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THE
FUNDAMENTAL PRINCIPLES

OF

PETROLOGY



McGraw-Hill BookCompany



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Metallurgical and Chemical Engineering P o we r



THE -FUND AMENTAL
PRINCIPLES OF PETROLOGY-



BY
DR. ERNST WEINSCHENK

PROFESSOR OF PETROGRAPHY IN THE UNIVERSITY OF MUNICH



AUTHORIZED TRANSLATION
(FROM THE THIRD GERMAN EDITION)



BY
ALBERT JOHANNSEN, PH. D.

ASSOCIATE PROFESSOR OF PETROLOGY IN THE UNIVERSITY OF CHICAGO



WITH 137 FIGURES AND 6 PLATES



FIRST EL-IT ION



McGRAW-HILL BOOK COMPANY, INC,
239 WEST 39TH STREET. NEW YORK



LONDON: HILL PUBLISHING CO., LTD.

6 & 8 BOUVERIE ST., E.G.

1916



Q &



COPYRIGHT, 1916, BY THE
MCGRAW-HILL BOOK COMPANY, INC.



THE. MAPLE. PRESS* YORK. PA



Oo U)e

of

Caroline Austin

this translation
is ~2><t6lcateo



357413



TRANSLATOR'S PREFACE

The need of a short presentation of the fundamental principles
of petrology as applied to geology has suggested the translation
of the first volume of Professor Weinschenk's Grundzuge der
Gesteinskunde. Without thereby expressing his belief or dis-
belief in the views of the author, the translator has tried to adhere
as closely as possible to the sense as well as to the mode of ex-
pression of the original. Only in the case of the Osann classifica-
tion has any additional material been inserted. The rules for
calculating analyses in this system are readily accessible to the
German student but not to others, and in view of the growing
use made of Osann's triangular diagrams by European geologists,
the addition was thought justifiable. Owing to the uncertainty
of shipments from Germany, it was impossible to obtain the original
electrotypes of the illustrations, consequently the cuts in this
translation were reproduced from the figures in the German
volume, and are not quite so sharp as they otherwise would have
been.

The translator is deeply indebted and very grateful to Dr.
E. A. Stephenson for critically reading this manuscript and for
various suggestions. He is also indebted to Miss Caroline A.
Duror for carefully reading it and for pointing out certain ambigu-
ous passages. The sad death of this brilliant and promising stu-
dent permits the writer to express his appreciation only in the
dedication of this translation to her memory.

ALBERT JOHANNSEN.

THE UNIVERSITY OF CHICAGO,
August 14, 1916.



