.5 and 

readings were taken at successive azimuths differing by 30°. The

551386 44 8
104 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

results obtained are plotted graphically above the profile itself. Numerically they are:

Apparent resistivity 

Bearing (ohm-centimeters)

North-south____ . _______________________ 16, 300 

N. 30° E____________._..__1____.______ 26, 000 

N. 60° E.__.______.._...._...__....______.__..__.., 37, 100

East-west-._______________________"______ 170, 000 

S. 60° E_______________________________ 134,000 

S. 30° E_-.._____-._.___._....._____.___....__._._. 14, 600

These results prove that the resistivity peak is real and that the 

resistivity is highly directional, being very low in a north-south direction and probably greatest in a direction somewhat south of east. This 

pattern suggests a tabular body of high resistivity, striking a little 

east of north. There is a similar but more subdued anomaly in profile 14 to the north, in the proper place to correlate with this one. 

The anomalies may represent two faults, or perhaps a fault zone. The 

high resistivity suggests mineralization with nonconductive minerals 

such as calcite, quartz, or fluorspar, and it would bear further investigation.

The high resistivity between stations 72 and 63 is correlated with 

similar highs in the profiles to the south. Faults are assumed to be 

present at stations 72 and 63.

The breaks at stations 52.5 and'47.5 also suggest faults, though 

their correlation is uncertain.

The sharp breaks upward at station 40.5 and downward at station 

35.5 both appear to be due to faulting. Faults near these two localities are shown on the areal maps by Stuart Weller (1920) and by 

Currier and Wagner (1937).

The sharp break at station 26.5 suggests faulting and may represent 

a branch of the Rosiclare fault. The main Rosiclare fault is crossed 

near station 22 and is reflected by a small break at station 22.5.

The sharp rise beginning at station 14 is similar to features found 

in other profiles to the south. It would suggest a fault with Fredonia 

limestone to the east were it not for the fact that Fredonia limestone 

is encountered west of this station in the east waU of the Eureka No. 

1 shaft. Although this break may be due to faulting, it is more prob-1 

ably due to the peculiarities of the Fredonia limestone.

PROFILE 14

The anomaly between stations 83 and 76 on profile 14 corresponds 

with that between stations 89 and 83 of profile 13. It may be due to 

a fault zone or block, and there may be a fault at station 82.5 and 

another at station 76.5.

The break near station 62 is thought to represent a fault.' It occurs 

near the western boundary of an alluvium deposit, loose sandstone
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 105

being noted to the west. The high resistivity to the east cannot be 

due to the alluvium, which has very low resistivity.

The small dip at statioa 56 may or may not be significant. 

, The break downward near station 48 is possibly due to a fault, 

though it cannot certainly be correlated with breaks on the adjacent 

profiles.

The small break between stations 35 and 34 occurs just about where 

the geologic map shows a fault with Menard limestone on the west 

and Tar Springs sandstone on the east.

The large break at station 27 coincides with a visible fault between 

Glen Dean limestone and Rosiclare sandstone.

The break at station 21.5 may be due either to a sinkhole in the 

Fredonia limestone or, as appears more likely, to a fault correlating 

with one at station 26 on profile 13.

The Rosiclare fault is crossed at station 8.5. The drop between 

stations 5 and 3 is apparently due to a transition from limestone to 

alluvium.

PROFILE 15

Profile 15 has a great many breaks, which are generally small, 

though many of them are larger than some breaks due to known 

faults. This profile is a quarter of a mile north of profile 14. Profile 

16 is a quarter' of a mile north of the west end of profile 15, but it 

swings southward and crosses profile 15 near station 52. Even these 

two nearest profiles cannot be satisfactorily correlated with profile 15, 

for they are at considerable distance from it and have few if any 

specific features in common with it.

There is a small break at station 136, near Three Mile Creek, 

but it is probably of no significance. >

Stations 121 and 123 are near an old open pit mine and two shafts, 

but there is no trace of disturbance op. that part of the resistivity profile.

Quartzite was noted at station 114.5, but the profile there is featureless. The breal at station 109.5 is very near a fault that is shown on' 

Weller's geologic map of Hardin County.

A fault that is sugg3sted near station 70 may correlate with the 

break near station 74.5 on profile 14.

The slight break at station 63.5 seems to correlate with a similar 

but more pronounced feature at station 62.5 on profile 14. It is possibly due to a fault.

A fault is mapped geologically near station 38, and there is a spring 

nearby, but the resistivity curve shows nothing.

Most of the ground between stations 40 and 15 is covered with 

alluvium. The geologic map indicates a complex pattern of faults 

in this vicinity, but the only one distinctly indicated on the resistivity 

profile is near station 12. This is evidently a fault between the Pales-
106 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

tine sandstone and the Fredonia limestone that is shown on the areal 

map of Hardin County.

PROFILE 16

Profile 16 was taken along the Golconda-Elizabethtown highway. 

Although the profile is plotted as if it were straight and parallel to 

the others, its eastern part cuts across a number of the other profiles; 

west of station 85 it runs nearly east and west. The course of the 

traverse and the positions of the stations are plotted on the base map 

(pi. 20). Only that part of profile 16 which extends eastward from station 145 is considered here, the western part being discussed on page 113.

The small oscillations in the curve near station 140 coincide with 

a local outcrop of Fredonia limestone, to which they are apparently 

due.

The W in the curve at station 111 is near a known fault of very 

large displacement.

The resistivity peak at station 102 is unaccounted for. It is asymmetrical with respect to a valley at station 100. There may be a 

fault near station 101, but no correlation has been made between 

profile 16 and profile 15, to the south.

Weller's map shows a fault near station 88, but the profile shows 

nothing here except a gradual rise.

The rise in the curve near station 68 seems to correlate with a 

similar feature in the profiles to the south, and it may mark the location of a fault.

The drop in the curve from stations 53 to 40 mostly coincides 

with the east slope of a ridge. It may be due to the effect of successively older formations encountered in passing eastward down the slope. 

The areal map shows a fault near station 39, but there is no indication 

of this fault on the profile.

The break ir the profile near station 35.5 strongly suggests a fault. 

The sharp rise at station 30 occurs at a known fault.

The break downward near station 26 may not be significant, though 

it is nearly in line with a similar but more pronounced feature at 

station 21 in profile 14, and with a break at station 26.5 on profile 13, 

so that these three breaks may well be due to a 'fault.

The peak between stations 23 and 19 is unaccounted for, but it 

may be due in part to underground openings, for there is a known 

cavern nearby. The Rosiclare fault is crossed near station 18.

The rise at station 3 occurs near an outcrop of Fredonia limestone, 

and is probably due to the transition from that rock to the Rosiclare 

sandstone.

PROFILES 1 AND 2

Although they lie in the eastern part of the Rosiclare district and 

cross the main Rosiclare fault, profiles 1 and 2 are not a part of the
PART 2. _STUDY BY EARTH-RESISTIVITY METHOD 107

system of. profiles hitherto discussed. Both are on the alluvium of the 

Ohio River flood plain, having been made in the hope of detecting faults 

that may lie buried beneath the alluvium. The electrode spacing on 

on them is 200 feet and the stations are 200 feet apart. The main 

Rosiclare fault changes its strike somewhat abruptly near the Good 

Hope shaft. To the north the strike is about N. 30° E., to the south 

it bends to approximately north-south. It was chieflly to determine 

whether there might be a hidden branch of this fault, striking about 

S. 30° W., continued under the alluvium, that traverses 1 and 2 were run.

Profile 1 shows only two breaks of any unusual magnitude a rise 

in resistivity between stations 8 and 9 and a drop between stations 13 

and 16. According to the mine maps the Rosiclare fault was crossed 

near station 16.

Profile 2 shows no pronounced changes in resistivity. There is a 

slight drop between stations 6 and 5, where the RosicUre fault is 

crossed. The even smaller break between stations 9 and 8 is about 

in line with the general course of the main fault to the north. There 

are other small features in this profile, notably between stations 15 

and 13 and between stations 3 and 2.

The conditions that affect these two profiles are in several respects 

unknown. In the nearby Annex shaft the alluvium is about 30 feet 

thick, and owing to its high electrical conductivity it blurs the resistivity effects due to the formations beneath. It is not known 

whether the alluvium is uniform in thickness or in texture, but probably it is neither. Areas where the alluvium was thin, so that the more 

resistive bedrock was near the surface, would be indicated by highs in 

the resistivity profiles, but so would included pockets of gravel. It is 

also not known whether, if there is a concealed southwestward extension of the main Rosiclare fault, the same or different formations occur 

on its two sides. The breaks between stations 8 and 9 of both profiles 

are in line with the general course of the main fault, and suggest that 

a branch having the same course may extend under the alluvium. 

But the evidence is inconclusive, and the problem must be regarded as 

still unsolved.

WESTERN AREA

The areal geology map of Hardin County by Stuart Weller (1920) 

shows one fault crossing Wallace Branch diagonally and another extending parallel to it. The resistivity work in the western area has 

been devoted principally, though not entirely, to making detailed 

measurements along this branch and following out what appears to be 

one of these faults.

PROFILE 17

The southernmost of the Wallace Branch profiles is No. 17. In 

going from west to east, one first encounters a drop in the resistivity
108 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

curve that extends from station 27 to station 21. This drop correlates 

with a more pronounced one that occurs in nearly all of the profiles to 

the north and that is interpreted as due to the principal fault in this 

valley, which apparently intersects profile 17 near station 23. Between 

stations 10 and 3 there is an anomalous rise in resistivity, which occurs 

on the east slope of a ridge, and the asymmetry of the resistivity profile as compared with the topography suggests faulting. A fault is tentatively placed at station 9, and another between stations 3 and 4.

PROFILE 18

The most outstanding anomaly in profile 18 is the abrupt drop, near 

the western end, from about80,000to less than 10,000 ohm-centimeters. 

The fault to which this drop is attributed is placed at station 25. In 

this instance the change in resistivity is entirely asymmetrical with 

respect to the topography, crossing Wallace Branch without any deviation. Between stations 10 and 4 there is a region of somewhat high 

resistivity, which appears to correlate with a similar region in profile 

17, and faults are placed near stations 14.5 and 6.

PROFILE 7

Profile 7 is one of the three profiles extending across both the eastern 

and western areas of the Rosielare district. Beginning at the west end, 

there is a very abrupt drop in resistivity between stations 116 and 114, 

which is correlated with similar features in profiles 17 and 18. A 

fault is indicated between stations 115 and 114. The small oscillations 

about station 107 occur near the eastern boundary of the Wallace 

Branch alluvium, and it may have no other significance, but it is tlte 

kind of feature that sometimes marks the crossing of a fault. There is 

some geologic evidence of faulting near station 104.

The next important anomaly, which occurs near station 79, strongly 

suggests a fault or fault zone in that vicinity.

PROFILE 10

Profile 10 is another of the long profiles extending across both the 

eastern and western parts of the district. The part of this profile extending westward from station 84 was run a second time, in order to verify 

some of the anomalies which appeared when it was first run and which, 

it was thought, might possibly be due to errors of measurement. The 

solid line curve, which ends at station 135, represents the original profile, run in 1932; the dotted-line profile, extending westward to station 

145, is the repeat line, run in 1934.

The most striking difference between the profiles is that westward 

from a point near station 110 they are about 100 feet out of phase;
. PART 2. STUDY BY EARTH-RESISTIVITY METHOD 109

for example, the notch close to station 116 on the original profile 

corresponds to the one near station 115 on the repeat profile. This 

difference is due to the fact that the electrode lines used for measuring 

the distance from the east end in the two traverses were of slightly 

different length, the one used in 1935 being the longer.

Another difference between the two profiles is that the resistivity 

values on the recheck profile are systematically less than the corresponding values on the original profile. The reason for this is that, in 

computing, a certain correction factor wais applied to the data obtained 

in the original traverse but not to those obtained in the recheck. 

