UNITED STATES DEPARTMENT OF THE INTERIOR 

Oscar L. Chapman, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

Bulletin 955

CONTRIBUTIONS TO ECONOMIC 

GEOLOGY, 1947

ORTON HALL LIBRARY

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1951
5)
CONTENTS

[The letters in parentheses preceding the titles are those used to designate the papers for advance

publication]

Page

(A) Drill-hole correlation as an aid in exploration of magnetite deposits of 

the Jersey Highlands, New York and New Jersey, by H. E. Hawkes 

and P. E. Hotz..._.._..._._.-._._.._.._.__.._.....___.._.._. 1

(B) Structural control of the gold deposits of the Cripple Creek district,

Teller County, Colo., by A. H. Koschmann_____________________ 19

(C) Phosphate deposits of the Deer Creek-Wells Canyon area, Caribou

County, Idaho, by Charles DeisS-_____________________________ 61

(D) Gold placers and their geologic environment in northwestern Park

County, Colo., by Quentin D. Singewald______________________ 103

(E) Fluorspar prospects of Montana, by Clyde P. Ross.--------------- 173

(F) Quicksilver deposits of the Bonanza-Nonpareil district, Douglas

County, Oreg., by R.E. Brown and A. C. Waters._______-__-___._ 225

ILLUSTKATIONS

PLATE 1. Detailed logs and drill-hole correlations, Hunterdon County,

N. J....-__........--._........--...-........_-_----.-. 14

2. Drill-hole correlations, Orange and Putriam Counties, N. Y ____ 16

3. Geologic map of Cripple Creek mining district, Teller County,

Colo., and area to the south___.___.___.__---__-_-_ In pocket

4. Map of Cripple Creek basin showing the subsurface brecciagranite contact, arrangement of veins, and the location of the 

principal shafts and tunnels--_------__-___-_---___-___ In pocket

5. Geologic map and sections of the Deer Creek-Wells Canyon

area, Idaho.._________________________________-_i____ In pocket

6. Columnar sections, lower part of phosphatic shale member,

Phosphoria formation, Deer Creek-Wells Canyon area-____ In pocket

7. Columnar sections, upper part of phosphatic shale member,

Phosphoria formation._______________________________ In pocket

8. Generalized geologic map showing bedrock geologic features 

pertinent to physiography and placers, northwestern Park 

County, Colo._-___..-__-___.____._-.-___-______.__. In pocket

9. Map showing ice boundaries during Wisconsin glacial stage,

Park County....__.___-__--_.___.____.-____.-_______ In pocket

10. Map showing placer workings and their related physiographic

features, Park County.______________________________ In pocket

11. Locations, elevations, and depths to bedrock of some prospect

holes on Snowstorm property, Park County.. ____________ In pocket

12. Detailed physiography with locations, elevations, and depths 

to bedrock of prospect holes in South Platte Valley near 

Fairplay, Park County-------_______________._--_-___ In'pocket

910926
IV CONTENTS

Page

PLATE 13. Detailed physiography with locations, elevations, and depths 

to bedrocl: of prospect holes in part of Tarryall Valley, Park 

County.-- ______________________________________ In pocket

14. A, Glacial cirques at head of Fourmile Gulch; B, Terrace No. 1, 

east of Fourmile Creek, Park County.__________________ 136

15. A, Terrace No. 2 gravel of Trout Creek resting on uneven surface of Benton shale; B, Cline Bench placer cut, Park County. 137

16. A, Alma placer from north end of town looking southeast toward 

northern part of placer workings; B, Close-up of productive 

gravel, Alma placer, Park County____._._._____.______. 160

17. A, Fairplay placer from north end of workings; B, View at south

end of workings, Fairplay placer, Park County.___________ 161

18. A, General view of East Butte from the south; B, View down 

Sage Creek from the top of Mount Royal, Sweetgrass Hills, 

Mont_---_--------__-----_____---._-________________ 192

19. Geologic sketch map of the Sweetgrass Hills _____________ In pocket

20. Geologic and topographic map of the Tootsie Creek fluorspar

deposits, Sweetgrass Hills_______-______-_____-_______ In pocket

21. A, View across Tootsie Creek with the Malvina cut in the foreground and the Chimneys in the middle distance; B, Close 

view of some of the fluorspar bodies in the Malvina pit --____. 193

22. Detailed map of the fluorspar exposures near the ridge crest on

the north side of upper Dry Creek, Sweetgrass Hills. _____ In pocket

23. Geologic and topographic map of the Spar fluorspar prospect,

Mineral County, Mont____________________________ In pocket

24. A, Brecciated area in beds of the Wallace (?) formation below 

the pit on the Spar fluorspar prospect; B, General view from 

the west showing the pit and quartz lens of the Spar fluorspar 

prospect._____________________________________________ 208

25. A, Northeast wall of the pit at the Spar fluorspar prospect, 

August 12, 1944; B, Detail of the wall of the Spar pit, September 11, 1944__._________________________ 208

26. View of the principal fluorspar exposure in the Boeing open pit,

near Austin, Mont-_-____-______-___--__________J____._ 208

27. Close views of the mineralized breccia at the prospect near the

Anaconda smelter, Deer Lodge County, Mont_____________ 209

28. Geologic and topographic map of the Silver Bow fluorspar

prospect, Silver Bow County, Mont__.,._________-_-- In pocket

29. Geologic map and section, Bonanza-Nonpareil mineralized

area, Douglas County, Oreg___._._..._.. . In pocket

30. Topographic map of Bonanza mine area___-_____________ In pocket

31. Geologic plan of Bonanza mine.______-_.____-_____--. In pocket

32. Plan and section, adit 12, and diamond-drill holes 11, 12, and

15, Bonanza mine. . -. _____-_______^___-__- In pocket

33. Geologic sections through ore bodies, Bonanza mine_ ____ In pocket

34. Longitudinal section, north ore body, Bonanza mine._____ In pocket

35. Geologic level maps, South Nonpareil mine. In pocket

36. Composite level map and sections, South Nonpareil mine. __ In pocket

37. Geologic plan, North Nonpareil mine__.__.__... -- In pocket

38. Cross section E-E', North Nonpareil mine________ In pocket
CONTENTS V

Page 

FIQUBE 1. Index map showing outline of pre-Cambrian magnetite belt in

New Jersey and adjacent States.._______________________ 2

2. Idealized block diagram showing structural relations of country

rock to ore, Jersey Highlands._________________________ 5

3. Idealized vertical section showing relation between foliation

angle in drill core and.true thickness...______________ 11

4. Index map of the Deer Creek-Wells Canyon area, Caribou

County,Idaho_-___-_-_-_--___--___--__-__-__-_.________ 62

5. Map of Colorado showing location of area of gold placers in

northwestern Park County.____-___-_-__ _-_-_-____-___- 108

6. Index map of part of Montana, showing the mapped fluorsparbearing areas and the approximate location of other known 

occurrences of fluorite___--_-_--____________--__________ 178

7. Map of part of northeastern Mineral County, Mont_______ 205

8. Sketch of fluorspar body in the pit at the Spar prospect, Mineral

County__---________---__-__--__.___________ 209

9. Map of the Boeing fluorspar prospect, near Austin, Mont_--____ 211

10. Index map of western Oregon__.__._______________________ 226

11. Columnar section of TJmpqua formation, Bonanza-Nonpareil

district, Douglas County, Oreg.___-_____-_-_-_____-___ 230

12. Section through ra'ise 2, on 370-foot'level, Bonanza mine.__._ 238

13. Topographic map of Nonpareil mine area___________________ 243
UNITED STATES DEPARTMENT OF THE INTERIOR 

J. A. Krug, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

Bulletin 955-A

* >

DRILL-HOLE CORRELATION AS AN AID IN 

EXPLORATION OF MAGNETITE DEPOSITS OF

THE JERSEY HIGHLANDS 

NEW YORK AND NEW JERSEY

BY

H. E. HAWKES AND P. E. HOTZ

Contributions to economic geology, 1947 

(Pages 1 17)

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1947

For sale by I he Superintendent of Documents, U. S- Government Printing Office, Washington 25, D. C.