vn



CONTENTS

PAGE

INTRODUCTION .........;...... ." . .... . .... 1

DEFINITIONS AND SUBDIVISIONS. -. . . . . . -. .5

THE SOLIDIFIED CRUST AND THE CRYSTALLINE SCHISTS ;,..... 9

The Formation of the Earth's Crust . . . 9

Characteristics of the Crystalline Schists . . . .^ 11

VULCANISM AND THE ORIGIN OF IGNEOUS ROCKS . ^\ 14

Older Theories of Vulcanism 14

Petrographic Observations on Vulcanism .1 16

Stiibel's Theory .-. . . . . . ... . . . . . 19

The Physical Character of Volcanic Magmas 21

The Outward Manifestations of Vulcanism 24

The Geologic Age of Igneous Rocks 27

THE COMPOSITION OF IGNEOUS ROCKS . . . . 30

Mineralogic Composition 31

Distribution of the Elements 33

Chemical Composition of Igneous Rocks 34

Physico-chemical Laws of the Magma 36

Action of Mineralizers . . ". 41

Magmatic Differentiation . .... . . . .'. .^f . 44

Complementary Dikes .. ... ..... <s. ..;... 48

Petrographic Provinces * . . .^ . : . . . 53

Theories of Magmatic Differentiation 54

Piezocrystallization 55

Injection of the Adjacent Rocks ...... 61

Type-mixing U</ . 66

Graphical Representations of Chemical Compositions .......... 68

ROCK WEATHERING . ...... . .... ^^< - . ^ ......... 73

Weathering in General . . . . ... . ../. .y. ...-' ,_.i './..... 73

Physical Weathering ./ . 74

Chemical Weathering ^ .............. 74

The Weathering Solutions ......... 79

The Weathered Residues ............ 80

Climatic Zones of Weathering ......;... 82

Chemical Weathering of Former Periods .... / .......... 83

Organic Weathering. (/ . 84

Rock-sculpture by Weathering . . . . . . . . . . . . 85

Denudation . . :i . . . . ' y" " ' ' ' ~ ' ' ^

THE XATURE OF THE SEDIMENTS ...... ^ IS. ............ 94

Composition of the Sediments ^ ....... 95

Mechanical Sediments 95

^Eolian Deposits i f ......... r ...... 97

Alluvial Deposits . ...... . . . -. . . . . . . . 99

Glacial Deposits . . . ...... . . . . . . .... - . I . . . 102

Chemical Sediments . . .- . . .- . - . . . . . .' . . . 103

Organogenic Sediments 106

ix



x CONTENTS

PAGE

Diagenesis Ill

Recent and Fossil Sediments 114

CONTACT-METAMOEPHISM ..' ^. 116

Agents of Contact-metamorphism 116

Contact-metamorphism Produced by Plutonic Rocks 119

Contact-metamorphism of Argillites 123

Contact-metamorphism of Carbonate-rocks 127

Contact-metamorphism of Basic Igneous Rocks 131

Serpentinization of Feldspar-free Rocks by Contact-metamorphism . . .132

Piezo-contact-metamorphism 134

Paragenesis of Contact-rocks , . . 136

Contact-metamorphism by Extrusive Rocks 137

POST-VOLCANIC PROCESSES 139

Post-volcanic Phenomena 139

Formation of Pegmatite 142

The Amygdaloids 144

Mineral-dikes and Ore-veins 146

Varieties of Rock Alteration 149

Kaolinization 149

Saussuritization, Uralitization, etc 15.1

Sericitization - 152

Serpentinization 152

Formation of Talc 153

Zeolitization 154

Other Alterations 1 54

Metasomatic Replacement of Carbonate-rocks 155

REGIONAL METAMORPHISM 156

Early Ideas Regarding the Crystalline Schists 157

Younger Crystalline Schists . 158

Variability of the Crystalline Schists 160

Gtimbel's Theory of Diagenesis 161

Theories of Regional Metamorphism 162

Plutonic and Hydrochemical Metamorphism 164

Latent Plasticity and Fractureless Folding 165

Dynamometamorphism 169

Facies of the Crystalline Schists . . . v 173

Summary 174

Granite and the Crystalline Schists 176

JOINTING AND TEXTURES 181

Appearance of Surface Exposures of Various Rocks 181

Jointing and Parting in Rocks 184

Megascopic Characters of Rocks 190

Internal Textures 195

Internal Textures of Igneous Rocks 196

Internal Textures of Contact-rocks and Crystalline Schists. ...... 199

Internal Textures of Sedimentary Rocks 201

Mechanical Textures . 202

Inclusions, Concretions, and Secretions 204

INDEX. . 207



THE FUNDAMENTAL
PRINCIPLES OF PETROLOGY

INTRODUCTION

Petrology, or the study of rocks, treats ^ the origin, present
condition, and decay of rocks. It traces their history through
every stage of their existence, and contributes to our knowledge
of the processes by which the earth has been brought to its
present state. Thus defined, petrology is one of the most
important fundamentals of geology, yet it has been greatly
neglected by past and present workers in the science. A geologist,
justly, is expected to have a thorough training in paleontology,
for without it successful geologic work is impossible. But the
requirement of proper paleontologic preparation has led to a
one-sided treatment of the subject, the petrologic side remaining
undeveloped.

