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,V'

GEOLOGY APPLIED TO
MINING

A CONCISE SUMMARY OF THE CHIEF GEOLOGICAL
PRINCIPLES, A KNOWLEDGE OF WHICH IS NECES- y^

SARY TO THE UNDERSTANDING AND PROPER
EXPLOITATION OF ORE-DEPOSITS

FOR MINING MEN AND STUDENTS

JOSIAH EDWARD SPURR, a.m

Geologist, United States Geological Survey; Consulting Geologist and Mining

Engineer to the Sultan of Turkey; Fellow of the Geological Society of

America; Member American Institute of Mining Engineers^

Member Washington Academy of Sciences, etc.

Second Impression

NEW YORK

THE ENGINEERING AND MINING JOURNAL
1905

TO NEWTORK
PUBLIC LIBRARY

152668A

TLLDEM FC

I'LT. DAT IONS I
S24 L I

The Engineering and Mining Journal

PREFACE.

The writer was led to attempt the present volume througn
the perception of how great a need there was, among
mining men and students, of some work stating concisely
those results of the science of geology which bear upon ore-
deposits. No work of this type exists, so far as the writer
is aware, in any language. The demand for such informa-
tion is great among men of the classes referred to; yet in
any of the available works on geology they find very little
of that for which they are searching, combined with a great
deal of that which, for the moment, is immaterial.

In preparing this work two points have been kept in
mind; first, to make statements as clear as possible, con-
sidering the technical nature of the subject; and, second,
t<i present the scientific facts accurately, and as fully as
absolutely necessary. Simplicity of language has been
constantly striven after, but it must be remembered that
it is impossible to discuss any technical matter without
using tenns peculiar to it.

This book, as it goes forth, is far from meeting the
author's perfect approval. It is a beginning, and it is

iv PREFACE.

believed that the demand is sufficient to warrant its imme-
diate publication. But it is the writer's purpose to work
steadily at its improvement and elaboration.

So sincere is his wish to furnish the desired information
to the large class for whom the book is intended, that he
asks for private communications from readers, stating
where they have not found the writing clear enough, or
asking information on questions not contained in this book.
Such suggestions will be a valuable aid to future enlarge-
ment and revision.

Although the writer addresses men who have had little to
do with geology as a science, or with the theory of that par-
ticular branch of geology with which the book deals —
namely, the study of ore-deposits — ^yet, before developing
his subject, he has deemed it necessary to anticipate a little.
With this purpose the first chapter has been inserted. For
a correct understanding of the science of ore-deposits, and
how the principles of geology may be practically applied to
economic advantage in finding and exploiting ore-bodies, it
is necessary, first of all, to have some ideas of what ore-
bodies are, and how they have formed. The study of the
processes of ore deposition has long been in a state of slow
growth ; within the past twenty years, however, it has been
more rapid and steady than heretofore, and the writer feels
justified in laying down certain principles.

Suggestions and criticisms of the most helpful nature in
regard to this work have been made by friends, who have
read portions of the rough draft of the manuscript. For
such aid grateful acknowledgment is due to Messrs.

PREFACE. V

T, Wayiand Vaughan, A. H. Brooks, tatd Waldemar Lind-
gren, of the United States Geological Survey. Especial
thanks are due to Mr. T. A. Rickard, Editor of the
Engineering and Mining Joumaly who carefully went over
the manuscript and made such trenchant suggestions as to
its revision that the general presentation was greatly alter-
ed and improved thereby.

J. E. Spurr.
Washington, D. C, Feb. 3, 1904.

CONTENTS.

CHAPTER I.

THE PROCESSES OP ORE-DEPOSITION. Page

Metamorphism, or Changes in the Earth's Crust 1

The Origin of Metaniorphic Rocks 1

Transformation of Igneous and Metamorpliic Rocks into

Sedimentary Rocks G

Special Metamorphic Processes Connected with Ores . 7
Processes of Ore-Concentration ......... 9

Concentration directly from Igneous Rocks, wliile Molten

or Cooling 9

Theory of Direct Concentration of the Basic Constit-
uents, During a State of Chiefly Igneous Fluidity

of the Rock 9

Theory of Concentration of the SiUcious and other
Constituents, in a State of Aqu«To-Igneous

Fluidity 13

Extraction of SiUcious and Other Constituents in
Solution in Waters Expelled from Cooling Rocks,
and Deposition in Foreign Rocks .... 15
Ore Deposits Formed Chiefly by Vapors . . . . 17

The Origin of Certain Hot Springs 18

Concentration by Underground Waters in General . . 19

Concentration by Surface Waters 20

Relative Work of Underground and of Surface Waters 21
The Mode of Ore-Deposition 22

CHAPTER II.

THE STUDY OP THE ARRANGEMENT OP THE STRATIPIED ROCKS AS
APPLIED TO MINING.

The Formation of Stratified Rocks

Formation of Sediments by Mechanical Agencies
Formation of Sediments by Chemical Agencies
Formation of Sediments by Organic Agencies .
Transformation of Sediments to Hard Rocks

The Physical Characters of Sedimentary Rocks

24
24
25
25
20
26

Vm CONTENTS.

Pa«e
The Chief Kinds of Sedimentary Rocks, Their Origin and Char-
acteristics 28

TJie Distinction between Bedding, Cleavage, Schistosity, and

Gneissic Structure 32

Different Geologic Periods during which Sedimentary Rocks

have Formed 35

Characteristics of the Different Fossils 43

The Order of Succession as Found in Actual Practice . . 52

Relation of Physical Characters to Geologic Age ... 53

Comparison and Correlation 56

Mode of Determining the Relative Age of Different

Strata 56

Mode of Correlating Similar Strata in Adjacent or

Separated Regions 57

The Association of Valuable Minerals with Certain Strata . . 58

General R,elations of Stratified Ores 58

Preferential Association with Certain Geologic Periods . 62
Preferential Association with Certain Kinds of Sediment-
ary Rocks 68

Contemporaneous Deposition of Ores and Strata ... 69
Selection of Favorable Strata for the Subsequent Deposi-
tion of Ores. 73

CHAPTER III.

THE STUDY OP IGNEOUS ROCKS AS APPLIED TO MINING.

Physical Characters of Igneous Rocks 79

The Different Kinds of Igneous Rocks 82

Classification of Igneous Rocks for Mining Men 83

Additional Definitions 88

Transitions between Different Kinds of Igneous Rocks . . 93

Forms of Igneous Rocks 95

General Relation between Igneous Rocks and Ore-Deposits . 99
Special Relation between Certain Igneous Rocks and Ore-
Deposits Ill

Advantages of Different Forms of Igneous Rocks . . .111

Advantages of Different Kinds of Igneous Rocks . . .112

Preferences of Certain Igneous Rocks for Certain Ores,

Displayed During the Cooling Processes . . .112
Preferences of Certain Igneous Rocks for Certain Ores,
Displayed by Selective Precipitation of Metals

from Solution 115

CONTENTS. IX

Page
Ore-bodies in the R61e of Igneous Intrusive Rocks . • . . .117
Igneous Rocks Intrusive Subsequent to Ore-Deposition . .118

CHAPTER IV.

THE STUDY OP DYNAMIC AND STRUCTURAL GEOLOGY AS APPLIED
TO MINING.

Part I. — General Conceptions and Mapping 120

Definitions 120

Folds and Faults 121

Effects of Erosion on Folded and Faulted Rocks . . J26

The Surface Mantle of Ddbris 130

The Systematic Working out of Geologic Structure . . 133

Strike and Dip 133

Recording Observations on Maps 136

Migration of Outcrops 139

Construction of Geologic Sections 142

Economic Results of Mapping and Cross-Sectioning . 145
Mapping and Sectioning of Igneous Rocks . .147
Part II. — ^Rock Deformation and Dislocation, and Their Con-
nection with Mineral Veins 148

Measurement of Folds and Faults 148

Folds and Faults as Loci of Ore-Deposition .... 164

Deposition of Ore in Folds 164

Deposition of Ore along Faults 169

Joints in Rocks 173

Ore-Deposition along Joints 175

Fractures and Fissures 177

Deposition of Ores along Fractures and Fissures . . 184
Shear-Zones or Crushed Zones, and Their Suitability for

Ore-Deposition 193

General Relations Between Rock Disturbances and Ore-
Deposits 194

The Intersection of Circulation Channels as Seats of

Mineralization 195

Rock Movements Subsequent to Ore-Deposition . . . 197
Dislocations Subsequent to Ore-Deposition as Seats

for Later Mineralization 198

Ribbon Structure 199

Faulted Faults and Their Relation to Ore-
Deposition 200

Rock Movements along Earlier-Formed Dikes . . . 203

X CONTENTS.

Pa««

Part III.— Placers 206

The Concentration of Gold in Placers 205

Concentration by Chemical Water-Action . . . 206

Concentration by Mechanical Water-Action . . . 208

Effects of Glacial Action 211

Various Kinds of Stream Grold-Placers 214

Beach Placers 218

Bench Placers 221

Old Placers 222

Fossil Placers 226

Re-concentrated Placers 227

Placers Other Than Gold-Placers 229

Residual Deposits 233

CHAPTER V.

THE STUDY OF CHEMICAL GEOLOGY AS APPLIED TO MINING.

The study of Ore-Concentration 235

The Shallow Underground Waters 237

The Work of Underground Waters in Dissolving Rocks . . 239
The Work of Underground Waters in Precipitating Mnierals . 242
Manner of Deposition in the Deeper Underground Regions . 244
Special Chemical Processes of the Shallow l^ndergroimd Waters 255

Zone of Weathering or Oxidation 256

Precipitation of Ores at the Surface 259

Precipitation of Ores in the Shallow Underground Zone . 265
Concentration According to Relative Solubilities . 265

Secondary Sulphide Enrichment 269

Features of the Process of Reconoentration of Pre-
existing Ores by Shallow Descending Waters . 272
Examples of Secondary Alteration by Surface Waters . 278
Manner in which Minerals are Precipitated by De-
scending Waters 285

Characteristics of Ore-Deposits Formed by Ascending and

by Descending Waters 289

Changes in Richness in Depth 293

Association of Minerals 299

Rock Alterations iis Guide to the Prospector 301

CHAPTER VI.

THE RELATION OP PHYHKMiHAI'HY TO MINING.

ILLUSTRATIONS.

Fig, Page

1. Map of Gap Nickel mine, Lancaster, Pennsylvania . . .11

2. Ideal sketch to illustrate unconformities 63

3. Cliff on Kuskokwim River, Alaska, showing; lateral tran-

sition between sandstones and shales 64

4. Occurrence of ore in a definite stratum, introduced subse-

quent to the stratum's formation. Rico, Colorado . . 75

5. Limestone beds of Derbyshire, with intruded igneous rock

traversed by veins 76

6. Dikes cutting granite, Cape Ann, Massachusetts ... 97

7. Primary pyrrhotite in augite 101

8. Conditions in a copper vein at Butte, Montana .... 116

9. Iron-ore bodies in Lola mine, Santiago, Cuba . . . .118

10. Folding of limestones and shales, Kuskokwim River, Alaska 122

11. Close folding of limy shales, on Yukon River, Alaska . .122

12. Overthrown folds 123

13. A monoclinal fold 123

14. Faults in strata near Forty Mile, Alaska 124

15. Reversed fault in Empire mine, Grass Valley, California . 124

16. Compensating faults, Omaha mine, Grass Valley, California 125

17. Eroded anticlinal range of deformation, Uinta Range, Utah 127

18. Simple fault-scarp at the Palisades, Yukon River . . . 129

19. Reversed erosion fault-scarp in the Lower Austrian Alps . 129

20. Bank of Glacial drift, Gloucester, Massachusetts . . .132

21. Figure illustrating strike and dip 134

22. Symbol for recording strike and dip 138

23. Diagram of a topographic base for geologic cross-sections . 144

24. Stereogram illustrating the total displacement of a fault . 154

25. Stereogram illustrating various functions of a fault . . 155

26. Stereogram illustrating the computation of a fault move-

ment, where part of the data is concealed .... 157

27. Stereogram of fault, where the lateral and perpendicular

separations are zero 158

28. Stereogram illustrating a bedding fault 158

29. Ideal vertical section of faulted stratified rocks, illustrating

fault fimctions 161

XU ILLUSTRATIONS.

Fig. Page

30. Diagram illustrating the relations of throw and vertical

separation, in the case of a reversed fault 161

31. Diagram illustrating the term offset as applied to a fault . 163

32. Auriferous saddle veins, Bendigo, Australia 164

33. Diagram showing occurrence of ore shoots in pitching arches

or folds of the strata, Elkhom mine, Montana . . . 166

34. Vein formation in the fractured apex of an anticline; New

Chum Railway mine, Bendigo, Australia .... 167

35. Deposition of ores in anticlinal folds, with barren synclinals,

West Side Vein, Tombstone District, Arizona . . . 168

36. Ore-deposition along faults, Bushwhacker-Park Regent

mine, Aspen, Colorado 170

37. Ore-deposition in the fissure along a minor fault, Eureka

vein, Rico, Colorado 172

38. Columnar jointing of basalt on Koyukuk Moimtain, Yukon

River, Alaska 174

39. Formation of ores along joints, Monte Cristo, Washington . 176

40. Sheet of glass cracked by torsional strain 178

41. Open fissure cutting and deflected by calcite vein, Mercur,

Utah 180

42. Granite quarry, showing increase of fractures and fissures

near the surface, Rockport, Massachusetts 183

43. Veins formed by the successive selection of difi'erent frac-

tures by mineralizing solutions, Ajax mine, Tintic, Utah 187

44. Disappearance or deflection of veins on passing from sand-

stone into shale, Bendigo, Australia 188

45. Deflection of veins in passing through slate, Bendigo,

Australia 188

46. Linked veins, Pachuca, Mexico 192

47. Ore shoot, Annie Lee mine, Cripple Creek, Colorado . . . 196

48. Ribbonstructureinquartz vein, Grass Valley, California . 201

49. Successive stages of faulting, Aspen, Colorado .... 202

50. Vein following a pre-existing dike, De Lamar, Idaho . . 204

51. "False bottom" of clay in gold placer deposit, Seward Penin-

sula, Alaska 210

52. Glacier-scooped basin containing auriferous glacial gravels,

Otago district, New Zealand 213

53. Irregular glacier-scooped depressions, filled with auriferous

glacial gravels, Otago district. New Zealand .... 213

54. Gulchplacer, Koyukuk district, Alaska 215

55. Ideal river, showing accumulation of auriferous bars . . 217

ILLUSTRATIONS . XUl

Fig. Pa«e

56. Section of beach placers, Nome, Alaska 220

57. Bench and valley placers. Blue Mountams, Oregon . . 221

58. Generalized section of an old placer * . 223

59. Contour map of Neocene bedrock surface, Grass Valley,

California . 225

60. Old auriferous gravels (Miocene), Otago district. New

Zealand 226

61. Platinum placers, River Iss, Ural Mountains, Russia . . 230

62. Section of tin placers, Siak district, Sumatra .... 231

63. Fossil in native silver as evidence of ore-deposition by re-

placement (of limestone) 249

64. Ore-deposition by replacement of schist along crushed zone,

Otago, New Zealand 249

65. Ore-deposition at the intersection of two circulation chan-

nels, Rico, Colorado 252

66. Deposition of iron ore by descending waters in joints and

pockets in limestone, Pennsylvania Furnace, Pennsyl-
vania 270

67. Close relation of galena zone to surface, evidence of depo-

sition of descending waters, Monte Cristo, Washington . 281

68. Ore in the roof formed by intersecting fractures, as evidence

of deposition by ascending waters, Bendigo, Australia . 291

69. Iron-ore deposit formed by descending waters, showing

constant relation to surface, Mesabi range, Minnesota . 292

70. Gold in pyrite and quartz. Thin section of ore magnified.

Grass Valley, California 300

CHAPTER I.
THE PROCESSES OF ORE DEPOSITION.

METAMORPHISM, OR CHANGES IN THE EARTH'S
CRUST.

The Origin of Metamorphic Rocks.

Is the earth's surface stable f

The seemingly stable crust of our earth undergoes slow
but stupendous alterations. In the course of our brief
lifetime we may not notice them; but, if we do, we marvel
at them. Such things as a river that has shifted its course,
a harbor that becomes choked with sand, or a mud island
that is washed away by the waves, interest us strongly.
Yet the researches of geology show that, during the long
succession of centuries, rivers which run from the uplands
to the sea may entirely remove mountains and spread them
out as sediments upon the ocean floor. In the course of
time these deposits may be lifted above the sea again to
form new land; for the crust of the earth is not quiet, but
is forever heaving up and down, expanding, contracting,
bending and breaking, converting sea-bottoms into dry
land, sinking mountains into the sea, and crumpling plains
into mountains. All this goes on with such undemon-

2 GEOLOGY APPLIED TO MINING.

strative slowness that those who live on the earth are
hardly made aware of these changes and are rarely dis-
turbed by them.

Example: Modern history records upward and downward
movements of the land at various points. It has lately
been ascertained that the whole region of the Great Lakes
is undergoing a slow tilting to the south-southwest. Meas-
urements, extending over a number of years, of the distances
between certain marks and the level of the lakes render it
probable that the region is being lifted on one side or de-
pressed on the other, and that the rate of change is such that
the two ends of a line 100 miles long and lying in a south-
southwest direction are relatively displaced four-tenths of a
foot in 100 years. The waters of each lake are rising on the
southern and western shores, or falling on the northern and
eastern shores, or both. At Toledo and Sandusky, the
water advances 8 or 9 inches in depth in a century. A
tract of land near Sandusky on which hay was made in
1828 is now permanently under water. In 3,500 years the
Falls of Niagara will cease to flow, as a consequence of this
movement.*

How may sedimentary rocks become metamorphic t

Regions which were once deeply buried may become part
of the surface by the removal of the overlying mass; and
study of the rock thus revealed gives an idea of what goes
on in the depths of the earth. Among the lessons thus
learned is the following: When sediments have accumulated
(as they may in the course of ages), to a depth of several
miles, the lower layeis may be affected by the weight of

* G. K. Gilbert, I8th Annual Report United States Geologioal Survey.
Part 11. pp. 601-645.

THE PROCESSES OF ORE DEPOSITION. 3

those above, by the internal heat of the earth and other
causes, so that chemical changes take place. The materials
begin to recrystallize, new minerals grow from the debris of
those in the sediments; and finally the rock becomes quite
different in appearance.

Sometimes we find such a rock with the marks of its
sedimentary origin still visible. Other rocks may be so
perfectly recrystallized that there is no direct evidence in
their structure that they ever were sediments, and we can
only determine this point in roundabout ways, as by
tracing the much altered rock into some less altered portion.
Such rocks are metamorphic; and they are chiefly divided
into schist and gneiss.

Example: In the northwest highlands of Scotland, on Ben
More and on Sgonnan More, movements in Cambrian con-
glomerates, sandstones and shales have produced extraor-
dinary changes. The conglomerate in its unaltered form is .
composed of rounded pebbles in a loose, gritty matrix.
Where subjected to movement the softer pebbles have been
crushed, flattened and elongated in the direction of move-
ment. In some cases they have been drawn out to such
an extent as to form thin lenticular bands of mica or horn-
blende-schist, flowing around the harder pebbles of quartz.
The original gritty matrix has been converted into a fine
micaceous or chloritic schist. Were it not for the presence
of the crushed schistose pebbles it would probably be
impossible to tell that this schist had a sedimentary origin.*

♦ B. N. Peach. J. Home, W. Qunn, C. T. Oough, L. mnxman, and H..M.
CftdffU, quarterly j9Wwa, Oeologicftl Swwty, Ygl, XLIV, pp. 431-432.

4 GEOLOGY APPLIED TO MINING.

How are igneous rocks formed?

The metamorphic rocks are related to another class of
crystalline rocks — the true igneous rocks. The igneous rock
has crystallized from a molten condition. At the surface
the formation of igneous rock is illustrated by lavas, but such
rocks are formed on a grander scale beneath the surface.
An igneous rock has generally a fairly constant texture,
and is composed throughout of the same minerals, which
are often about the same size, and lie in different attitudes.
These characteristics arise from the circumstances that the
mass has been fluid before cooling, so that all parts come to
have about the same composition; and since all parts have
cooled under nearly the same conditions, the resulting
minerals and structure are the same.

Why are metamorphic rocks often handed?

A true metamorphic rock has not been really fluid, in the
generally accepted sense of that word. At the most, the
effect of pressure and heat have made it slightly plastic, so
that it has yielded and slipped a very little. Therefore,
when it recrystallized, the materials did not move far in the
rock. If there were in the original sediments successive
layers of different nature, (such as dark ferruginous mud
beneath clean quartz sand), the recrystallized rock will
often preserve the banding; the mud will appear as a dark
layer of crystalline ferruginous minerals and the sand bed
will be represented by crystalline quartz.

Banded structure in metamorphic rocks may also be pro-
duced by more active crystallization along slipping planes
than in the rest of the rock.

THE PROCESSES OF ORE DEPOSITION. 5

May a metamorphic rock assume the characters of an
igneous rock?

The conditions which make a mass plastic and those
which make it fluid are not sharply separated. A rock
undergoing metamorphosis may become so plastic and so
thoroughly recrystallized that the result will be the same
as if the rock had slowly cooled from a molten state. Some
igneous rocks are known to have been thus formed, by slow
metamorphism, from sediments. When we can prove the
origin of such rocks, we often prefix the term metamorphic
to them — thus, metamorphic granite — ^but often we can-
not tell whether a granite is metamorphic or igneous, for
the characters are alike.

May an igneous rock assume the characters of a metamorphic
rock?

An igneous rock may, by becoming subject to conditions
of long-continued slight plasticity and pressure, acquire the
characters of a true metamorphic rock. A slight move-
merit takes place, generally along close-set parallel planes,
and here an active recrystallization and a re-arrangement
of the minerals occur, resulting in a banded structure. The
rock may lose all the traces of its essentially igneous
character, and become a gneiss or schist, indistinguishable
from one that has formed by the alteration of sediments.

Example: The crystalline schists and gneisses of the
Malvern Hills, in England, have been formed by the meta-

6 GEOLOGY APPLIED TO MINING.

morphism of igneous rocks. Shearing has taken place in
bands of varying breadth situated at irregular intervals.
The gneissic structure usually shades off on each side of the
zone into ordinary igneous masses (diorite, granite, etc.),
and within the zone itself the metaraorphism varies in
intensity. Proofs of mechanical forces l-esulting in shear-
ing are numerous. Hornblende crystals are drawn out
into ribbons, and feldspars are bent and broken. Fre-
quently black mica is formed along the shear-planes, so
that the rock splits into thin leaves whose surfaces glisten
with mica, while the interior may be dioritic. The chief
mineral changes are the recrystallization of feldspar, and
the production of biotite, muscovite, quartz and actinolite.*

Transformation of Igneous and Metamorphic Rocks
INTO Sedimentary Rocks.

Can igneous and metamorphic rocks be changed back to sedi-
mentary ones?

The earth's surface consists in part of igneous and meta-
morphic rocks. These rocks are attacked by the rain, the
sun and the frost; they are broken up by snow, by ice, by
glaciers, by landslides and by the roots of plants; and the
debris is carried down the hillsides into the valleys, and
along small streams into large ones, till it is emptied as sand
or mud into the sea, to become slowly solidified into sedi-
mentary rocks. The igneous or the metamorphic rock
may originally have been derived by recrystallization from

♦C. Calloway, Quarterly Journal, Geological Society, Vol. XLV, p. 476.

THE PROCESSES OF ORE DEPOSITION. 7

a sedimentfiiry one, so that the materials have undergone a
complete circle or cycle of change.

Can we find a beginning in the cycles of change?

So vast has been the period of time, during which such
processes have been going on, that there is scarcely any
rock of which we can say with certainty that it has not
been derived from another, of different nature. Still, we
can rarely be sure that an igneous rock has thus been
originated, and it is probable that many such rocks have
never been anything else, but have crystallized directly
from the molten interior. And since we can always trace
the sedimentary and the metamorphic rocks back into the
igneous originals — if we go back far enough — we may
regard the igneous rock as the beginning of the cycle.

Special Metamorphic Processes Connected with Ores.

Has the consideration of rock-changes a direct hearing upon

ore-depositsf

A rock is an aggregate of minerals; and with the trans-
formation of the rock the minerals undergo change. The
commonest rock-forming minerals are quartz, feldspar,
mica, hornblende and augite, these being made up of the
elements chiefly represented in the earth^s crust. The
rarer elements are also scattered through the rocks, and
occur in more or less abundant minerals. Certain of these
minerals, notably the heavy metals, have been put by man
to use in the arts, and it is especially with these that the
science of ore-deposits is concerned.

8 (JEOLOGY APPLIED TO MIKIXG.

What are the limits of the study of ore-deposits? .

Accurately speaking, the science of economic geology-
would embrace the study of the distribution of all the
elements, for practically all have some use; but it is the
rarer ones that it takes most intelligence and energy to
find and extract. For example, the most common of all
the elements (except oxygen) is silicon, which in the form
of sand is of great economic value. But it is so easy to find
and dig sand that small attention is paid to this element in
the study of economic geology. When we come to the next
commonest element, however, — a metal, aluminum — we
begin to pay closer attention. Clays, which are impure
silicates of aluminum, are sought after and studied for
pottery, porcelain, brick, tile, cement, etc.; and for the
manufacture of the pure metal and many other purposes we
seek and investigate deposits of highly aluminiferous min-
erals, such as bauxite and cryolite, corundum and emery,
and natural alum. Arriving at the next commonest ele-
ment — iron — ^we are fully in the domain of mining; and so
on down the list — calcium, magnesium, potassium, sodium,
titanium, carbon, phosphorous, manganese, sulphur,
barium, chromium, nickel, etc. The statement resolves
itself into this — that man finds artificial uses for all the
elements, and economic geology busies itself especially
with those which are most highly prized, and which are diffi-
cult to find in a sufficient degree of concentration, or in the
proper combination with other elements, forming minerals
which possess valuable properties. Especially does it
require study and effort to produce in quantities the rarer

THE PROCESSES OF ORE DEPOSITION. 9

elements, notably the less common of the heavy metals. On
that account the science in general, and this book in par-
ticular, will deal principally with these metals.

The rarer metals — tin, lead, zinc, silver, antimony, gold,
etc. — occur in small quantities nearly ever3rwhere in the
earth's crust — ^in rocks, in both fresh and salt surface
water, in underground water, and even in plants and in
animals. It requires rather exceptional conditions, how-
ever, to produce a mass containing such a proportion of
these as to render it profitable to make it the basis of
mining operations.

PROCESSES OF ORE-CONCENTRATION.

In what way does concentration of wluable elements take placet
We may conveniently divide the underground processes
of concentration into two classes — those which take place
within igneous rocks while they are still wholly or par-
tially molten or during their cooling period, and those
which are brought about chiefly through the action of
percolating waters in solid rocks.

Concentration Directly from Igneous Rocks, While
Molten or Cooling.

Theory of Direct Concentration of the Basic Constituents
During a State of Chiefly Igneous Fluidity of the Rock.

How may concentration take place in molten masses?

Petrographers have advanced the theory that in molten
masses the different elements tend to segregate. In this way

10 GEOLOGY APPLIED TO MINING.

it has been supposed that different rocks, such as granite
and diabase, may separate out of the same molten mass.
Granite contains much silica, diabase much magnesia and
iron.

Example: In west Cornwall the tin and copper veins are
associated with intrusive igneous rocks. These are granites,
greenstones, etc. In some cases it is found that the granite •
becomes less silicious toward the edges, a condition which
is supposed to have been brought about by segregation
while still molten. This granite is cut through by more
siHcious dikes, which, however, are evidently closely
related to the granite. The greenstones, which are in
smaller quantity, are altered basalts and gabbros, and
generally occur near the margins of the granite intrusions^
though not in the granite.

This geographical connection and the order of intrusion,
as worked out for the different rocks, favor the hypothesis
that the silicious and the basic rocks have originated by the
splitting up of an earlier molten mass of intermediate com-
position.*

With the iron of the basic rocks are generally small
amounts of some of the less common metals, in relatively
greater quantities than in the light-colored silicious rocks.
The metals are apt to be more abundant in some portions of
the dark heavy rocks than in others. Thus there may be
formed masses which are chiefly made up of metallic
minerals. By such a process of magmatic segregation
some iron deposits, some chromite (chrome iron) deposits,
some of nickel, etc., have been supposed to be formed.

* J. B. Hill, TranaactioM Royal Society, Cornwall, Vol. XII, Part VII, p. 579.

THE PROCESSES OF ORE DEPOSITION.

Example: In Lancaster county, Pennsylvania, the Gap
mine has, as chief metallic mineral, magnetic iron pyrite
(pyrrhotite), which contains sufficient nickel to render it
valuable as an ore of that metal. The ore occurs at the
contact of a lens-shaped mass of dark basic hornblende-
rock (amphibolite), which has been intruded (thrust up)
into mica-schists. This hornblende-rock is considerably
altered and when fresh had a different mineral composition,
being probably one of the very basic* rocks gahbro oc peri"
dotite,^ It is believed by J. F. KempJ that the pyrrhotite
which occurs in the outer rim of this basic intrusion is one
of the original minerals, crystallized out of the cooling rock
and segregated along the contact. (See Fig. 1).

IT7-rr

Mica Schist. Amphibolite. Granite. Ore.

Fig. 1. Generalized Map of Gap Mine, Lancaster, Pa., after J. F. Kemp.

Is this concentration in molten w^isses a very common and

important process?

It is held by many that not infrequently metals are so
highly concentrated in this way as to actually form ore-
deposits; and that when this is not the case, the process may

* The term basic is applied to igneous rocks rich in dark-colored heavy
minerals containing iron, and poor or wanting in quartz.

I For definitions of these rocks see pp. 86. 90.
"Ore-Deposits of the United States," p. 434.

12 CJEOLO(!Y APPLIED TO MINING.

still l>e important, in producing rocks which carry relatively
large amounts of the metals, though in a scattered condi-
tion. These scattered metals, through the agency of
further concentration, (by circulating waters, for example),
might give rise to ore-bodies; while the same agencies,
acting on rocks, poor or wanting in the metals, would not
contribute in that way.

Example: At Riddle's, Douglas county, Oregon, there is a
very basic rock, peridotite, made up of the minerals pyr-
oxene and olivine. The olivine contains a small percen-
tage of nickel, analysis having shown 0.26 per cent of oxide
of nickel. This rock has become thoroughly decomposed,
and has altered to serpentine. During the alteration of the
olivine, the nickel has separated out. Surface waters, per-
colating through the rocks within the zone of rock decay,
have taken the nickel into solution, and have precipitated it
as a coating on the walls of cracks and in small veins.
From this method of formation it has resulted that the
ores are richest at the outcrop, and diminish lower down,
till, on passing below the zone of surface decay, they dis-
appear.* In this case the ores have been concentrated in
their present form by surface waters, although in their
original condition in the fresh rock they were too sparsely
scattered to be noticeable; yet had it not been that there
was within the molten rock, previous to cooling, an unusual
proportion of nickel, the material would not have been at
hand for the waters to work upon, and no ore-deposits
would have been possible.

♦ Clarke and Diller. American Journal of Science. Series iii, Vol. XXXV,
p. 1483.

THE PROCESSES OF ORE DEPOSITION. 13

Theory of Concentration of the Silicions and Other Con-
stituents, in a State of Aqueo-Igneous Fluidity.
It is not so much dry heat which renders molten igneous
rocks fluid, as it is water combined with heat. In the
different kinds of molten igneous rocks, water is present in
different proportions. In general, the more silicious a
molten rock is, the more water does it contain. The rela-
tive order in which the different minerals crystallize in
granite, for example, cannot be explained by dry heat, but
only by admitting that the materials from which the
mineral formed were in a state of partial solution in water.
It is held by some writers that ore-deposits may originate^
under the combined influence of water and heat, in the
silicious igneous rocks.

How is' this process supposed to operate?

It has been found in the field that granites may pass
gradually into more silicious rocks composed of quartz and
feldspar, and that these may pass into quartz veins. It
has been held by some writers that such quartz veins have
a genetic connection with the granite. Their formation
has been explained by applying the theory of magmatic
segregation, as follows:*

The silicious rocks, such as the granites, may originate
by differentiation from a more basic magma. With the
further development of this process, quartz-feldspar rocks
may be formed; and, when the silica separates out from the
magma in a nearly pure state, quartz veins may result.

♦J E. SpuxT. *• Igneous Rocks as Related to Occurrence of Ores." Trans-
actiona American Institute Mining Engineers, Feb. and May, 1902, p. 21.

14 GEOLOGY APPLIED TO MINING.

Example: There are numerous veins and large masses of
quartz throughout the district of Omeo, AustraUa, in schists
or granular igneous rocks. The quartz is in places milky
in color, in others clear. In addition to the quartz veins
there are others of the same class which contain tourmaline
or feldspar, or muscovite (mica), or two or all of these
together in varying proportions, so that veins may be
extremely quartzose with but a small proportion of other
minerals, or may be so charged with them as to become a
variety of pegmatite. Study of the veins composed of
quartz alone, or quartz and tourmahne, shows that the
quartz has broken the tourmaline crystals, and penetrated
every crevice. The supposition that the quartz may have
been gradually deposited from solution around the tourma-
line crystals till the fissure was completely filled is negatived
by the observation that the tourmaline is not attached to
the walls of the veins, but '^floats'' free in the quartz.
These facts are explained by the author on the hypothesis
that the veins represent the residual silica of the granitic
rocks of the region, after the other minerals had crystallized
out, and that this residuum was squeezed out while in a
plastic state into every adjoining crevice.

These veins occur in a rich gold-quartz region; never-
theless, the author considers that the auriferous quartz
veins have had another origin.*

Since it is probable that the amount of water in the
igneous rocks increases in general with the increasing content
of silica, the end product of differentiation, from which the
quartz veins are crystallized, may be little more than highly
heated and compressed waters, heavily charged with silica

*A. W. Howitt, TrQn8(;K^i(m8 Royal Society Victoria, Vol. XXIII, pp.

THE PROCESSES OF ORE DEPOSITION. 15

in solution. Besides silica, other residual materials, left
over from the magma, may be present, among them gold;
so that the resulting quartz veins may be auriferous. Such
veins might have the same appearance as those formed by
ordinary underground waters.

Example: In the gold-bearing district of Silver Peak,
Nevada, are found quartz veins which pass gradually into
silicious granitic dikes, and seem to represent the silicious
extreme of segregation or differentiation of the granite.
Such veins usually contain a little feldspar and white mica,
while others to which a similar origin may be assigned, con-
tain none. Assays for gold and silver were made from two
of these veins. One contained 0.03 oz. gold and 0.13 oz.
silver, the other none.*

Extraction of Silicious and Other Constituents, in Solution,

in Waters Expelled from Cooling Rocks, and

Deposition in Foreign Rocks.

Are waters and vapors active when an igneoiis rock is in

process of cooling?

When an igneous rock begins to cool and harden, much
water, which has been a part of the molten material but
cannot form part of the rock, is pressed out. If the igneous
rock is at the surface, like a lava, this water, which is highly
heated, passes off in copious and long-lasting clouds of
steam. In the case of necks of molten rock, which feed
volcanoes, and of other bodies which have forced their

*H. W. Turner. Report for United States Geological Survey. (Trnpul^r
lished MSS.)

IG (iEOLOOY APPLIED TO MINING.

way up from the depths, through other rocks, but have
not succeeded in getting to the surface, the water is
forced into the adjoining rocks and, being under pressure,
may be either in liquid or vaporous form.

These waters are highly charged with various strong
vapors, and both water and vapors carry in solution much
mineral matter, among which may be the metals. It is
believed by many geologists that this mineral matter is
deposited while the solutioas are in process of circulation
through the rocks, and, further, that the metals may be
concentrated so as to form ore-deposits. Certain ore-
bodies found at the contact of an igneous rock with another
rock have been described as having this origin. Such
occurrences are termed contact-metamorphic ore-deposits.

The commonest kind of contact metamorphic ore-de-
posit is usually held to occur at the very contact of the
igneous rock. But contact metamorphism in general may
extend much further, forming an altered zone a mile or
more broad. Anywhere within this zone deposits of
metallic minerals, with the characteristics of contact-
metamorphic ore-deposits, may be found.

Example: The Dolcoath mine, in the Elkhorn mining dis-
trict, Montana,* lies in limestone, at a distance of over half
a mile from the granite, which has chiefly occasioned the
metamorphism of the district. Through this metamor-
phism the limestones have been recrystallized to marble,
the sandstones have become quartzites, the sandy and limy
shales are largely recrystallized to new minerals such as

*W. H. Weed, 22d Annual Report United States Geological Survey, Part II,
p. 606.

rilE PROCESSES OF ORE DEPOSITION. 17

pyroxene, garnet, epidote, etc. The ore-bearing stratum
of the mine was originally a bed of impure limestone, which
has been metamorphosed to garnet and pyroxene, with
spots of calcite. Associated with these gangue-minerals are
sulphide and telluride of bismuth, containing gold.

Ore-Deposits formed Chiefly by Vapors.

May some ore-concentrations he accamplished chiefly by

vapors?

Tin-veins are held by many writers to be usually formed
in this way. They are ordinarily confined to granite. The
explanation usually offered is that when the granite cools,
it shrinks, and crevices begin to open. Water escaping
from the hardening rock rises along these rents in the
form of vapor, and is accompanied by other especially
powerful vapors, such as chlorine and fluorine. The
vapors may carry tin and other mineral matters, which
they may deposit in the rents or in the porous walls, and
thus concentrate them sufficiently to make an ore-deposit.
Are tin-veins alone due to the action of vapors f

Others metals are known to be deposited by escaping
vapors. They have been found encrusting the mouths of
steam- jets (fumaroles) in lavas. Cinnabar, the ore of mer-
cury, and realgar, an ore of arsenic, as well as hematite, an
ore of iron, with copper and lead chlorides, are among the
metallic minerals which have been thus deposited at
Vesuvius. It is likely that some workable cinnabar de-
posits and even some of the other metals may have
been formed underground by vapors alone.

18 GEOLOGY APPLIED TO MINING.

The Origin of Certain Hot Springs.

What becomes of the water expelled from molten rock in cooling j
besides that which passes off in vapors at the surface?
We have seen that when intensely heated rock cools at the
surface, great quantities of the expelled waters pass off as
clouds of steam. As the crust slowly hardens, and the con-
gealing molten rock becomes further away from the surface,
the escaping waters will become cooler in their passage
upward. A stage will finally be reached when they will not
entirely flash into steam on emerging, but will remain
Uquid, though boiling and sending off a great deal of steam.
They will, in fact, emerge as hot springs, and it is probable
that the change from steam-jets (fumaroles) to hot
springs is the normal process of cooling volcanoes. As the
cooling progresses, these springs will lose in temperature,
volume and pressure, until finally they will in many cases
become ext net.

The water which is given off at the contact of an intrusive
mass of igneous rock, and which is frequently so active in
producing contact-metamorphism, must also exist after it
has accomplished these changes. We may suppose that if
there are any channels, such as are afforded by fissures or
faults, this water may find its way upward, and perhaps
even reach the surface.*

May such waters produce ore-depositsf

We have seen in considering contact-metamorphic ore-

* Springs having this origin may be called (following Professor Suess, of
Vienna,) juvenile springs, the term referring to the recent birth of the water
from the molten rock.

THE PROCESSES OF ORE DEPOSITION. 19

deposits that compressed vapoi-s and waters expelled from
solidifying igneous bodies are supposed to produce ores in
the adjacent intruded rocks; and that the vapors that
escape from volcanoes often deposit metallic minerals.
Therefore it may well be, also, that the hot waters which
succeed the vapors in the cooling of volcanic rock are
efficacious in concentrating ores. Ascending hot waters
are generally conceded to be the most powerful agents of
mineralization, and those hot waters which have the origin
above described should be especially active, for in addition
to their dissolving power, exerted on rocks which they
traverse, they may contain metals expelled in solution in
them from the crystallizing rocks from which they have
emanated.

Concentration by Underground Waters in General.

Does concentration cease when the rock is cold?

The work of concentration does not cease with the com-
plete cooling and hardening of the igneous rock. Rain-
water, falling upon the surface, is, in part, carried off in
rivulets and streams to the ocean; but probably the greater
part sinks below the surface. The underground water cir-
culates chiefly through natural channels, such as are offered
by any fissure or porous zone; but it also possesses the
power of working itself very slowly through most solid
rocks. From the moment these waters touch the surface
they dissolve substances from the rocks and precipitate
them again at other points. This work they do continu-
ously, and thus as far down as they penetrate there is a

20 GEOLOGY APPLIED TO MINING.

constant shifting of material. From the afHnity of like
minerals for each other, this shifting results in concentra-
tion; and where metallic minerals are concentrated, ore-
deposits are formed.

These waters, after sinking deeply, or nearing some body
of hot igneous rock, may be supposed to become heated,
and would then be still more powerful than before. They
may take up the unfinished work of concentration left by
the cooling processes of igneous rocks, and carry it to a
successful finish in the form of a workable body of ore; or
they may concentrate the metals sparsely scattered through
igneous, sedimentary or met amorphic rocks.

Is there any universal final stage of concentration?

These processes of concentration are never at an end.
With changing currents of water the ores are redissolved
and reprecipitated, changing their position and proportion.
An ore-body formed by deep underground waters may, in
consequence of the slow wearing away of the surface, finally
come to be exposed , or ^ ' outcrop . " Th en the shallow under-
ground watere may either make the ore poorer, or make it
richer, by dissolving and reprecipitating.

Even after a mine is opened, the work goes on, and
metals are often deposited on the walls of drifts, or encrust
tools which may be left in old workings.

CONCENTHATION BY SuHFACE WaTERS.

Surface waters have a twofold effect — chemical and
mechanical.

THE PROCESSES OF ORE DEPOSITION'. 21

How do surface waters act mechanically so 05 to concentrate

ores?

In large bodies of surface waters, as in streams or on the
shores of the ocean, the sediments, or finely ground mate-
rials worn from the rocks, become arranged according to
the relative weight and size, by the operation of the same
laws as those by which ores are concentrated in mills. In a
current of water the heaviest minerals sink first, and so are
separated from the lighter material, which is carried on.
When the sediment, which is thus transported by water,
has been taken from a decomposing rock containing valua-
ble minerals, such as gold, platinum and tin, these heavy
minerals become concentrated at certain points where the
current is too weak to carry them further, but is too strong
to allow most of the other materials to drop.

How do surface waters act chemically in concentrating ores?

There is no stream, however clear, which does not contain
dissolved mineral matter. This material may, on occasion,
be precipitated in large quantities, making sometimes a
deposit of economic value.

Relative Work of IlNDf:RGROUND and of Sitrface
Waters.

Ho\o do the mechanical and chemical activities of underground
waters compare xoith those of surface waters?
Underground waters move slowly through the rocks,
often occupying every available space, no matter how
minute. Ordinarily, however, they cannot unite into
l)odies of large volume like rivers and lakes, for they cannot

22 GEOLOGY APPLIED TO MINING.

find underground spaces large enough. This difference
makes their mechanical power practically nothing — for
they cannot carry mineral particles with them by the force
of their motion. But their chemical work is vastly more
important, for two chief reasons. The first is the greater
field of the underground waters, which work up and down,
and through and through a thick belt of rocks containing
small quantities of metals, while the surface waters only
skim the top of this belt. The second reason is that, by
virtue of the pressure and heat which the underground
watei-s fre(iuently attain, their power of solution, and hence
of concentration, is correspondingly increased.

The Mode of Ore Deposition.
Are ore-bodies formed by upward, downward, or laierally

moving xoaters?

Undergroimd waters move sometimes upward, some-
limes downward, sometimes sidewise; and, whatever their
direction, they have the power to dissolve and reprecipitate
mineral matter and hence to bring about concentrations of
ore. The ore-deposit formed by descending waters may
often be with difficulty distinguished from one formed by
ascending waters. Yet from the fact that heated waters
naturally rise, and that they are more capable of solution
than cold ones, it is probable that the most important
single class of ore-deposits has been formed by them.

How are ores deposited by waters in rocks?

Following the question as to what kind of underground
water has accomplished ore-deposition, the next inquiry

THE PROCESSES OF ORE DEPOSITION. 23

concerns the manner in which the ores in solution are
deposited in the rocks. The theories advanced by learned
men have perhaps exhausted all the possibilities of the
imagination as well as of reason. Three chief theories, now
each of them proved facts, have been most successful in
standing the test of time. These are the theories of suhsti-
iviion or replacement, of cavity-fUling, and of impregnation or
the filling of interstices (interstitial filUng).

Study shows that not one process is represented in the
average ore-body, but many. In most of them, one may
find excellent examples of the work of each of the three
processes above mentioned, and, even in a single hand-
specimen of ore, the same multiplicity of origin may be
displayed; although in general one process is chiefly active
in forming a certain ore deposit, and another process in a
second. Thus we have many typical replacement deposits
(among them many lead-silver ore-bodies in limestones)
and many typical fissure veins (where an open rift or fissure
has been filled by ore) ; yet, in the replacement deposit, one
may often find instances of fissure-filling; and, in the fissure
vein, examples of replacement.

CHAPTER 11.

THE STUDY OF THE ARRANGEMENT OF STRATI-
FIED ROCKS AS APPLIED TO MINING.

THE FORMATION OF STRATIFIED ROCKS.
Formation of Sediments by Mechanical Agencies.
How are sediments formed by mechanical agencies?

Rivers come down from their sources laden with mud and
dragging along pebbles on their bottoms; on reaching
the sea the coarse gravel is usually deposited near the
mouths of the rivers, while the finer material is carried
further on. Along the shore the waves attack the cliffs,
undermine them and finally cause them to break off, and
in this way new supplies of rock are produced, to be ground
into sand and mud by the churning of the surf. The
tides and currents sweep the material far out to sea or along
the coast.

In lakes, the material brought down by the streams like-
wise settles on the bottom.

Rivers work slowly to either side in their valleys; tliey
nearly always have a winding course, and at every curve
the current may be seen cutting under and removing the
bank on the concave side, and depositing sediment on the
opposite or convex margin, so as to produce a spit or bar.

ARRANGEMENT OF STRATIFIED ROCKS. 25

The result of many centuries of this cutting and rebuilding
is that the stream widens the valley, and that the valley
becomes covered with gravel and sand, which has been
washed, worked over, and abandoned by the stream.

Formation of Sediments by Chemical Agencies.
Are all sediments formed by broken and ground-up rockf

Besides the material which surface waters carry in sus-
pension, as mud, sand, or gravel, they contain substances
in solution. In limestone districts, for example, the waters
contain lime, and cooking utensils and boilers in which
such water is used become coated with this material, depos-
ited from the evaporating liquid. In nature this lime is
deposited similarly and on a large scale. In shallow lakes
and land-locked seas the water brought down by streams
from limestone regions may, after standing and evaporating,
precipitate lime on the bottom of the lake. This deposit is
generally lime carbonate (limestone) ; sometimes it is lime
sulphate (gypsum).

Is lime the only material chemically precipitated as sediment

in such cases?

Besides lime, other minerals are chemically precipitated
in ocean and lake waters, especially silica, but all in a far
less degree.

Formation of Sediments by Organic Agencies.
In lakes, seas and in the ocean, there live myriads of
animals that form their hard parts by extracting it from the
sea water. They absorb the mineral that is in solution and

26 GEOLOGY APPLIED TO MINING.

build it into their shells. It is generally lime that they
absorb and their shells are of lime carbonate. Such are all
of our ordinary shell-fish, as well as corals and a myriad of
others, famihar and unfamiliar. It is a famihar story how
corals live and die, leaving their limy shells behind; how
new animals build upon the skeletons of their ancestors,
and so on, till great masses are produced. In the same
way other shell-bearing marine animals may furnish
material which, Uttle by little, accumulates to great
thickness. There are thick strata which consist almost
entirely of oyster shells, and so on.

Transformation of Sediments to Hard Rocks.
How do these sediments become rocks and dry land?

By the warping and folding of the earth's crust, brought
about slowly during centuries of centuries, sediments are
brought out of the water and become part of the land.
They ordinarily harden with time. They may come to
occupy any position; it is as common to find sedimentary
rocks on the top of mountains as in the low plains.

THE PHYSICAL CHARACTERS OF SEDIMENTARY
ROCKS.

How can one distinguish sedimentary rocks from metamorphic

or igneous rocks?

Sedimentary rocks are distinguished from metamorphic
or igneous rocks by their physical characters. They are
often plainly fragmental — that is, they are made up of
broken, often waterwom fragments, large or small; or,

ARRANGEMENT OF STRATIFIED ROCKS. 27

like limestones, they are of a composition unknown in any
other than sediments. Fossils are almost infallible evi-
dences of the sedimentary origin of the rock which contains
them.

What is stratification or bedding?

This is another feature of sedimentary rocks. When we
cut a pit in the sand on the sea-shore, we see that the
material is deposited in layers, one layer, for instance,
being sand and another pebbles; or, if all the layers are of
sand, there is some slight difference, as of color.

This arrangement of successive layers is called bedding
or stratification. It arises from the fact that the material
laid down in water will ordinarily vary during successive
periods. All sedimentary rocks show this characteristic
more or less plainly. Each separate layer is called a bed
or stratum. Some rocks show distinct beds only a few
inches or even a fraction of an inch thick; these rocks have
been deposited under changing conditions — near the shore
of an ocean, for example, where the varying currents
brought about supplies of different kinds of detritus. In
other rocks the beds are thicker, and in some they may
extend through a thickness of many feet with a scarcely
perceptible stratification.

What is meant by the word ' formation ' as used in reference

to rocksf

The term formation is generally used by miners as a
name for any particular body of rock — thus a limestone
formation, etc. This use of the word is, in the writer's

28 GEOLOGY APPLIED TO MINING.

opinion, proper. Geologists use the word with a different
technical meaning, limiting its use in various ways.
Where rocks of different kinds and of different ages
lie one over the other, each belt, marked by certain con-
stant characteristics, is called a formation. In this sense
a formation may be only a few feet or thousands of feet
thick. Geologists often give a distinctive name to each
formation, for the sake of identifying it in description or
in mapping.

THE CHIEF KINDS OF SEDIMENTARY ROCKS,
THEIR ORIGIN AND CHARACTERISTICS.

What are the ordinary sedimentary or stratified rocks?

The ordinary sedimentary rocks are conglomerate, grit,
sandstone, quartzite, shale, slate, limestone, marble and
dolomite.

What are conglomerates ^ and what is their originf

Beds of pebbles, when cemented together, form con-
glomerate. They can be distinguished by the stratification
and the rounded pebbles.

What is a gritf

A coarse and impure sand, when hardened, is called a
grit.

What is a sandstone, and how does it originate?

When sand hardens so that the grains stick firmly
together it becomes a sandstone. Sandstone is of alW
colors, white or red being the most frequent. In pure

ARRANGEMENT OF STRATIFIED ROCKS. 29

sandstone the component grains arc entirely of quartz;
these rounded grains can usually be seen with the naked
eye. Sandstones arc porous, for between the grains there
are tiny spaces or interetices.

How does quartzite originatcf

The underground waters which usually permeate sand-
stones frequently bring silica, which they deposit in the
pores between the sand grains. After a period there may
result a solid quartz rock, or quartzite.

How can one tell qvxirtzite from vein quartz?

Sometimes quartzite is pure white, and very difficult to
distinguish from vein quartz, which has been deposited
entirely from underground waters. A close scrutiny,
especially with a magnifying glass, will often disclose the
faint outlines of the close-packed rounded grains of the
original sand. Even in thin sections under the microscope,
vein quartz and quartzite are often similar, but examination
generally shows the outlines of the sand grains in the
quartzite, marked by a rim of clay or iron.

What is the nature and origin of shales?

Mud beds, when somewhat dried, become clay. On
hardening, clay becomes shale, a rock distingushed by its
softness, its fineness of texture, and its easy sphtting into
thin sheets along the bedding planes. Shales may be of
any color, but are most frequently dark-colored, often
black.

30 GEOLOGY APPLIED TO MINING.

What is a slatef

When shale becomes still harder, it is called slate. The
property possessed by slates of splitting into thin sheets,
or fissUityf is usually due to pressure exerted upon the rock
subsequent to its deposition. Usually, also, the splitting
does not follow the bedding planes, though sometimes it
may.

How do limestones and marbles originate?

Limestone, dolomite and marble are intimately related.
They begin as the accumulation of the shells of marine
animals, often broken so as to form sand or mud; or, more
rarely, they are chemically precipitated. These deposits
harden into rocks. Limestones are of all degrees of com-
pactness, from the slightly consolidated shell-mud to the
dense semi-crystalline, generally dark blue, rock, which it
is sometimes difficult to recognize as sedimentary. When
limestones have been exposed to heat and pressure in the
earth's crust, they become crystalline, and are then marbles.
The fossils which are frequently found in limestones often
become obliterated when the marble state is reached, but
not always.

How does dolomite originate?

Lime-sands and limestones are generally carbonate of
lime, with some impurities. Magnesia (magnesium oxide)
has a great affinity for lime carbonate, and easily combines
with it to form dolomite, a mineral containing 54.35 per
cent calcium carbonate and 45.65 per cent magnesium
carbonate. Magnesium salts are present in most waters,

ARRANGEMENT OF STRATIFIED ROCKS. 31

especially in the ocean and in underground waters. Where
sea-water becomes land-locked, and the magnesium and
other salts concentrated, (as is the case in the Dead Sea
and in Great Salt Lake, for example), the lime deposits
which become precipitated are apt to be impregnated with
magnesium and to change to dolomite. Limestone rocks
that are permeated by underground magnesian waters
may be similarly altered.

