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Louis V. (Louis Valentine) Pirsson.

Rocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools online

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Online LibraryLouis V. (Louis Valentine) PirssonRocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools → online text (page 1 of 35)
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U C.



EOCKS

AND

BOOK MINEEALS

A MANUAL OF THE ELEMENTS OF PETROLOGY
WITHOUT THE USE OF THE MICROSCOPE

FOR THE GEOLOGIST, ENGINEER, MINER, ARCHITECT,

ETC., AND FOR INSTRUCTION IN COLLEGES

AND SCHOOLS



BY

LOUIS V. PIRSSON

LATE PROFESSOR OF PHYSICAL, GEOLOGY IN THE SHEFFIELD
SCIENTIFIC SCHOOL OF YALE UNIVERSITY



TOTAL ISSUE TWELVE THOUSAND



NEW YORK

JOHN WILEY & SONS, INC.
LONDON: CHAPMAN & HALL, LIMITED



OOPTBIQRT, 1906,
BY

LOUIS V. PIRSSON
Entered at Stationers' Ha



Stanbopc ipros

H.G1LSON COMPANY 1-25

BOSTON. U.S. A



PREFACE.



DURING the last fifteen years it has been one of the
writer's duties to teach the elements of Petrology to
students in various branches of Engineering, Mining,
Chemistry, Forestry, etc. The amount of time which
these students, in their undergraduate course, can devote
to the subject is limited and precludes any attempt to
give them such instruction in optical-microscopical
methods of research as would be worth while. The
subject has to be treated from the purely megascopic
standpoint, as indeed the vast majority of those who
have to deal with rocks in a practical or technical way
are also obliged to consider them.

In giving this instruction the author has long felt the
need of a small, concise and practical treatise in which
the rocks and rock-minerals are handled entirely from
this megascopic standpoint. In such works as exist
either the subject matter has not been brought down to
date to express our present knowledge of rocks, or it is
treated largely from the microscopical or chemical stand-
point, or the classifications used are based on microscopical
research and are thus not available for ordinary use, or
the rocks are treated incidentally with respect to some
other main purpose as in works on soils, ore deposits, etc.
The present work is an attempt to fill this need which the
writer believes is also felt in many other institutions in
which similar courses in Petrology are given. In addition
to this purpose its scope has also been somewhat enlarged
to meet the wants of many who have to consider rocks
from the scientific or practical point of view and who are
not in a position to use the microscopical method. It is
hoped that it may thus prove of service to field geologists,

iii



iv PREFACE

engineers, chemists, architects, miners, etc., as a handy
work of reference. Much of the theoretical side of
Petrology which has been developed during the last few
years, especially in the line of petrogenesis, does not
demand a knowledge of microscopical petrography for
its understanding, and the endeavor has been made to
present the elements of this in a simple manner. Although
the author has incorporated considerable original material
it goes without saying that a work of this character must
of necessity be mainly one of compilation. It would be
nearly impossible and in any case out of place in an
elementary treatise to give by reference the thousand
and one sources from which the material has been taken.
It should be mentioned, however, that in the description
of the minerals the writer has drawn freely upon the
mineralogies of Dana, Iddings and Rosenbusch and the
determinative mineralogy of Brush-Penfield. In the same
way in the treatment of the rocks the petrographies
of Rosenbusch and Zirkel and the geological text-book
of Geikie have been freely used.

Most of the illustrations have been prepared for this
work, but the wealth of material in the published reports,
bulletins, etc., of the United States Geological Survey
has also been freely used.

SHEFFIELD SCIENTIFIC SCHOOL OF YALE UNIVERSITY.
New Haven, Conn., Jan., 1908.



TABLE OF CONTENTS.



PART I.

Introductory and General Considerations.

Chapter Page

I. SCOPE OF PETROLOGY; HISTORICAL; METHODS OF STUDY 1
II. CHEMICAL CHARACTER OF THE EARTH'S CRUST AND ITS

COMPONENT MINERALS . . . 14



PART II.

Rock Minerals.

III. IMPORTANT PROPERTIES OF MINERALS 21

IV. DESCRIPTION OF THE ROCK-MAKING MINERALS ... 33
V. DETERMINATION OF THE ROCK-MAKING MINERALS . , 114

PART III.
The Rocks.