No one who is interested in unraveling the geology of a region
will find fault with a field geologist for having the most complete
knowledge possible of things paleontologic, but such knowledge
does not justify him in neglecting the assistance offered by
petrology ; knowledge especially indispensable to the working
geologist. Whoever would comprehend all the phases of geology
must give petrology the same weight as paleontology, and draw
upon each equally for his results.

The causes which led to the great preference given to paleon-
tology cannot be discussed here in detail. Broadly speaking, they
depend upon the fact that when paleontology started, all pre-
liminaries for rapid grow r th were found in the high stage of develop-
ment of zoology, and with this for a starting point, it needed but
to grow. Its methods of research, also, were relatively simple, and
did not first require apparatus and instruments of many kinds.
Thus, spreading over a prepared field, it captured the spirit of the
investigator by its surprising and easily reached results.

i



OF PETROLOGY



/ Petrography did not find such a road before it. Step by step
it had to prepare its own path; it advanced slowly, and was con-
tinually repulsed by many obstacles, until finally the introduction
of the microscope raised it to equal rank with the other sciences.
But with the introduction of microscopic methods a peculiar
change took place. While formerly none but geologists made use
of | petrographic-research methods, and these necessarily of the
simplest and most elementary kinds, an entirely new trend now
came about. Rocks were examined simply to determine their
mineral components, and no attention was paid to their geologic
relationships. Thus there arose a school, purely mineralogic
and entirely independent of geology, whose viewpoint extended
but little beyond the walls of the laboratory or beyond the field
of the microscope, and only rarely did a practical geologist dare
to enter the sacred precincts of microscopic petrography. The
unsuspected wonders then first revealed in the rocks, completely
fascinated those who had overcome the many difficulties of the
preliminary training necessary for microscopic petrography; yet
on account of their deficient geologic training, few petrographers
could solve geologic problems/ Paleontology, on the other hand,
gave an abundance of important results.

The purely mineralogic tendency did not persist a great while,
for petrographers soon realized that geologic research was as neces-
sary as microscopic observation. Geologists, however, had become
accustomed to the idea that this science was a hothouse production
which could not bear transplanting, and even at the present time
a large number of geologists regard with suspicion, if not with
direct distrust, the results obtained by petrographic research^ and
this aversion, instead of diminishing, increases more and more as
petrologists continue to destroy theories which had been con-
sidered fundamentals of geology^ The results of modern petro-
graphic research, however, are of such great importance to geology
that the attempt on the part of many geologists to exclude them
is an'injury to their science. 7

At the present time, petrography does not concern itself simply
with the microscopic examination of thin sections. Its horizon
has been broadened, and the greater as well as the lesser features
of the rocks have become the subject of its study. The micro-
scopic object, certainly, even yet remains an important working
tool of the craft, but observations in the field and the study of the



INTRODUCTION 3

geologic relationships of the rocks to each other have become of
just as great importance. While the great museums formerly
hesitated to allot even a little dark corner to petrographic collec-
tions, at the present time the rocks are given a place of equal rank
with other collections. Rock specimens which show the broad
geologic relationships are now valued most, and instead of being
tiresome, side-by-side arrangements of hand-specimens of uniform
size, these collections have come to rank among those that are
most stimulating.

Petrography, as the youngest branch of geology, is still far
from the stage where practically everyone holds the same views;
a stage which has been reached to such a great extent, at least
apparently, by the other branches of the science. Nevertheless,
geologists cannot afford to be antagonistic to the results of its
investigations, although this is the attitude taken by the authors
of some of the most recent geologic text-books.

