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DATE OUE
A TEX'f'EQOKiOF
GEOLOGY
BY
AMADEUS W. GRABAU
S.B., MASS. INST. OF TECHNOLOGY; S.M., S.D., HARVARD
PROFESSOR OF PALEONTOLOGY IN THE GOVERNMENT UNIVERSITY
OF PEKING, CHINA, AND PALEONTOLOGIST TO THE
CHINESE GEOLOGICAL SURVEY
FORMERLY LECTURER IN MINERALOGY AND IN GEOLOGY IN TUFTS
COLLEGE, PROFESSOR OF MINERALOGY AND GEOLOGY IN
THE KENSSELAER POLYTECHNIC INSTITUTE, AND
PROFESSOR OF PALAEONTOLOGY IN
COLUMBIA UNIVERSITY
AUTHOR OF "PRINCIPLES OF STRATIGRAPHY," "GEOLOGY OF THE NON-
METALLIC MINERAL DEPOSITS OTHER THAN SILICATES," " NORTH
AMERICAN INDEX FOSSILS " (WITH H. W. SHIMER), "GUIDES
TO THE GEOLOGY AND PALAEONTOLOGY OF NIAGARA
FALLS, OF EIGHTEEN MILE CREEK, AND OF
THE SCHOHARIE REGION," ETC., ETC.
PART I
GENERAL GEOLOGY
_. C. HEATH & CO, PUBLISHERS
BOSTON NEW YORK CHICAGO
COPYRIGHT, 1920,
BY D. C. HEATH & Cc
2iO
DEDICATED
TO
MY FORMER STUDENTS
WHO
IN THE NEW WORLD AND THE OLD
ARE TRANSMITTING, AUGMENTING, AND APPLYING
THE KNOWLEDGE OF THE
EARTH SCIENCE
IN THE ACQUIRING OF WHICH
IT HAS BEEN. MY PRIVILEGE
TO AID THEM
PREFACE
IN the preparation of this book, I have departed somewhat widely
from the prevailing order of treatment in current texts. Instead of
beginning with the destruction of rocks, it has seemed more logical to
give the student some knowledge of the rocks to be destroyed, and of
their character and origin. Instead of treating clastic rocks first and
igneous and other non-clastic rocks later, it has seemed more desir-
able to begin with those rocks from which elastics are largely derived,
before -dealing with the elastics themselves. Twenty years of experi-
ence as a teacher have convinced me that the average student admitted
to courses in geology receives too little instruction in minerals, and al-
though we generally recommend mineralogy as a desirable prerequisite,
few teachers can insist upon a preparation in this subject on the part
of the student. Yet without a knowledge of at least some minerals the
study of rocks is impossible, and few geological phenomena can be ade-
quately understood without at least a general knowledge of the rocks
which they affect. Students who are preparing to make geology their
life work, will in any case undertake a more extended study of minerals,
and they will turn to the excellent textbooks in that science now avail-
able, and some of which are listed on page 51. But the great majority
of students of geology come to this subject only with the desire to gain
some knowledge of the world they live in, of the material of which it is
composed, of the forces which have fashioned it, and of the laws which
have governed its development. They may do so from a desire to master
the secrets of nature for the material benefits to be derived from such a
mastery, or for the power which such a knowledge will confer upon them ;
or they may undertake the study of the earth, because they wish to
broaden their mental horizon and subject themselves to that stimula-
tion of the intellect, that deepening of spiritual perceptions, and that
awakening of dormant faculties, which others have found in a sym-
pathetic understanding of, and love for, the out-door world, and which,
in its fullest measure, is most frequently vouchsafed to the student of
geology. From whatever motive the" student approaches the subject,
he should be made to realize that his desires can best be attained,
if he keep in mind the maxim of La Rochefoucauld, "Pour Men sawir
une chose il faut en sawir les details." Detail does not appeal to the
vi Preface
average student, but a knowledge of a certain amount of detail is neces-
sary in any subject, if it is to be well understood, and in geology, as in
other sciences, no real understanding of principles and of phenomena
is possible without some conscientious devotion to detail. The book
of nature is unsealed only to him who is willing to learn the language
in which it is written.
I believe that at the ver}^ outset the student of the earth should sub-
ject himself to a moderate discipline in the elements of chemistry and
mineralogy at least. To those who can not devote the time to sepa-
rate courses in these sciences, Chapter IV may serve as guide for a series
<3f laboratory exercises, which may be carried on simultaneously with
the study in the classroom of the more general subjects treated in the
first three chapters. In all such studies the individual teacher must
select and amplify the subject matter; the text of Chapter IV is in-
tended primarily as a guide, while to the student, already familiar with
minerals, it may serve as a summary for convenient reference. In
its preparation, and especially in the selection of the mineral species in-
cluded in the tables, I have had the generous advice of the late lamented
Professor Alfred Moses of Columbia University, and that of Professor
L. Luquer of the same institution, while the experience gained in teach-
ing mineralogy for a number of years at the Rensselaer Polytechnic
Institute, at Tufts College and at the Museum of the Buffalo Society
of Natural Sciences has been drawn upon. For reasons already set
forth, I have next taken up the treatment of the igneous rocks. Many
years of teaching elementary students both in college and in the summer
sessions has satisfied me that while the study of rocks is carried on in
the laboratory the broader relations of those rocks are advantageously
treated at the same time in the lectures or classroom exercises. There-
fore I would recommend that Chapter VI be used entirely as a labora-
tory text, while Chapters VII to IX inclusive serve for simultaneous
classroom exercises, of course with proper material for illustration.
Chapter X is again best treated as a laboratory text on aqueous (chem-
ical) deposits, and here again each teacher will make his selection of
material as time and equipment permit. Chapter XI serves as the
accompanying text for lectures and classroom exercises, amplified and
illustrated according to the teacher's predilections and equipment.
It has been my experience, as no doubt it has been that of many other
teachers^ that the study of rocks of organic origin requires some under-
standing on the part of the student of the types of organisms which
are active in their production, and that the study of organic types
should not be relegated exclusively to the chapters on historical geology.
I therefore venture to hope that the introduction of illustrations of the
various types of rock-forming organisms will be welcomed by the teacher,
Preface
VII
and be of value to the student. Wherever possible I have selected
illustrations of material easily obtainable, as in the case of the mol-
luscan shells and several of the nullipores, which are common on our
Atlantic coast, and in our rivers and lakes, and in that of the bryozoans
and echinoderms (except the crinoids) which are easily obtained on our
coasts. Most of the corals too are the common species found in, or
easily added to, any collection. It is recommended that Chapter XII
be used both as classroom and laboratory text. The same thing ap-
plies to Chapter XIII, which deals chiefly with the plants that enter
into the construction of our peat and coal deposits, and specimens of
these can readily be obtained by any one. Here, too, laboratory work
with specimens and classroom exercises may go hand in hand.
The study of clastic rocks per se, treated in Chapter XVIII, may
again be carried on in the laboratory during the simultaneous treat-
ment of the forces which produce them and the phenomena of weather-
ing, erosion, transportation, and deposition (Chapters XV-XVII) in the
classroom and field. Although I do not suggest that these subjects be
entirely ignored during the earlier part of the course, where the teacher
may point out phenomena of weathering, stream and wave erosion,
etc., especially during field exercises, I believe that systematic study
is most satisfactorily deferred until non-clastic rocks, igneous phenom-
ena, and other forces which produce material from which clastic rocks
may in the first place be derived, have been considered. Other teachers
may not agree with me and they may find it desirable to transpose the
chapters. I have endeavored so to arrange the chapters that little
if anything will be lost in effectiveness by their transposition, and to
this end I have given repeated cross references. Therefore if the teacher
so desires, he may pass from Chapter III directly to Chapter XV, taking
up the intervening chapters in any order he deems fit.
I believe, however, that most teachers will agree with me that the
broader subject of the sculpturing of the lands (Chapters XXII-XXIII,
the essentially physiographic chapters) are best treated after, not only
the formation and original structural characters of the earth's crust,
but also its deformations have been studied. Although metamorphism
by igneous contact has been treated in connection with the igneous
phenomena, where it belongs, the main subject of metamorphism and
metamorphic rocks (Chapter XX) follows the treatment of deforma-
tions (Chapter XIX) and may likewise be treated largely as a labo-
ratory text. . .
Wherever possible, I have adopted the method of treating typical
examples of geological phenomena in some detail, with an abundance
of illustrations, because in my own experience, I have found that the
student grasps the details of isolated examples more readily and more
viil 'Preface
completely assimilates them than he does generalizations with illus-
trations drawn .from many sources, and from examples which are to
Mm little more than names. On this account I have devoted more
space than is usually given in textbooks to descriptions of well-known
volcanoes, such as Vesuvius, Etna, etc., to a few typical glaciers such
as the Aletsch, the Mer de Glace, the Malaspina, etc., to the Florida
reefs and the Great Barrier reef, and to such special examples of com-
plex rivers as the Niagara, the Genesee, and the Colorado. My choice
of the types has been partly influenced by their accessibility, and it
is my hope that the study of such accessible examples will awaken in
the student the determination to study them in the field, a possibility
any one may look forward to in these days of comparative lessening
of distances. The same motive of treatment of types rather than facts
and phenomena, has prompted me to devote what some may consider
an undue amount of space to the details of the great historic earth-
quakes. I believe, however, that the human interest which these phe-
nomena have, will appeal to the student, and he will duplicate my own
experience, that such a narrative will give him a deeper knowledge of
seismic phenomena than a categorical treatment of them could inculcate.
After due consideration and conference with the publishers, I have
decided to use the old style of endings for the geological systems and
periods, i.e., Cambrian, Ordovician; Silurian, etc., instead of the more
precise and uniform ending in ic (Cambric, Ordovicic, Siluric, etc.)
which I have always maintained, and still believe, to be superior, not
euphonically, but because they give uniformity to the terminology which
has the hit and miss characteristics of a language grown up uncon-
trolled, and which, is, therefore, not scientific. I am confident t hat in t he
future we shall adopt the system of uniform endings, and in my scientific
writings I shall continue to use it where I can persuade the editor to
permit it. In the present case, however, as the majority of teachers
cling to the old terminology, I have, though with some reluctance,
adopted it, more especially in view of the fact that it would make t hi*
use of the book more difficult if It did not conform to the language em-
ployed by the teacher. I desire, however, to have it distinctly under-
stood that I have not surrendered my belief in the superiority of the
more precise terminology, and that hereafter, as in the past, I shall
continue to advocate its use.
For substantial assistance in the preparation of this text I am under
obligation to a number of friends and colleagues. I have especially
enjoyed the freely given advice of my former colleagues at Columbia
University. The aid in the mineralogical chapter has already been re-
ferred to. Mr. Frederick K. Morris of Columbia read the proof of
Chapters XV to XVII inclusive and made many valuable criticisms
Preface
IX
and suggestions. A similar valuable office was performed by Professor
C. P. Berkey for Chapters XIX to XXI inclusive, and by Professor
D. W. Johnston for Chapters XXII and XXIII. To these gentlemen
my best thanks are tendered herewith, and the assurance that much
that may be valuable in these chapters is due. to their advice, while for
any divergence from their views, especially in the order and extent of
treatment, I take full responsibility. The entire proof was read by Dr.
Marjorie O'Connell of the American Museum of Natural History, and
again by my friend Mr. Ernest Welleck of the editorial staff of the
Popular Science Monfldy, for whose gratuitously given and extremely
effective service I gladly render full acknowledgment. My former
student Miss Mary Welleck, A.M., has been my assistant throughout
the arrangement of this text for the press, and has been of the greatest
service in securing illustrations. She has a number of block diagrams
entirely to her credit as acknowledged in the text, and also a number of
other illustrations, Mr. Frederick K. Morris has also contributed sev-
eral of his effective drawings, and has made special effort to secure
photographs as elsewhere acknowledged. To Miss Florence Holzwasser
of Barnard, I am also indebted for careful work in comparing manu-
script and copy, and to Messrs. C. J. Connelly and F. K. Morris and
to Dr. J. J, Galloway for reading part of the text and making suggestions.
To Miss Amy Hepburn, Librarian of Natural Science In Columbia,
and to the geological, botanical, and zoological libraries of that Uni-
versity I am under special obligation for freely furnished material for
illustrations. Prof. J. F. Kemp has generously loaned his portrait of
Werner, The United States Geological Survey has furnished a large
number of original photographs from which illustrations were made for
this book, and Dr. D. W/ Johnson of Columbia has placed his entire
collection of photographs at my disposal for selection of illustrative
material, and I hereby record my great indebtedness to Dr. Johnson
and to Dr. George Otis Smith, Director of the Federal Survey, for their
courtesy. To Dr. John M. Clarke and the State Museum at Albany
I am likewise under obligation for several illustrations in this part of
the text and for many more in Part II. The geological department of
Harvard University has generously loaned me a number of photographs
from the Gardner Collection through the chairman, Prof. R. A. Daly.
To Prof. J. B. Woodworth of that University I am also indebted for
several original photographs. The Alaska Engineering Commission
has also generously furnished a number of photographs of glaciers
through Mr. W. A. Ryan. Prof. Elizabeth Fisher of WeUesley College
has furnished several very effective photographs taken by herself and
others as acknowledged in the text. From Prof. W. H. Sherzer I have
received prints of Ms photographs of sand grains reproduced in Grabau
X
Preface
and Sherzer's Monroe Formation of Michigan. A number of photo-
graphs were taken by Dr. Marcus- 1. Goldman (U. S. G. S.) during a
trip with the author in England and Scotland under "the guidance of
Dr. Benjamin Peach. Several were taken by Mr, G. W. Stose (U. S. G. S.)
during an early trip in company with the author, in Nova Scotia. The
Philippine Bureau of Science, at Manila, Dr. Elmer D. Merrill, Di-
rector, has generously furnished the fine photograph of Mayon Volcano
reproduced in" the frontispiece. Prof. W. 0. Crosby has furnished a
number of illustrations used by him in his illustrated Museum Guide
(Dynamical and Structural Geology, ^Boston Society of Natural History,
,1892), and a number of photographs of geological features in Utah
have been received from Prof. F. J. Pack of the University of Utah.
To all these contributors my best thanks are given. To. The American
Museum of Natural History my thanks are gladly given for the fine
photograph of the eel-grass in the Annulate group, constructed by
Dr. Roy Miner and reproduced in Fig. 274, and for the photographs of
the Spine of Pelee (Figs. 106, 107) taken by Dr. E. 0. Hovey. The
American Geographical Society, through its director Dr. Bowman, also
generously loaned a number of cuts as elsewhere acknowledged, and the
Popular Science Monthly has furnished a number of photographs for
reproduction. Others to whom I am indebted for furnishing original
photographs are Dr. C. P. Berkey, Miss A. D. Savage, Dr. M. O'Con-
nell, Dr. Elsworth Huntington, the late Prof. C. S. Prosser, Dr. C. C.
Mook, and my brother Mr. P. L. Grabau. My former student Dr.
Bela Hubbard has prepared a number of photographs of rocks and rock
structures from original specimens and thereby put me under great
obligation. To Messrs. Dodd jMead and Co., Ginn and Co., Henry Holt
and Co., The Macmillan Co., John Wiley and Sons, and to Yale Uni-
versity Press, publishers of Military Geology, I am indebted for per-
mission to reproduce illustrations from books published by them. These
are acknowledged in the text, as are also the sources of other illustra-
tions, especially Kayser's Lehrbuch; Lake and Rastall, Textbook
of Geology; Le Conte, Elements of Geology; De Martonne, Geographic
Physique; Rosenbusch, Elemente der Gesteinslehre ; LyelTs Principles;
Ratzel, Die Erie; Gray's Botany; Davis, Erklarende Beschreibung
der Landformen; Haug's Traite; Merrill's Contributions to the History of
American Geology; Verrill and Smith, Invertebrates of Vineyard Sound;
Binney and Gould, Invertebrates of Massachusetts; and books by Wal-
ther, Haas, Bowman, Geikie, Zittel, Steinmann, Kriimmel, Murray,
Heim, Suess, J. M. Arms-Sheldon, and others. To the publisher of my
Principles of Stratigraphy, and of North American Index Fossils, Mr.
A. G. Seiler, I am indebted for permission to reproduce a number of
illustrations from these works. Prof. Moses generously permitted the
Preface - xi
reproduction of a number of illustrati6ns, < especially of crystal outlines,
from his Elements of Mineralogy^ published by D. Van Nostrand Co.
Finally, my sincere thanks are due to my publishers, Messrs. D. C.
Heath and Co., for their generosity in giving me a free hand in the
selection of illustrations, in placing no limit upon their number, and in
furnishing a considerable proportion of them.
NEW YORK, June 30, 1920.
CONTENTS
CHAPTER PAGE
I. INTRODUCTION . ... . . . . i
The Science of Geology. The Earth Viewed as a Whole.
II. .SUBDIVISIONS OF THE SCIENCE OF GEOLOGY . 13
Subdivisions of the Science in its Comprehensive Sense.
Subdivisions of the Science of Geology in its More
Limited Sense. Lithology or the Study of the Litho-
sphere.
III. METHODS or APPROACH IN THE STUDY or THE
EARTH 24
The Rise of Geological Observation and Interpretation.
The Field of Geological Observations. The Impor-
tance of Geological Literature.
IV. MATERIAL OF THE EARTH'S CRUST . . .38
The Chemical Elements and Their Primary Combinations.
Minerals.
V. ROCKS, THEIR CLASSIFICATION AND PRINCIPAL
TYPES . . . . . . . .64
Definitions. Age Relations of Rocks. Bed-Rock and
Mantle-Rock. Classifications of Rocks. The Unaltered
or Little Altered Rocks.
VI. THE PRINCIPAL TYPES OF IGNEOUS OR PYROGENIC
ROCKS . 84
The Igneous Magma. Formation of Igneous Rocks by
Cooling of Magma. Types pf Igneous Rocks Based on
Composition and Texture.
VII. MODERN VOLCANIC PHENOMENA . . . . 109
Distribution, Classification, and Development of.Vol-
canoes. Characteristic Forms and Activities of Typical
Modern Volcanoes. Classification of Volcanoes Ac-
cording to Type of Eruption and Form. Geological
Age of Volcanoes and Lava Flows.
xiv Contents
CHAPTER _ . PAGE
VIII.. STRUCTURAL CHARACTERS OF VOLCANOES, AND
OTHER IGNEOUS PHENOMENA. . . , 144
Extinct Volcanoes. Extinct Calderas and Sinks. Volcanic
Funnels and Pipes, Spines, Plugs, and Necks. Sheet La-
vas Formed by Fissure Eruption. Minor Phenomena
Generally Associated with Closing Stages of Volcanicity.
IX. FORM AND STRUCTURE OF OLDER IGNEOUS MASSES 188
Types of Older Igneous Masses. Contact of Igneous
Masses with Other Rocks.
X. THE AQUEOUS OR HYDROGENIC ROCKS ' . . 214
General Character and Varieties. The Textures of Aqueous
Deposits. The Principal Types of Aqueous or Hydro-
genie Deposits.
XI. MODE OF OCCURRENCE AND ORIGIN OF THE
AQUEOUS OR HYDROGENIC ROCKS , . .227
Types of Deposits. Sea- Water and the Evaporation Prod-
ucts and Chemical Precipitates Formed from It. Spe-
cial Conditions Favoring Deposition of Sea-Salts. An
Ancient Rock Salt Deposit Formed by Evaporation of
Sea- Water. Deposits of Salt by Concentration in La-
goons. Bar Theory of Ochsenius. Deposition of Salt
in Inland Desert Basins. Carbonate of Lime Deposits.
Other Chemical Deposits in the Sea. Chemical Depos-
its and Evaporation Products of Lakes. Chemical De-
posits and Evaporation Products of Rivers. Deposits
by Springs and Underground Waters. Mineral Veins.
XII. THE ORGANIC OR BIOGENIC ROCKS . . .269
Bioliths. Types of Organic Rocks or Bioliths. Deposits
of Carbonate of Lime by Plants. Foraminifera and
Foraminiferal Oozes and Limestones. Corals and
Related Reef-Building Animals. Characters and
Types of Modern Coral Reefs. Ancient Coral and
Coralline Reefs. Other Lime-Depositing Organisms.
Organic Deposits of Phosphate of Lime. Organic
Deposits of Silica.
XIII. THE ORGANIC OR BIOGENIC ROCKS: DEPOSITS
FORMED FROM THE ORGANIC TISSUES OF
PLANTS AND ANIMALS . . . . , 328
Deposits Formed from Vascular Plants. Altered De-
posits of Older Vegetal Material, Accumulation of
Contents xv
CHAPTER * AGE
Decaying Organic Matter from Animal Tissues, and
from Non- Vascular Plants. Alteration Products from
Organic Slime Produced by Non-Vascular Plants and by
Animal Tissues.
XIV. ATMOSPHERIC PRECIPITATES AND THEIR DERIVA-
. TIVES . . . . . . , . 354
Types of Atmospheric Precipitates. Compacting and
Modification of Snow. Glaciers. Ice-Caps. Conti-
- nental Glaciers. Icebergs. Causes of Ice Movement.
XV. DESTRUCTION OF ROCKS AND THE FORMATION OF
CLASTIC MATERIAL . . . . 390
Agents Active in the Formation of Fragmental or Clastic
Material. Processes of Erosion. Destructive Work
of the Atmosphere. Destructive Work of the Hydro-
sphere. Destructive Work of the Pyrosphere. De-
struction of Rocks by Movements of the Earth's Crust.
Rock Destruction by Glaciers. Rock Destruction by
Organisms.
XVI. TRANSPORTATION, SORTING, AND DEPOSITION or
CLASTIC ROCK MATERIAL . . . , 43^
Agents of Transportation and Sorting and Regions of
Deposition. Transporting and Sorting by Winds.
Deposition of Wind-Blown Sands. Dust Deposits.
Transporting and Sorting by Streams. River Deposits.
Glacial Transportation and Deposition.
XVII. TRANSPORTATION, SORTING, AND DEPOSITION OP ,
CLASTIC MATERIAL IN THE SEA . . . 509
The Geographical Subdivisions of the Sea. Bathymetric
Districts and Zones. Waves and Currents of the Sea.
Sources of Clastic Sediments Deposited in the Sea.
Transportation and Sorting of Clastic Material in the
Sea. Types of Clastic Deposits in the Sea. Summary
of Structures of Marine Clastics. Lateral Changes in
Fades and Overlap Relations of Marine Clastics.
XVIII. CONSOLIDATION OF CLASTIC MATERIAL ; TYPES OP
CLASTIC ROCKS 5^3
Consolidation of Clastics. Classification of Clastic Rocks,
Structural and Other Characters Used in Defining
Clastic Rock Types. Varieties of Clastic Rocks,
xvi Contents
CHAPTER PAGE
XIX. DEFORMATION OF THE ROCKS or THE EARTH'S
CRUST . . 582
Effects of Deformation. Types of Deformation Struc-
tures. Deformation by Folding. Structures Due to
Folding, Erosion and Renewal of Deposition. The
Causes of Folding. Deformation by Faulting. Topo-
graphic Features Due to Faulting. Minor Features
Accompanying Faulting. Other Structures Produced
by Deformation.
XX. METAMORPHISM AND METAMORPHIC ROCKS . . 642
Definition and Classification of Metamorphism. Activi-
ties of the Agencies Producing Metamorphism. 'Meta-
morphic Structures and Textures. Occurrence and
Age of Metamorphic Rocks. Types of Metamorphic
Rocks.
XXI. MOVEMENTS OF THE EARTH'S SURFACE AND
THEIR GEOLOGICAL EFFECTS . . . 655
Sudden Crustal Movements Earthquakes. Great
Earthquakes of Modern Times. Summary of Phenom-
ena Due to and Accompanying Earthquakes. Slow
Changes in Levels, Bradyseisms.
XXII. THE SCULPTURING OF THE EARTH'S SURFACE . 697
Initial Character of the Land Surface. The Erosion.
Cycle in the Sculpturing of the Land Surface. The
Erosion-Cycle on a Simple Coastal Plain. The Erosion
Cycle on Domes and Basins. The Erosion Cycle on
Anticlines and Synclines.
XXIII. THE SCULPTURING OF THE EARTH'S SURFACE
(Continued) . . . . . -747
The Erosion Cycle in a Faulted Region. Some Illustra-
tions of Complicated River Erosion. Land Forms
Due to Glacial Sculpture. The Sculpturing of the
Edge of the Land. Erosion Forms Produced by At-
mospheric Agencies.
INDEX . 825
PART I
GENERAL GEOLOGY
TABLE "OF THE ERAS AND PERIODS OF GEOLOGICAL TIME
PSYCHOZOIC OR QUATERNARY ERA
Recent or Holocene Period
Pleistocene Period
CENOZOIC OR TERTIARY ERA
Pliocene Period
Miocene Period
Oligocene Period
Eocene Period
Palseocene Period
MESOZOIC (SECONDARY) ERA
Cretaceous Period
Comanchean Period
Jurassic Period
Triassic Period
PALEOZOIC ERA
Permian Period
Carbonic or Pennsylvania!! Period
Mississippian Period
Devonian Period
Silurian Period
Ofdovician Period
Cambrian Period
PROTEROZOIC ANB ARCHEOZOIC ERAS
Pre-Cambrian Periods
(Algonkian arid Archaean)
OF GEOLOGY
PART I GENERAL GEOLOGY
CHAPTER I
INTRODUCTION
THE SCIENCE OF GEOLOGY
Definition. Geology Is the science of the earth in all its aspects,
except those which deal with the relationship of the earth to other
planets and to the sun of our solar system. That aspect of the
earth properly belongs to the science of Astronomy, for our earth
is one of the heavenly bodies with which that science is concerned.
It is true that this phase of the subject is often treated by the geol-
ogist under the name of astronomical geology, but in this book we
shall consider it as belonging primarily to the field of astronomy.
At the same time we shall recognize the importance to the student
of at least a general understanding of these astronomical relations
of the earth, and consider such an understanding a desirable pre-
liminary preparation.
Derivation of Terms. The term Geology is derived from the
Greek words ge (7?), earth, and logia (Xoywx), science, or logical
discourse. The science of geology was preceded by and in a meas-
ure grew out of the subject of geography, which literally is the
description of the earth (gmphein, y/>a<o.i/, to write) and was, of
course, chiefly confined to a description of. the earth's surface fea-
tures, and their significance in terms of human existence. But
since surface features are generally the expression of underlying
structure, and of the geological history of the region, it is evident
that geography cannot be divorced from geology, and that, to no
inconsiderable extent, the student of the geography of a region must
take account of its geological structure and history.
Scope of Geology. In its broadest sense, then, geology is the
science of the earth and all that pertains to 'it It is the study in
2 Introduction
detail of one of the planets in one solar system, by the inhabitants
of that planet, who are themselves a part of it. If other planets
of our solar system or those of other solar systems are inhabited,
the study of those planets by their inhabitants would correspond
to our geology it would be the science of a particular planet.
By us little can be ascertained regarding the structure and develop-
ment of other planets, except in an indirect way, and all investi-
gations along such lines fall properly into the domain of the astron-
omer, though he concerns himself primarily with interrelations of
the heavenly bodies. And thus we may consider that the study
of the physical universe, i.e., the cosmos, which may be called
the science of cosmology, has only the two primary divisions,
astronomy and geology. This may be summarized as follows :
COSMOLOGY
The science of the
1. Astronomy. The science of all the heavenly bodies,
including the earth, their character, distribution, interrela-
tions, movements, etc., and the laws which govern them.
2. Geology. The science which deals with the material,
structure, history, etc., of one of those bodies, i.e., the earth.
Such a view of the science of geology gives it an extremely
comprehensive scope, and we must recognize that in ordinary
parlance the term is used in a much more restricted sense
Nevertheless it is desirable to take this comprehensive view at
the outset, and to note the several subdivisions into which such a
broad science naturally falls, and with all of which the student
of any one division should have at least a general acquaintance.
We shall best get the proper view-point by first considering the
earth in its entirety.
THE EARTH VIEWED AS A WHOLE
Could we view the earth in. its entirety from some extraterrestrial
vantage point, we would recognize it as an oblate spheroid, that is,
a body approaching that of a sphere, but with its polar diameter
flattened and its equatorial belt somewhat inflated. By measure-
ment the polar diameter of the earth is found to be 7899.7 miles,
while the equatorial diameter is 7926.5 miles. If the observer in
extraterrestrial space is sufficiently removed from the earth's sur-
face, that surface would appear essentially smooth, as does the
surface of the moon to our unaided eyes ; the irregularities which
the dweller on the earth recognizes as mountains and valleys would
The Earth Viewed as a Whole 3
become of insignificant proportions. Were we to represent the
earth by an accurately scaled model 10 feet in polar diameter,
the equatorial diameter would exceed the polar by only a trifle
over T 4 7 of an inch, while the highest mountains on the earth's
surface would form elevations on the model less than -$% of an
inch in height. Thus viewed, the prominences which appear to
us formidable are after all of minor significance, and bearing this
in mind, we can understand that relatively slight bulgings or crum-
plings of the earth's surface may produce what to us appear as great
elevations. The wrinklings on the skin of a drying apple consti-
tute far more prominent elevations in proportion to the size of the
apple than do the highest mountain chains on the earth's surface,
when compared with the size of the earth, while the scratches and
notchings formed upon the surface of a glass marble, after a brief
play, form vastly greater depressions, in proportion, than do the
largest river canons, such as that of the Colorado, 01 the deepest
ocean depressions. Thus when the geologist finds evidence that
the summit of the high peaks of the Himalaya Mountains or the
top of the Apennine chain were once a part of the sea-bottom, he
no longer concludes that the sea once covered these mountains, as
was formerly supposed, but he infers rightly that these mountain
chains were raised by a wrinkling of the earth's crust or by an
upward warping which occurred at a time subsequent to that in
which this region was beneath the sea. The evidence from the
material of the mountain which leads the geologist to such a con-
clusion will be discussed in subsequent chapters, and the corrobo-
rative evidence from the structure of the mountains will also be
given. The chief lesson which it is intended to impress upon the
student at present is that surface features are relatively unimpor-
tant when we consider the earth as a whole, and that elevations and
depressions except those which we recognize as continental
masses and oceanic depressions are of local significance chiefly,
and may change from one to the other not once but many times.
There rolls the deep where grew the tree.
10 earth, what changes hast thou seen !
There where the long street roars hath been
The stillness of the central sea.
This is more than the expression of the poet's fancy ; it is a truth
which the observation of the earth and logical deductions from
these observations force upon the geologist's attention at every step.
4 Introduction
The Successive Inorganic Shells or Spheres of the Earth
The Spheres Open to Direct Study. The observer from his
extraterrestrial view-point will, however, note clearly that a large
part of the earth's surface to be precise, about 70.8% of it is
covered with water. This is the sea which is divided into a number
of oceans and surrounds all the lands, and, moreover, forms a
continuous body, the surface of which comprises approximately
137,070,000 square miles or 361.1 million square kilometers. The
land has a surface area of about 59,870,000 square miles or 148.8
million square kilometers, giving a total surface area for the earth
of 196,940,000 square miles or 509.9 million square kilometers.
Certain portions of the land surfaces are also covered by water
bodies, the lakes, which are not in direct connection with the sea.
Among these the Great Fresh- water Lakes of North America, and
the Caspian, a great salt-water lake, are the most prominent ex-
amples. These are to be classed as a part of the land, as conti-
nental water bodies, distinct from the sea. Moreover, the upper
layers of the land in nearly all parts are water-bearing, this water
filling the empty spaces of the soil, and occupying the pore-spaces
within the solid rock. This is the ground water, which is tapped by
wells, mines, or borings, or issues on the surface in springs which
feed the brooks or rivers, or form swamps, ponds, and lakes. It is
thus possible to speak of a nearly or quite continuous mantle of
water which completely covers some parts of the solid surface of
the earth and more or less completely saturates the exposed parts,
This mantle of water may be viewed as an aqueous sphere envelop-
ing the rock sphere of the earth and it is spoken of as the hydro-
sphere. It in turn is enveloped by the sphere of gas and vapor, the
well-known atmosphere.
These two spheres or shells constitute the outer layers of the
earth thus viewed in its entirety, and surround the more rigid mass
of the earth, with which we are most familiar. This consists of
solid rock, and of unconsolidated soil and other loose material, all
of which, except the surface film of organic matter, we may recog-
nize, on examination with a magnifier or by other means, as broken-
down rock material in a fine state of division. From observation
of soundings, dredgings, etc., and by inference, we know that; sim-
ilar material forms the ocean floor, and thus we recognize that be-
neath the hydrosphere is a sphere or a shell of rock material This
The Earth Viewed as a Whole
is called the rock sphere, or lithosphere, and in it are included all
of the soil and other unconsolidated rock material of the earth's
surface. How thick this shell of rock is, we have no means of know-
ing from observation. The deepest borings into the earth's crust
have penetrated only a little over a mile in depth, while the deeper
mines go less than two miles beneath the surface. But from logical
deductions of many observed physical facts, we conclude that this
shell has a thickness of at least 75 miles, and perhaps much more.
It is indeed generally held that the earth is solid rock to the core,
but there are those, who have held, and some who still "hold, that
the central portion of the earth consists of fluid or perhaps even of
gaseous matter.
There are thus three inorganic spheres, the atmosphere, the hy-
drosphere and the lithosphere 9 open to partial observation. The re-
lationships and relative
magnitudes are shown
in the following diagrams
(Figs, i a and 6).
The Inner Spheres.
From the observation
of many phenomena,
geologists and physicists
have reasoned that be-
neath the lithosphere,
other spheres character-
ized by certain peculiari-
ties may be recognized,
though their boundaries
are variable and probably
not very definite. One of these is the sphere or zone in which the
temperature of the earth is sufficiently high to permit the fusion
of rock if the pressure, which ordinarily keeps it solid by raising
the fusing point, were removed or lowered by some structural change
in the earth's crust. There may be regions in which essentially
permanent pools of molten rock exist within the crust, forming
feeders of volcanoes, but the. majority of such feeding areas are
more probably temporary. It is convenient to speak of this more
or less concentric zone as a distinct sphere and the name pyrosphere
has been applied to it.
Another equally indefinite sphere or zone, variously regarded as
FIG. i a. Diagram of the successive in-
organic spheres of the earth. The heavy
black line represents the lithosphere, including
the hydrosphere, taken as 75 miles. The
white semicircle above it indicates the atmos-
phere. C, center of the earth. The part in-
cluded between the two radii, 2 degrees apart,
is enlarged in Fig, i b.
Introduction
Height of Armosphere
100 miles
lying from 30 to 75 miles beneath the surface (according to our
estimate of the thickness of the lithosphere), is one of relative weak-
ness, between a strong external crust and a rigid central mass. Here
the rock yields most readily under long-
enduring strains of limited magnitude,
and earth-movements, resulting in the
deformation of the rocks, occur. This
zone or sphere, which has been called the
tectosphere (Murray) or the asthenosphere
(Barrell), may in part include the pyro-
sphere; nevertheless, it is convenient to
speak of it as distinct.
Finally there remains the central portion
of the earth, the centros'phere, which is by
far the largest part of it, and about which
we know nothing from observation and but
little from inference. From the known
rate of temperature increase in borings
and mines, it is inferred that the temper-
ature near the center of the earth may
be between ^ 200,000 and 350,000 degrees
Fahrenheit, a temperature so high that
were it not for the enormous pressure,
all the rocks there would probably be
vaporized. From the fact that the aver-
age specific gravity of the rocks which
constitute the earth's crust is only about
2.6 while that of the earth as a whole is
5.6, it is further inferred that the material
of the centrosphere is very heavy and it
has been calculated that if there were a
Pyr'o sphere
d.nd
Aarhenosphere
FIG. i b. Enlargement
of a part of the sector
shown in Fig. i &, to show
the relative thickness of
the crust of the earth, the
mean and greater depths
of the sea and the mean
and great heights of the
land. Approximate scale i
inch ==iio miles. On this
scale the two radii meet at
a distance of about 3 feet
from the surface of the
hydrosphere (black) this
point then representing the
center of the earth.
steady increase in the specific gravity of the material from the
surface toward the center, this would, at the latter point, become
n.2, which suggests the possibility of a central core of iron, if not
of gold and platinum or some other heavy metal. Because of
'this greater weight, the centrosphere is also spoken of as the bary-
sphere or heavy sphere.
. The Earth Viewed, as a Whole 7
The Organic Sphere or Biosphere
To these inorganic or lifeless spheres there is added the organic
sphere, or sphere of life, the biosphere (bios, /?*, life). It would
surprise the average student could he see the universality of this
organic shell of the earth. In arid out among the mass of the water
and of the air and upon and through the upper layers of the rocky
surface, the thread of living matter is woven, now constituting an
almost solid mass of tissue, as in the bodies of peat or in the dense
vegetation of a forest, OT the almost continuous tangle of seaweed
or layer of floating organisms in the sea ; again forming a network,
the meshes of which vary greatly in size, but are on the whole con-
tinuous, except where momentarily broken by the hand of man,
or by some abrupt, not to say cataclysmic disturbance, such as an
avalanche or outpouring of a mass of lava. But left for even a
short time, the steepest quarry wall, or the most precipitous cliff
formed by the sudden dislodgment of a mountain side or by the
sinking of a portion of the earth's crust, such as may take place
during an earthquake-producing disturbance, will be again taken
possession of by some form of plant, if not of animal life. Even
the stony lava field will be covered in time by a succession of organic
forms of increasing complexity. The apparently barren desert, too,
has its wide-meshed net of living beings, except perhaps where the
sands are constantly in motion ; and the presence of these organisms
is often shown by the countless tracks and trails which appear upon
the. surface of the sand on a dewy morning, or the sudden springing
up of vegetation from hidden seeds when moisture furnishes the
condition for expansive growth. And in the sea, life of some form
is never wanting long, here covering the bottom with a continu-
ous living carpet, there forming an ever changing web of floating
animal and plant life, through which the swimming world of animals
weaves an intricate pattern with the threads of its never ceasing
wanderings.
Thus it is perfectly in accord with the facts, when we speak of
a shell or sphere of living matter, the biosphere, which surrounds
the lithosphere and penetrates its upper layers as well as the hydro-
sphere and the atmosphere, but is distinct from all. That such a
biosphere has existed throughout most of the past ages of the earth's
history is clearly shown by the countless remains of the hard parts
of animals, such as the shells of mollusks, the bones of vertebrates
8
Introduction
and the like, which fill many of the rocky layers of the litho-
sphere, and indeed sometimes actually compose these layers.
For example, the white
chalk of the English and
French coastal regions is
literally made up of the
shells, and fragments of
shells, of organisms (Fora-
minifera, coccoliths, etc.),
as shown in the following
illustration (Fig. 2), and
many of our great lime-
stones are consolidated re-
mains of shells of animals
which formerly inhabited
the seas, or fresh- water
In the following
, N . ,
(Fig. 3) IS shown an
FIG. 2. Thin sections of chalk, as they
appear under the microscope, showing the
shells etc., of minute marine organisms
(chiefly Foraminifera and coccoliths) of
which the rock is composed. A, Chalk enlargement of a group of
from Sussex, England, enlarged 60 times. m i nute needle-like shells,
B. Chalk from Farafrah, Libyan desert, , r , . - , ..^
enlarged 60 times; the most characteristic each one of which was built
shells are Textularia (a) and Rotalia (5) . by a separate marine animal
C, Dried residue of milky chalk water with and of which a H me stone 3
coccoliths enlarged 700 times. (After , . . ,
Zittel ) traceable over wide areas
in the state of New York,
is almost entirely composed. From careful study, it has been de-
termined that a cubic inch of this rock, when pure, contains about
40,000 individual shells (J. M. Clarke).
Whole mountains are sometimes
made up of rock, which is largely, if
not entirely, the product of accumu-
lation of the hard structures of former
organisms. An example of this is
found in the famous Dolomites of the
Tyrol (Fig. 4), the chief rock masses
r ,. , * r i
of which are made up of calcareous
matter separated from a sea by marine (pteropod) shell Fragment
plants (Fig. 5), and the many centuries with numerous individuals
, . , / XJ . J enlarged three times; and a
of time during which the present forms specimen much enlarged.
of these peaks were carved from this (After HaH)
^ ,. ,.
FlG - 3- Styhohna fissu-
a minute molluscan
The Earth Viewed as' a Whole
IO
Introduction
rock have witnessed only a partial obliteration of the organic
structures, though no inconsiderable portion of the mass has been
removed.
The remains of animals and plants which
compose, or are included in, the rocks of the
earth's crust, are known to the geologist as
fossils (from the Latin fossilis, something that
is dug up) and their study forms an important
and integral part of the science of geology.
It is indeed the study of the constantly
changing elements of the biosphere, of which
the existing animals and plants form only the
pora porosa Schafh. most mo dern phase, that has made possible
A calcareous alga or .,,.,. - ,, , . , - ,. A ,,
marine plant which th( ; deciphering of the history of the earth.
was chiefly respon- This subject will be more fully considered in
sible for the making later chapters of this book,
of the rock out of ^ , . , , . , x -
which the Dolomites ^eds of coal > too > P omt to the former ex-
are carved. A, Nat- tensive accumulation of vegetable material,
ural size. B, en- an( j ^ Q ^ O f our Q y s h a i es an< J pools owes its
origin largely, and perhaps altogether, to the
distillation, during many centuries of time, of
pores, p ; r, constrict- buried organic matter which constituted a
ing rings character- . ' . .
istk of this genus. P ortlon ot the biosphere of former geological
periods.
^
larged 3 times, a.
canals which appear
upon the surface as
(After Steinrnann.)
Contributions to the Lithosphere from Other Spheres
We now see that the biosphere contributes no inconsiderable
portion to the growing lithosphere, and this is true to a certain
extent also of several of the other spheres. For not only was the
carbon of ancient plants, which, is now locked up in the earth's
crust as coal and oil, contributed by the atmosphere of the past,
but the water vapor of the air, freezing and descending as snow or
hail, or. crystallizing directly upon cold surfaces, forms a not unim-
portant, though in most regions very transitory, addition to the
solid rock of the earth. So, too, the conversion of water to ice forms
rock at low temperature, for ice is a rock. More permanent con-
tributions are made by the crystallizing of various salts, chief among
them the common salt (sodium chloride), which, as we shall see
later, may become buried by other rock material, and be preserved
The Earth Viewed as a Whole ' n
for millions of years as an essential part of the earth's crust. Thus
the salt now mined in New York State and in southeastern Mich-
igan was derived from the waters of an ancient sea, the deriva-
tion being by a roundabout process, which will be detailed later.
That this occurred in a period of time many millions of years ago
is amply shown by the ascertainable facts. So, too, the great salt
deposits of North Germany, which have furnished the world in the
past with most of its potash, are the product of the evaporation of
cut-off portions of an ancient sea which covered much of Europe,
though the period of its production is not quite so remote as that
of the best-known American salts. (See Chapters XXXIV and
XXXVIII.)
That the pyrosphere also contributes its quota to the rock mass
of the earth is abundantly shown by the vast extent of ancient
lavas and other igneous masses which now form a large proportion
of the solid crust of the earth and are visible to us as the result of
surface cooling in comparatively modern times, as in the case of
the great Columbia lava fields of Northwestern America and the
preserved fragments of similar flows on the British coast (Staffa,
Giants* Causeway, etc.), or which were uncovered by the removal,
during long geological epochs, of the overlying portions of the
earth's crust beneath which the ancient igneous masses assumed
their solid form.
Throughout the entire history of the earth, the lithosphere has
received additions by the cooling of molten rock material injected
into it or poured out on its surface, and this addition is in progress
even to-day.
Contributions to the Lithosphere from Outer Space
Finally it may be added that the earth's crust receives to-day,
and undoubtedly has received in the past, additions from the cosmic
spaces which surround our atmosphere. When a meteor reaches
the earth it forms an integral part of the unconsolidated or discrete
portion of the earth's crust, at least until it is gathered in by the
discerning collector of these messengers from the extraterrestrial
spaces.
Summary of Spheres
We may now summarize in tabular form the several spheres 01
shells into which the earth as a whole is divisible, and add to this
summary the derivation of their names.
12 " Introduction
A. The Inorganic Spheres, (arcfxupa) = sphere.
1. Atmosphere, Gr. atmos (<XT/AOS) = vapor or gas,
2. Hydrosphere, GiJiydor (vSwp) = water.
3. Lithosphere, Gr. Itihos (Xldos) = stone.
4. Pyrosphere, Gr. ^yr. (?r'p) = fire.
5. Asthenosphere, Gr. asthenos (cxa-^evT/s) = feeble.
6. Centrosphere, Gr. centron .(Kwrpw) = center,
or Barysphere, Gr. 6ary^ (papvs) = heavy.
B. The Organic Sphere.
7. Biosphere, Gr. Ji<w (/?>$) = Iff e.
CHAPTER II
SUBDIVISIONS OF THE SCIENCE OF GEOLOGY
IT is but natural that the mind of man, always eager to enter
into details, should have developed distinct branches of inquiry
into the character and history of the several parts of the earth
which is his home. In the beginning of such inquiry, a single mas-
ter mind may have encompassed the entire field ; but with the in-
creasing mass of detail, the individual range became limited to
special portions of the field, and in the course of time the several
lines of inquiry into the nature and history of our earth developed
into distinct sciences, each with its host of devoted searchers after
truth.
SUBDIVISIONS OF THE SCIENCE IN ITS COMPREHENSIVE SENSE
We may, then, at this point of our study, endeavor to ascertain
the natural subdivisions of the Science of the Earth, or Geology
in its broadest sense, and after that confine ourselves to those
branches which by common consent are reserved as the special
field of the geologist, as it is understood at the present time.
Atmology or Meteorology. From an inspection of the table of
spheres into which the earth may be divided, we form the natural
conclusion that the first line of cleavage would be in accord with
these natural subdivisions. The study of the atmosphere -has now
developed into a separate science, which is familiarly known as
Meteorology, because the meteors, or extraterrestrial bodies, the
familiar " shooting stars," which on entering our atmosphere be-
come luminous through friction and frequently reach the earth as
meteorites, have from the remotest days formed one of the chief
attractions for those whose gaze was turned away from the solid
earth. Thus atmospheric phenomena came to be designated as
meteoric phenomena and the science of these phenomena became
meteorology. But since the sphere with which this science is con-
cerned is the atmosphere, a more appropriate name for this science
itself is Atmology.
13
14 Subdivisions of the Science of Geology
Hydrology. That the hydrosphere, that is, the' oceans and
other water bodies, early attracted the serious-minded student
was but natural, since these water bodies form the natural high-
ways of commerce, and since they furnish so large a part of the
earth's population with the means of a livelihood. Thus the science
of the water, or Hydrology, was developed, which is variously sub-
divided again into the sciences of the oceans, or Oceanography, and
into the sciences of the lakes, the ponds, and the rivers. That some
of these, for example oceanography, have been developed to a
remarkable extent, is shown by the great oceanographic institutes
of the world, of which that of Berlin under Director Penck, and that
at Paris, under the direction of the Prince of Monaco, are the best
known. In America, oceanographic studies have chiefly been car-
ried on by the federal government and by private individuals,
among these the late Alexander Agassiz ; and there is a hydrographic
department of our government, the chief business of which is the
charting of our sea-coasts, lakes and rivers.
In England, too, oceanographic studies have been largely fostered
by the government, under whose auspices the famous Challenger
Expedition, for the exploration of the oceans, under the leadership
of Sir John Murray, was carried to successful completion. The
governments of other countries, too, have fostered exploring ex-
peditions for oceanographic research, these being generally known
by the name of the vessel employed in the expedition.
The student of geology, even in its narrower sense, must keep
abreast of these inquiries, at least to a moderate extent, for without
them many of his discoveries lack the clarifying light by the aid of
which he must interpret them; and in proportion as the researches
of the oceanographers and the other hydrologists become available,
the analysis of the structure and composition of the earth's crust,
the special field of the geologists in the modern sense, and the
interpretation of these facts in terms of earth history, become
more precise.
Lithology or Geology Proper. As has just been said, the special
field of inquiry of the modern geologist is the crust of the earth, or
the lithosphere. He is thus primarily a student of Lithology
though that term has also been used in the past in a narrower
sense, the study of rocks per se. It will be well, however, for the
sake of unity of terminology a great desideratum in every science
to adopt this term in its wider sense, namely, that which enoom*
The Science in its Comprehensive Sense 15
passes all the fields of inquiry which are concerned with the com-
position, structure, and history of the earth's rocky shell, the Utho-
sphere, generally assigned as the only legitimate field of the
modern geologist. Still, to-day more than ever, the geologist can-
not limit his inquiries to the facts disclosed by the rocks alone,
but must draw his conclusions with the aid of a wide knowledge
of the researches of the atmologist, the hydrologist, and even the
biologist.
Biology ; Palaeontology. Biology is the science of living
things. It is the study of the biosphere, and has perhaps been more
sedulously developed, and has a larger body of votaries, than any
other branch of the earth science. Biology, as currently under-
stood, limits itself to the study of modern living beings, the plants
and animals of to-day, and naturally falls into the two divisions, or
separate sciences, of Botany (Phytology 1 ), the study of plant life,"
and Zoology, 2 the study of animal life. Biologists ar,e, however,
aware of the fact that the modern world of plants and animals is
only a fragment of the great host of living beings which has in-
habited the earth from the remotest ages; it is the life record of
the youngest and shortest chapter in the history of the earth.
Throughout all the past ages, living forms existed upon the earth,
and their remains, as we have already seen, are embedded in the
rocks of the earth's crust as fossils. The study of these was at
first left to the geologist, but has now developed into a separate
science, that of Palaeontology (from the Greek palaios [TraAcuos],
ancient, onta [ovra], living beings, and logos or logia [Aoyta], science).
Other Divisions. When we come to the consideration of the
inner spheres of the earth, we must admit that, owing to our in-
ability to make direct observations, no separate sciences worthy
of that designation have yet been developed. It is true that a
science of earth disturbances or Seismology exists,* and that this
properly belongs to the study of the asthenosphere or tectosphere.
But the observations on which this science is based are chiefly
possible upon surface manifestations and the effects which these
produce, though inferences can also be drawn from the study
of disturbances which have occurred in the past, the effects of
which are recorded in the rocks. The same is true of the study of
the pyrosphere which can only be approached through observa-
1 Greek phyton [<j>vT6v]j plant. 2 Greek . so$n .\fvov], animal.
16 Subdivisions of the Science of Geology
tions of modern volcanoes (Vulcanology) and of the temperatures
in deep borings and mines. Of the interior of the earth we may
perhaps never know much through observation, and so deduc-
tions from physical phenomena alone must guide us. No separate
science of the centro- or barysphere has thus been developed.
From the foregoing, the student is led to the realization of the
truth that all true sciences are based on two great fundamentals,
observation and deduction. To observe and record facts alone does
not constitute the whole of the work of the man of science. Such
used to be the sole aim of the older naturalists, so called, who
dissented from the attitude of the ancient philosophers, because
these based their speculations on mental processes rather than ob-
servation of facts. To-day, however, we know that the inter-
pretation of facts, in the light of our growing knowledge of causes,
is as legitimate, indeed as important, a function of the student of
the earth as the observation of facts, and moreover a certain amount
of speculation always held in check by the appeal to facts is
not only desirable but necessary. The student of nature must be
a philosopher in the true sense of the word, and so long as he does
not lose sight of his fundamental base the observation of facts
his work will gain in value by allowing his reasoning faculties their
fullest play.
SUBDIVISIONS OF THE SCIENCE OF GEOLOGY IN ITS MORE LIM-
ITED SENSE. LITHOLOGY OR THE STUDY OF THE LITHOSPHKRE
Restricting our attention now to the more limited field to which
the modern geologist applies himself, namely the study of the ma-
terial, structure, and history of the earth's crust, or the lithosphere,
we may note at the outset that there are two phases or aspects of
this field, one or the other of which is cultivated more sedulously
by the geologist according to his predilections or circumstances,
but neither of which can be wholly neglected by the worker in
either field. These two aspects are the purely scientific, and the
practical or applied. The geologist who works in the pure science
field of his domain does so primarily for the intellectual satisfac-
tion derived from the discovery of facts and principles. His aim
is chiefly to search for nature's truths irrespective of their bearing
on human welfare, and his principal endeavor is directed toward
widening the boundaries of human knowledge and pushing forward
The Study of the Lithosphere 17
the frontiers of discovery. The chief aim of the practical geologist,
on the other hand, is directed toward making the forces and ma-
terials of the world available to man, to augment the welfare of
the human race, and to push forward the boundaries of civiliza-
tion. But this work, important and noble as it is, can only be
carried on successfully in proportion as the facts and laws of. the
science are discovered and demonstrated, and the practical geolo-
gist must forever depend on the worker in pure science, whose re-
ward too often is little more than the satisfaction which is to be
derived from a devotion to the search for truth. We shall for the
sake of convenience speak of the pure-science phase of our subject as
the scientific aspect of geology, and of the applied phase of geology as
geology in its relation to man. Each phase has its special subdi-
visions, to which attention may now be drawn.
I. The Scientific Aspect of Geology
Structural Geology. A logical analysis of this field of investi-
gation reveals a threefold aspect and three corresponding methods
of approach in study. In the first place the material of the earth's
crust, and the composition and the structure of this material, In-
vite attention. This portion of the subject is generally treated
under the caption Structural Geology. It takes account of the com-
position, form, and architecture of the earth's crust, and is primarily
an analytic branch of the science. From this point of view the
earth's crust may be considered under the following subdivisions,
given in the order of their magnitude.
Subdivisions of Structural Geology. These include the following
fields:
1. Chemical elements and ions.
2. The combination of these elements and ions into salts and
other compounds which either take on crystalline form or remain in
an uncrystalline (amorphous) condition. These compounds as they
occur in nature are, designated minerals.
3. The combination of crystals or fragments of the same or
different minerals into large masses, either bound together into a
more or less solid mass, or remaining in an unconsolidated condi-
tion. These are the rocks of the earth's crust, and for their study
a recognition of at least the principal component minerals is
essential.
i8 Subdivisions of "the Science of Geology
, 4. The association of rock masses into original or primary struo
tural units, which constitutes the primary architecture of the earth's
crust; or the impression upon it of secondary structures , through
deformation or other influences of an outside nature. This is struc-
tural geology in the narrower sense and has also been called
Geotectology. The original structure corresponds to the initial
architecture of a building, the stones or bricks of which correspond
to the rock masses of the earth's crust. The secondary structures
correspond to the changes subsequently made by additions, removal
or modification resulting from sagging with age, etc.