If a certain dolomite formation is everywhere of about
the same composition, the alteration has probably been
due to the first named cause; if the dolomite occurs chiefly
along water-courses, such as fissures in limestones, and is
of very irregular composition, the change has probably
been due to the latter agency. Marbles are very frequently
dolomitic or magnesian.

Example: In the lead and zinc mining region of south-
western Missouri, there are beds of magnesian limestone
or dolomite of Silurian age. This limestone is magnesian
wherever it outcrops and is evidently an original deposit.
On the other hand, there are irregular deposits of dolomite
immediately associated with the ore-bodies. This dolo-
mite is generally contiguous to the limestone wall rocks, and
appears to grade into them. Blocks of limestone are often
found covered with a shell of such dolomite, evidently
formed by the action of solutions containing magnesia upon
the limestone.*

How can one tell limestone from dolomite?

It is next to impossible to distinguish dolomite from
limestone by the appearance. In certain regions the

♦Arthur Winslow, Missouri Geological Survey, Vol. VII, p. 448.

:5*^ GEOLOGY APPLIED TO MINING.

dolomite will have a diiTerent appearance from the lime-
stone, being finer or coarser, or of a different color; but the
test will not hold good for another district. The best way
is to test with very dilute hydrochloric acid. A drop of
this on limestone causes a lively effervescence, while
dolomite is slightly or not at all attacked. This does not
apply to strong acid. A thin section of dolomite under
the microscope is like one of limestone, but may often be
distinguished by the tendency of dolomite to be in small
grains with perfect rhomboidal outlines, while calcite (lime
carbonate) is more frequently a mass of interlocking
irregular grains.

THE DISTINCTION BETWEP^N BEDDING, CLEAV-
AGE, SCHISTOSITY, AND GNEISSIC STRUCTURE.

What arc cleavage-planes?

When any rock, but particularly a shale, is exposed to
pressure by the movements of the earth's crust, it is apt to
break easily into sheets along certain planes determined
by the direction of the applied forces. These planes arc
called cleavage planes. They may lie at any angle to the
bedding planes, or may even coincide; they may be straight
while the bedding planes are folded; in short, the two sets
of parting planes have no relation to one another.

How can one distinguish between cleavage and stratification?

To be sure of stratification, one must look for layers

differing in texture or mineral composition. In the

case of conglomerates the pebbles typically have their

ARRANGEMENT OF STRATIFIED ROCKS. 33

longest diameter parallel to the stratification, for they have
come to rest upon their flat sides, and not on end; in fossil-
iferous rocks the fossil shells have their long axis parallel
to the stratification, for the same reason.* In short, any
evidence of the original position of the sediments must be
looked for. A rock may not split at all along its true
bedding planes, while it may separate perfectly along
cleavage planes which run across the stratification; but
the cleavage must not for that reason be mistaken for
bedding.

What are schists and gneisses?

Schists and gneisses have been mentioned in Chapter 1
as frequently the result of the metamorphism of sedi-
mentary rocks.

A schist is a highly metamorphosed slate, which has
become thoroughly crystalline, the minerals having a
parallel arrangement. The individual crystals are gener-
ally relatively small. Further crystallization may produce
larger crystals, and a less perfect parallel arrangement,
when a gneiss results. Gneiss, as commonly understood,
is composed of the same minerals as granite (feldspar,
quartz, mica, hornblende, etc.) and differs from it by reason
of the more or less marked arrangement of its minerals in
bands. With variations in the mineral composition
syenite gneiss, diorite gneiss, etc., are distinguished.

* An exception to this test of stratification planes is where rocks have been
stretched by movements of the crust. In that case it sometimes happens that
pebbles, and even fossils, are pulled out of shape, and so their long axes cease
to have any relation to the original stratification.

;U GEOLOGY APPLIED TO MINING.

possessing a mineral composition similar to that of the
igneous rock from which they have been named.

IIow do wc know that schists and gneisses may be formed

from sedimentary rocks?

Not infreciuently in schists, less often in gneisses, we
may find evidence of sedimentary origin, in the shape of
stratification, of pebbles, and (rarely) of obscure fossils.
In other j)laces no such evidence can be seen, and we
cannot doteu-mine the origin of the rock, (except perhaps,
!)y microscopic study), for schists and gneisses may also
be formed from igneous rocks.

What is schistosity and gneissic structure?

Both schists and gneisses have strong banding, resembling
stratification, but having no necessary connection with it.
In schists, where the crystals are well arranged in parallel
position, the rock splits very easily in the same direction,
especially in mica-schist, where mica is an abundant
mineral. This property is called schistosity. In gneisses,
where the parallelism is not so strong, resulting only in a
more or less marked banding, the banding is called gneissic
structure.

What are the different kinds of schists and gneisses?

According to the minerals which they contain, schists
and gneisses are given different names. With schists this
name is usually taken from the predominant mineral.
Thus, mica-schists are the commonest; and we have also
garnet-schists, hornblende-schists, etc.

ARRANGEMENT OF STRATIFIED ROCKS. 35

DIFFERENT GEOLOGIC PERIODS DURING WHICH
SEDIMENTARY ROCKS HAVE FORMED.

How long is it that sedimentary rocks have been forming?

Studies of geologists have proved that the stratified
rocks differ in point of age — that they have been con-
tinuously deposited during millions of years. At many
places the sedimentary beds are several miles in actual
thickness, and one can imagine what time must elapse to
allow so much sediment to accumulate.

What proves the fact of these great periods of time?

The best proof lies in the fossils which the sedimentary
rocks contain. By study of these remains or impressions
of animals and plants a good idea of the history of the
world has been obtained, and of the manner in which life
changed, in the course of periods compared with which the
historic period of man on earth is but as a day to a
century.

How did life begin and develop on earth?

We do not well understand the beginning of life, for in
the oldest rocks the traces of life are almost always de-
stroyed by metamorphism. But when we first find a good
record, theie were already mollusks, crustaceans and
worms; afterward fishes came in, and then reptiles; still
later mammals, and finally the highest type of mammals —
man. In the plant world there was a like gradual devel-
opment and change.

30 GEOLOGY APPLIED TO MINING.

Holo did the different geologic periods come to be so defined
and named?

The science of geology is new. Within the last hundred
years, geologists have studied, in different parts of the
world, the fossils of certain groups of stratified rocks, and
have applied names to the time periods covered by the
fossils they have there found. These names frequently
come from the name of the country where the rocks were
first studied — as Jurassic, from the Jura mountains (part
of the Alps); Cambrian, from Cambria (Wales); etc.
Afterward, in other parts of the world, rocks containing
similar fossils were also called Jurassic or Cambrian. Other
terms came from some real or supposed peculiarity of the
rocks of that period — thus. Carboniferous (meaning coal-
bearing). At first all coal was supposed to occur in the
rocks of this age; and conversely, all rocks of this age were
expected to contain coal. More recently both propositions
have been completely disproved, but the name remains.
Other names are left to us from earlier and cruder attempts
at age classification. Thus Tertiary and Quaternary
remain from a division of rocks into primary, secondary,
tertiary and quaternary. The first two of these terms have
been dropped, the last two retained.

Does the classification of geologic time by periods represerU a
natural system?

The classification is more or less arbitrary and might be
just as accurate if it were made up quite differently from
what it is. Between one period and another we must not

ARRANGEMENT OF STRATIFIED ROCKS.

imagine that there were sharp divisions. Life and the
deposition of sediments often passed smoothly and unin-
terruptedly from one period into another. However,
the classification is of accepted usage and enables general
understanding.

What are the names of the geologic periods of time?

Following are the chief divisions, beginning with the
oldest:

Archaean

Paleozoic

Mesozoic

Cenozoic

Cambrian

Silurian

Devonian

Carboniferous

Triassic

Jurassic

Cretaceous
[Tertiary
[Quaternary

No fossils. No
known life.

Each marked by
y characteristic
fossils.

Whit are the characteristics of rocks of the Archcean period?

The Archaean rocks contain no fossils and show no signs
of life. Although it is probable that life existed at that
period or during a portion of it, the traces have been
obliterated. The Archaean rocks are usually metamorphic.
They may be gneisses and schists, or massive crystalline
rocks, like granites They occupy large areas of the
earth's surface.

38 GEOLOGY APPLIED TO MINING.

What are the characteristics of rocks of the Cambrian periodf

The Cambrian rocks contain trilobites/ (often large),
certain generally very small brachiopods, worm-tracks,
some remains of fossil sponges, crinoids, etc.; also traces
of seaweeds.

What are the characteristics of rocks of the Silurian periodf

The Silurian rocks contain impressions of sea-weeds;
some terrestrial plants, among them some members
of the I^epidodendron^ family, having much the habit of
the spruce or pine tribe; trilobites' and many other crus-
taceans;* worms; very many graptolites'* (feather-like
animals); many moUusks," especially brachiopods' and
cophalopods ;" also many lamellibranchs,* corals and
crinoids.'" The earhest fishes also occur, some of them of
the shark tribe.

What are the characteristics of rocks of the Devonian periodf
The Devonian rocks contain sea- weeds, lycopods^*
(ground pines) and ferns, equisetse'"' or horse-tails, and
conifers.'' The animal life was varied. There were many
sponges and corals, crinoids, brachiopods, and other
kinds of mollusks, and a few trilobites. Fish are fre-
quently found, belonging to the shark and other tribes.
The Devonian has been called the Age of Fishes.

^ For the explanation of this and following terms, see pp. 44-51. ''See p. 51.
» See p. 47. * See p. 47. » See p. 45. • See p. 45. ' See p. 47. « See p. 46.
•Seep. 46. lo See p. 45. "Seep. 51. »» See p. 51. "Seep. 50.

ARRANGEMENT OF STRATIFIED ROCKS. 39

What are the characteristics of rocks of the Carboniferous

'period?

The Carboniferous was marked by an abundance of
vegetation, whose remains or imprints we often find as coal
or as fossils in the rocks. There were lepidodendron and
sigillaria/ and various ferns, conifers and calamites," (a
genus of horse-tails). As to the animal life, there was a
great abundance of crinoids or sea-lilies; also numerous
brachiopods, cephalopods/ etc. Besides fishes, remains of
amphibians* occur, some snake-like, some lizard-like, some
frog-like. On the land were insects, (cockroaches, etc.)
spiders and centipedes, and true reptiles — snakes, saurians,''
and turtles.

What are the characteristics of rocks of Mesozoic age?

The Triassic, the Jurassic and the Cretaceous periods
together constitute the Mesozoic age, called the Age of
Reptiles. In this age came the first mammals, the first of
the common or osseous* fishes, the first palms and angio-
sperms.'

What are the characteristics of rocks of the Triassic period?

The Triassic had neither the sigillarids nor the lepido-
dendrids of the Carboniferous era; but many cycads/
besides ferns, horse-tails, and conifers. As to animals,
some brachiopods and lamellibranchs were abundant;
also ammonites.* Fishes and reptiles, the latter including
the gigantic dinosaur,*" were also plentiful.

» See p. 51 . a See p. 51 » See p. 46. * See p. 48. * See p. 49. • See p. 49.
'Seep. 50. •♦Seep. 50. » See p. 47. "Seep. 49.

40 GEOLOGY APPLIED TO MINING.

What are the cJiaracteristics of rock of the Jurassic period?

The Jurassic contains, besides many characteristic
radiates/ sponges and moUusks, (brachiopods, lamelli-
branchs, cephalopods, etc.) remains of gigantic reptiles,
'ncluding the flying-reptiles or pterodactyls,^ the ichthy-
osaurus,' tortoises, etc.; also some mammals. Fishes
flourished.

What are the characteristics of rocks of the Cretaceous period?
The Cretaceous plants were marked by the first great
development of the angiosperms (including all plants with
a bark, except the conifers and cycads). This class
embraces the oak, willow, maple, etc. The smaller marine
animals have contributed shells in great variety and
profusion to the Cretaceous sediments. Rhizopods with
tiny shells (foraminifers^). were abundant, and constitute
most of the chalk beds. Sponges and corals were of great
importance. The oyster family flourished, and many
others. Sharks and other fishes were common; reptiles
were numerous, among them true sea-serpents, as much
as seventy-five feet long. Turtles lived, and also birds,
some of which possessed pointed teeth.

What are the characteristics of rocks of the Tertiary period?

The Tertiary is called the Age of Mammals, for during
this })erio(l mammals flourished. Yet most of the Tertiary
mammal species are now extinct.

The Tertiary beds often contain plant remains, belonging

J See p. 45. a See p. 49. =» See p. 49. * See p. 44.

ARRANGEMENT OF STRATIFIED ROCKS. 41

to species of oak, maple, dog-wood, magnolia, fig, palm, etc.
The mollusks comprise many species of oyster, clam, and
other lamellibranchs, but few brachiopods. Crabs, insects,
fishes, etc. were plentiful. Crocodiles, snakes, and turtles
abounded. There were many large mammals, including
now extinct species of elephant, tapir, rhinoceros, horse,
tiger, lion, wolf, peccary, etc. The remains of all of these
are found in the western United States. In the sea there
were whales, dolphins, seals and walruses.

Exam'ple: A quarry near Carson, Nevada, now used as a
State prison yard, is cut in grayish sandstones, whose
bedding is such as to indicate that they were deposited at
the mouth of an ancient stream. When the sandstone was
removed down to two shale layers, these were found liter-
a ly covered with the tracks of many species of birds and
mammals, including the mammoth, the deer, the wolf,
many birds, a horse, and tracks resembling those of a man,
but which may have been made by some animal. The
footprints and some associated bones indicate a probable
very late Tertiary age. These remarkable tracks were
exposed and trampled over by horses and men for eight or
ten years, without attracting any especial attention.*

What are the characteristics of rocks of the Quaternary 'period?
The Quaternary in called the Age of Man, for in this
period the human race began to flourish. During this age
came the GJacial Period or Tee Age, when a vast glacier or
glaciers covered the northern part of North America, with
their southern margin nmning irregularly across the northern
and central part of the present United States. These

* J. Le Conte, Proceedings California Academy of Science, Aug. 27, 1882.

42 GEOLOGY APPLIED TO MINING.

glaciers spared most of Alaska and some other areas, but
everywhere else ground off the cliffs and hills, and left,
on melting, vast deposits of boulders and gravels. Thus in
the eastern United States the southern and central por-
tions, which are unglaciated, are strikingly different from
the rocky, bouldery, often barren glaciated areas of the
north. In the Western States the glacier did not extend
far south of the present Canadian boundary.

In the early part of the Quaternary flourished many
great mammals, of species now generally extinct. These
comprised gigantic mastodons, elephants (mammoths),
lions, hyenas, bears, wolves, beavers, etc. Man probably
lived in the later Tertiary period, but evidence of his
existence is first complete in the Quaternary. He was
contemporary with the hairy mammoth and other extinct
species, which he has survived.

How can one tell to what 'period a given rock belongs?

The only reliable way to identify a stratified rock, as
belonging to one of these periods, is by study of the fossil
remains which it contains. These will tell in what stage
of the world's history the sediment was laid down. To
the paleontologist, who has made a careful study of extinct
animal forms, it is generally possible, on seeing a group of
fossils, to refer them to one of the great periods given.
But to the casual observer, the differences are not so
striking as to be retained without study and careful com-
parison. There are, to be sure, certain broad signs, which
he may use as guides, to a limited extent. Rocks con-
taining large numbers of trilobites, with few other fossils,

ARRANGEMENT OF STRATIFIED ROCKS. 43

are probably Cambrian, or Silurian, most probably the
former. Rocks with graptolites are probably Silurian.
Rocks containing plant remains, if these are largely reed-
like and otherwise unlike any of our common trees, are
probably Carboniferous, perhaps Devonian. If the plant
remains consist of leaves resembling those of our modern
trees, the rocks are probably Cretaceous or Tertiary. In
the same way, rocks containing fossil shells almost exactly
like those which are now occupied by living animals on the
sea-shore, are probably Tertiary, and in proportion as the
unlikeness increases, we can suspect an older age. A great
abundance of crinoids suggests the Carboniferous. Abun-
dance of sponges, corals and large brachiopods, with
fewer lamellibranchs, suggest the Carboniferous or Devo-
nian. Predominance of lamellibranchs indicates a probable
age not older than Triassic, etc.

With a little practice in observing rocks of known
age, one can tell ordinarily, although by no means always, a
Paleozic rock, a Mesozoic rock, or a Tertiary rock, from the
general look of the fossils, even if one cannot determine a
single species. Where a certain series of strata has been
determined by paleontologists, one can attentively examine
the fossils contained, and can then very likely recognize
beds of the same age in another part of the same district
or even in another district.

Characteristics of the Different Fossils.*
The fossils which have been referred to belong mostly to

* The descriptions and definitions under this head are adopted directly from
Dana's * Manual of Geology.'

44 GEOLOGY APPLIED TO MINING.

the animal kingdom ; some of them to the vegetable king-
dom.

How is the animal kingdom dividedf

The animal kingdom is divided into five sub-kingdoms.
Beginning with the lowest they are:

1. Protozoans.

2. Radiates.
3 Mollusks.

4. Articulates.

5. Vertebrates.

What are protozoans?

Protozoans are minute animals, (usually from a 100th
to a 10,000th of an inch in length )

They have no external organs save a mouth and minute
thread-like organs, and no digestive apparatus beyond a
stomach. The stomach and the mouth are sometimes
wanting. There is no heart or circulating system beyond
a palpitating vesicle.

What are rhizopods and foraminifersf

Among protozoans, the rhizopods are of especial interest.
The shells are usually much smaller than the head of a pin.
The most common kinds have calcareous shells called
foraminifere, and these have contributed largely to the
formation of the limestone strata. They consist of one or
more shells, and the compound kinds present various
shapes.

AUKANGEMENT OF STRATIFIED ROCKS. 45

What are radiates?

Radiates have a radiate structure, like a flower — that is,
they have similar parts or organs repeated around a vertical
axis. These animals have a mouth and stomach for eating
and digestion, and are widely diverse from plants, although
resembling them in their radiate arrangement of parts.

What are crinoidsf

Among the radiates, crinoids are animals like an nverted
star-fish or sea-urchin, standing on a stem like a flower.

What are graptolitesf

The graptolites were ancient, delicate plume-like animals
which belonged in the sub-kingdom of radiates.

What are mol usks?

MoUusks possess a soft fleshy bag, containing the stomach
and viscera. They do not possess a radiate structure nor
jointed appendages. - Similar parts are repeated on right
and left sides of a median plane, and not around a vertical
axis, as in radiates.

How are molliisks subdividedf
MoUusks are divided into:

1. Ordinary mollusks.

2. Ascidian mollusks.

3. Brachiate mollusks.

The ordinary mollusks are divided into!

1. The acephals, or headless mollusks, the head not

46 GEOLOGY APPLIED TO MINING.

being distinctly defined in outline; as the oyster
and clam.

2. The cephalates, having a defined head; as the snail.

3. The cephalopods, having the head furnished with

long arms (or feet); as the cuttle-fish.

What are lamellibranchsf

The acephals, or headless moUusks, are illustrated in the
group of lamelUbranchs. These common species are well
known as bivalves. One valve is on the right, and the
other on the left, of the animal. The clam and the oyster
are familiar examples.

What are gasteropods?

In the cephalates, one of the two groups are the gastp/ro-
pods. These are contained in univalve shells (shells all in
one piece). The animal crawls on a flat spreading fleshy
organ called the foot. The snail is a familiar example.

What are cephalopods?

Cephalopods, or cuttle-fishes, are of two kinds, one
having external shells and four gills; and another having
sometimes internal shells, but not external, and having
but two gills. The external shells are distinguished from
those of the gasteropods or ordinary univalves by nearly
always having transverse partitions — whence they are
called chambered shells. They may be straight or coiled,
but when coiled are usually coiled in a plane, and not a
spiral. The animal occupies the outer chamber of the
shell. The nautilus is an example.

ARRANGEMENT OF STRATIFIED ROCKS. 47

Modern cephalopods are almost exclusively naked
species, such as the cuttle-fish and squid.

What are ammonitesf

The ammonites are a genus of cephalopods, and have
very beautiful and often large coiled and fluted shells.

WhaJt are hrachiopods?

Among brachiate moUusks, brachiopods have a bivalve
shell, and in this respect are like ordinary bivalves. But
the shell, instead of covering the right and left sides, covers
the dorsal and ventral sides, or its plane is at right angles
to that of a clam. Moreover, it is symmetrical in form,
and equal on either side of a vertical line. The valves are
almost always unequal; the larger is the ventral, and the
smaller the dorsal.

WhaJt are articulatesf

Articulates consist of a series of joints or segments. The
legs, where any exist, are jointed, and there is no
internal skeleton. The articulates include worms, crusta-
ceans, and insects.

What are crustaceans?

The crustaceans have the body in two parts — the front
consisting of a head and thorax, the hinder part of the
abdomen. Crabs, lobsters, and shrimps are examples.

What are trilohitesf

Among crustaceans, the trilobites existed only in Paleo-
zoic time. They had jointed bodies with a crust-like

48 GEOLOGY APPLIED TO MINING.

exterior. They are more like the horse-shoe crab of the
Atlantic coast than any other living species.

What are vertebrates?

The fifth animal sub-kingdom comprises the vertebrates.
These have a jointed internal skeleton, and a bone-sheathed
cavity along the back for the great nervous cord, distinct
from the cavity of the viscera.

How are vertebrates divided?
The classes of vertebrates are:

1. Mammals, which are the highest branch of the animal
kingdom. They suckle their young and breathe with
lungs. Ordinary quadrupeds (four-footed animals) are all
mammals.

2. Birds produce their young in the egg form. They have
a heart of four cavities; they breathe by lungs, are covered
with feathers, and are adapted for* flying.

3. Reptiles produce their young in the egg form. They
breath by lungs, have a heart of three or four cavities; and
are naked or covered with scales. Examples are crocodiles,
Uzards, turtles, and snakes.

4. Amphibians produce their young in the egg form.
When young they breath by gills, and afterward by lungs
alqne. They possess a heart with three cavities, and are
naked or covered with scales. Examples are frogs and
salamanders.

5. Fishes usually produce their young in the egg form.
They possess a heart, usually of two cavities. They breathe
by gills, and are naked, or covered by scales.

ARRANGEMENT OF STRATIFIED ROCKS. 49

WhcU are ossecms fishes?

The osseous fishes or teliosts include nearly all modem
kinds, except the sharks and rays. They usually have
membranous scales. They are not known among fossils
before the Middle Mesozoic.

How are reptiles classified?

Reptiles are divided into snakes, saurians, and turtles.
The saurians vary in length from a few inches to fifty feet.

What were dinosaurs and pterosaurs?

Among the saurians the tribe of dinosaurs, reptiles ot
great size, now extinct, possessed some mammalian and
many bird-like characteristics.

Another group were the pterosaurs or the flying saurians.
The pterodactyls were the most common genus. The little
finger of the forefoot was excessively prolonged, and from
this a membrane extended to the tail and made a wing for
flying. The remaining fingers were short, and furnished
with claws. They had the habits of bats, and wings of a
similar character.

Whxd were ichthyosaurs?

The ichthyosaurs were a genus of the group of enalio-
saurs, or swimming saurians. They were gigantic animals,
10 to 40 feet long, having paddles somewhat like the
whale, long head and jaws, and an eye of enormous
dimensions.

50 (lEOLOGY APPLIED TO MIXING.

How is the vegetable kingdom divided?

The vegetable kingdom is primarily divided into crjrpto-
gams, which have no distinct flowers or proper fruit, (such
a.s ferns and sea-weed), and phenogams, having distinct
flowers and seed, (such as our ordinary trees and plants).

How arc the phenogams subdivided?
The phenogams are divided into :

1 . (lymnosperms. The plant has a bark, and grows by
an annual addition to the exterior of the wood, thus forming
rings of growth. The flowers are very simple, and the seed
naked. P]xamples are the pine, spruce, hemlock, etc.
(lynmosperms include: (1) Conifers. (2) Cycads. The
conifers include most of the common evergreen trees.
Their wood is simply woody fibre, without ducts. The
cycads had the habit of palms, while related to the pine
tribe.

2. Angiosperms. Growing by external annual rings,
Uke the gymnosperms; having regular flowers and covered
seeds. Examples are the maple, elm, rose, etc.

3. Endogens, having regular flowers and seed, but with
no bark and no rings of growth. In a transverse section
of a tnmk or stem, the ends of fibres are shown. Examples
are the palm, Indian corn, lily, etc.

How are the cryptogams subdivided?

The cryptogams are subdivided into:

1. Thallogens. Consisting wholly of cellular tissue;

ARRANGEMENT OF STRATIFIED ROCKS. 51

growing mostly in fronds without sterns, and in other
spreading forms; as, (1) Algae, or sea-weeds. (2) Lichens.
(3) Fungi, or mushrooms.

2. Anogens. Consisting wholly of cellular tissue; grow-
ing up in short, leafy stems; as, (1) Mosses. (2) Liver-
worts.

3. Acrogens. Consisting of vascular tissue in part, and
growing upward; as, (1) Ferns. (2) Lycopods (ground
pine). (3) Equiseta, or horse-tail; and including many
trees of the coal period.

What were lepidodendridsf

Among the fossil lycopods, the lepidodendrids (tall trees,
with the exterior embossed with scars in alternate order,)
were of many kinds. In foliage, they resembled the pines
and spruces of the present day.

What were sigillaridsf

The sigillarids differed from the lepidodendrids in having
the scars in vertical order. The trunk was woody, but not
firm within; and it had a large pith.

What were calamitesf

In the Paleozoic, the equisetse, or horse-tails, were re-
presented by plants called calamites. They had a reed-
like appearance, with jointed stem, and finely furrowed
surface.

53 geology applied to mining.

The Order of Succession as Found in Actual
Practice.

Is the succession of geologic periods , as previously stated ^
always shovm in the sedimentary rocks of a given diMrlctf
Not always ; indeed, not usually. Any one of the geologic
ages may not be represented at all, may be represented by
very thin beds, or by strata thousands of feet thick. Often
rocks of only a few of the geologic ages are present. Ter-
tiary beds may rest directly upon those of the Cambrian
age; or the Quaternary may rest upon the Archsean. In
fact, there is every possible combination.

WhxU is the reason for this lack of uniformity?

In a certain part of the earth's surface there may have
been no sediments during a certain age or ages. If the
region was a land area during any period this would be the
case, for stratified rocks are laid down in water. Again,
after the sediments were formed, during a certain period,
they may have changed into land, and have been stripped
off and carried away by the erosion of the rivers. Then,
when the land became again submerged and new strata
were deposited in their place, the new beds may have come
to rest on those of an age much greater than the series
usually found next underneath them.

What is an erosion gap and an unconformity?

In the case just mentioned, the line of contact between
the two series of strata will be irregular, for the newer beds
will have been deposited over the hills and hollows of the
older topography, although the stratification in both

ARRANGEMENT OP STRATIFIED ROCKS.

series may be parallel. This is called an erosion gap;
sometimes an unconformity by erosion.

Frequently, in the interval between the deposition of the
earlier series and that of the later one, there occur move-
ments in the earth's crust, so that the beds of the older
series are bent or folded; then the new series does not have
its stratification parallel, but rests discordantly on the bent
and worn edges of the old strata. This is a true uncon'
formity (Y\g.2).

Fig. 2. Ideal sketch to illustrate unconformities, a. Earlier line of unconform-
ity; b. Later line.

Relation of Physical Characters to Geologic Age.
Can one tell from the kind of rock what age it belongs to9

In general, the nature of the strata, whether limestone,
quartzite, etc., is of little value in determining to which of
the geological periods it belongs. A certain stage of the
Carboniferous, for example, may be represented })y a
sandstone in one district, a limestone in another.

Is a given bed necessarily of one kind of rock throughout?

A bed may pass laterally from one kind of rock into
another (as from a sandstone into a limestone), within a
space of a few miles or even much less.

GEOLOGY APPLIED TO MINING.

Example: On the Kuskokwim river, Alaska, the writer*
observed a series of interbedded sandstones and shales,
where the sandstone passes laterally into shale in a remark-
able way. A thick bed of sandstone splits into separated
beds which become alternate with beds of shale, and these
rapidly grow thinner as the shale beds increase in thickness
until the shale forms nearly the entire rock (Fig. 3).

Fig. 3. Sketch of cliff on Kuskokwim river, Alaska, showing sandstones and
shales passing laterally into one another.

What is the explanation for this transition?

If one considers sediments now being formed, one will
find such transitions common along the sea-shore. For
example, there is found at one point mud, at another sand,
at another gravel or cobbles, all being deposited at the same
time. When such a deposit is hardened and becomes
stone, it will show within a single bed the change from a
shale to a sandstone, and from a sandstone into a con-
glomerate.

* 20th Annual Report United States Geological Survey, Part VII, p. 126.

ARRANGEMENT OF STRATIFIED ROCKS. 55

Can strata he identified by their physical characters?

If only the foregoing exception is remembered as a guard
against over-confidence, it is possible to trace certain strata
a long distance by their physical characters, which they may
retain for hundreds of miles. - Sometimes the peculiarities
of certain strata are so marked that, when one finds in
adjacent districts rocks possessing these peculiarities, there
is a strong suggestion of identity of age.

Example: The lithological identity of the peculiar parti-
colored, thin-bedded shales, lithographic limestones and
quartzite of the Parting Quartzite series in Aspen, Colorado,
with Devonian strata described by Mr. C. D. Walcott from
Kanab creek, Utah, so impressed the writer, that other
conditions being favorable, he provisionally classified the
Aspen beds as Devonian, although the localities are
hundreds of miles apart. This correlation was afterward
borne out by the discovery of Devonian fossils in the
Aspen beds.

Yet correlation from physical characters alone, without
sufficient guardedness and auxiliary evidence, has fre-
quently led into the most awkward errors.

Does a certain kind of rock ever constitute beds of the same age
over large areas?

A certain character of strata may persist in some in-
stances over a wide region — even over large portions of
continents. Some physical features of strata of a certain
period seem to be of almost world-wide occurrence. In
rocks 'belonging to the Triassic, all over the world, there is

56 GEOLOGY APPLIED TO MINING.

an astonishing quantity of massive red sandstone; yet all
massive red standstones are by no means Triassic, and
conversely, all Triassic rocks are not red sandstone.

Comparison and Correlation.
Mode of Determining the Relative Age of Different Strata.

How can one determine the relative age of different series of

strata in contact with one another?

The relative age of a series of strata may often be deter-
mined by making reference to other strata, the age of which
is definitely known. The first^rule to be observed, is the
simple rule of superposition ^ by which the upper of two
series of strata, or of two beds, is usually the younger. This
is to be applied not only where the beds are horizontal but
where they are folded; it is easy to do this, except in the
rare case where the folding has been so great that some beds
have been overturned, and the normally lower ones come
on top. In this case the study of the folding, the fossils
present in the rock, etc., will afford the data requisite for
solving the problem.

This rule does not necessarily apply where a fault
separates the two beds in question.

How may the relative age of different series of strata^ not in

contact with one another, he judged?

Where two rocks are not found in actual contact, there
are other tests of their relative age. Of two rocks close
together, the more metamorphosed or hardened one is
probably the older; likewise, that one which shows most

ARRANGEMENT OF STRATIFIED ROCKS. 57

folding and faulting, and other evidence of disturbance, is
probably the older. One rock, such as conglomerate, may
contain pebbles which have been derived from the other:
in this case the conglomerate is clearly the younger. Many
such tests will present themselves to the careful inves-
tigator.

Mode of Correlating Similar Strata in Adjacent or Separated

Regions.

How are simUar strata in different localities identified as

SV£hf

This point has already been dwelt upon, but may be
briefly summarized.

The correlation or matching of similar strata in adjacent
or separated regions is usually done by means of the fossils
which they contain. If these fossils are nearly the same in
two different locaUties, the beds may be correlated; or, if,
although not belonging to the same species, they are known
to represent a similar stage in the development of life.

When fossils are lacking, the physical characteristics
may serve as a basis for correlation. The bed in question
may be traced wholly or partly from one district to another;
or, if its peculiarities are very striking, a correlation
between separated districts may often be made without
tracing out the intervening portions. Thin beds are
usually traced out and correlated by their physical charac-
teristics, which are apt to be more definitive in this case
than fossils.

58 GEOLOGY APPLIED TO MINING.

THE ASSOCIATION OF VALUABLE MINERAI^
WITH CERTAIN STRATA.

What is the commercial application of the knowledge of the
principles concerning strata?

Whenever valuable minerals are confined wholly or
partly to a certain bed or beds, the ability to recognize and
trace that bed in different localities often leads to the
discovery of new mineral districts and of new mines.

General Relations of Stratified Ores.

How is it that minerals are sometimes preferentially associated

with a certain sedimentary bed or beds?

The association of a valuable mineral with certain sedi-
mentary beds may be either primary or subsequent. That
is, the mineral may have originally been precipitated in
bed form, along with the other strata, or it may have been
introduced into the beds long after their deposition.

Among valuable minerals known to be deposited
originally in bed form, we may cite, besides coal,
some gypsum deposits, salt beds and many other
non-metallic minerals. In this way metallic minerals are
also deposited in a greater or less state of concentration.
Iron and manganese are deposited both in bogs and in the
ocean depths; and in some rich muds copper in slight
amounts and, to a certain extent, even silver and gold are
precipitated. Many of these first named deposits afford
workable minerals just as they are deposited; others,
especially those of the less common metals, require further

ARRANGEMENT OF STRATIFIED ROCKS. 59

concentration by percolating waters, but in the end the ore
will still be confined to the parent bed or its neighborhood.
Ores introduced by circulating waters into strata, subse-
quent to their formation, often choose one bed in preference
to another in consequence of some chemical or physical
pecuUarity favorable to deposition: and thus the resultant
ore-body takes on a bedded form.

Example: In Piemonte, near Brosso, Italy, are beds of
specular iron (hematite) and pyrite regularly interstratified
with beds of limestone and mica-schist. Nearby, at
Traversella, are deposits of pyrite, magnetite and copper
P3rrite in dolomite. At both these places the deposits are
confined to the flanks of an intrusive quartz-bearing diorite,
and the chief ore-deposits are accompanied by garnet or
homstone (altered and hardened limestone), together with
other metamorphic rocks. The conclusion has hence been
reached, that the ores are due to the influence of the
intrusive rock; that first gases, and later hot springs, both
emanating from the diorite, attacked and mineraHzed the
intruded strata. The limestone strata were more strongly
affected than the mica-schists and were replaced by ores,
so that they now are represented by ore-beds intercalated
in the schists. Strong fractures permitted the gases and
water to penetrate far from the diorite, so that the mineral-
ization was extensive.*

Is it important to tell whether a bed of ore is primary or

secondary?

Such a distinction is important, for the reason that in the
first case the ore will invariably follow its regular bed,

* V. Novarese, BuU. Com. Geol. Ital. Vol. XXXII, pp. 75-93, 1901.

60 GEOLOGY APPLIED TO MINING.

while in the second we must be always expecting it to
deviate from it or to occur in other forms. This latter
caution must also be maintained in regard to those primary
bedded deposits which have undergone secondary concen-
tration by circulating waters. These waters, besides con-
centrating the ore within the parent bed, are likely to
carry it out and to form ore-deposits at a distance from it.

What chief points must one keep in mind in following an

ore-bearing stratum?

In any case one of the chief things is to be able to trace
and recognize the same bed in different places. Whether
the occurrence of ore in bedded form is primary or sec-
ondary, it must be associated with a fairly constant
character of the rock. If the deposit is original, the con-
ditions which brought about the deposition of the same
valuable mineral in various places must have given rise to
other uniform physical characters. If it is secondary
the physical or chemical character, which determined
the precipitation of ore along a certain bed, must be
present wherever we can reasonably look for a continuance
of that ore If a limestone bed containing replacement
deposits passes laterally into a sandstone, the ore may be
poor or wanting in the sandstone. If a shale bed, which by
its impermeability or its organic matter has determined the
deposition of ores in or near it, passes into a sandstone or
limestone, again we must look for a change, and, very
likely, the disappearance of this ore-horizon. For this
reason, in tracing an ore-horizon, physical and chemical
points are among the most valuable means of identification.

AKIUXOEMENT OF STRATIFIED ROCKS. 61

and where such points fail the chances are that the ore-
horizon fails too.

Do ore-bearing strata ever extend as such for long dUtafwesf

Often a certain bed may be traced by its physical char-
acters hundreds of miles, and is everywhere a valuable
indication of contained ore.

Example: In the Lake Superior district, the ore-deposits
are confined to a certain set of beds, easily recognizable
by their peculiar physical characters. These beds make
up the iron-bearing formation — they are sometimes slaty,
sometimes massive, generally dark-colored rock — ^and by
tracing them vast ore-deposits are continually found.
This point is brought out in the following quotation from
Mr. Oscar Rohn.*

"It may be well to recall that the iron ore-deposits of the
Lake Superior district always occur in certain character-
istic formations, called iron-bearing formations, which are
associated with a series of conglomerates, quartzites,

slates, and vein stones So well understood

and so generally recognized is the association of ore-deposits
with a characteristic rock formation that in all well-
directed prospecting the limits of the formation are sought,
and underground work confined to the area within these
limits."

How may the presence of a known ore-bearing straium be
recognized in a place where it does not outcrop?
The relation of an ore-bearing bed to other beds lying

♦ Engineering and Mining Journal, Vol. LXXVI, p. 616.

62 GEOLOGY APPLIED TO MINING.

above and below should be studied and borne in niind.
Often where the ore-bearing bed is not exposed at the
surface, or is covered with surface debris, the recognition
of a bed having a known relation to the ore-bed leads to
the discovery of the latter and its contained ore-deposits.

Preferential Association with Certain Geologic
Periods.

Are certain minerals preferentially associated with certain
geologic periods?

To a Umited and unreliable extent, even in the most
favorable cases: generally not at all. Formerly this
association was thought to be very important, however.

A famous and able geologist, Sir Roderick Murchison, as
late as 1851 and 1859, made the statement that the chief
sources of gold were in Paleozoic rocks, particularly in the
Lower Silurian, — an opinion largely based on a fragmentary
knowledge of Australian geology. Since then, in AustraUa,
gold has been worked in Carboniferous, Devonian and
Silurian rocks, and even in the Triassic and Jurassic; in
South Africa the gold is largel}' in Devonian strata.

However, when we come to consider the world's great
mining regions, we observe that it is indeed the older
strata which in many cases carry the ores. The older
rocks have had more time than the younger ones and
more opportunity to become the seat of ore-deposition;
they have also been, in most cases, deeply buried by the
younger strata and so brought under such physical con-
ditions of heat and pressure as are conducive to ore-con-

ARRANGEMENT OF STRATIFIED ROCI^S. 63

centration; under these conditions they have been
pierced by intrusive roeks and subjected to all the ac-
companying processes of alteration and concentration.

To what geologic period do metalliferous veins generally

belong?

Metalliferous veins seemed to have formed at every
period of the world's history. As shown in the first
chapter, they are the result of concentrating by agencies
which have been active since the earliest age down to
the present day. The age of veins, as actually proven,
is extremely various, ranging from the Archaean through
the Paleozoic, Mesozoic and Tertiary. Passing over the
cases of veins and other ore-bodies belonging to older
periods, we may mention that, in America, the rich silver
districts of Leadville and Aspen, Colorado, and Eureka,
Nevada, are of Tertiaiy age. Moreover, in the Monte Cristo
district, Washington, the present writer has reached the
conclusion that the veins are chiefly Quaternary (Pleisto-
cene). At Steamboat Springs, in Nevada, the actually
escaping waters have deposited silica along the fissures
which they traverse. This siUca often contains sulphide
of mercury, in some quantity, and traces of gold. Similar
phenomena have been noted in other places.

Example: Workable deposits of cinnabar and stibnite
(sulphides of mercury and antimony) at Monte Amianta
and other places in Tuscany, Italy, have formed subse-
quent to early Pleistocene volcanic eruptions (andesite
and trach3rte). The cinnabar is found in the volcanic rock
as well as the associated sediments, and has formed by

64 ^ GEOLOGY APPLIED TO MINING.

preference in limestone beds. The stibnite is closely
connected with the cinnabar, and is associated with arsenic
sulphide (realgar), pyrite and Hmonite, and sulphur.
Most of the ore-deposits are intimately connected with
sulphur springs or emanations of sulphuretted hydrogen,
which are accompanied by incnistations of sulphur, and
silicious sinter which sometimes contains cinnabar.*

In opened mines and in placers it has been found that
surface waters still precipitate all sorts of metals, even gold,
from solution.

Example: The formation of lead and zinc sulphides is
now taking place in the Missouri mines. An instance may
be cited where an old tunnel near Joplin, driven through
shales, became filled with water and was left so for ten or
twelve years. In 1898, when it was re-opened, the surface
of the shales, on the roof and sides of the tunnels, was
found to be thickly encrusted with minute crystals of
blende. In places, the blende was deposited on the pick-
marks made when the tunnel was run.f

Can any statement he made concerning the m^st favorable

geological age for ore-deposition?

The ore-deposits of earlier ages have been exposed, along
with the containing rocks, to destruction by erosion; and,
even where the containing rocks have not been removed,
circulating waters have very often attacked the ores and
transformed them wholly or partly into secondary or sub-
sequent deposits. On the other hand, ores of compara-

*B. Lotti, ZeiUchrift fOr praktiache Geologic, 1901, pp. 41-46.
t W. P. Jenney, Transactions American Inatitute Mining Engineers, Oct.,
1902, p. 26.

ARRANGEMENT OP STRATIFIED ROCKS. 65

lively recent date, such as those of the Tertiary, will, in
mountain regions, have undergone only the right degree of
erosion for laying them open to the eye of man. So it is
possible that the younger ore-deposits will be found to
preponderate over the older ones, among the important
districts actually exploited.

That is to say, — other things being equals important ore-
deposits are rather more likely to occur in rocks belonging to
the older geologic ages than in the younger ones; and their dale
of formation is more apt to belong to the younger geologic ages
than to the older ones.

What is in general the age of primary bedded deposits?

In river beds and on sea-beaches, gold-placers, tin-
placers, etc., are today being formed, as they have been in
past ages. Most of the world^s productive placers are
Quaternary, for these are easily found and have not been
destroyed by erosion, as is the case with many of the older
surface deposits. Yet in California, British Columbia, and
Australia, Tertiary gravels, especially when protected by
overflows of lava, have afforded immense wealth. Similar
deposits exist in various older formations. Some author-
ities consider the Witwatersrand gold-bearing conglomer-
ates, which occur amid a Devonian formation, as beach
placers of that period; and placers of Cambrian and even
pre-Cambrian age have been described.

At the present day, also, iron, manganese, gypsum, etc.,
are being chemically precipitated in bed form in the bottoms
of lakes, seas and oceans; and this process is known to have
been active during the past ages.

66 GEOLOGY APPLIED TO MINING.

Is it true that different geologic ages have no special charac-
teristics as regards mineral deposits? -

It seems to be the case that in the different geologic ages
the general conditions varied more or less uniformly; and
in some of these ages the conditions for forming certain
mineral deposits were better than in others.

Are coal and oil deposits confined to certain geologic periods?

At certain periods of the earth^s history vegetation
flourished very rankly and swamps were very abundant,
enabling the preservation of accumulated layers of vegetable
matter, which thus became a part of the strata of that
period, and, by consolidation and metamorphism, have
turned into lignite, soft or hard coal. The Carboniferous
period was favorable to this process, and much coal was
formed then. It was thought, indeed, at one time, that
coal was formed only in this period, but since then it has
been found in quantity in other formations. In Virginia,
there is good coal in Triassic strata. In the Western
United States, great quantities of coal are found in the
Cretaceous, and on the Western coast, especially in Alaska,
it is abundant in the Tertiary. In Alaska we can see at
the present day the first process of coal formation in the
great areas of swamp-peat, which is often many feet thick,
and shows the closest resemblance in habit to the late
Tertiary lignites of the same region. When these peat-
swamps shall have been covered up by later strata, and
consolidated and changed by pressure, they will become
coal-beds.

ARRANGEMENT OF STRATIFIED ROCKS. 67

Thus we see that we cannot confine the formation of
coal-beds to any one geologic period. Yet we may still
regard the Carboniferous rocks as especially likely to
contain such deposits, while we should hardly expect to
find good coal-seams in the Cambrian and Silurian.

These favorable conditions, during a certain period, were
not world wide. At one place was marshy land, at another
the ocean. In the Carboniferous, over eastern North
America, there was much land, lagoons and flourishing
vegetation; in the western United States there was gener-
ally deep sea. So in the eastern area we ^nd coal plenti-
fully in the Carboniferous, especially in certain beds,
which can sometimes be traced for hundreds of miles; while
in the western area the Carboniferous was not especially
a coal-bearing period. Conversely, in the Rocky Mountain
region the Cretaceous is the great coal-bearing horizon; but
this is not the case in the eastern United States.

Oil is another mineral for which the conditions of forma-
tion have been more favorable in certain periods than in
others. Abundance of organic life during these periods
seems to have been the favoring circumstance, for the
organic matter has by its decomposition formed the oil.

Are there minerals other than coal and oil which were depos-
ited more abundantly in certain periodsf

In some periods, more than in others, there appear to
have been large areas of shallow evaporating seas, from
which certain mineral substances were precipitated. Thus,
both in the old world and in the new, Permian strata (the

68 GEOLOGY APPLIED TO MINING.

youngest part of the Carboniferous age) contain, in many
places, deposits of salt, gypsum, etc.

Is a knowledge of fossils of valine in identifying the same ore-
bearing straium in different districts?
Besides the identification of an ore-bed by its physical
characteristics, it is frequently possible to recognize it by
means of its contained fossils. The fossils are the only safe
evidence of identity in age, when the districts to be com-
pared lie some* distance apart. Beds having the same
physical appearance may, and doj occur in many ages,
while perhaps only in one did there exist the finer condi-
tions which were favorable to the production of deposits of
valuable minerals.

Example: Dr. Le Neve Foster relates that a French
inspector of mines. M. Meugy, hearing of the discovery of
phosphate of lime in a certain part of the Cretaceous in
England, and knowing from fossil evidence that beds of the
same age existed in France, concluded that the French
beds might also contain phosphate deposits — ^a conclusion
which was amply verified by prospecting.

Preferential Association with Certain Kinds of
Sedimentary Rocks.

Are certain kinds of minerals preferentially associated with

certain kinds of sedimentary rocksf

Certain kinds of rocks are sometimes preferred by certain
minerals for deposition; but there is no regular rule. The
association may be either primary or secondary.

arrangement op stratified rocks. 69

Contemporaneous Deposition of Ores and Strata.

In what cases is such association primary f

In coarse sediments which are evidently shallow water
or shore-formations, such as coarse impure sandstones,
conglomerates and clays, mixed with vegetable material
and plant remains, we may suspect coal or oil, or natural
gas.

Example: The petroleum-bearing strata of all periods
and of all parte of the world show, according to Dr. Rudolf
Zuber,* a remarkable resemblance in their formation and
composition. Everywhere they are bituminous clay-
shales, and variegated, mostly bright-colored, clays,
interstratified with sandstones and conglomerates. Lime-
stones, which may also occur in such series, contain tarry
materials, but rarely true petroleum.

There is a surprising resemblance between the red and
green clays and shales of an American Paleozoic oil-bearing
formation, with oil-bearing formations in the Jurassic of
Germany, and in the Eocene of Galicia. The oil-bearing
strata of the Upper Triassic in the western part of the
Argentine Republic are like the middle Tertiary (Oligocene)
oil-bearing rocks of the Carpathian Mountains; the green
oil-bearing shales, lying between the Jurassic and the
Cretaceous, in the northern part of the Argentine RepubliG,
cannot be distinguished from the Gahcian Eocene strata.
The oil-bearing strata at Baku in the Caucasus are identical
in appearance with the Carpathian OUgocene shales and
sandstones.

*Zeitschrift far praktische Geologic, March, 1898, p. 85.

70 GEOLOGY APPLIED TO MINING.

Deposits of salt, gypsum, etc., may be looked for espe-
cially in connection with red sandstones. This association
has been explained by the fact that when sea-water evapo-
rates, the first precipitate is oxide of iron (which gives the
red color to the rocks) ; this is followed by gypsum and then
by salt.

Can one admit such an associaiion for metallic mineralsf

It is as yet an open question as to whether copper may
not also be admitted to the same association. In Germany
the Mansfeld Kupferschiefer (copper-slate) is a thin bed of
bituminous shale lying between two thick deposits of
Permian sandstone. The ore contains, besides copper,
silver, lead, zinc, antimony, mercury, nickel and
cobalt. This Kupferschiefer outcrops over a large district
and frequently there is copper either in it or in the other
beds of the Permian. At St. Avoid and Wallerfangen,
(also in Germany), copper ores which occur in Triassic
sandstone, have been supposed to be contemporaneous
with the enclosing rock. In Utah, in the Silver Reef
district, red and gray Triassic sandstones and shales contain
bedded copper-silver deposits. In the Nacimiento mount-
ains in New Mexico copper ores occur in Triassic sandstones,
associated with plant remains. Similar deposits, although
without the organic remains, have been described by Prof,
W. P. Blake as occurring in Arizona. Prof. James F.
Kemp gives many examples of copper ores in Triassic and
Permian sandstones. ''Copper ores," he says, "are very
common throughout the estuary Triassic rocks of the

ARRANGEMENT OP STRATIFIED ROCKS. 71

Atlantic coast."* In the Permian of northern central
Texas there are three separate copper-bearing zones,
forming three Hnes of outcrop that extend in a general
northeasterly direction over a range of about three counties.

May not these bedded deposits of metallic minerals have been
introduced into the beds subsequent to their deposition?
Some observers have reasoned that the above deposits
were deposited contemporaneously with the other strata;
while others, finding evidence of the concentration of the
ores especially along fault-planes and other water-channels,
have beheved that the metals were introduced from
extraneous sources through these channels, and are only in
bed form because the layers of organic material served to
precipitate the metals in the ascending waters. The present
writer believes that the theory of original precipitation
contemporaneously with the strata has strong features to
recommend it, even though he regards a later concentration
by circulating waters as proven in many cases.

// these bedded deposits are even in part original^ where did
the metals come from, and how were they precipitated?

Organic matter is known to be a powerful precipitant of
metals from solution. Metals are known to exist in sea-
water, even gold. According to Phillips,t the waters of
the Mediterranean contain one centigram of copper to the
cubic meter. The same writer remarks that the "black
and usually very sulphurous matter deposited in basins

* 'Ore Deposits of the United States and Canada.' Fourth Edition, p. 223.
t 'Ore Deposits,' Second Edition, p. 132.

72 GEOLOGY APPLIED TO MINING.

where sea-water has been left to itself constantly contains
copper, and the same is generally true with regard to the
dark-colored gypseous muds of all ages." Luther Wagoner*
found that mud from San Francisco Bay contained gold
and silver; and that samples rich in organic matter con-
tained more than at other places. He concluded that
organic matter in mud reduces some silver from the sea-
water, and probably some gold.

Example: In the Sierra Oscura,- New Mexico, are red
sandstones and shales of probably Permian age. Some of
these red sandstone beds contain copper for an extent of a
number of miles. In a certain portion of the region there
are at least three distinct copper-bearing sandstones, in
which the ore is chalcocite and copper carbonate, dissem-
inated in minute grains. No dikes or igneous rocks of any
kind have been found associated with the copper-reefs or
the enclosing beds. They do not occupy lines of faulting;
and, indeed, were certainly formed before the main faults
of the district. The mode of occurrence of the ore in
regular beds, in part replacing plant remains, suggests that
the copper was deposited from the waters which deposited
the enclosing sediments. f

How can one explain the hypothesis that certain geologic
periods were more favorable for this process than others?
It may well be that during periods like the Permian and
Triassic, where the presence of great land-locked evapo-
rating shallow seas is shown by beds of gypsum, salt, etc.,
with impure red sandstones, the precipitation of metals in

* Transactions American Institute Mining Engineers, Vol. XXXI p. 807.
t H. W. Turner, Transactions American Institute Mining Engineers, Oct.,
1902.

ARRANGEMENT OF STRATIFIED ROCKS. 73

the muds was greater than at other times. In the first
place, the evat)oration of the sea-water would concentrate
the metals, along with the other substances in solution; in
the second place, the shore-line conditions would furnish
beds rich in organic matter, for the reduction and precipita-
tion of these metals.

Selection of Favorable Strata for the Subsequent
Deposition of Ores.

Subsequent bedded deposits, where the minerals were
introduced after the formation of the rocks, show frequently
a preference for one kind of rock.

Do sandstones especially attract the precipitation of ores?

Porous sandstones afford channels for waters, and many
tiny cavities which may be filled with precipitated minerals;
hence they are often selected for ore-deposition in prefer-
ence to adjoining strata.

Do schists especially attract the deposition of ores?

Schists are often selected for impregnation with valuable
minerals for the same reasons as sandstones; moreover,
shales, slates and schists containing organic matter often
act as precipitants to ore-bearing solutions, and hence
become the seat of ore-deposition in preference to other
beds.

Example: A small basin of coal-shales, near Belleville,
Jasper county, Missouri, carries beautifully preserved fossil
plants. The outer surface of the mass of shales, for a

74 GEOLOGY APPLIED TO MINING.

depth of about a foot, contains scattered crystals of blende,
with some galena and pyrite. The central portion does not
carry any mineral, the mass having been mineralized from
the outside, toward the interior, so far only as the mineral-
bearing waters could penetrate the dense plastic clay. The
crystals in their growth, have distorted otherwise perfect
fossil plants, giving evidence that the deposition of the
minerals was of later date than the embedding of the plant
remains.*

How does a stratum impervious to water cau^e the precipttOr-

tion of ores in bedded formf

A porous stratum may absorb the waters coming to it
from transverse fissures or other channels, may spread
these waters out, make their circulation sluggish, and so
induce the precipitation of ores. An impermeable stratum,
such as a bed of shale, may refuse all passage to solutions,
and compel them to slacken and spread out on its upper
or \mder side (according to whether the waters are ascend-
ing or descending), and likewise bring about precipitation.