It is a fact not to be overlooked that the student must have
had a broad preliminary training to reach the desired goal in
petrography; a training which generally takes more time and
effort than he is willing to give to a subject which he considers
simply a subordinate aid. For this reason it has been customary
for the instructor in geology to present to his students, as sufficient
for their purpose, only those facts which were known before true
petrography existed, and then to console himself, and excuse his own
superior attitude toward the whole science, by reflecting that the
numerous rock- types, which over-specializing petrographers hav,e
set up, have neither geologic significance nor justification; that the
new names which arise in such rapid succession have the ephemeral
character of day-flies; and that even among petrographers who
have made the science their life-work, but little unanimity yet
exists in regard to the most important questions affecting geology/
But what science has become great without having had its truths
promoted by opposition to its accepted views, and in what science
have the subdivisions of the classification and the setting up of
new names grown to such proportions as in paleontology, which,
in spite of this, is in such high favor with geologists?

The attempt here made to give a history of the rocks from the
standpoint of modern petrography, primarily for geologists, may
appear a somewhat thankless task. The presentation of the most
important results of petrographic research will encounter the



4 FUNDAMENTAL PRINCIPLES OF PETROLOGY

objection from the geologic side that their value to geologists is
out of proportion to the difficulty with which the beginner can
grasp and observe them, while on the other hand, the same pre-
sentation will be too generalized and too limited for the specialist
in petrography.



I. DEFINITIONS AND SUBDIVISIONS

LITERATURE

W. CROSS: "The Natural Classification of Igneous Rocks." Quart. Jour. Geol.

Soc. London, LXVI (1910), 470.
F. FOUQU ET A. MICHEL-LEVY: "Mineralogie micrographique des roches eruptives

franchises." Mem. carte geol. France, 1879.

A. MARKER: "Petrology for Students." 4th edition, Cambridge, 1908.
F. H. HATCH: "Textbook of Petrology." 5th edition, London, 1909.
J. P. IDDINGS: "Igneous Rocks. Composition, Texture, and Classification." 2

vols., New York, 1909 and 1913.

E. KALKOWSKY: "Elemente der Lithologie." Heidelberg, 1886.
H. ROSENBUSCH: "Elemente der Gesteinslehre." 3 Aufl., Stuttgart, 1910.
Idem: " Mikroskopische Physiographic der Mineralien und Gesteine." Bd. II,

"Massige Gesteine," 4 Aufl., Stuttgart, 1907.
J. ROTH: "Allgemeine und Chemische Geologic." Bd. II., "Petrographie," Berlin,

1887.
J. J. H. TEALL: "British Petrography, with Special Reference to the Igneous Rocks."

London, 1888.
CH. VELAIN: "Conferences de Petrographie." Paris, 1889.

E. WEIXSCHENK: "Grundziige der Gesteinskunde." II Teil. "Spezielle Gesteins-

kunde," 2 Aufl., Freiburg, 1907.

F. ZIRKEL: "Lehrbuch der Petrographie." 2 Aufl., Leipzig, 1893.

Rocks may be defined as mineral aggregates which, with more
or less constant composition, form geologically independent bodies
and an essential part of the earth's crust.

Nearly all rocks are aggregates of different minerals, that is,
rocks are compound or mixed. In the strict sense of the word,
simple rocks, which consist of but a single mineral, are extremely
rare. Mineral combinations, such as ore deposits, are not classified
as rocks on account of their usual subordinate development and
their less constant composition, although they possess the former
property in common with numerous dikes. There is no sharp
line of separation. Thus dark mica may not occur in the apophyses
from some granites, yet these aplites are still regarded as rocks.
The differentiation may have gone still farther; the feldspar also
may have disappeared, and the vein-filling may consist almost
exclusively of compact quartz. In spite of the absolute identity
in the mode of occurrence of the aplite and the quartz, the latter
is regarded rather as a mineral vein than a rock.

5



6



FUNDAMENTAL PRINCIPLES OF PETROLOGY



The constituents of a rock may have crystallized in the place
in which they occur. Such aggregates of authigenic (Gr. avdu, on
the spot, and 7171/0^0:1, to be born) individuals are called crystalline
rocks. On the other hand, those rocks whose allothigenic (Gr.
dXXo0i, elsewhere) components were derived from previously exist-
ing rocks, which were shattered and destroyed and later deposited
elsewhere in secondary beds, are called clastic (Gr. KXcwros, broken)
or fragmental rocks.