It was found in 1934 that condensers placed in the potential circuit 

improved the performance of the instrument, and made the readings 

somewhat more reliable than those of 1932.

The relative values in the two traverses, it will be seen, are nearly 

the same; the two profiles, in other words, are nearly of the same 

shape. A few of the minor oscillations on the original profile occur on 

the repeat in a somewhat subdued form.) but aside from this the two 

profiles agree in almost every detail. The recheck one of the very few 

that were made tends to show that the results of the work are in 

the main reliable. As nearly all the ups and downs of the original 

profile are identifiable also in the rech'eck, they must be considered as 

due in much greater degree to geologic features than to errors or to 

difficulties with the instrument.

Beginning at the west end of profile 10, there is the usual sharp 

drop followed by a region of low resistivity. -A fault is placed near 

station 143. The small oscillation in the vicinity of station 131 is 

near a point at which, according to Weller'smap, two branching faults 

intersect. The rise in resistivity between stations 122 and 117 is 

asymmetrical with respect to the topography, notably to a hill crest 

of Caseyville sandstone crossed near station 119. This fact suggests 

that a fault may occur near station 117; the suggestion is not verified, 

however, by the areal geology, inasmuch as Caseyville sandstone 

occurs on both sides of the hill.

PROFILE 19

From a point about 400 feet east of the top of Melcher Hill, profile 

19 continues down the eastern slope of that hill and across Wallace 

Branch. The resistivity rises steeply, though with minor reversals, 

from stations 10 to 3, and from station 3 to station 0 it steeply 

descends. The rise appears to be due entirely to the profile's crossing 

the beveled edges of different strata in going down the slope of Melcher 

Hill. The fault that appears in the profiles to the south is placed near 

station 0.5, being supposedly indicated by a notch in the profile 

at that point.
110 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

The small break between stations 13 and 12 occurs in the alluvium 

of Wallace Branch and may or may not be significant. Near station 

9 are some of the workings of the old Miller mine, which according 

to Weller's map is on a north-south fault. No other notable anomalies 

occur on this profile.

PROFILE 20

The region of high resistivity in the western part of profile 20 is in 

an area of Fredonia limestone, which contains local sinkholes, so that 

no significance is attached to the oscillations of resistivity in this area. 

The sharp drop in resistivity that reaches a minimum at station 13 is 

thought, however, to be due to a fault, which is placed between stations 14 and 15. The rise in resistivity near station 10 strongly 

suggests another fault. This anomaly does not, indeed, correlate with 

any on the profiles to the north and south, but the pattern of the 

known faults, when plotted on the map of the region, indicates that 

there may be cross-faulting in this vicinity. Definite proof of such 

faulting, however, would require more detailed mapping than has 

been done hitherto.

PROFILE 21

As there is an outcrop of Fredonia limestone at the west end of 

profile 21, no significance is attached to the drop in resistivity between 

stations 29 and 27. The sharp drop ending at station 21, however, 

is thought to be due to a fault between the Fredonia and younger, 

less resistant formations to the east. This inference is verified to a 

certain extent by the outcrops in Wallace Branch near station 22, 

where limestone crops out in the western but not in the eastern meanders of the stream. These meanders apparently swing back and forth 

across the fault. There are no anomalies farther east in this profile.

PROFILE 22

In the western part of profile 22 there are two somewhat abrupt 

drops, one between stations 4 and 5, which is followed by a rise in 

resistivity, and another between stations 9 and 12. These features 

appear to mark the crossing of two faults, one near station 5 and the 

other near station 10.5, which is close to the eastern border of a cemetery. The presence, of two faults is also suggested by the forking of 

Wallace Branch nearby. The westernmost fault would nearly coincide 

approximately with the western fork of Wallace Branch; the other, 

which is probably the fault indicated on profile 22 and the profiles 

farther north, would follow the general trend of the eastern fork. In 

the eastern part of profile 22 there are indications of faulting at various 

places between station 28 and the east end of the profile, which is at 

station 50. Specific features on the profile suggest a fault near station
PART 2. STUDY BY EARTH-RESISTIVITY METHOD HI

20, possibly another near 36, a third near 41, and a fourth between 46 

and 47 or it might be safer to say that a fault zone apparently extends 

about from station 36 to station 42. There are too few profiles to 

make possible any correlation of these anomalies. The profiles 

to the soutli do not extend far enough east. Profile 23, to the north, 

shows anomalies somewhat farther to the west than those on profile 

22. but it cannot be assumed that the anomalies on the two profiles 

are due to the same faults, for, according to Weller the faults in this 

vicinity strike northeastward.

PROFILE 23

On profile 23 to 'begin as usual at the west end the resistivity 

rises sharply between stations 60 and 59, east of which there are minor 

oscillations about a high value, and then a sharp drop east of station 

49. The significance of the rise at station 60 cannot be understood 

without more data for points farther west. The sharp drop between 

stations 47 and 46, however, appears to represent the eastern branch 

of the fault recorded farther south. The fault is placed at station 45, 

which is on the eastern fork of Wallace branch.

The anomalous rise in resistivity between stations 34 and 38 might 

suggest faulting. The rise occurs, however, where the traverse crosses 

a small hill capped with Bethel sandstone, which is underlain by the 

Eenault formation, and the rise is symmetrical with respect to the 

hill crest. Since the Renault formation ordinarily has a lower resistivity than the Bethel sandstone, this anomaly is probably due to the 

crossing of these formations in their ordinary stratigraphic sequence.

No further anomalies of any appreciable magnitude occur in this 

profile, though fault breccia was noted at station 4, «which is about 

400 feet east of the section corner. This is approximately the location 

of a fault mapped by Weller (1920).

PROFILE 24

The sharp drop in resistivity extending from station 5 to 11 on profile 24 correlates with similar features in the profiles to the south. A 

fault is placed near station 10, which again very nearly coincides 

with the eastern fork of Wallace Branch.

The oscillations between stations 19 and 25 occur in the area of 

the Bethel sandstone, which may account for them, but, since the 

oscillations are rather sharp, the possibility of faulting should not be 

disregarded.

PROFILE 25

All of the short profiles in the western part of the Rosiclare district 

that have been discussed already were run in 1934. Profile 25 is the 

only one that was run in 1932. At that time, some doubt was felt as to
112 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

the reliability of this profile, because it indicated such abrupt and so 

frequent changes from high resistivity to low. After the work of 1934, 

however, the changes in resistivity in the eastern part of profile 25 

were found to correlate well with the profiles to the south. The west 

end of profile 25 is about half a mile west of the Shetlerville road; 

the east end is at the north-south road along Wallace Branch.

Near the west end of this profile there is a region of moderately high 

resistivity, followed between stations 63 and 61 by a sharp drop to a 

region of low resistivity. This drop reflects the topography so closely 

that it is not possible to say whether there is a fault in this locality 

or whether the effect is due entirely to topography. Station 63 is at 

the crest of a hill, capped by Bethel sandstone, the steep eastern slope 

of which lies between stations 62 and 60. Between station 60 and 

station 50 the ground is low and undrained. The sharp break in 

resistivity between stations 62 and 61 may be due to a fault. The 

rise in resistivity east of station 50 may also indicate a fault, but it 

may have resulted merely from the eastward thinning of the alluvium. 

Practically the same feature occurs in corresponding positions on the 

two succeeding profiles to the north.

No importance is attached to the oscillations in the profile between 

stations 43 and 32, for here the traverse crosses an area in which 

there are many sinkholes. The sharp drop near station 31, however, 

and the corresponding rise between stations 26 and 25, appear to be 

more probably due to faulting. The intervening low stretch might 

then represent a graben, or down-dropped block of younger formations, 

between two blocks of Fredonia limestone; but a similar subsidence 

might result from a kind of pseudo-faulting, due to the caving in of 

a large cavern in limestone. There is some geologic evidence that 

such movements have occurred elsewhere in the district.

Another .pair of faults or pseudo-faults is indicated near stations 

19 and 14. The steep drop near the east end of the profile correlates 

with the fault that has been followed from the south. This is placed 

at about station 1.5 again only a little east of the east fork of Wallace 

Branch. There is some geologic corroboration of this fault. Near 

station 5 loose residual limestone occurs in the soil, and in the road 

at station 1 there is an outcrop of much-fractured sandstone, so that 

a fault must lie somewhere in between.'

PROFILE 26

Profile 26 extends eastward from Big Grand Pierre Creek to the 

north-south road on the half-mile line of sec. 24, T. 12 S., R. 7 E. 

The west end of this profile is farther west than those of the profiles 

immediately to the north or to the south.

The oscillations between stations 120 and 113 suggest faulting, 

and at station 118 what appeared to be fault breccia in sandstone
PAEfT 2. l&TTJDY BY EAIKTH-RESlISfriVTTT METHOD 113

was noted. Between stations 106 and 85 there are several sharp 

changes in resistivity that suggest faulting. Two of these, one between 

stations 106 arid 105 and the other near station 101, are in open fields 

and cannot be accounted for by topography. There is another break 

between stations 96 and 95, that may indicate a fault. The drop 

between stations 91 and 90 coincides with a steep sandstone escarpment, and is probably due to the change of resistivity as different 

strata are encountered at different elevations. The very sharp rise 

in resistivity between stations 86 and 85, about 300 feet west of the 

Pope-Hardin County line, probably represents a fault between the 

Hardinsburg and Bethel sandstones, which was mapped by Weller 

as passing a little west of the county line.

The region of low resistivity between stations 74 and 61 is of the 

same character as that noted in profile 25. It coincides with an undrained depression underlain by Rosiclare sandstone and Fredonia 

limestone. To the east the Rosiclare sandstone and the Fredonia 

limestone crop out on somewhat higher ground. The depression may 

represent another case of pseudo-faulting due to caving, but it seems 

most probably to be on a down-dropped fault block. The sharpness 

of the resistivity change at the western margin of the area suggests 

faulting, and the increase in resistivity at the eastern margin, though 

it might possibly be due to the transition from the alluvium in the 

depression to the more nearly bare rock farther east, is so abrupt as 

to indicate faulting.

The oscillations in the resistivity curve between stations 60 and 43 

occur in an area known to be occupied by Fredonia limestone and 

Rosiclare sandstone, with numerous sinkholes. A depression in the 

resistivity curve between stations 43 and 35 probably represents 

pseudo-faulting due to caving in of the Fredonia limestone. The 

resistivity "high" between stations 37 and 33 appears to correlate 

with a similar but more pronounced feature between stations 25 and 

19 on profile 25. A possible fault is indicated near station 30. '

The resistivity "high" between stations 25 and 15 appears to correlate with the similar "high" between stations 14 and 2 on profile 25, 

though its western boundary is not sharply determined.' The sharp 

break at station 18 may represent a fault. On the east side of the 

"high" the sharp slope downward near station 15 appears to be due 

to the Wallace Branch fault, crossed by the profiles to the south. 

The fault is placed near station 13.5, about 100 feet east of the northsouth road.

PROFILE 16

Profile 16, which follows the Elizabethtown-Golconda highway, is 

the longest continuous profile run in Hardin County. It contains 268 

stations, being therefore 26,800 feet, or about 5 miles in length.
114 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

The solid-line curve in the western part of the graph represents the 

profile as originally run. The very large anomaly between stations 

259 and 238 was so surprising that it was suspected of being'due to 

some kind of error or to some defect of the instrument. To determine 

if this were so the line was rerun from station 238 westward and then 

extended to station 268.. In the original line, the resistivity readings 

were all taken on one side of the concrete pavement; in the recheck, 

stations were taken at corresponding points on the opposite side of 

the pavement. The two curves, though showing minor differences in 

regard to secondary features, are similar in all essential respects. This 

result proves beyond a doubt that the anomaly is geologic, not instrumental. It also confirms the recheck on profile 10 in tending to 

show that the resistivity readings are generally reliable.

