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Full text of "Study of the earth: readings in geological science"

STUDY OF THE EARTH 

RE AD I N GS 1 N GEOLOGIG AC iSC 1 ENCE 



edited by 




}?if' 







UNIVERSITY 
OF FLORIDA 
LIBRARIES 







STUDY OF THE EARTH 



PRENTICE-HALL INTERNATIONAL, INC. 

London • Tokyo • Sydney • Paris 

PRENTICE-HALL OF CANADA, LTD. 
PRENTICE-HALL DE MEXICO, S.A. 



STUDY OF THE EARTH 

• READINGS IN GEOLOGICAL SCIENCE 



Edited by }. F. WHITE 

Department of Earth Sciences 
Antioch College 
Yellow Springs, Ohio 



Prentice-Hall, Inc. englewood cliffs, n.j. 






© 1962 by Prentice-Hall, Inc. 
Englewood CMs, N.J. 

All rights reserved. No part of this 
book may be reproduced in any form, 
by mimeograph or any other means, 
without permission in writing from 
the publishers. 

Library of Congress Catalog Card No.: 62-8547 

Printed in the United States of America 
8589S-C 

Second printing February, 1963 



Preface 



Study of the Earth brings together outstanding readings that are par- 
ticularly suited to an introduction to geology and related fields. Such 
readings, although educationally useful, are usually not available. 

The scope of this volume is broad, and should open up new vistas for 
most readers. It does not restrict the study of the earth to narrow special- 
izations, but helps make clear the unity and the wide range of problems 
that constitute geological science. 

The readings offer the following advantages: 

1) Stimulating articles, which might not otherwise be accessible, are 
conveniently at hand. The book may be used as a basic text or as a 
supplement. 

2) "Independent study" can be emphasized. In addition to valuable 
reading, the articles can be the basis for informed discussion, short reports, 
and lengthier papers. They also provide a point of departure for projects 
involving further independent study and research. 

3) The reader is introduced to scientific literature and becomes ac- 
quainted with distinguished scientists through their contributions. This, in 
turn, tends to stimulate more reading of scientific literature. 

4) The volume is designed to help the reader gain more meaning, inter- 
est, and perspective from his study of science. Terminology and description 
are subordinated to an awareness of the significant problems. Recent re- 
search and present problems are emphasized, but a historical setting is also 
given. 

5) The readings are at various levels. This is an advantage, for it pro- 
vides a challenge to even the most well-informed reader. The book does 
not contain highly popularized and overly-simplified material. 

6) The volume offers a freshness of approach and readability often 
lacking in the usual introductory book. 

I wish to express my appreciation to the many authors and publishers 
who have permitted inclusion of their material. For the purposes of uni- 
formity of style, minor modifications have been made, including the 
deletion of some illustrations. Unfortunately, all aspects of earth science, 
including many excellent papers, could not be included. 

J. F. W. 



Digitized by the Internet Archive 

in 2010 with funding from 

Lyrasis IVIembers and Sloan Foundation 



http://www.archive.org/details/studyofearthreadOOwhit 



Contents 



INTRODUCTION 

The Study of the Earth, /. F. White • 1 



THE LAW OF UNIFORMITY AND GEOLOGIC TIME 



James Button, Karl Von Zittel • 1 1 The Uniformity of Nature, Charles 
Coulston Gillispie • 18 Measuring Geologic Time, Adolph Knopf • 41 



THE EARTH MODEL PROBLEMS AND IMPLICATIONS 

The Interior of the Earth as Revealed by Earthquakes, I. Lehmann • 63 
The Radioactive Earth, Patrick M. Hurley • 73 The AMSOC Hole to 
the Earth's Mantle, H. H. Hess • 79 



CRUSTAL FEATURES AND PROCESSES 

The Crust, /. Tuzo Wilson • 89 Instability of Sea Level, Richard J. 
Russell • 105 Questions of the Coral Reefs, Norman D. Newell • 121 
How Volcanoes Grow, /. P. Eaton and K. J. Murata * 137 Coesite 
Craters and Space Geology, W. T. Pecora • 164 The Stratigraphic Pan- 
orama, Hollis D. Hedberg • 171 



PAST CLIMATES AND DRIFTING CONTINENTS 

A Theory of Ice Ages, Maurice Ewing and William L. Bonn '203 A 
Theory of Ice Ages II, Maurice Ewing and William L. Bonn '111 Car- 
bon Dioxide and the Climate, Gilbert N. Plass • 224 The Record of 
Climatic Changes as Revealed by Vertebrate Paleocology, Edwin H. Col- 
bert • 239 Bearing of Forests on the Theory of Continental Drift, Br. 
Ralph W. Chancy • 260 Rock Magnetism, S. K. Runcorn • 111 



viii CONTENTS 



THE HISTORY OF LIFE 



The Course of Evolution, George Gaylord Simpson '291 Theories of 
Evolution, George Gaylord Simpson • 301 Origin of the Amniote Egg, 
Alfred S. Romer 'BIO Origin of the Pacific Island Molluscan Fauna, 
Harry S. Ladd • 321 Origin of Life, Elso S. Barghoorn * 335 



ORIGIN AND EVOLUTION OF THE EARTH 

The Origin of Continents, Mountain Ranges, and Ocean Basins, George C. 
Kennedy • 349 Development of the Hydrosphere and Atmosphere, with 
special reference to probable composition of the early atmosphere, William 
W. Rubey • 363 The Origin of the Earth, Harold C. Urey • 386 



A.PPENDIX 

The Geologic Time Scale • 403 Selected References • 404 
Biographic Notes • 405 Glossary of Selected Terms • 406 



What stuft 'tis made of, whereof it is born, 
INTRODUCTION ^ ^^ *o learn. . . . — Shakespeare, The 

Merchant of Venice, Act I, sc. I 



The Study of the Earth 

• J. F. WHITE 



". . . the nature and system of the world have been discovered but lately. . . ." 

— Lucretius, De Rerum Natura, V (100-55 b.c.) 

IT IS DIFFICULT TO PROVIDE A BRIEF BUT MEANING- 
ful introduction to a topic as broad as the study of the earth. This 
introduction is meant to provide a framework to the study of geo- 
logical science: specifically its broader outlines, its importance, and 
its relationship to other fields of knowledge. The significance of the 
selections will be immediately apparent. 

Man has always contemplated his world and has sought explana- 
tions of how the earth began and how its various features were 
formed. Although many of his earlier explanations now seem fanci- 
ful, they served the needs of the time and seemed to explain satis- 
factorily the data at hand. The early Greeks viewed nature as a sub- 
ject worthy of philosophical speculation; many of their views still 
have an essentially modern outlook. However, it is only since the 
eighteenth century that the study of the earth has developed sys- 
tematically through the collection of verifiable data and the build- 
ing of scientific theories. Our most recent theories, which attempt to 
explain not only past findings but also current data, remain "true" 
until even more modern theories are developed to replace them. 

Past and present 

Although we have greatly enlarged our understanding of the earth, 
we neither pretend nor expect to know all the answers. Indeed, much 
of the increased understanding has only led to new questions, and it 
may well be that learning has no end. We find that the earth still 

1 



2 J. F. WHITE 

presents an enormous variety of problems, both large and small, of 
the present and of the past. We would like to better understand the 
large and small processes currently at work on and within the earth, 
and to know more about the present composition and structure of 
our planet. In addition, there are problems of the past which demand 
our investigation, including the process of evolution from more primi- 
tive forms. 

We have reason to believe that the earth has been undergoing a 
long and complex development, proceeding from a comparatively 
homogeneous and simple structure to one much more complex. Some 
five billion years ago, there was only a cloud of matter in the form of 
dust and gas. Later, an earth gradually evolved, and still later oceans, 
continents, and a primitive atmosphere formed. The first lifelike 
structure gradually developed, eons ago, from comparatively complex 
minerals. And a human animal appeared only an instant ago in 
geologic time, the improbable product of an incredibly long sequence 
of ancestral events which began with the appropriate-sized cloud at 
a certain distance from a star. Such are the broad but imperfectly 
understood outlines of the modern evolutionary view of the world, 
much of whose development has come through study of the earth. 

We are concerned, thus, with the past, present, and continuing 
development of our planet. This includes its scenery, minerals, conti- 
nents, oceans, atmosphere, internal nature, and life. None of these 
factors can be isolated, for even the origin of life has had profound 
effects on the composition of the atmosphere and the nature of the 
earth's surface. The study of the earth involves not only events and 
processes which we observe now, but also those acting through long 
periods of time. We cannot separate the past from the present, and 
the present becomes understandable only in relation to the past. 

Importance of study of the earth 

For a better understanding of the environment of which we are a 
part, we must study the earth, if only to cope with, control, and 
intelligently manage it. It may be surprising that we know relatively 
little about the nature of our environment. A more complete under- 
standing of the earth may be a matter of survival, since there is no 
necessity in the scheme of things that the human species survive. 

The unknown. It has been said that we know even less about 
the surface of our own planet than we do of the moon, primarily 



THE STUDY OF THE EARTH 3 

because of our inadequate knowledge of the vast area underlying 
the oceans. Exploration through an average depth of 12,000 feet of 
water is difficult, and only in recent years have we begun to make 
real progress in our investigation of the oceans and the land beneath 
them. 



Official photograph, U.S. 
Navy, Sept. 12,1952 

Volcanoes such as Bar- 
cena (Revillagigedo 
Islands, Mexico) sym- 
bolize the internal 
energy of the earth. Vol- 
canic activity appears to 
be a main process in the 
origin and evolution of 
the oceans, atmosphere, 
and lands. 




On the lands we can easily collect rock samples for study, but 
what of the rock material beneath the oceans? We know this material 
is different from the granitic rocks of which continents are made. 
But as little as we know of the thin film of materials forming the 
outer surface of the earth, we know even less about the much greater 
mass of the inner earth. For knowledge of this interior we have had 
to depend on indirect observations, such as the behavior of earth- 
quake waves passing through the earth's interior. We further assume 
that the earth's internal matter is similar to the meteoritic material 
coming from outer space. Clearly, an improved understanding of the 
interior of the earth is fundamental to our understanding of the 
earth's processes, properties, and origin. 

Since the earth is large, the number of questions that may be 
asked about its nature is correspondingly large. Do the continents 
and ocean basins remain fixed in their positions with respect to the 
equator and poles, or do the continents drift about over the face of 
the earth? What is the explanation of the large climatic changes 
that have taken place in the past; why did regions now on or near 
the equator once possess glaciers of continental extent? Do certain 





^> 



Photo from the Barringer 
Crater Company 

A visitor from outer 
space, not a nuclear ex- 
plosion, created this im- 
pressive crater, which is 
about one mile across. 
Studies of meteorites 
and their craters have 
given us valuable clues 
to the nature and com- 
position of the earth 
and other planets. 



of the large fracture systems of the earth provide evidence for the 
hypothesis of an expanding earth? What minerals are present under 
the ultra pressures and high temperatures of the interior of the 
earth? 

In addition to these, one of the oldest of questions still lacks a 
definite answer: How old is the earth? Our present calculations indi- 
cate an age of 4.5 x 10^ years or 4 billion, 500 million years. Another 
time-problem is the construction of an accurate calendar for the 
earth in order to correctly place and interpret past events. When did 
life appear, and how long has it taken for the various forms to evolve? 
What was the earliest life, and under what conditions did it originate? 
How swiftly did the oceans and the atmosphere form, and are they 
still forming? Progress in solving the problems of dating the past will 
help develop answers to these questions, and in the future, more 
knowledge, better theories, and new questions will allow us to im- 
prove our picture of the entire earth system. 

Modern thought. In the recent past, intellectuals as well as the 
common man had what now seem rather strange beliefs concerning 
the history and origin of their surroundings. For example, if men 
of the eighteenth century saw the Grand Canyon and wondered 
about its origin, they would picture a sudden, violent tearing apart 
of the earth's crust, a catastrophic or supernatural event during which 
the canyon was created. In fact, every feature of the world was 
thought to have formed suddenly by such means. Former beliefs 
concerning the records of past life were equally unusual. Some thought 
fossils were created by emanations from the stars; others thought 



THE STUDY OF THE EARTH 5 




Courtesy of the Buffalo 
Museum of Science and 
Irving Reimann 

Ancient but complex 
life is shown by this 
trilobite, a marine ani- 
mal of about 350 million 
years ago. A restoration 
of Terataspis which 
lived on Devonian coral 
reef near Williamsville, 
New York. 

they were the remains of hfe present before the time of the "Great 
Flood" and Noah's ark. 

It is apparent that men in the recent past viewed the earth far 
differently than we do today. This was due, not to any lack of 
reasoning, but to different fundamental assumptions or beliefs. There 
was no concept of change except by rapid and catastrophic or super- 
natural means. Once created, the earth and its life were viewed as 
unchanging and static, comparable to a giant mechanical clock that 
was made and set in motion by its creator some few thousand years 
ago. 

With the discovery of geologic time, time of unimaginable extent, 
and the clear formulation of the concept of uniformity (natural law 
is constant in time), the old views were no longer reasonable or even 
believable. In place of the static, young, and suddenly created earth 
and life, the revolutionary concept of a dynamic, ancient, ever- 



6 J. F.WHITE 

changing, and gradually developing world emerged. This revolution in 
thinking has not only been exceedingly significant in influencing the 
progress of science, but may be the most influential factor in the 
development of our modern thought. 

Man is building, at accelerating speed, a super-bridge of knowl- 
edge reaching to the stars. We can expect continued investigation of 
the earth, with its many unsolved problems, to contribute to the new 
knowledge that may again greatly change our view of the world. 

Economic implications. The inquiry into the nature of our planet 
has advanced not only our scientific knowledge and world view, but 
also the economic welfare of man. Foremost among the applications 
of the study of the earth is the discovery and exploitation of mineral 
resources. It is not an exaggeration to say that our present civilization 
owes its existence to the extensive use of mineral raw materials. At 
the present time, almost all of our energy requirements depend on 




Photo from Thermal Power 
Company, San Francisco 

Power from the earth's 
heat. Steam wells near 
San Francisco, Cal. 



THE STUDY OF THE EARTH 7 

the fossil fuels (oil, coal, and natural gas), and, for the future, much 
larger energy resources based on radioactive elements such as uranium 
are available. A further importance of minerals is that, with the ex- 
ception of agricultural products, almost everything we use is directly 
or indirectly made from mineral raw materials. 

We have heard much in recent years about the population explo- 
sion and the terribly impoverished conditions of the large majority 
of the human race in the underdeveloped regions of the world. Re- 
lated to these tragic facts of life is the problem of achieving a greater 
production of minerals, including an adequate supply of water. At 
the present time our minerals are being used in enormous amounts 
at ever increasing rates; with the exception of underground water, 
most of these can be regarded as non-replenishable. In the United 
States, where the population has approximately doubled in the last 
50 years, the production of minerals has increased more than 800 
per cent. If the economically backward areas are to be developed, 
and if population continues to grow at the present rate, the problem 
of exhaustion of known mineral resources seems to be merely a ques- 
tion of a short period of time. Research leading to a better knowledge 
of the occurrence and use of minerals is imperative if we are to solve 
these problems. 

Today, about 95 per cent of the world's people live on approxi- 
mately 25 per cent of the land. If we consider the lack of resources 
and the dense, growing populations, we see that this population 
distribution will need to be altered in the near future. Factors of un- 
favorable climate, rugged terrain, and poor soils are chiefly responsible 
for the present limited use of land areas. It seems clear that greater 
use can be made of our potential resources, if the appropriate research 
is done. It is possible that further investigation of climate, including 
increased knowledge of past climatic changes, may allow us to alter 
climates— even to increase the amount of rain in a desert region. 

Relationships to other fields 

Splitting knowledge into distinct and separate fields is artificial and 
arbitrary, for there are no sharp lines dividing man's knowledge. The 
study of the earth encompasses geological and other earth sciences 
(such as oceanography, meteorology, and geography) which are re- 
lated intimately to one another and also closely connected to other 
sciences. But the earth is only a part of a much larger complex— the 
solar system. The study of the solar system is part of astronomy, and 



8 J. F.WHITE 



CHEMISTRY 



PHYSICS 



BIOLOGY 



GEOLOGY 



EARTH SCIENCE 



ASTRONOMY 



GEOGRAPHY 



ENGINEERING 



Relationships between fields of study. 



here the interests of both fields merge. The basic building blocks of 
the earth are minerals (natural crystals) composed of elements and 
compounds, and chemistry as well as mineralogy began with the 
study of such mineral materials. The earth follows physical "laws," 
and here the interests of physics and the earth sciences are often 
closely related. The geographer is concerned with the physical environ- 
ment, primarily in its relation to people. How do mineral resources, 
soils, landforms, and climates influence population distribution, in- 
dustrial location, the planning of cities and regions, and the diff^erent 
kinds of culture? The study of past life and past environments, in- 



THE STUDY OF THE EARTH 9 

eluding climate ehanges, drifting continents, and rising and sinking 
lands, is of concern not only to geologists, climatologists, and other 
earth scientists, but also to biologists and anthropologists. Study in 
one field frequently contributes to the solving of problems in another. 
Finally, the study of the history of the earth is intrinsically related to 
the subjects of written history, philosophy and religion. 

Although the study of the earth is related to these other fields, 
and often makes use of techniques and concepts first developed in 
them, it has distinct and characteristic features of its own. First and 
of primary importance are the types of problems which constitute the 
field of study, for they are concerned with the material and structures 
in the earth itself. Second, the study is often of past events, proc- 
esses, and environments, giving the study of the past as much im- 
portance as the study of present conditions. 



THE LAW 

Amid all the revolutions of the Globe, the 
OF economy of nature has been uniform and 

her laws are the only things which have 
UNTFOR MTTY resisted the general movement. The rivers 

and the rocks, the seas and the continents 
, xTp. have been changed in all their parts; but the 

laws which direct those changes, and the 

rules to which they are subject, have 
(jiiOLCjCjilC remained invariably the same. — playfair, 

Illustrations of the Huttonian Theory (1803) 

TIME 



James Hutton 

• KARL VON ZITTEL 



AT A TIME WHEN WERNER WAS IN THE ZENITH OF HIS 
fame, during those seventies and eighties of the eighteenth century when 
young geologists were flocking to hear the wisdom from the lips of the 
prophet of geognosy in Freiberg, a private gentleman, living quietly in 
Edinburgh, was deliberating and writing a work on the earth's surface 
that will live for ever in the annals of geology as one of its noblest classics.* 
James Hutton, the author of the famous Theory of the Earth, was the 
son of a merchant, and was born in Edinburgh on 3rd June 1726. He 
received an excellent education at the High School and University of his 
native city. His strong bent for chemical science induced him to select 
medicine as a profession. He studied at Edinburgh, Paris, and Leyden, 
and took his degree at Leyden in 1749, but on his return to Scotland he 
did not follow out his profession. Having inherited an estate in Berwick- 
shire from his father, he went to reside there, and interested himself in 
agriculture and in chemical and geological pursuits. The success of an 

• This article is adapted from Karl von Zittel, History of Geology and Paleontology, 
trans. Maria M. Ogilvie-Gordon (Charles Scribner's Sons: New York, 1901), pp. 67-75. 
(ed.) 

* Abraham Gottlob Werner (1749-1817), Professor at Freiberg, was the most 
famous geologist of his time. Although he made many contributions to geology, he is 
remembered especially as the founder and influential teacher of the Neptunian Theory. 
This theory, in essence, taught that the earth originated by successive precipitations 
from original aqueous solutions containing all the material of the earth, (ed.) 

11 



12 KARL VON ZITTEL 

industrial undertaking in which he had a share afforded him ample means, 
and in 1768 he retired to Edinburgh, where he lived with his three sisters. 
He actively engaged in scientific inquiry, and enjoyed the cultural social 
intercourse open to him in Edinburgh. The literary fruits of his life in the 
country include several papers on meteorology and agriculture, and a 
large philosophical work. 

From his early days he had always taken a delight in studying the sur- 
face forms and rocks of the earth's crust, and had lost no opportunity of 
extending his geological knowledge during frequent journeys in Scotland, 
England, in Northern France, and the Netherlands. On his tours into 
the neighbourhood of Edinburgh he was often accompanied by his friends, 
who realised the originality of many of Hutton's views on geological sub- 
jects, and begged him to put them into writing. At last Hutton set him- 
self to the work of shaping his ideas into a coherent, comprehensive 
form, and in 1785 read his paper on the "Theory of the Earth" before 
the Royal Society of Edinburgh. Three years later it was published in 
the Transactions. 

The publication of the work attracted little favourable notice. This may 
have been due partly to the title, which was the same as that of so many 
valueless publications, and partly to the involved, unattractive style of 
writing; in larger measure, however, it was due to the fact that the learn- 
ing of the schools had no part in Hutton's work. Hutton's thoughts had 
been borne in upon him direct from nature; for the best part of his life 
he had conned them, tossed them in his mind, tested them, and sought 
repeated confirmation in nature before he had even begun to fix them in 
written words, or cared to think of anything but his own enjoyment of 
them. 

Hutton's work was projected upon a plane half a century beyond the 
recognised geology of his own time. Hutton's audience of geologists had 
to grow up under other influences than polemical discussions between 
Neptunists and Plutonists, and had to learn from Hutton himself to tap 
the fountain of science at its living source. 

In 1793 a Dublin mineralogist, Kirwan, attacked Hutton's work in 
ignoble terms, and the great Scotsman, now advanced in years, resolutely 
determined to revise his work and do his best by it. Valuable additions 
were made, and the subject-matter brought under more skilful treatment. 
In 1795 the revised work appeared at Edinburgh, in independent form 
and in two volumes. It was his last effort. Hutton died in 1797 from an 
internal disease which had overshadowed the closing years of his life. 

The original treatise of Hutton is divided into four parts. The first two 
parts discuss the origin of rocks. The earth is described as a firm body, 
enveloped in a mantle of water and atmosphere, and which has been 
exposed during immeasurable periods of time to constant change in its 
surface conformation. The events of past geologic ages can be most 



JAMES HUTTON 13 

satisfactorily predicted from a careful examination of present conditions 
and processes. The earth's crust, as far as it is open to our investigation, is 
largely composed of sandstones, clays, pebble deposits, and limestones 
that have accumulated on the bed of the ocean. The limestones represent 
the aggregated shells and remains of marine organisms, while the other 
deposits represent fragmental material transported from the continents. 
In addition to these sedimentary deposits of secondary origin there are 
primary rocks, such as granite and porphyry, which, as a rule, underlie the 
aqueous deposits. 

In earlier periods the earth presented the aspect of an immense ocean, 
surmounted here and there by islands and continents of primary rock. 
There must have been some powerful agency that converted the loose 
deposits into solid rock, and elevated the consolidated sediments above 
the level of the sea to form new islands and continents. 

According to Hutton, this agency could only have been heat; it could 
not have been water, since the cement material (quartz, felspar, fluorine, 
etc.) of many sedimentary rocks is not readily soluble in water, and could 
scarcely have been provided by water. On the other hand, most solid 
rocks are intermingled with siliceous, bituminous, or other material which 
may be melted under the influence of heat. This suggested to Hutton his 
theory that at a certain depth the sedimentary deposits are melted by the 
heat to which they are subjected, but that the tremendous weight of the 
superincumbent water causes the mineral elements to consolidate once 
more into coherent rock-masses. He applied this theory of the melting 
and subsequent consolidation of rock-material universally, to all pelagic 
and terrestrial sediments. 

In the third part it is shown that the present land areas of the globe 
are composed of rock-strata which have consolidated during past ages in 
the bed of the ocean. These are said to have been pushed upward by the 
expansive force of heat, while the strata have been bent and tilted during 
the upheaval. Hutton next describes the occurrence of crust-fissures both 
during the consolidation of the rock and during the elevation of large 
areas, and the subsequent inrush of molten rock or mineral ores into the 
fissures. He regards volcanoes as safety-valves during upheaval, which by 
affording exit at the surface for the molten rock-magma and superheated 
vapours prevent the expansive forces from raising the continents too far. 

The evidences of volcanic eruption in the older geological epochs are 
next discussed. Hutton expresses the opinion that during the earlier erup- 
tions the molten rock-material spread out between the accumulated sedi- 
ments or filled crust-fissures, but did not actually escape at the surface; 
consequently, that the older rock-magmas had solidified at great depths 
in the crust and under enormous pressure of superincumbent rocks. He 
calls the older eruptive rocks "subterraneous lavas," and includes amongst 
them porphyry and the whinstones (eq. trap-rock, greenstone, basalt, 



14 KARL VON ZITTEL 

wacke, amygdaloidal rocks); granite was also added in a later treatise. 
Hutton points out that the subterraneous lavas have a crystalline structure, 
whereas those that solidify at the surface have a slaggy or vesicular 
structure. 

In the fourth part, Hutton concentrates attention on the pre-existence 
of older continents and islands from which the materials composing more 
recent land areas must have been derived. He likewise discusses the evi- 
dences of pre-existing pelagic, littoral, and terrestrial faunas from which 
existing faunas must have sprung. But, he continues, the existence of 
ancient faunas assumes an abundant vegetation, and direct evidence of 
extinct floras is presented in the coal and bituminous deposits of the 
Carboniferous and other epochs. Other evidence is afforded in the silicified 
trunks of trees that occasionally are found in marine deposits, and have 
clearly been swept into the sea from adjacent lands. 

Hutton then sets forth, in passages that have become classic in geologi- 
cal science, the slow processes of the subaerial denudation of land-surfaces. 
He describes the effects of atmospheric weathering, of chemical decom- 
position of the rocks, of their demolition by various causes, and the con- 
stant attrition of the soil by the chemical and mechanical action of water. 
He elucidates with convincing clearness the destructive physical, chemical, 
and mechanical agencies that effect the dissolution of rocks, the work of 
running water in transporting the worn material from the land to the 
ocean, the steady subsidence of coarser and finer detritus that goes on in 
seas and oceans, lakes and rivers, and the slow accumulation of the 
deposits to form rock-strata. Hutton impresses upon his readers the vast- 
ness of the geological aeons necessary for the completion of any such 
cycle of destruction and construction. In proof of this, he calls attention 
to the comparative insignificance of any changes that have taken place in 
the surface conformation of the globe within historic time. 

Hutton was thus the great founder of physical and dynamical geology; 
he for the first time established the essential correlation in the processes 
of denudation and deposition; he showed how, in proportion as an old 
continent is worn away, the materials for a new continent are being pro- 
vided, how the deposits rise anew from the bed of the ocean, and another 
land replaces the old in the eternal economy of nature. The outcome of 
Hutton's argument is expressed in his words "that we find no vestige of 
a beginning,— no prospect of an end." 

When we compare Hutton's theory of the earth's structure with that 
of Werner and other contemporary or older writers, the great feature 
which distinguishes it and marks its superiority is the strict inductive 
method applied throughout. Every conclusion is based upon observed 
data that are carefully enumerated, no supernatural or unknown forces 
are resorted to, and the events and changes of past epochs are explained 
from analogy with the phenomena of the present age. 



JAMES HUTTON 15 

The undeveloped state of physics and chemistry in the time of Hutton 
certainly gave rise to several errors in connection with the origin of 
minerals and rocks. No geologist now would agree with the principle that 
heat has hardened and partially melted all sedimentary rocks, and just as 
little would he ascribe to heat the origin of flint, agate, silicified wood, 
etc. On the other hand, the recognised hypothesis of regional metamor- 
phism of the crystalline schists is an extension of Hutton's conception of 
the action of heat and pressure upon rocks. 

Hutton was the first to demonstrate the connection of eruptive veins 
and dykes with deeper-seated eruptive masses of granite, and the first to 
point out the differences of structure between superficial lavas and molten 
rock solidified under great pressure. In assuming that granite represents 
rock consolidated from a molten magma, Hutton laid the foundation 
of the doctrines of Plutonism as opposed to those of Neptunism. 

Again, no one before Hutton had demonstrated so effectively and con- 
clusively that geology had to reckon with immeasurably long epochs, and 
that natural forces which may appear small can, if they act during long 
periods of time, produce effects just as great as those that result from 
sudden catastrophes of short duration. 

Hutton's explanation of the uprising of continents, owing to the expan- 
sive force of the subterranean heat, was not altogether new, nor was it 
satisfactory. Neither had Hutton any clear conception of the significance 
of fossils as affording evidence of a gradual evolution in creation. Yet in 
spite of these disadvantages, Hutton's Theory of the Earth is one of the 
masterpieces in the history of geology. Many of his ideas have been 
adopted and extended by later geologists, more particularly by Charles 
Lyell, and form the very groundwork of modern geology. Hutton's genius 
first gave to geology the conception of calm, inexorable nature working 
little by little— by the raindrop, by the stream, by insidious decay, by 
slow waste, by the life and death of all organised creatures,— and eventu- 
ally accomplishing surface transformations on a scale more gigantic than 
was ever imagined in the philosophy of the ancients or the learning of 
the Schools. And it is not too much to say that the Huttonian principle 
of the value of small increments of change has had a beneficial, sugges- 
tive, and far-reaching influence not only on geology but on all the natural 
sciences. The generation after Hutton applied it to palaeontology, and 
thus paved the way for Darwin's still broader, biological conceptions 
upon the same basis. 

Hutton's scientific spirit and genial personality won for him many friends 
and adherents amongst the members of the Edinburgh academy. The 
most distinguished of these were Sir James Hall and the mathematician 
John Playfair. Hall (1762-1831) contested the validity of the opinion held 
by some of Hutton's opponents, that the melting of crystalline rocks 
would only yield amorphous glassy masses. Hall followed experimental 



16 KARL VON ZITTEL 

methods; he selected different varieties of ancient basalt and lavas from 
Vesuvius and Etna, reduced them to a molten state, and allowed them to 
cool. At first he arrived only at negative results, as vitreous masses were 
produced; but he then retarded the process of cooling, and actually suc- 
ceeded in obtaining solid, crystalline rock-material [Nicholson's Journal, 
No. 38, 1800). By regulating the temperature and the time allowed for 
the cooling and consolidation. Hall could produce rocks varying from 
finely to coarsely crystalline structure. And he therefore proved that under 
certain conditions crystalline rock could, as Hutton had said, be pro- 
duced by the cooling of molten rock-magma. Hall then put to the test 
Hutton's further hypothesis, that limestone also was melted and re- 
crystallised in nature. To this hypothesis the objection had been made 
that the carbonic acid gas must escape if limestone were brought to a 
glowing heat, and the material would be converted into quicklime. This 
was Hall's first experience; then he devised another experiment. He intro- 
duced chalk or powdered limestone into porcelain tubes or barrels, sealed 
them, and brought them to a very high temperature. The carbon dioxide 
gas could not escape under these conditions. The calcareous material was 
thus subjected to the enormous pressure of the imprisoned air, and car- 
bonic acid was converted under this pressure into a granular substance 
resembhng marble. Hall calculated from a series of successful experiments 
that a pressure equivalent to fifty-two atmospheres, or to a depth of sea- 
water 1,700 feet below sea-level, was necessary for the production of solid 
limestone, 3000 feet of depth for that of marble, and 5,700 feet of depth 
in order to reduce carbonate of lime to a molten state. 

These results were afterwards confirmed by other experimentalists. Thus 
Werner's theorv that cr\'stalline rock represented in all cases a precipitate 
from water was shown to be inadequate, and it was incontestably proved 
that crystalline rock might originate from molten rock when slowly cooled 
under pressure. 

Hall also conducted experiments on the bending and folding of rocks. 
He spread out alternate horizontal layers of cloth and clay, placed a weight 
upon them, and subjected them to strong lateral pressure. These and 
similar experiments have been often repeated within recent years, and it 
is well known that in this way phenomena of deformation can be arti- 
ficially produced which bear the closest resemblance to the phenomena 
of rock-deformation under natural conditions. 

Hall, in his desire to vindicate Hutton's theory, became himself one of 
the great founders of experimental geology. At the same time, John Play- 
fair,^ whose interest in geology had been roused by Hutton's companion- 
ship, became the enthusiastic exponent of Hutton's theory. 

^ John Playfair, bom 1748, in Bervie, Forfarshire, son of a minister, showed in his 
early years a remarkable genius for mathematics. He studied in Aberdeen and Edinburgh, 
in 1773 became minister in Bervie, in 1785 Professor of Mathematics in the University 



JAMES HUTTON 17 

It was Playfair's literary skill that opened the eyes of scientific men to 
the heritage Hutton had left for them. He did for Hutton's teaching what 
fifty years after was done for Darwin's doctrines by the gifted Huxley. The 
brilliant exponent and successful combatant, no less than the deep stu- 
dent and enlightened thinker, is required to establish a new system of 
thought, for such a system is always bound to be in a measure reactionary 
to older doctrines that have received the stamp of usage and authority. 

Playfair's Illustration of the Huttonian Theory (1802) is a lucid exposi- 
tion of the theory in the form of twenty-six ample discussive notes. Play- 
fair's work differs in no essential point from the views held by his master 
and friend, but many subjects which receive a subordinate treatment in 
the Theory of the Earth are brought into prominence by Playfair, and 
placed for the first time on a firm scientific basis. 

Among the subjects fully discussed are the uprise and bending of strata, 
the origin of crystalline rocks at low horizons of the crust and under very 
great pressure, and the occurrence of granite as dykes in various British 
localities. His treatment of valley and lake erosion is extremely able. 
And Playfair was the first geologist who realised that the huge erratic 
blocks might have been carried to their present position by former glaciers. 
His insight in this respect would alone have won for him a lasting fame, 
for the erratics on Alpine slopes and plains had long been observed by 
geologists and an explanation vainly sought. Playfair also studied the 
raised beaches on the coast-line of Scotland, and rightly concluded that 
they afforded evidence of an actual uprise of the land, in opposition to 
the views of Linnaeus and Celsius, who had explained a similar series of 
phenomena in Sweden as a result of the retreat of the ocean. Playfair gave 
the first complete account of the evidences of oscillations of level in 
European lands. 

Playfair's style is a model of clearness and precision, and his arguments 
are always thoroughly logical, and in agreement with physical laws. His 
Huttonian Theory was translated into French by C. A. Basset in 1815. 

of Edinburgh, and twenty years after Professor of Philosophy in the same University. 
Led by Hutton into the study of geology, he devoted his holidays to geological tours 
throughout Great Britain and Ireland, and in 1815 and 1816 made longer tours to 
Auvergne, Switzerland, and Italy; he died in 1819 in Edinburgh. 



The Uniformity of Nature 

• CHARLES COULSTON GILLISPIE 



We now propose to examine those changes which still take place on our globe, 
investigating the causes which continue to operate on its surface. . . This 
portion of the history of the earth is so much the more important, as it has 
been long considered possible to explain the more ancient revolutions on its 
surface by means of these still existing causes. . . But we shall presently see 
that unfortunately this is not the case in physical history; the thread of opera- 
tions is here broken, the march of nature is changed, and none of the agents 
that she now employs were sufficient for the production of her ancient works.* 

— Georges Cuvier ^ 

When we are unable to explain the monuments of past changes, it is always 
more probable that the difference arises from our ignorance of all the existing 
agents, or all their possible effects in an indefinite lapse of time, than that 
some cause was formerly in operation which has ceased to act. . . 

Our estimate, indeed, of the value of all geological evidence, and the interest 
derived from the investigation of the earth's history, must depend entirely on 
the degree of confidence which we feel in regard to the permanency of the laws 
of nature. Their immutable constancy alone can enable us to reason from 
analogy, by the strict rules of induction, respecting the events of former ages, 
or, by a comparison of the state of things at two distinct geological epochs, 
to arrive at the knowledge of general principles in the economy of our terrestrial 
system. — Sir Charles Lyell " 



2 



IF BUCKLAND FEARED THAT WITHOUT CATACLYSMS 

there was no God, Lyell was as fundamentally apprehensive lest, without 
uniformity, there was no scienccf He could feel no reverence for a law- 

• From Charles Coulston Gillispie, Genesis and Geology (Cambridge, Mass.: Harvard 
University Press, 1951), Chapter V. Reprinted by permission of the publishers. © The 
President and Fellows of Harvard College. 

* Georges Cuvier (1769-1832), the founder of vertebrate paleontology and compara- 
tive anatomy, was a leader of the catastrophist school. This school, dominant in the 
early 19th century, saw a succession of worlds that were separated by periods when 
nature was not natural, but supernatural and catastrophic. The present world began after 
the Biblical flood, the universal deluge, (ed.) 

f William Buckland (1784-1856), one of the foremost geologists of the early 19th 
century, was a catastrophist and strong supporter of the Mosaic account of the Flood, (ed.) 

18 



THE UNIFORMITY OF NATURE 19 

giver who kept amending the constitution of nature. For some reason or 
other, however, the catastrophist controversy never became so acrimonious 
as the Vulcanist had been. Professor Sedgwick might disagree profoundly 
with Lyell— in fact, he was almost certain to— but they remained fast 
friends.* Perhaps the incidental fact that the catastrophist-uniformitarian 
debate was carried on within the Geological Society instead of by con- 
flicting academies contributed to its air of scholarly good humor. Then 
too, Lyell's followers, and he with them, had once been diluviahsts and 
could display a certain amused and superior tolerance for the error of 
their own past ways. 

One may, perhaps, deplore the disappearance of subtitles in the twen- 
tieth century, for it is convenient to know what a book is going to say 
before one reads it. Like Buckland in the Reliquiae, Lyell firmly staked out 
his subject on the title page: Principles of Geology, Being an Attempt to 
Explain the Former Changes of the Earth's Surface, by Reference to 
Causes Now in Operation. Unlike Werner, Hutton, and Cuvier, Lyell 
was more the critic than the original investigator. A younger contem- 
porary later remarked, "We collect the data, and Lyell teaches us to 
comprehend the meaning of them." ^ Even before Lyell removed the flood 
from its accepted place in geological dynamics, however, a few scientists 
had begun to express reservations about its universal efficacy. Next to 
paleontological research, a field more popular with the diluviahsts, the 
phenomena of volcanic action and the structure of valleys were the sub- 
jects most interesting to geologists during this period. A number of points 
began to seem very dubious as a result of continually extended observa- 
tions. In the first place, it became increasingly difficult to refer the com- 
mencement of all so-called alluvial deposits to a single event or even to 
any one period. Rivers, too, appeared in many cases to have cut their 
valleys through successive strata and through lava flows of many different 
epochs, some of which were postdiluvian even by the catastrophist chro- 
nology. It was difficult to describe gently winding river beds as the result of 
the scouring action of a single torrent, which could more easily be sup- 
posed to have cut straight gorges in its violent retreat. Nor could mixtures 
of fresh and salt water deposits be explained as the kind of uniform suc- 
cession which a single flood, either salt or fresh, would have produced. 
Moreover, there was an increasing comprehension of the proper chrono- 
logical classification of "primitive" and "transition" rocks and of the vast 
ages which must haVe elapsed between their formation. Such suggestions 
were scattered, however, among a number of memoirs through which it 
would be profitless to chase them. It would be even less profitable to 
develop the arguments with which diluviahsts met the difficulties.'* 

Although these scattered objections were not pulled together into an 

* Adam Sedgwick (1785-1873), known principally for his work on the Paleozoic sys- 
tem, was a firm believer in catastrophic geology, (ed.) 



20 CHARLES COULSTON GILLISPIE 

integrated attack upon catastrophist assumptions in natural history until 
the publication of Principles of Geology, there were a few obscure critics 
who raised strident voices of dissent in the chorus of mutual congratula- 
tions which Buckland had touched off among geologists. Perhaps the 
Reverend John Fleming, a zoologist member of the Wernerian Society, 
set forth the closest approximation to an anticipatory statement of uni- 
formitarianism. He had to admit Buckland's success: 

This work [Reliquiae Diluvianae], like the "Theory" of Cuvier, has greatly 
contributed to render the science of geology popular, by bringing it into favour 
with the Church, and even securing the countenance of the drawing-room. The 
general reader has been charmed with the novel scenes which it discloses, 
while the Christian has hailed it with joy, as offering a valuable testimony to 
the authority of revelation.^ 

Such easy popularity was not sufficient excuse for error in the stern, 
Calvinist eye of the Reverend Dr. Fleming, however. For Buckland's geo- 
logical deluge was contradicted both by the evidence of revelation and 
by that of the rocks. In developing the latter objections. Dr. Fleming 
anticipated in outline the major points which Lyell expanded into his 
three-volume Principles: the gradual excavation of river valleys; the artifi- 
ciality of referring "alluvial" detritus, whether sediment or organic remains, 
to a single source; the philosophic gratuitousness of supposing that a dif- 
ferent order of forces had ever been called into play. Oddly enough 
though— in view of the attitude uniformitarians were to profess towards 
such reasoning— it was also on the ground of Mosaic testimony that Dr. 
Fleming took severe exceptions to Buckland's flood. For Cuvier and Buck- 
land destroyed every species; Moses saved two individuals from each. They 
substituted the antediluvian sea floor for the old land surfaces; Moses 
summoned and dismissed the waters from a never-changing earth. They 
described a sudden, transient, and violent torrent which left marks on 
every valley and gorge; Moses left word of a gentle stand of water rising 
placidlv for forty days. And the true flood left no traces except a rainbow, 
the only empirical sign God has ever given us.^ 

It was chiefly, however, upon the work of George Poulett Scrope, who 
had published his views in 1825 and 1826, that Lyell relied for much of 
the new factual material included in the Principles of Geology. Scrope had 
devoted his descriptive talents to the investigation of extinct and active 
volcanoes. His theoretical conclusions demolished the "craters of eleva- 
tion" conjured up by Werner's younger followers in their belated appre- 
ciation of the widespread incidence of volcanic formations. Scrope empha- 
sized the continuous nature of volcanic deposits, their presence in strata 
of every epoch, and the impossibility of classifying volcanoes according 
to whether they had been eruptive before or after a flood. Given time 
enough, one could account for all lava formations by volcanic action of 



THE UNIFORMITY OF NATURE 21 

an intensity no greater than that of the present, and Scrope exphcitly 
suggested that the same thing was true of every aspect of geological 
change/ 

Lyell, then, did not pull his method of interpretation out of thin air, 
nor single-handed revive the Huttonian attitude. In the quotation at the 
head of this chapter, Cuvier remarks that "it has long been considered 
possible to explain the more ancient revolutions ... by means of these 
still existing causes," and he regarded this as a doctrine which his work 
had overthrown. In 1825 Constant Prevost had dared to challenge Cuvier's 
authority, though no one paid much attention to him, and between then 
and 1830 he and Lyell undertook a number of extensive geological tours 
on the Continent, a type of journey very fashionable at the time among 
laymen as well as among scientists. Lyell's ideas seem to have formed 
rather suddenly. In 1825, for example, he knew Scrope only slightly and 
referred to his Considerations on Volcanos merely as "a very creditable 
work." ^ He did not then see in it implications which would upset dilu- 
vialist assumptions. "I was," he later wrote, "taught by Buckland the 
catastrophical or paroxysmal theory," and not until 1827 does his pub- 
lished correspondence begin to mention the existence of a definite "liberal" 
camp in geology.^ By the end of that year, however, he had delivered the 
manuscript of his first volume to the printer, who must have become a 
little annoyed with uniformitarianism, because, what with several more 
continental tours and continual changes in detail, Lyell did not get the 
book through the press until January 1830, though he later declared the 
main points of his theory to have been fixed before he wrote his first 
draft.^^ For so single-minded a work, the Principles came out in remark- 
ably haphazard fashion. Originally Lyell had planned two volumes. The 
counterattacks of the opposing school compelled him to modify his tactics, 
and he altered his plan so that each volume would not only cover its 
phase of physical history interpreted in terms of the present but would 
also meet the objections raised by the preceding installment. In the end, 
volume II appeared in January 1832, a second and revised edition of 
volume I in June 1832, and volume III in April 1833. 



The time had come, announced Lyell, for a proper science of the earth 
and of its inhabitants, and he proposed to set it forth. It was now for the 
first time possible to do so. The suspension, since around 1810, of all 
attempts to form cosmogonies had been a salutary reaction against the 
excesses of Neptunism. A host of industrious toilers had accumulated a 
great new body of data, and, by avoiding generalizations, "they in a few 
years disarmed all prejudice, and rescued the science from the imputation 
of being a dangerous, or at best but a visionary pursuit." ^^ They also pro- 



22 CHARLES COULSTON GILLISPIE 

vided Lyell with the raw materials for a book. What is the good, asked 
Lyell, of describing results, if their causes be necessarily a matter of inde- 
terminate speculation? There had, of course, been considerable progress. 
But instead of hearing that fossils are sports of nature, or rock strata the 
result of aqueous precipitation, we now hear of sudden and violent revo- 
lutions of the globe, called in by scientists more anxious to cut the Gordian 
knot of knowledge than to unravel it.^^ 

Whoever would unravel the tangled skein of phenomena which the 
face of the earth presents to view and discover a single, intelligible thread 
therein must accept this doctrine: 

that all former changes of the organic and inorganic creation are referrable to 
one uninterrupted succession of physical events, governed by the laws now in 
operation. . . . The principles of science must always remain unsettled so long 
as no fixed opinions are entertained on this fundamental question.^^ 

Hutton, indeed, had approximated the uniformitarian position, and as 
a result the study of the earth as a science dates from his work. But 
although Hutton had properly remarked that science could study only 
causes of the same kind as those observable in the dynamics of present 
changes, he had fallaciously allowed for a difference in intensity of opera- 
tion because his theory of thermal uplift postulated alternating periods 
of disturbance and repose.^^ Later generations had unfortunately taken 
advantage of this loophole to neglect the Huttonian attitude, and to bring 
in changes so catastrophic as to differ in kind. 

Lyell's strong insistence upon the distinctiveness of his own approach 
laid him open to charges of plagiarism levied by opponents who protested 
that, however infidel his system, it did not possess the merit of an original 
heresy. Even a reviewer who was sympathetic towards Lyell's general atti- 
tude made this point,^^ and indeed much of uniformitarianism was im- 
plicit in Hutton. For Lyell, too, science could not concern itself with 
origins of the universe, "a metaphysical question, worthy a theologian." ^° 
He built his synthesis on the methodological limitation that the past 
could be studied only by analogy to what natural agencies can accomplish 
in the present. Such theoretical originality as uniformitarianism possessed 
lay in its pushing the analogy to an identity, in its rigorous, undeviating 
insistence that existing forces, given time enough, account for the observ- 
able state of man's habitat. 

The three skillful and lucid volumes of the Principles of Geology were 
devoted simply to marshaling the evidence in support of this simple thesis, 
and since the contention required that there be no exceptions, the result 
came very close to being a Summa Geologica. But first, it was necessary 
to point out why geologists had been so long in finding out what their 
subject was, and why their work had so often been ill received. The in- 
herent difficulties of the science, Lyell thought, had rendered it pecu- 



THE UNIFORMITY OF NATURE 23 

liarly susceptible to the interpretations of ancient miraclemongers and 
their modern successors. The most ubiquitous stumbhng blocks were 
popular preconceptions in regard to the extent of past time. If one had 
to produce our world out of a hat only six thousand years old, one obvi- 
ously must call upon extraordinary deviations from the normal course of 
events, even though one might admit that nature now proceeds according 
to uniform laws. Aside from the authority of the Mosaic chronology, fur- 
ther obstacles arose from our unfortunate position as land animals, a 
situation eminently unfavorable for geological observation. Human beings 
inhabit only about a quarter of the globe, and that quarter the one which 
is the theater of decay. We know of, but cannot observe, the progress of 
new formations under the land and beneath the seas. A race of fish with 
human intellects, thought Lyell, would have built a proper, sound natural 
history much sooner than we have done.^^ 

Having stated his thesis and, as he thought, exposed the popular and 
theological prejudices against it, Lyell set himself and his readers to in- 
quiring how the vicissitudes which the earth's surface obviously had ex- 
perienced "can be reconciled with the existing order of nature," ^^ He did 
not, of course, deny the reality of change, but he insisted that all change 
had been uniform, proceeding in cycles in time rather like the orbits in 
space through which the planets swing. The climatic conditions of any 
given spot, for example, had varied with the continual shifting in the 
relative proportions of land and sea in that particular portion of the globe. 
Volume I devoted itself to describing the geological dynamics which occa- 
sioned such changes. Familiar examples of the mode in which the various 
agents behave were pointed out— the action of the atmosphere and of 
living organisms, of volcanoes and earthquakes, and above all of water. 
After each contemporary or historical illustration the point was made that 
the cumulative effects of such common forces had produced the phe- 
nomena which Cuvier and Buckland referred to cataclysms of an essen- 
tially miraculous character. In similar fashion, the second volume dealt 
with changes now in progress in the animate creation and showed them to 
be the only kind ever to have occurred. The last volume, in spite of the 
necessity for a number of digressions occasioned by objections to the first 
two, was largely descriptive. It incorporated the latest developments in 
chronological stratigraphy, paleontology, and physical geography, and it 
included Lyell's most important constructive contribution to the science 
in his identification and separation of the Pliocene, Miocene, and Eocene 
epochs of the tertiary period. ^^ 

Lyell professed to have derived his theory entirely from appearances, 
and no doubt he thought he had done just that. A crudely additive induc- 
tive approach still enjoyed an almost exclusive methodological vogue in 
1830, and though the chance that a hypothesis may be deduced from a 
brilhant intuitive flash would damn it out of hand no longer, such an 



24 CHARLES COULSTON GILLISPIE 

admission would have killed it then, even for its originator.^o Actually, 
however, after abstracting the central idea implied by Hutton and Play- 
fair, Lyell simply universalized the principle of uniformity and then ar- 
ranged the facts in accordance with it. The process necessarily involved 
some incidental special pleading. 

Lyell was, of course, perfectly aware that the flood was his chief enemy, 
because to many minds the diluvial theory alone seemed capable of afford- 
ing an explanation of natural phenomena in accordance with scriptural 
history.2^ And being chary of disturbing religious convictions unduly, he 
impugned the deluge explicitly in only one passage, and that one rather 
in the nature of a digression. Generally, he preferred the method of drain- 
ing the flood of its influence incidentally to the development of his larger 
interpretation. And where he does allude to the flood, what he objects to 
is its universality and its geological efficacy, not its existence. 

It had long been a question among the learned, even before the commence- 
ment of geological researches, whether the deluge of the Scriptures was uni- 
versal in reference to the whole surface of the globe, or only so with respect 
to that portion of it which was then inhabited by man. If the latter interpreta- 
tion be admissible, the reader will have seen, in former parts of this work, that 
there are two classes of phenomena in the configuration of the earth's surface, 
which might enable us to account for such an event. First, extensive lakes 
elevated above the level of the ocean; secondly, large tracts of dry land de- 
pressed below that level. ^^ 

That is to say, a lake like Titicaca, far above sea level, might burst its 
banks and flood the neighboring lowlands, or a very depressed land area, 
like the Valley Jordan, might be inundated by a break in the barriers sur- 
rounding it. Such, Lyell implies, was the Mosaic deluge. He admitted it 
to be undeniable, however, that recent naturalists had followed Buckland 
almost to a man in picturing the flood as violent, universal, and a pri- 
mary geological agency. 

But we agree with Dr. Fleming, that in the narrative of Moses, there are no 
terms employed that indicate the impetuous rushing of the waters, either as 
they rose or when they retreated, upon the restraining of the rain and the pass- 
ing of a wind over the earth. On the contrary, the olive-branch, brought back 
by the dove, seems as clear an indication to us that the vegetation was not 
destroyed, as it was then to Noah that the dry land was about to appear.^^ 

It is somewhat surprising to find the evidence of the olive branch in 
Lyell as well as in Kirwan, though one suspects that when Lyell intro- 
duced it, he had his tongue in his cheek. Lyell, however, never questioned 
the accuracy of the Pentateuch in its own realm, which was historical and 
religious. He did not even intend to discredit it as the description of an 
actual geological event, provided the event was interpreted simply as an 
incident in the regular course of nature, but he hoped the issue would 



THE UNIFORMITY OF NATURE 25 

not be pursued. "We have been led with great reluctance into this digres- 
sion, in the hope of relieving the minds of some of our readers from 
groundless apprehension respecting the bearing of many of the views 
advocated in this work." ^^ 

The subject of volume II, however, was not a digression. The whole of 
it was devoted to a discussion of the animate creation and the vicissitudes 
which species undergo. "To Geology . . . these subjects do strictly apper- 
tain"; 2^ and the basic question is, "First, whether species have a real and 
permanent existence in nature; or whether they are capable, as some 
naturalists pretend, of being indefinitely modified in the course of a long 
series of generations?" ^^ Lyell offered his readers an admirably clear and 
dispassionate exposition of Lamarck's theories. He was perfectly fair and 
perfectly sure that they were wrong, and his refutation of the doctrine of 
progressive development of life took the form of an equally clear precis 
of Cuvier's arguments.^^ 

Each species "was endowed, at the time of its creation, with the attri- 
butes and organization by which it is now distinguished." ^^ Only limited 
variations within a type have ever occurred. Each species, itself immutable, 
probably takes its origin from a single pair, such pairs having "been created 
in succession at such times and in such places as to enable them to mul- 
tiply and endure for an appointed period, and occupy an appointed space 
on the globe." ^^ Linnaeus had been mistaken in supposing that one cor- 
ner of the globe had once been set aside as a divine incubator; instead, 
life had obviously originated in a number of "foci of creation." Races of 
animals have, of course, become extinct and the globe repopulated by 
new creations from time to time, although it is somewhat unsetthng "that 
so astonishing a phenomenon can escape the observation of naturalists." ^^ 
As to the most important point, Lyell agreed with Bishop Berkeley, who 
"a century ago . . . inferred, on grounds which may be termed strictly 
geological, the recent date of the creation of man." ^^ But neither the 
appearance of man nor the disappearance of other species is to be con- 
sidered a break in the uniformity of natural variation. 

We cannot conclude this division of our subject without observing, that 
although we have as yet considered one class only of the causes (the organic) 
whereby species may become exterminated, yet the continued action of these 
alone, throughout myriads of future ages, must work an entire change in the 
state of the organic creation. . . . The mind is prepared by the contemplation 
of such future revolutions to look for the signs of others, of an analogous 
nature, in the monuments of the past. Instead of being astonished at the proofs 
there manifested of endless mutations in the animate world, they will appear 
to one who has thought profoundly on the fluctuations now in progress, to 
afford evidence in favor of the uniformity of the system, unless, indeed, we 
are precluded from speaking of uniformity when we characterize a principle of 
endless variation.^^ 



26 CHARLES COULSTON GILLISPIE 

It has often been suggested that Lyell was on the verge of hitting upon 
an evolutionary theory of organic nature, and it is true that, with benefit 
of hindsight, uniformitarianism in geology seems almost to cry out for 
evolutionism in biology. In this general and important sense, Lyell un- 
doubtedly prepared the way for Darwin. Lyell did not have the benefit of 
hindsight in the 1830's, however, and at the time he was forced to reject 
the idea that organic life had developed through modification of species, 
because the conception of a progressive approach to the present order of 
things, which Lyell referred to as "the ancient doctrine," ^^ was relied on 
very heavily by his opponents. This is not surprising when it is recalled 
that "progressive" is not necessarily the same as "evolutionary"— it all 
depends on how the progress comes about, whether by providential inter- 
ventions or by natural laws. 

Lyell did not perceive the possibility of amalgamating progress with 
uniformity by substituting transmutation of species for successive crea- 
tions. Instead, he tended to deny the progressive character of earth his- 
tory. The more subtle of the Mosaicists, on the other hand, urged that 
extraneous fossils and extinct vertebrates exhibit a continued development 
of organic life from the simplest to the most complicated forms. Sir Hum- 
phry Davy, and with him nearly everyone else, thought that "there seems, 
as it were, a gradual approach to the present system of things, and a suc- 
cession of destruction and creation preparatory to the existence of man." ^^ 
Lyell thought the recent creation of man to be indisputable, but the re- 
mainder of the proposition, "though very generally received, has no 
foundation in fact." ^^ The argument was a little weak here, and Lyell 
seems not to have been entirely comfortable with it. His theory had to 
account for the absence of the remains of mammalian quadrupeds in the 
more ancient rock formations. Lyell's explanation was not that species 
like lions and elephants had appeared only in recent ages, but that in 
the successive metamorphoses of older rocks, all traces of these larger, 
softer, and less durable terrestrial forms had been destroyed.^^ This fact, 
since we know how present causes destrov such relics, offers further proof 
of uniformity in the past population of the globe. 

One might well wonder why the absence of relics should be proof both 
of the uniform antiquity of other species and of the recent date of man's 
creation, but Lyell never noticed the inconsistency. His purpose was to 
demonstrate that our creation had not been an event so exceptional as 
to constitute a break in the continuity of nature: 

The introduction at a certain period of our race upon the earth, raises no 
presumptions whatever that each former exertion of creative power was char- 
acterized by the successive development of irrational animals of higher orders.^^ 

Comparison between men and animals strains the bounds of valid analogy. 
Though it was a new departure for the creative power to link "moral and 



THE UNIFORMITY OF NATURE 27 

intellectual faculties capable of indefinite improvement, with the animal 
nature," that it did so does not justify the assumption of any correspond- 
ing steps in a hypothetical progression of purely physical forms.^^ 

It might be thought that uniformitarians would be more uncompro- 
mising opponents of Darwin than catastrophists, but it did not fall out 
so in the event. Attitudes are more lasting than theories, and in any case 
Lyell was not likely to achieve a prestige as imposing as that of Moses. 
In later times, when Lyell ranged himself by Darwin's side, his earlier 
writings did indeed supply their opponents with a limited store of obso- 
lete ammunition. But if anything is more damaging than a Pyrrhic victory, 
it must be a Pyrrhic defeat; and though providentiahst critics of uniformi- 
tarianism did not prevail in the 1830's, they seized avidly on its incon- 
sistencies and gave a suggestive airing to notions of progressive develop- 
ment. It would not be too difficult to substitute natural selection for 
providential cataclysms and divine creations. 

For all that his attack upon scriptural geology was oblique, Lyell was 
thoroughly aware that his chief enemies would be "the ancient and mod- 
ern physico-theologians." ^^ The real purpose of his book was "to sink the 
diluvialists, and in short, all the theological sophists." '^^ He had before 
him, however, a horrid example of what might come from such an effort, 
and he was most anxious not to reawaken an uproar similar to the one 
which had greeted Hutton's theories. 

The mind of the English public was at that time in a state of feverish ex- 
citement. A class of writers in France had been labouring industriously for 
many years, to diminish the influence of the clergy, by sapping the foundations 
of the Christian faith, and their success, and the consequences of the Revolu- 
tion, had alarmed the most resolute minds.*^ 

Lyell, like most British scientists, never thought of himself as having any- 
thing in common with the tradition of rationalist skepticism. Quite the 
reverse, for Voltaire, who in Lyell's view had misinterpreted physics in 
order to ridicule the Scriptures, had also poked fun at the cultivators of 
geology, "regarding the science as one which had been successfully en- 
listed by theologians as an ally in their cause." ^^ No good would come 
of this sort of thing, either for science or religion, and Lyell desired each 
to return to its proper sphere, before they had hopelessly compromised 
one another once again. 

If ever Mosaic natural history could be set down without giving of- 
fense, thought Lyell, it would be in a historical sketch,^^ and he very 
much wanted to avoid giving offense. His letters began to express worry 
about the reception his volumes would meet with before he started writ- 
ing them. He may, he fears, have to sustain the episcopal wrath of the 
whole bench of bishops, newly roused to ire by the Reverend Mr. Mil- 



28 CHARLES COULSTON GILLISPIE 

man's History of the Jews. On the other hand, there is a hopeful chance 
that the furor over unfrocking Milman may create a diversion in his 
favor.^^ Tact, he urges his friends Mantell and Scrope, tact. Let them 
not run "unnecessarily counter to the feelings and prejudices of the age." ^^ 

Lyell attached great importance to preparing public opinion to accept 
his views. The traditional orthodoxy of the Quarterly Review made it the 
key organ in his campaign, and Lyell discovered in advance that Scrope 
would be his reviewer therein. A series of letters, written before the PrirL- 
ciples appeared, briefed Scrope on what to say. "If Murray has. to push 
my volumes, and you wield the geology of the Quarterly Review, we shall 
be able in a short time to work an entire change in public opinion." ^® 
The resultant article was, not unnaturally, eminently satisfactory; it turned 
one of the enemy's main batteries against him.'*'^ Lyell was not an unduly 
sensitive person, but he had a bad case of literary apprehensiveness as he 
saw his pages through the press. His concern cannot be laid to his tem- 
perament; it can only have arisen from his appreciation that, however 
carefully his argument sought to ignore the issue, its imphcations ran 
directly counter to a deep current of accepted opinion.^^ 

The question naturally arises, what of Lyell as a scientific thinker? 
It is, indeed, obvious that he did protest too much. All his opponents 
immediately pointed out that the "attempt to explain the former changes 
of the earth's surface by reference to causes now in operation" was in- 
trinsically no more objective than the effort to explain them by refer- 
ence to a comet, a flood, or whatever catastrophes might be indicated. 
Lyell gave himself away, they said, by his frequent use of the word 
"reconcile" and achieved only a patent over-reconciliation.^^ 

Geologically, of course, Lyell's critics were right. No one now holds 
such extreme views upon the uniform course of nature. As early as 1840, 
although the immediate issue as to the universal efEcacy of contemporary 
causes was not settled, neither did the problem taken simply as a geo- 
logical one provoke much discussion. Sir Roderick Murchison's The Si- 
lurian System, which was published in 1839, and which after Lyell's Prin- 
ciples was the next major contribution to the science to appear in Eng- 
land, seldom even alludes to the uniformitarian-catastrophist debate or to 
any theoretical controversies.^^ But the question had become very much 
more than a geological one, and the root of Lyell's ideas lay outside the 
bounds of that science, wide though they then were. By 1830, he wrote, 
in all branches of natural knowledge, and even in enigmas of the moral 
world, the advancement of learning was presenting more and more of 
the phenomena which an ignorant past had attributed to miracles, to 
demons, to divine interventions, or to other extraordinary agencies as 
merely manifestations of larger laws, more perfectly understood. 



THE UNIFORMITY OF NATURE 29 

The philosopher at last becomes convinced of the undeviating uniformity 
of secondary causes, and guided by his faith in this principle, he determines the 
probability of accounts transmitted to him of former occurrences.^^ 

Uniformitarian presuppositions, then, were simply those of optimistic 
materialism. It would take time, and Darwin, to demonstrate how hope- 
lessly Buckland's school was out of key with the times— witness Lyell's 
apprehensiveness and the Principles inclusion of certain Mosaic details. 
But however pervasive the hold of catastrophism in 1830, materialistic 
science had almost cut the ground from under materialistic theology, 
even then. Gratuitous LAell's assumption may have been, but it opened 
the way for scientific progress, while Buckland's blocked the very path 
he sought to tread. After 1859, the surviving catastrophists, although they 
had tO)'ed with ideas of organic progression in the 1830's, were to be 
found solidly behind Wilberforce; while the uniformitarians who were 
still alive supported Darwin and Huxley, despite volume II of the 
Principles. 

3 

The uniformitarian thesis was launched with considerable eclat, and it 
became immediately the object of widespread attention.^^ j^- (jf^j ^ot, 
however, win the universal and enthusiastic assent which had hailed Buck- 
land's magnum opus seven years earlier. Lyell's arrangement of the evi- 
dence, as it flowed from the press, wore the opposition down instead of 
overwhelming it, and chipped away the catastrophic positions in some- 
what the same fashion as that in which his rivers produced a gradual, if 
much less rapid, degradation of land surfaces. Even his opponents ex- 
tended the work their hearty approval insofar as its purely descriptive 
features were felt to be the most skillful and interesting presentation of 
the subject ever set before the pubhc. Before attacking its conclusions, 
most of them injudiciously and somewhat ostentatiously welcomed the 
opportunity to discuss theoretical first principles.^^ 

Adam Sedgwick, before attacking all of Lyell's main points, felt con- 
strained to express his appreciation of "the instruction I received from 
every chapter of his work, and of the delight with which I rose from the 
perusal of the whole." ^* The effort, too, to disarm religious opposition 
had been fairly successful, at least to the extent that no reputable scientist 
exploited the whirlwind of theological outrage which was blowing up on 
a cruder level of criticism. Lyell even attributed his election to a chair 
in King's College, London, to Conybeare's intervention with the bishops 
who controlled the appointment and who were told that Lyell's doctrines 
were "startling enough, but not . . . come by in otherwise than a straight- 
forward manner" or "from any hostile feeling towards revelation." ^^ The 
bishops were uneasy, but they managed to master their qualms. 



30 CHARLES COULSTON GILLISPIE 

Publishers hastened to take advantage of the renewed interest in natural 
history aroused by Lyell, and a host of new titles, or of new editions of 
old titles, were rapidly bought up as soon as they appeared on the book 
stands. Macculloch, Mantell, Conybeare and Philhps, Jameson, Bake- 
well, and Brande got out revised versions of their commentaries. Gran- 
ville Penn, Andrew Ure, Bishop Copleston, and John Faber, among others, 
appeared with the latest refinements of scriptural geology. Lyell's volumes, 
too, went through a number of editions. 

As the debate developed, the most prolific of Lyell's supporters was 
Gideon Mantell, a competent geologist who was not of an original turn 
of mind and whose work, therefore, was abreast of his time but never 
ahead of it. His books are useful as examples of the adoption and popu- 
larization of Lyell's ideas by the uniformitarian school. Mantell's discov- 
ery of the iguanodon was his major claim to fame. He was a surgeon by 
profession, a highly successful popular lecturer— receiving as much as 
twenty-five pounds for lectures at charity benefits ^^— and something of a 
social climber. He evidently hoped to emulate Sir Humphry Davy in 
realizing his social ambitions by achieving scientific eminence, and like 
Davy he was also sincerely interested in research for its own sake. Science 
even cost him his wife, who left him when his collection of specimens 
and fossils grew so large and so popular that it crowded the family out 
of their home, which had become virtually a public museum. 

Mantell got out a number of geological works written for the general 
public.^^ The Geology of the South-East of England, the expansion of 
an earlier work,^^ appeared in 1833. At this time Mantell's interpretations 
showed little evidence of uniformitarian influence. In his general sketch 
of the science he referred in conventional fashion to causes still in opera- 
tion which date "from the period when our continents and islands as- 
sumed their present form," and he distinguished between the alluvial and 
diluvial deposits overlying the tertiary formations— though he refused to 
commit himself as to whether the Biblical flood was responsible for 
diluvium.^^ In the Wonders of Geology, however, first published in 1838, 
Mantell gave a precis of Lyell's description of contemporary causes and 
asserted them to have been sufficient for all time. Formations were now 
classified as metamorphic, secondary, and tertiary, and the tertiary period 
was broken down into Eocene, Miocene, and Pliocene. Mantell still used 
the word "alluvial" for loose, water-borne accumulations, but "diluvial" 
no longer appears. 

There are a number of passages in the Wonders of Geology which il- 
lustrate how uniformitarianism seems (to the modern reader) almost to 
have demanded an evolutionary explanation of organic phenomena. If, 
for example, "naturalistic development" were substituted for "the Crea- 
tor" in the following sentences, they would read like a vague anticipation 
of Darwin: 



THE UNIFORMITY OF NATURE 31 

The fluctuating state of the earth's surface, with which our previous inquiries 
have made us famihar, will have prepared us for the disappearance of some 
species of animals; — and here another law of the Creator is manifest. Certain 
races of living beings, suitable to peculiar conditions of the earth, appear to 
have been created; and when those states became no longer favourable for the 
continuance of such types of organization, according to the natural laws by 
whioh the conditions of their existence were determined, the races disappeared, 
and were probably succeeded by new forms.^" 

For the uniformitarian school in the 1830's, however, the activities of the 
Creator still supplied a satisfactory hypothesis covering the evidence later 
explained by evolutionary theory. Mantell even refers to human skeletons 
which had been discovered in Guadeloupe encased in limestone and to 
human footprints found in a block of the same material in Missouri, but 
he regards this as proof that the formations were recent and not that man 
is ancient. The point is simply stated, not argued.®^ 

The Wonders of Geology was a uniformitarian text for laymen, and 
Mantell attached considerable importance to quieting the uneasiness of 
people who might have been misled by the hostility of uninformed theo- 
logians. When science and religion are properly understood and their 
spheres kept distinct, there is, he assured his readers, no conflict between 
them. There were very few scientists in Mantell's generation who did not 
make this point at some time or other, and like the great majority of 
them Mantell did not perceive that his position logically required the 
rejection of the whole framework of conventional natural theology. Rather 
than attempt any original contribution to the well-worked subject himself, 
he preferred to disarm suspicion by stating the views of the "eminent 
philosophers and divines" whose central opinions were so widely accepted 
that they had rescued geology from the "absurd and unfounded" charge 
of being inimical to Christian piety.^^ The philosophers and divines he 
relied on were, most of them, catastrophists like Whewell, Buckland, and 
Sedgwick. While rejecting their particular theories regarding the course 
of nature, Mantell did not hesitate to adopt their fundamental interpreta- 
tion of the meaning of nature. 

The new page in the volume of natural religion, which Geology has supplied, 
has been so fully illustrated by Dr. Buckland, in his celebrated Essay,^^ that 
I need not dwell at length on the evident and beautiful adaptation of the organ- 
ization of numberless living forms, through the lapse of indefinite periods of 
time, to every physical condition of the earth, and by which its surface was 
ultimately fitted for the abode of the human race. 

It is enough to point out that "we must believe, that every physical 
phenomenon which has taken place, from first to last, has emanated from 
the will of the Deity." ^^ Although Mantell's reader is repeatedly told 
that geology had nothing to do with the Bible, this does not seem to 



32 CHARLES COULSTON GILLISPIE 

have meant that science had no religious imphcations. All it meant was 
that Scripture had no scientific implications. 

Mantell adopted uniformitarian geological theories somewhat uncriti- 
cally, but he never rose above the more general presuppositions of the 
period. Henry de la Beche, on the other hand, in spite of many reserva- 
tions about the specific thesis of Principles of Geology, came much closer 
to accepting Lyell's central attitude towards science. De la Beche was the 
first director of the Geological Survey, founded in 1835, and like Mantell 
he wrote several popular texts and elementary manuals designed to assist 
the amateur observer towards a constructive enjoyment of his hobby,^^ 
Though not a uniformitarian, neither had De la Beche ever been a scrip- 
tural catastrophist. He represents, in fact, the tendency to ignore all 
questions of the sort— a tendency not yet very marked. For De la Beche, 
"The difference in the two theories is in reality not very great; the ques- 
tion being merely one of intensity of forces, so that, probably, by uniting 
the two, we should approximate nearer to the truth." ^^ He did, it is 
true, assume that there must have been successive creations of species, but 
in this he was simply expressing the current hypothesis. He did not relate 
it to the Biblical account.^^ De la Beche's own interpretations of the 
geological evidence were closer to the catastrophist than to the uniformi- 
tarian pattern, but entirely without Mosaic allusions or overtones, and as 
a result his writings were both temperate and, compared to the rest of the 
discussion, rather dry.®^ 

The orthodox opposition was more excited and, after an initial period 
of disorganization, took up its positions on lines of argument so well 
defined as to indicate careful staff planning. The catastrophist high com- 
mand centered in the universities. Buckland and Sedgwick still held the 
chairs of geology at Oxford and Cambridge. Daubeny was professor of 
chemistry at Oxford, Conybeare a fellow of New College, and Whewell 
senior tutor and later master of Trinity College, Cambridge. Nearly every 
meeting of the Geological Society appears to have resolved itself into a 
debate between Lyell's supporters and this "Oxford School of Geology." ^^ 
In general scientific circles, the Oxford school seems to have been thought 
the more reputable and the safer of the contending groups. Buckland was 
elected second president of the British Association for the Advancement 
of Science in 1832, and Conybeare the chairman of its geological section. 

One thing the Principles of Geology unquestionably accomplished. The 
book administered the coup de grace to the deluge. Few denied that Moses 
had indeed described an impressive flood, but as a primary, universal 
geological agency, it was abandoned. It is, of course, interesting that Lyell 
felt required to find a humble niche for it in his picture of a uniform 
past, and that he specifically comforted Bishop Copleston of Llandaff by 
assuring him that there was "no objection to his drowning as many peo- 
ple as he pleased on such parts as can be shown to have been inhabited 



THE UNIFORMITY OF NATURE 33 

in the days of Noah." ^'^ But the speed with which the ecumenical flood 
evaporated is starthng. As late as 1829 the period at which a flood operated 
was still regarded as central to chronological classification. Daubeny, it is 
true, did attempt to preserve some scope for violent aqueous action, but 
only in the formation of volcanoes and of valleys and not as a unique, 
world-wide event. He made this point in the course of a general argument 
to the effect that catastrophes greater than any we now see could be 
produced by present causes acting more intensely, and that he and Lyell, 
therefore, differed only on a question of degree. Although he thought it 
possible that "a doctrine in science may be true, although involving ques- 
tions that cannot be reconciled, at the time, to the statements of Scrip- 
ture," he also felt that his position, as opposed to Lyell's "has the further 
advantage of rendering the accounts of such catastrophes, which are 
handed down to us on the authority both of history and tradition, con- 
sistent with probability, instead of opposed to it . . . and thus, if not 
directly confirming the Mosaic history on this particular point, removing 
at least those obstacles to its reception that might exist, if we considered 
the event related as out of the course of nature." Scripture was not a 
source for science, of course. In the case of conflicting theories, however. 
Scripture may, Daubeny held, appropriately be used to tip the balance of 
probability one way or the other.'^^ 

The faithful were unable to take much satisfaction in so limited a 
catastrophe as Lyell's deluge allowed them, and most of them either lost 
interest in it or hastened to abandon it. Whewell, who, according to 
Lyell, "has more influence than any individual, unless it be Sedgwick," '^^ 
now contemptuously dismissed "those who have framed their geology by 
interpretations of Scripture." He still, however, allowed a limited validity 
to those 

geological speculations in which the Mosaical account of the deluge has been 
referred to; for whatever errors may have been committed on that subject, it 
would be as absurd to disregard the most ancient historical record, in attempt- 
ing to trace back the history of the earth, as it would be gratuitously to reject 
any other source of information.^^ 

But Whewell no longer felt inclined to introduce this particular evidence 
into the argument. 

Conybeare backtracked even more hastily, though less unreservedly. 
He had been so impressed with the Principles, and so disturbed by their 
implications, that he prevailed upon the editor of the Philosophical Maga- 
zine to run a series of pieces in which he took issue with all of Lyell's 
interpretations and with many of his specific illustrations.^^ These ar- 
ticles furnish the most complete statement of the catastrophist counter- 
offensive. Conybeare professed to adhere to the diluvial theory, but "only 
in a general and philosophical sense. Theologically, I am well contented 



34 CHARLES COULSTON GILLISPIE 

to let the Scriptural narrative rest on its appropriate moral evidence, and 
should only fear to weaken that evidence by mingling it with my own 
crude scientific speculations." This was a new fear with Conybeare, and 
he seems to have mastered it fairly rapidly, because his next sentence il- 
lustrates nicely the distressing necessity for reconciliation which arose from 
his religious beliefs— or were they doubts? 

I hold indeed, that Science, by exhibiting to us the independent evidence 
of analogous convulsions, may well be cited, as removing from that narrative 
all objections arising from alleged antecedent improbability: but whether the 
diluvial traces we still observe geologically, be the vestiges of the Mosaic deluge, 
or whether that convulsion was too transient, etc., to leave such traces, is 
quite another question.''^ 

Upon the deluge itself, Adam Sedgwick's apostasy was even more un- 
compromising and his recantation as president of the Geological Society 
almost ostentatiously manly: 

Having been myself a believer, and, to the best of my power, a propagator 
of what I now regard as a philosophic heresy ... I think it right, as one of 
my last acts before I quit this Chair, thus publicly to read my recantation. 

We ought, indeed, to have paused before we first adopted the diluvian 
theory, and referred all our old superficial gravel to the action of the Mosaic 
Flood. For of man, and the works of his hands, we have not yet found a 
single trace among the remnants of a former world entombed in these 
deposits.^^ 

The imphcation is significant. If relics of humanity had been found 
in the debris of a destroyed world, then we would have evidence that the 
destruction had been wrought by the Mosaic flood which, as we know, 
drowned a great many human beings. Sedgwick never abandoned the idea 
of sudden and universal geological catastrophes even though the Biblical 
deluge could no longer be one of them. 

Buckland, at this time, was preparing to meet the uniformitarian chal- 
lenge in his contribution to the Bridgewater Treatises. Much was hoped 
for from him by the trustees and devotees of that once popular series, and 
in the meantime rumors of the course his thoughts were taking were 
eagerly seized upon in scientific circles. Even his opponents never doubted 
Buckland's original sincerity. "Although I am convinced," wrote Lyell, "he 
does not beheve his own theory now, to its full extent, he beheved it 
when he started it." ^^ And later, Lyell hears that Buckland has changed 
his plan again, and that his "mode of reconciling geology and Genesis in 
his B. Treatise has been approved of by the Oxford Professors of Divinity 
and Hebrew!" ^^ As it turned out, Buckland never mentioned the deluge 
in his treatise. Sedgwick, indeed, made an abortive effort to account for 
it as a product of the paroxysmal earthquakes postulated in Beaumont's 



THE UNIFORMITY OF NATURE 35 

mountain-uplift theories.'^^ But uniformitarians had no difficulty in block- 
ing this new tack of the diluvialists.^" The deluge was finished. 