Certain rocks, namely the fluids and gases of the hydrosphere (Gr. vd wp, water,
<r<paipa, sphere) and atmosphere (Gr. dr/ws, vapor) which surround the solid part of the
lithosphere (Gr. \idos, stone) are neither crystalline nor clastic. Even some of the




FIG. 1. Massive granite, Grimselstrasse. (Gebr. Wehrli, Photo.)

rocks occurring as solid aggregates do not belong to these two great groups, since
they are built up of amorphous constituents formed in situ. Here belong the glassy
or hyaline (Gr. flaXos, glass) volcanic rocks, such as obsidian and pitchstone, and the
amorphous porodine (Gr. Tropos, pore, passage, diveiv, whirling) deposits from aqueous
solutions, the so-called colloidal gels. The latter are so subordinate, however, that
they would hardly be classed as rocks.

Crystalline rocks originated in various ways. If a primary
crust solidified over the earth, it must necessarily have had a
crystalline character. Molten masses, upon emerging from the
interior of the earth, crystallize; aqueous solutions not infre-
quently give crystalline precipitates; and the action of igneous
masses on originally clastic formations may produce crystalline
aggregates. It is, consequently, not possible to use crystallinity
as a basis for petrographic classification. It has been customary



DEFINITIONS AND SUBDIVISIONS 7

instead, to divide the rocks according to their mode of origin.
On this basis, three groups are distinguished.

(a) Igneous (Lat. ignis, fire) or eruptive (Lat. erumpere, to break
forth) rocks embrace those masses which were forced upward in
a molten condition from within the earth, and therefore also called
anogenic (Gr. avu, upward), or from their usual external appear-
ance, massive rocks (Fig. 1). They are authigenic or primary, and
are typical of the majority of crystalline rocks.

(b) Sedimentary (Lat. seder e, to settle) rocks include all rocks
which originated by precipitation from circulating surface water,
and all rocks whose components, transported by any means what-




FIG. 2. Stratified limestone near Wiesbaden. (Prof. Dr. Klemm, Photo.)

ever, were deposited from above. They are also called catogenic
(Gr. /card, down from) or, from their usual character, bedded rocks
(Fig. 2). The sediments are secondary and consist essentially of
allothigenic constituents. They are types of clastic or fragmental
rocks.

(c) The crystalline schists, finally, whose characteristic represen-
tatives were supposed to belong to the oldest formation of the
earth, the so-called Archean (Gr. apxcuos, ancient) or Azoic (Gr. a,
without, wri, life), form a group whose genetic relationships are
in many cases uncertain. They are therefore also called crypto-
genic (Gr. KPVWTOS. hidden) rocks. They consist, on the whole,
predominantly of authigenic constituents, and are in part pri-
mary and in part secondary rocks; the latter are usually much
altered.



8 FUNDAMENTAL PRINCIPLES OF PETROLOGY

Only the outermost beds of the earth's crust are known, the deepest openings being
less than one two-thousandth of the earth's radius. That which lies below is entirely
inaccessible, and we can only conjecture as to its condition.

The temperature of the earth increases with depth in the outer, accessible parts,
and while the temperature gradient differs greatly in different places, it averages about
30 per kilometer. Assuming a uniform increase from the surface downward, the
center would have-a temperature of about 200,000, the earth's radius being more than
6,000 km. There is no proof that this increase in temperature is uniform to the
center of the earth, since only a small percentage of the whole is known, but the
assumption that the interior has a high temperature is not unjustifiable.

We know, further, that the specific gravity of the earth as a whole is about 5.5,
or about twice as much as the average density of the accessible portion of the crust.
Within the interior, therefore, there must be present a great abundance of very heavy
minerals, most probably native iron, a constituent which also forms an important part
of meteorites, themselves fragments of destroyed worlds.