Beginning at the west end of profile 16, there is a sharp rise inresis1- 

tivity between stations 267 and 266. This seems almost certainly to 

be due to a fault, though its correlation with the profiles to the north 

and to the south is uncertain. It is inferred that a fault crosses near 

station 240, on the east side of the anomaly. The precise location of 

this fault is somewhat uncertain. Its being placed at station 240 is 

partly because this point is near the break between the high and low 

resistivities, partly because the point lines up well with corresponding 

features in the profiles on either side. There is little direct evidence 

in the surface geology to indicate faulting at this place, though Stuart 

Weller, on a preliminary geologic map of Hardin County, indicated 

a fault about here. At station 252 a small shear zone in sandstone is 

exposed in a road cut, but there is no noticeable break in the resistivity 

profile, so that the displacement here has probably been small.

The sharp rise in resistivity between stations 231 and 230 marks 

the eastern boundary of the undrained depression that has been noted 

in discussing other piofiles. This anomaly may or may not be a result 

of faulting. The oscillations in the resistivity curve between stations 

228 and 208 occur over Rosiclare sandstone and Fredonia limestone. 

The road cut between stations 206 and 205 reveals limestone, and 

farther east there is sandstone. From station 199 to 197 there is a 

road cut through massive Fredonia limestone. The low resistivity 

between stations 206 and 201 is apparently due to a down-dropped 

sandstone block, which has probably been let down by caving of the 

underlying limestone.

The sharp break downward in the resistivity curve between stations 

195 and 194 apparently represents a fault between Fredonia limestone 

on the west and an unknown formation on the east. Nothing that 

corresponds to "high" between stations 25 and 15 on profile 26 and 

the one between stations 15 and 2 on profile 25 appears on profile 16, 

though this profile is only 600 feet north of profile 26. The block of 

high resistivity apparently disappears in this short distance, which
PAKfT 2. STUDY BY EARTH-KESOBTIVITY MErrHOD 115

seems to indicate-a pair of converging faults or else cross-faulting. 

More detailed work would be required to determine just what has 

happened here.

The M-shaped oscillation on the profile between stations 181 and 177 

occurs in a road cut that exposes thoroughly shattered sandstone. 

This is just about on the strike of the main Wallace Branch fault, 

which was followed in the profiles to the south.

No further anomalies of any consequence occur on the part of this 

profile in the western section of the Rosiclare district.

PROFILE 27

Between stations 62 and 22 of profile 27 the resistivity varies so 

erratically that it is almost impossible to pick out significant features. 

The W-shaped oscillation at station 48 suggests a fault, and it is near 

a point where two faults intersect, according to Weller's geologic 

map. The oscillations from station 48 to station 22 are too erratic 

to interpret. The sharp drop between stations 20 and 19 suggests, 

a fault at the western boundary of the undrained depression, and the 

rise between stations 10 and 9 also suggests faulting. The intervening 

area of low resistivity correlates with a similar feature in profiles 16, 

26, and 25. The western boundary of the depression is thought to be 

almost certainly bounded by a fault; faulting on the eastern boundary 

appears less certain.

SUMMARY FOB THE BOSICLABE DISTRICT

Detailed discussion of the profiles of traverses run in the Rosiclare 

district has brought out a number of significant facts. It has been 

made evident, by applying the earth resistivity method, that many 

known faults are indicated by sharp resistivity anomalies, which in 

many instances can be used to locate these faults within approximately 

100 feet. Many known faults, on the other hand, have been crossed 

without producing in the resistivity readings any anomaly whatsoever.

Since many geological features other than faults can produce variations in resistivity curves, these curves should be interpreted only with 

regard to all available geologic information. Care has accordingly been 

taken, in the foregoing discussion, not to attribute to faulting those 

features in the resistivity curves that could be accounted for by other 

facts of the local areal geology or by topography. Only a few major 

faults not previously known are considered to have been moderately 

well established by the resistivity method. What appears to be a 

number of minor faults have been picked up on individual profiles, 

but, for reasons discussed in the introductory pages, their correlation 

from one profile to the next is very uncertain.

Another point brought out conclusively is that the work described 

hi this report can only be regarded as reconnaissance. Where de-
116 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

tailed information is required on a particular tract of land, the resistivity measurements must be much more closely spaced. One of the 

most important uses that can be made of the present survey is to indicate areas wherein more intensive prospecting, either by the resistivity method or by more direct methods, may be useful.

The more important features that have been brought out by the 

study of these profiles are shown on plate 20.

Eastern area. Beginning on the west side of the eastern area (pi. 

20), a possible fault is indicated just west of the township line between 

E. 7 E. and E. 8. E., on profiles 7, 8, and 10. It runs nearly northand 

south near station 65 on profile 7, station 90 on profile 10, and station 69 

on profile 8.

Another possible fault is indicated near station 4 on profile 5, station 

39.5 on profile 7, and station 42.5 on profile 8.

1 One of the longest faults in the eastern area that has been encountered in this survey is crossed at or near station 53.5 on profile 3, station 

30 on profile 4, station 23 on profile 5, station 96 on profile 6, station 

19 on profile 7, station 14 on profile 8, station 82 on profile 9, station 

29 on profile 10, station 78.5 on profile 11, station 74.5 on profile 12, 

station 72 on profile 13, station 62.5 on profile 14", station 62.5 on 

profile 15, and station 68 on profile 16. It strikes about N. 30° E. and 

is parallel in general to the main Rosiclare fault system.

Another major fault of similar strike is indicated near station 39.5 

on profile 3, and station 45 on profile 4. Just north of profile 4 this 

fault apparently divides into two branches. The western branch is indicated near station 38 on profile 5, station 79 on profile 6, and station 

3.5 on profile 7, a little north of which the fault which itself continues 

about N. 30° E., sends off a branch that strikes due north. The fault 

striking N. 30° E. is encountered at or near stating 4 on profile 8, 

station 75 on profile 9, and station 24 on profile 10. The northjrstriking 

branch occurs at or near station 1.5 (eastern number sequence) on 

profile 8, station 70 on profile 9, station 16.5 on profile 10, station 66 on 

profile 11, and station 62 on profile 12.

To return to the first bifurcation, near profile 4 there is some uncertainty as to the exact course of the east branch. It seems reasonable 

however to correlate station 45 on profile 5 with station 65 on profile 6, 

station 22 on profile 7, station 29.5 on profile 8, station 37.5 on profile 

11, and station 32 on profile 12.

Another fault, located partly on the basis of the resistivity profiles 

and partly on the basis of areal geology, is crossed at station 13.5 

(eastern sequence) on profile 8, station 2 on profile 10, station 40.5 on 

profile 13, and station 34 on profile 14.

Two short faults are indicated on the western parts of profiles 13 and 

14 by a rather unusual anomaly, the exact meaning of which is uncertain but which appears to merit further investigation. The western-
PAJOT 2. 6TUDY BY EARTH-RE SfLSITIVITY MErTHOlD 117

most of these faults is placed at station 88.5 on profile 13 and at station 

82.5 on profile 14. The other is placed at station 83.5 on profile 13 and 

at station 76 on profile 14, and it may cause the anomaly at station 70 

on profile 15.

The correlation of the anomaly near station 25 on profile 7 with the 

one near station 34 on profile 8 is rather uncertain, but these anomalies 

may represent an extension of the Argo-Blue Diggings fault system.

Another fault is indicated at station 40 (eastern sequence) on profile 8, at station 32 on profile 9, and at station 30 on profile 11, and it 

appears to be verified by a line of diamond-drill holes near profile 8. 

An anomaly at station 25.5 on profile 11 may represent the Rosiclare 

fault.

A rather pronounced break in the resistivity curve at station 23 on 

profile 9, at station 28.5 on profile 10, at station 16.5 on profile 11, 

and at station 15 on profile 12 is not satisfactorily accounted for. It 

may be due to faulting, but the available evidence appears to indicate 

that it is associated with the peculiar properties of the Fredonia limestone.

In an area a mile across that centers at Stone school, anomalies are 

so numerous and so irregularly distributed that correlation from one 

profile to the next is practically impossible, and the geologic map 

indicates complex faulting in this vicinity. Resistivity measurements 

in such an area must be more closely spaced than those made hitherto 

if they are to serve as a basis for.reliable interpretations. Only two 

or three remarks appear to be justified by the available data. Two 

similar anomalies which occur near the east ends of profiles 4 and 5 may 

be due to a fault. One of them is near station 113 on profile 4 and the 

other near station 107 on profile 5. There is also an isolated anomaly 

at station .99 on profile 5, which may or may not be significant.

Western area. The main purpose of running the profiles in the 

western area has been to delineate the fault system along Wallace 

Branch. Weller's map shows a fault along the valley of Wallace Branch 

but because of the alluvial cover the precise location of this fault was 

not known.

At least one fault has been followed along this valley for a distance 

of about two miles. For most of its length this fault appears to be 

flanked on the west by Fredonia limestone and on the east by the 

younger formations of the Chester series. At the points where it is 

believed to cross the traverse lines there has usually been a sharp 

rise in resistivity, and if these anomalies are rightly interpreted, the 

course of the fault along the valley of Wallace Branch has been deter- ' 

mined pretty accurately.

The fault is placed at station 23 on profile 17, station 24.5 on profile 

18, station 114.5 on profile 7, station 143 on profile 10, station 2.5 

on profile 19, station 14.5 on profile 20, station 21.5 on profile 21,
118 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

station 10.5 on profile 22, station 44.5 on profile 23, station 10 on 

profile 24, station 2.5 on profile 25, station 13 on profile 26, and station 179 on profile 16. On profile 22, there is also a suggestion of a 

west branch of this fault near station 5, but this fault is not identified 

in the profiles toward the north.

The Wallace Branch fault is indicated on the map (pi. 20) by a 

broken line in the vicinity of profiles 19 and 20, since the resistivity 

peak near station 9 on profile 20 is not matched on the profiles to the 

north or to the south. There may be some cross-faulting in the region 

between profiles 19 and 21, but in order to work this out properly, 

traverses that were more closely spaced would have to be made in 

this area.

Anomalies that have been tentatively correlated as representing a 

fault occur at station 10 on profile 17, station 14 on profile 18, station 

107 on profile 7, station 134 on profile 10, and station 8 on profile 

19. A fault near the old Miller mine is suggested near station 11 on 

profile 19.

The northeastward-trending fault drawn as crossing, profile 22 near 

station 47 and profile 23 at station 4 is based more on geologic than 

on geophysical evidence.

Another fault may cross profile 17 at station 4 and profile 18 at 

station 6.

In the parts of profiles 25, 26,16, and 27, that extend west of the Wal-» 

lace Branch Valley, there are suggestions of several faults, though most 

of the correlations from one profile to another are somewhat uncertain. 

A fault is indicated near station 48 on profile 27, in approximate 

agreement with the geologic map of Hardin County, according to 

which a fault crosses Big Grand Pierre Creek a little west of the western 

end of profile 26. There appears to be a fault near station 267 on 

profile 16. On profile 26 there are suggestions of two faults near 

stations 106 and 100.5 respectively, and of one between stations 96 

and 95, but the correlation between these is very uncertain. An 

anomaly near station 85 on profile 26 is almost directly south of the 

shear zone noted near station 252 on profile 16 and may be due to the 

same fault.

All of these four profiles, 25, 26, 16, and 27, show a region of low 

resistivity bounded rather sharply on both sides by regions of higher 

resistivity. The western boundary, which occurs approximately at 

station 62 on profile 25, station 75 on profile 26, station 240 on profile 

16, and station 20 on profile 26, is thought to be a fault. The eastern 

boundary of this same area, which occurs near station 48 on profile 

25, station 61 on profile 26, station 230 on profile 16, and station 9.5 

on profile 27, may not be a fault, but it probably is, for all the points 

noted lie on a nearly straight line, and the changes in resistivity 

appear too sharp to be due to a boundary of alluvium.
GEOLOGICAL SURVEY BULLETIN 942 PLATE 22

EXPLANATION

7

 O LINE OF TRAVERSE AND SELECTED STATION

NO.9 TRAVERSE NUMBER MAPPED BY GEOLOGIC METHODS

 -^ FAULT INDICATED BY SURFACE MAPPING

"--». PROBABLE FAULT INDICATED BY RESISTIVITY

^ *" POSSIBLE FAULT INDICATED BY RESISTIVITY

=== ROAD B MINE SHAFT

 TRAIL K OPEN CUT

* * FENCE >- TUNNEL

 BUILDING -.^ WATERCOURSE

SCALE IN FEET 

D ICOO 2000 3000

MAP OF THE CAVE IN ROCK DISTRICT, HARDIN COUNTY. ILL., SHOWING LOCATION OF 

EARTH-RESISTIVITY TRAVERSES AND FAULTS.