5. The surface forms resulting through the activities of destruc-
tive as well as reconstructive forces, and recognizable in the moun-
tains and valleys, the plains and plateaus, and other physical
features, generally considered under the subject of Physical Geog-
raphy. The term Geomorphology or the study of the details
of the earth's surface forms Has been commonly applied to this
branch of inquiry /when it is not merely descriptive or analytic,
but takes account of the history or genesis of the form as such. It
corresponds to the study of the form of a building as a whole, either
in its completeness or when changed to a ruin by destructive in-
fluences.
The study of most of these divisions of structural geology has
been carried to such detail that separate sciences have been devel-
oped. Thus the study of the natural substances, or minerals, has
become the science of Mineralogy, and that of the rocks the science
of Petrology (also sometimes designated as lithology in its narrower
sense), while the science of rock structures is Structural Geology in
the narrower sense, and that of the surface forms is Physiography r ,
and their relation to man is Geography.
In all such studies, while emphasis is laid on the analytical and
descriptive sides, the causal or dynamic side and the historical or
developmental side are given their due attention. This is expressed
by the preference of terms ending in ology (mineralogy, petrology,
geotectology, geomorphology) over those ending in graphy (petrog-
raphy, physical geography, etc.), which emphasize mainly the
descriptive and analytical side.
Dynamical Geology. - A second mode of approach is that which
lays the emphasis upon the forces that work upon and within the
earth to produce results, which, in this view, take a secondary rank.
This is Dynamical Geology and it naturally falls into the two great
The Study of the LIthosphere 19
divisions, the physical and chemical. The sciences of physics and
chemistry, in so far as they lay the stress upon the forces at work,
are the specialized development of these aspects. That chemistry,
in its analytical as well as its dynamic aspects, is of fundamental
importance to the geologist has generally been recognized, but the
importance of physics to the student of earth science has only re-
cently received- proper recognition by the establishment of geo-
physical laboratories.
Both chemical and physical forces may be viewed as constructional,
or those building up rock masses and rock structures, and as de-
structional, or those destroying them. A third view of these forces
is that which deals with their effects in modifying or deforming
the materials and structures, and this may be termed the reconstruc-
tional or deformational aspect.
Historical Geology or Geogenesis. The third method of ap-
proach, is the historical or evolutionary method, in which emphasis
is laid" upon development and the causes which underlie this de-
velopment. It is apparent that this phase of geology is the latest
and most specialized aspect, and that for its proper prosecution a
thorough preparation in the other two phases is needed. More-
over, since the history of life upon the earth is intimately bound
up with the history of the lithosphere, a knowledge of biology, and
especially of palaeontology, is indispensable for the prosecution
of any but the most general studies in earth history.
Several special branches have been developed within this field.
One of these deals with the origin or genesis of the rocks and their
structures, or the origin of the lithosphere. To this branch the
name Lithogenesis is commonly applied. Another branch considers
the character, arrangement, succession, order of formation, and
age relations of the stfatified rocks. This is the science of Stratig-
raphy, primarily a descriptive one. A third branch deals with the
succession and distribution of the organic remains, the petrifactions
or fossils in the rocks in so far as they have a bearing on the geolog-
ical history of the earth, i.e., the index fossils. This is the special
geological phase of Paleontology, which has also been designated
by the name Petrifactology (Haeckel). Again, there is the science
which deals with the development or genesis of the surface forms
of the earth not merely in the descriptive manner, but from the
view-point of origin. This, as already noted, is Geomorphology in
its proper sense, though it is more commonly known by the name
20 Subdivisions of the Science of Geology
of Physiography, which formerly had a very different meaning.
Finally there is the study of the changes in the earth's surface, or
- its geography through the successive ages of the earth's history,
together with the changes in climate and the dispersions and
migrations of the organisms and the causes which effected these.
This is the latest of the several aspects of Historical Geology and
is now termed Palceogeography or the geography of the past. In
this field the palaeogeography of the Pleistocene period has been
most extensively studied, and a separate branch, that of glacial
geology, has been developed. Geography, in the usual sense of the
term, is the geography of the modern or Holocene period of the
earth's history.
II. Geology in its Relation to the Welfare of Man
So far we have been considering geology in its pure science aspect,
that which appeals to the inquiring mind of man in search after
truth and knowledge, without ulterior motives of usefulness. There
is, however, another aspect of our science, and one which in recent
times has come strongly to the front. This is applied geology, in
which geological facts and forces are viewed in their relation to
the needs and requirements of man. As has already been intimated,
the application of any science for any purpose whatsoever is success-
ful in direct proportion to the profundity of knowledge possessed
by the applier. No successful exploration of geological products or
application of forces is possible without a thorough understanding
of the facts and principles of the science, and the student who
wishes to follow the applied side of his science should not fail to
make his preparation in the pure science side as broad and as pro-
found as circumstances will permit.
Among the earliest problems, to the solution of which geological
knowledge has been applied, are those of mining. Indeed, the
science of geology, in a measure, developed in response to the needs
of the miner for accurate knowledge of the conditions of occurrence,
distribution, and mode of origin of the valuable mineral deposits,
To such an extent has this been carried, that a separate branch of
mining geology has come into existence. Moreover, as our knowledge
increased and the possibility of more detailed application of our
science became apparent, special subdivisions of mining geology
have been developed, and it is found that individuals can profitably
The Study of the Lithosphere 21
devote themselves to the cultivation of a narrow field, to the prac-
tical exclusion of the others. Thus there have been developed the
branches of coal geology, of petroleum and gas geology, and of salt
geology, including the geology of potash, phosphates, nitrates, borax,
and other salts, and of bauxites and other aluminum ores. In
these branches the investigator confines himself to the problems
involved in the occurrence of these substances, which are chiefly re-
stricted to the stratified rocks. It is now well recognized that suc-
cessful search for such deposits can only be undertaken by one well
versed in the science of stratigraphy (including index fossils) and
structural geology, while a thorough understanding of the prin-
ciples of physiography and palaeogeography is almost indispensable.
The mining geologist who devotes himself primarily to the problems
of the metallic deposits must have not only a thorough knowledge
of mineralogy, petrology, and structural geology, but of dynamic
geology as well, and especially of the chemical and physical principles
involved in ore deposition. As many ores are also found in strat-
ified deposits, a knowledge of stratigraphy and of index fossils
is necessary, while an understanding of the principles of physiog-
raphy and palaeogeography will also be found of value in many
cases.
Geology, too, plays an indispensable role in the solution of many
important engineering problems. In the construction of the Cats-
kill aqueduct for New York City, a force of competent geologists
was constantly employed, and specialists were frequently called
upon for consultation. This same need was felt after the con-
struction of the Panama Canal had been undertaken, and a resident
geologist was appointed to supervise the later phases of construc-
tion. That many difficulties might have been avoided had such
supervision existed from the outset of the undertaking, is now
generally conceded.
Geological advice has always been employed in the construction
of great tunnels such as those piercing mountains or passing under
rivers and other water bodies. In many cases, too, the selection
of sites for bridges, dams, and other great engineering works has
been based on geological advice, while in other cases, where such
advice has not been sought or has been disregarded, disastrous
consequences have resulted. In consequence of the growing recog-
nition of the need of geological study in the undertaking of engi-
neering problems, the special branch of engineering geology has been
22 Subdivisions of the Science of Geology
developed. The geological engineer must be primarily a struc-
tural geologist and one who has a thorough grasp of the princi-
ples of dynamic geology, including hydrology as well, while physi-
ography too is of great importance to him. To a lesser degree a
knowledge of rock types, of stratigraphic principles, and of Index
fossils will be needed by him, and not infrequently a knowledge
of palaeogeography, especially that phase which deals with the
Pleistocene, or the problems of the glacial period, will be found of
the greatest value.
Finally there has been developed in recent years the special
branch of geology which deals with the problems involved in mili-
tary campaigns, and to this the name war or military geology has
been applied. Some of these problems are concerned with the
proper location of sites for camps, and trenches, and with water
supply and sanitation, and for these a knowledge of structural
geology, of rock types, of stratigraphy, and of glacial geology has
been found necessary. Other problems are of an engineering type
and require the preparation of the geological engineer. Again, the
problems involved in military operations need -for their solution
a well-trained physiographer and a competent meteorologist as
well. Problems concerned with naval warfare require the atten-
tion of one well versed in hydrology, especially that phase of it
which deals with the oceans, or oceanography.
There are other ways in which a knowledge of geology has be-
come useful to man, and as the science itself is developed new
channels of application Into which it may be directed will no doubt
be discovered.
III. The History of Geology
The student should further realize that the development of his
science, the history of geologic thought, cannot be neglected by
him. We profit by the mistakes of our predecessors as much as
we do by their achievements, and the history of the discovery of
facts and of the development of geological opinion since the days
of the Greek philosophers is fraught with lessons equal in Import
to those gained from the pursuit of the history of any other depart-
ment of human thought and endeavor. At this point It will be
desirable for the student to read the masterly sketch of this history
from the pen of Sir Archibald Geikie, the book entitled Found-
ers of Geology, and if possible follow this by a perusal of the older
The Study of the Lithosphere 23
"historical sketch by Sir Charles Lyell in volume I of his Principles
of Geology. For greater details the student is finally referred to
the History of Geology and Paleontology by Carl von Zittel,
translated into English by Maria Ogilvie Gordon. The history of
geology in America is adequately and fully treated by Dr. George
P. Merrill in his book, Contributions to the History of American
Geology.
CHAPTER III
METHODS OF APPROACH IN THE STUDY OF
THE EARTH
THE RISE OF GEOLOGICAL OBSERVATION AND INTERPRETATION
THE geologist is, above all things, an observer in the great out-
of-door world. The man whose horizon is bounded by the walls
of a city can never be a geologist,
though he may gain much scien-
tific knowledge from books and
from an inspection of collections
in museums and laboratories.
The true geologist, however, goes
directly to the earth and there
begins his inquiries. Not until
observations of natural facts and
phenomena were made in extenso
was the inquiry of the philoso-
phers regarding the earth and
its history placed on a scientific
basis. Scattered observations
and more or less accurate de-
ductions were made even in
antiquity. Thus Aristotle, in
the third century B.C., had a
very considerable understanding of the work of rivers and reasoned
correctly regarding the changes in the land and sea at former times.
The painter Leonardo 'da Vinci (1452-1519) correctly reasoned, from
an observation of the fossils found in the foothills of the Apennine
Mountains, that they were the shells of once living animals, though
they were generally regarded either as freaks of nature (lusus natura)
or as modern shells dropped by the pilgrims in their voyages across
these mountains. It is true that the significance of fossil sea shells
was recognized by the Greek philosophers but their explanations
24
FIG. 6. Georges Leopold Chretien
Frederic Dagobert Cuvler.
The Rise of Geological Observation
FIG. 7. Jean Baptiste Pierre Antoine
de Monet de Lamarck.
were generally ridiculed during the Middle Ages. It was, however,
not until the latter part of the eighteenth and the early part of the
nineteenth century that sys-
tematic investigations of the
rocks of the earth and their con-
tained fossils began, and from
this period we date the birth of
geology as a science. Scientific
geology arose more or less sim-
ultaneously in the different
countries of Europe. In France
Etienne Guettard (1715-1786)
and Nicholas Desmarest (1725-
1815) were among the first to
bring observation of facts to the
fore, while Buffon (1707-1788)
indulged in brilliant speculations
on the origin of the earth. Later
Alexander Brongniart (1770-
1847) investigated the rocks
around Paris, and Georges Cuvier (1769-1832, portrait, Fig, 6) and
the Chevalier de Lamarck (1744-1829, portrait, Fig. 7) described
their fossils. In Germany Abraham
Gottlob Werner (1750-1817, portrait,
Fig. 8), who is often called the founder
of German geology and who was Pro-
fessor at the Mining School at Frei-
berg i. S., exerted a prof ound influence
on geology especially by his teachings,
to which men flocked from all coun-
tries. Being mostly an observer of
the details of specimens and rarely
. venturing beyond his immediate sur-
roundings for field observations, many
of his geological deductions have
proved erroneous though his pupils
and followers, notably Leopold von
Buch, extended their observations
over wide areas and added much to the store of geological facts
as well as to its philosophy. Switzerland had its enthusiastic
\
FIG, 8. Abraham Gottlob
Werner.
26
Methods of Approach
FIG. 9. James Hutton, M.D.
student of the structure and history of the Alps in the person of
H. B. de Saussure (1740-1799), and Russia had in Pierre Simon
Pallas (1741-1811) its careful
student of the Ural Mountains,
and the rocks outcropping there
and elsewhere in the empire.
In Great Britain many men con-
tributed to the discovery of facts
and the interpretation of these.
Among them the first rank is
given to James Hutton (1726-
1797, Fig. 9), whose work marks
a turning point "in the history of
geology, for he insisted that
" the past history of the globe
must be explained by what can
be seen happening now, or to
have happened only recently/' 1
a dictum which has since become the very cornerstone of geology.
Button's great work, Theory of the Earth with Proofs and Illustra-
tions^ better known through the classic volume, Illustrations of the
Huttonian Theory, by his friend John Playfair (1802, Fig. 10),
which no student of geology should
neglect to read. In this work are
contained many of the fundamental
principles with which geologists
are concerned to-day, and they are
illustrated by a wealth of facts
gleaned by Hutton from his ram-
bles through Scotland and other
countries.
Another of the Scottish founders
of geology was Sir James Hall, to
whom we owe the origination of
experimental geology. The best
known, however, among the early
English geologists was William
Smith (1769-1839, portrait, Fig. n), who is generally called the
"Father of English Geology." He determined not only the correct
1 Geikie, Pounders of Geology, p. 299.
FIG. 10. John Playfair.
The Rise of Geological Observation
27
FIG. ii. William Smith.
successions of the English rock formations, and made the first
geological map of England, but gave to many of the formations
the names which they bear
to-day.
Finally, the student should
remember among English geolo-
gists the name of Sir Charles
Lyell (i797~ l8 75> portrait, Fig.
12), as that of a man who has
had the most profound influence
on geological thought. His
great work, the Principles of
Geology, has become a classic of
geological literature.
Among the men who exerted a
profound influence on American
geology in the early days of its
development, the names of William McClure (portrait, Fig. 13)
and Amos Eaton (portrait, Fig. 14) stand out prominently, Mc-
Clure, born in Scotland in 1763, became an American citizen near
the close of the century. In 1809 he published the first important
work on American Geology, in >.
which appeared the first geolog-
ical map of the Eastern United
States, and one of the first
geological maps of the country.
Amos Eaton (1776-1842, Fig.
14), born in. New York state, Is
known especially for his Index
to the Geology of the Northern
States (1818), which was the
first geological textbook pub-
lished in America, and in this
and subsequent works he laid
the foundation for the New
York geological system. Many
FIG. xa. - Sir Charles Lyell of the names of American for-
mations still current were first
applied by him. He also made the first geological map of New
YorM. The important influence which McClure and Eaton had
Methods of Approach
FIG. 13. William McClure.
on the development of American geology has been recognized by
the designation of the first two eras in the history of this science
in America as the Maclurean
(1785-1819) and the Eatonian
(1820-1829) (Merrill). Other
early American geologists will
be referred to in later chapters.
THE FIELD OF GEOLOGICAL
OBSERVATIONS
" Where, then/' the student
will ask, "can the facts of
geology be observed, and how
have they become available ? "
To get at the facts of structural
geology the student must go
to the rocks. True, the rocks
and the minerals and the fossils are brought to him in museums
and laboratories, and he will do well to begin his studies of selected
examples thus brought together and .capable of being examined
under the most favorable conditions. But the knowledge thus
gained must be amplified and correlated by repeated visits to the
home of the rocks, where alone their larger relations and their
true significance in the history
of the earth can be ascertained.
The Field 'for the Study of
Rocks and Rock Structures
Rock Exposures in Flat
Countries. The dwellers in
the interior of our country, or
the traveler on the broad plains
of northern Germany, of Rus-
sia, of Hungary, or of China,
will- find little opportunity to
get a view of the rocks which
underlie these regions, for an
almost continuous mantle of
FIG. 14. Amos Eaton.
soil and drift covers the solid rock. Only where rivers have cut
channels through the surface layer of loose material, the mantle-
The Field of Geological Observations 29
rock, or where quarries have been opened, or construction opera-
tions have necessitated excavation down to and into the solid bed
rock, is there an opportunity for observation of the rocks beneath
the mantle-rock. River valleys and gorges are therefore the
favorite resorts of the geologist of the plains, while quarries,
railroad cuts, and other excavations which expose the rock, like-
wise receive his attention. Borings and drillings for oil, gas, or
water often prove of use to him, though generally, except where
the diamond drill is used, and the core preserved, the record of
such borings is only of doubtful and minor value. Salt shafts,
such as those in central New York and that near Detroit, Mich.,
furnish excellent sections at the time of excavation. Fortunately
for the geologist of the plains, however, few regions of any great
extent are wholly covered by mantle-rock ; more commonly low-
lying ridges of rock, formed by resistant layers, are exposed above
the general surface of the soil and drift mantle. Such " ledges "
generally mark the edges of beds of hard rock which have a gentle
slope away from the edge of the ledge. When the ledge-forming
layer is very thick, a cliff of some height may result, and this
generally furnishes abundant opportunity for observation and
deduction.
All natural exposures of the rock are called outcrops, and the
outcropping ledges together with the exposures in the stream chan-
nels, especially those which cut across the cliffs, furnish to the geolo-
gist of the fiat countries his most satisfactory data. We may note
a few examples.
Illustration from New York State. The state of New York
furnishes an illustration of the types of rock outcrop in ledges re-
ferred to in the preceding paragraphs. Over the greater part of
the state the rocks are so gently inclined that they appear horizontal
to the eye, and it is only when they are seen in the cliffs of Niagara
gorge and along Lake Erie that their gentle southward descent
becomes noticeable. Such a section, considerably generalized, is
shown in the following diagram (Fig. is). 1 Where the beds end
in the air upon the north, the harder ones, such as limestones and
sandstones, form a series of low cliffs, while the ends of the softer
shale beds are. usually marked by broad flat-bottomed valleys. The
1 The usual method of drawing sections is to place the north end upon the right,
but this is here reversed, because the observer along Lake Erie views the cliffs from
the west, and therefore south is on his right.
Methods 01 Approacn
cu o
d CJ
d s
O g
O
X ^
O
If
"
largest and deepest of these valleys is occupied by Lake Ontario,
while others are filled by soil which conceals the rock. These cliffs
or escarpments generally have an abrupt northern face across the
,,,,, edge of the hard rock, and in these faces
. rf. P? eft & rA 7
quarries are generally opened.
These cliffs can be traced with more
or less interruption across New York
state from the Niagara River and Lake
Erie (Fig. 16) to the Hudson, although
they become modified because some of
the beds seen in the western part of the
state die out, or change in character and
new ones appear (Fig. 17). This is
shown on the geological map of the state
of New York, where a series of broad
color bands extends east and west across
the state. Each color band in general
represents one rock layer or group of
layers, and the width of the color band
indicates the amount of exposure of each
which would appear were all the cover-
ing soil and vegetation removed; or in
other words, the amount by which each
lower bed projects beyond the next higher
one which covers it.
Wherever streams have cut across this
series of beds, gorges are formed, in the
walls of which the cut edges of the rocks,
the soft layers as well as the hard ones,
are shown. The most striking examples
of such gorges are shown along the
Genesee River, which crosses the state
from south to north. In the section
from Rochester northward it cuts the
lower beds, the harder, strata producing
waterfalls. In the section between Portage and Mt. Morris, it
cuts the higher strata, and here too several waterfalls are formed by
hard layers. Between these two points many smaller tributary
streams have cut into the sides of the valley and exposed
the rocks. Here, too, are situated several deep shafts which go
The Field of Geological Observations
31
down vertically to the Salina salt beds and during the cutting of
which the succession of the rocky beds was ascertained.
A good understanding of the succession of this series of rocky
formations is obtained by the traveler who passes from the Adi-
rondack Mountains southwestward across the state to Elmira,
especially if he take advantage of the various sections "exposed
in the gorges of the streams and the banks of the Finger Lakes.
FIG. 1 6. Cliff of Devonian rocks exposed on the shores of Lake Erie, south
of Buffalo. Typical of rock exposures shown on the lake shore from Buffalo
to Cleveland. (The section shows Hamilton (Wanakah) shales at the base,
the projecting Morse Creek limestone, above which lie the Windom shales and
higher Devonian shales.)
It was by the study of these natural outcrops of the state, sup-
plemented by those made on the sections exposed during the cutting
of the Erie Canal in 1817-1825, that the foundation of American
geology was laid by such men as Amos Eaton, and by James Hall
and others associated with him on the geological survey of New
York state. .
Other Exposure of this Type. The type of outcrop just described
is found in most regions of flat-lying rocks in our own country as
32 Methods of Approach
well as abroad. Thus the geologist who starts from Baraboo, Wis-
consin, and proceeds southeastward to Lake Michigan at Mil-
waukee, will cross such a series of rocky ridges separated by soil-
filled valleys, and a similar experience, awaits the traveler across
Kansas from southeast to northwest, or the one who proceeds south-
ward across Oklahoma, or journeys from central Texas either north-
westward or southeastward. The traveler across England from
Liverpool to London also crosses such a series of rocky ridges, which
FIG. 17. View of West Hill, Schoharie, a terraced edge of the Helderberg
escarpment or cuesta, in eastern New York. Note the prominent cliffs formed
by the limestone members and the intermediate slopes formed by softer beds.
- (Courtesy of N. Y. State Geological Survey.)
in general extend in a northeast-southwest direction, and are formed
by a succession of nearly horizontal rock-layers with westward
facing cliffs, though these are not generally visible from the train.
Similar outcrops of nearly horizontal layers of rock surround Paris
in constantly widening circles on three sides. The edges of some
of these rocks form cliffs or escarpments facing outward. If one
were to represent the rock formations which underlie Paris, and
which have a shallow basin-like structure, by a nest of plates, the
smallest at the top, the successive rims of these plates would repre-
sent the encircling cliffs, while the location of Paris would be in
the center of the uppermost and smallest plate. Many rivers have
cut channels through the edges of these rock plates, while others
The Field of Geological Observations 33
flow in the depressions between successive rims. Thus numerous
and excellent exposures of the rocks are found, even though the
marginal portions between the edges of successive rock layers are
frequently covered by soil and vegetation. These numerous rock
exposures made possible the observations which gave the French
founders of our science the data on which to build their deductions,
and the cliffs which they form around Paris were of the utmost im-
portance in the conduct of the Great War so recently closed.
Outcrops in Mountainous Countries. It is, however, in the
elevated portions of the earth the mountains and chains of up-
lands that rock outcrops are most frequent on the surface of the
land. Here the soil and rock debris lodges only in the depressions,
while between these the ledges protrude and give opportunities
for observation. Here, too, deep canons are cut by the rivers and
glaciers and thus additional rock-walls are opened for observation.
German, Austrian, Swiss geologists have for the most part
been limited to such regions for their observations, and the wonder-
ful rock exposures of the Alps and other mountains have made it
possible for them to carry their studies along certain lines to great
lengths. Werner, the father of German geology, made most of
his observations in the subdued l mountain district of Saxony, es-
pecially the Erzgebirge, which has long been famous for its old
and extensive mining operations.
Since France and Italy also border on the Alps, and have mountain
Ranges of their own, the geologists of these countries were enabled
to avail themselves of the rocks and rock structures thus revealed.
British geologists, too, have been- able to some extent to resort to
this type of exposure, though in these moisture-enveloped islands,
as over parts of Scandinavia as well, the dense though low cover of
vegetation and the peat accumulations obstruct much of the un-
derlying rock, as all students of Irish geology know only too well.
The Highlands of Scotland, however, furnish many good opportu-
nities for observation, as do also many of the higher English dis-
tricts and especially the mountain region of Wales. Swedish geolo-
gists have frequently had to resort to digging through the surface
layers to get at the underlying rock, and it is not an uncommon
sight to see the Swedish geologist in the southern mterior
accompanied by a factotum, whose duty it is to wield pick and
shovel.
1 This term implies that the old mountains have been much worn down.
34 Methods of Approach
In European Russia and adjoining districts there are vast areas
of flat-lying rocks covered by soil and drift, so that, except along
the coast and in the river channels, outcrops of the bedrock are
difficult to find. But where these rocks are uplifted in the Ural
Mountains on the east, the Caucasus on the southland the Carpa-
thians on the west outcrops abound, and here the true relation-
ships of the rock formations may be ascertained..
In America, the New England uplands, the White, Green, and
Adirondack Mountains, and the Appalachian Chain furnish an
FIG. 1 8. Marsten Rock : View on the North Sea coast of England (Dur-
ham), showing characteristic erosion features in a detached mass of Magnesian
Limestone of Permian age, which was formerly united with the cliff on the left.
This is typical of the rocky character of much of the British coast, though the
kind of rock, the structures, and the erosion forms vary from point to point,
abundant series of rock outcrops for the eastern geologist. The
old and generally much-subdued mountain system, the rocks of
which may be traced by frequent outcrops from New York City
to the Highlands of the Hudson, and which can be followed south-
westward through New Jersey, Pennsylvania, and Maryland, and
indeed all the way down to the Carolinas, where it constitutes the
older Appalachian Chain, is a typical example of an elevated, though
for the most part not very mountainous, country, and here out-
crops abound. Many of our western mountains are especially well
adapted for geological observation, for here the aridity of the climate
prevents the growth of much vegetation, and the rock structures,
The 'Field of Geological Observations
35
frequently developed on a gigantic scale, are visible for many miles.
This makes the Rocky Mountains a veritable paradise for the
American geologist.
Outcrops upon the Coast. Of all the natural rock exposures,
however, those of the coast-line are the most attractive, and in
many respects the most satisfying. Wherever the sea-coast or the
shore of a large lake is formed by rocks which rise above the sur-
face of the water, the cutting work of the waves keeps the exposure
fresh. A tramp along a rocky sea-coast is replete with interest to
the geologist, and many of the choicest bits of geological observa-
tion have been made on such sea-cliffs. Great Britain, with its
wonderful rocky sea-coast, probably leads the world in the variety
and significance of rocks and rock structures there exposed (Fig.
iS). 1 No student of geology
can afford to. neglect the won-
derful English and Scottish
coast, which has furnished the
British geologists so many op-
portunities for the observation
of facts that there, more than
elsewhere, geological science
has advanced, since the days
of William Smith, with phe-
nomenal strides. This is
probably the reason why Eng-
lish geology quickly became
the standard of comparison
for other nations, in whose
home countries observation
was a more arduous task, be-
cause they did not include
such marvelous coast ex-
FIG. 19. PulpitRock, Nahant (Mass.).
One of the most picturesque and in-
structive rock sections upon the Atlantic
coast of New England. The rocks are
metamorphosed Cambrian shales and
limestones with a. great diabase sheet
(sill) intruded between the strata, and
the whole eroded by the waves working
chiefly upon the softer strata. (Photo
by A. W. G.)
posures.
Northern France, too, has a coast-line of great interest to the
geologist, and so has Norway, The coast-line of Germany, on the
other hand, is mostly sandy, and there is little diversity in the types
of the facts which it discloses.
The Atlantic coast-line of North America is for the most part a
sandy one. Only in New England, in the Canadian coastal prov-
1 See also Figs. 113, 114, 120, 121, 203, 408, 510. 5", 530-532, 719-721, 723 a, b.
36 Methods of Approach
inces, and in Newfoundland can be seen coastal sections compa-
rable to some extent to those of Great Britain. The northern re-
gions, however, are accessible with difficulty, and have only recently
been investigated. But the New England coast, and especially that
of Massachusetts (Fig. 19), is a Mecca for American geologists,
and many of the workers in American geology have had their pre-
liminary training through a study of that interesting region.
Regions for the Study ' of Dynamic Geology
Although the principles of dynamic geology, the workings of the
chemical and physical forces, may be studied to much advantage
in the laboratory such study, too, is incomplete without recourse
to the outdoor field. It is upon the sea-shore that some of the pro-
foundest lessons of erosion by waves, of transportation by currents,
and of deposition in quieter waters can be learned. Here, too,
the method of entombment of fossils and the formation of many
original structures, such as ripple-marks and the like, can be ob-
served. Rocky as well as sandy and muddy shores should be visited.
Shores of large lakes may serve as a substitute in inland regions,
but lakes have in addition many characters of their own. Ponds
and temporary pools also teach their lessons. River valleys and
gorges, rapids and waterfalls, brooks, and even the roadside gutter,
furnish lessons in dynamic geology, as do also the hillside, the
mountain slopes, and the elevated peaks, where rocks are shattered
by frost, and decay under atmospheric influence. Glaciers present
many illustrations of dynamic geology, while caverns and under-
ground channels have special lessons to teach. The deserts and all
regions where wind is at work furnish illustrations of the mechan-
ical activities of the wind, while pools and salt pans in arid regions
furnish illustrations of chemical activities and of precipitation of
salts through condensation of the water under evaporation. Springs,
too, illustrate dynamical activities, both physical and chemical,
and artesian wells, oil wells, geysers, and similar phenomena are
replete with them. Finally, volcanoes and other such phenomena
furnish the means for the study of igneous activities.
The great English geologist, Sir Charles Lyell, whom we some-
times call the " Father of Modern Geology," has said that the
geologist must be primarily a traveler he must go to other lands
than his own and so widen the scope of his experience. Werner,
Geological Literature 37
the founder of German geology, confined his observations mainly
to his limited Saxon district, and attempted to formulate from
these observations laws which should govern the rest of the world.
Naturally he fell into many and profound errors, so that to-day
scarcely one of his theories is held. Since his day German geologists
have, however, become great travelers, not only in their own but
in most other lands. As a result, their observations have become of
wide scope, and they have added much to geological knowledge.
British and American geologists have only recently begun to
follow the advice of Lyell, but already their efforts have been
crowned with considerable success.
Let the student of geology, then, come to realize that the value
of his deductions increases in proportion to the range of his obser-
vations, and that no single country or region of the world will give
him all he needs. The American student, owing to the wide ex-
tent and diversity of his country, is perhaps more favored in this
respect than is the geologist of any other nationality, but at present
only a limited portion of our country has become sufficiently ac-
cessible to make extended observations possible.
a
THE IMPORTANCE OF GEOLOGICAL LITERATURE
Finally, it must not be overlooked that the observations of our
predecessors are recorded in the literature of the science, and that
here we find a mine of information, the value of which cannot be
overestimated. No one can repeat all of the observations which
have been made in the past, even were such repetition desirable.
In addition to the laboratory and field, then, the student of geology
must go to the library, and a thorough understanding of the liter-
ature on his special field is of fundamental importance to the worker.
Besides special books on different aspects of the science, the student
should gain familiarity in the use of the official publications issued
by the governments of the various countries, the proceedings of
scientific societies, and the special journals devoted to geology and
kindred sciences.
CHAPTER IV
MATERIAL OF THE EARTH'S CRUST
THE material of which the crust of the earth consists is spoken
of as rock, a term which we shall presently define more precisely.
Rocks are in turn combinations of minerals or large aggregates of a
single material, and these are formed by the combination of chemical
elements, or by the union of those elementary combinations of
elements which are called ions. The study of chemical elements
and of their combination into ions and the union of these to form
other substances (salts, etc.), belongs in the domain of chemistry.
The study of minerals, their properties and occurrence, belongs
to the special branch of the earth science called mineralogy. An
elementary preparation in chemistry and mineralogy is necessary
to the student, and should be obtained by him if possible before
undertaking the study of geology. In this book we can give
only a brief summary of the more important elements and minerals
with which the student should have some acquaintance. The
important minerals which enter into the composition of the rocks,
or which themselves occur in rock-like masses, will be dealt with
somewhat more fully in the discussion of these rocks.
THE CHEMICAL ELEMENTS AND THEIR PRIMARY COMBINATIONS
Of all the chemical elements which enter into the composition
of the earth's crust, only a comparatively small number are of
importance in combining to form the more common minerals and
rocks. The principal ones are given in the following list from
F. W. Clarke, in which their relative importance is also indicated.
Some of these elements occur pure in nature and are then called
native elements. Among these are oxygen, nitrogen, sulphur,
carbon, and the metals gold, silver, copper, platinum, etc. The
majority of elements, however, form combinations among them-
selves, with the result that more or less stable compounds are
produced.
38
Chemical Elements and Their Combinations 39
The More Important Elements dnd Their Distribution
\
NAME OF ELEMENT SYMBOL
LlTHOSPHERE
93 PER CENT
OF WHOLE
HYDROSPHERE
7 PER CENT
OF WHOLE
AVERAGE
INCLUDING
ATMOSPHERE
Oxygen ... ...
Silicon ... Si
47-33
07 J A
85.79
$0.02
or 1 8^
Aluminum Al
7 8<
25.00
Iron . . . Fe
A f>
7-3
, T o
Calcium . . . Ca
4-5
7 A 7
Magnesium . Mg
5-47
2*7 A
5
3.22
2r\&
Sodium Na
.^4
2 46
.14
T TA
2 2 A,
Potassium ...... K
2 A^
r\A
o < - >
2 28
Hydrogen ...... H
22
* U 4
10 67
O C
Titanium Ti
46
95
x 2
Carbon C
IQ
OO2
4o
18
Chlorine Cl
06
2 Q7
20
Bromine ... , . Br
OO8
Phosphorus P
12
1 1
Sulphur i S
12
OQ
ii
Barium ... Ba
08
08
Manganese \ Mn
08
08
Strontium < Sr
O2
02
Nitrogen ... . i N
O3
Fluorine . Fl
IO
IO
All other Elements including
Gold, Silver, Platinum,
Arsenic, Copper, Lead,
Mercury, Nickel, Tin,
Zinc, Radium, etc. ...
.50
-47
Total j
IOO OO
IOO OO
IOO OO
Chemical Combinations
The following types of chemical combinations exist in nature
or are produced in the laboratory.
Oxides. Combination of an element with oxygen. Examples :
Silica (SiO 2 ) ; Carbon dioxide (CO 2 ) ; Iron oxide (Fe 2 3 ) ; Water
(H 2 0).
In the first of these, two parts of oxygen unite with one of silicon
to form silica or quartz ; in the second, two parts of oxygen in like
manner unite with one of carbon to form the gas carbon dioxide.
In the third example, three parts of oxygen unite with two of iron
to form the sesquioxide of iron ; and in the fourth example one part
of oxygen unites with two of hydrogen to form water.
40 Material of the Earth's Crust
Hydroxides. These are combinations of an element (metal)
or a group of elements with oxygen and hydrogen, the last two
in equal parts. Examples : Sodium hydroxide or caustic soda
(NaOH) ; Potassium hydroxide or caustic potash (KOH) ; Alumi-
num hydroxide (the mineral Gibbsite, A1(OH) 8 ). In this last
example it requires three parts of the (OH) group to satisfy the
combining power of one part of aluminum.
Oxyhydroxides. Like the preceding, but with an additional
molecule of oxygen. 1 Examples : Aluminum oxyhydroxide, the
mineral diaspore (AIO(OH)) ; Iron oxyhydroxide or goethite
(FeO(OH)). Both hydroxides and oxyhydroxides may also be
expressed as combinations of oxides and water (H 2 O) ; thus :
f A1(OH) 3 . 1, f A1 2 3 | f H 2 (Gibbsite)
2 \ Aluminum \ = \ Aluminum f +3 { Water
hydroxide J I oxide j ' I
I AIO(OH) ] f A1 2 3 1 | H 2 O (Diaspore)
2 j Aluminum r = { Aluminum r + 1 Water
I oxyhydroxide J I oxide J I
FeO(OH) 1 r Fe 2 O 3 1 f H 2 O (Goethite)
Iron oxy- [ = ) ^ ron f "^" I Water
hydroxide J I oxide J I
The hydroxides and oxyhydroxides also form bases with which
acids combine to form salts.
Acids. These are combinations of certain elements such as
chlorine, carbon, sulphur, silicon, etc., which are called negative
elements, or their oxides (negative ions), with hydrogen or with
the oxyhydrogen (OH) combination or radical. Examples:
Hydrochloric acid HC1, (Hydrogen chloride -f- water) ; Carbonic
acid, H 2 CO 3 = CO+2(OHJ; Sulphuric acid, H 2 SO 4 =SO 2 +2(OH).
Salts. A compound formed by the reaction between an acid and
a base (hydroxide or oxyhydroxide) with the simultaneous forma-
tion of water, is called a "salt.
Thus:
Na(OH) 1 f HC1 ] [ NaCl 1 f H 2 O
Sodium y -h { Hydrochloric \ = V Sodium [chloride f -f S Water
hydroxide J I acid J I or common salt j I
Ca(OH) 2 l f H 2 S0 4 ) f CaS0 4 1 [ 2 H 2 O
Calcium f + \ Sulphuric { =*'] Calcium [+ j Water
hydroxide J [ acid J I sulphate J [
1 More correctly derived from the hydroxide by the abstraction of water, as shown
on comparison ofr the formulas of Diaspore and Gibbsite, the former having two mole-
cules of water less.
Chemical Elements and Their Combinations 41
New salts may also be formed by the reaction, in solution, of a
strong acid upon a salt with a weak acid, when the weaker acid is
set free.
Thus:
SrCl 2 ] f H 2 S0 4
Strontium } + { Sulphuric } ~
chloride j [ acid J
SrS0 4 ] f 2HC1
Strontium } 4" \ Hydrochloric
sulphate J (, acid
Or they may be formed by the interaction of two salts iji solution
to form a less soluble salt.
Thus :
BaCi,]
Barium f Hh
chloride J
Na 2 SO 4
Sodium
sulphate
BaS0 4
Barium
sulphate
sNaCl
Sodium
chloride
The barium sulphate is insoluble and will be precipitated out.
Ions. ' When certain chemical compounds such as acids, salts
and the bases, are dissolved in water, they are believed to be dis-
sociated into two or more parts which are either the elements or
their simple combinations, and which are called ions. They
exhibit a marked behavior towards the passage of an electric
current through the solution; some, regarded <as charged with
positive electricity, being attracted by the negative electrode, and
others, regarded as negatively charged, being attracted to the
positive electrode. Examples are : .
Acids
I
HCL =H positive and Cl . . . . negative ions
,H 2 S0 4 =H 2 positive and S0 4 1 ... negative ions
Base KOH =K positive and (OH) . . . negative ions
Salts
NaCl =Na positive and Cl
CaS0 4 = Ca positive and S0 4
negative ions
negative ions
1 In dibasic acids the dissociation takes place in two stages. In fairly concentrated
solutions sulphuric acid dissociates wholly or in part as a monobasic acid. Thus:
H 2 S04=H+HS04. The second stage takes place when the solution is more dilute.
_ -j- a, , , . .
Thus: HS04=H+-S04. (Jones, H. C., The Nature of Solutions.}
42 - Material of the Earth's Crust
MINERALS
All native elements, oxides, hydroxides, oxyhydroxides, acids,
and salts which occur in nature in a solid state, are called min-
erals. They occur either in crystalline or uncrystalline (amor-
phous) form or both. Acids are rare as minerals, but native
elements and oxides are common, while hydroxides and oxyhy-
droxides are not infrequently met with. By far the larger number
of minerals, however, belong to the category of salts, among which
the dominant ones are the silicates formed by the combinations
of metals, etc., with silicic acid. The determination of minerals
depends upon the recognition of their physical characters as well
as of their chemical composition. There are many physical char-
acters of which the more important ones will be briefly summarized.
Crystalline Form
Most minerals assume definite forms in which certain planes
appear, which are found to have a definite relation to certain
imaginary lines or crystallographic axes (coordinate axes) about
which the crystal may be supposed to be built up. There. are
six systems recognized, based on the relative length and relation-
ship of the axes. In the systems with three axes, these axes
may differ in length, when they are designated by the letters a, b,
and c, respectively, the c axis being the vertical one. If a and b
are equal both are designated by the letter a ; if all three are equal
they are all called a. The- various faces of the crystal are read
with reference to the points at which they intersect the axis or
would do so if both were extended.
If, in a simple crystal of one set of planes, a plane intersects all
three axes (unit length), these axes being unequal, this plane is
given the symbol a:b:c (pyramid). If the two horizontal axes
are equal, it is designated '-a: a: c (tetragonal pyramid); if all
three axes are equal, it is designated a: a: a (octahedron). If
the plane cuts two axes and is parallel to the third, this parallelism
is indicated by the infinity sign (oo) and the formula becomes
a:b: *> c (prism); a: a: <&c (prism) or a: a: oa (dodecahedron)
as the case may be ; if it cuts only one axis and is parallel to the
other two the designation is a:ooj:oo or <*>a:b:<x>c (pina-
coids) ; oo a : oo J : c (basal pinacoids) ; a : oo a : oo c (second order
Minerals ' 43
prism) ; oo a : o a : c (basal plane) or a : <x a : oo a (cube) according
to the relative lengths of the axes. Planes cutting all three axes at
the unit length are called pyramid planes; those that cut the two
horizontal axes at the unit length and are parallel to the vertical
one, are called prism planes, while those that cut only one of the
horizontal axes, being parallel to the other and to the vertical one,
are called pinacoidal planes except in the case where the two horizon-
tal axes are of equal length (tetragonal system), when they are
called prism planes of the second order. Those which cut the c
axis and are parallel to the others are called basal pinacoids.
Finally, planes parallel to one horizontal axis and cutting the
other and the vertical one are called dome planes, except in the case
where the two horizontal axes are equal (tetragonal), when they
are called pyramid planes of the second order. Other planes may
occur which do not cut the axes at the unit length. These are
designated by the coefficient m for the first variation from the
unit length and n for the second. Thus with three equal axes we
may have planes with the formula a: a: ma (trigonal trisocta-
hedron), or a: ma: ma (tetragonal trisoctahedron) ; or finally,
a:na:ma (hexoctahedron) . The system with four axes has the
three horizontal ones equal and at angles of 60 with one another,
while the vertical one is at right angles to the others.
The Six Systems of Crystallization
L Isometric. Three axes of equal length or interchangeable
and at right angles to one another. Fundamental forms: cube
(a: Q a : oo a) ; octahedron (a : a : a) ; etc. (Fig. 20).
II. Tetragonal. Two horizontal axes equal and interchange-
able, the vertical one (c) of different length. All at right angles
to one another. Fundamental forms : tetragonal prism 1
(a: a: oo c} ; tetragonal pyramid (a:a:c); etc. (Fig. 21).
III. Hexagonal. Three equal horizontal or interchangeable
axes, forming angles of 60 degrees; a vertical axis of different
length at right angles to the horizontal ones. Fundamental forms :
hexagonal prism (a: a: co a : oo c) ; hexagonal pyramid (a : a :
QQ&: c) ; etc, (Fig. 22).
1 All the prisms require, of course, basal planes or pyramids to complete the solid.
44
Material of the Earth's Crust
FIG. 20. Isometric System. Principal Forms. The general symbols and tl
values of the coefficients for the figures given are added. (After Moses an
Parsons.)
Holohedral. (All planes developed.)
A. Octahedron. a: a: a.
B. Trigonal Trisoctahedron. a: a: ma
(01=2).
C. Tetragonal Trisoctahedron or Trape-
zokedron. a : ma : ma. (m 2.)
D. Hexoctahedron. a:na: ma.
E. Dodecahedron. a: a: oo a.
F. Tetrahexahedron. a :na: oo a. (n= 2.)
G. Cube or Hexahedron. a : oo a : oo a.
Hemihedra-L
(In this division only every alternate plai
is developed, thus giving only half the nur
ber of planes found in the corresponds
holohedral form. This is indicated by pr
fixing % to the symbol.)
H. Tetrahedron. J(a : a : a).
I. Deltohedron. %(a: a: ma).
J. Tristetrahedron. } (a : ma : ma
K. Hextetrahedron. J(a ; na : ma]
L. Pyritohedron. f (a : na : oo a")
Minerals
45
E
FIG. 21 ; Tetragonal System. Principal Forms.
(After Moses and Parsons.)
A. Tetragonal Pyramid,, ist order. a : a : c. 1
B. Tetragonal Pyramid, 2d order. a : oo a : c. 1
C. Ditetragonal Pyramid. a : na : c. 1
D. Ditetragonal Prism. a : na : oo c.
E. Tetragonal Prism, 2d order. a:ooa: coc (with pyramid of ist order
(p). na:na:mc).
F. Tetragonal Prism, ist order. a : a :"oo c.
D, E, F, show Basal Pinacoids oo a : co a : c.
1 When occurring in combination a unit length for c is selected and the formula be-
comes a : a : me.
V.
. .^.^-^^-^.-n
FIG. 22. Hexagonal System. Principal Forms.
(After Moses and Parsons.)
Holohedrai : (All faces developed.)
A. Hexagonal Pyramid, ist order. a:a:ca: c. 1
B. Hexagonal Pyramid, 2d order. a : na : na : c. 1
(=.2.)
C. Dihexagonal Pyramid. a : na : pa : c. 1
D. Dihexagonal Prism. a : na : pa : co c.
E. Hexagonal Prism, 2d order. a:na:na:cac. (= 2.)
F. Hexagonal Prism, ist order. a:a:caa:<x>c.
D, E, F, show Basal Pinacoids. oo a : co a : QO a : c.
Hemihedral. (Half the number of faces developed.)
G.* Rhombohedron, ist order. -|(<z : a : co a : c) - 1
H.- Scalenohedron. $(a : na : pa : c). 1
I. Trigonal Prism, ist order, J(d : a : oo a : oo c).
J. Ditrigonal Prism. J(& : na : pa :oo c).
I and J show Basal Pinacoids. 'co#:oo&:coa.:c.
1 me in combination ; p greater than n.
A6
Minerals
47
IV. Ortliorhombic. Three axes, all of unequal length, but all'
forming right angles with one another. Fundamental forms:
orthorhombic prism (a : b : oo c} ; orthorhombic pyramid (a : b : c) ;
etc. (Fig. 23).
J
c
FIG. 23. Orthorhombic System. Principal Forms in combination.
(After Moses and Parsons.)
A. Orthorhombic (unit) Pyramid (p.). a : b : c (or, na:b: me).
Orthorhombic (unit) Prism (m). a:b:<x>c (or, na:b:<&c).
Brachy-prism (/). na:b:o c. (n =2.)
Brachy-dome (/). oo#: b : c (orooa : b : 2 c).
B. Same forms as in A with addition of Basal Pinacoid (c): oo0:oo&:c.
C. Same forms as in B with, addition of two other pyramids (i). a : b : f c,
and (q). a : b : 2 c\ and two macro-domes (h).~~a : oo b : I c, and (k).~ a:aob':
2 c, and a macro-pinacoid (a). a :oo b : oo c.
V. Monoclinic. Three axes, all of unequal length, the hori-
zontal ones. at right angles to each other, the vertical one (c\
FIG. 24, Monoclinic System. Principal Forms in combination.
(After Moses and Parsons.) '
A. Monoclinic (unit) Prism (m). a:b:wc (or, na:b:<x>c).
Hemi-pyramid (negative p, positive v). a:b:c (or,na:b:mc).
Ortho Pinacoid (a). &:oo#:oo.
Clino Pinacoid (b). ooaibivoc.
Basal Pinacoid (c). oo a :oo b : c. **
^B. Same planes . as in A, except positive hemi-pyramid () and basal
pinacoids (c).
- C. Same planes as in A except positive hemi-pyramid ().
D. Unit prism; basal pihacoid; two positive hemi-pyramids v. and w.
(a:b:$c) and a clino-dome z = ( oo a : b : 2 c) .
Material of the Earth's Crust
inclined with reference to a, but forming a right angle with b.
Fundamental forms : monoclinic prism (a : b : co c) ; monoclinic
pyramid (a : b : c) (really 2 hemi-pyramids, a positive and a negative
one) ; etc. (Fig. 24).
VI, Triclinic. Three unequal axes all inclined with reference
to one another. Fundamental forms: triclinic prism (hemi-
prisms) (a : b : oo c)'; triclinic pyramid (a : b : c) (Fig. 25).
' FIG. 25. Triclinic System. Principal Forms. (After Moses and Parsons.)
A. Triclinic Pyramid. a:b:c } consists of 4 sets of 2 parallel planes each.
B. Hemi-brachy-dome (e). co a : b : c. 1
Mdcro-pinacoid (a). a : oo b : oo c.
Brachy-pinacoid (b). co a : b : o c.
Basal- pinacoid (c). oo a : oo b : c.
C. Hemi-macro-dome (d). a:<x>b: c. 1 Macro- (a), Brachy- (&), and Basal-
pinacoids (c).
D. Triclinic Hemi-p.rism(m). a:b:ccc. Macro- (a), Brachy- (b), and
Basal-pinacoid (c).
E. Combination of Macro- (a), Brachy- (6), and Basal-pinacoids (c).
Other Physical Characters
Cleavage. The ability of a mineral to split along one or more
planes parallel to actual or possible crystal planes is called cleamge^
and is an important aid in identifying mineral species,
Fracture. The mode of breaking in directions other than those
of cleavage is the type of fracture of the mineral It is -conchoidal
(Fig. 41) when it has rounded surfaces suggestive of a shell ; even
1 me in combination.
'Minerals 49
or uneven, when nearly plain, or rough and irregular; hackly or
splintery, when it has ragged sharp points and depressions, or
separates in a fiber- or splinter-like manner.
Tenacity. A mineral is brittle when it breaks into powder;
sectile, when small slices can be shaved off which crumble under a
hammer ; malleable when slices from it will flatten under a hammer ;
tough, when great resistance to tearing apart under strain or a
blow is shown ; ductile, when it can be drawn into wire.
Hardness. The resistance of a smooth plane, whether crystal,
cleavage, or fracture, to abrasion is called the hardness, and is
commonly determined by scratching the surface. It is expressed
in terms of a scale of ten common minerals (Mohs scale). Each
mineral will scratch all those softer than itself.
Scale of Hardness
1. Talc ' 6. Orthoclase
2. Gypsum (Selenite) 7. Quartz
3. Calcite ^ 8. Topaz
4. Fluorite 9. Sapphire
5. Apatite 10. Diamond
Minerals below 2.5 in hardness can usually be scratched with a
finger nail ; those below 6 by a pocket knife. Any mineral above
5.5 will scratch window glass. By these simple tests hardness
can be determined approximately.
Luster. The brilliancy or shine of a mineral is called its luster.
It is dependent upon the refractive power, transparency, and
structure of the mineral. The following types are recognized:
a. Metallic : luster of metals, gold, silver, copper, etc.
b. Non-metallic luster comprising :
Vitreous the luster of a fractured surface of glass; example,
quartz.
Adamantine the luster of uncut diamond, zircon, etc., due
to hjgh index of refraction.
Resinous the luster of resin ; example, sphalerite.
Greasy the luster of oiled glass ; example, elaeolite.
Pearly- the luster of mother of pearl; example, foliated talc.
Silky the luster of silk ; example, satin spar.
Dull without luster or shine of any kind; examples, chalk,
kaolin.
The prefix sub- is used to express a lesser degree of the partic-
ular luster; e.g. sub-metallic, sub-vitreous, etc.
50 Material of the Earth's Crust
Color. This depends on chemical composition and is variable ;
or on physical constitution, when a variety of color-changes with
the changes. in the direction of light is produced. These are:
Play of color (opal, labradorite) ; Iridescence, bands of prismatic
color; Tarnish, surface discoloration; Opalescence, milky or
pearly reflection ; Asterism, showing a star by reflected or trans-
mitted light, as in ruby, etc., or in some micas.
Streak. The color of the fine powder of a mineral is its streak.
It is obtained by scratching the mineral or rubbing it upon a smooth,
white, and hard surface. (Arkansas stone; streak stone.)
Transhicency. The capacity for transmitting light is the
translucency of a mineral. A mineral is transparent when objects
can be seen through it with clearness ; translucent, when it transmits
light, but objects cannot be seen; opaque when no light passes
through even the thin edges. Sub-transparent and sub-translucent
are also used.
Specific gravity. The weight of a substance divided by the
weight of an equal volume of distilled water (at 4 C.) is its specific
gravity. Exact determinations are made by fine balances, but
rough determinations can be made by weighing in the hand and
comparing with a mineral of equal size and known specific gravity.
Taste. Some minerals have a taste, such as astringent (alum) ;
salty (common salt) ; bitter (epsom salts) ; alkaline (soda) ; acid
(sassolite) ; cooling (niter) ; pungent (sal-ammoniac) ( '
Odor. On heating or burning, some minerals give off odors, of
which those of garlic (arsenic minerals), horseradish (selenium
minerals), or sulphur are examples. Fetid, bituminous, and
argillaceous (clay) odors also occur, the latter noticeable on breath-
ing upon the substance.
Feel. The response of a mineral to the sense of touch may
be smooth, soapy (talc), harsh, meager (aluminite), or cold, the latter
distinguishing gems from glass.
Other Characters. A few minerals are magnetic, and there is
great variation in transmission of heat-rays and of conductivity.
Various electric phenomena also exist.
Classification of Minerals
Minerals are classified on a chemical basis, and two distinct
methods have been employed which may in general be considered
as classifications; first, according to the acid radical (including
Minerals 51
oxides, etc.) and second, according to the basic radical. In the
first system the minerals are divided into the following classes : 1
.1. Native elements.
2. Sulphides, Selenides, Tellurides, Arsenides, Antimonides.
3. Sulpho salts.
4. Chlorides, Bromides, Iodides, Fluorides.
5. Oxides (Hydroxides, Oxyhydroxides).
6. Carbonates.
7. Silicates.
8. Titano-Silicates, Titanates.
9. Niobates or Columbates, Tantalates.
10. Phosphates, Arsenates, Vanadates, Antimonates.
11. 'Nitrates.