All this meant, however, was that catastrophism had been deprived of 
its most popular catastrophe. Upon the larger question of the relations 
between scientific theory, natural causation, and religious truth, the atti- 
tudes which had given rise to diluvialism remained stubbornly unaffected 
by the demise of that specific interpretation. Sedgwick, for example, 
worked up to his criticism of Lyell by way of some general remarks on the 
laws of nature and the comprehension of them. He was then president 
of the Geological Society. His attack, wrote Lyell, was the "severest," and 
the one against which he must put forth all his energies in the second 
volume of the Principles.^^ "1 believe," declared Professor Sedgwick, "that 
... all the primary modes of material action, are as immutable as the 
attributes of that Being from whose will they derive their only energy." 
The basic laws of nature, the law of gravitation, for instance, or of atomic 
affinity, are few and simple and not yet all discovered. Uniformitarianism 
confounded imperfectly comprehended appearances with some such basic 
law. It was a Ptolemaic view of earth history. "It assumes, that in the 
laboratory of nature, no elements have ever been brought together which 
we ourselves have not seen combined; that no forces have been developed 
by their combination, of which we have not witnessed the effects." ^^ 
It circumscribes, in other words, God's operations by our ignorance of 
them. At this level, Sedgwick was a penetrating critic. 

As an irreducible minimum, catastrophists required that theories of nat- 
ural causation admit of direct providential application. In this demand, 
never explicitly formulated, lay the root of all the troubles, and to satisfy 
it the Oxford school followed Sedgwick onto very treacherous ground. 
Conybeare had calmed down a little by 1832, when, as first president of 
the new British Association's geological section, he delivered his annual 
charge : 

No real philosopher, I conceive, ever doubted that the physical causes which 
have produced the geological phenomena were the same in kind, however they 
may have been modified as to the degree and intensity of their action, by the 
varying conditions under which they may have operated at different periods. 
It was to these varying conditions that the terms, a different order of things, 
and the like, were, I conceive, always intended to have been applied; though 
these terms may undoubtedly have been by some writers incautiously used.®^ 

Conybeare continued to feel fundamental reservations about the identity 
of causes in degree and in intensity, while admitting that the Supreme 
Lawgiver always moved in similar ways His wonders to perform. These 
reservations ultimately prevailed, to be sure, though scarcely in the way 
Conybeare presented them. Still, he painted an impressive and persuasive 
picture of a progressing global surface, of a progressive set of creations, 



36 CHARLES COULSTON GILLISPIE 

each more complicated than the last. Sciences even had developed in a 
logically necessary order of succession, from astronomy to geology. Grad- 
ually the globe had changed from a fluid spheroidal mass to a solid crust, 
itself still cooling in paroxysms less and less intense. This globe bore a 
different aspect in every age. As natural forces degenerated from age to 
age, the planet was inhabited by successively more advanced animal 
creations. 

Sedgwick too had discovered in his meditations upon the organic crea- 
tion the most insuperable objections to uniformitarianism. 

And I ask you, have we not in these things some indications of change and 
of an adjusting power altogether different from what we commonly understand 
by the laws of nature? Shall we say with the naturalists of a former century, 
that they are but the sports of nature? Or shall we adopt the doctrine of 
spontaneous generation and transmutation of species, with all their train of 
monstrous consequences? ** 

Lyell, to be sure, devotes a chapter to combating successfully the latter 
speculation. "A doctrine may however be abused," thought Sedgwick, 
"and yet contain many of the elements of truth." 

I think that in the repeated and almost entire changes of organic types in 
the successive formations of the earth — in the absence of mammalia in the 
older, and their very rare appearance (and then in forms entirely unknown to 
us) in the newer secondary groups — in the diffusion of warm-blooded quad- 
rupeds (frequently of unknown genera) through the older tertiary systems — in 
their great abundance (and frequency of known genera) in the upper portions 
of the same series — and, lastly, in the recent appearance of man on the surface 
of the earth (now universally admitted) — in one word, from all these facts com- 
bined, we have a series of proofs the most emphatic and convincing, — that 
the existing order of nature is not the last of an uninterrupted succession of 
mere physical events derived from laws now in daily operation: but on the 
contrary, that the approach to the present system of things has been gradual, 
and that there has been a progressive development of organic structure sub- 
servient to the purposes of life.*^ 

This weighty sentence took Sedgwick onto very thin ice indeed, but 
William Whewell in the British Critic skated even closer towards the final 
catastrophe of the cataclysmic creed: 

It is clear . . . that to give even a theoretical consistency to his system, it 
will be requisite that Mr. Lyell should supply us with some mode by which we 
may pass from a world filled with one kind of animal forms, to another, in 
which they are equally abundant, without perhaps one species in common. He 
must find some means of conducting us from the plesiosaurs and pterodactyls 
of the age of the lias, to the creatures which mark the oolites or the iron-sand. 
He must show us how we may proceed from these, to the forms of those later 



THE UNIFORMITY OF NATURE 37 

times which geologists love to call by the sounding names of the paleotherian 
and mastodontean periods. To frame even a hypothesis which will, with any 
plausibility, supply this defect in his speculations, is a harder task than that 
which Mr. Lyell has now executed. We conceive it undeniable (and Mr. Lyell 
would probably agree with us,) that we see in the transition from an earth 
peopled by one set of animals, to the same earth swarming with entirely new 
forms of organic life, a distinct manifestation of creative power, transcending 
the known laws of nature: and, it appears to us, that geology has thus lighted 
a new lamp along the path of natural theology.^® 

So there the question stood. It could not have been more clearly stated. 
Deluc, Kirwan, Conybeare, Sedgwick, and Buckland might almost be de- 
scribed as having written the last chapter in a historical interpretation to 
which Orosius and Gregory of Tours had contributed the early volumes. 
Their approach to natural history in the half-century of their scientific 
leadership disguised its lust for the catastrophic and the miraculous more 
successfully, perhaps, than the Seven Books Against the Pagans, but the 
compulsion to exhibit examples of divine intervention was strong. 

Neptunists and catastrophists set themselves a task which ultimately 
proved self-contradictory. They accorded complete philosophic validity to 
whatever results Baconian induction might bring them; and they also re- 
quired these results to display the structure and development of the ma- 
terial world as the history of an intending Providence with a moral pur- 
pose, as physical evidence not only of God's power but of His will and 
His immediacy. However firmly they might insist that Genesis was not 
designed to teach the truths of science, or the Geological Society to teach 
the truths of morality, still truth, as Sedgwick felt, could not be incon- 
sistent with itself. The central thread of interpretation became finer and 
finer. One by one its strands were broken and the weight of demonstra- 
tion put upon those remaining— the six days of creation, the six-thousand- 
year span of earth history, the birth of our present globe in a primeval 
diluvium, the antiquity and original parentage of species, the dynamical 
efficacy of divinely ordained cataclysms, the flood itself. Finally, the con- 
ception of a divinity who must continually interfere with his arrange- 
ments in order to prove himself a governing force depended upon the 
immutability of different manifestations of life. This was the one re- 
maining strand. Publicists of the school of theological science rushed to 
hang upon it, and of course they hanged themselves with it. In Sedgwick's 
view, 

Geology, like every other science when well interpreted, lends its aid to natural 
religion. It tells us, out of its own records, that man has been but a few years 
a dweller on the earth; for the traces of himself and of his works are confined 
to the last monuments of its history. Independently of every written testimony, 
we therefore believe that man, with all his powers and appetencies, his 
marvellous structure and his fitness for the world around him, was called into 



38 CHARLES COULSTON GILLISPIE 

being within a few thousand years of the days in which we hve — not by a 
transmutation of species, (a theory no better than a phrensied dream), but 
by a provident contriving power. And thus we at once remove a stumbhng 
block, thrown in our way by those who would rid themselves of a prescient 
first cause, by trying to resolve all phenomena into a succession of constant 
material actions, ascending into an eternity of past time.^^ 

Lyell, said the catastrophists, by dramatizing the necessity for getting from 
one form of life to another and for explaining the unique character and 
recent appearance of man, had offered the final proof for the incessant 
and ubiquitous application of God's creative powers to a universe which 
He had ordained, and the natural laws of which were few, perfect, and 
simple; beneficent when properly understood; and unchangeable (except 
by God). 

In the 1830's, however, no one, not even Lyell, pressed the argument 
to its logical conclusion. It will have been noticed that the anti-Mosaic 
schools also labored under the conventional obligation to point out how 
their larger understanding put God's works into a more sublimely neces- 
sary focus than the puerile and unscientific approach of Neptunist or 
catastrophist. Further, there were certain received empirical boundaries 
which Huttonians and uniformitarians themselves never thought to ques- 
tion. Play fair and Smith admitted a flood; Lyell's 1830 man was recently 
created and his animal species unique and more permanent than the 
progressive creative installments on which Sedgwick took insecure refuge. 
Even the attacks, then, upon the theories of religious materialism did 
not spring from a point of view fundamentally opposed to the idea of 
apprehending the divine through the sort of manifestations which guide 
an engineering project. Vulcanism and uniformitarianism were simply 
further stages in the retreat from the rigorous fundamentalism of Kir- 
wan and from the philosophic inspiration of Priestley. 



NOTES 

1. Cuvier, Theory of the Earth, p. 12. 

2. Lyell, Principles of Geology, I, 164-165. 

3. Quoted in Geikie, Founders of Geology, p. 404. 

4. By 1830, the differences over the interpretation of river valleys in particular had 
almost reached the proportions of a major issue. Two incipient schools, "Fluvialist" 
and "Diluvialist," had developed and had prepared the positions which uniformitarian 
and catastrophist would occupy once Lyell made the disagreement explicit and funda- 
mental. For the fluvialist papers and the immediate empirical background of the 
uniformitarian theory, see, for example, P.G.S., I, 89-91, 170-171; T.G.S., 2d series, 
II, 195-236, 279-286, 287-292, 337-352. For the anticipatory diluvialist counter- 
attack, see P.G.S., I, 145-149, 189-192; T.G.S., 2d series, I, 95-102; 11, 119-130. 

5. Fleming, "The Geological Deluge, as Interpreted by Bacon, Cuvier, and Professor 
Buckland, Inconsistent with the Testimony of Moses and the Phenomena of Nature," 
Edinburgh Philosophical Journal, XIV (1826), 205-239, p. 208. 

6. Ibid., pp. 209-215. Like many ex-Wernerians, Fleming had turned to zoology after 



THE UNIFORMITY OF NATURE 39 

"geognosy" became discredited. His chief work, Philosophy of Zoology, stood very 
firmly indeed upon the "real existence" of species in nature. 

7. Scrope, Considerations on Volcanoes, pp. iii-ix, 242-243; Geology of Central France, 
pp. vii-viii, 197-213. For evidence of some concessions to the prevailing catastrophist 
theories, see the former of these titles, pp. iv-v, 215-218. 

8. K. M. Lyell, Life of Charles Lyell, I, 163: Lyell to his sister, 4 December 1825. 

9. Ibid., I, 170: Lyell to his father, 10 April 1827. See also II, 6-7: Lyell to Whewell, 
7 March 1837. 

10. 'Ibid., II, 6-7; Lyell, Principles of Geology, III, vii-xviii. 

11. Lyell, Principles of Geology, I, 72. 

12. Ibid., Ill, 1-6. 

13. Ibid., 1, 144. 

14. Ibid., pp. 63-65. 

15. [W. H. Fitton], "Mr. Lyell's Elements of Geology," Edinburgh Review, LXIX 
(1839), 406-466. The Elements of Geology was a condensation and popularization 
of the Principles. On the question of Lyell's debt to Hutton, see V. A. Eyles, "James 
Hutton (1726-1797) and Sir Charles Lyell (1797-1875)," Nature, CLX (1947), 
694-695. 

16. K. M. Lyell, Life of Charles Lyell, I, 269: Lyell to Scrope, 14 June 1830. 

17. Lyell, Principles of Geology, I, 76-82. 

18. Ibid.,p. 104. 

19. Ibid., Ill, 52-61. 

20. Though he did not think himself guilty of violating it, Lyell would have agreed with 
Sedgwick's canon, promulgated in criticism of the Principles: "The language of 
theory can never fall from our lips with any grace or fitness, unless it appear as the 
simple enumeration of those general facts, with which, by observation alone, we 
have at length become acquainted" ("Presidential Address" [1831], P.G.S., I, 302). 

21. Lyell, Principles of Geology, III, 272-273. 

22. Ibid., Ill, 270. See also I, 89. 

23. Ibid., Ill, 271-272. 

24. Ibid., p. 272. 

25. Ibid., II, 179. 

26. Ibid., -p. 1. 

27. Ibid., pp. 2-35. 

28. Ibid., p. 65. 

29. Ibid., p. 124. 

30. Ibid., pp. 179-184. 

31. Ibid., p. 270. 

32. Ibid., pp. 156-157. 

33. Ibid., 1, 145. 

34. Ibid., quoted from Sir Humphry Davy, Consolations in Travel, dialogue iii. 

35. Lyell, Principles of Geology, I, 145. 

36. Ibid., pp. 145-153. 

37. Ibid., pp. 155-156. 

38. Ibid., p. 156. 

39. K. M. Lyell, Life of Charles Lyell, I, 271 : Lyell to Scrope, 14 June 1830. 

40. Ibid., pp. 309-311: Lyell to Scrope, 9 November 1830. 

41. Lyell, Principles of Geology, I, 65. 

42. Ibid., pp. 65-66; on Voltaire and geology, see also White, Warfare of Science with 
Theology, 1,229. 

43. K. M. Lyell, Life of Charles Lyell, I, 271 : Lyell to Scrope, 14 June 1830. 

44. Ibid., p. 263: Lyell to his sister, 26 February 1830. 

45. Ibid., p. 173: Lyell to Mantell, 29 December 1827. See also Mantell, Journal, p. 75 
(12 March 1830). 

46. K. M. Lyell, Life of Charles Lyell, I, 273: Lyell to Scrope, 25 June 1830. 

47. [Scrope], "Lyell's Principles of Geology," Quarterly Review, XLIII (1830), 411-469. 
For Lyell's satisfaction, see K. M. Lyell, Life of Charles Lyell, I, 309-311: Lyell to 
Scrope, 9 November 1830. 



40 CHARLES COULSTON GILLISPIE 

48. For a somewhat highly colored account of the theological opposition, both in the 
1830's and later, see T. H. Huxley, "The Lights of the Church and the Light of 
Science," in Science and Hebrew Tradition, volume VI of Collected Essays (New 
York, 1894), pp. 201-238. 

49. See, for example, Sedgwick, "Presidential Address" (1831), P.G.S., I, 304; Whewell, 
History of Inductive Sciences, III, 616-617; and see also Whewell's remarks in his 
review in the British Critic, IX (1831), 180-206, especially pp. 184-186, 202-204. 

50. Two vols.; London, 1839. Nor did Murchison's reviewer in the Edinburgh Review 
make any application of the work to interpretative controveries (LXXIII [1841], 
1-41). 

51. Lyell, Principles of Geology, I, 76. 

52. For the first three months, volume I of the Principles, of which 1500 copies were 
printed, sold fifty copies a week. After this, sales, instead of declining, picked up 
enough to necessitate a second edition, of which Murray expected to sell one thousand 
copies in a year, but which in fact was bought up before that. In 1835, a third edition 
of the whole sold 1750 copies in ten months, and a fourth edition of 2000 was sold 
out in 1836 (K. M. Lyell, Life of Charles Lyell, I, 311-312, 383, 449, 464). 

53. For example, W. D. Conybeare, who headed the catastrophist counterattack: "The 
great interest of this treatise seems to me to arise from its necessary tendency to force 
the current of scientific attention ... to certain points of theoretical inquiry, for the 
investigation of which the science has been for some time growing more and more 
mature" {Philosophical Magazine, VIII [1830], 215). See also Whewell in the 
BrifishCrific, IX (1831), 180. 

54. Sedgwick, "Presidential Address" (1831), P.G.S., I, 302. 

55. K. M. Lyell, Life of Charles Lyell, I, 316-317: Lyell to Mantell March, 183L 

56. Mantell, Journal, pp. 122, 129 (1 May and 13 November 1835). 

57. The Geology of the South-East of England (London, 1833); The Wonders of 
Geology; or, A Familiar Exposition of Geological Phenomena (4th ed.; 2 vols.; 
London, 1840); and The Medals of Creation; or. First Lessons in Geology, and in 
the Study of Organic Remains (2 vols.; London, 1844). 

58. Illustrations of the Geology of Sussex (London, 1822). 

59. Mantell, Geology of the South-East of England, p. 1 5. In the terminology of diluvialist 
geology in the 1820's, materials more recent than tertiary formations were classified 
as alluvial and diluvial. The former means loose, superficial accumulations deposited 
by the mechanical action of surface water since the time of the retreat of the sea. 
The latter refers to materials deposited during a major inundation. 

60. Mantell, Wonders of Geology, I, 115-116. 

61. I&zd.,pp. 69-77, 114. 

62. Ibid., pp. 5-7. 

63. The reference is to Buckland's Bridgewater Treatise (see above, p. 212) . 

64. Mantell, Wonders of Geology, II, 115-116. 

65. A Geological Manual (Philadelphia, 1832 [1st ed.; London, 1831]); and How to 
Observe (London, 1835). For De la Beche's work in establishing the Geological 
Survey, see F. J. North, "Geology's Debt to Henry Thomas de la Beche," Endeavour, 
III (1944), 15-19. 

66. De la Beche, Geological Manual, p. 32. 

67. De la Beche, Resedrc/ies in Theoretical Geology (London, 1834), pp. 240-241. 

68. Geological Manual, pp. 131, 158-165, and passim; see also his "Notes on the Ex- 
cavation of Valleys," Philosophical Magazine, VI (1829), 241-248; "Notes on the 
Formation of Extensive Conglomerate and Gravel Deposits," Ibid., VII (1830), 
161-171; and an anonymous criticism of several of his papers, ibid., pp. 189-194. 

69. K. M. Lyell, Life of Charles Lyell, I, 317-318, 330, 444; Geikie, Life of Murchison, I, 
203, 266-267. 

70. K. M.LyeW, Life of Charles Lyell, I, 317-318: Lyell to his sister, 7 April 1831. 

71. Charles Daubeny, "On the Diluvial Theory, and on the Origin of the Valleys of 
Auvergne," The Edinburgh New Philosophical Journal, X (1830-31), 201-229, pp. 
204-205. 

72. K. M. Lyell, Life of Charles Lyell, I, 312: Lyell to his sister, 14 November 1830. 



MEASURING GEOLOGIC TIME 41 

73. Whewell, History of Inductive Sciences, III, 601-602. 

74. Conybeare, "On Mr. Lyell's Principles of Geology," Philosophical Magazine, VIII 
(1830), 215-219; "An Examination of those Phaenomena of Geology which Seem 
to Bear Most Directly on Theoretical Speculation," ibid., pp. 359-362, 401-406; IX 
(1831), 19-23, 111-117, 188-197, 258-270. 

75. Ibid., IX, 190. 

76. Sedgwick, "Presidential Address" (1831), P.G.S., I, 313. 

77. K. M. Lyell, Life of Charles Lyell, I, 276: Lyell to his sister, 9 July 1830. 

78. Ihid., pp. 445-456: Lyell to Fleming, 7 January 1835. 

79. Sedgwick, "Presidential Address" (1831), P.G.S., I, 311-313; Lyell, Principles of 
GeoZogy, III, 272-273. 

80. K. M. Lyell, Life of Charles Lyell, \, 328: Lyell to Fleming, 29 August 1831. See also 
II, 3-5: Lyell to Whewell, 7 March 1837. 

81. Ibid., I, 318: Lyell to his sister, 7 April 1831. 

82. Sedgwick "Presidential Address" (1831), P.G.S., I, 300-301. 

83. Conybeare, "Report on Geology," British Association Reports (1831-32), I and II 
(bound in one), 406. 

84. Sedgwick, "Presidential Address" (1831), P.G.S., I, 305. 

85. Ibid., pp. 305-306. 

86. "Lyell— Princi^Zes of Geology," British Critic, IX ( 1831 ) , 194. 

87. Sedgwick, A Discourse on the Studies of the University (4th ed.; Cambridge, Eng- 
land, 1835), pp. 26-27. 



Measuring Geologic Time* 

• ADOLPH KNOPF 



IN RECENT YEARS OUR EARTH HAS BEEN AGING A BIL- 

lion years each decade. Between the beginning of the present century and 
1930, an age of the earth of 100 milhon years had become generally ac- 
cepted. In that year it was suggested that, in the light of the new dis- 
coveries of geology and radioactivity, the earth is at least 2000 milhon 
years old ( J ) . Now, we are envisaging an age of 4500 million years, and the 
end of the enormous lengthening of time appears to be in sight. Astron- 
omers had estimated that the universe began to expand 1860 million 
years ago. However, this figure became geologically unacceptable when 
it became apparent that it was less than the age of the oldest rocks on our 
own planet. Recently, the distance of the Andromeda nebula was re- 
determined at Palomar and was found to be twice as great as it had pre- 
viously been calculated to be. The distance of the Magellanic Cloud was 
also redetermined and set at twice the earlier figure. Since these distances 

• From Scientific Monthly (Nov., 1957), pp. 225-36. 

* For convenient reference, a Geologic Time Scale is given in the Appendix, (ed.) 



42 ADOLF KNOPF 

were the yardsticks for measuring the extragalactic distances, all distances 
were doubled. These findings, together with Hubble's rate of recession of 
the galaxies, indicate that the age of the universe is 4000 million years— 
a figure which is in much better agreement with the age deduced from 
the rocks of our earth than previous estimates had been. 

Our concept of geologic time has thus been increasing enormously, 
and this extension is a remarkable item in the history of ideas. At this 
point I may allude to the well-known estimate made by the Anglican 
Archbishop Usher, in 1654, that the earth was created in 4004 b.c. (Later, 
this estimate was improved upon and refined by the learned John Light- 
foot, vice chancellor of Cambridge University, the greatest Hebrew scholar 
of his day. He declared that God had created Adam out of the dust of 
the earth on the morning of Friday, 17 September at 9 o'clock. I have 
this information from Shotwell's absorbing book on The History of His- 
tory) (2). It was therefore something of a surprise to find Shakespeare's 
Rosalind saying, in As You Like It, which was produced in 1599, more 
than 50 years before the archbishop's pronouncement, "The poor world is 
almost 6000 years old, and in all this time there was not any man died 
in his own person, videlicet, in a love cause." Had Rosalind taken a 
modern elementary course in geology, she would have soon felt that she 
had made a grievous understatement! 

How ingrained had become the belief that the earth was 6000 years 
old is shown by the first proposal ever made to measure the age of the 
earth quantitatively. In 1715 the Astronomer Royal, Edmund Halley. 
wrote "A short account of the Cause of the Saltness of the Ocean; with 
a proposal, by help thereof, to discover the age of the world." He sug- 
gested that if the saltness of the ocean were measured at intervals of a 
few centuries, the rate of increase, and therefore the age of the ocean, 
could be determined. He lamented the ancient Greek and Latin authors 
had not handed down to us a record of the degree of saltness of the sea 
as it was some 2000 years ago, for, said he, "it can not be doubted but 
that the difference between what is now found and what then was, would 
become very sensible." Perhaps, he prophesied, the world would be found 
to be much older than many had hitherto imagined. The assumption 
that underlay Halley's proposal was that the ocean increases in saltness 
at a rate that is measurable in terms of human records. 

Nearly 2000 years elapsed, however, before a method was devised to 
measure the age of the ocean in years. In 1899 the brihiant Irish geologist 
Joly (3) estimated the age of the ocean, in years, as follows. The amount 
of sodium carried to the ocean each year by the rivers of the world is 
accurately known. If, then, we divide the amount of sodium in the ocean 
by the amount brought to the ocean annually, we have the age of the 
ocean — 90 million years. There is a very seductive simplicity about this 



MEASURING GEOLOGIC TIME 43 

estimate. However, in making it, several assumptions had to be made. 
The greatest is that the rate at which the rivers have been wearing down 
the continents and bringing sodium in the form of salt to the ocean has 
been constant. As a matter of fact, the earth has recently passed through 
an epoch of widespread mountain-making, as a result of which the conti- 
nents stand relatively high above sea level. The wearing away of the lands 
by erosion has therefore been speeded up, and the sodium that it thus 
released from the rocks is carried to the sea more abundantly than was 
the case during most of geologic time. How much faster is the present 
rate at which sodium is being delivered to the sea cannot be even roughly 
determined. However, because of the apparent logical rigor of this method, 
the figure of 90 or 100 million years for the age of the earth became 
generally accepted, and, in fact, long interfered with the favorable re- 
ception of far greater estimates based on new discoveries. 

CHRONOLOGY OF ROCKS OF THE EARTH'S CRUST 

The oldest method of measuring geologic time is by determining the 
thickness of the beds laid down during that time and multiplying the 
thickness by the rate at which these beds are supposed to have been de- 
posited. In 1905, Sollas (4) estimated that 265,000 feet of strata had ac- 
cumulated since the beginning of Huronian time, which was then thought 
to be near the beginning of geologic time. The figure of 265,000 feet was 
obtained by adding together the maximum thicknesses of the strata that 
were deposited during each of the successive geologic periods. Sollas was 
greatly impressed by Kelvin's estimate of 20 to 40 million years as the age 
of the earth. "Once more geology is put under bondage, not however as 
in her youth, tethered to a mere 6000 years, but free to roam through 
the ample magnitude of 30,000,000 years." By taking the rate of accumula- 
tion as 1 foot in a century, "as the evidence seems to indicate," Sollas con- 
cluded that more than 26 million years had elapsed during the time in 
which the 265,000 feet of strata were accumulating. 

Since nowhere has 265,000 feet of strata been laid down in one place, 
geology, in building up such a "geologic column," is obliged to use the 
methods of stratigraphy and paleontology. The application of these meth- 
ods to the geologic record has recently been presented by Stubblefield (5). 
From the beginning of the Cambrian onward, the sedimentray rocks con- 
tain fossils by means of which the sequence of the strata in time can be 
established. From this fact it follows that other means must be used in 
determining the age and succession of Precambrian rocks than those that 
are used for the Cambrian and younger rocks. For the Precambrian rocks, 
the methods of absolute age dating made possible by the numerous 
methods based on radioactivity have become essential; for the younger 



44 ADOLF KNOPF 

rocks, stratigraphy and paleontologic control have estabhshed a remarka- 
ble geochronology. It has been a chronology without years, however, and 
one of the chief purposes of this article is to show what progress in abso- 
lute dating has been made. 

The major time units since the Precambrian eon are the eras Paleozoic, 
Mesozoic, and Cenozoic. The strata that represent these eras are divided 
into systems, beginning with the Cambrian system. The time during which 
the strata that comprise a system were formed is called a "period." Most 
of the systems are subdivided on a paleontologic basis into "Lower," "Mid- 
dle," and "Upper." Thus, we have Lower, Middle, and Upper Cambrian 
and, in time phraseology. Early, Medial, and Late Cambrian; but the 
distinction between the strata and the time represented by them is more 
often honored in the breach than in the observance. It is, for example, 
widely customary to speak of "Lower Cambrian rocks" and "Lower Cam- 
brian time." 

The smallest unit of time is represented by a "zone." Formally, a zone 
is the smallest thickness of strata that is characterized by the presence 
of a distinctive flora or fauna. The same zone may range in thickness, 
from place to place, from a few inches to hundreds of feet. The associa- 
tion together of fossils of several species is more essential to the defining 
of a zone than is the presence of one or two particular species, because 
any one species may have a range in time, from district to district, on ac- 
count of migration or of differing environments. For convenience, each 
zone is named for a particular fossil (6). An ideal index fossil has four 
features: (i) It has a short vertical range (indicating that the lifetime of 
the species was short); (ii) it has a wide horizontal range; (iii) it is inde- 
pendent of lithic facies— that is, it may occur in sedimentary rocks of 
widely different composition; and (iv) it can be easily recognized (7, p. 12). 

For construction of a single time scale of world-wide applicability, 
zones are not suitable, because they are generally too local in geographic 
extent and in vertical range. "For correlation over long distances, where 
zones are horizontally too restricted and vertically too precise, a larger 
stratigraphic unit is required. It must correspond to groups of zones and 
be capable of universal extension by means of overlapping correlations, 
although based ultimately upon a standard zonal succession at a type 
locality or in a type area" (8). This larger stratigraphic unit is called 
a stage. 

These procedures have undoubtedly, so far, been most successfully ap- 
plied to the Jurassic system (7). This system has been divided, in Eng- 
land, into ten stages, from Hettangian (Jl, 9) to Portlandian (JIO), com- 
prising 58 ammonite zones, and a final stage, the Purbeckian (JH), which, 
having been laid down in fresh water, contains no ammonites and has 
been subdivided therefore on the basis of ostracods into three zones 
(7, p. 19). 



MEASURING GEOLOGIC TIME 45 

An urgent task for geology is to determine, in years, the length of the 
eras, periods, and "ages" (time spans of the stages) and, eventually of 
the zones. Not a single one of them— eras, periods, and ages, let alone 
zones — has yet been reliably determined. This statement is possibly sur- 
prising in view of the fact that almost any modern writer can produce a 
geologic timetable that gives precise datings and lengths of the eras and 
systems and even of some of the smaller subdivisions [Holmes {10); Kay 
(JJ); Schindewolf (U); Sonder (U)]. Sonder, in fact, gives the absolute 
lengths of the stages of the Permian, Triassic, Jurassic, Cretaceous, and 
Tertiary. These figures have been obtained in various remarkable ways. 
Ultimately, however, they are tied to three dates based on atomic dis- 
integration: 60 million years, the age of the pitchblende at Central City, 
Colorado; 220 million years, the age of the pitchblende at St. Joachims- 
thal, Bohemia; and 440 million years, the age of the uranium-bearing 
shale at Gullhogen, Sweden. The age of the Swedish shale is the only 
one of these that is paleontologically controlled, by the occurrence in the 
shale of Late Cambrian trilobites, which are correlated with the middle 
Franconian of the North American time scale [Howell and Lochman, 
Westergard, Berg (J2)]. The other two— Colorado and St. Joachimsthal 
—are less securely tied into the biochronologic scale. 

All other absolute ages have been derived from the three radioactive 
tie points by interpolation based on thickness of strata or by "reasoned 
guesses," to use the phrase employed by Simpson {13) in explaining how 
he constructed his absolute time chart for the Tertiary. Holmes in 1947 
(JO) built two time scales, called "A" and "B," "based on maximum 
thicknesses and control points fixed by lead-ratios." The B scale is re- 
garded by him as the more probable, but the geologic evidence appears 
to support more strongly parts of the A scale. There are three difficulties 
in building up such scales: (i) The boundaries between the systems are 
controversial— for example, between Devonian and Carboniferous, be- 
tween Triassic and Jurassic, between Cretaceous and Paleocene; (ii) the 
control points, except for that of the Swedish shale, are not precisely 
located; and (iii) the thicknesses of strata are not rehable measures of 
time. 

In 1905 Sollas (4) obtained 183,000 feet as the maximum thickness of 
strata accumulated since the beginning of Cambrian time; in 1931 Schu- 
chert (J4), in assembling the data for North America alone, got 259,000 
feet and expressed the conviction that, when the world's maximum thick- 
nesses have been compiled, these will total 400,000 feet. In 1947 Holmes 
{10) obtained 387,000 feet as a total. Kay {IS), in 1955, presented data 
much more nearly complete than any that had been previously assembled, 
and these aggregate at least 398,000 feet— a figure almost identical with 
that predicted by Schuchert in 1931. Although the total given by Holmes 
and the total based on Kay's data are substantially alike, they are summa- 



46 ADOLF KNOPF 

tions of items of considerably differing magnitudes; for example, the Si- 
lurian is credited by Kay with a maximum of 33,000 feet but with only 
20,000. feet by Holmes; the Oligocene, with 26,000 feet by Kay and with 
15,000 feet by Holmes; and the Miocene, with 14,000 feet by Kay and 
with 21,000 feet by Holmes. Kleinpell {16) gets 24,000 feet as a complete 
and unbroken sequence through the marine Miocene of California. When 
eventually a new summation of thicknesses is prepared— one that is based 
on the stages of the systems, paleontologically controlled— a much greater 
total than 400,000 feet will undoubtedly be obtained. 

The great differences in the estimates of maximum thickness of many 
of the systems manifestly indicate that thicknesses are unreliable measures 
of geologic time. As long ago as 1936 the conclusion had already been 
reached by Twenhofel {17) that estimates of time based on thicknesses 
of strata "are hardly worth the paper they are written on," and he presents 
detailed evidence in support of this revolutionary concept. Limitation of 
space prevents further marshaling of evidence here. 

The nearly insuperable obstacle that one encounters in using thick- 
nesses of rocks as measures of geologic time is the fact that the rocks 
generally give no internal evidence of the rate at which they were formed. 
Only a very few show a thin layering, or lamination, in which each lamina 
represents the sediment laid down in a year. Of the rocks that show such 
an annual lamination, those that have been studied most thoroughly are 
the Green River shales of Eocene age in Wyoming and Colorado. These 
annual layers— verves in their technical name— average less than 1/2000 
foot in thickness, and since the Green River shales are 2600 feet thick, 
the time represented by their accumulation is about 6 million years. Green 
River time, which is possibly, but far from assuredly, one-third of the 
Eocene, is the longest span of time that has so far been measured by 
means of data obtained from the sedimentary history of the rocks them- 
selves. This span of 6 million years is compatible with the great length 
of geologic time indicated by radioactive evidence, but there has as yet 
been no direct verification of the length of Green River time by radio- 
active methods. No one has yet measured the beginning and the end of 
Green River time by radioactive evidence, or even the beginning or end 
of the Eocene or of any other subdivision of geologic time. However, the 
methods of determining absolute ages have now become so numerous 
and are becoming so highly perfected that it will not be long before the 
lengths of the geologic time units will be accurately determined. 

HELIUM METHOD 

The helium method was the first of the methods based on atomic dis- 
integration to be used to measure geologic time. Helium was early rec- 
ognized to be a stable end-product of the radioactive transformation of 



MEASURING GEOLOGIC TIME 47 

uranium and thorium. Strutt (18) determined the amount of uranium 
contained in certain minerals (or the amount of thorium in thorianite) 
and the amount of hehum held in the minerals, and, having measured 
the rate of production of helium from uranium and thorium, he was able 
to calculate the "ages" of the minerals. These pioneer datings of minerals 
of geologically known ages showed that the ages determined from the 
helium content fell into the proper geologic time sequence. Some of the 
Precambrian minerals gave astonishingly high "ages," far higher than was 
then considered to be probable. For example, Precambrian zircon and 
sphene gave "ages" of 600 and 700 milhon years. The great age that was 
thus indicated was soon realized to be a minimum, because much of the 
helium formed in the minerals had leaked away. Zircon was found to re- 
tain only about one-third of the helium generated from the uranium and 
thorium contained in it. 

As the "lead method" of measuring geologic time grew in strength, the 
helium method, which gives only minimum values for the ages of miner- 
als, fell into disuse. In 1928 Paneth devised a technique whereby quanti- 
ties of hehum as small as 1/1,000,000 cubic centimeter could be accurately 
measured. In the new helium method based on this technique, only miner- 
als and rocks containing minute — one might say almost infinitesimally 
small— quantities of radioactive matter were selected. It was thought that 
the minute amount of helium generated in the mineral would be wholly 
retained in the mineral. Many rocks were examined by the new technique, 
and their ages, in years, were determined. Many of these ages appeared to 
be geologically acceptable. 

In 1940, however, the new hehum method collapsed. It was shown 
that if a rock is separated into its constituent minerals, there is marked 
difference in the ages given by the various minerals. For example, when 
the Palisade diabase was separated into its constituent minerals— plagio- 
clase, pyroxene, and magnetite — the plagioclase gave an age of 36 million 
years, the pyroxene, of 103 million years, and the magnetite of 134 million 
years. Manifestly the magnetite had retained more of the radiogenic helium 
formed within it than had the pyroxene and the plagioclase. When the 
magnetite was drastically purified by the removal of all adherent minerals 
(which are more radioactive than the magnetite), the indicated age of the 
magnetite was increased to 170 million years (19). Later work by Hurley 
{20) has cast doubt on the foregoing explanation, because determinations 
of age made before and after the minerals have been given an acid treat- 
ment suggest that all of them contain hehum commensurate with their 
ages. A granite that gave by the helium ratio an age of 68 million years 
gave, after an acid wash, an age of 200 million years. 

Because of such uncertainties about the helium age determinations, the 
method has again fallen into nearly complete disuse. One of the few 
recent determinations is that made by Gentner et al. (2J) in 1954 on a 



48 ADOLF KNOPF 

potassium salt from Alsace, of early Oligocene age; they obtained an age 
"of only 10 million years," but after allowing for loss of helium by dif- 
fusion and the speeding up of this diffusion by the formerly higher tem- 
perature of the potash-salt bed, the indicated age increased to 25 milHon 
years. Because of the assumptions and corrections that are necessary, this 
figure, which appears to be low for early Oligocene, does not carry much 
conviction. 

LEAD METHOD 

In 1905, Boltwood, of Yale University, suggested that lead is the ulti- 
mate product of the radioactive breakdown of uranium (22). This sug- 
gestion, sensational in its day, resulted from Boltwood's recognition that 
lead is invariably present in all uranium minerals. From the chemical 
analyses of 43 uranium minerals obtained from all parts of the world, 
Boltwood, in 1907, showed that the geologically older uranium minerals 
contain more lead than the younger minerals and that those of like geo- 
logic age have a like lead-uranium ratio. Boltwood then paid his debt to 
geology by giving us what has become known as the lead method of- 
measuring absolute geologic time {23). In this pioneer attempt he ven- 
tured to compute the ages of ten minerals. These ages ranged from 410 
million years for a uraninite from Connecticut to 2200 million years for 
another uranium-bearing mineral from Ceylon. These were stupendous 
figures, and they were not readily believed. Among those who soon ac- 
cepted them, however, were Joly, Holmes, and Barrell. 

In 1911 Holmes (24) began the great task of constructing an absolute 
geologic time scale. Boltwood had omitted to give the geologic ages of 
his analyzed radioactive minerals, and Holmes began to supply the de- 
ficiency. The amount of coordinated data was painfully small— that is, 
there were but few uraninites or other highly radioactive minerals whose 
geologic ages were accurately established and whose chemical composi- 
tion had been accurately determined. As a matter of fact, this difficulty 
is still with us, for uraninites and other highly radioactive minerals almost 
invariably occur in pegmatite dykes and veins. Consequently, their geo- 
logic age cannot, in the nature of things, be accurately determined. At 
this time also (1913), Holmes wrote the first of his illuminating accounts 
on the age of the earth {2S), which culminated, in 1956, in his paper 
"How old is the Earth?" {26). His answer is 4500 million years. The un- 
reserved acceptance in 1917 by Barrell— in his classic paper on "Rhythms 
and the measurements of geologic time" (27)— of the new and immensely 
longer time estimates based on radioactivity helped to pave the way for 
eventual acceptance of the longer time estimates. 

Because of the manifest reluctance of geologists and others to accept 
the immense figures based on atomic disintegration, A. C. Lawson, chair- 
man of the Division of Geology and Geography of the National Research 



MEASURING GEOLOGIC TIME 49 

Council, appointed, in 1923, a Committee on the Measurement of Geo- 
logic Time by Atomic Disintegration, "to see what it is all about." Under 
the able chairmanship of A. C. Lane, this committee, consisting of chem- 
ists, geologists, and physicists, actively stimulated research and promoted 
the fruitful cooperation between the investigators of the widely different 
disciplines that is necessary to solve the problems involved. Among its 
activities the committee published an annual report, in which the growth 
of the subject can be followed, and also, annually, a highly useful "An- 
notated bibliography of articles related to geologic time." 

Almost at the moment that the committee was getting under way, 
F. W. Clarke, chief chemist of the U.S. Geological Survey and author of 
the famous Data of Geochemistry, announced, "It is now plain that the 
uranium-lead ratio is of very questionable value in determining the age of 
minerals" (28). This reluctance on the part of Clarke to accept the great 
ages that were indicated by the uranium-lead ratios was undoubtedly due 
to the fact that he had been engaged for several decades in improving 
and refining Joly's estimate of 90 million years as the age of the ocean 
and had reached the figure 99,143,000 years, or in round numbers, 100 
million years. 

The lead method of determining absolute ages has, nevertheless, steadily 
grown in strength since it was first proposed by Boltwood. It has had 
some extraordinary and wholly unforeseen developments, but all of them 
have strengthened the method. In the first place, some years after Bolt- 
wgod^^nnounced that lead is the stable end-product of the radioactive 
disintegration of uranium, thorium also was found to yield lead as a stable 
end-product, and this fact has to be taken into account. Furthermore, 
uranium was discovered to consist of two isotopes, both of them radio- 
active; one has an atomic weight of 238 and is therefore called "uranium- 
238" and the other is of atomic weight 235, the now famous uranium- 
235. Both are generating lead, but at greatly different rates; the U^fL. 
produces lead six times as fast as the U-^^. Moreover, the atomic weights 
of the resultant leads differ. Uranium-238 produces a lead isotope of atomic 
weight 206, and U^^^ produces a lead isotope of atomic weight 207. 

Most of the radioactive minerals used in determining ages contain not 
only U^^^ and U^^^ but also thorium, which, as I have just mentioned, is 
~also producing lead. The thorium-derived lead has an atomic weight of 
>^08. Thus, when the chemist extracts the lead from such a radioactive 
mineral, the lead that he obtains— the so-called "radiogenic" lead— con- 
sists of a mixture of three isotopes of lead of atomic weights of 206, 207, 
and 208. It remained for the physicist to devise a means by which the 
proportions of these three leads can be determined. This was done by 
Aston, and the first mass spectrum of a radiogenic lead was obtained by 
him in 1929. It is now standard practice to have such a mass spectrum 
made in all reliable age determinations. 



50 ADOLF KNOPF 

When amounts of uranium and thorium in a mineral have been accu- 
rately determined, we have four sets of data from which the age of the 
mineral can be calculated. These data are the following: Pb^os/U^^s, 
p]3207/u235^ Pb208/'ph232^ ^^d Fh^^^Fh^^^. Whcn the four calculated ages 
agree, we can have full confidence in the indicated age. 

The ratio between the two radiogenic leads Pb^o^ and Pb^^e was re- 
garded, by Nier (29), as being the most reliable index of age. Since the 
two leads have very nearly identical chemical properties, their propor- 
tionality is not likely to have been altered by any geologic vicissitudes, such 
as weathering, oxidation, hydration, and leaching, that might have af- 
fected the mineral in which they occur. An age determination based on 
this ratio is called the "lead-lead" method. An advantage of this method 
is the fact that neither uranium, thorium, nor lead needs to be determined 
and that all the work can be done in one laboratory. Recently the un- 
precedented number of 96 age determinations was made by this method 
at the University of Toronto. But for definitive results, experience shows 
that uranium, thorium, total lead, and the isotopic composition of the 
lead must be determined. 

A recent example of age determination is afforded by the work done 
on uraninite from the Bob Ingersoll pegmatite, in the Black Hills, South 
Dakota {30). The analytical results in weight percentages are U, 64.55 ± 
0.64 (average of two determinations by different methods); Th, 2.93 ± 
0.04; and Pb, 17.01 ± 0.5. The ages are shown in Table 1. 

Three of the calculated ages agree, but that based on the Pbsos/Th^^s 
ratio is discrepant; the reason for this is not known. Wetherill et al. 
conclude that "when the U^ss-Pb^o^ and U^ss.pbsor ages agree for a fresh 
sample of uraninite, this age is probably the true age of the mineral." 

The lead method was greatly strengthened when, in 1938, Nier es- 
tablished the fact that all common lead— called also "ordinary lead" and 
"ore lead"— of whatever geologic age and provenance contains the isotope 

TABLE 1 

Ages of uraninite from the Black Hills, South Dakota (in millions of years) . 

Pb206/U238 pb207/U235 pb207/pb206 pb208/Th232 

1580 '' 1600 ' 1630 1440 

204 along with the predominant isotopes 206, 207, and 208. Since the 
isotope 204 is not of radiogenic origin, its presence in the lead formed 
within a radioactive mineral indicates that the radiogenic lead is "con- 
taminated"— in other words, that some common lead had become enclosed 
in the radioactive mineral at the time the mineral was formed. The con- 
taminating lead would make the calculated age too great, and it must be 
allowed for. To make the proper correction, especially if the correction is 
a considerable one, an isotopic analysis of the common lead that had been 



MEASURING GEOLOGIC TIME 51 

deposited in the same district and at the same time as the radioactive 
mineral must be used. The necessity for this rigorous requirement has only 
been recognized within the past several years. 

Nier also made the remarkable discovery that the relative abundances 
of the isotopes of common lead, regardless of geologic age and geographic 
source, differ considerably in spite of a nearly constant atomic weight of 
207.21. In a broad way, the older the leads are, the smaller is the total 
proportion of Pb^oe, Fh^^\ and Pb^os relative to ?h^^\ Manifestly, the 
common lead had been associated with uranium and thorium somewhere 
in the depths of the earth before it was deposited, some time later, as 
galena (PbS) in the place where we now find it. Thus, the common lead 
had become contaminated with radiogenic lead. 

By extrapolating backward to the time when the amount of admixed 
radiogenic lead isotopes was zero. Holmes (26) has obtained the composi- 
tion of the common lead when the substance of earth first became dif- 
ferentiated into crust, mantle, and core. That time was 4500 milhon years 
ago, and it can be regarded as marking the beginning of geologic time. 
Less hypothetical are the conclusions based on the isotopic composition 
of the common lead from the Rosetta Mine, Transvaal, Union of South 

TABLE 2 
Istopic composition of lead from the Rosetta Mine, Transvaal, Union of South Africa. 



Pb204 


pb206 


pb207 


Pb208 


Reference 


LOO 
LOO 


12.65 
12.58 


14.27 
14.11 


32.78 
32.77 


Russell etal. {31) 
Bate and Kulp {55) 



Africa. Its isotopic composition is given in Table 2. From these data Rus- 
sell et al. {31) computed the age of the Rosetta galena as being 2950 ± 
70 million years; Holmes and Cahen (32), as 3380 milhon years. 

Russell et al. think that galenas older than 1000 million years can well 
be dated by means of the isotopic composition of their leads, but Houter- 
mans, Geiss, Ehrenberg, and others have dated many leads that are much 
younger, even as young as late Tertiary. Generally, the age thus calculated 
does not coincide with the geologic age of the deposit in which the galena 
occurs. The suggestion has therefore been made that this age (p) denotes 
the time at which the ore-forming solution separated from the magma 
and the time, consequently, after which it was not subjected further to 
change by addition of radiogenic lead. Consequently, p can agree with the 
geologic age of the ore only if the ore had been immediately deposited. 
If the ore was not formed immediately after its constituents had sep- 
arated from the magma, then p (the "magmatic age" of the ore) is 
greater than the geologic age of the ore body in which it now occurs. This 
hypothesis can manifestly be improved geologically; at any rate, we can 



52 ADOLF KNOPF 

appreciate what a powerful tool the isotopic composition of common 
lead gives us in deciphering the origin of lead ore deposits. 

A variant of the lead method, devised in 1952 by Larsen, Keevil, and 
Harrison [33) as a rapid means of determining the age of rocks, is known 
as the "Larsen" or "lead-alpha" method. Zircon is the mineral that is 
chiefly used, on the theory that, because the atomic radius of zirconium 
(0.82A) differs so much from that of lead (1.32A), the zircon would con- 
tain no primary lead which it might have acquired during the magmatic 
consolidation of the igneous rocks in which it occurs. A spectograph is 
used to determine the amount of lead in the zircon, and alpha counters 
are used to determine the amount of helium given off per milligram of 
zircon per hour. The approximate age is given by 

t = CFh 



where C = 2480, Pb equals lead in parts per million, and a equals num- 
ber of alpha particles emitted per milligram, per hour. The results have 
proved to be uncertain, however. When the isotopic composition of the 
lead is ascertained, the four ages that are then calculable are, as a rule, 
highly discrepant. Zircon from the granite at Cape Town, Union of South 
Africa, gives the results shown in Table 3. 



Calculated ages of 


zircon 


from Cape 


TABLE 3 

Town, Union 


of South Africa 


(in 


millions of years). 


Pb206/U238 




Pb207/U 


235 




Pb207/pb206 




Pb- 


08/J'b232 


330 




354 






525 






237 



Nicolaysen (34) has recently made a careful study to determine the 
cause of these discrepancies. He concludes that "if these zircons crystal- 
lized 590 million years ago, and a constant diffusion coefficient has gov- 
erned the loss of lead isotopes throughout the history of the mineral, then 
the present pattern of 'discrepant' lead-uranium and lead-lead ages would 
result." 

The unreliability of the lead-alpha method, when applied to zircon may, 
in some cases, be due to the fact that the host rock (generally granite) 
may have been formed by the fusion of sedimentary rocks at the bottom 
of a down-folded geosyncline, or may have been modified by the melting 
of zirconiferous xenoliths. Some such explanation is indicated by the re- 
markable results obtained by Schuermann et al. (35) in processing 2000 
kilograms of the Lausitz granodiorite of Germany. Zircon was found to 
occur in two distinct varieties, one of which gives a provisional age of 
280 million years and the other, of 550 milhon years. 



MEASURING GEOLOGIC TIME 53 

RUBIDIUM-STRONTIUM METHOD 

Rubidium has long been known to be radioactive; it gives off beta rays 
and, consequently, it was known, from the theory of radioactive trans- 
formations, that nibidium-87 changes to strontium-87. In 1938 Strassmann 
and Walhng {36) isolated the radiogenic strontium from a rubidium- 
bearing lithium mica (lepidolite) that they had obtained from south- 
eastern Manitoba from a pegmatite known, by the lead method, to be 
about 2000 million years old. The strontium proved to be nearly 100 
percent pure Sr^^, whereas ordinary strontium consists of four isotopes, of 
which Sr^^ constitutes only 7.02 percent. The half-life of Rb^^ was cal- 
culated to be 6.3 X lO^'' years, and Hahn and Walling suggested that a 
new method was now available for dating rubidium-bearing minerals and 
rocks. They were optimistic about the potentiality of the strontium 
method, especially for determining the ages of ancient Precambrian rocks. 
More recently, as the result of the invention of refined techniques— the 
isotope dilution method, in particular— rocks as young as 60 million years 
have been measured. The strontium method has an advantage in that only 
a single transformation is involved in the change of Rb^'^ to Sr^'^. Another 
advantage is the fact that rubidium is widely distributed in potassium 
feldspars and micas, albeit in small amounts; this makes it possible to date 
many more rocks than is possible by the lead method. 

Ahrens, beginning in 1946, was the first to employ the strontium 
method and, in the succeeding years, made a large number of age meas- 
urements by optical spectrographic methods {37). This investigation 
showed extremely great ages in the older portions of the earth's crust, 
especially in Southern Rhodesia and adjacent regions— ages of between 
2000 and 3000 milhon years. In 1952, age determinations were first made 
(by investigators of the Department of Terrestrial Magnetism and the 
Geophysical Laboratory, both of the Carnegie Institution of Washington) 
by means of a new method in which stable isotope dilution and mass 
spectrometric techniques were used. Within a short time it became appar- 
ent that the rubidium-strontium method was giving much greater ages 
than those that were obtained by the lead method. The figure for the half- 
hfe of rubidium that was being used— as high as 6.42 X 10^° years— was 
found to be too great. If the rubidium-strontium ages of micas and micro- 
clines are calculated on the basis that the half-life of Rb^'^ is 5 X 10^° 
years, and if these are compared with the concordant uranium-lead ages 
obtained for uranium minerals that occur in the same pegmatites and that 
are therefore of the same age, excellent agreement is found, as is shown by 
Aldrich {38). Later in 1956, Huster and Rausch were reported to have 
determined, by direct counting experiments, that the half-life period of 
Rb^'^ is 4.9 to 5.0 X lO^^ years. 



54 ADOLF KNOPF 

A momentous advance in the use of the rubidium-strontium method was 
made in 1956 by Cormier et al. (39). Eight glauconites, from six different 
geologic horizons, were measured by a mass spectrometric isotope dilution 
method. The ages obtained range from 60 million years, for a Paleocene 
glauconite, to 470 million years for one of Lower Cambrian age. These 
ages have been computed on the basis of the newly accepted value for 
the half-hfe of Rb^"^ : T^^ = 5 X 10^° years. 

The Lower Cambrian glauconite was obtained from the Olenellus-hear- 
ing glauconite beds that constitute the top of the St. Piran sandstone on 
Mount Whyte, west of Lake Louise, Alberta, Canada (40). The great 
significance of the age measured— 470 million years— is that it is the only 
reliable absolute age determination we as yet have that is close to the be- 
ginning of Cambrian time. Since it is but a single determination, however, 
it is of only provisional value. It strengthens, however, the belief that the 
Cambrian began approximately 500 million years ago. 



POTASSIUM-ARGON METHOD 

In 1905, potassium was discovered to be feebly radioactive; it was found 
to emit beta rays. Later, in 1928, it was found to give off gamma rays as 
well. Not until 1937 was it discovered that all the radioactivity of po- 
tassium results from the decay of the isotope potassium-40, which con- 
stitutes but a minute fraction of the element potassium— approximately 
1/8400. The K**^ undergoes a dual transformation— one part decays to 
calcium-40 and one part to argon-40. How much changes to Ca^° and how 
much to A'**' (determined by the branching ratio e /0 ) has been diffi- 
cult to measure accurately. The latest figure for this ratio {41) is 0.1235. 
The half-life of K^o is 1310 million years. 

The conclusion of Weizsacker in 1937, from theoretical considerations, 
that A^" is one of the products of the radioactive disintegration of potas- 
sium was verified by Aldrich and Nier (42) in 1948. They showed that 
four potassium-bearing minerals, ranging in age from 200 million years to 
1600 million years, contained radiogenic argon in appropriate amounts, 
and they then suggested that a new method for determining the ages of 
rocks was possible. The technique for measuring the amount of argon that 
is radioactively produced in minerals has since been greatly refined, and 
thus the potassium-argon method of dating rocks was born. 

The potassium-argon method of dating minerals has several great ad- 
vantages. One is the abundance and wide distribution of potassium min- 
erals in the earth's crust— namely, potassium feldspar and biotite. The sec- 
ond, and enormously important, advantage is the fact that the geologic 
ages of many of the rocks that contain the potassium minerals can be ac- 
curately determined. If the rock that contains the potassium minerals also 



MEASURING GEOLOGIC TIME 55 

contains fossils or is associated with fossiliferous rocks, its geologic age is 
paleontologically controlled. 

A wholly unexpected discovery, made during the development of the 
potassium-argon method, was the fact that the potassium feldspars— or- 
thoclase and microcline (KAlSijOg)— retain only about 75 percent of the 
argpn that is generated within them, whereas biotite and other micas, de- 
spite their perfect cleavage, retain all or nearly all the argon formed within 
them. As a result of this discovery, investigators have turned, since early in 
1956, from using feldspar to using biotite in determining the ages of i^ 
neous rocks. 

A potassium-bearing mineral that is proving to be highly useful in dating 
sedimentary rocks is glauconite, K(Fe*^Al) (Mg,Fe^OSiAo(OH),. This 
is a mineral that forms in a marine environment; it is an "authigenic" 
product, formed contemporaneously with sedimentation. It is also proving 
to be highly useful in connection with the rubidium-strontium method de- 
scribed earlier. 

Numerous potassium-argon age datings have already been made in sev- 
eral laboratories in the United States, Canada, and Germany. The oldest 
rock so far dated is a cobble in the basal conglomerate of the Bulawayan 
system of Southern Rhodesia {43). The age (calculated by using x = 
0.55 X 10-9 yr-i and R = 0.085, where "R = 0.085" is an empirical calibra- 
tion constant that corrects for loss of argon) is 3310 million years. The 
basal beds of the Bulawayan system consists of thick conglomerate, com- 
posed mainly of granite boulders. The basal beds rest with conspicuous un- 
conformity on talc schists, intruded by granite like that of the granite 
boulders in the conglomerate. Accordingly, the Sebakwian system, which 
underlies the Bulawayan system, can tentatively be considered to be more 
than 3300 million years old. This age determination indicates that the 
Sebakwian rocks are the oldest rocks of the earth so far dated. The next 
essential step will be confirmation, by direct determination, of the age of 
the Sebakwian by more than one method— presumably by the potassium- 
argon and the rubidium-strontium methods— and by the use of several 
different minerals. , ^ 

The potassium-argon method has been used to determine the age of the 
Forest City, Iowa, meteorite— a bronzite chondrite. Wasserburg and Hay- 
den {44), using 0.085 as the value of the branching ratio R, found its age 
to be 4670 million years. The value 0.085, as was mentioned in the pre- 
ceding paragraph, had been obtained as a calibration constant in measur- 
ing the ages of feldspars; consequently, it is uncertain or unlikely that this 
value holds for bronzite and for other minerals for which it has been 
used. Fohnsbee et al. {45) determined the age of the Forest City meteorite 
as being 4240 million years by using the potassium-argon method, but they 
took R to be 0.11. According to Patterson {46) the age of the meteorite 



56 ADOLF KNOPF 

by the lead-lead method ("the most accurate method") is 4550 million 
years. 

The potassium-argon method has recently been developed to such an. 
extreme sensitivity that rocks that are less than 2 million years old have 
been successfully dated (47). The rhyolite at Sutter Buttes, California, 
was determined, on the basis of the argon-potassium ratio of its biotite, to 
be 1.57 million years old, and the geologically slightly younger andesite 
w^as found to be 1.69 million years old. Both absolute ages are virtually the 
same, being within the limits of experimental error. Both the rhyolite and 
andesite are known, from field evidence, to be Pliocene or Pleistocene. , 

The potassium-argon method thus gives promise of attaining a resolving 
power nearly as great as that of biochronology. In favorable circumstances 
—as, for example, the Jurassic system— the resolving power of biochro- 
nology is so great that it can distinguish no less than 58 world-wide am- 
monite zones, each of which is thought to represent a time-span of ap- 
proximately 500,000 years (48). The attainment of a correspondingly high 
resolving power by the potassium-argon method will be a great event in 
the history of geochronology. 

CARBON-14 AND OTHER METHODS 

The carbon-14 method of age determination devised by W. F. Libby in 
1947 is of great importance in dating the past 50,000 years (49). Neutrons 
produced by cosmic radiation react with atmospheric nitrogen at high alti- 
tudes to form radiocarbon (C^^), which then combines with oxygen to 
form carbon dioxide. Plants utilize the radioactive carbon dioxide along 
with the normal carbon dioxide; hence all living matter eventually con- 
tains radioactive carbon. The half-life of C^^ is 5570 years. Some ten labora- 
tories, scattered throughout the world, are determining ages by the radio- 
carbon method. Most of them are equipped to determine ages up to 
about 38,000 years; the extreme sensitivity of 53,000 years is reached by 
the laboratory at Groningen, Netherlands. In 53,000 years the radiocarbon 
has decreased to only about Vs of 1 percent of the minute amount that was 
originally present. 

Unlike the other radioactive methods of age dating, the radiocarbon 
method has not lengthened the previous estimates of geologic time but 
has cut down to one-half the long-accepted estimate of the length of post- 
glacial time, giving a date of 11,000 years ago as the beginning of the final 
retreat of the Wisconsin ice sheet. The radiocarbon method is useful only 
for dating Recent and late Pleistocene time— the last few^ moments j)f 
geologic time. 

Many other methods based on atomic disintegration have been proposed 
or are being developed, but it would take too much space to describe them 
here. One that is particularly desirable, since it would bridge the gap be- 



MEASURING GEOLOGIC TIME 57 

tween argon age datings and the carbon- 14 datings, has recently been out- 
hned by Arnold {SO). Beryllium-lO has been found to be a product of 
cosmic-ray bombardment in the atmosphere; it is a beta-ray emitter and 
has a half-life of 2.5 million years. This half-life period is long enough, if a 
method for using Be^*^ for radioactive age determinations can be developed, 
to date events in the Pleistocene and late Pliocene. The Pleistocene is con- 
ventionally considered to be 1 million years long, but this figure has not 
yet been confirmed by any objective evidence. 

ABSOLUTE AGES OF GEOLOGICALLY DATED 
MINERALS AND ROCKS 

Before 1956, only one absolute age determination had been made on 
paleontologically controlled material. That material was the Peltura zone 
of the remarkable marine oil shale in Sweden, which contains the uranium- 
bearing nodules known as "kolm"; it carries trilobites and other fossils, 
from which it is determined to be of very late Cambrian age. The kolm 
contains 0.462 percent uranium, which appears to have been precipitated 
out of the sea water and incorporated into the kolm at the time the 
kolm was forming. The isotopic composition of the radiogenic lead in 
the kolm was determined by Nier, in 1939, and yielded the very dis- 
concerting result that the age, based on Pb^^^/U^^^, is 380 million years, 
whereas that based on Pb^oT/p^soe jg 77Q million years. Now Nier, it 
must be recalled, regarded the figure given by the Pb^oT/p^soe j-^^jq 35 
being the least subject to error and hence the most rehable. For the 
kolm, however, the figure 770 million years was clearly too large. No an- 
swer to this paradox was forthcoming until Wickman (SI) proposed a 
solution. During the transformation of uranium to lead, one of the inter- 
mediate radioactive products is radon, a gas of half-life period of 3.82 
days. Consequently, the possibility exists that some of the radon may es- 
cape. If some does escape, the amount of radiogenic lead ultimately formed 
will be too small. Therefore, the age given by the ratio Pb^o'^/Pb^os ^^j] 
be too large, and the age given by the ratio Pb^oe/uass ^^j] ^e too small. 
By solving two simultaneous equations involving these quantities, the prob- 
able real age is found to be 440 million years. 

Three other absolute age datings have been fundamentally important 
in building up the absolute geologic time scale, but they are less securely 
placed in the geologic time column than is the Swedish kolm. One is the 
previously mentioned pitchblende from Central City, in the Front Range 
of Colorado. The mean of two closely concordant results obtained by 
Nier et al. {S2), in 1941, gives an age of 58 million years or, in round 
numbers, 60 million years. This figure of 60 million years has long been 
used, especially by Holmes and Stille, to date the Laramide revolution and 
hence the beginning of Tertiary time. However, the Laramide orogeny is 



58 ADOLF KNOPF 

now known to comprise eight or more phases. These phases extended 
in time from late Cretaceous to the end of the Ohgocene. The problem 
as now seen is: Which phase of the Laramide orogeny is dated by the 
pitchblende of Central City? According to T. S. Lovering, who has long 
studied the geology of Colorado in the field, the veins in which the pitch- 
blende occurs are related as aftereffects of the intrusion of a porphyry stock 
that cuts through a great thrust, 50 miles long, known as the Williams 
Range thrust. This thrust has affected strata as young as Fort Union, of 
Paleocene age. The pitchblende is therefore post-Fort Union and is re- 
garded as having been deposited at or near the end of Paleocene time {S3). 

The pitchblende from St. Joachimstal, Bohemia, constitutes another im- 
portant tie point. Nier (29) in 1939, using a pitchblende that contained 
42.3 percent of ordinary lead and 57.7 percent of radiogenic lead, ob- 
tained the figure of 227 million years as its age; since 1939, a slightly 
different value of the half-life of U^^^ has been adopted; this brings the 
Pb^o^/U^^^ age to 223 milhon years or, in round numbers, 220 million 
years. The various German authorities— Stille, von Bubnoff, and Weyl— 
regard the pitchblende as being of Saalian age— that is, of latest Early 
Permian ("Unter Rothliegend") age. 

The other valuable age-dating by the lead method, isotopically con- 
trolled, is based on a thorite from a pegmatite near Oslo, Norway. The 
calculated age, based on the Pb^os/Th^^^ ratio, is 224 million years; based 
on the Pb206/U238 ratio, it is 233 million years— in round numbers, 230 
million years. From the geologic evidence, the thorite is inferred to have 
been formed about the end of Early Permian time. 

From lead-uranium ratios, the end of the Ordovician is known to be, 
roughly, 350 milhon years ago, and the end of the Silurian, about 300 
million years ago. 

In 1956, 19 or more absolute age determinations on geologically dated 
material became suddenly available. These 19, as well as that of the kolm 
of Sweden, are shown in Table 4. They are listed according to their order 
in the geologic time scale, beginning with the oldest, of late Early Cam- 
brian age (470 million years) and ending with the Miocene (M4, the 
fourth of the six subdivisions of the Miocene). The corresponding abso- 
lute ages fall roughly into the proper sequence. The discrepancies point 
up the fact that the methods for determining absolute ages do not yet 
equal the resolving power of the biochronologic methods. 

The Miocene (M4) 21 million years, according to a potassium-argon 
determination made on glauconite (54), is out of hue with the ages deter- 
mined for the Oligocene. 

Particularly interesting are the two determinations of age made on the 
Hornerstown marl. An argon determination on glauconite by Wasserburg 
et al. (43) gives 50 million years, and a Rb^'^/Sr^'^ age determination on 
glauconite by Cormier et al. (39) gives 60 million years. Since the Homers- 



MEASURING GEOLOGIC TIME 59 



TABLE 4 
Age determinations of geologically dated minerals. 





Age 








Geological Age 


(millions 
of years ) 


Locality 


Mineral 


Method 


Miocene (M4) 


21 


New Zealand 


Glauconite 


Argon 


Oligocene (05) 


16 


New Zealand 


Glauconite 


Argon 


Oligocene (03) 


20 


New Zealand 


Glauconite 


Argon 


Oligocene (Ol) 


25 


Alsace 


Sylvite 


Argon and 
helium 


Eocene (E5) 


36 to 39 


New Zealand 


Glauconite 


Argon 


Paleocene 


50 


Hornerstown marl, NJ. 


Glauconite 


Argon 


Paleocene 


60 


Hornerstown marl, 
Clayton, NJ. 


Glauconite 


Strontium 


Paleocene 


47 


New Zealand 


Glauconite 


Argon 


Cretaceous 










"Late" Cretaceous 


62 


Clayton, N.J. 


Glauconite 


Strontium 


Maastrichtian (K12) 


70 


Navesink formation, 
Clayton, N.J. 


Glauconite 


Strontium 


Campanian (Kll) 


69 


Marshalltown 
formation, N.J. 


Glauconite 


Argon 


Cenomanian (K7) 


90 


Crowsnest volcanics. 
Alberta 


Feldspar 


Argon 


Albian (K6) 


138 


MacMurray, Canada 


Glauconite 


Argon 


Late Middle Devonian 










Givetian 


270 


Elk Point formation, 
Saskatchewan 


Sylvite 


Argon 


Late Middle Ordovician 


380 


Dubuque formation, 
Minn. 


Orthoclase 


Argon 


Early Ordovician 


375 to 381 


Stenbrottet, Sweden 


Glauconite 


Strontium 


Middle Upper Cambrian 


440 


Gullhogen, Sweden 


Kolm 


Lead 


Upper Cambrian 










Franconian 


440 


Sparta, Wis. 


Glauconite 


Argon 


Eariy Upper Cambrian 


401 to 413 


Central Texas 


Glauconite 


Strontium 


Late Lower Cambrian 


470 


St. Piran sandstone. 
Alberta 


Glauconite 


Strontium 



town marl is said to be an almost pure bed of glauconite, from 5 to 30 
feet thick, and to represent the whole of the Paleocene, future determina- 
tions of absolute age of the Hornerstown marl should be made on carefully 
selected material of accurately known stratigraphic position. 

The absolute age of the Albian, the sixth of the 12 or 13 stages that 
make up the Cretaceous system, is given as 138 million years (54), but this 
is obviously a misfit. 

Particularly interesting is a comparison of the absolute age of the kolm 
of Sweden and the recently determined absolute age of the Franconian 
glauconite of Sparta, Wisconsin. The paleontologic evidence indicates that 
the kolm and the glauconite are of the same, or of nearly the same, age 
— approximately middle Late Cambrian. The kolm, as was previously 
shown, is 440 million years old; the age of the glauconite, as it was de- 
termined by means of the potassium-argon method by Wasserburg et al. 



60 ADOLF KNOPF 

(43), is 440 million years. Paleontologic dating and absolute ages thus 
agree extraordinarily closely. The age given here— 440 million years— has 
been recalculated from the authors' data by means of the decay constants 
that are used in calculating the ages of the other glauconites listed in 
Table 4^ 

Finally, the absolute age dating of the glauconite from the Olenellus- 
bearing beds that make up the topmost portion of the St. Piran sandstone 
of Alberta is extraordinarily important, as was mentioned earlier. This ab- 
solute age of 470 million years, determined on rocks that contain fossils of 
known paleontologic age, is so far the nearest that we have to the dawn 
of the Cambrian period, which marks the beginning of the Paleozoic era, 
when the oceans began to team with living organisms of all the phyla 
except the vertebrates. 

SUMMARY 

The new evidence tends to strengthen the estimates that the Cenozoic 
era is approximately 70 million years long, the Mesozoic, approximately 
130 million years, and the Paleozoic, 300 milhon years. Before the be- 
ginning of the Paleozoic era there was a vast stretch of time, possibly 4000 
million years long. Eight-ninths of geologic time had already passed before 
there began that portion of the earth's history which is generally held to be 
the most significant [56). 

REFERENCES AND NOTES 

1. A. Knopf, in The Age of the Earth, Natl. Research Council (U.S.) Bull. No. 80 
(1931), p. 8. 

2. J. T. Shotwell, The History of History (Columbia Univ. Press, New York, 1939) . 

3. J. }oly, Sci. Trans. Roy. Dublin Soc. 7, No. 2, (1899). 

4. W. J. Sollas, The Age of the Earth and Other Geological Studies (Button, New 
York, 1905). 

5. C. J. Stubblefield, Advancement of Sci. 11, 149 (1954). 

6. L. J. Wills, The Physio graphical Evolution of Britain (Arnold, London, 1929), 
p. 62. 

7. W. J. Arkell, Jurassic Geology of the World (Oliver and Boyd, Edinburgh, 1956). 

8. , Am. ]. Sci. 2S4, 460 (1956). 

9. The symbol Jl is used as a mnemonic form to indicate that the Hettangian is the 
first stage of the Jurassic system. 

10. A. Holmes, "The construction of a geological time scale," Trans. Geol. Soc. Glasgow 
21, 145 (1947). 

11. M. Kay, North American Geosynclines, Geol. Soc. Amer. Mem. No. 48 (1951), 
p. 93; O. H. Schindewolf, Der Zeitfaktor in Geologic und Paleontologie (Schweitzer- 
bart'sche, Stuttgart, Germany, 1950), p. 22; R. A. Sender, Mechanik der Erde 
(Schweitzerbart'sche, Stuttgart, Germany, 1956), p. 64. 

12. B. F. Howell and C. Lochman, "Succession of Late Cambrian faunas in the Northern 
Hemisphere," /. Paleontol. 13, 115 (1939); A. H. Westergard, "Supplementary 
notes on the Upper Cambrian trilobites of Sweden," Sveriges Geol. Undersokn. 
Arsbok 41 (1947); R. Berg, "Franconian formation of Minnesota and Wisconsin," 
Bull. Geol. Soc. Amer. 65, 867 (1954). 



MEASURING GEOLOGIC TIME 61 

13. G. G. Simpson, "A continental Tertiary chart," /. Pdeontol. 21, 481 (1947). 

14. C. Schuchert, "Geochronologv, or the age of the earth on the basis of sediments and 
life," in The Age of the Earth, Natl. Research Council (U.S.) Bull. 80 (1931), 
p. 10. 

15. M. Kay, "Sediments and subsidence through time," in The Crust of the Earth, 
Geol. Soc. Amer. Spec. Paper No. 62 (1955), p. 672. 

16. R. M. Kleinpell, Miocene Stratigraphy of California (Am. Assoc. Petroleum Geol., 
Tulsa, Okla., 1938). 

17. W. H. Twenhofel, "Marine unconformities, marine conglomerates, and thicknesses 
of strata," Bull. Am. Assoc. Petroleum Geol. 20, 701 (1936). 

18. R. J. Strutt, Proc. Roy. Soc. London A83, 298 (1910). 

19. P. M. Hurley and C. Goodman, Bull. Geol. Soc. Amer. 54, 305 (1943). 

20. P. M. Hurley, ibid. 61,1 (1950). 

21. W. Gentner, K. Goebel, R. Prag, Geochim. et Cosmochim. Acta S, 124 (1954). 

22. B. B. Boltwood. Am. /. Sci. 20, 253 (1905). 

23. , ibid. 23, 77 (1907). 

24. A. Holmes, Proc. Roy. Soc. London ASS, 248 (1911). 

25. , The Age of the Earth (Harper, New York, 1913). 

26. , Trans. Edinburgh Geol. Soc. 16, 313 (1956). 

27. J. Barrel!, Bull. Geol. Soc. Amer. 28, 745 (1917). 

28. F. W. Clarke, The Data of Geochemistry, U.S. Geol. Survey Bull. 770 (ed. 5, 
1924), p. 322. 

29. A. O. Nier, Phvs. Rev. 5S, 153 (1939). 

30. G. W. Wetherill et al, Geochim. et Cosmochim. Acta 9, 292 (1956). 

31. R. D. Russell et al, Trans. Am. Geophys. Union 35, 301 (1954). 

32. A. Holmes and L. Cahen, "African geochronology," Colonial Geol. Surveys (Lon- 
don) 5, 32 (1955). 

33. E. S. Larsen, N. B. Keevil, H. C. Harrison, Bull. Geol. Soc. Amer. 63, 1045 (1952). 

34. L. U. Nicolaysen, Geochim. et Cosmochim. Acta 11, 41 (1957). 

35. H. M. E. Scliuermann et al, Geol. en Mijnbouw 18, 312 (1956). 

36. F. Strassmann and E. Walling, Ber. deut. chem. Ges. Jahrg. 71, 1 (1938). 

37. L. H. Ahrens, "Radioactive methods for determining geological age," in Physics and 
Chemistry of the Earth (McGraw-Hill, New York, 1956), vol. 1, p. 44. 

38. L. T. Aldrich, Science 123, 874 (1956). 

39. R. F. Cormier et al. Bull. Geol. Soc. Amer. 67, 1681 (1956). 

40. F. Rasetti, Middle Cambrian Stratigraphy and Faunas of the Canadian Rocky Moun- 
tains, Smithsonian Inst. Pubis. Misc. Collections 116, No. 5 (1951). 

41. G. W. Wetherill, "Radioactivity of postassium and geologic time," Science 126, 
545 (1957). 

42. L. T. Aldrich and A. O. Nier, Phys. Rev. 74, 876 (1948). 

43. G. W. Wasserburg, R. J. Havden, K. L. Jensen, Geochim. et Cosmochim. Acta 10, 
159 (1956). 

44. G. W. Wasserburg and R. J. Hayden, ibid. 7, 51 (1955). 

45. R. E. Folinsbee, J. Lipson, J. H. Reynolds, ibid. 10, 61 (1956). 

46. C. Patterson, ibid. 10, 230 (1956). 

47. G. H. Curtis et al. Nature 176, 1360 (1956). 

48. J. A. Jeletzky, Bull. Am. Assoc. Petroleum Geol. 40, 693 (1956). 

49. W. F. Libby, E. C. Anderson, J. R. Arnold, Science 109, 111 (1949); W. F. Libby, 
Am. Scientist 44,98 (1956). 

50. J. R. Arnold, Science 124, 584 (1956). 

51. F. E. Wickman, Geol. Foren. i Stockholm Forh. 64, 465 (1942). 

52. A. O. Nier, R. W. Thompson, B. F. Murphey, Phys. Rev. 60, 113 (1941). 

53. A. Knopf, "The geologic records of time," in Time and Its Mysteries (New York 
Univ. Press, New York, 1949), ser. Ill, p. 33; "Time in Earth history," in Genetics, 
Paleontology, and Evolution (Princeton Univ. Press, Princeton, N.J., 1949), p. 1. 