Example: In a series of interbedded sandstones and
shales at Rico, Colorado, occurs a bed relatively impervious
to water— the so-called ''blanket'* (Fig. 4). In the case of
the Enterprise mine, the blanket seems to have been
formed from the fine residue left from the dissolution of a
gypsum stratum, together with a breccia made by the
collapse of the overlying beds, as the gypsum disappeared.
The rocks have been seamed by nearly vertical fissures,
and mineralizing solutions rising along these have been

* W. p. Jenney, Tranaaction» American Institute Mining Enginetra, Ooi«
1902. p. 25.

ARRA.NGBMBNT OP STRATIFIED ROCKS.

checked by the blanket, and have deposited their ores in it,
on its under side. The ore-deposits, therefore, have a
constant relation to this particular bed, but the association
is a subsequent one, the ores having been introduced after
the deposition of the sediments.

Fig. 4. Longitudinal section through the Group tunnel, Enterprise mine, Rico,
Colo. After F. L. Ransome.

Why may ores he deposited in a rigid stratum in preference to

soft strata?

A certain stratum may, on account of its rigidity, be
favorable to the formation of open fractures, which cease in
neighboring soft beds. Hence veins formed along such
fractures may be confined chiefly to the more rigid strata.

Why is it that svhsequent ore-deposits frequently show a
prefererice for limestones over associated rocks?
limestone lends itself readily to the process of cavity-
making by the dissolving action of waters, more than any
other rock, and these cavities are sometimes filled up by
minerals. Yet true cave deposits are probably much rarer
than they were formerly thought to be. At Eureka, in

76 GEOLOGY APPLIED TO MINING.

Nevada, rich ores occur in caves, but it seems that the
caves have been formed by the shrinkage of ore-bodies,
attendant upon their alteration.

Limestone is easily replaced by metallic minerals, and
for this reason is frequently chosen above other rocks, by
circulating waters, as a locus for the precipitation of the
metals they carry. Silver-lead deposits, especially, prefer
Umestone. Iron deposits also frequently choose this rock.

Example: The superior suitability of limestones over
other rocks for replacement deposits, particularly of lead,
is shown in the case of the Derbyshire lead mines in
England. In this district there occur, in limestone,
intrusive sheets of igneous rock. Fractures traverse both
limestone and intrusive rock, and these fractures, together
with the joints and the bedding planes of the limestones,
have evidently furnished the channels along which the
ore-bearing solutions have circulated. The galena which
constitutes the ore, however, has been deposited only in
the Umestone, and in the intrusive rock the fractures are
barren. (Fig. 5.)

Fig. 5. Lead deposits, Derbyshire, England. After De La Beche. a.=lirae-
stone; b.=igneous rock ; c.=ore.

How may the chemical pecidiarities of certain strata induce
the precipitation of ores in them?
Besides the chemical suitability of limestone for replace-

ARRANGEMENT OP STRATIFIED ROCKS. 77

ment, and the precipitating action of beds containing
organic matter, other peculiarities of certain stratified
rocks may induce ore-deposition.

In shale beds there is always a considerable percentage of
iron. This usually combines with sulphur contained in
organic matter to form sulphide of iron (pyrite). Pyrite
has been shown by experiment to be active in causing other
metallic minerals, especially gold, to precipitate.

Example: At Ballarat, in Australia, there are certain
thin beds of slate containing pyrite. These are called
"indicators,"* for, whenever the auriferous quartz veins
of the district intersect them, the veins become uniformly
rich, even though in the remainder of their course they will
not repay working. The recognition and tracing out of
these indicator beds, therefore, becomes, perhaps, the
most important geologic work for mining men in these
districts. Even when the beds have been bent and broken
by earth movements their effect upon traversing lodes is
the same.

In surrif then, what sedimentary rocks are most likely to become
the site of ore-deposition from circvlating waters?
Among the stratified rocks, now one, now another, may
by its peculiarities induce its selection in preference to
adjoining strata, as a site of ore-deposition. The problem
admits of no empirical solution, but in each individual case
a comparison of the physical and chemical features of each
bed present may aid in deciding which is most favorable,

♦ T. A. Rickard. 'The Indicator Vein, Ballarat, Australia.' Tranaactioiu
American Institute Mining Engineers, Vol. XXX, pp. 1004-1019.

78 GEOLOGY APPLIED TO MINING.

and in discovering the main mineral belt. Shale beds con-
taining organic matter (especially if the beds be limy and
so easily adapted to the process of replacement) offer per-
haps the most favorable situation for ores. Next in
order comes limestones, while conglomerates are in general
not so favorable. Quartzites are the least favorable of
all, although even here ore-bodies are not infrequent.

CHAPTER IIL

THE STUDY OF IGNEOUS ROCKS AS APPLIED TO
MINING.

PHYSICAL CHARACTERS OF IGNEOUS ROCKS.

WhxU is an igneous rockf

A rock is an aggregate of minerals.

An igneous rock is one that has cooled from a molten
condition.

How can one tell if a rock is igneous or not?

An igneous rock bears marks indicating its origin, just
as the stratified and sedimentary rocks do. In the first
places since igneous rocks were not deposited in successive
layers in water, there is no true stratification. ♦

Do igneous rocks never have a handed structure?

In the case of lavas, when one flow after the other is
poured out, the accumulated rocks will be definitely lay-
ered, for between each flow there may be fragmental broken
material. Each flow also may, perhaps, differ sUghtly in
texture and composition from the earlier and the later
flows, and within itself the top and bottom will generally
be finer grained than the center, on account of having

80 GEOLOGY APPLIED TO MINING.

cooled more quickly. When an igneous rock flows in a
viscous, only incompletely molten condition, as it frequently
does, it may be drawn out into long bands differing more or
less from one another in structure, texture and compo-
sition. These bands may be a fraction of an inch or many
feet in thickness. This structure is flow-banding.

Does the crystalline structure afford a test for igneous rocks?

The structure of the igneous rock,when closely examined,
is the best test of its origin. In the first place, it is usually
crystalline. On hardening from a molten or otherwise
fluid condition solid substances tend to assume the beautiful
and often highly complicated geometric forms which are
peculiar to them. In the molten state all the elements are
intimately mingled — as consolidation commences they
begin to group themselves together according to their
affinities, and to form certain combinations which we call
minerals, each having a crystal form mathematically
distinct from that of unlike minerals.

Are all igneous rocks thus made up of crystalline minerals?

If the consolidation is extremely sudden, this crystal-
hzation has not time to take place (at least we cannoc
detect it, even with a microscope), or is shown only by
beautiful groupings of the rock materials, with no definite
separation into minerals. The rock, thus quickly hardened^
is termed volcanic glass or obsidian.

Example: Obsidian cliff, in the Yellowstone National
Park, rises in nearly vertical walls for 150 or 200 feet. It

IGNEOUS ROCKS. 81

has been formed as a surface flow, and is a natural glass,
the result of rapid cooling of a fused mass of rhyolitic lava.
This glass covers an area of about 10 square miles.*

Is this glassy condition of igneous rocks usual?

Nearly always the cooling is slow enough, even in the
case of lavas suddenly poured out, to allow at least the
beginning of crystallization. If the period of cooling is
relatively short, the crystals will be small in size, some-
times only perceptible with a microscope; if it is slower,
the crystals grow till they are very easily visible to the
naked eye.

How does one study an igneous rock, to distinguish it from

sedimentary and metamorphic onesf

When oniB observes such a rock, he observes its compact
texture (one crystal fitting into another without loss of
space); he notes the straight sharp geometric outlines of
some of the crystals (whose forms have not been interfered
with by intergrowths with neighboring crystals), in
distinction to the rounded or broken outhnes of the grains
of the sedimentary rock. He observes, also, that in
general the more prominent crystals do not have any
common direction of elongation; that is, they are not
parallel with one another, as are usually the crystals of
metamorphic rocks, but lie in all possible positions.

Can one always readily distinguish igneous rocks by this testf
There are exceptions to this rule, and in the field the

* Arnold Hague, 'Geological Excursion to the Rocky Mountains,' p. 350.

82 GEOLOGY APPIJED TO MINING.

observer will meet difficult cases. In some hardened rocks
where the individual grains arc not visible to the naked eye,
it may be hard to decide as to the origin, without a micro-
scope. There are, also, cases of igneous rocks where there
is a general parallelism of the crystals, though never on so
complete a scale as in metamorphic rocks. In the cases
referred to, the crystals, originally not parallel, have been
so arranged by flowage.

THE DIFFERENT KINDS OF IGNEOUS ROCKS.

How are igneous rocks classified?

Igneous rocks have been named in various ways, accord-
ing to various classifications. All these classifications are
more or less artificial, and each worker must select the one
which serves his purpose best. Various features have
served as main distinctions — chemical composition, mineral
composition, form, structure, geologic age, mode of occur-
rence, locality, etc. For one purpose a chemical classifi-
cation may be best, for another a classification as to mode
of occurrence, etc. The commonly accepted classification
is based partly on mineral composition, partly on chemical
composition, and partly on structure.

How are the different kinds of igneous rocks studied and

identified?

Since the adaptation of the microscope to petrographic
work, the science has been revolutionized; and now all the
real study is done with that instrument. As a result of the
detailed investigation thus made possible, the distinctions

ignUous rocks. 83

have in many cases been finely drawn, and the number of
rock names has rapidly multiplied.

Is it an easy thing to name an igneoiLS rock correctly in the

field?

Petrologists very commonly have difficulty in defining
a given igneous rock, without resource to their microscope.
To take lava-rocks, for example, one cannot always decide
without the microscope between many varieties of rhyolite,
phonolite, trachyte and andesite. The same is true of the
dike rocks, which have received a great number of special
names, now falling into disuse. In coarse-grained rocks,
the petrologist can be more certain, yet it sometimes
becomes difficult in the field to distinguish certain kinds of
granites, syenites and diorites from one another, and to
decide between some diorites and diabases.

CLASSIFICATION OF IGNEOUS ROCKS FOR MINING

MEN.

To what extent is it possible and necessary for the miner to

classify igneous rocks?

When such is the case, it is plain that the fine distinctions
of rock species are beyond the mining engineer and the
miner. Luckily, such distinctions, even could he make
them, would be of slight use to him. But broad demarca-
tions are necessary, and may be made upon physical
characteristics, without the microscope and chemical
analysis, and with only a slight knowledge of mineralogy.

84 GEOLOGY APPLIED TO MINING.

On what principles is such a practical classification based?

The classification the writer offers for this purpose is
based on: (1) Structure. (2) Mineralogical composition.
Rocks are first divided into granular, coarse porphyritic,
fine porphyritic, and glassy forms.

What is meant by a granular igneous rock?

The term granular is applied to a fairly even texture, the
constituent minerals being of nearly uniform size, and
generally interlocking.

What is meant by a porphyritic igneous rock?

Porphyritic rocks do not have their constituent minerals
of uniform size. There is a fine grained portion, which
may be dense and show no crystals to the naked eye, or
may sometimes be non-crystalline, like glass. This is the
groundmass. Through it are sprinkled crystals of larger
size, generally with perfect geometric outline, and often
separated from one another, so as to be completely sur-
rounded by the groundmass. These are porphyritic
crystals or phenocrysts, and the rock possesses porphyritic
structure.

In the coarse porphyritic structure, the groundmass is
crystalline, the individual minerals in it are usually visible
to the naked eye, or by the aid of a hand-lens, and the
porphyritic crystals are correspondingly large. In the
fine, porphyritic structure, the groundmass is fine, the
individual grains being difficultly or not at all discernible
to the naked eye; or it may be glassy.

IGNEOUS ROCKS. 85

What is meant by a glassy igneous rockf

Glassy rocks have no or few porphyritic crystals; neither
do they show any grains, even under the microscope — they
are smooth and homogeneous, like glass.

What is the relative abundance of these different kinds of

rockf

Granular rocks, coarse porphyritic and fine porphyritic
rocks are common; wholly glassy rocks are relatively
rare, and are found chiefl.y among the outpourings of
volcanoes.

Classification of Igneous Rocks.

A. Granular Rocks . Relatively coarse ; crystals of con-
stituent minerals easily visible to the naked eye, and all
of about the same size.

1. Granitic Rocks, Color gray, reddish or greenish.
Relatively light in color and weight. Quartz abundant,
while dark minerals (hornblende, mica, pyroxene, etc.)
form only a small portion of the rock. Mica apt to be
more abundant than in other granular rocks. Forms of
mineral grains in general short and blunt. Chief con-
stituent minerals, quartz, feldspar, mica, hornblende.

2. Dioritic Rocks, Of medium dark color and medium
weight; mottled, generally green, rocks. Quartz scarce
or absent; dark minerals (especially hornblende) fairly
abundant. Mica may be present, but is generally less in
amount than other dark minerals. Pyroxene may
occur. Grains of individual minerals have a tendency to
elongated forms, though they may be short. Constit-
uent minerals, feldspar, hornblende, mica.

3. Diahasic Rocks, Very dark and heavy, green of

86 GEOLOGY APPLIED TO MINING.

various shades, often black. No quartz, and very large
proportion of dark minerals. Mica almost always absent.
Pyroxene is usually very abundant, and there is often
olivine and hornblende. Magnetite in small grains (us-
ually invisible to the naked eye) is neariy always present.
Crystal forms generally elongated. Chief constituent min-
erals, feldspar, pyroxene, olivine.

4. PeridotUic Rocks. Color, very dark green or black,
darker and heavier than any of the forgoing. Are dis-
tinguished by the absence of feldspar. Often contain con-
siderable quantities of the metallic minerals (such as
magnetite, pyrrhotite, ilmenite, etc.) in small grains.
Chief constituent minerals, olivine, pyroxene, and horn-
blende. Any one of these, or any two, may in some cases
be entirely lacking, leaving the rock composed essentially
of two of the above-named minerals, or even one.

B Coarse Porphyritic Rocks. Are spotted with
well-formed crystals of the common rock-forming min-
erals, quartz, feldspar, mica, hornblende, pyroxene, etc.,
which are contained in a groundmass composed of
interlocking crystals of markedly smaller size than the por-
phyritic crystals.

1. Granitic Porphyry, Combines the coarse porphyritic
structure with the same physical and mineralogical char-
acters as the granitic rocks, as defined above. Chief
constituent minerals, quartz, feldspar, mica, hornblende.

2. Dioritic Porphyry. Combines the coarse porphyritic
structure with the same physical and mineralogical char-
acters as the dioritic rocks, as described above. Chief
constituent minerals, feldspar, hornblende, mica.

3. Diabasic Porphyry. Combines the coarse porphjni-
tic structure with the same physical and mineralogical
characters as diabasic rocks, as described above. Qiief
constituent minerals, feldspar, pyroxene, olivine.

IGNEOUS KOCKS. 87

C. Fine Pobphyritic Rocks. Like the coarse por-
phyritic rocks, but the groundmass is finer, so that the
individual crystalline grains in it are barely or not at
all visible to the naked eye.

1. Rhyolitic Rocks, These are chemically and miner-
alogically the same as the granitic rocks and the granitic
porphyry rocks, but differ in having the fine porphyritic
structure. Rhyolitic rocks are generally of light color
(white, light gray, pink, red, etc.) and of relatively light
weight. As porphyritic crystals they generally show
quartz, hexagonal in cross-section, and frequently short,
blunt feldspar. Crystals of dark mica are usual, and
often also hornblende; but the amount of dark minerals
is relatively small. The groundmass is generally rather
rough to the touch, and looks and feels somewhat like
broken coarse earthenware; the individual grains in it
are usually not distinguishable. Chief constituent min-
erals, quartz, feldspar, mica, hornblende.

2. Andesitic Rocks, These are chemically and miner-
alogically the same as the dioritic rocks and the dioritic
porphyry rocks, but differ in having the fine porphyritic
structure. In color the andesitic rocks are dark gray,
medium brown, dark red, etc. They are of medium weight.
Quartz is usually not found as porphyritic crystals, and
mica is not as common as in rhyolitic rocks. The por-
phyritic crystals are most apt to be feldspar and horn-
blende, often pyroxene. Dark minerals in general are
rather abundant. Groundmass generally slightly coarser
than with the rhyolitic rocks; the individual grains,
though they may be tiny, are often visible either to the
naked eye or through a hand-lens. Chief constituent
minerals, feldspar, hornblende, pyroxene, mica.

3. Basaltic Rocks. These are chemically and miner-
alogically the same as the diabasic rocks, and the diabasic

88 GEOLOGY APPLIED TO MINING.

porphyry rocks, but differ from them in having the fine
porphyritic structure. The porphyritic crystals are gen-
erally few, and do not differ so markedly in size from
the groundmass crystals as in the rhyolitic rocks and the
andesitic rocks. The groundmass is generally coarser
than in the andesitic and rhyolitic rocks; the individual
grains in it, though fine, can often be seen by the naked
eye. If they cannot, there are very likely no porphyri-
tic crystals to be seen. Basalts contain as a rule no
quartz or mica. They are usually black in color and
heavy. Where minerals can be distinguished in them,
they are usually pyroxene, feldspar, or olivine. Chief
constituent minerals, feldspar, pyroxene, olivine.

Additional Definitions.

Does this classification embrace all the rock names necessary

to a miner?

In the writer^s opinion, this is about as far as one who is
not a petrologist can safely go. There are, however, other
rock names frequently used by miners, on which observa-
tions will be made.

What is quartz porphyry?

This is familiar to mining men as one of the most impor-
tant rocks in TiCadville and other mining regions. The
name, formerly in good use by geologists, is being dropped
for granite porphyry or rhyolite porj)hyry, the former for
the coarse grained, the latter for the finer grained varieties.
The description of granite porphyry is that of quartz por-
phyry.

IGNEOUS KOCKS. 89

What 18' syenite?

This is a favorite term with miners. A syenite is a
granite without quartz. Like granite, it has a light color
and relatively light weight, contains relatively small
amounts of the dark-colored minerals, and is apt to con-
tain mica. It is not always easy to distinguish syenite
from diorite in hand specimens. Syenites are compara-
tively rare rocks.

What is trachyte?

Trachyte was formerly a much-used term with geologists.
In the Great Basin of Nevada, for example, enormous
quantities of volcanic rocks were classified as trachyte.
Now microscopic study has shown them to be mainly ande-
sites, and that none of them are trachytes. Trachyte is still
a popular term with miners, but now we know it to be a
comparatively rare rock. True trachyte bears the same
relation to syenite as rhyolite does to granite; it has the
chemical and mineralogical composition of syenite, but
with the fine porphyritic structure. It is, therefore, a
rhyolite without quartz.

What is phonolite?

Since this rock occurs in connection with the famous
gold ores of Cripple Creek, in Colorado, it has become well
known in the mining world. It is often difficult to identify
phonolite without microscopical or chemical tests. Phono-
lite contains, besides feldspar, nepheline, leucite, or both,
and pyroxene, sometimes hornblende. The colors are

90 GEOLOGY APPLIED TO MINING.

usually gray or green. Phonolites are also relatively rare
rocks.

It is popularly supposed that the metallic ring emitted
by fragments of some volcanic rocks is a test for phonolite;
but this is an antiquated idea. Rhyolites and other rocks
frequently give this ring.

What is amygdaloid?

Amygdaloid is a term applied to lavas which are cellular,
that is, are full of little holes or amygdules, which were
filled by steam at the time of consolidation. In the Lake
Superior region certain amygdaloidal basalts have their
amygdules filled with native copper, and so become ores
and of considerable interest to the miner.

What is doleritef

Dolerite is a term that has been used, sometimes instead
of diabase, sometimes instead of basalt.

What is gahbrof

The term gabbro is applied to certain granular rocks con-
sisting chiefly of feldspar and pyroxene. It thus falls
within the group of diabasic rocks, in the foregoing classi-
fication.

What is felsite?

Felsite is a general term applied to certain light-colored,
very fine-grained igneous rocks, chiefly altered rhyolites.
Strictly speaking, felsite is hardly an accurate term, and
most rocks so called may be proved to be rhyolite or

IGNEOUS BOCKS. 91

rhyolite porphjny. In felsites the porphyritic crystals are
small and few, or have become inconspicuous on account
of decomposition, and so are not visible to the naked eye.
The term is an allowable one, and, on account of its broad
definition, not difficult of application.

Example: Study of the felsites of Carodoc, Wales, show
microscopic structures which had been altered almost
beyond recognition, but which indicate that these rocks
were originally partly rhyolites and partly sedimentary
beds derived from the erosion of rhyoUtes (rhyoUte tuffs).*

What is greenstone?

Greenstone is a general name applied to certain igneous
rocks, generally rather fine-grained, of a general dark
green color. The term is used by geologists, especially
when no more accurate definition is possible in the field.
The greenstones are usually old rocks geologically, and the
green color is the result of thorough alteration. They are
diabases or diorite^, sometimes old andesites and basalts.
The term is admissible and of easy application.

Example: In northeastern Minnesota, on the eastern
part of the Mesabi iron range, are rocks which have been
called greenstones because of their general dark greenish
color. They are crystalline rocks composed usually of
hornblende and feldspar. Mineralogically, the rocks are
diorites, but they have been recrystallized from other
rocks, some of which were certainly diabases and andesites
and some probably diorites or gabbros.f

* F. Rutley, Quarterly Journal, Geological Society, Vol. XLVII, p. 512.
f U. S. Grant, 'Engineer's Year Book,' University of Minnesota, 1898, p. 54.

92 GEOLOGY APPLIED TO MINING.

What is pegmatite or giant granite?

Pegmatite or giant granite is a name applied to those
common dikes, generally granitic, where the grain is ex-
ceedingly coarse, individual crystals being frequently
several inches across.

What is serpentine rock?

This is a rock consisting partly or wholly of the dark-
green, greasy-feeling mineral serpentine. It is a metamor-
phic or altered rock, and in many cases is derived from
igneous, chiefly peridotitic rocks. The decomposition of
the olivine and pyroxene of peridotites usually affords
serpentine.

What is trap?

Trap is an old general name for dense, dark-colored dike
rocks. The term is still in use. A trap dike, more accu-
rately considered, may be made up of andesite, basalt,
diorite or diabase, etc.

What is breccia? *

Breccia is an Italian word, applied to crushed and broken,
yet still consolidated rock. A breccia resembles a con-
glomerate in being composed of coarse fragments packed
together; but in the former the pieces are sharp and angular,
in the latter rounded by water action, indicating their
origin. It is generally easy to recognize a breccia, but
sometimes it is difficult to tell how it originated. Where
there has been movement in a rock, as along the vicinity of

♦ Pronounced br^k-she-a>i, with accent on the first syllable.

IGNEOUS ROCKS. 93

a fault, a breccia is developed, called a friction breccia.
Lava, on the other hand, is often shattered by explosion
attending its eruption or by being forced into renewed
movement when partially hardened. The result is a
volcanic breccia.

What is pumicef

Pumice is a glassy lava, which at the time of hardening
was so full of steam-filled cavities that it now has a spongy
structure, and is so light that it often floats: it is a sort of
lava froth.

TRANSITIONS BETWEEN DIFFERENT KINDS OF
IGNEOUS ROCKS.

Are the divisions of igneous rocks, as given, sharply divided
from one another in point of mineralogical composition?
Igneous rocks form a connected series, with gradual
transitions from one of the artificial divisions above out-
lined to the other. This is the case in any classification.
No matter how many divisions are made, some rocks
will be found occupying the border lines. So it is very
possible, for example, to classify a rock in the field as a
diorite. which closer study would show to be a diabase.
The suffix "ic," as diorit-ic, in the foregoing scheme, ex-
presses a provisional determination, and saves one from
the charge of making hasty and faulty decisions.

Are the different igneous rocks separated sharply in point of
texture?
In point of texture there are all transitions between

94 GEOLOGY APPLIED TO MINING.

granitic rocks, granitic porphyry rocks, and rhyolitic rocks;
also between dioritic rocks, dioritic porphyry rocks and
andesitic rocks; and, again, between diabasic rocks,
diabasic poq>hyry rocks and basaltic rocks; in short,
between granular rocks, coarse porphyritic rocks, and fine
porphyritic rocks.

How did these different textures originatef

The difference in general seems to depend on the rapidity
with which the rock cooled, those which cooled more
slowly having had more time to crystallize, and hence pro-
ducing larger crystals and coarser rock texture. The
rapidity of cooUng depends in large part on proximity, to
the surface. Those which cool at the surface chill quickly;
while those that harden deep in the earth's crust retain
their heat for a long time. Therefore it is that the lavas
or surface rocks are almost wholly of fine porphyritic
structure. Small masses of molten rock thrust into older
harder rocks and there cooling (dikes), generally have the
coarse porphyritic structure, though often the fine porphy-
ritic. Large masses of rock cooling at a distance from the
earth's surface have generally the granular structure.
Cases may occur where a rock mass may be fine porphyritic
on the edges, where it cooled quicklj'-, coarse porphyritic
further in, and granular in the center, but in general rocks
are more or less homogeneous.

Are the different igneous rocks sharply separated in point of
compositionf
Similarly, a single rock mass may vary in composition, so

IGNEOUS ROCKS. 95

that, for example, it is a^diabase on the borders and a
diorite in the center; yet usually the different types of rock
are distinct in the field.

Example: The exceptional occurrence of different rock-
types as part of a single dike is shown at a locality in
Michigan, near Crystal Falls. Here, there has been
observed a dike, four feet wide, which cuts a mass of
gabbro. Near the center of the dike the rock is a granite,
containing biotite, while the sides consist of diorite without
any quartz. The two rocks are supposed to have separated
one from another while the mass was in a molten state.*

FORMS OF IGNEOUS ROCKS.

In what different forms do masses of igneous rocks occur?

The forms in which igneous rocks occur may be outlined
thus:

1. Fundamental.

2. Intrusive (Masses, dikes, sills or sheets.)

3. Extrusive. (Lavas.)

What are the fundamental igneous rocks and what is their

origin?

The fundamental igneous rocks underlie the oldest
stratified rocks. They are mostly Archsean granites and
form the floor on which the first recognizable sediments
were laid down. Many of them may be metamorphosed
and crystallized early sediments.

♦ J. M. Clements, Journal Geology, Vol. IV, No. 4, p. 377.

96 GEOLOGY APPLIED TO MINING.

What arc intrusive rocksf ^

111 the molten rocks, which exist below or within the
crust of the earth, important movements and migrations
occur. The plastic materia! is propelled upward by the
steam which it contains and by other causes, and so forces
its way into and through the hard rocks nearer the surface.
It enters these rocks along the line of least resistance —
along fissures, fault-planes, joints, crushed zones, or
bedding planes.

What is an intrusive mass?

The molten rock may come up in large volume, thrusting
aside or absorbing the rocks it enters (the intruded rock),
and thus forming a mass of irregular shape.

What is a dike of igneous rock?

Where the molten rock ascends along fissures or other
similar channels, it will form a zone, or a body with very
slight thickness as compared with its extent in other
directions. In form, a dike has the same characters as a
vein. Its boundaries are generally straight planes, and it
may dip away from the horizontal at all possible angles. A
dike may be an inch or a mile across, but it is usually
vastly longer than it is thick. (Fig. 6.) From a great
mass of intrusive rock there are usually smaller dikes
which run out into the surrounding formations.

What are sheets or sills of intrusive rock?

In a sedimentary rock, when the dikes run along the
bedding planes, and so are parallel, or nearly so, to the

lONBOUS ROCKS.

98 GEOLOGY APPLIED TO MINING.

stratification, they are called sheets or sills. A sheet or
sill may be thick or thin; there sometimes may be many
of them, alternating with beds of sedimentary rock.

How is it that we find fundamental and intrusive igneous

rocks aX the earlNs surface?

Although fundamental and intrusive rocks were originally
far below the surface, yet by the long process of erosion
the overlying mass has been stripped off and these rocks are
exposed to the hght of day. That is why we now find
them at the surface as often as we do those which were
poured out of volcanoes.

What are extrusive rocks or lavas?

Extrusive rocks are those which reach the surface by
means of the conduits above mentioned, and overflow either
from volcanoes, as explosive eruptions, with ashes and
scoriae, or as quiet wellings-out, either from volcanoes or
fissures. Beneath the surface are dikes which have been
the feeders, and the same lava may have spread out along
the bedding planes of the stratified rocks as sheets or sills.

Example: Examples of the pouring out of lavas from
volcanoes, with attendant showers of ash, pumice, etc., are
too well known to require specific mention.

An instance of the quiet welling out of a great mass of
lava along fissures is furnished by the Columbia river basalt,
which covers a great arid plain in Idaho, Oregon and
Washington. This lava consists of a series of flows from
20 to 150 feet thick, piled one on top of another. The
dikes which were the feeders to the flows are now in part
exposed in Oregon, by the erosion of the overlying rock;

IGNEOUS ROCKS. 99

and the lack of any ash or fragmental material, or volcanic
cones, shows that the eruption was quiet.* In Idaho a
total thickness of as much as 5,000 feet of this basalt is
shown by the Snake River canon, which has cut down
through it. Before the eruption of the lava there was in
this region a varied topography, but the mountain ranges,
deep valleys, and canons were all blotted out by the swiftly
succeeding flows, until only the very highest peaks still
show their heads.f

GENERAL RELATION BETWEEN IGNEOUS ROCKS
AND ORE-DEPOSITS.

Is there any relation between igneous rocks and ore-deposits?
There is the very closest connection. Probably nine out
of ten ore-deposits have some visible relation to a body
of igneous rocks. In general, also, a country free from
igneous rocks has a scarcity of ore-deposits.

What is the reason for this relation?

The reasons for this are, in part at least, known. All
igneous rocks contain metals. Iron, for example, is present
in every igneous rock in large amount — the percentage of
the whole rock being in some diabases and basalts 15 or 20
per cent., or even more. Manganese, also, is present in
most igneous rocks in noticeable quantities. When we
come to the rarer metals, we naturally do not find them in
such amounts; but chemists have proved the presence of

* W. Lindgren, 22d Annual Report United States Geological Survey, Part II,
p. 741.

t W. Lindgren, 20tb Annual Report United States Geological Survey, Part
III, pp. 91, 93,

A^*^^^

100 GEOLOGY APPLIED TO MINING.

such metals as copper, lead, zinc, nickel, tin, etc., in neariy
every kind of igneous rock. These metals usually occur as
constituents of the dark-colored silicates (hornblende,
pyroxene, dark mica, olivine, etc.) Even the rarest, such as
silver, platinum and gold, are similarly found, though in
small quantities.

Do metals occur in igneous rocks except as constituents of the

dark-colored silicatesf

The commoner metals occur in igneous rocks in the same
form that they do in ore-deposits — in the form of sulphides
and oxides. This has been proved in the case of iron pyrite,
the magnetic sulphide of iron (pyrrhotite), the magnetic
oxide of iron (magnetite), the magnetic oxide of iron with
titanium (ilmenite or titanic iron), the oxide of iron and
chromium (chromite), etc.

Example: In the diabase rocks of the Grass Valley dis-
trict, California, there are small, very abundant grains of
pyrite, pyrrhotite, and ilmenite, which occur within the
augite and feldspar of the rocks in such a way as to prove
that the metallic minerals are of primary origin (that is,
that they have crystallized out of the cooUng molten rock,
and have not been introduced subsequent to its consolida-
tion), and indeed were the first of the rock-forming minerals
to solidify. (Fig. 7.)

Do not the sedimentary rocks also contain disseminated
metals?

Chemical investigation seems to bear out the statement
that as a rule the sedimentary rocks, although they also

IGNEOUS ROCKS. 101

contain small quantities of the metals, yet are relatively
poorer in these than the igneous rocks.

Example: Mr. Luther Wagoner* has made a series of
delicate determinations of gold and silver in certain igneous
and sedimentary rocks of Cahfornia. Four specimens of
granite showed respectively the following weights in milli-
grams per ton: gold, 104, 137, 115, 1130; silver, 7660, 1220;

Fig. 7. Primary pyrrhotite in augite. Black, pyrrhotite; a. augite; b. uralite;t

c. chlorite. From W. Lindgren; 17th Annual Report United

States Geological Survey, Part II.

940; 5590. One specimen of syenite contained gold, 720;
silver, 15,430. A specimen of diabase contained gold, 76;
silver, 7,440. One of the basalt gave gold, 26; silver, 547.
The sedimentary rocks tested were three specimens of
sandstone and two of marble (one of the latter from Italy).
The sandstones gave respectively in gold, 39, 24, and 21; in

* 'Detection and Estimation of Gold and Silver.' Transactions American
Institute Mining Engineers, Vol. XXX, p. 798.
* A variety of hornblende.

102 (iEOLOGY APPLIED TO MINING.

silver, 540, 450, 320. The California marble showed gold,
5; silver, 212;. the Itahan sample, gold, 8.63; silver, 201.
Several assays in San Francisco Bay mud (containing some
organic material) gave gold, from 45 to 125. Two assays
of sea- water gave a mean gold 11.1 ; silver, 169.5 milligrams
per ton.

On the average, therefore, the granite contained 371
milligrams gold and 3852 silver; while, as before stated, the
syenite contained 720 gold and 15,430 silver; the diabase
76 gold and 7440 silver; and the basalt 26 gold and 547
silver. Taking the sedimentary rocks, the sandstones
averaged 28 gold and 437 silver; the marble, 7 gold and 206
silver; and the bay mud 85 gold.

Averaging the igneous rocks assayed, we find a mean of
gold 330 and silver 5547, while the mean of the sedimentary
rocks (sandstones and marbles) is 17 gold and 344 silver.
That is to say, the mean of the igneous rocks assayed con-
tains about 19 times as nuich gold and 16 times as much
silver as the mean of the sedimentary rocks.

Are disseminated metals equally abundant in different kinds

of sedimentary rocks?

In regard to the sedimentary rocks, it will be noticed
that the sandstones in these tests contained on an average
four times as much gold, and over twice as much silver as
the marbles; while the bay mud (which in hardening would
become shale) contained nearly 13 times as much as the
marble.

Are disseminated metals equally abundant in different kinds

of igneous rocks?

As regards the igneous rocks, if we take the silicious or
acid rocks (granite and^yenite) thus examined and compare

IGNEOUS ROCKS. 103

their mean with that given of the basic rocks (diabase and
basalt), we find that the silicious rocks showed nearly nine
times as much gold and about one and a half times as much
silver as the basic ones. It remains to be seen whether
further data would confirm these results.

What hearing has the presence of disseminated metals in
igneous rocks on the question of the relation between igneoiLS
rocks and ore-deposits?
The metals disseminated in igneous rocks often become

concentrated by various agencies.

1$ there no other reason for this relation?

There is another reason, and one perhaps as important,
why ore-deposits are so closely connected with igneous
rocks. These rocks retain their heat a long time before
cooUng. In the case of rocks that cool beneath the earth's
surface, it is safe to say that they keep their heat for cen-
turies of centuries. So all circulating waters passing
through them become heated, and the hot water having
less specific gravity than cold water (water expands on
heating) has a tendency to rise, and to appear at the surface
as hot springs. This hot water has a power of solution —
and hence a power of concentrating the disseminated
metals (whether in the igneous or in the stratified rocks)
into ore-deposits — many times greater than cold waters.
There are three phenomena frequently found to be con-
nected : igneous rocks, hot springs and ore-deposits. Often,
however, we find districts where the igneous rocks have

104 GEOLOGY APPLIED TO MINING.

long ago cooled, even far beneath the surface, and where
there are ancient ore-deposits, but no longer hot springs.

Is the presence of hot springs favorable to the finding of ore-
deposits?

The presence of hot springs in a country is favorable to
ore-deposits; but their absence cannot be taken as a sign
that such deposits do not exist. Hot springs are most
Ukely to occur in regions of younger eruptive rocks^ (Ter-
tiary, for example), where the under rocks are still hot;
and not so often in the older and perfectly chilled rocks.

How are the disseminated ores of igneous rocks concentraied,

before or during cooling?*

While the rock is still partially molten and fluid, the
different elements have some power of moving about, and
it is usually held that on account of the mutual attraction
of like materials they tend to group themselves and form
bodies more or less concentrated.

How is the process of magmatic segregation supposed to
effect the concentration of basic materials?

During the earlier period of cooling, the metallic sulphides
and oxides, which are among the first minerals to crystallize,
and are especially abundant in basicf rocks, may collect.
In some igneous rocks rich in iron, the iron becomes espe-
cially abundant in places and even may be sufficiently

I'The material of the following few pages follows closely certain portions of
Chapter I.

1 1. e. Dark colored, heavy rocks, containing a low percentage of silica.

IGNEOUS ROCKS. 109

concentrated to form an ore, though always retaining its
character of an original constituent. In Greenland,
masses of native iron have been found in basalt. Magnet-
ite, the magnetic oxide of iron, is sometimes sufficiently
abundant to form ore-bodies in this way. Such is the
case, for example, in Sweden, in Rhode Island, in the Lake
Superior region, in Canada and elsewhere.

Example: The titaniferous iron ores (magnetites) of the
Adirondack Mountains in New York are associated in all
cases with basic igneous rocks, which have been intruded
into older gneisses and crystalline limestones. The tran-
sition from the basic wall rock (generally gabbro) to ore
usually takes place gradually, but within a short space.
There is no ore along contacts, nor any evidence of the
formation of the ore after the consolidation of the rock.
The basic rock itself has been split up by segregation of its
essential minerals, so that some portions are almost entirely
of feldspar, while other portions contain large amounts of
pyroxene (making a gabbro). It is plain that the mag-
netite which forms the ores has been segregated, like the
feldspar and pyroxene, while the rock was in a partially
molten state; and it is possible that the high specific grav-
ity of the iron may have been influential in bringing about
this result.*

Are there other ore-deposits, besides those of iron, which are
thus known to he original, and due to magmatic seg-
regation of basic materials in igneous rocks?
The mixture of oxide of chromium and oxide of iron

*J. F. Kemp, 19th Annual Report United States Geologicyil Survey, Part
III, pp. 383-422.

106 GEOLOGY APPLIED TO MINING.

(chromite) is frequently found in tiny crystals in the
igneous rocks, and may be so abundant in certain parts that
it forms an ore. It may even form solid masses, crowding
out the other rock-forming minerals. Many of the known
chrome ore-deposits have been held to belong to this class.

Corundum, the oxide of aluminum, used chiefly as an
abrasive (the pure varieties are precious stones — sapphire
and ruby), has, in a number of cases, been found to be an
original constituent in igneous rocks, not only in small
quantities, but in those accumulations which are mined as
ores.

Nickel is found in fresh igneous rocks as part of a number
of minerals. Pyrrhotite, which is a magnetic sul-
phide of iron, very commonly contains nickel, and is one
of the principal ores; it is a not uncommon rock-forming
mineral. Large masses of nickeliferous pyrrhotite have
been explained by good authorities as original constituents
of igneous rocks; but others have questioned these con-
clusions.

How does the process of magmatic segregation effect the con-
centration of silicious inaterialsf

During the final stages of consolidation, heated waters,
steam and gases (containing silica, with earthy and metallic
minerals, in solution), which are left over from the cooling
mass, deposit their solid portions in the form of pegmatite
or quartz, as nests or veins in the hardening rock or in
some neighboring formation. It is held by some that
certain of the quartz veins having this origin contain suf-

IGNEOUS ROCKS. 107

ficient gold to render them ores; but this conclusion is not
yet universally accepted.

Do the residual sUicums solutions always form 'pegmatites and

quartz veins?

The residual silicious solutions, instead of forming definite
veins, may penetrate the rock with which the cooling
igneous mass is in contact, and there may deposit their
solid portions, usually by replacement of the original rock.

When the resulting altered rock contains ores, it is called
a contact metamorphic deposit.

Whal are the characteristics of contact metamorphic deposits?
The first mark identifying this class of deposits is their
location at the contact of an intrusive igneous rock with
another rock, or in evident close relation to it. But this is
not sufficient, for other deposits may in some cases be
formed along such a contact, circulating waters having
found this the easiest channel. In the true contact meta-
morphic deposit, mineralization has been accomplished by
materials pressed out of cooling rock. These materials
consist of heated waters and aqueous gas, mingled with
other gases of various kinds, and both the escaping waters
and the gases may carry in solution metallic and other
minerals, which they may deposit near the contact, in
concentrated form. Under these conditions certain min-
erals are characteristically formed which are rare in simple
hydatogenic* deposits. Such are minerals like fluorite,

* Water- formed.

108 GEOLOGY APPLIED TO MINING.

tourmaline and topaz, containing the volatile elements
boron and fluorine. Garnet is also a rather characteristic
mineral of these deposits. According to W. Lindgreu,*
a characteristic feature is the association of oxides of
iron with sulphides. f

Example: The ores at the old Tungsten mine, Trumbull,
Connecticut, have been formed at the contact of an igneous
rock (now metamorphosed to hornblende gneiss) and a
Hmestone into which the igneous rock was intrusive. The
ore occurs on the contact of these two rocks, in beds from
3 to 5 feet thick. It consists of quartz containing iron
pyrites, epidote, calcite, mica, and the wolfram minerals
(scheeUte and wolframite) for which the mine has been
worked. Zoisite, garnet, scapolite, hornblende, and mar-
casite, are also found. This ore-bearing contact zone was
due to the action of solutions at the contact of the intruded
igneous rock, these solutions being both heated and under
pressure. J

Associated veins are of pegmatite or vein quartz, and
contain, besides feldspar, muscovite and other common
minerals, topaz in large masses, fluorite, etc.

What are some of the special results of vapors and gases in
the residiud material under discussion, as regards deep-
seated deposits?
The deposits formed by the material expelled from cooling

igneous rock probably vary according to the nature and the

* 'Character and Genesis of Certain Contact-Deposits.* Transactioru
American Institute Mining Engineers, Vol. XXXI, p. 227.

t As contemporarily formed minerals.

t W. H. Hobbs. 22d Annual Report United States Geological Survey, Part
II, p. 13.

IGNEOUS ROCKS. 109

relative abundance of the gases. Pegmatites usually show,
from the presence of certain minerals containing well-
known gases in their composition, the presence and agency
of those gases in their formation. Tin-veins and veins con-
taining other valuable minerals, metallic and earthy, are
probably often formed largely by the action of abundant
gases, escaping from cooling granular rocks, deep below
the surface.

What characteristic evidence as to their origin do vein^ of this
class offer?

The characteristic sign of the origin of these veins is the
presence of minerals which do not easily form under simple
aqueous conditions, but are commonly produced by the
action of vapors. Among these minerals are tin-stone
(cassiterite, oxide of tin) itself, and others like tourmaline,
topaz, etc. Many apatite (phosphate of lime, with fluorine
or chlorine) deposits also probably belong to this class, as
the presence of the volatile elements (chlorine and fluorine)
in the mineral itself indicates. One of the commonest
minerals found in association with the apatite, that is,
scapolite, bears the same evidence, for the scapolites are
minerals whose composition is practically the same as the
feldspars, save for the presence of chlorine, indicating the
agency of this gas at the time of formation.

Are ores deposited also by vapors and ga^es escaping from

volcanic rocks at or near the surface?

At the surface, orifices emitting vapors and gases from
cooling volcanic rocks are termed fumaroles. From these

110 GEOLOGY APPLIED TO MINING.

gases valuable earthy and metallic minerals may be depos-
ited. This fumarolic activity may persist for ages.

Alunite or natural alum (sulphate of aluminum and
potash), for example, is formed in commercially valuable
quantities by the action of sulphurous gases on igneous rocks
containing potash and aluminum. Sulphur, deposited in
this way, is actively exploited in Italy and elsewhere. On
the walls of fissures in lava, where steam and other gases
escape, various metalUc minerals have been found which
are due to this action, such as specular iron (hematite,
oxide of iron), cinnabar (sulphide of mercury), realgar
(sulphide of arsenic), etc. Other ores very likely are
sometimes formed in this way.

Example: The Bassick mine is in Custer county, Colo-
rado. The rock of the region is gneiss, but an explosive
volcano has broken through, producing a pipe which is now
filled with rounded boulders, chiefly of volcanic rock. In
places these boulders are coated with rich metallic minerals.
The first coat consists of lead, antimony, and zinc sulphides;
an intermediate coat is of zinc sulphide, rich in silver and
gold; other coats are of chalcopyrite (sulphide of copper
and iron), and pyrite (sulphide of iron). Tellurides of the
precious metals also occur. These ores are very generally
considered by geologists to have been brought up in the
form of vapor during the fumarolic activity of the volcano.

IGNEOUS ROCKS. Ill

SPECIAL RELATIONS BETWEEN CERTAIN
IGNEOUS ROCKS AND ORE-DEPOSITS.

Advantages of Different Forms of Igneous Rocks.

What especial phases do the processes attendant upon cooling
show in fundamental igneous rocks?

Fundamental rocks are generally of coarse grain, showing
slow consolidation. Since they have cooled in many cases
at great depth, gases and vapors attending this process may
have formed characteristic ore-deposits. Tin-veins, for
example, are, as noted, generally confined to such rocks,
of granitic composition.

What advantages or disadvantages do extrusive rocks possess
for ore-concentration?

Extrusive rocks and hot springs are aUied occurrences;
yet, unless the flows of these rocks are very thick, they are
too near the surface for- hot-spring action to exercise its
best effort in producing mineraUzation ; for the heat of the
rocks disappears with comparative rapidity, and with it
the great concentrating power of the waters. Where the
flows are very thick, however, the central portions remain
warm for a very long time, and the hot-spring action is
prolonged and becomes more productive of results.

Fumarohc activity, properly speaking, and fumarolic
deposits are confined to extrusive rocks; while contact
metamorphic deposits are lacking.

112 GEOLOGY APPLIED TO MINING.

What advantages have intrusive rocks over others in bringing

about ore-depositionf

Intrusive rocks are the most favorable for promoting the
formation of ores. Like all igneous rocks, they contain
disseminated metals. On account of their being distant
from the surface, they cool with comparative slowness,
especially if they are in bodies of considerable size; and
thus the conditions for ore concentration are prolonged.

Intrusions come in contact with other igneous rocks and
with stratified rocks. While the igneous rocks are better
fitted than the sedimentaries for instigating the processes
of ore-deposition and for furnishing the disseminated metals
to the mineralizing waters, yet the latter are more suitable
for precipitating the dissolved metals. This is due to the
easy dissolution and replacement of the limestones, to the
tiny pores of the sandstones, which permits interstitial
deposition, and to the organic matter of the shales, which
often acts as a direct precipitant. Hence, where the two
classes of igneous and sedimentary rocks are intimately
associated, the most favorable conditions are realized.

Advantages of Different Kinds of Igneous Rocks.

Preferences of Certain Igneous Rocks for Certain Ores,
Displayed During the Cooling Processes.

Are the dark basic rocks more closely associated with ore-
deposits than the light silidous ores?
It has been found by chemists who have investigated the

metallic contents of fresh igneous rocks that the metals

IGNEOUS ROCKS. 113

were mostly present in the dark "ferro-magnesian"' min-
erals — hornblende, pyroxene, black mica, olivine, etc. This
being the case, we might expect ore-deposits to be more
definitely associated with the dark-colored basic rocks than
with the silicious ones. But this is only partly the case; the
preference seems to depend chiefly on the kind of metal.

Are some metals 'preferentially associated with light-colored

and sUicious rocks?

Tin is usually found in, or in relation with, granite; and a
general close connection with silicious rocks seems the case
with tungsten and molybdenum.

Example: In the Malay Peninsula tin deposits are found,
mainly on the western slope of the mountain range that
forms the backbone of the peninsula. This range is com-
posed largely of granitic rocks, with some limestone and
sandstone. The ore is cassiterite, associated with tourma-
Une, hornblende, tungsten minerals, magnetite, muscovite,
topaz, fluorite, sapphire, etc. Veins are found generally
in the granite, less frequently in the other rocks.*

Are some metals associated by preference with dark-colored

and basic rocks?

Chromium ore-deposits (chromite), for example, are
hardly found save in very basic rocks — peridotites.

When these peridotitic rocks decompose, they become
serpentine, which accounts for the chrome deposits fre-
quently occurring in serpentine rock.

Iron (generally magnetic, often containing titanium) also

*R. A. F. Penrose Pacilic Coast Miner Vol. VII, p. 340.

114 GEOLOGY APPLIED TO MINING.

forms ore-deposits in many basic rocks. As these rocks
are, by their very definition, richer in iron than the sihcious
ones, iron deposits in general may be allowed to exhibit a
certain preference for them.

Copper usually prefers basic rocks; on the other hand,,
there are many instances of rich copper deposits in silicious
rocks.

Example: In Cuba, serpentine is abundant among the
most ancient rocks. The serpentine is of igneous origin,
being derived from the alteration of dark, basic, igneous
rocks (such as peridotite). This rock is and has been
considered the most productive of metals among the for-
mations of the island. It contains large deposits of
copper, ores of iron and chromium, and gold.*

Platinum was formerly only found in placers. In Russia,
however, some years ago, the metal was found as an original
constituent in peridotitic rocks, and late inquiry in America
has fixed it as being in a number of such rocks. Prof.
Kemp has reported platinum in peridotite from the Tula-
meen region, British Columbia. It was also reported from
''fine-grained dark basaltic rock'* in British Columbia in
1895 by Mr. Carmichael,t assayer for British Columbia.
It seems, therefore, to be chiefly confined to the basic
rocks, and it is, indeed, an intimate associate of chromite.
Yet it has been found to occur, though less abundantly,
even in so silicious a rock as syenite.

* Fernandez de Castro, Hayes, Vaughan, and Spencer. 'Geological Recon-
naisance of Cuba,' 1901.

t Engineering and Mining Journal, Feb. 12, 1902, p. 249.

IGNEOUS ROCKS. 115

Most of the other minerals have, as far as known, slight
or no preference for certain igneous rocks.

Preferences of Certain Igneous Rocks for Certain Ores,

Displayed by Selective Precipitation of Metals

from Solution.

May ores show a preference for one igneous rock over another y
on account of the different effect of different rocks in pre-
cipitating ores from solution?

When ore-bearing solutions traverse a variety of igneous
rocks, there will be certain chemical reactions between the
so utions and the walls of the fracture which has afforded
them a channel. Where the rock is porous and permeable,
so that the solutions spread out and traverse it thoroughly
and slowly, there the opportunity for such reactions is very
great. In igneous rocks of different mineralogical and
chemical composition, the same solutions will react in
different ways, and the substances precipitated from solu-
tion as a result of these reactions will be apt to differ,'both
in quantity and quality. The result will be variable, and
will depend as much on the specific character of the
solutions (which vary greatly) as on the character of the
rock.

Thus it may happen that an ore-bearing solution will
form a rich deposit in one igneous rock and in another,
along the same fracture, very little. Moreover, on account
of the different character of solutions, a certain rock may
in one case be selected by preference for ore-deposition, and
in another case may be specially avoided.

116

GEOLOGY APPLIED TO MINING.

Example: At Butte, Montana, two granites of different
ages occur, one of which is ten per cent, more silicious than
the other. The less silicious granite contains a consid-
erable amount of hornblende and biotite, while the other
contains very little. An important class of copper ores
here have formed by replacement of the igneous rocks.
The fractures, along which circulated the solutions that
deposited the ore, cut both rocks. In the less silicious
granite, the veins are commonly rich in copper; in the more
silicious granite they are almost equally wide and strong,
but are lean, and composed chiefly of quartz, with com-

i'Vv'vl Granite ESS Aphte ^^ Qu*rtx

Fig. 8. Ideal plan of conditions in a copper vein at Butte, Montana, passing

from less silicious granite into silicious granite. After W. H. Weed.

paratively little pyrite and copper. Microscopic study of
the rocks show that in the process of replacement the horn-
blende was the first mineral to be altered to ore, indicating
that the nature of the solutions were such as to react most
readily with this mineral. It is believed that the presence
of the hornblende and other dark minerals in the less
silicious granite, and their absence in the more silicious
rock, determined the preferential precipitation of the ores
in the former.* (Fig. 8.)

* W. H. Weed, Tranaacliona American Institute Mining EngineerD, Vol.
XXXI. p. 643.

IGNEOUS ROCKS. 117

ORE-BODIES IN THE ROLE OF INTRUSIVE ROCKS.

Are metallic minerals ever thrust up in a molten condition in
the form of dikes, requiring no further concentration to form
ores?

It has been explained how certain metaUic minerals may
become segregated in molten masses so as to form ore-
deposits. If these metallic segregations, instead of remain-
ing where they originate, are disturbed by some movement,
and forced up into the rock above while still wholly or
partly fluid, it is conceivable that we should have dikes of
ore. The occurrence of iron ore (magnetite) in this form
has actually been reported.

Example: On Calamity brook, near lake Sanford, in the
Adirondack Mountains, are dikes of titaniferous magnetite
in anorthosite (a granular rock composed almost wholly of
labradorite, •a species of feldspar). The ore in the hand
specimen appears to be an exceedingly ferriferous gabbro,
and it contains inclusions of anorthosite, through which
run little dikes of pyroxene, garnet, and ore that end in
streaks of pyrites. The anorthosite inclusions are believed
to be masses of the country rock which were torn off during
the intrusion of the ore and about and through wl ich
gaseous action developed the little dikes, and streaks of
pyrites. Thin sections of the ore-dikes, studied under the
microscope, show coarsely crystalline aggregates of ilmenite
or titaniferous magnetite, pyroxene, and a little biotite.
Regarded as ores, they vary in richness, being sometimes
nearly pure magnetite, and again more than half siUcates.*

*J. F. Kemp, 19th Annual Report United States Geological Survey, Part
III, p. 412.