Price 25 cente

CONTENTS

Abstract________________________

Introduction. _ ___--_--__--.______.

Acknowledgments. ________________

Problem..... ____--_---_--___.___.

General geology.__--__-___-_-____.

Rock classification and correlation._. 

Technique of correlation__._____._..

Geologic core logging..__.___._.

Determination of true thickness.

Preparation of graphic log......

Division into units.-.--..-..-..

Method of correlation. _ __-.___. 

Detailed drill-hole correlation _.__...

Red-back magnetite property _ ..

Croton magnetite mine..-.-..-.

Turkey Hill mine___-_-_-_____.

Swayze mine..---_._______'___.

Application of methods.__________

Conclusions.. . ___--____.--____-__.

ILLUSTRATIONS

PLATE 1. Detailed logs and drill-hole correlations, Hunterdon County, 

N. J.: A, Sample of detailed logs, Turkey Hill mine; B, 

Drill-hole correlation, Turkey Hill mine; C, Drill-hole 

correlation, Swayze mine.______________________________

2. Drill-hole correlations, Orange and Putnam Counties, N. Y.: 

A, Red-back magnetite property, Orange County; B, 

Croton magnetite mine, Putnam County.-_______-__---___

FIGURE 1. Index map showing outline of pre-Cambrian magnetite belt in 

New Jersey and adjacent States.___--___--__-____--___._

2. Idealized block diagram showing structural relations of country 

rock to ore... _________________________________________

3. Idealized vertical section showing relation between foliation 

angle in drill core and true thickness....-----------------

Page 

1

Page

14

16

11

INSERT

i Samples of tabular geologic logs, Croton magnetite mine, Putnam County,

N Y-........-..._-............__---..__--__----..--_-.----.--- 8

in

CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1947

DRILL-HOLE CORRELATION AS AN AID IN EXPLORA^ 

TION OF MAGNETITE DEPOSITS OF THE JERSEY HIGHLANDS, NEW YORK AND NEW JERSEY

By H. E. HAWKES and P. E. HOTZ

ABSTRACT

Detailed studies of diamond drill cores from four magnetite deposits in the 

pre-Catabrian gneisses of the Jersey Highlands, New York and New Jersey, show 

that the gneisses adjoining the ore-bearing zones comprise pseudostratigraphic 

units of differing composition. These units are recognized in field examination 

of drill cores by (1) the relative proportion of dark to light minerals and (2) 

the presence of indicative mineral species as a basis for setting up characteristic 

units. The logging of the core is facilitated by the use of a tabular chart. After 

the logging of the core and determination of true thickness of the layers by 

measurement of the angle between the core axis and the foliation or layering 

of the gneiss, a graphic log showing the distinctive units on a true-thickness scale 

is prepared. The graphic logs of adjoining drill holes are then compared to 

determine the correlation of stratigraphic sections. Detailed studies of the 

stratigraphy and structure as revealed by drill cores give a much more nearly 

complete picture of the mode of occurrence of the ore than is otherwise possible, 

and when used as a guide in exploration they may save a considerable footage 

of drilling that would otherwise be wasted.

INTRODUCTION

Renewed exploration of the magnetic deposits in the pre-Cambrian 

rocks of the Highlands of southeastern New York, New Jersey, and 

Pennsylvania has recently been stimulated by wartime demands for 

additional sources of iron ore and by changes in the status of grade 

and reserves of Lake Superior ores. Old properties that have been 

inactive for many years are again being explored, both as war projects 

by the Bureau of Mines, United States Department of the Interior, 

and as long-term potential sources of ore by some of the major steel 

companies. Under the national strategic minerals program, the 

Geological Survey has been reexamining the general geologic environment of the ore deposits with special emphasis on principles of structural control that might be applied to future exploration problems.
2 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1947

As work is still in progress, this report is of a preliminary nature 

only.

 B. F. Leonard, of the Geological Survey, logged the core and made 

the geologic interpretations at the Red-bad?: property. The program 

has been under the general supervision of A. F. Buddington.

The data used in the present report were compiled from detailed 

geologic logging of core representing a total of 14,398 feet of diamond 

drilling at the Croton Magnetic Iron mine, Putnam County, N. Y., 

the Red-back prospect of the Sterling Group, Orange County. N. Y., 

and the Turkey Hill and Swayze mines near West Portal, Hunterdon

EXPLANATION

Magnetite belt

1 Red-back property

2 Croton mine

3 Turkey Hill mine

4 Swayze mine

FIGURE 1. Index map showing outline of pre-Cambrian magnetite belt in New Jersey and

adjacent States.

County, N. J. (See fig. 1.) The Croton mine was drilled by the 

Bethlehem Steel Co., and the others by the Bureau of Mines.

ACKNOWLEDGMENTS

The writers wish to express acknowledgment to Mr. Hugh Park,

of the Nipissing Mining Co., for permission to examine the Croton

core and to publish the resulting data; and to the Bureau of Mines

for permission to publish the results of the Red-back, Turkey Hill, 

and Swayze investigations.

PROBLEM

Most mining men familiar with the, pre-Cambrian gneisses of the 

Jersey Highlands have recognized a. general layered structure in the
DRILL-HOLE CORRELATION, MAGNETITE DEPOSITS 3

rocks. Not only are the ore bodies confined to fairly well defined 

tabular zones or "veins", but in many places the gneiss country rock 

also give.s the appearance of inter-layered alternating zones of contrasting rock types. This feature is so conspicuous that the gneisses 

were regarded by early writers 1 as the product of metamorphism of 

a great series of stratified sedimentary rocks. This view has been 

opposed by more recent workers 2 who consider that the gneisses are 

largely igneous in orgin and that the stratiform appearance is the 

effect of deformation in injection. Because of the prevalence of the 

theory of igneous origin, the stratiform character of the gneisses and 

its possible application to exploration problems have in recent years 

been largely overlooked. However, regardless of its origin, a 

stratiform habit of the gneiss country rock, at least in some described 

areas, has been reported as an. observed fact.3

Magnetite deposits in the Jersey Highlands commonly are restricted 

to more or less definite zones. If a succession of rocks adjoining 

the ore-bearing zone can be established in a given area, the exploration 

engineer can tell from an examination of the core already drilled 

how much farther he must drill to reach the ore-bearing zone and 

when to stop after passing through it. Also in faulted and folded 

areas, the development of a more accurate picture of the structure of 

the ore body will be facilitated. These and many other applications 

of stratigraphic methods would contribute toward a better understanding of the geology and consequently to a more efficient use of 

funds for exploration.