54. J. E. Lipson, Geochim. et Cosmochim. Acta 10, 149 (1956). 

55. G. L. Bate and J. L. Kulp, Science 122, 970 (1955). 

56. Following is a list of selected comprehensive accounts of measurement of geologic 



62 ADOLF KNOPF 

time: L. H. Ahrens, "Radioactive methods for determining geological age," in 
Physics and Chemistry of the Earth (McGraw-Hill, New York, 1956), vol. 1, pp. 
44-67; L. T.- Aldrich, "Measurement of radioactive ages of rocks," Science 123, 871 
(1956); H. Faul, Ed., Nuclear Geology, a Symposium on Nuclear Phenomena in 
the Earth Sciences (Wiley, New York, 1954); A. Holmes, The Age of the Earth 
(Nelson, London, 1937); A. Knopf, Ed., The Age of the Earth, Natl. Research 
Council (U.S.) Bull. 80 (1931); J. L. Kulp et al., "Present status of the lead method 
of age determination," Am. J. Sci. 2S2, 345 (1954); J. L. Kulp, "Isotopic dating 
and the geologic time scale," in The Crust of the Earth, Geol. Soc. Amer. Spec. 
Paper 62 (1955), pp. 609-630; }. P. Marble et al., "Symposium on the measure- 
ment of geologic time," Trans. Am. Geophys. Union 33, 149 (1952); K. Rankama, 
Isotope Geology (McGraw-Hill, New York, 1954); F. E. Zeuner, Dating the Past: 
an Introduction to Geochronology (Methuen, London, rev. ed. 3, 1952). 



THE EARTH 

All mountains, islands, and level lands have 

MODEL been raised up out of the bosom of the 

earth into the position they now occupy by 
PROBLEMS i^^ action of subterranean fires — lazzaro 

MORO, De Crostacei e degli altri marini 
A TajT) , Corpi che si truovano su Monti, Venice 

(1740) 

IMPLICATIONS 



The Interior of the Earth as Revealed 
by Earthquakes 

• I. LEHMANN 



THE DEEP INTERIOR OF THE EARTH IS INACCESSIBLE, AND 

no rays of light penetrate to let us see what is below the surface. But 
rays of another kind penetrate and carry with them their messages from 
the interior. The Earth has been found to have elastic properties that al- 
low movement set up at the source (focus) of an earthquake to radiate 
into the interior and to spread over the surface. In a strong earthquake 
the whole of the Earth is set vibrating. At some distance from the focus, 
depending on the strength of the shock, the movement is no longer per- 
ceptible, but sensitive seismographs can record the waves that emerge at 
the surface. The records provide data from which knowledge of the Earth's 
interior may be gained. From seismic studies we have learned that the 
Earth consists of a core surrounded by a mantle on which there is a crust; 
inside the core there is a small inner core (figure 1). 

Towards the end of the nineteenth century it was realized that earth- 
quake movements extend to great distances from the focus. Systematic 
recording of earthquakes began, and the very important results gained at 
an early stage greatly stimulated interest in this new science. 

The elastic waves that radiate into the Earth are of two kinds, having 
different speeds of travel: P waves [undae primae), in which the particle 
motion is longitudinal, and S waves {undae secundae) with transverse 
particle motion. The speed of S waves is roughly 60 per cent of that of 

• From Endeavour (April, 1959), pp. 99-105. 

63 



64 I. LEHMANN 




Fig. 1. Diagram show- 
ing mantle, core, and 
inner core of the earth. 



P waves. The arrivals of the waves are marked by groups of oscillations in 
the seismograms, and at moderate distances from the focus they usually 
stand out clearly. The seismogram of figure 2 was obtained at a distance 
of 18-6° from the focus.^ P and S appear on it. The large oscillations 
succeeding S are due to surface waves. 

We measure the arrival times of the waves today usually with an accu- 
racy of not less than 1 second, and deduce the travel times of the waves 
from focus to recording station, if the location of the focus and the time 
of occurrence of the earthquake can be found. When a fair number of 
travel times to points at different distances from foci at approximately the 
same depth (the normal depth is about 10-20 km) are available, time- 
distance tables can be set up, giving the travel times of each type of wave 
over the various distances. It is customary to present them as time-curves. 

The first attempts to construct time-curves for the P and S phases re- 
vealed that the average speed, as determined along the chords, increased 
with focal distance, indicating the wave velocity increased with depth. 
As a consequence of this the rays are not straight lines but have an up- 
ward curvature. 

At about 100° focal distance P and S became small and at somewhat 
greater distances could not be detected. Clear P phases reappeared at 
about 140°, but they had been delayed, so that their travel times did not 
fit on to the continuation of the time-curve for shorter distances. This ob- 
servation was made in 1906 by R. D. Oldham, who drew the conclusion 
that deep in the Earth there was a decrease of velocity, causing the rays 
to bend downwards so as to leave part of the Earth's surface in shadow. 
He attempted to calculate the depth at which the decrease of velocity oc- 

^ Distances in seismology are angular distances subtended at the centre of the Earth, 
as a first approximation taken to be a sphere. 



INTERIOR OF EARTH AS REVEALED BY EARTHQUAKES 65 

curred, but his observations were not good enough for this purpose; there- 
fore his results were very much in error. E. Wiechert, in Gottingen, had 
also come to the conclusion that the Earth had a core in which the ve- 
locity was smaller than in the surrounding mantle. It was the first great 
achievement of B. Gutenberg to establish this beyond doubt by means of 
records of distant earthquakes obtained on the Wiechert seismographs at 
Gottingen, and to calculate the radius of the core. His result did not differ 
much from later ones obtained from modern and more abundant data. 

Gutenberg made use of the time-curves for P and S up to about 103° 
established by Wiechert and Zoppritz, and of the wave velocities derived 
from them. At that time formulae had been developed by means of which 
transmission times to different distances could be calculated when the ve- 
locity as a function of the distance from the Earth's centre was known, 
and also formulae by means of which the wave velocities as a function of 
depth could be obtained when the time-curve was known. The ray emerg- 
ing at the distance where the P curve broke off would graze the core. 

The belated P waves observed from a little beyond 140° onwards had 
passed through the core, but the velocity of these waves in the core could 
not be derived from the formulae used for the mantle, for these formulae 
break down when there is a discontinuous decrease of velocity, as at the 
core boundary. But when a velocity distribution in the core was assumed 
the travel times could be calculated and Gutenberg varied his assumptions 
until the calculated travel times agreed with those observed. The velocity 
as a function of depth was then known for the whole of the Earth to a 
first approximation. 

On theoretical grounds it was to be expected that the rays would be 
reflected on reaching the surface of the Earth and would be reflected and 
refracted at discontinuity surfaces in the interior, such as the boundary 
of the core, partly as rays of the same kind and partly transformed into 
rays of the other kind (P into S and vice versa). Thus at great distances 
from the epicentre (the point on the Earth's surface directly above the 
focus) waves would be arriving along many different parths, producing 
oscillations in a seismogram and marking phases more or less prominent 
according to the energy carried. When the velocity distribution within the 
Earth is known, it is possible to calculate the transmission times along all 
the different paths. 

When earthquake records are examined a great many of the anticipated 
phases can be identified but some of those originally expected to be present 
are not found, namely all those that would have come as S waves through 
the core. Since phases may be present without being very clear, many 
years passed before it was definitely concluded that transverse waves were 
not transmitted through the core. At the surface of the Earth a fluid does 
not transmit transverse waves, and therefore we say that the core is fluid, 
although in other respects it may not resemble a fluid as we know it. 



66 I. LEHMANN 

The shadow zone for the P phase extends from about 105° to 143° 
epicentral distance. With modern highly sensitive instruments the P phase 
is found not to be completely absent in this range: in strong earthquakes 
it is usually faintly recorded. The appearance of P waves in the shadow 
zone may be due either to diffraction around the core boundary or to a 
spreading of the rays caused by a small gradual decrease of velocity just 
outside the core. 

In addition to this faint P phase there is, in the shadow zone, another 
later P phase that is faint at the smaller distances. On Gutenberg's original 
Earth model its presence could not be explained and it was vaguely 
ascribed to diffraction. However, as seismographs improved it was more 
and more clearly recorded, and an explanation was required. In 1936 the 
writer pointed out that the presence of a small inner core in which the P 
velocity was greater than just outside it would fully account for the oc- 
currence of the phase, for it would cause enough incident rays to bend 
upwards strongly enough for part of them to emerge in the shadow zone. 
Gutenberg and Richter accepted the existence of this inner core and cal- 
culated its radius and the velocity distribution in it. Later H. Jeffreys 
proved that diffraction could not account for the phase in question. 

Figure 3 is part of a seismogram recorded at an epicentral distance of 
70- 1°. PP, SS and PPP, SSS are, respectively, phases due to waves re- 




Fig. 2. The earthquake of 23rd July 1929 recorded at Copenhagen at 
epicentral distance 18.6°. 



vi^^Uaa-jv'"' ' •■«vv''~»-'^''<yvwj. 




'A^w/^''v'^~vv^ljy^ 



PP PPP 



Fig. 3. The earthquake of 30th June 1936 east of Kamchatka recorded at 
Copenhagen at epicentral distance 70.1°. 



INTERIOR OF EARTH AS REVEALED BY EARTHQUAKES 67 




Fig. 4. The earthquake of 1st March 1948 off the west coast of New 
Guinea recorded at Scoresby-Sund at epicentral distance 108°. 



fleeted once and twiee at the surface of the Earth. PS starts as P and is 
reflected as S. 

Figure 4 is part of a seismogram recorded at an epicentral distance of 
108°. P is here the P wave reflected at the boundary of the inner core. 
SKS starts as an S wave, traverses the core as a P wave, and is again an 
S wave after leaving the core. SKKS is also an S wave outside the core 
and a P wave inside, but it is reflected when, from inside, it meets the 
core boundary. Figure 5 shows the paths of the rays corresponding to 
some of the phases of figure 4. 

In figure 6 are seen the time-curves of the phases already mentioned and 
of a few others. PKP is the same phase as P. A great many more phases 
occur, especially at great epicentral distances. 

The fact that on the mantle there is a crust differing distinctly from it 
was shown by A. Mohorovicic in 1909; the boundary between crust and 
mantle is called the Mohorovicic discontinuity. The wave velocity in the 
crust is smaller than in the mantle underneath, and therefore the waves 
coming through the crust are refracted and bent upwards when they meet 
the mantle. There will be a range of distance within which both the re- 
fracted and the direct waves emerge, as indicated in figure 7; at a certain 
distance the refracted wave overtakes the direct wave because it travels 



Fig. 5. Rays from focus 
F to epicentre E at dis- 
tance 108°. 




68 I. LEHMANN 

faster in the lower layer. But its path is longer, and energy is lost on 
refraction, and therefore the corresponding phase in a seismogram will be 
smaller than that due to the direct wave. There therefore appears a small 
P phase succeeded by another much larger P phase. This was observed by 
Mohorovicic, who gave the correct interpretation. He was interested in 
finding the depth of the discontinuity but did not have the means of de- 
termining it with any accuracy. Actually it turned out to be extremely 
difficult to arrive at reliable values for the depth, though many different 
methods were employed. The best results have been obtained from explo- 
sions, which can be looked upon as artificial earthquakes. They can be 
timed with great precision, and the focus is exactly known; when they 
are well recorded at suitable distances more useful data are obtained than 
can be derived from earthquakes. It now seems to be established that the 
discontinuity is at a depth of between 30 and 40 km under most conti- 
nental areas. Under the deep oceans it is at a much smaller depth, only 
about 10-15 km under the water surface. 

While the evidence for the existence of the Mohorovicic discontinuity 
and of the other subdivisions of the Earth mentioned above is very clear, 
precise determination of their depths is difficult. It depends on very 
precise determination of the velocity variation throughout the Earth, and 
this in turn depends on precise determination of the travel times of the 
direct P and S waves and of some of the reflected and refracted waves. 
Much important work has been done along these lines, a great deal of it 
in the 1930s. In the course of this work it appeared that subdivisions of 
the mantle have also to be considered, but the evidence for them is not 
of a very precise nature, and their location is uncertain. 

It is a difficult and lengthy process to construct good time-distance tables 
or time-curves. To obtain accurate travel times we require to have the 
focus and the time of occurrence of the earthquake accurately determined. 
As a rule this cannot be done directly, because there are not enough ob- 
servations close to and around the epicentre. It is therefore necessary to 
make use of time-curves already in existence, and there is then the risk 
of transferring errors from these curves to the new travel times. When 
time-curves have been determined from them it may therefore be desirable 
to have the elements of the earthquakes redetermined and the whole proc- 
ess repeated. 

For the construction of the time-curves it has been customan,^ to use a 
graphical method, plotting travel times against distance and drawing a 
smooth curve through the cluster of points thus obtained. It is a somewhat 
arbitrary process, and, in common with other smoothing procedures, it is 
apt to smooth away or faultily to introduce changes of slope or curvature. 
This is serious, because it is such changes that indicate the existence of 
more or less abrupt changes of velocity or velocity gradient in the Earth. 
Many time-curves have been constructed in the course of time, and the 



INTERIOR OF EARTH AS REVEALED BY EARTHQUAKES 69 

PPS 



Fig. 6. Travel times for 

a surface focus (Jef- q 

freys-BuUen, 1940). 




20 40 60 80 100 120 140 160 180 
A (degrees) 




Mantle 



Fig. 7. Direct and re- 
fracted rays from focus 
F to epicentre E. 



existence of various so-called discontinuities has been derived from them. 
Two sets of time-distance tables or time-curves are now available which 
are far better than any previous ones. They are due to Gutenberg and 
Richter and to Jeffreys and Bullen. In 1928 Gutenberg published his 
Frankfurter Laufzeitkurven based on a large number of observations. In 
collaboration with C. F. Richter he greatly extended the work. In 1936 
they jointly published the first part of 'On seismic Waves', containing 
time-curves for a great many phases. Amplitude variation was considered 
for the fixing of the distances at which the curvature of the time-curve 



70 I. LEHMANN 

was either greater or smaller than usual. The amplitudes of the recorded 
waves should be relatively large at the distances where the time-curve bends 
strongly, and amplitudes should be small where the curve is straight. Am- 
plitudes were measured and used, and although very precise information is 
not derivable in this way, some useful indications were obtained. 

The readings of the seismic records from all over the world are collected 
and pubhshed in The International Seismological Summary' (I.S.S.), for 
which foci and times of origin of individual earthquakes are determined. 
For the reduction of the data down to 1928 inclusive the Zoppritz tables 
were used, but it became more and more apparent that the times given by 
these tables departed seriously from actual travel times. 

In 1928 Jeffreys began his very important work on travel times by a 
preliminary revision of the Zoppritz tables, using I.S.S. data. Later he 
undertook a thorough revision in collaboration with K. E. Bullen. A great 
quantity of data was used, and for the first time statistical methods were 
applied and the accuracy of the results obtained was evaluated. This im- 
plied difficult and extensive work because of the complicated processes in- 
volved. In part, new methods had to be developed. After the main work 
many special investigations followed, improving somewhat the first results. 
In 1940 the Jeffreys-Bullen (J-B) 'Seismological Tables' were published, 
and these are now being used for the I.S.S. 

From these tables Jeffreys calculated the variation of velocity with depth 
(figure 8). On the whole, the velocities of P and S waves increase down 
to the core boundary. Jeffreys found the velocity of P waves to be 5-6 
km/sec in the upper crust, below the sediments, and to be 13-6 km/sec 
at the core boundary, where it decreases to 81 km/sec. With increasing 
depth the velocity increases steadily in the outer core to about 10-4 
km/sec. In the inner core it is nearly constant at 11-2 km/sec. 

The velocity does not increase uniformly with depth all through the 
mantle. In the uppermost mantle the velocity increases slowly, but at a 
depth of a few hundred kilometres a strong velocity increase sets in, as 
indicated by a bending of the time-curve around 20° epicentral distance. 
Below a depth of about 1000 km the velocity again increases more slowly. 

On the whole, the velocities as derived from the Jeffreys-Bullen tables 
are probably not far wrong, but there are serious uncertainties. This is 
chiefly because it is so difficult to determine the slope of the time-curve 
accurately. The boundaries of the mantle regions in which the velocity 
gradients differ are indicated by changes of slope and curvature of the 
time-curve, but our data are not accurate enough for us to say exactly 
where these occur. It has been found particularly difficult to determine the 
velocity variation near the boundary of the inner core. 

A vast amount of data, in part more reliable, has accumulated since 
Jeffreys and Bullen constructed their tables. New seismographs have been 
developed, from the records of which the arrival times of the phases can 



INTERIOR OF EARTH AS REVEALED BY EARTHQUAKES 71 



14 
12 

u 

V 

"i" 8 

>^ 
■r. 6 



Fig. 8. Velocity as func- 
tion of depth according 
to H. Jeffreys, 1939. 



P^^^ 

S .. — 



1000 2000 



3000 4000 
Depth (km) 



5000 



6000 



be read with greater precision. In addition, explosion work has entered the 
picture. It has for a great many years been used for the exploration of the 
crust, especially for the finding of oil; since the second World War more 
effective explosives have been available, and many of the explosions have 
yielded results also for the upper mantle. Some large accidental explosions 
have also provided seismologists with new data. It was then found that 
the velocity just below the crust was greater than that derived from the 
Jeffreys-Bullen tables. Jeffreys drew attention to this, and in later work 
he provided corrected P tables for distances up to 30° for Europe. For, 
to complicate matters, it turned out that there were regional differences 
not only in the crust but also in the uppermost mantle. From the new 
tables it appears that the strong increase of velocity gradient in the mantle 
at first placed near 400 km depth is likely to occur at a much smaller 
depth, probably between 200 and 250 km. However, attempts to fix the 
depth accurately have as yet met with unsurmountable difficulties. 

The study of the Earth's interior is approached from many different 
directions. The upper mantle plays an important part in many investiga- 
tions, and geophysicists in various fields look to seismologists for precise 
and detailed information. Have we any means of supplying such in- 
formation? 

K. E. Bullen's presidential address to the International Association of 
Seismology and the Physics of the Interior of the Earth at Toronto in 
1957 had the title "Seismology in our Atomic Age." He pointed out that 
atomic explosions had far greater energy than the chemical explosions it 
is possible to use, and that they send waves deep into the interior of the 
Earth. Some atomic explosions have been recorded by seismographs, but 
there has been a reluctance by the authorities concerned to give prior in- 
formation about the exact location and time of the explosions. Though 
this has reduced the value of these explosions for seismological purposes, 



72 I. LEHMANN 

a few important results have been obtained, results that seemingly could 
not be derived from earthquake observations. There is no doubt that if an 
opportunity arose to record atomic explosions, by many seismographs 
placed at suitable distances from the source, information would be ob- 
tained that would help us to solve the problems that now confront us. 
We are here, as Bullen said, in a tantalizing position, for the tools we so 
much need exist, but they are not very likely to be placed in our hands. 
With ever-increasing fear of the perilous effect of atomic explosions, 
seismology can scarcely hope for any to be organized for its special pur- 
poses. However, if the intention to explode bombs for other purposes is 
made known beforehand and the location and time are communicated, 
as has been the case in some recent instances, it should be possible for 
seismologists to make some use of them. 

It is also well to remember that great masses of earthquake data are as 
yet unreduced and that, skilfully handled, they may yield fruitful results. 
New methods are also forthcoming in earthquake studies. The surface 
waves, not dealt with here, spread over the surface of the Earth, but they 
penetrate to some depth below it; in large earthquakes they may penetrate 
to considerable depth. The intense study of surface waves carried out in 
recent years, especially at the Lamont Geological Observatory, has pro- 
vided a new approach to the exploration of the crust and the upper mantle. 

While seismology teaches us a great deal about the interior of the Earth 
there is certainly very much more we should like to know. If we ask what 
are the materials inside the Earth, in what state they are, what is their 
density, and so on, seismology alone does not supply the answer. We 
have to look to other branches of geophysics and to other sciences such as 
geology, the physics and chemistry of the Earth's interior, and also to 
astronomy for additional information. Much attention has been given to 
pertinent questions in recent years, but the results are, in part, highly 
controversial. 

Important results on the density variation throughout the Earth were 
obtained by K. E. Bullen, The velocities of the seismic P and S waves 
depend on the density of the transmitting material and on the elasticity 
as characterized by the rigidity and the incompressibility. These three 
quantities cannot be derived from the two velocities, but estimates can be 
arrived at when information from various other sources is taken into 
account. Bullen finds that the density increases in the mantle from about 
3*3g/cm^ just below the Mohorovicic discontinuity to about 5'5 g/cm^ at 
the bottom of the mantle It then jumps to about 9*5 g/cm^ and increases 
to 11-5 g/cm^ at the bottom of the outer core. In the inner core there is 
a strong increase. The rigidity that represents the resistance to shearing 
stress increases in the mantle, until at the bottom it is nearly four times 
that of ordinary steel. In the outer core the rigiditv is quite small, and this 
is what we mean when we say that the core is fluid. On the other hand, 



THE RADIOACTIVE EARTH 73 

the incompressibility or the resistance to pressure does not change mate- 
rially at the boundary of the core. The pressure when evaluated was found 
to have reached about IV3 million atmospheres at the bottom of the 
mantle and about 4 million atmospheres at the centre of the Earth. Fol- 
lowing a long line of argument, Bullen arrives at the conclusion that the 
inner core is likely to be solid. 

The composition of the continental crust varies a great deal from one 
region to another. Granite is one of its main constituents, whereas this 
material is absent under the deep oceans, where the crust is much thinner. 
The upper mantle is believed to consist of ultrabasic rock rich in olivine. 
A transition is likely to take place, perhaps from one form of olivine to 
another, in the region where the seismic velocities increase more strongly 
than elsewhere, this region beginning at a depth of a few hundred 
kilometres. It has for a long time been believed that the core consists of 
iron and nickel, but recent investigations have led to the conclusion that 
the outer core possibly consists of material not much different from that 
of the mantle but transformed under the prevailing high pressure. The 
inner core is still believed to consist chieflv of iron and nickel. 



BIBLIOGRAPHY 

Oldham, R. D., Quart. J. geol. Soc. Lond., 62, 456, 1906. 

Mohorovicic, A., ]h. met. Obs. Zagreb, 9, I, 1910. 

Gutenberg, B., Nachr. Ges. Wiss. Gottingen, Math.-Phys. KL, 1, 1914. 

Gutenberg, B. and Richter, C. F., Beitr. Geophys., 43, 56, 1934. 

Jeffreys, H. and Bullen, K. E., Publ. Bur. Centr. Seism. Internat., A 11, 1, 1935. 

Jeffreys, H., Ibid., A 14, 1, 1936. 



The Radioactive Earth 

• PATRICK M. HURLEY 



WITHOUT THE HEAT FROM RADIOACTIVITY IT IS PROB- 

able that we would have had no atmosphere or oceans. Even if the ocean 
had existed, no land would have risen above it. Indeed, it is probable that 
the earth would have had a bare, rocky surface like the moon's, scorched 

• From How Old Is the Earth? by Patrick M. Hurley. Published by Doubleday & 
Company, Inc. Reprinted by permission. 



74 PATRICK M. HURLEY 

by the sun in daytime and bitter cold at night. You who read this book 
would never have been born. 

But the story of the earth is a story of heat. Throughout earth history 
large amounts of energy have been continuously expended in mountain 
building, volcanism, and other activities which have formed the continents, 
oceans, and atmosphere. Except for the actions of the surface agencies, 
driven by heat from the sun, the energy comes from the interior of the 
earth and must have been at one time in the form of heat. To try to 
explain the occurrence of this thermal energy, we must consider two prin- 
cipal sources: the heat inherited from the formation of the earth and the 
heat generated in the breakdown of radioactive elements. 

There are many arguments in favor of believing that the earth formed 
at a relatively low temperature. If this is true, a uniform distribution of 
the radioactive elements that we estimate were contained within the earth 
would have heated it sufficiently to have caused it to melt or partly melt. 
It is purely by chance that the sequence of events which we believe fol- 
lowed was such that the bulk of the earth stopped heating up again and 
remained fairly stable. 

These purely chance events are as follows. First, if the mantle of the 
earth is solid and there is no convection (transfer of heat by movement 
of iluid material) in it, it must lose heat by the slow process of conduction 
in the upper regions. If there is convection, from melting or otherwise, the 
heat can be rapidly transported to the surface. Therefore, the temperature 
can never get very much above that necessan' for melting. In the lower 
regions heat may be carried out by radiative transfer. 

Second, as soon as melting begins, there probably would be a migration 
of the molten radioactive substances upward because they crystallize at 
lower temperatures than compounds of magnesium, silicon, and iron. 
They would be forced upward in the liquid as the solid material settled 
downward. 

If in any region the heat-producing elements have not moved close 
enough to the surface, the temperature will rise locally to the melting 
point and a further upward migration will occur. Eventually they will 
have come close enough to the surface so that there will be no further 
melting. Enough of the heat generated will be lost by conduction to the 
surface so that a stable solid mantle remains. Gradually, following that 
time, the radioactive elements will decay slowly, and their heat production 
will diminish. This would tend to stabilize the mantle so that it is at some 
temperature below the melting point of its most fusible components. But 
if the chemistry of the radioactive elements had been otherwise and they 
had settled into the core, the earth would be continuously melting, losing 
heat by convection and solidifying again. 



THE RADIOACTIVE EARTH 75 




,234L J 1 1 238 



|Bi2IO| 



81 82 83 84 85 86 87 88 89 90 91 92 

ATOMIC NUMBER Z 

Fig. 9. Uranium 238 decays spontaneously to form thorium 234, which 
in turn breaks down into protactinium 234, and so on until the proces- 
sion stops at lead 206, which is stable. Some of the transformations are 
accompanied by alpha particle emission and some by beta particle 
emission. 



THE MAJOR HEAT-PRODUCING ELEMENTS 

Let us now examine the amount of heat given off by radioactive 
elements and estimate what abundance of these elements would cause 
melting in the mantle. The element uranium breaks down through several 
stages to form a stable end product, lead (see Fig. 9). As it undergoes 
successive transformations toward this stable end product, the isotope 
uranium 238 gives off 8 alpha particles as well as numerous gamma rays 
and beta particles. Summing up the energies of all these emitted particles 
and rays, we find that a total of 47.4 Mev (million electron volts) of 
energy has been expended for each atom of uranium 238 that breaks down 
to form an atom of lead 206. Since 1 Mev is equivalent to 3.83 X 10"^^ 
calories, it can be calculated that one gram of uranium in equilibrium 
with its daughter products is continuously giving off 0.71 calories per year. 
Similarly it can be calculated that the isotope uranium 235, of which atom 
bombs are made, is giving off 4.3 calories per gram per year when in 
equilibrium with its daughter products. Thorium and its series give off 
0.20 calories per gram of thorium per year. The only other important heat- 
producing element is the isotope of potassium, K'*". This gives off beta 
particles and gamma rays at a rate that yields 27 X 10"^ calories per gram 
of total potassium per year. 

Average granite and volcanic rock contain approximately the following 
amounts of these radioactive elements: 



76 PATRICK 


M. 


HURLEY 

Uranium 


Thorium 








parts per 


parts per 


Potassium 


Rock Type 




million 


million 


% 


Granitic rocks 




4 


14 


3.5 


Dark-colored 










volcanic rocks 




0.6 


2 


1.0 



Thus the radioactive components of the average granite can produce 
7 microcalories of heat per gram per year. Other rocks that make up the 
bulk of the crust produce somewhat less heat than granites, and it is esti- 
mated that the average rock in the crust above the Mohorovicic discon- 
tinuity probably produces about 2 microcalories of heat per gram per year. 

There is a measurable amount of heat continuously flowing to the sur- 
face of the earth. Measurements over continental areas have indicated that 
this amount averages about 1.2 microcalories per square centimeter per 
second. The amount of heat flowing in oceanic areas has been difficult to 
measure, but several measurements have been made. This is done by 
dropping a probe from a ship so it penetrates the mud on the bottom of 
the ocean for some distance. Refined temperature devices are then used 
to record the difference in temperature between two points on the probe. 
By determining the thermal conductivity of the mud, it is possible to 
calculate the amount of heat flowing upward from the earth into the 
ocean water. Surprisingly, it turns out that approximately the same amount 
of heat is flowing from the interior of the earth in the oceanic areas as in 
the continental areas; namely, about 1.2 microcalories per square centi- 
meter per second. 

From all these figures it can be calculated that the average continental 
crust down to a depth of 35 kilometers produces about Vi microcalories 
per square centimeter per second from the radioactive breakdown of 
uranium, thorium, and potassium. This is about one half of the observed 
heat flow to the surface. It means that only about one half of the heat 
flowing to the surface comes from a depth of greater than 35 kilometers. 

If you measure the amount of heat flow and estimate the thermal con- 
ductivity of the materials in the crust and below the crust, it is possible to 
estimate the increase in temperature with depth. Fig. 10 shows some esti- 
mates of the temperature-depth relationship. Note that the production of 
the heat in the crust greatly reduces the thermal gradient (rate of heat 
flow) at depth. It follows that the temperature at depth is very much less 
than would be expected if one simply measured the temperature in deep 
mines or other openings in the earth near the surface and extrapolated 
this information to great depth. In fact, if there were no radioactivity in 
the crustal rocks, the observed temperature gradients at the surface would 
require that the mantle be molten at a shallow depth; this is not in agree- 



THE RADIOACTIVE EARTH 77 



09 1.0 1.9 




TEMPERATURE (°C) 
1000° 2000° 




Fig. 10. The temperature gradient, or 
perature in a body, is proportional to 
portional to the conductivity. Near the 
the heat flow is 1.2 X IQ-^ cal/cmVsec 
degree C/sec. So the gradient is 1.2 X 
per cm or 17° per km. Heat-producing 
and any point below; the heat flow at 
the gradient. 



maximum rate of change of tem- 
the heat flow and inversely pro- 
surface of the earth, for example, 
; the conductivity is .007 cal/cm/ 
10-6/.007 = 17 X 10-5 degrees C 
elements lie between the surface 
depth is therefore less and so is 



ment with the known geological stability of the region. The facts, there- 
fore, support the hypothesis that the radioactive components of the earth 
are largely concentrated in the near-surface layers. 

By calculating the temperature at which materials would be molten at 
depths of 100 or 200 kilometers, it is possible to estimate how much radio- 
activity must be in the near-surface rocks in order to keep the temperature 
gradient within known bounds. Attempts to do this have indicated that at 
least 0.2 part per million of uranium and 0.7 part per million of thorium 
on the average must be in the rocks down to 100 kilometers depth under 
oceanic regions. Since these amounts of radioactive elements would supply 
much of the observed heat flow to the surface, there must be little heat 
left flowing from the interior. 

Thus we arrive at two important conclusions. The first is that very little 
of the original heat stored in the earth at the time of its formation is being 
lost, and, therefore, the earth is not cooling down at an appreciable rate, 
if at all. Secondly, we conclude that the major part of the earth's heat is 
coming from the breakdown of radioactive elements. Since almost all the 
breakdowns occur within the near-surface regions of the earth, it is rea- 
sonable to infer that some process has moved the radioactive elements to 
this location from a presumably homogeneous distribution at the time of 
the earth's origin. 



78 PATRICK M. HURLEY 

MIGRATIONS THROUGH THE MANTLE 

Again we see the need for some process to have brought up from within 
the earth the materials that make up the oceans and atmosphere and the 
radioactive elements. In support of this requirement, we see that uranium, 
thorium, and potassium do in fact belong to the group of elements that 
form compounds of rather low stability and therefore would be most 
likely to move to the outer part of the mantle in any process in which 
partial melting was involved. 

It is interesting that these conclusions do not violate the concept of an 
earth that is composed of materials similar to the iron and stone meteor- 
ites. The proportion of radioactive components in iron meteorites is very 
small indeed and would contribute a negligible amount to the heat of the 
earth if it were made of similar material. The amount of radioactive com- 
ponents in stony meteorites has been measured carefully, and it is a strik- 
ing coincidence that the amount corresponds very closely with that needed 
to give rise to the observed heat flow in the earth if it were uniformly of 
stony meteoritic composition. The fact that the radioactive components 
have migrated to the outer part of the mantle does not alter this interest- 
ing and supporting evidence. 

Thus we see an earth in which the central part is rather slowly chang- 
ing, if at all, in temperature and losing heat to the outside very slowly. 
Near the surface, however, there is an important balance in which the 
heat produced by radioactive elements can flow to the surface without 
causing melting unless the system is disturbed in some way. If at the 
margin of a continent the accumulation of sediments formed a low-con- 
ductivity blanket which also contained added amounts of heat-producing 
elements, this might be enough to cause melting at a depth of 100 or 200 
kilometers and give rise to volcanic activity and other effects related to 
mountain building. 

There has been much discussion and difference in opinion about the 
possibility of major convective overturns in the mantle down to the core 
boundary as a result of inhomogeneous distribution of heat sources. This 
process of convection could give rise to surface activity also and could be 
the cause of mountain-building events. In either case it is the heat from 
radioactivity that provides most of the energy for the dynamic events that 
have occurred at the earth's surface throughout geologic time. 



The AMSOC Hole to the Earth^s Mantle 

• H. H. HESS 

HISTORY OF THE PROJECT 

IN JULY 1957 IN WASHINGTON, D.C., THE EARTH SCIENCES 
panel of the National Science Foundation (NSF) had just completed two 
days of hard work evaluating some 60 odd projects requesting research 
grants. The difficult job completed we relaxed to reflect on our work. 
Many of the suggested projects were excellent and most were of high cali- 
ber. Walter Munk remarked, however, that not a single one of them was 
of such a nature that a really major advance in Earth Science would re- 
sult. He suggested the panel invent a project which might strike directly 
at the roots of a major problem and forthwith suggested a hole to the 
Moho. If the writer deserves any credit for past or future association with 
this project, it is that he took Munk seriously and prevented momentarily 
adjournment of the panel. In the few minutes remaining he proposed 
referring the project to the American Miscellaneous Society (AMSOC) 
for action. This was not a joke. It was done for several very good reasons. 
AMSOC has no constitution, no officers, no members, only founders, 
many of whom are distinguished scientists. Here was a society which could 
act immediately— no need to wait for the next council meeting to have 
the proposal referred to a committee which would report to the council a 
year later. Gordon Lill was appointed chairman of the committee to pro- 
ceed with the project and there followed the breakfast meeting in Cali- 
fornia mentioned by Bascom [1959] at which the specific plans were laid 
out. NSF being unable to grant funds to a society without officers or a 
membership list made it necessary that AMSOC get a reputable sponsor. 
The National Academv of Sciences-National Research Council very kindly, 
and I might say, courageously, gave the AMSOC committee a respectable 
home. NSF funds for a feasibility study were then forthcoming and, once 
in hand, Bascom was appointed Executive Secretary of the committee. 
The committee's history from this point on is one of record and can be left 
to historians. 

• From Transactions, American Geophysical Union (Dec, 1959), Vol. 40, No. 4. 

79 



80 H. H. HESS 



I doubt whether Walter Munk was cognizant of Frank B. Estabrooke's 
note in Science, Oct. 12, 1956. But whether he was or not, Estabrooke 
should be given credit for inventing the same project for pretty much the 
same reason at an earher date. 



THE METEORITE ANALOGY AND THE MANTLE 

In 1850 Boisse proposed that the Earth's interior might be analogous 
in composition to meteorites. It was a brilliant proposal for its day and 
in a general way it is probably correct. In detail, perhaps, it is today being 
taken too seriously. It depends on how good the meteorite sample is, con- 
sidering only those seen to fall, and how reliable a sample this is of the 
average composition of the body or bodies from which the meteorites were 
derived. The great majority of stony meteorites are analogous to volcanic 
rocks such as crystal tuffs, lithic tuffs, and breccias. Are we only getting a 
sample of the outer surface of the meteorite parent? Olivine nodules from 
the Earth's interior where ejected from a volcano as bombs are found to be 
friable, the crystals loosely held together. This is probably due in part to 
sudden decompression and in part to the lack of a cementing matrix. The 
mantle of the meteorite parent body might well behave in a similar man- 
ner upon being suddenly extracted from its environment. Were this the 
case the small particles resulting would be swept up by the Sun leaving 
only the somewhat more lithified stony meteorites and irons in orbits 
which might collide with the Earth. 

The decompressed densities of the inner planets (and the Moon) are 
not the same. This must mean that their compositions are not exactly the 
same. It may merely indicate a difference in the degree of oxidation of 
the iron present [see Ringwood, 1959, and many discussions by Urey]. 
It could mean that the initial bodies forming the solar system varied 
somewhat in composition or that their initial compositions have been 
changed by some process which caused losses to surrounding space. 

The uncertainties in the meteorite analogy cannot be resolved by further 
speculation, A hole or holes are required if we are to know something 
more definite about the chemistry of that 84 per cent of the volume of the 
Earth called the mantle. (The crust is less than one per cent of the Earth 
and we know quite a lot about it from observation. It seems quite rea- 
sonable to accept the meteorite analogy to the degree of postulating a 
Ni-Fe core.) A half-ton sample of mantle would probably give more spe- 
cific information than the 1400 odd meteorites now in collections. The 
meteorites could then be properly evaluated to give much additional in- 
formation. It seems foolhardy to put an enormous effort into attempts to 
sample the moon or even the planets without finding out what is 5 km 
below the sea floor. The information from the hole is necessary to attack 



THE AMSOC HOLE TO THE EARTH'S MANTLE 81 

in a well-reasoned manner what we wish to find out about the moon. 
Compared to space exploration the cost is small, 

CHOICE OF SITE 

Drilling a hole in the Moho in a continent is not at present possible be- 
cause it involves depths near 100,000 ft and temperatures too high for 
modern drilling equipment. The high temperatures also present difficulties 
in that many desirable measurements could not be made because the elec- 
tronic devices and electrical cables could not withstand such conditions. 
Oceanic islands were considered during the early stages of the feasibihty 
study. Aside from the fact that these are of volcanic origin, and hence one 
would be drilling into the underpinnings of a volcano, the depth to be 
drilled would be too great to reach the mantle. A small volcanic island ris- 
ing from the ocean floor in depths of 15,000 to 18,000 ft would be about 
40 miles wide at its base. Volcanic islands are in isostatic equilibrium. 
They represent loads in excess of the strength of the crust. If one assumes 
a density of 2.8 g/cc for the volcanic material this load would depress the 
Moho to 78,000 ft. Assuming the lowest reasonable density rather than the 
most probable one would give a depth in excess of 60,000 ft., Figure 1. 




Fig. 1. Section to show how volcanic load depresses the crust (vertical 
exaggeration x 2). The pressure at 25 km, at A and at B, is the same 
assuming perfect isostatic adjustment. One could drill at C and reach 
the mantle but to drill on the island would require a hole more than 
twice as deep. 

This leaves only one alternative, drilling a hole from a barge in the deep 
sea. The Moho can in many places be reached at depths of 30,000 to 
35,000 ft below sea level and only about half of this need be drilled, but 
the overlying water presents some unique problems. The ship must either 
be anchored or maintained in position by some sort of automatic position- 
keeping system. 

In choosing a site the weather conditions must be considered. In general 
this led us to look for one at a latitute less than 25°. It must, if possible, 
be within 500 miles of a port where supplies and repairs can be obtained 



82 H. H. HESS 

and exchange of personnel becomes feasible. The heat flow on the ocean 
floor must be less than two microcalories per cm^ per sec. Assuming a 
conductivity of 0.005 cgs units, two microcalories gives a gradient of 40°C. 
per km or 200°C. approximately at the Moho which is somewhat too high 
for existing well logging instruments. 

Two areas seem to fulfill the above conditions. One is about 120 miles 
north of San Juan, Puerto Rico, and the other south of Los Angeles from 
Guadalupe Island toward Clipperton Island. Seismic work is now under 
way to find places in these areas where a depth to the Moho is favorable 
and where the heat flow is comparatively low. 



DISCUSSION OF SCIENTIFIC OBJECTIVES 

Thus far the primary objective of the project, to sample the mantle, has 
been stressed. Lesser objectives of extraordinary interest and importance 
might be considered as by-products. An outline of the nature of the obser- 
vations and measurements to be made will serve to clarify the objectives. 

{a) If an authentic sample of the material below the M discontinuity 
were obtained, one could establish the following attributes for the following 
purposes: 

( 1 ) Density. The density of materials from the surface to the center of 
the Earth has been computed by Bullen and more recently by^ Bullard. 
These computations are based on the moment of rotational inertia of the 
Earth and are highly sensitive to the initial density assumed at the top of 
the mantle. If an exact figure could be given to this, the validity of the rest 
of the column would be greatly enhanced. The density values could also 
be used to great advantage in analyzing gravity anomalies in oceanic areas. 

(2) Composition, bulk, and mineral phases. If the composition and 
mineralogy of the top of the mantle were known, a much more valid Earth 
model could be constructed. High-pressure and high-temperature research 
could be concentrated on the type of material found rather than on some 
hypothetical preference. The validity of the meteorite analogy as a model 
for the Earth's interior could be tested. The hypothesis of a high-pressure 
phase of basalt existing below the discontinuity could be proved or dis- 
proved, or the olivine nodule (peridotite) hypothesis could be similarly 
examined. 

(3) Radioactivity. A better understanding of the heat budget of the 
Earth might be obtained if the radioactivity of the upper mantle were 
known. Perhaps some clue to explain the anomalously high heat flow from 
the floor of the ocean might result. 

(4) Age. Possibly the M discontinuity represents the primordial surface 
of the Earth and the rock material formed at the beginning of the Earth's 
history. If some means of determining its age could be found, the result 
might be highly significant. 



THE AMSOC HOLE TO THE EARTH'S MANTLE 83 

(5) Isotopes of Pb, and the total Pb and U. If primordial, the isotopic 
composition of the Pb corrected for the radiogenic Pb from U and Th 
present would significantly enhance the understanding of all Pb isotope 
age work. 

(6) After completing the hole, arrangements should be made to collect 
samples of gases or liquids leaking into the hole from the mantle rocks. 
Some clues might be obtained from the amounts and composition of 
these fluids as to the rate and character of additions to the hydrosphere 
and atmosphere from the interior of the Earth, and the past history of 
this process. 

{b) What is the layer immediately above the M discontinuity with a 
seismic velocity near 6.6 km /sec? While it is generally said to be basalt, 
there is no evidence to substantiate this hypothesis other than that the 
velocity is appropriate. It would also be appropriate for a variety of other 
materials. What is the origin of this layer? 

(c) The sedimentary column from the sea floor to the material men- 
tioned above could be sampled. Such a sample in the deep sea might give 
a complete sedimentary column stretching back to the beginning of the 
oceans. The fossil flora and fauna in this column back to the first appear- 
ance of life in the sea would be extremely interesting if it could be ob- 
tained. Or, perhaps, one would find that the oceans are relatively recent 
features on the Earth's surface. In any case here is a whole new world to 
explore. On the average, seismic information indicates about 1000 ft of un- 
consolidated sediment on the ocean floor. Layer 2 below this sediment has 
a seismic velocity ranging from 3.5 to 5.8 km/sec. This might consist of 
lithified sediments, volcanic rocks, or sedimentary rocks with igneous rock 
sills. 

(d) Over-all properties of the materials through which the hole passed 
could be measured to great advantage. 

(1) Thermal. One would like to obtain figures on the temperature 
gradient, conductivity, and a consequent better understanding of heat 
flow. 

(2) Seismic velocity. A seismic velocity log could be obtained which 
would form a better basis to understand seismic results at sea and perhaps 
test for seismic anisotropy in different directions around the hole. 

(3) Magnetism. The magnetic properties of the materials in the hole 
could be obtained. This would certainly lead to a much better means of 
interpreting the magnetic anomalies at sea. The direction and sign of the 
remnant magnetism of the rock samples progressively down the hole could 
be determined, perhaps shedding some light on paleomagnetic problems. 

(4) Electrical properties. Various types of electric logging could be 
done coupled with the laboratory measurements on the samples. 

Above are most of the obvious objectives but, no doubt, in probing into 
new and unexplored territory, the unexpected discoveries might play a large 
role in the final outcome. 



84 H, H. HESS 



PREDICTION AND SPECULATION 



Before a deep hole is drilled the best estimate of what may be encoun- 
tered must be made. The estimator takes the risk of being proved wrong. 

Figure 2 shows a hypothetical column through an oceanic section. This 
is not an average column such as has been published elsewhere [Hess, 
1955a] but one selected for comparatively shallow depth to the Moho. A 
section such as the one north of the Puerto Rico Trench might be ex- 
pected to be somewhat out of isostatic equilibrium thus giving the ob- 
served small positive gravity anomaly and consequently a pressure of 
12,090 kg/cm2 ^^ 40 km instead of the equilibrium pressure of 11,838 
kg/cm^ as estimated in Hess [1955a]. A similar situation might be ex- 
pected on the gentle rise west of the Acapulco Trench in the Pacific. On 



Km 


Woler 




Thickness Density kg/cm^ 
in Km g/cc 




40 X 103 412 ~] 
\^ 4 X 2 9 1160 J 

30 7 X 3 325 10208 
(30 7 X 3 243) (9956) 






Loyer 2 '^ 
Loyer 3 ^"'^Z, 
M — —'^ 

Mantle 

40 


/.*.% 


3 34 
3 325 

3 31 


12090 (11838) 







Fig. 2. Hypothetical crustal column where depth to the Moho is a mini- 
mum; Case 1 represents conditions on a gentle rise outside of a trench 
such as north of Puerto Rico where the column is slightly out of iso- 
static equilibrium (too high, small positive gravity anomaly) ; Case 2 (in 
parentheses) represents conditions where depth to the Moho is relatively 
small as a result of slightly lower than normal density in the mantle 
material. 

the other hand, the small depths to the Moho on the rise near Clipperton 
Island and northeastward probably are an isostatic-equilibrium situation 
with slightly lower density for the sub-Moho material, as is indicated in 
Figure 2 by the quantities in parentheses. The actual thicknesses and layer 
velocities at a site chosen would be determined by seismic work and will 
then have to be specifically considered. The present example represents an 
average set of conditions for a favorable site as deduced by available 
seismic information. 

The material of Layer 1, the unconsolidated sediments, is well known 
from cores obtained at sea and requires no further discussion. Layer 2 is 
very variable in thickness and in seismic velocity Vp which ranges from 
about 3.5 to 6.0 km /sec. No doubt its make-up is variable too. Presum- 



THE AMSOC HOLE TO THE EARTH'S MANTLE 85 
ably it consists of consolidated sedimentary rocks or volcanic rocks or both. 

The total thickness of Layers 1 and 2 is of the order of 1.3 km and if 
considered to be all sedimentary rock it is surprisingly small in amount 
considering present-day rates of sedimentation. Measurements made on 
cores of this commonly give a rate of about 1 cm /1 000 yr. A minimum 
rate seems to be about 1 mm/ 1000 yr. If the oceans are postulated to be 
three billion years old, this would mean 30 km of sediment at the faster 
rate or 3 km at the slowest. The quaternary may be abnormal in its contri- 
bution of sediment to the sea because of Pleistocene glaciation, but this 
argument does not seem particularly convincing to account for a rate 
perhaps 50 times normal. The most obvious alternatives are: (1) The 
oceans are relatively young. At 1 cm/ 1000 years the sediment could be 
accounted for if sedimentation only started in the Cretaceous. (2) The 
pre-Cretaceous sediments have in some manner been removed; for ex- 
ample, by incorporation into the continents by continental drift. ( 3 ) Non- 
deposition of any sediment over much of the ocean floor was a common 
attribute of the past. In any of these cases, those who expect a complete 
record far back to billions of years ago are doomed to disappointment. It 
will be extremely interesting when the well is drilled to find out which 
of these alternatives (if any) prove to be correct. In any case I would 
predict (though I am rooting against this prediction) that a very incom- 
plete section will be found. 

Layer 3 is commonly referred to as "the crust" and is generally con- 
sidered to be basalt. The reasons for calling it basalt are in part legendary, 
and in part based on its seismic velocity, which commonly ranges from 

6.4 to 6.8 km/sec. Other than its seismic velocity there is no compelling 
reason for concluding it is basalt. It may be basalt or it may be serpen- 
tinized peridotite (Fig. 3). It could possibly be some other rock type. 
Serpentinized peridotite is favored by the writer because such material has 
been dredged from fault scarps on the Mid-Atlantic Ridge in three places 
by investigators from Lamont Geological Observatory [Shand, 1949]. Ba- 
salts have also been dredged from this Ridge but could be attributed to 
debris from nearby seamounts or volcanic islands. The unique thing about 
Layer 3 is its comparatively great uniformity in thickness. Figure 4 is a 
plot of frequency of occurrence of thicknesses for this layer. This is based 
on all of the published seismic profiles in the deep sea plus some unpub- 
lished data of Raitt but omits those profiles which appear to be compli- 
cated by seamounts or islands in the immediate vicinity. Note that 81 per 
cent of the cases in the sample examined range in thickness from 4.0 to 

5.5 km. This range necessarily includes observational error which reason- 
ably could be considered to be ±0.5 km. Raitt (personal communication) 
finds that the average thickness of Layer 3 for all of his profiles, selected 
in much the same way as the data for Figure 4 were selected, comes to 
434 km. 

The surprising uniformity in thickness of Layer 3 requires that the bot- 



86 H. H. HESS 

g/cc 
3.40 



3.30 



3.20 



3.10 



300 



2.90 



2.80 



2.70 



2.60 















°Vy 


^0% 














// 
















Vz^u 












/ 


















/' 


)7o 














/ 
















/ 


75% 












- 1007 


A 


f 
)07o 































5.6 



6.0 



6.4 6.8 7.2 7.6 
Vp Km/Sec 



8.0 



8.4 



Fig. 3. Variation in velocity of Vp, density p , with per cent serpentization 
of peridotite; solid curve represents relationships for shallow depth and 
dashed curve an estimate of relationships at 15 km below sea level; solid 
curve based on laboratory measurements by J. Green at the California 
Research Laboratory, La Habra, except for sample of 100% serpen- 
tinized peridotite which was measured by Francis Birch at the Dunbar 
Laboratory, Harvard University; Birch's measurements were made at 
room temperature and pressures from to 10 kilobars. Green's measure- 
ments were made with variable temperature up to slightly more than 
200 °C. and pressures up to one kilobar. 



torn of the layer represent the position of an isotherm or past isotherm, 
and that this is a level at which a reaction or phase transition has taken 
place. If the layer were basalt flows, one would expect great variability in 
the thickness. Flows would be many times thicker near a vent or fissure 
from which they issued than at greater distances from their source. 

This leaves two alternatives: (1) that Layer 3 is basalt but that rocks of 
this chemical composition extend down into the mantle and are con- 
verted to eclogite (this was originally suggested by Sumner [1954] and 
also later by Kennedy [1956] and others), or (2) that the "crust" and 
material below are peridotitic in composition and an abrupt change from 
partially serpentinized peridotite to unserpentinized peridotite occurs at 
the Moho. The latter case would be consistent with Ringwood's [1958] 
model for the mantle and the writer's [Hess, 1955b], In this case the 
Moho under the oceans would represent some ancient time when the 
500°C. isotherm stood at this datum plane below sea level. It must be 
very much deeper than this today. 



THE AMSOC HOLE TO THE EARTH'S MANTLE 87 



65-7.0 


1 


3 


6.0-6.5 


1 






5.5-60 


1 






5.0-5.5 


1 






4.5-5.0 


1 






4.0-45 


1 


3.5-4.0 


] 


3.0-3.5 


1 







Fig. 4. Frequency histogram for thickness of Layer 3, the so-called 
"basalt" layer. 



Summary of predictions: 

(1) Layer 1 consists of unconsolidated sediments. 

(2) Layer 2 consists of consolidated sedimentary rocks with or without 
volcanics. 

(3) The so-called "basalt," Layer 3, is serpentinized peridotite such as 
has been dredged from the Mid-Atlantic Ridge. 

(4) The mantle will be peridotitic and the same composition as the 
olivine nodules of Ross and others [1954] and in Hess [1955b] and also 
the same composition as St. Paul's rock. Ringwood's model will be sub- 
stantially correct. 

(5) The sedimentary-rock section of Layers 1 and 2 will be very in- 
complete. 



CONCLUSION 

Geophysicists and geologists dealing with the solid Earth have tended 
to be conservative in their objectives. Excellent though their projects may 
have been, no one or group of them could possibly break through to com- 
pletelv new ground. Let us "take the bull by the horns" and find out 
what this planet upon which we reside is really made of instead of relying 
on ethereal analogies. This is a courageous project which deserves support. 
Besides this it fits Revelle's classic definition of good research, it will be 
fun to carry out. 



REFERENCES 

Bascom, W., The Mohole, Sci. Am., 200, 41-49 (1958). 
Hess, H. H., The Oceanic Crust, /. Marine Res., 14, 423-439 (1955a). 
Hess, H. H., Serpentine Orogeny and Epeirogeny, Geol. Soc. Am. Spec. Pap., 62, 391- 
408 (1955b). 



88 H. H. HESS 

Kennedy, G. C, Polymorphism in the Feldspars at High Temperatures and Pressures, 
Bui. Geol. Soc. Am., 67, 1711-1712 (1956) (abstract). 

Ross, C. S., M. D. Foster, and A. T. Meyers, The Origin of Dunites and Olivine-Rich 
Inclusions in Basaltic Rocks, Am. Mineralogist, 39, 693-737 (1954). 

Ringwood, A. E., The Constitution of the Mantle, III, Geochim. Cosmochim. Acta., 
15,195-212 (1958). 

Ringwood, A. E., On the Chemical Evolution and Densities of the Planets, Geochim. 
Cosmochim. Acta., IS, 257-283 (1959). 

Shand, S. J., Rocks of the Mid-Atlantic Ridge, /. Geol, SI, 89-92, 1949. ^ ^ 

Sumner, John S., Consequences of a Polymorphic Transition at the Mohorovicic Dis- 
continuity, Trdns. Am. Geophys. Union, 35, 385 (1954) (abstract). 

Urey, H. C, Diamonds, Meteorites, and the Origin of the Solar System, Astrophys. J., 
124,623-637 (1956). 



CRUSTAL 
FEATURES 

AND 
PROCESSES 



Go my Sons, buy stout shoes, climb 
mountains, search the valleys, the deserts, 
the sea shores, and the deep recesses of the 
earth. Look for the various kinds of 
minerals, note their characters and mark 
their origin. Lastly buy coal, build furnaces, 
observe and experiment without ceasing, for 
in this way and in no other will you arrive 
at a knowledge of the nature and properties 
of things. — SEVERiNus as quoted by 
Wallerius, Systema Mineralogicum (Vienna, 
1778) 



The Crust 



• J. TUZO WILSON 



MAN'S HOME-THE SURFACE OF THE EARTH-IS A SMALL 

and temperate shelter set in a vast and alien universe. The most remote 
oasis of the deserts is not to be compared to it for solitude. Well may man- 
kind glory in its fertile plains, its snow-topped pinnacles, its mighty oceans, 
for they are rare examples of moderation in a universe where extremes of 
heat and cold prevail. Through space too vast to comprehend there is 
darkness blacker than midnight and cold which is nearly absolute. For 
the most part space is empty, but at rare intervals dust as tenuous as the 
aurora lights up to the fiery glow of another Sun, a furnace hot with 
nuclear fire. 

Many theories suggest that around millions of other stars solar systems 
may revolve, but none can be seen. Within our own system no other 
place but the surface of the Earth is habitable. Certainly the other plan- 
etary bodies whose solid surface we can see— the Moon, Mars and Mer- 
cury-are not. 

The utter contrast between the surfaces of the Moon and the Earth, 
whose environments in space have been so similar, is particularly striking. 
The Moon's surface is dry and without air. On it there are no continents, 
no long ranges of mountains, and no active volcanoes, but instead a multi- 
tude of meteorite craters of all sizes, which are almost lacking on Earth. 

• From The Planet Earth, ed. D. R. Bates (New York: Pergamon Press Inc., 1957), 
pp. 48-73. 

89 



90 J. TUZO WILSON 

Various reasons suggest that the Earth's surface was once hke that of the 
Moon and that the Earth has developed its crust, its oceans and its atmos- 
phere, while the Moon has remained unchanged. 

On Earth the greatest miracle is life, but the combination of circum- 
stances which have made life possible is hardly less remarkable. An abun- 
dance of water and the emergence of dry land above it are the unusual 
attributes of the Earth to which we owe our existence. This favourable 
environment has developed because of two circumstances unique in the 
solar system and of great rarity in the universe. One factor has been that 
for several thousand million years the Sun's heat has maintained most of 
the Earth's surface in the narrow temperature range of liquid water. If the 
surface had been too cold it would have become solid and inert; if too hot 
it would have vaporized. Extremes of heat and cold are inimical to life in 
any form, so that we can be sure that suitable conditions for any kind of 
creatures, even ones very different from those we know, are rare. 

The second factor has been activity within the Earth. By good fortune, 
heat generated by the disintegration of radioactive minerals, combined 
with that given to the interior of the Earth during its early history, has 
provided energy for earthquakes and volcanism. Their activity has sufficed 
to uplift lands continuously above the eroding sea and maintain them as 
island homes. Moreover, it seems reasonable to conclude from the many 
sources of information which modern geophysical science has placed at 
our disposal, that the atmosphere, the oceans and the crust of the Earth 
have all been brought forth from the interior by volcanic and seismic ac- 
tivity during the planet's long history. Thus oceans and continents, with 
their vast ridges and trenches, valleys and mountains, have gradually been 
constructed on top of the original surface of the Earth, This now forms 
the base of the crust. It is hidden and only known to us from the echoes 
of seismic waves which it reflects. This view is a new one and not yet 
widely understood, but it seems forced upon us by our expanding knowl- 
edge. 

Consideration of the rate at which gases, steam and lava are poured 
forth by volcanoes has led to the idea that the atmosphere, oceans and 
rocks of the crust have all been produced by volcanicity. Studies of their 
composition and abundances strengthen this view. A rate not much 
higher than that at which volcanoes emit lava today would have sufficed 
to build the entire crust during the age of the Earth, which has recently 
been proved to be 4-5 thousand million years. This has made it possible 
to recount the growth of the world to its present state instead of merely 
describing its appearance. Geological features which were once a catalogue 
of details to be memorized by students are now beginning to take their 
places in an ordered story of evolution. But it is still a difficult story to 
tell, and will remain so until there has been time to fill the gaps and re- 
move uncertainties in our new kinds of information. It is important to 



THE CRUST 91 

emphasize the embryonic state of our new ideas about the Earth, for in 
this brief account we cannot dwell on the uncertainties, nor do justice to 
all the conflicting theories and suggestions, which for the present form 
part of the new and evolving history of the whole Earth. 

It seems simplest to begin with an account of the fracture systems 
which have controlled activity and guided the growth of the crust, then to 
discuss the rocks which furnish clues to many phases of its history, and 
finally to describe the great ocean floors and the growth upon them of 
submarine mountains, islands and continents. 

EARTHQUAKES AND FRACTURES 

The flow of lava and hence the building of the different parts of the 
crust is related to systems of fractures along which seismic and volcanic 
activity take place. Many parts of the Earth's crust have been fractured in 
the past. The faults mapped by geologists are scars that show where former 
displacements have occurred. Along active fractures, intermittent move- 
ments produce shocks called earthquakes, which are felt in the vicinity 
and recorded on sensitive seismographs all over the world. By studying 
these records and triangulating from the stations, seismologists can tell us 
when, where and at what depth any particular shock occurred. 

To collate and publish the data on earthquakes collected by all of the 
world's 600 seismological observatories, there is an organization called the 
International Seismological Summary, under the direction of Sir Harold 
Jeffreys of Cambridge. This information has been analysed by B. Guten- 
berg and C. F. Richter of California, who have shown in detail how all 
the world's important earthquakes are arranged along one of two narrow 
systems. Most of the world's volcanoes lie along these systems also, so 
that it is natural to suppose that active fractures provide the relief of pres- 
sure and the channels by which volcanic materials escape from the hotter 
interior of the Earth, 

The more active of these systems lies for the most part along continental 
margins and is here called the continental fracture system. It is formed of 
two belts which enfold the world in the shape of a great T. The stroke 
of the T extends along the Mediterranean region through the Alpine, 
Turkish, Persian and Himalayan Mountains, through Indonesia, New 
Guinea and other islands to New Zealand. The stem of the T springs from 
a junction in Celebes to encircle the Pacific Ocean through the Phil- 
ippines, Japan, Alaska, the Cordillera and Andes of the Americas, to 
Antarctica. 

The stem and western limb of the pattern each consist of a series of 
arcs joined end to end, which are but the surface expression of great 
conical fractures whose shape and position have been indicated by plot- 
ting the location of many earthquakes. Several of the cones extend to 



92 J. TUZO WILSON 

depths of 450 miles, which is over one tenth of the Earth's radius, but 

deeper shocks have never been recorded. 

The other principal fracture system is followed by the line of the mid- 
ocean ridges after which it is here named. The mid-Atlantic ridge, along 
which Jan Mayen Island, Iceland, the Azores, Ascension Island and Tristan 
da Cunha are peaks, is the best known part, but the mid-ocean fracture 
system is continuous and worldwide. M. Ewing of Columbia University 
has recently pointed out that the mid-Atlantic ridge turns and continues 
beneath the Southern Ocean south of Africa to connect with the Carlsberg 
ridge in the Indian Ocean, and thence south of Australia to join the prin- 
cipal ridges of the Pacific Ocean. Its pattern is irregular and not made up 
of a series of arcs like the continental system. All of the earthquakes along 
it are shallow— that is, less than 45 miles to their foci. Connecting these 
two principal systems and branching from them are many ancillary faults. 
Some of these are well known, both on land and on the sea floor, but the 
whole pattern has by no means been elucidated. 



The movement on fractures takes place a little bit at a time, giving rise 
to earthquakes. The displacement in a single earthquake is often several 
feet. For example, in the central zone of the San Francisco earthquake of 
1906, the whole surface of the Earth on one side of the fault was hori- 
zontally displaced by 21 feet relative to the other side. Fences, roads and 
houses lying across the fault were torn apart. 

Since only narrow belts about the Earth are seismically active, people in 
most parts of the world have never experienced a severe earthquake; but 
along the active fracture system people feel them every few weeks. The fol- 
lowing account of the great Assam shock of 1951, published in Nature by 
Captain F. Kingdon-Ward, gives some idea of the great forces released 
at the central region in a major earthquake. 

"Suddenly, after the faintest tremor (felt by my wife but not by me) there 
came an appalling noise, and the Earth began to shudder violently. I jumped 
up and looked out of the tent. I have a distinct recollection of seeing the out- 
lines of the landscape, visible against the starry sky, blurred — every ridge and 
tree fuzzy — as though it were rapidly moving up and down; but fifteen or 
twenty seconds passed before I realized that an earthquake had started. My wife 
shouted: "Earthquake!" before I did, and leapt out of bed. Together we 
rushed outside, I seizing the oil lantern which I placed on the ground. I was 
conscious of fearing that the tent would catch fire. We were immediately 
thrown to the ground; the lantern, too, was knocked over, and went out 
instantly. 

"I find it very difficult to recollect my emotions during the four or five 
minutes the shock lasted; but the first feeling of bewilderment — an incredulous 
astonishment that these solid-looking hills were in the grip of a force which 



THE CRUST 93 

shook them as a terrier shakes a rat — soon gave place to stark terror. Yet my 
wife and I lying side by side on the sandbank, spoke quite calmly together, 
and to our two Sherpa boys, who, having already been thrown down twice, 
were lying close to us. 

"The earthquake was now well under way, and it was felt as though a 
powerful ram were hitting against the Earth beneath us with the persistence of 
a kettle-drum. I had exactly the sensation that a thin crust at the bottom of the 
basin, on which we lay, was breaking up like an ice floe, and that we were 
all going down together through an immense hole, into the interior of the 
Earth. The din was terrible but it was difficult to separate the noise made by 
the earthquake itself from the roar of the rock avalanches pouring down on all 
sides into the basin. 

"Gradually the crash of falling rocks became more distinct, the frightful 
hammer blows weakened, the vibration grew less, and presently we knew that 
the main shock was over." 

The cause of the formation of fractures is not absolutely known. Some 
authorities believe that great but slow convection currents of a plastic 
nature occur in the mantle, but there has never been any direct evidence 
for the existence of these or any agreement about their nature. It is not 
clear why this flow should create fractures, nor have these theories been 
developed to a stage where they can explain the details of the Earth's 
surface as seen by geologists. A better theor}^ seems to be the much older 
one that the Earth is cooling and shrinking, and that as a result its outer 
parts crack in this rather special way. The emission of volcanic matter 
causes further contraction. 



KINDS OF ROCKS 

There are three principal classes of rocks. Those formed from lava are 
called volcanic rocks; those originally deposited on the sea floor and sub- 
sequently hardened are called sedimentary rocks; while those of either 
class which have been greatly recrystallized and altered are called meta- 
morphic or plutonic rocks. The name igneous is often used to cover all 
volcanic rocks and some of the plutonic rocks which most resemble them. 

Volcanic rocks 

Under this heading we will consider only those rocks which are known 
to rise as liquid lava along fractures. Their importance may be judged from 
the following simple calculation. In 1927 K. Sapper estimated the volume 
of all lava and ash poured out by volcanoes all over the world since a.d. 
1500. The average rate of one fifth of a cubic mile per year seems moderate 
enough, but consider the implications. If this rate had been constant dur- 
ing the total history of the Earth, enough lava would have been produced 



94 J. TUZO WILSON 

to cover all the continents with 18 miles of lava, but the continental crust 
is only 20 miles thick and the oceanic crust is smaller. Since the present 
rate is probably lower than that which prevailed in the remote past, it is 
likely that enough lava has been poured out to provide material for the 
whole crust. However, the lava is modified by processes to be described, 
before being incorporated into the crust. 

Andesitic volcanics—Andesite is the name of the most abundant type of 
lava emitted along the continental fracture system. It contains about 60 
per cent silica (SiOg), the remaining 40 per cent being made up of ele- 
ments common in many rocks, aluminium, iron, magnesium, calcium, so- 
dium and potassium. Most of the world's 480 volcanoes lie along the con- 
tinental system and emit mainly andesitic lavas, with lesser quantities of 
more siliceous lava called rhyolite (about 70 per cent silica), and of less 
siliceous lava called basalt (about 50 per cent sihca). It is these lavas, and 
chiefly andesite, which are believed to have supplied the materials out of 
which the continents have been built. 

Basaltic volcanics—The only group of lavas found along the mid-ocean 
fracture system and on the scattered volcanoes of the ocean floors are the 
less siliceous basalts and certain variants formed during their crystalliza- 
tion. Unlike andesites, basalts are found in all parts of the world, for they 
are emitted by volcanoes of both systems although subordinate to ande- 
sites in the continental system. 

The sources of lava— Basalts, without any andesites, flow from fracture 
systems which earthquakes show to be shallow, less than 45 miles deep, 
but andesites with a mixture of some basalt, flow from systems which 
earthquakes show to be up to 450 miles deep. It seems logical to explain 
this by suggesting that these lavas are derived from different layers within 
the Earth, the basalts originating in a shallow layer by partial melting of 
the mantle, while the andesites come from a deeper layer, bringing some 
basalt with them as they rise through the upper layer. 

Plutonic rocks 

The coarsely crystalline rocks which have formed at depth within the 
crust are given the name plutonic. Some of these are igneous rocks which 
have formed from trapped lavas which have cooled slowly. Others are vol- 
canic and sedimentary rocks which have recrystallized under high tempera- 
ture and pressure to form metamorphic rocks, many of which are called 
gneisses. Gneisses derived from sediments are the commonest rocks of the 
continental shields. In some cases the products of these two processes are 
so similar that their particular origin may be obscure. 

Among the igneous plutonic rocks, granite, granodiorite, and gabbro are 
the coarse-grained chemical equivalents of rhyolite, andesite and basalt 
respectively. 



THE CRUST 95 

Sedimentary rocks 

The classification of sedimentary rocks has always proved difficult, be- 
cause they are variable mixtures of precipitates and material worn or 
broken off other rocks. Traditionally the classification into rock types has 
been based upon texture and composition. Gravel and conglomerates are 
coarse, silt and shale are fine, sands are intermediate in texture. Everyone 
knows the chief constituents of sandstone and limestone. Shales are clay 
with an admixture of sand and lime. Less well known are arkoses, which 
consist predominantly of feldspar with quartz and a little mica, and grey- 
wackes which are a mixture of quartz and mica sometimes with a little 
feldspar. Much less common are black shale and coal, salt deposits and 
iron formations. 

For the purposes of broad regional description, such as are involved in 
this chapter, these classifications are not useful because several different 
rock types commonly occur together, T. D. Krynine and F. J. Pettijohn 
have shown that these associations are not random, and they have worked 
out genetic classifications, or fades, of rocks, based upon occurrence and 
origin. 

Borderland fades— This facies is sometimes termed graptolitic, from the 
graptolite fossils frequently found in its shales, but more commonly eu- 
geosyndinal, literally, 'more of a large earth downfold'. These sediments 
are those which are piled up along island arcs, swept into ocean trenches 
and accumulated in deltas and on continental shelves. They consist chiefly 
of shales and greywackes, the ill-sorted products of erosion of lavas and 
the finer material carried from continents. Since they accumulate along 
the borderlands of the continents in vast volumes and on the marginal 
ocean floors which are several miles deep, they form very thick sequences, 
often slumped and contorted. 

Rocks of this facies are by far the most abundant, but this is not readily 
apparent because most of them are below sea level and invisible until 
metamorphosed and uplifted into young mountains. By that process they 
are changed to plutonic rocks whose origin is disguised. Nor is the origin 
any more apparent when the mountains have been eroded to the gneisses 
of continental shields, although the average andesitic composition is pre- 
served throughout. Thus there is a cycle among the rocks in which lavas are 
broken down by weathering to sediments and sediments are metamor- 
phosed to plutonic rocks, some of which may be recycled by being eroded 
again to form more sediments. 

Platform fades— As the level of the ocean has fluctuated and as eugeo- 
synclines have weighed down the continental margins, shallow seas have 
often penetrated far inland over the continental crust. The North Sea, 
Hudson Bay and the shallow seas north of Australia are present day ex- 
amples. Minerals derived from the crust are washed by waves, cleaned and 



96 J. TUZO WILSON 

sorted, until the beds laid down consist at the base chiefly of pure sand- 
stone and grade up into shales and pure limestones. Evaporation and 
shallow water encourage the growth of corals where the climate is warm, 
and the formation of limestone. These platform rocks, widely exposed on 
every continent and full of fossils, are the stratigrapher's delight. They 
have come to be regarded as typical sediments, although they are in truth 
a rather special and ephemeral form which with the passage of time be- 
come eroded away and carried to more permanent resting places at the 
borderlands. 

These platform rocks grade into the borderland deposits, and at the 
junction may be preserved as wedge-shaped basins which are frequently 
called miogeosynclines, literally 'less of a large earth downfold'. 

Piedmont fades— Ahei great mountains are uplifted, they are rapidly 
eroded. Torrents sweep coarse, ill-sorted and undecomposed fragments, 
down into piedmont fans, into swampy basins on the inner sides of young 
mountains and into basins between ranges. These beds are predominantly 
red arkoses, but they also contain black shales, coal and occasionally cop- 
per-rich beds. Some examples are the Keweenawan rocks around Lake Su- 
perior, the Old Red Sandstone of Scotland, the Red Molasse of the Alps, 
and the Newark and Catskill series in the Appalachians. These rocks may 
be formed on top of miogeosynclines formed earlier in the same cycle. 

THE OCEAN BASINS 

The largest part of the crust is occupied by the world's ocean basins, 
which cover over 70 per cent of its surface, an area of 140 million square 
miles. Mapping this vast extent is an enormous and expensive task. It re- 
quires ships especially equipped and despatched for the purpose, since 
merchant ships have neither time nor facilities for exploration, and travel 
relatively restricted sea lanes. Most charts have been made by the world's 
navies, but other scientific work has been carried out by a hundred or so 
oceanographical expeditions. 

Soundings by lead and wire reached the deep ocean floors a century 
ago, but until as late as 1920 our knowledge of submarine topography 
was very scant. When the time-consuming method of sounding with lead 
and wire was replaced by modern echo-sounding methods, it became pos- 
sible to make continuous records of the time required for echoes to return 
to a ship from the sea floor. Properly scaled, these give profiles of depth. 
From them good charts have been prepared of many coasts and of the 
northern ocean floors. During the International Geophysical Year ships 
will be making great voyages to chart the unfrequented southern oceans. 

Scientific study of the deep ocean floor was initiated by the great Chal- 
lenger Expedition of 1872-6. In addition to making soundings, the expe- 
dition used dredges to collect grab-samples from the bottom. Later, corers 



THE CRUST 97 

were introduced, but they did not penetrate far until 1947, when B. Kul- 
lenberg of the Albatross Expedition devised a piston which used hydro- 
static pressure to help draw cores into the barrel. With such a device the 
Russians have cored as deeply as 100 feet into the floor of the Arctic 
Ocean, while other oceanographers have collected over 1,000 long cores 
from the deep oceans. 

Just as the sea floor reflects the signals of echo-sounding equipment and 
so reveals its depth, so do interfaces between layers of the crust reflect or re- 
fract back the stronger seismic waves generated by small explosions. Thus 
by dropping charges overboard at intervals, a ship or a pair of ships suitably 
equipped can receive these echoes and measure depths to layers within the 
crust. Other devices for studying the crust below the oceans are F. A. 
Vening Meinesz's method of determining gravity at sea, and Sir Edward 
Bullard's ingenious probe for measuring the rate at which heat is lost from 
the Earth by flowing out through the ocean floors. 

On the Earth's surface there are two main levels, that of the land plains 
and that of the ocean floors. The latter cover much larger areas and are 
about 3 miles below the general level of the land. We can think of the 
ocean floors as being close to the original surface of the Earth, only sep- 
arated from the mantle by an average of 3 miles of mud and lava flows. 

Not only do the ocean basins occupy a larger part of the crust than do 
the continents, but the topography of their floors is grander— the peaks are 
higher, the canyons deeper and the ranges longer than any on land. For 
example, the Hawaiian Islands rise 33,000 feet from the floor of the Pa- 
cific Ocean: the mid-ocean ridges form a continuous chain tens of thou- 
sands of miles long. No valleys on land in any way compare with the great 
trenches hundreds of miles long lying off island arcs at depths of from 
10,000 to 15,000 feet below the general floors of the ocean. The greatest 
known depth of 35,840 feet is reached in one of them, the Mariana Trench 
near Guam Island. Between these more striking features and covering much 
of the ocean floor are vast abyssal plains deep, flat and extremely smooth. 

It was once supposed that the deep oceans had remained dark, lifeless 
and unchanged, save for the finest rain of sediment, since the world be- 
gan; but new knowledge has quite dispelled this view. Across the ocean 
floor geophysicists have now traced great fractures, scarps and rifts, have 
found scattered volcanic peaks and ranges, and have charted canyons cut 
by slumps and flows of mud on the continental margins. From time to 
time earthquakes unleash huge mud slides on the continental slopes. On 
its own tremendous scale the ocean floor is slowly active, and the great 
features raised upon it are preserved in unseen majesty from the eroding 
effects of the atmosphere, each portraying its origin more clearly than do 
similar features on land. 

The continental blocks— Ower one quarter of the surface of the crust are 
reared the continental blocks. They are like solid rafts set in a solid sea. 



98 J. TUZO WILSON 

Nevertheless they may be said to float after a fashion, for their rocks are 
hghter than those of the ocean floor. In addition to rising 3 miles above 
the ocean floors their light roots of continental material sink to a depth 
of about 14 miles and depress beneath them the 3 miles of basalt lavas 
corresponding to the ocean floor. Thus the whole crust under the conti- 
nents is 20 miles in thickness and is in hydrostatic equilibrium with the 
oceans. 

The margins of the continents are flooded over by waters resting on the 
continental shelves. These may be anything up to 500 miles wide. Their 
edges are usually marked by a sharp increase in slope, frequently occurring 
at a depth of about 600 feet. It is believed that the shelves were cut to this 
depth during the last great glacial period when ice caps over much of the 
northern hemisphere lowered the oceans by this amount. The steep sides 
of the continental blocks are called the continental slopes. 

Island arcs and trenches— Lying in most cases off the margins of conti- 
nents are chains of island arcs, such as those off the coast of east Asia and 
in the West Indies. Seismically and volcanically they are the most active 
and mobile features of the Earth. Parallel with them along their convex 
sides are located all the deepest trenches in the oceans, so that together 
these features are part land and part ocean. They appear to indicate 
where continents are growing, and we will leave further discussion of 
them and of continents until later. 

The mid-ocean ridges — Apart from the continents, the greatest features 
rising from the ocean floor are the mid-ocean ridges whose extent has al- 
ready been described. The first discovered was the mid-Atlantic ridge, and 
it has only recently been shown to be connected with ridges in other oceans. 
These ridges are largely if not entirely composed of lava and volcanic 
debris and along them a concentration of shallow earthquakes has assisted 
in locating them and leaves no doubt that their volcanoes rise along a 
great fracture system. Great rifts and scarps which cut the volcanic rocks 
along the crest of the ridge show that movement and volcanism have al- 
ternated intermittently. 