118

GEOLOOY APPLIED TO MINING.

ItJNEOUS ROCKH INTUrSIVE SUBSEQUENT TOJ
ORE-DKPOSITION,

Mtuf 71 ot inirusive igneous rocks sometimes form later than an
ort-lmtli/^ tmd m*, though tlosdy dssociated with it, yet have^
no part in its formatimif
Jii.st as fii lilts iiiiiy be earlier than oreKleposition in a]
fertaiii case^ and may furnish the channels along which the
ore is concentrated, or may be later than ore-deposition,
may cut and displace tbe orc-ljody, and. far from being a .

r^r

J • \ I

i'^ ' i

'tj^

r^i^

-^^ '^M

Fig. 9. Iruii f>re-b<jdie# Oiernatite and niagne1i<^e), l^tln mine, Sanfin^fo prov-

inoe, Cuba. Blo^^k portion is ore, surrnu tided by porphyry,

Aftef Hayee, Vaughan^ and Spencer.

help in the ore-concentration, may be only a hindranci
and vexation to the miner— just so an intrusive igneous
rock may be earlier than ore-deposition and be largely
responsible for it, or may be later and cut it up and separate
it.

Where two igneous rocks are intruded at various times

IGNEOUS ROCKS. 119

in the same place, the ore-deposit resulting from the
influence of the first intrusion may be broken by the second.

Example: The hematite and magnetite iron ores of
Santiago province, Cuba,* have, since their formation, been
cut, floated up, and surrounded by intrusive masses of
porph3ny, so as to entirely alter their form (Fig. 9).

•Hayes, Vaughan, and Spencer. 'Geological Reconnaissance of Cuba/
1091. p. 81.

CHAPTER IV,

THE STUDY OF DYNAMIC AND STRUCTURAL
GEOLOGY AS APPLIED TO MINING.

PART L
GENERAL CONCEPTIONS AND MAPPING.

DEFINITIONS.

What is dynamic geology and structural geology f

Dynamic geology is a study of the physical forces which
produce changes in the earth^s crust. These forces (due
to the contraction of the earth from coohng, to migrations
of molten rock beneath the solid crust, or to the unequal
weight of different features of the surface, bringing about
unstable equilibrium) produce bending and breaking in the
rocks. Such disturbance is mainly noticeable in the
stratified rocks, the beds of which are forced out of their
original horizontal position into all manner of folds; or they
are even broken, with one part thrust past the other along
the hne of fracture. This last is called a fault. The study
of the arrangement or structure of these bent and broken
rocks is called structural geology.

DYNAMIC AND STRUCTURAL GEOLOGY. 121

FOLDS AND FAULTS.

What is the meaning of the term dipf

The inclination of a bed (or other geological feature
having a plane direction) is the dipy which is measured in
degrees from the horizontal.

Shovld we use the word hade instead of dip, in speaking of
veins?

The inclination of veins, dikes, faults, etc., is also called
hade, and is measured in degrees from the vertical. There
is, however, no need to have two opposing terms for any
one thing, and it is better to apply the term dip to the
inclination of the veins, faults, etc., as well as strata, and to
measure it in the same way.

Whai are the principal kinds of folds?

Folds are chiefly divided into two kinds, according to
whether they are open above or below. These are called
respectively syndines and anticlines.

A line drawn from the apex of a fold (the highest point
of an anticline or the lowest point of a syncline), midway
between the two sides or limbs of a fold lies in the axis
(Fig. 10).

If the axis is vertical, the two sides (or limbs) have the
same dip; if the axis is inclined the dips of the limbs will be
unequal, unless they are parallel, as when the folding is
intense and they have been jammed together, forming a
compressed or close fold. (Fig. 11).

122 GEOLOGY APPLIED TO MINING.

The opposite of a close fold is an open fold (Fig. 10).

What are overthrown folds?

In open folds the limbs normally dip in opposite direc-
tions; yet the folds may be such that the limbs incline in

Fig. 10. Folding of limestones and shales on Kuskokwim river, Alaska. After

J. E. Spurr,* a.=aniicline; b.=anticline overthrown at the apex;

c.=faulted anticline; dd.=synclines.

Fig. 11. Close folding in limy shales on Yukon river, Alaska, below Mission
creek. After J. E. Spurr.f

the same direction, though not necessarily at the same
angle. This constitutes an overthrown fold (Fig. 12). In
extreme cases the axis may assume a horizontal position.

* 20th Annual Report United States Geological Survey. Part VII, p. 127.
1 18th Annual Report United States Geological Survey, Part III, p. 177.

DYNAMIC AND STRUCTURAL GEOLOGY.

123

What is a monodinef

Where strata suddenly change from a horizontal to an
inclined position and then become horizontal again, a fold
with only one limb — a monocHne — is formed (Fig. 13).

Fig. 12. Overthrown folds; aa. anticlines; ss. synclines.

Fig. 13. Monoclinal fold.

What is a normal fault and what is a reversed fault?

Most fault planes have an inclination or dip between the
horizontal and vertical. When the rocks on the upper side
have moved down, relative to the rock on the under side,
the fault is called norma/. If the reverse movement has
taken place, the fault is called a reversed or thrust fault.

124

GEOLOGY APPLIED TO MINING.

The majority of faults are normal; but reversed faults are
frequent (Figs. 14 and 15).

What are compensating favltsf

Where a stratum or vein is faulted in many places it
sometimes happens that one fault will displace the bed, and

Fir. 14. Faults in strata, near Forty Mile, Yukon river, Alaska; nn.=nonnal
faults; r.^reverfied fault. After J. E. Spurr.*

Qu&n

■rzim flA

■mMmQuartz

Fig. IT). Reversed fault, lonRitudinal section, Empire mine. Grass Valley, Cali-
fornia. After W. Lindgren.t

another will bring it back to its original position. The two
faults are thus compensating. In other words, the block
comprised between the two faults has been moved out of
line, leaving the rest in place.

* 18th Annual Keport United States Geological Survey, Part HI, p. 177.
1 17th Annual Report United States Geological Survey, Part II, p. 263.

DYNAMIC AND STRUCTURAL GEOLOGY.

125

Example: The accompanying figure represents compen-
sating faults in the Omaha mine, Grass Valley, California.
These faults displace a vein about one foot wide, consisting
of quartz containing galena and iron pyrite, and other
sulphides, with some free gold (Fig. 16).*

What connection have faults with folds?
A monocline may easily pass into a fault. Faults along

*'^,'^*k?.*'-' "*'f>i'-

QUARTZ.

Fig. 16. Longitudinal section, showing fault, Omaha mine, Graha mine, Grass
Valley, California. After W. Lindgren.

the axes of folds are also common, for along the axes rocks
are weakened by bending and therefore liable to break.
The directions of faults are likely to conincide with those of
folds in the same region, for they both may originate as a
result of the same kind of pressure.

♦ Waldemar Lindgren. 17th Annual Report United States Geological
Survey, Part II, p. 243.

126 GEOLOGY APPLIED TO MINING.

EFFECTS OF EROSION ON FOLDED AND FAULTED
ROCKS.

What is erosionf

Erosion may be defined as the process of wearing away.
As applied to geology, it signifies the wearing away of the
rocks at the surface, chiefly by the action of streams.

Arc folds and faults lyisihle as such at the surface?

Deformation (as folding and faulting taken together may
be called) would affect the earth's surface precisely as it
does the rocks, were it not for the counteracting effects of
erosion. Erosion is a slow process, but it is continuous.
Folding and faulting is also a slow process — ^rarely spas-
modic. It goes on beneath our feet today so gently that
we do not notice it except where there is a slip in the gentle
mechanism and an earthquake results. Sometimes defor-
mation is more rapid in moulding the earth's surface than
is erosion; sometimes erosion is the more active. But
after deformation has stopped, owing to the easing of the
deforming pressure, erosion still keeps on, so in the end it
mostly has its own way and shapes the minor features of
the earth to suit itself. Therefore, folds and faults are
sometimes, but not usually, directly expressed as such at
the surface.

What are the effects of erosion and deformation in producing

topographic features?

Erosion and deformation are usually in opposition; the
latter lifts up mountains while the former is engaged in

DYNAMIC AND STRUCTURAL GEOLOGY.

127

wearing them down. Both forces, even if each were left to
itself, tend to produce irregularities— ridges and furrows —
in the earth's surface. Simple deformation makes upfolds
(anticlines) which are mountains, and downfolds (synchnes)
which are valleys. In the work of erosion rivers cut deep
trenches which are valleys, and the high parts left be-
tween are the mountains. The first named are mountains

Fig. 17. Graded anticlinal range of deformation. A generalized transverse sec-
tion of the Uinta range. Utah. After C. A. White.*

and valleys of deformation; the second, mountains and
valleys of erosion.

Example: The Uinta range, in Utah, as shown in the
accompanying figure (Fig. 17), is an anticlinal range, the
upfold in the strata corresponding with the topographic
dome. The great thickness of stratified rocks between the
dotted line on the figure and the present mountain tops (a
thickness greater than the height of the mountains above
the plains) has been stripped off by erosion. Still, if the

* 9th Annual Report United States Geological Survey, p. 694.

128 GEOLOGY APPLIED TO MINING.

present relief of the range is directly due to the upfolding of
the cnist, as geologists have held, this is a range of de-
formation.

Are mountains of erosion upfolds of the crust?

Mountains of erosion are not necessarily upfolds. Upfolds
tend to weaken the rocks so that they are more easily
washed away, leaving valleys, with synclinal mountains
between. Mountains may be composed of upfolds and
downfolds together; and they may trend diagonally or at
right angles to the trend of the folds.

In general, what relation has topography to folds?

As the result of these inharmonious processes, we may
expect to find the relief or topography bearing any con-
ceivable relation to the structure. In a hilly or mount-
ainous region the structure is sometimes suggested by the
topography, but more frequently the topography only
obscures its elucidation. It even frequently happens that
a highly folded region may have become by long erosion
topographically a plain.

What is the relation between topography and faults?

As with folds, so with faults. Faults break the earth's
surface and the moving of one rock past the other produces
a cliff or scarp (simple fault-scarp). Only recent faults
show this (Fig. 18).

The erosion which attacks a faulted surface may do one
of several things, dependent on the different nature of the
rocks on either side of the fault brought together by the

DYNAMIC AND STRUCTURAL GEOLOGY.

129

movement. There may result a scarp (erosion fault-
scarp), a gully or valley along the fault, or the fault may
not influence at all the outlines of the topography. An
erosion fault-scarp is produced when the rock on one side

Fig. 18. Simple fault-scarp at the Palisades, Yukon river, Alaska; a.=fault-
scarp; b.=fault. After J. E. Spurr.*

Fig. 19. Reversed erosion fault-scarp. Section in the Lower Austrian Alps.
After Bittner.

of the fault is softer, and so more easily worn away, than on
the other. If the rock on the upthrown side of the fault
is harder than that on the downthrown side, the scarp will

* 18th Annual Report United States Geological Survey, Part III, p. 199.

130 GEOLOGY APPLIED TO MINIKQ.

face the downthrown side, that is, it will simulate a simple
fault-scarp. This may be calleil a normal erosion fault-
searp. But if the rock on the downthrown side is harder,
it will eventually become higher by the wearing away of
the upthrown side, and the scarp will face this latter side.
This is a reversed erosion fault-scarp (Fig. 19).

Since therefore we cannot tell beforehand what relation the rock
structure will have to the topography, how are we to work
out the structure problems?
Stnicture can be satisfactorily worked out only by

reasoning from the attitude and position of the rocks as

they appear at the surface or outcrop.

THP] SURFACE MANTLE OF DEBRIS.

Do rocks outcrop all over the surface?

When we start out to study the geology of a district, we
find here one rock and there another; here a bed with a
certain inclination, there another bed inclining with a
different angle in another direction; then soil and forests
without outcrops, valley bottoms free from hard rock, ett;.
The underground rocks do not outcrop continuously save
in high mountainous regions.

Why do not rocks outcrop continuously?

Exposed rocks break up at the surface, under the influ-
ence of heat and cold, frost and thaw, rain and wind, the
roots of trees and plants, and the decomposing acids, chiefly
derived from vegetation, which soak down into the rocks
and attack them. The result is that such rocks crumble

DYNAMIC AND STRUCTURAL GEOLOGY. 131

into sand and clay. V^etation takes root, flourishes and
dies, and new generations of plants arise; thus a top loam
is formed, by the accumulation of vegetable remains. This
loose decomposed material, or soil, is often found in place,
directly over the solid rock whence it is derived. But, on
account of its looseness, it usually moves downhill, into
the valleys, and out towards the sea, in a steady but very
leisurely journey. Thus steep mountain tops become
stripped and expose only fresh rock, while their lower slopes
are covered thickly with coarse fragments, and the valleys
below are deeply filled with soil (wash or drift). Farther
down the vallejns, towards the sea, ths wash is apt to cover
larger and larger areas of soUd rock (bed-rock) until at last
it rests in the sea and there builds up a new series of sedi-
ments.

Besides the rocks that go to pieces slowly and thoroughly,
a great deal is broken up more suddenly and violently, by
rapid mountain streams.

Are glaciers active in making soil and gravel?

Thousands of years ago, there existed a great continental
glacier^ (like that which now covers much of Greenland
so deeply that we do not know where the land leaves off
beneath it and the sea commences) over most of British
North America east of the Rockies, and reached down into
the United States. Its southern limit extended on the
east into New Jersey, while on the extreme west it hard-
ly got below the present northern boundary of the United
States. In the mountains of some of our Western States
euch as Washington and Oregon, we still have local gla-

DYNAMIC AND STRUCTURAL GEOLOGY. 133

ciers, occupying valleys, broad mountain sides or coastal
slopes. Alaska contains many such glaciers, some of
which cover thousands of square miles.

Glaciers are powerful rock crushers and erosive agents,
and in their slow imperceptible forward flow they leave the
material, which they have crushed and mingled, either
beneath them or along their margins. Therefore the region
of the old continental glaciers (the glaciated area) is gener-
ally thickly covered by broken and mixed rock and soil
(glacial drift) from which the bed-rock peeps out only in
places (Fig. 20).

Does this covering of soil and gravel make the unravelling of

the structure difficultf

On the high mountains, where the rock is all exposed, it is
possible for an observer of ordinary keenness to perceive the
structure, unless it is complicated; but in a country where
outcrops are not abundant it is difficult to read even simple
structure.

THE SYSTEMATIC WORKING OUT OF GEOLOGIC
STRUCTURE.

Strike and Dip.

How shall one start to work out the structure of folded and

faulted rocks?

To work out the structure of a region, one must first learn

. to take the strike and dip of stratified rocks, for these rocks

furnish the best key to the disturbances which the crust has

undergone since their deposition. We know that they were

134 GEOLOGY APPLIED TO MINING.

laid down horizontally : hence, when we find them tilted art; a
certain angle, we know that the crust at this point has been
deformed to this extent.

How does one record strike and dipt

The dip has already been defined as the inclination of a
bed, measured in degrees, from the horizontal. The strike
is the direction of the outcropping edge of an inclined bed,
on a horizontal surface (such as it would be on a flat plain)

Fig. 21. The directions of strike and dip.

and is generally recorded in degrees from the north or from
the south (Fig. 21). The writer prefers referring all read-
ings so far as convenient, to the north point, — thus, N. 10®
E., N. 90° E., N. 75° W., etc. The direction of the dip is
invariably at right angles to the strike, but the inclination
may be to one side or the other of the line of strike. Hence,
in recording the dip, it is only necessary to note the general
direction (the exact direction being known from the strike)
and the angle of inclination. Thus, strike N. 60° E., dip
25° N. W. ; or strike N. 35° W., dip 6° S. W.

DYNAMIC AND STRUCTURAL GEOLOGY. 135

How accurately should strike and dip be read?

It is generally useless to read the strike and dip closer
than a degree; for the attitude of a bed generally varies
constantly, though often slightly, so that greater accuracy
in one place will not help in the broader problems.

How should the strike be read?

For this work a hand-compass, having a clinometer
(arrangement for reading the dip), is sufficient. To read
the strike, get the line of sight of the compass (the line
between the sights, or the straight edge of a square com-
pass) parallel to a line made by a horizontal plane cutting
the surface of a bed, and read the angle between this and
the north point of the compass needle. Thus, if this line is
30° to the right of the north point of the needle, (the obser-
ver facing in a northerly direction), its direction is N. 30° E.
magnetic.

The reading may be corrected subsequently so as to read
to the true north, by applying the known magnetic varia-
tion. The angle of this variation is to be added to the angle
read, or subtracted from it, according to whether the varia-
tion in the region under examination is to the east or to the
west of the true north. For example, in Maine at a place
where the variation (declination) is 18° W. (that is, where
the magnetic needle points 18° W. of true north) a magnetic
strike reading N. 30° E. would be corrected, by subtracting
the variation, to N. 12° E. true; in Oregon, at a place where
the variation is 2(fi E., the same magnetic reading (N. 30° E.
would be corrected, by adding the variation, to N. 50° E.
true.

13G GEOLOGY APPLIED TO MINING.

How should the dip be readf

To read the dip there is, in the clnometer. compass, a
little weight which hangs down and so is vertical. One side
of the square compass being held parallel to the dip of the
rock, the number of degrees of this dip from the horizontal
(or from the vertical, if one pleases) is registered on a scale
across which the weight swings. There are other clino-
meter arrangements, but for ordinary geological work this
simple one is as good as any. A graduated scale of degrees
pasted on the cover of a notebook, with a small coin at the
end of a thread, as weight, will also answer the purpose, the
straight edge of the book being held parallel with the dip in
measuring.

How can one best find the horizontal line?

In measuring strike and dip, it is best to judge with the
eye the general horizontal direction and greatest inclination
of an outcrop, and to hold the compass in the hand, away
from the outcrop, as near these average directions as
possible. The strike and dip usually vary much locally.
Sometimes it is easiest to scratch a horizontal Une on the
exposed face of a bed, to get the true strike. One may
remember that the dip is always the greatest inclination of
abed.

Recording Ors?:rvations on Maps.

What is necessary for the continvance of the work?

The next necessary thing is to have some sort of map. If
there is none on the proper scale, a small-scaled map may
be enlarged, and corrected as one works. Where there

DYNAMIC AND STRUCTURAL GEOLOGY. 137

is none at all, a sketch map will have to serve. For such a
sketch map, directions are easily found with a compass.
Distances are got by pacing, with or without a pedometer;
by an odometer attached to a carriage, or a cyclometer on a
bicycle. Elevations can be taken with an aneroid barom-
eter.

For accurate work, an accurate map is necessary. Such
a map is most easily made with a plane-table, a method by
which the surveying and the plotting go on simultaneously.
The principal points are determined by triangulation, the
elevations by means of vertical angles read with a transit,
(or aUdade of a plane-table) from the chief stations, and
the minor points sketched and re-sketched from the different
stations until they are approximately correct. For still
more accurate work the position and elevation of nearly all
the points are determined by stadia work. In these last two
ways are made the beautiful contoured maps of the United
States Geological Survey. Levelling may also be used for
determining elevations. A contoured map, or at least one
where the chief elevations are definitely recorded, is essen-
tial to any but the rudest of geological work, for it enables
the student afterward to read and reconstruct the topog-
raphy, without which the geology as exhibited on a plane
map, — a projection of the real surface on a horizontal
plane — can hardly be understood.

What does the student record on this map?

Upon this base-map the student should record every
outcrop which he judges necessary. In a complicated
country or where outcrops are few, often every rock

138 GEOLOGY APPLIED TO MINING.

exposed is necessary. But where there are many outcrops
of the same strike and dip and of the same kind of rock, or
where the structure is simple, many may be omitted.

The outcrops which are recorded may generally be
located on the map (especially a detailed map) by locating
the topography of the place in question — i. e., if the outcrop
occurs on top of a hill, and the top of that hill is shown on
the map, one can place the outcrop as closely as necessary.
Where there are few landmarks, it is often necessary to
locate outcrops instrumentally, by means of intersection
from two known points, or by a direction and measured
distance from some one known point, the result being
plotted on the map according to the scale used.

How are strike and dip recorded?

For recording the strike and dip, the following sign is
commonly used, the long line being the strike, and the
arrow, with the angle written close to it, recording the
direction and inclination of the dip (Fig. 22).

Fig. 22.

How should the different rocks be plotted?

The kind of rock may be written on the map, but it is
better to use an arbitrary sign for each rock, or, better still,
a color. A ])ox of colored pencils may be used for this, and
each color may be taken to represent one of the important
rocks of the district in question. For example, blue can be
used for limestone, brown for quartzite, red for granite,
and so on.

DYNAMIC AND STRUCTURAL GEOLOGY. 139

When all the data are thus plotted^ does it help our compre-
hension of the structure?

When all available outcrops have been recorded, the
general distribution of each color, representing its particular
formation, will be shown. In this way it often can be pre-
dicted, frequently with great accuracy, what rocks underlie
the coverings of soil and glacial drift or valley wash, where
there are no outcrops. By extending the line of strike in
different outcrops of the same formation till they come
together, the extension of beds under covering materials
can be made out with especial certainty; If there is great
and uniform disconnection along a certain line, between the
strikes of such outcrops thus extended, then the geologist
knows that this line is a fault-line, though it may not be
visible to the eye (on account of covering material); and
even the amount of discordance, or displacement of the
fault, can often be closely calculated.

Migration of Outcrops.

Do the lines of outcrop of veins ^ faults ^ etc., on the surface ^
always give an accurate idea of their direction?

One must cultivate some geometrical perception to grasp
the true attitude of beds, dikes, faults, etc., from the
puzzling lines of outcrops afforded by the ordinary topo-
graphic surface, especially where this is irregular. The
surface is a very uneven plane, which cuts these beds, dikes
and faults at all angles, and since they themselves he at aU
angles, the intersections may be infinitely varied. A bed
or fault having a straight strike may have an outcrop

140 GEOLOGY APPLIED TO MINING.

which will describe many kinds of curves when represented
on the geologic map. The problem is : Given a plane cutting
an uneven surface, where will be the intersection? — the
plane being the bed, dike or fault, and the uneven surface
the surface of the ground. Since the latter is always
changing, the problem does also.

What is the explanation of this outcrop migrationf

On a perfectly plane portion of the earth's surface,
another plane, such as a sedimentary bed, dike, vein or
fault, will outcrop as a staight line, whatever its dip. The
only plane land surface which we find in nature continuing
for a long distance is a horizontal plain. Here then a bed
will outcrop in a straight line, following the direction of
the strike. As soon as irregularities come in, the outcrop
abandons the straight line and wanders in irregular curves
and angles. This is so because the further up an inclined
bed is cut the further the outcrop moves horizontally in a direc-
tion opposite from the dip; the further down it is cut the
tnore the outcrop advances in the direction of the dip. The
amount of the outcrop migration depends on the dip of the
bed. In a vertical bed it is zero ; in a horizontal bed infinity.
It is necessary to be familiar with these laws, for often
when an outcrop occurs it is important to know, both in
geologic mapping and in practical exploring and mining
operations, what is its course over a topographically
irregular country, where slide-rock (talus), gravel wash,
glacial drift, or vegetation renders continuous actual obser-
vation impossible.

DYNAMIC AND STRUCTURAL GEOLOGY. 141

How can one estimate the amount of outcrop migration where
continuous observation is impossible?

By trigonometry, it is easy to find how much the outcrop
of a bed of given dip will advance with the dip out of the
line of strike, or retreat away from the dip out of it, with a
given heightening and lowering. The change in height
may be taken as the peipendicular side of a right trianglfj,
and the dip as the angle opposite the perpendicular. Then
the base is the horizontal migration of an outcrop (or the
actual migration as projected on to a horizontal map), and
the hypothenuse is the actual migration, as measured
roughly on the surface, in an airline between the two exten-
sions of two outcrops, and at right angles to the strike. The
first measurement (the horizontal migration), is exclu-
sively used in mapping; but a cross-section constructed
from the map shows the actual migration graphically. In
practical work an estimation of both the horizontal and
actual migration will be often of value.

The formulas are as follows:

Horizontal migration = change in height multipled by
the cotangent of the dip.

Actual migration = change in height divided by the sine
of the dip.

Taking the change in height as 1 :

Horizontal migration ^cotangent of dip.

Actual migration = the reciprocal of the sine of the dip.

Following is a table for the most important dips:

The figures are calculated for a change in height of 1 unit.
The concrete example of 100 feet has been taken.

142

OEOLOOY APPUfD TO UIKIKG.

Dip of bed,

dike, vein,

etc.

Change of

height in

topographic

surface

Horizontal
migration

of
outcrop.

Actual
migration

of
outcrop.

5°

100 ft.
100 ft.

infinity
1143 ft.

infinity
1147 ft.

10»

100 ft.

567 ft.

576 ft.

15°

100 ft.

373 ft.

386 ft.

2(y

100 ft.

275 ft.

292 ft.

25°

100 ft.

215 ft.

237 ft.

30°

100 ft.

173 ft.

200 ft.

35°

100 ft.

143 ft.

174 ft.

40°

100 ft.

119 ft.

155 ft.

45°

100 ft.

141 ft.

50°

100 ft.

84 ft.

130 ft.

55°

100 ft.

70 ft.

122 ft.

60°

100 ft.

58 ft.

115 ft.

65°

100 ft.

47 ft.

110 ft.

70°

100 ft.

36 ft.

106 ft.

75°

100 ft.

27 ft.

103 ft.

80°

100 ft.

18 ft.

101 ft.

85°

100 ft.

9 ft

100 ft.

90°

100 ft.

Oft.

100 ft.

CONSTRUCnON OF GEOLOGIC SECTIONS.

After plotting observations on maps, what is the next step?

The next step toward the comprehension of the structure
is the construction of vertical sections. Cross-sections (at
right angles to the line of strike) are the most serviceable.

DYNAMIC AND STRUCTURAL GEOLOGY. 143

Where should the cross-sections be placed?

These should be placed, first, where the surface outcrops
give the most thorough data, and, second, neither in the
most simple nor in the most complicated places (for the
first sections, at least).

How is the base for cross-sections constructed?

The base for such sections is taken from the topographic
maps. From a straight line, corresponding in length to the
length of the line taken on the map, perpendiculars are
drawn at the points where there are on the map data as to
relative elevation. These relative elevations are then
measured off on the perpendiculars, from the base line.
The elevation of the base line may be stated as related to
sea-level, where this is known, or to some other datum
plane; or the line may be given an assumed elevation.

Should the vertical scale be different from the horizontal?

Frequently the scale used for plotting these elevations is
greater (twice, three times, ten times as great) than that of
the base line — i.e., the vertical scale is greater than the
horizontal. This gives, in the sections, exaggerated topo-
graphy and exaggerated dips to the strata represented.
But in most cases it is better to have the vertical scale
the same as the horizontal, especially in large scale work
and in mining work; this gives a more accurate, even if
less accentuated, representation of the structure.

How is the outline of the topography obtained?
By drawing a line connecting the points thus marked out

144

GEOLOGY APPLIED TO MINING.

Fig. 23. Method of cons-
truction of a topographic
base for geologic cross-sec-
tions. Scale 1 inch»=1000
feet

Oil the verticaLs, a section of the
topography results (Pig. 23).

How are the geologic data put on this
topographic section?

On this section the geological data
found on this same line, on the geo-
logic map, are plotted, the stratified
rocks showing their dip. In places
where there are no data on this line,
outcrops not too far away may be
represented by prolonging their line
of strike till they intersect the sec-
tion line. As in the map, different
rocks may be represented by differ-
ent colors.

The inspection, on this section,
of the dips of a bed which outcrops
at various places generally allows a
correct reading of the structure, and
an easy deduction of the attitude and
location of the bed beneath other
rocks, away from its outcrops.
Thus the outcrops may be connected
and the complete structure repre-
sented. Mines, wells, bore holes,
etc., are all of the highest value for
supplementing and fixing the elu-
cidation of structure.

DYNAMIC AND STRUCTURAL GEOLOGY. 145

Cross-sections should be made at intervals, at a con-
venient distance apart. Often one well-established cross-
section helps in the working out of the neighboring ones.

Are longitvdinal sections ever advisable?

Longitudinal sections, parallel with the strike of the
stratified rocks and at right angles to the cross-section, are
sometimes valuable. Like the cross-sections, they are
taken vertically and are constructed in the same manner.
Cross-sections and longitudinal sections cross each other,
and at the lines of intersection should be identical, so, when
made independently, they are a valuable check on one
another. When the data for one section are incomplete,
an intersecting section may often supply it with data
which will enable its being worked out. Both cross and
longitudinal sections, when worked out, help to correct the
surface geological map, and to establish more accurately
the boundaries of the different formations, where these are
concealed.

Economic Results of Mapping and Cross-Sectioning.

What is the economic application of this mapping and cross-

sectioningf

In this way it can be ascertained what is the course of an
economically valuable bed, such as one of coal, iron, salt,
borax, oil or water, bedded veins of various metallic
minerals, etc. The approximate position of such beds can
be established beneath coverings of drift, and the proper
places for sinking shafts to find these outcrops can be

146 GEOLOGY APPLIED TO MINING.

determined. The sinking of shafts or the driving of tunnels
in barren rock, lying next to the bed sought after, can be
planned, and the distance that these drivings must be
pushed to reach the bed may be determined beforehand.
The same system, .more carefully carried out, enables the
tracing of an ordinary lode or vein outcrop under drift
coverings; it makes the searching for the underground
continuation of the surface outcrop of a vein, by means of
new shafts and tunnels, in many cases, a matter of close
calculation instead of guesswork.

What amount of geologic knowledge is necessary in order to be
able to make such valuable stvdiesf

To make such geologic studies it is necessary to be able
to recognize the different formations in the field — to dis-
tinguish sandstone, shale, conglomerate, limestone and
quartzite from one another, and to have as much of an
idea of igneous rocks as is given in the preceding chapter.
Close determination of the rocks is usually not necessary,
except for detailed work. The recognition of the relation of
the ores to a certain rock in the district in question, be that
rock what it may, and enough science to follow that important
rock both at the surface and under it, is the essential thing. It
does not concern the miner, in many cases, what the age of
the rock is. One may observe that in a certain silver-lead
district the ore-bodies are generally in or near a certain
shale-bed — the problem is to follow that bed everyivhere.
In another district, one may remark that the ores occur
chiefly near faults; "then the problem is to search for the

DYNAMIC AND STRUCTURAL GEOLOGY. 147

faults of the district, to study which have been the most
favored by ore-deposition, and to inquire into their hori-
zontal and vertical extent.

Mapping and Sectioning of Igneous Rocks.

Can one reason out the underground continuation and 'position
of igneous rocks in the same way as sedimentary rocks?
One must remember, in reasoning out structure, that
sedimentary rocks conform to one another — that is, their
bedding planes are parallel, for they were laid down one on
top of the other, on the sea-bottom. But igneous rocks are
not necessarily parallel to one another; neither do their
boundaries, unlike those of the sedimentary rocks, have
any constant direction. Thus it is difficult to reason out
accurately the outlines of an igneous body beneath the
surface; though the general ideas sketched in the last
chapter will usually enable an approximation. One can
decide whether the rock is a surface flow, the outcrop of a
dike or sill or irregular mass, or is a fundamental body, ami
so can draw his conclusions as to the underground exten-
sion. Generally the direction and dip of dikes can be
obtained from their outcrops; and a study of the fault, fold
and joint systems in the rock frequently throws some light
on the dike system also, for all are apt to be related.

PART II.

ROCK DEFORMATION AND DISLOCATION, AND
THEIR CONNECTION WITH MINERAL VEINS.

MEASUREMENT OF FOLDS AND FAULTS.

Do rock folds have only two dimensions?

Folds and faults must be thought of, not only as they are
represented on cross-sections, in two dimensions, but in all
their three dimensions. Think of a sheet of paper folded
and crumpled — the folds will not always be regular along
the strike, but there will be uneven ridges and hollows.
In geology a ridge, from which the rocks dip away on all
sides, so that every section is anticlinal, is called a dome;
a hollow, of which every section is synclinal, is a hasin. It
is possible, however, that an irregular fold may be anti-
clinal in one section ; and in another, at right angles to the
first, synclinal.

When can the displacement of a fault he estimxited?

Where there is a number of different rocks, such as
distinct beds, which the fault separates, it is only necessary
to match the beds on one side of the fault with the same
beds on the other side, to know approximately how much
they have been separated. The contacts of igneous rocks,

DYNAMIC AND STRUCTURAL GEOLOGY 149

or dikes, or faulted veins or ore-bodies, or even faulted
faults (the fault having cut through and displaced a pre-
existing older fault, as sometimes happens), may also be
matched on the two sides of a fracture to measure its
movement.

Is the separation of the parts of a faulted sedimentary bed
always an accurate measurement of the amount of displace-
mentf

A fault may lie in any plane (for example, it may be
parallel to a sedimentary bed or perpendicular to it), and on
this plane the direction of movement may be represented
by any conceivable line. It may even be parallel to the
plane of the sediinentary beds cut by the fault, in which
case the beds will not be separated, no matter how great
the movement; but a dike cutting these beds at right
angles will be, displaced by the whole movement of the
fault (Fig. 28). It is only when the plane of the sedimentary
beds is perpendicular to the direction of faulting that the
separation of the parts of a given bed is an accurate
measurement of the movement.

Do vertical cross-sections show accurately the displacement of
a fault?

One is apt to consider faults simply as dislocations of
sedimentary beds, and to assume that the amount of
movement which appears on vertical sections is the whole
displacement. The amount of displacement thus shown is
easily found graphically, and is valuable as showing the

160 GEOLOGY APPLIED TO MINING.

existence of a fault, and the break it makes in the sediment-
ary beds; but it does not necessarily convey an accurate
idea of the whole displacement.

Is a more accurate measure of fault-displacement necessary to
mining work?

Suppose that a spherical or lenticular or irregular ore-
body is cut in the middle by a fault, and one half of the
ore having been worked out up to the fault plane, it is
the question to find where the other half has gone. In this
case the separation of the strata (if the ore is in sedimentary
rocks) as seen in a vertical section, gives absolutely no clue
as to either the direction or the amount of displacement.
In mining work, therefore, we must study more closely.

Can we measure fault-displacement in homogeneous rock-
masses?

In homogeneous rock-masses the amount of movement in
faults cannot be ascertained or even approximately esti-
mated. The existence of a movement can be determined
by the record left on the slipping surface or surfaces, in the
shape of ground up rock, (fault-breccia), of polished and
scratched (striated) rock surfaces (sHckensides) etc. But
the amount of friction shown by grinding and rubbing is not
necessarily proportionate to the amount of movement, for
some faults with slight displacement have thick crushed
zones, while others of far greater movement show the
effects of friction to a slight degree only.

DYNAMIC AND STRUCTURAL GEOLOGY 151

Can we accurately measure fatUt-displacement in a heteroge-
neous rock massf
In a rock mass composed of different kinds of rocks, we

may measure with a greater or less degree of accuracy the

amount of movement.

Whxit are the chief aids in the work of measuring faults?

In mining geology it has been found that the more valu-
able aids are (besides sedimentary beds): igneous bodies,
such as dikes; veins; bodies of ore; pre-existing faults;
scratches (striae) on the fault plane, showing the direction
of movement; and the composition of the fault-breccia,
which may show, in some degree, the direction and the
amount of movement.

How do these things afford the necessary data?

The first four guides to displacement above mentioned
are appUcable because any continuous geologic feature,
when broken and displaced, may be matched in imagina-
tion by the observer.

As regards the striae, one rock moving past another along
a fracture will mark the other rock with grooves parallel to
the direction of movement. A given fault may have a
strike N. 45° E. and a dip of 30° to the north-west. On
this fault plane we may find that the striae are nearly hori-
zontal. We then know that along this fracture the faulted
portions moved horizontally past^each other. But we do
not yet know in which horizontal direction a given side
moved. Did the rock on the southeast side of the fault
move to the northeast or to the southwest? This can

152 GEOLOGY APPLIED TO MINING.

sometimes be told from a careful inspection of the strise.
Scratches that are narrow and deep at one end and become
broad and shallow at the other are usually caused by
movements toward the shallow side on the part of the
rock which did the scratching; and, conversely, indicate
movement toward the sharp end for the side which bears
the scratches.

Regarding fault-breccia, we may take as example a
faulted ore-body which leaves in the breccia a "trail'' of
ore, indicating the direction of movement. Concerning the
breccia as a test for the amount of movement, the following
may be said : If we find fragments of a certain rock, such as
a granite or a sandstone, in the fault-breccia at a point
where the wall rocks are both of different nature from
these fragments, then the movement must have been at
least as great as the distance from these fragments to the
nearest place where granite or sandstone forms one of the
walls of the fault.

Can faults be directly measured from the data in questionf

Sometimes the fault-movement may be directly measured
from the aids above mentioned; but more often it must be
calculated from at least two of them. The direct measure-
ment is possible, when the two parts of a separated ore-
body have been found on the two sides of a fault, or where
the intersection of a given dike with a given stratum has, in
the same way, been found on both sides, or where it is
otherwise possible to identify any given point on the two
sides. With zones of homogeneous rock, such as beds,
dikes and veins, the identification of any point is difficult

DYNAMIC AND STRUCTURAL GEOLOGY 153

— hence the unreliability of these bodies alone as registrars
of the true movement.

What are the functions of a fault movement, and how can they
be calculatedf

The following functions of a fault movement are im-
portant:

Dislocation and displacement are general terms, appli-
cable to any part or the whole of a fault movement. Each
of the functions defined below, and to which specific names
are given, may be called simply a dislocation or displace-
ment.

Total displacement is the distance which two points,
originally adjacent, are separated by the fault movement;
the line connecting these two points lies in the faultplane
in all straight faults. It is occasionally possible to deter-
mine the total displacement directly by such criteria as are
mentioned above ; but ordinarily it can only be calculated
or approximately estimated from some of its more easily
measured functions.

Example: The total displacement of a fault can best be
represented by a diagram. Fig. 24 shows a block of the
earth's crust, which is represented for the purpose of
illustration as being transparent. In the figure a portion
of a given sedimentary bed is represented, traversed by a
mineral-bearing vein (or it may be a dike of igneous rock).
This bed and included vein are cut by a given fault plane,
and the movement on the fault plane is such that the vein,
at its intersection with the bed, is separated in the direction
and by the distance a h. This distance is the real (maxi-

164

GEOLOGY APPLIED TO MINING.

mum) fault-movement, or total displacement. On the
earth's surface c is the outcrop of the fault plane, d of the
sedimentary bed, and e of the vein.

The lateral separation is ,the perpendicular or shortest
distance between the two parts of any continuous zonal
body (such as a sedimentary bed), which has been separated
by a fault, the distance being measured along the fault

Fig. 24. Stereogram illustrating the total displacement of a fault.

plane. The lateral separation may be measured in a
vertical, horizontal, or oblique line, according to the
attitude of the bodies between which it is measured, and
in any fault it may vary from zero to the total displace-
ment. The total displacement may often be calculated
from the lateral separation, since the latter is alwa)rs the
side of a right triangle of which the former is the hypotenuse.

DYNAMIC AND STRUCTURAL GEOLOGY 155

Example: The lateral separation of a fault is shown in
Fig. 25, where it is represented by the dotted line be, while
the total displacement is represented by the line ah.

The perpendicular separation is the perpendicular dis-
tance between the corresponding planes in the two parts of a
single body available as criterion (such as a sedimentary
bed), when this body has been separated by a fault, the
planes on each side of the fault being projected for the
purpose of measuring, if necessary.

Example: To illustrate the term perpendicular separa-
tion let us take Fig. 25. This is an ideal representation of a

Fig. 26. Stereogram to illustrate various functions of a fault. a6 is total dis-
placement; be is lateral separation; db is perpendicular separation.

portion of a sedimentary bed which has been faulted along
the short straight edges of the pieces, so that the pieces
come to occupy the relative position shown. Then the
perpendicular distance between the two separated planes,
represented by the dotted line dbj is the perpendicular
separation. If the fault was in the opposite direction, so
that the broken pieces were separated by a gap instead of
overlapping, then one of the planes would have to be
projected in order to measure the perpendicular separation.

15G GEOLOGY APPLIED TO MINING.

The perpendicular separation thus has a certain relation
to the lateral separation; for it constitutes a side of a right
triangle, the hypotenuse of which is the lateral separation,
except in the possible case where the perpendicular and
lateral separation coincide.

This mathematical relation makes it often possible to
estimate the lateral separation from the perpendicular
separation, and from the latter the total displacement.

Example: To illustrate the calculation of one of these
measurements from another, let us look again at Fig. 25,
where ah, the actual fault movement (being the distance
by which the two portions of the intersection of the dike
with the sedimentary bed are separated) is the total dis-
placement, be (drawn along the fault plane, perpendicular
to the edge of the faulted bed, and hence the shortest line
that can be drawn along the fault plane between the
broken edges) the lateral separation, and db, the perpen-
dicular distance between the planes of the separated
portions, the perpendicular separation. Then cdh is a
right triangle, as is bca.

Suppose a case that may often happen, that most of the
figure represented is concealed, as shown by the shading in
Fig. 26, only the light portion (which may represent an
outcrop or a mining shaft or tunnel) being displayed. We
may in any case measure the perpendicular separation.
Then, taking the angle of the fault plane with the faulted
stratum, we may calculate the lateral separation; for this
angle deducted from 90° gives the angle dbc (Fig. 25).
Then the perpendicular separation db divided by the cosine
of dbc equals be, the lateral separation Suppose, again,
the fault plane, as shown in Fig. 26, is scratched (striated)
or shows lines of dragged material, indicating the direction

DYNAMIC AND STUUCTUIUL GEOLOGY

157

of movement. The accurate angle of this direction of
scratching or dragging with a horizontal line drawn on the
fault plane may be subst^acted from 90° to give the angle
abc (Fig. 25). Then the already found lateral separation
6c, divided by the cosine of abc, gives the total displacement
ab.

Of .these three functions, the perpendicular separation is
most easy of measurement, and its value may vary from-

Fig. 26. Stereogram to illustrate the computation of a fault movement, where a
part of the data is concealed.

zero to the full amount of lateral separation. The lateral
separation is easier to ascertain than the total displacement,
and its value may vary from zero to the total dis-
placement. In Fig. 27 a case is illustratetl where
the lateral separation, the perpendicular separation, and
the vertical separation* of the faulted beds are zero; but

*Seep. 160.

158

GEOLOGY APPLIED TO MINING.

if an ore-body has been faulted as represented in the
figure, then the throw and the offset,* which in this
case coincide with each other and with the total displace-
ment, may be measured.

Fig. 27. stereogram of a fault in which the lateral separation, the perpendicular
separation and the vertical separation are zero.

Example: Fig. 28 is an ideal representation of that
class of faults where the movement takes place
along the bedding planes of stratified rocks — ^bedding
faults. With regard to the functions of such a fault, it
will be observed that, as far as the stratified rocks along
whose bedding the fault occure are concerned, the fault has
no perpendicular separation nor vertical separation; and
the other functions are usually impossible of measurement.

Fig. 28. stereogram illustrating a bedding fault.

Where, however, as is represented in the figure, a dike or
vein runs across the stratification and is displaced by the
fault, this affords opportunity for measuring the throw or
offset (which coincide in this case). Moreover, the perpen-

• See pp. 159, 162.

DYNAMIC AND STRUCTURAL GEOLOGY 159

dicular and lateral separation of the dike may be measured,
and perhaps the total displacement may be approximately
calculated or directly measured, as for example, between
the parts of the characteristic curve a (distance Oro),

The measurements which have been defined have no
constant direction, since they refer to fault movements
which are capable of infinite variation. In general geo-
logical work, however, it is often only possible to measure
fault movements along certain arbitrary planes. The
most valuable of these planes are, the earth's surface,
which may be considered a horizontal plane, and vertical
sections, into which available data are put, with the gaps
in the chain of information often theoretically filled out.
In such cases, where some dislocation is evident, but the
formation is so meager that it is not possible to know the
fault so accurately as to estimate even approximately its
total displacement, or lateral or perpendicular separation,
it is necessary to employ specific terms to designate the
known or estimated dislocation, although the relations of
these dislocations to the total displacement may be un-
known. For this purpose the terms offset, throw and
vertical separation may be used. The terms throw and
vertical separation are applied to the dislocation of a fault
as seen in a vertical section; the term offset j to the dislo-
cation as seen in a horizontal section, such as the earth's
surface may be considered to be.

The throw may be defined as the distance between the
two parts of any body available as criterion (such as a
sedimentary bed) when these parts have been separated

1()0 GEOLOGY APPLIED TO MINING.

by a fault, the distance being measured along the fault
plane, as shown in a vertical section.

The vertical separation is the perpendicular distance
between the intersection of any two parts of any faulted
body available as criterion (such as a sedimentary bed),
with the plane of a vertical section, the lines of intersection
l)eing pnijected if necessary for the purpose of measure-
ment. In perpendicular faults the vertical separation is
identical with the throw; in all others it is less than the
throw, but sustains a certain relationship to it, being one
side of a right triangle of which the throw is the hypotenuse.
Thus the vertical separation may vary from zero to the
full amount of the throw. The throw is always a part of
the total displacement, although with no definite relation-
ship to it, and varies from zero to the total displacement.

Example: Fig. 29 is an ideal vertical section of faulted
stratified rocks; ah is the vertical separation, ac the throw.
Suppose the whole belt occupied by the fault covered from
observation in some way : then the only evidence of faulting
which we have would be the fact that in different places we
find the same bed in different positions, and if we project the
different known parts of this bed, they will not meet.
This would be evidence of the probable existence of a fault,
but we would not know the direction, nor angle of it, and
so would be unable to measure the throw even approx-
imately. We would, however, be able lo measure the
vertical separation. If the fault represented in the dia-
gram were perpendicular to the strata, the vertical sepa-
ration would coincide with the throw; if it were horizontal
the vertical separation would be zero.

DYNAMIC AND STRUCTURAL GEOLOGY

161

Fig. 30 represents the relations of throw and vertical
separation, more diagrammatically, and in the case
of a reversed fault.

Fig. 29. Illustrating fault functions.

The vertical separation being measured, the throw may
be calculated, if the attitude of the fault is known; for the

Fig. 80. The relations of throw and vertical separation, in the case of a
reversed fault.

inclination of the fault (as shown in a vertical section)
from the horizontal, minus the dip of the faulted beds,

162 GEOLOGY APPLIED TO MINING,

equals the angle acb (Fig. 30.) (fee, the dip of the fault,
equals ecd, which, minus gcd, the dip of the bed, equals
acb.) Then the throw equals the vertical separation
dixided by the sine of acb.

The temi offset may be used to designate the perpendicu-
lar distance between the intersections of corresponding-
planes in the two parts of any faulted body available as
criterion, such as a sedimentary bed, with a horizontal
plan such as the earth's surface may be considered to be;
the planes being projected for the purpose of measuring,
if necessary. Like the throw, the heave or offset is a part
of the total displacement, but has no definite relationship
to it.

Example: Fig. 31 shows a horizontal surface plan, com-
prising a lake and rivers. The outcrop of the dotted bed is
displaced by the fault, and the offset of the fault is indi-
cated by the dotted lines. If it is desired to find the dis-
tance, along the outcrop of the fault plane, of the two parts
of the bed separated by the fault (a function which we may
term the horizontal throw), this distance may be calcu-
lated from the offset and the direction of the fault outcrop,
in the same manner as indicated for the vertical throw and
the vertical separation.

To sum up, there are six terms which may designate the
different parts of a fault movement, each term applying to a
measurement which varies in accuracy and proximity to
the total displacement in proportion to the available
amount of information. For general outline work where ac-

DYNAMIC AND STKUCTUKAL GEOLOGY. 163

curate data are not obtainable, the terms throw and vertical
separation, referring to the measurement of a fault at its
intersection with a vertical plane, and the term offset, indi-
cating the measurement of a fault at its intersection with a
horizontal plane, are adopted. The throw and offset are
parts of the actual fault movement, but of unknown value,
while the vertical displacement sustains a certain relation-

Fig. 81. Diagram illustrating the offset of a fault.

ship to the throw. Where more complete data are obtain-
able, the terms total displacement, lateral separation, and
perpendicular separation are adopted. The perpendicular
separation sustains a certain relationship to the lateral
separation, as the lateral separation does to the total dis-
placement.

KM GEOLOGY APPLIED TO MINING.

F( )L1)8 AND FAULTS AS LOCI OF ORE-DEPOSITION.

Deposition of Ore in Folds.
In what cases arc ores formed by preference in &ynclines or

anticlines f

Where there is a stratum impervious to water and that
stratum is folded with others, downward moving waters

Fig. 32. Auriferous Saddle Veins. New Chum Consolidated Mine, Bendigo, Aus-
tralia. C is quartz in apex of saddle. After T. A. Rickard.

will be arrested in the bottoms (troughs) of synclines
(dowTifolds), and upward moving waters in the tops of

DYNAMIC AND STRUCTURAL GEOLOGY. 165

anticlines (upfolds). If the waters are mineralizing, the
ores will be deposited by preference in these places. In a
given district the chief mineralization has generally been
brought about in large part either by upward or by down-
ward moving waters, so that the ores may be found either
in the anticlines or the synclines, as the case may be; and
once the law has been discovered it is easy to follow. For
example, if it is found that the tops of the anticlines are
likely to carry ore, all anticlines must be prospected.

Example: In the Bendigo gold-fields, Australia, aurifer-
ous quartz veins occur in highly folded Silurian sandstones
and slates.* The ore-bodies are apt to be especially large
and profitable at the apex of anticlines, forming so-called
"saddles" (Fig. 32), while in synclines similar deposits,
called "inverted saddles,*' though recognized, are rare
and unimportant.

Do folds need to be pronounced, in order thus to determine
ore-depositionf

Undoubtedly a strong fold in a relatively impervious
stratum is more favorable than a weak fold for producing
the localization of ores deposited by circulating waters.
Yet a slight flexure, such as a slight transverse trough or
arch in already highly folded and steeply dipping beds,
may determine ore-deposition and the location of an ore
body.

* T. A. Rickard, TransacHona American Institute Mining I'^nKineers, Vol.
XX, pp. 463 et 8eq.

1()(; GBOLOCJY APPLIED TO MINING.

Example: The ores of the Elkhorn mine, Jefiferson
county, Montana,* lie on the under side of the contact of
limestone (below) and hardened shale (above). These
strata dip 35° to 50° uniformly in the same direction,
forming part of the main fold of the region. In this fold
tlioro are several minor transverse cornigations, forming
arcl\(»s and troughs. The ores occur in two of the lesser
arclios, which pitch steeply, with the general dip of the
strata and unite near the surface to form a single broader
arch. Along the contact of limestone and hardened shale
\ho limestone has been crushed by slipping in the process of
folding. This ciushed rock formed the channel for uprising

Fiff. 38. Occurrence of ore shoots in pitching arches or folds of the strata. Elk-
horn mine, Montana. After W. H. Weed.

metalliferous solutioas. which were confined under the
arclies by tlie overlying relatively impervious hardened
slate, and there the ore was deposited (Fig. 33).

Why do oil and gas often occur at the summits of anticlinal
folds?

The same ])rinciple that arrests and accumulates ascend-
ing waters in the summits of anticlines holds good for other
fluids. Of great interest in this respect arc oil and gas,

* W. H. Weetl, 22d Annual Report United States Geological Survey, Part
II. pp. 492-495.

DYNAMIC AND STRUCTURAL GEOLOGY.

167

both of \¥ihich, forced upward under pressure, often
accumulate under anticlines, especially anticlinal domes,
even if the folds be very gentle and often barely percep-
tible. Borings for oil or gas are generally directed by
preference to these anticlines.

/« this the only reason why the crests of anticlines are often

selected as sites of ore-deposition f

In the proce&s of folding the tops of the anticlines are
the most pulled apart, the troughs of the synclines the
most compressed; hence at the tops of the anticUnes

Fig. 84. Vein formation iD the apex of an anticline. New Chum Railway mine,

Bendigo, Australia. M N\a apex of saddle occupied by

quartz. After T. A. Rickard.

there is Ukely to be a strong jointing and fracturing. This
permits the passage of waters and so determines a water-
course; and if the waters contain metals they may be
precipitated here.

Example: 1. In many of the saddles of auriferous
quartz in the Bendigo gold-fields Australia, mentioned
above, the vein penetrates upward through the beds along

168 GEOLOGY APPLIED TO MINING.

the axis of the anticlinal fold, in such a way as to indicate
that it has selected this position on account of the zone of
weakness. Fig. 34, showing an ore-body in the New
Chum Railway mine, is illustrative of a number of such
cases described by Mr. Rickard.

2. The mining district of Tombstone, Arizona, has as
rock formations a sedimentary series of limestones,
quartzites and shales, intruded by granodiorite* and
overlain by rhyolite. The sedimentary rocks are also cut
by many small dikes of granitic and dioritic rocks. The
series has been folded, producing anticlines, which are
often highly compressed, and occasionally faulted; and to

Fig. 35. Deposition of ores in anticlinal folds. Tombstone district, Arizona.
After John A. Church.

this folding and Assuring the rocks owe their ore. These
ores extend along the stratification, as bedded deposits, or
cut across it as true veins, or again have quite an irregular
shape; and all of these types run into one another. The
bedded deposits and the veins lie in general in the anticlines,
while the synclines are barren. Although the evidence
suggests that the ores have been deposited from uprising
waters, there is no impervious stratum which has arrested
the upward passage of the solutions and brought about
precipitation. In one anticline there may be as many as
three separate sheets of ore, one over the other.f

* A granular rock intermediate in composition between granite and diorite.
t John A. Church, Trantactions American Institute Mining Engineers, Vol.
XXXTII, pp. 3-37.