PART 2. STUDY BY EARTH-RESISTIVITY METHOD HQ 

RESISTIVITY PROFILES OF THE CAVE IN ROCK DISTRICT

The Cave in Rock district includes the northern three-quarters of 

sees. 3 and 4, T. 12 S., R. 9 E., most of sees. 33 and 34, and the southeastern half, with respect to a line drawn diagonally from the southwest to the northeast corner of sec. 27, T. 11 S., R. 9 E. (pi. 22). 

The area mapped barely overlaps the west sides of sees. 26 and 35, 

T. 11 S., R. 9 E.

The western part of the area is crossed from southwest to northeast 

by the Peters Creek fault zone, which has a northeasterly strike and 

a downthrow on the northwest. The downthrown block, which is 

capped by the Caseyville sandstone of the Pottsville group, is topographically the higher. Southeast of the fault zone lies a flat-topped 

hill, or mesa, of intermediate elevation, called Spar Mountain, which is 

capped with the Bethel sandstone of the Chester group. On the 

southeast and southwest, Spar Mountain is bounded by an erosional 

escarpment, and farther to the southeast and south lies an extensive 

area in which the country rock is the Fredonia limestone, of the 

Meramec group.

There is a distinct escarpment between the Caseyville sandstone 

and the Chester group along the line of outcrop of the Peters Creek 

fault zone, but this escarpment must be an erosional feature a faultline scarp rather than a fault scarp since the downdropped block, 

to the northwest, stands higher than the upraised block.

The known fluorspar deposits in this area occur principally in the 

tract of intermediate elevation southeast of the Peters Creek fault. 

The fluorspar is mined from bedding-replacement deposits that occur 

near the top of the Fredonia limestone and that crop out in the escarpment of Spar Mountain. Some of the mines have been developed 

through horizontal drifts or adits from the face of the escarpment; 

others are operated from vertical shafts on top of the hill.

In the summer of 1934, a few short experimental resistivity profiles 

were run in places known to be underlain by bedding-replacement 

deposits. These profiles revealed a number of unexpected, and to 

some extent unexplained, resistivity anomalies. It was therefore 

decided that in the summer of 1935 a controlled grid work of resistivity 

lines should be run in the area, not so much with the expectation of 

finding any particular features as with the desire to determine what 

could be found.

This grid work was confined chiefly to the area underlain by the 

Chester group between the Peters Creek fault and the area in which 

the Fredonia limestone crops out. Two series of traverses were run, 

one on approximately parallel lines extending northeast, and the other 

on shorter parallel lines extending northwest. These directions conform broadly with the major structural axes of the area. The second

551386 44 9
120 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

series of traverses extended far enough northwestward to cross the 

Peters Creek fault zone. The average spacing of the lines of traverse 

in this system was nearly 1,000 feet, while that of the lines running 

northeast was about 600 feet. Readings were taken at stations 100 

feet apart, and an electrode interval of 100 feet was used. The line of 

electrodes was at all times parallel to the line of traverse.

Topographic and areal maps (pis. 1 and 3) were made at the same 

time that the resistivity profiles were being run. The lines of traverse 

were located on this map by placing marked flags at intervals along 

each line, which were later located by the plane-table survey. The 

». faverse lines are shown on the base map (pi. 22).

DISCUSSION AND INTERPRETATION 

NORTHEAST-SOUTHWEST PROFILES

The Cave in Rock profiles (pi. 23) show much more erratic variation 

from one station to the next than the Rosiclare profiles. Unfortunately, as none of the traverses in the Cave in Rock district were repeated, 

there is no direct evidence as to how much of this erratic behavior 

is due to geologic and how much to instrumental causes. It is pertinent, however, to say that profiles 4, 5, 11, 12, 13, and 15 in the Rosiclare area, were run immediately after the completion of the work in 

the Cave in Rock area and with the same instrument; and the fact 

that these profiles do not show the erratic behavior characteristic 

of the Cave in Rock profiles tends to show that the causes of this 

behavior are chiefly geologic.

 In any case, the minor oscillations in the Cave in Rock profiles are 

too numerous to be interpreted individually or to be of any individual significance for our present purposes. In what follows, only the 

larger changes in the character of the resistivity profiles will be discussed.

PROFILE 1

In profile 1, the northwesternmost of those running northeast (see 

pi. 23), the changes of resistivity that seem to merit attention are 

those at stations 11, 20, 53, 80, 100, and 118. The anomaly at station 

11 occurs where the traverse is ascending the slope of Lead Hill, and 

is probably due to the transition from one formation to another or 

from a thick to a thin cover of soil above the bedrock. The anomaly 

at station 20 occurs on the north slope of Lead Hill, in an area containing many sinkholes. The steep rise in resistivity between stations 

51 and 55 occurs on the southwestern escarpment of Spar Mountain,, 

in which formations of the Chester group are exposed.

The sharp drop in resistivity at station 80 occurs on a very gentle 

slope. Only Bethel sandstone is mapped at this point, so that if there 

is a fault here it must have Bethel sandstone in both walls. No other
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 121

geologic feature has been observed here that would account for this 

anomaly.

The change in resistivity near station 100 is one of the few anomalies 

in the Cave hi Rock area that can be traced across a large number of 

successive profiles. It was formerly thought to be due to a fault, but 

comparison of the resistivity anomalies with the Wagner-Currier 

geologic maps (pi. 3) has shown that this series of anomalies invariably 

occurs at or near a northeastward-striking belt of the Paint Creek 

shale, a rock that has been found to be highly conductive.

The oscillations between stations 110 and 119 occur, in part, where 

the line crosses two small valleys, and they may be due to changes of 

resistivity with the irregular topography. One of the valleys is 

crossed at station 114, and it will be noted that the resistivity on the 

opposite sides of this valley is not quite the same. This asymmetry 

suggests that there may be a fault along the valley. The sharp drop 

near station 118, which occurs well beyond the creek valleys and out 

on level ground, seems best explainable as due to a fault. Beyond 

station 119 the resistivity values are almost constant.

PROFILE 2

Between stations 20 and 80 on resistivity profile 2 there is a great 

deal of erratic detail, which may be correlated in part with the many 

indentations of the topographic profile. There is a rather large drop 

in resistivity between stations 21 and 25, which occurs in the topographic saddle connecting Lead Hill with Spar Mountain, but this 

anomaly may be due merely to nearby sinkholes.

An increase in average resistivity begins at station 61. The change 

occurs at the crossing of a small valley, southwest of which the resistivity is high and northeast of which it is low, in a locality of gentle 

topographic relief. There is no direct geologic evidence of faulting 

at this place, and the only rock exposed nearby is Bethel sandstone; 

the topography, however, gives some suggestion of faulting. The creek 

valley that is crossed at station 61 trends northwest at that place, but 

about 1,000 feet farther down stream it swings rather sharply to the 

northeast. This is an unusual stream pattern for nearly horizontal 

strata, and it may be controlled by small cross faults.

There is another change in resistivity from high to low near station 

80, and this change, also, occurs in gentle topography. There is no 

satisfactory geologic explanation for it other than faulting, though 

there is no visible evidence of a fault.

The sharp drop in resistivity between stations 114 and 115 occurs 

where the profile crosses the same small stream that it crossed at station 

61, but the stream has made another sharp bend and is here flowing 

southeastward. In this vicinity the valley is rather steep-sided and
122 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

fairly symmetrical, and the strata dip very gently to the northeast, 

so that if the rocks on the two sides were the same the resistivity 

profile across the valley would also be symmetrical. According to 

the areal geology map (pi. 3), however, the strata on the two sides 

of the creek are not the same; Bethel sandstone alone appears on the 

southwest side, and Bethel sandstone overlain by Paint Creek shale 

and Cypress sandstone on the northeast side, apparently in normal 

stratigraphic sequence. The Paint Creek shale is probably responsible 

for the decline in the resistivity as the creek is crossed. The sharpest 

decline in resistivity, however, occurs between stations 117 and 119. 

As this is more than 200 feet from the outcrop of the Paint Creek 

shale, which dips away from these stations, that formation cannot 

be held responsible for this particular drop in resistivity. Two other 

possible causes appear to remain a fault parallel to the creek, in 

the Bethel sandstone, or a decline in the depth of the water table.

There is another sharp drop in resistivity between stations 122 and 

123, east of which the resistivity is low and very nearly constant. This 

same feature was observed at and east of station 119 on profile 1. Both 

breaks occur in open fields, and both are interpreted as being due to 

the same fault.

PROFILE 3

Between station 20 and station 30 on profile 3 lies a region of low 

resistivity, which is followed by one of high resistivity extending from 

station 30 to station 43. The transition occurs near station 30, which 

is approximately on the boundary between an area of deep soil and 

alluvium and a hill slope of bare rock. The resistivity drops between 

stations 43 and 51 and then rises again. Both the fall and the rise 

occur on uneven topography, where different strata crop out at different elevations along the line of traverse. If there has been faulting at 

this locality, its effect is obscured by the variations in resistivity that 

result from the combined effect of lithologic variety and uneven 

topography.

The small dip in the resistivity profile near stations 65 and 66 is 

unaccounted for. It occurs in flat country, and it cannpt definitely 

be correlated with like features on other profiles.

The large drop in resistivity between stations 84 and 88 appears 

to correlate with similar features occurring near station 80 on profile 

1 and station 80 on profile 2. Like these, it occurs in a region of gentle 

topography, where only Bethel sandstone is exposed, yet it marks the 

the division between an area of distinctly high and another of distinctly 

low resistivity, and seems best accounted for as a result of faulting.

In the remainder of the profile, there is a sharp break upward in 

the resistivity at about station 99, a break downward again at station 

106, another break upward at station 108, a break downward at
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 123

about station 114, and a sharp break downward between stations 

133 and 134. The break at station 134 evidently correlates with a 

similar feature at station 123 on profile 2 and station 119, profile 1. 

Breaks are more numerous in this part of the profile than in corresponding parts of the other two profiles, and their correlation is uncertain. The break at station 99 occurs where the profile crosses a valley, 

with respect to which it is asymmetric. The one at station 103 occurs 

where the same valley is crossed a second time, having made a band 

between these two stations. The break at station 108 occurs on a gentle 

slope where the profile runs parallel to the contour line. The break 

at station 115 occurs 100 feet southwest of the valley that was crossed 

at station 115 on profile 2, and as there the resistivity is high southwest of the valley and somewhat lower northeast of it. Here again 

the Paint Creek shale crops out in the northeast bank of the creek, 

but the major drop in resistivity occurs about 200 feet southwest of 

the nearest outcrop of Paint Creek shale. A small fault in the Bethel 

sandstone may possibly connect this anomaly with the corresponding 

feature between stations 114 and 115 of profile 2. Beyond station 

134, the same region of uniformly low resistivity that terminates 

profiles 1 and 2 is again encountered.