These ridges form a continuous system at least 40,000 miles long. They 
are often 200 miles wide and usually rise at least 10,000 feet from the ocean 
floor. Along the margins in some places are depressions, suggesting that 
the weight of the ridges has bowed down the crust on which they rest, so 
that in addition to the exposed parts they may have roots. No one has 
measured the depth or volume of these ridges, but it is very great and the 
volcanic activity that has built them is only sporadic and feeble. Clearly 
they have taken a vast length of time to accumulate — very likely most of 
the history of the Earth. The concept of uniformitarianism, that is, that 
the effect of natural laws on the Earth is constant, is a fundamental and 
sound one. The fact that these ridges are active and growing today sug- 



THE CRUST 99 

gests that this has been their behaviour in the past. The rates of growth, 
the scarcity of inert or abandoned ridges, and the impossibihty that any 
ridges once formed could disappear, all suggest that these mid-ocean ridges 
are very old and fundamental structures of the crust. 

The foci of the earthquakes along them are all at depths up to 45 miles, 
and none are deeper. A depth of 45 miles is well within the mantle 
and the temperatures there may be near the melting point of iron and 
magnesium silicates which probably constitute the mantle. The lavas 
along the mid-ocean ridges are basalt, which geochemists consider could 
be formed by partial melting of the mantle. It seems reasonable to believe 
that at times the fracturing below the ridges causes enough relief of pres- 
sure to allow pockets of lava to form. All these lavas have little gas, are not 
viscous and flow quietly out of the fractures. This accounts for the tran- 
quil nature of the volcanoes on Iceland, Hawaii and other mid-oceanic 
islands. From such lavas have the mid-ocean ridges been built. 

Ocean scarps— The fractures along the mid-ocean ridges, although they 
may be the chief ones, can hardly be the only ones on the ocean floor. 
During the past five years, R. Revelle, H. Menard and other oceanog- 
raphers sailing from California, have proved the existence of five great 
scarps running east and west for thousands of miles across the floor of the 
Pacific, spaced at regular intervals between San Francisco and the Gala- 
pagos Islands. These features are marked by cliffs up to two miles high, by 
lines of volcanic peaks and by changes in the nature of the sea floor on 
their two sides. For example, the floor may be smooth on one side of the 
scarp and fractured and covered with submarine peaks on the other side. 

1. Tolstoy has pointed out that a hne of sea mounts and scarps extend 
across the Atlantic Ocean from near Gibraltar through the Azores to the 
south side of the Grand Banks. Doubtless other scarps will be found, but 
in many parts of the oceans deep sea sediments may have largely buried 
them. 

Seamounts and guyots — Along these scarps and scattered elsewhere over 
the oceans are thousands of seamounts, that is, volcanic peaks which do 
not break the surface of the water. The pattern of their abundance and 
distribution is portrayed in the Micronesian Islands, the one region where 
such volcanic peaks appear as islands rather than submarine seamounts. 

A curious feature of seamounts is that the summits of many of them 
(called guyots) are flat and uniform. This cannot be an original volcanic 
feature, and H. H. Hess of Princeton has suggested that these seamounts 
formed as island peaks, became inactive and were long ago cut down to 
a former sea level. At first it was thought that the sea was once shallower, 
but opinion now is that the crust was not able to support these loads, and 
that they have slowly settled to their present depths. On some, corals have 
been able to build reefs at a rate equal to the settling and thus preserve the 
islands in the form of coral islands or atolls. Although the bases of many 



100 J. TUZO WILSON 

guyots are hidden in sediments, their frequent straight ahgnment suggests 
their connection with crustal fractures. 

Continental slopes, turbid currents and abyssal plains— That rivers de- 
posit much mud is made apparent in the rapid silting of harbours. Finer 
silt is swept out to sea and there slowly settles. One of the Spanish cap- 
tains wrote in 1518 of the Amazon, that it 'carieth such abundance of 
water and it entreth more than twenty leagues into the Sea, and mingleth 
not'; but the prodigious volume of the silt so carried was not measured 
until this century. Most of it settles close to shore upon the continental 
shelves and slopes, and indeed it is what they have largely been made of, 
as drilling for oil in the Atlantic and Gulf Coast shelves of the United 
States has shown. 

When detailed charts were first made of continental shelves, it was seen 
that they were scoured and furrowed as by gigantic slumps, and great 
canyons were discovered cut in their edges and extending to depths of 
12,000 feet or more. Laboratory experiments showed that it was possible 
for muddy flows to travel on the bottom beneath clear and lighter water, 
but there was some reluctance to abandon ideas of a still and silent sea 
bottom for one in which underwater flows cut canyons mightier than those 
of the Indus or Colorado rivers. 

The matter was settled by an ingenious explanation of the events which 
followed the Grand Banks earthquake of November 18, 1929. On that 
date at 8.32 p.m. the world's seismographs recorded a severe shock which 
shook the coast of Newfoundland and, according to records kept by the 
telegraph companies, instantly broke the six cables nearest to the focus. So 
much was normal and easily understood, but the telegraph companies' 
records also showed that at intervals during the next thirteen hours six 
other cables progressively farther from the focus were broken. Repair 
crews found that the breaks were not clean, but that scores of miles of 
cables were missing and that the broken ends were abraded and torn. 

The cause of this was a mystery until 1952, when B. C. Heezen and 
M. Ewing of Columbia University showed that if the shock which oc- 
curred on the continental slope had set a great slump in motion and 
stirred up turbid currents, these could have swept down the slopes to the 
deep abyssal plains on the ocean floor, breaking the cables as they reached 
them. The current would have reached a velocity of about 55 miles an 
hour soon after its start and would gradually have slowed down as it 
crossed the flatter ocean floor. Cores taken at the foot of the slope showed 
a succession of layers of sand, each grading up to finer silt and each inter- 
preted as the deposit laid down by one turbid current. Heezen and Ewing 
suggested that such currents are released whenever enough mud is piled 
up on a slope, at intervals varying from a few years to a few hundreds of 
years. 

Accurate charting of the floor of the north Atlantic Ocean has enabled 



THE CRUST 101 

the paths to be plotted along which these currents flow far over the ocean 
floors. By means of them much of the sediment carried by rivers and 
dumped on the continental margins is picked up again and transported 
to fill depressions. Much of this sediment must ultimately be washed into 
the deepest active trenches, there to wait metamorphism and uplift into 
young mountains. 

The echo of seismic waves reveals that the sediments in places on the 
deep, abyssal plains are thousands of feet thick, but some slopes are scored 
bare so that coring tubes break on hard lava. In places guyots rise abruptly 
from the abyssal plains, partly buried and partly protruding above the 
swirling currents of mud. On their tops no beds of sands dropped by the 
currents are found, but only thick uniform layers of finest clay settling 
from the undisturbed body of the ocean. 

Thus is an exciting story of activity on the dark floor of the ocean being 
unfolded. So far only a few regions have been sampled, but enough has 
been found to make the above account possible and reasonable. 

THE GROWTH OF MOUNTAINS 

It has been suggested that all the higher features on Earth have arisen 
directly or indirectly from volcanism occurring along one of two principal 
fracture systems. The mid-ocean system is the less active; it has not moved 
about because the ridges produced by it must have taken most of the 
Earth's history to grow. In contrast, the continental system produces lava 
so much more quickly that it takes only a few hundred million years to 
build high mountains. When it has built a great range like the Cordillera 
or the Andes, the evidence shows that eventually the range is abandoned 
by movement of a segment of the fracture system to some fresh location. 
Once active growth has ceased even great ranges fall prey to erosion by the 
weather, and are reduced to stumps like the Caledonian or Appalachian 
mountains, and finally to low lying provinces of Precambrian shields such 
as those of Finland and Canada. 

Thus the continental blocks are the scars left in places formerly occupied 
by the continental fracture system. This occasional migration of segments 
of the continental fracture system does not destroy the continuity of the 
system. A section at a time moves, like a meander in a river or like a by- 
pass introduced into a highway, without destroying the continuity of the 
belts about the Earth. Because of these piecemeal movements, the frac- 
ture system which is active at present is made up of sections of many 
different ages, and an evolutionary sequence can be pieced together from 
present day examples illustrating stages in its growth. The continental 
fracture system consists of linked elements, most of which are arcs, and 
it is in terms of the evolution of arcs that the growth of mountains and 
continents can best be discussed (Table 1). 



102 J. TUZO WILSON 



TABLE 1 
Stages in mountain building 



Stage 



Example 



Initial Event in Stage 



1 Island arc 

2 Active mountain arc 

3 Inactive mountain arc 

4 Province of a shield 



Aleutian Islands 

Coast Mountains of British 

Columbia 
Appalachian Mountains of 

New England 

Grenville province of Ca- 
nadian shield (contains 
several arcs) 



Formation of arcuate frac- 
ture 

Uplift and metamorphism 
of former island arc 

Migration of active fracture 
system to another loca- 
tion 

Gradual erosion 



The first stage in the formation of a new part of the continental frac- 
ture system is the fresh fracture of one or several new arcs. It is usual for 
them to form on the ocean floor not far from existing continental margins. 
Indeed, the centres of the arcs— not the arcs themselves— lie commonly on 
the contemporary position of the edge of the continent, as can be seen 
for the island arcs along the eastern coast of Asia from the Aleutian Is- 
lands to the Philippines. 

The structure of all these arcs is similar. Conical fracture zones indi- 
cated by earthquakes rise from depths as great as 450 miles, at first at 
angles of about 60° but at flatter angles near the surface. Where the frac- 
ture zones meet the surface they form ocean trenches. These include 
all the greatest deeps in the oceans. The Aleutian trench and the Japan 
deep are examples. The occurrences of the shallowest earthquakes beneath 
trenches shows that they are kept open by active movement in spite of 
the tendency of turbid currents and other deposition to fill them. In the 
case of arcs close to land, activity may not be enough to keep the trenches 
open, so that they may become filled with sediments which are literally 
squeezed to the surface to form an outer chain of sedimentary islands in 
the place of the trench. Such islands include Kodiak Island which replaces 
the Aleutian trench near Alaska, Trinidad, Tobago and Barbados islands 
off the West Indies, and Timor Island opposite Australia. 

At a fairly uniform distance of about 100 miles inside the trenches or 
the sedimentary islands, the main arc of volcanoes forms. Here, fed by a 
branch system of faults, the andesitic magma rises and accumulates, form- 
ing chains of small volcanic islands which grow to larger ones. Thus while 
the islands in the youngest arcs such as the Aleutian and Mariana Islands 
are the smallest, the islands in arcs of intermediate age are of larger size, 
like Okinawa which is two hundred million years old, and the oldest 
known arcs— Japan, New Guinea and New Zealand, all at least four or 
five hundred million years old— have the largest islands. 

As the volcanic islands grow in size, their lavas are rapidly eroded and 



THE CRUST 103 

deposited around the islands in great eugeosynclines which mingle with 
the sediments brought by rivers from the old continents to fill in the seas 
behind the arcs. The East China Sea, for example, is entirely shallow, for 
it has been filled partly by offshore volcanoes and partly by detritus re- 
moved from more ancient mountain ranges on the continent and poured 
into the sea by the Yangtze and the Hwang-Ho Rivers. 

For a few hundred million years, the growth of an island arc is gradual, 
but the transition from island arc to primary mountain range is marked 
by a profound change of a fairly rapid nature. What had been a great 
eugeosyncline and arc system is quickly transformed into metamorphic 
gneisses and granitic rocks, and at the same time raised high above sea 
level into great primary mountain chains like the western parts of the 
Cordillera or Andes. These chains preserve the double nature of the old 
island arcs, for what had been the arc of andesitic volcanoes is lifted high 
to become a range of granodiorite of the same composition, like the Sierra 
Nevada of California, while the part that had been an outer arc of islands 
of deep sea sediments is less uplifted to become such a range as the Coast 
Range of California, or in some cases remains as a trench like the deep one 
along the Pacific coast of the Andes. 

The cause of this transformation is still a mystery, but V. Saull of 
McGill University has made a most promising suggestion. He has pro- 
posed that the creation of sedimentary rocks is a process in which energy 
is absorbed from the Sun, and that a great pile of sediments under suit- 
able conditions can revert to igneous minerals, giving out heat in the 
process. This heat may cause the upper parts of young mountains to be- 
come mobile and to form intrusive rocks. Igneous and metamorphic min- 
erals, especially feldspars, may be less dense than the sedimentary ones 
from which they are made. This could explain expansion and the uplift 
of the mountains. After uplift, the primary ranges remain active for a 
time. Earthquakes continue and volcanoes again break through along the 
line of the old arc, as in the Cascade Mountains of northwestern United 
States. 

Gradually the activity becomes less, and after a period which usually 
does not exceed two hundred million years the ranges cease to be active, 
fresh fractures form elsewhere, the third stage is reached and the old moun- 
tains are slowly eroded away. The primary arcs of the Appalachians are in 
this stage. They now form low hills across central Newfoundland, the 
Maritime Provinces, New England and south through the Carolinas. Other 
parts of them are buried by coastal deposits. These hills are gneissic and 
metamorphic, with remnants of ancient volcanoes in places, as in New 
Hampshire, marking the hue of the volcanic arcs. 

The final stage is reached when the mountains are reduced by gradual 
erosion to parts of the basement in shields. By then the primary arcs have 
lost much of their character, and those of different ages becoijie hard to 



104 J. TUZO WILSON 

distinguish from one another. They were formerly all lumped together as 
Archaean rocks, but now age determinations are revealing ranges of dif- 
ferent ages, and faulted boundaries are being found between old systems. 

When analysed in this manner, shields are found to have been built up 
in zones, with progressively younger provinces towards the margins. In 
the central parts of each continent are one or several continental nuclei. 
All of these nuclei were formed between two and three thousand million 
years ago. They have quite different structures from later provinces and a 
high proportion of volcanic rocks, but the details of these areas are difE- 
cult to decipher and are inadequately known. Everything about them 
suggests greater activity, more volcanoes and conditions different from 
those of later times, but during the last two thousand million years moun- 
tain building processes seem to have resembled those active today. Before 
three thousand million years ago we have little record. It may be that the 
Earth was melting in places and was too disturbed for any record of the 
earliest parts of the crust to have been preserved for us to see. 

In addition to this sequence of primary mountains, there is another 
important group which arise as a secondary consequence of the first (Table 
I). These ranges are scarcely represented during the island arc stage but 
with the uplift of the primary arcs, the outer part of the miogeosyncline 
(that wedge of sediments formed where the sedimentary rocks of the 
platform meet the borderland), is also uplifted. As a result the rocks of the 
miogeosyncline slump inwards on to the continent and are crumpled and 
thrust into mountains of sedimentary rocks, which always lie on the conti- 
nental side of the primary arcs. The Rocky Mountains, the Carpathians 
and the eastern part of the Andes are examples. In all cases the volcanism 
and seismicity are minor in secondary mountains but the folding in them 
can be very intense, as in the Alps, formed where the primary arcs of the 
Apennine and Dinaric Mountains meet at a sharp angle. 

In older stages of evolution the secondary ranges are preserved in thick 
folded basins of little altered sedimentary rocks, which contrast with the 
plutonic rocks of the older primary ranges. The Valley and Ridge prov- 
ince of New York and Pennsylvania is a classic example which is secondar}' 
to the primary mountains of New England. In the oldest stage the sec- 
ondary ranges are preserved as basins of sedimentary rocks called Protero- 
zoic, which habitually lie along the continental side of the primary prov- 
ince of the Archaean to which they are related. 

Until age determinations were made, all the older zones were lumped 
together, all the primary mountains in one category (Archaean), all the 
secondary parts in another (Proterozoic). We now have enough age de- 
terminations to show that both categories contain rocks of many different 
ages, but we have not yet enough to trace all the boundaries which outline 
the different provinces. The matter is complicated by the widespread 
cover of platform rocks that hides the true continental structure over great 



INSTABILITY OF SEA LEVEL 105 

stretches of most continents. For example, the basement is exposed over 
much of Canada, but largely hidden in the United States. 

This is as far as we have space to take our interpretation of the history 
of the Earth's crust. It will be apparent that new discoveries in geophysics 
are demanding a reconsideration of much geological dogma handed down 
from the last century when no means existed for investigating the ocean 
floors, the Earth's interior, its age and the rates of geological processes, but 
it should be emphasized that to abandon some conventional geological 
interpretations does no violence to geological observations, which are 
usually sound and give us our most detailed knowledge of the Earth. 

In selecting conclusions from many which are still under debate, the 
desire to tell a connected story has been used as a guide, for the Earth's 
history cannot be a collection of disconnected facts. Its behaviour must 
have been governed by constant physical laws. In the immediate future 
advances will be rapid, and it is not unreasonable to hope that better un- 
derstanding will lead to practical assistance in prospecting for ores. To 
interpret the new results more scientists are needed who are equipped to 
understand both geology and physics. Geology and geophysics are but two 
aspects of the same search. They would never have been separated if geo- 
logical methods of observing the visible part of the Earth had not been 
developed so much sooner than the physical methods required to study 
the rest of it. 

Acknowledgement— In preparing this chapter the author has been much 
helped by the advice and assistance of Elizabeth Morrison and Michael 
Dence, whose aid is hereby acknowledged. 



Instability of Sea Level 

• RICHARD J. RUSSELL ^ 



ON A FIVE-FOOT GLOBE THE VERTICAL RELIEF BETWEEN 
the highest summit on land and the most profound deep of the ocean 
would approximate 0.09 inch. The depth to the main floor of the Pacific 
Ocean would be barely discernible, at a depth of about 0.02 inch below 
the surface of the geoid. The vertical relief along any great circle will be 

• From American Scientist (Dec, 1957) , pp. 414-30. 
1 A Sigma Xi-RESA National Lecture for 1956-57. 



106 RICHARD J. RUSSELL 

included within the most perfect possible circle having a five-foot diameter 
if the line has sufficient width to be visible a few feet away. The ocean 
basins have thus about the same outward convexity as the earth's surface. 

In the traditional terminology of the geologist, the lithosphere is in 
part separated from the atmosphere by a hydrosphere which is essentially 
similar in shape to a thin membrane which might be thought of as cov- 
ering a spherical balloon. The hydrosphere is interrupted in continuity 
by dry land, but its oceanic part forms a system that covers somewhat more 
than 70 per cent of the globe. 

The boundaries between the hydrosphere and its marginal lands are 
complex. For the most part they are the shorelines of the world's ocean 
system, with their many complicated ramifications. While the upper sur- 
face of the oceans is regarded as sea level, the mean position of that level 
varies considerably from place to place. The radial distance from the 
earth's center to the mean sea level lengthens near mountainous coasts, 
such as western South America, where gravitational pull distorts the ocean 
surface upward. Sea level varies temporarily according to tidal forces, 
barometric pressure, and changes in wind. 

Leaving aside the question of departures between geoid and water sur- 
face, changes in level between land and sea also depend on other factors. 
At a given place, mean sea level can be lowered as a result of subsidence 
of an area of ocean floor many thousands of miles away. The down-fault- 
ing of the trough of an ocean deep produces some minor effect along all 
coasts of the ocean system. The building of deltas or the deposition of 
terrigenous sediments around continental and island shores has a basin- 
filling effect, and hence tends to displace ocean surfaces toward slightly 
higher levels. During earth history there have been many secular changes 
such as these which have affected sea levels enormously, the greatest being 
volumetric growth of the hydrosphere itself. Our discussion, however, is 
not directed toward these long-term changes of level. It will concentrate 
on problems of more immediate interest and less theoretical nature. It will 
involve mainly the closing chapter of earth history, the time in which we 
are now living, including the recent past, with some mention of the im- 
mediate future. There have been several notable changes in sea level dur- 
ing the Quaternary. 

TODAY'S SHORELINES 

The most impressive fact concerning today's shorelines is their irregu- 
larity in outline. Along many coasts, shoreline distances exceed by many 
times the airline distances between separated places. Ranges of mountains 
or hills jut out to form promontories. Long arms of water extend back 
into the land to form gulfs, bays, and estuaries. Where an abundance of 
sediment is transported toward river mouths, the drowning is alluvial. 



INSTABILITY OF SEA LEVEL 107 

Valley flats extend back into the land with patterns which depend on the 
irregular configuration of valley walls. Few large deltas jut out into the 
sea, and their volumes are insignificant in comparison to the vast quantities 
of sediment that rivers transport to their mouths.^ Numerous coastal is- 
lands exist. In short, coasts have the appearance they would have if sea 
level should rise by several hundred feet during the next 50 centuries or so, 
and then remain stationary for several centuries. 




Fig. 1, Examples of coastal drowning. 



Even the smoother coasts exhibit evidences of drowning. Inland from 
the magnificent and comparatively straight beaches to the south of New 
York City we see the drowned topography which outlines the complex 
shores of Delaware and Chesapeake bays, and the ramifications of Albe- 
marle and Pamlico sounds. Texas and the adjacent part of Mexico display 
a similar and equally compound type of coast. The practically straight and 
smooth outer beaches of offshore, sandy islands, shield the highly irregular 
inner shores of Galveston, Matagorda, San Antonio, Corpus Christi, Baf- 
fin, and other bays, and Laguna Madre. The broadest coastal plains of 
South America are indented by such estuaries as the Amazon Valley and 
Rio de la Plata. 

It was a curious mistake that, in developing deductively a classification 
of shorelines, Gulliver- should have decided to regard as his two main 
classes the irregular shorelines, which he called suhmergent, and the 
smooth shores, which he called emergent. Evidence along the Mediter- 



108 RICHARD J. RUSSELL 

ranean coast of France completely refutes the Gulliver hypothesis. To 
the east of the Rhone Delta the coast exhibits practically all character- 
istics which he regarded as evidence of drowning. Provenge displays a 
succession of capes, estuaries, offshore rocky islands, and other expressions 
of coastal complexity, whereas, to the west of the delta, Languedoc ex- 
hibits all of the characteristic features of an emergent coast, according to 
Gulliver's criteria. Extending practically as far as the Pyrenees is a smooth 




,.v.v.^:>^J 







M Older Rock 
{Xvj Young Rock 



A -Aries 
M- Marseille 



Fig, 2. Rhone delta region. 



shoreline along which low, sandy offshore bars are separated from the 
mainland at most places by linear lagoons. But in this whole region the 
idea that land to the east of the delta is sinking, whereas to the west it is 
rising, is patently untrue because Pleistocene terraces are well developed 
both along the coast and inland along valleys which fail to show the tilt- 
ing effects demanded by the hypothesis.^ These alluvial benches were 
formed prior to the development of today's shoreline, and of necessity 
would reflect any uplift or depression which has been experienced along 
the shore. The terraces today stand as horizontally as when they were 
formed. The real explanation of shoreline patterns in southern France is 
not to be found in hypotheses of emergence or submergence of land. The 
complicated shores are those where waves encounter older, thoroughly con- 
solidated bedrock. The smooth shores occur where the coastal plain out- 
crop consists of younger and poorly consolidated bedrock. 

The "stern and rockbound coasts" of the world are those which best 
display the effects of drowning, for the reason that waves have been un- 
able to change them appreciably during the past several thousand years. 
Glacial sculpturing has altered some, with the general effect of deepening 



INSTABILITY OF SEA LEVEL 109 

old valleys and accentuating coastal irregularities, as along the coasts of 
Alaska, Norway, and southern Chile. The smooth-shoreline coasts are or- 
dinarily the fronts of alluvial plains or occur in places where comparatively 
unconsolidated rocks bear the brunt of wave attack. Under such condi- 
tions sedimentary particles are easily detached and rearranged. Wide 
beaches form readily, in some cases on the shallow bottoms seaward from 
the mainland, from which they may be separated by lagoons or tidal flats. 
Evidences of drowning are often observed. 

ALLUVIAL DROWNING 

Broad flood plains are characteristic of most rivers leading to the sea. 
For many years these were explained on an erosional basis. The rivers 
were pictured as having cut down their valleys to a baselevel established 
by the sea, after which their energies were directed toward lateral corrasion, 
or valley widening.^ The alluvium of flood plains was thought of as a thin 
veneer, resting on laterally planed bed rock.^ Within more recent years, 
however, the alluvium of many of these flood plains has been penetrated 
by borings, which in practically all cases reveal valley fill which is many 
times deeper than the deepest pools scoured along the river beds. In the 
case of the Lower Mississippi Valley the character of the bedrock topog- 
raphy which underlies the alluvium is comparatively well known, and 
contains river trenches several hundred feet deep, while the river is rarely 
over sixty feet, and in no case as much as 200 feet deep.® It is apparent 
that the accumulation of alluvium is a direct response to a rising sea level. 
The alluvial drowning which accompanies valley filling is not unlike the 
water drowning of estuaries. Both obscure a previously existing landscape 
which exhibited considerable topographic complexity. 

A deeply entrenched bedrock topography underlies the alluvium which 
is characteristic of the lower parts of valleys. As the alluvium is commonly 
saturated with water, its sediments occupy an environment of chemical 
reduction. Hydrogen sulphide gas is commonly liberated when borings are 
made in alluvium having considerable organic content. Bluish, greenish, or 
dark clays commonly contain pyrite and other minerals which form where 
the supply of oxygen is deficient. Immediately below the base of the 
reduced, alluvial section oxidized materials are commonly encountered, 
which are yellowish or reddish in color, and which contain iron-manga- 
nese nodules. Oxidation of the rocks below the alluvium took place above 
the water table of the old erosional topography which now lies buried 
beneath the fill. 

Alluvial drowning, in addition to extending up flood plains, is charac- 
teristic of many coastal plains. Individual summits of hills are at places 
isolated, so that they now rise in island-like fashion above the flats of deltas 
or coastal marshes. San Francisco Bay provides many examples. There is 



no RICHARD J. RUSSELL 

little difference in appearance between islands such as Alcatraz, Yerba 
Buena, and Angel, and hills such as El Cerrito, except that the former are 
surrounded by water and the latter by land. In the case of the Coyote 
Hills, toward the lower end of the Bay, the upland was an island less than 
a century ago but now rises as abruptly from encroaching alluvium as from 
the bordering waters of earlier dates. 

RAPIDITY OF RISING SEA LEVEL 

The western coast of Anatolia may be selected as exhibiting convincing 
evidence concerning the rapidity with which the last major rise in sea 
level has taken place. Some ten miles inland from the shore of the Great 
Meander River Delta, and not far above the ancient port of Miletus, is 
Bafa Lake, a body of water about 10 miles long and 30 fathoms deep. This 
lake lies in one of the main tributary valleys of the Meander and was an 
arm of the Latmian Gulf at the time of Herodotus. It owes its existence 
to the fact that the alluvial filling of the tributary valley could not keep 
up with that which advanced the front of the Meander Delta along the 
main valley. The advance appears to have been on the order of 10 miles 
during the last 25 centuries.'^ The alluvial fill of the main valley has formed 
a dam, behind which the lake was cut off and isolated. The recency of 
this history is indicated by the fact that Bafa is today so deep and that it 
retains some salinity which appears to be residual from the days when it 
was the arm of an estuary. A few miles to the north, in the Little Meander 
Valley, the old port of Ephesus now hes inland some four miles, for the 
reason that the Little Meander Delta has pushed its front forward that 
distance during less than 20 centuries. Insufficient time has elapsed for 
the establishment of anything like an equilibrium between sea level and 
alluvial accumulation in the valleys of western Anatolia. 

Less than 70 miles eastward from Istanbul is Sapanca Lake, another 
interesting illustration of topographic instability. The Gulf of Izmit and a 
long vallev to the east is the downthrown strip of earth's surface, bounded 
both to the north and south by active faults. The Gulf is an expression of 
drowning of the western part of this graben floor by water. To its east 
there has been some upwarping which has created a low divide between 
waters flowing into the Sea of Marmara through the Gulf of Izmit and 
those which flow along the Sakar}^a River system to the Black Sea. To the 
east of this divide is Sapanca Lake, in a shallow basin which owes its 
existence to the somewhat greater height of the upwarped land to the west 
and of the rapidly alluviating flood plain of the Sakar)^a to the east. At 
time of flood the Sakar^'a sends a branch into the lake, where it deposits 
sufficient sediment to form a delta. The curious thing about this delta is 
the fact that during most of the year, with normal and lower stages of the 
Sakarya, the flow is reversed, so that the stream which builds it leads out 



INSTABILITY OF SEA LEVEL 111 

of the lake, back through the delta, and into the river. A topographic 
anomaly such as Sapanca Lake will be short lived. Sakarya alluviation, 
which is responding to the rise in level of the Black Sea, created it, but 
soon will fill the Sapanca basin with sediment. 

In summary, shoreline irregularity and the alluvial filling of valleys indi- 
cate a recent general rise in sea level. Comparatively small areas of deltas 
and topographic instability along coasts, which is evidenced by rapid ad- 
vance of delta fronts and anomalous features such as Sapanca Lake, sug- 
gest that the rise in sea level has been rapid. 

DATE OF RECENT RISE 

There is now excellent evidence, based on dating by carbon isotopes, 
that during the last 5000 years, no significant changes of level between 
sea and land have occurred along the northern coast of the Gulf of Mex- 
ico.^ The main rise took place between about 18,000 and 5000 years ago. 
That there were reversals in trend, halting stages, and complications is 
most probable, but a widely accepted belief that significant changes in 
world-wide sea level have occurred during the last 50 centuries depends 
on evidence which needs thorough re-examination.^ Wood samples from 
the upper parts of Mississippi alluvium ordinarily have C-14 ages of less 
than 6000 years, while samples from the basal alluvium may be 18,000 
years old. 

PRE-RECENT SEA LEVEL 

The level of the pre-Recent seas which determined the now buried valley 
bottoms of the surface that lies below the sedimentary deposits of Recent 
age is best known along the northern coast of the Gulf of Mexico, for the 
reason that the subsurface of no other part of the world has been so 
thoroughly explored nor has yielded such a density of borings through the 
Recent-Pleistocene contact. Nor has any other region been the theater of 
more intensive geophysical investigation. Not only have the valleys and a 
broad expanse of coastal flat been investigated but also the submerged 
continental shelf. Oil wells have been drilled out to a distance of nearly 
30 miles from the shore. 

The pre-Recent trenches of the Mississippi, Neches, Sabine, Calcasieu, 
Pearl, and other rivers have been traced in considerable detail below the 
alluvium of the coastal flats and across the adjacent shelf.^*^ Each of these 
trenches leads to a Gulf of Mexico, which not only was much lower in 
level at the time they were eroded but which also was located considerably 
beyond today's coast, as much as 100 miles in western Louisiana. The best 
known trench, that of the Lower Mississippi, extends to a depth of about 
950 feet below sea level. That this was not the approximate level of the 



112 RICHARD J. RUSSELL 

Gulf is certain, for the reason that there has been local downwarping since 

the time the trench was cut. 

If the inland slopes of the pre-Recent valleys are projected outward as 
far as the shoreline of the time, with gradients in keeping with those of 
their landward parts, the indicated level of the Gulf of Mexico was about 
450 feet lower than at present. This latest estimate by Fisk and McFarlan 
must be considered as a minimum value for the reason that it is im- 
probable that the borings along the valley trenches actually reveal pre- 
cisely their lowest points. The estimate is slightly deeper than my 137 
meter approximation which was made in 1948.^^ 

A pre-Recent sea level of —450 feet is thoroughly in keeping with a 
depth of 100 meters or more of alluvial fill in the Rhone Delta and the 
30-fathom depth of Bafa Lake.^^ It is not inconsistent with the level of 
continental shelves. 

CONTINENTAL SHELVES 

Geologists have long recognized the fact that, from their standpoint, 
today's shorelines are not the significant boundaries of continents. The 
fundamental difference between ocean basin and continent lies in a con- 
trast in rock types. Granite and its derivative sedimentary rocks dominate 
the continents, while heavier, less siliceous, iron-magnesium-rich rocks 
dominate the deeper ocean basins. Certain islands, such as those off the 
coast of southeastern Asia or the great archipelago north of Canada, are 
detached fragments of continents. True oceanic islands consist of basic 
lavas which have been derived from typical ocean-basin magmas. The con- 
tinental boundaries of the geologist include the detached islands and also 
the continental shelves, which are particularly well developed around the 
Atlantic and the western side of the Pacific oceans. 

It has been more or less traditional to regard the depth of continental 
shelves as 100 fathoms. But their outer boundaries may have approximately 
the same location on charts of reasonable scale whether they are differ- 
entiated by 40, 100, or even 200 fathoms. The continental slope beyond 
the edge of the shelf is relatively steep, so that in most places there is 
no great distance between isobaths representing the depths mentioned. 
Most of the shelf area is actually closer to 50 than to 100 fathoms below 
sea level, but large areas are notably shallower. 

Curiously, the commonly held opinion concerning origin of the shelves 
is an erosional hypothesis. One hundred fathoms was regarded as wave 
base, or extreme limit to which wave action could plane the rock of con- 
tinents.^^ Although it is now known that currents capable of producing 
morphological changes exist at depths greatly in excess of 100 fathoms,^^ 
it is also known that wave action ordinarily produces little effect on un- 
consolidated sediments at depths of much over 6 fathoms. In borings 



INSTABILITY OF SEA LEVEL 113 

through beaches, particularly along the offshore barrier islands of smooth 
coasts, the upper 36 feet or less of section is ordinarily complicated struc- 
turally, with many evidences of rearrangement such as alternations be- 
tween the violent waves of storms and the tranquility of smoother waters 
demand, whereas materials at greater depth ordinarily present a less dis- 
turbed stratigraphy. The depth to marsh deposits along the Gulf Coast of 
the United States, in places where waves are moving beaches inland and 
across marshy coastal flats, is generally 36 feet or less. That wave action 
could plane continental margins to a depth of 100, or even 50, fathoms 
is wholly contrary to observational fact. In the case of the continental 
shelves, borings and seismic evidence ordinarily reveal deep sections of 
young unconsolidated or only moderately consolidated sediments, rather 
than the wide platforms [of] hard bedrock which was pictured under the 
erosional hypothesis. 

That the continental shelves are related to lower stands of sea level 
appears certain, but their depth should not be regarded as indicating 
the stand of the pre-Recent oceans. In some places they have been faulted 
or warped, but, more important, they generally have been sites of com- 
paratively heavy sedimentary accumulation. The Louisiana coast may 
represent a rather extreme case, but on the parts of the shelf farthest re- 
moved from the Mississippi trench, where downwarping has been at a 
minimum, it is usual to find that below water less than 30 feet deep the 
oxidized materials representing the pre-Recent surface are first encoun- 
tered at depths such as 550 feet. If the pre-Recent Gulf stood at —450, the 
regional subsidence has amounted to 100 feet, and the near-shore Recent 
sediment has a thickness of 420 feet or more. Along coasts which have 
been supplied less abundantly with sediment the continental shelf should 
more closely approximate the level of pre-Recent seas. 

One may speculate for a moment about the possibilities of finding 
important artifacts on the shelves. Sea coasts and coastal plains attract 
the heaviest concentrations of population today, and it is reasonable to 
believe that they did so in the past. When better methods are found for 
boring and excavating submerged sediments it is possible that archeology 
will experience an era of astonishing discoveries. In any event, discoveries 
await investigations below the blanket of Recent sediments, rather than 
the explorations of divers. 

CORAL REEFS 

Some eighty years ago, Darwin proposed the idea that the coral reefs of 
the Pacific and Indian oceans indicate a subsidence of islands, rather uni- 
formly over large areas of tropical oceans.^^ Since then a tremendous 
amount of information has been gathered concerning such things as the 
distribution of the several kinds of reefs, the depths of the inner lagoons 



114 RICHARD J. RUSSELL 

of atolls, and the nature of the rocks underlying the coral growths. Though 
the ideas of Darwin were attacked vigorously and fell into disrepute for 
some years, time has shown his observations to be essentially correct in 
most regards.^^ 

The majority of the reef-forming organisms live only at depths of less 
than 20-25 fathoms, but reef accumulations extend to much greater depths 
as a rule. Though certain cases of subsiding foundations must be recog- 
nized, reef thicknesses are best explained generally as being related, not to 
subsidence of ocean floors, as Darwin postulated, but to the general rise of 
sea levels. The latest episode of this history appears to be related to the 
Recent rise which has submerged the Pleistocene coastal plains and 
drowned the world's maritime shores. 

WHY HAS SEA LEVEL RISEN? 

The overwhelming evidence that sea level has risen both rapidly and 
recently demands an appropriate explanation. Normal geological process 
such as the filling of ocean basins by terrestrial detritus or the upwarping of 
some large area of ocean floor appear to be of the wrong order of magni- 
tude from the time standpoint. The melting of continental ice, however, 
fulfills exactly all requirements for answering the question. The change 
from waxing Late Wisconsin glaciation to the waning of continental ice 
appears to have occurred about 18,000 years ago.^'^ The idea that the re- 
turn of meltwaters elevated sea level is well over a century old, having 
been suggested by Charles Maclaren in 1842,^^ when few people were 
even aware that continental ice had spread broadly over the earth's lands 
during an epoch which had recently been named the Pleistocene.^^ 

The Pleistocene differs in many ways from the Tertiary epoch which 
preceded it. It was ushered in by the appearance of ice on continents and 
witnessed what may have been an unprecedented rapidity of mountain up- 
lift in many parts of the earth. A milder, less zonal pattern of climates 
gave way to a complex system with highly developed extremes, such as 
between polar and tropical, or desert and highly humid. But the most 
distinguishing feature was the introduction of cychc variations between 
stages of widespread glaciation and alternating interglacial stages, during 
at least one of which there was considerably less ice development on lands 
and polar seas than exist at present. The history of man coincides with this 
geologically abnormal epoch of time.^^ 

The pre-Recent topography was formed during the last major low-stand 
of sea level, which terminated about 18,000 years ago. During this latest 
glacial culmination some 27 per cent of the earth's land surface was cov- 
ered by ice. This may be compared to about 30 per cent at the maximum 
development of any Pleistocene glacial stage, or the 10 per cent which 
exists at present.^^ It is relatively easy to estimate the approximate area of 



INSTABILITY OF SEA LEVEL 115 

land which was once covered by Pleistocene ice, but the problem of de- 
termining its thickness has vexed students from the start. 

Maclaren considered the round figures of ice sheets north of 35°N, one 
mile thick, covering two-thirds of the dry land, and estimated a lowering of 
sea level by 800 feet. Under the assumption that one-eighth of the ice 
remains today, he estimated that sea level has experienced 700 feet of its 
ultimate 800-foot rise. Tylor ^ regarded the lowering of sea level as about 
600 feet. Penck^s considered evidence of Pacific atolls as demanding a 
universal stand of seas about 300 feet lower than at present, but thought 
that the volume of continental ice might possibly require withdrawal of 
from 320 to 650 feet of oceanic waters. If the average ice thickness was 
about 3240 feet, the amount should be on the order of 486 feet. Daly's 
1910 estimates were considerably lower, calling for a depression of sea 
level by 125 feet if the average ice thickness was 3000 feet, 167 for 4000, 
or 208 for 5000,"''' but in later works he inclined to regard the net lowering 
as being about 300 feet. Estimates on .the order of 485 feet by Nansen 
and Ramsay were criticized as being based on exaggerations of both ice 
area and thickness.^^ 

There is thus a long and distinguished record of opinion in favor of the 
hypothesis that sea level was lowered during the glacial stages of the 
Pleistocene, and raised according to the degree that continental ice re- 
turned meltwaters to the oceans. It is interesting that so much of the ar- 
gument concerning the major outlines of Ice Age history should be based 
upon observations from within the tropics. Students of coral reefs have 
become increasingly united as to the correctness of the idea of glacial con- 
trol.2^ It seems highly probable that the best way to estimate the average 
thickness of continental ice is to base the computation on the positions of 
pre-Recent valley floors, which appears to indicate a level of — 450 feet or 
slightly more. It is amazing how closely this conclusion agrees with the 
computations of Nansen and others. 

EARLIER CHANGES OF LEVEL 

There is now unanimous agreement that the Pleistocene witnessed sev- 
eral alternations between times of extensive ice cover and ice dissipation.^^ 
Most authorities agree that there were four major glacial stages and that 
the Recent is not the most pronounced of the interglacials. The evidence 
has been gathered mainly in parts of the earth which experienced glacia- 
tion, and hinges to a great extent on the interpretation of moraines and 
other depositional features which are of glacial origin. The problems of 
deciphering the record are admittedly difficult, and assistance from an ab- 
solute chronology, such as age determination by carbon-I4 ratios, sheds 
light only on the most recent part of the history. 

From the maritime coasts, however, come observations which appear 



116 RICHARD J. RUSSELL 

to furnish a better basis for identifying the major outlines of Pleistocene 
history in terms of changes of level between land and sea. The record along 
the northern shore of the Gulf of Mexico shows that each of the major gla- 
cial stages and its correspondingly low stand of the oceans resulted in creat- 
ing erosional surfaces similar to the pre-Recent surface. Each stage of de- 
glaciation was accompanied by the alluvial drowning of valleys, so that sedi- 
mentary deposits accumulated. These deposits provide an excellent record 
of the major outlines of glacial history. There were five of these major cy- 
cles of glaciation and deglaciation, with the last major rise in sea level in 
progress at present.^^ 

If the Gulf Coast region had remained completely stable during the 
Pleistocene the problem of separating the five successive systems of valley 
fining would be at least as complex as the interpretation of moraines 
farther to the north. But this was not the case. There was subsidence of 
the coastal region and uplift inland, so that each stratigraphic unit, as 
well as the surface of each alluvial unit, has been differentiated and pre- 
served. Each may be mapped either as a geological formation or as a 
topographical terrace. The record from each stage of waxing glaciation is 
similar to one that would originate if sea level were to start dropping today 
and within a few thousand years attain a level such as —450 feet. The 
alluvial deposits of the Lower Mississippi and other large river systems 
would become entrenched by their streams, leaving the present flood plains 
as elevated terrace surfaces. 

To whatever degree the alluvium of earlier stages escaped erosion, it 
remains as a formation representing a stage of the Quaternary. Each older 
formation, however, has been tilted Gulfward to a greater degree than any 
subsequent formation. In eastern Louisiana, for example, where the gra- 
dient of flood plains may be on the order of a few inches per mile, the 
youngest Pleistocene terrace slopes at about 5 feet, the others, 15, 25, and 
in excess of 35, respectively. In western Louisiana, where the Quaternary 
deformation has been less intense, the youngest terrace slopes less than 1 
foot per mile, the others, 1.5, 5, and 8 feet per mile. This convergence in 
slopes brings to the base of the section the oldest Pleistocene formation in 
territory toward the coast and across the shelf. Convergence in forma- 
tional or terrace slopes exists across a zone about 200 miles wide in Loui- 
siana. Northward, across Arkansas and at least as far up the Lower 
Mississippi Valley as Cape Girardeau, Missouri, the terraces stand above 
one another at comparatively regular intervals and slope Gulfward at ap- 
proximately similar gradients. In the latitude of Forrest City, Arkansas, 
for example, the highest and oldest terrace stands about 350 feet above 
the modern flood plain, and the others, at approximately 200, 100, and 40 
feet. These vertical intervals are characteristic north of Louisiana. An in- 
vestigation along the Brazos River of Texas resulted in a confirmation of 
Lower Mississippi Valley terrace history.^'^ A similar sequence of events 



INSTABILITY OF SEA LEVEL 117 

occurred along the lower Rhine and Rhone rivers. The classic terraces of 
the Rhone remain at constant vertical intervals upstream but converge as 
they flex downward beneath the Recent alluvium as they approach the 
coast.^ 

In the early stages of each episode of valley filling it is notable that 
gravel and coarse sand prevail in the sediments, whereas the upper and 
younger deposits are much finer-grained. As the gravels extend some dis- 
tance across the coastal plain it has been possible to trace the "basal con- 
glomerate" of each terrace formation in southern Louisiana by means of 
normal subsurface geological correlation techniques.^^ The base of the 
Quaternary approximates a depth of 2500 feet across much of the shelf. 

Louisiana, Rhone Valley, and Rhine Valley stratigraphic and terrace 
evidence appears to demand the recognition of five major stages of glacia- 
tion. Each of these undoubtedly witnessed many oscillations of lesser mag- 
nitude, and each was certainly accompanied by local variations in rates of 
ice accumulation or of recession and dissipation. Thus the student of mo- 
raines may attach great significance to Wisconsin substages, but in the 
record of the Gulf Coast the Late Wisconsin appears to be lumped in the 
erosional, low-sea-level time during which the pre-Recent surface was de- 
veloped. The Cary-Mankato may coincide with a change from coarse to 
finer sediments in the Lower Mississippi alluvial fill. 

PLEISTOCENE AND RECENT 

If the practice customarily employed by geologists for establishing di- 
visions of the time scale is applied to the Quaternary, it is logical to rec- 
ognize the Pleistocene as starting with the first significant accumulation of 
continental ice and the corresponding initial drop of sea level. An older 
regime which characterized the latest Tertiary came to an end, and the 
climatic, erosional, depositional, and diastrophic patterns of the Quater- 
nary were initiated. Thus, if the first Pleistocene glaciation be called the 
Nebraskan, it was the earliest accumulation of Nebraskan ice that ushered 
in the new epoch .^^ Patterns of marine deposition were upset when seas 
began lowering in level. While a hiatus was being established in the dep- 
ositional record across the shelves, continuous deposition was taking place 
on continental slopes beyond them, initiating what should be considered 
as the earliest of Pleistocene formations. It was not until Nebraskan ice 
waned that encroaching seas returned the theater of marine deposition up- 
ward and inland across the shelves, and eventually into valley systems. In 
these continental deposits the lowest Pleistocene marks a time somewhat 
later than that at the base of the uninterrupted marine section. 

For the reasons that it is customary to recognize the stage of the Quater- 
narv in which we are living by the name, Recent, and that it is desirable 
to follow the conventional stratigraphic methods of the geologist in sep- 



118 RICHARD J. RUSSELL 

arating the Recent from earlier stages, it is logical to define Recent as the 
time during which sea level has made its last general rise. This is es- 
sentially a definition proposed in 1872 by Reade.^^ Flint ^^ reviews the 
various proposals for differentiating the Recent, and criticizes most of them 
thoughtfully. Paleontology offers little help because there is nothing really 
distinguishing to separate Late Pleistocene from Recent faunas. Most defi- 
nitions of Recent are purely arbitrary, particularly when based on an 
event such as the first deglaciation north of the German coast of the 
Baltic, north of central Sweden, or north of the Niagara River. The merit 
of placing the break at the time that sea level started its last major rise is 
evident in a stratigraphic consideration of the problem. A well-defined and 
weathered surface began to be buried under a sedimentary cover, at an 
earliest absolute date on the outer margins of the shelves, then inland 
progressively until the alluvial drowning extended up valleys to form ex- 
isting flood plains. A new and distinct geological formation is in process of 
accumulation as a result of this last major coastal drowning. Thus consid- 
ered, neither the Recent nor earlier stages of Quaternary alluviation are 
strictly interglacial. Each began just after a major culmination of ice ac- 
cumulation and lasted through a substage of retreating ice and into the 
following interglacial stage. 



PRESENT AND FUTURE 

There is little reason to consider the present as Postglacial for there 
is much more extensive ice cover today than during most of the Pleisto- 
cene. The great portion of this ice rests on the little known land surface 
that we call Antarctica or on the bedrock of Greenland. In proportion, ice 
incorporated in valley glaciers of mountains, or existing in other scattered 
localities, such as Novaya Zemlya or Alaska, has comparatively small vol- 
ume. The stand of the seas depends primarily on the balance between ac- 
cumulation and melting of continental ice in the two main centers of 
storage. There is considerable probability that Antarctica has experienced 
but one accumulation of Pleistocene ice, but it is likely that the total 
volume has fluctuated considerably. 

Many investigators have advanced the idea that during one or more 
interglacial stages of the Pleistocene, sea levels stood considerably higher 
than now.^^ If so, it may be presumed that correspondingly less ice existed 
on the Antarctica and Greenland. Most of the evidence in favor of higher 
sea levels is provided by terrace and shoreline features which now occupy 
elevated positions. But the alternative possibility exists that continental 
margins and interiors have actually risen positively. If there were freshly 
created shoreline features widely distributed along maritime coasts at some 
comparatively uniform level, such as 200 feet, the argument that today's 



INSTABILITY OF SEA LEVEL 119 

sea level represents a lowering by that amount would be strong. On the 
other hand, if shoreline features stand at a variety of elevations, the sug- 
gestion is fairly conclusive that elevation has resulted from the differential 
elevation of rising land masses. The latter appears to be the case. Indian 
middens on the St. Lucia coast of California have been elevated as much 
as 600 feet. 

The fact that terrace surfaces display warpings that carry them below 
sea level for considerable depths along coasts of the Gulf of Mexico, the 
Mediterranean, the North Sea, and other places, whereas their inland parts 
are elevated by amounts varying up to several hundred feet, renders it 
difficult to determine whether Pleistocene sea levels ever attained eleva- 
tions in excess of those of the present day. That interglacial seas at times 
may have exceeded today's stands is possible, but not by the differences of 
level suggested by positions of higher terraces, for many of the surfaces 
are located well above the level which would be established if all conti- 
nental ice should melt. 

The recession of Recent glaciers appears to have been rapid until some- 
time such as 5000 b.c.,^^ and sea level must have risen rapidly. The aver- 
age rate may have approximated 3.5 feet per century for some 130 cen- 
turies. Since then the fluctuations have been minor, if judged by the be- 
havior of glaciers in the Alps and elsewhere. During the thirteenth and 
fourteenth centuries, climates became more severe in northern latitudes, 
glaciers waxed to a minor climax sometime after the first half of the 
eighteenth century and, it may be presumed, sea levels dropped slightly. 
During the twentieth century glaciers have waned and sea level has risen 
something on the order of 2.5 inches, mainly between 1930 and 1950. 

Whether the existing rise of sea level will continue is a matter of specu- 
lation. There is no very good reason to believe that it will, and some 
slight suggestion that the trend has reversed very recently. But all varia- 
tions either in ice volume or stand of the seas involve numerous and com- 
plicated reversals during the establishment of any major trend, and our 
time-base for forecasting future events is altogether too short to have 
much significance. 

If all continental ice melts, the net rise would bring the oceans to about 
the level of today's 200-foot contours.^^ Our coastal plains, with a large 
proportion of the most densely inhabited regions on earth, would be sub- 
merged by a depth approximating half of the present submergence of the 
continental shelves. This possible termination of the Quaternary would 
not likely result in any major catastrophies. One might hazard a guess that 
the rise of the oceans would not be faster than a foot or two per century 
at a maximum, so that populations would be forced to migrate inland and 
upslope at a rate which would be hardly noticeable, except in historical 
perspective. Deltas and coastal plains would be building outward, so that 
the total area of useful lowland might not be reduced appreciably. 



120 RICHARD J. RUSSELL 

BIBLIOGRAPHY 

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changes in the sea-level during the Glacial Period. Geological Magazine, 9: 392- 
399 (1872). 

2. Gulliver, F. P., Shoreline topography. Proc. American Academy of Arts and Sciences, 
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processes and shoreline development, 584 pp., New York (John Wiley and Sons), 
1919. 

3. Russell, Richard J., Geomorphology of the Rhone Delta, Annals, Association of 
American Geographers, 32: 149-254 (1942). 

4. Davis, William Morris, The geographical cycle, Geographical Journal, 14: 481-504 
(1899); The peneplain, American Geologist, 23: 207-239 (1899); Baselevel, grade, 
and peneplain. Journal of Geology, 10: 77-111 (1902); reproduced in. Geographical 
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1954. 

5. Fenneman, Nevin M., Floodplains produced without floods. Bulletin, American 
Geographical Society, 38: 89-91 (1906). 

6. Fisk, Harold N., Geological investigation of the alluvial valley of the Lower Mis- 
sissippi River, 78 pp., Vicksburg, Mississippi (Mississippi River Commission), 1944. 

7. Russell, Richard J., Alluvial morphology of Anatolian rivers. Annals, Association of 
American Geographers, 44: 363-391 (1954). 

8. Leblanc, Rufus J., and Bernard, Hugh A., Resume of late Recent geological history 
of the Gulf coast. Geologic en Mijnbouw, {N.W. ser), 16e jaargang 185-194 
(1954). 

9. Idem. The paper cited was one in a symposium. Quaternary changes in level, es- 
pecially in the Netherlands, in which most of the participants suggested minor 
fluctuations of more recent date. 

10. Fisk, Harold N., and McFarlan, Jr., E., Late Quaternary deltaic deposits of the 
Mississippi River, in. The Crust of the Earth, Geological Society of America, Special 
Paper 62: 279-302 (1955). 

11. Russell, Richard J., Coast of Louisiana, Bulletin de la Soc. beige de Geologic, de 
Paleontologie et d'Hydrologie, 57: 380-394 (1948). 

12. Russell, Richard J., op. cit., 1943, p. 57; 380-394 (1948). 

13. Johnson, Douglas W., Shoreline processes and shoreline development, 584 pp.. 
New York (John Wiley and Sons), 1919. 

14. Ericson, D. B., Ewing, M. Heezen, B. C, and Wollin, G., Sediment deposition in 
deep Atlantic, in. The Crust of the Earth, Geological Society of America Special 
Paper 62: 205-219 (1955). 

15. Darwin, Charles, On certain areas of elevation and subsidence in the Pacific and 
Indian oceans, as deduced from the study of coral formations. Proceedings, Geo- 
logical Society of London 2: 552-554 (1837). 

16. Davis, William Morris, The coral reef problem, American Geographical Society, 
Special publication 9: 596 pp. (1928). 

17. Suess, Hans E., Absolute chronology of the last glaciation. Science 123: 355-357 
(1956). 

18. Maclaren, Charles, The glacial theory of Professor Agassiz, American Journal of 
Science, 42: 346-365 {1842). 

19. Lyell, Sir Charles, Nouveaux elements de geologic, Paris (Pitois-Levrault et cie.), 
648 pp. (1839),ref. p. 621. 

20. Russell, Richard J., Climatic change through the ages, in. Yearbook of Agriculture, 
pp. 67-97 (1941). 

21. Flint, Richard Foster, Glacial geology of the Pleistocene epoch, 589 pp.. New York 
(John Wiley and Sons), 1947, ref. p. 207. 

22. Penck, Albrecht, Morphologic der Erdoberflache, 2 vols., Stuttgart (J. Engelhorn), 
1894,2:581,658-660. 

23. Daly, Reginald A., Pleistocene glaciation and the coral reef problem, American 
Journal of Science {4th Series), 30: 297-308 (1910). 



QUESTIONS OF THE CORAL REEFS 121 

24. Daly, Reginald A., The changing world of the Ice Age, 111 pp.. New Haven (Yale 
University Press), 1934, ref. on p. 48; and W. Ramsay, Fennia, 52: 48, 1930, and 
F. Nansen, The Strandflat and Isostasy; Videnskapsselkapets Skrifter, I, Mat-Naturv. 
Klasse, No. 11, 313 pp., Kristiania (1922). 

25. Vaughan, T. Wayland, Glacial control theory. Bulletin, Geological Society of 
America, 27: 41-55 (1916); Kuenen, Ph. H., An argument in favor of glacial con- 
trol of coral reefs. Journal of Geology, 59: 503-507, 1951. 

26. Fisk, Harold N., Depositional terrace slopes in Louisiana, Journal of Geomorphology, 
2: 181-200 (1939); Geology of Grant and La Salle parishes, Louisiana Geological 
Survey, Bulletin 10 (1939); idem.. Geology of Avoyelles and Rapides parishes, 18 
(1940); Russell, Richard J., Quaternary Surfaces in Louisiana, Comptes Rendus 
du Congress Internationale de Geologic, 2: 406-412 (1938); Quaternary history of 
Louisiana, Bulletin, Geological Society of America, 51: 1199-1234 (1940); Qua- 
ternary history of the Lower Mississippi Valley, Review of the Geographical Insti- 
tute, University of Istanbul, International Edition, 1: 3-10 (1954). 

27. Stricklin, Fred, Pleistocene terraces along the Brazos and Wichita rivers, central 
and north -central Texas. Louisiana State University, unpublished doctoral disserta- 
tion, 1953. 

28. Frink, J. W., Subsurface Pleistocene of Louisiana, in, Louisiana Geological Survey, 
Bulletin, 19: 369-419 (1941). 

29. Russell, Richard J., The Pliocene-Pleistocene boundary in Louisiana, Report of the 
Eighteenth Session, Great Britain 1948, International Geological Congress, IX: 
94-96 (1950). 

30. Reade, T. Mellard, The post-glacial geology and physiography of West Lanchashire 
and the Mersey estuary. Geological Magazine, 9: 111-119 (1872). 

31. Flint, Richard Foster, op. cit., pp. 438-443. " 

32. Ahlmann, H. W:Son, Glacier variations and climatic fluctuations, Bowman Me- 
morial Lectures, Series three, 51 pp.. New York (American Geographical Society), 
1953 

32. Gutenberg, B., Changes in sea level, post-glacial uplift, and mobility of the earth's 
interior. Bulletin, Geological Society of America, 52: 721-772 (1941); Kuenen, 
Ph. H., Sea level and crustal warping, in. The Crust of the Earth, Geological 
Society of America, Special Paper, 62: 193-204 (1955). 



Questions of the Coral Reefs 

• NORMAN D. NEWELL 



AFTER FINISHING A SURVEY OF THE GALAPAGOS ISLANDS, 
on October 20, 1835, H.M.S. "Beagle" began a long voyage across the 
Pacific. Coming, after some weeks, to the "Low or Dangerous Archipel- 
ago," the expedition's young naturalist noted that he saw "several of 
those most curious rings of coral land, just rising above the water's edge, 
which have been called Lagoon Islands. 

"A long and brilliantly-white beach," Charles Darwin recorded in his 

• From Natural History (Mar., 1959), pp. 118-32. 



122 NORMAN D. NEWELL 

Journal, "is capped by a margin of green vegetation : and the strip, looking 
either way, rapidly narrows away in the distance, and sinks beneath the 
horizon. From the mast-head, a wide expanse of smooth water can be seen 
within the ring. These low hollow coral islands bear no proportion to the 
vast ocean out of which they abruptly rise; and it seems wonderful that 
such weak invaders are not overwhelmed by the all-powerful and never- 
tiring waves of that great sea, miscalled the Pacific." 

The scene that young Darwin drew has been a favorite topic of ro- 
mantic writing from the time of Melville to the present. But, while such 
works have made the South Pacific legendary, they have also served to 
obscure the fact that more accessible coral seas lie near at hand — among 
the island archipelagoes and rocky shores of the tropical western Atlantic 
from Rio de Janeiro to Bermuda. Although the living reefs of the West 
Indies are small, post-Pleistocene newcomers when compared to many of 
the massive veterans of the Pacific, both deep borings and soundings of 
recent date indicate that some of the mightiest coral reefs ever known 
anywhere came into being in the Tertiary period, millions of years ago, 
along the southeast margin of the North American continent. 

The main architects of coral reefs are tiny colonial animals of the 
coelenterate phylum, related to the sea anemones and jellyfish. They are 
assisted in their construction work by certain lime-secreting red algae, 
whose cemented, calcareous skeletons have accumulated on shallow sea 
floors the world round through thousands of years to make a firm but 
very porous limestone. In past geologic periods, other groups— including 
certain sponges, mollusks, Bryozoa and blue-green algae— produced great 
reefs, but these are not now important as reef architects. 

Reef corals and algae require sunlight for growth. Consequently, they 
push upward toward the surface of the water, crowding together in pro- 
fusion at low tide level. Stragglers from these colonies may extend more 
than two hundred feet underwater, sparsely inhabiting rock ledges and 
talus slopes beyond the edge of the reef. 

There are many kinds of coral reefs, but the varieties that have long 
attracted scientific attention are the barrier reefs and atolls of the deep- 
ocean basins. Rising steeply to the surface above an ocean floor, thousands 
of feet deep, these reefs support rich, isolated communities of shallow- 
water organisms— mutually dependent plants and animals— living, as it 
were, in biological oases amid a desert of comparatively sterile, deep 
waters. 

It seems strange that the growth of reef corals is stimulated by strong 
surf. Yet, the living outer edges of reefs successfully resist the pounding 
of all but the most violent of breakers, even growing forward at times to- 
ward the waves. Indeed, the death and erosion of reef corals are most 
rapid in sheltered places: a reef community thrives best in strongly agi- 
tated surface waters. 



QUESTIONS OF THE CORAL REEFS 123 

Because the growing surfaces of mature coral reefs are essentially at sea 
level, reefs are sensitive to slight changes in sea level and resulting shifts 
in the relative distribution of land and sea. Fossil reefs are therefore re- 
liable datum points for ecologic interpretations of the sedimentary rocks 
in which they are found. In some places, ancient reefs— formed by algae, 
sponges, corals and other organisms, long buried under accumulations of 
sediments— contain large quantities of petroleum: it is becoming evident 
that a good proportion of the proved oil reserves of the world (nearly 
seven per cent of the total, if we except the non-reef oil fields of the 
Middle East) is contained in the porous rock of fossil reefs and associated 
lagoonal deposits. 

Scientific theories are inherently tentative "progress statements" about 
knowledge: they must be overhauled and modified from time to time as 
new evidence becomes available. In the search for knowledge, conflicting 
and seemingly contradictory theories, supported by opposing camps of 
competent investigators, rarely prove to be wholly right or wrong. This is 
true of the main theories that have been advanced to explain the origin 
of coral reefs. 

Many of the most obvious questions about coral reefs are not easily 
answered. For example, is the living reef only a thin veneer over a plat- 
form of eroded older rocks? Or is it the summit of a pile of skeletons of 
marine organisms, maintained at sea level by deposition over a subsiding 
foundation? These questions— the subject of lively and, at times, angry 
debate for nearly a century— formed the basis of the celebrated "coral reef 
problem." 

These questions now seem irrelevant. All living reefs are most probably 
thin growths resting on eroded surfaces of older rocks, some of which are 
fossil reefs. The West Indian reefs are most illuminating in this regard and 
they aid in a better understanding of all coral reefs. 

In common with many geological processes, the growth of coral reefs is 
too slow to be directly observed. Consequently, the history of a particular 
reef must be inferred from comparisons with other reefs in different stages 
of development and from studies of the biology of reef organisms and the 
processes of erosion and sedimentation around reefs. The study method is 
almost wholly deductive, rather than experimental, and rests on a basic 
premise of historical geology — "the present is the key to the past." 

The principal reef theories advanced by early investigators were infer- 
ences based on scanty biological and geological evidence. Since many 
crucial facts were— and, indeed, still are— lacking, some of these classic 
theories were in conflict. However, it is now becoming clear, as is the case 
so often in science, not only that the truth about coral reefs is a synthesis 
of many ideas once regarded as irreconcilable, but also that no single ex- 
planation can account for all coral reefs. 

Charles Darwin's observations of coral atolls, both in the Pacific and the 



124 NORMAN D. NEWELL 

Indian Ocean, stimulated him to formulate his "subsidence theory"— uni- 
versally recognized as a model of simplicity— along lines that had already 
occurred to him from what he had read of atolls. His Structure and Distri- 
bution of Coral Reefs, which appeared in 1842, was Darwin's first great 
work. 

Reef corals and algae, Darwin declared, become established in tropical 
seas, in favorable places provided by shallow sediment-free, rocky bottoms, 
frequently near the shore. If the sea floor subsides slowly, upward growth 
of the reef organisms may maintain the growing surface near sea level— 
the ceiling of growth for the reef-builders. Because growth is most rapid 
along the outer margin of a reef (and is inhibited along the shoreward 
margin by quiet waters, sediments and variable temperatures), the organ- 
isms occupying the inner part are unable to keep pace with subsidence. 
Thus, the outer, most rapidly growing part of the reef eventually becomes 
detached from the shore by a lagoon too deep or too muddy to support 
reef corals. Continued subsidence of a reef-encircled island leads to dis- 
appearance of the central island and formation of an atoll— a narrow ring 
of reef surrounding a lagoon that may range in maximum depth from 
about 30 to 250 feet. 

Darwin was acquainted with geological evidence of uplift in mountain 
regions, and relative subsidence in sedimentary basins, but the geologists 
of his day did not have a satisfactory explanation of these phenomena. It 
is now known from studies of variations in gravity that the low areas 
of the earth's crust, such as the ocean basins, are underlain by relatively 
dense, heavy rocks, whereas the higher areas, such as mountain ranges and 
the continents themselves, are underlain by lighter rocks. The resulting 
gravity equilibrium— or isostatic balance— between high and low areas is 
disturbed by erosion and transfer of sedimentary load from one area to 
another, and by the growth of coral reefs and volcanoes, which locally 
overload the crust and cause isostatic sinking. 

Darwin held that a coral reef might maintain its growing upper surface 
at sea level while the foundation slowly sank to depths of thousands of 
feet— thus resulting in very thick reef deposits, even though the reef or- 
ganisms are limited to water depths of less than three hundred feet (with 
an optimum at about fifteen feet). 

Darwin's pioneer work on coral reefs quickly found strong support 
from a young genius of nineteenth century American science, James 
Dwight Dana, who recognized that the lower parts of the river valleys of 
many reef-girdled islands are drowned; that is, they plunge beneath the 
sea and the shores are deeply embayed where the valleys disappear. This 
suggested to Dana that the islands had sunk (or sea level had risen), since 
the valleys were eroded. With Dana's endorsement, Darwin's subsidence 
theory was accepted and the stage was set for a famous controversy. 

Darwin's explanation of atolls was incomplete, however, because it did 



QUESTIONS OF THE CORAL REEFS 125 

not take into account the comparatively recent and great fluctuations in 
sea level produced by the waxing and waning of the Pleistocene conti- 
nental glaciers. A new principal— that of the glacial control of coral reef 
formation— was introduced before the end of the nineteenth century by 
the German geologist Albrecht Penck and was greatly amplified by Pro- 
fessor Reginald Daly of Harvard University, one of the great figures in 
American geology, renowned for his originality. 

Daly expressed the view that, at times of maximum glaciation, sea level 
must have been appreciably lower than the greatest depth of the present 
living reefs; that is, more than about 100 feet. Glacial cooling— and in- 
creased turbidity caused by wave erosion at the lowered sea level— killed 
off most of the reefs and deprived shores of the protection from erosion 
normally given by living reefs. He believed that erosion at the low glacial 
levels completely cut away small islands to form "banks," and produced 
broad erosional platforms around larger islands. As the glaciers melted and 
sea level and water temperatures slowly returned to normal, river valleys 
were drowned, the most favorable areas at the exposed edges of the ero- 
sional platforms were recolonized by corals, and new reefs grew upward 
about as rapidly as the rise in sea level. According to this theory, all liv- 
ing coral reefs are very young— less than ten thousand years old— and ex- 
tend no deeper than the level of the lowest stages of the Pleistocene sea, 
sav 450 feet below present sea level. 

Daly's glacial control theory enjoyed great popularity for many years 
and the essential importance to coral reefs of the effects of Pleistocene 
cooling and fluctuations of sea level is now well estabHshed. His case was 
weakened, however, by his insistence that great subsidence has not been 
involved in the origins of any living reefs. 

During the three decades before World War II, the subsidence theory 
was vigorously attacked by many leading authorities on coral reefs, es- 
pecially Daly, Vaughan and Gardiner, and a great forward step was made 
during this period by William Morris Davis, who stressed a fundamental 
difference between the oceanic reefs of the Indo-Pacific region, on the one 
hand, and the marginal belts of coral seas in the West Indies and else- 
where, on the other. Davis applied physiographic principles to the coral 
reef problem, as had Dana, concentrating his attention not so much on the 
reefs themselves as on the comparative differences between the cliffed 
shores of reef-free areas and the much less eroded, reef-protected shores. 
He demonstrated that the effects on coral reefs of the sea-level shifts of 
the Pleistocene glacial stages were great along the continental margins, 
but negligible in the deep-water basins of the tropical Pacific and Indian 
oceans. 

Davis showed how slow subsidence and, to a lesser degree, glacial 
changes in sea level had affected the great reefs of mid-ocean. At the same 
time, he pointed out that a majority— possibly all— of the pre-Pleistocene 



126 NORMAN D. NEWELL 

coral reefs in the West Indies and certain other "marginal" areas had been 
killed by the onset of the glacial changes. The living West Indian reefs- 
cited by Agassiz, Daly, Vaughan and others as evidence against Darwin's 
subsidence theory— are, in fact, postglacial and so young that for the most 
part they have not been involved either in measurable subsidence or in 
large changes in sea level. Hence, Davis declared, they are not closely com- 
parable to the really old barrier reefs and atolls of the western Pacific. 
Davis' may be termed the "synthetic" theory of coral reefs: it tailors the 
explanation to the local situation. 




A" 



..4t..C'. 




Darwin diagrammed his 
concept of reef evolu- 
tion, above. He held 
that coral growth kept 
abreast of a constant 
sea level as island sub- 
sided, to produce a bar- 
rier reef and a lagoon in 
early phases (top) and 
finally an atoll (bottom). 



DARWIN EXPLAINED OCEANIC CORAL REEFS 
BY SUBSIDENCE 



The most useful scientific theories are simple, but also adequate, expla- 
nations of natural phenomena. As regards this yardstick, Darwin's expla- 
nation of coral reefs— as stages in a continuous evolutionary sequence, lead- 
ing from fringing reefs to atolls— has long been considered an outstanding 
model. Well before Darwin, it was known that coral reefs are limestone 
prominences on the sea floor, built upward by the gradual accumulation of 
the skeletons of shallow-water corals and algae. Darwin knew that many 
reefs rise to the surface from very deep waters, extending far below the 
range where reef organisms can live. He concluded that the foundations 
of these reefs must have been sinking, while the upper portions grew up- 
ward — maintaining a position near sea level. Both the form and structure 
of reefs, Darwin held, could best be explained as a consequence of such 
upward growth during slow subsidence. As diagrammed, below, Darwin 



QUESTIONS OF THE CORAL REEFS 127 



2. Reef starts to grow 



1. Reefless new volcano. 





3. Growth matches 
subsidence. 



4. Atoll alone remains. 





128 NORMAN D. NEWELL 

recognized four main stages in reef evolution: a reefless, new island; a 

fringing reef; a barrier reef; and, finally, an atoll. 

Colonization of a new island's shore would be inhibited, at first, by ero- 
sion and sediments. But, eventually, a fringing reef would be established, 
protecting the shore from further wave erosion. Reef corals grow rapidly 
under favorable conditions: this growth keeps them close to sea level 
despite the island's persistent sinking. Subsidence, combined with upward 
reef growth, brings separation of the reef from the shore— in barrier form, 
enclosing a lagoon— and, finally, complete submergence of the central is- 
land, leaving only lagoon and atoll reef visible. For the more complex 
picture of reef growth along continental margins, where folding and up- 
lift are common, see the following. 

DALY STRESSED THE GLACIAL FACTORS 
CONTROLLING GROWTH 

Forty years ago, Reginald Daly pointed out that all preglacial coral reefs 
must have been exposed to the air and killed, as the fluctuating sea level 
reached new lows during the times of continental glaciation. In the tropics, 
shore lines previously protected by reefs would thus have been deprived 
of protection and eroded and planed by wave action to the new, low 
levels of the sea. The submerged terraces and banks over which present 
reefs grow, Daly believed, are the eroded stumps of preglacial reefs. Wil- 
liam Morris Davis, in turn, demonstrated that destruction and erosion of 
the coral reefs during these times of fluctuating sea level were not par- 
ticularly severe in the deepest parts of the tropical oceans, far removed 
from the influence of continental climate. Near the continents, however, 
as in the case of the West Indies, Davis found that the destructive effects 
were marked. These influences— of changes in sea level and of glacial cool- 
ing—are shown in the Virgin Islands. 

In this region, subsiding volcanic islands were originally flanked by pro- 
tective barrier reefs and shallow lagoons. With the onset of Pleistocene 
cooling and withdrawal of the sea, broad limestone coastal plains— with 
successive beach ridges and dunes — were exposed. The old barrier reefs 
were exposed and killed and the reef organisms were unable to re-establish 
a protective cover at these lower levels because of prevailing low tempera- 
tures. 

Melting of the continental glaciers and the rise of sea level to its present 
position, in turn, left traces of the old, cemented beach ridges— as rows of 
bottom prominences. Subsequent shore erosion of the volcanic islands was 
retarded by the establishment of very young, fringing reefs in shallow 
waters along the crests of the old, submerged beach ridges. These new 
reef growths agree in general form with barrier reefs. Other new reefs have 
grown from the rocky shoals adjacent to the island's cliffed headlands. 



QUESTIONS OF THE CORAL REEFS 129 



1. Barrier reef. 




2. Sea level drops. 




3. Reef is killed 




4. The sea rises. 



130 NORMAN D. NEWELL 

Daly's theory explains the extensive submarine terraces found in areas 
now marked by young and feeble fringing reefs. But, as Davis has shown, 
the limestone banks are, for the most part, ancient, drowned barrier reefs 
and lagoons— formed before the Pleistocene glaciation. Thus, a synthesis 
of Darwin's subsidence theory and Daly's glacial control theory is required 
to explain the features of many coral reefs in the West Indies. 

Since World War II, there has been a great revival of interest in coral 
reefs, sparked by the discovery of independent evidence about reef origins 
—evidence that could be obtained only by deep borings completely 
through the reefs to their underlying "basement." "I wish," Darwin had 
written, in the year preceding his death, to the American oceanographer 
Alexander Agassiz, "that some doubly rich millionaire would take it into 
his head to have borings made in some of the Pacific and Indian atolls, 
and bring back cores for slicing from a depth of 500 or 600 feet." The wish 
was realized, many years later, to an extent beyond Darwin's dreams. Two 
deep borings, completed on Eniwetok atoll in 1952, finally reached a vol- 
canic basement below reef limestone, but not until they had reached a 
depth of over four thousand feet. Thus Darwin's conclusion that atolls 
of the central basin of the Pacific Ocean rest on a subsiding foundation 
was finally confirmed. Eniwetok has subsided at an average rate of two 
millimeters each century for the past sixty million years. 

Fathometer soundings in the Pacific have also revealed the existence of 
hundreds of deeply submerged, flat-topped volcanic mountains, called 
"guyots," some of which are crowned by limestones and shallow-water fos- 
sils drowned millions of years ago by too rapid subsidence. The flat tops 
are interpreted as wave-eroded platforms, cut across newly formed vol- 
canoes before they sank under the sea during Cretaceous and early Ter- 
tiary times. Some guyots have also been identified on the floor of the At- 
lantic Ocean. 

Both these new findings make it clear that Darwin was basically right 
about the reefs of the deep oceans, and that his opponents went too far 
in denying that subsidence was an essential factor in the formation of the 
typical atolls and barrier reefs in the deep-water areas of the Indo-Pacific 
region. The discoveries of modern geophysics show that the crust beneath 
the oceans is relatively heavy, as compared to the continental areas, and 
we are no longer surprised that large areas of the ocean basins should be 
characterized by long-continued subsidence. The border areas between 
oceans and continents, on the other hand, are influenced by continental 
climate and terrigenous sedimentation, and they contain belts of crustal 
warping and folding. It is precisely in these areas that many of the coral 
reefs do not conform so well to the Darwinian concept of reef evolution 
by persistent subsidence. 

By far the greater part of scientific work on coral reefs has been done 



QUESTIONS OF THE CORAL REEFS 131 

in the deeper parts of the Indian and Pacific Oceans, and this fact has 
strongly colored our views. These reefs have many features in common 
that distinguish them from West Indian reefs, and a consideration of the 
differences between the two areas aids in drawing conclusions about the 
fundamental nature and genesis of all coral reefs. Let us briefly compare 
reefs of the two areas. 

The classic reefs of the Indo-Pacific area rise through thousands of feet 
of water just to the surface of the sea. Because they are formed of ce- 
mented skeletons, their upper slopes exceed the angle at which loose gravel 
or sand will come to rest, and the outer, advancing rim of the reef may 
descend as a sheer cliff for hundreds of feet. An average slope of 60° or 70° 
is not uncommon for the upper part of these coral reefs. 

At low tide, on a very quiet day, an observer on the reef edge can look 
down into unbelievably transparent, blue waters with a sensation of height, 
as though looking down from a cliff. When Captain Cook was exploring 
along the Great Barrier Reef of Australia, he found that he could not 
touch bottom with two hundred fathoms of line, although his ship stood 
within fifty yards of the reef edge. There are depths of a mile here, within 
half a mile of the reef. 

Large areas of the reef top, emergent or just awash at low tide, resemble 
a rough concrete slab, a few hundred yards wide, that extends as far as 
the eye can see. Only the hardiest reef organisms can stand daily exposure 
to the sun and air of the reef flat and the first impression, here, is one of 
desolation. Toward the seaward edge, where breaking waves continuously 
bathe the reef flat, the surface is covered by low corals and a pinkish, hard 
crust— formed by lime-secreting algae of the Lithothamnion group. At 
the very edge of the reef, the algal deposits form a low, hummocky ridge, 
which is very resistant to wave erosion. It has long been recognized that 
the surface of the inner part of the reef flat is an erosional plain— cut ap- 
proximately at low tide level in previously elevated reefs and island de- 
posits. It does not follow that all of the erosion is of recent date. Most 
probably, the reef flat lies not far from the normal interglacial level of the 
sea. The outer part of the reef flat is not an erosional surface, however; it 
is built up by the calcareous secretions of the hardiest species of corals 
and algae— organisms that can resist wave shock and stand exposure a 
few inches above low tide level, bathed by the splash of breaking waves. 
The reef platform is protected from wave destruction by the algal ridge. 
Without it, the flat would be destroyed about as fast as it forms. 