VI. GENERAL PETROLOGY OF IGNEOUS ROCKS 132

VII. DESCRIPTION OF IGNEOUS ROCKS 205

VIII. ORIGIN AND CLASSIFICATION OF STRATIFIED ROCKS . 275

IX. DESCRIPTION OF STRATIFIED ROCKS 293

X. ORIGIN, GENERAL CHARACTERS AND CLASSIFICATION OP

THE METAMORPHIC ROCKS 333

XI. DESCRIPTION OF METAMORPHIC ROCKS 351

XII. THE DETERMINATION OF ROCKS 398

INDEX . 409



ROCKS AND ROCK MINERALS



PART I.

INTRODUCTORY AND GENERAL.



CHAPTER I.

SCOPE OF PETROLOGY; HISTORICAL; METHODS
OF STUDY.

EVERYWHERE beneath the mantle of soil and vegetation
that covers the surface of the land lies rock, the solid
platform upon which the superficial debris of earth rests.
Here and there in mountain tops, in cliffs and ledges, we
see this underlying rock projecting from the soil and
exposed: we know that it must underlie the sea in the
same way. The outer shell of the earth then is made of
rock, which forms the foundation upon which rest all the
surface things with which we are acquainted. How thick
this zone of rock is we do not know, but upon it we live
and exert our activities; into it we penetrate for coal, oil,
gas, metals and other things upon which the material
features of our modern civilization depend. It is there-
fore of the highest importance to us, and the information
which we have acquired concerning it, by examination
and study, forms a valuable branch of human knowledge.

Petrology the Science of Rocks. Our knowledge of
all the various things which together make up that part
of the earth which it is permitted us to examine and study
and which has been comprehended under the heading of
Geology has now increased to such a degree that this
science has split up into a number of well-defined, sub-
ordinate branches or geological sciences. Thus Meteor-
ology is the science of the atmosphere, the summation of

1



2 ROCKS AND ROCK MINERALS

our knowledge of the causes and movements of winds,
storms, rain, the distribution of heat and cold, and in
general the study of the various factors that affect the
air and its movements and of the laws that govern them.
Physiography takes account of the surface features of
the earth, of the distribution of land and water and of the
agencies which are modifying them, the effects of climates
and the various causes which together produce the topo-
graphy which the earth's surface now exhibits. Paleon-
tology is the science resulting from the study of the remains
of past life upon the earth, as shown by the fossils inclosed
in the rocks, and teaches not only the different forms
which have existed but also seeks to discover the trans-
formation of one form into another and the various move-
ments or migrations of life upon the earth in past ages.

Petrology, in the same way, has now become a separate
branch, one of the geologic sciences. It comprises our
knowledge of the rocks forming the crust of the earth, the
results of our studies of the various component materials
which form them, of the different factors and the laws
governing them which have led to their formation, and
of their behavior under the action of the agencies to which
they have been subjected, and endeavors to classify the
kinds into orderly arrangement.

The terms Petrology and Petrography are not abso-
lute synonyms though often so used in a general way.
The former has been defined above; the latter more
particularly refers to the description of rocks and especially
with respect to their study by means of the microscope
as explained later thus microscopical petrography.
Petrology is used for the science in its broader aspects as
well and covers the geological and chemical relations of
rocks: thus strictly defined petrography may be said to
be a branch of petrology. The synonym Lithology has
become nearly obsolete. Petrology means the science of
rocks; lithology, the science of stones, and the word stone
is now used in a popular way for architectural and com-



METHODS OF STUDY 3

mercial purposes or to designate any loose piece of rock
of unknown origin.