Further, the pressure in the interior must be enormously greater than at the
surface, whereby the molecules, in the highly heated, presumably gaseous masses,
must be so closely packed that the gas does not occupy so large a volume as would
the same material in the solid state under less pressure. As a necessary consequence,
enormously greater stresses must exist in the interior than in the crust.



II. THE SOLIDIFIED CRUST AND THE CRYSTALLINE

SCHISTS

LITERATURE

J. N. v. FUCHS: "t)ber die Theorien der Erde." Munch, gelehrter Anzeiger, 1838.

ARCH. GEIKIE: "Textbook of Geology." 2d edition, London, 1885.

T. STERRY HUNT: "The Chemistry of the Primeval Earth." Geol. Mag., 1868.

J. HUTTON: "Theory of the Earth." 1795.

I.KANT: "Allgemeine Naturgeschichte und Theorie des Himmels-," 1755. Reprinted

in Ostwald's Klassiker der exakten Wissenschaften, No. 12.
P. S. LAPLACE: "Exposition du systeme du monde." Paris, 1796.
A. DE LAPPARENT: "Traite de geologic." 4th edition, Paris, 1900.
H. LENK: "Uber die Natur des Erdinnern." Erlangen, 1909.
CHAS. LYELL: "Principles of Geology." 12th edition, London, 1875.
R. MALLET: "On Volcanic Energy." Phil. Trans. Roy. Soc. London, CLXIII (1873),

I, 147.

C. F. XAUMANN: "Lehrbuch der Geognosie. 2 Aufl., Leipzig, 1858.
J. ROTH: " Allgemeine und Chemische Geologie." Bd. III. " Die Erstarrungskruste

und die Kristallinischen Schiefer." Berlin, 1890.
W. THOMSON: "The Internal Condition of the Earth, as to Temperature, Fluidity,

and Rigidity." Trans. Geol. Soc. Glasgow, VI (1891).
F. TOULA: "Die Verschiedenen Ansichten iiberdas Innere der Erde." Wien, 1876.

The Formation of the Earth's Crust. If we wish to familiarize
ourselves with the processes by which the rocks were formed, we
must study the earlier phases of the history of our solar system
when the earth, perhaps, was in the form of a molten mass sur-
rounded by a thick gaseous mantle. After a long period of time
the temperature of the surface may have been reduced so far that
a solid crust was formed over the molten interior; a crust which
was not scoriaceous like the upper surface of a lava-stream, but
uniformly crystalline like granite, on account of the extreme slow-
ness of the cooling and the enormous pressure of the heavy
atmosphere.

During this period, the temperature of the earth's surface Was
much above the critical temperature of water, and the present
ocean, then existing as vapor, formed the chief constituent of the
gaseous envelope. The atmospheric pressure, for this reason alone,
had risen to over two hundred atmospheres. In addition there
were innumerable other substances, gaseous at such high tempera-
tures, and their combined weight produced a pressure at the sur-

9



10 FUNDAMENTAL PRINCIPLES OF PETROLOGY

\

face such as is reached only deep within the earth at the present
time. By these means the molten masses became saturated with
many kinds of gases and vapors; the influence of which, as
mineralizers, will be described later.

Not only were the outer parts of the molten earth overloaded
with water-vapor and other gaseous substances, but they were
present in excess even to its innermost parts. The enormous
stresses produced by these gases, heated mucfy above their critical
temperatures, counterbalanced the enormous pressure of the at-
mosphere. In this manner the first coat of armor for the young
earth was formed, not uniformly and continuously, but often
disturbed by immense eruptions, often torn by molten masses
gushing forth from the deeps. As to how long the alternations
of solidification and fragmentation lasted, who would venture
a guess! As time passed the fluid center was more and more
firmly imprisoned by the solid shell, until finally the temperature
of the outer portion of the crust was reduced below the critical
temperature of water. The vapors then for the first time descended
as boiling rain upon the mantle built by volcanic forces.

The water, which became fluid under such high pressure at
365, was exceptionally active chemically, especially toward the


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