DYNAMIC AND STRUCTURAL GEOLOGY. 169

The result of folding has been to produce openings in the
anticlines, both between the strata and as cross-cutting
fractures; these openings have constituted channels for
rising watei's, and along them the ores have been deposited
where favorable opportunities, such as a chance for replace-
ment of the limestones, oflfered themselves (Fig. 35).

Deposition of Ore Along Faults.
Why are ores often formed on or near faults?

MineraUzation often takes place along fault zones be-
cause these ha\e afforded the most available circulation
channels for the mineralizing waters.

Example: An example is found in the Aspen district,
Colorado.* Fig. 36 is a section in the Bushwhacker-Park
Regent mine, showing this feature. The ores have chiefly
formed along a fault which makeS onl)' a comparatively
slight angle with the stratification, and especially at the
intersection of this fault with others. Thus the ore-bodies
are restricted to certain localities on the faults, while
other part.s of the faults are slightly or not at all miner-
alized. The reason for this is partly because some of the
faults originated subsequent to the chief period of oie-
deposition; but chiefly because the junction of two faults
made very favorable conditions for ore-deposition, as
explained in the paragraph on the principle of inter-
sections (See p. 196).

Are the largest faults the most favorable for ore-deposition?

The magnitude of the fault has no relation to the relative

likelihood of ore-deposits, for the favorable circumstance is

• Monograph XXI, United States Geological Survey, pp. 229-231.

iro

GEOLOGY APPLIED TO MINING.

the Assuring and crushing, producing channels of circula-
tion, and not the fault movement. Thus a very slight
fault may be far more thoroughly mineralized than a large

one.

Fig. 86. Cross-section of Bushwhacker-Park Regent mine, Aspen, Colorado.

Dark-shaded areas are ore-bodies. Heavy black lines are

faults. After J. E. Spurr.

Why, in a faulted country are the ore-bodies irregular and
ivhy do they often form rather near the faults than on themf
A fault is generally not a single plane; it is a zone of

close-set fractures, the movement being most intense along

DYNAMIC AND STRUCTURAL GEOLOGY. 171

a certain line and dying out slowly on both sides. Fre-
quently the rock on both sides of the fault-zone is thoroughly
wrenched and seamed with tiny cracks, even where it
appears solid to the naked eye. The mineral solutions are
more effective among slight fractures than in a large fissure;
thoroughly seamed rock is a very favorable place for ore,
because the solutions are checked and held in a way that
seems fitted for the working of the reactions which lead to
the precipitation of ores. If mineraUzation is slight along
the main fault fracture it may be considerable along some
of the auxiliary fractures, and in the strained rock near by.
This is especially the case in limestones, where great
deposits thus originate. Therefore the search for minerali-
zation along a fault plane, in districts where the two are
associated, should extend over a comparatively wide zone.

Eocample: 1. The veins of Rico, Colorado, as described
by T. A. Rickard and F. L. Ransome, are mostly along
fissures which have been opened by faulting. The dis-
placement of these faults, as shown on each side of the
veins, is, however, generally less than 10 feet. As a rule,
the more important faults of the region are not attended
by much ore. The ore-bearing fault is often a slight aux-
iliary slip, occurring beside the main plane of movement
of a larger fault, which is barren.

The result of dynamic strain in this region was the
development of planes or zones oT weakness, cracks and
fissures; and along these there was generally movement of
the rock on one side past the rock on the other. All of
these openings became the channels of circulating mineral-
bearing waters. In the smaller channels, however, (which
naturally were along the smaller faults) the circulation

173

GEOLOGY APPLIED TO MINING.

Sandstone. Sandy Limestone. Rhodo- Quartz. Zinc Cruised
Lime. chrosite. Blende. Rock.

Fig:. 37. Ore deposition in fissure along a minor fault. Section across the Eureka
vein, Rico, Colorado. After T. A. Rickard.

DYNAMIC AND HTRUCTURAL GEOLOGY. 173

was slow, SO that plenty of time was given for the processes
of deposition to act; while in the larger channels the circu-
lation was probably in many cases so rapid as not to allow
much precipitation along them. Fig. 37 shows a fissure
vein along a minor fault.

2. In the Queen of the West mine, Ten-Mile district,
Colorado, are sandstone and shale beds, with generally
conformable porphyry sheets. Faulting has taken place
along a series of parallel and closely contiguous planes, so
that the rock has been divided into thin sheets, each of
which has moved past the other a certain distance. In
the central part of the fissured zone the spaces between the
sheets have been filled with vein material, and the sheets
themselves decomposed, impregnated, and somewhat
replaced by it. The resulting condition is puzzling for the
miner who expects to find his ore bounded by well-defined
walls. There are here walls in abundance, but no one wall
can be followed continuously for any great distance.
Therefore it is the custom to run frequent cross-cuts away
from the main drift (which follows the central zone) and
these cuts disclose ore-bodies running parallel to this zone,
now on one side and now on the other, and often 15 or 25
feet distant from it.*

JOINTS IN ROCKS.

What are joints in rocksf

Joints are planes of fracture, or divisional planes, which
run through rocks. Few rocks are without them. Frag-
ments of broken rocks are often more or less completely

* S. F. Emmons, Transactions American Institute Mining Engineers, Vol.
XVI, p. 837.

1T4 GEOLOGY APPLIED TO MINING.

bounded by plane surfaces, whether the rock is igneous or
stratified. In stratified rocks one or two of the plane
surfaces are apt to be due to the stratification; the others
are joints. In igneous rocks all the planes are usually
joints.

How are joints produced?

Joints are produced by the application of force to the
rock. Earth movements may cause a strain or twisting —
the result is a system of cracks, such as we find when ice
or glass is put under such strains. According to the
nature of the stress, the number of joint systems vary,
together with their general direction and their direction
relative to one another, and the relative abundance of
joint cracks in the rock.

Fig. 88. Columnar Jointing of basalt, Koyukuk mountain, on the Yukon riyer,
Alaska, After J. E. Spurr.*

* 18th Annual Report United States Geological Survey, Part III.

DYNAMIC AND STRUCTURAL GEOLOGY. 175

Are all joints formed by regional strains f

Another kind of joint is formed by contraction. This
is found in most lavas and in many small dikes. The
heated rock shrinks and cracks on cooling. The resulting
jointing is usually systematic; it runs in the direction of
least resistance and hence is vertical to the planes of the
flow in lavas, and perpendicular to the walls in dikes.
Its effect is to divide the rock into columns — ^hence the
term columnar structure (Fig. 38). This columnar joint-
ing may also originate by reason of rocks shrinking through
chemical changes.

Ore-Deposition Along Joints.

WhcU advantage is there to the mining man in the study of

rock-joints?

Joint-planes have nearly uniform directions for long
distances, traversing even folded beds. Since they are the
record of strains in the earth's crust, their study should
never be neglected by the student of mining geology.
Frequently there is an observable connection between them
and fold and fault systems, mountain ranges, etc., in the
same district. Moreover, joint planes, especially when
closely set together, furnish a channel for underground
waters. Hence the joint systems may correspond to the
vein systems in a given district, and a study of the former
helps in exploring and exploiting to best advantage the
latter.

Example: The mining camp of Monte Cristo, in the
Cascade Range, Washington, is in a district where ninety-

176

GEOLOGY APPLIED TO MINING.

nine per cent, of the ores have formed along joint-planes.
These planes have furnished channels for circulating
waters and as a consequence the minerals which these
waters carried have been deposited near the joints (Fig. 39).
A study of the laws of jointing here is directly applicable
to the mineral veins, for every pecuUarity of the jointing is
copied by veins, with the added complication that the vein
(following the former or even present channel of easiest
circulation for waters) may pass from one joint to another.

SCALE or FEET

Fig. 39. Formation of ores along joints. Tunnel and vein exposures in vertical
cliflf , Glacier creek, Monte Cristo, Washington. After J. E. Spurr.

How should one study joints so as to arrive at an under-
standing of the system?

The strikes and dips of joints in various places should be
recorded on the map by the same symbol given for strati-
fied rocks; an accumulation of these records and their
combination usually enables one to comprehend the joint-
systems.

DYNAMIC AND STRUCTURAL GEOLOGY. 177

FRACTURES AND FISSURES.
What is meant by the term fractures as used in mining geology f

The teiin fractures is generally applied to cracks in rocks,
large enough to be distinctly visible to the naked eye.
The fractures may also come under the head of joints, in
which case they are joint fractures or joint cracks; or, if
there has been movement along them, they may also be
faults and fault fractures. As generally used, the term
denotes a break of an importance intermediate between a
joint and a fault, as these latter are most commonly
employed.

Fractures are, like joints and faults, the result of the
straining and cracking of rocks under pressures. A fault
plane is usually accompanied on both sides by parallel
fractures extending a greater or less distance away from it,
and there may or may not be faulting along them. In
regions where the rocks have been jointed by stress, one or
several of the systems may be so strongly developed as to
cause marked fractures, often a conspicuous feature in the
general appearance of the rocks.

In what way is this stress applied so as to form fractures and

fissures?

According to the way motion takes place in the crust,
certain portions may be stretched or compressed. By
both these methods fractures and fissures may originate;
in the former by the production of actual cracks from the
stretching; in the latter by the greater action of the com-
pressive strains along certain lines, there producing zones

178 GEOLOGY APPLIED TO MINING.

of more considerable crushing, shearing and faulting. It
is certain that open cracks and fissures may be formed
directly from stretching (tension) ; while, from the nature
of things, it is mpossible that openings are directly pro-
duced by compression. In the case of fracture zones
resulting from compression, however, release of the pressure
may allow them to open somewhat; they then become the
channels for circulating water's, and these may dissolve or
otherwise carry away the broken or ground-up material,
leaving a continuous opening or series of openings.

Twisting or torsion of a rigid body has been experi-
mentally found to produce systems of cracks intersecting at

Fig. 40. Sheet of glass cracked by torsional strain. After Daubree.

right angles. The force in this case seems to be tensional
(Fig. 40). This process probably takes place in the earth's
crust.

Earthquake shocks probably produce shattering and
fracturing of the rocks. This also is a sort of tensional
stress.

Eruptions and intrusions of volcanic material may cause
fracturing and fissuring of the rigid rocks through which
they pass, especially in the general neighborhood of the
surface.

DYNAMIC AND STRUCTURAL GEOLOGY. 179

What are conjugated fractures, and how are they formed?

It has been shown experimentally and by calculation
that compressive stress, applied horizontally in a given
direction (for instance, from north to south), will produce
two systems of fractures,' striking east and west, and
dipping respectively north and south at angles of about
45°. These two systems of fractures, parallel in strikes
but opposite in dip, are designated conjugated fractures.

Are fractures straight, or irregularf

As a rule, fractures, like joints and faults, being due to
stress, are straight, approaching as near as possible mathe-
matical planes. In rocks which are uniform in texture
and resistance, therefore, fractures generally deviate but
little from a straight course; but as nearly all rocks are
more or less irregular in these particulars, small and even
large irregularities will be found in actual fractures. The
same is true in fault planes.

Example: In the Mercur mine, Mercur district, Utah,
there is a system of open cracks and fissures, cutting the
limestones and also traversing a system of calcite veins,
later in age than the limestone, but earlier than the fissures.
These cracks often follow the course of a calcite vein,
which evidently offered less resistance than the hmestone.
Even where the vein and the fissure are perpendicular one
to the other, the latter is often deflected by the former
(Fig. 41). The accompanying sketch is from the wall of a
tunnel in the Mercur mine.

180

GEOLOGY APPLIED TO MINING.

Again, the visible crack which we call a fracture may be
a combined series of joints or fractures, each approaching a

-^^ ^^ r" ^- 1^

w^w

^ ^ r^ ^

.F^~Fr r^ ^^^"

i^O

r!ip'~rr^

^-^^-^ ^=^j=;

m-_)^_^ _'^ ^'

^"~F"

^ F^ F^^^^

C^cUcite. ZiTnvestoTie, , «^__-». ^

r*vA7"l rb=^ It I ipen. fissure..

Fig. 41. Open Assure cutting and deflectea by calcite vein, Mercur mine, Utah
After J. E. Spurr.*

mathematical plane, but together having an irregular
course.

* 16th Annual Report United States Geological Survey, Part II. p. 400.

DYNAMIC AND STRUCTURAL GEOLOGY. 181

How do fractures behave in different stratified rocks?

In non-homogeneous rock, as has been pointed out,
fractures, from whatever initial stress they may arise, will
tend to be deflected in the direction of least resistance. In
thin beds of homogeneous rock, such as a dense sandstone
or limestone, this direction is apt to be directly across the
bed, perpendicular to the stratification.

In a shale, on the other hand, the fracture will tend to
be deflected in the direction of the bedding, and in slates
in the direction of the cleavage. A fracture may entirely
cease on encountering a transverse fracture, the move-
ment being deflected and taken up by the latter.

Fractures are naturally most clean cut and persistent
in rigid rocks, like quartzites and igneous rocks. In soft
rocks like shales, any movement arising from stress may
be partly or wholly taken up by the yielding of the rock
near the disturbance, a giving way analogous to flowage.

Therefore, in passing from one stratum to another; such
as from a sandstone into a shale, fractures often change
both in direction and intensity. A strong fracture in a
sandstone bed may die out entirely in passing into a shale.

Are fractures and joints always persistent even in rigid and
homogeneous rocks?

Fractures, joints and faults may become less, and even
die out in rigid strata, where there is, nevertheless, enough
yielding progressively to take up more and more of the
dislocation.

182 GEOLOGY APPLIED TO MINING.

What are irnhricating fractures?

It frequently happens that when a fracture stops, the
continuation of the same line of weakness is shown by a
parallel fracture, a little to one side, beginning near where
the other leaves off, or overlapping. These may be called
imbricating fractures.

How are open fissures formed near the surface?

Even in greatly disturbed, folded, faulted and jointed
rocks, open fissures, larger than cracks, are relatively rare
and unimportant. At the very surface, the shrinkage of
volume in rock*^ resulting from chemical changes, and the
falling apart under the influence of gravity, cause many
fissures. But these conditions are characteristic only
near the surface. The rock encountered in mines is much
more soUd.

Example: The granite rocks of Cape Ann, Massachusetts,
especially as exposed along the shore, are traversed by
many open joint fissures and by joint fractures (Fig. 42).
At a very little distance below the surface, however, the
fissures disappear, and the fractures diminish greatly in
frequency, so that the rock found in quarries is a good
Imilding-stone.

lloio are openings below the sufface formed^ a)id to what depth

do they persist?

Fissures encountered in mines are not usually produced
directly ])y dynamic action, but are due to the dissolving
action of underground water circulating along faults,
fractures, or shear-zones. In easily soluble rocks, Kke

184 GEOLOGY APPUED TO MINING.

limestones, openings made in this way are especially large,
and a series of irregular connecting caves may result. In
loss soluble rocks, solution will generally not produce caves,
but only irregular widenings of the original crevice; the
resulting water channels will be straighter than in soluble
rooks, but will open out at one place, and contract almost
to nothing at another. In the case of the soluble rocks,
ojHMi spaoos are due almost wholly to solution; in the case
of the lUrtioultly soluble ones, the same may be the case;
but ill neither would the water have ordinarily been able
to gain access, so :v^ to accomplish its dissolving^ without
some preliminary channel due to rending. One can put
it jis an almost invariable rule, therefore, that fissures are
not open anil regular for long distances. They are rather a
string of coimeoteil openings of limited extent. This is
nooossarily true, for a regular open fissure any distance
uiulerground would soon be closed by the effects of gravity.
Irregular openings, with buttresses of solid rock between,
supporting the weight ctn both sides, can and do remain
open to considerable depths. At a certain ultimate depth,
however, it is supposed that the great pressure (com-
bined with increaseil fluidity of the rock, due to increase
of temperature) is sufficient to close even openings of this
sort.

Deposition of Ores Along Fractures and Fissures.

What is the application of the study of fissures and fractures
to the study of mineral veins?

Fractures and fissures become the channels for circulating

DYNAMIC AND STRUCTURAL GEOLOGY. 185

waters, and the seats of vein formation. Each one of their
characteristics, therefore, is characteristic also of a certain
class of mineral veins.

Do the veins in a given region ever follow definite systems in
regard to their trend?

Fractures and fissures, it has been pointed out, often
result from a regional strain in the crust, affecting large
areas alike. Thfey are, therefore, formed in definite sys-
tems, and where they become mineralized, the veins fall
into similar groupings.

Example: In the southwest of England, a series of
fissures runnhig north and south, or north-northwest and
south-southeast, traverses another series which runs in a
more east-and-west direction. The latter in Cornwall
contains the chief copper and tin ores, while the former
contains lead and iron. The east-and-west veins in the
west part of the region were formed before those that cross
them, for they are shifted, and their contents are broken
through by the latter.*.

What bearing has the irregular course of fractures on mineral
veins?

Mineralizing waters follow along fractures in all their
deviations. Along their course they often deposit ore,
both in the fractures and in the rock, by cavity-filling,
replacement or interstitial filling (impregnation).

* De La Beche, 'Geological Observer,' p. 659.

186 GEOLOGY APPLIED TO MINING.

Example: Many of the ore-bodies in the Tintic district,
Utah,* furnish excellent examples of veins formed along
circulation channels offered by successive fractures running
in different directions. The accompanying figure is from
the Ajax mine in that district. The country rock is lime-
stone, and the ores (which consist chiefly of pyrite, galena,
and enargite, carrying gold and silver, oxidation products
of these sulphides, and quartz and barite as gangue miner-
als) have been deposited as replacements of this rock, in
the neighborhood of the fractures. The fractures as a rule
are continuous past the point where they cease to be ore-
bearing, though this is not well represented in the diagram
(Fig. 43). This type of vein is an important and common
one, and grades into very irregular ore-bodies.

Does the peculiar behavior of fractures and joints in different
stratified rocks find an analogous behavior in mineral
veins?

Mineral veins in different kinds of stratified rocks show
a variation which corresponds exactly to the variations of
fractures under like circumstances.

Example: In the Bendigo gold-fields, Australia, the
auriferous quartz veins, largely in sandstone and shale,
show many irregularities illustrating these points. Fig. 44
shows the cross-section of an open cut behind the Victoria
Quartz mine in this region. Here is seen how veins in the
standstone stop abruptly on reaching the shale; how
others, stronger, persist into the shale, but are deflected in

* G. W. Tower. Jr.. and G. O. Smith, 19th Annual Report United States
Geological Survey, Part III, p. 724, et aeq.

DYNAMIC AND STRUCTURAL GEOLOGY.

187

1 ^

I-

188

GEOLOGY APPLIED TO MINING.

the direction of the bedding, and finally die out. The
deflections of veins in sandstone, passing through a slate
bed, is further shown in Fig. 45, from the same district.*

Fig. 44. Open cut ot Victoria Quartz mine, Bendigo, Australia, a, slate; &,
sandstone; c, gold quartz veins. After T. A. Rickard.

Fig. 45. Quartz veins in Confidence Extended mine, Bendigo, Australia.
After T. A. Rickard.

* T. A. Rickard. * The Bendigo Gold Field,' (Second paper). Tran9ac-
tions American Institute Mining Engineers, Vol. XXI, pp. 686-713.

DYNAMIC AND STRUCTURAL GEOLOGY. 189

Does the increase in the number and size of cracks and fissures
close to the surface have an influence upon mineral veins?
The cracks and fissures near the surface, opened up in
the manner previously described, veiy frequently become
the channels of mineralizing waters, and the sites of ore-
deposition, and are thus transformed into mineral veins.
Such veins, following the characteristics of the joints,
fractures or fissures along which they were formed, will be
largest and most numerous at the surface. Below the
surface they will become less in number, and will tend to
unite, forming a smaller number or even a single well-
defined strong vein, which persists to a considerable depth.
This occurrence is a matter of common observation among
miners, who often remark that a vein is "all broken up"
near the surface, and that it will get "more regular as it
goes down."

Under what conditions are branching surface veins, like those

described^ formed?

It follows, from the method of origin of the fractures in
which the veins were deposited, that the ores were brought
to their position and there laid down at a very slight
distance from the surface. The waters which are active
close to the surface are descending atmospheric waters, and
in a case of this kind it is usually these which have formed
the veins. The fewer and more regular veins attained in
depth are frequently the product of an earlier period of
deposition, which the surface waters have worked over and
re-deposited to form the surface ores, as will be explained
in the next chapter.

190 GEOLOGY APPLIED TO MINING.

What are filled depositsf

Veins or ore-bodies deposited in pre-existing cavities,
(not microscopic), whether caused by rending or solution
(generally by both) may be called filled deposits.

What is crustification?

Crustification is a banded structure produced by suc-
cessive deposition of different layers on the walls of an
opening; it is often visible in filled deposits.

Do all filled deposits show crustification?

Many deposits which have formed in open cavities show
no banding, but an irregular arrangement of minerals, or
are massive and homogeneous throughout.

7s all banding in veins, parallel to the walls, crustification?

Replacement deposits may often show banding in all
degress of perfection, sometimes simulating almost exactly
the crustification of filled deposits. This arises from the
existence of bands of different texture or chemical compo-
sition in the original rock. Certain of the bands may
induce the formation of a particular mineral during the
process of replacement, or at any rate may cause differences
of texture, even if the minerals deposited in the different
bands be approximately the same. Along fracture-zones
or shear-zones a very perfectly banded ore-deposit may
form by replacement, for the parts along the fracture
planes are replaced first, and afterward, more slowly and
under different conditions, the rock space between the
planes. This results in either physical or chemical differ-

DYNAMIC AND STRUCTURAL GEOLOGY. 191

ences, which are plainly visible as bands. Again, the
fracture crevices may be filled with vein matter, and the
sheets of rock between may be mineralized by the replace-
ment process: and thus a banded appearance results.

Example: According to S. F. Emmons there is in many
of the ore-deposits in the Gunnison region, in Colorado, a
noteworthy appearance of banded structure parallel with
the walls. Yet the evidence of thin sheeting of the country-
rock is so clear that it is probable this appearance arises
from the fact that some of the bands are the filling of
narrow fissures, and others a replacement of thin sheets of
the country-rock, the differing composition of the bands
resulting from the variation in the process of deposition.

Ribbon structure, described on p. 199, as a sheeting pro-
duced parg-Uel to the walls of a vein, by movement subse-
quent to its formation, may also be mistaken for crustifi-
cation.

What is a fissure vein?

The best type of the filled deposit is the fissure vein,
which may or may not show crust ification. Such a vein is
characterized by regular, straight walls, by a fairly constant
width, and by a definite direction of both strike and dip.
There is usually a sharp line of division between the vein
and the wall rock, such as is generally wanting in replace-
ment deposits.

Do ore-deposits in pre-existing cavities always form a distinct
class from other deposits?
Ore-deposits which have filled pre-existing cavities may

192

GEOLOGY APPLIED TO MINING.

also extend into the wall rock of the fissures, by replace-
ment or impregnation, and often no line can be drawn
between the ores formed by one process and those formed
by another, the two sorts forming a continuous body.

Whai are linked vei7isf

Linked veins are the filling of a series of branching and
reuniting fractures, of a peculiar type which can be best
shown by illustration (Fig. 46).

Fig. 46. Linked veins. Surface plan of vein system of Pachuca, Mexico.
Scale l-50,00a After E. Ordonez.

Example: In the mining district of Pachuca, in Mexico,
the veins follow a general east-west course, and are united
by diagonal branches. The peculiar character of each
branch is that it never crosses the veins that it unites.

DYNAMIC AND STRUCTURAL GEOLOGY. 193

Often two branches start from a vein at the same point and
run in opposite directions, so that one is apparently the
prolongation of the other; this circumstance has led some
miners to the belief in the crossing of the branches, and
has led them into serious mistakes.*

SHEAR ZONES OR CRUSHED ZONES, AND THEIR
SUITABILITY FOR ORE-DEPOSITION.

What are shear zones or fracture zones, and what influence

have they on ore-depositionf

When a rock mass is put under pressure by earth move-
ment, some parts of the rock, being weaker, will yield and
will be crushed, bent and broken to a greater extent. These
areas are apt to be fairly regular, and generally they form
pretty well defined zones of variable thickness, often with
obscure walls between them and the more solid rock. If
there has been movement along such a crushed zone, so
that the rock on one side has changed position noticeably
with the rock on the other side, it becomes a fault or a
fault zone. Often, however, there is hardly any noticeable
faulting, and in this case we may call it a shear zone, if the
rock has been crushed and sheared, or a fracture zone, if
it has only been especially intensely fractured. In either
case such a disturbed area or zone offers a channel for
circulating waters, and is very favorable to ore-deposition,
for the comparative slowness of circulation enforced by
the obstructed passage makes the percolation thorough,
and offers every chance for precipitation.

* E. Ordoiiez, Boletin del Institute Geologico de Mexico, 'El Mineral de
Pachuca,' p. 57.

194 GEOLOGY APPLIED TO MINING.

Example: The gold-quartz veins of Otago, New Zealand,
described by T. A. Rickard*, have formed largely in shear
zones, and the lodes show every variation from a condition
where the country rock (schist) forms the greater part, to
the entirely replaced stage, where the vein is clear aurif-
erous (juartz. They are found in channels but little
divideil from the main mass of the country rock, and the
schists themselves, beyond the lode boundaries, are often
auriferous. Probably certain belts of the schist, outside
of the lodes, are sufficiently mineralized to become mines.

CiENERAL RELATION BETWEEN ROCK DIS-
TURBANCES AND ORE-DEPOSITS.

Are regions of iifuii^turbed rocks favorable for ore-deposits?

In general a region of flat unfolded rocks is poor in ore-
deposits, a.s for example the region lying between the
Rocky Mountains and the Appalachians, as compared with
the folded region lying between the Rocky Mountains and
the Pacific. Where ore-deposits do occur in such a flat
region, they will often be found to be connected with some
minor disturbance.

Example: In southern Missouri and adjacent parts of
Kansas and Arkansas, the fiat Paleozoic strata, together
with underlying ancient crystalline rocks, have been
affected by a monoclinal uplift, elliptical in outline,
known as the Ozark uplift. On the sunmiit the bedding
planes are horizontal, while throughout the border areas
they are inclined away from the center. This disturbance
has produced fracturing, more pronounced along the

* Transactions American Institute Mining I'Jngineers, Vol. XXI, pp 411-442.

DYNAMIC AND STRUCTURAL GEOLOGY. 195

borders than on the tops, and the principal mining local-
ities (producing lead and zinc ores) are situated around
this border in such a way as to indicate that the mineral-
ization has been dependent upon the fracturing. In this
case the mineralization is thought to have been brought
about by descending waters, and not to have been con-
nected with igneous rocks or hot springs.*

What is the reason for the. general connection of ore-deposits

with disturbed rocksf

Mountains, igneous rocks, folded strata,, hot springs, and
ore-deposits are often all connected. The zones of
folding in strata lie along certain lines of weakness in
the crust. The relief of pressure caused by the giving
away of the strata in the folded region may cause a migra-
tion of the suppressed molten rock beneath the crust to
this zone; eruptions and intrusioas, accompanied by
further disturbances, fc^llow. Fractures and fissures are
fomied; by the influence of the igneous rocks hot spring
action is set up; and the igneous rocks themselves contain
disseminated metals which they supply to the circulating
waters. As a consequence of part or all of these conditions
various kinds of ore-deposits result.

THE INTERSECTION OF CIRCULATION CHANNELS
AS SEATS OF MINERALIZATION.

What are ore-shootsf

Veins or lodes are not usually equally rich throughout.
Poor or barren spaces of lode -separate ore-bodies irregular

♦ E, Hftwortb, Bulletin Geological Society America. Vol. II, pp. 231-240.

196

GEOLOGY APPLIED TO MINING.

in form or having more or less roughly a columnar shape.
The latter are called ore-shoots or chimneys (Fig. 47).
What is thr principle of intersection ds regards ore-deposits?
A large proportion of ore-shoots is fonned by the inter-
section of two water-courses. This may mean the inter-
section of two faults, of two joints, of a joint with a fault
plane, of joints or faults with a porous stratum, etc. The

.^ /

/!l ! V^

;;

k'1

('■• v

1«

»;/ /}y

Fig. 47. Ore-shoot in Annie Lee mine, Cripple Creek, Colorado, o, ore-shoot; 6,
dike in which shoot lies. After R. A. F. Penrose, Jr.*

principle is nearly the same throughout, and is explained in
Chapter V. It is at the intersection of two circulation chan-
nels, whether now or only formerly used by the solutions,
that one may look, in almost any district, for the richest
ore-bodies. In well-defined veins, the pockets or richest
portioas are apt to lie at the juncture of the main vein with
subordinate intersecting fractures or veins (feeders).

* 16th Annual Report United States Geological Survey, Part II.

DYNAMIC AND STRUCTURAL GEOLOGY. 197

ROCK MOVEMENTS SUBSEQUENT TO ORE-DEPO-
SITION.

Do rock movemerds occur sithsequent to ore-depositionf
^' In some cases we find a vein or ore-deposit entirely un-
affected by movements of the rocks after its deposition;
but generally there has been some subsequent disturbance,
producing folding, faulting, shearing, jointing, fracturing,
and fissuring, which affect the ore-body in the same way as
the enclosing rock. These movements may be very slight,
or they may be profound.

How do subsequent movements diminish the value, of an ore-

hodyf

The bending, breaking and separation of the part^ of an
ore-body or vein may make it difficult to follow it in
mining; or so expensive that the profit ^iH not pay for the
labor involved; or, sometimes, practically impossible.

Do movements subsequent to ore-deposition always decrease

the value of a vein or other ore-body?

Sometimes the disturbances may have an effect beneficial
to mining. The folding and faulting may so displace the ore-
body as to make it more accessible to mining operations
than it otherwise would be. Take the case of a coal or an
ore-bearing bed, for example which dips steeply into the
earth. The deeper such a bed is followed, the more
expensive and difficult becomes the mining. But if it
is folded and faulted so that it comes to the surface in a
number of different places, then it can be easily worked at
each of these.

198 geology applied to mining.

Dislocations Subsequent to Ore-Deposition as Seats
FOR Later Mineralization.

Is the foregoitig the only way that movements subsequent to ore-
deposition operate to increase the value of a vein?

An ore-body may be traversed by joints, or fissures,
which afford channels for waters to circulate, where other-
wise the openings have been completely cemented by ore
and accompanying gangue. These new openings may be
in time partially or wholly cemented up with gangue, or
with ore and gangue, and frequently the waters will work
over the old ore and reprecipitate it in concentrated form,
both in the fractures and in the vein near by. Thus
these portions may become the richest in the vein, and
perhaps the only portions that it will pay to work. Many
ore-shoots are of this origin. Again, the new solutions
may bring fresh metals from some outside source, which
they may deposit in or near the new fractures, either adding
them to the earlier deposited metals or depositing them
independently; and in this way also richer bodies may be
formed in the older vein.

In what direction are the subsequent fractures most likely to
occur with respect to the original veins or shoots?

Movements in rocks are Hkely to be very long continued,
though intermittent; and planes or zones of weakness
being once formed, renewed movements are likely to take
place along them. The openings along which ores are
formed are planes or zones of weakness, hence move-
ment may occur along them while the first deposition

DYNAMIC AND STRUCTURAL GEOLOGY. 199

is taking place, or after it has closed. Even though
the opening has been entirely cemented up by ore and
gangue, the regions of weakness will often remain as
such, because the parallel parting planes of the veins and
the encasing rock preserve the original slip-surfaces, and
because the brittle quartz, calcite, etc., of the vein may
be in many cases more easily broken than the tougher
and more yielding rock. Therefore, movements subse-
quent to ore-deposition are very likely to fracture the
veins parallel to their course, the fractures either lying in
them or alongside of them; and are also likely to renew
the fractures whose intersection with the main original
fracture and vein zone gave rise to ore-shoots. In this
way later parallel bauds in the main vein, of different
character (both as regards mineral composition and value),
may be produced; and the old shoots may be enriched by a
second deposition.

Ribbon Structure.

What is ribbon structure?

Movements in veins subsequent to their formation ma)'
produce a sheeting parallel to the walls, which may have
somewhat the aspect of original crustification, and may be
mistaken for it. This sort of banding is called ribbon
structure.

Example: The gold-bearing quartz veins of Nevada City
and Grass Valley, California, typically show sheeting or
ribbon structure, due to movement since deposition. True
original banding, or cnistification, is also found in these
veins, and often occurs in the same specimen of rock a.s the

200 GEOLOGY APPLIED TO MINING.

ribbon structure. Fig. 48 is a photograph of a specimen of
vein quartz containing gold-bearing pyrites from the
Providence mine.

Faulted Faults and Their Relation to Ore-Depo-
sition.

Are the faults of one period ever faulted by the favlts of a later

period?

Where there are a number of faults, developed at diflE^nt
peri()(is, the later movement may take place along the same
plane as the older one; thus along an old break the new
disturbance will continue the faulting and increase it.
Fault fissures which have Ijecome occupied by oi'e-bearing
veins often experience such renewal of motion, and we find
evidence of it in the crushed ore and vein material, which
may subse(|uently become re-cemented by new mineral
deposition, and yet will always show the angular outlines
due to breaking.

Again, the later faults may be developed along planes not
parallel to the old ones, and so cut and displace the old
faults in precisely the same way as the enclosing forma-
tion . Where ores have formed along both the earlier and
later faults, one vein may be found faulting another.

A specially complicated and likely case is where faulting
goes on for a long period slowly, and contemporaneously
with a persistent process of ore-deposition. The first ore-
deposits may be subsequent to the first folds and faults,
but they will be disturbed by the later movements ; yet these
later faults may be chosen for the seats of newer ore-deposits,
which may again be broken by still more recent movements,

202

GEOLOGY APPLIED TO MINING.

and so on. In such cases only careful examination of the
phenomena connected with each separate ore-deposit can
determine its age relative to the various displacements, and
serve as a guide to mining operations.

c i>

Fig. 49. Faulting in Smuggler and Molly Gibson mines, Aspen, Colorado, a.

Carboniferous shales; 6, porphyry (intrusive sheet); c, Carboniferous

limestone; d, Carboniferous dolomite; e, Devonian quartzite

series; /, Silurian dolomite; gr, Cambrian quartzite;

h, Archaean granite. After J. E. Spurr.

Example: A good example of successive faults acting in
different directions is found in the Smuggler and Molly
Gibson mines, Aspen, Colorado.* Study of the geology
here shows that first the rocks were folded and acquired
a steep dip (Fig. 49). Next came the development of the
Silver fault, nearly parallel to the bedding, but of such great

* J. E. Spurr, Monograph XXXI, United States Geological Survey, pp.
181-188.

DYNAMIC AND STRUCTURAL GEOLOGY. 203

displacement as to cut out the porphyry sheet h and the
limestone c, so that the shale a was brought into contact
with the dolomite d (Fig. 49 B). Subsequently came a
series of east-west faults dipping to the south (such as the
Delia fault) which faulted the Silver fault together with the
rock formations (Fig. 49 C). Finally there came a slipping
on the old plane of the Silver fault, which locally deviated
from that plane, and so constituted an independent fault
(Clark fault). The final result is shown in Fig. 49 D. The
Silver fault was formed before ore-deposition; the Delia
fault began to form before ore-deposition, but continued
after it. The Clark fault was formed after the main ore-
deposition, yet secondary more recent ores have, to a
sUght extent, formed along it.

ROCK MOVEMENTS ALONG EARLIER-FORMED
DIKES.

Why do veins sometimes form along earlier-formed dikes?

Following up the principle indicated in the fore-
going pages, we may remark that dikes of igneous
rock are usually intruded along . lines of weakness in
the rocks which they cut. In the same way as mentioned
in the case of veins, the zone of weakness, though to a cer-
tain extent cemented by the dike, is still apt to remain
weaker tham the rest of the rock. Any renewal of the
strains, therefore, is likely to produce a renewed fracturing
along this line, creating a new channel, which may become
the passage of circulating waters and in this way be again
cemented (this time by water action), and become a
mineral-bearing vein. The contact of the dike with the
fractured country rock is usually the weakest line; hence,
later veins are apt to occupy this position.

204

GEOLOGY APPLIED TO MINING,

Exampfe: The Black Jack-Trade Dollar vein, De Lamar
district, Idaho, consists of a quartz and orthoclase-feldspar
(valenciiiriite) gangue enchasing; ricli silver and gold
minerals in ^inall qiiantitiet^. The lower part of the vein
is situated at the contact of granite with a basalt dike a

6 e tL C h

Fig. 50, Vein fullowm;^ tlit? euiiirsie af a pre-exist i rig: dike Trade Dollar vein. He

Lamar district, Idaho. ti,"basalt dike; 6, i^raulte; c, vtJln quart:;.

After W. Liudgrtm.

few feet in t!uckness. The vein is separatetl from the basalt
by well-defined walls antl gouge, and often shows co nib-
structure.

There is evidence that fracturing and faulting have taken
place at the contact of the granite with the basalt dike,
involving a horizontal throw of 125 feet. Thus the old
fracture zone, which existed before the advent of the dike,
was reopened, and ga\'e passage to water-s which deposited
the vein* (Fig. 50).

♦ Wttldemar Limlgreti, 20tli Armiml Re^Kirt United Stateji Upolofiical
Survey, Pari Ml, pp. I.i4, 1(15, ete.

DYNAMIC AND STRUCTURAL GEOLOGY. 205

PART III.
PLACERS.

Why are placers considered under the head of dynamic

geology?

To dynamic geology belongs not only the study of rock
movements beneath the crust, but also those on the surface.
Thus under this head we naturally take into consideration
stratified ore-deposits, so far as these are chiefly of mechan-
ical origin.

The most important class of stratified deposits are the
placers. The name placer is applied to detrital deposits
of metals or valuable minerals, especially gold.

THE CONCENTRATION OF GOLD IN PLACERS.

What is the origin of placersf

Rocks at the surface are broken up by erosion and find
their way, by the power of gravity, aided by running water,
down into the valleys. In the highest valleys only the
coarser fragments remain, for the streams carry away the
smaller ones. Further down, as the stream current loses
its force, some of these smaller ones are deposited, and
only the still finer material is carried on, until nothing but
silt or mud is left. In a gold-bearing region, the veins are
broken up and sent on their journey in company with the
detritus of the enclosing rocks.

206 geology applied to mining.

Concentration by Chemical Water-Action.

How is gold freed from associated baser metals, in the surface

outcrops of veins?

Gold, in deposits not too close to the surface, generally
occurs in small quantities in intimate association with
metallic sulphides, such as pyrite (iron sulphide), galena
(lead sulphide), arsenopyrite or mispickel (sulph-arsenide
of iron), these sulphides being contained in quartz veins.
Near the surface, atmospheric agents attack the veins
chemically, and, if erosion is slow enough to let these
agents exercise their full influence in decomposing, dissolv-
ing, carrying away and re-depositing the various con-
stituents, the result is that the surface portion comes to
have a different character from the deeper part. The
sulphides are broken up and taken into solution; and the
metals thus dissolved are either carried quite away or are
re-deposited in deeper parts of the vein. But gold is soluble
with much greater difficulty than most other metals; hence,
when the sulphides which contained it are dissolved, it is
mostly left behind, in its native state, as free gold.

How is gold purified and chemically concentratedf

Where the surface rocks are decomposed, the gold,
mixed with the debris produced by erosion, may then be
already in the free state. Frequently, however, the
sulphides outcrop, or the gold is imperfectly separated
from other materials. Then in the gravels exactly the same
process goes on as we have described as occurring in the
vein outcrop, and (on account of the great porosity of the

DYNAMIC AND STRUCTURAL GEOLOGY. 207

gravels, permitting atmospheric waters to attack freely
every part), the ^ baser metals are carried away in solution,
and the gold is left behind or is dissolved and re-precipitated.
This is one reason why so much of the gold in placers, when
examined microscopically, shows unscratched or even
crystalline surfaces, indicating chemical deposition. Frag-
mental pieces of gold may receive fresh coatings from solu-
tions thus originating; or the solutions may deposit gold
upon fragments of organic matter, or metallic sulphides,
for these substances exert a precipitating effect.

It is even probable that gold already deposited in the
native state may be, to a slight extent, re-dissolved and
re-arranged.

Some observers, seeing the evidence of this chemical
action in placers, have concluded that gold might be intro-
duced into the placers from other localities, in solution in
surface waters. But it seems certain that the ruling influ-
ence is mechanical, and that chemical influence is only
auxiliary, producing further concentration and rearrange-
ment.

Why do gold placers often occur near mines containing chiefly
other metals, such as silver, lead, copper, etc.?
The fact that from all the metals carried from a vein to
the gravels, only gold survives, the rest being more or less
fully removed in solution, explains why many rich silver
regions, such as the Comstock and Leadville, were first
worked for their gold-bearing gravels. The ores carry a
certain proportion of gold, and it is this, freed more or less
completely from the other metallic constituents of the

208 GEOLOGY APPLIED TO MINING.

ore-deposits, which becomes placer gold. Even ores con-
taining only traces of gold may thus gi>*e rise to gold-
bearing gravels.

Concentration by Mechanical Water-Action.

How is gold mechanically concentrated in placers?

Particles of native gold in gravels, brought down into
the valleys by mechanical action, and freed from other
metals and often increased beyond their original size by
chemical action, are in the valleys still further mechan-
ically concentrated. Waters shift the gravels so that the
heaviest minerals, especially the gold, sink naturally to the
bottom; and, where there is not much disturbance by
running water, the gold particles seem to be able to work
downward through the loose gravel, probably during such
movement as the deposit imdergoes from percolating waters,
or from alternate thawing and freezing.

What is the pay-streak in gravels?

The result of the downward sifting of the gold is that
where the valley-deposits (sand, gravel, etc.) are porous,
by far the greater quantity of gold will be found at the very
bottom, in the few inches overlying the bed-rock. This is
called the pay-streak.

How is it that gold is often found in the bed-rock, under the

gravel?

The bed-rock itself, (especially if it is a shale or a schist,
and so affords cracks for the gold to work itself into), is

DYNAMIC AND STRUCTURAL GEOLOGY. 209

commonly rich for several inches in depth, and is taken up
by miners and worked with the gravels. This fact some-
times leads to the belief that the gold was originally con-
tained in these rocks; but generally the rock a little distance
below the surface will be found entirely barren, disproving
this supposition.

Why is the gold not especially concentrated in the pay-streaks

of certain placers?

Where the gravels are not sufficiently porous, the gold
cannot work itself down so well, and as a result it may
occur scattered throughout the whole deposit, though in
this case the gravel is relatively poorer per cubic yard than
the pay-streak on the bottom of porous gravels, the amount
of gold which is usually concentrated in the pay-streak
being distributed throughout the mass.

What is a false bottom, and how may it cause a second pay-
streak?

When there is, in the deposit, an impervious layer, such
as a clay seam, this will arrest the downward working of the
gold, and above it a pay-streak will be formed. A lava
bed, or a solid conglomerate, may play the same part.
Such an impervious layer is often called the false bottom,
from having the appearance of being the base of the
gravels. There may be several of these, with intervening
gravel beds, one below the other, each overlain by its pay-
streak; beneath, the real bottom may also have its pay-
streak.

210 GEOLOGY APPLIED TO MINING.

Example: Quartz creek, Seward Peninsula, Alaska,
contains gold-bearing gravels on its bottom and sides
(Fig. 51). The gold now (1900) being mined lies 2 or 3
feet below the surface, on what the miners call the bed-
rock, which is a blue clay, apparently intercalated in the
gravels. This blue clay afforded a floor upon which the
gold was concentrated. The real bed-rock at this point
has not been reached.

Fig. 51. False bottom of clay in a gold placer deposit. Cross-section of Quartz
creek, Kugruk, Alaska. Adapted from A. H. Brooks.*

What is the origin and significaiion of black sand and ruby
sand in gravels?

By the natural process of concentration other heavy
minerals are also collected, but, as none of them are so heavy
as gold, they are concentrated to a less degree. The
magnetite which is present in many rocks is concentrated,
and becomes the black sand or magnetic sand of miners;
the garnets found in many schists and other metamorphic
rocks form the ruby sand, etc. So, when a miner washes a
pan of gravel and gets a little black sand, his experience
generally tells him that the chances of gold are small, the

* United States Geological Survey, 1901, 'Reconnaissance in Nome
Region/ etc., Fig. 2.

DYNAMIC AND STRUCTUIUL GEOLOGY. ^ill

exact reason being that the materials in this gravel have
not been concentrated. In many regions the auriferous
veins are in rocks containing garnet; and the prospector
rightly concludes that the presence of ruby sand is a
favorable sign. On the other hand, the presence of either
of these sands does not necessarily indicate 'even a small
quantity of gold.

Effects of Glacial Action.

What foundation is there in the theory often held by miners,
that glaciers are responsible for many placer-deposits?
Frozen water, (snow, and especially ice), is a powerful
erosive agent. It fills crevices in rocks, and by its expan-
sion in freezing rends them apart; it accumulates in masses
on the steep hill sides in high mountains as mountain
glaciers; it moves down into the valleys as valley glaciers;
or, finally, piling up over mountain and valley, it forms a
great ice cap, or continental glacier. The slowly flowing
ice grinds away the rock on its bottom and sides, and
carries along on its surface what slides down on it from
cliffs above. So in glacier regions there is generally a
much greater abundance of surface debris than in un-
glaciated ones. It may be, therefore, that glaciers are
often effective in breaking up auriferous rocks; but the
cases where profitable placers are due entirely to glacial
action are probably few. This remark is made because it
is a favorite theory with miners "that the gold was brought
down by glaciers." Placers often occur in districts which
do not have any glaciers and never had any. It is true

212 GEOLOGY APPLIED TO MINING.

that many regions now bare of glaciers were formerly
covered with them, such as the great glaciated areas of
Canada and the North Eastern United States; but in each
place we must find the characteristic mark of glacier
deposits — unstratified drift or "till/' ice scratches, glacial
topography, etc., — before we can allow this factor to even
become possible in any theory of placer formation.

Besides the ground-up, unstratified drift which is the
product of the glacial mill, streams derived from the melting
of the glaciers, coming from their surface and below, work
over, rearrange and deposit the drift in more or less strati-
fied form. Such action tends to classify the minerals
present, but the process is generally incomplete as com-
pared with that accomplished by streams in valleys.
Hence, even stratified glacial deposits are not very favor-
able for placers.

Nevertheless, ordinary streams may take up material
supplied by glacial action and by "classifying" it so as to
shake the gold down to the bottom, produce good placers.
In some cases, even, the material ground out of auriferous
rocks by glaciers, and worked over by glacial streams,
may be rich enough in concentrated gold to be valuable.

Example: The Blue Spur placers, in the Otago district,
New Zealand, have been described by T. A. Rickard,*
who considers them due to the combined action of glacial
ice and glacial water. The deposit is probably as old as
the Eocene period, and belongs to the class of old placers,t
being above the bed of the present streams.

* Transactions American Institute Mining Engineers, Vol. XXI, pp. 432-436.
tSecp. 222.

DYNAMIC AND STRUCTURAL GEOLOGY.

213

The depression in which the auriferous gravels rest is
not that of an ordinary river valley, but is rather an
irregular cup-shaped depression in the schist (Fig. 52).
In the bottoms and sides of this hollow there are also other
smaller hollows in the bed-rock, such as are not usually

Fig. 52. Basin coutaining auriferous glacial gravels. Blue Spur placers, Otago
district, New Zealand, a, auriferous gravels; &, schist. After T. A. Rickard.

formed by running water (Fig. 53). Therefore, the basin
is believed to have been scooped out by a glacier, to which
conclusion the occurrence of large bouldere in the deposit,
derived from a locaUty 25 miles distant, lends support.
After scooping out the depressions, the glacier is sup-

Fig. 53. Irregular depressions filled with auriferous glacial gravels.
T. A. Rickard.

After

posed to have retreated, leaving the hollows to be occupied
by a lake. Materials ground out of the auriferous schists
by the glacier in the new position were carried into the
lake, where they accumulated in thick, coarse, rudely
stratified deposits. The gold gradually sifted its way to

214 GEOLOGY APPLIED TO MINING.

the lower portions. These deposits are now transformed
into hills by the erosion of streams, which first drained the
lakes, and then cut down through the lake sediments.

VARIOUS KINDS OF STREAM GOLD PLACERS.
What are guich placers?

Gulch placers are formed in the highest narrow valleys,
or gulches, of a river system. They usually head in hills
or mountains, and the material in their bottoms, though
rudely stratified, is coarse and shows only slightly the
effects of wear and transportation. In the extreme upper
p(jrtion of the gulch, where it heads in the bed-rock, gravel
is often wanting, but the amount of it increases as the
gulch gains depth. The gulches are generally more or less
V-shaped in outline; hence the width of the deposit is
slight. The gold, being near its place of origin, is com-
paratively coarse. The relative richness of various
gulches, even neighboring ones, varies greatly, according
to the richness of the rocks through which they cut.

Example: Myrtle creek, Koyukuk district, Alaska, may
be selected at random from a host of examples. The
gravels here rarely exceed 3^ feet in thickness, and overlie
mica-schist and slate, which stand on edge. The gold is
found principally on or near bed-rock, in the joints, fissures,
and cleavage crevices (Fig. 54).

What are the characteristics of broad valley placers?

The upper valleys or gulchas unite further down to form
larger and broader valleys. Here the stream flows in a
level plain of gravelly materials, which stretches back to

216 GEOLOGY APPLIED TO MINING.

the valley sides. As the stream wears away one bank and
builds up another, it changes its position, and so, at one
time it mns along one side of its valley, and at another
time the opposite side. In this lateral swinging it works
over, classifies and smooths the gravels of its flood-plain.
In auriferous regions these gravels become placers.

The valley gravels are in far greater quantity than the
gulch gravels; and, since with increasing distance from the
head of the stream the gradient of the stream usually
decreases, permitting increased deposition, their thickness
is comparatively great. On account of the more complete
work of the swinging rivers, the gold content is apt to be
more uniform than in the gulch placers; and since not only
the rich but the barren gulches have contributed their
material, this content is apt to be considerably less than the
rich but hmited gulch placers.

Where is the most gold apt to be found in valley gravels?

Although valley placei-s attain often to a considerable
depth, the statements made above concerning the working
downward of the gold in gulch placers, by reason of its
gravity, seem to apply here also.

7s the gold of broad valley placers of the same kind as that in

the gulch placers?

Naturally, in broad valley placers the gold is geJierally

of finer size than in the gulches.

■•
What are bar placers or bar diggings?

"When a stream runs through auriferous gravels and by

DYNAMIC AND STRUCTURAL GEOLOGY.

217

undercutting its banks brings down and works over large
quantities of these gravels, the gold undergoes a further

Fig. 55. Diagram of ideal river, showing accumulation of bars. Crosses show
the most favorable spots for the deposition of gold. After J. E. Spurr.*

concentration in the stream current. At places where the
current slackens to the right point, the heavy gold and the

■* 18th Annual Report United States Geological Survey, Part III, Fig. 24^

218 GEOLOGY APPLIED TO MINING.

coarse pebbles are deposited; further on, the fine gold and
the smaller pebbles and sand. On such a river "colors'*
(small flakes) of gold may be found everywhere in panning,
but the richest spot is where the most and the heaviest gold
has been deposited, on bars. Fig. 55 shows the spots along
such a stream where the gold will best accumulate. Bars
are always the first point attacked by the prospector in a
new count r>\ They are often very rich, but are quickly
worked out, and in general do not call for more approved
appliances than the cradle or the long-tom.

Example: In Alaska, river bars were worked as early as
1861, near the mouth of the Stikine river. Subsequently
gold was founrl in the bars of many of the other Alaskan
rivers. In 1885, bars on the Stewart and Lewes rivers were
worked, and soon afterward on I'orty Mile creek. Not
until 1SS7 did the pioneers advance from bar mining to
gulch diggings, in one of the branches of Forty Mile creek.
Since that time the practical exhaustion of the bars has
thrown all the enterprise into gulch mining, and even into
bench mining.*

BEACH PLACERS.

What are beach placers?

When a river, rising in a gold-bearing region, reaches
the sea with a slow current, it carries only fine mud or silt,
and the finest possible particles of gold. These gold flakes
almost float on the water; they are largely taken into
solution by the sea water, whence it comes that this water

* H. B. Goodrich, 18th Annual Report United States Geological Survey.
Part III, pp. 107-125.

DYNAMIC AND STRUCTURAL GEOLOGY. . 219

contains gold to the amount of about 11 milligrams to the
ton.* Part is probably deposited with the settling mud,
but the amount is very small and of no direct com-
mercial importance.

But where the rivers discharge into the ocean with a
strong current, they carry coarse rock fragments and gold
particles of considerable size, which are deposited on the
sea shore. The waves and currents work this material
sidewise till it forms beaches extending along the coast.
The surf, continually moving that portion of the material
which comes within its reach, often effects a concentration,
the gold being accumulated and much of the lighter
material swept away. Shore ice, especially in northern
regions, may also be sometimes an important agency in
working over the material. Thus beach placers are formed.

Sometimes the shore waves undercut a gravel bank con-
taining gold, and then concentrate the material in the
same way as before. This kind of beach placer differs
from that above mentioned, in that the rivers do not
directly contribute the gold to the beach; yet they do it
indirectly, for the gravels undercut by the waves have
generally been brought to this position by rivers — that is,
they are broad valley gravels, or they are old sea-shore
gravels brought down by former streams, and raised high
and dry by an uplift of the crust.

Example: Among the most famous beach placers are
those of Nome, Alaska, which caused a stampede of many

* Luther Wagoner, Tranaaciions American Institute Mining Engineers,
Vol. XXXI, p. 807.