Small patch reefs commonly occur in the sheltered lagoons. They pos- 
sess neither algal ridge nor reef flat. The upper surface corresponds to the 
tops of living corals, growing in water a few feet deep at low tide. Unlike 
the main reefs, many of which evidently are being cut down from higher 
levels, the solid interior of the patch reefs does not reach low tide level 
and there is no indication that they have been cut down from a higher 



132 NORMAN D. NEWELL 

level. These facts suggest that the patch reefs may generally be younger 
than the large seaward reefs. 

What, in contrast to this classic picture, do we see in the West Indies? 
By way of preface, it is generally accepted by geologists that sea level was 
appreciably higher during the last interglacial stage (some 100,000 years 
ago, when the polar ice caps apparently lost most of their ice) than it is 
today. Many believe that sea level was also higher three to four thousand 
years ago, but this contention is not yet established by unequivocal evi- 
dence, and is unsupported by observations in the tropical Atlantic. 

Fossil coral reefs some five to fifteen feet above present low tide level 
are widely distributed in the West Indies and elsewhere. Radiocarbon 
analyses of these fossil reefs in the Bahamas and the Florida Keys indicate 
that they are probably all interglacial in age. It is not known that any of 
these reefs have been completely planed down by marine erosion, but it 
may be assumed that smaller, patch reefs of the same age were destroyed 
during fluctuations of sea level. The present patch reefs of the West In- 
dies are clearly postglacial in age. 

Even a casual inspection of West Indian reefs shows that they are not 
like the seaward reefs of oceanic barriers and atolls. They form fringes 
along shoals, and rocky shores remote from the outer edges of deeper rock 
platforms and they never lie adjacent to deep waters. At their seaward 
edge, these reefs rarely extend into water sixty or seventy feet deep: below 
sixty feet, the gently sloping platforms bear only scattered heads of mas- 
sive corals, occasionally bunched to form low knolls. 

A majority of the West Indian reefs are confined to the windward sides 
of banks and islands and very few are exposed at low tides. Algal deposits 
are generally not important and considerable damage is done to the reefs 
during great storms and horizontal growth toward the open sea is every- 
where greatly retarded. From every indication, the West Indian reefs 
were formed at present sea level and they have not been appreciably af- 
fected by either crustal movements or changes in sea level. Thus, their 
maximum probable age cannot be greater than four to five thousand years, 
and many of them must be even younger. 

T. Wayland Vaughan, the foremost student of West Indian reefs, 
showed, in 1916, that the living reefs of this area are thin incrustations 
over eroded terraces, in some cases barely blanketing (but not concealing) 
old shore lines, beaches, and aeolian dunes formed at times of lower sea 
levels. Some of the reefs superficially resemble barrier reefs, while others, 
simulating atolls, form fringes around circular shoals. But generally they 
are too thin to mask the character of the underlying topography or even 
to conceal the underlying rocks. Vaughan judged the reefs to be only a 
few hundred years old, an estimate based on present growth rates of reef- 
forming corals. 

The best-developed reefs of the Bahamas, Jamaica and Florida usually 



QUESTIONS OF THE CORAL REEFS 133 

show three principal biotic zones— controlled, apparently, by differing con- 
ditions of turbulence and light at different depths. These are: an outer 
belt of massive corals, especially Montastrea annularis, lying at substrate 
depths between about thirty and sixty feet; an intermediate belt of elk- 
horn corals, Acropora palmata, in turbulent waters between about five and 
thirty feet; and an inner, rocky shoal— rising to the lowest tide level and 
characterized by the stinging hydroid, Millepora alcicornis, incrusting al- 
gae, seafans, and small massive corals. 

Low islands of the West Indies, like those of the tropical Indo-Pacific 
region, are generally formed of limestone, and a majority of the high is- 
lands are volcanic or are composed of folded sedimentary and metamor- 
phic rocks. Both kinds of islands in the West Indies are commonly sur- 
rounded by broad, shallow platforms (or "banks") of limestone with steep 
marginal slopes. Shallow banks, with or without islands, lie near the sur- 
face at many places in the Caribbean, the Bahamas and along the main- 
land coasts. They generally rise toward the shore in a series of low benches 
or steps from marginal depths of anywhere between twenty and sixty 
fathoms. Along reef-free rocky shores, the bottom lies near the maximum 
depth of strong wave abrasion, between about three and six fathoms. 

Numerous investigations of the continental shelf have been made along 
the north edge of the Gulf of Mexico since World War II. These have 
made use of automatically recording echo-sounding instruments, dredged 
samples and sediment cores. It is now known that mysterious pinnacles 
and ridges occur along the edges of submerged erosional terraces down to 
about sixty fathoms. These have been regarded as dead reefs, formed at 
low levels of the Pleistocene sea. It was thought that sea level rose so rap- 
idly, while the glaciers were melting, that the reef organisms were unable 
to maintain a growth position near the surface and the reefs had been 
drowned. 

Dredged samples do not support this interpretation, however. The rock 
of the pinnacles and the associated loose sediment only rarely contain 
examples of the principal West Indian reef-forming coral species. It seems 
probable that the northern part, at least, of the "West Indian province" 
—as the tropical, western Atlantic is called by biogeographers— was too 
cold for reef corals during these glacial stages, and that tropical biota did 
not become re-established in the Bahamas and in Florida until quite re- 
cently, perhaps only three to four thousand years ago. 

A more plausible explanation of the shelf-edge pinnacles and ridges is 
suggested by examination of the visible rocks of the islands of Bermuda, 
the Bahamas, and many of the offshore cays of Cuba, Puerto Rico and 
other areas throughout the West Indies. A few of these islands are formed 
of fossil coral reefs, but the majority are cemented beach and dune ridges, 
formed along successive shores of the fluctuating Pleistocene sea. Some 
of these old shore ridges, now submerged below sea level, rarely bearing 



2r 



s\^ 






-'' y-r^ 







\ 



/' 



Effective limit for reef 
/ corals, north and south 

of the equator, is pro- 
/ r, vided by an average 

21° c. ; water temperature of 

21° C, or higher. Thus, 

* the West Indian and 

\ , * V" Australasian reefs ex- 

x^^'^'^^'"-' *®"^ ^^^ about the same 

distance from the equa- 
tor. 

scattered, dead, reef corals, may be identical in character with the exposed 
ridges. 

The West Indian province is isolated from other tropical regions. Be- 
cause of this isolation, it contains distinctive animals and plants, and our 
knowledge of the manner in which West Indian biota came into existence 
is one of the triumphs of paleontology. The fossil record shows that, be- 
fore the Miocene epoch, the West Indies sea was populated by organisms 
similar to those of the Indo-Pacific and Mediterranean regions. Gradually, 
during the Miocene epoch (some twenty million years ago), the shallow- 
water forms of the western Atlantic were cut off on the east by a land 
barrier between the Mediterranean and Indian Oceans— and probably by 
deepening of the central Atlantic basin. Finally, during the Pliocene epoch 
(some six or eight million years ago), the Central American isthmus rose 

134 



QUESTIONS OF THE CORAL REEFS 135 







Deep boring at Eniwe- 
tok finally reached vol- 
canic rocks only after 
penetrating more than 
four thousand feet of 
reef deposits. Date of 
origin, established by 
Foraminifera, in the Eo- 
cene, about sixty mil- 
lion years ago. 





»aASf^.^>-/ag&?rg^tj- '^m^ -^mjiMiW^m&^^gj^^ 



136 NORMAN D. NEWELL 

above the sea, completely separating the Caribbean from the Pacific 
Ocean. 

Subsequent history of the West Indies has been characterized by a 
dwindling in the number of species of marine organisms. In contrast, the 
organisms of the much vaster Indo-Pacific region, with more varied eco- 
logic opportunities, have become progressively more diversified. The cli- 
max for West Indian corals (and probably the time of greatest reef-build- 
ing) was during late Oligocene and early Miocene times, some twenty to 
thirty million years ago. Since those times, Atlantic reef corals have been 
on the decline. The northern limit of coral reefs in the whole Atlantic re- 
gion has gradually shrunk toward the equator. 

Unfavorable changes in climate and sweeping alterations in the physical 
environment of the region generally have been deleterious for reef corals. 
The West Indian region was disturbed at the beginning of the Tertiary, 
and again late in the Miocene, by widespread mountain uplifts— resulting, 
both times, in widespread deposition of muddy sediments in the adjacent 
seas. These various factors— restriction of the West Indian area through 
isolation and cooling, and regional increase in the turbidity of the waters 
—have added up to a general deterioration of the West Indian reefs as 
compared to the mid-oceanic reefs of the Indo-Pacific, far removed from 
the influence of harsh continental climate and sedimentation. 

It is strange that the ups and downs of Pleistocene glaciation— which, 
according to Emiliani, may have cooled the Caribbean surface waters by 
as much as 5°C. below present temperatures, causing vast regional con- 
traction of the area of the West Indian biota— did not cause wholesale 
extinctions among the coral species. Instead, the fluctuations of sea level 
caused by the growth and melting of the continental glaciers may actually 
have stimulated the evolution of new coral species. One-third of the spe- 
cies of reef corals now living in the West Indian region are not known 
as fossils. However, this discrepancy may be attributable to nonpreserva- 
tion or inadequate search for fossils. While the Pleistocene climatic 
changes evidently did not deplete the West Indian reef community, the 
great fluctuations in sea level— ranging from four hundred and fifty feet 
below to some tens of feet above present sea level— did determine the sites 
favorable for reefs and greatly affected their modes of growth. 



How Volcanoes Grow 

• J. p. EATON AND K. J. MURATA 



SUMMARIZING THE STATE OF VOLCANOLOGICAL KNOWL- 

edge in 1952, Howel Williams (J)observed: "Much has been learned 
about the distribution, internal structure, and products of volcanoes, but 
pitifully little about the causes and mechanism of eruption." To remedy 
this deficiency he called for more intensive, continuous observations of well- 
chosen active volcanoes, with geophysics and geochemistry supplement- 
ing the traditional tools of geology. Current investigations of the U.S. 
Geological Survey's Hawaiian Volcano Observatory are much like those 
envisaged by Williams, and they are yielding an exciting new insight into 
the internal workings of volcanoes. 

No volcano has influenced our conception of the vital processes of active 
volcanism more than Kilanea. Geologists drawn to Hawaii by travelers' 
accounts of fantastic activity at this volcano were so impressed by what 
they saw that they framed whole theories of volcanic action around it. 
Even though its prime attraction, the renowned lava lake that circulated 
almost continuously within its great summit caldera for at least a century, 
was destroyed in 1924, Kilauea and its giant neighbor, Mauna Loa, have 
remained very active, one or the other having erupted about once in two 
years since that date. The comparative simplicity, the large size, and the 
frequent, voluminous, nonviolent eruptions of Hawaiian volcanoes make 
them ideally suited to illustrate the fundamental processes of volcanism. 
Here these processes can be studied safely and conveniently, in isolation 
from the great complications of structure and contaminating rocks that 
render most volcanoes so baffling. 

In 1823 William Ellis (2) found within Kilauea caldera "an immense 
gulf, in the form of a crescent, upwards of two miles in length, about a 
mile across, and apparently 800 feet deep. The bottom was filled with 
lava, and the southwest and northern parts of it were one vast flood of 
liquid fire in a state of terrific ebullition. . . ." Through the century that 
followed, visitors to Kilauea recorded successive infillings and collapses of 
Ellis' "gulf," as lava poured up through conduits beneath its floor and 
accumulated, crusted over, and partially congealed within it, later to be 

• From Science (Oct. 7, 1960), pp. 925-38. 

137 



138 J. P. EATON AND K. J. MURATA 

withdrawn into the depths or poured out through great fissures in the 

flank of the volcano. 

Continuous observation of the lava lake began with the establishment 
of the Hawaiian Volcano Observatory on the rim of Kilauea caldera in 
1912 (3). Detailed measurements of the height, size, and shape of the 
hquid surface of the lake (Fig. 1) as well as occasional measurements of 
its temperature and chemical analyses of the gases escaping from it were 
made from 1912 until the lake was destroyed by the eruption of 1924. 
The usefulness of seismograph and tiltmeter observations for deciphering 
unseen subterranean changes in the volcano was also demonstrated during 
these years when Jaggar (4) and his collaborators were collecting a wealth 
of data on Kilauea's baffling lava lake. 

SETTING AND GEOLOGY 

The geologic mapping of the Hawaiian Islands, carried out jointly by 
the U.S. Geological Survey and the Hawaii Division of Hydrography dur- 
ing the 1930's and 1940's, opened new dimensions in the study of Ha- 
waiian volcanoes (5). A thorough investigation of volcanic processes 
necessarily awaited an adequate geological description of the volcanoes. 
By mapping structures visible at the surface, by examining the shallow 
interior of the volcanoes in the sections exposed by faulting and erosion, 
and by studying very carefully the nature, variation, and distribution of 
the lavas composing the great Hawaiian shields, geologists have sketched 
the framework of the volcanoes' structure and history. 

Mauna Loa and Kilauea form the southern part of the island of Hawaii 
at the southeastern end of the Hawaiian Ridge, a great range of volcanic 
mountains rising from the floor of the Pacific Ocean and stretching 1600 
miles northwestward from Hawaii to Kure Island (Fig. 2, inset). Vol- 
canism appears to have progressed from the northwest toward the south- 
east along the ridge. Wave-wrecked volcanoes of the northwestern half of 
the ridge approach the surface as shoals or support low-lying coral atolls. 
Farther southeastward, remnants of volcanic rock rising in small islands 
still withstand the vanquishing sea. Only along the southeastern quarter 
of the ridge do the great volcanoes stand high above the sea, where they 
form the large inhabited islands of the Hawaiian group. Even here the 
evidence for migration of activity southeastward is strong, for volcanoes 
in the northwestern part of this group are deeply dissected, while Mauna 
Loa and Kilauea, still in vigorous activity at the southeastern end, are 
hardly marred by erosion. 

From its great length and narrow width it is apparent that the Hawaiian 
Ridge marks the course of a major fracture in the earth's crust through 
which lava has poured at different centers and different times to build 
the volcanoes that form it. The ridge rises from the axis of a broad swell 



Fig. 1. Lava lake in 
Halemaumau, 23 Janu- 
ary 1918. Floating is- 
lands of congealed lava 
are surrounded by mol- 
ten lava. In the fore- 
ground, an overflow 
from the lake has chilled 
to pahoehoe lava. In the 
background can be seen 
the wall of Kilauea cal- 
dera and the gentle 
slopes of the southwest 
rift zone of Mauna Loa. 




on the ocean floor and is flanked, near its southeastern end, by an ocean 
deep that runs down its northeast side and hooks around the south end 
of the island of Hawaii (6). 

Volcanoes of the ridge are built upon the simplest known section of the 
earth's crust (the Pacific basin is floored only by approximately 5 kilo- 
meters of basalt, covered by about 1 kilometer of sediments and resting 
directly upon the earth's mantle), and they are separated from other 
tectonically active regions by at least 2000 miles of seismically quiet ocean 
floor. Thus, in magnificent isolation, volcanic processes originating in the 
mantle raise the giant Hawaiian mountains to heights approaching 6 miles 
(10 kilometers) above the ocean floor. 

Hawaiian volcanoes bear little resemblance to steep-sided, central-type 
composite volcanoes like Fujiyama, in Japan. Rather, they are shaped like 
a warrior's shield, with a broad domical summit and gently sloping sides, 
and they attain enormous size. Mauna Loa rises more than 30,000 feet 
above its base on the ocean floor and has a volume of about 10,000 cubic 
miles. Even at sea level, about 16,000 feet above its base, it is more than 
70 miles long. The volcanoes are built almost entirely of thin flows of 
fluid basaltic lava, poured out chiefly from long fissures concentrated in 
relatively narrow rift zones. 

On surface evidence, rift zones appear to determine the location and 
shape of the volcanoes. Most commonly, each volcano has two principal 
rift zones meeting in the summit region at angles of 130° to 180°. The 
vertex of this angle usually points away from the unbuttressed flank of the 
Hawaiian ridge adjacent to the volcano. Rift zones are predominantly 
either almost parallel or more or less perpendicular to the axis of the ridge, 
but just how these zones are related to the fundamental fracture beneath 
the ridge is not clear. 

The summits of several volcanoes are indented by calderas formed by 

139 



140 J. P. EATON AND K. J. MURATA 

collapse of the surface rocks when support was withdrawn from below. 
Kilauea caldera, subcircular in plan and eccentrically set in the summit 
of the volcano, is ZVi miles long and 2 miles wide. Its floor, a low dome 
of lava flows that slope outward from Halemaumau, site of the old lava 
lake and principal vent of Kilauea, is almost 500 feet below the caldera 
rim on the northwest but level with the rim on the south. The present 
floor is about 600 feet higher than that depicted in a sketch by Maiden 
(7) in 1825. 

Along some rift zones, especially near their upper ends, are found other 
prominent collapse craters. The variation in size as well as the nature of 
pit craters, as these features are called, is well demonstrated by the "Chain 
of Craters" along the upper section of Kilauea's east rift zone. Here, pit 
craters range from the "Devils Throat," formed by a single collapse that 
left a pit 50 feet across and 250 feet deep with an overhanging lip, to the 
giant Makaopuhi (Fig. 3), the result of at least two episodes of collapse 
and two of flooding by lava that formed a gulf almost a mile across and 
900 feet deep. 

Prominent lateral faults, some of them submarine, flank several of the 
volcanoes. Notable among these are the Honuapo-Kaoiki fault system, 
which separates Kilauea from Mauna Loa and extends from just north 
of Kilauea caldera southwestward to the sea near Honuapo, and the Hilina 
fault system (Fig. 4), which drops a 30-mile-long segment of Kilauea's 
south flank abruptly toward the sea. Although the absolute movement 
on these faults cannot be specified, it is distinctly possible that the whole- 
sale uplift of the Hawaiian Ridge along such faults has been responsible 
for a significant fraction of its height. 

Hawaiian lava flows, both the smooth, glassy-skinned pahoehoe and the 
indescribably rough, clinkery-surfaced aa, are intricately broken by the 
processes that form them. The volcanic edifices built of these shattered 
flows are mammoth piles of rubble, shored up beneath the rift zones by 
thousands of thin, nearly vertical dikes of strong, dense basalt. Their bulk 
density, estimated from measurements of gravity across the Hawaiian 
Ridge (8) and in deep wells on the island of Hawaii, is no greater than 
2.3 grams per cubic centimeter, significantly less than the density, about 
2.8 grams per cubic centimeter, of an unvesiculated column of basaltic 
magma at depth. 

To judge from the historic and geologically recent behavior of Mauna 
Loa and Kilauea, Hawaiian volcanoes grow almost to their full size quite 
rapidly. Intervals between eruptions are only a few years or decades, and 
the flanks of the volcanoes are blanketed by new flows so frequently that 
erosion makes little headway. The lavas forming these primitive shields 
belong to the "tholeiitic" basalt series and differ primarily only in their 
content of olivine crystals. Although surging fountains of gas-inflated lava 
are often propelled hundreds of feet into the air by gas released from the 



HOW VOLCANOES GROW 141 

lava as it approaches the surface within the vent fissure, these eruptions 
show httle real explosivity and build only small cinder cones, spatter cones, 
and spatter ramparts around their vents. 
After the volcanoes reach maturity the interval between eruptions in- 



Fig. 2. Map ot Hawaii showing seismograph stations, tiltmeter bases, 
and the Kilauea lava flows of 1955 and 1960, The inset shows the entire 
chain, stretching 1600 miles northwestward from Hawaii to Kure Island. 

— I 1 1 1 \ 1 1 1 1 1 I I I I I I I I I I I I I I I r~3o^ 



PACIFIC OCEAN 




142 J. P. EATON AND K. J. MURATA 

creases, perhaps to a century or more, erosion begins to predominate over 
growth, and subtle changes appear in the chemistry and mineralogy of the 
lavas, which pass over into the alkalic basalt series. Eruptions become 
more explosive, building larger cinder cones around the vents. 

Even after Hawaiian volcanoes are overcome by old age and are trans- 
figured by profound erosion, occasional renewals of volcanism pour out 
additional lavas of the alkalic basalt series or even more highly differen- 
tiated lavas such as the felspathoid-bearing flows of Oahu and Kauai. 

The outstanding questions of the origin of magma, the mechanism of 
eruption, and the differentiation of magma are strongly interdependent, 
and any answer proposed for one must be compatible with data for the 
others. The mechanism of eruption plays a central role. It must account 
for how and by what path magma is brought to the surface, why the 
volcanoes erupt intermittently, and how volcanic structures such as rift 
zones, pit craters, and calderas are produced, and it must provide the intra- 
telluric environment necessary for the differentiation observed in the lavas. 



Fig. 3. Makaopuhi viewed from the west. A prehistoric lava pond, in the 
distance, was exposed by a later collapse in the foreground. The small 
pond of lava at the bottom of the deeper pit, 900 feet below the present 
rim, was poured into Makaopuhi in 1922. [R. T. Haugen, National Park 
Service.] 




HOW VOLCANOES GROW 143 

CURRENT INVESTIGATIONS 

To extend the physical description of the volcanoes to depth and to 
obtain information on the active processes within them, the methods of 
geology must be supplemented by those of geophysics and geochemistry. 
During the last few years the staff of the U.S. Geological Survey Volcano 
Observatory in Hawaii has been augmented, and its facihties have been 
expanded and modernized to equip it for the necessary multidiscipline 
attack on the problems of Hawaiian volcanism. 

A modernized seismograph network is giving us a better understanding 
of the internal structure of the volcanoes and is revealing some surprising 
evidence on processes within them. New instruments for measuring slight 
deformations of the earth's surface are providing information on the 
underground movement and accumulation of magma. Work at the Sur- 
vey's recently constructed Geochemical Laboratory is helping to unravel 
the mysteries of origin, underground history, and petrographic variations 
of Hawaiian lavas through a systematic, detailed study of the chemistry 
and petrology of the lavas and of the chemistry of the gases given off by 
the volcanoes during and between eruptions. 

EVIDENCE FROM GEOPHYSICS 

A variety of events within the volcanoes set up characteristic disturb- 
ances which are transmitted as elastic vibrations to the surface of the 
earth through the rocks composing the volcanoes and the crust and mantle 
of the earth beneath. These fleeting seismic pulsations carry vital infor- 
mation not only on the time, location, intensity, and nature of the events 
from which they spring, but also on the geologic structure and physical 
properties of the rocks through which they pass en route to the surface. 

To capture these important data, a network of very sensitive seismo- 
graphs is being developed in the Hawaiian Islands (Fig. 2). At the heart 
of the system four vertical-component seismometers, located in critical 
positions within a 15-kilometer radius of the observatory at the summit of 
Kilauea, transmit signals over telephone wires to the observatory, where 
four pens trace visible records of the motion of the ground at the seismom- 
eters. Seismographs in five other stations on the perimeter of the island of 
Hawaii provide critical additional data needed to locate earthquakes origi- 
nating in and beneath the volcanoes, and seismographs in one station on 
Maui and one on Oahu extend the network to the distances required to 
permit the delineation of the structure of the crust under the Hawaiian 
Ridge. 

Hawaii has earthquakes because it has volcanoes. In terms of numbers, 
practically all the earthquakes in the Hawaiian area occur in or beneath 



144 J. P. EATON AND K. J. MURATA 

the active volcanoes and are intimately associated with eruptions. A sig- 
nificant few, however, including most of Hawaii's largest, originate on 
lateral faults at some distance from the calderas and rift zones that give 
rise to so many quakes during eruptions. Although some earthquakes 
along the lateral faults originate at depths as great as 30 kilometers, most 
of them are relatively shallow. They appear to mark gross readjustments 
in the rocky basement in response to the slow growth of the volcanoes 
and to the internal forces that build them. 

Findings on the relation between travel time and distance for the strong 
seismic waves generated by large earthquakes on Hawaii and transmitted 
through the Hawaiian Ridge or refracted through the crust and mantle 
below to the most distant seismographs of the network are the data from 
which we can compute the "structure" of the earth beneath the volcanoes. 
Conventional interpretation of the travel-time curves indicates that there 
is a layered structure which represents a broad approximation of condi- 
tions along the Hawaiian Ridge. The implication of flat-lying, smooth con- 
tacts between discrete rock units should not be taken literally, especially 
for the portion of the structure lying above the level of the ocean floor 
surrounding the islands. 

The near-surface speed of the longitudinal wave, P, is surprisingly low, 
only about 3 km/sec, and testifies to the loose, rubbly nature of the flows 
composing the shields. From a moderate depth below the surface (here 
taken as about sea level) to a depth of several kilometers below sea level, 
the speed of P is about 4 km/sec. Below a depth of 3 kilometers the speed 
of P jumps abruptly to about 5.25 km/sec. The travel-time curves suggest 
that the speed of P increases still more, perhaps by a slow transition rather 
than an abrupt increase, to about 6.8 km/sec in the crust above the man- 
tle. At a depth of about 14 kilometers the speed of P jumps to 8.25 
km /sec, marking the top of the earth's mantle at the Mohorovicic dis- 
continuity. These data are plotted in Fig. 5 with those obtained by Raitt 
(9) from a seaborne seismic profile off the coast of Hawaii. Of special 
interest is the close correspondence in the depth to the Mohorovicic dis- 
continuity beneath the ocean and beneath the Hawaiian Ridge. It appears 
that the crust under Hawaii has been only slightly depressed by the enor- 
mous volcanoes built upon it. 

An accurate knowledge of just where earthquakes originate within the 
volcanoes is very important to our understanding of internal structure. 
Earthquakes do not occur at random but are concentrated in zones or 
along structures undergoing strain. Thus, from the earthquakes that occur 
beneath the Honuapo-Kaoiki fault system, which separates the southwest 
flank of Kilauea from Mauna Loa, we know that the system extends to 
a depth of at least 15 kilometers and that it is still very active. Likewise, 
earthquakes originate from near the surface to depths as great as 30 kilo- 
meters along the Hilina fault system just south of Kilauea caldera, but 



HOW VOLCANOES GROW 145 



Fig. 4. Hilina Pali fault 
scarp. This scarp, 1500 
feet high, has been al- 
most completely mantled 
by recent prehistoric 
lava flows. View is to- 
ward the southwest. 







farther east along this fault system earthquakes originating from depths 
greater than 10 kilometers are rare. Since about 1955, when a seismograph 
network capable of making reasonably accurate focal-point determinations 
was developed, the deepest earthquakes in the Hawaiian area have been 
recorded from a zone approaching a depth of 60 kilometers beneath the 
summit of Kilauea. In addition, thousands of quakes originate at shallow 
depths in the vicinity of Kilauea caldera when the volcano is swelling or 
shrinking in response to the movement of magma below. During the last 
two major eruptive cycles the east rift zone of Kilauea has produced only 
very shallow earthquakes, except very close to the caldera, and probably 
does not extend to a depth lower than the ocean floor. 

Insight into processes at work in the volcanoes can also be gained from 
the nature, sequence, or association of disturbances recorded on the seismo- 
graphs. Some of these disturbances are quite unearthquake-like and are 
apparently generated only by active volcanoes. When lava is pouring out 
at the surface during an eruption the entire region around the vent rocks 
gently to and fro as long as the vent is active. From seismographic 
evidence we know that this disturbance, called harmonic tremor from the 
sinusoidal nature of its seismic record, is generated near the earth's sur- 
face, probably by the rapid flow of magma through the feeding conduits. 
Because harmonic tremor rarely occurs when no eruption is in progress, 
its occurrence is excellent evidence that lava is streaming through con- 
duits underground. 

Great swarms of small earthquakes accompany several different proc- 
esses in the volcano. Unlike a large tectonic earthquake and its aftershocks, 
where one large quake is followed bv many smaller ones, the earthquakes 
in these swarms are uniformly small. The swarm usually begins slowly, 
rises to a maximum (in both average size and frequency of earthquakes). 



146 J. P. EATON AND K. J. MURATA 

and then dies off slowly or abruptly, according to the nature of the process 
generating the earthquakes. Moderate swarms of tiny, sharp, highly local- 
ized earthquakes accompany the extension of dikes toward the surface 
before eruptions. Such swarms cease abruptly when lava pours out at the 
surface. More impressive swarms of larger, shallow quakes scattered 
through the summit of Kilauea attend the rapid subsidence of the caldera 
and its environs when lava drains out through the rift zone of the volcano 
during flank eruptions. These swarms begin and end gradually. 

Occasionally great swarms of tiny-to-moderate, sharp earthquakes, total- 
ing several thousand during the few days they last, emanate from depths 
between 45 and 60 kilometers beneath the summit of Kilauea. These are 
the deepest quakes that occur in Hawaii, and they bear no immediate, 
obvious relation to events closer to the surface. Usually they are accom- 
panied by many hours of continuous, somewhat irregular tremor (spas- 
modic tremor) of weak-to-moderate intensity. The zone from which these 
disturbances stem is deep within the earth's mantle, three to four times 
deeper than the Mohorovicic discontinuity under Kilauea. Such activity 



Elevotion km 




Vp Daniily 
km/sec gm/cc 



^Density 
2 77 gm/o 



I 

/too 



1 
200 



parallel to nft zoni 
il eiaggeralad 2 lir 



Fig. 5. Schematic cross sections of an idealized Hawaiian volcano. 
Magma from a source about 60 kilometers deep streams up through 
permanently open conduits and collects in a shallow reservoir beneath 
the caldera. Occasional discharge of lava from the shallow reservoir 
through dikes that split to the surface constitute eruptions. Note the 
elongation of the volcano along the rift zones and the relatively slight 
depression of the Mohorovicic discontinuity beneath the volcano. Data 
for the oceanic cross sections on the right are from Raitt (9) and 
Worzel and Shurbet (13). 



HOW VOLCANOES GROW 147 

appears to mark the zone from which magma is collected and fed into 
the system of conduits leading to the heart of Kilauea. If the magma rises 
from greater depths, this is at least the deepest zone in which its upward 
migration is marked by detectable seismic disturbances. 

Whether Mauna Loa has a separate source of such activity beneath 
its summit we cannot yet say. No such source has been detected in the 
last five years, since sensitive seismographs have been in operation on 
Hawaii, but neither has Mauna Loa shown any sign of unrest during 
this interval. 

Although seismic disturbances disclose what is happening within the 
volcano and when and where these changes are occurring, they tell us 
very little about the likelihood that a particular disturbance will culminate 
in an eruption. Geophysical measurements of another sort, the measure- 
ment of tilting of the ground surface around the summit of the volcano, 
provide more direct evidence on the readiness of the volcano to erupt. As 
lava wells up within the volcano the surface of the ground above bulges 
upward and the flanks of the bulge tilt outward, and when an eruption 
pours thelava out at the summit or on the flank of the volcano, the ground 
above the emptying reservoir subsides. 

Before an eruption these changes are subtle and slow, and extreme care is 
required to detect them. Conventional tiltmeters are sufficiently sensitive, 
but they are so strongly influenced by accidental local vagaries of earth 
structure and weather that their records are unreliable. To provide high 
reliability as well as high sensitivity and to make it possible to set up many 
low-cost tilt-measuring stations, an unconventional tiltmeter employing 
permanent tilt bases and an ultrasensitive, portable, water-tube leveling 
system has been developed. Successive relevelings at a tilt base, which 
consists of three permanent piers set in the ground at the vertices of an 
equilateral triangle 50 meters on a side, can detect tilting of the earth's 
surface as slight as 1 milhmeter in 5 kilometers {10). 

CASE HISTORY: KILAUEA ERUPTION, 1959-60 

Even while the water-tube leveling system was being refined and tested 
between November 1957 and August 1958, preliminary readings on an 
experimental tilt base at Uwekahuna showed that the ground surface was 
tilting steadily outward from the caldera. By October 1958, measurements 
at additional tilt bases newly installed in a ring around the caldera revealed 
that the entire caldera rim was tilting outward. Analysis of tilting around 
the summit of Kilauea detected by the expanding network of tilt bases 
between October 1958 and February 1959 indicated that the entire sum- 
mit region was swelling as magma slowly welled up from the depths and 
accumulated a few kilometers beneath the south rim of the caldera. 



148 J. P. EATON AND K. J. MURATA 

After the occurrence of several moderate earthquakes just southeast of 
the caldera on 19 February, the swelling stopped, and from May until 
August the summit of the volcano subsided slowly. Then a great swarm 
of deep earthquakes and associated tremor from a source about 55 
kilometers deep and a few kilometers northeast of the Kilauea caldera 
kept Hawaiian seismographs in almost constant agitation between the 
14th and 19th of August (Fig. 6). Magma moving into conduits beneath 







Fig. 6. A swarm of deep earthquakes and spasmodic tremor that orig- 
inated about 55 kilometers beneath Kilauea caldera on 16 August 1959. 
Such activity appears to mark the movement of lava into the conduits 
beneath Kilauea. This seismogram was recorded on smoked paper at the 
observatory, 14 kilometers from the desert seisometer that detected 
these disturbances. 



Kilauea during this episode made itself felt at the surface shortly, for 
rapid swelling of the volcano resumed between August and October 
(Fig. 7, inset A). 

In its early stages, swelling of Kilauea took place with little or no seismic 
accompaniment. Lava rose from the depths and streamed slowly toward 
the shallow reservoir. At most, occasional intervals of weak harmonic 
tremor, originating perhaps 5 to 15 kilometers beneath the surface and 
lasting about half an hour, marked the lava's upward migration. 



HOW VOLCANOES GROW 149 

In the months preceding the 1959 outbreak of Kilauea there was no 
general increase in seismic activity, as there had been before the 1954 
eruption. The first suspicious sign appeared during September 1959, when 
a series of very shallow, tiny earthquakes began recording on the North Pit 
seismograph on the northeast rim of Halemaumau. By the first of Novem- 
ber, quakes of this swarm exceeded 1000 per day, but they were so small 
they barely were recorded on other seismographs only one mile away. A 
hurried remeasurement of tilting at bases around the caldera during the 
second week of November revealed that dramatic changes were in progress: 
the summit of Kilauea was swelhng at least three times faster than during 
previous months (Fig. 7, inset A). In mid-afternoon on 14 November 
earthquakes emanating from the caldera suddenly increased about tenfold 
in number and intensity. At frequent intervals during the next 5 hours the 
entire summit region shuddered as earthquakes marked the rending of the 
crust by the eruptive fissure splitting toward the surface. Then, at 8:08 
P.M., the lava broke through in a half-mile-long fissure about half-way 
up the south wall of Kilauea Iki crater, just east of Kilauea caldera. 
Abruptly the swarm of earthquakes stopped, and seismographs around the 
caldera began to record the strong harmonic tremor characteristic of lava 
outpouring from Hawaiian volcanoes (Fig. 8). 

During the next 24 hours the erupting fissure gradually shortened until 
only one fountain remained active. But then the rate of lava outpouring, 
which had decreased as the erupting fissure shortened, began to increase 
again, and it continued to increase steadily until the fountain died out 
suddenly on 21 November. The 40 million cubic yards of lava poured into 
Kilauea Iki crater filled it to a depth of 35 feet, slightly above the level of 
the vent. 

Seismographs and tiltmeters warned that the eruption was not over. 
Feeble harmonic tremor that persisted after the fountain died was soon 
augmented by a growing swarm of tiny, shallow quakes such as preceded 
the eruption; and tiltmeters, which recorded a rapid deflation of the 
shallow lava reservoir while the fountain poured out its lava, revealed 
that the volcano was being inflated rapidly once more (Fig. 7). At 1:00 
A.M. on 26 November the main vent of the first phase of the eruption 
revived. By 4:35 p.m. an additional 4.7 milhon cubic yards of lava had 
poured into the pond, increasing its depth to 350 feet and raising its 
surface high above the level of the original vent. Again the fountain died 
abruptly, and this time lava began to pour back down the vent. By 12:30 
p.m. the next day 6 mdlion cubic yards of lava had disappeared from the 
lake, leaving a black ring of frozen lava 30 feet above its receding surface. 

During the following three weeks 14 more eruptive phases of shorter 
and shorter duration but with increasingly vigorous fountaining took place 
at the Kilauea Iki vent (Fig. 9). The highest fountain was measured dur- 
ing the 15th phase, on 19 December, when a column of incandescent, gas- 



150 J. P. EATON AND K. J. MURATA 



East — West Component of Tilting 
at Uwekal)una 
May 1959 to April I960 




Till.rg rot. 2 5 . lO'" ' s 
165 inches pe' mile i 



Tilting Pattern Aug 15 to Oct 16 
Tilting Pottern Oct 16 to Nov 13 



Tilting Pottern Jon 21 to Feb 5 



Fig. 7. Ground tilting at stations around Kilauea caldera associated 
with the 1959-1960 eruption. The east-west component of tilting at 
Uwekahuna shows the swelling and collapse of the summit of Kilauea 
as a function of time. Westward tilting (up) corresponds to swelling, 
and eastward tilting (down), to collapse. Inset A illustrates the pattern 
of tilting around the caldera during two periods of swelling. Inset B 
illustrates the pattern during collapse. Note the 40-fold difference in 
scale between A and B. 



inflated lava Jetted to 1900 feet, by far the greatest fountain height yet 
measured in Hawaii. At its highest stand, at the end of the eighth phase, 
the lava pond was 414 feet deep and contained 58 million cubic yards of 
lava. At the end of each phase the fountain died abruptly, and from the 
2nd to the 16th phase, a mighty river of lava surged back down the vent 



HOW VOLCANOES GROW 151 

as soon as the fountaining stopped (Fig. 10). Of the 133 milhon cubic 
yards of lava spewed out into Kilauea Iki crater during the eruption, only 
48 million cubic yards remains in the 367-foot-deep pond. The other 85 
million cubic yards poured back underground almost as soon as it col- 
lected in the Kilauea Iki lava pond, where its volume could be so con- 
veniently measured. 

Tiltmeters around Kilauea caldera showed that the volcano was swelhng 
rapidly as phase after phase of the eruption delivered its lava to the sur- 
face and then swallowed it up again. When surface activity ceased at 
Kilauea Iki on 21 December, far more lava was stored in the shallow reser- 
voir beneath the caldera than when the eruption began (Fig. 7). It 
appeared that Kilauea was in an unstable state and that further activity 
was very likely. 

During the last week of December a swarm of small earthquakes began 
to record on the seismograph at Pahoa. By means of a sensitive portable 
seismograph the source of these earthquakes was soon traced to the east rift 
zone of Kilauea, about 25 miles east of the caldera, near the site of the 
first outbreak of the 1955 eruption (Fig. 2). The magma that inflated the 
summit region most probably exerted pressure on the plastic core of the 
rift zone, and earthquakes revealed where the rift zone yielded and where 
dikes began to extend toward the surface. 

Early in January the frequency and size of earthquakes from the east rift 
zone increased, and the region from which they emanated moved on toward 
the sea. On 13 January the village of Kapoho was rocked by frequent, very 
shallow earthquakes, and by nightfall a graben 0.5 mile wide and 2 miles 
long that contained about half of the town had subsided several feet. At 
7:30 P.M. the earthquake swarm gave way to harmonic tremor, and the 
flank eruption broke out along a fissure 0.75 mile long near the center of 
the subsiding graben, a few hundred yards north of Kapoho and nearly 30 
miles east of the summit of Kilauea. 

During the next five weeks nearly 160 million cubic yards of lava poured 
out of the vent north of Kapoho and reshaped the topography of the 
eastern tip of Hawaii (Fig. 2) . As the flow from the vent to the sea 2 miles 
away gradually built higher and higher, lava crowded out of the natural 
channel that initially confined it. Sluggish flows spread laterally from the 
main channel, destroying almost all of Kopoho, south of the vent, and 
most of the village of Koae, north of the vent. Dikes 15 to 20 feet high, 
built in a futile attempt to confine or divert flows that threatened a resi- 
dential community along the seashore 2 miles southeast of Kapoho, were 
completely overwhelmed, and the lava moved on to destroy a portion of 
that community. 

On 17 January, only four days after the flank eruption began, the summit 
of Kilauea began to subside precipitously as lava began to drain from 
beneath the caldera and to move through the rift zone toward the Kapoho- 



152 J. P. EATON AND K. J. MURATA 








UWEKAHUNA 

short-period 

vertical 

Nov 14, 1959 

Shallow 
dike- splitting 
swarm 



Eruption began 






Harmonic 
tremor 

, I minute 



Fig. 8. Seismogram showing a swarm of earthquakes immediately pre- 
ceding the eruption in Kilauea Iki, followed by harmonic tremor caused 
by lava streaming through the erupting fissure near the surface. This 
seismogram is from a short-period vertical seismograph at Uwekahuna. 






vent (Fig. 7, inset B). By the end of January a strong swarm of shallow 
earthquakes was in progress at Kilauea caldera, where the brittle surface 
rocks were failing under the rapid and severe deformation caused by con- 
tinuing subsidence (Fig. 11). On 7 February an unseen fissure broke 
through into the still liquid core of the 300-foot-deep pond of lava erupted 
into Halemaumau in 1952, and the floor of Halemaumau settled about 150 
feet as the hquid beneath it drained away. A smaller area in the center of 
the floor dropped an additional 200 feet, but it was partially refilled by 
sluggish flows of viscous lava draining from under the subsiding crust of 
the pond around it. 

By the first of April, when rapid subsidence and the swarm of earth- 
quakes it caused had ceased, tiltmeters around the summit indicated that 
the ground surface above the shallow reservoir that was deflated during 
the flank eruption had sunk about 5 feet. The total volume of collapse at 
the summit (the volume swept out by the surface of the volcano as its 
summit subsided), estimated from tiltmeter data, is close to the total 
volume of lava erupted at the surface. 



Fig. 9. Five-hundred- 
foot lava fountain in 
Kilauea Iki crater at 
7:00 a.m. on 5 Decem- 
ber, 1959. Note the new 
cinder cone at left of the 
fountain and the lake of 
fresh lava 400 feet deep 
in the foreground. The 
west wall of Kilauea 
caldera and the south- 
east flank of Mauna Loa 
are in the background 
of the picture, which 
was taken with the 
camera facing west. 




Comparisons of temperatures and silica content of the lava erupted at 
Kilauea Iki and at Kapoho provide additional data on the underground his- 
tory of Hawaiian lava. Temperatures measured in the core of the fountain 
at Kilauea Iki were consistently above I120°C (measured with a hot-wire 
optical pyrometer and uncorrected for departure from black-body radi- 
ation). During a single phase of the eruption the temperature of the lava 
usually increased from about 1120°C near the beginning of the phase to 
about 1150°C near the end. The maximum temperature was measured 
during the fourth phase, when 1190°C was recorded. During early phases 
the silica content of the lava varied between 46.3 and 49.5 percent, but 
after the fourth phase it stabilized at about 46.8 percent. Petrographically 
the lava is a tholeiitic picrite basalt, consisting of olivine phenocrysts set 
in a fine-grained ground-mass of plagioclase feldspar, pyroxene, and glass. 

The lava erupted during the first two weeks of the flank eruption closely 
resembled the lava erupted in the same region in 1955. These lavas are 
tholeiitic basalts, poor in olivine but containing abundant phenocrysts of 
plagioclase feldspar and pyroxene. The silica content was about 50 percent, 
and the temperature was only 1050° to 1060°C, fully 100°C cooler than 
the lava at Kilauea Iki. After the second week the lava emerging from 
the Kapoho vent began to change; the silica content dropped, and the 
temperature increased. During the last week of voluminous lava eruption 
in February the temperature reached a maximum of 1130°C and the com- 
position approached that of the lava erupted at Kilauea Iki. 

It seems quite probable that the lava poured out during the first two 
weeks of the flank eruption had remained stored in the rift zone since at 
least 1955, if not since 1924, when lava drained from the summit into the 
east rift zone but failed to reach the surface. The chemical composition 
and mineralogy of this lava reveal a degree of differentiation that is unusual 
for Kilauea. The last lava erupted at Kapoho petrographically resembles 

153 



154 J. P. EATON AND K. J. MURATA 

Kilauea Iki lava, and it is entirely possible that magma moved from the 
summit reservoir, down through the rift zone, to the Kapoho vent during 
the course of the flank eruption. 

ORIGIN OF THE MAGMA 

Although the geophysical evidence presented above permits us to trace 
the movement of magma through the volcano, it does not suggest why 
nor how magma enters the volcano at depth and rises through it to 
heights approaching 10 kilometers above the ocean floor to pour out 
at the surface. The "ascensive force of the lava," as it was called by Dana 
(IJ), was attributed by Daly (12) to the lower average density of the 




Fig. 10. A river of lava 
pouring back into the 
Kilauea Iki vent at 7:30 
a.m. on 19 December 
1959. The top of the 
cone is 400 feet higher 
than the vent. The pic- 
ture was taken with the 
camera facing south. 



column of lava as compared to that of the crust of the earth above the 
zone in which the lava begins its journey to the surface. New information 
on the structure of the earth's crust beneath the Pacific basin requires 
that we revise the details of the model presented by Daly. We suggest 
that the crust here is much thinner than he believed it to be, and few 
geologists would now subscribe to the view that there is an eruptible 
basaltic glassy substratum underlying a crystalline crust. In principle, 
however, no better explanation of the ascensive force has been offered than 
that proposed by Daly. 

If we assign densities to the molten lava column and to the various 
earth layers reported by Raitt for the Pacific basin in the Hawaiian region, 
we can compute the minimum depth at which lava can enter the volcanic 
system and be forced to the summits of the volcanoes. The densities given 
in Fig. 5 for the layers in Raitt's oceanic crust are those of the standard 
oceanic crustal gravity section adopted by Worzel and Shurbet (13). 



HOW VOLCANOES GROW 155 

For the average density of the basaltic lava column we shall adopt Daly's 
estimate of 2.77 grams per cubic centimeter. Balancing the densities of 
the lava column and the crust, we find that to raise the lava % kilometers 
above sea level the lava column must extend at least to a depth x below 
sea level, where x = 32.34 -f 5.54 z kilometers. Thus, to raise lava to the 
summit of Kilauea (1.2 kilometers), the lava column must extend to a 
depth of at least 39 kilometers below sea level; and to raise lava to the 
summit of Mauna Loa (4.2 kilometers), it must extend to a depth of at 
least 57 kilometers. These figures are in good agreement with the depth at 
which, according to the evidence of swarms of deep earthquakes and 
tremor, lava is fed into the Kilauea system. 

Data from still another quarter, the study of surface waves of large 
earthquakes, throw additional light on the origin of Hawaiian lavas. Recent 
analyses of the dispersion of Rayleigh waves crossing the Pacific basin 
reveal that the rigidity of the mantle decreases somewhat at a depth of 
60 kilometers (H). In view of the two other lines of evidence suggesting 
that Hawaiian magma originates at about this depth, it seems reasonable 
to conclude that the softening of the mantle at 60 kilometers is caused 



Fig. 11. Seismogram showing a swarm of shallow earthquakes caused 
by rapid subsidence and deformation of the summit of Kilauea. This 
swarm lasted for several weeks. The seismogram was recorded on a 
short-period vertical seismograph at Uwekahuna. 




156 J. P. EATON AND K. J. MURATA 

by partial melting of a peridotite mantle to yield an eruptible basaltic 
fraction. Perhaps, to go back to glassy substratum like that postulated 
by Daly but of higher density, the cooling and the consequent partial 
crystallization of a noneruptible, dense, glassy mantle drives off a lighter 
basaltic fraction that can be erupted to the surface. 



MECHANISM, COMPOSITION, AND KINETICS 
OF ERUPTION OF THE LAVAS 

Let us recapitulate the evidence on the mechanism of eruption pre- 
sented above and examine, by following the magma on its course through 
the volcano, how that mechanism explains surface geologic features. When 
magma enters the deep conduit beneath Kilauea (a portion of the funda- 
mental fracture beneath the Hawaiian Ridge that is currently active) it 
begins a slow ascent through the heated depths toward the cooler crust 
and volcanic pile above. The movement of magma into the conduit at 
depth is relatively slow and steady, being governed, perhaps, by the rate 
at which the magma can be separated from the mantle and funneled into 
the open conduit. After leaving the upper portion of the mantle and 
traversing the basaltic layer that floored the ancient ocean, the magma 
emerges into the lighter, weaker rocks composing the volcanic pile and 
collects in a reservoir only a few kilometers beneath the surface. Up- 
welling of lava and consequent inflation of the high-level reservoir are 
slow processes that continue for months or even years prior to an eruption. 
Mounting pressure within the expanding reservoir finally drives the magma 
into dikes that split the frozen crust above the reservoir. When one of 
these dikes breaks through to the surface, an eruption ensues; the reservoir 
shrinks, and the pressure within it decreases as lava is discharged. 

Occasionally magma from the main reservoir is driven laterally into 
the mobile core of a rift zone, and failure of the confining rocks at some 
point along the rift results in a flank eruption, sometime miles from the 
summit of the volcano. Discharge of lava at a low elevation along a rift 
zone can cause a much greater drop in reservoir pressure than can result 
from a summit eruption. The volume of flank eruptions and the conse- 
quent reservoir deflation and ground-surface subsidence are much larger 
than for summit eruptions. 

Rift zones, like the central reservoir, appear to be relatively shallow 
structures. They are zones split by countless dikes seeking to discharge 
lava at a low elevation through a long channel that cuts the cold crust in 
competition with other dikes that provide shorter channels through the 
cold crust to higher elevations near the summit. Concentration of these 
dikes in a zone and the ultimate generation of a molten rift-zone core 
result from the tendency for each dike to heat the rocks around it and 



HOW VOLCANOES GROW 157 



TABLE 1 

Chemical composition of typical Hawaiian rocks 
(these compositions are plotted on Fig. 12) . 







Tholeiitic basalt series* 




Alkalic basalt series t 


Compound 
















A 


B 


c 


D 


E 


F 


G 


SiOo 


50.94 


50.08 


46.59 


62.23 


50.09 


62.19 


43.28 


AI2O3 


12.97 


13.73 


6.69 


12.03 


19.49 


17.43 


14.43 


FeoOs 


1.95 


1.32 


2.20 


5.55 


0.73 


1.65 


0.70 


FeO 


8.96 


9.79 


10.46 


4.76 


8.47 


2.64 


10.92 


MgO 


10.68 


7.89 


21.79 


2.05 


4.33 


0.40 


11.68 


CaO 


9.88 


11.50 


7.41 


4.25 


6.92 


0.86 


11.22 


NaoO 


1.99 


2.18 


1.33 


3.20 


4.82 


8.28 


2.49 


K2O 


0.37 


0.56 


0.28 


1.36 


1.93 


5.03 


0.83 


H2O+ 


0.12 


0.02 


0.37 


0.33 


.32 


0.39 


0.05 


H2O- 


0.04 


0.00 


0.04 


0.52 


.08 


0.14 


0.03 


Ti02 


1.78 


2.60 


1.83 


2.18 


2.47 


0.37 


4.12 


P2O5 


0.21 


0.26 


0.11 


0.01 


0.78 


0.14 


0.31 


MnO 


0.17 


0.17 


0.18 


0.43 


0.15 


0.32 


0.13 


CO2 


0.04 


0.01 








0.02 




Cr203 






0.13 






tr. 


0.10 


NiO 






0.12 










SO3 






0.00 






0.00 


0.20 


Total 


100.10 


100.11 


100.53 


99.211 


100.58 


99.93§ 


100.5411 



* (A) Tholeiitic olivine basalt, Mauna Loa, at highway at south boundary of Waiakea 
Forest Reserve, 2.65 km northwest of the Olaa sugar mill, island of Hawaii. Analyst, 
L. N. Tarrant (3J). (B) Tholeiitic basalt, Kilauea, splash from lava lake, 1917, island 
of Hawaii. Analyst, L. N. Tarrant. Reanalysis of a previously described sample. New 
analyses published with permission of H. A. Powers (19). (C) Mafitic gabbro porphyry, 
Kilauea, Uwekahuna laccolith in the wall of the caldera, island of Hawaii. Analyst, 
G. Steiger (32). (D) Granophyre, Koolau Volcano, quartz dolerite dike at Palolo 
quarry in the southeastern part of Honolulu, island of Oahu. Analyst, K. Nagashima 
(33). f(E) Hawaiite (andesine andesite), Mauna Kea, elevation 2700 feet, on north- 
west flank near Nohonaohae, island of Hawaii. Analyst, H. S. Washington (16). (F) 
Trachyte obsidian, Hualalai, Puu Waawaa, island of Hawaii. Analyst, W. F. Hillebrand 
(15). (G) Picritic alkalic basalt, Haleakala Volcano, lava flow of 1750(?) on the south- 
west slope near Makena, island of Maui. Analyst, M. G. Keyes (34). |Includes 0.31 
SrO. § Includes 0.03 BaO and 0.04 Zr02. || Includes 0.05 BaO. 

lessen the freezing effect of the cold crust on later dikes that follow nearby 
paths. 

Rapid, severe deflation of the central reservoir or of its lateral protru- 
sions into the rift-zone cores can lead to the collapse of the ground surface 
by withdrawal of support from below. This process, which is especially 
severe for flank eruptions far down the slopes of the volcano, seems to be 
responsible for the formation of pit craters and calderas. 

Basalt occupies a key position in modern theories of petrogenesis, and 
most, if not all, other kinds of igneous rocks are considered to have their 
ultimate origins in basaltic magmas. Thus, the chemical differentiation of 
basaltic magmas is a fundamental geochemical problem that has occupied 
the attention of many investigators throughout the world. Study of this 
differentiation in basaltic areas on the continents is complicated by the 



158 J. P. EATON AND K. J. MURATA 

ever-present possibility that basaltic magmas may become contaminated 
by the diverse rocks that make up the crust of the continents. In the 
Hawaiian province, with its simple basaltic substratum, the possibility of 
such contamination is minimal, so magmatic differentiation may be in- 
vestigated here with confidence. 

The work of Cross {IS), Washington (J6), Macdonald (17), Went- 
worth and Winchell (18), and Powers (19), among others, has disclosed 
a wide range in chemical composition among Hawaiian basaltic lavas and 
\Las established the broad outline of genetic relationships among rocks 
of different composition. Analyses of typical examples of the different types 
of Hawaiian rocks are given in Table 1. 

The division of basaltic rocks into a tholeiitic series and an alkalic series, 
first made for the basaltic rocks of Scotland by Bailey and others (20), 
is also useful in the study of the Hawaiian rocks, as was recently shown by 
Tilley {21 ) . As emphasized by Macdonald {17), the fundamental primitive 
magma of Hawaii is tholeiitic olivine basalt (Table 1, sample A). Sample 
A closely approximates the average composition of tholeiitic lavas from 
the currently active mature volcanoes Kilauea and Mauna Loa, and this 
general type of lava makes up the great bulk of each of the Hawaiian 
Islands. Rocks of the alkalic basalt series are produced in lesser quan- 
tities in the declining stages of volcanic activity and, on the island of 
Hawaii, characteristically occur as mantles over the tholeiitic shields of 
the extinct or late-stage volcanoes Mauna Kea, Kohala, and Hualalai. 

The analyses in Table 1 pose the fundamental geochemical problem of 
explaining the differentiation of primitive tholeiitic magma to produce the 
other types of rocks with such greatly different composition. An adequate 
theory must not only satisfy the chemical criteria but must also correlate 
existing information on the relative amounts of the different types of 
rocks, their sequence of eruption, the melting and reaction relationships 
among the constituent minerals, and the kinetics of ascent and cooling of 
molten magmas. 

All investigators of Hawaiian basalts since Cross {15) have emphasized 
the role of kinetics of eruption in controlling the extent and nature of 
differentiation of basaltic magma, but they have not agreed on the precise 
mechanism of control. Particularly, the mechanism of transition from 
tholeiitic to alkalic magmas during the life cycle of a volcano has remained 
in doubt. Our studies suggest that the transition is mainly the result of 
progressively more favorable conditions becoming established for exten- 
sive fractional cr}^stallization of pyroxene during the later stages of a 
volcano, when magmas rise and cool very slowly and eruptions become 
very infrequent. This dynamical-chemical relationship is here discussed 
briefly with the aid of Fig. 12. 

Of the many different ways in which analyses of basaltic lavas may be 
plotted for study, the one shown in Fig. 12 offers the great advantage of 



HOW VOLCANOES GROW 159 

indicating the compositions of the three major minerals of the lavas — 
namely, pyroxene, plagioclase feldspar, and olivine. In this diagram 
differences in chemical composition are directly interpretable in terms of 
differences in the proportions of the three minerals. The diagram was 
originally derived by plotting the composition of 150 basaltic rocks from 
Hawaii and the British Hebridean province, and it has been published in 
full elsewhere (22). The skeletonized version is presented here for the 
sake of simplicity and clarity. 

The parallelism in composition between the tholeiitic basalt series 
(C-a-A-B-b-D) and the alkalic series (G-c-E-d-F) is well shown in Fig. 12. 
Both series have olivine-rich members {C-a-A and G-c) and a group of 
closely related differentiates with progressively increasing content of 
silica {B-b-D and E-d-F). In the tholeiitic series, this group includes rocks, 
such as granophyre (D), that are rich in quartz, whereas in the alkahc 
series even the most sihceous member (trachyte F) is free of quartz but 
is rich in alkalic feldspar. 

Molten tholeiitic magma of composition A, rising toward the surface, 
cools and first precipitates olivine [(Mg, Fe), SiO^] crystals, which grow 
rapidly in size to a diameter of several millimeters (23). Olivine, having 
a greater specific gravity, tends to sink in the molten magma. This simple 
act of separating the crystal from the melt in which it formed changes 
the composition of the melt along the hue A to B, and the composition 
of the underlying magma that receives the settling olivines, along the line 
A to C. Thus originate two complementary types of lavas, tholeiitic basalt 
(B) which is poorer in olivine, and picritic basalt (C) which is richer 
in olivine, than the parent magma. It should be noted that a shift in 
composition anywhere in the diagram involves such a fractional crystalli- 
zation of one or more minerals. 

There is a limit to changing the composition of the melt by settling of 
olivine because, at around point B, olivine precipitation ceases, and with 
decreasing temperatures augitic pyroxene [(Ca, Mg, Fe^^, Fe^+) (Si, 
Al)20o] begins to crystallize. If the rate of cooling is very gradual and 
pyroxene is crystallized fractionally, the composition of the residual melt 
will move along BE into the zone of the alkalic series. If the cooling is 
rapid, as in the currently active volcanoes, plagioclase feldspar [(Ca, Na) 
(Al, Si) AlSigOJ soon starts to crystallize along with pyroxene, and 
the fractional syncrystallization of the two minerals yields residual melts 
with tholeiitic compositions along B-b-D. Therefore, the rate of ascent 
and hence cooling of the magma within the temperature range of the 
initial crystallization of pyroxene is of utmost importance in the differenti- 
ation of basaltic magma. 

The spectacular eruptions of Kilauea and Mauna Loa permit us to 
observe tholeiitic lavas in the making. As indicated in Fig. 12, however, 
only a part of the tholeiitic series is represented among the lavas of 



160 J. P. EATON AND K. J. MURATA 

these two volcanoes. Compositions between b-D apparently require a 
somewhat slower regimen of cooling than that experienced by materials 
that reach the surface, and rocks with such compositions may be crystalliz- 
ing at depth within the two volcanoes. In the deeply dissected Koolau 
Volcano on Oahu and in Tertiary volcanoes of the British Hebrides, 
such rocks are found characteristically as dikes, sills, and other intrusive 
bodies. The entire tholeiitic series of rocks, therefore, appears to be a 
product of conditions that prevail in basaltic volcanoes that erupt vigor- 
ously and frequently. 

Kilauea and Mauna Loa erupt on the average every few years. The re- 
duced vigor of volcanoes that have reached the stage of producing alkalic 
lavas is illustrated by Hualalai on the island of Hawaii and Haleakala on 




Q^ Tholeiitic basalt series 
(1^^ Alkalic basalt series 



Quartz 



Fig. 12. Diagram showing interrelationships among typical Hawaiian 
volcanic rocks as manifested by their composition with respect to mag- 
nesia and alumina-silica ratio. Open circles, rocks of the tholeiitic basalt 
series listed in Table 1 ; solid circles, rocks of the alkalic basalt 
series. Tholeiitic olivine basalt (point A) is the primary magma of 
Hawaii; all other rock types are derived from it by fractional crystalliza- 
tion of the different minerals and the resulting changes in the com- 
position of tholeiitic and alkalic magmas are as follows: Olivine loss; 
A-B and c-E; olivine gain; A-C and c-G; pyroxene plus plagioclase loss; 
B-b-D and E-d-F; pyroxene loss; a-G, A-c, B-E, and b-d. The zone 
enclosed by a dashed line marks the range in composition found in 
tholeiitic lavas of the currently active volcanoes of Kilauea and Mauna 
Loa. 



HOW VOLCANOES GROW 161 

the island of Maui. One hundred and sixty and about 210 years, re- 
spectively, have passed since these volcanoes last erupted (24). The more 
sluggish and halting ascent of the magma in such volcanoes allows the 
very slow cooling that is necessary for fractional crystallization of pyroxene. 

The general derivation of alkalic magmas through fractional crystalliza- 
tion of pyroxene is shown in Fig. 12, starting from four illustrative points 
[a, A, B, and b) in the tholeiitic series. There are differences in the details 
of the fractional crystallization process along the four paths, but dis- 
cussion of these differences will be deferred to a subsequent article. Within 
the alkalic series itself, the same fractional crystallization of olivine and of 
pyroxene and feldspar takes place as in the theoleiitic series and accounts 
for the parallelism in composition between the two series. In general, 
the olivine and pyroxene that are fractionally crystallized from the cooler 
alkalic magmas are richer in ferrous iron. 

The world-wide problem of the origin of tholeiitic and alkalic basalts 
is being actively investigated by many petrologists, some of whom favor a 
separate derivation of the two compositional series from different depths 
in the mantle of the earth. Our studies suggest, rather, that the composi- 
tion of basaltic rocks is primarily a function of the rate of ascent and 
cooling of a single fundamental magma. With the geological, geophysical, 
and geochemical techniques now available at the observatory located on 
an active volcano, it should be possible to obtain experimental verifica- 
tion of this interesting relationship between kinetics of eruption and com- 
position of erupted lavas, at least within the tholeiitic basalt series. 

VOLCANIC GASES 

In Hawaii, volcanic gases are manifested most spectacularly during an 
eruption in the effervescing fire fountains, which squirt a pulsating stream 
of molten lava up to heights of a thousand feet and more. In other vol- 
canic regions, such as Indonesia {2S), they give rise to more explosive 
and deadly phenomena like nuee ardente eruptions. A typical composition 
(in volume percent) of Hawaiian magmatic gases, as established through 
the work of Shepherd {26), Jaggar {27), and Naughton and Terada {28), 
is as follows: H^O, 79.31; CO3, 11.61; SO^, 6.48; N^, 1.29; H„ 0.58; CO, 
0.37; S,, 0.24 CI2, 0.05; A, 0.04. The proportions of the constituents vary 
over a certain range, and Ellis (29) has shown that the variations are 
largely accountable in terms of shifts in gas equilibria with changing 
temperature. The role of gases in controlling the state of oxidation of the 
magma requires thorough investigation {30). 

Volcanic gases, in whole or in part, represent primordial materials 
reaching the surface of the earth for the first time. Thus, over the span 
of geological time the accumulation of such gases from innumerable 
eruptions determined the evolutionary course of our atmosphere and 



162 J. P. EATON AND K. J. MURATA 

hydrosphere. The new Geochemical Laboratory is equipped with a mass 
spectrometer for rapid analysis of gases, and a program of systematically 
analyzing all volcanic exhalations has been started. 

SUMMARY 

Hawaiian volcanoes offer an unmatched opportunity for studying 
the mechanism of eruptions and the diffentiation of primitive tholeiitic 
basaltic magma. They are located near the center of the Pacific basin, 
more than 2000 miles from the nearest region of active tectonism, and the 
story of their origin and continuing activity is one of pure volcanism. 
Because their lavas experience a minimal exposure to contamination by 
heterogeneous crustal rocks as they rise to the surface, fractional crys- 
tallization plays the dominant role in producing changes in the chemical 
composition of the lavas extruded at different stages in the life cycle 
of the volcanoes. 

The enormous size, relatively simple structure, and frequent voluminous 
eruptions of Hawaiian volcanoes all permit the effective use of seismographs 
and tiltmeters in delineating their internal structure and in detecting the 
movement and accumulation of magma within them. Other more general 
geophysical investigations of the Pacific crust and the mantle below pro- 
vide additional evidence on where Hawaiian magma originates and how 
it is driven to the surface. 

The ultimate cause of volcanism is the fundamental instability of the 
crust and upper mantle of the earth. About 60 kilometers beneath the 
Pacific the rocks of the mantle yield a fluid fraction with the composition 
of tholeiitic basalt. The density of this basaltic magma fraction is less than 
the average density of the 50 kilometers of mantle (peridotite?), 5 kil- 
ometers of basaltic crust, and 5 kilometers of water that lie above it, and 
if the opportunity arises it can be squeezed to the surface by the weight of 
the material above. The fundamental fracture beneath the Hawaiian Ridge 
has tapped this source of magma and provides the avenue through which 
it can escape to the surface. 

Lava rising through the fundamental fracture beneath Kilauea accumu- 
lates slowly in a shallow reservoir only a few kilometers beneath the 
caldera. At irregular intervals dikes project upward from the expanding 
reservoir, and if the expansion and consequent pressure within the reservoir 
are great enough, the dikes break through to the surface and discharge the 
accumulated lava in an eruption. 

Geochemical studies show that while the volcanoes are vigorously active, 
the most striking variation in their lavas is the content of olivine. Rapid 
delivery of magma to the surface permits only slight cooling underground, 
and the only mineral that is fractionally crystallized in significant amounts 
is olivine, which is depleted from some flows and concentrated in others. 



HOW VOLCANOES GROW 163 

When activity declines and magma wells up from depth much less rapidly, 
it remains in the shallow reservoirs for increasingly longer periods of time. 
Here the magma cools so slowly through the temperature range in which 
pyroxene crystallizes that this mineral, as well as the early-formed olivine, 
settles out of the melt and is immobilized on the floor of the reservoir. 
Such separation of pyroxene "desilicates" the tholeiitic parent magma 
and changes its composition to that of alkalic basalt, the predominant lava 
of the declining stage of Hawaiian volcanism. The temperature, composi- 
tion, and rate of ascent of the basaltic magma to the surface, therefore, 
are closely interrelated, and the study of the complex interrelationships of 
these geophysical and geochemical factors constitutes the fascinating work 
of observing how volcanoes grow. 

REFERENCES AND NOTES 

1. H. Williams, Quart. /. Geol Soc. London 109, 311 (1954). 
2 .W. Ellis, Journal of William Ellis, A Narrative of a Tour Through Hawaii in J 823 
(Hawaiian Gazette Co., Honolulu, new ed., 1917). 

3. Founded, and initially financed, jointly by the Massachusetts Institute of Tech- 
nology and The Hawaiian Volcano Research Association, the Hawaiian Volcano 
Observatory was transferred to the U.S. Government in 1917. Since 1948 it has 
been operated by The U.S. Geological Survey with the encouragement and support 
of The National Park Service. Publication of this article is authorized by the 
director of The U.S. Geological Survey. 

4. T. A. Jaggar, Bull. Seismol. Soc. Am. 10, 155 (1920). 

5. H. T. Stearns and G. A. Macdonald, Hawan Div. Hydrog. Bull. 9 (1946). 

6. E. L. Hamilton, Bull. Geol. Soc. Am. 68, 1011 (1957). 

7. W. T. Brigham, B. P. Bishop Museum Mem. 2, No. 4, 379 (1909). 

8. G. P. Woollard, Trans. Am. Qeophys. Union 32, 358 (1951). 

9. R. W. Raitt, Bull. Geol. Soc. Am. 67, 1623 (1956). 

10. J. P. Eaton, Bull. Seismol Soc. Am. 49, 301 (1959). 

11. J. D. Dana, Characteristics of Volcanoes (Dodd, Mead, New York, 1890). 

12. R. A. Daly, Igneous Rocks and the Depths of the Earth (McGraw-Hill, New York, 
1933). 

13. J. W. Worzel and G. L. Shurbet, "Gravity Interpretations from Standard Oceanic 
and Continental Crustal Sections," Geol. Soc. Am. Spec. Papers No. 62 (1955), 
pp. 87-100. 

14. J. Dorman, M. Ewing, J. Oliver, Bull. Seismol. Soc. Am. SO, 87 (1960). 

15. W. Cross, "Lavas of Hawaii and their Relations," U.S. Geol. Survey Profess. 
Papers No. 88 (1915). 

16. H.S.Washington, Am. /.Sci. 6, 339 (1923). 

17. G. A. Macdonald, "Petrography of the Island of Hawaii," U.S. Geol. Survey 
Profess. Papers No. 214D (1949) . 

18. C. K. Wentworth and H. Winchell, Bull. Geol. Soc. Am. 58, 49 (1947) . 

19. H. A. Powers, Geochim. et Cosmochim. Acta 7, 77- (1955) . 

20. E. B. Bailey et al., "Tertiary and Post-Tertiary Geology of Mull, Loch Aline, and 
Oban," Mem. Geol. Survey, Scotland (1924) . 

21. C. E. Tilley, Quart. /. Geo/. Soc. London 106, 37 (1950). 

22. K. }. Murata, Am. /. Sci. 258-A, 247 (1960). 

23. H. I. Drever and R. Johnston, Trans. Roy. Soc. Edinburgh 63, 289 (1957). 

24. G. A. Macdonald, Catalogue of the Active Volcanoes of the World Including 
Solfatara Fields: pt. 3, Hawaiian Islands (International Volcanological Association, 
Naples, 1955). 



164 W. T. PECORA 

25. R. W. Van Bcmmelen, The Geology of Indonesia: vol. lA, General Geology 
(Government Printing Office, The Hague, Netherlands, 1949). 

26. E. S. Shepherd, Am./. Sd.35-A, 311 (1938). 

27. T. A. Jaggar, ibid. 238, 313 (1940). 

28. J. J. Naughton and K. Terada, Science 120, 580 (1954). 

29. A.J. Ellis, Am. J. Sd. 255, 416 (1957). 

30. F.E.Osborn, ibid. 257, 609 (1959). 

31. G. A. Macdonald and J. P. Eaton, U.S. Geol. Survey Bull. No. 1021-D (1955), 
p. 127. 

32. R.A. Daly,/. GcoZ. J 9, 289 (1911). 

33. H. Kuno, K. Yamasaki, C. lida, K. Nagashima, Japan. J. Geol. and Geography, 
Trans. 28, 179 (1957). 

34. H.S. Washington and M.G.Keyes, Am. /.Sci. J 5, 199 (1928). 



Coesite Craters and Space Geology* 

• W. T. PECORA 



THE TERM "SPACE GEOLOGY/' ADMITTEDLY A HYBRID 

term, has nevertheless received the sanction of current usage among 
geologists to signify the extension to extraterrestrial objects and domains 
of those concepts and techniques of study hitherto employed in geology, 
the study of the earth. 

The recent discovery of coesite, a high pressure polymorph of silica, at 
Meteor Crater, Ariz., reported in GeoTimes in the preceding issue (p. 37) 
and by Chao, Shoemaker, and Madsen in Science ( J ) can be of great sig- 
nificance in recognizing impact craters on earth caused by meteoritic falls. 
Coesite had been known only as a dense form of silica synthesized in the 
laboratory; and a determined search for a natural occurrence was culmi- 
nated by its discovery in sheared Coconino sandstone by the U. S. Geo- 
logical Survey in June of this year. 

EXPERIMENTAL DATA 

Coesite was first made in 1953 by Dr. Loring Goes, Jr., in the laboratory 
of the Norton Company, Worcester, Mass. Working systematically and 
without fanfare, Dr. Goes succeeded in synthesizing a great many complex 
mineral substances, among them a high density form of silica now known 
as coesite in his honor. He produced this compound at pressures of about 

• From GeoTimes, V. No. 2 (Sept. 1960) . 

* Publication authorized by the Director, U.S. Geological Survey. 



COESITE CRATERS AND SPACE GEOLOGY 165 

35 kilobars and in the temperature range of 500°-800° C. Those of us who 
heard Goes speak of his experimental results, or who visited his laboratory, 
were impressed with his humility and his dogged persistence in creating 
one mineral after another in his specially designed high pressure apparatus. 
MacDonald (2), working as a guest at U.C.L.A., in 1955, repeated Goes' 
success in synthesizing coesite by using the high-pressure "squeezer" de- 
signed by Griggs. At the "Bush Gonference" in the Fall of 1955, Mac- 
Donald presented to members and guests of the Geophysical Laboratory a 
theoretical discussion of the quartz-coesite equilibrium and the possible 
existence of rocks at great depth in the crust that are chemically equivalent 
to granite or basalt but composed of denser mineral phases than, for 
example, those characteristic of rocks at shallower depth or at the surface. 
Led by this suggestion, perhaps, a general search for coesite in eclogitic 
rocks was made but without success. More recently, Dachille and Roy (3) 
of Pennsylvania State University and Boyd and England (4) of the Geo- 
physical Laboratory synthesized coesite and redetermined the quartz- 
coesite equilibrium curve over a wide range of temperatures and pressures. 
From the accumulated evidence of the experimentalists, it seemed un- 
likely that coesite could form at anything but very great depths in the 
earth. It would be expected to invert to a less dense form of silica enroute 
to the surface in geologic time. 

Shock experiments on single crystals of quartz by De Garli and Jamie- 
son (5) failed to produce coesite at pressures of 380 kilobars (calculated) 
but changed the quartz to an isotropic substance having many of the prop- 
erties of a glass. Passage of shock waves through substances can locally raise 
the pressure much higher than can be reached with present static load 
devices. Unfortunately shock wave pressures have durations of milliseconds 
and the actual pressure cannot be measured directly but must be calcu- 
lated. The mystery lies in the realization that if up to 380 kilobars under 
shock conditions in the laboratory coesite was not formed from quartz, how 
was it formed at Meteor Grater? Goesite was also looked for but not found 
in rocks at nuclear explosion craters and in specially designed laboratory 
experiments involving hypervelocity impact, where high pressure shock 
waves were also generated. 

NATURAL OGGURRENGE 

Goesite, quartz, and fused silica glass coexist in specimens of sheared 
porous, Goconino sandstone at Meteor Grater. Goesite is concentrated on 
quartz grain boundaries and in fractures in quartz grains. If the composi- 
tion of the glass (lechateherite) could be accurately determined, one 
could estimate the approximate minimum temperature necessary to sinter 
the rock. The pressure parameter, unfortunately, cannot be estimated from 
the equilibrium curves because if, as suggested by petrographic relations. 



166 W. T. PECORA 

the transformation of quartz to coesite preceded the wholesale sintering 
phenomenon, we are dealing with a shear phenomenon of very short 
duration followed by a peak in the thermal reaction. The very high 
pressures required to transform quartz to coesite (above 20 Icilobars) 
could thus have been induced at grain to grain contacts through the 
general action of a shearing force induced by severe shock. 

The abundance of coesite-bearing sandstone fragments within the crater 
and as fall-out debris in the vicinity around the crater nevertheless points 
up the mechanics of transformation of quartz to coesite as a force of first 
order magnitude. If this force was imparted by the passage of a shock wave 
generated by the impact of a meteorite, the mineral transformation must 
still have been made in the matter of a fraction of a second. 

Shoemaker (6) in his structural analysis of Meteor Crater made a special 
study of the "inverse stratigraphy" and fall-out debris in and around the 
crater. From his observations he is convinced that the significant features 
are best explained by shock wave phenomena. Dietz (7) reached a similar 
conclusion to explain shatter cone structures produced in meteorite im- 
pact craters. Solution of the mystery of coesite formation by shock involves 
reaction rate, duration and peak of the shock wave, secondary wave effects, 
temperature gradient, and brecciation phenomena. And faith! Intuitive 
reasoning will eventually find the key to the mystery that has stimulated 
timely research by Shoemaker, Chao, Dietz, and many others. 

ACCOUNT OF DISCOVERY 

About three years ago, E. M. Shoemaker of the U. S. Geological Survey 
remapped Meteor Crater in connection with his study of cryptovolcanic 
structures and nuclear explosion craters on behalf of the Atomic Energy 
Commission. During this study the Barringer Crater Company, owner of 
Meteor Crater, permitted his access into the pit, and acquisition of speci- 
men material — a rare privilege. The similarity between nuclear explosion 
craters and Meteor Crater led Shoemaker to the development of his 
philosophy of impact mechanics that was presented as part of his doctor's 
dissertation at Princeton University. 