Before sfcratigraphic methods can be applied in the solution of 

exploration problems, the exigence of a definite succession in the 

gneiss adjoining a magnetite deposit must first be demonstrated by 

careful and detailed mapping or by examination of cores from the 

first holes drilled. Then a practical and generally applicable method 

for identifying and correlating the successions must be developed.

1 Rogers, H. D., Description of the geology of the State of New Jersey, a final report, 

New Jersey Geol. Survey, p. 13, 1840. Kitchell, William, New Jersey Geol. Survey, 2d 

Ann. Kept., for 1855, p. 131, 1856. Cook, G. H., Geology of New Jersey, New Jersey Geol. 

Survey, p. 44, 1868. Brltton, N. L., and Merrill, F. J. H., New Jersey Geol. Survey, Ann. 

Kept, for 1885, p. 36, 1886.

2 Nason, F. L., New Jersey Geol. Survey, Ann. Kept, for 1889, pp. 54-56 ; questions 

sedimentary origin of gneisses but does not actively support igneous origin. Spencer, A. C., 

U. S. Geol. Survey Geol. Atlas, Franklin Furnace folio (no. 161) p. 8, 1908. Bayley, W. S., 

Iron mines and mining in New Jersey ; New Jersey Geol. Survey, Final Kept. Ser., vol. 7. 

p. 126, 1910. Bayley, W. S., U. S. Geol. Survey Geol. Atlas, Passaic folio (no. 157) pp. 5-6, 

1908. Berkey, C. P., and Rice, Marion, Geology of the West Point quadrangle, N. "Y. : 

New York State Mus. Bull. 225-26, p. 110, 1921. Fraser, D. M., in Miller, B. L., Northampton County, Pa.. Geology and Geography; Pennsylvania Geol. Survey, 4th Ser., Bull. 

CNo. 48, p. 1«9, 1939.

3 Wolff, J. E., The geological structure in the vicinity of Hibernin, New Jersey, and its 

relation to the ore deposits : New Jersey Geol. Survey, Ann. Rept. for 1893, pp. 359-69, 

with map, 1894. Westgate, L. G., The geology of the northern part of Jenny Jump Mountain in Warren County, New Jersey : New Jersey Geol. Survey, Ann. Rept. for 1895, pp. 

21-61, with map, 1896. Bayley, W. S., U. S. Geol. Survey Geol. Atlas, Passaic folio (no. 

157), p. 6, 1908.
4 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1947

GENERAL GEOLOGY

The belt of pre-Cambrian gneisses which underlies the Jersey Highlands extends in a northeasterly direction from eastern Pennsylvania 

through northern New Jersey into southeastern New York. (See 

tig. 1) The formations on the flanks of the gneiss belt are sedimentary rocks of Paleozoic and Mesozoic age. The pre^Cambrian 

complex includes medium-grained crystalline gneisses composed primarily of qu'artz, feldspar, hornblende, biotite, and pyroxene in varying proportions. Locally, marble and mafic mineral aggregates 

(skarn), interpreted as replacements of marble, are associated with 

the gneiss. The entire gneiss belt is characterized by layering and

gneissic foliation which in general strikes uniformly northeast and 

dips southeast. Superimposed on this is a linear structure, indicated 

by the alignment of rod-shaped minerals and by axes of minor folds in 

foliation surfaces. The linear structure lies in the planar structure 

and is commonly inclined toward the northeast at gentle to moderate 

angles. The structural trends are amazingly uniform throughout the 

gneiss belt.

The stratiform character of the rocks appears very definite and 

undoubtedly reflects in some manner the past history of the gneisses 

the primary composition and structure, and the type and degree of 

subsequent metamorphism.

The structure of the magnetite ore bodies that occur in the gneiss 

is intimately related to the structure of the adjoining country rock. 

The four deposits studied in detail for this report and, as far as can be 

inferred, most of the v other deposits of the Highlands are localized 

within very definite layers or ore-bearing zones which are comf ormable 

with the structure of the country rock. Ore within the ore-bearing

zones occurs as "shoots", which have the f orm of flattened and greatly 

elongated pods in which the short axis is normal to the plane of the

ore-bearing zone and the long axis parallel to the linear structure of 

the enclosing gneiss. In some places the shoots may pinch down to 

negligible thickness or may terminate. Although these distinctive

ZOlies are the exclusive hosts for the ox-e shoots, magnetite ig not always

present in them, and the zones must then be identified by other lithologic or mineraloglc features. (See fig. 2.)

Most of the local areas of mineralization in the Highlands apparently contain only one ore-bearing zone, though some of the larger 

mines have two parallel ore-bearing zones, each with its complex of

individual shoots. In some areas the apparently random distribution of deposits has been interpreted as complex folding of a single

ore-bearing zone.4 One persistent pencil-shaped shoot is interpreted

4 Nason, F. L., The geological structure of the Ringwoocl Iron mines, New Jersey : Am. 

Inst. Min. Eng, Trans. vol. 24, p. 509, 1895.
DRILL-HOLE CORRELATION, MAGNETITE DEPOSITS 5

as a localization of ore on the axis of a fold.5 The intimate relation 

of ore to certain definite layers of gneiss has been emphasized in almost all published descriptions.6

Gneiss containing elongate mineral grains

Interlayered gneiss 

Strike and dip of foliation

Pitch of lineation 

FIGURE 2. Idealized block diagram showing structural relations of country rock to ore.

Little is known about the genesis of the iron ore. Modes of origin 

that have been advocated in the literature include metamorphism of 

bog ore," concentrations of magnetite sand, segregations of iron de8 Colony, R. J., The magnetite iron deposits of southeastern New York; New York State 

Mus. Bull. 249-50, pp. 99-105, 1923.

6 Cplony, R. J., op. cit. Bayley, W. S., Iron mines and mining in New Jersey: New 

Jersey Geol. Survey, Final Report Ser., vol. 7, 1910. 

735194 47 2
6 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1947

rived from the metamorpliism and subsequent redistribution of the 

components of basic igneous rocks, igneous injection of the ore as 

magma, and hydrothermal and pneumatolytic deposition.

ROCK CLASSIFICATION AND CORRELATION

Several widely divergent systems of classification for the Highlands gneisses have been proposed by workers in the field. The system that has received the widest'acceptance is that sponsored by 

Spencer T and Bayley,8 who divide the gneiss complex into three units 

depending 0n composition: (1) The potash-rich Byram gneiss, (2) the 

soda-rich Losee gneiss, and (3) the magnesia- and iron-rich Pochuck 

gneiss. The three gneisses, according to Bayley, are predominantly 

igneous, although the Pochuck may contain some metamorphosed 

sediments. The pre-Cambrian marble (Franklin limestone) is regarded as part of an older sedimentary series into which the gneisses 

were intruded.

Berkey and Rice 9 make somewhat similar division of the gneisses 

in their study of the gneiss belt in the West Point quadrangle, New 

York. Colony 10 , in discussing the magnetite deposits of southeastern 

New York, accepts the classification of Berkey and Rice but points 

out the profound influence that the lithology and structure of the 

original sedimentary series had on the composition and structure of 

the later instrusions.