220 . GEOLOGY APPLIED TO MINING.

thousand prospectors in 1900. These are examples
of the type of beach placers described in the last
paragraph. In this region the rocky hills are bordered
along the sea by a flat coastal plain, composed of
auriferous gravels brought down by rivers and spread out
under the sea as a marine shore deposit, at a time when the
land was at a lower level than at present (Fig. 56). Sub-
sequently the land was uplifted, and the marine gravels
became transformed into the present shore plain. The
strong waves of this region cut back the gravel and wash it
away, and concentrate the gold, forming rich, but limited
deposits. Therefore the gold in these placers has been
successively concentrated by waters more than once.

Fig. 56. Diagrammatic section of beach placers at Nome, Alaska. After
Schrader and Brooks.* *

Do beach placers extend seaward under the waier?

Beach placers, Uke bar placers, are almost invariably,
from their nature, shallow and hence short-lived. They are
confined to a narrow strip along the beach, for, even when
the gold has been derived from auriferous gravels forming
the shore, these older gravels will be relatively much
poorer, and either will not pay for working at all, or must be
worked on a larger scale at a much smaller profit per ton.
That portion of the gravel seaward from the surf-beaten

* United Stat^ Geological Survey, 'Reconnaissance in Nome Region, 1901/

DYNAMIC AND STRUCTURAL GEOLOGY.

221

zone will not have undergone the concentrating action of
the surf, and will also ordinarily contain a very much
smaller proportion of gold.

BENCH PLACERS.
What are bench placers or bench diggings?

A river valley often shows along its sides shelves, terraces,
or benches, part of the old river bottom when the stream
was at a higher level, in which bottom it has cut itself a
newer and deeper channel (Fig. 57). If the rock region is
gold-bearing, and especially if there is gold on the bars and
in the valley gravels of the present streams, then gold may
be also looked for in the gravels lying on these high benches.

Fig. 57. Bench and valley placers. Section across Rye valley, Blue Mountains,
Oregon. After W. Lindgren.

Examples: 1. In the Nome district, Alaska, the sides of
the present stream valleys are covered with gravels and
marked by terraces or benches, representing former levels
of the down-cutting streams. Such benches occur at
elevations of from 10 to 100 feet above the present stream.
The gravels on them are known to be auriferous.*

2. Rye valley. Blue Mountains, Oregon, is cut in tilted
Tertiary lake beds. On its sides is a series of river-cut
terraces, on which lie later (Pleistocene) gravels, which are

♦Alfred H. Brooks, ' Reconnaissance in Nome Region,* etc., United
States Geological Survey, 1901.

222 GEOLOGY APPLIED TO MINING.

gold-bearing (Fig. 57). There are six or eight of these
benches, of which the largest is several hundred feet wide.
The gravel is coarse and well-rolled. In the bed of the
valley are still younger gravels, also gold-bearing.
The gold is fine, and is richest at the bottom of
the gravel deposits.*

OLD PLACERS.
What are old 'placers?

Sometimes the earth's surface is disturbed by crustal
movement. In some cases the movement may be a
general elevation or depression, while in other cases a gentle
tilting is produced, so that one portion of a given region
is relatively more elevated or depressed than another.
Again, the disturbance may be quite irregular, producing a
warping of the crust. After such movements the rivers
change their velocity and often their direction, adjusting
themselves to the new slopes of the surface.

A certain river may be running rapidly down a steeply
sloping country, and, on account of its strong current, it is
steadily cutting its bed deeper into the rock. A gentle
crustal movement occurs, and the lower portion of the river
experiences more upUft than the upper. The gradient is
changed; the current becomes sluggish; the stream ceases
to cut into the rock, and most of the detritus which is
brought down is not carried out to sea as formerly, but is
smoothed out by the stream along the valley. Thus the
valley becomes more and more deeply covered with

* W. Lindgren, 22d Annual Report United States Geological Survey,
Part II. p. 788.

DYNAMIC AND STRUCTURAL GEOLOGY.

223

gravels, and hence more and more shallow, and it may end
by being entirely filled up.

On the other hand, take a region of sluggish streams
which have filled up their valleys with gravels, and think of
the region being slightly tilted so that the streams begin to
run rapidly. If the tilt is in the direction of the old streams,
the new ones will have much of the former direction, but, if
it is in other directions, the new rivers may flow at right
angles to the old, or even in the opposite direction, for a
country whose rivers have filled up their valleys will be
nearly flat and will permit the streams to change their beds

Fig., 58. Generalized section of an old placer, with technical terms as used in

California, a, volcanic cap; 6, upper lead; c, bench gravel; d, channel

gravel; c, bed-rock; /, rim. Adapted from R. E. Browne.*

easily. In any case, new channels are cut. When this is
accomphshed, the old river gravels will be left between the
new valleys, and, in proportion as the latter are deeply
cut, the former becomes relatively high above them.
These old gravels may have been placers, and, when thus

* Report California State Mineralogist, 1890, p. 437.

224 GEOLOGY APPLIED TO MINING.

left on or under the hills above the present valleys, they
may be called old placers (Fig. 58).

Examples: 1. The auriferous gravels of the Sierra
Nevada, in California, are classic examples of old placer
gravels. They represent Tertiary streams w^ich had
entirely different valleys from those of the present day.
These old gravels have been sufficiently explored, by drift
mining and in other ways, to show in general how the
Tertiary valleys lay and in what directions the streams
ran; moreover, the general surface of the Tertiary land,
between the valleys, can be ascertained. The accom-
panying map by W. Lindgren* (Fig. 59) shows the topog-
raphy of this period, with the present topography beneath
in fainter lines. From this map it may be seen that while
the features of the Tertiary (Neocene) topography showed
prominent relief, the surface was much less cut by deep
ravines than at present. In general the present streams
run at right angles to the former ones.

After the old auriferous gravels were laid down, great
flows of volcanic rock filled up the valleys, and, as the
tilting took place at about the same time, the new streams
followed entirely independent sources. They have now
cut down their beds so as to expose the Tertiary gravels at
many points.

2. In the Otago gold district. New Zealand, described by
T. A. Rickardf, the rivers flow rapidly, and in large part
run through narrow gorges which they have recently cut.
They have excavated their courses down below the level of
their ancient valleys, which were mostly broad and filled

* 17th Annual Report United States Geological Survey, Part II, PI. II.
t Transactions American Institute Mining Engineers, Vol. XXI, p. 411.

DYNAMIC AND STRUCTURAL GEOLOGY.

225

g
I

^ .2

> ^

osS
•o a

, O

22G

OEOLOOY APPLIED TO MINING.

with alluvium. The gravel in these old valle3r8 (of lower
Miocene and Eocene age) forms the placer diggings of the
present day (Fig. 60).

Old placers, being generally old broad valley placers, are
commonly very thick, and contain the most gold in the
lower part, the main pay-streak being as a rule just above

PLAN SECTION

Fig. 00. Old auriferous gravels (MiooeneX Otago district, New Zealaad.
After T. A. Rickard.

bed-rock. On account of their position, they are well
adapted for hydraulic mining, and on account of their
usually easy drainage by tunnels, for drift mining.

FOSSIL PLACERS.
What is meant by the term fossil placersf

The old placers proper are usually of Tertiary age; they
are plainly river deposits belonging to an age just preceding
the present, and, save for thin lava beds or barren gravel
deposits, they are not covered by younger beds. But
placer gravels may be of any age. They may be hardened
into rocks (conglomerates), and be folded and faulted so as
to lose all evidence of their original relation to any stream.

DYNAMIC AND STRUCTURAL GEOLOGY. 227

They may be deeply buried by the accumulation of later
beds; and, when again exposed by erosion, they may out-
crop either in the mountains or in the valleys. But they
will often still retain the gold that was in them origi-
nally, and may be profitable for mining; or, when they
are attacked by erosion, the gold will accumulate in
stream bottoms to form a new generation of placers.

The number of instances where fossil placers permit of
productive mining is not so large as might be expected; but
it is probable that, in many cases not yet recognized,
modem placers derive their gold from old conglomerates
which are of this nature. Solid rocks must contain many
times more gold than loose gravels to be equally profitable.

Example: On Pole creek, a branch of Cherry creek,
Madison county, Montana, a thick conglomerate (maximum
thickness 500 feet) lies unconformably below Cambrian
beds, and above Archaean gneisses and schists. This con-
glomerate seems to be auriferous throughout its extent, and
the gold in it has been explained as the result of mechanical
concentration on the shores of the pre-Cambrian ocean.
It is thus a fossil beach placer of pre-Camhrian age.*

RE-CONCENTRATED PLACERS.f

Any one of the classes of placers above mentioned may
have derived its material wholly or partly from older
placers. Thus the gold may have to pass through several

* A. N. Winohell, Tranaactions American Institute Mining Engineers, Feb.-
May, 1902.

t This term was first used by Alfred H. Brooks. 'Reconnaissance in the
Cape Nome fiegipn/ AlMka, United States Geological Survey, 1901. p. 149.

228 GEOLOGY APPLIED TO MINING.

successive concentrations, at different times, before it can
render a placer profitable. A gulch placer may represent
the re-concentrated remains of a bench placer, and a bar
placer may be a re-concentration of the gulch placer.
Similarly a fossil placer may be attacked by erosion and
its gold concentrated anew. Numerous other combina-
tions have commonly occurred.

Example: On the river GallikO; in Macedonia, in the
district of Kilkitch, are placers worked at least as early as
the time of Philip of Macedon, the father of Alexander
the Great. According to modern ideas, they are very
low grade, and are washed only by a few peasants. The
original source of the gold is in veins which contain silver-
bearing galena, carrying small quantities (usually only
traces) of gold. These veins have been extensively eroded,
the silver and lead have been dissolved and carried away,
and the gold has been left in fine particles in the gravels.

During the late Tertiary period a great thickness of such
gravels accumulated in the broad valleys of the Tertiary
rivers. Although the gold in these gravels is very scarce
it is nevertheless everywhere present, and the deposits may
be considered as broad valley placers.

In these old gravels certain definite ancient stream beds,
usually marked by coarser gravel, can be distinguished, and
in such channels the gold is much more abundant than
elsewhere. These are probably accumulated bar or
stream placers formed in streams which ran through the
broad valley deposits, and derived their gold from them.
This is the second stage in the concentration.

Since the formation of the gravels the land has been
slightly uplifted, and streams have cut down through them.
The placers worked to-day are the sands in the present

DYNAMIC AND STRUCTURAL GEOLOGY.

stream-beds. A small part of the gold in these placers is
derived directly from the veins, but most of it comes from
the older gravels. Where the stream cuts one of the above-
mentioned ancient stream channels, the deposit is rela-
tively rich. This is the third stage of concentration, and
only at this point is the gold sufficient in quality to be
worked.

PLACERS OTHER THAN GOLD PLACERS.

What are the characteristics of platinum placers?

The geology of platinum placers is like that of gold
placers. By its resistance to atmospheric agents of
decomposition, platinum, like gold, retains its integrity
while other minerals are decomposed and dissolved; and
by reason of its great weight, it is left behind in streams
where lighter material is carried away. Thus a natural
concentration is effected. Platinum is often found with
gold in placers, a circumstance to be explained by their
common resistance to disintegration, and their common
great specific gravity.

Example: The platinum placers of the Ural Mountains,
in Russia, have been the most productive in the world.
Along the Tura river and its tributaries the placer gravels
have a width of from 400 feet to half a mile. The gravel
varies from 8 to 24 feet in thickness, and is overlain with
peat about 4 feet thick. The richest gravels are those
directly above the bed-rock, not exceeding 4 feet in thick-
ness. The pebbles are nearly angular, with frequent large
boulders, and are entirely due to river action. Gold and
platinum occur together in the placers (Fig. 61).

230

GEOLOGY APPLIED TO MINING.

Fig. 61. Platinum placers. Plan of a portion of ttie river las, northern Ural
Mountains, Russia. After A. Zaitseff.*

What are th£ characteristics of tin placers?

A large proportion of the world's supply of tin occurs in
placers, usually called stream-works. The metal is in the
form of the oxide, cassiterite, a heavy black mineral that
does not have a metallic appearance, but has a dull stony

♦ *Die Platiniagerstatten am Ural.'

DYNAMIC AND STRUCTURAL GEOLOGY.

231

look. It is, however, nearly as heavy as metallic iron,
and in gravels tends downward. Besides this, it is with
great difficulty attacked by decomposition. Tin placers
are made up from the debris of tin-veins in the solid rock,
and wherever one of these deposits is found the other may
be looked for.

Example: In the district of Siak, Sumatra, alluvial tin,
often in workable quantities, is found in the gravels in the

-0.8 FT.

YELLOW CUY —

ANGULAR QUART2 AND CARBONIZED WOOD 0.7 FT.

FINE GRAY CLAY 0.15 FT,

PAY GRAVEL 0.20 FT.

^ TOUGH GREY CLAY -

-10 TO 11.00 rr

, YELLOW DECOMPOSED ROCK — — - 1 TO 4.00 FT.

t IMPURE 8AN08TONE, BED-ROCK

Fig. en. Section of tin placer, Kotta Ranah, Siak district, Sumatra.
After C. M. Rolker.

stream valleys.* The bed-rock consists of impure sand-
stones and quartzites. with some silicious granite con-
taining muscovite and tourmaline — minerals frequently
associated with tinstone. The stream channels are covered

232 GEOLOGY APPLIED TO MIKING.

by a shallow gravel deposit, growing shallower toward the
heaiis of the streams, till bed-rock is reached. The tin-
bearing stratum consists of angular quartz gravel and
tinstone. The pay-streak is only a few inches thick, and
is overlain by a bed of quartz gravel containing little tin-
stone; and this by sandy clay and surface vegetable
accumulation. It is underlain by a tough gray clay,
which, as in the case of gold placers, has formed a false
bottom or false bed-rock, by its imper\'iousness. It is
probable that the tin is derived from tin-bearing quartz
veins in the underlying sandstone* (Fig. 62).

What is the origin of diamond placers?

In many parts of the world diamonds accumulate in the
stream sands and are recovered by washing. The sands
are made up of the debris of the diamond-bearing rock.
In this case the diamond owes its preservation not to its
specific gravity, which is not great, but rather to its hard-
ness and freedom from decomposition, which preserve it
when other minerals disintegrate and are washed away.

What other minerals are concentrated in placersf

Among others, monazite, a phosphate of the cerium metals
(usually cerium, lanthanum, didymium) which has of late
years been sought after for use in the making of mantles of
incandescent gas burners, has thus been obtained. In
rocks it occurs scattered in small crystals and is not com-
mercially available; but it is capable of being separated
from the gravels. In this case the mineral has a consid-

* C. M. Rolker, Transactions American Institute Mining Engineers, VoL
XX. pp. 50-84.

DYNAMIC AND STRUCTURAL GEOLOGY. 233

erable specific gravity — equal to that of magnetic iron
ore — and this operates chiefly in the concentration in the
gravels. Like other placer minerals, it is not easily
attacked by atmospheric agents of decomposition.

RESIDUAL DEPOSITS

What are residual' or rooted deposits?

Residual or rooted deposits form in weathered and
softened portions of the sohd veins (root deposits). In
some countries where erosion has not been very active in
sweeping away the surface accumulations, decomposed
rock in place extends to a great depth. In this loose
surface material the gold, changed to its native state by the
atmospheric agents above mentioned, shows a tendency
to work downward. The concentration is aided effectu-
ally by the winds, which blow away the finer and the
looser material and leave the heavier particles. The rain
serves the same purpose and removes material both
mechanically and in solution. Such deposits are not
necessarily in stream valleys, but are found in flat countries
or on the side of slopes; the parent or root deposit in the
solid rock is never far distant, and may directly underlie
the residual deposit. On account of the mechanical and
chemical concentration which it has undergone^ the residual
deposit is very often richer than the vein beneath.

Example: The placers of Kotchkar, in the Ural Moun-
tains, are mostly found immediately under the sod and in
intimate relation with the outcrops of the auriferous veins.

234 GEOLOGY APPLlEp TO MINING.

Their course does not depend on the surface configuration,
but coincides with that of the veins, into which they grad-
ually pass, at a depth of from 20 to 75 feet.*

Are rooted or residual deposits, other than those of gold,

known to occur?

Iron ores are sometimes formed in this way, some bodies
of phosphate of lime, and other mineral deposits.

* N. Wyssotzky. 'Les Mines d'Or de Kotchkar.' Memoires du comit^
Geologique. St. Petersburg. Vol. XIII. No. 3. p. 211.

CHAPTER V.

THE STUDY OF CHEMICAL GEOLOGY AS APPLIED
TO MINING.

THE STUDY OF ORE -CONCENTRATION.

The study of the chemical laws by which many geologic
changes take place is of important economic value. The
knowledge of these laws is beginning to throw greater
light upon the nature, probable extent and probable value
of a given ore-deposit, so that features can frequently be
predicted, the foreknowledge of which is of immense value
to the miner.

As already explained, the rock-forming elements are
constantly in a state of movement and change. The
waters which traverse the rocks of the crust are perhaps the
most powerful agents accompUshing the change; they are
continually dissolving, transporting and re-depositing. In
the process of solution they select certain elements in
preference to the rest, and in the process of precipitation
they concentrate them. Thus, concentrations of most of
the elements, to a greater or less degree, are brought about
in various places. In the case of the valuable rarer miner
als, we call such concentrations ore-bodies.

236 GEOLOGY APPLIED TO MINING.

The important thing, then, is to inquire how these
metals migrate and segregate — what is the agency and what
are the qualities by virtue of which this is accomplished.

What is the work of mechanical and chemical processes,

respectively, in ore-depositionf

The mechanical agencies have been discussed, in the
first chapter, in the chapter on stratified rocks, and in the
chapter on dynamic geology. They are almost wholly
at the earth's surface. Chief of them is water, or rather
gravity aided by water; to a very slight extent, wind also.
Particles of many kinds of minerals are shaken up and
"classified,^'* in moving water, flowing downward in
streams to the sea under the influence of the earth's gravity;
and along sea margins, in waters moved by tides or set in
motion by the winds.

The chemical agencies are extremely active by sea and
land, on the earth's surface and under it They are very
largely responsible, by their disintegrating action, for the
breaking up of the surface rock into small pieces, which are
sorted over by mechanical agencies so as to form
detrital ore-deposits. Moreover, by themselves alone they
accomplish the chief work of ore-concentration.

What is the principal agent in the chemical concentration of

ores?

Many substances are chemically active in the migration
of materials to form concentrations, but the chief vehicle

* This word is used in its technical meaning as regards artificial ore-concen-
tration.

CHEMICAL GEOLOGY. 237

for all these is undoubtedly water. Being dissolved in
water, these substances acquire the property of motion,
and so can exercise their concentrating forces, which
otherwise must have been powerless. Outside of solution
in water, the chief method by which these substances can
acquire the power of motion is by volatilization — changing
into the gaseous state, or by passing into solution in gases.
Of all the gases which thus play a part, water-vapor or
steam, is, again, probably by far the most important.

THE SHALLOW UNDERGROUND WATERS.

What is the source of the ground-wcUerf

Most of the water that falls on the earth as rain or snow
sinks into the rocks; the rest is evaporated or flows into
lakes or into the ocean. Thus there is a great body of
subterranean water.

Whai is the level of ground-wcUerf

If we sink a well, at first we encounter no water, but at
some depth the water oozes into it from every crevice in
the soil or rock, and after gathering, it stands at a certain
level. This has been called the level of ground-water; it is
near the surface in wet regions, and deeper in drier regions.
After a rain the water level rises and is often at the surface.
When there is little rain it sinks, so that many wells go dry.

The surface of ground-water is not horizontal in a hilly
country, but follows in a general way the topographic
surface, though it is less accentuated, being further from
the surface on hilltops and often practically corresponding

238 GEOLOGY APPLIED TO MINING.

with it in the valleys. Yet a well sunk on the top of a hill
will generally find water long before reaching the level of
the valley.

la the ground-water everywhere present in the rocks, mid to

what depths does it extend?

The distinction of the rock above and below the ground-
water surface has been insisted upon by the latest and best
writers on ore-deposits. They have even carried the
theory so far as to give to the subject an ideal aspect. It
has been conceived that below the ground-water level all
the openings in the rock, of whatever kind, are saturated
with water, whence the phrase "sea of ground-water'' has
originated, which sea is conceived to extend to a great
depth — several miles. But it is possible that this concep-
tion is a little too much generalized. In many deep mines,
the water is nearly all encountered in the upper levels, and
the deeper portions are often dry, even dusty This
suggests that the standing body of ground-water does not
in general extend to a depth of 10,000 feet, or anything
like it, but that its limit is something like 2,000 feet, and in
many regions 500 feet.* Some writers are of the opinion
that the amount of water which descends into the earth
below 2,000 feet is slight, and that it only attains great
depths by comparatively large fissures, which are excep-
tional. Numerous cases of deep, perfectly dry mines,
which find water (frequently warm) on tapping some
fissure, support these ideas.

* J. F. Kemp, Tran9action9 Amerioan Institute Mining Engineers, Vol.
XXXI, p. 187. eto.

CHEMICAL GEOLOGY. 239

The penetration of ground-water into rocks is exceed-
ingly irregular. In one place it sinks deeply and freely by
means of rock-openings and in another it is almost com-
pletely shut out by dense and impervious rocks. It is
probable that the universal presence of ground-water is
characteristic chiefly of a comparatively shallow surface belt,
below which the water which has not been again drawn off
at the surface, at a lower level, or has not been used up in
hydration processes, is concentrated into the larger fissures.

Example: Prof. Vogt* has written of several deep mines
in Norway in which the lowest pump-station is only about
250 meters from the surface. In one of them, the water
for use in drilling below that level has to be carried down.

THE WORK OF UNDERGROUND WATERS IN
DISSOLVING ROCKS.

How can underground waters dissolve rock-minerals on an

extensive scale?

Deep-seated waters are usually ascending, and not
infrequently heated. Surface water, n a cold state, is
capable of dissolving many materials from the rocks
through which it passes Heating the water augments its
activity in dissolving materials, and permits it to take into
solution a greater quantity of foreign matter; pressure in
general has a similar effect. Moreover, some of the mate-
rials taken into solution by water, or mechanically held in
it in small separate particles, increase its solvent power to a

* Tranaadiant American Institute Mining Engineers, Vol. XXXI, p. 165.

240 GEOLOGY APPLIED TO MINING.

great degree. The most important of these substances is
probably carbon dioxide (or carbonic acid), which is espe-
cially powerful in attacking, and taking into solution,
materials but slightly soluble in pure water, such as quartz
and many silicates, as well as metallic sulphides. Hydro-
gen sulphide is also abundant in many ascending waters, as
in sulphur springs. By uniting with a little oxygen, the
gas changes into sulphuric acid, which transforms many
difficultly soluble salts into easily soluble sulphates. Alka-
line solutions and especially alkaline sulphides in solution
assist the solution of gbld and certain metallic sulphides,
such as those of silver and copper. Certain substances in
solution are especially favorable to the solution of certain
other salts. P'or example, gold is soluble in ferric sulphate,
in alkaline iodides, in sodic and potassic chloride,* in sodic
carbonate,t sodic sulphide,! sodic sulphydrate, etc. Iodine
or chlorine in solution unites with certain metals, such as
gold, and makes an easily soluble iodide or chloride. From
common salt (sodium chloride), in saline waters, hydro-
chloric acid may be formed by ferric sulphate or sulphuric
acid; and manganese oxides, operating upon this hydro-
chloric acid, would produce free chlorine, which might then
act on metals as described above.

In fact, the processes of solution in waters are almost
endless and many of them are very complex. The essential
thing to understand is that in all waters these processes are

* Egleston, Tranaactioru American Institute Mining Engineers, Vol. VIII,
p. 455.

t Doelter, 'Chemische Mineralogie,' Leipzig, 1890.

X Becker, Monograph United States Geological Survey, Vol. XII, p. 433.

CHEMICAL GEOLOGY. 241

active and thus that all waters are capable of taking most
mineral matters into solution; and that deep-seated waters,
on account of the generally greater temperature and
pressure, as well as on account of the intimate way in
which they pass through rocks, and the length of time that
they spend in passage, are more powerful solvents than
surface waters. Therefore, given metals to dissolve, such
as are found in desseminated form in most igneous rocks
and in some sedimentary ones, and given waters circulating
through these rocks, we may be sure that the underground
waters will become charged to a certain extent with the
metals, as well as with most of the other mineral substances
which they encounter in transit.

Do we know, ds a matter of fact, that underground waters do
carry mineral matter in solution?

It is not only by theory that we come to this conclusion.
In many waters, it is true, the amounts of some of the
rarer elements (such, for example, as the precious metals)
held in solution, are so small that we are unable to detect
them chemically; but in other waters analyses prove their
presence. Rain water, having been evaporated, has
become purified by a natural process of distillation; but
ground-waters always contain small amounts of the
elements of the rocks or soils they have traversed. Many
waters contain iron, which they deposit as soon as they
become exposed to the oxidizing effect of the atmosphere;
thus, in many swamps and stagnant pools we find a red
deposit of hydrated iron oxide. In limestone regions, the
waters contain a considerable amount of lime. Cold

242 GEOLOGY APPLIED TO MINING.

springs may contain many different salts in solution, such
as various earthy and alkaline carbonates, sulphates, and
chlorides, silica, etc. The hot springs which issue at the
surface, or are tapped by mining explorations underground,
are still more highly charged with these substances.

What rare mineral elements and metals are found in under-
ground waters?

A familiar example is the natural lithia water, which
occurs in springs and contains a considerable quantity of
the salts of the element lithium, not abundant in nature.
This is used for medicinal purposes. Of more immediate
interest to the subject of ore-deposition is the established
presence of the salts of arsenic, antimony, zinc, lead, tin,
etc., even of gold, in many mineral springs.

Examples: 1. The deep waters from the 2,000 foot level
of the Geyser mine, Silver Cliff, Colorado, were found by
W. F. Hillebrand to contain silica, alumina, iron, manga-
nese, lime, strontium, magnesium, potassium, sodium
and lithium compounds, together with carbonates of lead,
copper, and zinc.

2. The springs at Rippoldsau and Kissingen, southwest
Germany, have been found to contain tin, antimony,
copper and arsenic*

THE WORK OF UNDERGROUND WATERS IN
PRECIPITATING MINERALS.

Are minerals precipitated from solution in concentrated form?
That materials are deposited from underground waters

* F. Poaepny, 'Genesis of Ore-Deposits,' p. 43.

CHEMICAL GEOLOGY. 243

we know from actual experience. These materials include
not only the commoner ones, such as calcium, sodium,
silica, etc., which are deposited as tufa around the outlets
of springs; but also the less common elements. The
waters that bear the minerals obtain them slowly, picking
them up here, there, and ever3rwhere, but it is not every-
where that the proper conditions occur for precipitation.
The result is that where the favorable conditions do occur,
a good deal of the materials in question are likely to be
precipitated, for an endless supply of water comes, each
bringing and contributing its mite. In this way concen-
tration is effected, and from a state of dissemination in the
rock, so thinly spread that the most expert chemist detects
it with difficulty, lead may be concentrated to form enor-
mous bodies of galena, or gold may be concentrated to
form such great nuggets, each worth a fortune, as have
been found in Australia. It becomes necessary then for
the investigator to gain, so far as he may, some idea of
what these conditions are.

Why are ores especially apt to be concentrated along water-
courses?

In the first place, it has been pointed out that, other
things being equal, ores are most likely to occur along
water channels, such as a fracture or set of fractures, a
porous bed, etc. This is primarily because in such water-
courses an enormous volume of water continually passes,
and to a less degree because here the conditions are more
favorable to deposition than elsewhere. Into unfractured
or otherwise relatively impervious rock, even if the condi-

244 GEOLOGY APPLIED TO MINING.

tions for deposition are of the best, only a small amount of
water attains, and consequently not enough metals are
ever brought to form an ore-deposit. But, where the
conditions favorable for deposition occur along a water-
course, then the supplies of materials are so great that
eventually a large body of metallic minerals may accu-
mulate.

MANNER OF DEPOSITION IN THE DEEPER UN-
DERGROUND REGIONS.

In what forms arc metals usually deposited in the deeper

underground zone?

Most of the metals are deposited in the deeper region
(and to a great extent also in the shallow underground
region) as sulphides. Under special conditions, other
compounds are precipitated, such as carbonates and
silicates. The deposition of zinc silicates at Franklin
(New Jersey) is an illustration of the latter. Metals may
also be precipitated in the native form, as is probably often
the case with gold, platinum, copper, etc.; they may form
rarer combinations such as arsenides, tellurides, etc., or
they mayj)e deposited as oxides.

Under what conditions are metals precipitated as oxides in

the deeper underground regions?

We generally think of the formation of oxides as charac-
teristic of the surface, and of the sulphides as the natural
products of the deeper regions. This is true, as a rule, for
sulphides commonly change into oxides during the pro-

CHEMICAL GEOLOGY. 245

cess of weathering. But the opposite extreme of con-
ditions from those prevalent near the surface, the most
intense heat and pressure, and the presence of strong
solutions and vapors may, also, produce oxides. An oxide
of iron, hematite, is often deposited from gases in volcanoes.
Magnetite and hematite are also found in the contact-
metamorphic deposits, formed by the highly concentrated,
heated, and compressed solutions and vapors given off
from cooling masses of molten rock.

Within molten but slowly solidifying rocks, oxides of
iron (magnetite and titaniferous magnetite), and probably
oxide of iron and chromium (chromite, chrome iron), etc.,
are accumulated to such an extent as to form ore-bodies.
At Franklin Furnace, New Jersey, oxide of zinc (zincite)
and oxide of iron, zinc and manganese (franklinite), have
been formed under conditions of great depth, heat, and
pressure, as indicated by the uncommon minerals with
which they are associated — ^minerals usually found in
contact-metamorphic ore-deposits — and by the highly
metamorphosed condition of the beds in which they lie.*

Oxides formed under these conditions may be associated
with sulphides formed at the same time, as is the case in
some contact-metamorphic ore-deposits.

What causes the precipitation of sulphides from soltUionsf

One of the most important causes is the presence of
organic matter, in shales, sandstones or limestones.

* J. F. Kemp, 'Ore-Deposits of the United States.' p. 267.

246 QEOLOOT APPLIED TO MINING.

How does organic matter caitse ore-depositionf

First and, probably, most important, the carbon of the
organic matter may unite with the soluble sulphates of the
metals, and, by abstracting the oxygen from these salts,
cause the formation of sulphides which, being relatively
insoluble, are precipitated. The carbon and the oxygen
unite to form carbonic acid, which is carried on by the
water in solution, and immediately assists in the dissolving
power of this water. Second, nmch organic material
contains sulphur, and by decomposition this sulphur may
become sulphuretted hydrogen. A rotten egg is a familiar
example, emitting the disagreeable peculiar odor of this
gas. When this sulphuretted hydrogen comes into con-
tact with soluble salts of the metals it combines with
them and precipitates the salts as sulphides. For ex-
ample, a solution carrying copper and iron chlorides would
be precipitated by sulphuretted hydrogen as chalcopyrite,
hydrochloric acid being the other result. * This acid goes off
into the water and aids further solution and concentration.
Most beds containing organic matter give off a certain
amount of sulphuretted hydrogen. In fact, certain shales
and Hmestones have this peculiarity so marked that the
former are called by the Germans stink-shales (stink-
schiefer) and the latter by ourselves, fetid limestones.

What other causes of ore-precipitation are there?

Another important motive for deposition is operative
when metallic salts in solution come in contact with rock
minerals with which they can combine to form a mineral

CHEMICAL GEOLOGY. 247

compound more stable and less soluble than that already
existing. This is in accordance with a fundamental law of
chemistry, and the result is the very important process of
replacement.

In what manner do replacement deposits form?

The replaced rock is dissolved by the water and for each
particle taken up a particle of vein material is deposited in
its place. Microscopic study shows how insidiously the
solutions can work their way through the apparently solid
rock. They circulate along even the tiniest cracks and
between the crystals. From there they penetrate into the
interior of the individual grains, first following the cleavage
planes. The rock undergoing displacement may be
sprinkled wdth disconnected crystals of the ore, the channels
by which the ore-bearing solutions have come being invis-
ible. Finally most or all of the rock may be removed,
leaving a solid ore-mass.

How can one recognize a replacement deposit?

From the condition of the formation of such a deposit one
usually finds all stages from the rock sprinkled with ore-
minerals to the solid ore. There is a usual absence of
banding, and the ore-bodies are apt to be irregular, with
ill defined boundaries. Yet, since they usually follow
some water channel, they may have in one or more
directions definite extensions, and so may be classified
from the standpoint of form, either as disseminations,
irregular masses, shoots (pipes or chimneys), or veins.

248 GEOLOGY APPLIED TO MINING.

Sometimes the ore-mineral occurs as a pseudomorph, after
one of the original rock minerals — ^that is, it has the peculiar
crystal form of that mineral — which is conclusive proof of
replacement. One may also sometimes find fossils com-
pletely changed to an ore-mineral or even to a native
metal.

Example: The lead-silver deposits of Aspen, Colorado,
have formed chiefly from replacement from limestone and
dolomite.* Mineralization began along fractures (often
microscopic) resulting from some rock movement, and
often attendant upon actual faulting. From the fractures
the metallic minerals penetrate the adjoining rock. Fossils
have been found which are imbedded in the ore, or have
been so changed as to form a part of the ore. Fig. 63
shows a mass of pure native silver from Aspen. In this,
part of a perfect fossil shell is firmly fixed. At another
place a fossil was found completely turned to zinc and lead
sulphides and carbonates.

Are replacements always in limestones or dolomites?

Replacement deposits also occur in difficultly soluble
rocks, like quartzites and granites. Large ore-deposits in
such rocks chiefly due to this process are abundant; and,
even in many deposits where the ore has formed in pre-
existing cavities, large or small, replacement will be found
to have been a very important auxiliary process.

Examples: 1. The formation of auriferous quartz veins
by replacement of schist along zones of crushing, in the

* Monograph XXXI, United States Geological Survey, pp. 206-236.

CHEMICAL GEOLOGY.

249

district of Otago, New Zealand, is described by T
Rickard.*

The sketch (Fig. 64) covers a width of 5 feet. Instead
of forming a narrow clean-cut crack or fissure, the soft
schist is traversed by a crushed zone bounded by parallel
fractures. The parts between the two lines of fracture
form the beginning of the ^^muUocky reef'' of the Austra-
lian miner, — that is, a lode carrying a large proportion of
included country rock. Percolating waters deposit their

Fig. 63. Fossil imbedded in native sil-
ver, Aspen, Colorado. From J. E.
Spun*.

Fig. 64. Type of lode in Otago, New-
Zealand . The crushed zone between
aa and 66 may be entirely replaced.
After T. A. Rickard.

quartz first along the lines of parting, and we get a twin
system of veins; and, if the action be continued further,
the intervening filling is also silicified. It may seem a long
step from a lode where the larger part of the gold-bearing
material is crushed rock, to a massive vein of clear aurif-
erous quartz, yet the difference is one of degrees only, being
due to the variable extent to which the quartz has replaced
the rock.

* Transactiona American Institute Mining Engineers. Vol. XXI, p. 418.

250 GEOLOGY APPLIED TO MINING.

2. The ore-deposits of Monte Oisto, in the Cascade Range,
Washington, have formed* chiefly by replacement of the
granitic rocks and andesites in which they occur. The
mineralizing waters have come into the rocks along joint
planes, and have then replaced the wall rock, frequently
forming bodies of solid ore (chiefly sulphides, such as iron
pyrite, arsenopyrite, blende galena, etc.) several feet in
thickness. From the joints the waters make their way into
cracks of microscopic dimensions, and so, little by little,
attack every part of the rock. Of the original minerals of
the granitic rocks (tonalite) and of the andesite (these
minerals comprising chiefly quartz, feldspar, hornblende,
mica, magnetite and augite) all except the quartz are
decomposed, and new quartz, calcite, pyrite and other
sulphides grow gradually in their place. With a moderate
proportion of sulphides in the altered rock, it becomes an
ore, with a quartz and sometimes a calcite gangue.

Does the presence of metallic minerals bring aboiU the pre-
cipitation of others?

Metallic oxides and sulphides, already existing in a rock
or vein, may act as precipitants of dissolved metallic salts.
Sulphides often change their composition in doing so, and
form a compound richer in the metallic base than before.
Thus chalcopyrite, a mineral containing 34.5 per cent of
metallic copper, may be transformed into bornite, con-
taining 55.5 per cent. Iron sulphide (pyrite) may unite
with copper solutions to produce chalcopyrite, etc. In
auriferous quartz veins the gold is very commonly con-

* J. K. Spurr, 22J Annual Report United States Geological Survey, Part II,
pp. 831-33.

CHEMICAL GEOLOGY. 251

tained in the iron pyrite, which in many cases seems to
have acted as a precipitant.

Sulphides may also induce the precipitation of other
sulphides, particularly of similar sulphides, without, so far
as we know, any chemical reaction.

How are ores precipitated by the mingling of solutions?

Since different water currents traverse different rocks,
composed of various elements, and are subjected to different
conditions of heat, pressure, etc., the mineral solutions
contained in each one ^dll rarely be identical with those in
another. Whenever two currents meet, therefore, the
mingling of the solutions must bring about certain chemical
reactions and some materials are likely to be precipitated.
Where waters containing free sulphuretted hydrogen meet
others containing soluble metallic salts, for example,
metallic sulphides will be precipitated. There are a host
of other reactions which may take place. This is the
chemical explanation for the principle of intersections,
defined in Chapter IV.

Example: In the Newman Hill deposits, at Rico, Colo-
rado, as described by J. B. Farish, T. A. Rickard, and F. L.
Ransome, the ores commonly occur chiefly on the under
side of "blankets,'' which are impervious beds interstrati-
fied in the shaly sandstones of the region. These imper-
vious beds have originated in several different ways, but in
every case consist of a decomposed mass through which the
ascending mineralizing solutions were unable to pass, and
so spread out and deposited the metals. Yet the under
side of these blankets is not uniformly mineralized; there

CHEMICAL GEOLOGY. 253

the vertical lode and that afforded by a porous zone under-
neath the blanket. Solutions ascending in the fissures
not only found their upward progress barred by the imper-
vious shales, but entered a porous zone traversed by later-
ally moving solutions, which effected the precipitation of
the ores.

Are ores precipitated by decrease of temperature and pressure
in ascending waters?

In proportion as underground waters become hotter and
under greater pressure, their power of solution increases;
therefore when they become cooler and under less pressure,
on nearing the surface, their power of solution decreases,
and they are obliged to precipitate some of their burden.
This statement demands certain qualifications. For
example, the laws of solution are different for different
materials; and a temperature which is most favorable for
the solution of a certain salt may not be so favorable as a
lower temperature for the solution of a certain other. But
the general principle above stated holds good, and is
influential in determining the deposition of ores. For-
merly, indeed, it was held to be more important than it is at
present, and was called upon to explain most ore-deposition
from ascending waters. But now that we recognize the
great importance of deposition by mingling of solutions,
by contact with organic matter, and by replacement of
rocks, or by contact with already precipitated metallic
minerals, we see that it must be rather rare that an ore-
body is formed by release of temperature and pressure alone.
Yet in many cases where we have evidence that some other

254 GEOLOGY APPLIED TO MINING.

cause has been active in ore-deposition, it is probable that
this also has been an important factor, without which the
precipitation might not have taken place at this point.

For example, a rising column of heated and compressed
water may meet with solutions of a different nature at the
intersection of another circulation channel; but no precipi-
tation takes place. Further up, where the waters are
cooler and under less pressure, they may again meet solution
similar to those met below, from another channel, and here
abundant precipitation, resulting in the formation of an
ore-body, may occur.

Example: The most familiar example of deposition by
this cause is in the masses of sinter which are thrown down
at the mouths of hot springs, when these emerge into the
cool and free open. These deposits consist largely of silica
and carbonate of lime; but other substances occur in them
in greater or less quantity. Iron, arsenic, tin, and many
other metals have been found under such conditions.

At what depth below the surface are ore-deposits formed

mainly by ascending waters?

It is probable that some ore-deposits are formed by
ascending waters veiy near or practically at the surface.
Such metallic elements as the waters carry in solution, and
which they have not precipitated lower down, can hardly
fail to be precipitated when coming into surface conditions.
The question then is, how deep may these deposits be
formed? On that point we have evidence in different
mining districts that ore-bodies may be formed at least
two or three miles below the surface.

CHEMICAL GEOLOGY. 255

Example: In the Tintic mining district, Utah, there is a
series of older sedimentary rocks which in Mezozoic time
were lifted above the sea and folded. Soon after this time
the ore-bodies were formed. Measurement of the thickness
of the stratified rocks found in some parts of the region,
but worn away from the ore-bodies now being worked,
shows that these ore-bodies were mainly formed at least
12,000 feet below the surface.*

SPECIAL CHEMICAL PROCESSES OF THE SHALLOW
UNDERGROUND WATERS.

Are the chemical processes of the shallow underground waters
the same as those of the deep underground watersf
The chemical processes of the shallow underground
waters (mostly descending from the force of gravity) are
usually considerably different from those of the deep under-
ground waters (mostly rising from hydrostatic pressure,
heat, contained gas, etc.); but this is not always true.
From the mere inspection of the results of the chemical
action of a given current of water, it is usually impossible
to tell the source or the direction of movement. Sulphides,
for example, are deposited equally well by ascending, by
descending, and by horizontally moving waters.

What are the peculiar chemical effects of shallow waters?

Waters coming from the surface contain certain gases in
solution derived from the atmosphere or the decay of vege-
tation, which gases are not so abundant in deeper waters.

♦Tower & Smith. 19th Annual Report United States Geological Survey,
Part III. p. 715.

25() GEOLOGY APPLIED TO MINING.

Therefore, the chemical effects may also be different.
Water coming from the surface contains more oxygen and,
as a rule, less sulphuretted hydrogen than do the deeper
waters. Surface waters attack the rocks through which
they pass, and take nmch of the material into solution.
Silicates, sulphides, etc., are decomposed and, in part,
altered to oxides and carbonates. Some of these changes
involve an access in bulk, by the addition of the oxygen
and carbonic acid from the waters. In some cases this
increase is so great as to cause minar foldings in pliable
banded rocks, and brccciation in rigid ones. Other chemi-
cal changes involve a shrinkage, the amount of material
that passes into solution being so large that the net result
is to cause the rocks to contract.

Zones of Weathering or Oxidation.

What is the process of weathering?

At the surface, where this contraction is most active, it
results in disintegration and crumbling of the rock; further
down, in a loss of strength and cohesion, in enlargement of
openings, and in the development of cracks (often not
actually opened, but only potential), into open fissures.

This general process is called weathering, from being so
conspicuous at the surface, where the rocks are exposed to
the atmosphere, or weather; and the zone of rocks affected
by it (a zone roughly parallel to the surface configuration)
is called the weathering zone.

Example: In a tropical country, like Brazil, the surface
weathering of rocks is vastly more active than in cooler

CHEMICAL GEOLOGY. 257

climates. Decomposition to a depth of 100 feet is common,
and sometimes extends more than 300 feet. The daily
range of temperature sometimes amounts to more than
100° Fahrenheit. These changes cause the rocks to crack
and to admit moisture and the acids which bring about
rock decay. The hot season is the rainy season, and
waters falling upon the hot rocks have their temperature
raised to about 140°, which makes them more efficacious ;
and the rainfall is very large. The chief acids which aid
in the decomposition are carbonic acid, nitric acid, and
especially the organic acids derived from the decay of the
abundant plant and animal life.*

Is the weathered zone the same as the zone of oxidation?

This practically corresponds with what is called espe-
cially in consideration of ore-deposits, the zone of oxida-
tion, being that belt where the highly oxygenated surface
waters have altered the sulphides more or less thoroughly
to oxides, carbonates, etc.

Are chemical processes active in the rearrangement of ores in

the zone of oxidation?

They produce very marked results. Rocks near the sur-
face are physically shattered; hence, waters gain access to
every pcJrtion. The shrinkage resulting from the first
chemical reactions of these waters carries on the work.
The rock crumbles (unless it is immediately swept away by
the streams) into a kind of coarse sand, and the waters have
an opportunity to search thoroughly every part of it.
Much of its substance is dissolved and re-precipitated or

♦ J. C. Branner, Bulletin Geological Society America, Vol. VI I, pp. 255-314.

2dS geology applied to mining.

carried a^-ay. When re-precipitated it may still be near the
surface, or may be far below. In any case the groupings
change. Scattered amounts of the metals may be con-
centrated by this process.

Whai is the iron hat {iron cap) or gossan of an ore-body , and
how is it formed f

In most ore-bodies iron forms an important part, even
when the chief values are in some other metal. Many
copper ores, for example, consist wholly or in part of chal-
copyrite, the sulphide of iron and copper. During and
after the weathering and oxidation of the surface parts of
these deposits, the metals other than iron may be leached
out, carried down and re-precipitated, leaving the iron
(with quartz and other gangue minerals, if they were pres-
ent in the original deposit), in the form of a soft yellow
limonite, or sometimes as hard brown limonite,or hematite
(oxides of iron). This iron covers the valuable ore, which
is found by sinking through it. In Germany it is called
the iron hat, in Cornwall gossan, and in America generally
iron cap (or capping). A strong iron cap is a favorable
sign of a large ore-body beneath.

Example: *At the Cobre copper mines, in the province
of Santiago, Cuba, the principal lode and the chief courses
of the ore are indicated at the surface by spongy quartz and
iron oxide, with highly colored clays. Inmiediately
beneath, or in the clays, were found oxides, sulphides and

* Professor Ansted. Quoted by Hayes, Vaughan, and Spencer, 'Geoloi(*
ical Heconnaissance of Cuba/ 1001.

CHEMICAL GEOLOGY. 259

carbonates of copper. Further down, these all change to
copper pyrite (sulphide of copper and iron), which maybe
regarded as the original ore, the carbonates and oxides of
copper having been derived from it by oxidation, and the
iron cap formed by the leaching of the copper out of the
iron and quartz. The oxidized zone extends down nearly a
hundred feet.

Precipitation of Ores at the Surface.

How are ores precipitated, from solution in surface waters, in

swamps?

Water containing iron finds its way into swamps, and on
standing there for a while the oxygen of the air combines
with the iron carbonate in solution. Carbonic acid is
evolved, and hydrated iron oxide; the latter forms a scum
on the surface or sinks to the bottom. Successive pre-
cipitates may accumulate till a considerable deposit is
formed. These '*bog-ores," as they are called, have been,
and are still, much exploited for commercial purposes.
They include not only the ores formed at the present day,
but those of past ages. The latter, like coal beds, have
been covered by later sediments and so preserved till now,
when they are often exposed by erosion.

Example: In the Three Rivers district, Quebec, Canada,
extensive deposits of bog iron ore have formed and are now
forming. The iron contained in the streams is deposited
in swamps, streams, and lakes, wherever the water is for a
time stationary, or choked with vegetation. Beginning as
a light film, the ore gradually accumulates to thick crusts,
and in couree of time a very considerable amount accumu-

2H() GEOLOGY APPLIED TO MINING.

latei^, SO that in places it is dug out for commercial use.
This iron has been used since 1730.*

A re ores precipitated from the waters of lakes and oceans?

Waters entering oceans, lakes, etc., carry mineral matter
which may be finally precipitated on the bottom. In con-
fined lakes or inland seas, important deposits of common
salt, gypsum, magnesium and potassium minerals, etc., are
formed. In the sea, manganese solutions are precipitated
on the bottom in concretionary form, as has been proved
by dredging. Much of the commercial manganese has this
origin, the manganiferous layers of old sea sediments being
attacked by land waters, subsequent to uplift and erosion,
and the manganese being thereby more highly concen-
trated. Other metals, such as copper, and even silver and
gold (chemical analysis has proven the presence of these
and many more in common sea water) are probably pre-
cipitated in small quantities in the sediments accumulating
in the sea bottoms and along the shores; and these slightly
metalliferous layers, after uplift, may yield to land waters
a material which, after further concentration by them, will
form workable ore-bodies.

Example: In the Paleozoic region of Georgia occur ores
of manganese, which are found in connection with only
three formations — a limestone, a dolomite, and a quartzite.
The manganese is in clays residual from the decay of the
rocks, and has evidently been concentrated from a dis-
seminated state in these beds or overlying strata now

* K^sum^ by J. F. Kemp, 'Ore Deposits of the United States/ p. 90.

CHEMICAL GEOLOGY. 261

removed by erosion. It is supposed to have been derived
originally from silicates in crystalline rocks, from which
it was taken by streams in solution to the sea, where the
Paleozoic strata were being deposited, and was precipitated
in them. Long afterwards, when these beds were again
part of the continent, surface waters dissolved and concen-
trated the manganese so as to form ores.*

Are mineral deposits ever formed at the surface by the evapora-
tion of underground watersf

A special and peculiar phase of the circulation of water in
rocks, and especially in soils close to the surface, is depend-
ent upon the surface evaporation. Where evaporation is
strong, the surface would quickly become entirely dry were
it not that moisture from deeper down rises to take the
place of that removed. This action is particularly strong
in hot and arid climates. The mineral content of the
evaporated waters is left on the Surface, forming the crust
of salt or '' alkali " famiUar in desert regions. In natural
hollows or basins in the topography, beneath the surface of
which the groundwater accumulates (even though it is not
abundant enough to stand long above the surface), so much
material is brought to the surface that the incrustation is
often of economic value. Such deposits consist chiefly of
salt, borax and soda. In some cases it is possible that
useful accumulations of certain metals may be formed in
this way.

Example: In western Colorado, deposits of uranium

*T. L. Watson, Tranaaction9 American Institute Mining Engineers, Feb.,
1903.

262 GEOLOGY APPLIED TO MINING.

and vanadium occur in Jurassic strata. The chief mineral
is a vanadate of uranium and potassium, called carnotite.
The ore occurs disseminated through sandstone, as irr^u-
lar bunchy pockets in this rock, or along the contact of
sandstone with shale. The ore bunches have the appear-
ance of being impregnation deposits, formed by solution
along planes of easy circulation, frequently bedding planes.
The most interesting fact concerning them is their super-
ficial character. They are flat-lying streaks which in some
cases disappear into unmineralized sandstone when
followed only a few feet underground. In places the ore
has formed along crevices plainly due to recent surface
movement, showing it to be not only superficial but very
recent.

It is supposed that the ore exists in very small amounts
in the sandstone and that the surface deposits have been
concentrated from this condition; and it seems extremely
likely that this concentration has been effected by the
strong evaporation of a semi-arid climate, continually
removing the moisture from the surface and leaving the
dissolved contents behind in the rocks.*

By a similar process of evaporation incrustations may be
formed in caverns.

Example: Nitrates are frequently found in cavern
earths. A large amount of saltpeter (nitrate of potash)
was taken from the Mammoth Cave in Kentucky during
the war of 1812, and from caverns in Alabama and Georgia
during the Civil War, for the manufacture of gimpowder.
Investigation of these deposits points to the conclusion that
the nitrates were brought in by water percolating through
the soils above the caves and were deposited on the floors.

* F. L. Ransome. American Journal Science, Fourth Series, Vol. X, pp. 121-
130.

CHEMICAL GEOLOGY. 263

Currents of air, passing in and out of the caverns, removed
the water, leaving the salts in the cave earth. The accu-
mulation of salts occurs only in caverns where the inflow
of surface water does not exceed in amount the water
removed by evaporation. In wet caves the soluble salts
are washed onward with the water bearing them, and so
are not deposited. Nitrates deposited under overhanging
cliffs have the same origin. As to the source of the nitrates,
vegetation furnishes continually, during its decay, a small
amount of nitric acid.*

Are there other instances of the precipitation of ores from
surface waters?

Waters containing phosphoric acid, derived, for example,
from the dung of sea fowls, may change limestones or lime
marls lying near the surface from hme carbonate into lime
phosphate, of great value as a fertilizer. Numerous other
examples might be cited. In gold placers there is, as pre-
viously noted, certainly some solution and redeposition of
the gold by the surface waters, even though gold is com-
paratively resistant to solution in general.

Are minerals secreted from surface waters by living organisms?

The precipitation of lime and silica from solution in
sea water by incorporation into the shells of marine animals
is of vast importance. By the accumulation of such shells
on the sea or lake bottoms originate the majority of lime-
stone deposits. A tiny fresh water organism that itiakes
its shell out of iron has been discovered, and the accumu-

* W. H. Hess, Jaurrua of Geology, Vol. VHI, p. 129.

204 GEOLOGY APPLIED TO MINING.

lation of these has been held to be important in forming
some iron-ore deposits.

Are ares precipitated from surface waters by organic matter?

Precipitation by organic matter plays an important part
at the surface, as well as underground.

In the sea, in a certain broad zone somewhat remote from
shore and yet not in the greatest depths, the precipitation
of silicate of iron (glauconite), is accomplished largely
through the agency of organic matter, and through the
accumulation of this glauconite, and its subsequent alter-
ation and re-concentration, iron ore-deposits have been
formed.

Example: In eastern Texas are found beds of limonite
(hydrous oxide of iron) associated with marine glauconitic
sands. The silicate of iron was formed beneath the sea,
probably chiefly through the agency of tiny organisms,
which precipitated the iron and silica, either from fine mud
washed out from the land, or from the solution in the sea
water, or both. After deposition the beds were lifted up
and l)ecame dry; then the surface waters decomposed the
glauconite. The iron was changed to oxide, and on further
concentration (by the surface waters) formed limonite
beds.*

Metallic gold, in placer regions, is frequently found in
grass roots, having been precipitated there by the reducing
action of organic matter. Pieces of wood, etc., in placers,
have the same effect.