12. Borates.
13. Uranates.
14. Sulphates, Chromates, Tellurates.
15. Tungstates, Molybdates.
16. Oxalates, Mellitates. (Salts of organic acids.)
17. Hydrocarbon compounds.
Tables of Important Minerals
In the following tabular list, arranged essentially according to
the basic radical, the more important minerals are given, with a
brief characterization of their essential features. For more
details the student is referred to the textbooks cited below.
1. A. J. MOSES AMD C. L. PARSONS. Elements of Mineralogy, Crystallog-
raphy, and Blowpipe Analysis. 5th edition, 1916. N. Y., D. Van Nostrand
Company.
2. DAHA-FORD. Manual of Mineralogy. i$th edition. John Wiley and
Sons, N. Y. 1912.
3. H. A. MIERS. Mineralogy. An introduction to the scientific study of
minerals. Macmillan and Co., London. 1902.
4. A. H. PHILLIPS. Mineralogy. The Macmillan Co., N. Y. 1912.
5. A. F. ROGERS. Introduction to the Study of Minerals. McGraw Hill
Book Co., N. Y. 1912.
6. DANA, EDWARD S. A Text Book of Mineralogy, etc. John Wiley and
Sons,N.Y.
V- J- D- DANA. The System of Mineralogy, Descriptive Mineralogy by
E. S. Dana. John Wiley and Sons, N. Y.
8. W. O. CROSBY. Tables for the Determination of Common Minerals,
Chiefly by their Physical Properties. Boston. Published by the Author.
i Dana, J, D. and E. S. : The System of Mineralogy. 6th ed.
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CHAPTER V
ROCKS, THEIR CLASSIFICATION AND PRINCIPAL TYPES
' DEFINITIONS
A ROCK may be defined as a mineral mass or an association of
minerals, which in their natural occurrence form an essential
part of the earth's crust. This distinction is not a very precise
one, especially when the material of the rock consists of only one
mineral. Thus the'calcite in a vein would be considered a mineral,
while essentially the same material in a bed of marble would be
considered a rock. We shall see, however, as we proceed with the
discussion of the -rocks, that in practice the distinction between
rock and mineral can as a rule be readily made.
AGE RELATIONS OF ROCKS
It will be useful at this early stage of our study to recognize the
fact that the rocks of the earth's crust are of various ages. Some,
like the rocks which make up the Adirondack Mountains, Pikes
Peak in Colorado, the Highlands of the Hudson, the Scottish High-
lands, the main mass of Finland, and a great part of central France,
etc., are very old ; others, like those of the " puys " which are
scattered over the central French region, the basalts of the Columbia
and Snake River plateaus of the northwestern United States, the
rocks immediately underlying Paris, London, Vienna, and Berlin,
and the rocks of southern Florida are very young, though not all
of the same age. It is possible to divide the history of the earth
into a number of periods and eras, just as human history can be
divided. But whereas the successive periods of human history are
measured by centuries at the most, those of the pre-hurnan earth
history are measured by hundreds of thousands if not by millions
of years. And as we can refer the monuments and buildings of
human origin to their successive periods in human history, often
from the character of these monuments and buildings themselves,
64
Bed-Rock and Mantle-Rock 65
so we can refer most of the different rocks and rock structures of
the earth's crust to their respective geological periods, or to the
period of the earth's history when they came into existence as rocks.
It is desirable that the student should begin to familiarize himself
at this point with the names of the different periods of earth history
FIG. 26. Ledge of glaciated rock overlain by glacial drift, showing the
sharp contact line between the bed-rock and the mantle-rock. New York City.
(F. K. Morris, Photo.)
as given in the table in Chapter XXIV of this book. At a later
stage of his studies he will learn by what means it becomes possible
to refer rocks and rock structures to their proper period. When in
the succeeding pages we refer to the geological age of any rock mass
the student should consult the table until he has become familiar
with the succession of the periods. (See p. xviii.)
BED-ROCK AND MANTLE-ROCK
In general we may distinguish between the solid or bed-rock and
the unconsolidated rock or rock material, which latter is commonly
called the mantle-rock because it covers or mantles the bed-rock
which everywhere underlies it and projects through it as ledges.
The mantle-rock is of course much younger than the bed-rock
upon which it rests. In the northern United States and Canada
and the northwestern part of Europe, the mantle rock generally
rests abruptly upon the solid or bed-rock, the contact line between
the two being commonly sharp (Fig. 26). In many other por-
66 Classification and Principal Types of Rocks
tions of the earth, however, there is a gradation between the two,
the mantle-rock becoming more stony downwards, and passing into
rotten rock and finally into fresh bed-rock. This relationship
will be discussed more fully in a subsequent chapter ; the present
one will only take account of the solid rock which forms all except
the surface film of the crust of the earth.
CLASSIFICATIONS OF ROCKS
General Principles of Classification. Classification of natural
objects may be made either on a natural or on an artificial basis.
A natural classification is based on the origin and development
.of the objects classified or on. their genesis. Such a classification
is therefore called a genetic one, and it alone has a permanent scien-
tific value. It is true that a genetic classification can be made
only when the genesis or origin of the objects classified has been
determined, and that therefore the artificial systems of classifica-
tion will always precede the scientific or genetic one. The artificial
classification is generally based upon the possession in common, by
the objects classified, of a single character or, at most, a few char-
acters. Thus whales were formerly classified with fish because,
like these, they lived in the water and had a fish-like form. Their
true relationship is, however, more closely with elephants, both
being mammals. The eel-grass of our coast is not a grass, but
belongs to the family of water lilies, although its leaves are grass-
like. Rock-salt and sandstone have little in . common, though
generally classified together as sedimentary rocks ; both may have
been deposited in lagoons near the seashore, and therefore in a
sense are sediments.
In general, it may be said that the progress of any science is
indicated by the replacement of the original artificial by the
natural or genetic classification. Thus as the study of plants
developed into the science of botany, the original artificial classi-
fication of plants, based on the number of stamens of the flower,
and on other superficial characters, became superseded by the
natural classification, which is based upon community of origin
of the members of the same group ; and in zoology, we find that
artificial classifications, based on superficial resemblances or the
possession of a common character, are constantly discarded as the
true genetic or natural relationships of animals are becoming more
fully understood.
Classifications of Rocks 67
A Convenient Artificial Classification of Rocks
A convenient classification of rocks, which has come into general
use, recognizes three fundamental divisions, as follows :
1. Igneous Rocks. Rocks which result from the cooling of a
molten mass, or magma, either upon the surface of the earth, or
within the crust at greater or less depths. An example of the
first is basaltic lava ; of the second, granite.
2. Sedimentary Rocks. Rocks which were formed (a) as me-
chanical sediments in water or air, (b) as chemical precipitates and
evaporation products from solution in water, and (c) as deposits,
formed by organisms either in water or air. Examples of these
are: (a) sandstone, shales, etc.; (b) rock-salt/ gypsum, cave
deposits (stalactites), etc.; (c) coral and shell limestones, chalk,
guano beds, coal. The three groups here indicated are commonly
made subdivisions of the sedimentary rocks, and designated re-
spectively as follows: (a) mechanical sediments, (6) chemical
sediments, and (c) organic sediments.
3. Metamorphic Rocks. These rocks were originally members
of one or the other, of the preceeding divisions, but have become
sufficiently altered by natural agencies, such as pressure, heat,
and so forth, so that for the most part the original characters are
obliterated or lost, and new and special characters are added.
Examples , of such metamorphic rocks are : (a) gneiss which
may be derived from an original granite, but may also have origi-
nated from another rock, even from a sediment ; (b) mica schist,
which may have been derived from a shale or an impure sandstone
or some other rock ; (c} marble, which is derived from some form of
limestone that may have been of organic, of chemical, or of me-
chanical origin ; (d) graphite, which may be derived from coal
beds or from carbonaceous shales, etc.
Such a classification is sufficiently serviceable for all practical
purposes, but it cannot be called a scientific classification, because
in the second and third group are included types of very diverse
origin; for the three groups mentioned* under sedimentary rocks
have little or nothing in common, except that they are sediments
in a very broad and indefinite comprehension of that term. More
correctly speaking, they are not igneous rocks, and not pro-
nouncedly metamorphic rocks, and this is almost their, only claim
to a grouping under a separate division. The metamorphic group,
68 Classification and Principal Types of Rocks
too, includes, as we have seen, rocks of very diverse origin, but
since it is not as a rule possible to determine, except perhaps after
prolonged and careful study and analysis, if then, what the char-
acter of the original rock was, it will probably always be necessary
to-retain this group as a matter of convenience.
Principles of a Natural or Genetic Classification of Rocks
While the practical worker may find the preceding classification
sufficient for all needs, and while in the succeeding chapters we
may frequently refer to these convenient three types, the student
should nevertheless understand the principles of a natural classi-
fication, and so. far as it is practicable we shall base our subsequent
discussions upon such a natural classification. For in geology
as in other sciences, logical thinking is of the first importance,
and logical thought is best fostered by the most rigid adherence to
exact methods of classification, and the only exact classification
of natural objects is one based upon genetic relationship, that is,
upon community of origin of the members of each group.
At the outset of our endeavor to understand the natural relations
of rocks we must clearly comprehend two fundamental principles.
The first of these is, that in this natural world of ours all things
are subject to continued change, even though that change may be a
very slow one, so slow that the years of a man's life or those of many
successive generations are of insufficient length to permit the
recognition of such changes. All rocks are constantly undergoing
modification, and though sometimes these changes may be rapid,
as in the coking of coal in a burning mine, or the change of clay to
brick in a kiln, the more usual method of change is a slow and grad-
ual one. In a certain sense all rocks may be considered as meta-
morpkic or altered rocks, a view strongly held by some geologists.
Extreme rnetamorphism, such as produced the common types which
are usually spoken of as metamorphic rocks, is only a phase of the
general alteration or rnetamorphism which all rocks undergo, and
this phase is characterized by the greater or complete obliteration
of the characters which the rock possessed during its early history.
This more pronounced change may have been brought about by
the greater intensity of the activities responsible for it, or by their
longer continuance in time, or by both.
The second principle, a knowledge of which is fundamental to
our understanding of rocks as well as all other natural objects,
The Unaltered or Little Altered Rocks 69
is that related types are not separated as a rule by sharp lines of
demarcation, but that always and everywhere in nature gradation
is the predominant rule.
Thus a sandstone may grade into, a shale, on the one hand, and into a lime-
stone on the other, and shale and limestone may intergrade, though of course
there are always types which retain the characters of one or the other with
sufficient distinctness to make their classification possible. A granite may
grade into a syenite, and this into a diorite ; and again, a granite and a diorite
may approach each other so closely in character that classification becomes
difficult. In like manner a sandstone may have so many characters of a schist
that it becomes a difficult question to which of the two divisions it belongs.
A shale may grade into a slate and a granite into a gneiss. With slightly meta-
morphosed rocks, classification is often very difficult, though a thoroughly meta-
morphosed rock is easily recognized as such.
Extreme metamorphism is only a later or final phase of the
change which all rocks are undergoing, and in. a natural classifica-
tion such end-products are to be placed with the group from which
they are derived. Two gneisses, for example, one of which is
derived from a granite and one from some type of sandstone, are
not related to each other any more than two human beings are
related because they speak the same language, or obey the same
laws of civilization. As before said, however, the practical diffi-
culty of determining the original character of a metamorphic
rock must be reckoned with, just as in the study of thoroughly
civilized human beings of different but entirely disguised nation-
alities, the practical difficulty of ascertaining the nationality of
each (assuming that the individuals refuse or are unable to disclose
it) must.be taken into account. In this respect thoroughly meta-
morphosed rocks hold the same relation to their ancestral type,
as thoroughly Americanized individuals descended from different
nationalities hold to their ancestors.
It thus becomes necessary for us to study first the unaltered or
but slightly altered rocks, the history of which is ascertainable
from the characters which they retain.
THE UNALTERED OK. LITTLE ALTERED ROCKS
When we study the unaltered or slightly altered rocks, we again
note that they fall into two quite readily recognizable divisions.
The first of these comprises rocks which have been produced from
material not originally rock, for example, by cooling from a lava,
70 Classification and Principal Types of Rocks
crystallizing from a solution in water, and the like, a group
which in a broad sense may be spoken of as produced in a chemical
manner. The other division comprises rocks which are made up
of fragments or particles of other rocks, such as sandstone made of
grains of sand, a conglomerate made of pebbles of various kinds
and of sand, and others like these. This is the group of fragments!
or clastic rocks (Latin: clastus, broken) which are always made
from some preexisting rock that has been broken down into discrete
particles, which then are recombined either directly or after more
or less assorting. The first group, on the other hand, is that of the
non-fragmental or non-clastic type, which is not formed from frag-
ments of other rocks, but from non-rock material. In the regular
order of formation this type would appear first, though it is per-
fectly possible that non-fragmental rocks may originate from
material which itself was derived from other rock, by melting,
solution, or vaporization of such a rock and subsequent resolidifica-
tion. But in this case there is an intermediate non-rock stage,
while the material of the, fragmental or clastic rocks is always rock,
' though it may be broken into the finest particles. We shall con-
sider the characteristics of each group briefly.
The Non-Fragmental Rocks
(Endogenetic .Rocks)
The non-fragmental rocks may be properly regarded as contri-
butions to the lithosphere from three of the other spheres ; namely;
the hydrosphere, the pyrosphere, and the atmosphere. This
contribution may be direct, as in the case of the hardening of a
lava from the pyrosphere, the separation out of salt from the water
of the hydrosphere, by the concentration of that water or by
chemical reactions, or the separation of snow or hail-stones by
the solidification of the water vapor from the atmosphere. Each
of these spheres may thus in a measure be credited with the genera-
tion of these respective rocks, and it is therefore possible to speak
of these three types, respectively, as : pyro genie, generated by the
pyrosphere; hydrogenic, generated by the hydrosphere; and
atmogenic, or generated by the atmosphere. Instead of these
names we may of course speak of these deposits as of igneous,
aqueous, and. atmospheric origin, respectively, recognizing the fact
that the rocks' contributed by the atmosphere direct, that is, the
snow and hail, are of an evanescent character.
The Unaltered or Little Altered Rocks 71
In addition to these three types, the direct contributions of
three of the inorganic spheres to the lithosphere, there is an
indirect contribution of material from two of these spheres by the
agency of organisms. Both animals and plants take lime from the
sea-water to build their shells and other calcareous structures, and
other organisms take silica from sea-water to build their hard parts.
Deposits of such organically secreted lime and silica are very
important in the construction of the lithosphere, and form, as a
rule, readily recognizable types of rocks.
Again, carbon is taken by plants from the carbon dioxide of the
atmosphere, and the plant tissues built up from it may often become
compacted into beds of coal which form important members of
the rock series of the lithosphere. Here again is an organically
formed rock, the material of which is obtained from the atmosphere.
Such organically formed rocks are the contribution of the bio-
sphere to the lithosphere, and in conformity with the method of
designation which implies such a relation to that sphere, they may
be called biogenic rocks.
We may then summarize in the following table these four funda-
mental types of non-clastic rocks, rocks which are produced essen-
tially by chemical (including physiological) reactions from non-
rock material. We term these rocks endo genetic because they
are formed by forces in a measure inherent in the material of the
spheres which produce them.
THE. FOUR TYPES OF ENDOGENETIC (NON-CLASTIC) ROCKS
1. Pyrogenic or Igneous : Produced by the pyrosphere.
2. Hydrogenic or Aqueous : Produced by the hydrosphere.
3. Atmogenic or Atmospheric: Produced direct by the atmosphere.
4. Biogenic or Organic : Produced by the biosphere from material taken
from the hydrosphere or atmosphere.
We may consider these rock types from another point of view.
Were we to imagine the rock material of the earth deprived of its
solid character, that which is essential to the constitution of a rock,
we should have to think of it as. existing in one or more of three
possible forms.
i. It may be turned into a molten mass like lava, by the applica-
tion of heat, and so constitute an igneous magma, in which all the
minerals which go to the making of the rock would become as it
were dissolved in one another, and the compounds dissociated into
7^ Classification and Principal Types, of Rocks
their ions. It would then become a part of the py'rosphere, and
in this state most of the rocks of the earth are believed to have
existed at an earlier period according to the believers in one hy-
pothesis of the earth's origin (see Chapter XXIX) when the earth
as a whole was a molten mass. Whether this' theory is correct or
not, the fact that many of the rocks of the earth's crust were at
one time in this state cannot be doubted. By solidification igneous
or pyrogenic rocks are* formed.
2. The rocks of the earth may be turned into a condition of
vapor by the application in most cases of still greater heat.
In such a state they would become a part of the earth's atmosphere.
The same hypothesis of earth-origin holds that this was a condition
of the entire rock mass of the earth at a still earlier period, and that
from this condition of vapor was separated at a later period the
molten material of the earth, and later still the water of the hydro-
sphere. Again, the truth of this hypothesis is not essential to the
recognition of the fact that some at least of the rocks of the earth
were formerly in a condition of vapor, and that they were separated
from the earth's atmosphere either by direct condensation, as we
see to-day in the formation of snow, or by the work of organisms,
such 'as has resulted in the separation of the carbon of the at-
mosphere, which now constitutes our coal beds, by the agency of
plants. Rocks derived by direct condensation from the atmosphere
are atmogenic rocks, while those separated by the activities (physio-
logical) of organisms, are biogenic or organic rocks.
3. Finally, we may think of the rocks of the earth, or at least
some of them, as dissolved in the universal fluid envelope of the
earth the water and so become a part of the hydrosphere.
If rock material, thus held in solution, is separated out from the
water by direct condensation, by chemical reactions, or by elec-
trolytic action, aqueous or hydrogenic rocks are produced. If, how-
ever, the dissolved material is separated out by the agency of
organisms, as in the formation of limestone masses on coral reefs
by polyps and lime-secreting seaweeds (algae), it becomes an
organic or bio genie rock.
There are no other primary states than those of fusion, solution
in water, or vaporization, into which the rocks of the earth may be
changed, nor are there any other known ways in which rocks are
formed from the three states of primary dissociation except by
direct precipitation or separation or by organic agencies. Hence
The Unaltered or Little Altered Rocks 73
the four rock types the igneous or pyrogenic, the aqueous or
hydrogenic, the atmogenic, and the organic or biogenic are the
FIG. 27. Diagram showing the interrelations of the Endogenetic Rocks.
only primary types recognized. These relationships are shown in
the preceding diagram. (Fig. 27.)
The Fragmented or Clastic Rocks
(Exogenetic Rocks)
These are the rocks which are made up of fragments of other
rocks, which may range in size from dust particles of microscopic
dimensions to boulders many feet in diameter. For their produc-
tion it is evident that preexisting rocks should be broken into frag-
ments, and that these fragments should become recemented or
bound together again in some manner. The methods by which
rocks are broken into fragments, and those by which the fragments
are recemented will be taken up later, but we may here use as
an illustration of this type of rock one of the artificial rock-making
processes carried on by man, and which differs primarily from the
natural process in the rapidity with which it is carried forward.
This is the process of manufacture of rubble concrete for paving
74 Classification and Principal Types of Rocks.
and other construction. Rocks like the trap of the Palisades, an
ancient igneous rock, are quarried and passed through the stone-
crusher, where they are reduced to rock rubble or particles of small
dimensions. They are assorted into different sizes by screening,
and bound together by a cement, which is a mixture of lime, alu-
mina, and silica, and of sand, which is the finer rock material obtained
either from the screening, or more commonly from natural sand
banks. The result is a clastic rock, but an artificial, one, and this
is produced in a few days, whereas a similar rock produced in
nature 'might require as many centuries or millenniums for its pro-
duction. But trap rocks or other igneous rocks are not the only
ones used in the making of rubble concrete, nor are igneous rocks
,the only source of clastic material in nature, though they are often
the most common one. All rocks, igneous, aqueous, and organic,
and clastic rocks as well, are broken into fragments by natural
agencies, and from these fragments new clastic rocks are made.
For the rocks of the earth's crust are being constantly reworked,
all of them, whether clastic or non-clastic, furnishing material for
the formation of a younger bed of clastic rock. Let us take an
example for illustration from the eastern United States.
There is in the foothills of the Catskill Mountains a great deposit
of bedded clastic rocks known to the arts as Hudson River .Blue
Stone, and used for the manufacture of flag-stones for sidewalks
in New York City and elsewhere, for curbings, and for many other
purposes. To the geologist this rock is known as a member of the
Middle and Upper Devonian series of rocks, of which we shall learn
more in the future. Examination under the microscope shows
that this rock is composed of ifiall particles, some of which are
themselves small fragments of clastic rocks (these are called das-
toliths) and this shows that this particular blue-stone rock is made
up of material derived in large part, if not wholly, from an older
clastic rock which was broken into fragments, assorted according
to size by natural agencies, and recemented to form new clastic
rocks, the " Blue Stone " being made up of the assorted finer par-
ticles only. From the character of the small fragments of clastic
rock (the clastoliths), and from a study of the structural and age
relations of the " Blue Stone " to the rocks of greater age, it has
been possible to determine that the material of the " Blue Stone "
was derived from the so-called Hudson River formation, which crops
out to the east of the Blue Stone area and forms in part the Taconic
The Unaltered or Little Altered Rocks 75
Mountain range along the New York-Massachusetts boundary
line. This rock is of much greater age than the " Blue Stone,"
belonging to the Ordovician period of the geological series (see
table, Chapter XXIV) . Similar examination of this Ordovician clas-
tic rock shows that it in turn was derived from a still older clastic
rock, this time of Pre-Cambrian age, some of which can be seen in
the ledges' of Manhattan Island. This clastic rock finally was de-
rived from still older igneous and metamorphic rocks (Berkey).
A second example from the central region of our country may be
given. In Ohio, Michigan, and Western Ontario is a great bed
of very pure sandstone, a clastic rock made up almost entirely of
small, well-rounded grains of quartz sand bound together. This
rock is so pure that it is used for glass-making in Toledo, Ohio. It
is known by the name of the Syhania sandstone, and it belongs
to the Silurian division of the geological series. Careful study
has shown that the grains were originally a part of a still older
sandstone, called the Saint Peter sandstone, which belongs to the
Ordovician division and covers a large area in the Mississippi
Valley region and east of that in Michigan, Canada, etc., though
its outcrops are found only in restricted areas. This rock was in
turn derived, at least in part, from the destruction of a still older
sandstone, the so-called Potsdam sandstone of Cambrian age,
which crops out farther to the north, in Wisconsin, Minnesota, etc.
This sandstone finally was derived from the still older granites,
gneisses, etc., which are seen in the Canadian region to the north.
Thus the Sylyania represents the third generation of sandstone,
the Saint Peter the second, and the Potsdam the first.
Finally we may note that the moetern sands of the Libyan desert
are derived from the breaking up into its component grains of an
older sandstone, the Nubian, and of the rock from which the
sphinx has been cut (Fig. 28). If these sands become bound
together into a sandstone at some future time, this sandstone will
be of the second generation.
We see, therefore, that clastic rocks may be of different genera-
tions. The first generation is always derived frorri some igneous
or other non-clastic rock or the metamorphosed product of such
a rock. The later generations are derived successively from older
elastics, though new contributions from the crystalline source
may also be made. This formation of a new and younger clastic
rock from the material of an older one may be compared with the
76 Classification and Principal Types of Rocks
construction, in more recent times, of man-made structures and
monuments from the stones obtained by the demolition of older
human monuments or structures.
The cycle of change which includes the successive generations of
a clastic series may be brought to an end by the melting of the
FIG. 28. The Sphinx, cut from a rock ledge and surrounded (and for-
merly partly buried) by desert sands derived largely from the destruction of a
sandstone in other parts of the Libyan desert. The Great Pyramid in the
background is covered by slabs of Nummulitic limestone.
elastics, and by their incorporation into a new igneous .mass,
or by their pronounced metamorphism. Then a new cycle of
formation of clastic rock will begin.
The Agents Active in the Breaking Up or " Clastation " of Rocks
We may now consider the chief agents which are active in the
breaking up or clastation of rocks, that is, those responsible for
the formation of clastic material from which clastic rocks are
formed by consolidation. We may classify the several types of
clastic rocks according to the agent which produces the material,
or which arranges it in the form in which it becomes consolidated.
The methods of clastation will be considered in a later chapter,
The Unaltered. or Little Altered Rocks 77
The Atmosphere as a Rock Breaker. In the first place, rocks
are broken up or " clastated " by the atmosphere acting either
chemically, by the action of the various gases and vapors of the air
upon the rock, or mechanically, as in the case of freezing moisture,
or by the wind. Clastic material thus produced may be called
atmodastic material, and rocks formed by the consolidation of such
clastic material may be called atmodastic rocks. As clastic material
is often accumulated in certain localities after transportation by
the wind, i.e., the atmosphere in motion, and as this material has
generally a very definite form and structure, we may further dis-
tinguish wind-arranged or wind-deposited clastic material, such as
is found in sand-dunes. When this is consolidated into a rock it
becomes a wind-formed or eolian rock, a rock which may also be
called anemoclastic from the Greek anemos (avc/xos), the wind. 1 There
are many examples of such wind-formed, eolian, or anemoclastic
rocks to which we shall call attention again in a later chapter.
We may mention here as examples the recent dune-rock of the
Bermuda Islands, and the much older sandstones of the White
Cliffs in the Colorado Plateau region. The first of these was
formed in the modern or Holocene period, the second in the Jurassic
period, and this has retained its structure ever since that time,
though much of the original rock, which had a far wider distribution,
has been worn away again from large areas.
The Hydrosphere as a Rock Breaker. The second great
agent which accomplishes the destruction of rocks is the hydro-
sphere. This destruction, so far as it bears on our present point
of view, is mechanical in its nature, although chemical work, the
solution of rocks (such as limestone, salt, etc.) is also performed
by the hydrosphere. Such solution, however, results in the
reincorporation of the rock material into the material of the hydro-
sphere from which in turn it may be deposited as an aqueous or
hydrogenic rock (stalactites and other cave deposits). Such a
rock is, however, not a clastic but a " genie " rock, and the student
should learn to make the proper distinction between these two.
The mechanical work of water is manifested in streams, where
the current moves the rock fragments which it has broken from the
ledges, and grinds them down as it carries them along. It is also
seen on the seashore, where the waves produce an analogous effect.
1 We have this same root in the word Anemone, the name of the wind-flower, -1
anemometer, the instrument for measuring the velocity of the wind, etc.
in
78 Classification and Principal Types of Rocks
But in addition to the breaking off of the rock fragments and the
grinding of them to sand and pebbles, moving water in streams
and on the sea-coast assorts clastic material, however produced,
into grades of various sizes, and more or less according to the weight,
and therefore the nature, of the material. Moreover, clastic
material deposited by and in the water will generally be char-
FIG. 29. Gorge of the Genesee River below the Lower Falls at Portage,
N.Y., showing in the opposite banks, the cut edges of the stratified rocks (shales
and sandstones) which formerly were continuous across the gorge.
acterized by a bedded structure ; that is, it will exhibit, if seen in
section, a succession of layers or strata one. above the other, each
one of which was at one time the topmost layer (Fig. 30). Such
deposits are called. stratified, and of course, at any on*e time, only the
top of the topmost stratum is visible. Where a river, however, has
cut a gorge into an older stratified series, or where the waves on the
sea-coast or lake shore again partly wear away such a series which
has been lifted above the level of the sea by natural disturbances,
or rises above the lake-level, the cut edges of such strata can be
seen. Thus on the opposite banks of the Genesee River (Fig. 29)
we see the cut edges of the successive strata which were formerly
continuous across the chasm. On the sea-coast at Atlantic High-
lands, N.J., we likewise see the edges of the strata which were
exposed because the waves cut laterally into the old deposit which
The Unaltered or Little Altered Rocks
79
had been previously uplifted ; and a similar exposure of successive
strata is seen on the shore of Lake Erie (Fig. 16, p. 31) and on
many other shores (Fig. 30).
In addition, to the bedded structure called stratification, which
is characteristic of water-laid deposits of clastic material (and to
some extent of hydrogenic deposits also), there are other features
FIG. 30. Sea cliff of the south coast of Helgoland, showing the edges of
the stratified rocks .of which the island is composed, exposed by wave cutting.
The strata are of Permian age and dip towards the north, but appear to be
horizontal in the sea cliff. (After E. Haase, from Walther.)
by which water-laid elastics may be recognized. These will be
more fully discussed in a later chapter. At present it is merely
desired that the student should recognize that water-laid deposits
have definite characters. Some of these may point to a deposition
of the material on a river flood-plain, 'an alluvial fan; or a delta;
some to deposition in lakes, ponds, or playa basins, and some to
deposition in the sea. These last may be regarded as the most
typical examples of water-laid elastics, and they are generally
characterized by the inclusion of the shells and other remains of
marine organisms. A sandstone or mud-rock with marine fossils
(Fig. 31), such as may be obtained from numerous localities the
world over, will serve as a typical example of a water-laid clastic.
In accordance with the general method of naming such deposits,
those under consideration may be termed hydrodastic. They
8o Classification and Principal Types of Rocks
may also be called aqueous elastics or sediments, in distinction from
aqueous precipitates or concentrates, which belong to the group
of hydrogenic deposits.
The Pyrosphere as a Rock Breaker. As we have seen, the
pyrosphere contributes pyrogenic or igneous material, which on
FIG. 31. Photograph of a slab of sandstone filled with marine fossils. (Oris-
kany sandstone, N. Y.)
cooling and hardening forms a part of the solid crust of the earth.
It "also breaks up material already rock, and this is generally
brought about by explosive activities such as
are characteristic of most volcanoes. By these
activities the rock, whether an older lava Qr a
rock of other origin, located in and around the
crater, may be shattered and the material
thrown high into the air, to descend as a shower
of ashes or larger particles. Such material,
which is readily recognized by its character, is
called pyroclastic material, and when bound to-
gether to form a rock, this becomes a pyro-
FIG. p. Fault- clastic rock. Examples: volcanic tuff, volcanic
breccia. . _ r '
agglomerate, etc.
Shattering of Bfocks within the Lithosphere by Movements.
When one part of the lithosphere moves over or against another
The Unaltered or Little Altered Rocks
part, as happens when the earth's crust suffers disturbances (see
Chapter XXI) the rock along the plane of movement is shattered
or ground fine, and clastic material with very definite character-
istics is produced. As such a movement of adjoining rock masses
generally produces a displacement, or fault (see Chapter XIX), the
shattered or ground-up material is commonly called a fault-breccia
(Fig. 32). A more general term for such material is autoclastic. }
because it is produced by the self-destruction or breaking up of the
rocks of the earth's crust.
The Biosphere as a Rock Breaker. That growing plants,
such as trees, arising from seeds which lodged in a fissure of the
rock, can by expansive growth shatter that rock, has been fre-
quently observed (Fig. 33). Animals, too, break rocks. Thus
where a spring issues
upon a level surface in
a more or less arid
country, vast herds of
hoofed animals will con-
gregate to drink, and
their constant stamping
of the rock surf ace will
break it and produce
dust and sand which
mingles with the water
and which on drying
may be carried away
by the wind. Thus
hollows are produced
in the surface of the
land, and these may
be filled with water
and so produce ponds.
Many such are known to exist in western North America, in
north and central Africa, and elsewhere. Fish and other animals
in the sea will break off branches of coral from reefs (biogenic rock)
and grind these to powder to obtain for nourishment the animal
matter which surrounds these coral masses. Thus much coral
sand and mud is produced, and this is clastic rock material of
strictly organic origin.
But by far -the greatest destroyer of rock is man. We have seen
FIG. 33. A huge gravestone, broken and
displaced by the growth of the roots of a birch-
tree. Hannover. (After Walther.)
82 Classification and Principal Types of Rocks
how rock is broken down to be used for the making of artificial
rock, that is man-made rock (rubble-concrete, etc.). As man is a
part of the organic world or biosphere, his work must be classed
with that of other organisms. Clastic material thus produced
by organisms, and the rocks,- whether natural or artificial (i.e.,
man-made), made from these, may therefore be called biodastic.
Summary of Clastic Rocks
We have thus five main groups or classes of clastic rocks, each
produced by one of the spheres and named after it. (See the dia-
gram, Fig. 34.) In general the rock is regarded as belonging to_
FIG. 34. Diagram of the interrelations of the Exogenetic Rocks.
that particular class, the agent of which has placed upon it its
characterization stamp. Tims the material of a water-laid rock
may have been originally produced in another way than by aqueous
erosion it may have been produced by weathering, after which
it was sorted by and deposited in the water. Thus water has
stamped it as undeniably an aqueous clastic or hydroclastic rock.
If it is possible, as it sometimes is, to determine how the material
The Unaltered or 'Little Altered Rocks 83
originated before it was deposited by and in' water, this may serve
to further characterize the rock as of a special division in the hydro-
clastic group. The types of clastic rocks then are :
1. Atmodastic rocks. Rocks produced by atmospheric destruc-
tion of rocks, and consolidated without rearrangement. Example :
consolidated laterites.
i a. Anemo clastic rocks. Eolian rocks. Material of variable
origin, transported, assorted, and deposited by wind. Example :
eolian sandstone.
2. Hydroclastic rocks. Rocks produced from material eroded
or sorted by, and deposited by and in water, whether by rivers
(fluviatile) in lakes (lacustrine) or in the sea (marine). Examples :
fossiliferous marine sandstones or shales.
3. Pyrodastic rocks. Rocks formed from material which has
resulted from shattering of older rocks by volcanic explosions.
Examples : volcanic tuff, volcanic agglomerate.
4. Autodastic rocks. Rocks formed of material shattered or
ground by movements within the earth's crust. Example: .fault-
breccia.
5. Biodastic rocks. Rocks produced from material broken or
arranged by animals (more rarely by plants) and by man. Includes
artificial rocks. Examples : some consolidated muds from coral
reefs, rubble-concrete, etc.
In the succeeding chapters we will consider first the commoner
types of rocks, in which the original characters are retained, and
their structures, leaving the metamorphic derivatives for a future
chapter.
CHAPTER VI -
THE PRINCIPAL TYPES, OF IGNEOUS OR PYROGENIC
ROCKS
THE IGNEOUS MAGMA
THE term molten magma is applied to molten rock material or
igneous fluid, which is formed or exists within that part of the
earth which we have called the pyrosphere, and which, it will be
remembered, interpenetrates the lithosphere and the asthenosphere
(tectosphere). When the magma reaches the surface of the earth,
either in a volcanic eruption, or through large fissures in the earth's
crust, as in parts of Iceland to-day, it is called & fluid lava, from
which by solidification in cooling a solid lava or lava-rock is pro-
duced. Such a rock mass constitutes an extrusive or effusive
igneous rock mass. When the magma does not reach the surface,
but cools within the crust of the earth, into which it has risen or
become intruded to a greater or less extent, either upon or by the
" formation of fissures, or otherwise, it forms an intrusive igneous
mass. Finally, a magma 'may be conceived as cooling essentially
where it was formed, and to produce a deep-seated or abyssal igneous
rock mass.
Outcrops of Igneous Rock Formed by Solidification from Magmas
It is obvious that only the surface-flows or lavas will be accessible
to man immediately after cooling, arid of these generally only the
superficial portion. All the other igneous rock masses are located
within the earth's crust, and buried beneath the surface rock masses,
sometimes to very great depths. Some of those which closely
approach the surface may be reached by deep borings or by mining
operations, but this can occur only in exceptional cases. Some of
these may also be conceived of. as brought into view. on the face
of a great dislocation-block of the earth's crust, where one side
of the broken crust is lifted and the other depressed. This too,
84
The Igneous Magma . 85
however, is probably so exceptional that it may be considered,
in the absence of other modifications, to be practically negligible.
In general, it is only after prolonged erosion, which results in the
removal of much of the surface-covering of the rock, that intrusive
masses become exposed, while deep-seated masses may require
the removal of many thousands of feet of covering rock before they
become visible. This removal of great covering rock masses
requires, of course, long periods of time, and it thus becomes
evident that the igneous rock masses, other than surface lavas,
which are now visible in outcrops, are of great age, and that their
formation by cooling has taken place at remote periods of time.
Igneous intrusions and deep-seated masses which are now being
formed are entirely invisible to us, and will not be exposed until
many ages have passed by, if ever. Nevertheless there are indirect
ways in which we can infer the existence of igneous masses beneath
the surface, which are now undergoing the process of solidification,
and to some of these we shall refer later.
Characters and Composition of Igneous Magmas
It is not possible to gain a complete knowledge of the composition
of an igneous magma from the composition of the igneous rock
which has resulted by solidification from that magma, because
the magma contains in -addition to the substances which make
up the solid rock formed from it, large quantities of volatile gases
which are expelled upon cooling and relief of pressure. The volatile
gases which are 'expelled under these conditions are, water vapor,
carbon dioxide, hydrochloric acid, sulphurous vapors, and the like.
Such expulsion of volatile matter is shown by all surf ace . lavas
from which clouds of steam arise, and such steam on analysis proves
to carry with it many of the other gaseous emanations. Others
quickly condense upon the surface and may form salts of various
kinds, which either encrust the surface of the cooling lava,
or are carried away in solution by the condensing waters from the
water vapors. Sometimes the expulsion of gas and vapors is so
rapid that violent explosions result, and this is indeed the chief
cause in the production .of pyroclastic material. In practically
every volcanic eruption there is produced a cloud of vapor and
gases which carries upward vast masses of finely divided rock
material the product of the explosive action and often rises
86 Principal Types of Igneous or Pyrogenlc Rocks
to heights of many miles above the volcano (Fig. 35). When
gases and vapors alone issue from a fissure in the earth, the
phenomenon is spoken of as a fumarole. Fumaroles are commonly
associated with de-
clining volcanic ac-
tivities. The gases
and waters of cool-
ing magmas within
the earth's crust may
escape through fis-
sures not penetrated
by the magma or
opened since cooling
began, and these
FIG. 35. Volcano in eruption, showing the cloud
of steam and ashes- projected high into the- air.
may reach the sur-
face as mineral
springs, either hot or cold. In their upward passage, such waters
and gases may deposit mineral matter which they carried, and this
is believed by many to account for vein and other mineral deposits.
The non- volatile material of the magma which solidifies to form
the lava or other igneous rocks consists predominantly of only a
small number of substances, chief among which is silica (SiO^),
which, however, is present in variable amounts, according to the
nature of the magma." In addition to this there are the oxides of
the metals aluminum (Al), iron (Fe), magnesium (Mg), calcium
(Ca), sodium (Na), and potassium (K). These, too, vary in
amounts in the different magmas. On solidification th.e silica
unites with them to form various silicate minerals, which in the
coarser-grained igneous rocks can be readily distinguished.
In a general way there can be recognized a gradation in the com-
position of the magmas from a point where silica is most abundant,
sometimes forming 75 per cent of the whole mass, with alumina
and potassium next, by an increase in sodium and later in calcium,
magnesium, and iron, and a reduction in silica to a point where this
constitutes 50 per cent or less (rarely 35 per cent) of the entire mass.
With this occurs a reduction in the oxides of potassium, sodium,
and aluminum, sometimes to the complete or nearly complete
elimination of some of these. The end high in silica, etc., is called
the acid end of the series ; , the other end is the basic end. Rocks
formed from the acid portion of an igneous magma generally have
Formation of Igneous Rocks by Cooling of Magma 87
light-colored and light-weight minerals predominating ; those
formed from the basic portion have mainly dark-colored and
heavy minerals. Acid magmas (and lavas) are generally very
stiff or viscous even at high temperatures (2000 C. or over), and
their contained gases escape with difficulty, and often with explo-
sive violence, as the magma approaches the surface. Basic mag-
mas (and lavas), on the other hand, are more fluid even at much
lower temperatures (1300 C.), and on this account the gases
escape more readily, and explosions are less common. The lavas
of Vesuvius and Mont Pelee are examples of the first, those of
Kilauea in Hawaii, of the second.
FORMATION or IGNEOUS ROCKS BY COOLING. OF MAGMA
With the cooling of the non-volatile part of the magma, solidifi-
cation takes place and igneous rocks are produced. The kind of
rock will of course vary with the variation in the composition of
the magma, which must be considered to be the character of first
importance. Next to this is the rate of cooling, which determines
the grain or texture of the rock, and which in turn is influenced
by the relative position of the magma while cooling.
Influence of Rate of Cooling. Texture
It is a well-ascertained fact that a rapidly cooling magma will
tend to produce a mass of glass, and that in proportion as the rate
is slower will there be opportunity for the growth of mineral
crystals, the size of which is, in general, proportional to the slow-
ness of the cooling. When the
entire rock becomes a mass of min-
eral crystals, it is said to be holo-
crystalline. Between this and the
glassy type in which there are no
crystals, there are all gradations.
The relative size and arrangement
of the component crystals of a
rock form its texture.
Primary Textures. Two gen-
eral types of primary texture may
be Distinguished, the homogeneous or uniform, and the hetero-
geneous or porphyritic. In the first all the crystals of each mineral
FIG. 36. Porphyry ; typical por-
phyritic texture.
88 Principal Types of Igneous or Pyrogenic Rocks
are essentially of uniform size, though the size varies with the
mineral. In the second, or porphyritic type (Fig. 36), on the other
hand, the crystals of certain of the essential minerals (generally
the feldspars) are of two sizes, one large and more or less fully and
perfectly formed, the other small and less perfect. The larger
crystals, which are scattered through
the mass, are called the phenocrysts;
the finer-grained mass, together with
other minerals, also in small grains,
forms the ground-mass.
Secondary Textures. The tex-
tures of the ground-mass and of the
homogeneous textured rocks are
called secondary. When the grains
or crystals are all nearly uniform,
FIG. 37. Granitic texture the texture is said to be granitic or
(holocrystalline), slightly granular (Fig. 37), and it may be
enarge * coarsely or finely granular, i.e., the
texture is coarse-grained or fine-grained so long as the individual
crystals are distinguishable. But when the crystals are no longer
distinguishable, the texture becomes dense or felsitic, while a step
further brings us to the glassy texture in which no crystals are
developed.
Relation of Textures to Kinds of Magmas
In general, the basic magmas, being more fluid, will tend to form
' coarser crystals, the resulting rocks therefore being more com-
monly coarse-grained. Glassy textures are correspondingly less
frequently developed. Acid magmas, on the other hand, tend to
develop the finer textures more frequently, and here glassy rocks
are common.
The presence of much gas and water vapor, too, tends to increase
the power of crystal forming, and along certain fissures, presumably
the pathway of escape of these gases and vapors, the texture is
often of exceeding coarseness. The most common examples are
the dikes or other masses of pegmatite, a rock composed sometimes
of huge crystals of feldspar, quartz, and mica, the latter mineral
being obtained in large plates from this rock, which is the chief
commercial" source of this mineral.
Formation of Igneous Rocks by Cooling of Magma 89
Relation of Texture to Place of Cooling and Bulk of Magma
As the coarseness of grain is in proportion to the slowness of
cooling, it is evident that, other things being equal, whatever
lowers the rate of cooling will cause an increase in the size of the
crystals, and of the coarseness of texture. Thus a magma cooling
within the earth's crust, especially at great depth, will of necessity
cool slowly, and so become a coarsely crystalline rock. Again,
the central part of the magma will cool more slowly than its outer
part, which is in contact with the cooler
enclosing rock. In like manner small masses
will cool more rapidly than large ones, and
this is especially the case when the igneous
mass is a thin sheet transecting other and
cooler rocks (dikes (Fig. 38), sills, etc., see
beyond). There is, however, one modifica-
tion of this general rule, which should be
noted, and that is the conditions under which
porphyritic rocks appear. In these the fine FIG. 38. Small
ground-mass indicates rapid cooling, but the dike of basalt show -
presence of the large phenocrysts shows that
these crystals grew to their full size before granular texture,
the ground-mass solidified. In other words,
the phenocrysts were floating crystals in a still semi-fluid matrix.
Typical porphyries (Fig. 36) (with fine-grained ground-mass) are
most common in certain lava flows (as in the typical trachyte
from Drachenf els, Germany), somewhat less so in the smaller in-
trusive masses, and still less common, and indeed rather rare, in
the great subterranean masses. All "types of igneous rocks may,
however, show a porphyritic texture.
Classification of Igneous Rocks
From what has been said up to this point, it appears that the
principles which underlie the classification of igneous rock are
relatively simple. The first general division is made upon a chem-
ical basis, which in the rocks is expressed by the mineral species
present. Further subdivisions are based upon the texture of the
rock, for the same magma may furnish rocks of different textures
as the result of cooling at different rates under different conditions.
9 o Principal Types of Igneous or - Pyrogenic Rocks
Since, as above stated, the character of the magma is most readily
ascertained from the minerals which crystallize from it, chief
attention is ordinarily given to these minerals. Furthermore,
since these are readily recognizable only in the coarse-grained or
holocrystalline products of cooling of the magmas, it is customary
to designate the rock groups based on chemical and mineralogical
characters by the name of the typical holocrystalline member of
each group. Before we. consider these types, however, we must
briefly review the essential minerals which enter into the construc-
tion of these rocks.
The Essential Minerals of Igneous Rocks
We 'may in general distinguish three groups of essential minerals
which make up the bulk of the igneous rocks. These are the
following, arranged in each case in the order from acidic to basic.
They are: (i) Quartz, (2) The Feldspars and Feldspathoids, and
(3) the Micas and Ferro-Magnesian Silicates.
Qtiaxtz (SiO*). -This generally occurs in glassy fragments of
irregular and usually of angular form, more rarely in. crystals.
It is easily recognized by its hardness (see pp. 49 and 61).
The Feldspars. In composition these are all double silicates
of .aluminum and an alkali metal (K,Na, etc.) or an alkaline .earth
(Cst,Mg, etc.) or both. In general the feldspars are divided into
orthoclase 1 or the feldspar with right-angled cleavage, and plagiodase
or the feldspar .with oblique cleavages. The most acid feldspar con-
tains only potassium in addition, to both alumina and silicic acid
(potash feldspar or orthoclase), the most basic only lime in place of
the potash (lime feldspar or anorthite). Between the two stands
the soda feldspar (albite) where sodium is the metallic base besides
aluminum. Between the pure soda and the pure lime feldspars are
a number of intermediate types, consisting of different proportions
of both soda and lime ; and between the pure potash and soda feld-
spars there are also mixtures of the two.
These relations may be expressed in the following table, where the
three main or (theoretically) pure types of feldspars are each
expressed by a molecular symbol and the mixed types by formulae
which indicate the proportions of each.
" * Mtcroline is a potash feldspar like orthoclase, but crystallizing in the triclinic
system, and therefore of the plagiodase type.
Formation of Igneous Rocks by Cooling of Magma 91
Table of the Feldspars
PRINCIPAL OR
PURE TYPES
MIXED TYPES
COMPOSITION
(Chemical Fonnula)
COMPOSITION
(Molecular Symbol
or Formula)
ORTHOCLASE
{Monoclinic)
Orthoclase
(Potash
Feldspar)
K 2 A1 2 3 6 SioJ
or i
KAlSisOs
Or
Anorthoclase
. . i
O^Aba
to
OnAb 4 . fi
OJ
'S
CO
o
is
"o
<
AlUte
(Soda Feldspar)
Na 2 O - A1 2 3 6S;0 2
or i
NaAlSigOs
Ab
to
(AbgAni).
Oligoclase
AbeAni
to
Ab 2 Ani
^AGIOCLASES
(Triclmic)
(Andesine)
Ab 8 An 2
to
Ab4An 3
Labradorite
AbiArti
to
AbiAn 2
1
tn
^
%
rt
Bytownite
AbiAns
to
AbiAne.
Anorthite
(Lime Feldspar)
CaO - A' A 2 SiO 2
or
CaAUSiA
An
to
(AngAbi)
In ordinary rock determination it is very difficult to recognize
the intermediate feldspars, though this may be done with more or
less precision when a thin slice of the rock is placed under the micro-
scope, and examined by the use of polarized light. From chemical
analysis, however, which gives the amount of each substance pres-
ent, it is possible to calculate the types of feldspar (and other min-
erals) present in the rock, a calculation which must, of course, be
checked by microscopic examination. The plagioclase feldspars
usually .show fine parallel (twinning) striae on some faces.
92 Principal Types of Igneous or Pyrogenic Rocks
In general, it may be said that orthoclase, albite, and ollgoclase
are characteristic of acidic rocks, or rocks high in silica, with ortho-
clase in the most acidic, while labradorite, bytownite, and anorthite
characterize the basic rocks, with the last at the most basic end
of the series.
The Feldspathoids. Under this designation are placed double
silicates of aluminum and the alkalies or alkaline earths, which
have many characters in common with the feldspars besides the
general similarity, in their composition, but differ in crystal form
and some other characters. Theoretically we again have three types,
the potash, soda, and lime feldspathoids, but actually the first two
are more commonly intermixtures. This is shown in the following
table. In the order of their importance as rock constituents, these
minerals are nephelite, leucite, and melilite, the last being very
rare.
Table of the Feldspathoids
MINERAL
COMPOSITION
(pure)
, USUAL MODIFICATION
Leucite
K 2 0-Al 2 3 .4Si0 2
or
KALSi 2 6
Some Na 2 O replaces
part of K 2
Nephelite
4 Na 2 4 AlaOs 9 Si02
or (almost)
NaAlSi0 4
Some K 2 and CaO
replaces part of Na 2
Melilite
i2CaO-2Al 2 3 '9Si0 2
or
Ca 3 Al(SiO 4 )3
In general, the occurrence of leucite in igneous rock implies a
magma rich in potash ; nephelite, one rich in alumina and soda ;
and melilite, one poor in silica and alumina but rich in lime.
Melilite occurs mainly in a few rare basalts.
Common Mica and the Ferromagnesian Minerals. The
name ferromagnesian silicates is given to the minerals (usually dark
in color) which include in their composition both iron and magne-
sium. They comprise the dark mica, biotite, the hornblendes, pyrox-
enes, and olivine. The common or white mica (muscomte) has neither
iron nor magnesium, but is a silicate of potassium and aluminum
Formation of Igneous Rocks - by Cooling of Magma 93
together with some hydrogen. Its usual formula is K(OH)-
A1 2 O3-* 28162, or HKAl2Si2Os and it is commonly called the potash
mica. As might be inferred, it occurs in rocks where other potash
minerals are common, such as granites/pegmatites, etc. It is also
common in metamorphic schists.
In the ferromagnesian silicates we trace a gradation in com-
position from biotite, or black mica, in which potash is present,
through the amphiboles and pyroxenes, in which the potash is re-
placed by lime, to olivine, where no alkali or alkaline earth is pres-
ent. This is shown in the following table where muscovite is also
placed at the acidic end and magnetite, the pure iron oxide, at the
basic end.
Table j}f the Ferromagnesian Minerals and Their Two End Members
MINERALS
COMPOSITION
USUAL MODIFICATION
Muscovite
H 2 KAl 3 (SiO 4 )3
to
(HK)Al 2 (Si0 4 ) 2
Without iron or magne-
sium.
Biotite
, (HK) 2 (MgFe) 2 Al 2 (Si0 4 ) 3
(nearly)
Addition of magnesium
and iron ; reduction of
alumina and silica.
AmpMbole
(Hornblende)
Ca(MgFe) 3 (SiO 3 ) 4
(in general)
Substitution of , calcium
for hydrogen, potassium,
and aluminum, though
some of the last may be
present.
Pyroxene
(Augite)
Ca(M g Fe)(Si0 3 ) 4
(in general)
Olivine
(MgFe) 2 Si0 4
Omission of calcium, some
reduction of silica.
Magnetite
Fe 8 O 4
Omission of silica and mag-
nesium.
It must be understood that the amphiboles and pyroxenes are
more complex in composition than here stated, and that there are
a number of varieties of each, differing in composition. It is, how-
ever, not necessary that these be considered here. The common
94 Principal Types of Igneous or Pyrogenic Rocks
amphiboles and pyroxenes crystallize in the'monoclinic system, but
there are also orthorhombic members of each group. ^ The chief
means of distinction is the (prismatic) cleavage form, that of the
pyroxenes being nearly at right angles, and that of the amphiboles
forming angles of nearly 120 and 60 degrees, respectively, as shown
Amphibole
FIG. 39. Basal sections of crystals of Pyroxene and AmpHbole, showing
characteristic differences in outline, and cleavage as seen under the microscope.
A characteristic interference figure is shown in Pyroxene'. (After Moses and
Parsons.)
in the above outlines (Fig. 39). In general, it may be said that
whereas muscovite occurs chiefly in the most acidic rocks, biotite
and hornblende indicate greater basicity, pyroxenes still greater
basicity, while olivine is characteristic only of the very basic igneous
rocks. When pyroxene and olivine are present, free :quartz is
usually absent.
Secondary or Accessory Minerals. There are many minerals
which occur in small quantities in igneous rocks but are not neces-
sary constituents of them. They are called accessory minerals,
and their presence is most frequently detected by the microscope.
Zircon and titanite are good examples of these. By alteration
many other accessory minerals are formed from the primary or
original ones.
Order of Crystallization
In general the order of crystallization of the minerals from an
igneous magma follows the order of their basicity, the most basic
separating out first, the most acid last. Thus olivine will form in
perfect crystals from a basaltic magma; the other minerals being,
less perfectly crystallized. The pyroxenes and the hornblendes
separate out before the feldspathoids and the feldspars, while
Types of Igneous Rocks 95
the quartz, if there is any free silica remaining, separates out last,
filling the interstices between the other minerals. For that reason
quartz is practically never in perfect crystal form in granites or
other igneous rocks which contain it, but forms irregular grains,*
the shape of which is determined by the form of the cavities left
between the other minerals. It has a crystalline structure, how-
ever, though not a crystal form.
TYPES or IGNEOUS ROCKS BASED ON COMPOSITION
AND TEXTURE
The table on page 96 summarizes the more important types of
igneous rocks, beginning with the most acidic on the left, and ex-
tending to the basic types on the right. It will be seen that there
are certain composition groups based upon the kind of feldspar (or
feldspathoid) present, the kind of ferromagnesian mineral, and the
presence or absence of free quartz or olivine. _ Each of these com-
position groups includes a series of rocks ranging in texture from
coarse (at the bottom) through fine and dense or felsitic to glassy (at
the top). The coarser-grained rocks are generally formed as deep-
seated masses, which have become exposed by erosion. The in-
termediate types occur mainly as intrusive masses, while the dense
and glassy types (including the cellular) are primarily formed from
surface flows. As the mineral character of the several groups is
most readily ascertained in the case of the coarse-grained varieties,
the various groups will be considered under these respective types.