PROFILE 4

From station 20 to a point near station 38, profile 4 extends across 

a low flat area, containing Fredonia limestone overlain by deep soil, 

and from there to station 57 it is on the southern escarpment of Spar 

Mountain, station 57 being at the south brow of the mountain, on 

its capping Qf Bethel sandstone. All of the changes in resistivity 

between stations 20 and 60 can be attributed to variations in the 

depth of overburden above the bedrock and to the outcropping of 

different strata along the face of the escarpment. The rise in resistivity between stations 64 and 65, the drop near station 70, and the 

rise again near station 79 all occur on gently rolling topography, 

where much of the land has been cultivated. Bethel sandstone 

crops out at the surface, but there are a few sinkholes formed by caving 

of the limestone beneath. The sharp variations in resistivity along 

here do not correlate very well with the adjacent profiles, and they 

may be due to underground solution channels. The break near 

station 79 may possibly represent a fault along a small valley which 

the traverse crosses at this place.

The sharp drop in resistivity between stations 92 and 93 appears to 

correlate with a similar drop near station 86 on profile 3 and stations 

80 on profiles 1 and 2. This correlation impjlies the existence of 

either a crooked cross fault or a combination of intersecting faults 

separating a region of higher resistivity to the southwest from a region 

x of lower resistivity to the northeast.
124 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

It is almost impossible to make any rational interpretations of the 

erratic variations of resistivity northeast of station 93. From stations 

93 to 105 the topography is nearly flat; a part of the land was formerly 

cultivated. At about station 109 the profile crosses a small valley, 

and at station 117 it crosses a stream in a larger V-shaped valley. 

From stations 122 to 135 the ground is again almost level.

The rise in resistivity between stations 106 and 109 occurs near 

the point where the profile crosses a small valley, in the bottom of 

which there are outcrops of Paint Craek shale, and on both sides of 

which there is Cypress sandstone. This rise is probably due to the 

influence of the shale.

The profile is asymmetrical with reference to the stream crossed 

at station 117, showing successively high resistivity, a drop to low 

resistivity, and a rise again on the-northeast side of the valley.

In this locality the geologic map (pi. 3) shows that Paint Creek 

shale crops out on both sides of the valley, near station 116 on the 

southwest side and near station 118 on the northeast side. The shape 

of the resistivity profile does not very closely correspond to this outcrop pattern. There is no evident influence of the shale on the southwest side, and station 120, on the northeast side, is well up on the hill 

approximately 60 feet above the place where the map (pi. 3) shows 

Paint Creek shale. Since there is little or no correspondence between 

this and the adjacent profiles, the significance of these variations is 

not clear.

PROFILE 5

The part of profile 5 between stations 20 and 56 extends from the 

deep soil cover over the Fredonia limestone across the successive 

strata that crop out along the escarpment. The Bethel sandstone is 

encountered near station 52. ' There is a sharp drop in resistrvity 

between stations 57 and 59. The break in the traverse between stations 62 and 70 ^s due to a pond. Northeast of station 73 the resistivity rises sharply.

The reason for the area of low resistivity between stations 59 and 

73 and for its sharp boundary on the two sides is by no means clear. 

Part of this "low" occurs in an undrained basin containing sinkholes. 

In this area a bedding-replacement deposit of fluorspar is being mined 

at a depth of somewhat more than 100 feet, and there are enough 

underground workings and diamond-drill holes in the locality to rule 

out the possibility of any faults of more than a few feet displacement. 

The surface formation is Bethel sandstone, beneath which the Renault limestone is exposed in a nearby sinkhole. It may be that a combination of thick water-saturated soil in the depression and the 

proximity to the underlying Renault limestone is responsible for 

the low resistivity.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 125

From station 89 to station 96 there is a steady decline in resistivity. 

The northeast end of this decline can be accounted for by the presence 

of the Paint Creek shale, which is more conductive than the Bethel 

sandstone, but the beginning of the decline, which is several hundred 

feet from the shale, cannot be thus accounted for. This drop in resistivity correlates with similar features on the profiles already discussed where no Paint Creek shale is known to occur, and it suggests 

a possible fault whose location is somewhat uncertain but which is 

probably near station 93 or station 94. Between stations 96 and 111 

the traverse extends across an area of low relief, which is mapped as . 

Cypress sandstone. Across this area the resistivity values oscillate 

with rather large amplitude, for reasons that are not clear. A creek in 

a rather steep-sided V-shaped valley is crossed at station 115. According to the geologic map (pi. 3), the bottom of the valley is in Bethel 

sandstone, above which on both sides is the Paint Creek shale, and the 

rock of the uplands on both sides is Cypress sandstone. The Paint 

Creek shale is somewhat lower on the northeast side of the valley than 

on the southwest side, but this fact is attributed to regional dip. The 

profile across the valley is asymmetrical, showing high resistivity immediately southwest of the creek and low resistivity for about 500 

feet to the northeast. This difference can be partly accounted for .by 

the lower elevation of the Paint Creek on the northeast side of the 

valley, but it may be partly due to the fact that on the northeast side 

of the main valley the traverse follows the outcrop of the Paint Creek 

along a tributary valley for a distance of two or three hundred feet. 

The rise in resistivity beyond station 122 very nearly coincides 

with the boundary between the Paint Creek shale and the overlying 

Cypress sandstone.

PROFILES 6, 7, and 8

Profiles 6, 7, and 8 are short, experimental lines run during the summer of 1934, when it was not planned to carry out the more extensive 

grid work of traverses that were run during the following season.

Profile 6 is a short line, of only nine stations. It shows, in general, 

a steep decline in resistivity from a maximum value at station 1 to 

a minimum at station 9. A small valley is crossed at station 6, where 

there is a slight oscillation in the resistivity curve.

On profile 7 the resistivity is high at the southwest end, but declines 

sharply, reaching a low value at station 5, and it rises'to a high value 

between stations 13 and 17. The "low" very nearly correlates with a 

similar area of low resistivity noted in profile 5 and is probably due 

to the same cause. .

On profile 8 the resistivity rises from station 1 at the southwest end 

to station 12, drops abruptly between stations 12 and 13 from a high 

value to one less than half as great, and then oscillates about the lower
126 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

value. The sharp drop occurs at the same small valley that was 

crossed at station 6 on profile 6. The valley apparently follows a fault, 

which, incidentally, correlates with the drop in resistivity at about 

station 94 on profile 5. In each instance, the Paint Creek shale crops 

out from 100 to 300 feet northeast of the apparent location of the 

fault and dips away from the fault. This line of anomalies cannot, 

therefore, be attributed to the influence of the Paint Creek shale.

PROFILE 9

The line forming the basis of profile 9 is* unfortunately located, in 

that most of it extends along the southeastern scarp of Spar Mountain, so that it continually rises and falls in passing over the eroded 

edges of the various formations there exposed. The variation of 

resistivity resulting from this cause alone is sufficient to obscure any 

anomalies that might be due to faults. The region of low resistivity 

between stations 60 and 70 correlates with the corresponding regions 

in profiles 5 and 7 already discussed. The sharp rise in resistivity 

between stations 94 and 95 is just about on the strike of the supposed 

fault that crosses the preceding profiles, yet it may be due wholly to 

transition from the Renault limestone to the Bethel sandstone along 

the face of the scarp.

PROFILE 10

Profile 10, which is only 2,000 feet long and extends from station 30 

to station 50, is all in an area of Fredonia limestone in which there are 

scattered sinkholes. The resistivity values fluctuate up and do\\n 

about a mean value of about 60,000 ohm-centimeters. The variation 

is of the kind commonly encountered in sinkhole topography and probably is caused merely by the irregularity of the surface of the Fredonia limestone.

PROFILE II

Profile 11 extends along the southeastern base of Spar Mountain. 

It is chiefly on the Fredonia limestone but crosses the outcrop 

of the "sub-Rosiclare sandstone" zone in two places. Toward the 

northeast end it extends into the area of the Renault formation. The 

small peak in the resistivity curve between stations 27 and 23 is about 

where the outcrops of the sub-Rosiclare sandstone is crossed. The 

sandstone is crossed again between stations 20 and 15 and causes another 

peak in the resistivity curve. The lows are over Fredonia limestone. 

The rise toward the northeast end of the curve is over the Rosiclare 

sandstone, the Levias limestone, and the Renault formation.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 127

PROFILE 12

Profile 12 is parallel to and about 600 feet southeast of profile 11. 

It is almost featureless throughout, evidently because of a deep covering of wet ground and soil and alluvium.

NORTHWEST-SOUTHEAST PROFILES

In discussing the variations of the profiles (pis. 22, 23) that trend 

northwest, the southeast end of each profile is considered first.

PROFILE 13

From station 1 to station 13 the resistivity values on profile 13 

oscillate about an average value of 70,000 ohm-centimeters. Between 

stations 13 and 18 there is a region of low resistivity, followed by a 

return to about the previous average from station 18 to station 24. 

Then comes an abrupt drop, which continues to station 43, northwest 

of which the resistivity again rises sharply.

Between stations 1 and 33 the profile is on rock mapped as Cypress 

sandstone. Through most of the length of the profile the topography 

is of very slight relief and the land largely under cultivation. The drop 

in resistivity between stations 13 and 18 indicates possibly a small 

fault block. The sharp drop between stations 24 and 25 occurs where 

the profile crosses a small creek with low valley walls. The region of 

low resistivity to the northwest is the same as that shown on the northe astern sections of profiles 1 and 2. The only probable geologic 

explanation for this sudden drop appears to be that this creek follows 

a fault. A short distance downstream the rock is much jointed, as if 

by local disturbance. The sharp rise in resistivity at station 44 coincides 

with the Peters Creek fault, with a rolling surface of Hardinsburg sandstone to the southeast and a steep escarpment of Caseyville sandstone 

to the northwest.

PROFILE 14

The southeast end of profile 14 is designated station 6. This 

point is a few hundred feet outside of the area mapped but is a short 

distance over the edge of the scarp. The low resistivity between stations 

 3 and 6 is probably due to the influence of the Paint Creek shale, 

and the succeeding rise to that of the Cypress sandstone. The drop 

in resistivity near station 8 is symmetrical with reference to a small 

V-shaped valley, and is probably due to the fact that the influence of 

the Paint Creek shale is stronger in the bottom of the valley than on the 

sides. The drop in resistivity close to station 20 is near the crossing of 

a rather steep V-shaped valley. The valley walls are of Cypress sandstone, but the Paint Creek shale should be only 20 or 30 feet deeper 

below the valley floor, so that this drop also may be due to the influence of the Paint Creek shale.
128 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

A drop in resistivity northwest of station 24 correlates with a similar 

drop near station 24 on profile 13. It cannot be easily explained 

except by faulting, since it occurs on almost level ground in an open 

field.

PROFILE 15

Profile 15 begins, at the southeast, on Bethel sandstone, then 

crosses the the outcrop of Paint Creek shale near station 2, after which 

it traverses Cypress sandstone. The low resistivity between stations 2 

and 6 is evidently due to the influence of the Paint Creek shale.

In the small valley crossed at station 15, a narrow strip of Paint 

Creek shale again crops out, probably accounting for the low resistivity 

at this locality. Beyond station 18 the resistivity again rises sharply, 

the rise coinciding with a small fault, discovered by Wagner, between 

the Cypress and the Bethel sandstones.

Between stations 32 and 34 the resistivity again declines sharply. 

At first this drop was thought to be due to a fault, but it is more probably 

due to the Paint Creek shale, of which the areal geologic map (pi. 3) 

shows an outcrop between stations 33 and 35.

The sharp rise beyond station 45 is caused by the Caseyville sandstone which is exposed just west of the Peters Creek fault.

PROFILE 16

The southeast end of profile 16 is on Cypress sandstone. The drop 

in ^resistivity immediately following, and the low values of 

resistivity between stations 3 and 19, are evidently due to the Paint 

Creek shale, which the traverse crosses along this part of the profile. 

The traverse continues over a stretch underlain by a thin layer of Cypress sandstone, which causes only a slight rise in resistivity, and then 

crosses the Paint Creek shale again. The more marked rise in resistivity between stations 19 and 20 probably marks the transition from 

Paint Creek shale to the underlying Bethel sandstone.