This system and classification the threefold division of the gneisses 

and the separation of igneous and metamorphosed sedimentary rocks 

is not well adapted to detailed geologic problems, such as drill-core 

logging and large-scale underground and surface mapping. A distinction between soda and potash feldspars in the field may be difficult 

and unreliable. A distinction between dark gneisses of igneous 

origin and rocks of similar appearance but of sedimentary origin is 

still more difficult. The most important objection to this classification 

is its failure to distinguish between certain variations in the mineral 

composition of the rocks that have proved to be reliable keys to the

lithologic units,

The system which has been found most effective in logging drill cores 

is essentially a variation of Spencer's system in which some of his 

fundamental lithologic distinctions are retained, but where the use of 

rigid formational categories is avoided.

Two lithologic features have been found that are relatively per- sJstent laterally and that can be used for identifying corresponding

1 Spencer, A. C., op. cit., pp. 4-6.

8 Bayley, W. S., U. S. Geol. Survey Geol. Atlas, Passaic folio (no. 157), pp. 4-6, 1908. 

Bayley, W. S., Iron mines and mining in New Jersey ; New Jersey Geol. Survey, Final Kept. 

Sen, vol. 7, pp. 117-129, 1910. Bayley, W. S., U. S. Geol. Survey Geol. Atlas, Raritan 

folio (no. 191), pp. 7-9, 1914.

9 Berkpy, C. T., and Rice, Marion, op. cit, pp. 28-29.

10 Colony, R. J., op. cit, pp. 39, 49-54.
DRILL-HOLE CORRELATION, MAGNETITE DEPOSITS 7

layers, in adjoining drill sections, namely, the relative proportion of 

dark to light minerals and the presence of indicative mineral species. 

The principal dark minerals rich in magnesia, iron, and lime are hornblende, biotite, and pyroxene, and the principal light minerals rich in 

silica, soda, potash, and alumina are quartz and feldspar. The diagnostic mineral species are diopside, garnet, and carbonates.

Correlations based upon a large number of other compositional and 

textural variations were tried and found to be unreliable, or at least 

inconclusive. A statistical study of the distribution and relative 

abundance of pegmatite in the drill core showed no correlation whatever with mineralization or with any particular rock type or unit. The 

presence of brown or black biotite, the ratio of hornblende to augite, 

and the presence of conspicuous sphene all failed to identify a given 

unit well enough to permit correlation between drill sections.

TECHNIQUE OF CORRELATION

The procedure given below for establishing unit correlation has 

been developed as a result of detailed geologic analyses of drill cores 

from the four properties studied. It will probably be subject to 

revisions, and the authors will welcome any suggestions for improvements.

The procedure may be outlined as follows:

1. Preparation of detailed geologic logs of the core.

2. Computation of the true thickness of the layers of the gneiss 

from the angles of foliation with respect to the drill core, whose inclination is known.

3. Preparation on separate strips of paper of a graphic log of the 

detailed geology of each hole, using a true-thickness scale.

4. Division of the graphic log into units that are characterized by 

some unique or predominant feature, such as distinctive rock type or 

mineral species.

5. Comparison of the graphic logs of adjoining holes to determine 

the correlation of stratigraphic sections.

The descriptive system used in preparing the geologic logs involves 

only such features as can be seen with the naked eye. Although it 

lacks the precision of microscope analysis, it has the advantage of 

obtaining in the field practically all the information necessary for 

working out correlations. Features that can be determined adequately 

only with the microscope, such as the composition of the feldspars or 

the ratio of feldspars to quartz, are not used.

The task of core logging may be further simplified by-the use of 

a tabular chart. (See chart, facing p. 8.) Longhand recording of 

detailed and complete geologic notes of drill cores is often exceedingly
8 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1947

tedious. Without a constant reminder, such as is provided by the 

chart, some of the textural and compositional features that should 

be recorded in a complete description may readily be forgotten. 

Furthermore, significant changes in a rock sequence may be obscured 

in the confusion of a large number of notebook pages.

The chart, which may be prepared in mimeographed or printed 

form, contains spaces in which almost all the pertinent geologic data 

can be indicated by check marks or. other simple forms of notation. 

Where longer descriptions are desirable, reference can be made to 

notes in the field book, although in actual practice this is rarely 

found necessary.

The descriptive terms and notation used in the chart are defined 

below in accordance with their usage in the present investigation. 

With some of the terms the special problems involved have made it 

desirable to diverge slightly from standard usage in the geologic 

literature. For example, "gneiss" is used for all nonpegmatitic silicate rocks containing less than 50 percent of dark minerals, regardless of whether the gneissic texture is well developed or not.

<c

BOCK TYPE

The presence of a rock type is shown on the chart by a check mark. Where 

a given section is composed of thin layers of more than one rock type, figures 

inserted in the appropriate columns indicate the percentage of each type, the 

total percent being distributed over two or more columns.

Pegmatite. Rock in which the minerals are coarse-grained, the grains frequently as much as an inch or more in diameter. .Pegmatite in the Jersey 

Highlands is commonly composed of quartz and potash feldspar and minor 

amounts of dark minerals like those in the gneisses.

Light gneiss. Rock that contains less than about 25 percent of dark minerals.

Dark gneiss. Rock that contains more than about 25 percent of dark minerals, 

but less than 50 percent.

Mixed gneiss. Rock showing alternating thin layers of more than one rock 

type. Where these rock types are markedly contrasting, individual descriptions 

of each component are recorded, and the percentage of that component rock 

type is entered in the proper column.

Amphibolites. Typically a massive homogeneous hornblende-feldspar rock 

containing more than 50 percent and less than 85 percent of dark minerals.

Sltarn. Rock composed entirely or almost entirely of dark minerals, principally pyroxene but also to a lesser degree hornblende and biotite and locally

garnet,

Marble Rock composed predominantly of calcite or dolomite. 

Ore. One of the above rock types that is estimated to contain magnetite in 

commercial quantities.

TEXTURE

intermediate textures are indicated on the chart by checks in more than one

column.

Pegmatitic. Average diameter of crystals greater than 0.4 inch. 

Coarse-grained. Diameter between 0.15 and 0.4 inch.
PROPERTY Croton HOLE LOCATION 18860 N.

PUTNAM COUNTY N.Y. 17272 E. INCLINATION

BEARING. 