Definition of a Rock. By the term " rock," geologically
speaking, is meant the material composing one of the
individual parts of the earth's solid crust, which, if not
exposed, everywhere underlies the superficial covering
of soil, vegetation or water which lies upon it. The
popular understanding of this term, that it denotes a
hard or firm substance, is not, geologically, a necessary
one, for a soft bed of clay or of volcanic ash is as truly a
rock as a mass of the hardest granite. Moreover it
implies within limits, which will be explained elsewhere,
a certain constancy of chemical and mineral composition
of the mass recognized as forming a particular kind of
rock. Thus the chance filling of a mineral vein by variable
amounts of quartz, calcite and ores is not accepted by
petrographers as forming a definite kind of rock. The
term is also used with different meanings; it may be
denotive of the substance forming parts of the earth's
crust, as quartz and feldspar arranged in a particular
manner are said to form a rock granite or it may
refer to the masses themselves and thus possess a larger,
geological significance. In a general way the former
may be said to be a petrographic, the latter a geologic
usage. When used in this broader geologic sense the
mass recognized as an individual kind of rock must
possess definite boundaries and show by its relations to
other rock masses that it owes its existence to a definite
geological process. The absolute size of the mass is not
involved in this, for a seam or dike of granite cutting rocks
of other kinds may be as thin as cardboard or a mile in
thickness.

Composition of Rocks. Rocks are sometimes defined
as aggregates of one or more minerals, but this is not a
broad enough or wholly correct definition. Rocks may
be composed entirely of minerals or entirely of glass or
of a mixture of both. Minerals are substances having



4 ROCKS AND ROCK MINERALS

definite chemical compositions and usually of crystalline
structure; glasses are molten masses chilled and solidified
without definite composition and structure. Rocks
composed wholly of minerals may be simple or compound,
that is, the rock may be formed of one kind of mineral
alone, as for example, some of the purest marbles which
consist of calcite only or of a mixture of two or more like
ordinary granite which is made of grains of quartz, feld-
spar and mica. These subjects are treated more fully in
later chapters.

History of Petrology. The science of geology may be
said to have commenced when rocks as objects of inves-
tigation began to be studied. As the individual minerals
composing rocks, or contained in their cavities, were
investigated by chemical means and by the goniometer,
the science of Mineralogy and its related subject, Crys-
tallography, began. At first the difference between
rocks and minerals was not very clearly perceived; very
dense rocks composed of mineral grains so fine that they
could not be distinguished by the eye or magnifying lens
were thought to be homogeneous substances, and similar
in their nature to minerals. This continued in many
cases even down to the middle of the last century.

As the knowledge of the composition and properties of
minerals grew it was seen in the case of the coarser
grained rocks that they were composed of aggregates of
these mineral grains, and according to the kinds of the
component minerals many common rocks had already
received names early in the last century. As investiga-
tion proceeded and geological science grew many new
combinations were discovered and the list of named
rocks increased, and it may be remarked here that these
early geologists, armed only with a simple lens, were
exceedingly keen observers and made many surprisingly
correct observations on the mineral composition of quite
fine grained rocks. Various schemes of classification were
proposed, some of them containing admirable features,



METHODS OF STUDY 5

but the dense varieties defied the means of investigation
then at command, and in great part their composition,
properties and relations to other rocks remained unknown.
About the middle of the last century Sorby, an English
geologist, showed that, by a suitable method of operation,
very thin slices of rocks could be prepared, and by
the study of such thin sections under the microscope the
kinds of component mineral grains could be made out,
their properties and relations to one another, the order in
which they had been formed, the processes to which they
had been subjected, and many other interesting and
important features discovered, and that it was possible
to do this even in the case of the densest and most com-
pact rocks. This method of investigation was imme-
diately taken up, especially in Germany by Zirkel and
others, and with its advent a new era in the study of rocks
and the science of Petrography may be said to have
begun. A flood of knowledge regarding rocks and
especially of the minerals composing them began to rise
and has kept on increasing to the present day. The
study of the properties of transparent minerals under the
action of polarized light received a great impulse, and
the facts discovered have in turn been of immeasurable
service in the investigation of rocks by this method.
Thus at first attention was directed chiefly to the mineral-
ogical side of petrology; the kinds of minerals of which
rocks are composed and their properties were considered
of first importance, and this is reflected in the schemes of
classification devised at this period. Later, the chemical
composition of rocks, both in mass and as shown in their
component minerals, their origin and the relations of the
different varieties to each other began to attract more
attention and have been regarded as of increasing impor-
tance down to the present day. This increasing impor-
tance of these aspects of the subject is also seen in the
weight placed upon them in the more recent schemes of
classification proposed for the igneous rocks, those formed



6 ROCKS AND ROCK MINERALS

by the solidification of the molten masses coming from
the earth's interior.