In May I960 one of Shoemaker's specimens was sent to the Geological 
Survey Laboratory in Washington, D. C, where E. C. T. Chao made a 
detailed petrographic examination. He noted that in thin section the 
quartz grains were strongly sheared and imbedded in a fine-grained matrix 
which George Merrill in 1907 reported to be opal or some sort of silica 
glass. But Chao observed that the index of refraction of the matrix ma- 
terial was higher than that of the fragmented quartz. Suspecting either a 
most unusual glass or a mixture of very fine grained minerals, he made a 
standard X-ray powder pattern film of the matrix material. Quartz lines 
were readily recognized on the film and the additional lines were identified 



COESITE CRATERS AND SPACE GEOLOGY 167 



.^-^> 




Fig, 1. Air photo of Me- 
teor Crater (courtesy John 
S. Shelton, Claremont, Cal- 
ifornia). 



Fig. 2. View of the lunar 
crater Copernicus taken 
from the 100-in. telescope 
at Mount Wilson Observ- 
atory (courtesy Mount 
Wilson Observatory). Co- 
pernicus is 56 miles in 
diameter, about 11,000 ft. 
deep and with a rim about 
3,300 ft. high. 





4- 




I INCH 



Fig. 3. Experimental cra- 
ter formed by hypervelocity 
impact. Work undertaken 
under arrangement be- 
tween the U.S. Geological 
Survey and the Ames Re- 
search Center of NASA. 



168 W. T. PECORA 

as coesite— an exciting discovery. Ed Chao then proceeded to obtain 
the optical constants of the mineral as further evidence. 

To verify the identification and to cover all loopholes, Survey colleagues 
Brian Skinner, Joseph Fahey, and Harry Bastron then came through with 
able assistance. Skinner fortunately had, for comparison, a powder pattern 
he made some time ago of synthesized coesite obtained from F. R. Boyd. 
Brian immediately made and analyzed some diffractometer runs on the 
quartz-coesite mixture and, later, on the purified coesite, thus corroborating 
Chao's original identification. Fahey employed his talents in chemically 
treating samples with HF to obtain a purified concentrate of coesite, 
which Bastron then analyzed spectrochemically to report 99+ percent 
Si and less than one percent other cations. While these surgeons were 
slicing away, the news was telephoned to Shoemaker at Menlo Park, where 
he and Beth Madsen then identified coesite by X-ray study in other speci- 
mens of sheared Coconino sandstone collected from different parts of 
the crater. The telephone was used frequently by Chao and Skinner in 
discussing the discovery with Joe Boyd who, with Gordon MacDonald, 
later examined the sample material and the raw laboratory data during 
a visit to the Survey. 

As well as any other example, this account illustrates the great im- 
portance of the X-ray and telephone to modern mineralogists. Like 
Merrill's original specimens in the U. S. National Museum, coesite-bearing 
Coconino sandstone from Meteor Crater probably can now be unearthed 
in many museum collections throughout the world. 

It is a legend among the Hopi Indians that one of their gods descended 
from space in fiery grandeur to rest beneath the ground at the site of 
Meteor Crater. Rock flour (finely powdered white silica) was gathered 
by them at the crater and used in their ceremonies. Thus the Indians, 
long before geologists, were the first collectors of coesite. 



DIAMONDS AT METEOR CRATER 

A. E. Foote in 1891 reported the occurrence of fine-grained diamonds 
in iron meteorites found in this region. Lipschutz and Anders (8), Univer- 
sity of Chicago, as a result of their study of diamond-bearing meteorites, 
supported H. H. Nininger's contention that the diamonds near Canyon 
Diablo were formed in the meteorites by a shock wave upon impact 
with the Earth rather than in a parent lunar or planetary body that later 
disintegrated. They cite metallographic evidence to show that these iron 
meteorites were reheated to about 950° C. for 1 to 5 seconds after they had 
attained their present, small size. 

Sintered coesite-bearing rock at Meteor Crater would indicate a tem- 
perature maximum probably in excess of 1,000° C. with a pressure mini- 



COESITE CRATERS AND SPACE GEOLOGY 169 

mum probably in excess of 20 kilobars. The conditions attending coesite 
formation support the hypothesis of Nininger and Lipschutz and Anders. 

IMPACT METAMORPHISM 

The formation of coesite and diamond through meteoritic impact leads 
us into the broader concept of "impact metamorphism." Perhaps dense 
phases of other minerals will also come to light in a restudy of this and 
other impact craters. It seems a sure bet that coesite will be identified at 
other crater localities in quartzose rocks. The question that confronts us is 
whether or not coesite will prove to be an "index mineral" only of impact 
craters because of the pressure-temperature requirements. One might ex- 
pect yet to find coesite in fault zones or other deformed rocks that suffered 
repeated shearing. This is the task of the field geologist, certainly, but 
not without the mineralogist as his hand maiden. 

LUNAR CRATERS 

The investigations by Shoemaker and by Dietz have rekindled interest 
in the belief that some prominent lunar craters may well be craters in- 
duced by falls of meteorites many times larger than that which occurred 
at Meteor Crater, Ariz. The photogeologic map of Copernicus (displayed 
at Copenhagen) by Shoemaker and Hackman illustrates features now 
recognized in impact craters. Experiments on hypervelocity cratering by 
scientists of the Ames Research Center of the National Aeronautics and 



Fig. 4. Naturally fused 
sample of Coconino 
sandstone from Meteor 
Crater, Ariz., contain- 
ing large amounts of 
silica glass (lechatelie- 
rite). 



wm 


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■ 


■ 


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^^^^^H 


■ 


Bl 


^^H^^^^^^l 


" '1 


^^^^^1 


^ 


^^ 


^^^^^^^^H 


:" 1 


^^^^^^^^^v 






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170 W. T. PECORA 

Space Agency in collaboration with Survey scientists have been started 
in an attempt to explore this phenomenon and to provide necessary data 
for more precise formulation of mechanics of cratering by impact. If some 
lunar craters are indeed impact craters, and are formed in rocks containing 
free silica, then coesite and other dense mineral phases may well have 
formed on the moon. 

Craters on the earth formed by meteorites hitting this planet at high 
velocity involve a geologic process heretofore little known. In addition to 
the land form produced we have much to explore in the areas of rock 
deformation and impact metamorphism. Many circular depressions of 
uncertain or of stated volcanic origin on earth will yet prove to be impact 
craters. It is encouraging and reassuring to state that space geology, there- 
fore, really begins at home on earth, 

RIESKESSEL CALDERA 

During revision of this manuscript for GeoTimes Ed Chao received in 
the mail specimens of "suevite" (pumaceous tuff) and "shocked" granite 
astutely collected by Gene Shoemaker near the rim of the Rieskessel 
caldera in Bavaria, Germany— a depression many times larger than Meteor 
Crater. Coesite was identified in the specimen by Chao. The geologic 
features of the Rieskessel caldera, whose floor measures 13 by 15 miles 
across, are summarized by Williams (9), Bucher (10), and Dietz {11). If 
coesite indeed proves to be an index mineral specific to impact craters, 
rather than of shock-shear origin of whatever energy source, the startling 
coesite discovery at Rieskessel has great significance. Rubble on the floor of 
the Ries Plain is overlain by lake sediments of late Miocene age. This, then, 
the second of the "coesite craters" known, would be the first recorded in 
the pre-Pleistocene record. Perhaps the Shoemaker-Chao combination will 
reveal other coesite craters. We hope that geologists the world over are 
also stimulated to look for coesite and thus increase our knowledge of im- 
pact craters on planet Earth. 

REFERENCES 

1. Chao, E. C. T., Shoemaker, E. M., and Madsen, B. M., 1960. The Erst natural 
occurrence of coesite from Meteor Crater, Ariz.: Science, v. 132. 

2. MacDonald, G. J. F., 1956, Quartz-coesite stability relations at high temperatures 
and pressures: Am. Jour. Sci., v. 254, p. 713-721. 

3. Dachille, Frank, and Roy, Rustum, 1959, High-pressure region of the silica isotypes: 
Zeitschrift Krist., v. Ill, p. 451-461. 

4. Boyd, F. R. and England, J. L., 1960, The quartz-coesite transition: Jour. Geo- 
physical Research 65, no. 2, p. 749-756. 

5. De Carli, Paul S., and Jamieson, John C, 1959, Formation of an amorphous form 
of quartz under shock conditions: Jour. Chem. Physics, v. 31, no. 6, p. 1675-1676. 

6. Shoemaker, E. M., 1959, Impact mechanics at Meteor Crater, Ariz.: U.S. Geol. 
Survey open file report. 



THE STRATIGRAPHIC PANORAMA 171 

7. Dietz, Robert S., 1960, Meteorite impact suggested by shatter cones in rock: 
Science, V. 131, p. 1781-1784. 

8. Lipschutz, M. E., and Anders, Edward, 1960, The record in the meteorites W. 
Origin of diamonds in iron meteorites: The Enrico Fermi Institute for Nuclear 
Studies. The Univ. of Chicago. EFlNS-60-32. 

9. Williams, H., 1941, Calderas and their origin: Univ. of Calif. Pub., v. 25, p. 239- 
346. 

10. Bucher, W. H., 1933, Volcanic explosions and overthrusts: Amer. Geophys. Union 
Trans., 14th Ann. Mtg., p. 238-242. 

11. Dietz, Robert S., 1959, Shatter cones in cryptoexplosion structures: Jour. Geol., 
V. 67, p. 496-505. 



The Stratigraphic Panorama* 

• HOLLIS D. HEDBERG 



Abstract: Stratigraphy means literally the descriptive science of strata. It deals 
with the composition, form, arrangment, distribution, succession, and classifica- 
tion of rock strata and it also involves the interpretation of these features in 
terms of mode of origin, environment, age, history, relation to organic evolu- 
tion, and relation to other geologic concepts. Stratigraphy concerns itself with 
the complete picture of the rocks of the earth's crust as strata of various kinds 
and the significance of these strata in the earth's geological development. 

There are many branches of stratigraphy, depending on the particular features 
of rock strata under consideration. One of the most important is chrono- 
stratigraphy which deals with the age determination and age classification of 
strata. Its basic purpose is to interpret the history of the earth through the 
chronologic sequence of its rock strata. 

The principal means used to work out chronostratigraphy are ( 1 ) the physi- 
cal interrelations of strata, (2) the relation of strata to sequence of organic 
evolution, and (3) radioactivity age determinations. Valuable supplementary 
evidence of age or chronostratigraphic position can be supplied by other fea- 
tures of rock strata and other geologic phenomena such as lithology, mineralogy, 
ore deposits, chemical composition, paleomagnetism, paleoclimatology, changes 
in sea level, orogeny, igneous activity, and unconformities. However, few of 
these can be proved to have had effects which were distinctly recognizable, 
identical in character, and synchronous over the whole world. Coordinated 
utilization of all possible lines of relative and absolute age determination and 
time correlation offers the best promise for continued progress in chrono- 
stratigraphy. 

• From Geological Society of America Bulletin, Vol. 72 (April, 1961), pp. 499-518. 

* An Inquiry into the Bases for Age Determination and Age Classification of the 
Earth's Rock Strata— Address as Retiring President of The Geological Society of 
America. 



172 HOLLIS D. HEDBERG 

Joined to the problem of the dating of strata and the estabhshment of their 
sequence with respect to earth history is the task of chronostratigraphic classi- 
Ecation. The record of 4 thousand milhon years, written in milhons of cubic 
miles of strata, is too vast to be comprehended as a whole and it is necessary to 
break it down into smaller more practicable units. The only adequate reference 
standards for the scope of these chronostratigraphic units are specifically desig- 
nated intervals of rock strata — stratotypes. 

The fundamental unit of world-wide chronostratigraphic classification is the 
system. The systems, largely established in Western Europe during the first half 
of the last century, were originally thought to constitute "natural" units with 
respect to earth history. In view of the local and rather haphazard manner in 
which most of them originated and the primitive state of world geological 
knowledge at that time, it is difficult now to see them as "natural" divisions of 
world-wide extent. Nevertheless the belief is still supported by many, including 
the USSR Stratigraphic Commission, that the systems are marked off by a 
concurrence of major events in geologic history and major changes in the course 
of organic evolution. They assume that all lines of stratigraphic evidence con- 
verge to form "natural" divisions of strata with respect to time and that hence 
separate kinds of stratigraphic classification — lithostratigraphic, biostratigraphic, 
etc. — are unnecessary. 

On the other hand, investigations by other competent workers of the evi- 
dence for world-wide "natural breaks" in either the diastrophic record or the 
record of organic evolution have resulted in strong judgments to the contrary. 
The conclusion seems reasonable, regardless of what may be proved eventually, 
that it has not yet been demonstrated that world-wide "natural breaks" in the 
general character and continuity of strata exist at the scale of the presently 
accepted geologic systems nor that the evidence at the boundaries of the present 
systems is such as to allow them to be considered as the "natural" world-wide 
division points of the chronostratigraphic scale. Rather, the evidence suggests 
that our geologic systems are only arbitrary chronostratigraphic units in a con- 
tinuum characterized by intricately overlapping and not necessarily coincident 
changes in the many and various properties and attributes of rock strata and 
that their principal significance is that of standard units of chronostratigraphic 
reference, independent of other kinds of stratigraphic classification. Regardless 
of opinion on the relation of these units to events of earth history, the critically 
important point is that the systems and their major subdivisions should be tied 
down by international agreement to specifically designated and delimited 
sequences of rock strata — stratotypes — so as to provide a uniform basis of 
definition for everyone. 



THE STRATIGRAPHIC PANORAMA 173 



CONTENTS 

Introduction 173 Igneous activity 187 

General scope of stratigraphy 174 Unconformities 187 

Chronostratigraphy 175 Chronostratigrapliic classification ... 188 

Principal bases for determination of Our present chronostratigrapliic 

age or chronostratigraphic posi- units 190 

tion 176 Concept of world-wide "natural 

Physical interrelation of strata. ... 177 breaks" 191 

Relations of strata to sequence of Evidence in diastrophic record for 

organic evolution 178 world-wide "natural breaks" . . . 192 

Radioactivity dating of strata 180 Evidence in record of organic evo- 

Other indicators of age or chrono- lution for world-wide "natural 

stratigraphic position 182 breaks" 193 

Lithology 182 Summation of evidence regarding 

Mineralogy and ore deposits 184 world-wide "natural breaks" ... 195 

Chemical composition 185 Views of USSR Stratigraphic Com- 

Paleomagnetism 185 mission on chronostratigraphic 

Paleoclimatology 185 classification 195 

Changes in sea level 186 Summary and conclusions 197 

Orogeny 186 References cited 199 



INTRODUCTION 

The other day I was flying across the United States. When we left New 
York the sky was completely overcast, and looking down through the 
plane window I could see nothing but clouds. But later on, as we pro- 
gressed westward, there appeared, with increasing frequency, little breaks 
or rifts in the clouds, and I amused myself by trying to see what there 
might be in these occasional glimpses of landscape that could tell me 
where I was geographically in this journey across the continent. What was 
there in the topography, the drainage pattern, the vegetation, the culture, 
of each of these individual views that might help me identify my position 
in the over-all panorama of my trip? And was there anything in the simi- 
larities or differences in the succession of landscapes that might allow me 
to group the scenery into natural provinces— to classify it geographically? 

As I flew over this vast and varied country and with more or less success 
identified our geographic location from these occasional vistas of the earth 
below, it occurred me to wonder with what success could one determine 
stratigraphic position in a journey through geologic time, viewing only in 
isolated occurrences the sequence of rocks making up the earth's crust. 
Would I be able to identify Silurian and Devonian, for instance, as nat- 
ural units in this stratigraphic panorama, or would they be as artificial and 
diflBcult to distinguish as Indiana from Illinois or as the Kansas-Colorado 
line? Well, it is something on the order of this stratigraphic game that I 
propose to explore with you tonight— to explore, and to examine with you 
some of the implications of our results on stratigraphic philosophy. 



174 HOLLIS D. HEDBERG 

GENERAL SCOPE OF STRATIGRAPHY 

First of all, what is stratigraphy? Literally (from stratum and graphia), 
the word can be said to mean "the descriptive science of strata," and I 
see no need to depart from this simple definition inherent in the world it- 
self. Stratigraphy, therefore, as applied to geology, deals with all rock 
sti'ata and all aspects of rocks as strata; and a geological stratum may 
be defined simply as a layer of rock, unified by possessing certain chai:- 
acters or attributes distinguishing it from adjacent layers. The separation 
of a stratum from adjacent strata may commonly be marked by visible 
planes of bedding or parting, but strata may also exist with less visually 
perceptible boundaries— always, however, with boundaries that represent 
horizons of change— change in lithology, in mineralogy, in paleontology, 
in chemical composition, in age, or in anything else. Stratigraphy involves 
the composition, form, arrangement, distribution, succession, and classifi- 
cation of rock strata in normal sequence. Further, it involves the inter- 
pretation of these features of rock strata in terms of origin, environment, 
age, history, relation to organic evolution, and relation to other geologic 
concepts. Moreover, since in the larger sense the whole earth's crust is 
stratified, all classes of rock— igneous and metamorphic as well as sedi- 
mentary — fall within the general scope of stratigraphy. Thus we have in 
stratigraphy a broad and magnificent field which concerns itself with 
the complete picture and understanding of the layers of the earth's crust 
in all the aspects in which they manifest themselves. 

This is indeed a broad concept of stratigraphy which I have given you, 
and it is true that it touches upon almost all other branches of geology; 
but the point to remember is that stratigraphy deals with rocks as strata 
and involves these other branches only as they apply to rock strata and only 
to the extent to which they apply to rock strata. (This, for example, is 
the difference between lithology and lithostratigraphy, between paleontol- 
ogy and biostratigraphy). 

I am well aware that there are many who would confine stratigraphy to 
the age relations of strata, and some who would even go further and would 
confine stratigraphy to the age relations of strata as worked out by fossils. 
Now I would be among the first to grant that the determination of the 
age relations of strata is one of the most important objectives of stratig- 
raphy—but my point is that it is not the only objective. There are other 
important and coordinate fields of stratigraphy also. I would be among 
the first to grant that fossils constitute one of the most useful means of 
working out the age relations of strata— but again not the only means. 
Much has been learned about the age relations of Precambrian and other 
relatively barren strata without any help from fossils. 

The most pressing objective in the work of many stratigraphers may be 



THE STRATIGRAPHIC PANORAMA 175 

not the determination of the age of strata— the assigning of them to the 
Eocene, Ohgocene, or Miocene— but the determination of the hthologic 
characteristics of these strata, the dehneation and classification of them 
as three-dimensional lithologically unified bodies— lithostratigraphic units 
—regardless of their geologic age. Those of you in the petroleum industry 
will know how vitally important is such work, and, incidentally, just be- 
cause this branch of stratigraphy— lithostratigraphy— happens to have a 
rather direct commercial application is certainly no reason for considering 
it outside the pale of stratigraphy, or for considering it an ignoble objec- 
tive, or a sort of preliminary exercise, a protostratigraphy, or only a means 
to the end of true stratigraphy, as some seem inclined to do. (It is just 
possible that commercial utihty and geological science are not mutually 
exclusive!) 

The picture of the earth's crust stratified according to variations in 
lithology is as much true stratigraphy as the picture of the earth's crust 
stratified according to geologic age. Both are valuable concepts in them- 
selves and both are essential parts in our understanding of earth history. 
Likewise, the picture of the stratigraphic distribution of fossils in the 
earth's crust is valuable not only for the aid it gives in determining the 
age of strata, but also as an indicator of changes in life environment or 
paleontology. The classification of the earth's strata with respect to mode 
of origin is as much stratigraphy as the classification of the earth's strata 
with respect to time of origin. There are as many phases of stratigraphy as 
there are ways in which strata can be classified. Any attempt to restrict the 
term to less than the broad basic meaning implied by its etymological roots 
not only is confusing but, in addition to serving no conceivably useful 
purpose, actually has a harmful and cramping eflFect on geological think- 
ing. 

CHRONOSTRATIGRAPHY 

So much then for the general scope of stratigraphy. That branch of 
stratigraphy that has to do with the age and age relations of strata may be 
called chronostratigraphy, and it is this with which we are primarily con- 
cerned here. While I have emphasized that chronostratigraphy is not the 
whole of stratigraphy and that there are many other branches, each with 
its own particular objectives, still almost all the criteria on which these 
other stratigraphic fields are based have also some bearing on the determi- 
nation of position with respect to geologic time and thus also play a role 
in chronostratigraphy. 

If we could look over the stratigraphic panorama— the whole picture 
of the rock strata of the earth's crust—, what would be the evidence we 
could find to help us to determine chronostratigraphy, to help us fix the 
age relations of strata relative to each other and relative to the course of 



176 HOLLIS D. HEDBERG 

earth history? And what might be the evidence we could find in the se- 
quence of rock strata, once properly worked out, for subdivision of this 
history into chapters or units with respect to geologic time— for chrono- 
stratigraphic classification? 

Looking at it another way, if we had been viewing from an earth satel- 
lite the development of the rock strata of the earth's crust from earliest 
geologic time to the present, what features would we have seen impressed 
in these rock strata that might now afford a clue to recognition of their 
proper sequence in time of origin, especially if, as is usually the case, we 
could now see only isolated bits and fragments of the total picture at any 
one place. Just as the only occasional rifts in the clouds in our transconti- 
nental air journey let us see only bits and fragments of the total geographic 
panorama? And what basis might these imprints of earth history in the 
rock record give for the recognition of different ages or periods in this 
history? 

This is the stratigraphic problem before us, and remember that, above 
all, it is truly stratigraphic. We as stratigraphers are interested in the age of 
strata, not so much the age of the rock matter itself which, you must re- 
member, is largely as old as the earth itself (with the exception of a 
probably quite minor amount of cosmic matter that has accumulated 
since the original formation of the earth). New strata have been con- 
tinually added to the earth's crust ever since the dawn of geologic history — 
added and destroyed, added and destroyed. And these strata are new, as 
strata, even though they were formed merely by the reworking and re- 
arranging of old material that was already there — breaking up old rock 
and depositing it to form new, melting old rock and cooling and crystal- 
lizing it to form new, metamorphosing rocks in place to make new rock 
strata out of old. It is strata and their character in which we as stratig- 
raphers are principally interested, not so much the constituent rock mat- 
ter itself. And, when we speak of geologic age, we are talking not of the 
age of the rock matter but the age of strata— the age of certain layers of 
the earth's crust. 

PRINCIPAL BASES FOR DETERMINATION OF AGE OR 
CHRONOSTRATIGRAPHIC POSITION 

Let us first look at the means we have for determining the age of the 
earth's strata, either the relative age of strata with respect to each other 
or their absolute age expressed in millions of years, or both. Three princi- 
pal means stand out: 

(1) Determination of relative age by the physical interrelations of 
strata 

(2) Determination of relative age by relation to the sequence of or- 
ganic evolution 



THE STRATIGRAPHIC PANORAMA 177 
(3) Determination of absolute (and relative) age by radioactive 
methods. 

Physical interrelation of strata 

Probably no criterion of relative age is more simple and more positive 
than that afforded by the superposition of strata, although there is often 
a tendency to forget, or even scorn, its role in the course of attention given 
to more complex methods. Relative stratigraphic position is an obvious 
key to relative age. In any normal sequence of sedimentary rocks, each 
succeeding bed upwards is younger than that on which it rests. This is 
the fundamental and classic concept of relative age determination, and 
all other methods have been founded on its vahdity as a starting point. 

In any local exposure of undisturbed strata relative chronostratigraphic 
position is thus usually readily apparent. However, as we all know, diffi- 
culties arise when strata are highly disturbed— overturned or overthrust— 
when a younger igneous rock is implaced by intrusion within a sequence 
of older strata; when a relatively mobile older rock, like salt or gypsum, 
has been injected into or has flowed out over younger strata; when lateral 
changes in facies or thickness occur, and when unconformities are present. 
Even under these conditions, however, careful studies of field relations and 
contacts may reveal relative age. 

Perhaps the greatest impediment to assignment of strata to their cor- 
rect relative chronostratigraphic position by means of the simple law of 
superimposition is lack of continuity in exposure. It is then that the supple- 
mentary tool of stratigraphic correlation enters — when the separation of 
exposures is such as to prevent actual tracing of beds. Stratigraphic corre- 
lation is the determination, through similarities in character, of mutual 
correspondence in stratigraphic position between beds at two or more 
separated points. Correlation may be based on correspondence in lithology, 
in fossil content, in electric-log character, in geologic age, or in any other 
property of a stratum. Such correlation may or may not be an exact time 
correlation of the strata involved but is always a useful aid to relative age 
determination. 

The development of the art of correlation has been one of the great 
contributions of the petroleum industry to stratigraphic knowledge. Micro- 
paleontology, heavy minerals, electric logs, gamma-ray logs, and many other 
specialized techniques have been utilized very successfully. Lowman 
(1949, p. 1964-1967) explained very well how this simple approach to 
determining relative stratigraphic sequence has been used throughout the 
broad Gulf of Mexico coastal region by building outward from well section 
to well section a network of purely empirical stratal correlations regard- 
less of chronologic or facies implications. This is the approach that has 
been and is being used in numerous other areas all over the world both 
by means of well sections and by means of outcrop sections. As a result of 



178 HOLLIS D. HEDBERG 

some 2 million holes drilled for oil during the last century, plus progress 
in techniques of correlation, it is now possible with assurance to determine 
the relative stratigraphic position of strata over many vast basin areas of 
the world without regard to their relation to any standard geologic time 
scale. As drilling is pushed more extensively into offshore areas and more 
complete sections are found, correlation ties over still more extensive 
regions will become possible. It is not at all inconceivable that even inter- 
continental sequences eventually might be tied together by this means 
alone. 

Noteworthy also in the determination of relative chronostratigraphic 
position through observed relations of strata is the contribution of aerial 
photography to the tracing of beds and whole stratigraphic sequences 
from one area to another. No one who has seen the broad sweep of con- 
tinuous bands of strata shown by air photos along certain uplifted moun- 
tain fronts can doubt for a moment the tremendous aid that these have 
given to the direct lateral extension of stratigraphic sequence— an aid lack- 
ing to our early stratigraphers. 

Finally, special mention should be made of the contribution of geo- 
physics in the tracing of strata not only through electric-log correlation of 
well sections, but also through the interpretation of seismic-reflection and 
refraction data over areas where neither outcrops nor wells are present and 
to depths beyond the reach of the drill. 

All in all, the tools for the working out of stratigraphic sequence by the 
direct tracing or simple correlation of strata are far, far more powerful 
than they were in the days of the early stratigraphers, and it might be 
quite impressive if we could see now how far these direct methods alone 
would have carried us toward the establishment of world-wide chrono- 
stratigraphic order. 

Relations of strata to sequence of organic evolution 

That our present-day knowledge of the sequence of strata in the earth's 
crust is in major part due to the evidence supplied by fossils is a truism. 
Merely in their role as distinctive rock constituents, fossils have furnished 
one of the best and most widely used means of tracing beds and correlating 
them. However, going far beyond this relatively simple but highly useful 
contribution to stratigraphy, fossils have furnished, through their record 
of the evolution of life on this planet, an amazingly effective key to the 
relative positioning of strata in widely separated regions and from conti- 
nent to continent. 

In the first place, fossils have led to the separation of the strata of the 
earth's crust into two great divisions— a younger Phanerozoic division in 
which fossil evidence is abundant and the fossils include relatively highly 
organized life forms, and an older Cryptozoic (Precambrian) division in 
which fossil evidence is scanty or lacking and most of the record found to 



THE STRATIGRAPHIC PANORAMA 179 

date is of relatively simple primitive life forms. Moreover, within the 
Phanerozoic rocks, where the record is good and the forms complex, the 
progress of organic evolution has been traced in great detail along a multi- 
tude of hues and through innumerable varied and constantly changing 
forms, thus making possible an ever-increasingly detailed relative age posi- 
tioning of strata throughout the world by their fossil content, once the 
general sequence was established. 

Stubblefield (1954) has stressed the relationship of paleontology to 
stratigraphy and the historically close interdependence of the two. Only as 
the stratigraphic succession could be known through direct observation of 
the superposition of beds could the evolutionary sequence of fossil forms 
be established, and only as this sequence was established could the local 
rock sections of widely separated regions be tied together in proper order 
of age. 

The outstanding success with which paleontology has been applied to 
the relative dating of Phanerozoic strata during the last 150 years is such 
that to the outside observer it might seem that no obstacles remain to the 
most detailed world-wide determination of the chronostratigraphic ar- 
rangement of these rocks. That such is not the case is of, course, evident 
to most biostratigraphers. Only an infinitesimal fraction of the life of the 
past has become available to us for study. A large part of the strata even 
of Phanerozoic age are nonfossiliferous or only very sparingly fossiliferous; 
on the other hand, the biostratigrapher is sometimes locally embarrassed 
by a great wealth of fossils of different types which have evolved at quite 
different rates. Then, again, the fossil remains found in swamp deposits are 
not those of river deposits or marine shore lines or deserts or deep-sea 
oozes, and the extension of time surfaces between these different environ- 
ments on the basis of evolutionary sequence of fossils is .difficult. Condi- 
tions for the preservation of fossils have been highly variable, leaving 
barren gaps on the one hand and on the other allowing reworking and 
redeposition of forms. Finally, unconformities, hiatuses, and structural 
complications have frequently confused interpretation of the fossil record. 

The Crvptozoic or Precambrian deserves a special word from the bio- 
stratigraphic viewpoint. For a long time this seemed like a paleontological 
no-man's land, but during the last decades important progress has been 
made toward penetrating the secrets of life retained in the scanty record 
of these strata. It now appears that there is reasonable evidence that life 
was already existent at the time of the oldest known rocks on earth — 
some 3 to 4 billion years ago (Ahrens, 1955). The traces in these earliest 
rocks are indeed scanty— finely disseminated carbon or graphite, and some 
limestones—, but in Precambrian rocks of somewhat lesser age stromatolites 
and other algal remains have been found all over the world, limestones 
are not uncommon, and even coal and traces of petroleum have been 
noted. What could be more challenging for biostratigraphers than the 



180 HOLLIS D. HEDBERG 

eflPort to build up our knowledge of life during these long dark ages, con- 
stituting nine-tenths of the earth's span of existence, before, so to speak, 
the fossil record burst full blown upon the scene. The recent discovery 
at Ediacara in South Australia, discussed by Glaessner (1960), of an 
abundant soft-bodied fossil fauna of coelenterates, annelids, and other 
forms (including some known only from the Precambrian of South Af- 
rica and England) in a Precambrian sequence that grades upward without 
a break into Lower Cambrian strata with hard-shelled fossils is an indi- 
cation that we may yet learn much more about Precambrian life. 

Radioactivity dating of strata 

The discovery early in this century that certain elements contained in 
the rocks of the earth's crust are in continuous radioactive disintegration 
to form other elements or isotopes at a rate that is not only constant but 
also rapid enough to be measurable opened up to geologists a vista for 
dating rocks which still seems almost too fortunate to be true. Stratig- 
raphy owes a tremendous debt to those geoscientists and institutions all 
over the world who, in spite of the laborious, intricate, and painstaking 
techniques necessarily involved, and in spite of repeated discouragements, 
have persisted in efforts to develop, refine, and apply methods of radio- 
active dating. Although work in this field is most certainly still in its 
earliest stages and still complicated by many uncertainties, the results to 
date already appear to have contributed a reasonably reliable concept of 
the general magnitude of geologic time and to hold forth assured promise 
of much more accurate measurements and detailed dating in the future. 

Up to now the most useful stratigraphic results appear to have come 
from the uranium-lead (also thorium-lead and lead-lead), rubidium- 
strontium, potassium-argon, and carbon- 14 methods— each particularly 
useful under appropriate circumstances. Problems of evaluating factors 
both of inheritance and of leakage of products are still troublesome, but 
concordant results obtained by different methods on the same rock or by 
the same method on different minerals in the same rock particularly 
inspire confidence. 

The Holmes radioactivity time scale of 1947 recently has been revised 
in the light of new data, by Arthur Holmes (1960) and by J. L. Kulp 
(1960), with gratifying accordance in conclusions. However, as Holmes 
says (p. 203), 

"To meet requirements of a reasonably accurate time-scale, measured ages need 
to be far more reliable, closely correlated, and evenly distributed throughout 
the periods than has yet proved to be practicable." 

Paul (1960) has recently supphed a critical evaluation of the current 
status of radioactive dating. 



THE STRATIGRAPHIC PANORAMA 181 

It is important always to remember that what radioactive determina- 
tions give us is not a simple direct reading of the age of a rock. Instead 
they give certain physical data on the isotope composition of certain min- 
eral crystals within a rock which, only after certain assumptions and cer- 
tain allowances have been made, may permit an interpretation of the 
number of years that have elapsed since the birth of the crystals. This in 
turn may further allow an interpretation of the dating of the process re- 
sponsible for the generation of the crystals, which, depending on circum- 
stances, may be more or less indicative of the age of the inclosing rock 
specimen, and which finally may from its field relations allow an age 
interpretation of the strata with which the rock specimen was associated. 
Thus the dating of minerals in a granite gives the date of its crystalliza- 
tion, which may not always be the date of intrusion; the dating of biotite 
in a schist may give the age of the metamorphism responsible for con- 
verting a certain stratum to a crystalline schist, but not the age of the 
original stratum itself; the dating of authigenic minerals in a sedimentary 
rock dates the diagenetic process that produced these minerals, but does 
not necessarily give the date of deposition of the now-inclosing sediment; 
and the dating of detrital minerals in a sediment obviously is the dating of 
rock material older than the sedimentary stratum itself. All these datings 
are, of course, extremely valuable in themselves as indicators of the timing 
of certain geological events, but their limitations as direct indicators of 
age of strata should be recognized. 

(Incidentally, what is the age of stratum of mica schist formed by the 
metamorphism in Silurian time of a Cambrian shale derived from the 
weathering products of a Precambrian granite? Is it the age of the shale — 
Cambrian—, the age of the granite products— Precambrian— , or the age 
of the metamorphic process— Silurian?) 

While radioactive methods are almost unique in their potential ability 
to contribute absolute age values expressed in years or millions of years, 
still it is probably their contribution of evidence for relative ages, supple- 
menting evidence supplied by other means and regardless of absolute age 
values, that is most important to us. Radioactive dating offers a possible 
check on the many uncertainties remaining in the relative age assign- 
ments of Phanerozoic rocks and offers the major hope for working out on 
a world-wide basis the relative age relationships of the great mass of 
Precambrian rocks, representing 90 per cent of geologic time, where fossils 
are scanty or lacking and where structural complications and metamor- 
phism allow the direct observation of original sequence in only local inter- 
vals. Inexact as the radioactive methods may be at the present time, there 
is tremendous comfort in the thought that almost all rocks are endowed 
with little radioactive clocks of some sort, ticking away and storing up 
time records which we shall probably one day be able to interpret much 
more exactly than now. 



182 HOLLIS D. HEDBERG 



OTHER INDICATORS OF AGE OR 
CHRONOSTRATIGRAPHIC POSITION 

So much for the principal bases of age or relative age determination- 
superposition of strata, organic evolution, and radioactivity. Of these only 
superposition is independent. Organic evolution and radioactivity, while 
independent now, were established originally only with the help of some 
preknowledge of sequence. It may therefore be worthwhile to consider if 
there are other lines of evidence which, now that a general sequence has 
been worked out, may aid in recognizing chronostratigraphic position. 
Even though they may not reflect irreversible changes to the extent that 
changes through organic evolution are considered irreversible, still other 
features may be helpful under certain circumstances in identifying posi- 
tion in the chronostratigraphic scale. 

Lithology 

First, let's look at simple lithology. There are probably few of us who 
have not, at some time, looked at some new section of rocks and remarked 
"Looks like typical Triassic" or "I don't know why, but this just looks like 
Cambrian." Well, how much really is there in the lithologic character of 
strata which is significant of their age? We have come a long way from 
Wernerism but is there, perhaps, still some measure of truth in the 
thought that rock types vary with geologic age? See discussions by Rubey 
(1951, p. 1114). Fairbridge (1954), and Pettijohn (1957, p. 682-690). 

I suppose such an investigation might well start with a look at Pre- 
cambrian lithologies as compared with those of Phanerozoic strata. While 
the principle of uniformitarianism has been accepted by most as extending 
back through the Precambrian in at least a general way, still a number of 
observers have from time to time pointed out supposedly distinctive fea- 
tures of Precambrian rocks. In general, however, the results of increased 
information on the Precambrian all over the world have tended to negate 
rather than to support generalizations on distinctive lithologies. In a re- 
cent address, Hawkes (1958, p. 315) has concluded that in general the 
rocks formed in Precambrian time were similar to those being formed to- 
day, but that the proportion of undifferentiated sediments, graywackes 
and arkoses, was higher, in keeping with the thought that with advancing 
geological time repeated cycles of weathering and deposition would tend 
to result in increased amounts of fully differentiated sediments— quartzites 
and argillaceous rocks. However, he stresses that the important fact is that 
these differentiated types do occur to some extent even in the oldest rocks. 
Thus he mentions phyllites, quartzites, and limestones in the most ancient 
rocks of South Africa, and the records from other continents now include 



THE STRATIGRAPHIC PANORAMA 183 

a number of observations of sedimentary carbonates and quartzites from 
within their older Precambrian strata {e.g., Armstrong, 1960). 

Gill (1957, p. 186-187) has suggested that quartzites and limestones 
might theoretically be expected to be lacking in the oldest rocks not only 
because weathering conditions of earliest Precambrian time may not have 
been favorable to a complete breakdown of rock materials into mineral 
fractions, but also because coarse-grained quartz-bearing rocks may have 
been lacking to supply quartz sands, and because limestones could not be 
deposited until the primitive oceans were saturated with CaCOg. At the 
same time, however, he concludes that "if there is anything truly dis- 
tinctive about Archean sediments it has still to be defined." 

Probably the most widely mentioned example of a distinctive Pre- 
cambrian rock is that of the banded siliceous iron-bearing strata recorded 
from the Lake Superior region, Labrador, Scandinavia, Russia, South Af- 
rica, Mauretania, India, Australia, Brazil, Venezuela, and elsewhere. The 
extensive development of rock of this type only in the Precambrian is 
indeed impressive but there have been lesser developments of somewhat 
similar rocks in later times, and detailed comparison of the Precambrian 
occurrences would probably show very considerable differences in rocks 
popularly considered the same. Harold James (1960, p. 107) has warned 
against thinking of these iron-bearing strata as having very specific age 
significance even if they are confined to the Precambrian, since radioactive 
dating suggests that the time interval between rocks of this type in the 
Lake Superior Precambrian may be as long or longer than all of the 
Phanerozoic. 

A greater abundance of dolomites and magnesian limestones in propor- 
tion to ordinary limestones in the early Paleozoic and late Precambrian as 
compared to later strata has been suggested by Daly (1909) and others. 
Chilhngar (1956, p. 2266) concludes that 

"there is no simple relation between the Ca/Mg ratio and the age of carbonate 
rocks. There is, however, a general decline in abundance of dolomites (or 
increase in the average Ca/Mg ratio) in going up the geologic column, with 
superimposed periodic fluctuations of calcitic and dolomitic limestones." 

A number of other rock types seem characteristic of or limited to 
certain parts of the Phanerozoic column, but are also quite clearly related 
in origin to plant or animal life and may thus perhaps be considered only 
as further instance of age dating through organic evolution. For example, 
while coaly matter is known as far back as the Precambrian, extensive 
deposits of coal would be indicative of Carboniferous or younger age. 
Likewise, while traces of petroleum seem to have originated in Precam- 
brian rocks (James, 1960, p. Ill), its indigenous occurrence in major 
quantities is limited to Phanerozoic strata. Chalk is supposed to be 
typically Cretaceous, and diatomite is known only from the Tertiary, and 



184 HOLLIS D. HEDBERG 

even the Precambrian siliceous iron ores may be related to organic life. 
In any case, extensive developments of the rock types mentioned do fur- 
nish some general suggestion of chronostratigraphic position through 
lithology alone. 

Some other rock types, while not limited in chronostratigraphic range, 
have come to be known popularly as particularly common in certain parts 
of the geologic column— Permian and Triassic red beds, Tertiary mottled 
claystones, Jurassic radiolarian cherts. Tertiary and Cretaceous lignites, 
Permian evaporites, etc. We know, however, that these are the lithologic 
products of certain environments and we know that these environments 
were not universal in any of the above-mentioned periods nor were they 
by any means confined to these periods. 

Mineralogy and ore deposits 

Certain minerals and ores appear to show some relation to geologic 
age. Thus glauconite is common throughout the Phanerozoic systems but 
has only rarely been reported below the Cambrian and then only in the 
late Precambrian. Likewise, many have noted that most of the world's 
gold comes from the older Precambrian of Canada, South Africa, Aus- 
tralia, India, and Brazil. Much of the world's sedimentary copper comes 
from the Permian and Triassic, and many of the widespread Triassic red 
beds are characterized by an unusually high copper content. Miholic 
(1947) has commented that nickel ores are predominantly in Precam- 
brian rocks, tin in the Paleozoic, lead and zinc in rocks of Late Paleozoic 
to Early Tertiary age, and mercury in the younger Tertiary. However, he 
attributes this distribution in part to the fact that differential erosion has 
exposed a larger proportion of the high-temperature (deeper) metallic 
veins in the older rocks, while low-temperature deposits (shallower) have 
been relatively better preserved in the younger rocks. He also attributes 
it in part to steeper geothermal gradients in the past. 

DeRoever (1956, p. 125) notes the absence of the mineral lawsonite in 
rocks formed by pre-Mesozoic metamorphism, and the relative scarcity of 
glaucophane and crossite in those rocks. He sees 

"not only an evolution of life during the history of the earth but also some 
change in the character of the metamorphic mineral assemblages produced 
during the main phases of regional metamorphism of the various orogenic 
epochs." 

Many sedimentary-rock petrographers have noted a general tendency 
toward simplification of heavy detrital-mineral suites with increasing age 
of the enclosing rock. {See particularly Pettijohn, 1941.) 



THE STRATIGRAPHIC PANORAMA 185 

Chemical composition 

There have been repeated inquiries into the possibility of a systematic 
change in general chemical composition of sediments with time. However, 
Rubey (1951) is impressed with the evidence for long-range constancy 
in composition both of the atmosphere and of sea water and concludes 
(p. 1111) that while 

"the composition of both sea water and the atmosphere may have varied some- 
what during the past . . . the geologic record indicates that these variations 
have probably been within relatively narrow limits." 

Nanz (1953) has compared the chemical composition of Precambrian 
shales with that of shales of younger eras and finds a progressive decrease 
in AI0O3, FeO, total iron, K2O, and carbon with decreasing age. He also 
finds a progressive increase of CaO, PoO-, CO,, and SO3, which he thinks 
may be related to the development of life. The other consistent variations 
he attributes to "progressively coarser textures in the younger samples." 
Briggs (1959) comments on a progressive decrease in ferrous /ferric ratios 
beginning about 2 billion years ago; he attributes the decrease to the 
change from a previous reducing atmosphere to a progressively more ox- 
idizing atmosphere as a result of the development of plant life. Polder- 
vaart (1955) has given an interesting discussion of the chemical evolution 
of the earth's crust. 

Paleomagnetism 

Studies of the earth's paleomagnetism have suggested a highly intriguing 
prospect for dating rock strata. If the remanent magnetism of a rock is a 
record of the position of the earth's magnetic poles at the time of forma- 
tion of the rock, and if there has been a major shifting of the poles 
throughout geologic time (as studies seem to indicate), then, utilizing 
what we already know of the age sequence of strata as an initial guide, 
we would have in paleomagnetism a widely available means of further 
extending our original age dating. Evaluation of this method will have to 
await further investigation and further checking of the validity of some 
of the assumptions made, but many papers are already appearing in which 
conclusions on dating are based on paleomagnetic evidence. {See Cox 
and Doell, 1960, for a recent review of paleomagnetism). 

Paleoclimatology 

There is plentiful evidence in the earth's strata of major climatic changes 
in various regions during the course of earth history, and there is reason 
to believe that some of these changes may have been the result of extra- 
terrestrial causes or other causes of a nature able to affect the climate of the 
earth as a whole. However, as Dorf (1960) remarks, climates are not 



186 HOLLIS D. HEDBERG 

themselves subject to fossilization, and the clues we have to ancient cli- 
mates must be derived from their imprint on the geologic record— fossil 
plants and animals, glacial deposits, evaporites, red beds, coal-swamp de- 
posits, and various other reflections of climate. Of particular importance 
to our knowledge of world-wide climatic changes is the steadily increasing 
mass of data on ocean temperatures of the past yielded by Urey's oxygen- 
isotope geological thermometer. In spite of the masking effect of normal 
latitudinal variations and other regional factors affecting climate, there 
appear to have been variations in temperature and precipitation in the 
past sufficiently general to furnish aid in the world-wide dating of strata. 

Changes in sea level 

One of the classic concepts of historical geology is that of a rhythmic 
alternation of world-wide transgressions and regressions of the sea. {See 
discussion by Dunbar and Rodgers, 1957, p. 305-306.) To the extent that 
this concept is valid it should provide an important means of relative 
positioning of strata all over the world with respect to a standard se- 
quence, once this has been estabhshed. The recognition and correlation 
of alternations of marine and nonmarine sediments, of shallow-water and 
deep-water deposits, and of transgressive and regressive facies should then 
constitute a simple key to the fitting together of local sequences into a 
chronologically ordered whole. Certainly this has already been done suc- 
cessfully over quite extensive regions. However, although sea level is es- 
sentially accordant the world over and although the addition of water 
volume locally is rapidly transmitted as a rise in sea level the world over, 
still the suspicion lurks that many of the effects in the rock record are due 
neither to changes in the total volume of sea water nor to general changes 
in the form of the ocean basins, but rather are the result of local changes 
in the relative vertical position of sea and land in specific coastal areas. 
It seems evident that local vertical movements of the solid crust both 
on the continents and in the ocean basins have been so great and so 
variable geographically in relation to time as to leave much less order in 
the world-wide rock record of marine transgressions and regressions than 
some theorists might hope to see. Moreover, there is no reason to expect 
that the sediments of one transgression will have differed distinguishably 
from those of another. Gignoux (1936, p. 494-495) has brought out in 
excellent manner the caution with which one must look at even so widely 
accepted a transgression as that of the Late Cretaceous. 

Orogeny 

Another classic concept of historical geology is that periodic world-wide 
orogenies "punctuate" the record and through their effects on sedimenta- 
tion, erosion, igneous activity, and rock deformation furnish potential 
guides to chronostratigraphic position. Here, again, this is indeed true over 



THE STRATIGRAPHIC PANORAMA 187 

broad areas, but as will be discussed later, it is doubtful that the nature 
of orogenies has been such as to have left similar and synchronous effects 
world-wide in the rock record. 

Igneous activity 

There has perhaps been some over-all decrease in igneous activity in the 
earth's crust since the beginning of the rock record, and some broad 
variations throughout geologic time, but nothing which would in itself 
be a very reliable guide to world-wide dating of strata. Similarly, orderly 
changes in composition of igneous rocks depending on time of origin have 
been noted for specific provinces, but there appears to be little basis for 
conclusions on a world-wide variation in igneous-rock composition with 
geologic age. It is of interest, however, that J. T. Wilson (1952) has re- 
marked that the oldest rocks in North America, Australia, and South 
Africa are greenstone volcanic rocks which he thinks may be contem- 
poraneous and may represent a distinct division of early Archean time. 
The relation of sediments to periods of intrusion has of course frequently 
been utilized in regional dating. Hess (1955) has demonstrated the value 
of the dating of serpentines as a clue to the dating of the birth of moun- 
tain systems. 

Unconformities 

The relation of strata to major unconformities is a widely useful key to 
approximate chronostratigraphic position and one on which age determi- 
nations are frequently based. Its utility is great in a general way in inter- 
regional studies, but it should always be borne in mind that no known 
unconformities are world-wide, and in many respects ari unconformity is 
one of the poorest time markers since by its very nature the age of a 
surface of unconformity commonly varies drastically from place to place. 

Many other lines of investigation may also furnish useful supplementary 
evidence for determining the relative age or position of rock strata. Weeks 
(1958, p. 3-5) has made an interesting list of phenomena suggesting a 
high degree of world-wide parallelism of geologic events. However, useful 
as all of these, as well as those here mentioned, may be in local or regional 
time correlations, few can be proved to have had distinctively recognizable, 
identical, and contemporaneous effects on rock strata everywhere over the 
whole world. 

Finally, we may still ask, what is there in just the general so-called 
"ravages of time" which might leave an imprint on the earth's rock strata? 
We all grow old with the years, and try as we may the results are pretty 
hard to hide. We usually have little difficulty, just at a glance, in telling 
a young man from an old man. Is there any way we can tell young rocks 
from old rocks just by their look? Unfortunately the answer seems to be 



188 HOLLIS D. HEDBERG 

—only in so far as the longer span of existence of old rocks will have al- 
lowed the results of more experiences to have been impressed upon them. 
Thus old rocks will in general be more consolidated, more indurated, more 
recrystallized, more deformed, more intruded, and more generally "beat 
up" than young rocks, but this is still all a matter of experiences suffered, 
not age, and the Cambrian blue clays of Russia still "look" much younger 
than Tertiary schists in the Alps. 

So much then for our means of age determination of strata, our means 
of determining chronostratigraphic position, the means we can use for 
putting all the strata of the earth's crust into their proper sequential posi- 
tion with respect to geologic time and even expressing their geologic age in 
terms of years or millions of years. We have seen that superposition of 
strata, organic evolution, and radioactive disintegration constitute our prin- 
cipal tools, but that there are many other features of rock strata which, 
once the general order becomes clear in any one place, may be helpful 
in extending our dating elsewhere, and we have seen that new and useful 
methods still continue to be developed. 

We have made tremendous progress in the short 2 centuries in which 
geologists have worked at this task, but it is evident that we still have far 
to go. None of our methods is infallible, all have their defects and limita- 
tions, and, even with all put together, huge doubts and uncertainties still 
remain. However, the future is full of promise, and I think we can con- 
fidently expect the rapid progress of the past to continue into the future 
if we recognize stratigraphy for the broad field that it is, and if we leave 
the way open for the co-operative utilization of all hnes of stratigraphic 
evidence (presently known or to be developed) to contribute to this para- 
mount goal— the working out of chronostratigraphy. 



CHRONOSTRATIGRAPHIC CLASSIFICATION 

The dating, or determination of the relative position, of the earth's 
strata is the fundamental task of chronostratigraphy, but it is still not in 
itself the whole story. We have also the very practical problem of how to 
handle and utilize the results of our work. Our objective is simple. In its 
largest sense the purpose of chronostratigraphy is to interpret the history 
of the earth through the sequence of its strata. But history requires bench 
marks, reference points, dates, and divisions. The record of 3 or 4 billion 
years of time written in millions of cubic miles of strata is too vast a 
proposition for us to handle as a whole. We must get down to smaller 
units for practical concepts. We must implement age determination with 
age classiEcation. And how should this be done? 

History fundamentally involves time. With respect to time of origin 
there is only one true order of strata, one true sequence, and this is re- 



THE STRATIGRAPHIC PANORAMA 189 

lated to only one time. And there is only one kind of time.^ I have no 
patience with the claim that organic evolution measures one kind of time 
and radioactive disintegration another. We may speak of relative age 
and absolute age, but they are relative or absolute with respect to one and 
the same kind of time. What we do have is several different means of age 
determination, and, as we have said, our best hope of success is in the 
mutual interplay of these methods, in the combination of the contribu- 
tions that each can make. To achieve this success, therefore, we must use 
units of reference to which all of these methods can apply. 

What are such units? What are these standards of measurement which 
we can employ, these units of reference for geologic history? Well, in com- 
mon history we use years or centuries, and we have seen that also in 
geologic history our radioactive methods can be used to interpret age in 
terms of years. However, years are not marked off for us on the clock face 
of organic evolution. Here it is sequence of life forms that is our measur- 
ing scale. In the method of superposition of strata it is number and se- 
quence of beds that form our scale. And in other methods it is change 
in certain other properties on which our concept of relative age depends. 

What then is the common denominator by which we can bring all 
these indicators of relative or absolute age together? I know of only one, 
and that is the old, simple, and classic one of the rocks themselves — 
designated intervals of rock strata— stratotypes, if you will. It seems to me 
that these must be our fundamental standards of reference for earth his- 
tory, and the basis of our age classification. 

The history of the earth, with all its varied events, is written for us only 
in the sequence of rock strata making up the earth's crust. These strata 
carry the stor}^, such as we can know it, like pages in a book. This book is 
already printed— without our help and without our advice. We can still 
divide it into chapters to suit ourselves, if we wish, but we can do this 
only by dividing it into groups of pages. There may be endless argument 
among us as to what events in the story should be the bases for the chap- 
ters, depending on individual interests and individual viewpoints, but the 
pages will remain the same regardless of how we group them. And, like 
the pages of the book, so the strata of the earth are our only fixed basis 
of reference for chapters in the history of the earth— for the divisions of 
our chronostratigraphic scale. 

Some may wish to base the major chapters or divisions — our geologic 
systems— on changes in organic evolution, others on diastrophic events, 
others on paleoclimatic changes, others on radioactive-age dates. But these 
are all intangible concepts whose scope may vary with opinion, or with 

1 Subsequent to presentation of this address, Preston Cloud has kindly called my 
attention to J. B. S. Haldane's letter in Nature (vol. 15, no. 3888, p. 555, May 6, 1944) 
referring to Milne's interesting concept of two different time scales. This does not, 
however, mean more than one kind of time. 



190 HOLLIS D. HEDBERG 

new discoveries, or with new determinations. If we fix the basis of a sys- 
tem, or a series, or a stage, as a designated section (or sections) of rock 
strata, then we all have a common standard of reference which in its type 
can mean only one specific interval in the time scale to any of us regard- 
less of our ever-changing interpretation of geologic history. This is not a 
freezing of what we measure, as some have claimed (Bell, 1959), but a 
freezing of the units by which we measure. And I think this constancy is 
what we want in any standard of measurement. Then we can proceed to 
extend our systems and series, and stages, throughout the world as best 
we can to the extent that the sum total of our means of time correlation 
allows, with the assurance that we are all working toward the same objec- 
tive within the same guide hues (Hedberg, 1959, p. 676). 

Our present chronostratigraphic units 

The history of our presently existing named chronostratigraphic units is 
interesting. I shall comment here only on those units of major rank and 
supposed world-wide extent— the so-called systems— and on these only very 
briefly, paraphrasing some comments of mine many years ago (Hedberg, 
1948). 

Most of our named systems were born in the early part of the last 
century. These divisions originated largely in western Europe at a time 
when the science of stratigraphy was in its earliest infancy, when the 
stratigraphic sequences of only a very small part of the earth's crust were 
known. Some of the systems were originally based on lithologic features 
thought to characterize rocks belonging to a particular interval of geologic 
time; others were simply designations apphed to observed rock sequences 
in certain geographical areas; still others were introduced later as compro- 
mises to include intermediate disputed strata. In general the bases for 
original definition were remarkably varied and haphazard; their order of 
establishment was without any relation to their position in time sequence; 
and certainly they were not the result of any preconceived master plan 
for chronostratigraphic zonation of the earth's strata as a whole. Quoting 
from Stubblefield of Great Britain (1954, p 153), where many of these 
systems originated, 

"They were defined gradually and on variable and mostly empirical bases. 
Though some degree of paleontological unity . . . was sought, the dividing lines 
in general were taken at such major physical breaks, or changes in bulk 
lithology, as seemed to have regional significance." 

See also Rastall (1944), R. C. Moore (1955, p. 547, 571), Spieker (1956, 
p. 1803), Weller (1960, p. 39), and others. 

However, granting that the named systems of our standard scale were 
created more or less at random in different places at different times, yet 



THE STRATIGRAPHIC PANORAMA 191 

surprisingly enough they have worked quite well. From a multiplicity of 
originally proposed systems certain ones have emerged and have been ac- 
cepted by stratigraphers in general because they have proved useful ref- 
erence standards for geological time on a world-wide basis. Naturally, they 
are imperfect in many respects, and we might divide the chronostrati- 
graphic section quite differently if we had it to do all over again today. 
However, perish the thought that we should confuse the great work of 
the past by drastically changing our system divisions and their nomen- 
clature now! On the contrary, I believe that we can continue to live quite 
comfortably with what we have. Only we must understand what it is 
that we really have inherited in our present systems— perhaps no more nor 
less than rather arbitrarily chosen reference units for expressing geologic 
age— and we must sharpen and define them better in terms of type sec- 
tions of rock strata so that they may better serve this simple and most use- 
ful function, regardless of any other significance they may have. 

Concept of world-wide "natural breaks" 

Historically, most of the systems in their locality of origin contrasted 
strongly with adjacent ones through outstanding differences in lithology, 
structure, or fossil content, and their authors beheved their boundaries to 
be indicative of "natural" division points in earth history. {See Hedberg, 
1948.) Perhaps even more than the originators themselves, their imme- 
diate successors assumed these systemic units to represent distinct world- 
wide steps in earth history. Later work has now shown that most of the 
supposedly sharp breaks in the type areas were actually due to local 
changes in environment, or to unconformities and hiatuses that left gaps 
in the local stratigraphic sequence. Sedimentary sections have now been 
found elsewhere in the world filling in many of those gaps and completing 
a more orderly and continuous sequence of fossils, on a world-wide basis, 
than was originally believed to exist. However, following the impetus of 
these early ideas, the highly attractive belief has persisted that the systems 
are "natural" units marked off by relatively abrupt world-wide changes in 
earth history and in the evolutionary sequence of life forms. Although 
such good fortune might seem to some of us little less than miraculous in 
the light of the way the systems were originally established, still a co- 
incidence of the present systems with these so-called "natural" breaks is 
accepted by so many, and would be so convenient if true, that its validity 
becomes of critical importance in its effect on our whole stratigraphic 
philosophy. {See good discussion in Dunbar and Rodgers, 1957, p. 302- 
307.) The question is: should we identify rocks throughout the world with 
a particular system on the basis of a certain concept of world-wide events 
or characteristics supposed to mark the "natural" limits of that system? 
Or should we be content to identify rocks throughout the world with a 



192 HOLLIS D. HEDBERG 

particular system on the more empirical basis of time correlation to the 
best of our ability with an established type of that system, regardless of any 
preconceived notion of what its world-wide characteristics or relations to 
earth history should be? 

Evidence in diastrophic record for world-wide "natural breaks" 

Let us briefly consider first the evidence for world-wide diastrophism as 
a "natural" basis for the separation of the present systems, especially since 
diastrophism might be expected to be reflected both in hthology and in 
the sequence of life forms. 

In a masterful address before this Society 12 years ago, Gilluly (1949) 
shook the theory of periodic world-wide orogeny to its mountain-building 
roots. Gilluly (p. 562) noted that this theory had previously been ques- 
tioned by "Shepard (1923), Berry (1929), Von Bubnoff (1931), Arkell 
(1933), Woodring (1938), Spieker (1946)" and others. In a stimulating 
paper with the intriguing title, Palaeozoic, Mesozoic, and Kainozoic; a 
geological disaster, R. H. Rastall (1944, p. 163), speaking in England, had 
said, 

"Another point that is in much need of emphasis, particularly in this country, 
is that periods of orogeny are far from being world-wide. As a matter of fact 
they are distinctly local. When comparing the tectonic history of northwestern 
Europe with any area in the southern hemisphere, it would probably be much 
nearer the truth to say that the major revolutions alternate rather than synchro- 
nize. ... It seems probable that orogeny was always going on somewhere in 
the world and still is." 

More recently, Spieker (1956) has again vigorously and indignantly 
belabored the theory, Arkell (1956, p. 641) has said, 

"So far as our knowledge goes at present, it does not point to any master plan 
of universal, periodic, or synchronized erogenic and epeirogenic movements. 
The events were episodic, sporadic, not periodic. There was no 'pulse of the 
earth'." 

Gignoux (1955, p. 248) in rejecting Stille's periodicity of orogeny has said, 

"Considered in the light of global unity, geological phenomena have obeyed 
neither the baton of an orchestra nor the rule of a geometer." 

Likewise Tyrrell (1955, p. 411), after discussing Sonder's geological cycle 
(geosynclinal phase, transgression phase, orogenic phase, continental phase) 
says, 

"The use of Sonder's scheme in this connection does not imply its full ac- 
ceptance, or that it is applicable all over the earth. ... It does not take into 



THE STRATIGRAPHIC PANORAMA 193 

account the fact that the earth's crust consists of dissimilar units, with different 
histories, and most probably with differing and non-synchronous geological 
cycles. Moreover, geological cycles do not always coincide with the standard 
geological eras. The great Russian platform, for example, has remained almost 
undisturbed since Precambrian time, and must have experienced a quite differ- 
ent cycle of events from those of geosynclinal and orogenic regions such as 
western Europe and eastern North America." 

King (1955, p. 723) favors the thought 

"that orogeny and epeirogeny were episodic rather than continuous; that epi- 
sodes affected fractions of continents rather than whole continents or all con- 
tinents; and that episodes may be expressed in one place by compression of the 
crust, in another by tension, at one place by orogeny, at another by epeirogeny." 

Hawkes (1958), Holmes (1960), Kulp (1960), and others have recently 
brought out evidence opposing the idea of abrupt periodic world-wide 
orogenies of a type that might serve as boundary markers for systems. 
Gastil (1960), however, does see a concentration of radioactive age dates 
at approximately 200 million-year intervals which he suggests represents 
a cyclic distribution of broad periods of crustal adjustment. 

Evidence in record of organic evolution for world-wide "natural breaks" 

Let us now turn to the evidence for "natural breaks" on a world-wide 
scale in the record of organic evolution. The extremely valuable sym- 
posium on the Distribution of evolutionary explosions in geologic time, 
organized by Lloyd Henbest and published in the Journal of Paleontology 
(1952, p. 298-394), represented an effort to examine the evidence for ab- 
rupt world-wide changes in the paleontologic record which might appear 
to be the reflection of periodic world-wide diastrophism. I quote from Hen- 
best's conclusions (Henbest, 1952, p. 317): 

"The evidence from stratigraphic paleontology . . . indicates that the diastrophic 
theory and some of its corollaries that are applied to stratigraphy represent 
greatly oversimplified and exaggerated inference. ... As we progress in filling 
gaps in stratigraphic records and reconstructing the history of the earth, the 
lines of evolution will emerge as paramount and the gigantic synchronous pulses 
will become more diffused and obscure as integral properties of earth processes." 

J. S. WiUiams (1954, p. 1604-1605) suggests, 

"World-wide 'natural' boundaries do not exist between geologic systems. A 
'natural' boundary may exist in the limited area of the type section and in 
other areas, but not in all and probably not in most regions. Despite what 
seems to the writer to be rather general recognition of this situation, it seems 
that most geologists more or less unconsciously think in terms of some kind of 
a lithologic, faunal, or diastrophic break at systemic boundaries in local areas." 



194 HOLLIS D. HEDBERG 

I quote also from Hawkes (1958, p. 318) who, summing up the evi- 
dence, including that from the Henbest symposium and the more recent 
Crust of the earth symposium, says, 

"It is recognized that there is a sensitive connexion between faunas and their 
environment. Changes in the temperature, composition, depth, circulation, 
muddiness, and the like of water have an influence on the nature, extent, and 
rate of speciation of marine animals. If, as seems highly probable, such environ- 
mental changes have been in progress continuously on the earth throughout 
geological time there would be no reason to expect organic evolution to show 
special advances at three arbitrarily chosen periods such as the supposedly brief 
episodes of the Caledonian, Hercynian, and the Alpine revolutions — and it is 
now conceded that this is not supported by the evidence. The long-entertained 
idea that bursts of evolution coincided with these three episodes is on the way 
out." 

On the other hand, Schindewolf (1954), Newell (1956, p. 97), and 
many others see abrupt paleontological changes at the boundaries of the 
erathems which are "real, approximately synchronous, and are recog- 
nizable at many places in different parts of the earth." So the subject is 
still far from being closed. 

Perhaps the most striking of all supposed "natural breaks" is that still 
enigmatic discontinuity that separates the highly fossiliferous rocks and 
complexly organized life of the Phanerozoic from the barren or very 
sparingly fossiliferous rocks and primitive life forms of the Precambrian. 
Even here, however, conclusions must be reserved until we better under- 
stand its nature. In many places sedimentary strata go down from the 
base of Cambrian fossils concordantly and in apparently continuous se- 
quence into the Precambrian. Vicissitudes in the preservation of fossils 
and in environments of deposition probably make the "base of the Olenel- 
lus zone" or the "base of Cambrian fossils" far from a world-wide 
isochronous surface, and the supposed sharpness of the break may be 
greatly exaggerated. 

I have mentioned the Ediacara fossil occurrence in the Precambrian of 
Australia and I feel certain that before long other Precambrian finds will 
be made which tell us still more about this presently mysterious abrupt 
appearance of complex hfe forms in our rock record. In the meantime, 
there is every reason to push on with the chronostratigraphic classification 
of the Precambrian to the best of our ability on whatever objective bases 
we can find. I would say amen to Harold James' conclusion (1960, p. 
113) that, 

"although the immense duration of time and the lack of diagnostic fossils are 
formidable obstacles to overcome, the problems of stratigraphy and correlation 
in the Precambrian can and must be solved. Despite the difficulties, the Pre- 
cambrian is not a world apart; it contains the same kind of rocks and reveals 



THE STRATIGRAPHIC PANORAMA 195 

the same kinds of geologic processes known from the record of younger eras; 
the same principles apply and the same rules must be used. And as with rocks 
of the younger eras, stratigraphy and correlation are the very essence of under- 
standing the geologic record." 

Summation of evidence regarding world-wide "natural breaks" 

Passing on from the evidence of diastrophism and organic evolution with 
regard to world-wide "natural breaks," we have already looked briefly at 
the age significance of lithologic character, mineralogy, chemical composi- 
tion, changes in sea level, igneous activity, unconformities, and other rock 
characters and features affecting rocks, and have found only a little therein 
on which to base a reliable world-wide chronology and much less on which 
to base "natural" world-wide chronostratigraphic divisions. Chmatic 
changes and any other phenomena that may possibly result from extra- 
terrestial influences might oflFer particular promise of sound world-wide 
"natural" time-stratigraphic divisions; but here again evidence for recog- 
nition must he in the strata themselves, and such evidence seems still too 
scanty and conflicting to furnish us yet with any very practical basis of 
classification. 

Far be it from me to attempt to pass judgment on the mighty mass of 
objective data which must be evaluated to determine if, and if so when 
and how, great world-wide "natural breaks" or revolutions may have taken 
place in earth history with sufficient impact and abruptness to have left 
their mark so clearly and so sharply in earth strata as to constitute the 
basis for "natural" world-wide chronostratigraphic divisions; and far be it 
from me to attempt to pass judgment on whether such breaks, if they do 
exist, coincide with our presently accepted systems. If this is what you 
expected me to do, you will be disappointed. All I can do is to rely on the 
work of those who have studied the evidence much more thoroughly and 
much more understandingly than I. I do not find agreement among them, 
but I do find so much well-reasoned judgment in opposition that I must 
conclude, regardless of what may eventually be proved, that it has not yet 
been demonstrated that world-wide "natural breaks" in the character and 
continuity of our strata exist at the scale of the presently accepted geo- 
logic systems, nor has it been demonstrated that the evidence at the 
boundaries of the present systems is such as to allow them to be con- 
sidered as the "natural" world-wide division points of the chronostratig- 
raphic scale. 

Views of USSR Stratigraphic Commission on chronostratigraphic 
classification 

Some of you here may agree with my conclusions, some I know will 
not; but for those of you who do, it may be well to show that I have by 



196 HOLLIS D. HEDBERG 

no means been setting up a straw man to attack in your presence. As 

evidence thereof, I present the 1960 conclusions of the official USSR 

Stratigraphic Commission (Interdepartmental Stratigraphic Committee 

of the USSR, 1960) purporting to represent the thinking of a vast group 

of distinguished geologists on the other side of the earth: This says in 

summary: 

1. The basic aims of stratigraphy are the age correlation of rocks and 
the construction of a single reference scale of the geochronologic divisions 
and corresponding stratigraphic divisions for the entire Earth, based on 
so-called "natural" steps or stages in the history of the physical develop- 
ment of the Earth and the evolution of organic life. 

2. The subdivisions of this single, so-called "natural" stratigraphic scale 
should be based on the totality of all lines of evidence, and therefore 
separate kinds of stratigraphic classification such as lithostratigraphic, bio- 
stratigraphic, and chronostratigraphic are unacceptable. 

3. The dividing of the history of the earth into the so-called "natural" 
steps is made possible by the irreversibility of geologic phenomena and by 
their periodicity, most clearly manifested in the alternation of long-con- 
tinued stages of slow and gradual evolutionary development with shorter 
stages of rapid transformation in the face of the earth concomitant with 
great rearrangements in the internal structure of the earth's crust. This 
periodicity is also manifested in alternations of large transgressions and 
regressions of the sea, in corresponding changes in the course of organic 
evolution, in changes in the process of sedimentation and denudation, in 
changes in paleogeography, in changes in igneous and metamorphic ac- 
tivity, and in large tectonic movements of wide geographic range. 

4. The presently recognized systems are "natural" divisions generally 
characterized in their lower parts by a sequence from continental to marine 
transgressive deposits and in their upper part by regressive deposits. Their 
boundaries are frequently characterized by angular unconformities, strati- 
graphic breaks, abrupt changes of facies, and evidence of igneous activity. 
The systems are paleontologically distinctive and are successively marked 
by the appearance and wide development of new hfe groups of major rank. 

I think it is at once evident that the Russian approach to chronostrati- 
graphic classification differs considerably in fundamental philosophy from 
that which I have favored in this discourse. The Russians would start with 
the assumption of the existence of world-wide "natural" steps in the 
historical development of the rocks of the Earth's crust, more or less 
accordantly reflected in all lines of stratigraphic evidence, and would then 
aim to fit the earth's strata as well as possible into these steps. I would 
favor a more objective approach, which would start by classifying strata 
independently with respect to each of several kinds of stratigraphic cri- 
teria without the preconceived conclusion that any or all of these would 



THE STRATIGRAPHIC PANORAMA 197 

show accordance with each other or with any "natural" over-all chrono- 
stratigraphic grouping of strata. If they did, I might rejoice, but if they 
didn't I would feel no urge to make them do so. I would try to make 
chronostratigraphic units as significant of earth history as possible, but I 
would define them purely on the basis of type sections— stratotypes— and 
then would extend them throughout the world strictly on the basis of 
empirical time correlation with the stratotype, utilizing the sum total of 
available evidence of any kind for determining time equivalence. The 
Russian course is deductive; that which I favor is inductive. 

In a sense the Russian view retains somewhat the influence of the old 
catastrophists. Opposed to the idea of the essential continuity and uni- 
formity of change for the world as a whole is their idea of periodic world- 
wide breaks reflected in world-wide changes in strata and their contents. 
Opposed to the idea that the boundaries of the generally accepted systems 
are rather arbitrary points is their view that they mark "natural" world- 
wide division points. Their concept is an attractive one and one which has 
long appealed to stratigraphers. It should not be passed over lightly, and 
the only point I wish to make is that I (and perhaps you) have not yet 
been convinced that there is valid objective evidence for it, nor have I 
(and perhaps you) yet been convinced that there are valid theoretical 
reasons why it should be so. Certainly it is something that will eventually 
be demonstrated either true or false as our knowledge of world stratigraphy 
develops; meanwhile, it is fortunate that both the Russians and we recog- 
nize the same standard systems and the need to establish for these, natural 
or artificial as they may be, designated type sections (stratotypes). 

SUMMARY AND CONCLUSIONS 

Probably few if any of our systemic boundaries are adequately defined 
in type areas of the present time, and stratigraphic progress is impeded by 
gaps and overlaps at these boundaries and by futile controversies over the 
placement of strata. Our crying need is for the careful designation and 
universal acceptance of limits in continuous, type, or reference sections 
which can serve as standards for these systems. Without such definition 
we will have only endless argument and continued chaos. To obtain them 
for these world-wide units will require world-wide co-operation— the estab- 
lishment of stratotypes by international commissions of qualified stratig- 
raphers whose findings will have the support of an authoritative inter- 
national geological body. With a universally accepted base of reference 
once fixed, these systemic boundaries can then be extended through time 
correlation by any means available to us— fossils, radioactive age determina- 
tions, and what not— around the world as best we can, but always with 
a standard reference to which we can return in case of doubt or contro- 



198 HOLLIS D. HEDBERG 

versy. Our systems thus delimited may be for many of us simply standard 
and universally understood units of chronostratigraphic measurement and 
record, with nothing more holy about them than there is holiness in the 
mile, the foot, or the meter. For others, they may be sacred chapters in 
earth history. It really matters not too much which, as long as we agree 
on the means of definition and the means of extension. 

Overlaps we shall have to eliminate by assignment to one or the other 
of the overlapping systems. The gaps, as they are filled, can also be as- 
signed to one or the other of the adjacent systems by adding to their 
standard reference sections, or even by giving new names to the inter- 
mediate strata if the situation justifies. The resolving power of our means 
of age determination and time correlation is not so fine but that there will 
probably always be a greater or lesser no-man's land of strata of uncertain 
systemic assignment with varying distance away from the designated type 
or reference sections, but such is to be normally expected, and there is no 
need to strain facts to make sharp divisions of that which it is beyond our 
power to divide. {See also Williams, 1954.) The succession of strata on 
the earth provides a spectrum of age similar to the spectrum of light, and 
there is no more crime in referring to strata of uncertain age position be- 
tween type Jurassic and type Cretaceous as Jurassic-Cretaceous than there 
is in referring to a color midway between blue and green wave lengths 
in the hght spectrum as blue-green. In fact this is much more accurate 
and scientific than to insist on a label of either blue or green or Jurassic 
or Cretaceous when there is no more proof of one than the other by 
reference to the type. 

The picture of stratigraphy typified by the Russian viewpoint and also 
accepted by many others is a beautiful one, and perhaps its validity may 
one day be demonstrated to those of us who prefer for the present to pro- 
ceed more cautiously. On the other hand, I cannot but think that the 
more purely objective concept, which I have outlined, of a stratigraphy 
that aims to delineate the earth's strata the world over, just as they are 
found, with respect to as many of their many features as may be of inter- 
est or utility, and then proceeds to the drawing of only such conclusions 
on earth history as the established facts justify is also a beautiful picture. 
I cannot help but repeat a quotation from the anniversary address of Sir 
Cyril Hinshelwood (1958, p. x) to the Royal Society, to which Professor 
Hawkes (1959) has called attention, because it seems so appropriate to 
the history of this matter of stratigraphic classification. It reads as follows: 

"What the true seeker after knowledge in his heart desires is some sim- 
ple design which he feels must underlie the facts. . . .The search for prin- 
ciples which are aesthetically satisfying seems often frustrated by the 
complexity of nature; and the conflict between imagination and austere 
regard for truth seems often to result in the passage of scientific theories 



THE STRATIGRAPHIC PANORAMA 199 

through three stages. The first is that of gross over-simphfication, reflect- 
ing partly the need for practical working rules, and even more, a too- 
enthusiastic aspiration after elegance of form. In the second stage the 
symmetry of the hypothetical systems is distorted and the neatness marred 
as the recalcitrant facts increasingly rebel against conformity. In the third 
stage, if and when this is attained, a new order emerges, more intricately 
contrived, less obvious, and with its parts more subtly interwoven, since 
it is of nature's and not of man's conception," 

So, in conclusion, for my part I should prefer to continue to think of 
the stratigraphic record as a panorama— a picture that has changed con- 
tinuously and profoundly as it was unrolled, but one in which the changes 
in its various individual features overlap and intergrade and so blend that 
in the ensemble it is all one scene. Some parts, to be sure, are concealed 
from us today, but when we fully know the crust of our earth, both on the 
continents and under the oceans, the chances are that in one place or 
another the gaps in the rock record will be filled. Just as there is no sedi- 
ment deposited unless there has been erosion somewhere, so there is no 
sequence of rocks in the earth's crust that is not somewhere equivalent 
to an unconformity or hiatus, and no unconformity or hiatus in the local 
record that may not somewhere be represented by deposits. Just as local 
clouds may obscure the continuity of our geographic panorama as we fly 
across the continent, so locally we have breaks and gaps in the rock record, 
but, for the earth as a whole, there still exists the continuous stratig- 
raphic panorama. 

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252, p. 683-694. 