60°

N. 50° W. LOGGED BY P.E.H. 

DATE 7/18/44

Depth 

in feet)

132.0

136.0

144.0 

146.0

.J 6-Op_ 

" 166.0 

I69.0_ 

_ I79.0_ 

180.0

202.0

._2q4.cT

_240.0 

253.5

255.0

266.0

 o 

a>

 a 

a>

01

u.

i

i

i

j

(

Core recovered

i 

>

3 "O 

3 0

B ;»

0

Recovery (percent)

'E

3

u 

o

cc

A

B

C

Pegmatite

Rock type

Ol 

w

'5 

o

y

40

o

a>

5

c

O

y

y

60

y

y

y

y

y

y

y

y

y

y

 o

0)

X

1

in in

5

c

y

y

Amphibolite

c 

o

JH 

(A)

y

Marble

0)

6

Texture

Grain size

Pegmotitic

y

y

grained Coarsey

y

y

y

y

y

.Medium- grained

y

y

y

y

y

y

©

y

y

y

y

y

y

 a 

a>

c '5

i

0)

c 

il

y

Homogeneous

y

y

y

y

y

y

y

y

y

y

Inhomogeneous

y

y

y

y

y

Fabric and 

structure

Massive

y

y

y

y

y

y

y

y

y

y

y

Foliated

y

y

y

(2)

0

y

(/)

0)

(2)

Lineoted

y

y

 o

V

o

y

y

y

©

Contorted

y

Schlieren Sheared

y

Mineral content

Light

N

O 

0

y

T

y

y

y

y

y

y

y

y

Feldspar

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

Dark

Colcite Diopside

1

1

1

1

Hypersthene

0)

3 

<J

1

2

2

y

i

i

i

2

1

Hornblende

3

2

2

3

3

o 

"o

03

1

1

T

T

y

T

1

4

o 

j5

o 

in

y

Garnet

a>

c

<l>

a.

2

y

2

2

y

2

2

3

3

2

Apatite

dark minerals Percent, 

15

25

30

3

30

85

50

40

8

50

3

20

25

3

30

Secondary

Chlorite

?

J

?

 o 'a.

Weathering products

Opaque

Magnetite

T

T

10

T

15

<u

OL

T

y

Pyrrhotite

y

y

T

T

y

/

y

5

T

Chalcopyrite

y

y

T

T

T

y

T

T

Notebook reference page) (No. and 

2-26

2-26

Remarks

Average foliation angle-. 36° 

Computed dip-- 84° SE.

Lineation, parallel dip

Pyroxene, dirty green

Pyroxene, dirty green

Slickensided; seamed with chlorite

Feldspars, pink

Foliation

£

a. 

a> O

153 

156 

186 

192 

202 

226 

245 

252 

264

0)

c

35 

30 

35 

40 

50 

20 

35 

25 

50

HOLE 4' INCLINATION 75°,IN SAME CROSS SECTION AS HOLE I

_405.0,

415.0

438.0

_____

488.5

501.0

531.0

540.0

565.0

613.0

617.0

655.0

662.0

ft

15

y

35

y

y

y

y

y

50

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

/

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

(/)

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

y

0

i

i

2

2

2

2

1

1

1

1

1

1

1

2

Z

J

2

2

r~

y

y

i

2

y

y

3

3

3

3

3

3

y

3

y

3

y

20

20

20

T

30

T

35

C<J

20

15

30

y

y

y

y

y

y

T

J

J

T

T

T

T

J

T

T

T

T

T

2-28

2-28

Pyroxene, dirty green

Rock appears spongy because of

Pyroxene, dirty green

420

460

488

522

ODU

587

595

600

610

635

645

45

25

15

50

25

35

0

60

15

IO

15

EXAMPLE OF DATA SHEET USED TO TABULATE CORE DATA, CROTON MAGNETITE MINE, PUTNAM COUNTY, N.Y.

(SEE TEXT FOR EXPLANATION) 735194 O - 47 (Face p. 8)
DRILL-HOLE CORRELATION, MAGNETITE DEPOSITS . 9

Medium-grained. Diameter between 0.04 and 0.15 inch. 

 Fine-grained. Diameter less than 0.04 inch.

Homogeneous. Having uniform grain size.

Inhomogeneous. Grain size varies locally within limits indicated in the 

grain-size columns.

FABRIC AND STRUCTURE

Fully developed features are indicated by simple check mark. Faintly developed feature of fabric or structure is indicated by a check mark enclosed 

in a circle.

Massive. Showing little or no foliation or lineation.

Foliated. Showing a planar fabric defined by planar orientation and distribution of dark minerals or by thin layers of different character.

Lmeated. Showing a linear fabric defined by orientation of elongate minerals, 

principally hornblende but to a lesser degree biotite.

Layered. Composed of contrasting layers of half an inch or more in thickness.

Contorted. Locally showing extreme changes in the attitude of the foliation 

or layering.

Schlieren. Elongate masses containing conspicuous dark minerals in a lightercolored matrix.

Sheared. Cut by secondary schistose zones.

MINERAL, CONTENT

The relative abundance of the various dark minerals is noted on the chart 

with numerals to indicate the order of abundance, the number 1 indicating the 

most abundant, the number 2 the next in abundance, and so on; accessory 

minerals are indicated by check marks, and minerals present only as a trace by 

"T". The following list of common minerals of the Highlands gneisses gives the 

features by which they may be most readily distinguished in hand specimens.

LIGHT-COLORED MINERALS

No attempt is made to indicate the relative abundance of quartz or feldspar; 

their presence is indicated by a check mark.

Quarts. White to colorless, hard (scratches glass), distinguished from feldspar by lack of cleavage.

Feldspar. Generally conspicuous cleavage; potash feldspar is white to salmon pink, soda-lime feldspar white to deep green or gray, and albite-twinning 

lamellae may be visible.

DARK-COLOKED MINERALS

Caleite (including dolomite). Soft; effervesces directly or in powdered form 

with dilute acid; commonly indicated by leached character of enclosing rock. 

Caleite is classified as a dark mineral because of its genetic relationship with 

diopside.

Diopside. Color white, light gray, pale green, dirty green, or dull greenish 

gray; commonly occurs in rounded grains; distinguished from feldspar by 

"woody" appearance of cleavage surfaces and from augite by color. Color as 

a criterion for distinguishing diopside from augite is purely for convenience in 

field identification; microscope determination of some pyroxene here included 

with diopside shows that it should strictly be classed as augite.

Hypersthene. Color tawny to brassy; otherwise similar in appearance to 

augite.
10 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1947

Augite. Color uniformly deep green; distinguished from hornblende by its 

color and stubby crystal form, from chlorite by its greater hardness, and from 

diopside by color.

Hornblende. Color commonly black ; good cleavage ; rodlike crystal form.

Biotite. Easily distinguished from other dark minerals by its micaceous 

cleavage; both brown and black varieties apparently belong ,to the same continuous series, in which the shade of color is a reflection of variation in chemical 

composition.

Garnet. Color pale pink to rose red; hard with rounded grain outlines; distinguished from sphene by absence of yellow or brownish color tinge and lack 

of diamond shape.

Sphene (titanite). Color yellow to deep tan, in places shows diamond shape; 

distinguished from garnet by color and form.

Apatite. In rounded to subrounded granules; clear, colorless to pale green; 

greasy fracture surfaces; distinguished from quartz and feldspar by greasy 

appearance.

Other dark minerals. Rarely tourmaline (coal black; high luster; hard, 

brittle) and allanite (dark brown; commonly surrounded by alteration halos 

resulting from radioactive emanations) occur in pegmatites and quartz veins.

SECONDARY MINERALS

Chlorite. Soft, deep green; flaky; commonly occurs in sheared zones and 

locally as an alteration product of augite or hornblende.

Epidote. Occurs primarily as a secondary pistachio-green alteration in lime 

feldspars; also occurs in veinlets.

Weathering products (limonite, kaolin) : Indicated by brown staining, leaching, and softening of rocks in upper 100 feet of drill holes.