Microscopic examination is generally necessary to ascertain the
characters of the finer-grained and dense types, though typical
examples may generally be recognized without such aid.
The Granite-Rhyolite Series
This contains the most acidic of the common igneous rocks.
Common types are : granite, rhyolite, quartz-felsite, obsidian, and
pumice, and the porphyritic varieties of these.
Granite. The coarse-grained members consist primarily of
orthoclase feldspar and free quartz. When these alone are present,
as is sometimes the case, the rock is called a Unary granite. Gen-
erally, however, black mica (biotite) and often muscovite and some
hornblende are present, these being readily recognized by their
dark color, while the biotite is distinguished from the hornblende
96 Principal Types of Igneous or Pyrogenic Rocks
"I
Principal Ign
ed
X
'""'
j ^
as
<P
3*9
Withou
Olivine
s|
&!
ta
Witho
Olivi
SSVH
Andesite pumice
Andesite perlite
Andesite obsidia
ASSV1O
f Fel
Pla
C
A
Biotite (or) (and)
Hornblende
etimes with Augite
Wit
Qu
itic
ite
II !|
fc^i-a-
.5^!'
56:
t
-.
j
2 I 8
: 2
i ft
Diorite
Porphyritic
diorite
Porphyri
granite
Types of Igneous Rocks 97
by its scaiiness and softness. The orthoclase is commonly recog-
nized by its pinkish or flesh color and by the smooth cleavage planes
which it shows, as well as the more or less marked crystal form.
The quartz is always in irregular, glassy masses, breaks with rough
fracture and, as a result, looks darker than the feldspar by reflected
light.
Granites vary by the replacement of some of the orthoclase by
acid plagioclase while hornblende may increase in amount, forming
a hornblendic granite. With this may go a reduction in the
quantity of free quartz present, when the rock approaches a
syenite in composition, while with the increase in plagioclase the
quartz-diorites or diorites are approached. In rare cases, too,
augite may occur, showing an approach to gabbro. Among 'the
common accessory minerals are magnetite, zircon, and garnet. A
porphyritic texture is sometimes developed by the formation of
large and more or less perfect crystals of orthoclase, which fre-
quently show a twinned character (Carlsbad twins), recognizable
by the fact that one half of the
crystal appears darker than the
other in reflected light, the shade
being reversed with change in the
position of the light.
Pegmatite. This is the name
given to a coarse variety of a
granite which occurs in vein or
dike-like masses, and generally
consists of large crystals of ortho-
clase (sometimes acid plagioclase)
occasionally up to a foot or more
in diameter, large masses of quartz,
and large plates of mica (musco-
vite). The quartz and feldspar
are sometimes found intergrown in such a manner that the surface
of the feldspar seems to be scattered over with small, irregular,
dark masses of quartz, appearing not unlike cuneiform characters.
On this account such a mixture of feldspar and quartz is called
graphic granite (Fig. 40).
Rhyolite and Quartz Felsites. These have essentially the same
mineralogical composition as granite, though the association may
vary more, producing greater variety. In color the rocks are gen-
FIG. 40. Graphic Granite;
slightly reduced.
98 Principal Types of Igneous or Pyrogenic Rocks
erally light gray, with yellows, and pale reds, and occasionally darker
shades. The texture varies considerably, from very finely crystal-
line to dense or felsitic, when the rock consists of minute masses
of quartz and feldspar often with more or less glass. A cellular
structure may also be developed to a slight degree in rhyolites
formed from surface flows. In many rhyolites, especially if they
are porphyritic, quartz can be recognized (commonly as small
double-sided pyramids), giving the surface a rough feel. Pheno-
crysts of feldspar also occur in the porphyritic types. When the
phenocrysts of quartz and feldspar make up about half the mass
of the rock it is called a rkyolite-porphyry; when it constitutes most
of the rock it is called a granite-porphyry and marks an approach
to granite. In both the ground mass is generally dense or felsitic.
The name rhyolite is given to this rock because of the flow
, structure often exhibited (Greek, petV=flow). ,
The Acid Glasses. These include obsidian, pitchstone, perlite,
and pumice. In these the excess of silica, which on crystallization
would produce free quartz, can generally be recognized only on
analysis, except when phenocrysts are developed. As the magmas
which produce syenites on crystalliza-
tion also form glasses of similar ap-
pearance, though poorer * in .silica, it
is evident that it is not possible to
determine, except by analysis^ whether
a given glass belongs to that or to the
granite series.
Obsidian. This is a homogeneous
glass with a low percentage of water.
FIG. 41. Obsidian, showing It is black to red in color, with trans-
conchoidal fracture. lucent edgeSj and a ^^0^1 f r a c .
ture, so called because the surface of
the fracture is generally marked by a series of concentric lines, like
the growth-lines of a shell (Fig. 41).
Pitchstone. This is like obsidian, but contains from 5 to 10
per cent of water. It has commonly a sheen or luster suggestive
of resin, and its colors are commonly reds and greens.
PerKte, or pearl stone (Fig. 42). This is glass composed of nu-
merous rounded masses, with concentric structure like the coats of
an onion, formed by contraction in cooling and separated by larger
straight cracks. It usually contains from. -2 to 4 per cent of water.
Types of Igneous Rqfclc
99
Pumice (Fig. 43). This is a cellular or porous glass, its charac-
ter being due to the liberation of gases on cooling. It may be re-
garded as the consolidated sur-
face froth of the lavas.
All the glasses form from sur-
face flows of lava, and the move-
ment or flowing, after partial
solidification, is commonly shown
by the occurrence in them of
layers of denser or more stony
material, in which minute crys-
tals of feldspar and quartz are de-
* veloped. Often these are arranged
in rosettes of radiating structure
to which the name spherulites is
given (Fig. 44). Cavities due
to the expansion of steam or
gases are also formed, which are generally spherical and often
contain crystals of various minerals (topaz, quartz feldspar,
garnet, etc.). -These are. called lithophysa or stone bubbles
(Fig. 45), and they vary in size up to an inch in -diameter.
Typical localities for obsidian are the Lipari Islands and Yellow-
FIG. 42. Perlite. Thin section
under* the microscope, enlarged
30 diameters. Hlinik, Hungary.
(After Rosehbusch.)
FIG. 43. Pumice. Surface of a
hand specimen.
FIG, 44. Spherulitic Obsidian.
stone Park; for pitchstone, Meissen near Dresden, Saxony,
the Island of Arran, west Scotland, and Silver Cliff, Colorado;
while the best known localities for perlites are in Hungary.
ioo Principal Types of Igneous or Pyrogenic Rocks
Devitrified Old Glasses. Volcanic glasses of very early geologi-
cal time have generally undergone a change by the development in
them of excessively minute crystals of feldspar and quartz. From
FIG. 45. Lithophysge in Lithoidite of Obsidian Cliff, Yellowstone National
Park. Slightly reduced. U. S. Geol. Sur. Bull. 150.
this they lose their glassy appearance and resemble felsites, which
name is commonly applied to them. The^ also have been called
petrosilex. They are not uncommon in the old lava flows of the
New England states and elsewhere.
The Syenite-Trachyte Series
This series is primarily characterized by deficiency in silica, so
that no free quartz is formed on crystallization. It includes syenite,
trachyte, felsites, and glasses.
Syenite. This rock typically consists of orthoclase and horn-
blende, without quartz. When biotite is present, the rock is called
mica syenite. In practically all syenites some of the orthoclase
is replaced by plagioclase, and this may occur to such a degree that
it makes up half of the feldspar, when the rock approaches a diorite,
and is called a monzonite. Augite too may replace the hornblende,
occurring with orthoclase and forming an augite syenite. Finally,
Types of Igneous Rocks 101
quartz syenite, with a small amount of free quartz, shows a transition
from granites. With the appearance of nephelite, they pass into
nephelite syenites. Porphyritic syenites have large crystals of feld-
spar. Common accessory minerals are magnetite, zircon, and
apatite. The name syenite is derived from the ancient Syene (now
Assuan) in Egypt, where the rock is, however, a hornblende granite
which was formerly used for obelisks. As now used the name was
first applied to a granite rock almost without quartz near Dresden
(Plauen'sche Grund, or Plauen Gorge).
Trachytes and Felsites. These have essentially the same com-
position as the syenites, of which they form the crypto-crystalline
representatives with the same relationship that rhyolite holds to
granite. Trachytes (Fig. 46) differ from rhyolites in the absence
or great rarity of quartz. Biotite is in general the most abundant
dark mineral, but hornblende and augite also occur, forming varie-
ties. The texture varies from felsitic||i the true felsites to strongly
and coarsely porphyritic, and not infrequently the rock is somewhat
cellular. Felsites, on account of their dense structure, cannot
readily be distinguished from rocks of the same type in the granite
or even in the diorite group. In typical trachytes, which are gen-
erally porphyritic, the ground-mass is made up of fine rods of ortho-
clase arranged more or less parallel and in flowing lines. This is
the characteristic trachyte texture, which can often be seen with
the naked eye in the coarser crystalline varieties. Large pheno-
crysts of a clear vitreous variety of orthoclase (called sanidine) are
characteri3tic of the porphyritic trachytes.
When the phenocrysts are so abundant as to constitute about
half the mass of the rock with felsitic ground-mass, it is called a
trachyte porphyry. When phenocrysts form the bulk of the rock,
while the ground-mass becomes more coarse-textured, the rock is
called a syenite porphyry and marks the transition to syenite.
The name trachyte is derived from its rough or harsh surface
feel (Greek, T/XXXVS, rough). The most typical trachytes come from
the peak of the Drachenfels on the Rhine. (See map, Fig; 98.)
The Glasses of the Syenite Series. Because the fusing point
of these more basic rocks is lower than that of the granite series,
being about 2000 F. (1100 C.) for trachytes as compared with
about 2200 F. (1200 C.) for the rhyolites, and 2250 F. (1240 C.)
for granites, they remain liquid longer, and hence crystallization
occurs more generally with less frequent formation of glasses.
io2 Principal Types of Igneous or Pyrogenic Rocks
When glasses are formed, they are indistinguishable, except by
analysis, from those of the granite series.
The Nephelite-Syenite Phonolite Series
These differ from syenites chemically in the greater amount of
soda, and mineralogically in the partial substitution of nephelite
(eleolite) for the feldspar.
Nephelite-Syenite. This corresponds to syenite except for
the presence of nephelite or sodalite, of both or of leucite. In some
cases (Litchfield, Maine) the feldspar is wholly plagioclase. Zir-
con is a usual secondary mineral.
Phonolites. (Klingstein, so called because of its ringing sound.)
These rocks are generally dense and finely crystalline, seldom
vesicular or glassy. They are
mostly dull green or gray in
color, and when light colored
they are not readily distin-
guished from trachytes except
by the microscope, which
shows the presence of the
nephelite. The chief feldspar
is orthoclase, while the com-
mon dark mineral is augite,
hornblende being rare. A
peculiar character is the fact
. that the rock breaks into thin
slabs which have a musical
ring under the hammer. The
rock is frequently porphyritic,
both augite and feldspar
phenocrysts appearing in a
dense ground-mass. When the phenocrysts are very abundant the
rock is called phonolite-porphyry, and when in excess, it becomes a
nephelite-syenite porphyry. With increase in orthoclase and decrease
in nephelite, the phonolites pass into trachytes. . (See Fig. 46.)
Glasses of the Nephelite-Syenite Series. These are still rarer
than those of the preceding case, since the fusing point of phonolites
(somewhat less than 2000 F. or 1090 C.) is still lower than that
of trachytes. Phonolite obsidians are known from the Peak of
Teneriffe.
FIG. 46. Trachyte-phonolite show-
ing typical trachitic texture of the
ground-mass and a large phenocryst.
The Rhon, Germany. Enlarged 24
diameters, seen under crossed nicols.
(After Rosenbusch.)'
Types of Igneous Rocks 103
The Quartz- Diorite Dacite Series
The members of this series differ from those of the granite series,
chiefly in the substitution of plagioclase for orthoclase. The plagio-
clase can be recognized by the striated character of the cleavage
surface.
Quartz-Diorite. This resembles' granite, but is darker and
heavier. Acid plagioclase, quartz, hornblende, and (or) biotite
are the essential minerals. When biotite predominates the rock
is called a quartz-mica-diorite. These rocks form a transition from
granites to diorites, the intermediate forms being called grano-
diorites.
A typical locality for quartz-mica-diorite is found in the Cortland
Series near Peekskill, N. Y., one for the hornblendic quartz-diorite
in the Yellowstone Park.
Dacite. This rock is difficult to distinguish from rhyolite except
by the use of the microscope. It has the same relation to quartz-
diorite that rhyolite has to granite. When the texture is finely
felsitic and non-porphyritic, it can only be termed felsite. When
porphyritic, the dacites are recognized by the striated surfaces of
the plagioclase phenocrysts, which predominate. Glasses and cellu-
lar texture are not uncommon.
When phenocrysts constitute up to half the mass of the rock, it
becomes a dacite-porphyry. When they are in marked excess over
the ground-mass, it becomes a quartz-diorite-porphyry. The name
is derived from the old province of Dacia, now Transylvania (Sieb'en-
biirgen),, before the war a part of Hungary, which is a typical local-
ity. . Dacites generally occur with andesites.
Glasses of this Series. The glasses of this series are more com-
mon than those of the syenite series, for the fusing point is only a
little less than that of the granite series. They are generally in-
cluded with the glasses of the next series.
The Diorite- Andesite Series
This differs from the preceding in the absence of free quartz (in
the crystalline members) and from the syenite series in the substi-
tution of acid plagioclase for orthoclase.
Diorites. These are granitoid rocks, the chief feldspar of which
,is acid plagioclase, and they are rich in hornblende. When biotite
largely replaces the hornblende, the rock is called mica-diorite.
IO4 Principal Types of Igneous or Pyrogenic Rocks
Again, augite may replace part of the other dark minerals, forming
an augite diorite, which is a passage-rock to gabbro. Typically,
the feldspar is in excess of the dark minerals, but in other cases
these may lead in the mineral constituents. A rock of diorite compo-
sition may also arise by metamorphism of gabbros with the change
of the augite to hornblende. Porphyritic diorites, occurring in
dikes, have been called camptonites. Characteristic secondary
minerals are: magnetite, titanite, and apatite. The name diorite
was given to this rock because of the striking contrast between
the light and dark minerals of which it is composed.
Andesites. These are the fine-textured to felsitic members of
the diorite series. The acid plagioclase feldspars are the most
abundant minerals, but quartz is rare or absent. Biotite, horn-
blende, and augite are the dark minerals, the last two predomi-
nating over the biotite. The general colors'of the rocks are grays or
greens. Typical andesite is felsitic, sometimes cellular, and com-
monly porphyritic. The felsitic ground-mass consists of micro-
scopic rods of feldspar, forming a felt-like aggregate. From trachyte
and dacite it can generally be distinguished only by its darker color,
owing to the greater abundance of the ferromagnesian silicates.
According to the predominant dark mineral we have mica andesite,
hornblende andesite, augite andesite, etc.
In the porphyritic varieties, the phenocrysts are mostly feldspar ;
when very abundant (up to half the mass) they form an andesite-
porphyry; when in excess, a diorite-porphyry. These show an
increasingly coarser ground-mass, and grade into diorites proper.
With increase in orthoclase, andesites pass into trachytes, and
with increase in the dark and more basic ferromagnesian silicates
and a decrease of feldspars, they pass into basalts ; with addition
of quartz they pass into dacites. The type localities for these
rocks are in the Andes Mountains, and they are abundant and
widespread in the Pacific Coast region of North America, especially
in the old volcanic cones of Mt. Hood, Mt. Shasta, Mt. Rainier,
and others.
The Glasses of the Diorite-Andesite Series. Andesites fuse
at temperatures around 2000 F. (1100 C.), which is about the
same as for trachytes. Like these, therefore, they do not i readily
form glasses, and most of those referred to andesites are probably
referable to the dacites. The glasses of this type which are recog-
nized are : andesite-obsidian, andesite-perlite, and andesite-pumice.
Types of Igneous Rocks 105
They are distinguished from the more acidic glasses only by chem-
ical analysis, or by their field relations to recognizable types of
andesites or dacites. Andesite obsidian has been obtained from
Clear Lake, CaL, andesite perlite from Eureka, Nev.
The Gabbro-(Pyroxenite, Peridotite)- Basalt Series
This includes all the basic igneous rocks, the coarser-grained
members of which range through gabbro, olivine-gabbro, pyroxenite
and peridotites, while the fine-grained ones form diabases and basalt.
The surface flows are represented by scoriaceous, ropy, or other
lavas, and more rarely by glasses (tachylite, Peele's hair, etc.).
Gabbro and Olivine Gabbro. - These are generally coarsely
crystalline dark rocks, composed chiefly of basic plagioclase and
monoclinic pyroxenes, the dark silicates typically predominating.
There are, however, varieties which are almost wholly composed
of coarse crystalline labradorite (Canada, Adirondacks) with little
or no pyroxene. These are also called anor tho sites ; where the py-
roxene is of the ortho rhombic varieties (enstatite, bronzite, hyper-
sthene), the rock is called norite. Gabbro may contain some horn-
blende and biotite. When olivine is present the rock becomes
olivine-gabbro, olivine-norite, etc. In rare cases nephelite becomes
an important mineral in some basic gabbros, forming the rock
themlite, which occurs in the Crazy Mountains, Montana. Gabbros
have a wide distribution.
Pyroxenites and Peridotites. By the decrease of the plagioclase,
the gabbros pass insensibly into pyroxenites and peridotites, which
are usually found associated with the gabbros, but also occur in-
dependently. Pyroxenites generally contain little else than pyrox-
ene, which maybe orthorhombic (enstatite, bronzite, hypersthene) or
monoclinic (diallage, augite), but these are not readily distinguish-
able by the unaided eye, though the orthorhombic pyroxenes show a
bronze luster. Frequent accessory minerals are hornblende, magne-
tite, and pyrrhotite. When olivine is added the rock becomes a
peridotite, and when olivine predominates it becomes a dunite, of
which the nearly pure olivine rock of North Carolina is an example.
Sometimes magnetite is so abundant as to make the rock almost an
iron ore. Porphyritic peridotites have been called picrites, especially
when occurring in dikes. Sometimes hornblende is abundant, and
it may even form a rock by itself, which is then called amphibolite.
io6 Principal Types of Igneous or Pyrogenic Rocks
Pyroxenites and peridotites change with age and by metamor-
phism into serpentines.
Diabase. This*is a transitional rock between the gabbros and
the Basalts, being fine-grained but holocrystalline and differing
in detail of texture from the gabbro. It consists of basic plagio-
clase, augite, and often olivine, the first occurring as elongated rec-
tangular rods arranged in an interlacing manner, while the other
FIG. 47. a, Basalt, showing typical diabasic structure of lath-shaped crys-
tals of plagioclase, Burney Falls, Shasta Co., Cal. b, Bronzite diabase,
York, Pa., showing lath-shaped plagioclase crystals. Both sections are seen
under crossed nicols enlarged 24 times (after Rosenbusch).
minerals are packed in between them in irregular masses. This is
brought about by the fact that the plagioclase crystallized out first,
contrary to the usual order of crystallization, and that the ferro-
magnesian minerals had to adapt themselves to the remaining
spaces. This diabasic texture (Fig. 47) is sometimes modified into
an ophitic one, where the rods of plagioclase are included in large,
coarsely crystalline masses of pyroxene. Diabase is common as
an intrusive rock, both as dikes and sills.
Basalt. These are the basic fine-grained or dense igneous rocks
which form the surface flows of the gabbros, pyroxenites, and peri-
dotites. Typically the texture is dense or felsitic, often cellular,
and the rock may be characterized by .almond-shaped steam holes
which are subsequently filled with secondary . calcite, giving the
rock an amygdaloidal structure (Fig. 48). Sometimes these cav-
ities are filled with copper, as in the Lake Superior region. For-
Types of Igneous Rocks
107
FIG. 48. Amygdaloid.
phyritic basalts show phenocrysts of augite and more cpmmonly
olivine. Such .porphyri tic olivine basalts are also called melaphyres.
The felsitic ground mass is composed
of microscopic augite, plagioclase and
magnetite crystals. Sometimes a little
glass is present. With abundant de-
velopment of phenocrysts, of augite
and olivine, the rock becomes a basalt
porphyry and with excessive develop-
ment of phenocrysts a gabbro porphyry
is produced.
In rare basalts nephelite or leucite
may replace the feldspar, giving a
variety of types, distinguishable only
under the microscope.
Augitites and Limburgites. These
are rare basaltic rocks, with little or no feldspar, which correspond
to the pyroxenites and peridotites, respectively.
. The Basic Glasses. These represent surface formations of the
basaltic flows, but* are on the whole not common. Some of the
porous or vesicular scoria is glassy, but for the most part it has a
dense stony or felsitic texture. Typical basic glass, corresponding
to obsidian, is known by the name of tachylite, while a fibrous vari-
ety found in the crater of Kilauea is called Pele's hair.
Special Field Names
Several names are commonly applied to the basic igneous rocks
when the exact character cannot be determined in the field.
One of these is dolerite, used for rocks, which may be diorites or
gabbros, but in which the dark mineral is undeterminable. An-
other is trap, in common use for dark, dense basalts or diabases.
It is derived from the Swedish trappar, a stairway, because the
sheets of this rock sometimes form successive steps in the land-
scape. Old trap or basaltic rocks which have, by alteration, de-
veloped sufficient of the mineral chlorite to give them a greenish
cast, are called greenstones.
io8
Modern Volcanic Phenomena
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CHAPTER VII
MODERN VOLCANIC PHENOMENA'
EXCEPT in the case of comparatively young volcanic eruptions,
the structures and relationships of igneous masses can be studied
only in regions where these masses have been uncovered by erosion
of the rocks which formerly covered them, and their origin can be
determined only from the available facts. We shall first consider
those masses the formation of which are open to observation or
which have been formed so recently that there can be no doubt as
to their origin.
DISTRIBUTION, CLASSIFICATION, AND DEVELOPMENT OF
VOLCANOES
Volcanoes are found in many parts of the world at the present
time, but their most extensive distribution is around the borders
of the Pacific, where they form a " chain of fire " which surrounds
that ocean. The map on the opposite page shows this and their
occurrence elsewhere. Along the western border of the Pacific
more than 150 active volcanoes are distributed through a length
of 16,000 kilometers, while about 100 border the eastern margin of
that ocean from the Aleutian Islands to Tierra del Fuego. Typical
and perfect examples of the former are Fujiyama in Japan (Fig. 50)
and Mayon in the Philippines (Frontispiece), while Bogosloff and
Popocatepetl may serve as examples of the latter. Within the
Pacific and arising from its floor are other active volcanoes, such
as those of the Hawaiian Islands, near its very center, and those of
the southwest coral reef regions.
On the Atlantic borders, active volcanoes are not common, being
chiefly confined to the Antillean region in the west, the Icelandic in
the north, and the Azores, Canaries, and Cape Verde Islands in
the east. These are in reality local or isolated groups of volcanoes,
except those of Iceland, which are the last manifestations of vol-
canic activity in a vast volcanic field which extends from Greenland
on. the west to Siberia on the east, and which began far back in
109
no
Modern Volcanic Phenomena
FIG. 50. Fujiyama, a perfect volcanic cone in Japan.
FIG. 51. Map of the Naples volcanic region, with the three principal
volcanic centers, Vesuvius, the Phlegraean volcanic field west of Naples
(Pampi Flegrei), and the Island of Ischia; together with the submarine vol-
canoes. (After J. Walther, from Rated.) .
Distribution, Classification, and Development in
the geological history of the region. In the Mediterranean are
three important fields of volcanic activity known since ancient
times : that around Naples, that of Sicily and its islands, and that
of the Grecian archipelago. The Naples volcanic field (Fig. 51)
comprises Vesuvius with Monte Somma on the east of Naples,
the Phlegraean volcanic field with a number of vents including
FIG. 52. View of- Stromboli from the northwest.
Kayser's Lehrbuch.)
(After A. Bergeat, from
Monte Nuovo, the Solfatara, the Pozzuoli district, etc., on the west,
and the volcanic island of Ischia on the southwest, besides many
smaller islands, including Capri on the south. The Sicilian region
includes Mount Etna in the northeastern part of that island, and
the volcanoes of the Lipari or ^Eolian group of islands north of
Sicily, among which are Vulcano and Stromboli (Fig. 52), the latter
called the " Lighthouse of the Mediterranean/' because of the
Fiq. -53. Volcanoes of the great Rift Valley of East Africa. (After Moore.)
flashes of its explosions, given off at intervals of from i to 20 min-
utes. In the Grecian archipelago the island of Santorin has been
a scene of volcanic activity for more than 2000 years. Volcanoes
are also active around the Indian Ocean, especially on the Islands
which form its eastern margin. Active volcanoes exist in the great
" Rift Valley " of East Africa, in which lie many of the large lakes
of that region (Fig. 53), and recently extinct volcanoes nrrnr along
ii2 Modern Volcanic Phenomena
the northern margin of the Indian Ocean in Arabia, while more
ancient volcanic activities are recorded in India.
Active, Dormant, and Extinct Volcanoes
Volcanoes may be classed as active when eruptions are occurring
at intervals ; as dormant, when they have been quiescent for cen-
turies, though the possibility of a new eruption exists ; and as ex-
tinct, when volcanic activity has entirely ceased and the volcano is
undergoing destruction by atmospheric agencies. There is of
course every gradation between these ; -some that would be classed
as dormant may never be revived, while others, after a long sleep,
burst forth again, as in the case of Vesuvius at the beginning of
the Christian Era. It should be clearly understood that volcanism
is not a phenomenon restricted to the present, but that volcanic
activities, often on a much grander scale than those of to-day, have
been going on during all the periods of the earth's history. Many
of the older volcanoes and their products, after suffering more or
less destruction by erosion, were buried under more recent deposits,
and have been reexposed only in part as the result of the latest
phases of erosion.
Formation of New Volcanoes in the Historic Period
Most of the modern volcanoes came into existence before the
time of recorded history, but a few have been formed during the
historic period, and their growth and development has been wit-
nessed by man. The most noted of these are : Monte Nuovo, in the
Bay of Baiae near Naples (1538) ; Jorullo, Mexico (1759) ; Pochutla,
Mexico (1870) ; Camiguin, Philippine Islands (1871) ; a new cone
of the Ajusco group, Mexico (1881) ; the New Mountain of Japan
(1910) ; and the submarine cones of Sabrina and Graham Islands
(1811 and 1831). A few of these may be considered in some detail.
Monte Huovo. This volcano (Fig. 54) arose in the Phlegrsean
fields west of Naples on Sunday, September 29, 1538, beginning
about one o'clock in the morning. It appeared mainly on the site
of the ancient Lake Lucrinus, which itself was regarded as the
crater of an older but extinct preexisting volcano, filled with water.
Incandescent gases burst open the earth, and within a week a cone
made up of ejected ashes, cinders, and large stones, but no lava,
was built up to a height of 440 feet above the level of the sea. The
Distribution, Classification, and Development 113
depth of the crater Is 421 feet, reaching within 19 feet of the sea-
level, and its basal circumference, about 8000 feet. The eruption
was preceded on the day and night before by. about twenty earth-
quake shocks in Pozzuoli and the neighborhood, and after the erup-
tion it was found that the sea had receded for some distance, owing
FIG. 54. Monte Nuovo, formed in the Bay of Baiae, Sept 29, 1538.
(After Lyell.) i. Cone of Monte Nuovo. 2. Rim of crater of same.
3. Thermal spring called Bath of Nero, or Stufe di Tritoli.
to the elevation of the region, and the strand was covered with
quantities of dead fish, as well as dead birds.
The cone of Monte Nuovo is very regular, and is composed of
layers of pumaceous fragments and ashes, and of trachytic blocks,
the whole partly consolidated and dipping away from the crater
at angles of 26 to 30 degrees. Fragments of marine shells and
Roman bricks were also included in the beds as a result of the
disturbance of the region by the explosions.
Jorullo. This cone arose on the night of September 28, 1759,
at a point 35 miles distant from any then existing volcano, and
120 miles from the sea. It appeared on a level plain 2000 to 3000
feet above the sea in the state of Michoacan, Mexico. A north-
easterly fissure opened in the midst of the sugar and indigo fields
of this plain, and ashes and rocks were thrown to a great height,
and on falling these built up six conical hills ^on the line of the chasm,
the smallest 300 feet high, while Jorullo itself was built up to 1600
feet above the olain, or 4.26$ feet above sea-level. Great streams
114
Modern Volcanic Phenomena
of basaltic lava were poured forth from Jorullo, these including
fragments of granitic rock. The ejection did not cease until Febru-
ary, 1760. Twenty years later the lava was still hot enough, a
few inches below the surface, to light a cigar. This eruption, too,
was preceded by -earthquakes and subterranean rumblings, which
began in the June preceding. There has been no eruption in this
region since.
Camiguin. This volcano also started from a fissure in a level
plain on one of .the small islands north of Luzon in the Philippines.
Beginning in 1871, it continued to be active for four years, by which
time it had reached
a height of about
1 800 feet.
Sabrina Island.
This submarine vol-
canic cone appeared
above the waters' of
the Atlantic in the
Azores group, off the
coast of St. Michaels,
on June 13, 1811,
and rose to a height
of about 300 feet
above the sea, gain-
ing a circumference
of about a mile.
Being, however,
largely made of unconsolidated material, it has since been washed
away again. As observed from the nearest cliff on St. Michaels,
the explosions resembled a mixed discharge of cannon and mus-
ketry and were accompanied by a great abundance of lightning.
The appearance of the eruption above water is shown in the
accompanying figure from a sketch made at that time (Fig. 55).
Graham Island (Isle Julia). This island, which existed for only
about three months, rose as a submarine cinder cone in 1831 in the
Mediterranean between the southwest coast of Sicily and that pro-
jecting part of the African coast where ancient Carthage stood. A
few years before the appearance of the island, soundings at this
locality showed a depth of water of 100 fathoms. Premonitory
shocks were Mt on June 28 over the spot and on the adioininff
FIG. 55. The volcanic eruption which formed
Sabrina Island in the Azores, June 13, 1811. (After
DeLa Beche.)
Distribution, Classification, and Development 115
coast of Sicily. About July 10, a column of water 60 feet high and
800 yards in circumference was seen rising from the sea, followed
soon after by dense clouds of steam which rose 1800 feet. Eight
days later the same
observer noted at
this spot a small
island 12 feet high
and with a crater at
its center, from FIG. 56. Supposed section of Graham Island,
which volcanic mat- (After C. McLaren, Geology of Fife and the
ter and immense Lot 7 hia s ' P L *i, Edin -> l8 39; from Lyell's Prin-
copies.)
clouds of vapor were
ejected, while the sea round about was covered with floating
cinders and dead fish. By the end of July the island had become
from 50 to 90 feet in height and three fourths of a mile in circum-
ference, while on August 4 it was reported above 200 feet high and
three miles in circumference. After this it began to dimmish in
size, owing to the erosion by the sea, decreasing to two miles
in circumference by August 25, and to three fifths of a mile and a
maximum height of 107 feet by September 3, when the crater was
about 780 feet in circumference. On September 29 the island was
reduced to a circumference of only about 700 yards, and toward
the close of Oc-
tober it had dis-
appeared except
for a small point
of sand and
scoriae. By the
commencement of
1832 there was
only a shoal, with
a mass of igneous
rock which ap-
FIG. 57. Graham Island, as it appeared on Sept. parently filled the
29, 1831. The apparent bedding planes sloping towards center of the
the center of the volcano are not such in reality. ^ rpkri f TT-IA o-^
(From Lyell's Principles.) - Vent ' ^ ap ~
pearance and sup-
posed structure of this island and of the cone, which rose thus
about 800 feet above the sea floor, are shown in the above
figures reproduced from Lyell (Figs. 56, 57). The lava core or
n6
Modern Volcanic Phenomena
neck, which probably 'never' appeared above sea-level, now forms
the highest part of the submerged remnant of the volcano. The
fragmental material Included, besides volcanic ash and cinders,
blocks of limestone, dolomite, and sandstone.
Several other submarine eruptions of this type have been re-
corded from various sections of the Atlantic and from the Medi-
terranean.
CHARACTERISTIC FORMS AND ACTIVITIES OF TYPICAL MODERN
VOLCANOES
Of the many modem volcanoes, a number which have been ob-
served for long periods of time may be described somewhat in de-
tail so that the student will get a grasp of the essentials of the struc-
ture and activities of volca-
noes. We shall begin with a
type in which liquid lava is
the chief product of erup-
tion, then consider types in
which both lava and frag-
mental material . are pro-
duced, and enter into the
building of the cone, and
finally consider some ex-
amples which are purely
explosive, with the produc-
tion of only fragmental ma-
terial. The nature and com-
position of the fragmental
material will be considered
more at length in a subse-
quent chapter.
Before proceeding, the
student should clearly un-
derstand that the hill 'or
mountain which constitutes
the volcano is built up
around an opening or vent
from the material ejected
FIG. 58. Volcanic bomb. Vesuvius,
eruption of 1872. (After Ratzel, Die
Erde.)
from this vent, and that it
does not represent an upris-
Forms and Activities of Typical Modern Volcanoes 117
ing or an upheaval of a part of the earth's surface, as was at one
time supposed to be the case. Three kinds of material are ejected
from volcanic vents ; of these, two or all may be present. The
first type comprises gases and water vapors mingled with fumes of
other, substances ; this is always present. The second is liquid
magma or lava, and the third consists of the shattered lava and
shattered rocks resulting from the explosive activities, comprising
masses of all sizes, from large lava balls or bombs (Fig. 58), more
or less spherical, or elliptical from their rotary motion through the
air, to fine lava particles or lapilli, and fragments or blocks of
older igneous or other rocks, and dust produced by the shattering
of these. In general we speak of such material as volcanic ashes
and cinders. The eruptive activities range from the quiet upwell-
ing or bubbling-up of liquid lava to the most violent explosion due
to the sudden expansion of the gases and vapors.
Kilauea (and Mauna Loo) of the Hawaiian Islands
The entire group of the Hawaiian Islands (Fig. 59) in the mid-
Pacific is a series of volcanic cones built up from the sea-bottom in
former times. Most of these volcanoes are now extinct, and many
FIG. 59. Map of the Hawaiian Islands, showing the principal craters-
(After Dana.)
of them are undergoing erosion ; but a large, active crater, that of
Kilauea (4000 feet above sea-level), exists on the east side of the
volcanic mountain of Mauna Loa (13,675 ft. high) and about twenty
miles from its summit, which is also marked by an active crater.
The crater pit of Kilauea (Fig. 60) is rudely oval in form, with a
circumference of about nine miles, and its floor is formed by a rough
stony crust of solidified lava resting upon a vast column of molten
rock which arises from an unknown depth in the crust of the earth.
In places, this floor is broken by lakes of liquid lava, red to white
n8
Modern Volcanic Phenomena
hot, and set into boiling activity by the ebullition of gases (Figs.
61, 62). From cracks in the floor and on its margin jets of lava are
FIG. 60. View of outline of the crater of Kilauea from Volcano House.
(After Button.)
frequently projected to great heights, and some of this material,
blown like spray by the wind, is drawn out into slender, hair-like
fibers. This has become known as Pele's hair, so named after the
goddess of the mountains.
FIG. 61. The Lava Lake at one side of the crater of Kilauea.
The margin of the crater is formed by a precipitous cliff which
varies in height from time to time. This is due to the fact that
the lava column which supports the floor rises as the pressure in-
creases below, until a point is reached where the wall may be rup-
tured, and the liquid lava flows out, whereupon the column dimin-
120
Modern Volcanic Phenomena
ishes, carrying the floor downward, until it may be 700 feet below
the edge of the crater rim. The rate of rising may be as much as
100 feet in a year, but in modern times the lava has not overflowed
the rim, but issues from lateral fissures due to cracking or fusion,
the floor approaching only to within 300 feet of the edge of the
FIG. 63- A lava stream falling in cascades over a. cliff into the sea,
Hawaiian Islands.
crater. During the eruption of 1840 the lava first appeared on
the side of the mountain five miles from the main crater, after which
it issued at successively lower levels.
Similar conditions exist in the crater of Mauna Loa proper, but
here the top of the lava column is nearly 10,000 feet above that of
Kilauea.
The lava of these volcanoes is very liquid, being of an extremely
basic character and forming basaltic rocks on cooling. Because
of its liquidity, it will continue to flow for a long time ; some of the
lava flows, of Hawaii are thirty miles in length. When it reaches
a cliff, the lava cascades over it like a waterfall (Fig. 63). The
surface of such lava streams often shows local wave-like advances,
which produce the appearance of a series of crushed pillows piled
one against the other. This type of surface form is known in Hawaii
Forms and Activities of Typical Modern Volcanoes 121
as Pahoehoe (pron. pa-hoi-hoi) (Fig. 64), and it is seen in older lavas
of this type in many regions of the world. A ropy surface, having
the appearance of coils of heavy rope, is also commonly produced.
Compare also with Fig. 66.
As the lava stream moves along, it sweeps away forests in its
course, and carries away masses of rock and soil covered with vege-
FIG, 64. Sluggish lava flow forming pillow lava or " pahoehoe " on
Mauna Loa.
tation (Fig. 65). Sometimes a stream will part around a mass of
such rock and, reuniting, enclose it as an island. On reaching the
sea, the lava plunges into it with loud detonations and becomes
shivered into millions of particles of glass, which may be thrown
in clouds into the air. The light from such an eruption has been
visible for over a hundred miles at sea, and at a distance of forty
miles fine print could be read at midnight (Dana).
When the crust of the lava stream has cooled, the interior mass,
still in a molten condition, will flow on, leaving a tunnel behind.
Such tunnels are common in Hawaii, and their roofs are frequently
122
Modern Volcanic Phenomena
FIG. 65. End of the lava flow -of 1881 on Mauna Loa. Note the trees
which were killed but not consumed, and those which escaped.
incrusted with lava pendants or -stalactites up to 20 or 30 inches in
length, while on the floor corresponding drip mounds or stalag-
mites of lava are
found. Small " spatter
cones" may also arise
on its surface (Fig. 66).
The form of these
basic lava volcanoes is
very characteristic,
being a very flat cone
of vast dimensions. As
they arise from the sea-
bottom, only a small
part is visible, the total
height of Mauna Loa
being more than 30,000
feet, although consider-
ably less than half of
this height is seen.
The summit is nearly
FIG. 66. Lava tunnel formed by the cool-
ing of the outer surface of the flow, after
which the lava within flows out, leaving the
tunnel. On the surface of the flow a spatter
cone was built up. Hawaiian Islands.
flat for several square miles, and the slopes of the sides do not
average more than seven degrees.
Forms and Activities of Typical Modern Volcanoes 123
Craterless Volcanoes of Viscous Lava
When the lava is very viscous, such as is characteristic of very
silicious (acidic) lavas, it may happen that the lava mass rises as a
dome-like swelling from the surface, producing a mound or hill of
lava which is not characterized by a summit crater. The volcano
of Chimborazo in Ecuador (20,498 feet above the sea) appears to
FIG. 67. Map of the ruin of the cone of Santorin, in the Greek Archipelago.
(From Kayser's Lehrbuch.)
be of this type, and an eruption of this character occurred in
1866 which formed the Isle of Asphroessa-at Santorin in the Grecian
Archipelago (Figs. 67, 68). A gradual rising of the bottom of the
bay occurred, until the island appeared, which apparently was due
to slow upward and outward pressure by steam, which was escap-
ing at every pore through the scoriaceous lava surface. The red-hot
lava could be seen through the fissures, and the whole mass was
undulating and swaying from side to side, sometimes appearing
to swell to nearly double its size, and to throw out ridges like moun-
tain spurs. At last a broad , chasm appeared across the top of the
cone, accompanied by a tremendous roar of steam, while rocks and
124
Modern Volcanic Phenomena
Fia 6S _ Bird's eye view of the Gulf of Santorin, during the volcanic
eruption of February, 1866, looking west. (From Lyell's Principles.)
a. Therasia. b. The northern entrance, 1,068 feet deep. c. Thera.
' d. Mt. St. Elias, rising 1,887 feet above the sea, composed of granular lime-
stone and clay-slate; the only non-volcanic rocks in Santorin. e. Aspronisi.
/. Little Kaimeni (Kaymeni or Ksemense). g. New Kaimeni (Nea Kaymeni
or Ksemense). h. Old Kaimeni (Palcea Kaymeni or K^menae). i. Asphro-
essa. k. George.
ashes mixed with steam were thrown to heights of 50 to 100 feet,
masses of this material, 30 cubic feet in bulk, falling at distances of
600 yards from the new crater. Then the activity subsided, the
cone was lowered, the crater closed in, and after a few minutes of
quiet the process recommenced.
FIG. 69-0. Grand Puy of Sarcoui,
composed of trachyte and rising be- FIG. 69 b. Experimental illustra-
tween two breached scoria cones ; a tion of the mode of formation, of vol-
typical example of a pustular cone or canic blister cones composed of viscid
volcanic blister formed of highly lavas,
viscous lava. Auvergne, France.
An example of such a blister cone or volcanic dome, now extinct, is
found in the Grand Puy of Sarcoui (Fig. 69 a) in the old volcanic
district of Central France, This mountain is a mass of trachyte
Forms and Activities of Typical Modern Volcanoes 125
lava having the appearance of an inverted cup, without a crater,
and was apparently formed from a mass of viscous lava which was
forced upward to form a blister upon the surface of the earth. Blis-
ters of this kind have been reproduced experimentally, as shown
in the preceding figure (Fig. 696).
Vesuvius
A very different type of volcano is represented by Vesuvius,
probably the best and longest known of active volcanoes. The
FIG. 70. Map of Vesuvius with its lava streams up to 1872. The darker
are the later, and lighter the earlier flows. Scale about i : 250,000. After
Le Herr and others. (From Kayser's Lehrbuch.)
present cone, which lies east of Naples, is surrounded on three sides
by the rim of the ancient cone, now called Monte Somma (Figs.
70, 71), which was perfect up to the first century of the Christian
126
Modern Volcanic Phenomena
Era. That volcano had been dormant for so long that its slopes
were clothed with vineyards and gardens and dotted over with
villas, while at
the foot of the
mountain lay the
populous cities of
Herculaneum and
Pompeii. Even
the interior slopes
of the crater, of
which only a part
remains in Monte
Somma, were
covered with wild
vines, so Plutarch
tells us, while the
floor of the crater
was a sterile plain.
On this plain,
from which there
was only a single
outlet, a break in
the wall of the
crater, the gladi-
ators of Spartacus
encamped in 72
B.C., while the
praetor Clodius
guarded the out-
let and attacked
Spartacus by low-
ering his soldiers
into the crater
over the precip-
itous walls.
In the year 63
A.D. the first evi-
dence of the re-
awakening of the
volcano made
Forms and Activities of Typical Modern 'Volcanoes 127
itself felt in an earthquake which damaged the cities in the vi-
cinity. From that time to 79 A.D. slight shocks were frequent,
and in August of that year they became more numerous and
violent, finally terminating ' in the first great historic' eruption.
As described by the younger Pliny, a dense column of vapor first
arose vertically from the crater, spreading out laterally so that the
Ruins of Pompeii.
upper part resembled the head, and the lower the trunk, of a pine
tree. Flashes of light, vivid as lightning, at intervals pierced this
cloud, and ashes began to fall even upon the ships at distant Mi-
senum, shoaling the sea in places and burying Herculaneum and Pom-
peii (Fig. 72). The violent explosions shattered the crater rim,
of which only a part remains in Monte Somma, and the later cone
of Vesuvius proper was built upon the floor of the old crater, sur-
rounded on three sides by its rim. No lava appears to have been
ejected at this time, the material being all of the pyroclastic or
fragmental type, such as lapilli, sand, and fragments of older lava.
The first lava stream recorded from Vesuvius flowed in the erup-
tion of 1036 ; which was the seventh since the reawakening of the
volcano. Another eruption occurred in 1049, and still another in
1138 or 1139; after this the volcano rested for 168 years, though
two minor vents opened at distant points, one at Solfatara, near
Pozzuoli (Bay of Baiae), in 1198, and the other on the island of
128
Modern Volcanic Phenomena
Ischia in 1302. Then in 1306 a minor eruption of Vesuvius took
place, after which this volcano again became dormant for 325 years
or until 163 1 , with one slight eruption in 1 500, During this interval
the Sicilian volcano Etna was, however, in constant eruption, while
within the Phlegraean volcanic field west of Naples arose the new
volcano Monte Nuovo in 1538 (ante, p. 112). Between 1139 and
1631, or for 492 years, there had been no violent eruption, and the
FIG. 73. Eruption of Vesuvius in 1872. (After photograph from Ratzel.)
crater, which was five miles in circumference .and about a thousand
paces deep, had its sides covered with brushwood forests frequented
by the wild boar, while cattle grazed on its floor. Three small
pools of water remained upon the floor of the crater, one being hot
and bitter, another more salty than the sea, and the third hot but
tasteless. Suddenly, in 1631, the floor and sides of the crater were
blown to fragments which the wind scattered, and in December of
that year seven streams of lava poured forth from the crater, over-
whelming several villages on the flanks and at the foot of the moun-
tain, one of which, Resina, had been partly built over the ancient
site of Herculaneum. Great floods of mud, from the condensed
vapor and the ashes, also poured down the sides of the volcano and
did much damage.
Forms and Activities of Typical Modern Volcanoes 129
After a brief rest, the eruptions were renewed in 1666, since which
time they have occurred almost constantly, with only short inter-
mittent periods of quiescence (Fig. 73). The last great eruption
occurred in 1906. This began with premonitory explosions in 1904,
while during the whole of 1905 a narrow stream of lava flowed from
a fissure in the cone (Fig. 74). On April 4, 1906, began the last
great eruption, which was inaugurated by the appearance of a
cloud of dust, carried aloft by the gases, and assuming the shape of
FIG. 74. Looking into the crater of Vesuvius ; hot lava sending up clouds
of steam.
a cauliflower. At the same time several lava streams broke out
at successively lower levels in the side of the cone. Three days
later (April 7) occurred a violent 'explosion, and a dust cloud arose
vertically into the air for a height of four miles, this dust falling
in such quantities upon the roofs of the houses in the near-by towns
as to cause their collapse. Larger streams of lava also issued from
various openings, one of them reaching the town of Boscotrecase
and destroying it. The main lava stream descended the steeper
slopes at a rate of somewhat less then two miles an hour, but
flowed at a much lower rate on the gentler slopes. The lava had a
temperature of more than 2000 F., but owing to the rapid cooling
130
Modern Volcanic Phenomena
on the surface it did not burn up the trees with which it came in
contact, but charred them, and sometimes broke them off by its
weight and carried them along on its surface.
FIG. 75. Inside the crater of Vesuvius. Note the stratified appearance of
the wall of ashes and cinders, and the slopes of loose material.
Both the old cone of Monte Somma, and the later cone of Vesu-
vius, which has a height of about 4000 feet, are built up of layers
of cinders, ashes, and lava (Fig. 75) . These have a steep inclination,
dipping away from the rim of the craters in all directions at angles
of 26 to 40 or more. These layers are cut vertically by numerous
fissures, which are filled with hardened lavas, forming dikes which
132
Modern Volcanic Phenomena
bind the entire mass together. Much of the consolidation of the
fragmental material is also due to the fact that it falls often in a
half -fused condition, and the heat from the volcano tends to bind the
particles together. Material carried to greater distances, however,
remains incoherent. Since the last eruption (1906) much gullying
by erosion has occurred on the slopes of the cone (Fig. 76). (See
also Fig. 86 a, p. 142.)
Etna
This famous volcano, in the eastern part of the island of Sicily,
rises almost 11,000 feet above the sea, and has a nearly circular
FIG. 77. Map of Etna and the Val-del-Bove, or Valle-del-Bue. After
map of Italian general staff. (From Ratzel.) The orientation of this map
is such that north is on the right. The coast line runs in a direction east of
north.
base with a circumference of 87 miles, while its lavas cover an area
almost twice as great (Fig. 77). The lower part of the cone is cul-
tivated ; higher up are forests, and the upper part is a barren lava
133
134
Modern Volcanic Phenomena
waste, which terminates in. a sort of tableland, from which arises
the principal cone, noo feet in height. From this cone sulphurous
vapors and steam constantly arise, and lavas are emitted at fre-
quent intervals. Viewed from north or south the cone is very sym-
metrical ; but on the east it is cut by a deep valley, the Val-del-Bove
or Valle-del-Bue, which is a vast amphitheater four or five miles in
diameter, and is enclosed by precipices between 3000 and 4000 feet
high at the upper end. This valley was probably formed in part
by explosions and in part by subsidences (Fig. 78).
During the eruptions of Etna, which have been known to be more
or less continuous since the fourth century B.C., the upper cone
has repeatedly been blown away or has been partly engulfed by
subsidences, being
renewed again each
time by upbuilding
from lava and frag-
mental material.
Where shown in sec-
tions, the structure
is that of stratified
layers dipping away
steeply from the
crater, but more
complicated than in
Vesuvius.' Numer-
ous dikes intersect
the layers vertically
and bind them to-
gether, such dikes
of the Val-del-Bove
FIG.
79. Dikes at the base of the Serra del
Solfizfo, Etna. (After Lyell.)
being especially well shown in the walls
(Fig. 79)-
The formation of such dikes was illustrated by the eruption of
1669, when the whole top of the mountain collapsed. At this time
a fissure six feet broad and of unknown depth opened in the side
of the mountain, extending north and south for a length of 12 miles
from the plain of St. Lio to within a mile of the summit. Five
other parallel fissures opened one after the other. The incandes-
cent glow from these fissures showed that they were filled up to a
certain height with lava which on cooling produced transecting dikes.
Near the town of Nicolosi, which lay near the base of the wooded
Forms and Activities of Typical Modern Volcanoes 135
region, about 20 miles from the summit of Etna, and which was de-
stroyed by preliminary earthquakes, a gulf opened from which sand
and scoria were thrown, which built up a 'subordinate cone, Monte
Rossi, about 450 feet high (see map, Fig. 77).
The great lava stream of this eruption overflowed fourteen towns
and villages, some having a population of between 3000 and 4000
souls, and finally reached the walls of Catania by the sea (Fig. 77)
15 miles away. It^ accumulated against the walls of this city,
which were 60 feet high, and finally flowed over them, but without
FIG. So. View of a part of the Val-del-Bove with parasitic cones and steep
lava streams. The main crater, emitting steam, is in the background,
(After Sartorius and Lasaulx, from Kayser's Lehrbuch.)
destroying them, falling on the inside in a series of fiery cascades
which overwhelmed part of the city. It covered the first 13 miles
of its journey in 20 days, but required 23 days for the last two miles.
Sometimes it moved at the rate of 1500 feet an hour, at other times
only a few yards in several days. When it finally reached the sea,
it was still 600 yards broad and 40 feet deep. Its surface was
generally solid rock, but the hot liquid interior broke this surface
and flowed on to be in turn chilled with a repetition of the
process. The course of this lava stream is shown upon the map
(Fig. 77).
The formation of lateral cones or monticules from which two
136
Modern Volcanic Phenomena
eruptions proceed for every one that issues from the main cone,
is a very characteristic feature of Etna, more than 200 such cones
being known. One of these (Monte Minardo, east of Bronte, see
map, Fig. 77) reached a height of over 750 feet. When new open-
ings form, the lava from these may surround and even bury the
old monticules, so that the volcano is covered with numerous, more
or less buried, extinct, parasitic cones.
In the eruption of August, 1852, to May, 1853, two new cones
opened close together near the head of the Val-del-Bove, rising in
1 6 days to a height of about 500 feet (Fig. 80). The lava poured
down the Val-del-Bove, in places completely filling it from side to
side, so that it became a barren waste, no longer able to support the
cattle from which it had derived its name.
Mont Pelee
This volcano, on the island of Martinique in the West Indies,
became violently active in May, 1902, the volcano Soufriere on St.
Vincent going into activity at almost the same time. The erup-
tion of Mont Pelee was characterized by violent explosions, pre-
FIG. 81. View of the volcano Mount Pelee, on Martinique, showing the
spine (a) with a larger view of the same (&). (From E. de Martonne in
Geographic Physique.)
ceded by small premonitory symptoms. There was no actual out-
pouring of lava, which was completely shattered, and the mass of
minute, incandescent rock particles was carried aloft by the highly
heated gases, forming dense" fiery clouds, which not only rose into
the air, but rushed like a stream through a gap in the crater down
the slopes of the mountain into the sea, overwhelming and destroy-
ing all life/including all but two individuals of the 30,000 inhabit-
Forms and Activities of Typical Modern Volcanoes 137
ants of the town of St. Pierre. This ejection of incandescent
clouds continued for several months. Another remarkable feature
of this eruption was the formation of the great spine, which will
be again considered somewhat later (Figs. 81, 106, 107).
Krakatoa and Bandai-San
The volcano of Krakatoa forms an island in the straits of Sunda,
between Java and Sumatra in the East Indies (Fig. 82). It sud-
denly became active on August 26 and 27, 1883. This activity
FIG. 82. Map of the straits of Sunda in the East Indies, showing the
location of the volcano Krakatoa and the rift lines which center in it. (After
R. D. H. Verbeck, from Ratzel, Die Erde.)
began with a series of premonitory convulsions, after which the
greater part of the island was blown away by a succession of terrific
explosions, the detonations of which were heard more than 150
miles away. A mass of material estimated at a bulk of almost one
and one-eighth cubic miles was thrown into the air in the form of
lapilli, ashes, and the finest dust some of it to, the height of seven-
teen miles and the air waves generated by the explosion traveled
westward carrying the dust with them, and are supposed to have
passed three and a quarter times around the earth (82,200 miles)
before they died away. For many months after the eruption the
dust in the air caused a series of brilliant sunsets all over the earth.