* R. A. F. Penrose. Jr.. First Annual Report Texas Geological Survey.

CHEMICAL GEOLOGY. 265

Example: On one of the tributary streams of the Galliko
river in Macedonia, exceedingly little gold can be got from
the gravels, and what is obtained is very fine, but, in some
localities, if the grass and turf over which the water occa-
sionally flows be pounded up and washed in a gold-pan, a
much larger quantity of coarser gold is obtained.*

Precipitation of Ores in the Shallow Underground

Zone.

Turning away from the precipitation of ores at the very
surface, let us look at the facts of their precipitation in
rocks near the surface. It has already been explained that
in the process of weathering the superficial portions of the
rocks are largely taken into solution, as well as much of
the rock lying within a moderate distance from the surface.

Concentration According to Relative Solubilities.

How do the different solvhilities of metallic minerals bring

about their selective concentration?

Some materials are more soluble than others, hence some
valuable metals, for example, are taken with difficulty into
solut on, and, when in solution, are not carried far before
being precipitated. Others are more easily soluble and are
carried further. The result is that in ore-deposits which
have been greatly affected by weathering and the accom-
panying action of surface waters concentration of metals
according to their relative solubilities is very great.

* Observations by the writer.

266 GEOLOGY APPLIED TO MINING.

In the case of a mineral not easily soluble how may concen-
tration take placet

The concentration may take place by the removal in
solution of the more easily soluble minerals with which it
was originally associated. This forms residual deposits,
which are often important. For example, phosphatic lime
nodules in limestones are frequently concentrated at or
near the surface by the removal in solution of the more
easily soluble carbonate of lime in which they were em-
bedded; and often only this surface portion can be worked,
the unaltered portions containing too small an amount of
phosphate. Iron carbonate or limonite nodules in lime-
stone are concentrated by the same process into workable
iron ore at the surface. Outcrops and weathered portions
of gold-bearing veins are often richer than the unoxidized
portions below, for much of the rock has been removed in
solution, while the gold has been attacked to a less extent;
hence the percentage of gold in the weathered and oxidized
rock is greater than in the unaltered portions.

Example: In the gold belt of the Blue Moimtains, in
eastern Oregon, the gold-quartz veins, which carry free
gold, are more or less oxidized to a depth of from 100 to 300
feet, and this zone is generally richer than the unaltered
ore below. At one mine (the Sanger, on Eagle creek,) the
uppermost 100 feet showed a narrow vein yielding $25 per
ton, while below the vein widened, and the average values
were reduced to $12 per ton.*

* W. Lindgren, 22d Annual Report United States Geological Survey, Part
II, p. 611.

CHEMICAL GEOLOGY. 267

Are minerals taken irUo solution re-precipitaied in concen-
trated form?

Concentration by solution and re-precipitation is a com-
mon process. In places where there is only one mineral
of importance this mineral may be compactly precipitated
in the zone of surface waters. Great iron ore-deposits have
been made in this way; and concentrations of the more
valuable metals are frequent.

Example: 1. In the Tintic district, Utah* (described by
Tower and Smith), there is a good example of the enrich-
ment of ores in the oxidized zone. The ores contained in
the sedimentary rocks in this district (mostly limestones)
are completely oxidized to a depth of several hundred feet,
and partially to the lowest points reached in the mine
workings. Surface waters have decomposed the original
sulphides. The metals thus attacked have been largely
taken into solution and re-deposited as new minerals. The
metallic minerals of the original deposit are sulphides and
sulpharsenides, principally pyrite, galena, enargite and
silver sulphide. These have been changed to oxide of iron,
sulphate and carbonate of lead, hydrous arsenates, and
arsenites of copper, oxides of copper, native copper, chlo-
ride of silver and native silver. During these processes the
various metals have largely been segregated, and form
distinct deposits, so that there are great bodies of ore con-
taining principally lead, or copper, or silver.

In the veins in the igneous rocks, in the same district,
the oxide ores carry about twice as much silver and lead as
the sulphide ores, there being nearly a corresponding
decrease in iron and silica. These segregations of the

* 19th Annual Report United States Geological Survey, Part IIL

2G8 GEOLOGY APPLIED TO MINING.

metals result from differences in the solubilities and stabili-
ties of the various minerals.

2. In the Red Cliff mining district, Colorado, the ores
occur at two distinct horizons. The first horizon is in
Lower Carboniferous limestone, beneath an intrusive sheet
of rhyolitic rock, and the ores are replacements of the lime-
stone by iron pyrite and silver-bearing jgalena on an im-
mense scale. The oxidation of these sulphides to sulphates
and oxides may be well observed. The second horizon is
from 200 to 300 feet lower, geologically, on the top of a
white Cambrian quartzite. The ores are smaller in volume
and more irregular in distribution, but are very much
richer. They are fine ochreous material, largely basic iron
sulphate, containing silver and gold. There is good
ground for assuming that these metals have, in part at
least, been leached from the ore-bodies of the higher horizon,
by solutions of iron sulphate.*

When a number of different metals are thus worked over does a
definite arrangement result?

Where a number of metals are attacked by surface
waters, the result of their differences in solubility is the
formation of rough mineral belts. These follow the surface
in general, and each is characterized by a preponderance of
certain metals or minerals. For example, in deposits con-
taining lead, zinc, and copper, the effect of descending
waters may be to separate the metals into zones, the lead
(galena) being above, and zinc (blende) below. Fre-
quently there is a third and still lower*one characterized by

* Franklin Guiterman. Proceedings Colorado Scientific Society, Vol. Ill,
1890. Supplement; S. F. Emmons, 'Geological Excursion to the Rocky
Mountains,' p. 417.

CHEMICAL GEOLOGY. 269

copper; and a fourth, characterized by iron, has been
observed. There are irregularities in these zones, and the
different minerals commonly occur together, even in the
same hand-specimen; but in a broad way they are often
well . defined.

Secondary Sulphide Enrichment.

What is the meaning of the term secondary sulphide enrich-

mentf

The working over and re-concentration of an earlier ore-
body into richer sulphides by descending waters has been
called secondary sulphide enrichment.

What is the secondary sulphide zone?

In many regions secondary sulphides occur in a more or
less definite zone, underlying the oxidized zone and over-
lying the primary ores (generally also sulphides). The
metals are leached out of the oxidized ores by descending
waters and carried downward to the unoxidized suphides,
where they are themselves precipitated as sulphides, often
by the direct influence of the primary ores. Such secon-
dary sulphides are commonly richer than the primary ones.

In the ca^es where ores are precipitated as sulphides, from
descending waters, where does the sulphur come from?
The decomposition of less stable sulphides is sup-
posed to furnish the necessary sulphur. Even where no
large body of older sulphides exists, disseminated sulphide
of iron (pyrite) may occur as it does in many sedimentary

210

GEOLOGY APPLIED TO MINING.

and most igneous rocks, even in those apparently fresh.
Other possible sources of sulphur are sulphur-bearing
waters, and the sulphur frequently present in sedimentary
beds containing organic matter. Soluble metallic sulphates
in surface waters may sometimes be reduced to sulphides.

May an ore-body be formed wholly by descending waters where

none already exists?

In the case where no older deposit exists, ore may still
be ormed by descending surface waters, provided that the
rock through which the waters percolate has a sufficient
quantity of disseminated minerals.

Example: The formation of iron ore-deposits by descend-
ing waters has taken place in certain parts of central and

Fig. 66. Deposition of irou ore by descending waters, in the bedding and joint
seams of limestone; and nodular iron ores in residual clay in the hollows
of the limestone. Section at the Pennsylvania Furnace ore-
bank, Pennsylvania. After T. C. Hopkins.

eastern Pennsylvania. The ores are oxides, largely limo-
nite. They occur as rounded or elongated fragments, with
residual clay, in irregular deposits in cavities which extend
from the surface down into beds of limestone, clay or sand-
stone (Fig. 66). The original source of the iron is in Paleo-

CHEMICAL GEOLOGY. 271

zoic shales and limestones, where it is disseminated in the
form of carbonate, with some sulphide and silicate. The
segregation of the diffused iron into the ore lumps is brought
about by descending surface waters. The metal has been
dissolved by the organic and carbonic acids of these waters,
and precipitated in concentrated form, in its present
position, in part as oxide, and in part as carbonate, which
has subsequently been oxidized. Weathering of the rock
leaves the ores embedded in residual clays.*

Are ore-bodies formed entirely by descending waters likely to
be so important a^ are secondary sulphide enrichments?
Except in the case of iron and some other of the com-
moner ores, such ore-masses are usually smaller and leaner
than those formed by secondary enrichments. Moreover,
at a moderate depth below the surface the mineralization
is apt to fail to such an extent as to make the deposit un-
workable. This depth varies with the character of the
rock. Where the rock openings are small and closely set
together, the belt of mineralization will follow more closely
the surface and will often extend only a distance of several
hundred feet; strong fracture zones or fissures, however,
may carry the surface waters and their effects locally much
deeper.

Features of the Process of Re-concentration of
Pre-existing Ores by Shallow Descending Waters.

Are the ores concentrated by shallow descending waters all
leached from the present oxidized zone?
Whether or not the concentration by descending waters

•T. C Hopkins. Bulletin Geological Society of America, Vol. XI, pp. 475-502.

272 GEOLOGY APPLIED TO MINING.

acts upon the previous ore-body, the results are not simply
those which can be obtained from a given belt of oxidized
rock at a given period. In most places thousands of feet
of rocks — often several miles — have been removed by
erosion to lay bare the present surface. As the surface
rocks are stripped away, they may contribute a part of
their metallic contents to the rocks below, and so the surface
zones of mineralization continually migrate downward,
keeping pace with the erosion. The result is that we may
have, below the oxidized zone, not alone the concentrated
metals of that zone, but contributions from many ancient
oxidized zones, long since swept away.

Are regions with the deepest zones of weathering or oxidation
most likely to he attended by rich concentrations by de-
scending waters?

This consideration leads to the comparison of oxidized
zones and zones of metal concentration by surface waters.
Oxidation is a slow process. Hence, in countries of little
erosion (which are necessarily those of little moisture)
oxidation has plenty of time, and extends, partially at least,
to great depths, in spite of the fact that atmospheric waters
are the chief agents of oxidation. In countries of great
rainfall and erosion, the zones of oxidation, though rapidly
formed, may be swept away as rapidly, so that it hardly
penetrates below the surface. Yet in the latter case the
zone of concentrated ores may be more rich than where the
oxidized zone attains unusual development, for the thick-
ness of rock removed in a given time by erosion is many

CHEMICAL GEOLOGY. '273

times more than in the arid region, and therefore larger
quantities of metals are brought into solution and made
susceptible of re-precipitation and concentration.

What are the conditions determining the concentration of ores
by descending waters in the surface zones?

In different cases the concentration of pre-existing veins
by surface waters varies, whether the newly-formed min-
erals take the form of oxides, chlorides, carbonates, or
sulphides.

The conditions which determine this variability of effect
are divisible into three classes.

1. Relative quantity of solutions and slope of land
surface.

2. Chemical and physical nature of ores.

3. Chemical nature of solutions.

How does the relative amount of rain- and snowfall^ and the
surface slopes, affect this process?

The factor determining the relative quantity of solutions
is climate. On this depends the amount of moisture pre-
cipitated. Other things being equal, a large quantity of
water will do more work in dissolving and re-precipitating
mineral matter than a small quantity.

Another important circumstance is the relative rapidity
with which that portion of the waters which remains on the
surface wears away the rocks. This condition is depend-
ent, with a given quantity of rainfall, upon the surface
slopes.

274 GEOLOGY APPLIED TO MINING.

We may consider the chances in four different kinds of
country:
Moderate to well-watered country with steep slopes.
Moderate to well-watered country with slight slopes.
Arid country with steep slopes.
Arid country with slight slopes.

What arc the chances for this process in a well-waiered country

with steep siopesf

In a well-watered country, the level of ground water will
be high, and, as this corresponds in general to the upper
level of sulphides, the zone of oxidation will be relatively
shallow. If the slopes are steep, the surface will wear
rapidly, so that the oxidized zone may even be removed as
fast as it forms, and the sulphide zone may almost or quite
come to the surface.

In this case the abundance of waters will tend toward a
complete rearrangement of metals in the superficial zone,
but the rapid wearing away is apt to interfere with this
process, and the concentrated metals which outcrop at the
surface are largely removed and lost.

What is the usual effect of descending surface waters in

a well-watered country with slight siopesf

If the slopes are slight, and the supply of water abundant,
the oxidized zone will be well marked and thoroughly
altered, though relatively shallow. The rearrangement of
metals according to their relative solubilities will be com-
paratively complete. Under such conditions, gossan, or
iron capping, is common. According to the solubility of the

CHEMICAL GEOLOGY. 275

minerals and metals in the orginal deposit, the gossan may
be exceptionally rich; or very poor, with rich ores below,
in the enriched oxidized ores and in the enriched secondary
sulphide zone.

WhM are the characteristic effects of descending surface waters
in an arid region with steep slopes?

In an arid region, where the supply of water is small,
the changes wrought are characteristically incomplete. In
the zone of surface alteration oxidized and unoxidized ores
occur side by side, both often occurring together at or
near the outcrop. Therefore the oxidized zone, and the
zone of secondary sulphide enrichment, are not so well
defined as in regions of heavier rainfall.

The level of ground water being low or wanting, the zone
of partial oxidation extends far down. For the same
reason, the secondary sulphide zone is apt to be indistinct,
and sometimes, perhaps, not separable from the oxidized
zone.

In sum, the degree of rearrangement is not likely to be
so complete as in a well-watered region, but the enriched
zone is likely to be as thick or thicker, and will be more of
the oxidized than of the secondary sulphide nature.

Where the slopes are steep, the occasional rainfalls or
snow meltings have great power to strip the surface, and
carry it down to the valleys; and if this stripping is not so
active as in a well-watered country, the same scarcity of
water limits the rapidity of ore-concentration in the surface
Eone. Here, then, the zone of surface rearrangement is apt

2T6 GEOLOCiY APPLIED TO MININCi.

to he relatively not so thick, and at the same time is mcom-
plete.

In an arid region with slight slopes what are the effects of
descending surface waiersf

Where the slopes are slight, in an arid country, the wear-
ing away will not keep pace with the alteration in the sur-
face zone. Hence, in the course of time, the rearrangement
of minerals will extend to a very considerable depth.

What are the most favorable conditions for oxide or sulphide
concentration near the surface?

It seems that great precipitation is more favorable to
this result than aridity, and the slight slopes to steep ones.
Most favorable is the combination of slight slopes and
abundant precipitation; the combinations of abundant
precipitation and steep slopes, and of slight precipitation
and slight slopes, are perhaps equally favorable one to
another; but in the first case the oxidized zone will be
slight, and the secondary sulphide zone important, and in
the second the reverse will be true.

How does the chemical and physical nature of veins affect their
superficial concentration by descending surface waters?

The rearrangement depends upon the facility with which
the ores are taken into solution and re-deposited, and so
easily soluble ores should be more quickly and completely
rearranged than minerals which are difficultly soluble.

CHEMICAL GEOLOGY. 277

Quartz veins, for example, containing free gold, (which
is relatively difficultly soluble), should not be expected to
show so much rearrangement as copper ores, which are
relatively easily soluble.

The physical conditions of the veins also largely govern
this action in the surface zones. If the metals are, from
the nature of the vein, easily accessible to surface waters,
the result will be more complete than if they are not readily
attacked. This may depend on the original characters of
the vein, or on subsequent conditions. For example, veins
consisting largely of metallic minerals are much more
quickly attacked than those where the metallic minerals
are small in amount and locked in gangue, such as quartz.
Also veins that have been shattered or fractured since their
formation are easier of attack than those which are un-
broken. The actual outcrop of a vein is almost always
shattered by changes in temperature, so that this is a
specially favorable field for alteration by surface- waters.

How does the character of the solvents contained in descending
surface waters affect the nature of ore-concentration by
therriy and what determines their character?

The character of the solvents is often as important as the
relative solubility of the metallic minerals. In one region
where a given solvent is present, a given metal, easily
attacked by it, may be readily dissolved and re-deposited;
in another case the same metal, for the Jack of such solvent,
may remain comparatively little altered. In descending

278 GEOLOGY APPLIED TO MINING.

surface waters the character of the contained solvents may
depend on the nature of the ores in the vein, on the nature
of the gangue, of the wall-rock, of the soil, or of surface
deposits of various kinds. Through all of these descending
waters must pass.

Examples op Secondary Alteration by Surface
Waters.

1. Gold quartz veins in a country of steep slopes and
abundant precipitation. Gold belt of the Blue Mountains
of eastern Oregon.*

Typical gold-quartz veins (that is, quartz veins contain-
ing gold, which is generally associated with iron pyrite
scattered through, and embedded in, the quartz) are, it
seems, not always easily affected by surface waters. In
the first place the great mass of quartz protects, to a large
extent, the relatively small quantity of pyrite from the
air and surface waters.

In a moist country, where oxygen and carbonic acid are
the chief reagents contained in the waters which sink
below the surface, gold is little affected. The waters
oxidize the pyrite, and the dissolved iron is carried off.
Ferric sulphate, which may be one of the products of the
oxidation of the pyrite, can dissolve gold; but either this
action is slight, or the gold is almost immediately precipi-
tated again, for experience shows that the bulk of the gold
stays in the free state in the oxidized outcrops.

It even remains there, where erosion is very weak, after
the outcrop has crumbled to soil, and forms residual placer

*Waldemar Lindgren, 22d Annual Report United States Geological
Survey, Part II, p. 611, etc.

CHEMICAL GEOLOGY, 279

deposits (rooted deposits). Frequently such deposits are
rich.

Where the slopes are steep and the rainfall abundant,
as in the Blue Mountains of Oregon, the surface debris is
swept away too soon to permit any accumulation of the
kind above mentioned. The water level is high in this
region, and oxidation extends down from 100 to 300 feet,
and is then only partial. The partially oxidized surface
zones sometimes show twice as much gold per ton as the
unaltered lower portions; in other cases there is very little
difference. An increase of gold and a decrease of silver in
the oxidized zone was noted in one case. This is to be
explained by the leaching out of a portion of silver by
oxidizing waters. Under ordinary conditions silver is more
easily soluble than gold.

There is no observable zone of enriched secondary sul-
phides in this instance.

2. Veins carrying lead, zinc, copper and iron, with gold
and silver, with a relatively small amount of gangue, in a
region of great precipitation and very steep slopes. Dis-
trict of Monte Cristo, Cascade Range, Washington.

This district has been described by the writer;* and the
suggestion made that the ore-deposits as a whole may have
been formed by downward moving solutions. But as the
waters would have had the same effect upon a body of
earlier ore, formed in some other way, the case still serves
to illustrate the action of descending surface waters under
these conditions.

The climatic and surface conditions are practically the
same as those in the Blue Mountains of Oregon, already
cited. But while, in the Blue Mountains, concentration in
the partially oxidized gold quartz vein is slight, and a zone

* 22d Annual Report United States Geological Survey, Part II.

280 GEOLOGY APPLIED TO MINING.

of sulphides formed by descending waters is not recognized
as existing, in the Monte Cristo ores the sulphides deposited
by descending waters form remarkably strong and com-
plete zones. The difference is plainly due to the different
characters of the minerals involved.

In the Monte Cristo district, as in the Blue Mountains,
there is no zone of complete oxidation. Sulphides outcrop
at the surface. The zone of even tolerably complete oxida-.
tion does not extend more than a depth of ten feet any-
where, and generally is lacking.

Enonnously abundant surface waters, keeping the
ground-water level close te the surface most of the year,
have dissolved the lead, zinc, copper, iron, silver, gold, etc.,
from their original positions, carried them down, and
re-precipitated them as sulphides. The minerals are pre^
cipitated roughly in the order of their relative solubility, the
least soluble being carried the least distance. Thus the
upper zone is characterized by lead (galena), gold and
silver, and the lower limit of galena follows the contour of
the surface, some 100 to 150 feet below it (Fig. 67). Below
this there are some less regular, but still definite, zones
characterized successively by zinc (blende), copper (chal-
copyrite), and iron and arsenic (arsenopyrite and pyrite).
The sulphides near the surface carry an average of 0.95
ounces gold and 12 ounces silver to the ton; at some dis-
tance (a few hundred feet) from the surface, the pyrite and
arsenopyrite contain an average of 0.6 ounces gold and 7
ounces silver.

A maxinumi of 600 feet is assigned for the vertical dis-
tance between the surface and the bottom of the copper
zone.

3. Copper pyrite ores (or iron pyrite carrying some
copper) in a well-watered country with moderate slopes.
Ducktown, Tennessee, etc.*

*W. H. Weed. Tranaactiona American Institute Mining Engineers, VoL
XXX, p. 449; J. F. Kemp, id. Vol. XXXI, p. 244.

CHEMICAL GEOLOGY.

281

282 GEOLOGY APPLIED TO MINING.

The general conditions permit of a thorough, though not
especially deep zone of oxidation, and a high water level,
below which sulphides exist.

The solvents or reagents in such surface waters are chiefly
(besides the water itself) oxygen and carbonic acid. Iron
sulphide, whether free (in the form of pyrite, marcasite, or
pyrrhotite), or in combination with copper sulphides as
copper pyrite (chalcopyrite) is easily attacked by the oxy-
gen of surface waters, forming iron oxide (limonite), and the
sulphur becoming sulphuric acid. Iron sulphate is also
formed. The copper sulphide maybe transformed into cop-
per oxide in the same way, but it generally goes into solu-
tion, chiefly as copper sulphate, and passes downward. On
reaching unaltered sulphides, the soluble copper salt is pre-
cipitated, forming copper-bearing sulphides, which grow
progressively richer with the continuation of the process,
the iron being taken away in the forrri of the soluble sul-
phate. Pure copper sulphides, like chalcocite (copper
glance), is even formed. The iron released by precipitation
of the copper sulphide in part finds its way further down in
the earth and is there again precipitated, as pyrite.

When this process has gone on for a long time, as it has
in this case, where the slopes are gradual and the wearing
away does not move faster than the progress of the altera-
tion, there will be a surface zone where the oxidized iron ore
(limonite) will be left with the quartz of the original gangue
(made spoiigy by the dissolving out of the or'ginal sul-
phides), with only small amounts of copper carbonates or
oxides. This is the characteristic gossan or iron hat.
Beneath this will come the secondary zone of very rich
copper sulphides, chalcocite and bornite, with pure chal-
copyrite. The zone is apt to have considerable extent.
It constitutes the principal ore-bearing horizon of many
copper mines of the kind described. Beneath, the pure

CHEMICAL GEOLOGY.

copper sulphides will disappear, and the proportion of
copper in the pyrite will grow less and less, till the original
pyrite with a small percentage of copper — the unaltered
ore — comes in permanently.

4. Ores of lead, zinc and copper, with silver and gold,
in an arid region with slight slopes. Horn Silver mine,
Utah.*

This is practically the mineral composition of the Monte
Cristo deposits, but the climate and the surface condition
are directly reversed. On theoretical grounds it has
already been pointed out that a well- watered country with
steep slopes and an arid country with slight slopes were
about equally favorable for the concentrating action of
downward tending surface waters; and the evidence here
seems confirmatory.

The mine is situated at the foot of a mountain, on the
edge of a desert valley. The footwall is limestone, the
hanging wall, andesite; the gangue is quartz and barite.
The workings are down 1,200 feet. Oxidation is only
partial; galena outcrops on the surface, mixed with lead
and sulphate; yet this zone of partial oxidation extends
down to the lowest depths reached.

Going down on the ore-body, changes in the ore occur.
The upper ores are lead ores, mainly lead sulphate (angle-
site) with sulphide (galena), some carbonates and oxides
of lead, and horn and ruby silver (silver chloride and
sulphide of silver and antimony). No zinc and copper are
found. This class of ore persists down to 400 feet; at 400
and 500 feet more arsenic and antimony are found, and a
little zinc. Further down, zinc increases, until at 700
feet there is an enormous amount, generally in the form
of carbonate or silicate; a little lead is associated with it.

* S. F. Emmons, Trantactuma American Institute Mining Engineers, Feb.,
1901.

284 OEOLOOY APPLIED TO MINING.

At 650 feet copper begins to come in, and extends down to
750, but not to 800 feet. The ore is largely chalcocite
(copper sulphide) with a good deal of galena. The lower
levels contain no copper, zinc or lead.

The results are, then, practically the same in these semi-
oxidized ores as in the sulphide ores of Monte Cristo,
though the mineral zones are somewhat broader. In the
Horn Silver mine oxidized minerals and sulphides have
evidently been deposited side by side, the small amount of
water at any given time permitting this. For example,
ruby silver is generally in such cases a secondary mineral,
and would normally occupy a deeper zone than the oxidized
ores, in districts where the supply of water was abundant
and there was a definite and high water level. In the
copper belt the sulphide chalcocite is probably secondary,
yet it occurs with -carbonates and silicates. The mineral
zones, therefore, are zones of partial oxidation, to a less
degree of secondary sulphide deposition; and a separate
belt of secondary sulphide deposition very likely does not
exist.

The chlorine abundant in the waters of dry climates
shows its effect in the silver chloride. The other reagents
were probably oxygen from the air, which converted the
lead sulphide into sulphate; carbonic acid, very likely
derived from the limestone footwall, which produced the
lead and zinc carbonates; and silica, from the solution of the
quartz gangue, which produced the zinc silicates.

How deep does the zone of concentration of oxidized or sulphide
ores by descending surface waters generally extend?
In the xMonte Cristo district, Washington, just cited, a
maximum of 600 feet of sulphide enrichment was esti-
mated, and 300 feet is probably a nearer average. In the
case of the Horn Silver mine, Utah, the concentrated ores

CHEMICAL GEOLOGY. 285

(mingled oxidized and sulphide ores) extended down to
750 feet. At the De Lamar mine, Nevada, the gangue is
quartz, the metallic mineral pyrite and perhaps some form
of telluride, and the values gold and silver. The ennch-
ment extends down 700 feet or somewhat more. The ore
here is all oxidized.* In the pyrite deposits of southern
Spain and Portugal the surface zone rich in copper usually
extends some 300 feet down, below which the pyrite con-
tains very little copper.f

At Ducktown, Tennessee, the lower limit of the zone of
rich copper minerals, formed by concentration of the
original lean magnetic pyrite (pyrrhotite) is about 100 feet
below the surface, and the zone is thin, the iron hat or gos-
san occupying the greater part of this distance. At the
Independence mine, Victor, Cripple Creek district, Colo-
rado, the zone of secondary precipitation and enrichment
of ores by surface waters extends in general some 400 or
500 feet below the surface. J

Manner in which Minerals are Precipitated by
Descending Waters.

In what forms are metals usually precipitated by shallow

descending waters?

Deposits by descending waters may be oxides, sulphates,
carbonates, chlorides, sulphides, etc. Those minerals

* S. F. Emmons, Transactions American Institute Mining Engineers. Feb.;
190L

t Klockmann, Zeitschrift fiir prakiische Geologic, 1895.

t T. a. Riekard, Engineering and Mining JmirncU, Vol. LXXIV, No. 26,
p. 850.

286 GEOLOGY APPLIED TO MINING.

which have been aflFected by the oxidizing process arc
converted into some compounds containing oxygen, whether
it be sulphate, carbonate, or oxide, even if they were
originally sulphides.

Copper sulphide in the oxidized zone will be largely
converted into copper carbonate (malachite or azurite) or
cuprite and tenorite (red and black oxides). Copper
sulphate may also be formed, but being soluble in water
will not as a rule be precipitated, but will be held in solution
until by some reaction the copper is precipitated in another
form. But sulphate of lead is relatively insoluble, hence
this mineral (anglesite) is frequent in the oxidized zone of
lead ores, as well as the carbonates (cerussite) and the
oxides (minium, litharge, etc.). Chlorides are formed in
the oxidized zone, by the actions of waters containing
chlorine or alkaline chlorides in solution, and the metallic
chlorides that are relatively insoluble are precipitated.
Silver chloride or horn silver (cerargyrite) is a familiar
case. Easily soluble chlorides such as those of iron
and gold are not found to any great extent.

Under what conditions are ores most likely to he deposited as
chlorides J in the oxidized zone?

In arid regions there is little or no free drainage to the
ocean, and the surface and ground waters are largely
removed by evaporation, leaving their solid compounds
behind. Chlorides of sodium (common salt) magnesium,
etc., form part of this residue. They have been leached
out of the decomposing rocks in small quantities, or

CHEMICAL GEOLOGY. 287

supplied by hot springs, but on continued accumulation
and concentration by evaporation become important.
Such is the origin of the salt pans and alkali flats, as well
as the saline lakes which occupy the depressions of desert
regions. In these arid tracts alkaline chlorides are abund-
ant in the shallow underground waters which percolate
through the ores, and the results are likely to be chlorina-
tion of the metals, the precipitation in the weathered zone
of the insoluble chlorides, and a more or less thorough dis-
solving out of the soluble ones.

Example: In the dry tracts of Arizona, New Mexico and
Nevada, where salty incrustations, due to the causes
sketched above, are found in nearly every valley depres-
sion, the chloride of silver is noticeably abundant and
frequent in the weathered zone of ore deposits.*

What causes the deposition of sulphides by descending waters?
Deposition of sulphides from descending waters is often
brought about by the reduction of soluble metallic salts by
contact with already existing sulphides. For example, a
copper solution coming in contact with crystallized pyrite
(iron sulphide) may be reduced, so that the sulphide of
copper and iron (chalcopyrite) results. Renewed copper
solutions acting upon this chalcopyrite may change it into a
sulphide richer in copper, such as bornite. Still renewed
solutions may change the bornite to chalcocite. Chalcopy-
rite contains 34.5 per cent, copper, bornite 55.5, and chal-
cocite 77.8 per cent, so that the quantity of copper is
greatly increased.

* R. A. F. Penrose, Jr., Journal of Geology, Vol. II, No. 3.

2HH (4E0L0GY APPLIED TO MININO.

Eocample: Chalcocitc is the principal ore in the great
copper district of Butte, Montana, though bomite and
enargite arc common. The chalcocite forms coatings on
the other metallic minerals in such a way as to show that
it was one of the latest minerals to crystallize. As depth
is gained the percentage of pyrite and enargite increases in
comparison with that of chalcocite, so that while the first
thousand feet of ores averaged 8 or 10 per cent copper, the
second thousand averaged about 6 per cent. The chalco-
cite has been formed by a chemical reaction between copper
sulphate in solution in descending waters and the iron
pyrites and other primary sulphides lying below. By
imitating the conditions in the mines, chalcocite has been
produced artificially.*

What is the action of organic matter in the shallow under-
ground zonCy as regards the precipitation of sulphides f
Precipitation by the action of organic matter is very
important in the shallow water zone, both near the surface
and at considerable depths, and takes place in the same
way that has been described for the deeper underground
regioas. Mine timbers (especially those in old mines) may
precipitate metals from solution in mine waters. Dr.
Raymond has reported a case in a New Mexican mine,
where the eye of an old pick has been filled with galena
(sulphide of lead) by the reducing action of the wooden
handle which once occupied this position. This is only an
example of what must often occur on a large scale when
descending solutions come in contact with beds of shale or
other sedimentaries containing organic matter. The

* H. v. Winchell, Bulletin Geological Society of America, Vol. XIV. pp.
269-276.

CHEMICAL GEOLOGY. 289

reactions are much the same as in the case with ascending
solutions. (See p. 246.)

Are metcUlic minerals deposited by descending waters as

replacements or as cavity fillingsf

In respect to manner of deposition, the metals borne by
the shallow, generally descending, waters, may be precipi-
tated in the same way as those in the deeper waters. Like
them, they may be, and perhaps oftenest are, deposited by
replacement, preferably of limestone, frequently of some
other rock. They may occupy the tiny openings of a
porous rock, or cavities formed either by fracturing or
dissolution.

CHARACTERISTICS OF ORE-DEPOSITS FORMED BY
ASCENDING AND BY DESCENDING WATERS.

Is it of practical value to know whether a given ore-depomt was
formed by ascending or descending waters?
This knowledge is often of economic importance.

How can this point be ascertained?

Let us take a case where metalliferous solutions are
stopped in their circulation by a relatively impervious
stratum, a decomposed dike, or other rock mass, or whatso-
ever it may be, and where as a consequence of the spreading
out. and detention of the solutions, ore-deposition takes
place. If the ore-deposits are conspicuously placed on the
under contact of such an impervious body, it is a fairly safe
index of ascending currents; if on the upper side, of descend-

290 GEOLOGY APPLIED TO MINING.

ing. The case is emphasized in folded strata, for the
upward-tending solutions will be confined chiefly in the
tops of anticlines, and the downward moving ones in the
troughs of synclines, and here the ore-deposition will by
preference take place.

Example: In the Bendigo goldfield, Australia, described
on p. 165, the auriferous quartz veins occur by preference at
the apex of anticlinal folds in the stratified beds (saddle
reefs) (Fig. 32). Deposits in the synclinal folds (inverted
saddles) are rare and imimportant.

Where ore-bodies are formed at an intersection of circu-
lation channels, be those channels joints, faults, porous
beds, or any combination of these, they will often be found
to form by preference either on the upper or the lower side
of such intersections. They will be either in the troughs
formed by the two channels converging downward and
meeting; or in the roof, formed by the channels converging
upward and meeting. The former case is generally an
indication of descending waters, the latter of rising ones.

Example: The typical false saddle, auriferous quartz
veins of the Bendigo goldfields, Australia, drawn by T. A.
Rickard, are, as shown (Fig. 68), formed at the inter-
section of a joint a a with a bedding plane. The fact that
the ore-body has formed in the roof rather than in the
trough of the intersection may be regarded as indicating
that the auriferous quartz was deposited by ascending
waters.*

* Transactions American Institute Mining Engineers, Vol. XX, p. 469.

CHEMICAL GEOLOGY. 291

When ore-bodies show constant and evident relation to
the surface, being strong on the outcrop, but shallow
and becoming impoverished with depth, it is an evidence
of formation by descending waters. (Seep. 280 and Fig. 67.)

rv:V-:-..| 8AND8TONE ^HJSLATE ^^gUARTl

Fig. 68. Ore formed by intersecting fractures, a a is fracture cutting across
stratification. After T. A. Rickard.

waters, as is shown among other things by their intimate
connection with the surface. On the Mesabi iron range,
near the north shore of Lake Superior, in Minnesota, great
masses of iron ore lie at the surface, under glacial drift
(Fig. 69). The iron was originally a marine precipitate,
disseminated through a sedimentary rock, from which
condition it has been concentrated into commercially

Example: Many iron deposits are examples of this. \

The Lake Superior iron ore-bodies are due to descending J

292

GEOLOGY APPLIED TO MINING.

valuable ore-deposits, in favorable places, by descending
surface waters.

The presence of cavities crusted with stalactites and
stalagmites of ore indicate a downward movement of the
waters at the time these stalactites and stalagmites were
deposited, and very likely during all the ore-deposition.
Since, however, in ore-deposits formed mainly by ascending

Fig. 69. Iron ore-deposits showing constant relation to surface (formed by des-
cending waters). General section at Biwabik, Mesabi iron range.
After H. V. WincheU.*

waters there are apt to be minor stalactitic growths,
formed by downward tending surface waters of a later
period, short duration, and relatively slight efficiency, one
should guard against the sweeping application of this test.
Many ore-deposits are formed by the combined effects of
ascending and descending waters, often acting at different

* 20th Annual Report Minnesota Geological and Natural History Survey.

CHEMICAL GEOLOGY. 293

periods. Ore-deposits formed by ascending waters may,
as already described, be worked over and redeposited by
descending waters. Therefore, one must beware of
applying evidence obtained from a single ore-body to all
the ores of a district; and one should especially avoid
taking the evidence of the shallow ore-deposits near the
surface, where there is frequently strong evidence of the
work of descending waters, to apply necessarily to deeper
ores.

What practicU deductions follow the solution of the question
as to the deposition by ascending or descending waters?

The efficiency of downward tending waters is greater
near the surface, and therefore on going down a moderate
distance the ore deposited from such waters is apt to
become impoverished and fail rapidly; while a deposit by
ascending waters is likely to be much more deep-seated
and regular.

CHANGES IN RICHNESS IN DEPTH.

What changes do ores, deposited by descending waters, show
in depth?

Ores due primarily to descending waters can be counted
on to become poorer in depth, as remarked above. Where
an earlier ore-deposit has been worked over and concen-
trated by descending surface waters, the values will be
often less below the enriched shallow zone.

294 GB0L06T APPLIED TO MIXING.

What general changes may ores deposited by ascending waJters

show in depth?

Concerning the great class of ore-deposits due to ascend-
ing waters, it is e^ident, that, being limited bodies,
they will have a top and a bottom somewhere; also
that at some point, probably intermediate between the
top and the bottom, they will be largest and probably
richest. These ore-deposits would ordinarily not be
revealed to the eye of man were it not for the removal
of the overlying rocks. Therefore, the wholly fortuitous
circumstance of the level of the plane of erosion at the time
of the discovery of the ore-body determines whether they
will become stronger or weaker in depth. Erosion may
reveal only the top of a deposit, and it will grow richer
below; or it may reveal the bottom, and it will grow rapidly
poorer; or it may cut some intermediate level, and the vein
may hold its own for a long distance do\ni, with about the
same strength and richness. These conditions are apt to
be fairly uniform over a considerable district, so far as the
ore-deposits formed at a single period are concerned. So
in one district the ore characteristically grows weaker in
depth, in another stronger.

What changes may sedimentary ores show in depth?

In the case of sedimentary ore-bodies, which have been
folded so as to dip at a high angle, it is plain that depth can
have no effect in determining richness or poverty.

How deep may a vein exterid?

Under favorable circumstances, where there is a strong

CHEMICAL GEOLOGY. 295

water channel, and constant facilities or factors favorable
to ore-deposition all along it, it is probable that the resulting
vein may extend to a great d^th, perhaps in extreme cases,
two or three miles below the original surface. Where such
a vein is being worked, it may be profitable as far down as
the present methods of exploitation can be pushed.

Example: The gold-quartz veins of Nevada City and
Grass Valley,* California, show, in general, a continuity
down to the greatest depths worked. Many of the smaller
veins diminish and disappear in depth; but the larger ones
hold their own. In one district there is exposed, by mining
and irregularities in the topography, a vertical distance of
3,500 feet, within which there is no evidence of change in
the character or quality of the ore; in another place the
same truth holds for a vertical distance of 2,600 feet.

These veins must have been formed at a depth of several
thousand feet below the surface, and a great part of their
original extent (the upper portion) must have been worn
away by erosion.

In ore-deposits due to ascending waters^ may the character of
the minerals change with depth?

There may be a change in the character of mineral in
depth, for ores may be deposited according to their relative
solubility, by ascending waters, in much the same way
(though probably not so regularly and definitely) as by
descending waters.

* W. Lindgren, 17th Annual Report United States Geological Survey, Part
II, p. 162.

296 GEOLOGY APPLIED TO MINING.

Example: In the case of the Dolcoath mine in Cornwall,
which has probably been formed by ascending waters,
copper is relatively more abundant in the upper zone, tin
in the lower one.

May ore-deposits formed by ascending waters ever exfend
great distances downward without change in the character
of the oref

In some cases the character of the ore may remain about
the same through a considerable vertical range.

Example: The silver-lead deposits in Aspen, Colorado,
show about the same characteristics through a known verti-
cal range of over 3,000 feet.* Here the ore-deposition has
been largely along a bedding fault. This has usually, on
one side, a bed of shales, which have probably contributed
toward precipitating the ores as metallic sulphides. The
beds are steeply upturned, so that for considerable depths
uniform conditions for deposition have obtained.

Is it possible that any veins continue downward indefinitelyf

Owing to the pressure exerted by gravity, it is doubtless
more difficult for a fissure to stay open in depth than near
the surface. The tendency is to press the sides together
and close the opening. At a certain depth, it is probably
the case that the pressure and the plasticity resulting from
this, together with the increase of heat, makes it impossible
for fissures, fractures, or other openings to exist. Such
depth has been variously estimated at from 16,000 to
33,000 feet.

CHEMICAL GEOLOGY. 297

Is this theoretical downward limit of any practical importance?
This limit is far below the depth that can be attained in
mining. But some veins are very old, and even the com-
paratively recent ones (such as the Tertiary veins) are old
enough to have had in many cases several thousand feet
of rocks, which overlay them at the time of their formation,
removed by erosion. Hence, it is very possible that we
may find some veins diminishing and even disappearing in
depth.

Why is it that veins may sometimes diminish in size and
value below the zone of oxidation?

Near the surface, as already described (p. 182), openings
tend to become large and numerous. To be sure of the
persistence of a vein which has depended for its origin upon
a fissure, or system of fractures, one must first explore it
down below the zone of oxidation. Many a vein, large and
promising near the outcrop, will be found to dwindle to
quite insignificant proportions before arriving at this point;
biit, if the vein is still strong here, there is good reason for
expecting that it may continue so to a good depth, other
conditions being favorable.

Is there any relation between the horizontal and the vertical
extent of a vein?

There are all kinds of fractures and fissures, little and big,
transitory and permanent. It is a saying of some miners
that a vein will go down as far in depth as it extends hori-
zontally on the surface in outcrop. This saying seems to

298 GEOLOGY APPLIED TO MINING.

have some sound principle behind it; and practically the
same criterion has been applied by W. Lindgren in the case
of the gold quartz veins of Grass Valley and Nevada City,
California. Mr. Lindgren remarks:* "In considering the
probable permanency of a given vein, its general character
must be taken into consideration. Continuous well-
defined outcrops and wide bodies of quartz are in general
good indications of the maintenance in depth, as is also
any evidence of strong faulting and movement. . . .
A fissure which can be definitely proved to extend only a
short distance will in all probability be found to be corre-
spondingly limited in depth."

May a vein change in value in depth, on passing from one

rock into another?

In the case where a fissure or fracture system, along
which mineralization has taken place, passes from one kind
of rock into another, it may change its character so as to
influence strongly the character of the vein. For example,
a strong fissure in limestone may die out entirely on meeting
a bed of shales. This is an influence exerted by the
mechanical properties of different rocks.

Where a vein or other ore-deposit depends largely for
its existence upon the chemical properties of the rocks
through which it passes, the same changes may be expected.
For example, a replacement deposit in limestone may be
expected to be poor or to stop entirely when followed down
into quartzite or granite, although the physical conditions

* 17th Annual Report United States Geological Survey Part II, p. 163.

CHEMICAL GEOLOGY. 399

(the fracture zones and water channels) may be as good
in the latter rocks as in the limestones.

In sum, what changes may he expected in depth?

There is, therefore, no general rule as to whether veins
and other ore-deposits grow richer or poorer in depth.
Sometimes they may grow richer, sometimes poorer; some-
times they may grow richer, then poorer, then richer again;
and sometimes the distribution of values may be fairly
equable to a considerable depth. Each case must be taken
up and investigated separately, before the probabilities can
be arrived at.

ASSOCIATIONS OF MINERALS.

Can the 'presence of a given metal in an ore-deposit ever

indicate the probable presence of another?

Owing to chemical affinities (the fact of having similar
properties of solution and deposition, etc.) there are asso-
ciations more or less marked between different minerals in
ore-deposits; and this association, once understood, may be
of practical advantage to the miner.

What are some of these associations?

I^ad and silver are closely associated. Most silver is
obtained from argentiferous galena, and most galena con-
tains a greater or smaller amount of silver.

Lead, zinc and iron, most commonly in the form of
galena, blende and pyrite, but also in other forms, are
intimately associated, and often occur together.

300

GEOLOGY APPLIED TO MINING.

Copper and iron, whether in the common form of chalco-
pyrite (sulphide of copper and iron) or otherwise, are
closely associated. Pyrite may contain a variable amount
of copper sulphide, increasing up to the pure chalcopyrite
(34.5 per cent copper).

Lead and barium (the latter in the form of barite, or
heavy spar) are also frequently associated.

Tin and a number of rare elements are usually associated,

Fig. 70. Thin sections of ore from Omaha mine, Grass Valley, California. Mag-
nified 17 diameters. Light areas, quartz; shaded areas, pyrite;
black areas, gold. After W. Lindgren.*

though in any given case (as is, indeed, true for the other
associations cited) the combinations may not be found.
Among the metals and minerals of commercial value which
often occur in company with tin are tourmaline, fluorite,
topaz, lithia mica and wolfram.

* 17th Annual Report United States Geological Survey, Part II, PI. V.

CHEMICAL GEOLOGY. 301

Gold is frequently found in veins of quartz, and is espe-
cially associated with iron pyrites in this case (Fig. 70).

Do non-metallic minerals form associations in this way?

Another class of minerals, associated because they are
all easily precipitated from evaporating waters, are salt,
gypsum, anhydrite, and frequently borax, etc. These
minerals often occur also in red rocks, for which the expla-
nation is probably that the oxide of iron which colors the
rocks was also a chemical precipitate accompanying evapo-
ration.

The association of organic minerals, such as petroleum,
mineral asphalt or bitumen, natural gas and ozocerite
(mineral wax), is natural, for these are all the product of
the various distilling and other processes operating upon
sediments rich in organic mineral. Wherever one of these
minerals occurs, others of the same group should be sus-
pected in the region.

ROCK ALTERATIONS AS GUIDE TO THE PROS-
PECTOR.

Whai guides do the oxidation processes of veins afford the

prospectorf

The chemical peculiarities of ore veins and associated
rocks may often furnish the prospector a guide to the
presence of ore, even where there is little or none in sight.
Most oxidized outcrops of veins are marked by a decom-
posed mass, colored reddish from the oxide of iron which

302 GEOLOGY APPLIED TO MINING.

has been formed by decomposition •from the iron present
in the unaltered vein. This outcrop may look like any-
thing but a metal vein, but the experienced prospector
recognizes the sign, and not only samples and assays the
decomposed, often clayey material, but begins making
excavations to see what the stuff looks like a little further
down. Green stains on such an outcrop may indicate
copper, nickel or chromium. Bright blue, green, and red
stains together usually mean copper. Bright red stains
may come from either lead or copper, or from rarer metals.

Where the outcrop of a quartz vein is cavernous or cellu-
lar, and rusty and crumbling, this generally points to the
former presence of iron pyrite and other sulphides, which
have been dissolved out by the weathering. An assay is
then always advisable, for in the dissolution of the metallic
sulphides a large part of the contained gold is often left
behind, so that this sort of quartz is often very good ore,
and it is, besides, free milling. In such quartz the cubic
cavities are often exact casts of the dissolved pyrite crystals.
In depth these sulphides are reached, and the ore becomes
more refractory, the gold which nature has separated in the
weathered zone being difficult of separation by metallur-
gical processes.

Whxii indications of mineralization are afforded by certain

rock alterations?

Less well known to the prospectors than the indications
afforded by oxidation are those given by the alterations in
the wall rocks which often accompany ore-deposits. In

CHEMICAL GEOLOGY. 303

general, if a rock has a thoroughly altered, softened or
decomposed appearance, it testifies to the former searching,
dissolving and precipitating effect of chemically active
solutions, and in so far it is a favorable sign. Rock thus
decomposed, whether it was originally granite or quartzite,
05 anything else, is often called by the general and incorrect
term porphyry, especially when in an almost clayey state.
An abundance of iron pyrite, in a softened igneous rock,
is generally a good sign.

Is this rock alteration always of the same type?

The exact nature of this decomposition and alteration
varies, dependent upon the character of the rock thus
affected, and upon the chemical nature of the waters which
have usually done the work.

Example: The ores of the Silver City and the De Lamar
districts, Idaho, occur as quartz veins, carrying frequently
high values of gold and silver. The wall rocks in the differ-
,ent mines are granite, basalt and rhyolite. There is good
evidence that the veins were deposited by ascending hot
waters. The granite has been comparatively little altered
by these waters, the basalt more so, the rhyolite intensely.
The granite is often nearly fresh close up to the vein, though
occasionally it is softened and changed by the alteration of
its feldspar to pale yellowish-green, clayey or micaceous
products (sericite).

The basalt is altered in a zone along the vein, which, as
a rule, is not wide, but of very irregular extent. In the
first stages of alteration, it has a dull, earthy, dark-green
color, and contains small cubes of pyrite. When further

304 GEOLOGY APPLIED TO MINING.

altered, the rock becomes bluish-green or yellowish (due
to the development of the mineral epidote).

The rhyolites are the most altered of all, the rocks for
many hundreds of feet from the veins being changed.
The chief process has been silicification, so that the rock
often keeps its hardness and has a deceptively fresh appear^
ance. It sometimes contains also pyrite, for example, at
the De Lamar mines, where this mineral is very abimdant
Sometimes the rock has been softened, and near the veins
some streaks have been entirely converted to white or
yellowish clay (kaolin).*

How are rocks altered near tin veins?

In the neighborhood of tin veins in granite (the usual
occurrence) the rock is changed, largely by the removal of
its feldspar, and the deposition of some new minerab.
especially white mica (muscovite), in its place, to a rock
called ''greisen." This preserves a granitic appearance,
but is made up essentially of quartz and muscovite.

What does the silicification of rocks signify?

The alteration of rocks (preferably limestones, although
to a less extent other rocks) to silica is a favorable sign.
The resulting rock is jasperoid. Miners call it quartz,
quartz-rock, flint, chert, hornstone, jasper, etc. Ore-
bearing solutions generally contain a large amount of
silica, and where ores are deposited (and often where they
are not) this silica replaces the country rock in the neigh-
borhood of water channels. Ore-deposits are not in-

* W. Lindgren, 20th Annual Report United States Geological Survey, Part
III, pp. 124. 177.

CHEMICAL GEOLOGY. 305

frequently surrounded by a sort of rough shell of such
jasperoid.

Example: In the Tintic silver-lead-copper mining dis-
trict, Utah, (described by Tower and Smith)* the most
common form of chemical alteration is the substitution of
silica for lime, which when complete, forms jasperoid. In
this change the structure, texture, and color of the original
limestone are usually retained. The jasperoid is not only
intimately associated with the ores, but forms large bodies
which have little or no metallic content.

* 19th Annual Report United States Geological Survey, Part III.

CHAPTER VL

THE RELATION OF PHYSIOGRAPHY TO MINING.

What is physiographyf

Physiography is a study of the features of the earth'3
surface and the causes by which they were made to assume
their present shape. By this study we arrive at the com-
prehension of the processes by which mountains, hills,
valleys, lake basins, etc., were formed. These processes
include movements of the earth's crust, both faults and
folds, and the erosion of rivers, glaciers, seas, lakes, etc.
The application of physiography to mining is perhaps of
limited extent, but often appears unexpectedly.

Which are more favorable to ore-deposits, mountains or

plains?

In general, mountains are more favorable to ore-deposits
than are plains, because mountains are regions of disturb-
ance. Here the rocks are usually folded, igneous rocks are
likely to occur, faults and fractures are developed by the
movements, and all conditions necessary to ore-deposition
may be present.

In plains, on the other hand, the rocks are more likely
to be undisturbed, and igneous rocks and fracture zones
are less likely to occur.

RELATION OF FEIYSIOGRAPIIY TO MINING. 307

In mountainous regions, also, the underground water
circulation is generally more vigorous, on account of the
greater differences in elevation. Water sinking into the
rocks at the top of a steep mountain, for example, and
emerging at the foot, will circulate vigorously through the
intervening space. In flat countries this element is practi-
cally absent.

Are ore-deposits ever found in plains?

Some plains are the roots of old mo\intains, smoothed
out and leveled by continuous and persistent erosion; and
in such places one may find the ore-deposits that were
fonned in the old mountains. This is especially likely in
the case of such veins as were originally deposited at a con-
siderable depth below the surface, such as gold quartz veins
and tin veins.

Moreover, it is possible, even in flat-lying rocks, that the
condition of circiilation, the supplies of disseminated metals,
and the conditions for deposition, may be so favorable as
to determine the formation of ore-bodies.

What is a fault topography ^ and why may it sometimes serve as

a guide to ore-deposits?

There is in each district a distinct type of topography,
determined by the natural distribution, relative hardness,
etc., of the stratified and igneous formations which make up
the rock mass. Faults have a peculiar effect, in breaking
in upon this type of topography with irregularities, which
are partly due to unequal erosion caused by lines of weak-
ness developed along fault zones, and partly to the unequal

308 GEOLOGY APPLIED TO MINING.

erosion produced by the checkering of rocks of different
degrees of resistance, by the faulting. If the faulting be
complex (especially if there be two or more systems of
intereecting faults), it may produce a very irregular minor
topography, often marked by low rounded hills, and very
striking to the eye of the trained geologist, as deviating
from the conventional type. Such districts, especially if
they are in a region where igneous rocks occur, are fre-
quently favorable for ore-deposits. The areas of complex
faulting may often be regarded as the outcrop of a sort of
rock column, where the openings are so large as to induce a
concentration of the water circulation. The mining camps
of Leadville and Aspen, in Colorado, and Pioche and Eureka,
in Nevada, are good examples of this kind.

Single faults are also frequently connected with ore-
deposition, and may be traced from the topography. A
cliff or scarp may result from the direct dislocation of the
surface by a fault, or from the unequal erosion of rocks
brought together by it. In other cases the fault is marked
by a gully or valley.

Do veins ever produce characteristic forms in the topographyf

Sometimes veins are harder than the rocks in which they
occur, and -in this case erosion leaves them as ridges of an
unmistakable nature. Such is often the case with quartz
veins and other hard veins. On the other hand, a vein may
wear away, when exposed to the weather, more rapidly
than the rock, and then is marked by a straight groove in
the surface.