Classification of Rocks. According to their mode of
origin and the position of the masses with respect to the
earth's crust and to each other, rocks naturally divide
themselves into three main groups, divisions which are
recognized by practically all geologists. These are the
igneous rocks made by the solidification of molten material;
the sedimentary or bedded rocks formed by the precipita-
tion of sediments in water, to which may be added the
small group of seolian or wind formed deposits, and the
metamorphic rocks, those produced by the secondary
action of certain processes upon either igneous or sedi-
mentary ones by which their original characters are
wholly or partly obscured and replaced by new ones and
which are therefore most conveniently considered in a
separate group. This grouping will be used in this work,
and each group with its further subdivisions, their char-
acters, relations, etc., will be treated by itself.

Summarizing then what has just been stated, we have:

I. Igneous Rocks, solidified molten masses.
II. Sedimentary Rocks, precipitated sediments.
III. Metamorphic Rocks, secondary formed from
I and II.

Field and Petrographic Classifications. The sedimentary
rocks are classified in two ways, in one they are sub-
divided according to the kinds and fineness of the mineral
particles which compose them, in the other according to
the geological age, as shown by their position and fossils,
in which the sediments were laid down. The first is the
petrographic, the second the geological, or more strictly
the historical classification, and in this work these rocks
are treated only according to the former method. In
classifying them they have, so far, been simply divided
according to the properties mentioned above, and as they
have not yet been the subject of the detailed research



METHODS OF STUDY 7

which their importance demands, the simple classifica-
tion adopted by geologists in the field has been followed
by the petrographers.

With respect to the igneous rocks and to a lesser degree
the metamorphic ones the case is different. The use of
the microscope in the study of thin sections has shown
that rocks which may appear absolutely identical, either
in the field or as one simply compares hand-specimens,
may be composed of entirely different minerals, or their
chemical analysis may prove them to be fundamentally
different in chemical composition. They may thus be
quite different rocks deserving separate names and places
in any classification in which all of the essential char-
acters of rocks are considered, and yet outwardly to the
eye there may be no hint of this. There have arisen two
useful terms, megascopic (from the Greek ju,eyas great)
and microscopic, the first descriptive of those characters
of rocks which can be perceived by the eye alone or aided
by a simple pocket-lens, and the second referring to those
which require the use of the microscope on thin sections.
It is obvious that a classification which is based upon
microscopic characters as much as upon megascopic ones
cannot be used in determining rocks in the field. It may
be more correct and scientific, but in the nature of things
it cannot be of general application and use. This subject
will be treated of more fully in the section devoted to the
igneous rocks, and it is sufficient to say here that the
object of this work is to supply a field classification
based upon the megascopic characters of rocks to be
determined by the eye or pocket-lens, aided by a few
simple means for the determination of minerals. In
addition many important facts regarding rocks and
especially igneous ones have been discovered in these
later years which are not dependent upon their classifica-
tion or microscopic study, and it is intended to give some
account of these in a simple general way.

Microscopical Petrography. Although this volume is



8



ROCKS AND ROCK MINERALS



not based upon the microscopical method of research it
will be of interest to indicate briefly how this is conducted
and the sort of results obtained by it. To prepare the
thin sections or slices of rock for study, a chip of the
material as thin as can be obtained is taken. It should
be for ordinary purposes about an inch in diameter and
of firm unaltered material. It is first ground flat on a
metal plate with coarse emery powder and water and then
very smooth on another plate with very fine powder. It
is then cemented by the aid of heat to a piece of glass
with Canada balsam and the other side ground down with
the coarse emery until it is as thin as cardboard, or as
far as it is possible to carry the operation safely with the
coarse powder. It is then in a similar way ground down
with the finer powder and finally finished on a glass plate
with the finest flour of emery until, in the case of dark,
dense rocks, it becomes so thin that ordinary print may
be read through it. It is then transferred, after melting
the cementing balsam, to a microscopic object glass slide,
enveloped in balsam, a thin cover glass placed over it, and
it is then ready for use. The professionals who make a
business of preparing such sections save much time in the
coarser work by the use of sawing disks with diamond
dust embedded in them or by using car-
borundum powder on disks or endless
revolving wires. They become very expert
in the final grinding and prepare sections
whose thickness is quite uniformly about
WITTJ f an inch, experience having shown
that this is best for general work. The
appearance of one of these finished sections
is shown in Figure 1. By this process the
minute mineral grains composing the
densest and blackest of basaltic lavas
become transparent and may be deter-
mined under the microscope.
The microscope used in petrographic work differs from




Fig. i. Thin rock
section



PLATE 1.