138
Modern Volcanic Phenomena
Dust fell in large quantities on the decks of vessels 1600 miles dis-
tant, three days after the eruption, and tracts of deep water were
made so shallow from this 'dust as to become unnavigable. Great
sea-waves (tsunamis) were generated, one of which was estimated
Crater Perbuatan
VerMen I
I Crater Danan
Ancient Volcano
Verlatent
'Sea Level
FIG. 83 a. Map and section (on. line AB) of Krakatoa before the explo-
sion of 1883. After Verbeck. i, older andesite; 2, younger andesite;
3, basalt; T, Tertiary basement. (From Kayser's Lehrbuch.)
to have risen 100 feet, and these destroyed 1295 towns and villages
along the shores, killing 36,380 people. By their force a large ship
was carried inland for a mile and a half and left stranded 30 feet
above sea-level. Great blocks of stone, weighing from 30 to 50
tons, were also carried inland for two or three miles. Altogether
this was the, most stupendous manifestation of volcanic activity
Forms and Activities of Typical Modern Volcanoes 139
known in modern times. The appearances before and after the
explosion are shown in the maps and sections here given (Figs. 83
a, 6) and the main remaining mass in Fig. 84.
Bandai-San in Japan was a volcanic cone 2000 feet high and had
been dormant for a thousand years. Suddenly in 1888 the greater
part of the cone was blown away by a terrific series of explosions
FIG. 83 b. Map and section (on line CD) of Krakatoa after the explosion
"of 1883. i to 3, same as in preceding; 4, product of latest eruption.
(From Kayser's Lehrbuch.)
\
which continued through a period of less than two hours, leaving
only a remnant which terminates in a cliff about 1500 feet high
(Fig. 85). This explosion is believed to have been 4ue to the per-
colation of surface waters into the volcanic interior and the forma-
tion of steam, which caused the disruption. No lava flows were
observed in this eruption, and the volcano has since been in-
active.
140
Modern Volcanic Phenomena
FIG. 84. View of the Rakata of Krakatoa, the chief remaining fragment
of an older eruption, showing the numerous dikes which bind the mass together.
(After Judd from Ratzel, Die Erde.)
FIG. 85. Section of the Bandai-San. (After Sekiya.) The dotted line
shows the part destroyed by the explosion of 1888.
CLASSIFICATION or VOLCANOES ACCORDING TO TYPE OF
ERUPTION AND FORM
We have now seen something of the mode of eruption of several
distinct types of volcanoes in various parts of the earth. Ac-
cording to their mode of eruption, we may classify them as the
quiet type on the one extreme, represented by the welling up and
pouring out of liquid lavas, as in Kilauea, and the explosive type,
represented by Krakatoa and Bandai-San, where shattering of
Classification of Volcanoes 141
rock material occurs, but without the outpouring of liquid lava.
In. the milder examples of this type a cinder cone is built up
(Figs. 86 a, b). Between these two stand the types which show both
kinds of eruption, with the result that beds of volcanic ashes and
lapilli alternate with beds of solidified" lava. Here we place Vesu-
FIG. 86 a. Cinder Cone, Arizona. Young cinder cone on left, late mature
cinder cone on right. The young cone and lava flow are but a few hundred
years old and are located on the northern edge of the Flagstaff, Arizona, topo-
graphic sheet. (Photo by D. W. Johnson.)
vius and Etna, explosive eruptions being more marked in the former
and lava eruptions in the latter.
Comparison of Form. Comparing the form of the cinder cone
with that of a pure lava cone, we see a striking difference. The
former, illustrated by the wonderfully perfect cinder cone of Mayon
in the Philippines (Frontispiece) has steep slopes, the angle being
determined by the nature and coarseness ' of the fragmental ma-
terial. The lava cone, on the other hand, especially that composed
of basic lava, is broad and relatively low, though the crater may be
situated at a great height above the base. The slopes are very
gentle, and the top generally a plateau. This is illustrated by
Mauna Loa in the Hawaiian group.
In the diagrams on page 143 is shown a comparison of a number
of modern volcanoes and craters drawn approximately to scale
(Fig, 87).
142
Modern Volcanic Phenomena
Geological Age of Volcanoes and Laya Flows 143
3
4
.rfttfs^ /**-__
5Xm
FIG. 87. Cone sections of various types of volcanoes, i. Vesuvius.
2. Lake Laach. 3. Rocca Monfina. 4. Lago Bracciano. 5. Krakatoa.
6. Peak of Teneriffe. 7. Mauna Loa. (From Kayser's Lehrbuch.)
GEOLOGICAL AGE OF
VOLCANOES AND
LAVA FLOWS
The geological age of
a volcano can be deter-
mined from the age of the
associated formations. It
is obvious that a lava
flow is always younger
than the formation upon
which it rests, and older
than that which covers it.
In Fig. 88 is shown a lava
sheet which rests upon
river gravels of Pleisto-
cene age and is therefore
younger than these. The
extensive erosion which
it has suffered, indicates
that it is probably of late FlG< 8 8. Lava flow over Pleistocene gravel,
Pleistocene age. Utah. (Photo, bylF. J. Pack.)
CHAPTER -VIII
STRUCTURAL CHARACTERS OF VOLCANOES, AND
. * OTHER IGNEOUS PHENOMENA
THE structural character of volcanoes is revealed in cones that
have become extinct, for in these the parts are not only more easily
accessible, but dissection has often revealed the internal character
as well.
EXTINCT VOLCANOES
There are many regions where volcanoes have been active in
the recent geological past, and in such cases enough of the form
and character of the volcanoes, now extinct, is still retained to
enable one to recognize them readily. Such recently extinct and
partly dissected volcanoes are not only of interest as showing
former distribution of volcanic activity, but they have an added
value because their erosion has revealed many features which in an
active volcano are not open to view. Thus the study of the
recently extinct volcanoes supplements that of active ones.
Extinct Volcanoes of Central France
One of the most notable fields of former volcanic activity, and
one that has played a prominent part in the history of the science
of yolcanology, lies in the central part of the great area of crystalline
and younger rocks which makes up the so-called Massif Central
of France, and which is bounded on the north by the Paris basin
of Mesozoic and Tertiary rocks, on the southwest by the basin
of the Garonne, and on the east by the valleys of the Rhone and
Sadne. The center of this massif (Fig. 89) is dissected by the
river Allier, which flows north into the Loire, and it is along the
western border of the Allier valley, which is bounded by a fault,
that the main volcanic district is located, while a second one lies
144
Extinct Volcanoes
on the southeast, in the region of the headwaters of the Allier and
the Loire.
The younger eruptive rocks belong to several geological epochs,
namely the Pleistocene, the Pliocene, and the Miocene. Of these
the youngest, of Pleistocene age, form the famous Chaine des Puys,
FIG. 89. Geological map of Auvergne (after Michel-Levy, M. Boule
and Glangeaud). Scale about i : 1,400,000. Light gray, gneiss and granite.
Black, Carbonic beds (mostly confined to the Loire valley). Dark gray,
Tertiary trachytes, basalts, etc. Fine lines, Quaternary eruptives of the Puys.
White, Tertiary and Quaternary. (From Kayser's Lehrbuch.)
a group of extinct volcanoes which still retain to a remarkable
degree their form and general character. South of this lies the vol-
canic district of Mont Dore, and still farther 'south that of Cantal
both of Pliocene age. To the east of the latter and between
the Allier and the Loire is the Chaine du'Velay, of Pliocene basalts,
and east of this a series* of Miocene eruptive hills (Mezenc and
Megal).
146 Structural Characters of Volcanoes
The Ctiatne des'Ptiys. This is best approached from "Clermont-
Ferrand, which lies to the east of the highest of these old volcanic
hills, the Puy de Borne (Fig. 90), from the summit of which an
inspiring panorama of these silent volcanic cones, some sixty in
number, may be seen. They extend for a distance of about 90
kilometers north from the Mont Dore region. The Puy de Dome
itself is not a perfect volcano, but is formed by a central dike or
neck of trachytic rock (called by the French domite), but the great
majority of the cones show each their cup-shaped crater at the top,
FIG. 90. Puy de D6me, an extinct volcano in the Chaine des Puys, central
France. (Photo, by D. W. Johnson.)
and produce a volcanic topography rivaling that of the surface of
the moon (Fig. 91). The lava of these volcanoes consists of olivine
basalts and andesites, with abundant slags, scoriae, and pumaceous
material all rich in ferro-magnesian minerals, and containing from
50 to 58 per cent of silica. The outpourings belong to the middle
and later Pleistocene time. Besides the perfect crater cones there
are, however, great intumescences, or dome-like blisters of trachytic
rock without craters, and these represent the type of up-swellings
of viscous acidic lavas already discussed. Such an one is the
Grand Puy of Sarcoui, shown in the distance on the extreme right
of our illustration (Fig. 91), 'and in outline on page 124 (Fig. 69 a).
Extinct Volcanoes
147
The Massif of Mont Dore. This mass, south of the Puys,
represents the remnant of a great volcano, active during Pliocene
time, but since then partly destroyed by erosion. While this makes
possible the detailed study of the various parts of the volcano,
and gives us an opportunity to observe the successive eruptions
and their effects, it also makes a more difficult problem for the
student to comprehend, for it must be borne in mind that the
Structural Characters of Volcanoes
/ w. : m<S^
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\fefe*
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if 8..-..
-
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many peaks and prominences
now seen are not separate vol-
canoes as in the Puy district,
but erosion remnants of larger
masses, and the imagination
must be drawn upon to restore
again what is missing and so
get a picture of the whole vol-
cano as it was during its prime.
In the adjoining diagram (Fig.
92) is represented such a view
of the relationships of the dif-
ferent volcanic rocks which
make up this volcano as would
be obtained were the entire
massif cut through its highest
part (Sancy) like a round cake
cut through the middle, and half
of it removed so that the cut
side of the other half is visible.
Such a cross-section, as it is
called, is built up from innu-
merable local observations which
are then connected by logical
inferences. This section shows
that the foundation of the vol-
cano is ancient granite and
gneiss, and that it was built up
on an almost level surface (a
peneplane) which had pre-
viously been cut across the old
foundation rocks by natural
agencies. Upon this floor lies
a mass of andesitic tuff, the
product of the first eruptive
activity (upper Miocene). Then
(early Pliocene) followed an
eruption of trachyte porphyries
through the Sancy vents and
that of the Capucin, the flows
Extinct Volcanoes 149
of which extended laterally for some distance, finally thinning
away. The next eruptions (middle Pliocene) were those of the
Puy de Pailleret and the Puy de Cliergue on each side of Sancy,
these lavas on cooling forming hornblendic andesites. Then came
the late Pliocene eruption of the Plateau basalts, which cut and
rest upon the others, and this was followed by local Pleistocene
eruptions of the basalts and the formation of cinder cones which
correspond to those of the Puy Chain. The succession of events is
clearly indicated by the relationships, especially the superposition
of the various lavas and pyroclastic products, and it will be ob-
served that the eruptions proceeded from acidic trachytes (to-
gether with rhyolites, phonolites, etc.) to basic types, i.e. basalts.
The tuffs often carry impressions of the vegetation of their time
from which their geological age can be determined.
The Cantal. This Tertiary volcanic massif lies south of Mont
Dore and is connected with it by a basaltic plateau. It forms the
most prominent of the Pliocene volcanoes, and it also shows much
dissection, so that the structure and succession of eruptive events
can be determined. Its diameter is from 60 to 80 kilometers, and
its mass about ten times that of Mont Dore. As in the latter case
there are many elevated peaks, the highest of which, the Plomb-
du-Cantal, rises 1858 meters. These are all erosion peaks of
parts of older eruptions, only one the Puy de Griou (1694 meters)
representing a late, though not the latest, eruption from the cen-
ter of the volcano, but even it does not show the original height
of the mountain. In Fig. 93 two cross sections are shown, one
from northwest to southeast, the other at right angles to it, and
both passing through the center of the old volcano. These are
reconstructed in. the same manner as that of Mont Dore, namely,
from numerous local observations and the combination "of these.
The succession of eruptive events here is of similar character to
that of Mont Dore, the volcano resting upon an old erosion floor
of gneiss upon which were locally deposited beds of Oligocene sedi-
ments (4, ing. 93), which are especially well shown on the south-
west. Through these came, first, eruptions of older basalt (5)
(not positively known, though suspected^at Mont Dore) and this
was followed by eruptions of acid trachytes and phonolites with
trachytic tuffs, still of Miocene age (6). These locally rest upon
the gneiss, or upon the Oligocene sediments, and still again, in one
part of the section, upon the older basalts, showing that they
ISO
Structural Characters of Volcanoes
succeeded these. Then was thrown out from the central orifice
a mass of breccias, cinders, etc., of andesitic and basaltic material
(7), intruded by dikes and interbedded with sheets of andesite,
this also forming a capping rock (8) and marking the great early
Pliocene eruption from the central orifice. Through these, in
middle Pliocene time, was forced the dike of phonolite which forms
the Puy de Griou, and finally came the eruption of the last basalt,
Extinct Volcanoes
.152
Structural Characters of Volcanoes
Extinct Volcanoes 153
the so-called Plateau basalt, because the great plateaus of the
region are capped by it.
If the student has succeeded in gaining a clear conception of the
succession of events in these two dissected volcanoes, he may next
attempt an analysis of the two cross sections of the southeastern
district given in the next two diagrams (Figs. 94 and 95), where, as
a result of eruption through numerous vents, a more complicated
structure is produced. After this he will be ready to analyze
FIG. 96. Map of the volcanic areas and fracture lines of Central France,
the former shaded, the latter in broken lines. Fault lines are shown solid.
(After Marcellin Boule.)
sections of older volcanic districts, including those of our own
country, as given in the various folios of the Geological Atlas of the
United States.
Arrangement of these Volcanic Centers Along Lines of Fracture
in the Earth's Crust. From the study of the volcanic region of
Central France it has become apparent that all of these volcanic
manifestations are located along lines of fracture in the earth's
crust, these fractures making possible the rise of the lavas and
gases which have produced the phenomena. The map of the
region (Fig. 96) shows the fractures ascertained and their relation
154
Structural Characters of Volcanoes
to the volcanic manifestations. This is a very general arrange-
ment of volcanoes the world over, and will be referred to again
later on,
The Extinct Volcanoes of the Rhine Region
Extinct volcanoes and volcanic activity during Tertiary time
are shown in a number of localities in the mountainous region
through which the river Rhine has cut its famous gorge. The
most impressive of these are the Seven Mountains (Siebengebirge)
FIG. 97. Volcanic landscape of the Siebengebirge after a photograph from
the Rodderberg. (From F. Ratzel, Die Erde.) These low mountains are
slightly dissected volcanic peaks of Tertiary age. The high peak on the left
is the Drachenfels. (See map, Fig. 98.)
on the right bank of the river in the Cologne region not far from
Bonn (Fig. 97). Like the Tertiary volcanoes of France, these
show only in part their former character, erosion having modified
them to a considerable degree. Nevertheless, it can be recognized
that they represent a series of eruptions, which, like those of France,
proceeded from acidic to basic lavas.
The eruptions began in Miocene time, and the volcanic masses
were built up on the old erosion surface of the Devonian shales
Extinct Volcanoes
and sandstones, which form the cliff of the Rhine gorge (map,
Fig. 98). The first outpouring resulted in the formation of light-
colored trachyte of which the typical trachyte of the Drachenfels,
already referred to in the discussion of that rock (p. 101), was the
product. Others of the hills of this region were also formed by this
_
Basalt Diluvium Alluvium
FIG. 98.- Map of the volcanic district' of the Rhine (Siebengebirge).
(After Laspeyres, from Walther.)
eruption. The next eruption was of more basic lava, resulting in the
formation of andesites, of which rock another group of these hills is
composed. Finally, very basic lavas came to the surface, forming
basalts, the hills of this rock being scattered among the others,
while dikes of the basalt cut the older andesites and trachytes.
This last eruption occurred in Pleistocene time. There is thus a
156 Structural Characters of Volcanoes
succession from acid to basic lavas as in the Auvergne district.
One of the last formed of these volcanoes is the Rodderberg,
situated on the west bank, of the Rhine between Mehlem and
Rolandseck. It consists of basaltic scoriae which in places rest
upon, and have by their heat altered and partly fused, some of
the older river sediments of the Rhine, and its crater, still perfectly
recognizable, is filled with a deposit of wind-blown dust or loess
and has now become the site of a thriving farm (the Broichhof).
EXTINCT CALDEKAS AND SINKS
The term caldera has often been applied to large craters, such
as those of Kilauea, but it has recently been suggested that these
FIG. 99. Cinder Cone within the Crater Lake, Oregon. A volcano
built within the basin of a sink. (Photo by D. W. Johnson.)
be spoken of as sinks, because they are formed by subsidences of
the lava column, and that the name caldera be restricted to explo-
sion craters or hollows such as that formed at Krakatoa (Daly).
Both sinks and calderas are known which were formed by past
volcanic activities in a region not subject to such disturbances at
present. An example of an older sink is Crater Lake, Oregon
(Figs. 99, 100 a-d}. This occupies the site of a former volcano,
which has been named Mount Mazama (Fig. 100 cf) and the sum-
mit of which has collapsed. From this summit glaciers descended
probably during the Pleistocene glacial period, which scoured and
polished the sides of the volcano, as is shown' by the marks still
Extinct Calderas and Sinks
10 TRAVEL-GUIDE MAP '"
CRATER LAKE F NATIONAL PARK
OREGON
1 i Scale 2 3
FIG. 100 a. Map of Crater Lake, National Park, Oregon. (U. S. G. S.)
i S 8
Structural Characters of Volcanoes
FIG. ioo b. Map of Crater Lake, Oregon. (U. S. G. S.) Heights and
soundings in feet. (Copied from de Martonne.)
remaining on the oftter slopes of the lake rim. Thus the col-
lapse of the mountain summit is shown to have been a recent one,
and appears to Have followed upon an extensive outpouring of
6lacir Pic Llo Rock
FIG. ioo c. Profile section of Crater Lake National Park. (U. S.' Dept.
Interior.)
lava. Many old calderas are found in various parts of the world,
those of the Eifel district in western Germany (Fig. 101), where
they are known as Maare, being the most typical (Figs. 102,
103). These are readily recognized by their circular character,
Extinct Galderas and Sinks
159
and by the fact that around their margins are extensive deposits
of scoriae and even of small volcanic bombs together with
the fragments of shale and sandstone blown from the craters
(Fig. 104). Less frequently are lava flows of basalt trachyte or
FIG. ioo d. Section of Crater Lake and its rim, with, the probable outline
of Mount Mazama. Structural details generalized. (Vertical and horizontal
scale the same.) Smithsonian Institution.
phonolite associated with them, which, together with the f rag-
mental material, built up crater cones. One of the most typical and
largest of these hollows is now occupied by the beautiful Laacher
Lake (Fig. 105). The lapilli from these explosive eruptions form
, FIG. 101. Map of the volcanic district of the Eifel. (After von Deschen.)
Be. Bertrich; Da. Daun; Dr. Dires; Ge. Gerolstein; Gi. Gillenfeld; H.Hilles-
heim; Klb. Kelberg; Ma. Manderscheid; U. Ulrnan; Maare in tyack; vol-
canic rocks, shaded. (From Kayser's Lehrbuch.)
160 Structural Characters of Volcanoes
FIG. 102. Gmiinden Maar, Eifel.
An explosion crater converted into a lake.
FIG. 103. Schalkenmehren Maar, Eifel.
A Tertiary explosion crater converted into a lake.
Volcanic Funnels, Pipes,. Plugs, and Necks 161
extensive deposits of " sand "along the left bank of the Rhine,
readily visible from the train.
Taff-
WctU
FIG. 104. Ideal section through a Maar of the Eifel showing the old crater
funnel and pipe, with the lakelet in the upper part, and the wall of tuff and
scoriae surrounding it. (From Kayser's Lekrbuch.)
VOLCANIC FUNNELS AND PIPES, SPINES, PLUGS, AND NECKS
Funnels and Pipes. From the mouth or rim of the crater
of the volcano the slope is generally inward, forming a funnel-
shaped depression to its bottom, this constituting the normal
crater. This differs from the crater of Kilauea, which is a sink
with practically perpendicular sides. The funnel is continued
FIG. 105. The Laachersee (Lake Laach) Northwest Germany, occupying
an old explosion crater. (After Walther.)
downward into the depths of the earth's crust as a more or less
cylindrical tube or pipe, which is the main conduit or vent through
which* the lava reaches the surface. (See Fig. 87, p. 143.)
The Spine of Mont Pelee (Figs, 106, 107). We have, of
course, no direct means of knowing from observation that this tube
162 Structural Characters of Volcanoes
is in reality a cylindrical one, nor that it penetrates vertically
through the country rock upon which the volcano is built. That such
is the case, however, maybe inferred from the remarkable phenome-
non which accompanied the eruption of Mont Pelee in Martinique
FIG. 1 06. The great "spine" of Mont Pelee, Martinique, from the east.
From the old summit plateau, the basin, of X'Etang Sec. The spine rises ap-
proximately 358 meters above the old crater rim in the middle foreground.
(Photo by E. 0. Hovey, March 25, 1903; courtesy of American Museum of
Natural History.)
in 1902, when a columnar mass of extremely viscous of solid lava
was pushed up 700 to 1000 feet above the crater, reaching a height
of over 5000 feet above the level of the sea. This remarkable
Volcanic Funnels, Pipes, Plugs, and Necks 163
column appears to have been the plug of lava which filled the pipe
of the volcano, and which lacked the proper fluidity to flow over
the edge of 'the crater, but hardened in the pipe and was pushed
upward by the pressure of the gases beneath, retaining essentially
the form of the tube in which it had solidified. The growth of
this spine of Mont Pelee was .actually witnessed. It began in
October, 1902, and reached its maximum elevation in seven months.
After that it slowly crumbled away under the influence of the
FIG. 107. The spine and upper part of the new cone of Mont Pelee, Mar-
tinique, from the north ; from the crater rim. (Photo by E. 0. Hovey, March
26, 1903 ; courtesy of American Museum of Natural History.)
atmosphere and its own weight, and from the explosion of gases
beneath it.
Plugs and Necks. If we speak of the tube which descends from
the base of the crater as the volcanic pipe or vent, the solidification
of the lava in this pipe forms the volcanic plug. When this plug
is pushed upward and becomes visible, as in Mont Pelee, it con-
stitutes a volcanic spine. When it becomes visible as the result of
the removal of the enclosing mass of the rock material which*
constituted the volcanic cone, it becomes a volcanic neck. Necks
are often left as the only relief feature of a volcano, because of the
164 Structural Characters of Volcanoes
solid nature of the lava which has hardened in the tube or pipe,
and the more readily erodible character of the material which
constitutes the rest of the volcano. Frequently, too, a cross-
section of a plug still within the pipe is seen when the country
has been worn down until both volcano and projecting neck have
been removed. Again, in an erosion cliff, a vertical section of a
part 'of such a volcanic plug may sometimes be seen, showing
the relation which it assumed to the country rock on cooling.
From such sections the form of the plug, and hence that of the
original pipe, may be ascertained. Theoretical considerations,
too, would lead us to infer, first that the upward path of the
heated gases, vapors, and lavas is most likely a direct one, and
secondly that the passage-way would become a more or less cy-
lindrical one, even though it was part of an irregular fissure in
the first place.
The French experimental geologist, Daubree, was able to show
that gases and vapors under high pressure, when forced through
fissures in limestone, granite, steel, or other substances, con-
verted their passageway into a cylindrical canal. From this we
may argue that the pipe or passageway of some volcanoes was
formed by the advancing heated gases and vapors which are
liberated from the magma deep down in the earth, and which,
under a pressure of thousands of atmospheres, are forced to find
their way to the surface through fissures, which they enlarge
to cylindrical canals and thus prepare the way for the uprising
lava masses.
Volcanic Necks and Exposed Plugs of Old Volcanoes
As we have seen, the term volcanic neck is applied to the hardened
mass of lava which filled the upper part of the pipe of the volcano
and which has been modeled out in relief by the erosion of the
material of the cone which formerly surrounded it. This is to
be distinguished from a volcanic plug, which is the mass of lava
hardened in the, pipe which is seen to be still surrounded by the
material through which it passed, whether this is the material of
the cone, or the country rock beneath, on which the volcano was
*built up.
In districts of comparatively recent but extinct volcanic activity,
volcanic necks are not uncommon. Some of the finest examples
Volcanic Funnels, Pipes, Plugs, and Necks 165
are found in the Tertiary volcanic district of Haute-Loire, France,
the eastern part of the volcanic region of Auvergne already referred
to. Here, near the town of Le Puy, is the famous Rocher St.
Michel (Fig. 108), an almost perfect example of a volcanic neck
from which all surrounding material of the cone has been removed.
This old neck, the position of which is shown in the section on
p. 152 (Fig. 95), is not a uniform mass of lava, however, but is rather
FIG. 108. Rocher St. Michel, in the Bassin du Puy, south central France.
The modeled-out neck of an extinct volcano, now crowned by a chapel. For
location and relation to other rocks of the region, see the' section, Fig. 95, page
152. (After Tempest Anderson, Volcanic Studies in Many Lands.)
made up of fragments of volcanic rock bound together by igneous
material, and represents the product of an explosive eruption, the
fragments having fallen back again into the throat of the volcano
where they .were solidified. Thus this neck represents the filling of
the upper part of the old volcanic vent, and although it no longer
shows the perfect original form of this vent, having been subjected
to narrowing by erosion near the top, it may still be regarded as
nearly typical of such structures. Its summit is to-day crowned
by a chapel. The neighboring and higher Rocher Corneille
Is, however, an erosion remnant of a brecciated lava mass,
1 66 Structural Characters of Volcanoes
probably from this same volcano, and rests on horizontal strata
(Oligocene).
Typical examples of volcanic necks from our own country are
the Leucite Hills of Wyoming, and peaks of the Mount Taylor
region of New Mexico (Fig. 109) . Another example, from Colorado,
is shown in Fig. 1 10. In diameter such necks may vary from several
hundred feet to several miles, and the material may be either solid
lava or fragments of the same bound together," that is, a breccia
of lava fragments. When the necks have been exposed to atmos-
pheric influences for some time, especially in a dry climate, they
FIG. 199. Great Neck, a volcanic neck in the Mount Taylor region, New
Mexico. (Photo by D. W. Johnson.)
are apt to crumble, and a mass of loose debris will accumulate
around them, forming a conical hill or tepee butte, from which the
summit of the much reduced neck may project. Such a hill must
not be mistaken for the original volcanic cone which surrounded the
central plug of lava before it was modeled out in relief as a neck.
It is merely a conical heap of fragmental material forming talus-
slopes around the central core, from the destruction .of which the
material was derived.
Not all projecting neck-like masses of volcanic rock, however,
can be interpreted as old volcanic necks. We have already seen
that the Rocher Corneille at Le Puy, France, is an erosion remnant
of a brecciated lava sheet resting on older rocks, although its form
is not unlike that of the old neck of the Rocher St. Michel close by
(see Fig. 95). The famous Devil's Tower, or Mato Tepee, of
Volcanic Funnels, Pipes, Plugs, and Necks 167
Wyoming (Fig. in) has long been, and by some Is still, regarded
as an ancient volcanic neck, but others consider it as a tower-like
erosion remnant of an old lava sheet intruded beneath the sur-
face (probably a laccolith, see beyond). The peculiar columnar
structure of the rock of this tower favors the last interpretation,
as will be more fully shown in the next chapter. The determina-
tion of the neck character of such a mass depends, of course, on
the possibility of showing that the igneous rock continues down-
ward through the rocks of the earth's crust, whereas an erosion
remnant of a lava sheet or intruded mass would rest upon the
FIG. no. Conical butte formed by a typical volcanic plug, a pillar of
basalt formed by cooling in the vent of an extinct volcano, and modeled out
in relief as a neck by erosion of the volcanic material which formerly surrounded
it. It is now surrounded by basaltic debris due to weathering, and this forms
talus-slopes. One mile north of Adair station, Colorado and Southern R. R.
Elmoro quadrangle. Colorado. (G, W: Stose, photo from U. S. G. S.)
rocks of the crust which are continuous beneath it, and would
not necessarily have direct connection with the deeper parts of
the earth.
Volcanic plugs, i.e., the hardened lava which still fills the old
pipes, are exposed in many regions in horizontal or in ^vertical
sections as the result of erosion. Practically all of these are found,
not within the old volcanic cone, for sections of such cones are
seldom if ever exposed, but in the rock beneath, upon which the
now vanished volcano had been built. They therefore represent
the lower part of the filling of the old volcanic pipe or conduit,
and those that are exposed belong to the older volcanoes of the
i68
Structural Characters of Volcanoes
world. The presence of the solid lava plug is of course inferred
in all uneroded volcanoes, and indeed it is a necessary part of an
extinct example. The part it takes in the determination of a
, volcano-like hill as an extinct volcano is illustrated by the classical
case of the Kammerbuhl, a small hill in northern Bohemia, which
played a leading role in the days when geologists still discussed
the question of the origin of beds of basalt, one group, the follow-
FIG. in. -Mato Tepee, Devil's Tower, Wyoming, a supposed volcanic neck
showing vertical columnar structure. (Photo by N. H. Darton, U. S. G. S.)
ers of Werner, holding that this rock was precipitated from water,
while another group argued for its volcanic origin.
The hill in question (Fig. 112 a) is composed mainly of cinders and
altered sediments, with a small basalt stream on one side. Werner
and Ms followers contended that the material of this hill and others
like it was formed by the combustion of beds of coal, which had
not only burned the older slates and the younger rock material,
but had also in part melted a layer of basalt of aqueous origin, and
so produced the cinders. The poet Goethe, however, believed
Volcanic Funnels, Pipes, Plugs, and Necks 169
FIG. 112 #. The Kammerbuhl, an old volcanic
hill in Bohemia.
this hill to be an extinct volcano, and argued that if a tunnel were
driven into it the hardened plug of lava in the old pipe would be
encountered near its center. To prove the correctness of the view
of the poet-naturalist,
his friend Count Cas-
par von Sternberg
had the tunnel driven
in 1837, with the re-
sult that the volcanic
plug was found and its
connection with* the
basalt layer proved
(Fig. 112*). This
virtually ended the
controversy about the
origin of basalt.
Sometimes old lava plugs may be modeled out in relief by the
erosion of the older sediments which were penetrated by the vol-
canic pipe. In such a case the resulting hill is essentially a neck,
not readily distinguishable, except perhaps in the material of
which it consists, from necks produced by the removal of the
volcanic cone only. An example of such an older neck is found
in Edinburgh, Scotland, where it forms the famous peak known
as Arthur's Seat, near Holyrood Castle. The material of this
peak consists of hardened basaltic lava and fragmental rock in
which not infrequently are found pieces of wood from the trees
which clothed the
slopes of the ancient
volcano. These
wood fragments,
xnow silicified, were
buried in the frag-
mental material
which filled the old
throat of the vol-
cano, and which
FIG. 112 b. Section of the K'ammerbuhl, show-
ing the probable former outline of the volcano and
the old volcanic plug. #, metamorphic rocks;
bj basaltic scoriag; c, plug of basalt; d, stream of
basalt; e, alluvial beds..
probably extends to some depth because of the sinking of the old
lava plug on solidifying.
Sections of volcanic plugs in the older rocks are often exposed
by erosion. An illustration of one such from the coast of Ireland
iyo
Structural Characters of . Volcanoes
is here given in Fig. 113 a, and a second, still joined to the basaltic
lava sheet, in Fig. 113 b. The basaltic lava penetrates the chalk,
which was altered to marble by the heat of the basalt. It is prob-
able that these are
not the plugs filling
the pipes of sepa-
rate volcanoes, but
that they represent
fissures through
FIG. 113 <*.- Section of volcanic plug (basalt) in which lava reached
chalk. Coast of Antrim, Ireland. (After Geikie.) the surface in the
form of a great
she^t, as described later. A similar plug may be seen on the
Nova Scotia coast at Wasson's BluS, not far from Parrsborough.
Here the volcanic material penetrates old sandstones and gypsum
A A A AA A, A A A A] A|
Basalt
FIG. 1135. Marmorization of chalk beds by basalt. Island of Rathlin
on coast of Antrim, Ireland. (Leonard.) The marble is dotted. (From
Kayser's Lehrbuch.)
beds of late Palaeozoic age, and belongs to the eruptions of Triassic
time which produced the great lava sheet of Cape Blomidon and
adjoining regions.
Sections of old volcanic plugs are
abundant in some districts. Thus on
the famous shore of County Fife in
Scotland, no fewer than eighty are
found in a space twelve miles in length
by from six to eight in width. These
plugs (Fig. 114) penetrate sandstones,
shales, limestones, and coal beds, and FIG. 114. Ground r plan
are consequently of more recent date section of the P lu s of an old
,, ' , . j j 1. _ ,-. volcano on the shore of St.
than the 'periods durmg which these MonanS) Fifej Scotland.
sediments were deposited (Mississip- (After Geikie.)
Volcanic Funnels, Pipes, Plugs, and Necks 171
pian, see Chapter XXXVI). The sections
of the plugs are roughly circular or ellip-
tical, varying in size, and from them dikes
and sheets of igneous material not infre-
quently penetrate the adjoining rocks, after
the manner of the radiating dikes seen
in modern volcanoes, as already described.
The plugs of Fife are not placed along
lines of great fissures, as might be sup-
posed, but penetrate the rocks apparently
without any relationship to its structure.
That necks are, however, ranged along
great lines of fissures in the earth's crust
is frequently found, and is seen in the
alignment of some of the necks of the
Auvergne region in France (see ante) and
elsewhere, and may be inferred from the
arrangement of the volcanoes of Iceland.
Here the lava first welled up through a
recognizable fissure in the earth's crust,
which subsequently was partly closed by
the hardening of the lava, so that only the
broader portions remained open. These
were converted into volcanic pipes, above
which individual volcanoes were built up
(Fig. 115). The Japanese volcanoes, too,
seem to be arranged along great lines of
fissures, and this is probably true for the
majority of existing volcanoes, as is in-
dicated by their linear arrangement (Fig.
116).
The material filling the volcanic vents,
and therefore the material of necks and
plugs, ranges from basalt to rhyolite, ac-
cording to the acidity of the volcanic
mass. When deeper portions of the necks
are exposed they show rocks of coarser
crystalline type, even gabbro and grano-
phyre, which have resulted from slow
cooling. In other cases fragmental ma-
TJ2
Structural Characters of Volcanoes
FIG. 116. Volcanoes in Nicaragua, showing linear arrangement apparently
along a fissure line. This line is parallel to the trend of the mountains and
ridges, and to the coast line. (After Karl von Seebach, from F. Ratzel, Die
Erde.)
Miocene Volcano
Jura,
,''"* Trias
' -**"
FIG. 117. Diagrammatic section through the Freiburg region (L Br.)
showing the former and present topography and the extensive erosion. (After
Th. Lorenz, from Kayser's Lehrbuch.)
Volcanic Funnels, Pipes, Plugs, and Necks 173
terial (agglomerate and tuff) fills the ancient vent, as in the case
of the Nova Scotia plug already cited. A mixture of both may
occur, and the structure may show evidence of successive erup-
tions through the same vent, and the formation of a cylinder of
lava within a cylinder. Finally, as already noted, the lava may
enclose fragments of the wall rock, generally from a deeper hori-
zon. The relationship of a plug to the old volcano in a much-
eroded region is shown in the section on the preceding page
(Fig. 117).
FIG. 1 1 8. Diamond mine in old volcanic plug, South Africa.
In South Africa, old volcanic plugs have become of great eco-
nomic importance, for it is in them that the great diamond mines
are located; These plugs, which are often of funnel form, broaden-
ing upward in diameter to 300 and in exceptional cases to 685
meters, are composed of brecciated rock, the so-called "blue
ground" which contains the gems (Fig. 118). Whether the dia-
monds represent the crystallized carbon of wood included in the
plugs as in, those of Scotland, or whether the carbon is of sub-
terranean origin, is an open question. The character of the dia-
mond-bearing material suggests that it may have risen from great
depths.
174
Structural Characters of Volcanoes
SHEET LAVAS FORMED BY FISSURE ERUPTION
Not all lava reaches the surface through volcanic pipes above
which volcanoes are built. Indeed, we have seen that some of
these pipes, as in the
Icelandic volcanoes, are
merely the reduced ex-
pression of large fissures
which opened in the
earth and through which
the lava at first reached
the surface, after which
volcanoes were built up
on those spots where
the fissure became
broken up into discon-
nected pipes. In some
of the grandest erup-
tions of molten rock,
generally basalt, no volcanoes are built up, or at least not until
long after the eruption begins. Instead, great fiat surfaces of lava
,
FIG. 119^ Map of the Columbia and Snake
River lava fields. (After Bowman.)
FIG. 119 &. " Gordon Craters," Malheur County, S. E. Oregon. Irregu-
larities in surface of recent lava, produced by pressure. (Photo by Russell
U. S. G. S. Courtesy of D. W. Johnson.)
Sheet Lavas Formed by Fissure Eruption 175
lava plains or plateaus are produced. -"Fissure eruption, in
which the lava wells up along the entire length of the fissured, is
seen to-day in Iceland. Along the great Eld Cleft, lava has
welled up in historic times, spreading on both sides of the fissure
to form a flat basalt plain, 270 square miles in extent. Along the
southern prolongation of the fissure, however, where it was nar-
rower, a row o.f low
cones has been
formed, through iso-
lation of a series of
vents.
One of the largest
lava plains of this
type is the Columbia
and Snake River
plain, which covers
an area of 200,000
to 250,000 square
miles in Washing-
ton, Oregon, Idaho,
and California (Fig.
ago). The lava
apparently spread
over a region of
varying topography,
filling the valleys
FIG. 119 c. Canon in the lava of the Snake
River plateau.
and burying the
smaller hills, sur-
rounding the larger
ones, and penetrating the valleys in their sides. A succession of
outpourings occurred, sometimes at short intervals, sometimes long
enough apart for the production, by weathering, of old soil beds
upon the preceding sheets, which were occasionally covered with
forests before the next lava sheet was poured out. The present
surface of this lava plain is in many places barren and desolate
beyond description, but in some places low cones rise above it,
formed probably during the later stages of volcanic activity, or the
surface is broken by pressure (Fig. 119 6). Where canons are cut
by rivers, the nature of the material is well shown (Fig. 119 c).
Another extensive flat lava plain formed by fissure eruption
176 Structural Characters of Volcanoes
covers Central India, and forms what is known as the Dekkan
Plateau. This has a present area of some 200,000 square miles, but
probably was originally much larger. The thickness of the basalt
sheet, generally spoken of as the Dekkan Trap, is in places more
than a mile. No cones have been built upon its surface.
Remnants of an equally extensive lava sheet of this type are found
in western Europe, where they occur on the west coast of Ireland
(Giant's Causeway, etc.), Scotland, Staff a, the Orkney and Faroe
Islands, and perhaps Iceland. The active work of the sea, aided
no doubt fay the weather, has removed large parts of this plateau,
FIG. 120. Basaltic columns of the Giant's Causeway, on the coast of Ireland. .
while others have probably subsided beneath the sea-level by
recent disturbances of that part of the earth's crust. This lava
sheet, too, is composite, and beds with plant remains, from which
the age of the eruption can be ascertained, lie between successive
flews./ *
Remnants of much older (Triassic or Jurassic) lava flows probably
of this type are found in New Jersey (Watchung Mountains), in
the Connecticut Valley of Connecticut and Massachusetts, and
along the shores of the Bay of Fundy in Nova Scotia. In New
Jersey and elsewhere the peculiar character of the base of the
sheet sometimes indicates that the lava overflowed bodies of
Sheet Lavas Formed by Fissure Eruption 177
standing water, with the result that steam pipes and a comminuted
basal structure were produced, the mud from the bottom of the
lake or pond being forced up into the lava. In cavities thus pro-
duced many famous mineral deposits (zeolites, etc.) were subse-
quently formed. This structure is also found in lava sheets of
basic volcanoes.
A characteristic feature of' many of these basaltic sheets is the
development of prismatic columns, which generally stand vertical,
FIG. 121. Near view of a portion of the columnar basalt of the Giant's
Causeway, coast of Ireland. The upper surfaces of the transverse joints are
seen to be convex in most'of the columns, but in a few cases they are concave,
holding small pools of water.
as so finely shown in the Giant's Causeway on the north coast of
Ireland (Figs. 120, 121). Sometimes, however, these columns are
curved, as is seen in some parts of the island of Staffa off the Scot-
tish coast, while in other parts they are vertical as at the entrance
to Fingal's Cave (Figs. 122 a-d}. This columnar or basaltic joint-
ing is also seen in some of the basalt sheets of the Columbia lava
plateau and the basalt of the Watchung Mountains, and is in-
deed a fairly common structure in basaltic lavas. The name joint
is unfortunate, as these structures have little in common with
the true joints, which are fractures in the earth's crust, and of
178
Structural Characters of Volcanoes
FIG. 1 22 a.; General view of the FIG. 122 5. The island of Staffa,
[sland of Staffa from the sea. Note showing the 'basalt columns capped
the columnar basalt capped by massive by massive basalt on the left and an
basalt. Entrance to Fingal's Cave eroded surface of basalt columns on
near the center of the view. (Photo the right. Curved columns in the
by author.) distance. (Photo by author.)
^FiG. 122 c. FingaPs Cave on the island of Staffa, showing the columnar
jointing. This is a sea-cave, eroded by the waves which remove the columns.
The floor of the cave is covered by sea-water even at low tide, though a nar-
row path has been constructed on one side. The roof of the cave is formed
by the non-columnar capping-mass of basalt, as is shown in Fig. 122 d. (See
further: Chapter XXIII, and Figs. 720, 721.) (Photo by author.)
Sheet Lavas Formed by Fissure Eruption 179
secondary origin. The columnar structure, on the other hand,
is a primary structure, and is caused by a radial contraction of the
FIG. 122 d. Wall and roof of FingaFs Cave, Staffa, showing the cross-
jointing of the columnar basalt, and the irregular appearance of the massive
basalt which forms the roof of the cave. (Photo by author.)
cooling lava mass about a series of equally spaced centers (Fig. 123).
As a result six-sided prisms are produced. A characteristic feature
of these prisms is the curved form of the horizontal or transverse
FIG. 123. Diagram illustrating the formation of contraction prisms.
The centers of attraction are connected by solid lines. The prisms formed are
dotted. (From Grabau, Principles of Stratigraphy.)
planes, which generally separate them into component blocks
(Figs. 121, 122 d). t Prismatic structure, though best developed
in basalt, is not confined to this rock, but occurs even in such acid
lavas as obsidian (Fig. 124). The position of the columns is, in
iSo
Structural Characters of Volcanoes
general, at right" angles to the surface of the mass in which they
are developed, while diverging and curved columns are often de-
veloped just below the surface of the lava sheet. Examples of
FIG. 124. Columnar jointing in obsidian, Obsidian Cliff, Yellowstone
National Park. (U. S. G. S.)
such curved columns are seen, as before noted, in parts of the
island of Stafifa and in parts of the basalt sheets of the Watchung
Mountains, New Jersey.
Minor Phenomena
181
MINOR PHENOMENA GENERALLY ASSOCIATED WITH CLOSING
STAGES or VOLCANICITY
There are a number of igneous phenomena which are manifested
upon the surface of the earth to-day, which are regarded as pri-
marily associated with the dying stages of volcanicity. These may
be enumerated in the
order of their importance
as follows :
a. Solfataric action.
Zc Fumarolic activity.
c. Mofettes and effer-
vescent springs.
d. Mud-volcanoes.
e. Geysers.
'/. Hot springs.
Solfataric action.
The volcano of Solfatara
(Fig. 125), in the Phle-
graean fields near Naples,
represents a dying stage
in volcanicity, giving off
steam and gases only,
since its last eruption in
1198. The crater of Sol-
fatara is very wide, but
its walls are only about
100 feet high, while its
floor is marshy and salt-
encrusted, with occa-
sional pools of boiling
water. On one side, at the foot of the crater wall, a jet of steam
escapes from an opening and rises to a height of 6 or 7 yards. In
Iceland, Java, New Zealand, the Andes, and elsewhere, -such
solfataric vents, as they are called, are common, indicating the
dying condition of those particular volcanoes. Besides the steam
many other gases are given off, these including hydrochloric acid
gas, sulphur dioxide, sulphureted hydrogen, ammonium chloride,
and, in the case of the Italian Saffioni, boracic acid, which forms
the source of the important borax industry of Tuscany.
FIG. 125. The Solfatara at Pozzuoli, Italy.
The steam and gases issue in one corner of
the nearly extinct volcano.
i&2 Structural Characters of Volcanoes
Fumaroles. These are typically emanations of steam, from
fissures, or from cooling lava surfaces, but they also vary, espe-
cially with the decrease in temperature of the lava or other source.
Above 350 C. only dry fumaroles exist, these giving off chiefly
anhydrous chlorides. Between this and a temperature of 100 C.
fall the acid fumaroles, which give off hydrochloric acid and sulphur
dioxide, with some steam. At about 100 C. occur the alkaline
fumaroles, which give ofi steam and ammonium chloride, and
FIG. 126. Active mud-volcanoes near Volcano Lake, Cerro Prieto, Delta
of the Rio Colorado. The largest of the group, seen in the distance, is now
quiescent. (Courtesy of the American Geographical Society, Broadway at
157 St., New York. From the Geographical Review.) (For location, see map,
Fig. 166.) j:
below 100 C. fall the cold fumaroles with nearly pure steam.
The famous valley of Ten Thousand Smokes, in the Katmai Penin-
sula of Alaska, is a prominent example of fumarolic action on a
large scale.
Mofettes. These give off only carbon dioxide, nitrogen, and
oxygen at the temperature of the atmosphere. Caves in which
such gases are given off exist in many volcanic districts ; and they
are sometimes exhibited to tourists with the lowering into them of
dogs or other luckless animals, which quickly become unconscious,
Minor Phenomena
after which they are drawn up and revived. The carbon dioxide,
on account t of its weight, remains near the bottom of the cave.
Effervescent springs, i.e. water charged with C(>2, occur in many old
volcanic regions.
Mud-Volcanoes. Among the subordinate phenomena asso-
ciated with or comparable to igneous activities, is the formation of
mud-volcanoes (Fig. 126). These are cones built up of mud with
small craters at the summit, resembling miniature volcanoes.
Their height varies from a few feet to a hundred feet or more,
FIG. '127. The Great Geyser Basin of the Madison River in Yellowstone
National Park. (Guyot.)
and their activity is either constant or intermittent, quiet or ex-
plosive. They are formed by highly heated steam or by gases,
which rise through a superficial layer of mud and mginate from
underlying lava beds or from chemical reaction. The mud is built
up into cones of the volcanic type. As the material is soft, how-
ever, these cones are readily destroyed by rains, etc.
Examples are known from the Colorado Desert, from lower Cali-
fornia, the Yellowstone Park (the Paint Pots) and elsewhere In
this country. They also occur at Baku on the Caspian Sea and in
the Crimea, where some of them rise to 250 feet in height, and
are provided with apical craters.
Geysers and Hot Springs. Geysers are springs of hot water
which erupt violently at intervals, with periods of quiescence be-
tween. They have essentially the same relation to ordinary hot
184
Structural Characters of Volcanoes
springs that volcanoes of intermittent explosive eruptions have to
those of quiet and constant lava flows. Geysers are abundant in
Iceland, in New Zealand, and in the Yellowstone Park, where there
are about 100 active examples
and more than 3000 hot
springs (Fig. 127). A geyser
consists typically of a more
or less circular basin sur-
rounded by a rim of silicious
material (geyserite) , and this
commonly forms the upper
part of a cone comparable to a
volcanic cone and its crater.
From the center of the basin
a pipe of circular section and
with smooth wall descends,
this and the basin being filled
with water. In the typical
Great Geyser of Iceland, the
cone is about 1 20 feet in di-
ameter and 13 feet high, while
the crater-like basin at the
top is 60 feet in diameter and
5 feet deep. The central
tube or pipe has a diameter
of about 10 feet, with smooth,
cylindrical, vertical walls.
The temperature of the water
which fills the pipe and basin
ranges from 75 to 90 C., but at a depth of 70 feet in the pipe
the temperature is about 130 C. The eruption occurs at almost
twenty-four hour intervals, and the water of the basin is thrown
to a height of nearly 100 feet. The characters of the other geysers
are similar, though differing in detail. The appearance of the
Giant Geyser of the. Yellowstone is shown in Fig. 128, and of Old
Faithful in Fig. 129. Diagrammatic sections of the two main
Icelandic examples, " The Geyser " and " Strokr," are given in
Fig. 130.
The eruption is due to the heating, above the boiling point, of the
water at a depth, while at the same time th^ nrfisanrft of t"hA mlmrm
FIG. 128. Giant Geyser of the Yellow-
stone. (After Hay den.) The eruptions
of this geyser occur at intervals of 7 to 12
days, and last for full 60 minutes at
each eruption. The column Is thrown to
heights of 200 to 250 feet.
Minor Phenomena
of water keeps it from changing to steam'. With increased heating,
however, the point is reached where the water will change to steam
in spite of the pressure, and the expansion of the steam raises the
FIG. 129. Old Faithful Geyser in eruption, Yellowstone National Park.
(Photo by D, W. Johnson.) The eruptions of this geyser occur at intervals of
60 to 75 minutes, each eruption lasting 4 minutes. The height to which the
column is thrown is 125 to 150 feet. The eruptions are heralded by loud
rumblings, with spasmodic outbursts of jets 10 to 20 feet in height, then the
column is suddenly thrown up with a loud roar, maintaining a-height varying
from 90 to 150 feet for two or three minutes with occasional steeple-shaped
jets rising still higher, the jets varying, and giving off greal rolling clouds of
steam; then the jets gradually decrease in altitude, and in five minutes the
eruption is over, the tube apparently empty but emitting occasional puffs of
steam for a few minutes longer. During the eruption the water falls in heavy
masses about the vent, filling the basins surrounding it and running off in all
directions. The estimated discharge is 3000 barrels at each eruption.
column of water, and so lowers the pressure, with the result that a
large amount of steam is suddenly formed from the super-heated
water, and the eruption takes place. The source of the water is
believed by some to be the ground-water, and by others it is re-
garded as new or juvenile water, given off by subterranean igneous
masses in the process of cooling. The heat is probably in all cases
of volcanic origin. The geysers of Iceland and New Zealand are
situated in regions of still active volcanicity, while those of the
i86
Structural Characters of Volcanoes
Strokr
FIG. 130. Semidiagrammatic sections of the Icelandic geysers, "The
Geyser" (the type) and " Strokr." Strokr has a funnel-like pit 36 feet deep
and 8 feet across, expanding into a saucer-like basin. The tube is generally
filled to within 6 feet of the top with clear water, which boils furiously, owing
to the escape of great bubbles of steam coming from two openings in opposite
sides of the tube. When the eruption took place, the jets rose in a sheaf-like
column to a Height of 100 or more feet. Eruptions took place at very irregular
and long intervals; but they could be produced in a short time by "putting a
lid on the great kettle," by dumping in large pieces of turf. "The Geyser''
is a pool of limpid green water, whose surface rises and falls in rhythmic pulsa-
tions. The usual temperature is 170 F. (76.6 C.) or 200 F. (93.3 C.) but
varies, being greater immediately before eruption. The shallow saucer-like
basin is about 60 feet across and slopes gentry to a cylindrical shaft 10 feet
in diameter, forming the pipe of the geyser; this being about 70 feet deep.
The tube is very regular. Before an eruption, bubbles of steam entering the
tube suddenly collapse with loud but muffled reports and a disturbance of the
quiet surface of the water. During this "simmering," the water rises in dome-
like mounds over the pipe and overflows the basin, running down the terraced
slope and wetting the cauliflower-like forms of sinter that adorn it. Domes of
water rise in quick succession, and finally burst into play, followed by a rapid
succession of jets increasing in height until the column is 90 to 100 feet high,
accompanied by dense clouds of steam. This lasts a few moments and then
ceases, and the basin is empty and apparently lined with a smooth coating of
white silica. The great geysers of the -Yellowstone surpass this in magnitude
of eruption, but not in beauty. (Weed.)
Minor Phenomena 187
Yellowstone are in a region where volcanoes have recently become
extinct.
The hot waters commonly carry silica is solution, and this is
deposited partly by the cooling and partly by the aid of low organ-
isms, especially algae, and so the cone of silicious sinter is built up
(Fig. 131). The quiet flowing hot springs more commonly deposit
carbonate of lime. These deposits will be discussed in a subsequent
chapter.
Geysers are commonly affected by earthquakes which disturb
the arrangement of the rocks of the region. The Icelandic geyser
FIG. 131. Spike Geyser, Witch Creek, Yellowstone Park. Showing an
exceptionally fine mass of silicious sinter (geyserite) built up around the basin.
The sinter shows botryoidal surfaces, (Photo J. P. Iddings, from U. S. G: S.)
Strokr is said to have come into existence during the earthquake
of 1789, and the earthquake of 1896 put an end to its activity.
It has not been in eruption since, A new geyser or hot spring ap-
peared after the shocks of the first day (Sept. 15) of the earth-
quake of 1896, this spring throwing water and rocks to an- estimated
height of 600 feet, but in a few hours it subsided to a height of 10
or 12 feet. It ceased to flow altogether after ten days.
CHAPTER IX
FORM AND STRUCTURE OF OLDER IGNEOUS
MASSES
TYPES OF OLDER IGNEOUS MASSES
LEAVING now modem volcanoes and those which have so recently
become extinct that their characteristics can still be readily rec-
ognized even though modified by erosion, we must next turn our
attention to those rock masses and structures of igneous origin
which were formed at a depth in the earth's crust, and have become
visible only as the result of erosion of the overlying rocks, or through
dislocation of the earth's crust. These may be grouped under the
following divisions: i. Dikes; 2. Stocks; 3. Sills; 4. Laccoliths
and related types; 5. Bysmaliths; 6. Bosses; 7. Batholiths.
The first five groups are classed as intrusive or hypabyssal masses,
to which group the volcanic plugs may also be referred, especially
that part which penetrates the older rocks beneath the volcano.
Bosses and batholiths are classed as deep-seated or abyssal igneous
rocks, and they may in general be regarded as representing the
hardened pools of igneous material from which both the intrusive
and the extrusive or volcanic masses are derived.