OPAQUE MINERALS

Where the mineral is present in significant quantities, the estimated percentage 

by volume is entered in the appropriate column; a check mark (V) indicates that 

it is present in minor quantities, and a T that it is present as a trace.

Magnetite. Black, metallic luster; most readily identified by its magnetic 

properties.

Pyrite. Golov uniformly brassy; hard, brittle; nonmagnetic.

Pyrrhotite: Color paler and more bronze-colored than pyrite; softer than 

pyrite; magnetic.

Chalcopyrite. Color golden yellow with faint olive-green tinge; softer than 

pyrite.

DETERMINATION OF TRUE THICKNESS

The true thickness of the rocks penetrated in a drill hole can be determined, provided (1) that the strike and dip of the rocks are fairly 

uniform and (2) that the hole is straight or is one whose vertical deflection is known, and is directed normally to the strike.

In figure 3, if the dip of the rocks (B) , as determined from measurements )osures of the inclination of foliation OF layering in local rock

and the inclination (A) of the drill hole are known, the true thickness 

(t) of the section cut by the drill can be computed. The true thickness 

is the product of the drilled thickness (dt) and the sine of the angle
DRILL-HOLE CORRELATION, MAGNETITE DEPOSITS 11

(C) that the foliation or layering makes with the axis of the core 

(dt sin G = t). The angle may be determined by simple graphic construction, or, if two angles dip of foliation and inclination of drill 

hole are known, the unknown angle is 180° minus the sums of the 

known angles (G = 180 A B). This angle is referred to here as 

the "foliation angle".

In the absence of local rock exposures, however, the dip may not be 

accurately known and some alternative method of determining foliA Inclination of drill hole 

B Dip of foliation

C Foliation angle 

dt Drilled thickness

t true thickness

FIGUHE 3. Idealized vertical section showing relation between foliation angle in drill

core and true thickness. \

ation angle must be sought. For that reason a special study of the 

Croton, Swayze, and Turkey Hill cores was made to determine whether 

direct measurements of the foliation angle with respect to the core axis 

in core specimens were reliable. This study showed that although individual observations varied widely, the arithmetic mean of a large 

number of dips computed from measurements of the core-foliation 

angle checked surprisingly well with the observed dips in rock exposures. With three exceptions the dip computed from directly measured foliation angles differed from the dip measured on rock 'exposures 

or on cross-sectional drill-hole diagrams by 10° or less. In most of the 

measurements the difference was less than 4°.
12 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1947

The following tables summarize these data:

Comparison of computed dip with observed dip of rocks in the Croton, Turkey

Hill, and Swayze mines

Croton magnetic iron mine, Putnam County, N. Y.

Drill 

hole 

No.

1 

2 

3 

4 

5 

' 6 

7 

8 

9

1011

X

Average 

 Tunne

Inclination 

of drill 

hole

60° NW.

45° NW. 

60° NW. 

75° NW. 

45° NW. 

45° NW. 

58° NW. 

60° NW. 

45° NW. 

65° NW. 

65° NW.

Number 

of angles 

measured

35 

32 

22 

48 

22 

22 24 

27 

23 

67 

25

Mean of 

measured 

angles

36° 

55° 

52° 

29° 

 40° 

50° 

38° 

39° 

37° 

33° 

34°

of 15 dip observations in Theall

Dip measured on did 

through drill holes 1 a 

Dip measured on dio 

through drill holes 7 a 

Dip measured on dio 

through drill holes 9 a

pside zone, section 

nd 4.. ............ ..^.

pside zone, section 

ndlO 

pside zone, section 

nd-11 : 

Computed 

dip

84° SE. 

80° SE. 

68° SE. 

76° SE. 

85° SE. 

85° SE. 

84° SE. 

81° SE. 

82° NW. 

82° SE. 

81° SE.

82° SE.

81° SE. 

85° SE. 

78° SE.

Turkey Hill property, Hnnterdon County, N. J.

Drill 

hole 

No.

8 

9 

10 

11

Inclination 

of drill 

hole

90° 

90°

60° NW. 

65° NW.

Number 

of angles 

measured

36 

33 

GOqq

46

Average of six dip observations 01 

3,000 feet southwest of drilled are

Dip rnes 

of dril

Mean of 

measured 

angles

43° 

44° 

70° 

67°

i outcrops 

a-.._.__

isured on bottom of ore zone, section

Computed 

dip

47° SE. 

46° SE, 

50° SE. 

48° SE.

50° SE.

40° SE.

Swayze property, Hunterdon County, N. J.

Drill 

hole 

NO.

1

4

5

6

Inclination 

of drill 

hole

40° N.

42° S.

60° S.

52° S,

Number

of angles

measured

£7

47

8

30

28

Mean of 

measured 

angles

42°

53° 51°

48°

Dip measured on north ore zone, section of

drill h

Computed

dip

87° N!

85° N.

69° N.

80° N.

79° N.
DRILL-HOLE CORRELATION, MAGNETITE DEPOSITS 13

It may happen, as at the Croton and Red-back properties, that a 

grouping of layers into natural units will be directly observable from 

an examination of the tabular chart. This is illustrated by the sample 

tabular drill logs of drill holes 1 and 4 at the Croton mine where the 

limits of the diopside unit are indicated by the extent of diopside-. 

bearing rocks.

In the Turkey Hill and Swayze drill cores, however, the identity of 

the various units was not immediately apparent from a cursory study 

of the geologic logs. Here, the details of the geology were plotted 

for each drill hole on individual strips of cross-section paper, using a 

true-thickness scale. Special symbols were vised to indicate variations in the two diagnostic characteristics of the lithology the relative proportion of dark to light minerals and the presence of diopside or garnet. The rock types tabulated at the Swayze and Turkey 

Hill mines were light gneiss and pegmatite, dark gneiss, and skarn, 

all of which could be conveniently represented by various symbols. 

Where individual members of the sequence were less than 2 or 3 feet 

thick, it was found impracticable to plot them separately, and a 

symbol for "mixed" gneiss was used. The presence of diopside was 

shown by a heavy line parallel to the lithologic section, Detailed 

graphic logs are illustrated in plate 1, A.

DIVISION INTO UNITS

In general, experience has shoAvn that minor members of the gneiss 

sequence are not persistent laterally and cannot be positively identified 

in adjoining drill holes. The only exceptions were the 5-foot garnetbiotite gneiss layer, which appears in drill holes 1A, 2, and 3 at Redback (pi. 2, A) indicated lateral extent 1,500 feet and the 4-foot 

marble bed below the ore zone at Red-back indicated lateral extent 

3,900 feet.

However, marker zones a few score to several hundred feet thick 

and composed of many minor layers occur at all the properties studied. 

In contrast to the component layers, these marker zones or units maintain fairly consistent thickness and lithology along the strike. The 

most persistent and reliable marker units contain either a diagnostic 

rock type, skarn or amphibolite, or a diagnostic mineral species, diopside or garnet. These units serve as keys to the rock suocession and 

form the framework around which the rest of the sequence is built.