The drop in resistivity near station 32 occurs at a northeastwardflowing creek flowing over sandstone in which there are prominent 

joints that strike N. 60° E. The Paint Creek shale, as shown on the 

geologic map (pi. 3), crops out between stations 33 and 34. At the 

time these traverses were being run, this break in resistivity was 

thought to represent a fault along the creek. A comparison of the 

resistivity results with the areal geologic map, however, makes it 

appear probable that this anomaly is due to the influence of Paint 

Creek occurring in normal stratigraphic sequence.

The sharp rise northwest of station 46 is due to the influence of the 

Caseyville sandstone. However, it can hardly represent the Peters 

Creek fault which, according to the geologic map, crosses the profile
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 129

near station 43 with Hardinsburg sandstone in the east wall and the 

Clore limestone, Degonia sandstone, and Kinkaid limestone in the west 

wall.

PROFILE 17

From station 7 to station 37, according to the areal geologic map, 

profile 17 crosses Bethel sandstone. Between stations 18 and 20 there 

is a large drop in resistivity, beginning about 100 feet northwest of a 

small creek and the resistivity rises again somewhat gradually from 

stations 22 to 29. There is no visible reason for this low area of resistivity unless a fault, possibly the one shown on the geologic map, extends far enough to cross the profile near station 19, where it would be 

parallel to the creek and near its left bank. The low resistivity could, 

in that case, be due to the influence of the Renault limestone, which is 

moderatively conductive, under a thin covering of Bethel sandstone, 

and the rise in resistivity to the northwest could be due to the thickening of the Bethel sandstone in that direction. The sharp drop in resistivity between stations 33 and 34 also occurs in the Bethel sandstone 

area, and might also be due to a fault. At station 32, prominent joints 

striking N. 65° E. were noted. The bedding here strikes N. 65° E. and 

has a dip 8° NW., which corresponds to a 14-foot change in elevation 

per hundred feet of horizontal distance. Should this dip persist, it 

would cause such an increase northwestward in the thickness of the 

Bethel sandstone, as would easily account for the observed rise in resistivity. * i

The downward break in the profile between stations 36 and 37 coincides, again, with the outcrop of the Paint Creek shale as shown on the 

areal geologic map (pi. 3).

The rise in resistivity at station 48 is evidently due to the influence 

of the Caseyville sandstone. The two branches of the Peters Creek 

fault, crossed at stations 41 and 43, had very little influence on the 

resistivity values.

PROFILE 18

Profile 18 begins at the southeast, in a region of high resistivity. 

The resistivity drops to a considerably lower level near station 15, 

which is on the axis of a small valley, and suggestions of faulting along 

this valley have been noted in some of the other profiles.

The sharp drop in resistivity between stations 39 and 40 occurs at a 

point where, according to the geologic map, the profile crosses a branch 

of the Peters Creek fault which here brings Cypress sandstone, on the 

northwest side, against Bethel sandstone. The Cypress sandstone is 

probably underlain at no great depth by the Paint Creek shale, and the 

shale is evidently responsible for the low resistivity on the northwest 

side of the fault. The geologic map shows three more faults between 

stations 40 and 50, but they are not indicated by the resistivity values,
130 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

which are all uniformly low. The tiny break between stations 47 and 

48 might be due to one of these faults, but it would not be noticed if the 

fault were not otherwise known to be present. The sharp rise in 

resistivity beyond station 51 is apparently not caused by a fault but by 

Caseyville sandstone cropping out in normal stratigraphic sequence.

PROFILE 19

Profile 19, one of the short experimental profiles run in 1934, has no 

features that appear to be significant. It does not show any drop in 

resistivity corresponding to that near station 15 on profile 18.

PROFILES 20 AND 21

Profile 20 is another of the short profiles taken in 1934. Profile 21 

was run in 1935. Together they make a nearly continouus profile, for 

the northwest end of profile 20 and the southeast end of 21 almost 

coincide.

The southeastern part of profile 20 shows very low resistivity along a 

stretch of 800 feet, in which it crosses the undrained depression. This 

depression had a similar effect of several of the profiles trending northeast. Beyond station 10 the resistivity rises rather sharply, probably 

because the rock, with its thin cover of soil, beyond the depression, is 

much less conductive than the deep wet soil within the depression.

There is a sharp drcp in resistivity between stations 35 and 36, near 

a northeastward-trending valley on both sides of which the surface formation is mapped as Bethel sandstone. A somewhat similar feature 

was noted near station 34 on profile 17, and the resistivity values oscillate rather widely at the corresponding part of profile 18. These three 

anomalies may represent a fault parallel to the Peters Creek fault 

zone.

The final drop in resistivity between, stations-41 and 42* suggests a 

fault near station 41, and this is precisely where the geologic map shows 

one of the faults of the Peters Creek fault zone. The map shows another of these faults near station 44 and a third at station 46, but 

neither is indicated on the resistivity profile, unless a small break between stations 47 and 48 represents one of them.

PROFILE 22

Profile 22, which is another of the short preliminary profiles, closely 

resembles'profile 20, showing low-resistivity for the first nine stations 

to the southeast and then a rise to a higher resistivity level on the 

northwest. Mine and drill data indicate that there is no fault of more 

than a few feet displacement in this locality, so that the change in resistivity is probably due to the transition from deep wet soil to dry, 

almost bare Bethel sandstone. Where the resistivity is low the Renault
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 131

limestone, which is relatively conductive, is much nearer tne surface 

than on higher ground to the northwest.

PROFILES 23 and 24

Profile 24, one of the short preliminary profiles, shows the same 

transition from low to high resistivity that was noted in the foregoing 

profiles.

Profile 23, which overlaps 24 by about three stations, is one of the 

the later ones. From stations 15 to 41, this profile oscillates within a 

relatively narrow range, then drops abruptly between stations 41 

and 43. The geologic map shows one of the branches of the Peters 

Creek fault, with Bethel sandstone on both sides, at station 39.5 and 

another branch at station 42, with Bethel sandstone to the southeast 

and Tar Springs sandstone to the northwest. This latter fault is probably the one indicated by the drop in the profile, the first one producing- no distinguishable effect. A third fault is shown on the geologic 

map near station 46, but the resistivity profile shows only a very 

slight rise at this point. Between stations 55 and 68, the profile 

oscillates about a low resistivity value. The topography between 

stations 57 and 72 is fairly flat. This western part of the profile 

extends beyond the area mapped geologically, and also beyond the 

adjacent profiles. There is a suggestion of possible faulting west of 

Peters Creek, between stations 57 and 68. The sharp rise in resistivity 

between stations 72 and 73 reflects the transition from shale to conglomerate of the Caseyville sandstone; there apparently is no fault 

at this place.

PROFILE 25

Profile 25 is about 2,000 feet southwest of profiles 23 and 24. Between stations 14 and 35 it passes over a small hill, the crest of which 

it crosses near station 26. The hill is capped with limestone of the 

Renault formation, and the Levias limestone, Rosiclare sandstone, 

and Fredonia limestone crop out on its flanks. As the strata here 

have only gentle dips, the same formations should be traversed on 

opposite sides of the hill, and the resistivity values should accordingly 

be more or less symmetrical with respect to its crest. It was found, 

however, that the resistivity is high on the southeast side of the hill 

and low on the northwest side. There is a sharp drop in resistivity 

near station 23, then an oscillation, and a further drop between stations 33 and 34. Granting that these variations of resistivity may be 

due in part to variations in the thickness of the overburden, it remains 

possible that tliere is a fault near station 23 and another near station 33.

Beyond station 35 the traverse crosses rather low and gently rolling topography. Between stations 45 and 46 there is a small but 

abrupt rise in resistivity. It is here that the traverse crosses Peters
132 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

Creek, and loose quartzite and fault breccia are exposed in the bank of 

the stream. The geologic map (pi. 3) does not show any fault at this 

place, but the "high" evidently reflects an unmapped member of the 

Peters Creek fault zone.

PROFILE 26

Profile 26 crosses tae topographic saddle connecting Lead Hill with 

Spar Mountain. The ridge crest is at station 30. Except for a little 

Rosiclare sandstone at the highest part, only Fredonia limestone is 

exposed in this saddle. Between stations 20 and 45 the profile shows 

a rather erratic variation of resistivity, with a general descent. A short 

distance northeast of the line of traverse, near station 25, there is a 

minor fault with a strike of N. 65° E., at which quartzite is exposed; 

and only a short distance southeast of the traverse Fredonia limestone, 

on the other side of the fault, is found hi the dump of an old shaft. 

This fault may extend to profile 25 and may account for the anomaly 

noted near station 23 on that profile.

There is a break downward in profile 26 between stations 35 and 37. 

Loose boulders of what appear to be brecciated sandstone are scattered 

over the surface near station 35, and there may be a small fault extending from this point to the vicinity of station 33 on profile 25.

There is another break downward in resistivity near station 40, and 

here the geologic map shows a branch of the Peters Creek fault, separating Fredonia limestone on the southeast from Bethel sandstone on 

the northwest. It shows another fault, between Bethel and Cypress 

sandstones, near station 45, and this fault probably accounts for the 

rise in resistivity northwest of that station.

Between stations 50 and 51, about where the traverse crosses Peters 

Creek, there is again a sharp break downward in the resistivity. Fault 

breccia was observed in the hillside at station 49.

SUMMARY AND CONCLUSIONS FOR THE CAVE IN ROCK DISTRICT

The resistivity work in the Cave in Rock district was carried out in 

a spirit of inquiry, its purpose being to see what could be discovered in 

in an area of minor faults rather than to discover any particular geologic features. The resistivity values were found to vary somewhat 

more erratically in this area than hi that around Rosiclare, but, by 

disregarding the lesser variations and focusing attention upon the 

larger, it is possible to detect a certain degree of correlation between resistivity and structure. It has been found, for example, that 

certain major faults in the Peters Creek fault zone were marked by 

rather abrupt changes in the value of the resistivity. Other faults, 

however, because the rocks on the opposite sides are nearly equal in 

resistivity, are not indicated by the slightest break in the resistivity 

profile.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 133

It has been found that the Paint Creek shale, which is only about 

15 to 20 feet thick, is so good an electrical conductor that wherever its 

outcrop is crossed there is a large drop in resistivity. It may even 

have a marked effect on resistivity where it is overlain by the Cypress 

sandstone, if the sandstone is not too thick. So notable is the effect of 

the Paint Creek shale that its line of outcrop striking northeastward 

across the northwest corner of sec. 34, T. 11 S., R. 9 E. was mistaken 

when the profiles were being run for a major fault, a mistake that was 

corrected only after careful study of the resistivity data in comparison 

with the geologic map.

An undrained depression hi the southwest corner of sec. 34 contains 

an area of abnormally low resistivity about 1,000 feet in diameter. 

Mining and drill-hole data indicate that this area is not bounded by 

faults, though one small fault of less than 10 feet displacement is encountered in the underground workings beneath the depression. The 

evidence indicates that the low resistivity in this depression, as compared with that of the surrounding area, is due jointly to its greater 

proximity to the conductive Renault formation and to its being more 

deeply mantled with an especially wet residual soil.

Several of the resistivity anomalies in the district, however, are 

regarded as probably due to minor faults that had not been detected hi 

the course of the geologic mapping, These obscure faults, on which the 

displacements cannot be more than a few tens of feet, have already 

been discussed at some length in analyzing the profiles. Considerable 

uncertainty exists in many cases regarding the correlation from one 

profile to the next. What seem to be the most probable correlations 

of these features is indicated on the map (pi. 22), but it should be 

remembered that these tentative correlations are not the only ones 

possible.

In the area bounded by profiles 1,4,13, and 16 the resistivity values 

suggest faulting in a somewhat irregular pattern. If such faulting has 

occurred, the small faults could well act as channels for mineral-bearing 

solutions, which would make this area a possible source of fluorspar, 

occurring eitherinveins or in bedding-replacement deposits. One small 

fault was discovered in this locality in the course of the areal mapping, 

and the presence of others is suggested by the resistivity data. These 

faults being bounded on both sides by the same formation, would have 

"been difficult to detect by geologic mapping unless outcrops of the fault 

breccias had been encountered.