Intrusive or Hypabyssal Igneous Masses
Dikes. In modern volcanoes, as we have seen, the eruption
does not always take place through the crater, but a fissure may
open in the mountain side through which the lava wells out, and
upon it there are generally built up subsidiary or parasitic cones or
series of cones, the so-called monticules. This fissuring of the moun-
tain side is due to the fact that the material of the mountain, which
is often in large part volcanic ash only, is shattered with compara-
tive ease by the pressure of the rising lava, while the hardened plug
of lava, which fills the throat of the crater, cannot be lifted except
by a much greater force.
188
Types of Older Igneous Masses 189
When lava hardens in the fissures formed in the sides of the
volcano, a dike is produced. This is so called, because erosion
often removes the soft tuffs and other material, leaving the hardened
lava mass projecting as a wall. Fine examples of such dikes cutting
stratified tuffs are seen on the flanks of the Sicilian volcano Etna,
especially in the Val-del-Bove, as already noted (Fig. 79, p. 134).
Such dikes are also found in many districts of recently extinct
volcanoes, but they are not confined to the cones of volcanoes. In-
FIG. 132. West Spanish Peak, Colorado, from the northwest. In the fore-
ground, a large dike and several small dikes weathered-out in relief. (After
Stose, from U. S. G, S.) (See map, Fig. 138, p. 195.)
ded, where eruption takes place through fissures in the earth, with-
out the building of volcanoes, the hardening of the lava in the fissure
produces a dike. As, in such cases, the country rock is often much
older, the contrast which the dikes form with it is generally very
marked. Of course, such dikes will not become visible until the
lava sheet of which they formed the vents is removed, or until a
marginal section is cut into the igneous mass by the sea or other
agent with the consequent uncovering of the dike.
Over much of western North America the great lava sheets are
still in place, and the dikes which connect with them are not visible.
Over much of Great Britain, however, the basalt sheet of similar
age, and which probably extended to Iceland, has been removed by
1 90 Form and Structure of Older Igneous Masses
erosion, In part by the sea, and in part by atmospheric agencies.
As a result, the manifold types of rocks which make up the British
Isles are found transected by numerous dikes of basaltic material,
most of which have no apparent relation to any existing masses of
solidified lava. Some of these have been traced for a distance
of 60 or even 90 miles. Dikes connecting with remnants of the
basaltic sheets are also exposed on the seashore. In some cases
the dikes, being more resistant than the enclosing rock, have with-
FIG. 133. View of the great dike, running north from West Spanish Peak,
Colorado. This dike originally cut nearly horizontal strata which have weath-
" ered away, leaving a continuous wall 100 feet high. The horizontal markings
along the side of the wall indicate the original contact with the stratified rocks.
(Stose, photo; from U. S. G. S.)
stood erosion, while the country rock has been worn away. As
a result, they stand above the country like stone walls, a fact which
first gave rise to the name dike, since they are not unlike the arti-
ficial dikes built in some portions of western Europe, especially the
Netherlands, to keep out the sea from the low-lying lands (Figs.
132-134).
Along the border of the great trap sheet which forms the Dekkan.
Plateau of Central India, where the sea or other agents have eaten
away a part of the outer margin of the sheet, dikes are not infre-
quently exposed, penetrating the basement rock upon which the
trap sheet lies. For the most part, however, the dikes cannot be
traced directly to the trap, though the connection is indicated by the
Types of Older Igneous Masses
191
similarity of the material. Dikes of probably much older origin
are found abundantly along the Massachusetts coast, and to a
lesser extent along the Maine coast and elsewhere. Here they gen-
erally consist of the type of basalt known as diabase (see p. 106),
a lava which has cooled with sufficient slowness to permit the devel-
opment of recognizable crystals. We must, therefore, conclude
that these dikes represent the deeper portions of fissures filled by
lava, where, on account of the depth below the surface at that time,
the cooling was slow. Often a columnar structure is found in these
FIG. 134. Devil's Wall (Teufelsmauer) near Oschitz, Bohemia. A basaltic
dike, 25 km. long, 2 meters thick with horizontal prismatic joints. Weath-
ered in relief, wall-like. (From Kayser's Lehrbuck.)
dikes, the columns extending across them from wall to wall (Fig.
134), Where the sea has access to them after they have become
exposed on the surface from the erosion of large masses of rock,
these columns are frequently removed by the waves, leaving a
deep, parallel-sided fissure, into which the sea penetrates with great
force during storms and at high tide. Sometimes caverns are
formed by the removal of the lower columns only 5 when the com-
pression of the air in these caverns under the impact of the wave
will result in producing a regular series of water spouts after each
inrush of the waters, Such phenomena are common on dike-in-
fested coasts and are known as " spouting horns/ ' etc. Chasms
left by the removal of dikes are characteristic features of the rock-
bound New England coast (Fig. 135). Many other igneous rocks
192 Form and Structure of Older Igneous Masses
besides basalt and diabase occur in the form of dikes, and it may be
inferred that a majority of them never reached the surface as
molten lava but cooled in blind fissures of the earth's crust. Their
present exposure is therefore brought about by removal, through
erosion, of the rock which once concealed them. Among the more
frequent occurrences are those of granite (aplite, etc.), and especially
of the coarse or giant granite, pegmatite (Fig. -136) . These pegmatite
dikes are often of great width, and form the source of many rare
mineral deposits, besides yielding feldspar, quartz, and mica in
sufficiently large masses to be com-
mercially valuable. In some cases dikes
of this and other kinds have branches,
and sometimes they are extremely ir-
regular, having perhaps eaten their
way into the country rock. Such ir-
regular dikes are also spoken of as
igneous veins, though they have little
in common with true veins (see p.
265).
The essential character of dikes
may be summed up by saying that
FIG. 135. An eroded dia- they are generally of considerable
base dike in granite; west side }j nea ] extent, continuing sometimes
MalT L "tftot^of^ for many miles, and of uniform width,
chasm the dike is seen covered ranging from a fraction of an inch to
by boulders which were formed man y f ee t. When they cut bedded
by the waves from the dike . ' , * -, , ,
material. At present the chasm ^ta, they may do so at any angle,
is above high-water mark, show- but in general, dikes have an approxi-
ing recent elevation of the coast, mately vertical position/though this
(After Shaler, U. S. G. S.) -/ , , * ' f
. may be changed by subsequent move-
ments of the entire mass. Around old volcanic centers dikes
are often radially arranged, as shown on the island of Mull, west
coast of Scotland (Fig. 137), and in the ancient volcanic center of
the Spanish Peaks in Colorado (Fig. 138), the dikes of which have
already been referred to (Figs. 132, 133), Frequently dikes of differ-
ent ages intersect one another, in which case the younger can be
recognized by the fact that it cuts the older. In the broader dikes
the rock texture of the marginal portion is commonly finer than that
of the center, because more rapid cooling took place where the
igneous rock was in contact with the cool country rock which it
Types of Older Igneous Masses 193
penetrated. This is especially well recognized in the case of the
dike rocks of coarser grain. Sometimes the marginal texture is
even glassy, and the color and composition may also vary from the
margin to the center. The wall rock of dikes and the basement '
rock of lava flows frequently show a certain amount of alteration
due to the heat of the lava mass. This contact metamorphism
(see p. 207) is seldom very extensive, however, dying out a short
distance from the igneous mass, especially when this cooled rapidly
near the surface. Lava streams have indeed been known to flow
over ice masses without completely melting them. This argues
FIG. 136. -Pegmatite dike in crystalline dolomite, New York City.
for a comparatively rapid cooling of the lava exposed on the surface
of the earth.
Stocks. These are dike-like intrusions which are of short ex-
tent, sometimes more or less regular and cylindrical, at others ir-
regular. They are similar to volcanic plugs or cores, such as fill
the conduits of old volcanic pipes, but differ from them in not reach-
ing the surface, though the natme has been indifferently used for
the intrusions of plug-like character of moderate size, even those
that are in reality the filling of ancient volcanic conduits. It is
true that it is not always possible to determine in any particular
case whether the intrusion reached the surface and formed a vol-
cano, or whether it extended only part way up into the rocks of the
194 Form and Structure- of Older Igneous Masses
earth's crusts. In such a case the name stock >is best applied to
these structures. In character they partake of the deeper portions
of volcanic plugs or pipe fillings, and to some extent of those of
dikes as well, especially if they are irregular.
6
FIG. 137. Map and section (on AB) of the Island of Mull, west coast of
Scotland. (After Judd.) i, Non- volcanic basement beds; 2, granite;
3, basalt flows; 4, gabbroitic dikes ; 5, acid flows; 6 } volcanic tuffs and breccias.
Note the radiating and branching dikes.
Intrusive Sheets or Sills. -In many regions of the world we
find sheets of basalt and other igneous rock, the chemical and min-
eralogical characters of which indicate that they were formed by
the cooling of a magma, often with considerable slowness, and which
are interbedded with rocks of a clastic character, the latter evi-
Types of Older Igneous- Masses
.195
dently of non-volcanic origin. Some of these have been intruded
between the layers, but in other cases igneous sheets of this type
may be old flows, which were poured out over a surface composed
of horizontal strata, and which were subsequently covered by other
strata of non- volcanic origin. It is important that the two types
be distinguished. In the cases of interbedded lava sheets, we
should expect to find evidence of this succession of deposits
(stratification) not only in the igneous sheet itself, but also in the
enclosing rock.
iii^&siapaS
N./ ~VS, ^^^^^M-J, v^XX
FIG. 138. Map of a part of the Spanish Peaks region, Colorado, showing
the numerous dikes radiating from the volcanic necks or ancient centers of
volcanic action. (See Figs. 131, 132, pp. 189, 190.)
Lava streams have very definite and recognizable surface charac-
ters, as we have seen, and these are different from the structures
found in the bottom of the sheet. In most cases, not only is
the surface form of a sheet distinct (ropey, pillowy, rough, etc.),
but the lava itself is compact, or even glassy near the surface, and
commonly layers of steam holes, either empty or filled secondarily
by mineral deposits, are found for some distance down from, and
parallel to, the surface. The structure of the enclosing rock also
is distinctive, for whereas the under layer over which the lava was
poured out shows some effect from the heat of the stream (meta-
morphism), and may actually have' furnished- fragments which are
included in the lava mass, the overlying layer will show no such
contact phenomena, for the lava will, in most cases, have cooled
196 Form and Structure of Older Igneous Masses
sufficiently, before being covered by sediments, not to produce any
such effect. A still more convincing argument of the contem-
poraneity of the lava
flow is furnished when,
as is frequently the case,
fragments of the lava are
included in the overlying
stratum, having been
broken from the surface
of the sheet before or at
the time of the deposi-
tion of the covering layer.
Weathering of the sur-
face of the sheet and the
formation of soil layers
is also a clear indica-
tion of the exposure . of
the lava sheet for a time
before it was buried by
sediment. Such old soil
surfaces are seen in nearly
all of the fine series of
ancient lava flows which
are to-day exposed on
the east coast of Scotland
(Fife), and the length of
time of the exposure is
shown by the fact that
roots and stumps of an-
cient trees (Calamites) are
found in these old soils,,
and were completely
buried by the sediments
which followed. A suc-
cession of such ancient
forest beds is found in
FIG, 139. Section of Amethyst Moun-
tain, Yellowstone National Park. The
mountain consists of a base of Archaean
granite and Carbonic limestone, oyerlain dis-
conformably by 2700 feet of Tertiary strata,
chiefly of volcanic origin. The coarse beds
are conglomerates and breccias, and alternat-
ing with beds of finer material are sandstones
and shales bearing the abundant silicified re-
mains of fossil forests. There are at least
fifteen successive forests, indicating that num-
ber of volcanic outbursts, separated by suf-
ficient -time to allow the growth of forest
trees varying in diameter of trunks from two
to ten feet. ..(After Holmes.) For photo-
graph of several of these trees see illustration
in Chapter XLV.
the series of lava flows
exposed by erosion in the Yellowstone National Park (Fig. 139).
Intruded igneous sheets or sills, however, show not only a basal
but also an upper igneous contact, and, indeed, may include frag-
Types of Older Igneous Masses 197
ments of the overlying as well as the underlying strata. More-
over, the sheets themselves show a similar fine-grained or dense
character on both surfaces, while toward the center they become
more coarsely crystalline. Such sheets are evidently gf more re-
cent origin than the enclosing rocks, and were intruded between
and parallel with them/forcing them apart, and cooling thus within
the earth's crust without ever reaching the surface. Such intru-
sive sheets are also called sills, 1 and they are well represented by
FIG. 140. Near view "of the Palisades of the Hudson, showing jointed
trap (simulating columnar jointing) at top and talus slopes below. (Photo by
D.W.Johnson.)
the rocks which now form the Palisades of the Hudson River op-
posite New York City, though these constitute only a small part
of the former extent of the intruded sheet (Fig. 140). This is sev-
eral hundred feet in thickness and has been traced for a distance of
about 100 miles. A similarly extensive example, well known to
foreign geologists, and indeed the type from which the term sill
has been derived, is the " Great Whin Sill " of the north of England.
This can be traced for a distance of 80 miles between the enclosing
rocks, its resistance to erosion helping to produce the great cliff
known as the Pennine escarpment, which bounds the Vale of
1 The name sill was originally applied by the miners in the north of England to
any prominent or hard projecting bed or stratum. The type of the volcanic sills is
the great Whin-sill of northern England mentioned below, this being a prominent bed
or sill of whin-stone, a name given to any hard, fine-grained rock, such as basalt,
guartzite, etc.
198 Form and Structure of Older Igneous Masses
Eden on the east (Fig. 141). This sill varies from 20 to 150 feet
in thickness, the average being from 80 to 100 feet, and it forms
a prominent ledge wherever exposed in
section (Fig. 142). It covers an area
probably not less than 1000 square
miles in extent. Other sills, mostly
of lesser thickness and extent, are found
in widely distant regions of the world.
Among these may be mentioned the
one forming Salisbury Crags in Edin-
burgh, Scotland, which overlooks Holy-
rood Castle, and which figured in the
disputes over the origin of basalt in
Werner's time (Fig. 143 c).
Through erosion, the sill is exposed in
various ways as shown in the following
diagrams (Fig. 143). Sills which were
originally intruded between horizontal,
strata may become inclined by the arch-
ing of these -strata, as in the case of the
Palisade sill (Fig. 143 a), or they may
even become folded with the strata.
A characteristic feature of sills, and
one which aids greatly in distinguishing
them from contemporaneous flows, is
seen in the lack of absolute conformity
to the enclosing strata. Though in any
given locality this conformity may be
unquestioned, it will be found that on
tracing the sill for some distance, it gen-
erally breaks across some of the layers,
passing either to a higher or lower level,
and there continuing for a time parallel to
the enclosing strata (Fig. 144) . The rocks
of the sills are generally massive or not very coarse-grained, though
those of the center of large sills, such as that of the Palisades, are
sometimes of moderate coarseness. They are generally darker-
colored and denser-grained on both upper and lower margins, and
the change in grain toward the center is often a very regular one.
In the tunnels which have been cut through the Palisade sill, it has
Types of Older Igneous Masses
199
been possible to ascertain, from the size of the grain, the distance
from the upper or lower margins at any selected locality. Colum-
nar structure is not a characteristic feature of sills, the apparent
FIG. 142. View looking up Hilton Beek, northern England, showing the
out-crop of the Whin Sill, which forms the prominent cliff on either side, and
rests on Carboniferous limestone. (Photo by the Author.)
columnar structure of the Palisade sheet being due to a series of
subsequently formed intersecting fissures or true joints.
Laccoliths. The Henry Mountains of Utah represent rem-
nants of a peculiar type of intruded mass which, instead of spread-
FIG. 143. Diagrammatic sections of volcanic sills, a, Palisades of Hudson;
6, Whin Sill, North England; c, Salisbury Crag in Edinburgh, Scotland.
ing out between the strata, is localized, each separate intrusion
swelling into a semi-lenticular or dome-like mass, space for which
is made by the lifting into an arch of the rocks which overlie the
200 Form and Structure of Older Igneous Masses
FIG. 144. Base of Palisade diabase, showing lateral ascent of the diabase
across the strata of the Newark group. Kings Point, Weehawken, N. J.,
looking west. (From photographs, U. S. G. S.)
intruded mass (Figs. 145, 146). These structures were first de-
scribed by an American geologist, the late G. K. Gilbert, and
the type was by him designated a laccolith. Other laccoliths
FIG. 145. Ideal restoration of the laccoliths of Mt. Holmes, Henry Moun-
tains, Utah.
have since been found in many parts of the world, the best known
examples among these being in Colorado and Montana. Lac-
coliths become visible by the erosion of the covering rock, and
FIG. 146. Ideal cross-section of the laccoliths of Mt. Holmes, after restoration.
then they constitute hills of igneous material, thick in the exposed
center, but thinning away in all directions where they pass be-
neath the remnants of the original covering sheet or interpenetrate
Types of Older Igneous Masses
201
it in a series of wedges (Figs/ 147, 148). It is important to bear
in mind that where the contact of the thin edge with the cover-
FIG. 147. Hesperus Mountain, showing wedges and sheets of trachyte
intruded into the shales, from the laccolith of Mt. Moss.
ing rock is exposed around the margin of the hill, this contact is
seen to be an igneous one, clearly proving the rock to be intrusive.
Moreover, it must be shown that the igneous rocks rest upon rock
FIG. 148. Ideal section of La Plata Mountain, Colo., showing the sup-
posed original form of the laccolith of Mt. Moss. The line a, a is the present
profile which cuts Hesperus Mountain and Mt. Moss.
of a different type, and that they are not deep-seated igneous masses
which have eaten their way into the overlying rock (bosses, etc.).
When a laccolithic intrusion has been completely isolated by erosion
so that no part of the original mass is in contact with the overlying
202
Form and Structure, of Older Igneous Masses
rock, the recognition of the laccolithic origin of the mass becomes
very difficult, and the proof of such an origin is sometimes incon-
clusive. The Mato Tepee or Devil's Tower of Wyoming (Fig. in,
p. 168) has also been interpreted by Jaggar as the remnant of a lac-
colith rather than a volcanic neck.
Small laccoliths may be lexposed in cross-section in a cliff, when
their character is undoubted. Laccoliths range in maximum
thickness from less than a hundred to several thousand feet,
and their diameter varies from a few hundred yards to several
N.W.
FIG. 149 a. Section of Corndon Hill, in Shropshire, England; the type of
the phacolith. (After Harker.)
miles. As in the case of. sills, which may be regarded as the
extreme in one direction of laccoliths, the upper and lower margins
are generally finer grained and may be darker-colored and
lower in silica content, besides being richer in ferro-magnesian
minerals, than the center. Columnar structure is sometimes de-
veloped, the columns standing vertically. This is shown in
the Mato Tepee, already referred to as probably- an erosion
remnant of a thick laccolith.
Phacoliths. Corndon Hill of Shropshire, England (Fig. 149 a),
appears to represent a peculiar form of lenticular igneous intrusion
in which, however, instead of forcing the overlying strata upward
into a dome, these masses occupy the
axes of both upward (anticlinal) and
downward (synclinal) folds of the
enclosing strata. These cavities, it is
inferred, were formed as the result of
the spreading of the strata by lateral
compression during the folding, and
thus the igneous masses merely oc-
cupy the spaces made for them by
other agencies, instead of actively forcing the strata apart (Fig.
149 i). Or it may be that the compression of the stratified rocks
into folds produced lines of weakness along which the igneous mass
FIG. 149 b. Diagram illus-
trating the formation of phaco-
liths. (After Harker.)
Types of Older Igneous Masses 203
found it easy to enter. Such types of intrusions have been called
phacoliths (Harker) (Greek phacos, <a/<os, a lentil).
Chonoliths. Another type of this intrusion has been described
from the Aletschhorn, a mountain in the Aar Massif of the Alps,
and from Ascutney Mountain, N. H. Here, during the folding
of the strata, there were formed irregular, instead of lenticular,
cavities into which the lava from below found its way. To a greater
or less degree the igneous mass may also have forced apart the rock
in an irregular manner, partaking of the characters of both the
laccolith and the phacolith. The most striking feature of such
FIG. 150. - Section of the Mt. Holmes bysmalith. (After Iddings.)
an igneous mass is, however, the great irregularity of the spaces
which it occupies. To such masses the name chonoHth (ckoanolitk)
has been applied by Daly, because the cavities formed, acted as a
mold into which the igneous rock was poured (Greek choanos,
Xo'avo? or x^* a mold). It is obvious that small intrusions of this
type grade into igneous veins.
Bysmaliths. Still another type of igneous intrusion has been
found in the old volcanic center of the southern end of the Gallatin
Range in the Yellowstone National Parkin northwestern Montana
(Fig. 1 50) . Here the mass which forms Mt. Holmes h^ the nature
of a huge core, resembling a giant volcanic neck, but connected
either with no surface flow, or with flows of only secondary signifi-
cance. It is a laccolith in which the upward force was so great as
to rupture the overlying rock mass, and carry it upward for a great
204 Form and Structure of Older Igneous Masses
distance. This is therefore a large plutonic plug or core which has
forced its way upward as a compact mass into the overlying rock,
and the contact of this mass with the rocks around its margin shows
the evidence of such upward movement. On this account it has
.been called a bysmalith l (Iddings), a rock rising from the depths
(Gr. /?wr<ros, the deep). A bysmalith represents the other extreme
of laccolith formation, with the exaggeration of the vertical dimen-
sions. Bysmaliths are also called plutonic plugs in distinction from
volcanic plugs, which are the filling of pipes that reach the surface.
They differ from stocks in the manner of origin, having forced their
way into the rock by lifting the obstruction in their path, while
stocks are intrusive into fissures, which they may widen and alter
by pressure and otherwise.
It is evident that the recognition of the particular type of igneous
intrusion which any given mass represents, can only be determined
from careful examination of both the mass itself and of the enclos-
ing rock, .and that many cases may occur where erosion has ren-
dered the interpretation at best a doubtful one. If the student
keeps in mind the types here given and the essential characters of
each, he may, by elimination, in most cases be enabled to reach a
conclusion regarding the nature of an igneous mass with which he
becomes confronted in the field. Extended examination of all the
exposed parts, however, and especially of their contacts with the
enclosing rock, will be necessary before such an interpretation is
possible.
It must be emphasized again, that the types so far discussed show,
from the nature of the rock, that they have cooled at some depth
below the surface, but that they do not belong to the great mass of
deep-seated igneous material which formed the reservoir, so to
speak, from which these intrusions were fed. This group of deep-
seated rocks will be discussed in the next section.
Deep-Seated or Abyssal Igneous Masses
In many portions of the world, especially in such regions of an-
cient rock as eastern Canada, the Adirondack Mountains, parts of
Sweden and Finland, and elsewhere, igneous rocks of coarse grain
(holocrystalline) are found, which may be interpreted as a part of
the reservoir of igneous material from which ancient volcanoes
1 More corrrectly byssolith.
Types of Older Igneous Masses
205
were fed, and from which the other types of intrusions (the hypa-
byssal) emanated. That these are now exposed upon the surface
is due to prolonged erosion, which has removed great thicknesses of
overlying rock beneath which the magma cooled. At the same
time these igneous magmas forced or ate their way to some extent
into the overlying rocks, which, therefore, when still preserved, show
an igneous contact, that is, a contact of a cool with a molten rock
mass. These igneous masses consist of granite, syenite, diorite,
gabbro, or of the more
basic types of rocks, and
they have been divided
on the basis of their form
and size into bosses and
batholiths.
Bosses. These are
deep-seated igneous
FIG. 151. Ground-plan of a granite boss,
with the ring of contact metamorphism.
a, sandstones, shales, etc., dipping at high
angles in the direction of the arrows ; b, zone
or ring within which these rocks are .meta-
morphosed; c y granite, sending out veins or
apophyses into b.
masses which show a
dome-like surface rather
than the form of a plug,
and their section as re-
vealed by erosion is a
more or less circular one
(Fig. 151). They are sur-
rounded by other rocks, often sediments, which show alteration
from contact with the heated igneous mass, and such altera-
tion appears often in concentric zones around the boss, the most
strongly altered zone being next to the igneous mass.
Batholiths. These ate huge bosses of very irregular form, the
exposure of which can sometimes be traced over many square
miles. They are particularly characteristic of the older parts
of the earth's crust, and they have a very variable relation
to the rocks into which they are intruded. The granite head-
lands of Cape Ann, Mass., of Mount Desert, Maine, and of
Halifax, Nova Scotia, are examples of more or less eroded rock
masses of this type.
Since these masses cooled very slowly, they became coarsely
crystalline, while the prolonged heat greatly affected the rocks with
which they came in contact. In batholiths, as in bosses, we may
generally trace a series of zones of alteration in decreasing intensity
from the igneous mass outward, those immediately in contact with
206 Form and Structure of Older Igneous Masses
the igneous mass showing most profound alteration, while each
zone generally has developed minerals peculiar to it.
Subordinate Igneous Masses
Apopliyses. This name is applied to offshoots from any in-
trusive igneous mass whether a dike, sill, laccolith, or deep-seated
magma. Apophyses are generally irregular in form and die out
in a short distance. Sometimes they may have the character of
small dikes for some distance of their extent. They form the surest
means of distinguishing an intruded mass from a surface flow.
Contemporaneous Veins. As all magmas contain more or less
water-vapor, this may become locally segregated during the pro-
cess of solidification by crystallization of the magma, thus ren-
dering portions of the magma very fluid, because of the abundance
of the water in it. If the main mass of the magma which has al-
ready solidified but is still highly heated, is fissured, as may happen
especially near the margin of the mass, this more liquid magma will
flow into the fissures, and there solidify. Because the acidic min-
eral combinations will be the last to form, these contemporaneous
veins, as they are called, will consist of increasingly lighter minerals,
toward the outer part of the cooling mass. Thus it will happen that
a magma which solidified to form a dark, dioritic rock, for example,
will become intersected near its margin with irregular veins of light-
colored rock, consisting mainly of orthoclase, feldspar, and quartz,
and still farther, near the margin, by more or less pure quartz veins.
This is well shown in the rock ledges of the Massachusetts coast
some distance north of Boston, where such a dark dioritic rock is
interpenetrated in all directions by veins filled with a pinkish, fine-
grained, granitic rock (chiefly orthoclase and quartz), forming a
very striking contrast, from which that portion of the coast has
received its name of " Marble Head."
Pegmatite Veins. Biy the local concentration of much water-
vapor and the gases from the solidifying of the main mass of the
igneous body, exceptionally fluid magmas may be produced which
contain much silica and the substances which go to make up the
acid minerals, together with the rarer mineral substances of igneous
magmas. This very fluid magma will occupy fissures and cavities
in the main mass, from which it is differentiated, and will also be
injected into fissures in the adjoining ,rock. Slow solidification
Contact of Igneous Masses with Other Rocks 207
produces coarse crystals, often many feet or yards in diameter, and
these will be largely the more acidic (potash and soda) feldspars,
the lighter-colored micas, and much free quartz. Such a coarse
rock of acidic minerals is known as a pegmatite, and in it the inter-
growth of quartz within the feldspar produces the peculiar structure
known as graphic or pegmatitic structure (see p. 97, Fig, 40). Rocks
of this type are known as pegmatites, but they are not always of
such coarse texture. In size too, the pegmatite masses may vary
frc-m dike-like intrusions hundreds of meters across, to veins only
a few millimeters thick. In such pegmatites there are commonly
found many minerals formed of the rarer element?, most common
among which are tourmalines (commonly the black variety, but
also the red, green, or blue gem types), huge crystals of spodumene,
of beryl, etc., and not infrequently many metals as well.
CONTACT OF IGNEOUS MASSES WITH OTHER ROCKS
Igneous Contact
By the term igneous contact we mean the junction which has
been produced by a mass of igneous magma while still hot with the
rock over which it is poured out, if it is a lava, or with the rock of
the earth's crust into which it is intruded. In the first case the
cold rock over which the lava flow is poured is called the basement
rock, in the second case the rock into which the igneous mass is
intruded is called the country rock. In either case the older rock
may be assumed to have been cold when the hot magma came into
contact with it, and the structures which have resulted from such
a coming together of highly heated with cold rocks, are contact
phenomena, and the alterations produced in either rock are called
contact metamorphism.
In the case of surface flows of lava, the phenomena of contact
metamorphism are seen only at the base of the lava sheet, though
a certain change is produced where the surface of the sheet is in
contact with the atmosphere, or with water in the case of a sub-
marine outpouring of lava, and these changes in the lava sheet may
also be referred to as metamorphic changes in a very literal inter-
pretation of the term.
Contact metamorphism is very slight in the case of most intru-
sive sheets or sills, in laccoliths, and generally in dikes, though in
the case of such large sills as the Palisades it may extend for many
208 Form and Structure -of , Older Igneous Masses
yards from the contact. In all cases it must be remembered that
in intrusive masses the contact phenomena are found on all sides
of the intruded mass. Around volcanic plugs and stocks contact
phenomena are well marked, especially in the former, where there
was continuous or repeated passage upward of igneous material
with surface eruption. Again, where dikes are numerous and close
together, much contact metamorphism is observable. But such
phenomena are most marked in the large, deep-seated igneous
masses of coarse grain, which cooled slowly and which, therefore,
subjected the adjoining rocks for a long time to the heat of the
igneous body and the action of the gases given off from it.
Kinds of Contact Metamorphism
We may distinguish two kinds of contact metamorphism, that
produced upon the igneous mass itself from contact with a cool
wall rock, and that produced upon the wall or country rock. The
first is spoken of as an endomorphic, and the second as an exomorphic,
change.
EndomorpMc Effects. The effect produced by the contact of
a magma with the wall or country rock upon the resulting igneous
rock itself is in the first place a change in texture along the con-
tact, as we have already seen. This is shown by finer grain or even
a glassy character of the igneous rock at the junction with the wall
rock. A porphyritic texture with fine ground-mass and coarse
phenocrysts may also develop as the result of more rapid cooling.
When, however, the enclosing rock is thoroughly heated by the
igneous mass, as in a volcanic conduit, no perceptible change may
result in the marginal portion of the igneous mass on cooling, while
the effects on the wall rock itself (the exomorphic effects) are the
more marked. . _ ;'
In the second place, new minerals, not found in the main mass
of the igneous intrusion, may be formed near the contact, from
the chemical activities of vapors and gases which tend to be ex-
cluded from the main mass as it solidifies and to escape toward the
margin of the mass and thence into the surrounding rock. In
granitic intrusions tourmaline is not an uncommon mineral thus
formed.
Exomorphic Effects. The effects of the heated intrusion upon
the wall or country rock are, however, the most marked. Among
Contact of Igneous Masses with Other Rocks 209
these the most notable are the baking or hardening and toughening
of the rock near the contact, from the heat, and its frequent change
to a more crystalline condition. Next in importance is the develop-
ment of new minerals on the contact zone, these being generally
formed by the chemical activity of the gases and vapors which
enter the rock and constitute the mineralizers.
The width of the zone subject to contact metamorphism varies
with the size and the heat of the igneous mass, and with the amount
of mineralizing gases and vapors given off. It also varies with the
character of the enclosing rocks, some being more easily altered than
Bothers, while some are more permeable to heat and mineralizing
vapors than others. In general, older igneous rocks into which
younger ones are intruded, are less altered than are sediments of
chemical, organic, or clastic origin, in which the chemical composi-
tion is such as to permit ready change, or in which the conduc-
tivity, texture, and other characters allow easy entrance of the
heat and the vapors. The special effects on a few types of these
may be noted.
Effects on Limestones. Limestone is the general name applied
to rocks consisting of carbonate of lime or of carbonate of lime and
magnesia. They may be of aqueous (chemical), organic, or of
clastic origin, as more fully discussed in later chapters. Lime-
stones are seldom pure, there being commonly an admixture of clay
or of silica in the form of flint, chert, intimately admixed grains
(sand) , or other particles. The first and most general effect of the
igneous mass upon limestone is the crystallization of the latter,
with the result that a marble is produced. Great masses of marble
are, however, not produced in this manner, but by more extended
(regional) metamorphism, during mountain-making disturbances
(see Chapter XX). Changes in the mineral character also occur
by recombination of the various substances present. Thus if silica
is present, the lime will combine with it, with the separation of
carbon dioxide, and a lime silicate (the mineral wollastonite) is pro-
duced. The change may be expressed in the following formula :
CaC0 3 + Si0 2 = CaSiQs + C0 2
Carbonate of ' Silica Lime silicate Carbon
Lime (Wollastonite) dioxide .
If the limestone is magnesian, then a double silicate of lime and
magnesium is produced, this giving a mineral , of the pyroxene
group, known as diopside, abundant crystals of which are found in
210 Form and Structure of Older Igneous Masses
the northern part of the city of New York (Inwood region), where
the magnesian limestones are penetrated by numerous pegmatite
dikes (see Fig. 136, p. 193). The formula expressing this change
may be written as follows :
Ca Mg(C0 3 ) 2 +2 Si0 2 = Ca Mg(SiO 3 ) 2 +2 CO 2
Lime magnesium Silica Lime magnesium Carbon
carbonate silicate (diopside) dioxide
When both clay and quartz are present, a double silicate of lime
and alumina may result, the aluminum being furnished by the clay.
This new compound will then crystallize out in the form of the
mineral garnet, while both water and carbon dioxide are given off.
The following formula expresses this change :
3 CaC0 3 -f B4Al 2 Si 2 9 + Si0 2 = Ca 3 Al 2 Si 3 Oi 2 +3- C0 2 + 2 H 2
Lime carbonate Clay Silica Lime aluminum Carbon Water
(Calcite) (Quartz) silicate (Garnet 1 ) dioxide
Good illustrations of the formation of garnets, sometimes of con-
siderable size and in large numbers, but of no great value, may be
seen in the impurer parts of the same limestones (Inwood marble)
in the northern part of New York City, near the contacts with
dikes.
Many substances may be carried into the limestones by gases
and vapors, and so produce a variety of minerals. Those men-
tioned are, however, the most important.
Effects on Mud-Rocks. This is a general name applied to sedi-
ments composed most commonly of microscopic particles of quartz
and of clay, sometimes the one and sometimes the other substance
predominating. Lime particles may also be very abundant and
intimately disseminated among the clay and quartz, and other
substances in a fine state of division may also occur. Such mud-
rocks may be massive, or they may have a fine and irregularly
bedded or laminated structure, when they are called shales. By
baking along the contact, such a mud-rock may be changed into a
dense mass comparable to artificially baked clay (porcelain),
forming a rock called porcelanite (porcellanite) or a horn/els, which
may have the appearance and hardness of a dense basalt or other
igneous rock and may be mistaken for such. Another feature pro-
duced in such mud-rocks, a short distance from the contact, is a
series of spots or knots of mineral matter or even of crystals of
1 TMs is the variety Grossularite. There are several others of different composi-
tion, See Table, p. 62.
Contact of Igneous Masses with Other Rocks 211
minerals, among which the commonest is a silicate of alumina known
by the mineral name of andalmite (Al 2 SiO 5 ) ; other minerals may
also be developed. v
Effect on Quartz Rocks (Sandstones, etc.). When the country
rock consists largely of quartz, commonly in fine grains (sandstone)
the effect of the intrusion x>f an igneous mass is not so marked as
in the case of other rocks. Close to the contact, the sandstone
may be hardened into a quartzite, and if clay, lime, or other mineral
substances are present, new minerals may be produced,
Alteration by Gases and Vapors
Where the volcanic activity has subsided into the solfataric or
fumarolic stage, with the emission only of vapors and gases, the
country rock around the vents and along fissures penetrated by
these gases and vapors may be profoundly altered. New minerals
may be produced in this zone of alteration and deposits of older
minerals may be enriched by the addition of new material from
the gases and vapors. In this wise, important mineral and ore
deposits may be produced, to some of which reference will again
be made in a later chapter.
Ancient Igneous Masses in Sedimentary Contact with the
Overlying Rock
The granite mass of Pikes Peak, in Colorado, differs from the
granite masses previously discussed in the fact that the contact
with the overlying rock is not an igneous, but a sedimentary one.
To be sure, when the granite was formed by the cooling of a deep-
seated igneous magma, it was in igneous contact with the over-
lying rocks of that time. But these covering rocks were entirely
removed by erosion, and later sediments (sand, then limestone)
were spread over -the eroded granite surface, and at a much later
date still, after the core of the Rocky Mountains was elevated^
these younger rocks were again partly removed by erosion, and in
the various canons which cut the lower slopes of , the mountains
these sediments are seen to rest upon the eroded surface of the
granite (Figs. 152, 153).
Evidently the relationship thus seen between the granite and
the sediments does not permit our classifying the Pikes Peak granite
mass as belonging to any of the igneous types so far discussed, and
212
Form and Structure of Older Igneous Masses
the name abyssolith has been proposed for it by Grabau, An abysso-
lith, then, is a mass of rock, generally granite, or one of the more
basic rocks of this type,
which was originally a
boss, or a batholith, or
may even have been a
bysmalith, laccolith, or
stock, which has been
exposed by erosion, then
covered by sediments
which are, of course, un-
affected by the igneous
mass because it was cool
at the time, and after a
more or less domelike
uplift the sediments were
again eroded from the
surface of the dome, re-
exposing the igneous core
around which unaltered
FIG. 152. Contact of the stratified basal
Palaeozoic beds and the granitic basement rock
of the Pikes Peak mass, in Williams' Canon,
Colorado. (Photo by Author.)
sediments crop out. In
addition to the Pikes
Peak region we may note as an example of this type the Black
Hills Dome, with its center of old igneous rocks. Many small
I inch * 10 feet.
FIG. 153, Details of contact of Palaeozoic sediments and Pre-Palasozoic
granites in Williams' Canon, Colorado. (After Crosby.)
examples of such resurrected igneous rocks of dome-like form sur-
rounded by unaltered sediments are known from this country as
well as from Europe.
Contact of Igneous Masses with Other Rocks 213
Relative Age of Igneous and Enclosing Rock
If we then recognize the nature of the contact between an ig-
neous mass and the sediments which once covered it, we can de-
termine the relative ages of the two series. If the contact is
igneous, the sediments are older than the intruded igneous mass.
If the contact is a sedimentary one, the igneous mass is the older,
and there is a long-time interval lost, between the two an in-
terval during which erosion removed the older covering rocks, with
which the mass was in igneous contact, and this erosion occurred
before the sediments now seen in contact with the igneous mass
were deposited. This shows how important it becomes to deter-
mine whether a contact is igneous or sedimentary.
CHAPTER X
THE AQUEOUS OR HYDROGEMC ROCKS
GENERAL CHARACTER AND VARIETIES
Source of the Material. Practically all natural waters contain
mineral matter in solution, the common illustration being ocean
water, every liter of which contains about 35 grams of mineral
matter, more* than 27 grams of this being common salt (sodium
chloride), w|iich means that every cubic mile of sea-water contains
about 131,5126,000 short tons of this important substance in solu-
tion. Sea-water is therefore spoken of as salty or saline, and this
saltiness is readily recognized by taste. The other dissolved sub-
stances are also called salts, but their presence is not so readily
recognized. Some water bodies, like the Baltic Sea, contain only
one fourth or less of this amount of salts in solution, and such waters
are called brackish. The waters of the Hudson River some dis-
tance above New York City are brackish, because they are formed
by a commingling of the fresh water from the upper river with salt
water entering from the sea.
When the water contains so little substance in solution that it
can be used for drinking purposes (whether it is contaminated by
organic matter or not) it is called fresh water, but fresh water in
nature always contains some substances in solution, lime usually
predominating, this when present in sufficient quantity forming
hard water. When carbonate of soda and similar substances are
present in such quantities as to render the water unfit for human
consumption, it is called alkaline, and all travelers in the semi-arid
regions of western North America are familiar with such water.
Finally there are water bodies like Great Salt Lake, the Dead Sea,
and others, in which the quantity of common salt in solution ex-
ceeds many times that in the ocean water, and such waters .are.
spoken of as super-saline, or as brines. Brines may also be pro^
duced by partial evaporation of ocean water, just as brackish waters
are produced by the dilution of ocean water.
214
General Character and Varieties 215
Separation of Material. When the substances which waters
hold in solution are separated out so as to form solid material, this
material constitutes rock masses to which in general we apply the
name of salts, after the example -of the commonest of these, the
ordinary salt. If the separation is produced by organisms, the
product is called organic salts, and these belong in the division
of organic or biogenic rocks, where they will be discussed. If
the separation is by inorganic activities, the product is a normal
aqueous or hydrogenic rock, to which "the present chapter is
devoted.
Separation by inorganic means is accomplished in a variety of
ways, of which the following are the most important :
1. Separation by the condensation or complete evaporation of
the water by heat, drying winds, etc., during which process a stage
is reached when the water becomes saturated, that is, it holds in
solution as much of a given salt as it can hold for that temperature
and pressure. If that stage is passed, the excess of salt separates
out, often in a more or .less crystalline form. The point of satura-
tion varies with the nature of the salt, and when two or more salts
are present in the solution, they will not only have separate and dis-
tinct saturation points, but the presence of each is likely to in-
fluence the saturation point of the others, either lowering or raising
them. Complete separation of all the salts will occur upon com-
plete evaporation of the water. Such salts are called evaporation
products, or briefly, evaporates. 1
2. Separation of salts from the solution by the force of attrac-
tion which ctystals or particles of mineral matter exert on material
of the same kind in the solution (generally a saturated one) in which
those crystals or particles are immersed.
3. Separation by the abstracting of the solvent or substance
which holds the salt in solution.
4. Separation by chemical reaction between minerals in solution
in the water, and other substances introduced from extraneous
sources, either as gases or as solutions, and the consequent forma-
tion of new and less soluble compounds which are then precipitated.
5. Separation by electrolysis.
1 Strictly speaking, this is also a chemical combination of the dissociated ions, which
occurs at the moment of solidification. Such salts are therefore also precipitates, but
it is well to keep this type distinct from that resulting throne^ reactions with newlv
216 The Aqueous or Hydrogenic Rocks
As illustrations of the first class we may cite the separation of
salt when sea-water is evaporated or when incrustations of salt
are formed from a solution of that substance which is allowed to
stand for a time in a warm room. The second method is illustrated
by the crystallization of the alum of a saturated solution around a
crystal of alum introduced into that solution, or the formation
of rock candy from a. saturated solution of sugar. Precipitation
through abstraction of a solvent is shown by the deposits of lime on
the inside of boilers and tea-kettles where " hard " water is used,
the solvent, which is carbon dioxide, being driven off by the heat.
It is also shown by the precipitation of lime around the mouths of
springs, in which the water escaping from under pressure loses some
of the carbon dioxide which was the solvent of the lime in the water.
Examples of precipitation of salts by the addition of substances in
solution which by chemical reaction produce a less soluble com-
pound, or by the passing of gases through a solution, are familiar to
all workers in chemical laboratories, while the electrolytic method
is one much practiced in the arts.
The first three methods cited are most commonly observed in
nature. The precipitation by the addition of reagents, whether
liquid or gaseous, is chiefly found in the formation of lime deposits
under the influence of ammoniacal gases. Since these are, however,
in nearly all cases produced by the direct activities or the indirect
influence, through decay, of organisms, such deposits are best
referred to the organic or biogenic group, and they will be discussed
there in this book. Electrolytic processes in nature are still little
understood, but that they are going on cannot be doubted. Some
of the inclusions of salt in the pore spaces of marine sediments have
been explained in this manner.
Most precipitates from an aqueous solution are called salts, no
matter how they are formed or what their composition. There
are, however, some simpler substances, such as the oxide of silicon
or silica (Si0 2 ) and others, which cannot properly be called salts.
(See Chapter IV.) One of the essential characteristics of a salt is
its purity of composition, though it is true that under certain con-
ditions precipitation of several salts may take place simultaneously,
thus producing what is called a eutectic mixture. When pure, the
material is in reality a single mineral mass, as for example, the min-
eral Halite or rock salt ; but because of its extensive development
it is treated as a rock.
The Textures of Aqueous Deposits 217
Classification of aqueous precipitates according to composition. It
is evident that chemical composition is the primary basis on which
aqueous precipitates must be classified. Furthermore, most salts look
very much alike except when they are crystallized, or when, as in
special cases, color, hardness, and weight make distinction possible.
In the following pages will be given some of the more important and
common salts and oxides, the classification being made, for the sake
of convenience, upon the basic element in the composition.
THE TEXTURES OF AQUEOUS DEPOSITS
The texture of aqueous precipitates is either crystalline or non-
crystalline the latter also being designated amorphous. Crystals
of all sizes and degrees of perfection may form, the most perfect
FIG. 154 a. Oolitic limestone silicified. (Photo by B. Hubbard.)
being those least interfered with by other crystals. The amor-
phous texture may be either in the form of separate or discrete
particles, which may or may not be tied together subsequently, as
in oolites, or it may be a solid or concrete mass.
Discrete particles. These comprise the two following textures :
(a) Oolitic texture (Fig. 154 a, V), characteristic of oolites. The
particles are small spheres, generally with a nucleus and with radial,
or with concentric or zonal, structure, the size suggesting the roe
offish; typically lime carbonate.
2l8
The Aqueous or Hydrogenic Rocks
(6) Pisolitic texture (Fig.
155), characteristic of piso-
lites. The spherules are of the
size of a pea or larger. These
are also chiefly lime carbonate.
Concrete Masses. These
include the following textures :
(a) Botryoidal (Fig. 156),
with grape-like rounded sur-
faces.
(b) Banded, in layers which
in section show a banded
structure.
(c) Laminated, in thin
FIG. 154^. Thin section of Jurassic
oolite showing the characteristic zonal
structure. Brown Jura, Schonberg, near layers or lamina.
Freiburg I. B., Germany. Enlarged 24
diameters. (After Rosenbusch.)
(d) Scaly, composed of
small scale-like masses.
(e) Fibrous, in slender hairlike fibers, often elongated crystals.
(/) Tufaceous (Figs. 157, 158), porous as in calcareous tufa.
(g) Concretionary (Fig. 159), in large more or less spherical masses.
FIG. 155. Photograph of a specimen of pisolite somewhat reduced, (Photo
by B. Hubbard.)
The Textures of Aqueous Deposits 219
FIG. 156. Botryoidal structure in calcium carbonate deposits.
FIG. 157. A fragment of calcare- FIG. 158. Bird's nest and eggs
ous tufa. Reduced. (Photo by B. " petrified " or covered with a deposit
Hubbard.) of lime carbonate, by submersion
in tufa-depositing spring-water. Re-
duced. (Photo by B. Hubbard.)
FIG. 159. Concretionary limestone ; basin of ancient Lake Lahonton. (After
Russell.)
220 The Aqueous or Hydrogenic Rocks
THE PRINCIPAL TYPES OF AQUEOUS OE. HYDROGENIC
DEPOSITS
Among the many precipitates or other deposits formed directly
from aqueous solutions, a certain number is found in sufficiently
large quantities to be treated as rock material, while others are im-
portant as sources of valuable substances or are themselves of
economic value. Their essential mineral characters have already
been given in the tables in Chapter IV,
V on-Metallic Aqueous Deposits
Rock Salt Chloride of sodium (NaCl). This is the most important as.
well as the most abundant mineral substance obtained from ocean water. It
commonly occurs in beds, which may have a thickness of a hundred feet or more
but generally are only a few feet thick, forming a succession of beds separated
by gypsum, anhydrite, limestone, dolomite, or clay, or more rarely by other
mineral matter. Such deposits are formed by concentration of sea- water either
in basins cut off from the sea by elevation or by the formation of a barrier, or
in lagoons behind a bar with continued supply of sea-water through an inlet,
as more fully discussed in the next chapter. Salt is also separated from lake
basins in arid regions by the concentration, through partial evaporation, of the
water. Finally, salt deposits are formed in desert basins from salt disseminated
through the rocks in the rims of those basins, as more fully discussed in the next
chapter.
Rock-salt deposits are seldom very continuous, having in most cases a lens-
like form. Salt is also extensively manufactured by the evaporation of sea-
water in shallow salt gardens or sea salinas, which are located on nearly all shores
where the climatic and other conditions are favorable. Natural as well as ar-
tificial brines, formed by the solution of old salt beds in the earth's crust, or of
disseminated salt, are also extensively used in the manufacture of salt. Salt is
mined in central New York, in southern Michigan, in Louisiana, and in a few
other localities in the United States. Extensive salt mines exist in Austria,
Galicia, Rumania, and elsewhere in the Old World. The salt mines of North
Germany are worked chiefly for their potash deposits. Hills or mountains of
salt are found in many of the dry regions of the world, in northern Spain
(Cardona), Algeria, Persia, and elsewhere.
The chief uses are for domestic and dairy purposes and for chemical industries.
Gypsum. Hydrous sulphate of lime (CaSCX - 2 HsO). This is a common
associate of salt deposits, being found beneath beds of rock-salt of marine origin
(sometimes replaced by anhydrite) and always separating from sea- water, which
undergoes concentration before the rock-salt separates out. Such rock gypsum
is commonly massive and sometimes impure, but pure white gypsum (alabaster)
also occurs. It also occurs as crystals (selenite), scattered through mud and
sand deposits, especially in desert regions. A fibrous variety (satin spar) occurs
in veins. Gypsum is seldom formed in extensive beds in desert regions except
. Principal Types of Aqueous Deposits 221
where sea-water evaporates. It is also formed as an alteration product, gen-
erally of limestones, by waters carrying sulphuric acid ; large beds of gypsum
result^ in this way, those of New York state being an example. Anhydrite
deposits are also changed to gypsum when surface waters come in contact with
them. ^ Extensive deposits of gypsum are found in the " Red Beds " of the west-
ern United States, and in Kansas, Oklahoma, and Texas. It is found in Nova
Scotia and in the Paris Basin, where it has been and still is extensively quarried
and burned into Plaster of Paris ; also in many other parts of the world.
Raw gypsum is ground and used as a natural fertilizer (land plaster), to re-
tard the setting of cement, and for many chemical purposes. When burned
or " calcined " at 350 F. it loses most of its water (CaS0 4 - JHaO), and is
ground into the familiar " Plaster of Paris." When this is mixed with water
gypsum is again formed. It hardens rapidly and is used expensively in the
arts for molding, statuary, etc., and for stucco work.
Anhydrite. Calcium sulphate (CaSO 4 ). This is distinguished from gyp-
sum by its greater hardness and specific gravity. Its color is often also more
grayish. It is formed as a primary deposit from sea- water in cut-off basins,
especially when the water contains an excess of chlorides. The largest known
deposits thus formed are in northern Germany, where they underlie the salt and
potash beds. Anhydrite slowly changes to gypsum, by taking on water, with
an expansion of the mass. It is of little economic importance.
Carbonate of Lime. Calcite, Aragonite, Limestones, etc. (CaC0 3 ). The
great bulk of the deposits of carbonate of lime, including the limestone beds, is
of organic or of clastic origin, but there are a number of lime deposits which be-
long to the hydrogenic division. The most extensive of these is probably cal-
careous tufa (Fig. 157), which is formed by springs issuing in limestone regions
and depositing the excess of lime which they hold in solution. Calcareous tufa
is mostly porous^nd light, incrusting not infrequently mosses and other plants
and also other^bjects. Some massive deposits of this
type, however, are known, constituting the so-called
" Mexican Onyx." Compact carbonate of lime de-
posits are also formed in caves as stalactites and
stalagmites. These usually have a banded structure,
showing the successive addition of layers. That of
the stalactites is concentric around the longitudinal
axis, which in the early stages is formed by a delicate
tube.
Beds of limestone are sometimes built up from large
rounded masses or " concretions," which have re-
sulted from the deposition of carbonate of lime, gen-
erally around a nucleus. Sometimes these are FIG. 160. Concre-
formed as original masses of limestone, as in the tionarymagnesian lime-
limestone deposits of Lake Lahonton (Fig. 159), or f one ^TT^?^
an older limestone of this type which occurs in the ^^
Permian series (Magnesian limes tone) of northeastern ph t )
England (Fig. 160). In other cases these concretions
occur in shale beds, forming distinct layers, which when they become confluent
by ^continued growth form bands of limestone. When separate they often
222 The Aqueous or Hydrogenic Rocks
FIG. 161 a. Septarium or
Turtlestone. A concretion from
calcareous shales. The fissures
of the interior were rilled with
calcite veins, which became
exposed after erosion and
weathering of the surface of
the concretion; about \ natu-
ral size. (B. Hubbard, photo.)
take on disk-shaped or spherical forms, while a series of radial cracks are de-
veloped in the interior with the growth of calcite veins. t Such concretions are
called septaria (Figs. 161 a, 5). All car-
bonate of lime deposits effervesce readily
with dilute hydrochloric acid, this being the
most distinctive test.
Dolomite. Carbonate of lime and mag-
nesia ((CaMg)COs). This differs from the
pure carbonate of lime deposits in its greater
hardness and in the fact that it effervesces
only in strong hydrochloric acid. It is some-
times a primary deposit in basins where
anhydrite is forming, with which it becomes
more or less intimately mixed. It may also
be formed as an original precipitate in por-
tions of the sea in which the solution has be-
come concentrated. Many dolomitic lime-
stones are, however, of secondary origin, the
original magnesia content, which was derived
for the most part from calcareous algae, etc.,
being proportionately increased through the
solution of carbonate of lime by ground water. Secondary deposition of mag-
nesium carbonate 'also takes place from solutions in the circulating ground
water. Pure dolomite contains 54.35% CaC0 3 , and 45-65% MgCO 3 .
Magnesite. Carbonate of Magnesia (MgCOs). This is harder and more
compact than dolomite, and dissolves with effervescence only in hot hydro-
chloric acid. It occurs as a crystalline mineral, as replacement of dolomite,
and also. as an amorphous, earthy, hard, compact mineral, probably a colloidal
precipitate. It is often concretionary with
conchoidal fracture, appearing like un-
glazed porcelain, this type being usually
derived from the alteration of serpentine
and other magnesian rocks. Beds, of mag-
nesite are found associated with gypsum in
fresh-water limestones of France.