Once the key units have been delimited, the remainder of the section 

may be subdivided, if desirable. Further groupings may be made on 

the basis of variations in the relative proportion of light and dark
.14 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1947

gneiss. In many places a relatively sharp boundary may separate a 

section which is composed predominantly of dark gneiss from one that 

is predominantly light gneiss, and here the delimitation of the unit 

is relatively easy. Where the transition between dark and light gneiss 

is spread over an appreciable thickness, selection of the boundaries 

of the units is more difficult and to a certain extent arbitrary. Examples of both types of boundaries are1 represented on the Turkey 

Hill and Swayze diagrams. (See pi. 1. Upper boundaries of units 

B, E, and F, Turkey Hill sections.)

METHOD OF CORRELATION

Adjoining cores are now correlated by matching the graphic logs, 

with proper attention of course to the expected position of the various 

units inferred from geometric projections. Key units are first 

matched, and the adjoining units examined for similarities .in thickness and lithology. If the correspondence is not good, a minor revision of the boundaries of the units or possibly a reexamination of 

the core may solve the difficulty. When this fails, the possibility of 

complication by faulting, folding, or lateral variation in metamorphic 

effects or original composition should be considered.

A forecast of the position of the ore zone in a hole where drilling is 

still in progress can be made if one contact of a key unit is identified 

and correlated with an adjoining drill section. This method was applicable at all four of the properties studied, as key horizons were 

penetrated above the ore zone in almost all the holes.

DETAILED DRILL-HOLE CORRELATION

The four properties studied do not, of course, represent all the 

types of gneissic structure and lithology which may be expected in the

Jersey Highlands. In none of these areas is there much calcareous 

rock, and it is possible that a somewhat different structural environment will be found in the neighborhood of the magnetite deposits that 

occur in marble.

Sillimanite and graphite were not present in Significant Quantities 

in any of the cores examined. These minerals are known to be present 

in the rocks adjoining Other magnetite ore bodies, and they may prove

to be valuable, like garnet and diopsidfi, ftS dkgnOStiC lOTalS,

The f our*properties show certain notable contrasts. The Red-back 

area is the only one that contains garnet as a diagnostic mineral. Diopside may be found at both Croton and Turkey Hill as a diagnostic 

mineral, but is absent or nondiagnostic at Swayze and Red-back, The 

Croton section is composed predominantly of relatively dark gneiss, 

the Red-back section predominantly of light gneiss, and the Swayze 

and Turkey Hill sections of a complex interlayering of the two.
GEOLOGICAL SURVEY BULLETIN 955 PLATE I

SOUTHEAST

ODH II

200-

100-

80-

60-

40-

20-

O-1

DDH9

/ '/'

iW.<S

I

t Diopside

Top of

Ore-bearing zone 

UNIT D

<$&£

./ xTX

 (Horizon defined

 <by first appeor- 

\once of skarn

Diopside 

/

;}T;5j; 'Horizon defined by

- r'''''* ^ top of long section 

l/- [of light gneiss

I Horizon defined by 

bottom of long section of light gneiss

NORTHWEST

A. SAMPLE OF DETAILED LOGS, TURKEY HILL MINE

EXPLANATION

Light gneiss

Mixed gneiss

Skarn

DDK It

TRUE "THICKNESS

FEET 

7CO-(

600-1

500-

4OOEXPLANATION

Light gneiss with dark hornblende 

and biotite gneiss

B f>.j.\~*d

r\-'..>.>:>.1

Augite and diopside skarn, dark 

hornblende, pyroxene, and biotite 

gneisses, minor marble, top defined 

by first appearance of skarn

Light gneiss and pegmatite

Ore-bearing zone; mixed and 

dark gneisses; magnetite

100-

50-

O-1

ODH 9

A ::

Light, dark, and mixed gneisses with

subordinate coarse-grained pyroxene

skorn; top defined by light gneiss

zone of variable thickness

Light gneiss and pegmatite

^____0 ODH 8 

Diamond-drill t-.ole

Compare generalized graphic 

logs of holes 9 and II with 

detailed logs at left

SHAFT

\

\ DDH 8 (vertical) 

\,DDHIO

,DDH II 

(62°)

(60°)

.SHAFT

nDH9(verticol) 200 FEET

B. DRILL-HOLE CORRELATION, TURKEY HILL MINE

DDH I

DDH 2

SOUTH

ORE-BEARING 

ZONE

Datum

Possible faultDDH 5

DDH 6

NORTH 

ORE-BEARING

SURFACE PROJECTION OF 

NORTH ORE-BEARING ZONE

DDH 6 

(50°)

(40°)

SURFACE PROJECTION OF 

SOUTH ORE-BEARING ZONE

(45°)

EXPLANATION

Dark hornblende gneiss

South ore-bearing zone: coarse green 

augite skarn, magnetite

Light gneiss with subordinate dark 

hornblende gneiss

North ore-bearing zone: dark 

hornblende gneiss

"Mm

Exclusively light gneiss and pegmatite

*____oDDH 5 

Diamond-drill hole

C. DRILL-HOLE CORRELATION, SWAYZE MINE

DETAILED LOGS AND DRILL-HOLE CORRELATIONS, HUNTERDON COUNTY, N.J.

735194 O - 47 (Face p. 14)
DRILL-HOLE CORRELATION, MAGNETITE DEPOSITS 15

Bed-back magnetite property. The geologic section at the Redback property (pi. 2, A) differs from those of the,other areas in the 

presence of "marker horizons," relatively thin layers in the sequence 

that can be identified in two or more drill holes. Conspicuous marker 

layers are a garnet-biotite gneiss layer (unit C) and a layer of marble 

beneath the ore zone (unit H). The other key units are characterized 

by garnet-bearing rocks (unit E), amphibolite (unit F) and skarn 

(the ore zone, unit G). The remainder of the sequence is built around 

this framework of marker layers and units.

The ore-bearing zone in all the drill holes contains local concentrations of magnetite, though in some of the sections only in negligible 

quantities. The ore apparently favors pyroxene skarn as a host, 

though some disseminated magnetite occurs in the marble.

Croton magnetite mine. The principal key unit at the Croton property (pi. 2, B) is the diopside zone, unit B, which appears in the same 

position in all the holes for a strike length of 3,700 feet. Apparently 

the unit pinches or dies out south of^ drill hole 6, as it does not appear 

in drill holes 2, 3, and 5..

Unit D, predominantly light gneiss, adjoins the ore-bearing zone 

on the west side and provides a secondary key to the rock succession. 

In drill holes 2, 3, and 5, where the diopside zone is absent, the eastern 

boundary of unit D is the only distinctive horizon, as units A and C 

merge and are indistinguishable in the absence, of the diopside zone.

The ore-bearing zone (unit C) is a thick zone of fairly uniform 

dark hornblende-pyroxene gneiss through which concentrations of 

magnetite are scattered without any system or control that could be 

determined. The barren parts of the zone are identical lithologically 

Avith unit A.

Turkey Hill mine. Unit B, containing augite and.diopside skam 

with admixed dark and minor light gneiss, is the most distinctive unit 

of the Turkey Hill drill sections (pi. 1, B). Parts of this unit appearing in drill holes 8 and 10 have been subjected to intense secondary 

shearing and alteration, which have largely destroyed the original 

texture and appearance of the rock In drill hole 11 marble occurs 

with the diopside rock of unit B.