METHODS OF STUDY 9

that ordinarily employed in being furnished with a
variety of apparatus arranged for studying the mineral
sections in polarized light. Underneath the table or
stage which carries the section is a nicol prism of calcite
which polarizes the light coming upward from the reflect-
ing mirror below before it passes through the section,
that is, the vibrations of the ether which produce the
phenomenon of light instead of occurring in all directions,
as in common light, are reduced to a definite direction in
one plane.

Another nicol prism called the analyzer is fitted in the
tube above the object lens so that the effects produced by
the mineral particles on the polarized light, as it passes
through them, can be tested and studied. The nicols
can be also removed so that the effects of ordinary light can
be seen. Other arrangements are provided for strongly
converging the light as it passes through the minerals
and for testing the results produced in a variety of ways.

Subjected to such processes the transparent minerals
of the rocks exhibit a great variety of phenomena by
means of which the different species may be definitely
determined. Crystals or fragments of crystals of an
almost incredible degree of minuteness may be studied
with high powers, their properties examined and the par-
ticular variety of mineral to which they belong made out.
In order to use this means of studying rocks a good
knowledge of general Mineralogy, of Crystallography, of
Optics, and in particular of the optical properties of the
rock-making minerals is essential. Owing to the cost of
the apparatus, the technical knowledge required in its
use and the difficulty of making thin sections, it is obvious
that this method of studying rocks can never become a
popular or general one, but many of the results which
have been attained by it are easily understandable and
may be mentioned.

Results of Microscopic Research. By the method
described above the kind of mineral or mineral grains



10 ROCKS AND ROCK MINERALS

making up the most compact and dense rocks may be
determined; whether the rock is of sedimentary or igneous
origin can be told and, if the latter, the general order in
which the mineral varieties have crystallized from the
molten fluid. It can be seen whether the crystal particles
are clear and homogeneous or if they contain inclusions
of various kinds, facts which often throw light on their
origin and history; whether they are fresh and unchanged
or have been decayed by the action of the elements and
altered wholly or partly into other substances; whether
they have been subjected to the enormous pressures of
mountain building in the crust and are strained and
crushed or not. It is possible to tell at once if a rock
contains more or less glass associated with the mineral
grains, and if it does, to thus learn with certainty its
igneous origin and the fact that in all probability it is a
surface lava, glass, in the nature of things, being almost
entirely confined to such rocks.

Furthermore, if the grains are not too microscopically
fine it may be possible, not only to determine the kinds of
minerals they are, but to measure their areas or diameters
in a given section, ascertain from this the relative pro-
portions of the different kinds of grains present, and then,
when the chemical composition of the component minerals
is known, compute the chemical composition of the rock
mass as a whole, a factor, especially in the case of igneous
rocks, often of great importance in scientific classification
and in other ways.

These are some of the more important features of
rocks which may be discovered by their microscopic
study, and they are sufficient to illustrate the value of the
method in aiding geological research.*

* The following works in which rock-making minerals and rocks
are treated and classified according to the results of microscopical
research in a more or less extensive and detailed way may be men-
tioned: Rock Minerals, by J. P. Iddings (Wiley and Sons, New York).
Quantitative Classification of Igneous Rocks, by Messrs.Cross,Iddings,



METHODS OF STUDY 11

Megascopic Study of Rocks. Although the microscope
is necessary for the complete investigation of rocks many
of their important features may be observed without its
use. In the case of the coarser grained ones, those
where the size of the grains is one-sixteenth of an inch in
diameter or more, the component minerals can usually
be identified by the aid of the lens or by simple means.



Online LibraryLouis V. (Louis Valentine) PirssonRocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools → online text (page 1 of 35)