Apatite and Phosphate Hock. Al-
though the most extensive deposits of phos-
phate of lime are of organic origin, there
are some that must be considered as purely
aqueous or hydrogenic deposits. Among
these are the apatite veins and the marine
concretions of phosphate around an or-
ganic nucleus. Phosphate of lime occurs
also as a secondary replacement of lime-
stone. Apatite crystallizes in hexagonal
prisms, etc., and is readily recognized by
its form, hardness, and color (see Table in Chapter IV). Rock phosphates
are generally amorphous and compact. The pure mineral (tncalcium phos-
FIG. 161 b. A weathered sep-
tariunij showing the mineral,
which filled the fissures, left in
relief, thus producing the typical
" septarium ' ' structure. (Photo
of a specimen in Columbia Uni-
versity, by B. Hubbard.)
Principal Types of Aqueous Deposits ' 223
phate) contains 45.8% of phosphoric acid ^Os). The principal use of
phosphate is as a fertilizer.
Potash. Salts. There are a number of evaporation products which are pri-
marily salts of potash and are important sources of this substance. Among the
more common of these are Sylvite, the chloride of potassium (KC1), a very
soluble, soft, transparent, milky reddish or yellowish mineral, commonly mixed
with rock salt; Carnallite, the hydrous double chloride of potassium and
magnesium (KC1 MgCla 6 HaO), a colorless or snow-white or variously
colored salt, easily soluble, and an important source of potash ; and Kainite,
the compound chloride of potassium and sulphate of magnesium, with water
(KC1 ' MgS04 3 H 2 0), colorless to deep blood-red, and chiefly an alteration
product. The chief use of potash is for agricultural purposes and in chemical
works,
Trona. Sodium carbonate (Na 2 C0 3 NaHC0 3 * 2 H 2 0). This is a com-
mon evaporation product of alkaline lakes. It is glassy or transparent
-when crystallized, but usually forms a white salt. Important American
localities are Searle's Marsh, Owens and Mono Lakes, CaL, and Soda
Lake, Nevada, while some Russian and Hungarian lakes, and the Natron
lakes of Egypt west of Cairo, represent foreign localities. It is used for
domestic and chemical purposes, but much of the commercial trona is arti-
ficially produced. On account of its solubility it is not generally found
in the older rocks.
Mirabilite. (Glauber salt.) Hydrous sulphate of sodium (Na2SO*
10 H20). This is a crystalline, granular salt, usually colorless, and readily
soluble in water. It is formed, especially in winter, by certain saline bodies,
such as Great Salt Lake", Utah, and the Kara Bugas Gulf on the east coast of
the Caspian Sea. On the floor of the Kara Bugas a bed of mirabilite, estimated
to contain 1000 million metric tons, has formed but it is not yet exploited. In
Wyoming and New Mexico occur beds of this salt mixed with epsomite, natron,
and common salt (halite). In some cases the deposit is 15 feet thick and covers
an area of 100 acres. l( ,
Glauberite. Double sulphate of sodium and calcium (Na 2 S0 4 CaS0 4 ).
This is a white, gray, yellow, or red mineral, a little harder than common salt
(2.5-3). It occurs in many playa lakes (Borax Lake, Searle's Marsh, Death
Valley), and in the salt deposits of Germany, Spain, Austria, Sicily, etc. It also
occurs in a number of Tertiary deposits in Spain.
Soda Niter, Chile Saltpeter. Nitrate of sodium (NaNO 3 ). This is a
white to reddish brown, gray, or lemon-yellow salt, commonly impure, when it
is called caliche. It is abundant in the desert tracts of western Chile and else-
where in South America and other parts of the world.
Borax Salts. These include Borax -or. Tinkal, sodium borate (Na 2 B 4 7 -
10 H 2 0), a colorless or yellowish or green to gray, soluble mineral, somewhat
harder than comrnon salt; Colemanite, the hydrous borate of calcium (CaB 6 O u
.YHj5)I"'genSatiy*a" massive, glassy or colorless, more or less transparent min-
eral, found as a bed from five to 20 feet thick in San Bernardino Co., CaL, and
elsewhere; and Uhxtoe, the double borate of sodium and calcium with water
(NaCaB 5 9 8 H 2 0), which occurs as white balls of fibrous material and satiny
luster (cotton balls). Borax salts are found in volcanic regions, such as the
224 The Aqueous or Hydrogenic Rocks
famous " saffioni," i.e., fumaroles, In the volcanic region of Tuscany; in hot
spring and lake deposits of volcanic districts, as in Tibet, etc., and the Coast
Ranges of California, and in playa deposits, as in Death Valley and elsewhere
in the western United States. Colemanite
is the chief American source of borax.
Silica. Oxide of silicon (Si0 2 ) frequently
with water. The most familiar form of this
is quartz in crystalline or amorphous form,
which occurs in veins, geodes, etc., colorless
to variously colored ; also as addition to
quartz grains in sandstones, enlarging them
to fill all the interstices. It occurs exten-
sively as a porous, white, and light spongy
FIG. 162. Concretion of deposit of hydrous quartz, around geysers
flint from the chalk beds of ( see Fig I3I) p I87 ) 5 an( i fe h ence known as
England. About one fifth nat- geyser ite or silicious sinter. Again, it occurs
' (Ph0t as concretions of fl int in Chalk beds (Fig '
Hubbd as concreons o n '
U ar 162), or as chert nodules or layers in lime-
stone. These types are compact and have a conchoidal fracture, with a black
or dark brown color on the freshly fractured surface. Flint and chert are
secondary concentrations through solution and redeposition of disseminated
silica particles of organic origin. (Fig. 163).
Glauconite. Greensand. This is a complex silicate of potassium and iron
(KFeSi 2 6 H 2 0). It consists mainly of dark green grains, generally mixed
with impurities, found in strata of Cretaceous and other formations, and is form-
ing at the present time along the margin of the continental shelf in the Atlantic
Ocean and elsewhere. It promises to be an important source of potassium, in
which the New Jersey deposits are especially rich. They also contain much
phosphorus.
Metallic Aqueous Deposits
Manganese Ores. Nodules of oxide of manganese occur in many parts
of the deep sea in the region of red clay deposits (Fig. 174). They are
more or less rounded or irregular masses of black color. Oolitic pyrolu-
site (Mn0 2 ), an iron-black mineral of metallic luster, occurs interstrati-
fied with other beds in certain localities. A hydrous oxide of manganese
(wad) occurs as an amorphous earthy deposit in bogs, generally associated
with iron ores.
Limonite. Bog iron ore, hydrous oxide of iron (2 Fe 2 3 3 H 2 0). This
occurs in massive, botryoidal, earthy, or porous masses of yellow or ochery color
and yellow streak. It forms deposits in bogs and swamps, in shallow water, in
depths above 12 feet, from the oxidation of carbonate of iron carried into the
bog in solution. The ores are always mixed with sand or earthy impurities.
There are several other ferric hydrates, some with less and some wjth more
water; e.g. Gothite (Fe 2 3 . 2H 2 O). Oolitic limonites occur in older strata,
where they form an important source of iron, though of low percentage. Suet
Principal Types of Aqueous . Deposits 225
are the minette ores of Lorraine and Luxemburg, formerly one of the chief domes-
tic sources of iron in Germany. These are believed by many to have resulted
from the oxidation of oolitic siderite or glauconite.
Hematite. Red oxide of iron (Fe 2 3 ). This ranges in color from deep
red to reddish brown and black with red streak. It occurs interbedded with
shales and limestones in the Palaeozoic formations of Germany, France, Bohemia,
FIG. 163. Quarry-wall of Cummings Cement Mine, Akron, Erie Co,, N. Y.
The upper bed (a) is a very cherty limestone (Corniferous) the chert nod-
ules standing out in relief as the result of weathering; (5) Onondaga lime-
stone without chert; (c) Akron dolomite, 7 feet thick, with fossils of upper
Silurian Age. Between it and the Onondaga limestone (Middle Devonian)
is a hiatus or break in succession involving the whole of the lower Devonian,
which is absent. The two formations are disconformable ; (d) Bertie water-lime
mined for natural cement. (Courtesy N. Y. State Museum.)
and the United States, where the (Clinton) iron ores of New York and the Appa-
lachian region form a characteristic example. Many of these beds appear to
be replacement of limestone by the iron; in other cases the iron ore seems to
be a primary deposit. .
Siderite. Carbonate of iron (FeC0 3 ). This important iron ore occurs
in crystallized form in veins cutting limestone and other rocks, and is readily
recognized by its perfect rhombohedral cleavage, vitreous to pearly luster,
greenish to brownish color, and translucent to subtranslucent character. It is
more common, however, as an interbedded rock in the older sedimentary series,
226 The Aqueous or Hydrogenlc Rocks
being known as clay-Iron-stone, spherosiderite, or black band. Clay-iron-stone
has a dense or fine-grained structure, forming concretions, which often include
FIG. 164. Clay-iron-stone concretion, Connecticut valley. (After
Gratacap.)
organic remains (Fig. 164). The ""black band" forms continuous layers in
the formations which carry coal.
CHAPTER XI . '
MODE OF OCCURRENCE AND ORIGIN OF THE
AQUEOUS OR HYDROGENIC ROCKS
TYPES OF DEPOSITS
EACH of the several water bodies of the earth may form precipi-
tates of mineral matter, and hence we may classify these deposits
under the following heads :
A. Marine deposits, or those formed in the sea and its depend-
ent water bodies.
B. Lacustrine deposits, or those formed in lakes, ponds, fresh-
water marshes, salinas, playas, etc.
C. Flumatile deposits, or those formed by rivers in their beds, or
on the flood-plain or delta surfaces, except those formed in lakes
along the river course, or at its mouth.
D. Terrestrial deposits, or those formed by springs and by the
ground water in fissures, caverns, cavities in the rock, etc. To
these belong many important deposits of mineral matter.
The several types may grade one into the other, but their main
characteristics are quite distinct.
SEA- WATER AND THE EVAPORATION PRODUCTS AND CHEMICAL
PRECIPITATES FORMED PROM IT
Amount of Salt in Sea-Water. As has been noted in the
ceding chapter, the oceans, which are the large bodies of sea-water
lying between the continents, contain the normal salt water, in
which about 35 grams of salt occur in every liter of water. Since
a liter of pure water weighs 1000 grams, the quantity of salt is
essentially 35 per thousand by weight, which is expressed by the
formula 35 per mille (or 35^). This corresponds, of course, to
3.5 per hundred or 3.5 per cent (3.5%), but since the difference in
salinity between different water bodies is often very slight, and
227
228 Aqueous or Hydrogenic Rocks
because in brackish and fresh waters the actual quantity of mineral
matter in solution is very small, it is more satisfactory to express
the quantity in permillages than in percentages.
The quantity of salt in solution determines the salinity of the
water. Thus the average salinity of the ocean water is 35 per mille,
(3-5 P er cent )> varying somewhat for the different oceans, for differ-
ent parts of the same ocean, and for different depths. On the other
hand, the salinity of the Red Sea surface waters is 38.8 per mille,
while that of the surface waters of the Black Sea is only 18.3 per
mille, which is due to the fact that this water body is almost entirely
cut off from the rest of the sea, and that it receives many fresh-
water rivers. Finally, the average surface salinity of the Baltic
Sea is only 7.8 per mille, whereas if the water of this enclosed basin
is taken as a whole, it is somewhat more saline because of the
greater salinity of the deeper layers. Even then, however, it is
only 10 per mille. The Baltic Sea, moreover, shows a remarkable
gradation in the salinity of its waters from west to east. Where it
joins the North Sea at the Skager Rack, the salinity is 34 per mille,
but in the Kattegat it is only 22 per mille. Thence it gradually
decreases eastward and northward until the waters near the heads
of the Gulf of Finland and that of Bothnia arq essentially fresh.
Composition of the Sea-Salts
Although, strictly speaking, the material held in solution by the
water of the sea is not in combination as salts, such as are produced
on evaporation, but rather in the form of ions, the basic elements
and the acid radicals being separated, nevertheless it is customary
and convenient to consider them as combined into the form of
salts. Among these, common salt or sodium chloride makes up the
bulk of the material, being nearly 78 per cent of the total mass of
salt, or over 27 per mille of the salinity (which is taken as 35 in round
numbers), In the following table the composition of the sea-water
salts is- given in the form of such combinations, together with the
permillage of each in normal sea-water, and the number of short
tons in a cubic mile of sea-water.
With the calcium carbonate are included the small quantities
of other salts present, such as the iodine, lithium, manganese,
and phosphorus salts, and the silver, gold, nickel, and other metals
which are present in minute quantities in the solution.
Sea- Water and Its Products
TABLE OF THE COMPOSITION OF SEA-SALTS
PERMILLAGE
PERCENTAGE
OR ACTUAL
TONS oi 1
SALT
SYMBOL
OF TOTAL
SALTS TAKEN
WEIGHT IN
GRAMS PER
2000 LBS. EACH
PER CUBIC MILE
AS 100
LITER OP SEA
OF SEA WATER
WATER
i. Sodium Chloride
NaCl
77-75S
27.213
131,526,080
2. Magnesium Chloride
MgCl 2
10.878
3.807
18,399,360
3. Magnesium Sulphate
MgS0 4
4-737
1-658
8,012,480
4. Calcium Sulphate .
CaS0 4 -
3.600
1.260
6,089,440
5. Potassium Sulphate .
K 2 S0 4
2.465
0.863
4,169,760
6. Calcium Carbonate .
CaCO 3
o-345
0.123
583 3 5 2
7. Magnesium Bromide
MgBr 2
0.217
0.076
367,360
100.000
35.000
169,148,000
Common Salts Produced by Evaporation of Sea-Water
The two principal salts which are produced by the evaporation
of sea- water are the common salt, sodium chloride (NaCl), and
gypsum or calcium sulphate, with two molecules of water (CaSCX
+ 2 H 2 0) . By local oversaturation with lime, calcium carbonate
(CaC0 3 ) may also separate out, but this is more commonly pro-
duced by chemical reaction. When the evaporation has gone very
far, other salts, such as those of magnesium, and finally potash salts,
separate out. Common salt and gypsum are obtained on many
sea-coasts, either by evaporation of the water by artificial heat, or
by conducting the sea- water into large shallow "pans," that is,
fields surrounded with dams and having a hard, flat bottom. When
the outlet of the pan is closed, evaporation takes place under the
influence of the sun and drying winds, and after a while gypsum
separates out. When most of this has been deposited, the water
is conducted to another pan, where evaporation continues until a
large part of the common salt (sodium chloride) has separated out.
Then the remaining dense brine, which is called the mother liquor,
is drawn off and either returned to the sea or further evaporated
for the rarer salts. In this manner a large part of the world is
supplied with its domestic requirements of salt, though vast quan-
tities of this commodity are also obtained from inland salt deposits,
which are the product of evaporation either of sea-water or of in-
land salt-bearing waters, in former geological periods.
230 Aqueous or Hydrogenlc .Rocks
Experiments in Evaporation of Sea-Water
In 1849, the Italian chemist, J. Usiglio, published the results of
experiments which he had made at Cette on the south coast of
France. lie had taken 5 liters of the sea-water from the Medi-
terranean and evaporated this, keeping an exact record of the point
which the evaporation had reached when separation of the several
salts took place, and determining the amount of the various salts
separated at the successive stages.
No separation of salts occurred until the water was evaporated to nearly
one half its volume, when the iron oxide and a part of the carbonate of lime of
the sea-water separated out. Later still, when the original 5 liters of the water had
been reduced by evaporation to about one liter, the remainder of the carbonate
of lime, together with the hydrous sulphate of lime, or gypsum, was precipitated.
More than 84 per cent of the total amount of gypsum contained in the sea- water
was deposited before the water became dense enough to allow separation of the
common salt (NaCl), the remainder of the gypsum being thrown down with
that salt and with various amounts of magnesium salts (MgSC>4 and .MgCl-j),
and finally with sodium bromide. A significant fact was that no sodium sepa-
rated out until the evaporation had reduced the original 5 liters to less than
half a liter, or to less than one tenth the original volume. At that time the
amount of solid matter still held in solution in the half liter of water remaining
was about 184.4 grams, which would correspond to 368.8 grams per liter, or a
salinity of 368.8 per rnille, whereas the salinity of the original sea-water was
about 38.5 per mille. 1 From this we must conclude that a water body must
reach this high degree of salinity by evaporation before salt can be deposited
in nature. Since there are to-day no known large bodies of water with such a
salinity, it follows that extensive salt deposition is not going on to-day by simple
evaporation of large bodies of sea-water. To be sure, there are many small and
shallow marginal lagoons on the sea-coast, and especially on the shores of more
or less enclosed salt-water bodies, such as the Black and Caspian Seas, and Great
Salt Lake, where evaporation goes far enough to precipitate salt but this is,
as a rule, only in comparatively small amounts, though commercially important.
Complete evaporation of the 5 liters of sea water was not achieved by Usiglio,
for his experiments ceased when the volume had been reduced to about 81 cubic
centimeters. This remaining dense "mother liquor " retained all of the potash
salt of the original sea- water in solution, together with some of the sodium and
magnesium salts. The amounts present were as follows :
NaCl 1 2. 942 5 grams
MgS0 4 9.2725 grams
MgCl2 15.8 200 grams
NaBr 1.6500 grams
KC1 2.6695 grams
Total 42. 3545 grams
1 It is probable that actual separation of salt (NaCl) begins at a somewhat lower
salinity, for at the stage here noted something over 3 grams of salt had already
separated out.
Conditions Favoring Deposition of Sea-Salts 231
This corresponds to a salinity of 522.9 grams per liter or 522.9 per mille (52.29
per cent) .
The salts of the mother liquor are precipitated only at very high or very
low temperatures, and from this we must conclude that potash deposits in nature
are formed only under exceptional conditions.
SPECIAL CONDITIONS FAVORING DEPOSITION or SEA-SALTS
Modern Examples
From the foregoing it becomes apparent that the first requisite
for the deposition of sea-salts in nature is the concentration of the
sea-water under the influence of conditions which favor evapora-
tion, such as the heat of the sun arid, above all, drying winds. That
such evaporation cannot go on in the open ocean is apparent, and
so we must look to bodies of sea-water cut off from the main oceans.
The Caspian Sea (Fig. 165). This, the largest isolated salt-
water body of the earth, may in many respects be considered typical.
It lies within the region of drying winds, and is partly surrounded
by deserts. Evaporation has gone so far that its surface is 85 feet
below sea-level, yet the salinity of its water is only 12.94 per mille,
or a little over one third that of normal ocean water. 1 This is due
to the fact that a large amount of fresh water is brought in by the
Volga and other rivers tributary to it, so that in spite of the evapo-
ration, the salt content is very low. As we shall see later, much of
the original salt has been specially concentrated in the Kara Bugas
Gulf and other dependent bodies, and some salt has no doubt been
deposited on the bottom of the lake and then preserved by a cover-
ing layer of impervious material (gypsum, clay, etc.).
The Black Sea. This nearly, but not quite, isolated mediter-
ranean water body has a surface salinity of only 18.3 per mille, but
the lower layers are denser, so that the average salinity of the water
as a whole is 22.04 per mille. Obviously no salt can be deposited
in the open parts of this sea.
There can be no question, however, that were it not for the supply
of fresh water, both the Black and the Caspian seas would have a
high salinity, while the Baltic would be nearer to sea-water in that
respect. Indeed, should the fresh-water supply be entirely cut off
from the Caspian, continued evaporation would result in the sepa-
ration of most of its salt upon the bottom of its basin, and eventu-
1 Average of five analyses made in 1878.
232
Aqueous or Hydrogenic Rocks
ally only a layer of mother liquor would remain to cover these de-
posits, and under special conditions this mother liquor might also
be forced to part with its salts. That such evaporation of large
FIG. 165. Map of the Caspian Sea and the salt lake region north of it.
enclosed bodies of sea-water has occurred in the past history of the
earth is indicated by the nature of the salt deposits now found en-
closed in the rocks of the earth's crust, as will be seen presently.
To-day such deposits are formed in local " salt pans " along the
Conditions Favoring Deposition of Sea-Salts 233
margin of the Black Sea, the Red Sea, and especially in the Ran of
Cutch on the west coast of India, deposits which for centuries have
supplied the natives with salt.
FIG. 166, Map of the Colorado Desert with the Salton Sink and Pattie
Basin. (After Sykes, from McDougal, Am. Geog. Soc. Bulletin.)
The Salton Sink (Fig. 166). This is a great depression at the
head of the Gulf of California, surrounded on three sides by high
mountains which shut out the moisture-bearing winds, especially
on the Pacific side. The valley is separated from the Gulf of CaH-
234 Aqueous or Hydrogenlc Rocks
forma by the delta of the Colorado River. The lowest part of its
floor lies 273,5 ^ eet below sea-level, and is occupied by a small lake,
which is surrounded by extensive salt deposits. It is generally held
that this depression was formerly a part of the Gulf of California
and was cut off from it when the Colorado built its delta. Under the
influence of the drying winds which descend from the Coast Ranges,
the cut-off portion of the sea- water evaporated, and much of the
salt was deposited on the floor of the basin, which was converted
into a desert. Some of the salt may have been previously removed,
when the Colorado drained into this basin and converted it into
a fresh- water lake, which stood 40 feet 'or more above sea-level, as
shown by old shore lines. Recently the Colorado has several times
reentered this basin and enlarged the central lake.
Effects of Condensation of Sea-Water on Its Animal Life
A moment's reflection will show that in the process of evapora-
tion and concentration -of the cut-off portion of the sea-water, all
the animals which lived in that water would be killed, and their
remains would sink to the bottom or be cast upon the shores. The
shells of the Mollusca, the homy coverings of Crustacea, and the
bones and teeth of fish and other vertebrates would be embedded
in the layer upon which the salt later comes to lie. Thus a very
definite and restricted fossiliferous substratum is produced for
salt deposits of this type, and this will furnish a criterion by which
ancient salt deposits can be interpreted. ' If the change in salinity
is gradual, because the water body subject to evaporation is
large, extensive fossiliferous deposits may be formed, including
important beds of limestone, before the water is dense enough to
kill the organisms. After that the water will remain essentially
lifeless (though there are certain forms of animals which live only
in strong brines), and the deposits formed in it will be barren of
organic remains. An exception to this may, however, occur if the
sea should break into the basin again, flooding it with normal sea-
water, and bringing in with it the normal sea-fauna. Then, if the
basin is again cut off from the sea, evaporation will set in with the
repetition of the series of evaporation deposits.
Order of the Deposition of Sediments and Salts
If the basin whose waters become subject to evaporation is
large, the waters, as they shrink, will leave a succession of de-
An Ancient Rock Salt Deposit 235
posits of various kinds around the margin. Along the shores would
be found sands and clays which farther out might merge into or-
ganic limestone. As the sea-water becomes concentrated, the or-
ganically formed limestones would come to an end, and chemically
formed limestones and dolomites would take their place. Gypsum
or anhydrite deposits follow next, and finally, as the water becomes
very saline and the area much contracted, salt is deposited. Last
aOrg*ruc Limestone f'^T I JE, ^ 5 * 1 / * nc * lf SChemic*! Limestone
6 Mother bquerSdirs An d Dolomites
FIG. 167. Diagrammatic cross-section of a cut-off basin, and the deposits
formed in it after complete evaporation.
of all, the mother liquor salts are precipitated under favorable con-
ditions, but only in the central area of the "original basin, where
the last of the water lingers. The relationships of these several
deposits are shown in the preceding diagram (Fig. 167), which is
entirely schematic. It must be emphasized that normal salt de-
posits formed from evaporating sea-water must always be under-
lain by a layer of gypsum or its anhydrous equivalent, the mineral
anhydrite.
AN ANCIENT ROCK-SALT DEPOSIT FORMED BY EVAPORATION
or SEA-WATER
Among the many salt deposits within the earth's crust which
were formed during earlier geological periods and preserved through
burial by later deposits, a certain number can best be explained as
formed by the evaporation of cut-off portions of the sea in the
manner above outlined. This means, of course, that they have the
essential characteristics which we have seen are the normal accom-
paniments of salt deposits formed from complete evaporation of a
cut-off body of sea-water. The most notable example is found in
the great salt deposits of North Germany (Magdeburg-Halber-
stadt region), most widely known, as the Stassfurt deposits, though
this is only one of the localities where these salts are mined. Their
peculiar interest lies in the fact that they have associated with
them the rare potash salts which, we have seen, are precipitated
236 Aqueous or Hydrogenic Rocks
from the mother liquor on complete evaporation of the sea-water.
Before the World War these deposits furnished by far the largest
amount of potash to the commerce of the world, and their abun-
dance is such that they can supply the entire world at the present
rate of consumption for perhaps 2000 years to come.
If we take a section through these deposits from top to bottom, as revealed
by the numerous boreholes and by mines in operation, we find the following char-
acteristic succession :
At the base lies a variable thickness of limestones and dolomites, which in
some sections contain old reefs formed by organisms which inhabited these
waters while they were still connected with the sea, and for some time after.
This limestone is known as the Zechstein, and it takes its definite place with the
salt deposits and with the underlying red sandstones (Rothliegendes) , in the
series of successive formations which were made during the later portion of
the Palaeozoic era of the earth's history.
Above the Zechstein limestones and dolomites lies a formation of anhydrite
and gypsum 100 meters in thickness, and this is followed by the salt beds. These
average 245 meters in thickness and are subdivided by about 3000 layers
of anhydrite, the so-called annual rings. Above this follow the mother liquor
salts which have given these deposits their great value. They include more
than 30 rare minerals, the more abundant, as a rule, in layers or strata, the whole
averaging from about 60 to over 90 meters in thickness. The most important
of them from a commercial point of view are, of course, the potash salts.
Above these mother liquor salts occurs a second series, beginning generally with
a salt clay containing remains of marine animals, followed by anhydrite (30-80
meters) and rock-salt, with about 400 annual rings of polyhalite, which is a com-
plex hydrous sulphate of calcium, magnesium, and potash. The series is closed
by a succession of minor layers of red clay, anhydrite, rock-salt (about 40 meters),
anhydrite, and red clay, the last forming the top of the deposits. Through-
out the entire series, except in the basal Zechstein limestones and the salt clay
at the middle, remains of organisms are wanting, except a few plant fragments,
which were blown into the water body. It appears then that both series of
deposits, the lower as well as the upper, represent the succession which we have
seen is characteristic of a progressively drying up cut-off portion of the sea.
Moreover, as we shall see later, they differ from the succession of salt deposits
formed in other ways, and they may, therefore, unhesitatingly be interpreted as
accumulations of the first type. The regular intercalation of the anhydrite
layers in the lower salt suggests that they are due to seasonal fluctuations in the
density of the water a periodic slight reduction in the salinity during a moister
period putting a temporary end to salt deposition and permitting the forma-
tion of anhydrite, which is separated from less concentrated waters rich in salt.
If these changes were of annual recurrence, as seems likely, it would appear
that the formation of this salt mass took three thousand years. Each annual
ring is on the average about 7 millimeters thick, while the salt layers with which
they alternate are from 8 to 9 millimeters in thickness.
After the deposition of most of the mother liquor salts, a period of fluctua-
An Ancient Rock Salt Deposit 237
tion occurred, and salt with polyhalite layers was formed. This alternation,
too, is probably seasonal, but shows that the brine was now more highly con-
centrated and of changed composition, and it is probable that regular changes
in -temperature were most influential in producing the succession of deposits.
The higher anhydrite and salt series indicates that the sea-water again filled
the basin, then it was cut off anew, and again underwent complete evaporation.
It is also quite probable that the waters of these basins were enriched by salt
brought in- solution by intermittent streams from the surrounding hills, as is
the case in many modern desert basins, and that, because of this excess of salt,
anhydrite, rather than gypsum, was deposited.
We cannot, however, settle the question of origin from the de-
posits of North Germany alone. If they were formed by the dry-
ing up of an enclosed sea, there should be neighboring salts and
sediments which should show such variations as we would expect
nearer the shores of this water body, and these deposits should
be- of the same age. The method of searching out and determining
formations of like geological age in different parts of the world will
be dealt with later. Suffice it to say here that such age determina-
tion and correlation of formations in different countries is quite
possible by the aid of fossils, by the relationship of the beds to one
another, and by other criteria.
Taking, then, such deposits of the same age in other parts of
Europe, we find first that the mother liquor salts fail as we proceed
away from the North German region, where apparently was the
center of concentration. Salt is still found in a number of sections *
even in eastern England. Here, however, magnesian limestones
prevail, some of them showing a peculiar structure suggestive of
chemical deposition (Fig. 160, p. 221). Limestone formed of a
restricted number of organic remains elsewhere makes up the de-
posit of Permian age in England. Finally, in the north and west of
England, only red sandstone deposits represent the Permian, these
being formed near the old shore-line of the Permian basin, and in
part above it, by rivers which brought sands from the uplands and
dropped them in their lower shallow courses (Fig. 168).
In the other direction, toward the Ural Mountains of Russia, a
change in the character of the Permian formations may also be ob-
served. At firtet limestones predominate, but farther east these
are largely replaced by .sandy and clayey beds, among which coal
seams are found, which indicate a swamp-land condition.
It seems practically demonstrated, then, that these ancient salt
deposits with their valuable beds of potash salts were formed by
238 Aqueous or Hydrogenic Rocks
the drying up of a large body of sea-water which had become sepa-
rated from the main ocean by the formation of a land barrier. This
water body appears to have extended from central England on the
west nearly to the Ural Mountains on the east. Its southern
FIG. 1 68. Map of the Zechstein Sea of Permian time in northern Europe,
showing the approximate outline of that water body, and the mountains of that
period. $= Berlin; 'Bi.-Breslau; DT.^ Dresden; Rei. = Halberstadt; Hb.=
Heidelberg; Hl= Halle; L = Leipzig; M= Magdeburg; Mb = Marburg; Th=
Thorn; W^Wesel.
boundary was north of the Danube, and its northern in the region
of the Baltic Sea of to-day. A significant corollary is that the
climate of North Europe was very much dryer at that time than
it is to-day, for at present no such complete evaporation would take
place. This will be referred to again in a later part of this book.
DEPOSITS OF SALT BY CONCENTRATION IN LAGOONS. BAR
THEORY OP OCHSENITJS
Complete evaporation of a salt lake is only one way though
a most effective way of producing salt deposits. A second
method consists in the concentration of the sea-water in a nearly
shut-off lagoon in regions of arid climate. As a typical example of
this we may select the Kara Bugas Gulf (Fig. 169), a bay which
lies on the eastern coast of the Caspian Sea, from which it is sepa-
rated only by a sand bar, across which a narrow strait maintains con-
nection with the Caspian. On the other sides the gulf is surrounded
by deserts, and there are no streams entering it. Since the Caspian
Sea is a large salt-water body, though of lower salinity than the
ocean, the Kara Bugas Gulf is nearly as satisfactory an illustration
as a similar bay on the open sea would be.
Deposits of Salt by Concentration
239
Because of the narrow inlet from the Caspian, sufficient water
is not supplied to counterbalance the evaporation over the surface
of the Kara Bugas Lagoon, and so a slight diSerence of level is
FIG. 169, .Map of the Kara Bugas (Karabugas) Gulf or Adji-darja (salt
water), on the eastern border of the Caspian Sea.
produced, the Kara Bugas surface being sufficiently lower to cause
a constant inflowing current of water from the Caspian (Fig. 169 a).
Since the salt thus carried in solution is not removed, the waters of
240
Aqueous or Hydrogenic Rocks
the lagoon become more and more saline. A determination made
some time ago showed a salinity of 285 per mille (28.5%), while
that of the Caspian, from which the salt was abstracted, was
only 12.94 per mille (i.294%). 1 This is a sufficient concentration
for the deposition of some salts, but not of the common salt, which
is not forming at the present time on the bottom of the gulf. An
FIG. 169 a. Cross-section of the Caspian Sea and the Kara Bugas Gulf,
from Baku across the inlet to the northeastern shore of the Gulf. Vertical scale
greatly enlarged. Depths given in meters. The level of the Kara Bugas is
slightly lower than that of the Caspian.
extensive bed of sodium sulphate or glauber salt has, 'however,
formed on the bottom of this bay, and much gypsum is being de-
posited.
It is evident that as concentration progresses, ordinary salt will
be deposited, if, indeed, beds of salt do not actually underlie the layer
of glauber salt, having been formed during a period of former greater
concentration. It is also evident that so long as the connection
with the Caspian is maintained, no mother liquor salts will be pre-
cipitated, since that requires nearly complete evaporation. It is
not likely that such separation and complete evaporation of the
Kara Bugas waters can take place so long as it remains separated
from the Caspian by only a sand bar, for the lowering of the water
in the Gulf would create a sufficient inward current to keep the
inlet open, and even enlarge it. Therefore, while salt deposits of
considerable thickness may accumulate on the bottom of such a
gulf as the Kara Bugas, no potash salts can be formed in it.
The Caspian, like the ocean, abounds in animal life. Thousands
of fish, many seals, and other animals are carried through the nar-
rows across the bar and into the Kara Bugas, where they are killed
by the high salinity of the water. Their carcasses float about and
later sink to the bottom, or are cast upon the shore, portions of
which are literally covered with dead fish which furnish food for.
migratory birds. Shells of dead'mollusks, especially the cockle
(Cardium edule),, which lived in these waters before they became
1 It must be remembered that the salinity of the ocean water is 35*0 per mille or 3.5%.
Deposits of Salt by Concentration . 241
concentrated to their present degree, occur in enormous numbers on
the shore of the Kara Bugas, and equally large numbers are buried
with the fish remains in the mechanical sediments on the bottom of
the gulf. These sediments are, therefore, highly fossiliferous, and
should they harden into rock, they would -constitute beds of fossil-
iferous clays and sandstones in close association with the salt de-
posits. Moreover, while salt deposits are forming in the Kara
Bugas, normal sediments free from salts are forming in the Caspian
in close juxtaposition to the Kara Bugas, and these, too, are fossil-
iferous. All of these factors must be kept in mind when we at-
tempt to use this example, or others like it, in the interpretation
of the history of older salt deposits.
From its simplicity this example has gained wide currency as an explanation
of the origin of rock-salt deposits in all parts of 'the world. The theory was
first developed, though not originated, by the German chemist, Professor Karl
Ochsenius, and it is commonly spoken of as the " Bar Theory of Ochsenius."
While it explains many an ancient salt deposit, especially the great series of
Tertiary salts in the region of the present Carpathian Mountains and elsewhere,
it does not satisfy the conditions found in other older salt deposits, especially
those of the United States, for which another mode of origin must be postu-
lated, as will be shown in a later section.
An Older Salt Deposit Formed According to4he Bar Theory
Perhaps the best example of an older rock-salt deposit which
can be explained by the Bar Theory was found in the salt beds of
the Bitter Seas on the peninsula of Suez '(Fig. 170). Before the
Suez Canal was cut, these lakes were of very high salinity, and on
the bottom of at least one of them an immense bed of rock-salt had
formed. This has, however, been entirely redissolved by the fresher
waters of the canal. A characteristic feature of this salt bed was
not only the presence of many layers of gypsum, but also of innu-
merable layers of clay in which were embedded the shells of mollusks
such as now live in the Red Sea near by, while similar shells were
entombed in the sediments which formed on the bottom of the
Red Sea during the period of salt deposition, though these sedi-
ments are free from salt.
We have here a deposit which satisfies all the requirements of
the lagoon and bar example of the Kara Bugas, namely, a circum-
scribed area, presence of numerous organic remains in the salt-
bearing series, and the association in a neighboring area (the Red
242
Aqueous or Hydrogenic Rocks
Sea) of normal deposits with the same fossils but with no salt. In
the present case it is known that these Bitter Lakes formerly con-
stituted an extension of the Gulf of Suez and the Red Sea, forming
the ancient Heroopolitan Gulf. The
silting up of the mouth of this gulf,
which was not yet complete during the
sixth century before Christ, formed
the bar which cut off the lagoon from
the remainder of the gulf. According
to some authorities, this bar appears to
have been the site of the crossing of
the Red Sea by the Israelites in" their
exodus from Egypt. With the forma-
tion of this bar, which now constitutes
the southern margin of the isthmus of
Suez, the conditions favorable to the
deposition of the gypsum and salt were
produced. Repeated overflow from the
Red Sea supplied the waters and the
mud in which were buried the or-
ganisms which could live here until
the water became too salty. Then
their shells and other remains sank to
the bottom; a layer of gypsum was
formed over them, and then a layer of
salt, covered by a more or less imper-
vious layer which prevented re-solution
when next the waters of the Red Sea poured in again. In the
waters thus freshened, new organisms developed from the larval
stages brought by these waters, and a new cycle of deposition was
inaugurated. The mother-liquor, however, never evaporated in
this lagoon, but remained behind, forming the bitter waters of the
lakes, which, however, have lost much of their character since the
letting in of the sea-water by the canal.
DEPOSITION OF SALT IN INLAND DESERT BASINS
Salt deposits are forming in many portions of the world .to-day,
where no direct connection with the sea exists. The inland salt
lakes and salinas are in some cases shrunken bodies of fresh water
formerly of greater extent. This is the history of Great Salt Lake
FIG. 170. Map of the
Bitter Lake of Suez and tjie
Suez Canal.
Deposition of Salt in Inland Desert Basins 243
of Utah, which is only a remnant of a much larger fresh-water lake
Lake Bonneville. By evaporation and concentration of the
water in the deeper part of the basin, the salinity was increased,
and finally in some cases reached the concentration necessary for
the deposition of salt. Much additional salt is constantly brought
into the lake by the streams which feed it. Naturally the question
as to the origin of the salt in these waters arises. The answer is that
it is leached out of the rocks within the drainage area of the basin.
Not all rocks contain sodium chloride, but this is generally present
in clastic and other rocks which have ben formed on the bottom
of the sea in former geological periods. These rocks include an-
cient sea-water, often highly concentrated, within their pores, where
it is hermetically sealed
up and is set free only
when the rocks are sub-
sequently exposed to
atmospheric decay and
erosion. Then the con-
nate water, as it is
called, evaporates, but
the salt remains behind
to be redissolved by the
surface waters and
carried away. If it is
carried into an inland
drainage basin from
which there is no escape,
except by evaporation, salinas and even extensive salt deposits
are formed. Such areas, covered with glistening white rock-
salt or with irregular salt masses, are found in the desert basins of
central Asia (Figs. 171 a, J), and indeed are not uncommon in many
other desert areas. So long as the supply of salt lasts, such a de-
posit will continue to grow by periodic additions, and thus beds of
salt of great thickness may form. If silt is brought in during a
period of flooding, or if sand or dust is blown across the salt plain,
a layer of clastic sediment may form over the salt, and this in turn
may again be covered by pure salt deposits. As a rule, however,
gypsum is not deposited in such basins, not because the connate
waters imprisoned in the rock did not contain it, but because after
it is set free it is less readily redissolved by the surface waters than
FIG, 1710.- The irregular salt surface of
the Salt Plain of Lop, Eastern Turkestan.
(After a photograph by Ellsworth Huntington.)
244
Aqueous or Hydrogenic Rocks
is the common salt, and that which is dissolved is likely to separate
out again from the waters before they reach the central basin. . In
conformity with this we often find the sands which surround such
basins filled with gyp-
sum crystals which have
grown from the ground
water as it passed
through the sands,
carrying the more
soluble sodium chloride
to the central basin.
In like manner the
potash salts will not, as
a rule, reach the central
basin, for though they
are very soluble, the
FIG. 171 &. The southern edge of the Salt
Plain of Lop, in Turkestan. (From a photo-
graph by Ellsworth Huntington.)
fine particles of the soil
of the desert among
which the water must find its way to the central basin have a
great affinity for the potash and will, by some still little under-
stood process, separate this substance from the water during its
passage. Hence the water which reaches the central basin will
contain little else than pure sodium chloride, and therefore only
FIG. 172. Silver Peak Marsh, Nevada, a typical playa (Photo F. R. Porter
from U. S. G. S.). ,
pure rock-salt deposits are formed. Gypsum may, however, result
from the alteration of limestones formed of lime-mud and dust
which is washed or blown over the salt. In this manner gypsum
beds may be formed above the salt beds, whereas in normal
marine deposits of salt, or in those due to the evaporation of cut-
Deposition of Salt in Inland Desert Basins 245
FIG. 1 73 . Professor Johannes
Walther, widely known for his
investigations of desert phe-
nomena.
offs, the gypsum will underlie the salt. The importance of these
facts in the determination of the origin of older salt beds is very
great. Another fact that must not
be overlooked in desert salt de-
posits is that the parting and en-
closing layers of sediment will con-
tain no marine organisms. They
will, -indeed, be for the most part
entirely free from organic remains.
A few desert organisms and mi-
gratory birds may, however, become
entombed in these deposits or even
in the salt itself, but their terres-
trial, character is readily recognized
by the expert. Huntington reports
finding, in the salt of Lop Nor in
eastern Turkestan, a dead plover
which had been preserved in the salt
for centuries. Around the border
of some of the Persian salinas a
zone of mud is sometimes found,
in which are entombed the bones of animals which came to drink
of the salty water .and perished there.
Beyond these salinas in all directions we pass into the region of
desert sands and dust deposits, and these have very definite char-
acteristics which can be recognized even after they have hardened
into rock beneath a cover of other deposits. Thus the geologist
will generally be able to recognize in the rocks associated with the
ancient desert salts the structures which clearly indicate that
origin. The appearance of a typical American salt-marsh or
play a is shown in Fig. 172.
Among the geologists who have made extensive investigations
into the origin .and mode of deposition of desert salts, the foremost
rank must be assigned to Professor Johannes Walther of the Uni-
versity of Halle. (Portrait, Fig. 173.)
An Ancient Example of a Desert Salt Deposit
In the central part of the state of New York, in western Ontario,
and in southern Michigan, occur ancient rock salt deposits, all of
which were formerly and some of which are still buried under thou-
246 Aqueous or Hydrogenic Rocks
sands of feet of rock, a considerable portion of the latter being of
marine origin. The rock-salts themselves, however, which rest
in some places upon marine beds of Lower Silurian (Niagaran) age
and in others upon non-marine sediments, are in some places covered
by marine beds of Upper Silurian age (Monroan) and are associated
laterally with deposits of clastic material which shows all of the
features of sediments found in modern deserts. Moreover, no
fossils are found in these deposits nor in those which separate the
several salt beds of the series from one another, nor are there any
Middle Silurian beds of marine origin to be found within hundreds
of miles of the salt deposits. Thus it appears that these very an-
cient salt beds of Middle Silurian age were formed in a desert which
then occupied much of the area now covered by the Great Lakes
and adjoining territory (see further, Chapter XXXIV).
No potash deposits are found associated with these salts, and
from what he has learned so far, the student will realize that there
is little likelihood of the finding of these salts unless it can be shown
that the basal salt beds of some sections are not of desert but of
marine origin, resulting from the drying up of the last remnant of
the Niagaran sea which preceded the desert period.
There is, however, one important fact which seems to argue
against such a marine interpretation of the basal beds, and that is
the absence of gypsum or anhydrite beds berieath the salt. Indeed,
the entire series of Salina salt deposits lacks the foundation layers
of gypsum or anhydrite, though gypsum overlies the salt in a
number of localities. Much of this is, however, known to have
been produced by the alteration of former limestone beds which
were invaded by sulphur-bearing waters.
CARBONATE OF LIME DEPOSITS
Although some carbonate of lime separates out on evaporation
of the sea-water, this is of such small amount that it practically
disappears in the evaporation series produced along the seacoast
and from cut-off bodies. Nevertheless, carbonate of lime deposits
are formed in the sea, not so much by evaporation, though
such an origin may be ascribed to some deposits, as by the
force of attraction of other particles of lime in sea-water saturated
with lime carbonate, or by the abstraction of the solvent carbon
dioxide through agitation of the water. Chemical Drecioitation
Carbonate of Lime Deposits 247
of lime also takes place and is perhaps the most common mode of
lime deposition in some places aside, from that due to organic
action. This precipitation, however, is due largely to the forma-
tion of ammonium carbonate by the decay of organic matter, and
this ammonium carbonate reacts with the lime sulphate or other
lime salts in the sea-water, producing calcium carbonate, which
is precipitated, and an ammonium salt which remains in solution.
The reactions may be written in the following way :
CaS0 4 4
- (NH4) 2 C0 3
= CaC0 3 +
(NH4) 2 S0 4
Calcium
Sulphate
Ammonium
Carbonate
Calcium
Carbonate
Ammonium
Sulphate
Or again
CaCl 2 ,
f (NEL^COs
= CaCO 3 H
h 2NH4C1
Calcium
Chloride
Ammonium
Carbonate
Calcium
Carbonate
Ammonium
Chloride
Where lime is precipitated by such reactions, it often forms more or
less spherical or irregular masses or nodules, to which the name
concretions is applied. Such concretions have been dredged from
many portions of the sea, and they appear to be especially common
where organic matter which has reached the floor of the ocean
undergoes a process of decay. Such areas are, however, not uni-
versal because the organic matter on the ocean bottom is generally
devoured by bottom-feeding animals before the decay progresses
far. It is only where the character of the water, or the tempera-,
ture, is such that bottom-feeders are scarce or absent, that 'decay
of stray organic matter can take place.
Deposition of lime due to the attraction of other lime particles
is illustrated by the hardening on the ocean bottom of the loose lime-
sands and muds worn from the coral and other organic limestone
masses in the sea. It is a general fact that wherever lime-mud,' or
sand, forms upon the sea-floor, this is soon bound together by the
filling in, between the particles, of lime derived from the sea-water.
This seems to take place most actively in warm regions, where the
amount of lime in the sea-water is above the average. As a result,
the floor of the ocean in such regions is a hard surface to which
various stationary marine animals attach themselves, while others,
such as certain worms or sponges, bore into this rock to a certain
depth.
On the surface of ancient limestone beds we often find the
marks of animals which had become cemented to it. Such cemen-
248 Aqueous or Hydrogenic Rocks
tation could of course take place only if the surface were of
sufficient firmness, and this indicates that the old deposits of
lime-sand, or mud, from which these beds were formed in the sea ;
hardened by further separation of lime, so that the animals living
there could become attached to it.
Another illustration of such Hme deposition is seen in the coating of lime
around grams of quartz, basalt, or other sand, or around fragments of shell,
etc., which are found on the shores of the Island of Gran Canada in the Canary
Islands. These coated grains are more or less spherical and of the texture
called oolitic (see p. 217). By further deposition of lime between the grains
they are bound together into a solid rock, an oolite, which is quarried at low tide.
The water here is warm, having throughout much of the year a temperature
above 20 C, and there is much lime in solution. Still another interesting
example of lime deposition is seen in certain Mexican lagoons where insect
eggs are coated with lime, producing a series of rounded oolite grains. Most
of the grains of oolitic character are, however, produced by the activities of
bacteria or minute algae in the sea, and on this account must be classified as of
organic origin. They will be more fully discussed in a later chapter.
OTHER CHEMICAL DEPOSITS IN THE, SEA
Small quantities of other substances are deposited in the sea
as the result of certain chemical reactions, or through attraction
by material of like composition. The most important of these
are phosphatic concretions and concretions of oxide of manganese,
both of which hav.e a -wide distribution over the sea-bottom. A
third group of such deposits forms grains of the green mineral
glauconite, of which " green-sands " are made. : -
Phosphate of Lime. This is produced by many marine animals
which take the phosphoric acid either directly from the sea-water,
or from their food. The phosphate of lime is built into certain
hard tissues, as the shells of some brachiopoda (Lingula), the bones
and teeth of fish, etc., and it is also present in the excrements of
fish and other animals. Such particles accumulating on the sea-
bottom have the power of attracting to them more phosphate and
precipitating it upon their surfaces, which thus become a nucleus
around which phosphate concretions are built. Such phosphate
concretions are found on the ocean floor in many localities at mod-
erate depths. They are also found in many old limestones, in
which they are generally scattered. By the weathering of this
limestone the nodules which are left behind are concentrated into
beds which are rich enough to be worked. By solution of the
Other Chemical Deposits in the Sea 249
phosphate and by redeposition in veins or on the walls of cavities,
or by replacing limestones upon which they rest, rich deposits of
phosphate are produced.
Manganese Concretions. Concretions of oxide of manganese
with oxide of iron, clay, and other substances are also found on the
floor of the deep sea in
many localities (Fig. 174).
The manganese and iron
form concentric layers
about some nucleus, which
may be the tooth of a
shark, or some other sub-
stance. It is not fully
known whether the man-
ganese is derived from the
sea-water in which it is
present in very small
quantities, or whether it is
derived from the decom-
position of basic volcanic
rocks on the floor of the , FIG 174- -Nodule of oxide of manganese
__ from Red Clay of abyssal ocean bottom,
ocean. Manganese con- (After J. Murray.)
cretions are also found in
ancient marine deposits, and in some cases at least may represent
concentration of scattered nodules by the weathering of the rock
which contained them.
Glauconite. Still another deposit formed by chemical means
on the sea-floor, is the mineral glauconite, which, when abundant,
forms beds chiefly composed of small grains of this mineral. On
account of their green color such beds of glauconite grains are
commonly called " green-sands." Chemically the mineral is an
impure hydrous silicate of iron and potassium, and it is commonly
formed from fine mud which fills the interior of small shells of
Foraminifera (Fig. 196, p. 275), partly by the reaction with the
products of decay of the organic matter in these shells, and partly
by reaction with the substances in the sea-water. The whole
process is too complex to be further discussed here, and should be
taken up again in a more advanced course. 1
1 See A. W. Grabau, Principles of Stratigraphy, pp. 670-673, and the literature there
cited.
250 Aqueous or Hydrogenic Rocks
Beds of green-sands are not uncommon in older marine and other
deposits. The most characteristic examples are found in the strata
of Cretaceous age which crop out in New Jersey and Maryland.
Sometimes by exposure the iron of the green-sand is changed to
an oxide, and ochery or red beds will be produced. Such- is the
vividly red sand bed which is so prominent in the section at Atlantic
Highlands, N. J., and which underlies the town of Red Bank, to
which it has given the name. Beds of green-sands are also common
in the Cretaceous strata of southern England,- where they are
generally spoken of as the Green Sands. They also' occur in France
and elsewhere. Some beds of green-sands are, however, found in
deposits which accumulated elsewhere than on the sea-floor.
CHEMICAL DEPOSITS AND EVAPORATION PRODUCTS OF LAKES
Lacustrine Deposits
Lakes may be classed as fresh- water, alkaline-water, salt- water,
and brine lakes. The salt-water and brine lakes have already been
referred to, and it has been shown that the deposits in these are
mainly, pure salt (sodium chloride), and in some of the larger ones,
like Great Salt Lake, also sodium sulphate or mirabilite. The Cas- -
pian must be differentiated from salt .lakes of smaller size, as it is
more properly a portion of the sea which has been cut off. Hence
the 'deposits there are generally like those formed on the sea-coast.
Composition of Lake Water
The composition of lake water varies of course in an almost endless manner,
no two lakes having water of exactly the same composition. Nevertheless,
it is possible to select certain types or averages of groups, which represent in a
general way the mineral substances present in such waters. Fresh-water
lakes are of course the most abundant, and as their composition, varies to a less
degree than that of the other lakes, it is possible to give a general average.
This is shown in the first column of the annexed table, in which the average also
includes that of river waters, which are not essentially different from those of
lakes. In the other columns the composition of a typical alkaline water (Owen's
Lake, California), saline water (Lake Corongamite, Victoria, Australia), and
brine (Dead Sea at 200 meters depth) are given, and the composition of ocean
water is again given for comparison. In this table the composition is expressed
in terms of. ions rather than of salts, which is the more accurate way of state-
ment.
Evaporation Products of Lakes
251
TABLE OF THE COMPOSITION or LAKE WATERS
FRESH
WATER
ALKALINE
WATER
SALINE
WATER
BRINE
OCEAN
WATER
NAME OF ION
Average
Salinity
Salinity
213.7
Salinity
46
Salinity
251-1
Average
Salinity
per mille
per mille
per mille
per mille
per mille
Carbonic oxide . . .
C0 3
35-15
24.55
Trace
0.207
Sulphuric oxide . . .
S0 4
12.14
9-93
1.65
0.22
7.692
Phosphoric oxide . . .
P0 4
O.I I
Trace
Boron oxide ....
B 4 7
0.14
__
-
Trace
Chlorine . .
Cl
r 68
24 82
en 10
67 ft/t
Bromine ......
Br
oy-o*'
O 22
1)7.04.
T 1 C
55- 2 9 2
o 188
Nitrogen oxide . ". . .
N0 3
0.90
0.45
* -75
Lithium .
Li
Trur-**
Calcium
Ca
10 OO
o 02
O T 7
I 68
1-ff\*7
Magnesium
Ms
3 A T
O OI
u.j-3
n fj*j
j A 2
.197
3*79
Sodium ,
Na
p VO
38 oo
*' 1 i
2r 07
IO OO
7^5
?o r'n^Z
Potassium
K
2 12
i 62
o 84
T *?n
o u Oyo
i 106
Iron oxide . ..
Aluminum oxide . . .
FeA 1
A1 2 3 /
2.75
0.04
1.79
Trace
Silica
Si0 2
II 6?
o 14.
Trace
Trace
Arsenic oxide . . . .
AsA
0.05
Trace
100. OO
IOO.OQ
IOO.OO
IOO.OO
Deposits of Fresh-Water Lakes
The three most important types of deposits formed from the
waters of fresh-water lakes and ponds are those of carbonate of
lime and carbonate and oxide of iron.
Carbonate of Lime. This is by far the most important con-
stituent of lake waters, and for that matter, of fresh-water bodies of
all kinds. Nevertheless, the actual quantity is smaller in fresh than
in sea-water. As will be seen from the analysis a cubic mile of fresh-
water contains on the average only 360,915 short tons of calcium car-
bonate (CaC0 3 ), whereas a cubic mile of sea-water contains 583,520
short tons of this mineral in solution. It must be remembered, how-
ever, that sea- water contains a vastly larger quantity of other sub-
stances, of which common salt forms 131,526,000 tons, while in
average fresh-water it forms only about 19,656 short tons per cubic
mile. Moreover, the total quantity of mineral substances dissolved
in the sea is 169,148,000 short tons per cubic mile, in fresh-water only
854,100 short tons ; that is, the sea contains about 200 times as
2 S 2
Aqueous or Hydrogenic Rocks
much mineral matter in solution. The mineral matter of fresh-
water bodies is precipitated by three methods: evaporation,
chemical reaction, and organic secretion. The last belongs to the
topic of organic deposits.