The interpretations here presented are not offered as conclusive. 

They do little toward answering directly the question where to look 

for fluorspar,; they are most likely to be useful indirectly, by showing in 

what area it may be best worth while to prospect further by more 

direct methods.
134 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

APPENDIX 12

THEORY OF THE WENNEK METHOD OF MEASURING 

EARTH RESISTIVITY

ELECTRICAL RESISTANCE

OHM'S LAW

If an electric current, 7, (fig. 23) is made to flow through a resistance, there will be a difference of electrical potential, or voltage, E, 

between the opposite ends of the resistance. 'jit is found by experi1111111

AAAAAAFIGURE 23. Electrical circuit for measuring the resistance R.

ment that the ratio of the potential difference to the current is constant, at any constant temperature, for any particular resister.

Tfl

~r =R (a constant), (1)

where the resistance of the body, R, is expressed in ohms when E is in 

volts and /in amperes. That is, a resistance of one ohm will cause a

FIGURE 24. Measurement of the resistance of a rectangular block to the flow of current parallel to the y axis.

potential drop of one volt for a current of one ampere. This relation 

is known as Ohm's law.

RESISTANCE OP AN EXTENDED BODY

The resistance of a body varies with its size, shape, and composition, 

and with the direction in which the current flows through it. Consider 

a rectangular block of material, as shown in figure 24, with the X, Y,

u This appendix is intended to help those engaged in electrical earth-resistivity work to interpret their 

problems.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 135

and Z axes of coordinates taken along one set of its edges. Let a 

current /flow through the block parallel to the Y axis, and let Ebe the 

voltage drop between its ends.

Under these conditions it is found experimentally that' the total resistance of the block is directly proportional to its length parallel to 

the direction in which the current flows, and inversely proportional to 

the area of its cross section normal to the direction of flow. Hence,

-j^=R=pj-, (2)

where p is a constant of proportionality, which differs in value for 

different materials. 

If

x=y=z~l centimeter,

then the block would be a centimeter cube and

(3)

Thus p, which is known as the specific electrical resistivity for a given 

material, is the resistance offered by a unit cube of that material to the 

flow of an electric current in a direction perpendicular to one pair of 

its faces.

Solving equation (2) for p we obtain

or, in words: This has the dimensions

. resistance X length2 . , , Resistivity = - ; - = resistance X length. 

length

When the resistance is measured in ohms and the length in centimeters, the resistivity is in ohm-centimeters. If the length is 

measured in meters, in inches, or in feet, the resistivity is in ohmmeters, ohm-inches, or ohm-feet, respectively. One system of units 

is converted to another 1>y means of the same factor that is used in 

converting the corresponding unit of length. These factors are given 

in the table following.

Conversion factors for electrical resistivity

Ohm-centimeters

1

100

2.5400

30. 480

Ohm-meters

0.01

1

.025400

. 30480

Ohm-inches

0. 39370

39. 370

1

12

Ohm-feet

0. 032808

3.2808

. 083333

1

551386 44- -10
136 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

Some materials have different resistivities in different directions, 

and these are said to be anisotropic. Isotropic materials, with regard 

to the flow of electricity, are those which have the same resistivity hi 

all directions. A body is both homogeneous and isotropic as regards the 

conduction of electrictiy when its specific resistivity is the same at 

all points and in all directions.

RESISTANCE OP HALF SPACE TO CURRENT PROM SINGLE

ELECTRODE

In the case of earth-resistivity measurements the conductor, instead 

of being hi the nature of an elongate cylinder (such as a wire) or prism, 

is a mass of practically infinite extent, on the lower side of abounding 

surface that may broadly be regarded as horizontal. For purposes of 

analysis, we shall assume that the topographic surface is a smooth 

plane surface of infinite extent, and that the earth beneath is of infinite 

depth and isotropic as regards electrical properties.

Let us imagine that we insert a single electrode into this ideally 

homogeneous ground and cause a current 7 to flow through the elecFIGXJRE 25. Radial flow of electric current into half space from a single hemispherical electrode (only

quarter space shown).

trode into the ground. Since, by hypothesis, all equal radial segments 

hi the ground leading away from this electrode have the same resistance 

the current will distribute itself equally among them and hence will 

flow away radially and equally hi all directions (fig. 25).

Since the current will everywhere flow radially away from the electrode, the conductor can be thought of as being composed of a series 

of concentric hemispherical shells having the electrode as their common center.

Let r0 (fig. 26) be the radius of a small hemisphere centering at the 

electrode. Let ri} ra , rz, ____________rw be respectively the outer

radii of successive shells at increasing distances away from the electrode. 

The differences between these successive radii can be made as small 

as one likes and may be thought of as infinitesimal.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 137

Let ARi be the resistance of the shell whose inner and outer radii 

are rQ and r1} A#2 that of the shell between ^ and rz, and A#» that of 

the shell whose inner and outer radii are rn-j and rn. Let Rn be 

the resistance of the region between r0 and rn .

FIGURE 26 Resistive hemispherical shells about a hemispherical electrode (only a quarter of a sphere

shown).

RESISTANCE OP A SINGLE SHELL

The resistance A Rv of the shell between radii rQ and rt corresponds 

to the resistance of a rectangular prism, since the flow of current is 

everywhere perpendicular to the surfaces of the shell.

Let p be the specific resistivity'of the medium. Then, for a thin 

hemispherical shell,

Resistance=specific resistivity x^icknessof^hell^

area of shell

or

U3

(4)

'2irrn rn_x 27r\r»_1 r'nj)

TOTAL RESISTANCE

The total resistance, Rn from r=r0 to r=rn can now be obtained 

by adding the resistances of each of the shells:

(5)

By combuiing equations (4) and (5) we get

rn-i r

13 The product r\ ra is intermediate between ri» and ro1, etc.
138 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

in which all of the terms except the first and last cancel, leaving

Now if we let V0 be the potential at the surface r=r0, and Vn be that 

at the surface r=rn, and 7 the current flowing, then by Ohm's Law

or the potential drop from rQ to rn is

The potential Vn at rn is

TWO CURRENT ELECTRODES

Suppose that we vary the above case, where there is only a single 

electrode, by attaching a second electrode to the ground at a distance

FIGURE 27. Determination of the potential at an arbitrary point P.

3a from the first. Now through one electrode we let a current 7 flow 

into the ground, and through the second we let an exactly equal current 

flow out. We wish to know the potential at any point P in the conducting medium.

Let 61 (fig. 27) be-the electrode through which the current flows 

into the ground, and <72 the one through which it flows out. Let r\ be 

the distance of a point P from Cl} and r2 its distance from <72 . Let + VQ 

be the potential at radius r0 from Ci, and V0 that at radius rQ from 

<72 . Let Vi be the potential at P due to the current flowing from Ci, 

and V2 that due to the current flowing to C2 . Let V be the total 

potential at P.
PART 2. STUDY BY EARTH-RESISTIVITY METHOD 139 

s

Now potential is a scalar quantity, and hence adds algebraically. 

Therefore

V=V1 +V2 . (8) 

By equation (7)

and

Adding, we get

or

r« /1 i \

(9)

EQUIPOTENTIAL SURFACES AND CURRENT FLOW LINES

When the current /is kept constant the potential at any point P depends only upon the parameters rt and r2 . A surface at all points of 

which the potential is the same is known as an equipotential surface. 

If we set V = Vc = a constant, then equation (9) becomes the equation 

of the equipotential surface passing through the point P.

To any given value of the potential from -f- V0 at Ci to V0 at C2 

there corresponds a separate equipotential surface. The shapes of these 

surfaces may be obtained by investigating the family of equations

 

MM 

 =const. (11) ^ '

TI r2

In the vicinity of one of the electrodes, either

T^TI or r-i»T2 .

In such an instance the effect of the remote electrode becomes negligible and equation (11) is reduced to either

1 -1

 = const., or -=const.

n ' r2

in the vicinity of C\ or C2 , respectively. Thus the closer to either 

electrode, the nearer the equipotential surfaces approach hemispheres 

in shape.

Halfway between the-electrodes, TI and r2 are equal, and the equipotential surface here becomes a vertical plane bisecting the region 

between the two electrodes.
142 FLUORSPAR AREAS IN HARDIN COUNTY, ILL.

all the others are kept constant. Tims ^ means " the rate at which p

varies when a is changed, z, y, and a being kept constant."

Assuming that we are working on a level topography, we may distinguish the following cases of variation: 

Case I. Suppose that

a"-n "55-°-

(16)

These equations tell us that the resistivity as computed by the Wenner equation is independent of position, of azimuth, and of electrode 

spacing. This can only be true in a medium that is homogeneous, isotropic, and of infinite extent. A large and deep body of well-mixed 

water would practically fulfill these conditions.

Case II. Suppose that

dx

(17)

da^

These equations imply that the medium is made up of homogeneous 

horizontal layers, but that these layers differ in resistivity. In this 

case measurement of p for a series of 'different values of a enables one 

to tell something about the depth to certain layers.

Case III. Suppose that

dp =

dp. da:

(18)
GEOLOGICAL SURVEY BULLETIN 942 PLATE 24

THE Gl^H-ROONEY TYPE OF INSTRUMENT USED IN FIELD WORK.

PART 2. STUDY BY EAKTH-RESIOTIVITY METHOD 143

These equations express the most general case of inhomogeneity. 

They mean that if we take readings at successive stations in either the 

.XT-direction or the F-direction the value of p changes, then if we take 

readings at different azimuths about a single point the value of p 

changes, and that if we vary the electrode spacing the value of p changes. 

It was such a general problem as this that was encountered in the work 

in Hardin County. There, the quantity principally investigated was

£-, with a such that the lines were parallel to the .XT-direction (line of 

traverse) and a was kept constant at 100 feet. Occasionally >r- was

investigated in order to determine the strike of some possible fault.

APPARATUS USED

The actual conditions are somewhat more complex than those indicated schematically in figure 29, for the following reasons: At the contact of the potential electrodes (Pi and P2 ) with the ground electrolytic action takes place as in a galvanic cell, producing a difference of 

potential sometimes as high as a volt between the potential stakes. As 

this effect is frequently much greater than the potential difference arising 

from the flow of the current in the ground, it must be eliminated before 

the latter can be determined, and the apparatus must be so constructed 

as to accomplish this object.

The instruments used in the work here described (pi. 24) were of the 

Gish-Rooney type but were built from a design by Mr. F. C. Farnham, 

of the Missouri Bureau of Geology and mines. The instruments are 

compact and of light weight, requiring but one operator to make observations.

The wiring of the apparatus is shown in figure 30. The current is 

supplied by a 90-volt radio B-battery housed in the instrument itself. 

Provision is made for plugging in an auxiliary "booster" battery for 

cases where, owing to dry ground or some other cause, extra voltage is 

needed to drive enough current through the ground. The current flows 

through a carbon rheostat and then through a milliammeter. The 

milliammeter is a Weston panel-type instrument having two ranges, 

0 to 15 and 0 to 150 milliamperes. Unless too-resistant ground makes 

it necessary to employ a smaller current, the current is adjusted by 

means of the rheostat to an even 50 milliamperes at each reading.

After leaving the milliammeter the current flows through one unit 

of a double commutator, thence into the ground through one .of the 

current electrodes, and out again through the other. It then flows 

back through the other half of the commutator to the battery. This 

double commutator consists of two units mounted on a common 

shaft, which is geared to a hand crank. When the crank is turned the 

direct current from the battery is made to alternate in the ground,
EQUIPOTENTIAL SURFACES

FIQUEE 29. Vertical section along the line of the electrodes showing the lines of fl

homogeneous and isotropie groun
PARIT 2, &TTJDY FT BlAETH-RESIOTIYITr MEfTEGQD 145

with a frequency of about 12 cycles per second. The alternating potential difference thus set up between the potential electrodes is rectified 

by the second unit of the commutator, and is fed to the potentiometer 

as direct (nonalternating) potential difference. The potentiometer 

used for measuring the difference of potential, E, is a Leeds and

POTENTIOMETER

o-i 110 mv.

o-i5oma./o-i5ma

MILLIAMMETER

ELECTROLYTIC 

CONDENSER 

25 n F

^

C| PI GROUND P2 C 2 

FIGUBE 30. Wiring diagram of the earth-resistivity instrument.