200 HOLLIS D. HEDBERG 

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THE STRATIGRAPHIC PANORAMA 201 

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^ -^^ ■'■ In the face of geological opinion of today 

the writer has been forced by the sheer 
CLIMA 1 ES weight of such evidence into accepting some 

form of Continental Drift to explain the 
AND "face of the Earth"; indeed a world without 

some form of crustal drifting would appear 
■QjD Tp'pjjs^Q to him as unreal as one lacking in biological 

evolution. — alex l. du toit, Our Wander- 
CONTINENTS ^"^ Continents (1937) 



A Theory of Ice Ages 

• MAURICE EWING AND WILLIAM L. DONN 



THIS ARTICLE IS A PRELIMINARY REPORT OF NEW IDEAS 

related to the origin of glacial climates; it is based largely on observations 
made during the last 20 years. Glacial climates pose two problems: (i) the 
striking alternations during the Pleistocene epoch of glacial and interglacial 
stages and (ii) the even more striking change from the warm nonglacial 
chmate, which prevailed generally from the Permian to the Pleistocene, to 
the cold and glacial conditions of the Pleistocene and Recent. 

If it is difficult to answer the second question, it is even more difficult 
to solve both problems on the basis of a single theory. The present study 
(J) offers an explanation for the alternations in climate during the 
Pleistocene and proposes an explanation for the change from nonglacial 
to glacial climates. 

PLEISTOCENE GLACIAL AND 
INTERGLACIAL STAGES 

First we wish to develop the following principal points of the glacial- 
interglacial theory. 

1) The melting of an Arctic ice sheet (such as exists at present) would 
increase the interchange of water between the Atlantic and Arctic oceans, 
cooling the North Atlantic and warming the Arctic and making it ice-free, 
thus providing an increased source of moisture for the polar atmosphere. 

• From Science (June 15, 1956), pp. 1061-66. 

203 



204 MAURICE EWING AND WILLIAM L. DONN 

2) Two factors would then favor the growth of glaciers: (i) increased 
precipitation over arctic and subarctic lands and (ii) changes in atmos- 
pheric circulation, the latter also resulting from the warmer Arctic and 
cooler Atlantic oceans. 

3) The lowering of sea level would greatly decrease the interchange of 
water between the Atlantic and Arctic oceans, which, together with the 
cooling effect of surrounding glaciers, would reduce Arctic surface tem- 
peratures until abrupt freezing occurred. The fairly sudden reversal of 
conditions favorable to glacial development would terminate the growth 
of glaciers abruptly. 

4) As continental glaciers waned, the sea level would rise, causing an 
increased transport of surface waters northward until the Arctic ice sheet 
melted once again, completing the cycle. 

5 ) Temperature changes in the surface waters of the Arctic and Atlantic 
oceans are thus the causes of, rather than the consequences of, the waxing 
and waning of continental glaciers. 

Abrupt change in Atlantic deep-sea sediments. Using radiocarbon meas- 
urements, Ericson et al. (2) and Rubin and Suess (3) have established 
dates as far back as 30,000 years ago for cores of deep-sea sediments. Using 
paleotemperature measurements based on the oxygen isotope ratio, Emili- 
ani (4) determined the temperature of the water in which pelagic Fora- 
minifera lived; his findings covered a time interval represented by the 
longest cores available. Suess (5) then combined these two sets of results 
by plotting the radiocarbon ages for three deep-sea cores from the equa- 
torial Atlantic and Caribbean against the paleotemperatures of Fora- 
minifera tests. The resulting graph may be interpreted as showing a 
temperature decline of 1°C per 11,000 years for the interval from 90,000 
to 11,000 years before the present. Temperatures then began an abrupt in- 
crease at a rate of about 1°C per 1000 years from 11,000 years ago to a few 
thousand years ago. For the last few thousand years, temperatures have 
remained about as high as the maximum value that was reached during 
all Pleistocene interglacial stages (see Emihani, 4, Figs. 2 and 3). 

The curve given by Suess for Core A 172-6 shows the abrupt tempera- 
ture change beginning about 16,000 years ago as compared with the change 
shown at 11,000 years in the other cores. Since the time scale for this core 
depends on only three points, considerable interpolation is required, and 
since Emiliani shows that a strong solution of carbonates occurred in a 
number of places, linear interpretation is likely to be unreliable 

In addition, faunal studies by many investigators show that a well- 
marked change in most cores indicates the termination of the Wisconsin 
age. In 1935, Schott (6) described the faunal break in many equatorial 
Atlantic cores, which was later noted in more recent longer cores— for 
example, by Bramlette and Bradley in 1940 (7), Phleger, Parker and 
Pierson (8), Ovey (9) and Schott {10, 11). Most recently, radiocarbon 



A THEORY OF ICE AGES 205 

measurements on a number of cores described by Ericson and Wollin 
(12) from the Atlantic Ocean and Caribbean Sea show that this charac- 
teristic break {13), marking a change from cold to warm pelagic fauna, 
occurred 11,000 years ago (2). Cores taken in the Gulf of Mexico show 
a similar faunal break (J4), although as yet no quantitative temperature 
or time measurements have been made. However, on the basis of the 
fauna change, it is possible to correlate the break in the Gulf of Mexico 
with that described in the Atlantic Ocean. 

Cores from the Gulf of Mexico. Many sediment cores 30 to 40 feet long 
have been taken on the Mississippi Cone {IS), which spreads over most 
of the deeper part of the Gulf of Mexico from a vertex just off the Missis- 
sippi Delta. These cores, which show a layer up to 2 feet thick of fora- 
miniferal lutite, indicating warm water, overlie with a sharp transition a 
layer of essentially unfossiliferous lutite and silt, the bottom of which 
was never reached. Pending radiocarbon measurements, it seems reason- 
able to extrapolate the date of 11,000 years ago for the bottom of the 
foraminiferal layer. In cores taken on the nearby Sigsbee Knolls, which 
rise 20 fathoms above the cone and above the reach of turbidity currents, 
Foraminifera of the cold-water type are abundant throughout the silty 
layers {14). It is concluded that the extreme scarcity of Foraminifera in 
the silty layer of the Mississippi cone is due to dilution from a great influx 
of elastics in Wisconsin time. The sediments give no evidence of any 
climatic change except that 11,000 years ago. 

Further, the beginning of rapid postglacial rise in sea level is indicated 
by the abrupt decrease in clastic deposition on the Mississippi cone about 
11,000 years ago, when the drowned rivers retained their clastic sediments 
instead of discharging them into the Gulf {IS). 

The result of a very recent investigation of turbidity current deposits 
made by Heezen and Ewing {16) shows that an abrupt rise in sea level 
must have taken place about 11,000 years ago; this event is also taken to 
mark the close of the Wisconsin glacial stage. 

Conclusions from Atlantic and Gulf data. On the basis of these data, we 
regard 11,000 years ago as the date of the most recent significant tempera- 
ture change in the Atlantic Ocean, and also as marking the end of the 
Wisconsin glacial stage. Additional breaks in isotopic temperatures (4) 
and faunal types (7, 8, 11, 12) occur in many of the Atlantic cores which 
penetrate most of the Pleistocene. We regard these as marking the transi- 
tion from earlier glacial to interglacial stages. 

The data show that this fairly abrupt increase in temperature in the 
surface layers of the Atlantic Ocean about 11,000 years ago was the most 
significant temperature change of the past 60,000 to 80,000 years, although 
similar changes occurred earlier in the Pleistocene. Because the beginning 
of glacial retreat which closed the last Wisconsin substage (Mankato) 
occurred about 11,000 years ago— for example, Fhnt (J7)— it appears that 



206 MAURICE EWING AND WILLIAM L. DONN 

the oceans became warm abruptly at the time retreat commenced and 

remained warm during retreat of the ice. 

We propose to show that the temperature of the surface layer of the 
ocean, rather than external conditions, regulated the climate of the land. 
Specifically, we suggest that the alternating warm and cold stages of Pleisto- 
cene climate are the effects of fairly abrupt alternations between warm 
and cold conditions of the upper layer of the Atlantic and Arctic oceans. 
We are thus led to consider the cause of the pronounced fluctuations of 
the temperature of the Atlantic and Arctic oceans. 

Arctic Ocean cores and the influence of the Arctic. Three short cores 
(9, 14, and 16 centimeters long) and one long core (81 centimeters long) 
were taken in the deeper part of the Arctic Ocean by A. P. Crary and 
studied by D. B. Ericson. The upper 20 centimeters of the Slcentimeter 
core contain abundant Foraminifera (Globigerina) , which decrease in the 
next underlying 10 centimeters. The long section below 30 centimeters 
consists of lutite, with granules and pebbles distributed sparingly through- 
out, and shows no Globigerina except for a few near the bottom. Several 
radiocarbon measurements on the Foraminifera of the 16-centimeter core 
(18) gave an age from 18,000 to 23,000 years. Other cores, collected else- 
where in the Arctic by Crary, show thin sediment zones above the fora- 
miniferal layer. The presence of this upper foraminiferal zone, with no or 
only thin overlying sediment, indicates an abrupt end of conditions 
favorable to the growth of Foraminifera and suggests that the Arctic 
Ocean was open during the Wisconsin glacial stage. Further, the coarse 
fragments in the longer lower section of the core are attributable to 
ice rafting under conditions that provided numerous icebergs free to 
move in open water. This condition would be met by an ice-free Arctic 
during Wisconsin time. The absence of Globigerina in the lower section 
of the core, except for the very bottom, can be explained by dilution by 
clastic sediments; and the presence of Foraminifera in the upper portion 
can be explained by a decrease in the amount of elastics. This is analogous 
to the similar, well-founded observations on cores from the Gulf of Mex- 
ico that were described in a preceding paragraph. 

Based on the possibility that the Arctic Ocean can undergo both ice- 
free and ice-covered stages, we propose the following assumption: When 
the Arctic Ocean is ice-covered, surface temperatures in the Atlantic in- 
crease and continental glaciers decline; when the Arctic is open, surface 
temperatures in the Atlantic decrease, and continental glaciers develop. 
With this assumption in mind, let us examine the process by which an 
ice-free Arctic can bring about a glacial stage. 

Consequences of an ice-free Arctic Ocean. Given an open Arctic Ocean, 
the resulting large increase in absorption of insolation is well known. The 
present circulation of water within the Arctic Ocean would be greatly 
increased (19) since the wind stresses would then be applied directly to 



A THEORY OF ICE AGES 207 

the water. It is postulated that a marked increase in the interchange of 
water between the Arctic and Atlantic oceans would occur which would 
tend toward equalization of temperatures, cooling the Atlantic and warm- 
ing the Arctic. With the added effect of greater absorption of insolation, 
the ice-free condition of the Arctic Ocean would be maintained against 
the cooling effect of increased evaporation. 

The present mean temperature of the surface of the Arctic Ocean in 
January is — 35°C and in July, about 0°C, The vapor pressure of water (or 
ice) at 0°C is 4.6 millimeters of mercury; at 3°C, 5.7 millimeters; and for 
ice at— 30°C, 0.3 milhmeter. The change to an ice-free condition would 
necessarily require an increase of winter ocean surface temperatures by at 
least 35 °C. By extrapolating from the afore-mentioned data, it can be 
seen that the vapor pressure would increase by a factor of about 50 in the 
winter season, while the summer ocean surface temperature and vapor 
pressure would be only a little increased. The principal effect of an open 
Arctic is thus the providing of greater moisture during the long polar 
winter. 

Although it is difficult to make a detailed analysis of the consequent 
changes in atmospheric and oceanic circulations and the results thereof, 
the following general conclusions seem reasonable. 

1) The increased evaporation from an open Arctic Ocean, particularly 
in winter, would increase the precipitation over adjacent cold land areas, 
where lack of precipitation, rather than high temperatures, at present 
prevents the growth of glaciers (for example, Stokes, 20). 

2) The present polar high, with clockwise circulation, would be replaced 
by a low-pressure area with counterclockwise circulation because of the 
contrast between the warm water and the surrounding colder land. The 
resulting reinforced counterclockwise circulation of the Arctic Ocean 
would tend to increase further the interchange of Atlantic and Arctic 
waters. Judging from the present North Atlantic circulation. 

3) The semipermanent North Atlantic low would be displaced roughly 
10 to 20 degrees southward. Increased zonal flow of air around the northern 
portions of this low would transport cool moist air over eastern North 
America. At the same time, the presence of relatively warm water on all 
sides of the continents would promote the development of cool continental 
highs. 

4) Winter and summer conditions over the continents would become 
more similar as a permanent continental ice sheet developed, with a 
resulting migration of the present polar front southward. 

5) The contrast between the cold northern land areas and the warmer 
open Arctic Ocean would result in a second, although weaker, polar frontal 
zone surrounding the polar low. 

6) A second belt of storms would therefore be present, providing some 



208 MAURICE EWING AND WILLIAM L. BONN 

nourishment for growing glaciers at the northern margin, in addition to 
the stronger nourishment at the southern margin. 

7) The Atlantic Ocean would be cooled by lowered air temperatures 
induced by continental glaciers and by the abrupt cooling effect of in- 
creased interchange with the Arctic Ocean already mentioned. 




Fig. 1. Chase's map of 
hypothetical pressure 
pattern during a glacial 
age with an ice-free 
Arctic Ocean. Air mo- 
tion is clockwise about 
high-pressure areas (H) 
and counterclockwise 
about low-pressure areas 
(L). Continental highs 
correspond to present- 
day cold winter situa- 
tions. 



J. Chase has given a general confirmation of these views in an analysis 
(see Fig. 1) that followed a presentation of the ideas of this paper at a 
colloquium at the Woods Hole Oceanographic Institution {21). His 
analysis is based on a study of extreme conditions shown on historical 
weather maps and represents hypothetical mean isobars during a glacial 
stage having an open Arctic Ocean. 

Termination of a glacial stage. The glacial stage would be brought to 
an abrupt close by the development of a new Arctic Ocean ice sheet. 
Along most of an Atlantic Ocean profile, through either Iceland or Spitz- 
bergen, the sill depth between the Arctic and Atlantic oceans is between 
200 and 300 fathoms, and much of this is less than 50 fathoms, a depth 
generally accepted as the probable value for maximum Wisconsin decrease 
in sea level (see Fig. 2). When the lowering of sea level reached about 
50 fathoms, a serious reduction in the interchange of water between these 
oceans would occur. The reduced inflow of warm Atlantic water, together 
with the cooling effect of the continental glaciers, would eventually allow 
a new Arctic Ocean ice sheet to form. The interchange of water would 
then be reduced to even less than its present-day value, owing to the re- 



A THEORY OF ICE AGES 209 

duced sea level. Thus, with the present radiation balance and the slow 
return to the current atmospheric circulation pattern, there would be no 
tendency to remove the Arctic Ocean ice sheet until sea level returned to 
at least its present position. A reversal of the phenomena described as 
consequences of an ice-free Arctic Ocean would consequently occur. Al- 
though gradual wastage of the continental glaciers would follow, the 
warming of the Atlantic surface water would be more rapid because of 
an abruptly diminished interchange with the Arctic. 





NORTH ATLANTIC PROFILE THROUGH SPITSBERGEN 




NORTH ATLANTIC PROFILE THROUGH ICELAND 



Fig. 2. Two profiles across the North Atlantic Ocean indicating the re- 
striction of Arctic-Atlantic interchange that would result from a lowering 
of sea level about 50 fathoms (hatched area) across either section. 
Although a narrow deep zone exists west of Spitzbergen, this is known 
to be a region of return flow from the north. Depths are in fathoms. 

If we consider the last glacial stage, we find that the studies of deep-sea 
cores indicate that 11,000 years ago is the date of the end of Wisconsin 
time. Furthermore, the end of Wisconsin glacial conditions in the ocean 
corresponds with, and according to our theory, caused the end of the 
Mankato substage on the continents. The glacial retreat between the 
Tazewell and Mankato glacial substage maxima (for example, Flint 17) 
is only a minor fluctuation. 

The interglacial stage. The transition from glacial to interglacial con- 



210 MAURICE EWING AND WILLIAM L. DONN 

ditions in the Atlantic Ocean would be simultaneous with the time of 
maximum continental glaciation (neglecting minor fluctuations). As the 
continental glaciers wasted, their coohng effects would diminish. The 
consequent rise in sea level would slowly enlarge the cross section of the 
channel between the Atlantic and Arctic oceans, providing an increase in 
the northward transport of warm water, as is occurring at present. Even- 
tually a condition would be reached where the Arctic ice sheet would 
disappear. 

Considerable evidence has been given by Berezkin (J9) and by Crary, 
Kulp, and Marshall {22) which indicates that the Arctic Ocean has been 
warming recently. If that trend continues, open water over the entire 
Arctic Ocean might occur within a few centuries, with consequent glacia- 
tion in northern latitudes. 

The presence of five temperature maxima, marking past interglacial 
stages, all at about the same temperature as the present (4) is strong 
evidence that an internal, self-regulating mechanism controlled the climate 
during the Pleistocene. Emihani's graph of temperature versus time (4, 
Fig. 3) gives compelling evidence of oscillations of the system between 
two quasi-stable states, with the significant external conditions remaining 
constant throughout. This implies that present temperature is at the 
maximum value expected for an interglacial stage and that a decrease in 
temperature marking the onset of the next glacial stage may be expected 
within some few thousands of years. 

In describing Pacific cores, Emiliani notes (4, p. 561) far less conspicu- 
ous temperature variations than in cores from the Atlantic. He explains 
this on the basis of greater vertical circulation in the Pacific, which seems 
to be a rather ad hoc solution. The observed uniformity in Pacific tem- 
peratures, however, is an expected consequence of the theory proposed 
here, in which strong temperature changes should be limited to the Atlan- 
tic and Arctic oceans. These observations also imply that some mecha- 
nism, involving only the Atlantic and Arctic oceans, is the most reasonable 
solution for the Pleistocene climatic variations. 

Interpretation of Wisconsin glaciers. Our theory provides a new explana- 
tion for the Scandinavian and Siberian ice sheets. According to Flint (23), 
this sheet had a maximum thickness of about 10,000 feet, tapering off 
strongly to the north and east, and less strongly to the south. The com- 
bined lateral extent was from about 5°W at the British Isles to about 
110°E at the Taimyr Peninsula. In view of the long continental path and 
the prominent mountain barriers to the south and west, it seems difficult 
to imagine nourishment from storms arriving from these directions. How- 
ever, nourishment from the north would be provided by an ice-free Arctic 
Ocean, but would be almost impossible with the ice-covered ocean usually 
assumed for the glacial age. The steep north slope of this ice sheet further 
supports the theory that the source of precipitation was to the north. 



A THEORY OF ICE AGES 211 

The Laurentide ice sheet, which covered nearly 5 milhon square miles 
at the Wisconsin maximum (23), extended westward to meet the Cor- 
dilleran glaciers, and eastward to a hne seaward of the present Atlantic 
coast, with its southern boundary along the Missouri and Ohio rivers. 
The maximum thickness has been estimated by Flint {23) to be about 
10,000 feet. Although the northern boundary is not well known, it has 
been assumed to be thin, but recent observations by G. Hattersly-Smith 
{24) give evidence of very severe glaciation on Ward Island and northern 
Ellesmere Land at about 83°N. Here again, as with the northern and west- 
ern margins, it is difficult to explain the sources of nourishment on the 
basis of present-day circulation and an ice-covered Arctic Ocean. However, 
the modified circulation described here provides for sources of precipita- 
tion from the Atlantic and the Arctic oceans, in addition to that coming 
from the south. 

Finally, it is well known that the areas of the northern hemisphere 
covered by Pleistocene glaciers are centered roughly at the northeastern 
coast of Greenland, near the strait through which Atlantic and Arctic 
waters interchange (see, for example, Flint, 23, plate 3). For the most 
part, glaciation in other areas was minor and was controlled directly by 
mountains. The distribution of Pleistocene glaciers again indicates the 
strong influence of both the Arctic and North Atlantic oceans on Pleisto- 
cene continental glaciers. 

Climatic optimum. Evidence for open water in the Arctic Ocean in 
post- Wisconsin time has been accepted by many writers (for example, 
Brooks, 2S) and attributed to the "climatic optimum" or "thermal maxi- 
mum" which many climatologists believe prevailed during the long in- 
terval from about 7000 to 2500 years before the present. The evidence is 
found in part on islands and remote shores where correlation with estab- 
lished chronology is difficult. We suggest that this evidence for an ice-free 
Arctic pertains to the open Arctic we have postulated for Wisconsin time 
rather than to the climatic optimum. 

Other evidence for a climatic optimum is found further south on the 
continents, where it is correlated reliably with post- Wisconsin chronology, 
Although the climatic optimum is correctly dated here, we believe that it 
is a minor climatic fluctuation because it left no conspicuous evidence in 
marine sediments (4, 12). Also, according to Fisk {26), there is no evi- 
dence of higher sea level than the present in the Gulf of Mexico during 
all of post-Pleistocene time. 

Early man in the Americas. The facts about early man in the Americas 
support the idea of an ice-free Arctic during Wisconsin time and hence 
during earlier glacial stages. According to recent prevailing opinion— for 
example, Eiseley, (27)— early man reached Alaska from Siberia in great 
numbers during late Wisconsin time. The usefulness of the accepted land 
bridge between Siberia and Alaska would have been very limited if the 



212 MAURICE EWING AND WILLIAM L. BONN 
Arctic Ocean had been ice-covered and the chmate far colder than at pres- 
ent. The Denbigh Fhnt complex (northwestern Alaska) has been esti- 
mated from geologic correlations to correspond to warm periods either 
earlier than 12,000 years ago or about 8500 years ago {28, 29). Based on 
direct observations of the Denbigh flint work, Giddings (29) concludes: 
"The Bering Strait region was already a culture center at the time of 
deposit of the Denbigh flint layer." Giddings further notes "that most of 
the early flint techniques were distributed primarily on a broad band cen- 
tering at the Arctic Circle; they seldom strayed south." 

We believe that these observations refer to the time of the relatively 
warm and open Arctic Ocean prior to 11,000 years ago. The imphcation 
of a long established culture in the arctic region conflicts strongly with 
the conventional concept of a Wisconsin ice sheet continuous from the 
North Pole to the Ohio River. If the Arctic Ocean were open in Wiscon- 
sin time, we should expect evidence of settlements along most of the 
shores of the Arctic, contemporaneous with those in Alaska. Giddings (29) 
has already pointed out a similarity between cultures for the Denbigh 
complex and settlements in northern Siberia. 

About 11,000 years ago, the break-up of the ice permitted such rapid 
migration of Arctic population southward that the southern tip of South 
America was reached in a few thousand years {27, 30). The initial avenue 
from Alaska was the high plains east of the Rocky Mountains {27, 31), 
which would have no mountain barrier if it commenced in the low un- 
glaciated area north of the Brooks Range and fronting on the Arctic 
Ocean. Following glacial retreat at the close of the Wisconsin stage, the 
route along the high plain east of the Rockies would have opened, while 
that from northern Siberia would have closed as sea level rose and ice 
formed in the Arctic Ocean. Thus, as early man migrated southward, 
continued migration from northern Siberia was cut off. 

INITIATION OF PLEISTOCENE GLACIAL CLIMATE 

Although the theory we have presented attempts to provide an explana- 
tion for the alternations of climate during the Pleistocene epoch, it cannot 
give an explanation for the initiation of cold Pleistocene climate. A solu- 
tion to this problem is offered now. 

Reconsideration of the hypothesis of pole-wandering. Following the 
recognition of the extent and distribution of Pleistocene glaciers, many 
scientists sought an explanation of glacial climates in terms of major shifts 
in the positions of the poles. Much of the early work was summarized in 
1883 by Hann (32), who believed that great secular changes in climate 
could only be accomplished by changes in the earth's axis of rotation. 
Kelvin and other physicists demonstrated that significant pole shifts would 
be impossible in view of the accepted evidence for high rigidity of the 



A THEORY OF ICE AGES 213 

earth, thereby directing most subsequent studies toward alternative ex- 
planations. However, although they used different bases for their hypoth- 
eses, Koppen and Wegener {33, 34) and Milankovitch (35) never aban- 
doned this idea. 

In recent years attention has again been directed toward this hypothesis 
as it became clear that the earth could not be considered as a completely 
rigid body. Thus, Vening Meinesz (36) concluded that, "The forces caus- 
ing tectonic orogeny which are probably exerted by sub-crustal currents 
must have been amply sufficient for a shift of the poles," and he assumed 
a pole shift of many degrees as the basis of his explanation of the major 
fracture pattern of the earth's crust. Runcorn (37) indicated that the 
earth's surface could undergo large displacements relative to the interior 
as a result of convection currents. In considering the direction of mag- 
netic fields indicated by studies of paleomagnetism, Runcorn {38) be- 
lieves that the variation in these fields could be fully explained by pole 
wandering. 

Using a different approach, Jardetzky (39) recently reevaluated the work 
of Milankovich, concluding ". . . there was possible a slow secular dis- 
placem'ent of the crust in space, which was progressive during all geologic 
periods. The cause of the rotation of the crust is the existence of a moment 
of centrifugal forces acting on the crust and due to the asymmetry of the 
distribution of masses in the outer shell." 

The possibility that adequate forces exist to produce relative movement 
between the earth's surface and the interior has led us to reopen the ques- 
tion of the effects of pole wandering on secular changes in climate. It 
should be noted that the poles wander, according to present conception, 
in a relative sense. The differential movement between an outer shell and 
the interior results in different points on the surface assuming the posi- 
tions of the poles. 

Climatic consequences of pole migration. The poles are presently lo- 
cated in positions of extreme thermal isolation, in marked contrast to the 
conditions that would prevail if both were in the open ocean. If the North 
Pole were located in the North Pacific (for example, 35°N and 180°W) 
and the South Pole at the antipodes of this, in the South Atlantic Ocean, 
the free interchange of water with the polar regions would preclude for- 
mation of polar ice caps. The free interchange would further tend to 
equalize temperature extremes both geographically and seasonally. A re- 
sulting weak and uniform latitudinal temperature gradient would occur, 
in contrast to the present zonality. This kind of climate must have pre- 
vailed between the Permian and Pleistocene glaciations (and probably 
during the long intervals between other glacial periods), according to 
inferences made from the geologic record by all authorities. 

Based on different investigations, Kreichgauer {40), Koppen and Wege- 
ner (33), Milankovich {3S), Koppen {34), and Creer et al {41), have all 



214 MAURICE EWING AND WILLIAM L. BONN 
placed the North Pole in the North Pacific Ocean for a long interval 
beginning with the Cambrian. Although Milankovitch gave no dating, the 
other investigators estimated that the pole arrived in the Arctic Ocean 
during the Tertiary. On the basis of the worldwide distribution of corals 
of various ages, T. H. Y. Ma (42) concluded that sudden displacements 
of the solid earth shell with respect to the interior occurred. He also lo- 
cated pre-Cretaceous pole positions at distances of more than 90 degrees 
from their present positions in order to reconcile the fossil record with 
the appropriate climate, and concluded that abrupt shifts of the earth's 
crust during the Tertiary then carried the poles to their present locations. 
From studies of rock magnetization, Hospers (43) concluded that pole 
migration since Eocene time could have amounted to 10 degrees. It is pro- 
posed here that the migration of the poles from an open-ocean environ- 
ment to the thermally isolated arctic and antarctic regions resulted in the 
change from the warm equable climate to the glacial climates of the 
Pleistocene. 

Assuming that the North Pole migrated into the Arctic Ocean, the 
coohng effects of high latitudes would have become concentrated in this 
region owing to the isolation of the Arctic from the other oceans. In the 
same way, the migration of the South Pole from the freely circulating 
southern oceans to the Antarctic continent would have concentrated cool- 
ing effects over the land. Both polar regions became sources of cold 
"polar" air that contrasted strongly with the warm air from equatorial 
regions. The Pleistocene and Recent climates, characterized by marked 
zonality, were thus established. Growth of glaciers requiring for the most 
part only ample precipitation on cold continental regions (for example, 
Haurwitz, 44) was greatly favored by this climate. The Pleistocene type 
of climate may thus be expected to continue as long as the poles remain 
near their present thermally isolated positions. 

The motion of the poles was probably somewhat intermittent. If we 
consider convection to be the mechanism producing this motion, orogenies 
would be good indicators of convectional activity. The beginning of rapid 
polar motion would coincide with the major orogenies at the end of the 
Tertiary, and possibly also at the end of the Cretaceous. The climatic 
oscillations within the Pleistocene were far too rapid to be related to move- 
ments of the pole in and out of the Arctic region. 

As a consequence of the theories proposed, the principal alternations 
between glacial and nonglacial stages would occur in the arctic. Relatively 
minor changes would be expected in the antarctic, resulting primarily from 
the slight warming and cooling of the Atlantic Ocean. Despite these minor 
changes, such as the present decrease in antarctic ice, the theory requires 
a secular increase from zero thickness at the beginning of the Pleistocene 
glacial epoch. There is some evidence that this has occurred. In many parts 



A THEORY OF ICE AGES 215 

of the world, phases of high sea level are recorded by elevated beaches. 
Although there is some disagreement about correlations, many authors— 
for example, Zeuner (43)— identify five or six such beaches, at elevations 
up to about 100 meters. Zeuner showed that a graph of beach elevation 
against time is approximately linear. He recognized that "it seems prob- 
able that this straight line represents a more or less continuous drop of 
sea level in the course of the Pleistocene on which the oscillations due to 
glacial eustasy were superimposed." It is now suggested that this appar- 
ently secular decrease in sea level, with attendant preservation of the 
beaches showing successive decreasing sea level maxima, can be accounted 
for by the secular growth of an antarctic ice cap. Numerous esitmates that 
the total decrease in sea level due to present ice caps is about 60 meters 
have been made; an additional decrease of about 50 meters can be at- 
tributed to thermal contraction of the sea water, if the present mean ocean 
temperature is taken as about 10°C below the Tertiary mean. We can 
thus provide for a secular decrease in sea level of about 100 meters, which 
seems to account for the highest of the elevated beaches. 



CONCLUSIONS 

The theories of the origin of the Pleistocene glacial climate and of the 
glacial and interglacial stages proposed here are in complete harmony with 
the doctrine of uniformitarianism. No external influences or catastrophic 
events are required to initiate or maintain these conditions. It is postulated 
that some mechanical process has caused the poles to migrate to positions 
very favorable for the development of glacial climates. The major changes 
within the Pleistocene are considered here to have resulted primarily from 
the alternations of ice-covered and ice-free states of the surface of the 
Arctic Ocean. 

For the most part, this article pertains to the Pleistocene glacial epoch 
and the warm interval between the Permian and the Pleistocene. Although 
little is known about possible glacial and interglacial stages during the 
Permian and Proterozoic glacial intervals, the initiation of such intervals 
could have been a consequence of the same mechanism as that proposed 
here for the initiation of the Pleistocene. The "warm" periods prevailing 
during the long intervals between the times of glaciations before as well as 
after, the Permian could also be explained, according to the theory pro- 
posed, as a consequence of the location of the poles in regions of freely 
circulating oceans. 

The consequences of the ideas presented are that the Pleistocene chmate 
will continue while the poles maintain their present locations and that 
the Recent epoch can be considered as another interglacial stage. 



216 MAURICE EWING AND WILLIAM L. DONN 



REFERENCES AND NOTES 



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4. C. Emiliani J. Geo/. 63, 538 (1955). 

5. H.E.Suess, Science i23, 355 (1956). 

6. W. Schott, Wzss. Ergeb, deut. atlantischen Expedition Meteor 3, 3 (1935). 

7. N. M. Bramlette and W. H. Bradley, US. Geol. Survey Profess. Paper 196 (1940). 

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9. C. D. Ovey, Roy. Meteorol. Soc. Centennial Proc. (1950). 

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1 1 . , Heidelberger Beitr. Mineral. Petrog. 4 ( 19 54 ) . 

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13. In many of the deep-sea basin areas continuity in deposition is destroyed by the 
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past cannot be determined with precision. However, when it became possible to 
take, or select for study, cores undisturbed by such action [D. B. Ericson et al., 
Geol. Soc. Amer. Spec. Paper No. 62 (1955), pp. 205-220], the pattern of deposi- 
tion where a layer with warm-water fauna overlies a layer with cold-water fauna was 
unmistakable. A second complicating factor is introduced by animals, whose filled 
burrows frequently disturb sediment contacts. Allowance must be made for this 
effect in estimating the rate of change of sediment type. 

14. M. Ewing and D. Ericson, "Studies of cores from the Gulf of Mexico," Prog. Rept. 
Lamont Geol. Observatory (1955). 

15. , Topography and Sediments in the Gulf of Mexico (American Assoc. 

Petroleum Geol., in press) . 

16. B. Heezen and M. Ewing, Bull. Am. Assoc. Petroleum Geol., in press. 

17. R. F. Flint, Am. J. Sci. 253, 5 ( 19 5 5 ) . 

18. W. Broecker and J. L. Kulp, unpublished. 

19. V. A. Berezkin, Morskoi sbornik 4, 105 (1937). 

20. W.L.Stokes, Science 122,815 (1955). 

21. We are grateful to J. Chase for this general confirmation, which was sent in a 
personal communication on 28 June 1955. 

22. A. P. Crary,J. L. Kulp, E. W.Marshall, Science 122, 1171 (1955). 

23. R. F. Flint, Glacial Geology and the Pleistocene Epoch (Wiley, New York, 1947). 

24. G. Hattersly-Smith, Arcfic 8, 1 (1955). 

25. C. E. P. Brooks, Climate through the Ages (McGraw-Hill, New York, 1949). 

26. H. N. Fisk, personal communication. 

27. L. C. Eiseley, Anthrobol. Soc. Washington IS (1955). 

28. D. Hopkins and J. Giddings, Jr., Smithsonian Institution Misc. Collections 121, 
11 (1953). 

29. T- Giddings, Jr., Sci. American 190, 6 (1954). 

30. W. R. Hurt, Jr.,Am.Anfic/uify J8, 3 (1955). 

31. E. Antevs, "The Quaternary of North America," in Regionale Geologie der Erde 
(Akademische Verlagsgesellschaft, Leipzig, 1941 ) . 

32. ].Hann, Handbuch der Klimatologie {\SS3). 

33. W. Koppen and A. Wegener, Die Klimat der geologischen Vorzeit (Berlin, 1924). 

34. W. Koppen, Mefeoro/, Z. 106-110 (1940). 

35. M. Milankovitch, Handbuch der Geophysik (Gebr. Brontrager, Berlin 1938), vol. 
9, pp. 593-698. 

36. F. A. Vening Meinesz, Trans. Am. Geophys. Union 28, 1 (1947) . 

37. S. K. Runcorn, Advances in Physics 4, 4 (1955) . 

38. , Endeavour 14, SS (1955). 



A THEORY OF ICE AGES II 217 

39. W. S. Jardetzky, Trans. Am. Geophys. Union 30, 6 (1949) . 

40. D. Kreichgauer, Die Aquatorfrage in der Geologic (Steyl, Kaldenkirchen, 1902). 

41. K. M. Creer, E. Irving, S. K. Runcorn, J. Geomagn. Geoelect. Kyoto, in press. 

42. T. Y. H. Ma, Bull. Geol. Soc. China 20, 343 (1940); Research on the Past Climate 
and Continental Drift (Privately published, Taiwan, 1952). 

43. J. Hospers, /. Geo/. 63, 1 (1953). 

44. B. Haurwitz and J. Austin, Climatology (McGraw-Hill, New York, 1949) . 

45. F. Zeuner, Dating the Past (Methuen, London, 1950) . 



A Theory of Ice Ages II 

• MAURICE EWING AND WILLIAM L. DONN 



IN A RECENTLY PROPOSED THEORY OF ICE AGES (J) WE 
formulated the thesis that (i) the Pleistocene Ice Age was initiated when 
the North and South poles migrated into the thermally isolated locations 
of the Arctic Ocean and Antarctica, respectively, and that (ii) fluctuations 
of glacial with interglacial climate during the Pleistocene epoch were con- 
trolled primarily by alternation from an ice-free to an ice-covered state of 
the surface waters of the Arctic Ocean. According to this theory, the local 
terrestrial conditions of thermal isolation and adequate precipitation, 
rather than broad, world-wide changes of terrestrial or extraterrestrial ori- 
gin, should be emphasized as the causes of Pleistocene glaciation. 

FURTHER NOTES ON THE NORTHERN HEMISPHERE 

Despite the feeling of some authorities that the effects of an open 
Arctic Ocean would be quantitively insufficient to cause the amount of 
glaciation that existed, the validity of the theory seems to be illustrated 
by present conditions in the Arctic and Antarctic regions. 

Thus, the unexplained glacial conditions which have continued in 
Greenland since the Pleistocene contrast very sharply with the present 
ice-free condition of northern Canada at the same latitudes. The signifi- 
cants geographic difference between Greenland and northern Canada is 
their location with respect to the North Atlantic Ocean. As a result of the 
location of Greenland, there is enough moisture in its atmosphere to cause 
sufficiently heavy precipitation of snow for the maintenance of glacial 
conditions, whereas the very scanty precipitation at the same latitudes in 

• From Science (May 16, 1958), pp. 1159-62. 



218 MAURICE EWING AND WILLIAM L. BONN 

Canada results in the present lack of glaciers there. Also, the precipitation 
in the southern part of Greenland is much heavier than that in the north- 
ern part. Hence, an open Arctic Ocean during the Pleistocene seems to 
be the only geographic condition which could have produced glacial con- 
ditions in northern Canada equivalent to those in Greenland today. Fur- 
ther, the effects of the combination of thermal isolation and adequate 
precipitation can be seen from a comparison of present conditions in the 
Arctic and Antarctic areas. The thick Antarctic icecap contrasts sharply 
with conditions in the Arctic Ocean area, with the exception of those in 
Greenland. This can be explained by (i) the more complete thermal 
isolation of Antarctica than of the Arctic (the condition in the Arctic is 
the result of the small interchange of water between the Arctic and At- 
lantic oceans, without which interchange the Arctic Ocean would have a 
permanently thick frozen cover); and (ii) the availabihty of moisture from 
the surrounding open oceans for snow precipitation on Antarctica. Such 
precipitation is very slight over the nearly completely landlocked Arctic. 

Greenland is similar to Antarctica in being thermally isolated, in being 
bounded largely by open water, and in having an icecap equivalent in 
thickness to that of the icecap in Antarctica. Greenland is also similar to 
Antarctica in that its icecap was probably maintained with little change 
during the Pleistocene interglacial stages. The following observation re- 
corded by Charlesworth (2, vol. 1, p. 94) certainly supports this: "The 
Pleistocene ice sheets had a maximum thickness . . . which significantly 
enough is roughly that of the modern ice sheets of Greenland and Ant- 
arctica." In view of these conditions, we may expect the Greenland and 
Antarctic icecaps to be preserved, with minor fluctuations, as long as the 
Poles are located in their present positions. 

Thus, the present contrast between Greenland and northern Canada 
and that between the Arctic and Antarctic regions, which result from local 
conditions, are comparable to contrasts between glacial and interglacial 
stages and make it a plausible conclusion that the latter are also the 
results of restricted terrestial changes rather than of global or extrater- 
restial causes. 

Further evidence that there was formerly a source of precipitation in the 
Arctic region lies in the position of the glacial divide, as determined from 
indicators of ice movement and glacial rebound. On the basis of indicators, 
J. Tuzo Wilson (3) shows that the divide ran approximately east-west 
through central Canada, except where its course was controlled by topog- 
raphy. Earlier notions based on the highest elevations covered by ice, as 
summarized by Flint (4), and deductions based on the theory that the 
source of nourishment was to the south (5), have placed this ice divide 
much further to the south. Also, detailed studies of ice movement in 
Alberta (6) show that movement was from the north rather than from 
the northeast, as had previously been supposed. 



A THEORY OF ICE AGES II 219 

Although Lee et al. have applied the above data on motion indicators 
to late glacial conditions (7), the data of glacial rebound suggest, also, 
that the North American Wisconsin ice divide lay in the vicinity of Hud- 
son Bay, thus giving independent supporting evidence for the existence 
of a source of precipitation to the north of the terminal moraine line in 
the eastern half of North America. The evidence for uplift northward is 
best given by the elevated beaches of the present Great Lakes and the 
ancient Lake Agassiz (2, vol. 2, p. 132; 4, pp. 250-251; 8). An uplift 
of six to eight inches per 100 miles per century is given for these areas. 
Uplift determined from elevated areas around Hudson Bay reaches a 
maximum of 1000 feet (2, vol. 2, p. 1321) and is continuing at present 
(9) at an undetermined rate. If the data from the Great Lakes region is 
extrapolated through Hudson Bay, it seems clear that a continuous thick- 
ening of ice occurred from the present hinge line northward to the Hud- 
son Bay region. 

Blake's study (13) indicates that rebound in Labrador is less than that 
around Hudson Bay and supports the location of the divide shown by 
Wilson. 

Glacial rebound on northern Ellesmere Island and Ward Hunt Island 
(about 83°N) varies from 100 to 200 feet along the shore to at least 600 
to 700 feet further inland {II), thus approaching the magnitude of the 
uplift at Hudson Bay, far to the south. This suggests and supports still 
further the argument that there must have been a source of moisture in 
the Arctic region. 

THE SOUTHERN HEMISPHERE 

We wish to elaborate here on the statement in part I of this discussion 
( I ) to the effect that Pleistocene glaciation in the Southern Hemisphere 
regions other than Antarctica and the sub-Antarctic islands was mainly 
limited to high elevations and was consequent upon the general cooling 
produced by the much greater change in the Northern Hemisphere. 

From the excellent summaries of Charlesworth (2, vol. 1, p. 44; vol. 1, 
p. 1322) and Flint (4), it is noted that in South America glaciers ex- 
tended along the Andes, with a few gaps, from Cape Horn to Sierra 
Nevada de Santa Marta in Colombia. The glaciers broadened considerably 
on Tierra del Fuego and on the plains east of the mountains in Patagonia. 
Pleistocene glaciation in Africa was confined to the Atlas Mountains of 
French Morocco and the high mountains of Equatorial East Africa, both 
of which areas have perennial snow fields today. In Australia, barely 150 
square miles in the Australian Alps were glaciated, and upland areas of 
South Island (New Zealand), and of Tasmania (both south of 40°S) 
were extensively glaciated. Thus, except for Tasmania and the smaller 
region in Austraha, it is noted by Charlesworth (2, vol. 1, p. 44; vol. 2, p. 



220 MAURICE EWING AND WILLIAM L. BONN 

1322) that Pleistocene glaciers were merely extensions of the glaciers that 
remain today in New Zealand, the Andes, and Africa. Further, with the 
exception of Auckland and the Macquarie Islands, the sub-Antarctic is- 
lands also have existing glaciers which were more extensive in Pleistocene 
time. 

A moderate lowering of the snow line in the Southern Hemisphere, 
which will confirm the foregoing reconstruction of glacial conditions, is 
expected to result from the global cooling produced through the effects of 
glacial and pluvial conditions in the Northern Hemisphere upon the 
radiation and heat budgets of the earth. 

A planetary decrease in the amount of absorbed insolation would re- 
sult from a rise in albedo of the Northern Hemisphere. This would follow 
from the greater reflectivity of the ice in glaciated regions and of the 
clouds in the pluviated regions (the latter to be described in detail below.) 

The areas in the Northern Hemisphere which were ice-covered during 
the Pleistocene glacial stages and are ice-free today cover 10.7 million 
square miles and are distributed around a latitude of 60°N. If we assume 
a mean cloudiness of 60 percent for this region in both glacial and non- 
glacial stages, the albedo in the remaining 40 percent would be raised 
from 10 to 70 during a glacial interval. If 300 calories per square cen- 
timeter per day {12) is taken as the mean insolation received at the 
surface at a latitude of 60°N, then the resulting decrease in insolation 
available for absorption in the glaciated areas is 2.0 X 10^^ calories per 
day. 

Further, as will be described in detail below, about 12 million square 
miles of arid regions were well watered (pluviated) during glacial stages. 
The mean cloudiness of these regions, which are distributed around lati- 
tudes of 30°N and 30°S, is about 20 percent at present. If an 'increase in 
cloudiness to 60 percent (a figure based on present equivalent areas) dur- 
ing the glaciopluvial stages is assumed, the albedo of these desert regions 
would increase from 15 (for sand) to 80 (for clouds). If 470 calories per 
square centimeter per day (IS) is taken as the mean surface insolation, 
the increased albedo would result in a decrease of absorbable insolation of 
4 X 10^^ calories per day for the pluviated zones. A total reduction of 
6.0 X 10^^ calories per day would thus occur for the combination of 
glaciated and pluviated regions. (The difference in albedo between sea ice 
and rough water in high altitudes is so small that no significant change 
in this estimate would occur if the Arctic Ocean were open during a 
glacial stage, as postulated by our theory.) This is a significant percentage 
of the direct insolation of 85 X 10^^ calories per day for the entire earth. 
It is noteworthy that the terrestial changes described seem capable of re- 
ducing the radiation budget of the earth without reliance upon extrater- 
restrial changes, and thus of producing a sufficient degree of cooling to 
bring about glaciation in the Southern Hemisphere. It is also noteworthy 



A THEORY OF ICE AGES II 221 

that the pluviated regions are at least as important as the glacial areas in 
promoting global cooling. In the foregoing calculations, no account has 
been taken of the small effect of absorption in the atmosphere. 

Although it is generally admitted that uplift of land areas result in cool- 
ing of such regions, it should also be noted that a minor contribution to 
the general cooling of the lands would also result from the glacial low- 
ering of sea level, since a change of 300 feet in sea level produces an 
average change in temperature of 1°F. 

ANTARCTICA 

The field evidence available for an estimate of the effect of the conti- 
nental ice budget on Antarctica during Wisconsin time is scanty and 
inconclusive at present. Similarly, the conclusions about the Antarctic ice 
budget that can be derived from existing theories of glaciation are quite 
ambiguous. Thus, the Antarctic icecap appears to be in approximate 
equilibrium at present with regard to height {13) and lateral extent. Yet 
evidence in the form of exposed glaciated mountain areas exists to indicate 
that there was a former higher equilibrium stand of the icecap. Most au- 
thorities place this higher stand in Wisconsin time. Theories of glaciation 
require the assumption that, for the most part, the ocean and air surround- 
ing Antarctica were cooler during glacial stages. Such conditions would 
produce a decrease in snow precipitation over Antarctica which would 
more than offset the decrease in wastage which results from lowering of 
temperatures. It is difficult to conceive of there having been glacial growth 
on frigid Antarctica during times when the surrounding environment was 
cooler than it now is. It seems more reasonable to suppose that former 
higher levels of the icecap were a result of growth during interglacial 
stages or even during the more recent climatic optimum. Possibly the 
continuing study of Antarctica will provide information for the dating of 
this higher stand; this is at present an unsolved problem. 

PLUVIAL STAGES 

The effect of the Pleistocene conditions of moisture in presently arid 
areas is second in importance only to the contemporaneous glaciation in 
higher latitudes. The major desert areas, which are today uninhabited 
barren wastes, although they occupy a very large part of the temperate 
zones, were formerly fertile, well-watered lands (J4). These areas, which 
were often covered by very large lakes, include the Sahara and Arabian 
deserts, the desert of central Asia, and the Austrahan Kalahari, the North 
American, the Atacama, and the Patagonian deserts. No theory of glacia- 
tion and no investigation of Pleistocene glacial stages would be complete 



222 MAURICE EWING AND WILLIAM L. DONN 

without an explanation of the pluvial stages and their relation to glacia- 

tion. 

Although there is a considerable amount of evidence which suggests 
strongly that pluvial and glacial conditions occurred simultaneously, the 
most positive evidence for this comes from Lake Lahontan in western 
North America (IS). 

The Lahontan data refer only to the end of the last glacial stage, but 
very strong evidence for glacial-pluvial simultaneity comes from observa- 
tions around the Caspian and Black seas. According to P. F. Fedorov, 
every transgression of the Caspian Sea which occurred during glacial ad- 
vances of Pleistocene time coincides, without exception, with a regression 
of the Black Sea (16); hence, it seems that pluviation was contem- 
poraneous with glacial lowering of sea level throughout the Pleistocene 
period. 

The predominant cause of present-day deserts is their location in either 
the belt of subtropical calms (the horse latitudes) or in the trade wind 
zone marginal to this belt; in these zones the dry air moves equatorward, 
becoming warmer and thereby able to carry increased amounts of moisture. 
A secondary cause is the location of these deserts on the lee sides of 
mountains and along coasts bathed by cool ocean waters. Some desert 
areas are the result of a combination of all these causes. 

The higher stands of many lakes and rivers during the glaciopluvial 
stages were the result of the snows and meltwater of adjacent glaciers. 
But the largest of these pluviated regions, including most of the present- 
day deserts, were so remote from glaciated areas that the cause of pluvial 
conditions must be other than simple proximity to glaciers. The fact that 
there has been widespread rainfall in the past over broad areas which are 
not only arid at present but which lie in climatic zones where conditions 
are basically unfavorable for the formation of rain in significant amounts 
indicates strongly that a fundamental modification of the atmospheric cir- 
culation must have occurred during the glaciopluvial stages. 

In part I of this discussion ( I ) , the theory was advanced that the pres- 
ent north-polar high-pressure area is a reversal from a polar low, which 
resulted from the contrast in temperature between the relatively warm, 
open Arctic Ocean and the surrounding cold, glaciated continents. Fur- 
ther, it was stated that the Iceland low-pressure area, which at present 
weakens in summer and intensifies in winter, probably migrated southward 
during glacial stages. By an extension of the reasoning involved, it is pos- 
sible to construct a model of modified circulation which could account for 
the pluvial conditions that have been described for the present major 
desert areas. The critical changes in circulation, which have been de- 
scribed, in principle, by a number of investigators in the past, are outlined 
below. 

1) During a glacial stage, the Iceland "low" of the North Atlantic 



A THEORY OF ICE AGES II 223 

would migrate southward and would maintain present winter intensity all 
year as a result of the perennial temperature contrast between the cold 
glaciated continents and the relatively warm ocean. Increased storm in- 
tensity and frequency would therefore persist throughout the year, the 
paths of the storms being deflected far to the south of the present paths, 

2) At present, the belts of subtropical calms (the horse latitudes) are 
located at approximately 30°N and 30°S and show greater intensity over 
the oceans in summer than in winter. During a glacial stage, this zone 
would also migrate southward in the strongly glaciated Northern Hemi- 
sphere and would probably weaken over the oceans because of the per- 
sistence of cold conditions over the continents. 

3) The combination of an icecap extending into the middle latitudes, or 
present Temperate Zone, plus the southward migration of both the Ice- 
land low and the horse latitudes would result in the southward displace- 
ment of the entire zone of the prevailing westerlies wind belt and hence 
of the entire belt of migratory cyclonic storms which predominate in this 
belt. These storms would consequently travel well into the regions which, 
at present, are deserts because they lie in the dry horse latitudes and 
adjacent areas, 

4) Owing to the changes described above, polar air masses originating 
over the icecaps in the middle latitudes would tend to meet the extremely 
moist equatorial air much more frequently than at present, thereby gen- 
erating very intense storms which would yield the very high precipitation 
characteristic only of hurricanes today. 

5) Although in general it would be cooler than at present as a result of 
widespread global cooling during a glacial stage, the low-pressure doldrum 
belt would become relatively stronger through contrast with the very cold 
belt of the middle latitudes. Further, this belt, now located north of the 
equator in the vicinity of continents, would probably be displaced some- 
what to the south of the equator as a result of the pronounced cooling 
of the northern continents. This would tend to increase the amount of 
moisture over the present desert regions of the low southern latitudes of 
South America and Africa, thus increasing precipitation over the deserts 
of South Africa and the west coast of South America. 

6) As a result of the present monsoon pattern in southern Asia and the 
Indian Ocean, the doldrums are located over Austraha during the north- 
ern winter. With glacial conditions existing over the northern continents, 
the present winter-type pattern would tend to become semi-permanent, 
bringing considerably more moisture and precipitation to Australia. It is 
a well-known fact, established from the fossil record [see Benson {17)], 
that, during the Pleistocene, large fauna with tropical affinities inhabited 
Australia. This and the pluvial conditions of central Australia can be 
explained by the theory of the change in circulation; the small high-alti- 
tude glacier of southern Austraha could have existed in much the same 



224 GILBERT N. PLASS 

manner as do equatorial glaciers on the mountain areas of Africa and 
South America at present (J8). 

REFERENCES AND NOTES 

1. M. Ewing and W. L. Donn, Science 123, 1061 (1956). Owing to an oversight, 
the value given (page 1066) for the thermal contraction of the oceans from a 10°C 
drop in temperature, during late Tertiary is about eight times too large. 

2. J. K. Charlesworth, The Quaternary Era (Arnold, London, 1957). 

3. J. T. Wilson, glacial map, in R. F. Flint, Glacial and Pleistocene Geology (Wiley, 
New York, 1957). 

4. R. F. Fhnt, Glacial and Pleistocene Geology (Wiley, New York, 1957) . 

5. W.F.Tanner, Science 122, 642 (1955). 

6. C. P. Gravenor and R. B. Ellwood, Research Council Alberta {Can.) Prelim, Kept. 
57-1 (1957). 

7. H. A. Lee, B. Craig, J. G. Fyles, Geol. Soc. Am. Abstr. (1957), pp. 90, 91. 

8. L. V. Pierson and C. S. Schuchert, A Textbook of Geology (Wiley, New York, ed. 
3, 1929 ) , p. 302; B. Gutenberg, Bull. Geol. Soc. Am. 52, 743 ( 1941 ) . 

9. J. T. Wilson, personal communication. 

10. W. Blake, Jr., Science 121, 112 (1955). 

11. G. Hattersley-Smith, Arctic 8, 26 (1955); R. L. Christie, Geol. Survey Paper Can., 
56-9 (1957). 

12. F. A. Berry, E. Bolloy, N. Beers, Eds., Handbook of Meteorology (McGraw-Hill, 
New York, 1945). 

13. R. Revelle, H. V. Sverup, W. Munk, Abstr. in Trans. Am. Geophys. Union 36, 31 
(1935). 

14. For numerous references supporting and documenting this statement see 2, vol. 2, 
eh. 41. 

15. W. S. Broeker and P. C. Orr, "The radiocarbon chronology of Lake Lahontan and 
Lake Bonneville," Bull. Geol. Soc. Am., in press. 

16. P. V. Fedorov, "Quaternary stratigraphy and history of the Caspian Sea." Isvest. 
Akad. Nauk S.S.S.R., Ser. Geol. No. J ( 19 57 ) . 

17. W. N. Benson, Rept. Australian New Zealand Assoc. Advance Sci. ISth Meeting 
(J92J), pp. 45-128. 

18. This article is Lamont Geological Observatory Contribution No. 288. 



Carbon Dioxide and the Climate 

• GILBERT N. PLASS 



SCIENTISTS HAVE LONG BEEN FASCINATED WITH THE 
problem of explaining variations in the climate. For at least nine-tenths of 
the time since the beginning of recorded geological history, the average 
temperature of the earth has been higher than it is today. Between these 

• From American Scientist (July, 1956), pp. 302-16. 



CARBON DIOXIDE AND THE CLIMATE 225 

warm epochs there have been severe periods of glaciation which have 
lasted a few milhon years and which have occurred at intervals of roughly 
250,000,000 years. Of more immediate interest to us is the general warm- 
ing of the climate that has taken place in the last sixty years. 

Theories of chmatic change are exceedingly numerous. Is it possible that 
any one of these theories can explain most of the known facts about cli- 
mate? The most widely held theories at the present time call upon varia- 
tions in the solar energy received by the earth, changes in the amount of 
volcanic dust in the atmosphere, and variations in the average elevation 
of the continents. Although it is entirely possible that changes in each of 
these factors may have had an influence on the earth's chmate at par- 
ticular times and places, none of these theories alone seems able to ex- 
plain a majority of the known facts about world-wide climatic variations. 

Although the carbon dioxide theory of climatic change was one of the 
most widely held fifty years ago, in recent years it has had relatively 
few adherents. However, recent research work suggests that the usual 
reasons for rejecting this theory are not valid. Thus it seems appropriate 
to reconsider the question of variations in the amount of carbon dioxide 
in the atmosphere and whether it can satisfactorily account for many of 
the world-wide climatic changes. 

Because of the relatively low temperatures at the earth's surface and 
in the atmosphere, virtually all of the outgoing radiation from the earth 
to space is in the infrared region of the spectrum. Thus it is important to 
know which constituents of the atmosphere absorb in the infrared. The 
three most abundant gases in our atmosphere are oxygen, nitrogen, and 
argon. However, none of these three gases absorb appreciably in the rele- 
vant spectral region in the infrared. If these were the only gases in our 
atmosphere, our climate would be considerably colder than it is today. The 
heat radiated from the surface of the earth would not be stopped in its 
passage out to space with the result that the earth's surface would cool 
rapidly. 

Fortunately for us, three other gases occur in our atmosphere in rela- 
tively minute quantities: carbon dioxide, water vapor, and ozone. Unlike 
the more abundant gases, all three of these rarer gases absorb strongly 
over at least a portion of the infrared spectrum. The concentration of 
carbon dioxide in the atmosphere is about 0.03 per cent by volume; it is 
fairly uniformly mixed as high as accurate measurements have been made. 
Water vapor and ozone also exist in very small concentrations in the 
atmosphere, but the exact amount that is present varies with time and 
place. 

The infrared absorption properties of carbon dioxide, water vapor, and 
ozone determine our climate to a large extent. Their action has often 
been compared to that of a greenhouse. There the rays of the sun bring 
the heat energy in through the transparent glass. However, the outgoing 



226 GILBERT N. PLASS 

heat energy from the plants and other objects in the greenhouse is in the 
infrared where glass is largely opaque. The heat energy is fairly effectively 
trapped inside the greenhouse and the temperature is considerably warmer 
than outside. 

In a similar manner the temperature at the surface of the earth is con- 
trolled by the transparency of the atmosphere in the visible and infrared 
portions of the spectrum. The incoming radiation from the sun in the 
visible portion of the spectrum reaches the surface of the earth on a clear 
day with relatively little attenuation since the atmosphere is transparent 
to most frequencies in the visible. However, in order to have a warm cli- 
mate, this heat energy must be held near the surface of the earth and can- 
not be reradiated to space immediately. The atmosphere is opaque or 
partially opaque to a large range of frequencies in the infrared because 
of the absorption properties of the three relatively rare gases described 
above. Thus radiation emitted by the earth's surface cannot escape freely 
to space and the temperature at the surface is higher than it would be 
otherwise. The atmosphere has just the same properties as the glass in 
the greenhouse. The carbon dioxide theory states that, as the amount of 
carbon dioxide increases, the atmosphere becomes opaque over a larger 
frequency interval; the outgoing radiation is trapped more effectively near 
the earth's surface and the temperature rises. The latest calculations show 
that if the carbon dioxide content of the atmosphere should double, the 
surface temperature would rise 3.6°C. and if the amount should be cut 
in half, the surface temperature would fall 3.8°C. 

The carbon dioxide theory was first proposed in 1861 by Tyndall. The 
first extensive calculations were necessarily done by very approximate 
methods. There are thousands of spectral lines due to carbon dioxide 
which are responsible for the absorption and each of these lines occurs in 
a complicated pattern with variations in intensity and the width of the 
spectral lines. Further the pattern is not even the same at all heights in 
the atmosphere, since the width and intensity of the spectral lines varies 
with the temperature and pressure. Only recently has a reasonably ac- 
curate solution to the problem of the influence of carbon dioxide on 
surface temperature been possible, because of accurate infrared measure- 
ments, theoretical developments, and the availability of a high-speed elec- 
tronic computer. 

The fact that water vapor absorbs to some extent in the same spectral 
interval as carbon dioxide is the basis for the usual objection to the carbon 
dioxide theory. According to this argument the water vapor absorption is 
so large that there would be virtually no change in the outgoing radiation 
if the carbon dioxide concentration should change. However, this conclu- 
sion was based on early, very approximate treatments of the very complex 
problem of the calculation of the infrared flux in the atmosphere. Recent 
and more accurate calculations that take into account the detailed struc- 



CARBON DIOXIDE AND THE CLIMATE 227 

ture of the spectra of these two gases show that they are relatively inde- 
pendent of one another in their influence on the infrared absorption. 
There are two main reasons for this result: (1) there is no correlation 
between the frequencies of the spectral lines for carbon dioxide and water 
vapor and so the lines do not often overlap because of nearly coincident 
positions for the spectral lines; (2) the fractional concentration of water 
vapor falls off ver}' rapidly with height whereas carbon dioxide is nearly 
uniformly distributed. Because of this last fact, even if the water vapor 
absorption were larger than that of carbon dioxide in a certain spectral 
interval at the surface of the earth, at only a short distance above the 
ground the carbon dioxide absorption would be considerably larger than 
that of the water vapor. Careful estimates show that the temperature 
changes given above for carbon dioxide would not be reduced by more 
than 20 per cent because of water vapor absorption. 

One further objection has been raised to the carbon dioxide theory: the 
atmosphere is completely opaque at the center of the carbon dioxide 
band and therefore there is no change in the absorption as the carbon 
dioxide amount varies. This is entirely true for a spectral interval about 
one micron wide on either side of the center of the carbon dioxide band. 
However, the argument neglects the hundreds of spectral lines from car- 
bon dioxide that are outside this interval of complete absorption. The 
change in absorption for a given variation in carbon dioxide amount is 
greatest for a spectral interval that is only partially opaque; the tempera- 
ture variation at the surface of the earth is determined by the change in 
absorption of such intervals. 

Thus there does not seem to be a fundamental objection to the carbon 
dioxide theory of climatic change. Further the temperature changes given 
by the theory for reasonable variations in the carbon dioxide amount are 
more than enough to cause noticeable changes in the climate. It is not 
usually appreciated that very small changes in the average temperature 
can have an appreciable influence on the climate. For example, various 
authorities estimate that, if the average temperature should decrease from 
1.5 to 8°C., the glaciers would again form over an appreciable fraction of 
the earth's surface. Similarly a rise in the average temperature of perhaps 
only 4°C. would bring a tropical climate to most of the earth's surface. 

Before discussing in detail the carbon dioxide theory of climatic change 
it is first necessar}' to study the various factors that enter into the carbon 
dioxide balance, including the exchange of carbon dioxide between the 
oceans and the atmosphere. 

The largest loss of carbon dioxide from the atmosphere is due to the 
process of photosynthesis which uses about 60 X 10^ tons per year. In a 
steady state precisely the same amount of carbon dioxide is returned to 
the atmosphere each year by all the processes of respiration and decay of 
plants and animals, provided only that none is permanently lost in the 



228 GILBERT N. PLASS 

form of new coal, oil, and other organic deposits. At the present time, at 
least, this loss is very small (0.01 X 10^ tons per year) and can be neg- 
lected for all practical purposes. If this steady state of absorption and 
emission of carbon dioxide by the organic world is disturbed, for ex- 
ample, by a sudden increase of carbon dioxide in the atmosphere, it is 
known that the amount used in photosynthesis would then increase. How- 
ever, after a very few years the processes of decay and respiration would 
also have increased. Since an average carbon atom that has been used in 
photosynthesis returns to the atmosphere from the biosphere in about 10 
years and virtually all of the carbon atoms return in 250 years, it follows 
that the factors influencing the carbon dioxide balance from the organic 
world would again be in balance in a very few years. 

The two most important contributing factors from the inorganic world 
are the release of carbon dioxide from the interior of the earth by hot 
springs, volcanoes, and other sources and the formation of carbonates in 
the weathering of igneous rocks. They happen to be nearly in balance to- 
day. The first one adds and the second subtracts about 0.1 X 10^ tons per 
year to the atmosphere. Thus it appears that as far as natural factors are 
concerned, the amount of carbon dioxide taken out of the atmosphere 
is very nearly equal to the amount returned to it. The specific numbers 
given in this section are only order of magnitude estimates. The values 
given here are averages of some of the more careful estimates. 

Recently, however, man has added an important new factor to the 
carbon dioxide balance. As first pointed out by Callendar, the combustion 
of fossil fuels is adding 6.0 X 10^ tons per year of carbon dioxide to the 
atmosphere at the present time and the rate is increasing every year. To- 
day this factor is larger than any contribution from the inorganic world. 
Thus today man by his own activities is increasing the carbon dioxide in 
the atmosphere at the rate of 30 per cent a century. The possible influence 
of this on the climate will be discussed later. 

The oceans contain a vast reservoir of carbon dioxide; some of it is 
in the form of dissolved gas, but it consists mostly of carbonates in various 
degrees of ionization. From the known dissociation constants for sea water, 
it is possible to calculate the atmospheric carbon dioxide pressure that is 
in equilibrium with a given amount of carbonate in the oceans. At the 
present time the carbon dioxide pressure is about 3 X 10"^ atm.; there 
are 2.3 X 10^^ tons of carbon dioxide in the atmosphere and 130 X 10^^ 
tons of carbon dioxide and carbonates in the oceans. Thus the oceans con- 
tain over fifty times as much carbon dioxide as the atmosphere. If condi- 
tions should change, the oceans can add to or subtract from the amount in 
the atmosphere. 

Kulp has recently shown from radiocarbon determinations that the deep 
ocean waters at the latitude of Newfoundland were at the surface 1700 
years ago. This suggests that it may take tens of thousands of years for the 



CARBON DIOXIDE AND THE CLIMATE 229 

waters of the deep ocean to make one complete circuit from the surface 
to the bottom and back. Only the surface waters of the oceans can absorb 
carbon dioxide directly from the atmosphere. Since there is very little 
circulation between the surface waters and the ocean depths, the time for 
the atmosphere-ocean system to return to equilibrium following a disturb- 
ance of some sort is at least as long as the turnover time of the oceans. 
Thus, if the atmospheric carbon dioxide amount should suddenly increase, 
it may easily take a period of tens of thousands of years before the at- 
mosphere-ocean system is again in equilibrium. 

Let us next examine some of the variations in the atmospheric carbon 
dioxide amount in past geological epochs and their correlation with the 
climate as deduced from the geological record. It is interesting that a 
large number of these climatic variations can be explained simply and 
naturally by the carbon dioxide theory. 

During the last glacial epoch of perhaps a million years' duration, four 
distinct periods of glaciation separated by warmer interglacial periods 
have long been recognized. Recently Wiseman has studied the sediments 
of the deep ocean floor and has found evidence for ten distinct tempera- 
ture minima within the last 620,000 years. It appears that a fundamental 
property of a glacial epoch is to have a climate that is continually fluc- 
tuating. The glaciers advance and then recede and repeat the cycle sev- 
eral times before the end of the glacial epoch. No other theory of climatic 
change seems able to explain in a simple and straightforward manner these 
continual oscillations in climate during a million-year epoch of glaciation. 

In order to understand these oscillations let us consider the figure where 
the equilibrium pressure of the carbon dioxide in the atmosphere is 
plotted against the total amount of carbon dioxide in the atmosphere- 
ocean system. These curves were calculated as described above with the 
additional assumption that the average temperature varies as predicted by 
the carbon dioxide theory. Curves are shown when the oceans have 0.90, 
0.95, and 1.00 times their present volume in order to allow for the fact 
that the ocean volume decreases during a period of glaciation. 

The present value for the carbon dioxide pressure (3 X 10"^ atm.) and 
the total amount of carbon dioxide in the atmosphere-ocean system 
(1.32 X 10^4 tons) is marked with the letter "P" in the figure. Let us 
suppose that a million years ago the carbon dioxide balance was upset and 
that the total amount of carbon dioxide in the atmosphere-ocean system 
was reduced 7 per cent to 1.23 X 10^^ tons and that it remained Gxed at 
this new lower value throughout the ensuing glacial period. Let us further 
assume that if the average temperature should fall 3.8°C. that great ice 
sheets would again form and cover sizable portions of the continents. With 
the reduced carbon dioxide amount the atmosphere-ocean system would 
finally come to equilibrium at the point "G" in the figure. The new at- 
mospheric carbon dioxide pressure would be 1.5 X 10"* atm. This would 



230 GILBERT N. PLASS 




TOTAL CARBON DIOXIDE AMOUNT IN ATMOSPHERE -OCEAN SYSTEM CtONS) 



The equilibrium pressure of carbon dioxide in the atmosphere as a func- 
tion of the total amount of carbon dioxide in the atmosphere-ocean 
system. Curves are shown when the oceans have 0.90, 0.95 and 1.00 times 
their present volume. The present value for these quantities is indicated 
by the letter "P." The line between "G" and "N" represents a possible 
oscillation of the climate during a glacial period. 



reduce the surface temperature by 3.8°C. according to this theory; this 
would be sufficient to start a period of glaciation. 

Let us assume in agreement with the estimates of glacial authorities 
that the glaciers contain about 5 per cent of the water of the oceans when 
the ice sheets have reached their maximum development. Since only small 
amounts of carbonates are held permanently in glacial ice, the loss of this 
water by the oceans means that the oceans now contain too much car- 
bonate for their reduced volume. They release carbon dioxide, thus in- 
creasing the amount in the atmosphere. 

The atmosphere-ocean system again reaches equilibrium at the point 
"N" in the figure some tens of thousands of years later. This point rep- 
resents the equilibrium conditions when the ocean volume is 95 per cent 
of its present value and the atmospheric carbon dioxide pressure is 2.5 X 
10"^ atm. However, when the carbon dioxide pressure reaches this value, 
the average surface temperature rises to virtually its present value. It is 
then too warm to maintain the glaciers and they start to melt. This process 
probably takes thousands of years, but finally the oceans return to their 
original volume. Now the oceans do not contain sufficient carbonates for 
their increased volume; the atmosphere-ocean system is no longer in equi- 
librium. The oceans absorb additional carbon dioxide from the atmos- 



CARBON DIOXIDE AND THE CLIMATE 231 

phere until after tens of thousands of years the system is again near 
equilibrium at the point "G" in the figure. The reduced atmospheric car- 
bon dioxide pressure now causes the surface temperature to fall 3.8°C. 
and another ice sheet starts to form. This cycle continues indefinitely as 
long as the total carbon dioxide amount in the atmosphere-ocean system 
remains fixed at 1.23 X 10^^ tons. The period for one complete cycle 
depends on the rate of circulation of the oceans, but may be very roughly 
estimated as 50,000 years or more. 

The climate must continually oscillate from a glacial to an interglacial 
period until the total carbon dioxide amount is again increased by a change 
in one of the factors in the carbon dioxide balance. When the total carbon 
dioxide amount is reduced slightly below its present value, there is no 
stable state for the climate; it must continually oscillate. On the other 
hand, if some event should greatly reduce the total carbon dioxide amount 
(perhaps by 30 per cent or more), a permanent period of glaciation with- 
out these oscillations would be possible. In order to explain the various 
stages in this cycle more clearly, specific numbers have been assumed. How- 
ever, it may be verified easily that none of the conclusions that have been 
reached depend in a critical way on the particular numbers that were 
chosen. It should also be pointed out that, if there is sufficient time in the 
various stages of the cycle for the oceans to come to equilibrium with cal- 
cium carbonate, the form of the curves in the figure is somewhat changed, 
but none of the conclusions reached above is essentially altered. 

In addition to lower temperatures, increased precipitation is also neces- 
sary for the formation of extensive glaciation. Most theories of climatic 
change have found it very difficult to explain this increased precipitation. 
For example, in the variable sun theory, a decrease in the sun's radiation 
reduces the surface temperature. However, this also reduces the energy 
available to drive the general circulation of the atmosphere. A decreased 
circulation presumably means decreased cloud formation and precipita- 
tion. In order to account for the increased precipitation an ingenious, but 
unconvincing modification of the variable sun theory states that glacial 
periods result from an increase in the sun's radiations. The slightly in- 
creased average temperatures are suDDOsed to be compensated by the 
greater precipitation. 

The carbon dioxide theory provides a simple, straight-forward explana- 
tion for the increased precipitation during a glacial epoch. One of the 
parameters that determines the amount of precipitation from a given cloud 
is the radiant loss of heat energy from the upper surface of the cloud. 
If this radiation loss increases, the temperature at the upper surface of 
the cloud decreases. This increases the temperature difference between the 
upper and lower surface of the cloud, which in turn increases the convec- 
tion in the cloud. Because of these more vigorous convection currents, it 
is more likely that rain will fall from the cloud. Thus on the average there 



232 GILBERT N. PLASS 

is more rainfall from a given cloud if the radiation loss from its upper 
surface increases. 

According to the carbon dioxide theory there is a smaller than normal 
amount of carbon dioxide in the atmosphere when glaciers are beginning 
to form. Not only the surface of the earth, but also the upper surface of a 
cloud is cooler, since they can lose heat energy more rapidly to space. 
Recent calculations show that the upper surface of a cloud at a height 
of 4 km is 2.2°C. cooler when the carbon dioxide pressure is half the pres- 
ent value. Further the upward flux of radiation that strikes the lower 
surface of the cloud is larger when the carbon dioxide amount is reduced; 
thus the lower surface of the cloud is warmer than before. Thus, the 
larger temperature difference between the upper and lower surfaces of the 
cloud causes increased convection in the cloud; the level of precipitation 
should increase appreciably. Thus, according to the carbon dioxide theory, 
colder and wetter climates occur together. 

There is considerable geological evidence that extensive outbursts of 
mountain building occurred several millions of years before each of the 
last two major glacial epochs. Again the carbon dioxide theory seems to 
be the only theory that suggests a reason for the time lag between these 
two events. During a major period of mountain building, tremendous 
quantities of igneous rock are exposed to weathering. In mountainous 
country the zone for the active disintegration of rock extends much farther 
beneath the surface than it does in flat country. The weathering of ig- 
neous rock changes it into carbonates, thus removing carbon dioxide from 
the atmosphere. 

The explanation of the time lag in terms of the carbon dioxide theory 
is that large quantities of carbon dioxide are removed from the atmosphere 
by the increased weathering after a period of major mountain building. 
After some millions of years, the carbon dioxide content of the atmosphere 
is reduced sufficiently to bring on a period of glaciation. From estimates 
of the increased weathering that occurs after the uplift of a mountain 
range, it is found that the time lag is of the order of a million years. 

However, during an epoch of mountain building greatly increased 
amounts of carbon dioxide must be released from the interior of the earth 
into the atmosphere through volcanic vents and hot springs. Additional 
millions of years are required to use up this additional carbon dioxide by 
the process of weathering. Thus the actual time interval between the on- 
set of an epoch of mountain building and the ensuing glaciation can be 
considerably greater than a million years, if large additional quantities of 
carbon dioxide are released from the interior of the earth. Indeed, if these 
amounts are very large, weathering would be unable to reduce the atmos- 
pheric carbon dioxide content to a sufficiently low level to cause a glacial 
period. In fact some periods of mountain building have not been followed 



CARBON DIOXIDE AND THE CLIMATE 233 

by extensive glaciation. Such theories of glacial change as the variation in 
the amount of volcanic dust in the atmosphere and the change in the 
average elevation of the lands have found it difficult to explain why the 
glaciers do not form immediately after the uplift of a major mountain 
range. 

During the geological history of the earth the amount of carbon dioxide 
lost from the atmosphere in the formation of coal, oil, and other organic 
deposits has varied widely. This loss is relatively minor today. On the other 
hand it would be especially large during a period such as the Carboniferous 
when there where extensive marshes and shallow seas. At the end of the 
Carboniferous the atmospheric-carbon dioxide content may have been re- 
duced to a very low level because of the tremendous quantities that had 
been used in the newly formed coal and oil deposits. It is perhaps sig- 
nificant that the glaciation at the end of the Carboniferous may have 
been the most severe in the earth's history. 

Radiocarbon dating indicates that recent changes in climate have been 
contemporaneous in both hemispheres. In the last fifty years virtually all 
known glaciers in both hemispheres have been retreating. According to 
the carbon dioxide theory, such changes in climate should occur at the 
same time in both hemispheres. The exchange of air between the two 
hemispheres is relatively rapid. Even if the atmospheric carbon dioxide 
content should increase suddenly in one hemisphere through a variation 
of some factor that enters into the carbon dioxide balance, the amount in 
the two hemispheres should again be equal in a relatively short interval on 
the geological time scale, perhaps no more than a few decades. It should 
be mentioned that it is possible to have glaciation in one hemisphere and 
not the other even though the atmospheric carbon dioxide amounts are 
the same. If one hemisphere has extensive mountain ranges and the other 
is relatively flat, glaciers could spread from the mountainous region of one 
hemisphere, whereas they would be unable to form on the more level 
land of the other hemisphere at the same average temperature. 

The carbon dioxide theory has given plausible explanations for the be- 
ginning of a glacial period and of the climatic oscillations that occur dur- 
ing a glacial period. What increases the total carbon dioxide amount suf- 
ficiently to bring a glacial period to an end? One possibility is that the 
rock weathering is slowly reduced because of the increasing flatness of the 
land. In addition extensive glaciation probably reduces the rate of weather- 
ing for the fraction of the land surface that is covered by the glaciers. 
Thus, as the loss of carbon dioxide from the atmosphere for weathering 
decreases as a glacial epoch nears its end, the amount of atmospheric car- 
bon dioxide slowly increases until finally the surface temperature is too 
high to allow further growth of the glaciers. An extensive period of moun- 
tain building has occurred at intervals of roughly 250,000,000 years during 
the earth's history and a glacial period has followed in each case during 



234 GILBERT N. PLASS 

the time interval when sufficient carbon dioxide was removed from the 

atmosphere. 