Evaporation. Lake waters are, as a rule, very far from being
saturated with carbonate of lime, and for this reason such a sub-
stance will not be de-
N -* posited as an evaporation
product until much or all
of the lake water is
evaporated. An excep-
tion to this is the depo-
sition of carbonate of
lime in the form of cal-
careous tufa in the mar-
ginal, pools or on beaches
of great fresh-water lakes
situated in dry climates,
but constantly supplied
with water by a large
river. Such tufa forms
by local complete evapo-
ration of the water which
lies in shallow marginal
pools above the ordinary
water-level and is re-
plenished at intervals by
spray and- the waves.
The spray which satu-
rates the sands of the
beaches may also, on
rapid evaporation, leave
behind carbonate of lime
to bind the sand grains
together. Such deposits of calcareous tufa are found in the old
lake beaches on the slopes of the Colorado or Salton desert up
to 40 feet above sea-level. These were formed when the Colorado
River drained into the basin and kept it full of water up to. that
level, in spite of the rapid evaporation which was taking place in
the dry climate of the region, and which has completely dried
FIG. 175. Map of Lake Bonneville and its
present remnant, Great Salt Lake of Utah.
Evaporation Products of Lakes
2 53
FIG, 176. Terraces and shore-lines of Lake Bonneville, near Wellsville,
.Utah, showing contrast between littoral and subaerial topography. (After
Gilbert.)
FIG. 177, Abandoned shore-lines of Lake Bonneville. North end of Oquirrh
Mountains, Utah. (Photo by F. T. Pack.)
254 Aqueous or Hydrogenic Rocks
out that basin since the Colorado has become diverted to the
Gulf of California. The nearly complete evaporation of lake
water is illustrated by the history of lakes Bonneville and Lahonton,
which in an earlier period of the earth's history occupied large
FIG. 178. Map of ancient Lake Lahonton and some of- the present residual
water bodies.
areas in the western United States (Figs. 175, 178), and the old
shorelines of which are still traceable (Figs. 1.76, 177). The product
of evaporation of the water of these lakes- was chiefly carbonate
of lime in the form of calcareous tufa, of which several types were
formed. This covers the older rock surfaces over wide areas
(Fig. 176) and forms layers of tufaceous limestone, alternating with
Evaporation Products of Lakes
255
deposits of sands and gravels, which it- sometimes cements. In
places these deposits form a mass over 50 feet in thickness, though
elsewhere they are represented only by an average thickness of
20 to 25 feet.
Three types of calcareous tufa are found within the basin of old Lake Lahon-
ton, each type belonging to a separate period of formation. The first and oldest
is called a lithoidal tufa because it is rock-like, cementing the old gravels of the
JT IG I79 Thinolithic tufa or Thinolite, from Lake Lahonton Basin. (After
RusseU.)
lake floor, and it not infrequently contains shells of fresh-water Mollusca. The
next type, formed after an interval of exposure, is known as thinolitUc tufa,
(Fig. 179), and consists of a series of large prismatic crystals six to eight inches
long and almost half an inch in thickness. These form a layer from six^to
eight feet thick where best developed. The final type of tufa is called dendritic
(Figs. 159, i8g), from its, branching structure, and this is the most abundant,
256
Aqueous or Hydrogenic Rocks
covering the old rocks of the lake with a deposit from twenty to fifty feet thick.
Sometimes this formed dome-shaped or mushroom-like masses up to five or
six feet in diameter (Fig. 159, p. 219), and where these are crowded they often
assume a polygonal outline
resembling paving blocks.
Internally these masses
have a more or less radiate
structure.
Significance of lime de-
posits of this type. Lime-
stones like the above, due
to evaporation, indicate a
dry climate during the
period of their formation.
If such limestones are cov-
ered by other deposits,
which later harden to rocks,
they will form a member of
a series of stratified rocks
similar in general appear-
ance to many that are found
in the older parts of the
earth's crust. If in such
an older series the limestone
member can be determined
by its peculiar character-
istics to have originated as
an evaporation product of
an old lake basin, a definite
conception of the physical
conditions of that region at
the time of the formation of
the limestone bed is gained. It is therefore important that the particular
characteristics of such limestone beds should be understood. -At the same
time it must be remembered that with the passage of time, such a limestone
will undergo more or less change, so that the old characters are to a greater
or less degree obliterated or altered. Nevertheless, enough may remain to
indicate the origin of the limestone, and from the adjoining formations and
those lying beneath and above it additional evidence for or against the evapo ra-
tional origin of such a limestone may be obtained. What may prove to be an
example of this type formed in a past geological period (Permian), is the bed of
Magnesian Limestone exposed on and near the coast of Durham in England,
which shows a structure not unlike the spherical structure of the dendritic tufa
of Lake Lahonton (Fig. 160, p. 221). The internal structure is, however, more
strongly crystalline, which may be due to its greater age. (Fig. 18, p. 34.)
Chemical precipitation. This will take place in stagnant lakes
and ponds, the waters of which contain much carbonate of lime in
FIG. 1 80. Tufa-domes, shore of Pyramid
Lake, a remnant of Lake Lahonton. Dendritic
tufa. (After Russell.)
Evaporation Products in Lakes
257
solution, and which moreover contain decaying organic matter.
This decay, as in the ocean, will produce ammonium carbonate,
which will tend toward precipitating the lime. It may be doubted,
however, that much lime is precipitated in this manner from fresh
water, since the presence of carbon dioxide, which is also formed
as the product of decay, would tend to keep the lime in solution.
Organic separation. The most common method of abstraction
of lime from fresh water is that by the activities of organisms, both
animal and plant. But deposits so formed belong to the group
of organic rocks, and their discussion will be deferred to another
chapter.
Iron Carbonate and Oxides. Stagnant swamps are often
covered By an iridescent filnvwhich is the result of the oxidation,
on contact with the atmosphere, of iron salts which were contained
in the water. Such iron salts are originally in the form of ferrous
carbonate (FeC0 3 ), which in mineral form is the ore siderite. This
iron carbonate is derived from iron salts in the soil and has resulted
from the decomposition of iron-bearing
rocks (the ferro-magnesian silicates of
igneous rocks), the leaching being per-
formed by rain and ground waters which
have taken up carbon dioxide (COs) from
decaying vegetation. ' When such a so-
lution of iron carbonate is brought into
a lake or swamp, one of two things will
happen. When there is much decaying
organic matter in the swamp, this will
appropriate all the available oxygen,
and then the iron is deposited in the
form of the carbonate. This is gen-
erally impure, being mixed with, the mud
held in suspension and carried down with
the iron carbonate. This mixture is most
frequently deposited around some object
which forms the nucleus of an iron-stone
concretion, as such structures are called
(Fig. 181). Such iron-stone or siderite
concretions 'when sufficiently pure form ores of iron, and they are
commonly found in formations in which coal (the product of the
partial decay of the vegetation) is also found.
FIG. 181. Clay-iron-
stone concretion split in
two, showing the impression
of a fossil fern (Neurof-
teris). Mazon Creek, 111.
(Photo by B. Hubbard.
Specimen in Columbia Uni-
versity.)
258 Aqueous or Hydrogenic Rocks
If, however, vegetation is not abundant, the carbonate will be
changed to the oxide, by contact with the air and with the oxygen
dissolved in the water, and in such cases an iron oxide will
be deposited. This is commonly in the form of limonite, the
yellow iron ore, in which water is present, and the loose, porous
masses of this mineral which form on the bottom of such ponds are
called bog-iron-ore. Such bog-ores are generally most abundant
near the margins of the ponds and swamps and are often wanting
near their centers. Sometimes such bog-ore forms very rapidly,
some Swedish lakes having deposited layers several inches thick
in twenty-six years. ' -
The decomposition of the iron carbonate in the solution and its
oxidation to insoluble iron oxide is aided by, and in some cases
largely due to, the work of minute organisms in the water, the
so-called " iron bacteria." Such deposits might be referred to the
organic group, were it not for the difficulty of distinguishing them
from purely chemical deposits such as those described.
Alkaline Lakes and Their Deposits
Under this heading are classed lakes in which carbonate of soda
plays a more important part than carbonate of lime, which is
generally present in small quantities only. The relatively great
preponderance of the C0 2 radical, and the much reduced chlorine
content, further differentiate typical alkaline lake waters from those
of saline lakes, in which sodium chloride is the dominant salt in solu-
tion and the carbonates are insignificant in quantity. Sulphates
and chlorides, however, are generally present, and one or the other
or both may be abundant, making complex alkaline waters. In
some cases, too, potassium may be an important element, exceeding
even the sodium, as in Albert Lake, Oregon.
Some of the principal water bodies in which deposits of this and
similar types are formed are :
Sodium Sulphate. Laramie Lakes, Wyoming
Soda Lake, Cal.
Sevier Lake, Utah
.Mono Lake, Cal.
Owen's Lake, Cal.
Searle's Lake or Marsh, Cal.
Albert Lake, Oregon
, Altai and Domoshakovo Lakes, Russia
Evaporation Products of Rivers 259
Sodium Carbonate. Soda Lake, etc., Nev,
Owen's Lake, Cal.
Searle's Lake, Cal.
Natron Lakes, Egypt
Lakes in Hungary, Armenia, Venezuela, etc.
Borax. Searle's Lake, Cal.
Death Valley, Cal.
Soda Niter (Chile Saltpeter). Desert Lakes of Chile
Searle's Lake ,
Potassium Nitrate (Saltpeter). Cochabamba, Bolivia
Different salts may be deposited at different times by these
lakes. Thus Searle's Lake or Marsh deposits both carbonate and
sulphate of sodium and borax and niter as well. From Owen's
Lake, California, both carbonate and sulphate of sodium are ob-
tained. Common salt (sodium chloride) is also an accompaniment
of the deposits in many of these lakes.
The greatest deposits of niter known in the world are found in
the Atacama and Tarapaca deserts of Chile. The amount, has
been estimated at 254,760,000 tons and is found at elevations
exceeding 2000 feet above the sea and from 50 to 100 miles from
the coast. The crude sodium nitrate is known as caliche and is
associated with anhydrite, gypsum, epsomite, halite, and other
minerals. The origin of these and of the potash nitrates of Bolivia
is not fully understood, but they were deposited from solution in
drying basins. 1
Deposits of Saline Lakes and Brines
These lakes deposit chiefly sodium chloride or common salt,
especially those in which the waters are a brine, as is the case in
most inland salt lakes. Great Salt Lake, . Utah, the Dead Sea,
and numerous Russian and Siberian lakes serve as examples.
They supply vast quantities of common salt for domestic and other
purposes.
CHEMICAL DEPOSITS AND EVAPORATION PRODUCTS or RIVERS
. (Fluviatile Chemical Deposits)
These are of comparatively rare occurrence and are confined
chiefly to regions of arid climate. As carbonate of lime is the
*For fuller discussion, see A. W. Grabau, Geology of the Non-metallic Mineral De-
posits, etc. Vol. I, Chapter XIII. McGraw-Hill Book Co.
260 Aqueous or Hydrogenic . Rocks
main mineral constituent of the river water and of that of fresh-
water lakes, it forms the chief, indeed practically the only, im-
portant chemical deposit of fresh water.
In Bahia, Brazil, the rivers which flow in and over the older
limestones are highly charged with lime in solution, and under the
influence of the tropical sun, partial evaporation of the water and
precipitation of the lime takes place. Thus deposits of lime rang-
ing up to zoo feet in thickness have been formed, and they often
contain plant remains as well as shells of river and land mollusks
of species still living in the region. Angular as well as water-worn
fragments of other rock are also included, and at times the mass
becomes brecciated through local disruption and cementation of
the fragments. Similar deposits are also formed around waterfalls
of rivers in limestone regions in various parts of the world.
On the broad flood-plains of many tropical rivers and .on their
deltas are formed crusts of limestone, which are precipitated from
the over-charged river water from the limestone hills. In Mexico
and the southern United States these are known as tepetate, while
the nodular limestone masses embedded with the sediments of the
Indus and Ganges, in northern India, are known as kankar. Such
limestone nodules of chemical origin are very characteristic of
river sediments in arid regions, and their occurrence in older sand-
stones leads us to ascribe a similar origin to these deposits.
DEPOSITS BY SPRINGS AND UOTERGROUND WATERS
Lime Deposits of Springs. In limestone regions, the under-
ground water is generally strongly charged with carbonate of lime
in solution. Where this water reaches the surface in springs,
the relief of pressure and the escape of carbon dioxide combine to
cause the precipitation of some of the carbonate of lime as a
flocculent material, which later hardens to solid stone. Where
leaves, mosses, or other organisms are bathed by this spring water,
they are covered by a deposit of lime or are included as fossils in
a mass of calcareous tufa or travertine. In ordinary spring water,
the deposition of lime goes on rather slowly, but sometimes the
growth of the travertine deposit is very rapid. At the Baths of
San Vignone, in Tuscany, for example, travertine is deposited at
the rate of six inches a year, while at San Filippo, in Sicily, the rate
is one foot in four months. Here a hill, a mile and a quarter long
Deposits by Springs and Underground Waters 261
and a third of a mile broad, has been formed by such deposits,
the height being at least 250 feet.
The water of hot springs generally carries more mineral matter
in solution than that of cold springs. The cooling of the water
on reaching the surface
is also very conducive
to lime deposition.
Hence we have here ex-
tensive travertine de-
posits, as shown in the
terraces, dams, and
basins of the Mammoth
Hot Springs of the Yel-
lowstone , (Fig. 182).
The origin of the ter-
races is illustrated in
the subjoined diagram
(Fig. 183). '
A peculiar form of
lime: deposit from
springs is the well-known
onyx marble, or. Mexican
onyx, which occurs interbedded with normal tufas in Arizona,
Mexico, California, and elsewhere in America, and. in North
Africa, Persia, and elsewhere in the Old World. This is a com-
pact, highly crystalline, and often beautifully variegated lime-
stone of a semi- translucent character, much used for decorative
purposes, the con-
struction of soda-
water fountains, etc.
It is frequently found
resting on crystalline
rocks in regions de-
182. Portion of the Sinter Terraces
of Mammoth Hot Springs, Yellowstone Na-
tional Park.
FIG. 183. Diagrammatic section of Sinter
Terraces formed by the water of hot springs.
The rim of the terrace is built up most rapidly
because as the water overflows it cools quickly at
this point and deposits its mineral matter. Series
of terrace-basins are thus formed. (From Kay-
ser's Lehrbuch.)
void of other lime-
stones, and this sug-
gests that the lime
and the water which
brought it to the sur-
face were of deep-seated or magmatic origin, that is, derived from
hot igneous masses within the crust of the earth.
262 Aqueous or Hydrogenic Rocks
No beds of this deposit have actually been observed in the
process of making, but from the fact that it is generally enclosed by
beds of normal tufa, we may assume that these deposits were
formed in temporary pools or lakes where standing water prevented
the rapid escape of C0 2 , the rock thus becoming compact instead
of porous, as is the case in ordinary tufa formed on the land.
OoHtes and Pisolites Deposited by. Springs, At the famous
springs of Carlsbad, in Bohemia, carbonate of lime is deposited
in the form of spheroidal, discrete masses of the size of a pea, and
hence forming an accumulation, of particles which when bound
together into a rock would constitute a pisolite. As the water
rises in the springs it holds in suspension minute mineral fragments
such as quartz or feldspar, which then receive a coating of lime
precipitated from the water. As the particles are turned over
and over in all directions like a pith-ball in a fountain jet, the
coating will be uniform all over, and spheroidal masses are pro-
duced. Sometimes gas and air bubbles form the nuclei around
which the lime is deposited, thus forming spheroids with a hollow
center. The water of these springs is probably of deep-seated
volcanic (magmatic) origin, from which source the lime is also
derived. This is indicated by the fact that there are no known
beds of limestone through which these waters ascend.
While no doubt both oolites and pisolites have been formed in
the past by springs, the great majority of these deposits now found
in the rocks of the earth's crust were formed in standing bodies of
water through the influence of organic matter either directly, by
the physiological activities of living organisms, such as bacteria
or minute algas, or by ammonia generated by decaying organic
matter. They will be discussed in a subsequent chapter.
Lime Deposits in Caves. The best-known types of lime
deposits from ground- water are found in caverns. Two types of
structures are generally recognized, the stalactite (Fig. 184),
depending from the roof of the cavern or from some projecting
edge, and the stalagmite, which forms on the floor of the cavern,
building up a mound or pyramid, or forming a hummocky lime-
stone floor (Fig. 185),
The formation of stalactites may be observed in tunnels and
underground chambers, the roofs of which consist of blocks held
together by lime-mortar, as well as in limestone caverns. Percolat-
ing ground-water will dissolve a part of this lime, and when a drop
Deposits by Springs and Underground Waters 263
of this lime-charged water reaches the tunnel and is suspended from
the roof, rapid evaporation will cause the formation, around the
drop, of a thin "shell of lime. The pressure of the percolating
FIG. 184. Compound stalactites and stalactic-sheets in Luray Cave,
Virginia. (From U. S. G. S.)
water will cause the breaking of this film of lime, leaving only a
small ring on the roof. The original 'drop falls, and a new one,
equally charged with lime, suspends itself from the bottom of the
lime ring. By constant repetition, a slender delicate tube is formed
by the successive additions of minute rings of lime, and this is
the basis of the stalactite. Other water, running down the outside
of this tube, will thicken and strengthen it by the addition of
successive layers of lime. In this process the lower end of the
264
Aqueous or Hydrogenic Rocks
initial tube is soon closed, and after that the stalactite remains
a solid icicle of carbonate of lime. Neighboring stalactites may,
from close juxtaposition, become confluent and form broad sheets
or curtains of lime which are often beautifully banded. Such
sheets of lime depending from an edge in the roof or along the
line of a crack are shown in Luray Cavern, Virginia, where they
form one of the striking features of this beautiful cave (Fig. 184).
Stalagmites are built up on the floor of a cavern by the evapora-
tion of the water which drops from the roof and generally from the
FIG. 185. Stalactites and stalagmites in Marengo Cave, Indiana. Note
the numerous small stalactites which depend from the roof of the cave, and
confluence of the larger stalactites with the stalagmites to form columns.
end of the stalactite. The continued evaporation of this water
leaves a minute quantity of. lime, which is gradually built up into
a mound, and this becomes steeper and steeper as it increases in
height, and finally forms a conical or even columnar mass beneath
the stalactite, with which it may eventually become joined into a
continuous column. This is well shown in the above- view of
the interior of Marengo Cave in Indiana (Fig. 185). Many such
columns may result in the cutting off of chambers and galleries
from the original caverns. On the margin of the stalagmite Sec-
ondary dependent stalactites are often formed, wherever a higher
portion of the stalagmite projects cap-like beyond the lower.
Thus highly complex and picturesque structures are produced.
Mineral Veins 265
Where the water spreads laterally before evaporating, an extended
sheet of stalagmite material is formed, and the lateral confluence
of many such sheets may result in the production of a stalagmite
floor. In many of the limestone caverns of southern France and
other parts of Europe, which during Palaeolithic time were in-
habited by the people of the Old Stone Age, implements and even
the bones of these prehistoric people are found embedded in the
clay of the cavern floor, over which not infrequently a cover of
stalagmitic material has been formed.
Basins are also formed in caverns where water, holding the lime
in solution, runs over a ledge. The edges of such a ledge cause the
water to break into ripples as it overflows, and this permits the
escape of some of the carbon dioxide which holds the lime in solu-
tion. In consequence, this lime is deposited at the edges of the
ledge, and so a rim is gradually built up, which holds back more
and more of the water in a permanent pool. Many such pools are
found in limestone caves such as Luray, where the conditions for
their formation are favorable. They are analogous to the sinter
terraces of the hot springs (Fig. 183).
MINERAL VEINS
Of all the deposits formed by waters circulating in or rising
through the crust of the earth, the mineral veins are the most
important to man. The mineral-bearing waters which form the
veins are probably in most cases hot, and they may even be in the
form of vapors. These waters or vapors deposit their load of
mineral matter either upon the walls of cavities or fissures, or by
replacing the rock material
alter it along their passage-
way, which may be a
minute crack . or other
avenue of escape. The
first group is called fissure
veins, the second replace-
ment veins or deposits.
Fissure Veins (Fig. FIG. i860.. Section of the Prinzen Lode,
I8 6 a). -Deposition in ^reiberg. a, blende; , quartz; c, fluorite;
J . ,*',. * ' tf, barrte; e, pynte; /, calcite.
fissures is brought about
by the cooling effect produced by the walls upon the solution, or
by chemical reaction with the material of the wall rock. Thus
266
Aqueous or Hydrogenic Rocks
an acid solution coming in contact with a limestone would become
neutralized and lime salts inclosing other minerals and metallic
substances might be deposited. Fissure veins have the form of
sheets, or films, of min-
eral matter cutting the
rock. If there is a suc-
cession of deposits, the
vein will be banded par-
allel to the wall-rock,
and the central portion
may be filled with crys-
tals. Veins of ore min-
erals or metallic sub-
stances contain, besides
this material, quantities
of other minerals such
as quartz, calcite, barite,
etc., and these constitute
the gangue material of
the miner. Thus native
gold is commonly found
^ of - t
to . & . . '
but quartz veins without
gold or other metal are
much more common. So, too, are veins of calcite and other min-
erals of little commercial value.
In rocks containing many veins there can often be recognized
several distinct series of successive origin. The relative age of
veins can be determined from the fact that the younger veins are
continuous across the older ones which they intersect. Some-
times cavities of irregular shape In the country rock form the site
of mineral and ore deposits, these differing from true fissure veins
mainly in their form and extent. Deposits of this kind are called
cavity-filled ore deposits (Fig. i86&).
Replacement Deposits (Fig. iS6c}. Instead of filling a pre-
viously existing fissure or cavity with its deposits, the mineral-
bearing waters or vapors may deposit their material as they pass
through the rock, by dissolving mineral particles of the country
rock and filling their places with new mineral matter. Such a
replacement of one mineral by another would go on, molecule by
FIG 186 b. -Transverse section of the great
ore chamber in the Emma mine, Utah.
i inch = 159 feet.
Mineral Veins
267
molecule, until an area of the rock is so re-
placed -by, or impregnated with, valuable min-
eral matter as to become an important ore
deposit. Frequently, of course, the replace-
ment and fissure type of deposit may inter-
grade, for the walls of an open fissure may
also be partly replaced by mineral matter.
Veins in which ores are scattered, i.e. lean
veins, may become locally enriched by the sub-
sequent solution and local concentration of the
valuable mineral matter. The movement of
the ores is generally from higher to lower
levels. Concentration may also occur by the
weathering or solution and removal of the
gangue material.
Placer Deposits. Concentration of val-
uable minerals may also occur at points distant
from the original vein. Thus quartz veins
carrying gold may be broken up by the
weather and the fragments washed away by
the streams. On account of the greater
weight of the gold, this will be concentrated
in favorable areas and so form the well-known placer deposits
(Fig. 187).
FIG. 1 86 c. Plan
of a tin lode at East
Wheal Loyell Mine,
Cornwall, England.
A, B, leader; C, C,
granite impregnated
with tin ore ; D, D,
granite.
FIG. 187. Hydraulic mining of Placer deposits (gold-bearing gravels),
Colorado.
268 Aqueous or Hydrogenic Rocks
Source of the Vein Minerals. The material in solution in the
vein-forming waters or vapors may be derived either by solution
from the rocks with which surface waters, descending into the
earth, come in contact in their circulation through the deeper
portions of the earth's crust, deriving their material chiefly from
FIG. 187 a. Park City, Utah, a typical mining camp. (Photo by F. J. Pack.)
rocks which overlie the point of deposition (descending solu-
tion), or the waters and vapors circulating through the rocks
laterally may dissolve their ore material and deposit it on reaching
the fissure (lateral secretion). Again, water derived as emana-
tion from deep-seated igneous masses (juvenile or magmatic
water) may carry upwards in solution the mineral substances
derived from these masses and deposit them in the higher fissures
(ascending solutions). This last mode of formation is regarded
by many as the most typical origin of mineral veins.
CHAPTER XII
THE ORGANIC OR BIOGENIC ROCKS
BlOLITHS
BY organic rocks, using that term in its strictly limited sense, we
understand those additions to the lithosphere which have resulted
from, or are the product of, the direct physiological activities of
organisms, both animal and plant. Rocks secondarily derived from
organic deposits, such as beds of limestone made of fragments
worn from coral reefs, have sometimes been included under this
heading, but they do not belong here, being strictly of fragmental
or clastic origin. Only those deposits which are formed in place
by organisms or which are largely built up of such material which
has been transported and has accumulated without much wear,
can be included here. Of course it must be recognized that a lime-
stone of shells or corals, which is strictly an organic limestone, may
pass laterally by degrees into one of fragmental origin. Gradation
exists everywhere in nature, but we are now concerned with the
study of types which may be readily recognized. The true organic
rocks are conveniently termed bioliths.
TYPES OF ORGANIC ROCKS OR BIOLITHS
At the outset we must distinguish two groups of material of or-
ganic origin which enter into the formation of rocks. The first
is the stony material, either carbonate of lime (with some-
times phosphate of lime) or silica, which animals and plants take
chiefly from the water in which they live, and in which these min-
erals were dissolved. This they precipitate upon or within their tis-
sues to build shells, corals, bones, and other hard structures. Min-
eral matter thus formed may be called organic precipitates. The sec-
ond group consists of the soft tissues of organisms, such as the flesh
,of animals and the tissues of plants, the latter made up in large part
of the substance called cellulose. Such organic tissues, as they
269
270 The Organic or Biogenic Rocks
may be called, are much more perishable than are the organic pre-
cipitates, which are generally preserved during long periods of
geological time without undergoing much alteration. Organic
tissues, on the other hand, undergo decay as soon as death has en-
sued, and if not protected, they will quickly disappear by changing
into gaseous and other matter. When protected, however, by
burial, the change is commonly incomplete, and a product, largely
composed of carbon and hydrogen, remains behind. The least
altered of such products may form beds of coal; the more completely
altered products form various bitumens. Because these substances
are all more or less subject to consumption by fire, they have
also been called caustoliths or caustobioliths (KCLVVTIKOS = capable of
burning). In the present chapter we shall consider the stony de-
posits of plants and animals, and in the next one the deposits formed
by the organic tissues and the materials resulting from their decay.
Kinds of Rock Material Produced by Precipitation of Mineral
Matter from Solution by Organisms
Here again we may distinguish two main types according to the
composition of the material precipitated, namely, the calcareous and
the silicious bioliths. There are others, such as certain iron oxide
deposits, which are formed by the agency of organisms, but they
are of minor importance. The calcareous group may again be
subdivided into those in which the material is carbonate of lime
(calcite, aragonite, etc.), with more or less magnesia, and those
in which it is largely phosphate of lime.
DEPOSITS OF CARBONATE OF LIME BY PLANTS
Deposits of carbonate of lime are formed by plants as well as by
animals and are among the most abundant precipitates in the sea,
though important ones are also formed in fresh water. Only two
groups of plants are active in precipitating carbonate of lime, the
Bacteria and the Algae, and in each group only a limited number
are active in this way.
Lime Deposited by Bacteria-
Bacteria are microscopic plants of extremely simple organiza-
tion, but of almost universal distribution and vast abundance.
Certain bacteria (called denitrifying) which live in the warmer
Deposits of Carbonate of Lime by Plants 271
portions of the sea effect the reduction of nitrates in the water
to ammonia, which, with carbon dioxide, produced by other bacteria,
forms ammonium carbonate. This reacts with the calcium sul-
phate in the sea-water, and the result is the formation of calcium
carbonate. The reaction is:
CaSO 4 + (NBLOsCOa = CaC0 3 +
Calcium Ammonium Calcium Ammonium
Sulphate Carbonate Carbonate Sulphate
The calcium carbonate separates out
in the form of small spherical or
elongated grains, which accumulate
as a mass of such discrete particles
and form a deposit of oolite. Such
deposits are forming to-day in great FIG. 188. Pseudomonas
abundance off the Florida coast and * Hme-predpitating bao
,.,,,. i . ^ teria: greatly enlarged. (After
near the islands of that region. The Kellermann.)
most common form of the denitrify-
ing bacteria has been named Pseudomonas calcis, in allusion to
the fact that it precipitates lime. It is illustrated in Fig. 188,
greatly enlarged.
Algtz and Algous Limestones
The term alga (plural algce) is applied to one of the lowest divi-
sions of plants. Most algse inhabit the sea, though many also live
in fresh water. They range in size from microscopic forms to the
giant kelps of the Pacific Ocean, which sometimes grow several
hundred feet in length, and are important because t they contain
both potash and iodine. A number of algae precipitate lime-car-
bonate upon and in their tissues, and so form stony structures,
either as distinct masses or as incrustations of rocks, shells or
other substances. To such structures the general name nullipore
is applied, and while they occur in cooler waters as well, they are
most common in tropical seas, where they constitute an impor-
tant agent in the building of coral reefs. Many reefs consist, to
the extent of more than half of their mass, of these organisms.
Another important fact to be noted is that many, if not
most, of these lime-secreting algae also separate out a considerable
amount of magnesium and precipitate it as the carbonate. Con-
sequently, the rock resulting from such algous accumulations will be
a limestome rich in magnesium carbonate and may approach a
272
The Organic or Biogenic Rocks
dolomite in composition, especially when, by subsequent leaching,
the proportional amount of calcium carbonate is reduced. An
example of such a rock, formed in the ancient Triassic sea, is seen
in the peaks called the Dolo-
mites, from the character of the
rock, and which are situated in
the Alps of the Tyrol (Fig, 4, p. 9).
The rock is known to have been
largely built up from lime-
secreting algae of the genus Diplo-
pora (Fig. 5, p. 10).
Among -the more important
types of nullipores, we may men-
tion only a few in addition to
the Diplopora.
Lithothamnium (Fig. 189).
This forms irregular masses with
knobby and sometimes leaf-like
surface features. It abounds on
modern coral reef sand also formed
extensive limestone masses in
former periods.
Halimeda (Fig. 190). This is
a form of much more plant-like appearance, having structures
resembling a stem and leaves, covered with lime, and brittle when
dry. It grows in the pro-
tected lagoons of coral reefs
and other regions.
Corallina (Fig. 191).
This is a pink, jointed plant,
remotely resembling a finely
branched coral or hydroid and
common below low tide on
all our North Atlantic coasts
(Corallina zone). When the
plant is dead and dry, the
color becomes white.
Chara (Fig. 192 a), Be-
sides these and many other
marine nullipores, there are attached to rock.
FIG. 189. Two modern species
of LUkothamnium, a lime-secreting
alga, which plays an important part
in the building of modern coral-
reefs. Isle Maurice. (After
ZitteL) - -
FIG. 190. Halimeda tuna, a lime-
secreting green alga from the modern sea;
Deposits of. Carbonate of Lime by Plants 273
some which live in fresh and mineral-spring waters. The common
fresh-water form is the ston&wort or Chara, found in fresh-water
lakes of limestone regions, It is a green alga, but appears gray
from the amount of lime precipi-
tated upon its surface. When dry,
it is white and very brittle. When
abundant, it forms deposits of marl
on the lake-bottom. Some older
limestones, such as some of those of
Tertiary age which underlie the city
of Paris and crop out some distance
from it, are made of the crushed and
more or less compacted, limy fila-
ments of this alga. Their origin
FIG. 191. Corallina, sp. A modern
lime-secreting alga, i, entire plant, natural
size; 2, a small branch enlarged.
FIG. 192 a. Chara vulgaris,
Linn. A modern lime-secreting
alga, growing in fresh water. An
important marl and limestone for-
mer. (From Haas, Leitfossilien.)
from this plant is recognized by the abundance in them of the little
ridged globular vessels, about the size of a pin-head, which were the
spore-bearing cases of the plant. These are readily recognized by
the peculiar spiral bands which surround them (Fig. 192 J). Such
274
The Organic or Biogenic Rocks
limestones also commonly inclose the shells of fresh-water snails
and other mollusca.
Filamentous algae are also active in hot springs, separating out
the lime carbonate, which then builds up the mounds and basins
often found around these, as in the Yellowstone region. It is diffi-
FIG. 192 b. Char a wlgans, L. A
recent calcareous alga (fresh water) ;
spore-vessel with corona. Enlarged.
This is frequently found in great num-
bers in fresh-water limestones, show-
ing their mode of origin. (From
Ha^is, LeitfossiUen.)
FIG. 193. A coccolithophora.
A mass of coccoliths; a marine
pelagic plant of low order covered
with calcareous plates. (Greatly en-
larged. After Murray.)
cult to determine in any case what part of the lime of such hot-
spring basins is built up by purely hydrogenic means (setante,p. 261),
and to what extent algae are responsible. The oolites of Great
Salt Lake and of other highly saline waters have also been regarded
by some authorities as largely
due to the growth and lime-
secreting habit of microscopic
algae (Rothpletz).
Coccolitlis, etc. Finally,
we may mention certain float-
ing organisms in the sea, gen-
erally regarded as extremely
low types of plants called
CoccoUthophores (Fig. 193),
which are covered with an
armor of plates. These plates,
according to their form, are
called coccoliths , discoliths,
cyathoUths, etc. When covered
with rods they are called rhabdoliths, and form a rhdbdosphere
(Fig. 194). These structures are found in calcareous oozes which
FIG. 194, Rhabdosphere. Much en-
larged. (After J. Murray.)
Foraminifera and Foraminiferal Oozes
27S
FIG. 195. Globigerina biilloides.
A modern pelagic foraminiferan,
with expanded pseudopodia. (After
Wyville Thompson.) *
remain on the floor of the deeper parts of the oceans and which
are largely composed of minute shells of Foraminifera (Globigerina
ooze), to be described next.
FORAMINIFERA AND FORAMINIFERAL OOZES AND LIMESTONES
The name Foraminifera is given to one of the classes of the
lowest group of animals, the Protozoa, in which each animal
secretes a small shell of carbonate
of lime, to which successive cham-
bers are added as the animal
grows, all the chambers being
occupied by the living animal
tissue. Many of the shells are
pierced by holes, as in the modern
Globigerina (Fig. 195), through
which delicate threads of living
matter (pseudopodia} project,
which serve to collect food.
There are many varieties of
these shell-bearing Foraminifera in the modern ocean (Fig. 196).
Globigerina Ooze.
Globigerina is the
most common among
the floating organisms
in the upper layers
of the sea-water. Its
shell consists of a
number of chambers
of increasing size,
the whole forming a
globular mass (Fig.
196, 2). Upon the
death of the animal
these shells slowly
sink to the bottom
(this requiring from
three to six days) , and
FIG. 196. Modern Foraminiferal types. Much y- they are not dis-
enlarged. i,0rbulina; 2, Globigerina; 3, Rotalia; , , f u p
4, Polystomella; 5, Catcarina. (After Neumayer, soivecl a S am m e
ErdgeschicUe; from Ratzel, Die Erde.) process, as happens in
276
The Organic or Biogenic Rocks
very deep water, they will accumulate upon the floor of the ocean
as a Globigerina ooze (Fig. 197), made up largely of this shell, but of
others as well and of coccoliths and other organisms, including non-
calcareous types. This ooze is most abundant in depths between
2500 and 4500 meters, the percentage of lime carbonate decreasing
from 70 per cent in the lesser to
50 per cent in the greater depths,
where more of the shells have been
dissolved. Nearly 30 per cent of
the area of the sea-floor is covered
with this Globigerina ooze, its
greatest distribution being in the
Atlantic and its least in the Pa-
cific, with the Indian Ocean in-
termediate (Fig. 198).
An Older Globigerina Limestone.
An example of a limestone now
exposed above sea-level, but formed
as a Globigerina ooze in deep water,
is found on the Island of Malta in
the Mediterranean. The age of
this rock is older Tertiary (Oligo-
cene), but nearly 40 per cent of
the species whose shells compose
this rock still live in the neighbor-
ing waters of the Mediterranean.
Most of the minute shells of which
the rock is composed are those of Globigerina. Scattered among
them are nodules of phosphate of lime similar to those found in
the deeper ocean waters of to-day. Altogether, this limestone,
now a solid rock, represents admirably a former deep-sea deposit
of Globigerina ooze, which, in the course of time, has solidified and
been lifted above sea-level by earth-movements of the kind to be
discussed in a subsequent chapter. Such old Globigerina limestones
occur in other districts as well.
Shallow Water and Terrestrial Foraminiferal Deposits
On tropical coasts, especially those of coral islands, shells of dead
Foraminifera often accumulate in large quantities, but these are
only exceptionally the shells of Globigerina, other forms which live
FIG, 197. Globigerina ooze,
from the deep sea, enlarged about
thirteen times. (After Murray and
Renard.) Besides the foraminif-
eran shells there are pteropods,
ostracods, and other organic struc-
tures.
Foraminifera and Foraminiferal Oozes 277
278
The Organic or Biogenic Rocks
in shallow water predominating. Owing to the lightness of these
shells, they are often carried far inland by the wind, forming dunes
FIG. 199 a. Foraminiferal shell,
Miliola type. (Spiroloculina badensis
d'Orbigny. Miocene, Baden.) Lateral
and top views. Important limestone
builder. (From Haas, Leitfossttien.)
FIG. 199 b. A foraminiferal shell
of the Miliola type. (Biloculina in-
ornata d'Orb. Miocene, Baden.) Note
that each new chamber covers all pre-
ceding ones. Two views and section.
Important limestone builder.
and even extended deposits chiefly composed of them. In the
western part of India (Kathiawar Peninsula) such a limestone,
called the Junagarh limestone, from the city .of that name which is
built upon it, overlies the
Deccan trap at a distance
of thirty miles from the sea.
It has a thickness probably
exceeding 200 feet, and its
cross-bedded structure in-
dicates wind transportation
(see Chapter XVI). It is
almost entirely made up
of foraminiferal shells and
other lime particles, with
only from 6.5 to 12.5 per
cent of silicious material.
The chief foraminiferan
shell of this rock is known
as Miliola (Figs. 199 a, &),
on which account the rock
is called Miliotic limestone.
Such limestones are found
on -the Arabian peninsula
and elsewhere, and in Tertiary deposits as well.
Chalk. This is a white, soft, friable rock, which consists of
minute shells and fragments of shells of Foraminif era, of coccoliths
FIG. 200, Thin sections of chalk as
seen under the microscope. A, chalk from
Sussex, England, enlarged 60 times;
B, chalk from Farafrah, Libyan desert,
enlarged 60 times; C, dried residue of
milky chalk-water with coccoliths, enlarged
700 times; a, Textularia globulosa; b, Ro-
talia (Discorbina) marginata. (After
Zittel.)
Foraminifera and Foraminiferal Oozes
279
and of other calcareous structures, all of them exceedingly minute.
A properly prepared slide (Fig. 200) of the material, from which
the finest dust has been washed out, shows, under the microscope,
a number of scattered shells of Foraminifera, of which a form of
triangular outline and composed of a double row of constantly
increasing chambers is the most abundant. This form, known as
Textularia globulosa (Fig. 200 a), lives to-day in the estuary of the
Dee River near Chester, England, and, like the other common species
FIG 201. Wave-cut cliff in chalk beds near Dover, England. From D. W.
Johnson's Shore Processes. By permission of John Wiley & Sons. The chalk
is in large part composed of microscopic shells and other calcareous organic
structures as shown in section under the microscope (Fig. 200). See also the
view of the Chalk Cliffs on the French coast, Fig. 713.
of the chalk (Rotalia marginata, Fig. 200 J), is therefore a shallow-
water species and not a surface floater (plankton) as is Globigerina.
From this and other facts it appears that the chalk is not a deep-
water deposit, as is the Globigerina ooze, but was formed in shallow
water, and the absence in it of quartz sands and of clays must be
accounted for by assuming that the lands which could supply such
material were too low to affect the deposits.
The chalk forms extensive beds over northern France and Bel-
gium and the south and east of England. These beds were once
continuous across the Channel and over much of the North Sea,
280
The Organic or Biogenic Rocks
while at the same time they extended as far to the northwest as
northern Ireland. The cliffs which they now present to the sea
and inland are the result of subsequent erosion (Fig. 201 ; see also
Chalk Cliffs. of Fecamp on the
French coast, Fig. 713). The
flints in the chalk (Fig. 162,
p. 224) are the result of sec-
ondary segregation of silica which
originally was scattered through
it, and which originated from the
FIG. 202. Shell of a Nummulite
cut transversely and in part hori-
zontally. Enlarged. (Group of
Nummidites lucasana Defr. Eocene,
Bavaria. From Haas, Leitfossilien.)
silicious skeletons or other parts
of marine organisms (Radiolaria,
sponge spicules, etc.) . At certain
levels in the chalk, beds of marine shells or other organisms are
found, indicating a temporary cessation of the chalk-forming con-
ditions and the inauguration for a time of normal beach or shal-
low sea deposition. The possibility that some chalk beds may be
formed by the drifting inland of shells and fragments of lime by
wind,- analogous to the Miliolitic limestone of India, has been
suggested.
Nummulitic Limestone. Over large areas of southern Europe
and northern Africa, and in parts of Asia as well, occur thick de-
posits of limestones which are largely. or almost wholly composed
of disk-like or button-like bodies, varying in size from that of a
pinhead to an inch or more in diameter. From their resemblance
to coins, these bodies have long
been known as Nummulites.
When worn, broken, or cut, they
show a characteristic internal
structure, with regular division
into chambers (Fig. 202), and they
are recognized as belonging to the
class of Foraminif era, of which
they constitute a remarkable,
gigantic, but now wholly extinct
FIG. 203. A fragment of Num-
mulitic limestone from the Pyrenees.
The nummulites are shown in sec-
tion, and of natural size. (After
Haas, Leitfossilien.}
type. These rocks all belong to
the early Tertiary period, and in
Egypt they have been quarried since the days of Herodotus and
before, and they were extensively used in the facing of the Great
Pyramid (Fig. 28, p. 76). A section of such limestone from the
Foraminifera and Foraminiferal Oozes
281
Pyrenees is shown in Fig. 203. These large Foraminifera probably
lived in shallow water, as do their nearest modern relatives.
Some of the limestones of our
Gulf Coast States (Vicksburg
limestone), and those of the West
Indies and elsewhere, are largely
composed of related Foraminifera.
One of these, on the island of
Cuba, is made up entirely of the
shells of such a form (Orbitoides) ,
somewhat larger than a pinhead ;
an enlarged photographic view of
one of these is reproduced in
Fig. 204. Similar limestones of
great thickness occur in Jamaica.
It is not impossible that these
were formed after the manner of
the Miliolitic limestone of India (Junagarh Limestone) described
above,. the shells being blown inland from the coast. This is sug-
gested by the almost total absence of other organisms.
Fusulina Limestone. 7 Another type of limestone, also formed
of large foraminiferal shells, is found in the upper Palaeozoic series
(Pennsylvanian and Permian) of western North America, Europe,
FIG. 204. OrUtoides (Lepidocy-
clina) kempi, O'ConnelL Enlarge-
ment of a single shell in section.
Cuba. (After M. O'ConneU.)
: FIG. 205. A polished piece of Fusulina limestone of the Carbonic. En-
larged nine times. On left sections cut the Fusulina transversely, on the right
obliquely to the longitudinal axis, (From Haas, Leitfossitien.)
282
The Organic or Biogenic Rocks
and Asia. An enlarged section of such a limestone is shown in
Fig. 205. These foraminiferal shells frequently resemble a kernel
of rice ; they are elongate and spindle-shaped (Fusulina, Fig. 206)
JT IG 2^ _ FusuLina cylindrica, a typical foraminiferal shell forming rocks
in the later Palaeozoic. A group natural size, and a single shell much en-
larged and partly sectioned to show interior. (After Kayser.)
or more or less like a football in form (Scfavagerina, Fig. 207), but
seldom more than a fraction of an inch in greatest diameter. They
are restricted to that part of the geological series, becoming extinct
with the close of the Palaeozoic era, though a form of similar appear-
A B
FIG. 207. Schwagerina verbecki, Geinitz. A, diagrammatic view; B, plan of
structure ; a, natural size. (After Schwager.)
ance, but very different structure, occurs in Tertiary rocks. In
general, the Fusulina is like a Nummulite with the axis of coiling
greatly elongated. Since the Fusulinae indicate the horizon of
the oldest extensive coal deposit, their recognition is of importance.
This will be more fully discussed in a later chapter.
CORALS AND RELATED REEF-BUILDING ANIMALS
Corals and Coral Polyps. The name coral is applied to the hard
structures (usually of carbonate of lime), built by delicate and
Corals and Related Reef-building Animals 283
as a rule small animals, which live only in normal sea-water and
mostly in regions of tropical or subtropical climates. The animals
are called polyps, and they are more or less cylindrical, fleshy, but
very delicate organisms, closed at the bottom, but having a central
opening, the mouth, at the top, around which there are one or more
rings of tentacles. In the
simpler forms, which are
known as hydroid polyps, the
entire interior of the body-
cylinder is hollow and con-
stitutes the stomach ; but in
the coral polyps proper (Fig,
208), the interior is variously
modified, chief among the
modifications being a series
of fleshy plates which extend
from the bottom to the top
of the cylinder and divide
the inner cavity into a num-
ber of radial chambers.
Not all polyps secrete a
hard structure, but those that
FIG. 208. Vertical section through a
polyp greatly enlarged (Astroides caly-
cularis), (After Lacaze-Duthiers.) Mouth
surrounded by tentacles and 'beneath it
the "stomatodaeurn. " The fleshy mesen-
teries are shown, and the calcareous
septa which lie between them in position.
In the center of the bottom is the
columella arising from the calcareous
basal plate (Sk). , (From Hass, Leit-
fossilien.}
do so precipitate the lime
from the sea- water in and
upon the outer layers of their
body, especially at the base
of the cylinder. These hard
structures begin as needles or
spicules of lime, which in some
groups, the gorgonias, seldom
or never unite into a solid
mass, but remain scattered in the fleshy parts of the body and are
left as lime needles upon the decay of the flesh. In other groups,
however, these needles are so numerous and crowded that they
become welded into a solid, more or less porous, stony mass, the
true coral. Of this, two types may be recognized, the solid rod
and the star coral The first of these is built by a colony of
polyps which are bound together by a solid fleshy substance into a
cylinder of living matter, over the surface of which the individual
polyps are scattered. The hollow central axis of the mass is
284 The Organic or Biogenic Rocks
filled by the carbonate of lime which this fleshy mass separates
from the sea-water, and when this central calcareous rod is stripped
of the surrounding fleshy substance, it appears as a much branch-
ing solid structure of carbonate of lime, colored a deep pink or
red in the most familiar types.. This is the precious coral of com-
merce, from which coral beads and ornaments are cut. In the
more common, but less familiar gorgonias, the sea-fans and sea-
whips, so abundant on most coral reefs, this central axis is horny
instead of calcareous, but is otherwise much of the same charac-
ter. The lime secreted by the gorgonias, as already stated, is de-
FIG. 209. A simple cup-coral (Caryophyllia cyathus), attached to the
sea-bottom. (After Dana, Corals and Coral Islands, by permission of Dodd,
Mead & Co.)
posited as lime needles or spicules in the fleshy mass which sur-
rounds and builds the horny axis. These spicules are sometimes
of importance in the formation of modern limestones.
Far more abundant than the group just described, and of more
importance in the formation of organic limestones, are the star
corals. These are so called because they show upon their surfaces
one or many, generally more or less depressed, circular, oval, or
polygonal areas or cups, each of which contains a radial series of
vertical plates, which converge toward the center of the cup and give
the appearance of rays from a central star. -These rays are called
the septa, and they correspond to the radial fleshy plates within the
body of the coral polyps by the base of which they are deposited.
Corals and Related Reef-building Animals 285
We may recognize simple corals with only one septa-bearing cup,
which then is either circular or oval (Fig. 209), and compound corals,
in which many such cups occur side by side, separated by inter-
vening limestone material, when their outline is circular or oval,
FIG. 210. Compound coral head (Astrcea pattida), with polyps partly ex-
panded and partly contracted. The expanded polyps show the tentacles
which surround the mouth ; the contracted polyps show the polygonal outline
from crowding. (After Dana, Corals and Coral Islands, by permission of Dodd,
Mead & Co.)
(Fig. 210) or closely crowded, when they assume more or less polyg-
onal forms (Fig. 21 1). But always the septa radiate from the
center of each cup to its margins. Sometimes the cups are so mi-
nute and separated by such broad intervals of spongy lime matter,
that the mass has a more or less homogeneous appearance, the cups
FIG. 211. A compound coral head with crowded prismatic corallites
(Acervularia ananas). The specimen represents a worn pebble, formerly a
part of a larger head. (From Kayser.)
being recognized only on careful examination, as they are small.
Often they form closely crowded tubes (Madrepora, Fig. 212) ; at
other times a massive branch is covered with a closely-set series of
minute cups (Porites, Fig. 213), In still other coral heads the cups
286
The Organic or Biogenic Rocks
>r calices are confluent, producing sinuous valleys and ridges
[Fig. 214).
Some of the ancient corals which were important as limestone
makers consist of a series of tubes arranged either in a loose, more
FIG. 212. Reef-coral, Madrepora palmata, natural size, and a and b slightly
enlarged calices. (After A. Agassiz; from Ratzd, Die Erde,}
or less chain-like series (chain coral or Haly sites, Fig. 215) or closely
crowded and taking on a columnar prismatic form from crowding
(honeycomb-coral or Favosites, Fig. 216, and Columnaria). In
these corals the septa are generally very short, or they may be
Corals and Related Reef -building Animals 287
represented by vertical rows of spines, or again they may be absent
altogether. Instead, the tubes are divided by numerous horizon-
FiG. 213. A massive branching coral (Porites mordax) with very small calices;
an important reef-builder. (After A. Agassiz ; from Ratzel, Die Erde.)
tal partitions which are often so closely crowded as to give the tube,
when broken lengthwise, a finely cellular structure (Fig. 216).
FIG. 214. Two small heads of a brain-coral. (Maandrina.) a, with
soft parts; b, corallum. Slightly reduced. (After Brehm; from Ratzel,
Die Erde.)
288
The Organic or Biogenic Rocks
FIG. 215. The chain coral. (Holy-
sites cafenularia, E. and H.) Silurian ;
with enlargement of a few corallites.
This is an important index fossil of
the Silurian, and an important lime-
stone builder as well. (From Haas,
Die Leitfossilien.)
FIG. 216. A characteristic a-sep-
tate compound coral. (Pawsites got-
landica.) Silurian. The columns are
prismatic and their walls pierced by
regular pores. Internally the tubes
are divided by horizontal plates or
tabulae. This is an important lime-
stone builder in the Paleozoic. (From
Haas, IMtfossilien,)
FIG. 217. A modern -hydrocoralline (Milleporaakicornis). Important as
a reef-builder. (From Dana, Corals and Coral Islands, by permission of Dodd,
Mead & Co.)
Corals and Related Reef-building Animals 289
Hydroid Polyps and Hydrocorallines. Finally the hydroid
polyps which, it will be remembered, have no internal fleshy radi-
ating plates, sometimes build calcareous coral-like structures in
which, however, the cups are merely holes in the more or less solid-
appearing limestone mass (Mdllepora, Fig. 217) which forms the
structure secreted by them and to which the name hydrocoralline
is applied.
Such hydrocorallines often form important and extensive
portions of coral reefs, the form known as Millepora (referring
FIG. 218. Fragments of large masses of stromatoporoids, which were im-
portant reef-builders in the Palaeozoic. A, Stromatoporella tuberculata, a
weathered fragment showing the hummocky surface, and the successive layers
(Devonian); B, Stromatopom antiqua, a transverse polished section of a
fragment showing the coarse concentric layers (Silurian). fc (After Nicholson;
from Grabau and Shimer, North American Index Fossils.)
to the thousands of pores on the surface, i.e. the cups)
abounding on certain modern coral reefs, while another, the Stroma-
topora, sometimes makes up the main portion of ancient (Paleo-
zoic) coral reefs, growing in masses up to ten feet in diameter. It is
readily recognized by the concentric arrangement of the succes-
sive layers of which this mass is composed (Fig. 218). Each layer
will, on microscopic examination, show a very definite structure
such as is found only in limestone deposits of organic origin. With-
out careful examination of details, however, the student will not
be able to distinguish the numerous kinds of Stromatoporas from
one another, nor will he be able readily to distinguish them from
ancient limestone masses of similar concentric structure built by
marine or even by fresh-water plants of low order (Algae, e.g. Cryp-
tozoon, etc.).
290 The Organic or Biogenic Rocks
CHARACTERS AND TYPES OF MODERN CORAL REEFS
Corals commonly grow associated in regions of the sea where
minute food particles are abundant, where the mean temperature
of the coldest months does not usually fall below 21 degrees C.,
or the minimum annual temperature below 1 8 C. and where a favor-
able hard bottom, free from, silt, exists for their attachment. For
it must be noted that the coral polyps are not free-moving or float-
ing organisms (except in their larval stages), but attach themselves
to the sea-bottom and lead a sedentary existence. By such asso-
ciation in growth of corals, together with other lime-secreting or-
ganisms, a reef is built up which rises toward the level of the sea,
and may grow so high that it is exposed at very low tides for a short
period of time. Coral reefs must be distinguished from coral islands
(see Fig. 220), which represent a stage subsequent to the reef when,
by wave action, the broken-off dead coral-masses and coral-sands
are heaped up to such an extent that they permanently project
above the water. Reefs, on the other hand, are always submerged
except for the short period of lowest tides already referred to. Mod-
ern coral reefs are chiefly confined to the region limited by the
sSth degree north and south latitudes, the Bermuda Islands, bathed
by the warm Gulf stream, being the chief exception to this, lying
in latitude 32 N. This distribution is partly due to the inability
of polyps to separate out much lime from sea- water in colder regions,
and also because whenever ice forms in winter the coral polyps,
always growing near the surface of the sea, are readily destroyed
by such ice. On this account we may confidently assume that
ancient coral reefs required similar warm temperatures for their
formation, and if we find that rocks now exposed in the Arctic re-
gions are formed from ancient coral reefs, as are those on the North
Siberian islands, we must assume that at the time of their forma-
tion this region had a more tropical climate.
In the second place, reef-building corals flourish best in shal 