Northrup instrument reading to about 1 millivolt over a range of 0 to 

1,110 millivolts.

The purpose of the double commutator is to eliminate the effect of 

the polarization of the potential electrodes, together with the effect of 

any stray ground currents that may exist. It was often found that 

despite the commutation the galvanometer of the potentiometer was 

too unsteady for a good reading. In such a case, the polarization 

between the two potential electrodes invariably proved to be a large
146 FLUORSPAR AREAS ENi HAIRIDIN COUNTY!, ILL.

fraction of a volt. In order to eliminate this difficulty an electrolytic 

condenser was placed in series-with the potentiometer. This condenser completely eliminated any effect of electrode polarization. The 

condenser used had a capacity of 25 microfarads, but subsequent 

tests indicate that a much smaller capacity possibly as small as 1 

microfarad would have done just as well.

The electrodes were of cold-rolled steel, 24 by % inches, driven about 

a foot into the ground.

The wire used for the lines was 18-gauge, 20-strand copper, singleconductor Tyrex of 0.2 inch outside diameter. Because of the low tensile strength of copper there was some difficulty with open circuits, and

FIGURE 31. Stamp and die for putting copper sleeves used for markers on insulated wires.

wire having a mixture of steel and copper strands would be preferable. 

The lines were attached to the stakes by means of spring clips of the 

type used in charging storage batteries.

The instrument itself occupied a position at the center of the line 

of electrodes. For a value of a equal to 100 feet, the potential lines 

were each 50 feet long, and the two current leads each 150 feet long.

Reels can be used for the lines, but for an electrode spacing of 100 

feet it was found much more convenient to roll the lines up over the 

hand and elbow.
PART 2. .STUDY BY EARTH-RESISTIVITY METHOD 147

For some purposes it is convenient to have permanent markers on 

the lines, making them usable as measuring tapes. Figure 31 shows 

the method used for putting markers on the lines. Blanks of hardened, 

light-gauge copper were cut so as to fit snugly around the line. After 

being stenciled with the proper number, the blanks were placed on the 

steel die and were driven down with the stamp. Into the sleeve so 

formed the line was placed and the sleeve crimped on tightly by means 

of the crimper. Markers made in this way proved entirely satisfactory.

INDEX

Page

Abstracts._____._________.___ 1-2,73 

Accessibility of the area.--.._..---_--.._ 2-3

Acknowledgments__._-.____ ____ 4,93 

Alluvium, electrical resistivity of. __ - ___ 89

Barite, occurrence of..__________ 27,32,62,72 

Bastin, E. S., quoted._____________ 64 

Bedding replacement deposit, definition and

useofname._.__._____ 8 

Benzon Fluorspar Co., property of--------43,57-60,

pi. 16

prospecting by..._.___.___.__. 60 

Bethel sandstone, character and distribution

of 17

Blanket deposit, use of name. ____. __ 8 

Butts, Charles, quoted. _______.___ 54

C. M. Miller mine, description of______ 64 

Calcite, character and occurrence of. -. 24,26,31-32,

35, 57, 58 

origin of.._..--.------___.--._..... 9

Caseyville sandstone, character and occurrence

Of.... . . .-. . ...... 13-14,21

Cave in Rock district, accessibility of_--.-. 2-3 

earth-resistivity profiles of.. 119-133,pis. 22,23 

industrial importance of..... _. _____ 4-5

location of.._____.:.__..____ 2,92,119 

See also particular subjects.

Cave in Rock mine, description of __ 62-63, pis. 6,7 

Cerussite, occurrence of._....... .... 14,33

Chalcopyrite, character and occurrence of.. 24,27,33 

paragenesis of_....._.......__.... 39-40

Chattanooga shale, occurrence of.... - ___ 5

Chester group, character and distribution of.. 17-21 

Chester Martin prospect, features of_...... 64-65

Cleveland mine, description of-....__--- 58, pi. 16

Clore limestone, character and occurrence of. _ 20 

Coon-tail spar, features of._______...30, pi. 10 

Crystal Fluorspar Co_---------_.-_----- 56-57

Crystal mine, description of....----------.--. 27,

42,43,45,56-57,72, pis. 5, 13, A, 14, 15,A 

Cypress sandstone, character and distribution

of ..... .................... 18

Defender mine. See Green-Defender mine. 

Degonia sandstone, character and occurrence

of 20

Devonian rocks, occurrence of. __ 5,6

Earth resistivity, factors affecting.. 76,77,82-90

measurement of, apparatus for-... 79-

80,143-147, pi. 24

method of-.... ...... 77-79,80-82,134-143

profiles of, in C ave in Rock district ... - 119-

133, pis. 22,23

in Rosiclare district____ 94-118, pis. 20,21

Page

Electrical resistivity. See Earth resistivity. 

Eureka prospect, features of.. - .._

F. E. Martin prospect, features of_______ 24 

Faulting, geophysical evidences of---------_ 29,

94-118,120-133, pis. 20, 21

topographic evidences of--- ____..- __ 29-30

Faults, descriptions of- __________ 8,24-28,74

relation of, to fluorspar deposits______ 76

tracing of...--_.._____________ 90-92

variations in electrical resistivity due to... 82-86

Field work-._ _.. . 3-4,92-93,119-120

Fluorite, character of_ ___________ 31

See also Fluorspar. 

Fluorspar, character of.~...._ _ ___ __ 31,76

deposition of .____________ 9-10,36-39 

grades of. _____________ 35,46,68-69 

mining and milling of. ___..-- 65-67, pi. 18 

production of._________ 4,69-70,71, pi. 19 

prospecting for, methods of._______ 75-77 

suggestions for.......________ 52-55,72

reserves of________________ 70-71,72 

shipping points for...___________ 2,3 

uses of -..-_. _ ..--..____ 68-69 

Fluorspar deposits, classification of-____ 8,74-75 

depth to. . ..._ 9-10,21

minerals of _________________ 30-33 

origin of.-- ........................... 47-52

paragenesis of_ _.____________ 36-40

stratigraphic and structural relations of _ 21,

22-23,30,40-46,76, pis. 4,5,8,15, B

texture of.......__....._33-35, pis. 6,7,9-12

Folds, relation of, to ore bodies_____22-23,41-42' 

Fredonia limestone member, character and

distribution of ___________ 14-15 

electrical resistivity of._._______ 89-90 

fluorspar in..______________.. 40,71,72

Galena, character and occurrence of .,. 24,33,58,64 

paragenesis of..________.____. 9,39-40 

Geologic history of the deposits_______ 47-51 

Geophysical evidences of faulting. 29,94-118,120-133 

Geophysical prospecting, methods of...__ 75-76 

Gish-Rooney apparatus, description of. 79-80,

143-147, pi. 24

Glen Dean limestone, character and distribution of-__-.... ------__ 19

Golconda limestone, character and distribution of-____---.-. .. 18 

Goose Creek, fluorspar on________.... 64

Green-Defender mine, description of.,57-58, pi. 16 

Greenockite, occurrence of 27

Hardinsburg sandstone, character and distribution of________.__... 18-19

149
150 INDEX

Heavy spar. See Barite. 

Hicks dome, features of_

Page

5,47

Igneous rocks, occurrence of...._------_.. 6,8

Illinois-Kentucky fluorspar field, geologic features of..-___-------------- 5-10

Kinkaid limestone, character and occurrence of- 20-21

Lead adit, description of- 43,58-59, pis. 5,13, B, 17, A 

Lead Hill, features of_.___________._ 12 

Levias limestone member, character and distribution of-----------.-------.. 16-17

fluorspar in.....--.-..-.-_.......___ 27,72

Location of the area.---,-.- 2-3,73-74,92,94,119

McWade fault, features of._____.____ 28 

Mahoning Mining Co., operations by..._ 71 

Marcasite, character and occurrence of - 33 

paragenesis of._.___.___._.- 39-40 

Menard limestone, character of------_----- 20

Milling, methods of ..---.----- -. 65-67, pi. 18

Minerva Oil Co., operations by ______ 71-72 

Mines and prospects, description of._------- 55-65

Mining, methods of-.-. _________ _ 65 

Mississippian series, description of___.- _ 14-21

Ohm's law, definition of _________. __ 134 

Ore, milling of ..................... 65-67,pi. 18

mining of ______..__-___..... 65

Origin of the fluorspar deposits. _ - ___.... 47-52 

Oxford pits, description of__._._...._ 60

Paint Creek shale, character and distribution of. 17-18

electrical resistivity of. __ - _ - -... _ 133

Palestine sandstone, occurrence of----------- 20

Paragenesis of fluorspar deposits- ____.... 36-40

Pennsylvanian series, character of ----------- 21

Peters Creek fault zone, description of_...... 24-28

Pittsburgh Fluorspar Products Co., property

operated by. -----_.....--- 63-64

Production from the area-____ 4,69-70,71, pi. 19 

Prospecting, methods of-------__---------- 75-77

suggestions for_----------__------ 52-55,72

Quartz, character and occurrence of- _ 32-33, pis. 6, 7 

deposition of-__----------------------- 9,39

Renault formation, character and distribution of-.-._ _ -. .. 17

fluorspar in.______-__.__..... 27,71,72 

Resistivity. See Earth resistivity.

Page

Rocks of the area, age of----- - ----..-.. 5,7

character and distribution of -. - - _ _..... 5-8,

13-21,74-75, pi. 3

electrical resistivity of .. 74-75,76,77,86-90,133 

Rose property, fluorspar at-!.........._... 54

Rosiclare district, earth-resistivity profiles of

94-118, pis. 20,21

location of...-.__.____.__._... 74,92,94 

Rosiclare sandstone member, character and"

distribution of..---_--------_ 15-16

St. Louis limestone, character and distribution of -------------------_-- 14

fluorspar in --_----_ -----------____- 10,14

Ste. Genevieve limestone, character and distribution of.-_-___-__--_------- 14-17

fluorspar in_-___-----._______-_.__._ 30,40

Smithsonite, character and occurrence of___ 33

Sphalerite, deposition of .- - .. __... 9.30-«@

occurrence of..__.._...T-~... .. 24,27,33,71

Stratigraphy, of the area. __ .. _... 6,7,13-21,74

relation of, to fluorspar deposits____.. 30,40

Structure, effect of, on earth resistivity.---_ 82-90

of the area...... ----.....5,21-30,74, pis. 4,5

relation of, to fluorspar deposits__...-. 21-23, 

41-46, 76, pis. 4, 5

Sub-Rosiclare sandstone, character and distribution of,_-___----._--_----. 15,38

fluorspar in_...................._..... 71

Sulfldes, deposition of..-----------.----.--- 9.31MO

Tar Springs sandstone, character and distribution of-__.___-------___... 19

32 cut, description of--. _-_-- -__------ 58

Topography, effect of, on earth resistivity,-.- 86-90 

of the area.--------------- 10-13,29,119, pis. 1,2

Towns of the area,.-_____.___.___ 2-3

Veins, description of.-..- 9-10,42-43,45-46, pi. 15, B 

Victory Fluorspar Mining Co., propertfes of- 60-62,

pis. 15, B, 17, B 

Vienna limestone, character and occurrence of. 20

Waltersburg sandstone, character and occurrence of_.---.--.--._-----_- 20

Weathering, effect of, on earth resistivity.-_ 89-90 

Wenner method, description of------- 77-79,134-143

West Morrison adit, description of--.- 59-60, pi. 16

Zinc, occurrence of__.. .. . __ . -.. 72

Zinc blende. See Sphalerite.

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