The top of the ore-bearing zone, unit D, is defined by a relatively 

abrupt change from the predominantly light gneiss of unit C to the 

predominantly dark gneiss of the ore-bearing zone. The bottom of 

the ore-bearing zone is marked by light gneiss of variable thickness 

separating it from unit E below. The separation is not well marked, 

as the general lithology of units D and E is somewhat similar. The 

contact of unit E with unit F is also indefinite, as it is marked only 

by a gradual transition from predominantly dark to predominantly 

light gneiss. The zone which contains the ore in drill holes 9 and 10
16 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1947

is barren however in drill holes 8 and ,11, and would not have been 

recognized except for its appropriate position in the rock column.

Swayze mine. Two ore-bearing zones may be found at the Swayze 

property (pi. 1, C). The south ore-bearing zone (unit K) is defined 

by an abundance of coarse-grained, deep-green augite skarn. The 

limits of the zone are well defined, as the adjoining units are devoid 

of this rock type. Small concentrations of magnetite occur in unit K 

in both holes.

No ore was cut in unit M, but the identity of this unit with the north 

zone that contains the ore was inferred by its position with respect to 

the previously mined north ore body. It is not a distinctive unit, and 

the correlation shown in the diagram is highly speculative.

APPLICATION OF METHODS

If the layered gneisses of the Jersey Highlands had few structural 

complexities and if local structural trends were completely known, the 

expected position of the ore-bearing zone could be accurately projected 

for any proposed diamond-drill hole. Unfortunately this is not generally possible. The application of stratigraphic methods has proved 

useful in more accurately predicting the position of the ore-bearing 

zone", as shown by the following examples.

When drilling at the Swayze property (pi. 1, C) began, there was 

considerable doubt as to the strike and dip of the ore structure. No 

bedrock is exposed within half a mile of the mine, and a determination of the strike of the ore by dip needle was not feasible because Ol

artificial magnetic disturbances. The rocks exposed within a 5-mile 

radius uniformly strike northeast and dip 45° SE. Nevertheless, the 

old mine reports and the recollections of men who had worked in the 

mine indicated a northwest strike and a vertical to northeast dip. 

Holes 1 and 2 were drilled on the assumption of a northwest strike. 

Correlation of the distinctive lithology of unit K and compilation of 

foliation angles measured in the core specimens showed this assumption to be correct, barring the remote theoretical possibility of

horizontal la.yex-in.gf. J\^e st, result of the ooxifii'mtitioia of tile northeast

dip by measurements of foliation angles in the core, subsequent drilling

was laid out to approach the ore-bearing zone from the northeast

Tather than the southwest side.

At the Turkey Hill property (pi. 1, B), the badly sheared section in 

the upper parts of drill holes 8 and 10 suggested a cross fault. As the

ore-bearing zone in drill hole 11 might be offset by the supposed fault,

it was therefor© desirable to ascertain from stratigraphic evidence

where the position of the ore-bearing zone was to be expected. Identification of skarn-bearing unit B served to locate the relative position
GEOLOGICAL SURVEY BULLETIN 955 PLATE 2

NORTH

TRUE THICKNESS

FEET 

SOO-i

400-

300-

200-

100-

50-

0 '

SOUTH

EXPLANATION

Felsic gray gneiss, migmatite, and granite

Biotite gneiss and hornblendebiotile gneiss

Garnet-biotite gneiss

Mica gneisses (brown and black biotite), 

in part hornblendic and pyroxenic

Garnet-biotite gneiss except in hole 2,

where unit is predominantly chloritized

biotite gneiss

Amphibolite and pyroxene-hornblendebrown biotite gneiss

Mafic pyroxene-hornblende-brown 

biotite gneiss

Ore-bearing zone: magnetite in pyroxene 

rock(skarn), underlain by mafic pyroxenehornblende-brown biotite gneiss

H LE1-E3J 

Impure (silicated) marble

A. RED-BACK MAGNETITE PROPERTY, ORANGE COUNTY.

Undifferentiated

 __oDDH 2

Diamond-drill hole

TRUE THICKNESS

FEET 

700-1

600

500-

400-

300

200

100

50-

EXPLANATION

DISTINCTIVE OR PREDOMINANT ROCK TYPE

Dark hornblendc-augite gneiss, subordinate layers of biotite gneiss 

and amphibolite

B P!

Diopside-biotite gneiss

Ore-bearing zone: dark augite-hornblendc gneiss, subordinate biotite 

gneiss and amphibolite; magnetite ore

Drill holes in 

some section

Light quartz-rich gneisses, subord 

note layers of mafic gneisses

^_-_o° DH 8 

Diamond-drill hole

SOUTHWEST

Drill holes in 

some section

DDH 3

DDH 5

Amphibolite

Datum -

Correlation using top of light gneiss 

as datum

Correlotion using bottom of diopside zone as datum 

B. CROTON MAGNETITE MINE, PUTNAM COUNTY

DRILL-HOLE CORRELATIONS, ORANGE AND PUTNAM COUNTIES, N.Y
DRILL-HOLE CORRELATION, MAGNETITE DEPOSITS 17

of the hole in the stratigraphic sequence, and the ore-bearing zone was 

penetrated in its normal position beneath unit B.

At the Red-back property (pi. 2, A) the expected position of the 

ore-bearing zone in drill hole 10 was determined by geometric projections from the surface and from adjoining drill hole. 11. In the 

absence of a detailed analysis of the core, the drill hole was stopped a 

short distance below what was believed to be the bottom of the ore 

zone. Subsequent detailed geologic logging showed that the hole had 

not passed through the ore-bearing zone.

Stratigraphic logging is useful also in identifying the ore zone where 

it contains no magnetite to distinguish it from adjoining layers. 

Wasteful drilling in the barren footwall rock can be avoided when the 

ore-bearing zone is passed and no chance remains of finding more ore.

Naturally, every mineral deposit has a somewhat different environment and presents different geologic problems. However, an intelligent application of stratigraphic methods should help in- solving many 

of those problems and in the long run save more than enough mone}' 

to offset the time and trouble of making the correlations.

CONCLUSIONS

The following conclusions apply specifically to the four magnetite 

deposits that were studied in detail, but it may be reasonably inferred 

that similar conditions prevail at many other magnetite-bearing areas 

in the Jersey Highlands.

(1) The gneissic complex has a stratified character, and the magnetite deposits are restricted to certain layers.

(2) In general, the individual minor layers are not laterally persistent, but broader units comprising homogeneous groups of minor 

layers and containing a distinctive lithologic or mineralogic feature 

'are persistent and can be used as stratigraphic horizon markers.

(3) In the absence of more reliable data and provided (a) the strike 

is known to be approximately normal to the drill section, (b) there is 

no complex folding, and (c) the course of the drill hole is known, a 

reasonably accurate determination of the dip of the gneissic foliation, 

layering, and ore structure can be obtained by taking closely spaced 

measurements of the foliation angle in core specimens and computing 

the dip from the average of. those measurements in each hole.

(4) Detailed examinations of drill cores for systematic succession 

of lithologic units and for structure give a more nearly complete 

picture of the mode of occurrence of the ore than is otherwise possible, 

and when used as a guide in exploration may save a considerable 

footage of wasted drilling.

6