RELATION OP PHYSIOGRAPHY TO MINING. 309

Examples: 1. In the Bendigo gold quartz r^ion,
Australia, the quartz veins are more resistant to erosion
than the enclosing Silurian slates and sandstones, and
hence, are left at the surface as projecting ridges or ledges.*

2. In the region around the Copper Queen mine, Arizona,
a relation can be traced between the surface of the ground
and the underlying ore-bodies. The ore-bodies, having been
eroded faster than the enclosed rocks, form well-marked
depressions, t

When may a knowledge of the characteristic erosion forms of
different kinds of rock aid in locating ore-depositsf
Where a given rock in a district is known to be preferably
selected for ore-deposition this rock may often be recog-
nized by the peculiar topography of the country. For
example, a limestone weathers quite differently from a
quartzite and produces a distinct topography; a shale or
slate is again different. Eruptive rocks may generally be
distinguished from sedimentary by the difference in the
topographic forms produced.

How are deposits of soluble minerals sometimes indicated in

the topography?

In the case of stratified soluble minerals, especially rock
salt, the dissolving out of portions of the salt bodies under-
ground by circulating waters often causes a series of de-
pressions or sinks along or near the line of outcrop, by

* T. A. Rickard, Transactions American Institute Mining Engineers, Vol.
XX, p. 318.

t James Douglas, Transactions American Institute Mining Engineers, Vol.
XXIX. p. 637.

310 GEOLOGY APPLIED TO MINING.

which the salt zone maybe recognized. Such sinks exist In
the neighborhood of the great salt deposits of Stassfurt,
in (Jerniany, and the settling of the rock due to the under-
ground caverns has often involved the partial ruin of
villages.

Are ore-deposits more likely to occur at the tops of hills or at
their basesf

Professor Van Hise* has explained that in regions where
deposits were made by descending waters during the exist-
ence of the present topography ore-deposition is likely to
be greater at the crests, or beneath the upper slopes, where
the quantity of descending water is greatest. Deposits by
ascending waters, he explains (always restricting this
observation to such ore-deposits as were formed during the
present topography) are more likely to occur beneath the
valleys, or beneath the lower slopes, for here ascending
springs usually emerge. Third, ore-deposits which have
been first fonned by ascending waters and subsequently
enriched by descending waters will be on the slopes, prob-
ably in many cases nearer the valleys than the crests.

* Tran8ctctiona American Institute Mining Engineers, Vol. XXX, p. 27 et. aeq.

INDEX.

Page
A

Acrogens, defined 51

Adirondack Mountains, magnet-
ite 105

ore-dikes 117

Age of rocks containing ore-de-
posits 65

Age of rocks, how told. . . 42, 43, 56, 57

Age of veins 63

Alabama, saltpeter 262

Alaska, coal 66

glaciers 133

Kojmkuk district 214

Kuskokwim River 122

Nome 219,221

Slacers 218
eward Peninsula 210

Yukon River . . 122, 124, 129, 174

Algse, defined 51

Alkali flats, origin 261

Alterations of rocks, guides to

prospecting 302

Alum 109

Aluminum, ores ^

oxide 106

sulphate 110

Alunite 109

Ammonites, defined 47

Amphibians, defined 48

Amphibolite 11

Amygdaloid 90

Andesite, alteration 91

ores deposited from 63

replacement of 250

Andesitic rocks, defined 87

transitions 94

Angiosperms, defined 50

Anglesite 286

Annydrite, associated minerals . . 301
Animal kingdom, division of ... . 44

Anogens, defined 51

Anorthosite, defined 117

Ansted, Prof., cited 258

Anticlinal folds, oil and gas in . . . 166

Anticlinal mountains 127, 128

Anticlines 121

apex of 165

fracturing in 167, 168

ore-deposition in . 164,165,167,168

Antimony m springs 242

Italy 63

Mansfeld, Germany 70

Antimony sulphide, deposited by

fumaroles 110

Apatite in veins 109

Apex of anticlines 165

Aqeo-igneous fluidity 13

Archaean granites 95

rock.H, characteristics of . ... 37
veins in 63

Page

Archaean period 37

Arches in strata 166

Ar^^entine Republic, oil 69

Arid climates, superficial altera-
tion of ores 283

Arid region, chlorides in 286, 287

Arizona, Copper Queen mine .... 309

Tombstone 168

Arkansas, lead and zinc 194

Arsenates, derived from arsenides 267

Arsenic in sinter 264

in springs 242

with copper 267

Arsenic sulphide 110

deposited from fumaroles ... 17

Italy 64

Arsenides 244

Arsenites, derived from arsenides 267

Arsenopyrite. auriferous 206, 280

Articulates, defined 47

Ascending solutions, ore-deposits

by 19, 74, 165. 166, 168

Ascending water ore-deposits,

changes in depth

. . . 289, 290, 291, 294, 295, 296

criteria 289, 290, 291

Aspen, Colorado . 169,170.202,203, 248

age of ores 63

parting quartzite 55

Asphalt mineral, origin 301

Association of minerals 299, 301

Atmospheric waters, ore-deposi-
tion by 189

Augite in diabase 100

in rocks 7

Auriferous quartz-veins 77

Australia. Ballarat 77

Bendigo

164, 165, 167, 186, 188, 289, 309

Omeo 13

Tertiary placers 65

Australian gold, age of containing

rocks 62

Azurite 286

Baku, oil-bearing strata 69

Ballarat, Australia 77

Banded structure, igneous rocks . 79

metamorphic rocks 4

veins 190, 191

Bars, formation of 217

Bar placers 216

Barite 300

as gangue 283

Ba.Halt, alteration 91

altered to green.stones 10

Columbia River 98

columnar jointing 174

gold and silver in 101. 102

314 INDEX.

Page
Basaltic rocks, defined 87

transitions 94

Basaltic structure 174

Basic igneous rocks, gold and sil-
ver in 103

Basic rocks, connection with cer-
tain metals 113, 114

connection with ore-deposits. 112

Basic, term defined 11, 104

Basin, structural 148

Bassick mine, Colorado 1 10

Bauxite 8

Beach placers 218, 219

fossil 227

reconcentrated 219, 220

seaward extent 220

Becker, G. F., cited. / 240

Bed, defined. 27

Beds of 6re, association with cer-
tain rock-characters 60

identification by fossils. ..... 68

primary and secondary 59

Bed form, minerals occurring in . . 58
Bedded deposits, subsequent ....

71,73,74,75

Bedded ores, precipitation 71

Bedding, explanation 27

faults 158, 159

Bed-rock, of placers 223

placer gold on 208, 209

Bench placers 221

Ben More, metamorphism 3

Birds, defined 48

Bismuth, sulphide 17

telluride 17

Bittner, cited 129

Bituminous shale 69, 70

Black sand in gravels 210

Blake, W. P., cited 70

Blende in coal-shales 73

deposited in mine workings . 64

zone near surface 268, 280

Bog iron 241,259

Borax, associated minerals 301

formed by evaporation 261

Bornite 282

formation 250, 287

Boron, minerals containing 108

Brachiopods, defined 47

Branner, J. C, quoted 257

Brazil, weathering 256, 257

Breccia, defined 92

due to chemical charges .... 256
British Columbia, platinum 114

Tertiary placers 65

Brooks, A. H., cited . 210, 220, 221, 227

Browne, R. E., cited 223

Butte ore-deposition 116

Cadell, H. M., cited 3

Calamites, defined 51

Calcite in contact metamorphic

deposits 108

in dolomite 30

in metamorphic rocks 17

California, auriferous gravels. . . . 224

gold in rocks 101

California, Grass Valley. 100, 124, J
Nevada City and Grass Val-
ley 197,295

old placers 223

Tertiary placers 65

Cambrian 37

fossils of 38

lack of coal in 67

origin of term 36

placers 65

rocks 43

Cambrian period 37

Cambrian sediments, nietamor-

phismof 3

Canada, magnetite 105

Three Rivers district 259

Carbonate of iron deposits, origin. 266
Carbonate of lime, deposition at

surface 25

Carbonates, deep formation 244

formation at surface 256

Carbonic acid, in rock-weathering 257

in waters 240

Carboniferous, coal :..... 66

fossils of 39

gold in. . . 62

origin of term 36

rocKS 43

Carboniferous p>eriod 37

Carmichael, Mr., cited 114

Carnotite 262

Carpathians, oil . 69

Cassiterite 109, 113, 230

Castro, Fernandez de, cited 114

Caivern deposits 75, 262

Cavities, ore-deposition in 190

Cavity filling 33

Cephal9pods, defined 46

Cenozoic era 37

Cerargyrite 286

Cerussite 286

Chalcocite 287

formation 282, 288

secondary 284 .

in Sierra Oscura. 72

Chalcopjo-ite, alteration to born-
ite 250

deposited by fumaroles 110

formation 287

formed from pyrite 288

zone near surface 280

Changes in depth, ores 12

Channels, ore-deposition along . . 243
Chemical action of surface waters

20,21

Chemical action of underground

waters 22

Chemical agencies, ore-concen-
tration by 236, 237

Chimneys (ore) 196

Chlorides, deposited from de-
scending waters 285

in oxidized zone 286

Chlorine, as solvent. 240

in dry climates 284

in minerals 109

in vapors from granite 17

Chloritic schist 3

INDEX.

315

Page
Chrome iron. Seechromite.
Chromite; magmatic segregation . 106

relation to basic rocks 113

Chromium deposits 113

Chromium oxide, magmatic dif-
ferentiation 245

Chromium stains on outcrops . . . 302

Church, John A., cited 168

Cinnabar, deposited from fuma-

roles 17,64,110

Monte Amiata, Italy 63

Qarke, F. W., cited 12

Clays, composition of 8

uses 8

Clements, J. M., cited 95

(cleavage, distinction from strati-
fication ; .. .' 32,33

Cleavage planes, defined 32

Climate, relation to ore-deposi-
tion 273, 274, 275, 276

Clinometer 135, 136

Clough. C. T., cited 3

< 'oal, age of 36, 66

association with certain stra-
ta 69

formation 66

shales 73

Cobalt, Mansfeld, Germany 70

Colorado, Aspen

.... 169, 170, 202, 203, 248, 308

Bassick mine 110

Cripple Creek 89, 196, 285

Devonian 25

Gunnison region 191

Leadville 308

Leadville and Aspen 63

Red Cliflf district 268

Rico 54,171,172,251

Silver Cliflf 242

Ten Mile district 173

uranium and vanadium .... 261

Colors of gold 218

Columbia River basalt 98

Columnar structure 175

Concentration, by specific gravity 21
by waters, subsequent to
magmatic segregation. ... 12

from solution 267

of ores 20, 21

of valuable elements 9

Conglomerate, defined 28

gold-bearing 65, 226, 227

oil-bearing 69

passing into sandstone 54

suitability for ore-deposition. 28

Conifers, defined 50

Conjugated fractures 179

Connecticut, tungsten mine 108

Contact-metamorphic o r e - d e -

posits 16. 107, 108, 245

Contact-metamorphism 16

Contraction during alteration . . . 256

Contraction in rocks 175, 182

Copper, arsenic compounds with . 267

association with iron 300

Butte, Mont 116

in igneous rocks 100

in muds 58

. « . Pago

Copper, in Permian strata. . . 70, 7 1 , 72

in red sandstones 70

in sea-water 71

in springs 242

Lake Superior 90

precipitation in sediments . . 261
preference for basic rocks ... 114
replacing plant remains .... 72

secondary sulphides 282

stains, on outcrops 302

veins. Cornwall 10, 185, 296

zone of 268

gossan 258

Traversella, Italy 59

Copper carbonate, formation of. . 286

Sierra Oscura 72

Copper minerals, superficial alter-
ation 282

Copper oxide^ formation 286

Copper sulphide, oxidation 286

Cornwall, copper and tin 296

veins 10

vein system 1 85

Correlation of strata 67

Corundum 8

magmatic segregation 106

use of 106

Cretaceous, coal in 66

fossils of 40

Cretaceous period 37

Crinoids, age indicated by 43

defined 45

Cripple Creek, Colorado 89, 196

Cross-sections, construction. .143, 144

Crust, movements of 1

Crustaceans, defined 47

Crustification 190

Cryolite 8

Cryptogams, defined 50

Crystallization during metamor-

phism 6

in granite 13

in i|;neous rocks 100

Crystalline structure, igneou.s

rocks 80,81

Crystals, growth of 80

Cuba, copper 258

iron ores 1 18, 119

serpentine 114

Cuprite 286

Cycads, defined 50

Cycles in rock-formation 7

Ddbris, surface 130, 131

Decomposition of rocks, guide to

prospector 303

Deformation, eflfects 126

De La Beche, cited 76, 185

Depth, changes in richness

293,294,295

changes in value 248, 299

of ore-deposition 254

Descending waters, deposition by
22, 175, 189, 270, 271, 273, 279, 289
ore-deposits, changes in

depth 293

ore-deposits, criteria. 289.290, 291

316

INDEX.

Pa«e
Deflcendingwatera, sulphide depo-
sition by 287

Devonian, fossils of 38

Kold-bearing conglomerates . 65

jcold in 62

m Colorado 55

rocks, how distingnishe<l. ... 43

Devonian period 37

Diabase, alteration 91

Diabasic porphyry* define<l 86

rocks, transitions 94

gold and silver in 101, 102

metallic minerals in 100

segregation of 10

transition to diorite 95

Diabasic rocks, defined 85

transitions 94

Diamond placers 232

Differentiation of dike 95

Diller, J. S., cited on nickel ores . . 12

Dike, defined 96

of ore 117

veins formed along 203, 204

Dinosaur, age of 39

defined 49

Diorites, alteration 91

connection with ores 59

gneiss 33

quartz-bearing 59

transition to diabase 95

Dioritic porphyry, defined 86

transitions 94

Dioritic rocks, defined 85

Dip, defined 121

how recorded 134, 138

reading of 136

Disintegration, surface 257

Dislocation of fault, defined 153

Displacement of fault, defined. . . 153

Doelter, cited 240

Dolcoath mine, Montana 16

Dolerite, defined 90

Dolomite, distinction from

marble 31.32

origin of 30,31

Dolomitic marbles 31

Dome 148

Douglas, James, cited 309

Drift 131

Dynamic geology, definition .... 120

E
Earthquakes, fractures produced

V 178

Economic geology, scope 8

Egleston, T., cited 240

Elements, in the earth 8

segregation in molten masses 9

Elkhom district, Montana 16

Elongation of pebbles in conglom-
erate 3

Emery 8

Emmons, S. F., cited 173, 268, 285

Enargite 288

England, Derbyshire 76

vein systems 185

Enrichment in oxidized zone .... 267
Enrichment, sulphide 269

Eocene, oil 69

. placers 212, 213, 226

Epidote as alteration product . . . 304

in contact metamorphic de-
posits 108

Equiseta, defined 51

Erosion, defined 126

gap in sediments 52, ^

work of 98

Eruptions, producing fractures . . 178
Eureka, Nevada, age of ore 63

cave-deposits 75

Evaporation, ore-deposition by . . 261
Expansion during alteration .... 256
Extrusive rocks 96

advantages for ore-depo-
sition Ill

connection with hot springs . Ill

defined 98

False bottom, tin placers 232

False saddle 290

Farish, J. B., cited 251

Faults, accurate measuring 150

as shown in vertical cross-
sections 149

bedding 158, 159

compensating 124, 125

definition 120

dying out of 181

estimation of 148

existence shown 150

how detected 139

in heterogeneous rocks 151

in homogeneous rocks 150

means of measuring 151

measured by displacement of

strata 149

normal 12, 13, 24

ore-deposition on 169, 172

post-mineral 203

pre-mineral 203

relation to topography

128,307,308

reversed 123, 124

Fault-breccia, indicating move-
ment 152

Faulted district, suitability for

ore-deposition 308

Faulted faults 200, 201, 202

Faulting, different oeriods of . 200, 202

effect upon values 197

reversal of 200

Fault-movement, computation of 157
fractures of 160, 161, 162

Fault-scarp 128, 129, 130

reversed 129

simple 129

Fault-zone, nature of 170, 171

ore-deposition in ... 170, 171, 173

Feldspar, altered to sericite 203

in gneiss 33

in igneous rocks

85,86,87,88,89,90

in veins 14, 15, 204

Felsite, defined 90

in Wales 91

INDEX.

3ir

Page

Ferric sulphate as solvent 240, 278

Ferro-manganese minerals, metals

in. 113

Filled deposits 190

Fishes, defined 48

Fissility in shales 30

Fissures, cause of 182, 183

extent 298

near surface 189

open 182

ore-deposition in 184, 185

origin 184

Fissure-eruptions, lavas 98

Fissure-filling by ores 23

Fissure vein 172, 191

Flow-banding, igneous rocks .... 80
Flow-structure, igneous rocks ... 82

Fluorine, in minerals 109

in vapors from granite 17

Fluorite, dissociation with tin. . . . 300
in contact metaroorphic de-
posits 107

in tin veins 113

Folding, effect upon values 197

Folds, close 121

connection with ores 194

how joined 120

kinds of 121, 122

open 122

ore-deposition in 164, 165

overthrown 122, 123

relation to topography 128

three dimensions of 148

Foraminifers, age of 40

defined 44

Formation, meaning of term .... 27

Forty Mile creek, Alaska 218

Fossil placers 226, 227

Fossils as evidence of geologic age 35

as guide to age 42

changed to ore 248

in shales 73

significance of 27

stretching of 33

use in identifying ore-beds . . 68

Foster, C. Le Neve, cited 68

Fractures, compound 180

conjugated 179

course of 179

definition 177

dyin^ out of 181

imbricating 182

in different strata 181

in hard strata 75

ore-deposition along 185, 186, 243

origin 177, 1 78

reversal of 199

subsequent to veins 198

Fracture-zones, influence on ore-
deposition 193

Fragmental rocks 26

Friction breccia 92

Free-gold in outcrops 278, 302

Fumaroles, change to hot springs. 1 8

Monte Amianta, Italy 64

ore-deposition by. ... 17, 109, 110
Fumarolic action, confined to

extensive rocks Ill

Page

Fundamental igneous rocks 95

connection with ore-deposi-
tion Ill

exposure of 98

G
Gabbro, altered to greenstone ... 10

containing ore 11, 105

defined 90

Galena, deposited in mine work-
ings 64

in coal-shales 73

zone of 268, 280, 281

Galicia, oil 69

Gangue minerals 186

Ciap mine, Pennsylvania 11

Garnet in contact metamorphic

deposits 108

in gabbro 117

in metamorphic rocks 17, 59

in placer gravels 210, 21 1

Garnet-schists 34

Gas, natural, association with

certain strata 69

in anticlinal folds 166

ore-deposition by

39,106,107,108,109

origin 301

Gasteropods, defined 46

Geologic age^ length of 35

Geologic i3eriods 37

association with certain ores. 62

how named 36

Georgia, saltpeter 262

Germany, Kupferschiefer 70

oil 69

Rippoldsau and Kissingen . . 242

Stassfuth 310

Gilbert, G. K., cited 2

Glaciers, connection with placers

211,212,213

effect of 211,212

erosive power 131

Glacial period 41

Glass, volcanic 80

Glassy igneous rock, defined 85

Glaucomte 264

Gneiss 3, 33, 108

derivation from igneous

rocks 34

kinds of 34

Gneissic structure 6

defined 34

Gold, concentration at surface . . . 206
concentration in gravels .... 208
concentration in surface

water 21

enrichment by oxidation . . . 266

formation of 206

geologic pel iods found in . . . 62

in arsenopyrite 280

in bay mud 72

in contact metamorphic de-
posits 17

increase by oxidation 279

in granitic quartz veins 15

in igneous rocks 100

in muds 58

318

INDEX.

Page

Gold, in pyrite 300 301

in quartz veins 301

in sea-water 71, 102, 219

in serpentine ; 114

in springs 242

in A^ey placers 216

precipitatpd by pyrite. . . .77, 251
precipitation by organic

matter 264

precipitation in placers 207

precipitation in sediments . . 260

residual deposits 266

rooted deposits 233, 234

solubility of 206, 240, 279

Gold-quartz veins, Otago 194

possible origin 15, 107

Goodrich, H. B., cited 218

Gossan 258, 282

Granite, connection with tin . . 17, 113

giant 92

j?old and silver in 101, 102

m Cornwall 10

metamorphic influence of . . . 16

replacement by ores 116

replacement of 248

s^re^ation in 10

transition to quartz veins. . 13, 15

Granitic porphyry, defined 86

Granitic porphyry rocks, transi-
tions 94

Granitic rocks, connection with

tin-veins Ill

defined 85

replacement of 260

transitions 94

Granodiorite, defined 168

Grant, U. S., cited 91

Grammar igneous rocks, connec-
tion with ore-deposition . . 109

defined 84,85

Graptolites, age indicated by. . . . 43

defined 45

Gravels in river valleys 25

Great Lakes, crustal Movement . . 2

Greenland, native iron 105

Greenstones in Cornwall 10

defined 91

origin 10

Greisen, defined 304

Grit, defined 28

Groundmass, in igneous rocks ... 84

Ground-water, depth of 238

level 237

source of 237

Guiterman, Franklin, cited 269

Gulch placers 214, 215

Gunn, W., cited 3

Gymnosperms, defined 50

Gjrpsum, associated minerals. ... 301

beds 58

deposition at surface 25

deposition in lakes 260

in Permian rocks 68

in red sandstone strata 70

residue from 74

Page
H

Hade, use of word 121

Hague, Arnold, cited 81

Hayes, C. W., cited 114, 119

Haworth, E., cited 195

Heave of fault 162

Heavy spar 300

Hematite, Cuba 119

deposited by f umaroles

17,116,245

Hematite, Piemonte, Italy 59

Hess, W. H., cited 263

HiU,J. B.,cited 10

Hinxman, L., cited 3

Hobbs, W. H., cited 108

Hopkins, T. C, cited 270, 271

Horn silver 283, 286

Hornblende, containing metals . . 100
in contact metamorphic de-

po.sits 108

in gneiss 33, 108

in Igneous rocks . 82, 86. 87, 88, 89

in rocks 7

in tin veins 113

Hornblende-schist 31, 34

Home, J., cited 3

Homstone, definition 59

Hot Springs, connection with ex-
trusive rocks Ill

connection with igneous

rocks 103

connection with ore-deposits

103, 104

cooling of 18

deposition of minerals 63

mineralization by 59

origin 18. 20, 103

substances contained 242

Hot waters, producing ore-depo-
sition • 19

Howitt, A. W.. cited 14

Hydatogenic deposits 107

Hydrogen sulphide in waters .... 240
Hydrogen sulphide. See sulphur-
etted hydrogen.

Ichthyosaurus, age of 40

defined 49

Idaho, basalt 98

De Lamar district 204

Silver City and De Lamar . . . 303

Igneous rocks, cause of fluidity . . 13

characters of 4

classification. 82, 83

connection with hot springs . 103
connection with ore-deposits

99,103

cooling 94

crystalline structure 80

defined 79

derived from metamorphic. . 5

distinguishing charactera ... 81

forms of 95

fractures in 181

glassy form 80

mapping and sectioning .... 147

metamorphism of 5, 6

Page

Igneous rocks, naming of 83

Qiigin 4

stimulating ore-deposition . . 112

textures 94

transformation to sediments . 6

transitions 93, 94

water in 13

Ilmenite in igneous rocks 86, 100

Imbricating fractures 182

Impervious clay, forming pay-
streak 232

Impervious strata, relation to ore-
deposition. .73,164,166,166,209, 253
Impregnation deposits, camotite. 262
Impregnation of rocks by ore. ... 23

Indicators, Ballarat 77

Intersections, ore-deposition at. . 169
Intersections, principle of. . .196, 251

Interstitial filling by ores 23

Intrusions, producing fractures. . 178
Intrusive igneous rocks, defined . 96, 96

Intrusive mass, defined 96

Intrusive ore-bodies 117

Intrusive rock 108

advantages for ore-deposi-
tion 112

ores derived from 69

subsequent to ore-deposition 1 18

Iodine as solvent 240

Iron, association with copper. . . . 300
connection with basic rocks .

113,114

deposits by descending

waters 270

derived from glauconite .... 264

in bays 58

in diabase 10

in igneous rocks 99

in sinter 254

in waters 241

Lake Superior 291, 292

ma^matic segregation 106

native 106

precipitation by organic

matter 264

residual deposits 266

relative importance 8

rooted deposits 234

segregation of 10

zone due to surface waters . . 269

Iron cap 258

Iron carbonate 266, 271

Iron ores, Cuba 118, 119

Lake Superior 61

Piemonte, Italy 69

Iron oxide 110

deposited from fumaroles. . . 17
magmatic differentiation of . 285

Iron sulphate as solvent 268

auriferous 268

Italy, Monte Amianta, ores .... 63
Piemonte, iron and copper

ores 59

sulphur 110

Jasperoid 304, 305

Jenny, W. P., cited 64, 74

INDEX. 319

Page

Joints, columnar 174, 175

defined 173

dying out of 181

how studied 176

ore-deposition on. ... 18, 175, 176

origin of 174

relation to veins 176

Jurassic, gold in 62

oil in 69

origin of term 36

Jurassic period 37

fossils of 40

Juvenile springs 18

Kansas, lead and zinc 194

Kaolin, due to rock decompo-
sition 304

Kemp, J. F., cited

. . 11, 70, 105, 114, 117, 238, 245, 280

Kentucky, saltpeter 262

Klockmann, F., cited 286

Kuskokwim river, Alaska 54

Labradorite 117

Lakes, ores precipitated in 260

I^akersediments 24

Lake Superior, copper 90

iron ores 61, 291

Lamellibranchs, age indicated by. 43

defined 46

Lateral separation of fault

164,155,156,167,168

Lavas, defined 98

flow-structure 79, 80

nature 4

Lead, association with silver ....

association with zinc 299

deposits, Derbyshire 76

in Igneous rocks 100

in springs 242

Mansfeld, Germany 70

veins, Cornwall 186

zone near surface 268, 281

Lead, and zinc ores, Missouri 195

Lead carbonate 267

Lead sulphate 267

formation 286

Lead sulphide, deposited by fu-

. maroles 110

formation in mine workings . 64
Leadville, Colorado, age of ore. . . 63

Le Conte, J., cited 41

Leucite in igneous rocks 89

Lepidodendrids, defined. ....... 61

Level, changes of ' . . . . 2

Life, beginning and devek>pment

of 36

Lignites, Alaska 66

Limbsoffold 121

Lime, in waters 25, 241

replacement by silica. . . . 304, 305
Lime carbonate (See also calcium

carbonate) 30

as sinter 254

change to lime phosphate . . . 263
deposition at surface 26

320

INDEX.

Page
lime carbonate, in dolomite .... 30

Lime phosphate 68, 109

origin 263

rooted deposits 234

Lime sulphate deposition at sur-
face 25

Limestones, change to lime phos-
phate 263

chemical dei>o8ition 25

containing cinnabar 63, 64

containing ores 59

destruction from dolomite . 31, 32

fetid 246

in oil-bearing strata 69

magnesian 31

metamorphism of 16

origin 30, 263

replaced by ore 59

selected by ore-deposition . .

75,76,78

Limonite, Monte Amianta, Italy . 64

Lindgren, W., cited

99, 108, 125, 201, 204, 221, 222,

224, 225, 266, 278, 295, 298, 300, 304

Linked veins 192

Lithia mica, association with tin . 300

Lithium in springs 242

Longitudinal sections 145

Lotti, B., cited 64

Lycopods, defined 51

Macedonia, gold .228. 229, 265

Magmatic differentiation in dike . 95

Magmatic segregation

10, 13, 104, 106, 106, 107

Magnesia in diabase 10

Magnetic iron pyrite (See alse pyr-

rhotite) 11

Magnetic variation 135

Magnetite, Cuba 119

dikes of 117

in igneous rocks 86, 100

in i>]acer gravel 210

in tin veins 113

magmatic segregation 105

Magnesian marbles 31

Magnesium in waters 30

Magnesium carbonate in dolomite 30
Magnesium salts, deposition in

lakes 260

Malachite 286

Malay Peninsula, tin 113

Malvern Hills, metamorphism ... 6

Mammals, denned 48

Mammoth, period of 42

Man, perit)d of existence 42

Manganese, deposits of 260

m igneous rocks 99

in oceans 58, 260

Manganese oxide, deep formation 245

Manner of ore-deposition 23

Mansfeld, Germany, ores 70

Mapping, economic results 145

geological, how done 138

of igneous rocks 147

Maps, how made 137

use of 136,137

Page

Marble, dolomitic 31

gold and silver in 102

origin 16, 30

Massachusetts, Cape Ann 182, 183

Mechanical action of surface

waters 21

Mechanical agencies, ore-concen-
tration by 236

Mercur, Utah 179

Merciuy, Mansfeld, Germany. ... 70

Mercury sulphide 110

deposited from fumaroles ... 17

Italy 63

Mesabi range, Minnesota . 91, 291, 292

Mesozoic era 37

fossils of 39

vein formation 63

Metallic minerals deposited from

fumaroles 17

Metalliferous veins, period of

formation 63

Metals in ferro-magnesian min-
erals 113

in rocks 7, 10, 99, 100

Metals, rarer, occurrence 9

Metamorphic processes connected

with ores 7

Metamorphic rocks, banding of . . 4

defined 3

derived from igneous ,5

origin of characteristics 4

transformation to igneous . . 5
transformation to sediments 6

Metamorphism, contact 16

of conjslomerates 3

of sediments 2, 16

Meteoric waters, heating of 20

producing ore-deposition ... 19

Mexico, Pachuca 192, 193

Mica, containing metals ........ 100

in contact metamorphic de-
posits 108

in quartz veins 14

in rocks ... 7, 33, 85, ,86, 87, 88, 89
Mica schist, associated with ores . 59

origin 3

Michigan, Crystal Falls. 95

Microscope, petrographic 82, 83

Migration of outcrops 139, 140

estimation of 141, 142

Mine-workings, ore-deposition in

20,64,288

Mineral wax 301

Mineral zones near surface 268

Mineralization by vapors 16

Mineralizing solutions, chemicals

175,176

Minerals, associations of . .58, 299, 301
Mingling of ore-bearing solutions

261,252

Minnesota, Mesabi range .91, 291, 292

Miocene placers 226

Mispickel, gold in 206

Missouri, Belleville 73

lead and zinc 194

Joplin district 64

origin of dolomite 31

Molluskfl, defined 46

INDEX.

331

Page
Molybdenum, connection with

silicious rocks 113

Monocline, defined 126

Monazite in placers 232

use of 232

Montana, Butte district 116, 288

Dolcoath mine 16

Elkhom mine 166

fossil placers 227

Monte Amianta, Italy, ores 63

Monte Cristo. .63,176,260,279,280, 281
Monte Cristo, Washington, age of

ore 63

Mountains, favorab^eness for ore-
deposition 306, 307

origin 127, 128

Movements subsequent to ore-
deposition. 197

Murchison, Sir Roderick, men-
tioned 62

Muscovite in contact metamoi^

phic deposits 108

in greisen 304

in quarts veins 14, 15

in tm veins 113

Native metals 244

Native silver 267

Aspen 248

Neocene placers 224

Nepheline in igneous rocks 89

Nevada, De I<amar mine 285

Eureka district 63, 76, 308

Pioche 308

silver chloride 287

Silver Peak. 15

Steamboat Springs 63

Tertiary fossils 41

trachyte in 89

New Jersey, Franklin Furnace. 244,245

New Mexico, copper 72

silver chloride 287

New York, Adirondack^ 105

New Zealand, Otago

.... 194, 212, 213, 224, 226, 249

Nickel, Gap mine 11

in igneous rocks 12, 100, 106

Mansfeld, Germany 70

Oregon 12

segregation of 10

stains on outcrops 302

Nitrates, cavern formation 262

Nitric acid, in rock-weathering . . 257

organic orijsin 263

Novarese, V., cited 59

Obsidian 80

Oceans, ores precipitated in 260

Offset of fault 159, 162. 163

Oil, association with certain

strata 69

in anticlinal folds 166

Oil deposits, preference for cer-
tain geologic periods 67

Old placers 222, 223, 224. 225, 226

Oligocene, oil 69

Olivine, alteration of 92

containing metals 100

in igneous rocks 12, 86, 88

nickel-bearing 12

Omeo, Australia, veins 14

Ordonez, E., cited 192

Ore-bearing strata, dimensions of. 61

Ore-bodies, defined 235

shrinkage of 76

Ore-concentration, conditions . . . 243
Ore-deposition by hot springs ... 63

by juvenile waters 19

by release of pressure . . . 253, 254

chemical agencies 236, 237

favorable conditions for . 112

in mine-workings 20

manner of 22, 23

mechanical agencies 236

on lowered temperature . 253, 254

recent '. . 288

Ore-deposits, connection with hot

spring 103. 104

connection with igneous

rocks 99

connection with rock dis-
turbances 199

contact metamorphic 16

date of formation 65

depth of formation 254

Oregon, basalt 98

Blue Mountains 221, 266, 278

nickel ores 12

Ores, association with certain geo-
logic periods 62

changes in depth 12

contemporaneous with strata 69
deposited after volcanic

eruptions 63

deposited by surface evapo-
ration 261

derived from intrusive rock . 59

re-deposition of 189

selective precipitation 115

Ore-shoots .- 195, 196

Organic acid in rock-weathering. 257
Organic matter, precipitation by

71, 72, 73^245, 246, 264, 288

Organic minerals, origin < 301

Organic sediments 25, 26

Otago, New Zealand 194, 212, 213

Outcrop, ore-bodies 20, 302

migration of . . . 139, 140, 141. 142

of rocks 130, 131

selection of 138

Orthodase as gangue mineral . . . . ' 204

Osseous fishes 49

Oxidation, depth of 279. 280

guide to prospector 301

of ores 257.258

relation to ore-concentration

272.273

zone of 257

Oxides, association with sul-
phides 245

deposited by descending wa-
ters 285

formation at surface 256

formation at depth 244

o22 INDEX.

Page
Oxidised zone, concentration of

metals from 272

depth of 269

enrichment in 266, 267

erosion of 272

minerals in , 286

Osark uplift, influence on ore-
deposition. 194

Ozocerite, origin . 301

Pachuca, Mexico 192

Paleozoic, oil in 69

veins in 63

Paleozoic era 37

Pay-streak in placerH 208, 209, 232

Peach, B. N., cited. 3

Peat-swamps, Alaska 66

Pebbles, stretching of 33

Pegmatite, defined 92

formation 106, 109

Pennsylvania, Gap mine 11

iron ores 270

Penrose, R. A. P., cited

...........113,196,264,288

Peridotite, alteration 12, 114

composition 12

containing ore 11

Peridotitic rooks, alteration 92

containing chromium 113

defined, , 86

Permian, copi)er in .70, 71

mineraLs in , ^ • • 67

Perpendicular separatioi) of fault

166,166,157,158

Petroleum, origin 301

Petroleum-bearing, strata 69

Phenocrysts, defined 84

Phenograms, defined. . , 50

Phillips, i, A., cited. 71

Phonolite, defined 89

Phosphate of lime, England and

France 68

in veins 109

origin 263, 266

rooted deports. ........ . . 234

Phosphoric acid, organic origin . . 263

Physiography, defined 306

relation to mining 306

Piemonte, Italy, iron and copper

ores 69

Placers, age of 65

beach 218,219

bench 221

black band 210

broad valley. . ....:........ 214

defined ..;...... 206

diamond 232

Eocene .212,213

false bottom 208, 209

fossU 226, 227

gold 207,214,216

mechanical origin 208

new generations of . 227

old 222. 223, 224, 225, 226

origin 205, 207

.pay-streak 208, 209

Page

Placers, platinum. . , ....... .229

precipitation of gold In . . 206, 207

reooncentrated 227, 228, 229

ruby sand 210

second pay-streak in 209

solution of gold in . !. 263

Tertiary 226, 228, 229

tin 230.231,232

volcanic cappinjg of 223, 224

Plains, ore-deposits in 306, 307

Plant remains, age indicated by . . 43
Platinum, concentration in sur-
face water 21

in rocks. . .....'.. .100, 114

placers 229

Pleistocene, veins in 63

Porous strata, effect on ore-depo-
sition. '. 74, 253

ore-deposition in „ 243

Porphyritic crystals 84, 91

Porphyritic rocks, defined . .84,86,87

transitions 94

Porphyritic structure 84

Porphyry 173

incorrect use of term 303

quartz 88

Portugal, pyrite deposits 286

Posepny, F., cited 242

Post-mineral faults 202, 203

Potassic chlorate as solvent ..... 240

Potassium nitrate '. . 262

Potassium salts, deposition in

lakes 260

Potassium sulphate 110

Pottery, materials for 8

Pre-Cambrian auriferous con-
glomerate 227

Pressure, decrease of 263, 264

Primary association of strata and

minerals 68

Primary ores ......' 269

Primary ore-beds, secondary con-
centration in 60

Principle of intersection 196, 261

Protozoans, defined 44

Pseudomorphs 248

Pterodactyls, age of 40

defined 49

Pumice, defined^ 93

Pyrite, alteration to chalcopyrite. 260

as precipitant of gold 77

auriferous. . 300, 301, 302

deposited by fumaroles 110

formation in shale 77

gold in. 206

in altered rock, isignificailce. 303

in coal-shales 73

in contact metamorphic de-
posits 108

in Igneous rocks . . ......... 100

Monte Amianta, Italy 64

Piemonte, Itialy ' 69

precipitant of gold 251

Pyroxene, alteration of 92

containing metals 100

in igneous rocks . 86, 87, 88, 89, 90

in metamorphic rocks 17

in periodotite 12

Page

Pyrrhotite, copper-bearing 285

Gap mine 11

in iffneous rocks 86, 100

nickeliferous 106

Quartz in rocks 7, 33, 85, 86, 87

Quartz-feldspar rocks, origin .... 13

Quartz porphyrjr, defined 88

Quartz vein, auriferous 77

distinction from quartzite . . 29

magmatic origin 106

outcrop 302

possible origin 14

transitions into granites .... 13

Quartzite. defined 28, 29

fractures in 181

origin of 16

replacement of •.-.••• 248

suitability for ore-deposition. 78

Quaternary, fossils of 42

origin of term 36

veins in 63

Quaternary period 3"^

Radiates, defined 45

Rainfall, relation to ore-concen-
tration 273

Ransome, F. L., cited. 171,251,252,262

Rarer elements in rocks 7

Rarer metals, occurrence 9

Realgar, deposited by furoaroles . 110

Monte Amianta, Italy 64

Recent ore-deposition 288

Reconcentrated placers . 227, 228, 229
Red sandstones, connection with

minerals 70

copper in 72

Replacement of andesite 250

of granite 116, 250

of hornblende 116

of lime 304,305

of limestone 39, 76

process of 247

of rock 23,248

of schist 194

Replacement deposit, marks of . . .

247.248

Reptiles, classified 49

defined 48

Residual deposits 233, 234, 278

manganese 260

origin 266

Rhizopods, defined 44

Rhode Island, magnetite 105

Rhyolite, alteration 91

defined 87

glassy 81

transitions 94

Ribbon structure. . . 191, 199, 200, 201

Rickard, T. A., cited 77. 164,

165, 167, 171, 172, 188, 194. 212,
213, 224, 226, 249, 251, 290, 291, 309

Rico, Colorado 74, 171, 172

Riddles, Oregon, nickel ores 12

Rigid stratum, selected for ore-
deposition 75

Him-rock (in placers) 223

INDEX. 323

Page

Rivers, change of bed 24, 222, 223

River sediments 24

Rock, defined by 79

Rock-forming minerals 7

Rohn, Oscar, cited 61

Rolker, C. M., cited 231, 232

Rooted deposits 233, 234, 279

Ruby. 106

sand in gravels 210

silver 283,284

Russia, platinum 114

Ural Mountains 229, 230, 233

Rutley, F., cited 91

Saddle, false 290

Saddle-veins 164, 165

Salt, associated minerals 301

deposition of 260

in Permian 68

in Ted sandstone strata 70

Salt-deposits indicated by to-
pography 309

Salt flats, origin 261

Saltpeter, formation 262

Sand as an ore . . . ., 8

Sandstone, connection with min-
erals. 70

copper in 72

defined . 28

fractures in 181

l^ld and silver in 101

impregnation deposits 262

in Triassic. ,. . . • 55, 56

metamorphism into quartz-
ite 16

metamorphism into schists . . 3

oil-bearing 69

passing into conglomerate ... 54

passing into shale 54

selected for ore-deposition . . 73

Sapphire 106

in tin veins 113

Saurians 49

Scapolite in contact metamorphic

deposits 108

in vems 109

Scheelite 108

Schist 3

defined 33

derivation from igneous

rocks 34

derivation from sediments . . 3, 34

kinds of 34

replacement of 194, 248, 249

selected for ore-deposition . . 73

silicification of 249

Schistosity, defined 34

Schrader, F. C, cited 215, 220

Sea-shore, concentration of ores

on 21

Sea-water, gfold in 102, 219

metals m , 71

Secondary association of strata

and minerals 58

Secondary concentration, condi-
tion dependent on ... . 276, 277
in primary ore-beds 60

324 INDEX.

Page
Secondary sulphide enrichment . . ^9

Secondary 8ulphide8 269, 282

Sectioning of igneous rocks 147

Sections, \-ertical, construction of

143.144

Sedimentary ores 260

Sedimentary rocks, advantages

for ore-deposition 112

asscKiation with certain min-
erals 68

chosen for ore-deposition ... 77
derived from igneous and

metamorphic rocks 6

gold and silver in 101. 102

kinds 28

phj'sical characters 26

succession 52

Sediments, chemical 25

ele\'ati9n of 26

formation of 24

lateral transitions 53

organic 256

transformation to hard rocks 26

Segr^ation in granite 10

in molten masses

magmatic 104. 105. 106, 107

of mckel . 11

Sericite^ alterat ion f n>m feldspar . 308

Serpentme 92

C^lba 114

deri\'ed from i>eridotite 12

Sgonnan More, metamorphism . . 3

Shales, defined 29

fractures in 181

iron in 77

metamorphism of 3, 16

oil-bearing 69

passing into sandstone 54

selected for ore-deposit ion .. 78

Shaler N. S.. cited 182. 183

Shallow underground waters (See

also vadose waters) 255

Shearing in metamorphic rocks . . 5, 6

Shear zones 193

influence on ore-deposttion.

193,194

Sheet, intrusive, defined 96

Sierra Oscura, New Mexico 72

Sif^illarids, defined 51

Silica, as sinter 254

deposited from surface

waters 25

in granite 10

Silicates, decomposition at sur-
face 256

metallic 244

Silicious constituents, concentra-
tion of 13

Silicious dikes, Cornwall 10

Silicious igneous rocks, ores in .13, 102
Silicious rocks, connection with

tungsten and molybdenum 113

Silicon as an ore 8

Silification near veins 304

Sill, intrusive, defined 96

Silurian, gold in 62

lack of coal in 67

fossils of 38

Page

Silurian period 37

Silurian rocks, how distinguished. 43

Silver and gold in rocks 101, 102

association with lead 299

chloride 267

decrease bv oxidation 279

deposited by fumaroles 110

in arsenopyrite. 280

in igneous rocks 100

in muds 58, 72

in sea^water 102

Mansfeld, Germany 70

Peak, Nevada, gold-quarts

veins 15

precipitation in sediments . . 260

solubility 279

Silver-lead deposits in limestone . 76

Sinter, formation 254

Slate, defined 30

Slickensides 160

Slopes, relation to ore-deposition .

274.275.276

Smith, G. O., cited

186, 187, 255, 267, 305

Suda, formed by evaporation. . . . 261

Sodic carbonate as solvent 240

Sodic chloride as solvent 240

Sodic sulphide as soh'ent 240

Sodic sulphydrate as soh'ent .... 240

Solfataras, Italy 64

Solubilities, concentration accord-
ing to 265

Soluble minerals, indicated by

topography 309

Spain, pyrite deposits 285

Specular iron, deposited by fuma-
roles 110

Piemonte, Italy 59

S|>encer, A. C, cited 119, 190. 191

Springs, hot. See Hot Springs.

Spurr, J. E., citeti

13. 54, 122. 129, 169, 170, 174.

176, 180, 202, 217, 248. 250, 265, 281

Stains, mineral 302

Stalactities of ore, signification . . 292
Stalagmites of ores, signification . 292

Steam from lavas 15, 17

Steamboat Springs, Nevada 63

Stibnite, Monte Amianta, Italy. . 63

Stink-shales 246

Strata, association with \'a!uable

minerals ., 58

contemporaneous with ores . 69

correlation of 67

identification by physical

characters 65

ore-bearing, dimensions of . . 61
{persistence of characteristics. 65
Stratification, absence in igneous

rocks 79

distinction from cleavage . . 32, 33

explained 27

Stratified rocks, fractures in 181

veins in 186

Stratum, defined 27

selected for ore-deposition . . 74

Streams, concentration in 21

Stream-works 230

INDEX.

325

Stretching of pebbles and fossils . 33
Striae on fault-planes ... 150, 151, 152

Strike, defined 134

how recorded 134. 138

reading of 135

Structural geology, definition ... 120
Subsequent fractures, course of . . 198

in veins 198

substitution of ores for rock . 23

Suess, Edouard, cited 18

Sulpharsepides 267

Sulphates, deposited by descend-
ing waters 285

derived from sulphides 268

reduced to sulphides 270

Sulphide enrichment 269

Sulphides, association with oxides 245
contemporaneous with oxides 284
decomposition at surface . . . 256

deposition of 244, 245, 287

secondary 269

Sulphur, deposited from fuma-

roles 64, 110

Monte Amianta, Italy 64

origin 269,270

Sulphuretted hydrogen (See also

hydrogen sulphide) 246

precipitation by 251

volcanic 64

Sumatra, tin 231

Superficial alteration of copper

ores 282

Superficial enrichment, depth of .

284,285

Superposition of strata, rule of . . . 56

Surface changes 1,2, 222, 223

fissures near 82, 183

veins formed near 189

surface slopes, relation to ore-
concentration. 273

Surface waters, eflFect in ore-depo-
sition. 20,255,256

Swamps, precipitation of ores . . . 259

Sweden, magnetite . . . ., 105

Syenite, containing platinum. . . . 114

defined 89

^old and silver in 101 , 102

Syenite gneiss 33

Synclines . 121

ore-deposition in . . . *. 164

Talus 140

Teliosts 49

Telluride, bismuth 17

Tellurides 244

deposited by fumaroles 110

Temperature, decrea.se of ... . 253. 254

Tennessee, Ducktown 285, 288

Tenorite 286

Tertiary, coal in 66

fossils of 40

oil in 69

origin of term 36

placers in 224, 226, 228, 229

topography 224

veins in 63

period 37

Tertiary 224

Texas, copper 71

limonite 264

Thallogens, defined 50

Throw 159, 160, 161, 162

of fault 162

Tin, associated metals 300

concentration in surface wa-
ter 21

connection with granite .... 113

Cornwall 296

in igneous rocks 100

in sinter 254

in springs 242

in veins 109, 232

Tin oxide 109, 230

Tin placers 232

Tin veins, alteration of rocks near 304
connection with granitic

rocks Ill

Cornwall 10, 185

formation 109

origin 17

Tintic, Utah 186

Titaniferous iron 105

Titanium in iron 113

Tonalite, Monte Cristo 250

Topaz, association with tin ... 113, 300
in contact metamorphic de-
posits 108

in veins 109

Topography, how produced

relation to faults 128

relation to folds

relation to ore-deposition . . . 273

Total displacement of fault

^ • ,: 153, 154, 157

Tourmaline, association with tin .

231,300

in contact metamorphic de-
posits 108

in veins 14, 109, 113 *

Tower, G. W., Jr., cited

186, 187. 255, 267, 305

Trachyte, defined 89

ores deposited from 63

Trap rock, defined 92

Triassic, coal in 66

conditions favoring ore-depo-
sition 72

copper in 70

fossils of 39

gold in 62

oil 69

Triassic period 37

Triassic rocks, persistence of char-
acteristics 55, 56

Trilobites, age of 38, 42

defined 47

Troughs of synclines 164

Tuff, defined 91

Tungsten, connection with sili-

cious rocks 113

in tin veins 1 13

Turner, H. W., cited 15, 72

U
Uintarange, Utah 127

326 INDEX.

Page

Unconformity in sediments 53

Underground waters, chemical

work 22

mechanical action 21

Uralite 101

Ural Mountains, platinum . . . 229, 230

rooted gold deposits 233

Uranium, formed by evaporation

261,262

Utah, copper in 70

Horn Silver mine 283, 285

Mercur 179

Tintic 186, 255, 267, 305

Uinta range 127

Vadose waters, concentration by. 12

Valley gravels 25

Valley placers 214, 216

Valleys, origin 127

Value, changes in depth 298, 299

Vanadium,formed by evaporation 262

Van Hise, C. R., cited 310

Vapors, deep-seated, ore-deposi-
tion by 108, 109

forming ore deposits 17

aqueous 16

Vaughan, T. W., cited 114, 119

Vep;etable kingdom, divisions ... 50

Vems, age of 63

banding 190,191

ehanges in depth 298, 299

deflection of 186, 188

downward extent

294,295,296,297

dying out of 186

enrichment by oxidation .... 297
influence on topography. . . . 308

linked 192

relation between horizontal

» and vertical 297, 298

relation to joints 175

superficial 189

transitions into granites .... 13

Vein-systems 185

Vertebrates, defined 48

Vertical separation

157, 158, 159, 160, 161, 162

Vesuvius, fumarolic ore-deposi-
tion 17

Virginia, coal 66

Vogt, J. H. L., cited 239

Volcanic breccia 93

Volcanic eruptions, ores deposited

after 63

Page

Volcanic glass 80

Volcanic pipes 110

Volcanoes, eruptions. 98

W
Wagoner, Luther, cited. . .72, 101. 219

Wides, Caradoc 91

Walls of vein, multiplicity of ... . 173

Washington 131

Washington, basalt. ... 98

Waters, chemical work 22

effectof 239,240

expelled from cooling rocks . 15

in rocks 13, 239

mechanical action 21

ore-deposition by 19, 21, 22

precipitation from 243

solvent power 241

Watson, T. L., cited 261

Wax, mineral origin 301

Weathering, zone of 256

Weed, W. H., cited 116, 166, 280

White, A. A., cited 127

Winchell, A. N., cited 227

WincheU, H. V., cited 288, 292

Winslow, Arthur, cited 31

Witwatersrand and gold-bearing

conglomerates 65

Wolfram, association with tin ... 300

Wolframite 108

Wolfram minerals 108

Wyssotzky, N., cited 234

Yellowstone Park, obsidian 80

Zaitseff, A., cited 230

Zinc, associated with lead 299

in igneous rocks 100

in springs 242

Mansfeld, Germany 70

zone near surface 268

Zinc-oxide, deep formation 245

Zinc sulphide, deposited by fuma-

roles 110

Zoisite in contact metamorphic

deposits 108

Zone of crushing 249

of mineral deposition . 269,280,284
of secondary sulphide ...... 269

of weathering 256

Zuber, Rudolf, cited 69

THE
NATURE OF ORE DEPOSITS

DR. RICHARD BECK

Professor of Geology and Economic Oeology,
Freiberg Mining Academy

TRAMBLATET) AND REVISED BT

WALTER HARVEY WEED

U. S. Geological Survey

An intelligent dissertation on Ore Deposition, writ-
ten by an eminent expert on the subject and translated
and thoroughly revised to date by Walter Harvey Weed,
of the U. S. Geological Survey, from the original
German of Dr. Richard Beck. Treating in an author-
itative style the nature and methods of occurrence of ore
deposits in all parts of the world, together with detailed
descriptions and characteristic cross-sections of veins,
ore bodies and ores from many famous mines of ancient
and modem times. The data embodied in this important
manual distinctly applies to local conditions, and the
almost inexhaustible amount of general and precise
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Two volumes, octavo, cloth binding, profusely illustrated,
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GENERAL CONTENTS.

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Chapter II. — ^Discovery. Chapter X. — Ventilation.

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Chapter IV. — ^Excavations— Ex- Chapter XII. — Access.

plosives. Chapter XIII. — Dressing.

Chapter V. — Support— Timber- Chapter XIV. — Legislation.

ing. Chapter XV. — Condition of tha

Chapter VI. — Exploitation. workmen.

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Chapter III.— Rolls. Chapter XVI.— Fine Sand and Slime
Chapter IV. — Steam, Pneumatic and Concentrators.

Chap1?r"H^^S^.^y Stamp. ^^^^^ ^^^"''^^r "*""• P

Chapter VI.— Pulverizers other than Chapter XVIII.— Miscellaneous Pro-

Gravitv Stamps. cess of Separation.

Chapter Vll. — Laws of Crushing. Part III. — Accessory Apparatus:

Part II. — Separating, Concentrating Chapter XIX. — Accessory Apparatus.

or Washing: Part IV. — Mill Processes and Man-
Chapter VIII. — Preliminary Washers. agement :

Chapter IX. — Sizing Screens. Chapter XX. — Summarv of Princi-

Chapter X. — Principles of Screen pies and Outlines of Mills.

Sizing. Chapter XXI. — General Ideas on

Chapter Xl. — Classifiers. Milling.

Chapter XII. — Laws of Classifying by Appendix, Tables and Other Useful

Free Settling in Water. Information. Index.

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THE

SAMPLING and ESTIMATION
OF ORE IN A MINE

By T. A. RICKARD

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This most excellent book is a reprint with revision and
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GEOLOGY APPLIED TO
MINING

JOSIAH EDWARD SPURR, A.M.

The aim of the author in this most excellent dis-
sertation on the physical history of the earth is to
intelligently impart to the reader a comprehensive study
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IN TWO STYLES I

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