What is the reason for the recent temperature rise that is found through- 
out the world? Will this trend toward warmer climates continue for some 
time? The carbon dioxide theory may provide the answer. We have dis- 
cussed the burning of fossil fuels which is adding more than 6x10^ tons 
per year of carbon dioxide to the atmosphere. If all of this extra carbon 
dioxide remains in the atmosphere, the average temperature is increasing 
at the rate of 1.1 °C, per century from this cause. Since 1900 a careful 
study of world temperature records shows that the average temperature 
has been increasing at roughly this rate. Of course, the agreement between 
these numbers could be merely a coincidence. 

As the concentration of carbon dioxide in the atmosphere increases, 
there are two factors in the carbon dioxide balance that can change. First 
the oceans absorb more carbon dioxide in order to come to equilibrium 
with the larger atmospheric concentration. However, only the surface 
waters can absorb this gas and because of the slow circulation of the 
oceans, it probably takes at least ten thousand years for this process to 
come to equilibrium. Whenever the carbon dioxide amount is increasing 
an upper limit for the amount absorbed by the oceans can be found at any 
time by assuming that the atmosphere-ocean system is always in equilib- 
rium. The actual amount absorbed by the oceans will be considerably less 
than the amount calculated in this manner for at least several centuries 
after a sudden increase in the atmospheric carbon dioxide amount. In the 
first few centuries the surface ocean waters can absorb only a relatively 
small fraction of the additional carbon dioxide. 

The second factor that can change is the amount used in photosyn- 
thesis. A higher level of photosynthetic activity can be supported by the 
increased carbon dioxide amount. As previously discussed, this process tem- 
porarily withdraws some of the additional carbon dioxide from the at- 
mosphere into the organic world. However, in a relatively few years the 
increased rates of respiration and decay bring this process back into equi- 
librium and only a relatively small amount of carbon dioxide is perma- 
nently lost from the atmosphere. Thus it appears that a major fraction of 
the additional carbon dioxide that is released into the atmosphere remains 
there for at least several centuries. 

Even if there may be some question as to whether or not the general 
amelioration of the climate in the last fifty years has really been caused 
by increased industrial activity, there can be no doubt that this will be- 
come an increasingly serious problem as the level of industrial activity in- 
creases. In a few centuries the amount of carbon dioxide released into the 
atmosphere will have become so large that it will have a profound in- 
fluence on our climate. 

After making allowance for industrial growth, a conservative estimate 



CARBON DIOXIDE AND THE CLIMATE 235 

shows that the known reserves of coal and oil will be used up in about 
1000 years. If this occurs, nearly 4 X 10^^ tons of carbon dioxide will 
have been added to the atmosphere; this is seventeen times the present 
amount. The total amount in the atmosphere-ocean system will have in- 
creased from 1.32 X 10^* tons to 1.72 X 10^^ tons. Even if the atmos- 
phere-ocean system is assumed to be in equilibrium at the end of the 
thousand year period, the atmospheric carbon dioxide pressure will be 
3 X 10"^ atm., which is ten times the present value; the corresponding 
increase in the temperature from this cause will be 13.4°C. If it is further 
assumed that there would be sufficient time for the calcium carbonate 
to dissolve and come to equilibrium in the oceans, the atmospheric pres- 
sure will be 1.1 X 10"^ atm. and the temperature rise 7.0°C. This last 
figure is a lower limit for the temperature rise that will occur because of 
man's industrial activities; the actual temperature rise must be larger since 
there will be insufficient time for these various equilibria to be established. 
Our energy requirements are increasing so rapidly that the use of nuclear 
fuels will probably not change materially the rate of use of the organic 
fuels. 

Unfortunately it is difficult to obtain any direct evidence for the carbon 
dioxide content of the atmosphere during past geological epochs. In fact 
it is not even certain from direct measurements whether or not the carbon 
dioxide content has increased in the last fifty years. A plot of such meas- 
urements can be fitted nicely with a linear curve that increases by 10 per 
cent in that time interval. However, the probable error for most of the 
measurements is so large that this result is not very firmly established. 
Because of its importance to the climate, regular measurements of the 
atmospheric carbon dioxide content should be started at several different 
countr)' locations and continued for a number of decades. Since the at- 
mospheric carbon dioxide content varies somewhat with the past history 
of the air mass and the time of year, a number of measurements are nec- 
essary in order to obtain a reliable average. The present predicted rise of 
3 per cent a decade could be easily observed with the present techniques of 
analvsis. As to the carbon dioxide content of the atmosphere at earlier 
periods, only general discussions of the various factors that effect the car- 
bon dioxide balance can be given at the present time. It is possible though 
that we will be able to calculate the carbon dioxide amount of a past 
epoch from measurements of the ocean temperature and the rate of car- 
bonate deposition during that epoch together with further studies of the 
atmosphere-ocean equilibrium. 

There is some interesting evidence which suggests that the carbon di- 
oxide content of the atmosphere was once much larger than at present. 
It is known that plants grow more luxuriantly and rapidly in an atmos- 
phere that has from five to ten times the normal carbon dioxide amount. 
In fact carbon dioxide is sometimes released in greenhouses in order to 



236 GILBERT N. PLASS 

promote growth. Since plants are perfectly adapted to make maximum use 
of the spectral range and intensity of the light that reaches them from the 
sun for photosynthesis, it seems strange that they are not better adapted 
to the present carbon dioxide concentration in the atmosphere. The sim- 
plest explanation of this fact is that the plants evolved at a time when the 
carbon dioxide concentration was considerably higher than it is today and 
that it has been at a higher level during most of the ensuing time. Higher 
temperatures than today during most of the earth's history would have re- 
sulted from this higher carbon dioxide content. In fact the geological 
evidence shows that warmer climates than today have existed for at least 
nine-tenths of the time since the Cambrian period. 

Further evidence as to the carbon dioxide amounts in the past is pro- 
vided by the pH of sea water. There is a definite pH value associated with 
a given atmospheric carbon dioxide amount when the atmosphere-ocean 
system is in equilibrium. Further, many marine animals are very sensitive 
to the pH value, the higher marine animals being more sensitive in gen- 
eral than the lower. For example, herring are killed if the pH changes 
by more than one-half unit; lower marine animals such as sea urchins, 
diatoms, and algae cannot tolerate pH changes of more than one unit. 

This suggests that the pH of the oceans has not varied by more than 
these amounts since the time when these animals evolved or at most that 
the pH has changed extremely slowly so that these animals could evolve 
to live in the changed environment. However, even with the stringent re- 
quirement that the pH of sea water should not change by more than one- 
half unit, the atmospheric carbon dioxide amount can still vary by a factor 
of fifty and maintain equilibrium between the atmosphere and the oceans. 
Thus very large changes in the atmospheric carbon dioxide amount can 
occur without influencing either marine or land animals; still larger varia- 
tions would even be possible over time intervals sufficiently long to allow 
the animals to adapt to their new environment. 

All calculations of radiocarbon dates have been made on the assumption 
that the amount of atmospheric carbon dioxide has remained constant. 
If the theory presented here of carbon dioxide variations in the atmos- 
phere is correct, then the reduced carbon dioxide amount at the time of the 
last glaciation means that all radiocarbon dates for events before the re- 
cession of the glaciers are in question. 

Variations in the concentration or distribution of any gas that absorbs in 
the infrared portion of the spectrum can influence the surface temperature 
in the same manner as we have already discussed for carbon dioxide. Ozone 
and water vapor are the only two other gases that absorb in this region and 
also exist in the atmosphere in sufficient quantities to have an appreciable 
effect. Few suggestions have been made that relate variations in the con- 
centration of these two gases to the climate, since these changes do not 
seem to be related directly to definite geological factors. However, recent 



CARBON DIOXIDE AND THE CLIMATE 237 

calculations have shown that variations in the distribution of ozone can 
appreciably change the surface temperature. Normally the ozone con- 
centration has a maximum in the stratosphere with relatively small 
amounts at lower altitudes. Vertical air currents occasionally bring some 
of the ozone down from the stratosphere, thus greatly increasing the con- 
centration at lower altitudes. This is sufficient to increase the surface tem- 
perature from radiation effects by several degrees Centigrade. 

The relative humidity as a function of altitude is continually changing 
and a similar effect on the surface temperature exists for water vapor. 
These relatively rapid variations in temperature are superimposed on those 
from carbon dioxide alone. The latter variations are relatively constant 
over long time intervals compared to the former. However, water vapor 
can also have an effect over long time intervals, since the amount that 
can be held in the atmosphere decreases very rapidly as the temperature 
drops. During a glacial period the atmosphere has a smaller capacity to 
hold water vapor; for this reason the infrared heat energy from the earth's 
surface can escape more easily to space. Thus the influence of water vapor 
on the infrared absorption tends to reduce the surface temperature still 
more once a glacial period has started. The increased cloud amount dur- 
ing such a period also acts to reduce the surface temperature by reflecting 
the incoming solar radiation back to space. Therefore the temperature 
decrease during a glacial epoch is probably somewhat greater than is cal- 
culated from the carbon dioxide effect alone. 

A very large number of different theories of chmatic change have been 
proposed. As more evidence about past climatic change is obtained, each 
theorv has to meet continually more rigorous tests in order to explain 
the known facts. Each of the major theories of climatic change predicts 
a different temperature trend during the remainder of this century. A com- 
parison of these predictions with the actual record at the end of the cen- 
tury will provide an important test of these theories. 

The variable sun theory predicts that the temperature will decrease for 
some decades. The maximum of the eighty-year period in the sun-spot 
cycle probably occurred in 1947. Thus the total energy received from the 
sun including the ultraviolet should decrease for some decades when the 
records are averaged over the shorter periods in the cycle. On the other 
hand a continued increase in the average temperature could be justified 
by the variable sun theory only if measurement showed a corresponding 
increase in the solar constant. 

Changes in the average elevation of the continents clearly cannot be 
used to explain any variations in the climate over a period of a few cen- 
turies. However, the volcanic dust theory predicts appreciably lower tem- 
peratures for a few years following volcanic activity that throws large 
quantities of dust into the atmosphere. The last such explosion was when 
Katmai on the Aleutian Islands erupted in 1912. More volcanic explosions 



238 GILBERT N. PLASS 

of this kind must occcur before sufficient data can be obtained to cor- 
relate with the predictions of this theory. At the present time it is entirely 
possible that volcanic dust creates small perturbations in the climate while 
the general trend is determined by some other factor. 

On the other hand the carbon dioxide theory is the only one that pre- 
dicts a continually rising average temperature for the remainder of this 
century because of the accumulation of carbon dioxide in the atmosphere 
as a result of industrial activity. In fact the temperature rise from this 
cause may be so large in several centuries that it will present a serious 
problem to future generations. The removal of vast quantities of carbon 
dioxide from the atmosphere would be an extremely costly operation. If 
at the end of this century the average temperature has continued to rise 
and in addition measurement also shows that the atmospheric carbon 
dioxide amount has also increased, then it will be firmly established that 
carbon dioxide is a determining factor in causing climatic change. 

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The Record of Climatic Changes as 
Revealed by Vertebrate Paleoecology 



• EDWIN H. COLBERT 



THE NATURE OF THE EVIDENCE 

THE RECORD OF CHANGES IN THE EARTH'S CLIMATE DUR- 

ing past geologic ages can be derived from various lines of evidence, and 
of these the history of the vertebrates, as revealed by their fossil remains, 
is of particular importance. In the first place, the evidence afforded by the 
backboned animals through geologic time is important because it covers a 
vast expanse of earth history. True, the record of the vertebrates is not 
quite as extensive in time as that of the invertebrate animals, but the 
difference is not great. These two lines of evidence, combined with those 
of paleobotany and of stratigraphy, cover the whole time span during 
which there is a record of life on the earth. 

The evidence of fossil vertebrates is important also because it covers 
the widest possible range of ecological environments. Vertebrates have 
lived at all levels in the oceans, and at almost all levels on the land. They 
have lived in continental waters and in upland environments of exceeding 
dryness. They have lived in the air. Because of these wide geological 
adaptations the vertebrates can be used in conjunction with all other 
forms of life, both plant and animal, in the interpretation of past climates. 

The vertebrates are useful too because many of them have been very 
sensitive to climatic conditions, and therefore to the changes in climates. 
In this respect they may, of course, be compared with the invertebrates 
and with plants. In all forms of life the ecological tolerances vary greatly 
from group to group, often down to the species level, so that when the 
evidence is properly evaluated and weighted it is frequently of the utmost 
importance because of the implications that can be drawn from it con- 
cerning climatic conditions in past times. 

In the discussion of the problem at hand attention should be called to 
the use of the word implications. It must be realized that the evidence 

• From Climatic Change: Evidence, Causes, and Effects., ed. Harlow Shapley (Cam- 
bridge, Mass.: Harvard University Press, 1953), pp. 249-71. Reprinted by permission 
of the publishers. © The President and Fellows of Harvard College. 

239 



240 EDWIN H. COLBERT 

of the vertebrates as it bears upon past climates is for the most part in- 
direct evidence. We assume that ecological conditions (and by extension, 
climatic conditions as well) were thus and so because of the presence of 
certain animals in the sediments. This assumption rests upon a basic 
precept that must be accepted at the outset, if our interpretation of past 
climates upon the evidence of vertebrate paleoecology is to have order 
and vahdity. The precept is that within limits the past is to be inter- 
preted in terms of the present. Of course this maxim is commonly accepted 
in the fields of geology and paleontology— so much so that it is generally 
taken for granted— but it is being emphasized here because it is of un- 
usual significance in the present study. 

In line with this assumption we suppose that animals of the past were 
more or less similar to their modern relatives in physiologic requirements, 
and hence in their ecological tolerances. Consequently we assume that 
fishes, amphibians, birds, and mammals of the past were more or less like 
their modern relatives in their life processes, in their habits, and in the 
environments within which they were contained. From this we are then 
able to draw conclusions as to the environments and the climates in which 
certain animals lived by a comparison of environments and climates in 
which their modern relatives live. 

Of course such a method of interpretation must be used with great 
caution and with as much insight as possible, because animals are never 
identical in their needs. But it is reasonable to suppose, for instance, that 
the temperature limitations under which reptiles of the past lived would 
be roughly similar to temperature limitations for modern reptiles; and to 
extend the argument to greater detail, it is reasonable to think that the 
ecological conditions favorable to crocodiles in the past would be more or 
less similar to ecological conditions under which modern crocodilians live. 
On the other hand, the possibilities of closely related animals living in 
strikingly different environments must always be kept in mind. One need 
only cite the limitations of modern elephants to tropical and subtropical 
climates as compared with the extensions of some of their closely related 
cousins, the extinct mammoths, into arctic realms. Examples such as these 
call attention to the need for careful interpretation of fossil evidence. 

In the attempt to interpret past climatic conditions from the verte- 
brates, evidence other than that of the fossils is used whenever possible. 
In many instances such evidence is lacking or, if present, is of little value, 
but in other cases there is material that, when taken in conjunction with 
the evidence of the fossils, throws considerable light on the ecologic con- 
ditions in which the animals lived. 

The physical characteristics of the sediments are often helpful. Thus 
the nature of the rocks may indicate whether the animals that they contain 
lived in fresh or salt water, in muddv rivers and estuaries, or in clear 
lagoons, in swamps or uplands, in deserts or regions of much vegetation. 



THE RECORD OF CLIMATIC CHANGES 241 

Of course the nature of the sediments is not so important in an interpreta- 
tion of vertebrate hfe as it is for the invertebrates, especially the marine 
invertebrates, since the vertebrates are generally rather independent ani- 
mals that can move from one place to another. Even so, the sedimentary 
record must be carefully analyzed as an adjunct to the study of the fossil 
bones. 

Very commonly— possibly in a majority of cases— invertebrates and 
plants are not found along with the vertebrates, but when they are they 
may supplement the backboned animals in an interpretation of the eco- 
logical conditions of the time and place being studied. Also, such things 
as fossil footprints, raindrop impressions, mudcracks, and so on should be 
mentioned. The list might be extended. Again it is necessary to insert a 
word of warning. In using fossil vertebrates and the correlative evidence 
of sediments, plants, footprints, and the like, on^e may fall into the error of 
reasoning in a circle. Ecological conditions may be inferred from the pres- 
ence of certain vertebrates in the sediments and in turn the presence of 
the vertebrates may then be interpreted in the light of the inferred eco- 
logical conditions. This is a danger that must be kept in mind, and gen- 
erally it will not trap the alert paleontologist. Various lines of evidence can 
quite legitimately supplement each other, back and forth, without intro- 
ducing the pitfall of circular reasoning. Only the unwary are apt to be 
caught. 

METHODS OF COMPARISON 

In practice, the interpretation of probable past climates from the evi- 
dence of fossil vertebrates rests largely upon a study of the distributions 
of vertebrates during past ages. In following such a study attention is given 
not only to the occurrences of various types of vertebrates in the sediments 
of the earth, but also to the distribution of entire faunas, since the con- 
clusions that are to be drawn from a complete fauna may often have 
greater validity than those based upon single genera or species. Faunas 
and the elements that compose them are studied in space and in time, in 
their distribution over the surface of the earth and in their succession 
through the strata that constitute the sedimentary history of the earth. In 
each geologic period, and so far as possible within the subdivisions of 
each period, the assemblages of animals and the individual animals them- 
selves are analyzed for the information that they may give as to the envir- 
onments in which they lived. What were the conditions of temperature, of 
humidity, of light, of periodic variations, and of air and water currents 
that prevailed when the animals and the faunas to which they belonged 
were living? To answer these questions, contemporaneous animals and 
faunas of the past are compared with each other, and as far as is possible 
they are also compared with related animals and with similar faunas living 



242 EDWIN H. COLBERT 

at the present time. When close relations do not exist, some attempt is 
made to draw analogies from which reasonably valid conclusions can be 
established. 

All of this results primarily in a restoration of local environments, and 
from the composite relations of many environments broad conclusions as 
to general climatic conditions can be reached. 

The test of these conclusions rests in part upon the manner in which 
they accord or fail to accord with the general climatic picture of the past 
that has been drawn upon the basis of all the data available. If the evi- 
dence of vertebrate paleoecology is more or less in accord with other lines 
of evidence, it would seem probable that all the data, and the conclusions 
drawn from these data, are valid. If serious discrepancies are apparent, 
then it becomes necessary to evaluate the various lines of evidence bearing 
upon the problem of past climates. In this case the error may rest either 
with the evidence of the vertebrates, or at least with the interpretation of 
such evidence, or it may be that the vertebrates throw new light upon the 
problem at hand and show that some of our previous concepts have been 
at fault. 

Perhaps at this place it might be well to outline the general sequence 
of climates during past geologic ages, as developed from many lines of 
evidence. As set forth by Brooks in his definitive text. Climate Through 
the Ages, the succession from Cambrian to recent times is as follows: 

Pleistocene Glaciation in temperate latitudes 

Phocene Cool 

Miocene Moderate 

Oligocene Moderate to warm 

Eocene (including Paleocene) Moderate, becoming warm 

Cretaceous Moderate 

Jurassic Warm and equable 

Triassic Warm and equable 

Permian Glacial at first, becoming moderate 

Carboniferous Warm at first, becoming glacial 

Devonian Moderate, becoming warm 

Silurian Warm 

Ordovician Moderate to warm 

Cambrian Cold, becoming warm 

The striking thing about this table is the preponderance of warm and 
moderate climates that prevailed through the greater part of earth history 
since Cambrian times. According to Brooks's analysis, the Paleozoic era 
began with cold climates, but from then until the present, the history of 
the earth has been characterized by warm and moderate conditions except 
for two major glacial interludes— one in the late Paleozoic and one in the 
Pleistocene. 



THE RECORD OF CLIMATIC CHANGES 243 

How does the evidence of the vertebrates accord with this picture? To 
check this, let us now turn to a brief survey of vertebrate paleoecology. 

THE EVIDENCE OF MIDDLE PALEOZOIC VERTEBRATES 

The record of chmatic changes as based upon the evidence of extinct 
vertebrates begins with the Silurian period. It is certain, of course, that 
the vertebrates originated long before Silurian times, and they must have 
been well established in the faunas of the Cambrian and Ordovician, but 
in those distant days they were very likely small unarmored animals that 
would not be preserved as fossils in the sediments. Indeed, isolated scales 
from sediments of Ordovician age give undisputed proof of the fact that 
primitive vertebrates were then living on the earth— probably in streams 
and ponds. 

Adequate remains of early vertebrates first appear, however, in sedi- 
ments of Silurian age, so it is at this stage of earth history that the verte- 
brate record really begins. And while the Silurian record is definite, it is 
not, for the most part, abundant, nor is it very widely spread. Conse- 
quently, we must look to the sediments of Upper Silurian times, to those 
of the stage known as the Downtonian, and especially to the rocks of the 
following Devonian period, to find the fully documented beginning of 
vertebrate history. For this reason our discussion will begin with the 
Devonian period, a geologic age that represents about the middle point of 
Paleozoic history. 

The Devonian period has often been called the "Age of Fishes" because 
of the abundance of early aquatic vertebrates in many Devonian faunas, 
and also because of the dominant position that these early vertebrates 
enjoyed at that time. This was the period of the first great evolutionary 
radiation of vertebrates. Primitive jawless vertebrates, the ostracoderms, 
abounded in many faunas over the world. Contemporaneous with them 
were the armored placoderms, representing an "experiment" in vertebrate 
evolution that was destined to early failure. In addition to these early 
vertebrates, which cannot properly be designated as "fishes" (as we gen- 
erally use the term), there were the early ancestors of our modern fishes, 
primitive sharks, ancestral bony fishes, early lungfishes, and the important 
crossopterygian fishes that were to give rise to the first land-living am- 
phibians. At the very close of Devonian times the first amphibians ap- 
peared, and the vertebrates came ashore and invaded a new environment. 

The Devonian vertebrates represent for the most part animals that 
lived in continental fresh waters, or in relatively shallow estuaries that led 
to the sea. This evidence, along with correlative facts, seems to indicate 
that the vertebrates had their origins in fresh water and went through some 
of their early history in such a habitat. It was only at a later date that they 
invaded the oceans. 



244 EDWIN H. COLBERT 

This view is especially important in the attempt to interpret past cli- 
mates, since it gives evidence of continental conditions. And in the inter- 
pretation of climates the development of life on the continents and along 
the continental borders must be given particular attention, since this is 
perhaps a more accurate guide to past climates than is the development of 
marine life. There is no intention at this place to decry the importance 
of the marine evidence; certainly marine invertebrate faunas are very im- 
portant because of their wide distributions, and especially because of the 
sensitivity of many marine organisms to temperatures. Yet as we know 
from experience, the ocean is a vast world of its own, less affected by cli- 
matic changes than the land. Life on the land, even the life of continental 
waters, is bound to be more sensitive to changing conditions in the atmos- 
phere than life in the oceans. 

Devonian vertebrates are especially well known from northern Europe 
and from North America, and in these regions various faunas have been 
found that represent a succession of life through the period, which in 
Europe extends down into the Downtonian stage as well. In Europe the 
*'01d Red Sandstone" of Scotland is rightly famous, and fossils from this 
extensive deposit have been collected for much more than a hundred years. 
In addition to the Scottish region, Devonian vertebrates are known from 
the Shetland and Orkney Islands, from the Baltic states, from Germany, 
and from Russia. They are even found as far north as Spitzbergen. 

The Arctic occurrence of Devonian vertebrates is further illustrated by 
the presence of very important faunas in eastern Greenland, where the 
earliest amphibians, of upper Devonian age, are found. Devonian verte- 
brate faunas can be traced from Greenland to the south, through Quebec 
and into New York and Ohio. To the west they are found in Wyoming 
and Arizona. 

Our knowledge of Devonian vertebrates in other parts of the world is 
scanty, but well-known faunas are found also in New South Wales, Aus- 
tralia, and even Antarctica. 

From this it can be seen that the early vertebrates were widely dis- 
tributed, and it is reasonable to think that if the fossil record were more 
complete we would know many numerous faunas in other regions be- 
tween the extremes of Greenland and Australia where now the record is 
blank. At any rate, it is safe to assume that Devonian vertebrates were of 
almost world-wide extent, and because of the close relationships that char- 
acterize the individual elements of the various faunas, it is reasonable to 
suppose that the environmental conditions under which these faunas lived 
were roughly the same. 

Most of the Devonian "fishes" were obviously the inhabitants of rather 
shallow waters, living either in continental streams or ponds or in estuaries 
and bays along the edges of the continents. These faunal occurrences give 



THE RECORD OF CLIMATIC CHANGES 245 

evidence of a warm-water environment, where the conditions of Hfe were 
controlled to a large extent by the climates that were prevalent over con- 
tinental areas. Because of this it is reasonable to assume, on the basis of 
vertebrate evidence, that the Devonian period was a time of widely spread 
equable climates, a period of uniformity over most of the earth's surface. 

A striking character of the Devonian vertebrate faunas is the preva- 
lence in them of air-breathing or choanate fishes; of lungfishes and cros- 
sopterygians. The presence of these fishes in such relative abundance in 
many of the Devonian localities is a clear indication that the waters in 
which they lived were subjected to frequent periods of great reduction 
or drying up. This conclusion is reached by analogy with the recent lung- 
fishes. (The one known species of modern crossopter)'gian is a specialized 
marine form, and properly it cannot be used in an interpretation of the 
habits of the fresh-water or estuarine Devonian lungfishes.) The recent 
Australian lungfish (the most primitive of the modern forms) is able to 
withstand periods of drought, when the streams or ponds in which it lives 
become much reduced and the water foul, by coming to the surface and 
breathing air. The South American and African lung fishes are able to bury 
themselves in the mud, to withstand several months of complete dryness 
as air-breathing vertebrates. 

Evidently environmental conditions were advantageous to air-breathing 
fishes in Devonian times, and this would point to the fact that, while 
climates may not have been arid, there were probably dry seasons, during 
which bodies of fresh water became considerably restricted. Indeed, the 
periodic alternations of wet and dry seasons can be correlated closely with 
the rise of the amphibians from crossopterygian fishes. It was the urge in 
these fishes to venture from one pool of water to another during dry sea- 
sons that marked the first steps in the invasion of land by the vertebrates 
—a process that was to culminate in the appearance of the amphibians. 

It is interesting that the first amphibians known to us have been found 
in the upper Devonian sediments of eastern Greenland. Modern amphib- 
ians have very definite limits of temperature tolerance, and taking into 
account the anatomical and physiological characteristics of this class of 
vertebrates one can only assume that the same was true of the amphibians 
of past ages. Therefore the presence of early amphibians in the upper 
Devonian of eastern Greenland leads to the conclusion that this part of 
the world enjoyed relatively warm climatic conditions in middle Paleozoic 
times, when the earth climatically was a relatively uniform planet. Grada- 
tions in climate from equator to the poles were probably very gradual and 
were not marked by extremes. It is probable also that the temperature 
diflPerences between seasons were not so strikingly varied as they are at the 
present time. 



246 EDWIN H. COLBERT 



THE EVIDENCE OF UPPER PALEOZOIC VERTEBRATES 

The invasion of the land by vertebrates, which began with the close of 
the Devonian period, was thoroughly established in the next phases of 
geologic history, that is, in the lapse of time that is designated abroad as 
the Carboniferous period and in this country as the Mississippian and 
Pennsylvanian periods. The amphibians evolved along various lines of 
adaptation in Mississippian times, and this evolution continued into the 
Pennsylvanian. The first reptiles— derived from certain amphibians— ap- 
peared in this latter period. 

This period was the time of the great coal forests in the Northern Hemi- 
sphere. There is abundant evidence from the sediments and from paleo- 
botany to show that during the Carboniferous (to use the European term 
as a comprehensive designation for all of this portion of Paleozoic history) 
great tropical swamps covered extensive portions of the Paleozoic con- 
tinents, and in these lush, swampy forests the early land-living vertebrates 
abounded. Climates must have been warm and moist throughout a great 
part of Carboniferous times to support the abundant vegetation of ancient 
club mosses, lepidodendrons, horsetails, and ferns that constituted the 
forests of those distant days, and such climates and environments were 
most conducive to the development of the early cold-blooded tetrapods, 
the amphibians, and reptiles. 

These early amphibians and reptiles are known mainly from North 
American and European localities. They are found, for instance in England 
and Scotland, and in Bohemia and France, while on this continent they 
have been found in the Allegheny region of Pennsylvania, West Virginia, 
and Ohio, in Illinois, in New Brunswick, and in Nova Scotia. The resem- 
blances between some of the faunas of North America and Europe are 
very close, and indicate not only closely parallel climatic conditions in 
these separate regions, but very probably a land connection that allowed 
a relatively rapid diffusion of animals from one region to the other. 

The evidence of the vertebrates indicates that climates of the Carbon- 
iferous continued without much break from those of the Devonian. As in 
the earlier period the climates of the earth were in general rather uniform. 
Tropical floras and the animals that one might expect in such associations 
of plants lived at fairly high latitudes— as far north as 45°. Geologic evi- 
dence indicates that there were mountain uplifts toward the end of 
Carboniferous times, and these were accompanied in the Southern Hemi- 
sphere by extensive glaciations. Profound as these events may have been, 
they seem not to have affected the late Carboniferous faunas of the North- 
ern Hemisphere to any appreciable degree. Amphibious and primitive 
reptiles continued to prosper in the swamps and streams of northern 



THE RECORD OF CLIMATIC CHANGES 247 

Europe and America; evidently environments were not greatly changed in 
those regions. 

The Carboniferous was the period of amphibian dominance. Perhaps 
the picture as we see it is somewhat unbalanced as a result of facies 
developments. Thus, the Carboniferous vertebrates that are especially 
well known are those of the coal-forest deposits, and hence they represent 
the animals that were living in the swamps of those days. Perhaps there 
were numerous reptiles living in upland regions that have not been pre- 
served in the fossil record. While this is a distinct possibility, the evidence 
seems to indicate that reptiles were generally small and minor elements of 
the faunas of that day. Certainly Carboniferous environments were favor- 
able to amphibian life. 

With the advent of Permian times, however, there were definite 
changes in climates and environments, and in the land-living vertebrate 
faunas. The mountain-making and the southern glaciations that had been 
initiated in late Carboniferous times continued, reaching a climax in the 
early part of the Permian period. The world was becoming a planet char- 
acterized by varying climates and environments. All of this favored the 
emergence of the reptiles, which then became the dominant animals of 
the earth, even though there was probably some active competition be- 
tween these animals and some of the largest and most aggressive of the 
amphibians. 

The one great advantage that the early reptiles enjoyed over the am- 
phibians with which they were contemporaneous was the method of 
reproduction. The advent of the reptiles was marked by the appearance 
of the protected amniote egg— an egg capable of development away from 
water, and this was an event of great importance in the evolution of the 
vertebrates, for it completely emancipated the land-living tetrapods from 
dependence upon the water. The amphibians were forced to return to the 
water to breed, because the unprotected amphibian egg could develop 
only in a liquid medium, but with the appearance of the amniote egg the 
reptiles were able to reproduce and live entirely on dry land. Conse- 
quently, these animals became much more independent than the amphib- 
ians had been, and many new ecological possibilities were made available 
for their evolutionary development. One must keep this important fact in 
mind in connection with the uplift of continental masses in Permian times. 

Permian vertebrate faunas, with reptiles the dominant and in most 
cases, the most abundant animals comprising the assemblages, but with 
amphibians generally present in goodly numbers, are also known from 
many continental regions. Lower Permian faunas are abundantly repre- 
sented in the so-called red beds of Texas and adjacent states, while closely 
related faunas are found in Europe, especially in France. Middle Permian 
faunas are found in some parts of Europe, while upper Permian assem- 
blages are known from Scotland and from the Zechstein of Germany. 



248 EDWIN H. COLBERT 

There is a remarkable series of Permian deposits on the Dvina River in 
northern Russia, containing a succession of faunas ranging from middle 
through upper Permian age, and beyond. The famous Karroo series in 
South Africa carries vertebrate faunas that begin with the middle Permian 
and extend through the remainder of this geologic period and on into the 
Triassic. Permian faunas are found in other regions of Africa as well, and 
in addition in Asia and Australia. Some Permian vertebrates have been 
found in Brazil. This survey gives an idea of the world-wide extent of the 
land-living tetrapods in Permian times. 

In many regions the Permian continental sediments in which verte- 
brates are found are marked by their striking and often brilliant red colors. 
Many theories have been advanced to explain red beds, and it would be 
fruitless to discuss them here. There is much reason to think, however, 
that the continental red beds of Permian age represent in many instances 
a deposition of sediments under conditions of alternating moist conditions 
and drought. The evidence of the vertebrates would seem to be in accord 
with such an explanation. 

In view of the distribution of vertebrate faunas this theory of Permian 
climates is more logical than the idea of widely spread desert conditions 
that has been invoked by so many students of geologic history. It is hard 
to reconcile the indications of abundant reptilian and amphibian faunas 
with permanent desert environments. For instance, the Permian faunas 
of Texas, New Mexico, and Oklahoma contain mixed assemblages of fishes, 
amphibians, and reptiles. Although some of the reptiles in these faunas 
may have lived very well under desert conditions, it is difficult to imagine 
the amphibians as existing very far from water— at least for a part of each 
year— while the presence of fresh-water fishes certainly is an indication of 
streams and ponds. Even some of the reptiles must have lived in close 
proximity to water, as is indicated not only by the morphological evidence 
of the animals themselves, but also by the nature of the sediments in which 
they have been preserved. Therefore one comes to the conclusion that 
these animals lived in a region where water was near at hand during a 
part of the year, even though some months may have been rather arid. In 
this connection Olson has recently suggested, upon the basis of his long 
and careful studies of Texas faunas, that some of the early Permian animals 
of North America lived in a delta region. The resemblance of some of the 
European faunas of early Permian age to those of North America sug- 
gests that environmental conditions in the Old World may have been 
rather similar to those of Texas and adjacent regions. 

When we come to look at the Permian vertebrates of the Karroo series 
(South Africa), a somewhat different picture emerges. Here we find a 
preponderance of reptiles, many of them of large size, and most of them 
obviously active in a reptilian way. Amphibians are much less evident in 
the Karroo faunas than they are in the early Permian faunas of Texas or 



THE RECORD OF CLIMATIC CHANGES 249 

of France. In the Karroo assemblages there are numerous large herbivorous 
reptiles, and associated with them are many carnivorous forms. Here is an 
ecological relationship of herbivore and carnivore that has been paralleled 
many times among later land-living tetrapods. In fact, Watson has com- 
pared the Karroo reptiles with the plains-living mammals of Tertiary 
times, and has suggested that South Africa in the Permian period was not 
unlike North America in the days when herds of camels and horses, with 
their attendant predators, roamed the high plains. The parallel is an apt 
one. Because of the resemblance of the Dvina faunas of Russia to the 
Karroo faunas, it is reasonable to think that in northern Europe similar 
conditions were typical of middle and upper Permian times. 

From such evidence it is apparent that the environmental conditions 
under which the continental Permian faunas lived were varied, but were 
marked on the whole by their "upland" nature. Evidently there were dif- 
ferent types of climates and it is quite probable that there were marked 
alternations of seasons. 

What about temperatures? Here the evidence of the Permian reptiles 
is particularly important. Many large reptiles are known in the Karroo 
faunas in South Africa at a latitude of about 30°S, while Permian tetra- 
pods are found at similar latitudes in South America and in Australia. 
Large Permian reptiles closely related to the South African forms are 
found along the Dvina River of Russia just below the Arctic Circle, at a 
north latitude of about 65°. Therefore one is led to the conclusion that in 
much of the Permian period temperatures were similar over a broad belt 
of the earth, extending up and down toward what we now call the Arctic 
and Antarctic regions. Temperatures over this broad belt of the earth must 
have been warm to temperate, but never very cold. 

The reason for this assumption is founded upon our knowledge of 
reptilian physiology and temperature tolerances. Modern reptiles have 
varying temperatures that correspond roughly with the temperatures of 
the environments in which they live. Their temperature tolerances cover 
rather narrow ranges, and it is not possible for them to survive body 
temperatures that go above or below the limits of their ranges of tolerance. 
In this modern world of definite climatic belts all reptiles find the environ- 
ments most suitable to them in the tropical, subtropical, and temperate 
regions of the earth, while the large reptiles inhabit only the tropics and 
subtropics. 

The large crocodihans and turtles, and the largest lizards and snakes, 
find the optimum conditions for life and growth in a band around the 
equatorial part of the earth and bounded north and south more or less by 
the 30th parallels of latitude. North or south of this tropical and subtropical 
band the winters are too severe for such large reptiles; they are unable to 
protect themselves against the cold. The smaller reptiles that live in the 
more northerly and southerly regions exist through the winters by bur- 



250 EDWIN H. COLBERT 

rowing into the ground or by retiring into subsurface dens, where they 
endure several months in a stage of suspended animation. 

It is reasonable to think that the large reptiles of Permian times were 
not markedly different from large modern reptiles in their physiological 
requirements. Consequently, the presence of various reptiles as large as 
big dogs, sheep ponies, and even oxen in the faunas of South Africa and 
northern Russia is an indication that temperatures in these widely sepa- 
rate regions were warm to moderate, but never really cold. Varied as the 
climates may have been because of seasonal changes and the alternation 
of wet and dry periods, the earth enjoyed comparatively uniform tempera- 
tures during most of the Permian period. There was an interlude of south- 
ern glacial climates at the beginning of Permian times, but the effects of 
this cold period had disappeared by the middle Permian, when large 
reptiles extended their ranges into the southernmost tip of the African 
continent. 

THE EVIDENCE OF LOWER MESOZOIC VERTEBRATES 

The Permian is represented at the present time in many regions by red 
beds, and the same is true of the Triassic. In fact, continental red beds are 
especially characteristic of the beginning of Mesozoic history, and they 
can be found in widely separated parts of the earth, in western North 
America and along the Atlantic seaboard, in Scotland and central Europe, 
in South Africa, in Yunnan and other parts of western China, in India, and 
in southern Brazil. As in the case of the Permian sediments, many geolo- 
gists have interpreted these widely distributed Triassic red beds as indica- 
tive of broad desert regions at the beginning of Mesozoic history, but 
again, as in the case of the Permian sediments, the included faunas indi- 
cate that moist conditions were prevalent in many regions. It would seem 
that the Triassic was a time of alternating wet and dry seasons, which led 
to the oxidation of iron in the accumulating sediments and the production 
of the characteristic red colors. 

The labyrinthodont amphibians that were so prominent in the Permian 
faunas continued into the Triassic period, where they went through a final 
stage of evolution marked by extreme adaptations for life in the water. 
Therefore, wherever Triassic labyrinthodonts are found, it seems safe to 
assume that there were streams and ponds in abundance, and climates 
must have been fairly moist. The distribution of the Triassic labyrintho- 
donts is remarkably wide. These large and highly specialized amphibians 
are found in all of the continental regions, as far north as northern Russia 
and as far south as South Africa, southern Argentina, and eastern Australia. 
Moreover, they extend beyond the continents to the north, to eastern 
Greenland and to the island of Spitzbergen. Consequently, their latitudinal 
extent is from 80° N to about 40° S. This spread is indeed a very broad 



THE RECORD OF CLIMATIC CHANGES 251 

belt, for which fairly uniform conditions must be assumed. The labyrintho- 
donts, as said, lived in moist environments, and it must be supposed that 
the temperatures of their habitats were moderate. 

The evidence of the Triassic reptiles is similar to that of the labyrintho- 
dont amphibians. Reptiles in the faunas of that geologic period are, like 
the amphibians, widely spread, from northern Russia and Scotland on the 
north to South Africa and southern Brazil on the south, with many locali- 
ties in between. It has already been pointed out how temperature toler- 
ances of reptiles are such that these animals are unable to exist in regions 
of extreme temperatures unless they are able to protect themselves by 
going underground or by seeking refuge in temperate waters. Therefore 
the presence of large Triassic reptiles at high and low latitudes is a pretty 
clear indication that temperatures were moderate over much of the earth's 
surface at the beginning of Mesozoic history. Such large reptiles would 
have been unable to seek protection underground, and therefore the en- 
vironments in which they lived were never really cold. It is probable that 
tropical, subtropical, and warm temperate climates extended from the 
equator toward the poles, with climatic variations brought about mainly 
by the alternation of wet and dry seasons. 

It will be remembered that the Karroo series of South Africa was cited 
in connection with the discussion of Permian vertebrates. This series of 
sediments continues from the Permian into the Triassic, seemingly with 
no major break; here is one locality on earth where we can see a merging 
of Paleozoic and Mesozoic history as a single, continuous story (which, of 
course, would be true of the entire geologic record if we had it completely 
preserved). Large reptiles were present in the Permian phases of the 
Karroo series, and they continued into the Triassic portions of South 
African history. For instance, the dicynodont reptiles, many of con- 
siderable size, continued into the Triassic, while some of the mammallike 
Triassic theridonts were good-sized reptiles. Early dinosaurs appear in 
the Triassic of South Africa, as they do at the beginning of Mesozoic 
history in other regions such as central Europe, North America, and 
western China. The presence of these first dinosaurs, some of them 20 
feet or more in length, is an indication of mild cHmates and an abundant 
food supply. 

The dicynodont reptiles, which were so characteristic of the Karroo 
of South Africa, spread to many other parts of the world in Triassic 
times, and gigantic dicynodonts are found in southern Brazil, in the Chinle 
Triassic sediments of North America, and in China. We might mention 
also the Triassic phytosaurs— crocodilelike reptiles of large, and often of 
gigantic size. They are found in western and eastern North America, in 
central Europe, and in central India, and in all of these regions the 
phytosaurs are remarkably similar to each other. 

All of this evidence from the early Mesozoic land-living vertebrates 



252 EDWIN H. COLBERT 

supports the concept of moderate temperatures throughout the world in 
Triassic times. There is additional support for the picture of Triassic 
climates as here drawn from marine reptiles of the Triassic. In this 
geologic period the first ichthyosaurs or fishlike reptiles arose, to com- 
mence a line of reptilian evolution that was to continue through the extent 
of Mesozoic times. Certainly the ichthyosaurs must have been warm- 
water animals, and it is interesting to see that these animals lived not only 
in such latitudes as central Europe, western North America, and Timor, 
in the East Indies (almost on the equator), but also in the extreme north, 
in Spitzbergen. 

THE EVIDENCE OF MIDDLE AND UPPER 
MESOZOIC VERTEBRATES 

With the close of Triassic times and the opening of Jurassic history 
there were broad areas of desert conditions in some parts of the world. 
For instance, the early Jurassic continental sediments of the southwestern 
United States are marked by extensive exposure of eolian sandstones that 
could have been deposited only as great desert dunes. As Jurassic history 
progressed, however, these arid conditions, wherever they had been estab- 
lished, gave way to generally moist climates. 

The evidence would seem to indicate that most of the Jurassic period 
was a time of uniformity seldom equaled in the history of the earth. 
Lands were generally low and frequently covered with swamps, while 
there were extensive marine incursions over the continents. Climates 
must have been very uniform and benign, because this was a period of 
luxuriant vegetation and a trend to giantism among many of the reptiles 
that dominated the earth and the seas. 

Vertebrate faunas of Jurassic age are not so numerous nor are they so 
widely distributed as are the faunas of Triassic or of Cretaceous age. 
Perhaps this is partly because of continental submergence and the ad- 
vancement of shallow-water seas over the continental platforms. Cer- 
tainly our knowledge of land-living Jurassic tetrapods is restricted for the 
most part to the upper Jurassic, and to a few areas. In this respect it 
should be mentioned that the fossil record of marine reptiles from the 
Jurassic is as widely distributed as the record of land-living vertebrates. 

There are two great upper Jurassic faunas, one, the Morrison fauna 
from western North America, the other, the Tendaguru fauna from 
East Africa. These faunas are remarkably similar to each other, and they 
are characterized by the prevalence in them of gigantic dinosaurs. In 
both of these faunas there are many huge, plant-eating sauropod dino- 
saurs, armored stegosaurs, and the smaller herbivorous camptosaurs; and 
balanced against these plant-feeders, the giant carnivorous theropod 
dinosaurs, the predators of those distant days. There are other reptiles 



THE RECORD OF CLIMATIC CHANGES 253 

as well— crocodiles and turtles being especially noteworthy— but the 
dinosaurs are dominant. The presence of such great numbers of large 
dinosaurs in the Morrison and Tendaguru faunas is an indication of the 
prevalence of swamps in which the sauropods lived and of the luxuriance 
of vegetation on which those dinosaurs fed, which in turn is an indication 
that climates were very mild, and for the most part probably tropical or 
subtropical. The occurrence of such similar faunas in North America and 
East Africa betokens the wide spread of tropical conditions over much 
of the land surface of the earth. 

In England and Europe the upper Jurassic sediments contain not only 
the large dinosaurs so characteristic of this period of geologic history, 
but also giant ichthyosaurs and plesiosaurs, which were marine reptiles. 
Evidently in this part of the world there were low islands or restricted 
land areas with seaways between them, thus bringing about the deposi- 
tion of land-living and aquatic reptiles in the Jurassic sequence. Again, 
it would seem reasonable to suppose that the climates were essentially 
similar to those of North America and Africa. 

As for marine vertebrates, it should be pointed out that icthyosaurs 
and plesiosaurs have been found in the Jurassic of eastern Greenland at 
a north latitude of some 70° or more. Another point that should be made 
here is that in the Jurassic Solenhofen hmestones of Germany are found 
the skeletons of pterosaurs or flying reptiles, and the remains of the first 
birds as well. Studies of the limestones indicate that they were probably 
deposited in a coral lagoon, which would indicate tropical seas as far 
north as 50° during the Jurassic period. Jurassic limestones in England 
are also interpreted as having been formed by coral reefs. 

From all of this evidence one can picture the Jurassic period as a time 
of extraordinary climatic uniformity over much of the earth, when tropi- 
cal and subtropical conditions extended far toward the polar regions. It 
was probably a period during which there was a little differentiation of 
seasons, in contrast to the preceding Triassic period. 

The transition from Jurassic to Cretaceous times was marked by the 
beginnings of mountain uplifts that were to bring an end to the uniformity 
of middle Mesozoic environments and initiate the varied conditions that 
were to mark in ever-increasing measure later geologic history. Intense 
mountain foldings, the Nevadan Revolution, took place along the Pacific 
border of North America. Continental uplifts began that were to con- 
tinue through the Cretaceous period and beyond. Of course these changes 
were gradual, so that in effect the beginning of Cretaceous times was on 
the whole much like the closing stages of Jurassic times. 

That the Cretaceous period was a time of uplift is indicated by the ex- 
tensive land-living vertebrate faunas characterizing this phase of geologic 
history, especially toward the end of Cretaceous times. Cretaceous sedi- 
ments containing land-living tetrapods are found at various localities on 



254 EDWIN H. COLBERT 

all of the continental land-masses, and as exploratory work continues addi- 
tional localities come to light with each passing decade. Consequently, our 
knowledge of Cretaceous tetrapods is much more extensive and varied 
than is the information we have about the Jurassic amphibians and rep- 
tiles. In this respect it should be pointed out that much of what we know 
about the Cretaceous land-living vertebrates is based to a large degree 
upon faunas of upper Cretaceous age. 

Faunas containing Cretaceous land animals are found as far north as 
northern Europe and western Canada and as far south as South Africa, 
Australia, and Patagonia. Moreover there are many faunas from the conti- 
nental land masses in between these limits of latitude. The marine reptiles 
of the Cretaceous period, notably the ichtyosaurs, plesiosaurs, and mosa- 
saurs, were as widely spread across the face of the earth as the land-living 
forms— an indication that warm seas extended far into the northern and 
southern latitudes of the globe. From this it is evident that environmental 
conditions were sufficiently uniform to allow the successful existence and 
evolution of great reptiles over a large part of the earth's surface. 

There is, however, ample evidence from the reptiles that the earth in 
Cretaceous times, especially in the upper Cretaceous, was a more varied 
planet that it had been during the Jurassic period. For instance, none of 
the dinosaurs of the Cretaceous attain the extreme size seen in the Jurassic 
sauropods, even though in some lines, such as that of the giant carnivo- 
rous dinosaurs, the culmination of phylogenetic size was not reached until 
the end of the Cretaceous period. Sauropods continued into Cretaceous 
times, but in this later geologic period they were reduced in bulk as com- 
pared with their Jurassic forerunners. Most of the other dinosaurs of the 
Cretaceous, especially the dominant ornithischians, were only moderately 
large as compared with the Jurassic giants. This change would indicate 
that environmental conditions probably were not so favorable to giantism 
as in the preceding geologic period which in turn means that the climates 
were perhaps not so uniformly tropical or subtropical and plant life not so 
luxuriant as they had been in Jurassic times. 

While the consideration of plants is not rightly within the scope of the 
present discussion, it might be pointed out at this place that one of the 
great events of the Cretaceous period was the modernization of floras. It 
was during this stage of geologic history that the angiosperms made their 
appearance, so that the forests, which hitherto had consisted of com- 
paratively primitive ferns and gymnosperms, now assumed a modern as- 
pect by reason of the varied deciduous trees composing them. The pres- 
ence of angiosperms in Cretaceous floras is an indication, it has been 
suggested, of alternating seasons. The occurrence in Alaska and Green- 
land of such plants as figs, breadfruit, palms, and cycads leads to the con- 
clusion that subtropical and temperate climates extended far beyond the 
middle portions of the earth during these final stages of Mesozoic history. 



THE RECORD OF CLIMATIC CHANGES 255 

Not only did the flowering plants give a modern appearance to Creta- 
ceous life, but also some of the land-living animals, contemporaries of the 
dinosaurs and pterosaurs, were essentially modern in form and relation- 
ships. In this category we find the crocodilians, turtles, lizards, and snakes, 
all of which had their beginning in the earlier phases of Mesozoic history, 
and all of which survived the end of the Cretaceous period to live into 
modern times. Also the first placental mammals appear in Cretaceous sedi- 
ments. This is significant, because it represents the beginning of evolu- 
tionary radiation among the animals that were to be freed from the old 
reptilian dependence upon external temperatures. Cretaceous mammals 
were comparatively insignificant, but they set a pattern for vertebrate life 
that was to develop after conditions were no longer favorable for the 
continuation of the dinosaurs and the other dominant reptiles of the 
Mesozoic. 

To sum up, the evidence of Cretaceous vertebrates shows that the end 
of the Mesozoic era was a time of rather varied climates, possibly with a 
definite alternation of seasons, but with cold climates as we know them 
either very much restricted to the polar regions or completely nonexistent. 
The wide distribution of giant Cretaceous reptiles shows that tempera- 
ture conditions favorable to ectothermic vertebrates were general over 
most of the continental regions of the earth. To our modern eyes it was 
still a world of genial climates. 

THE EVIDENCE OF LOWER CENOZOIC VERTEBRATES 

The transition from Mesozoic to Cenozoic times was one of the great 
crises of earth history. Mountain uplifts constituting the so-called Lara- 
mide Revolution had begun with the close of the Cretaceous period, and 
these continued into Tertiary times to build the great mountain chains of 
the modern world. Because of these crustal disturbances, and very likely 
because of other factors as well, climates and environments were affected 
to such an extent that there was a great change in the life of the earth, 
especially among the land-living vertebrates. A wave of extinction swept 
away the many reptiles that had been dominant in Mesozoic times, leav- 
ing only the crocodilians, the turtles, the snakes and lizards, and a few 
rhynchocephalians to carr}' on the history of reptilian life. The mammals 
that had been so insignificant during the Cretaceous period suddenly 
expanded with explosive effect, to occupy the various ecological niches 
that had been vacated by the once-dominant reptiles. Modern birds ap- 
peared in all their variety. From this point on the land and the air, and 
to some extent the sea as well, were to belong to the warm-blooded 
vertebrates, the birds and mammals. 

This fact introduces a complication in our attempt to interpret past 
climates from the evidence of the vertebrates. As has been shown, the 



256 EDWIN H. COLBERT 

amphibians and reptiles are especially valuable to the student of environ- 
ments because of their temperature limitations. The distribution of ex- 
tinct faunas containing large reptiles is an indication of the possible distri- 
bution of warm chmates in past times. 

But the mammals and birds, being endothermic, are not such effective 
indicators of the conditions under which they lived. True, we know that 
certain mammals and birds at the present time are confined to tropical 
areas, while others live in the polar regions of the earth. By analogy, it 
can be assumed that extinct forms closely related to modern tropic-dwell- 
ing mammals (or birds) also lived in such environments, and of course the 
same line of reasoning can be extended to inhabitants of other climatic 
belts. But there are pitfalls here, as pointed out earlier in this communica- 
tion. Modern elephants are tropical and subtropical animals, but their 
close relatives, the extinct mammoths, were temperate and arctic animals. 
The same is true of modern rhinoceroses and some of the extinct rhi- 
noceroses. Modern musk oxen are arctic mammals, but it may well be 
that some of the extinct musk oxen lived in temperate regions. 

The picture is complicated still further by the fact that many mammals 
and birds have very wide ranges. For instance, the modern puma or 
cougar of the Western Hemisphere ranges from the snows of Canada to 
the tip of South America, while his cousin, the Old World leopard, ex- 
tends from northern China to the southern part of Africa. Among the 
migratory birds, individuals cover vast expanses of latitude twice each year. 
All of these facts must be kept in mind when evaluating the distribution 
of animal life during Cenozoic times. 

There are still the amphibians and the reptiles, especially the latter, to 
aid the student of past climates during the Cenozoic. These animals were 
as much at the mercy of their environments as their Mesozoic relatives 
and forebears. And the student of Cenozoic faunas is of necessity guided 
to a considerable extent by the associations of the animals with plant life 
of that time. 

Even though the extinction of the dinosaurs and the sudden appearance 
of many groups of mammals were almost instantaneous events in terms 
of geologic time, the actual processes of extinction and replacement by 
new forms must have been slow in terms of years. It is hard to believe 
that climates at the beginning of the Cenozoic were widely different from 
those that characterized the close of Mesozoic times, although there is 
evidence of some cooling during the early stages of Cenozoic history. 

The mammals of the Tertiar,/ period that one finds in middle and north- 
ern latitudes are of the sort that one might expect in warm climates, al- 
though again it must be repeated that this criterion is to be used with 
great caution. In early and middle Tertiary times there were various herbiv- 
orous animals living in the middle and even the northern latitudes, and 
they must have depended for their sustenance upon rather luxuriant vege- 



THE RECORD OF CLIMATIC CHANGES 257 

tation of the type that one would expect in moist subtropical or temperate 
forests. Such were the early horses, rhinoceroses, and tapirs, so widely 
spread in Tertiary times, the ubiquitous mastodonts, and other large 
browsing animals like the amblypods and titanotheres. These animals in 
themselves are not definitive, but taken in conjunction with the paleobo- 
tanical evidence they contribute to a concept of fairly uniform tempera- 
tures extending in a wide belt around the earth. The evidence is reinforced 
by the freqent presence of crocodilians in association with such mammals. 
Consequently we come to the conclusion that although climates were 
gradually changing, the earlier part of the Cenozoic era was still marked 
by much of the uniformity that was so typical of Mesozoic times. There 
must have been varied environments at this stage of earth history, and 
there were probably variations in climatic conditions. But in spite of 
environmental and climatic differences from place to place, the definitely 
zoned climatic belts, so familiar to us at the present time, apparently did 
not exist. 

EVIDENCE OF UPPER CENOZOIC VERTEBRATES 

It is not until upper Cenozoic times that the evidence for an earth 
with climates more or less like those known to us begins to emerge. This 
is apparent for the most part from clues other than those afforded by the 
vertebrates— from paleobotanical evidence, from direct evidence of the 
sediments, and so on. But the vertebrates do give some help on this prob- 
lem. 

The climates of the earth were still fairly warm as late as the beginning 
of the Pliocene period. In sediments of that age there are alligators found 
in the northern Great Plains of North America, which means that tem- 
peratures must have been at least warm-temperate at that time. On the 
whole, however, the mammalian faunas of late Miocene and Pliocene 
times give an indication of the development of varied environments and 
zonation in climates. In the later stages of the Tertiary period we can 
see the development of steppe forms, of animals that lived upon the high 
plains where temperatures at times were certainly severe. Plains-living 
horses and many kinds of deer and antelopes are prominent in the faunas 
of that time. This may be in part an expression of facies developments, 
but nevertheless it is an indication too of the fact that climates were 
becoming more extreme than they had been, perhaps since late Paleozoic 
times. 

Near the end of the Pliocene there are indications of definite climatic 
cooling in the Northern Hemisphere, intimations of the first great glacial 
advance of Pleistocene times. These indications come from evidence other 
than that of the mammals, but the mammals none the less are in accord 
with such evidence. 



258 EDWIN H. COLBERT 

The history of dimates during the Pleistocene period is so well known 
that it needs little elucidation here. It is well authenticated by the glacial 
evidence, and there is not much that can be added from the evidence of 
the mammals. There were clearly four glacial advances in the Northern 
Hemisphere, with interglacial periods of warmth and moisture. The mam- 
mals of that time receded and advanced with the glaciers, so that during 
periods of glacial maxima boreal forms like musk oxen and mammoths, 
woolly rhinoceroses and reindeer, lived in latitudes that are now temper- 
ate. In the periods of glacial retreat the mammals advanced to the north, 
so that horses and antelopes lived in Alaska. We are now living in an 
interglacial period in which glaciers are still diminishing, and there is evi- 
dence even in recent times of the movement of mammals from one re- 
gion to another. It is a mere detail, but an interesting one, that certain 
mammals like the armadillo and the cacomistle, generally considered as 
"southern" in North America, are now spreading their ranges to the north. 

With the advent of Pleistocene times the patterns of chmatic succes- 
sion and climatic variations as we know them became set. The climates 
of the earth became sharply zoned, from the tropics of the equator through 
the temperate zones to the boreal climates of the poles. Definite alterna- 
tions of seasons were established— wet and dry seasons in the equatorial 
regions, hot and cold seasons in the higher latitudes. But this climatic 
pattern, so generally accepted by us, is far from typical in the history of 
the earth; in fact, the present climates of the world make up an atypical 
pattern, quite at variance with what has held through much of geologic 
history. 

CONCLUSION 

So far as past climates can be interpreted from the record of fossil ver- 
tebrates, it would appear that during much of earth history the world 
has enjoyed uniformly warm, equable climates over most of its surface. 
There have been digressions from this pattern of uniformity at times, and 
various complications in it, but the general picture of past vertebrate life 
is that of warmth-loving animals living over wide ranges of latitude, from 
the southern tips of the continental land masses through the middle lati- 
tudes to regions as far north as the Arctic Circle. 

The earliest vertebrates of middle Paleozoic times are found in many 
northern regions where climates are now severe. Yet these early verte- 
brates must have been the denizens of warm or temperate— certainly not 
cold— waters. 

At the close of the Devonian period the first known amphibians ap- 
peared in eastern Greenland. These animals certainly lived in other re- 
gions as well, although still unrepresented in the fossil record, but their 
presence in a land so far north is an indication of the mild conditions that 



THE RECORD OF CLIMATIC CHANGES 259 

must have prevailed over much of the earth at that time. The story is 
continued through upper Paleozoic times, when first the amphibians and 
then the reptiles become well established in characteristic faunas from 
the equator to far northern and southern regions. In late Carboniferous 
and early Permian times the climatic conditions probably became more 
extreme than they had been, and it seems likely that varied, if not rigorous, 
climates continued through the Permian into the Triassic period. Yet 
even so, the Triassic was sufficiently warm that amphibians and reptiles, 
notably sensitive to temperature conditions, lived as far north as Spitz- 
bergen and Greenland and as far south as the extent of the southern 
continental land masses. 

Middle and upper Mesozoic times were periods of relatively great uni- 
formity, when climatic and environmental conditions were favorable to 
the development of giant reptiles that spread over the face of the earth. 
In the Cretaceous period, however, there were subtle but definite changes 
in environments, leading finally to the extinction of the great reptiles that 
had been dominant since late Paleozoic times, and leading to the estab- 
lishment of the mammals as the dominant land animals. 

In early Cenozoic times it is probable that there was some cooling, but 
climates were not greatly different from those typical of late Mesozoic 
days. During the course of later geologic history definite changes took 
place. Climates became zoned to a greater degree than they had been 
zoned before, and environments showed great variations from the equator 
to the poles. To such changes in climates the mammals and the birds 
readily became adapted, but the ectothermic amphibians and reptiles, be- 
ing unable to adapt themselves completely to climatic extremes, were 
restricted in distribution. These events marked the emergence of the mod- 
ern pattern of climates, and of the distribution of animals in relation to 
these climates, to culminate in the world as we know it today. 

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260 DR. RALPH W. CHANEY 

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Bearing of Forests on the Theory 
of Continental Drift 



• DR. RALPH W. CHANEY 



FORESTS OR CONTINENTS - WHICH OF THESE HAVE 

moved over the surface of the earth during the past? This question 
arises when we consider the fossil forests of the north, where long winters 

• From Scientific Monthly (Dec, 1940), pp. 489-99. 



THE THEORY OF CONTINENTAL DRIFT 261 

with sub-zero temperatures make it impossible for trees to live to-day. It 
again comes to mind when we uncover in the rocks of the western United 
States petrified logs and leaf impressions of trees which now exist only in 
the tropics. Such records of past life establish the fact of great changes 
during earth history. But whether these changes have involved migrations 
of forests southward or movements of whole continents northward is a 
question on which paleobotanists and geophysicists are not always in 
agreement. 

On first thought it seems more probable that the forests have moved 
rather than the continents. The span of a human life is too short to wit- 
ness major changes, but we instinctively feel, as implied by such expres- 
sions as "solid ground" and "everlasting hills," that the continents on 
which we live are the epitome of permanence. Many of us have witnessed 
changes in forest distribution, largely, it is admitted, through man's clear- 
ing of woodlands for other uses. Such superficial observations and reac- 
tions can scarcely be weighed seriously in a question involving world-wide 
changes during scores of millions of years. It is necessary to turn to the 
fossil record for the solution of a problem which had its beginning long 
ages before man came to live upon the earth. 

The hypothesis of continental drift, as presented by Wegener, assumes 
the original massing of the existing land masses into an aggregate termed 
Pangaea. Subsequently the American continents are thought to have 
broken off and drifted to their present position. As Wegener states in his 
book, "The Origin of Continents and Ocean Basins," the starting point 
of this idea of continental union and dispersal was the close correspond- 
ence of the coasts of Africa and South America. This suggested that they 
had once been joined and that they subsequently drifted to opposite sides 
of the Atlantic Ocean. Evidence was also presented for the fusion 
of North America with Europe. Wegener concluded that as recently as 
the geologic period preceding our own, there was only a narrow inland 
sea separating these continents, and that the Atlantic Ocean as we now 
know it did not come into existence until the period in which we live. 
Although his discussion of north-south movements involves some contra- 
dictions, Wegener definitely indicates his behef that the position of the 
continents with relation to the north pole has also changed widely in later 
geologic time. Writing more recently, duToit makes the following state- 
ments in his book, "Our Wandering Continents": "From the Cretaceous 
onwards we can accept a series of polar 'shifts,' ... A general movement 
at first north, then north-east, thereafter north again and finally east, 
modified to some extent by the continued divergence of the two conti- 
nents"; and "Indeed, from the mid-Palaeozoic onwards the lands must 
have crept northwards for thousands of kilometers to account for their 
deduced climatic vicissitudes. Such, indeed, constitutes the most telling 
demonstration of the reality of Continental Drift." 



262 DR. RALPH W. CHANEY 

The paleobotanist approaches the question of continental drift versus 
forest migration with an attitude which has been current among students 
of the earth sciences since Hutton and Lyell, over a centur)' ago, put 
forth the doctrine that the present is the key to the past. Viewing the 
vegetation of to-day from the pole southward, we note gradual changes 
from boreal to temperate and from temperate to tropical forests. Dwarfed 
spruce, willow and birch on the Alaska tundra give way to maple, elm or 
redwood at middle latitudes, and these in turn disappear as fig, laurel and 
palm attain dominance in Central America. This change in modern for- 
ests southward we interpret as largely a response to rising temperatures. 
Figs may not live near the arctic circle because of the severe winters; the 
trees of the north can not meet competition with forest giants in the 
tropics. The result is a zoning of vegetation which enables a student of 
modern plants to estimate the approximate latitude and temperature of his 
position from the character of the forest. Similar zoning characterizes the 
forests of Eurasia as well. This must of necessity be the case if temperature 
is the primary factor in plant distribution, since— with certain modifica- 
tions to be discussed later— temperature is a function of latitude. Fig. 1 
shows the distribution of several of the major floristic units in the northern 
hemisphere, together with the isotherms for the winter season. This is the 
season most significant to our discussion, since minimum temperatures 
largely determine the northward limits of forest distribution. 

The paleobotanist finds evidence that forest zoning can be traced back 




Fig. 1. Distribution of January isotherms and vegetation in the northern 
hemisphere. 



THE THEORY OF CONTINENTAL DRIFT 263 

for tens of millions of years, to the epoch known as the Eocene. There 
are abundant fossil records of Eocene plants in the northern continents 
which make possible the reconstruction of a zone of subtropical forests, 
as indicated by the black circles in Fig. 2. At each of the localities so 
marked, there have been found leaves, fruits or stems of plants which re- 
semble those now living in the tropics or on their borders. Some of the 
more common of these plants are the avocado {Persea), chumico {Te- 
tracera), fig (Ficus), magnolia (Magnolia) and nipa palm (Nipadites). 
Their fossil leaves are relatively large and thick, like those of modern 
plants which live in warm regions. A slab of fossils and the modern forest 
containing similar living trees are pictured in Figs. 3 and 4. Our conclu- 
sion that such fossil floras indicate subtropical temperatures is based upon 
the assumption that plants of the past had essentially the same habitat 
requirements as their nearest living relatives. Single species taken by 
themselves would not justify such an assumption, but when most or all of 
the members of a fossil forest indicate warm living conditions, we may 
conclude with confidence that this forest lived south of the zone of winter 
frosts. 

On our map several circles along the northern fringe of the Eocene 
subtropical zone are white in their northern halves. This indicates that the 
fossil floras which they represent were transitional in composition between 
subtropical and temperate forests. The latter, shown by white circles, oc- 
cupied a latitude averaging 55 degrees, and were made up largely of plants 
which live to-day in regions where the temperature is intermediate be- 
tween tropical and boreal. Some of the more common members of this 
temperate flora are basswood (Tilia), chestnut (Castanea), elm [Ulmus), 
hornbeam {Carpinus), maple (Acer), oak (Quercus), redwood (Sequoia), 
sycamore (Platanus) and walnut (Juglans). Fossil remains of these trees 
are found widely in Eocene deposits of Alaska, Greenland, Spitzbergen 
and northeastern Asia. A slab of redwood twigs is shown in Fig. 6, and 
adjacent to it a picture of the coast redwood forest of California. 

Still farther north, where trees are now stunted or wholly absent, there 
are several localities where the vegetation of the Eocene was limited al- 
most entirely to boreal plants such as birch (Betula), poplar (Populus), 
spruce (Picea) and willow (Salix). These are indicated by ovals on our 
map, and are not so numerous as in the other zones due to inadequate 
information regarding fossil plants in extreme high latitudes. . . . The 
zonation of these northern floras and of those farther south is closely sim- 
ilar to that of corresponding modern forests. Vegetation of a given climatic 
type is at approximately the same distance from the north pole in Eu- 
rasia as in North America, from which we conclude that these continents 
were grouped about the north pole in essentially their present position as 
far back as Eocene time. 

A striking difference between the Eocene distribution of these floras and 



264 DR. RALPH W. CHANEY 





Fig. 3. Rainforest of 
Guatemala. (Latitude 
16° north.) Similar to 
subtropical Eocene 
floras at middle lati- 
tudes. 



Fig. 4. Fossil leaves of 
magnolia. From the 
Eocene found in Ore- 
gon (latitude 44° north). 
Reduced one third. 



266 DR. RALPH W. CHANEY 

their present occurrence is that in every case they ranged farther north 
in the past. The subtropical forests, now located within 36 degrees of the 
equator, ranged beyond 51 degrees north latitude; the temperate forest lay 
20 degrees north of the center of its modern range; and the boreal forest, 
extending into regions where trees no longer can live, had outposts 20 
degrees north of the latitude in which it is best developed at the present 
time. The subsequent migration of these forests southward to their pres- 
ent positions we interpret as due to climatic change,— a gradual lowering 
of temperature which made it impossible for them to survive in the north. 
Supporting this idea of a climate becoming colder during later geologic 
time is the evidence of fossil shells; marine molluscs of types now charac- 
terizing warm seas ranged as far north as Alaska as shown by their occur- 
rence there in rocks of Eocene age. Mammals to-day limited to the 
warmer parts of the world also lived well to the north of their present 
homes. It is not within the province of this paper to consider the causes 
of such reduction in temperature, but the fact of its change seems to be 
well established by the fossil record of organisms which lived both on the 
land and in the sea. The resulting shift of forests southward for equal 
distances in North America and Eurasia (see Figs. 2 and 1) indicates 
that as far back as Eocene time these continents were grouped around 
the north pole in their present relative positions. The latter point is 
worthy of emphasis, since the consensus of opinion among exponents of 
continental drift places the pole at approximately 45 degrees north lati- 
tude and 170 degrees west longitude during the Eocene. By this they do 
not necessarily mean that the position of the axis of rotation has been 
altered, but rather that the continents had a different relative position 
around the poles; on Wegener's map North America was turned so that 
the present Pacific Coast faced northward instead of westward; Europe 
lay off to the south, with Spitzbergen at latitude 40 degrees and Green- 
land at about 30 degrees. The walnuts, oaks and redwoods which make 
up so large a part of the fossil flora from these localities now live in com- 
parable latitudes, and a hypothesis which has moved them northward thus 
meets the known facts of forest distribution during the Eocene. The sub- 
tropical floras farther south in England and France contain figs and mag- 
nohas equally suited to the latitude 65 or 70 degrees south of the pole, as 
based on this concept of continental drift. 

But when we come to the western hemisphere and examine the position 
of the corresponding North American forests, strange inconsistencies are 
at once apparent. The Eocene flora of Alaska would have lived only 15 
degrees away from the north pole, at a latitude much too high for tem- 
perate forests if the climate as postulated was like that of to-day; the sub- 
tropical flora of Oregon would have lived about 30 degrees south of the 
pole, at a latitude now too severe for the best development even of a 
temperate flora. It is apparent that in settling the problems of fossil floras 



THE THEORY OF CONTINENTAL DRIFT 267 

on their own continent, European exponents of the theory of continental 
drift have condemned our American forests to retroactive frost and freez- 
ing. The character and distribution of Eocene forests in North America 
definitely refutes the suggestion that the northern continents have changed 
their positions around the pole during later geologic time. They lay in 
essentially the same latitudes as floras in Eurasia which contain similar or 
identical fossil species, and were distributed in zones governed as they 
now are by their distance from the existing north pole. Any explanation 
of changed climatic distribution since the Eocene must apply to all the 
continents of the northern hemisphere, rather than to a particular area 
selected because it seems best to fit a hypothesis. 

There is an equally fundamental objection to the map of the Eocene 
continents as postulated in Wegener's Pangaea. As indicated above, there 
was no Atlantic Ocean separating North America from Europe during 
that epoch. In the absence of an ocean, no current like our modern Gulf 
Stream could have carried warm waters to the shores of Scandinavia as it 
does to-day. The effects of the Gulf Stream upon living forests in north- 
western Europe may be seen by reference to Fig. 1. Trees which are char- 
acteristic of central Europe range northward beyond the Arctic Circle 
along a shore to which are brought the warm waters from the Gulf of 
Mexico. The northward turning of isotherms in this region is an expres- 
sion of the milder air temperatures which result from this current. In the 
Pacific Ocean there is likewise a response to the warmer climate resulting 
from the Japan current, for temperate forests extend farther north along 
the coast of Alaska than in the interior. These relations of ocean currents 
to land temperatures may be summarized by stating that shores are warmer 
than continental interiors, especially on the windward sides of the con- 
tinents and in winter. At this season isotherms turn northward over the 
oceans, southward over the continents, in the northern hemisphere. 

It is obviously impossible to draw isotherms based on direct observation 
of Eocene temperatures, for weather bureaus were not functioning sixty 
million years ago, nor were there ships at sea to radio information regard- 
ing the oceans. But by drawing lines known as isoflors we may approxi- 
mate the positions of Eocene isotherms. These lines connect floras of the 
same general composition, which are assumed to indicate, as do similar 
floras to-day, essentially the same climatic background. The isoflor con- 
necting the localities where subtropical floras have been recorded, as shown 
by Fig. 9, swings up the west coast of Europe into England, then turns 
southward into France and trends in a southeasterly direction, with a bulge 
north over the Black Sea, across Eurasia to the coast of central China. 
Here it turns northward along the coast of Japan, reaching the coast of 
western America in Washington and Oregon, swinging southeastward to 
Tennessee, and thence north across the Atlantic to the British Isles. The 
Eocene isoflor connecting temperate floras likewise swings far to the north 



Fig, 5. California red- 
woods similar to those 
that grew as far north 
as the Arctic Circle in 
the Eocene Age. 




Fig. 6. Fossil redwood 
leaves from Eocene de- 
posits of St. Lawrence 
Island, Alaska, less than 
200 miles from the Arc- 
tic Circle. 



268 



THE THEORY OF CONTINENTAL DRIFT 269 

on the western coast of Europe to Spitzbergen, thence southward across 
Russia to Korea and southern Siberia, turning northward around the shore 
of the Pacific to Alaska, trending southeasterly across Canada, and north- 
ward again in the Atlantic on both sides of Greenland. Fewer fossil locali- 
ties are available for the boreal isoflor, but it also swings northward over 
oceans and southward across continents. So closely do the Eocene isoflors 
correspond in position to the modern winter isotherms of the northern 
hemisphere that we may assume they have essentially the same significance 
as indicators of minimum temperatures. And since they swing northward 
over the oceans as now constituted, southward over the continents as we 
know them to-day, we are forced to the conclusion that these ocean basins 
and continental platforms must have stood in essentially their present 
positions as far back as Eocene time. Again there is direct contradiction 
of the hypothesis that the northern continents have moved since the 
Eocene, and that the Atlantic basin has resulted from the gap formed by 
the cleavage of the New World from the Old. There must have been an 
Atlantic Ocean between North America and Europe at the time our fossil 
forests were living, else why should we have evidence of an Eocene equiva- 
lent of the Gulf Stream in the northward turning of the isoflors between 
Greenland and Scandinavia? Plotting the Eocene fossil plant localities on 
Wegener's Pangaea, the isoflors would have run in a nearly north-south 
direction rather than in parallel lines around the poles as do isotherms 
to-day, and as isotherms must always have run if heat from the sun has 
been the controlling factor in earth temperatures and plant distribution. 



V 


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Fig. 9. Distribution of Eocene isoflors in the northern hemisphere. 



270 DR. RALPH W. CHANEY 

We conclude that the evidence of Eocene floras, made up of close 
relatives of living trees whose climatic requirements are well known, 
strongly refutes the hypothesis of continental drift during later geologic 
time. The question of drift at an earlier date in earth history must be 
answered by reference to the nature and distribution of plant fossils in 
older rocks, and need not be considered here. But for tens of millions of 
years, since life on the earth has been similar to that of to-day. North 
America and Eurasia have occupied their present position with relation 
to the north pole and the ocean basins. During this latest chapter of Hfe 
history, forests have migrated southward in response to changing climate, 
over continents whose stability through the ages seems well established. 



LIST OF OLDER TERTIARY LOCALITIES OR FORMATIONS 

(Shown on Fig. 2) 

Cool temperate 
(1) Taimyr River, Siberia; (2) Boganida River, Siberia; (3) Tschirmyi, Siberia; (4) 
Tas-takh Lake, Siberia; (5) New Siberia Islands; (6) Banks Island; (7) Bathurst 
Island; (8) Ellesmere Island; (9) Grinnell Land. 

Temperate 
(10) Iceland; (II) Sabine Island, Greenland; (12) Spitzbergen; (13) Lozva River, 
Siberia; (14) Simonova, Siberia; (15) Fushun, Manchuria; (16) Kisshu-Meisen and 
Ryudu, Korea; (17) Possiet Bay, Siberia; (18) Khabarovsk, Siberia; (19) Dui, Sakhalin; 
(20) Naibuchi, Sakhalin; (21) Shitakara and Ishikari, Hokkaido, Japan; (22) Korf 
Gulf, Siberia; (23) Commander Islands; (24) Anadyr River, Siberia; (25) St. Lawrence 
Island; (26) Kobuk River, Alaska; (27) Eska Creek, Alaska; (28) Chignik, Alaska; 
(29) Cape Douglas, Alaska; (30) Port Graham, Alaska; (31) Central Yukon Valley, 
Alaska; (32) Berg Lake, Alaska; (33) Kupreanof Island, Alaska; (34) Great Bear River, 
MacKenzie; (35) Atanekerdluk, Disko Island, Greenland. 

Subtropical 
(36) Antrim County, Ireland; (37) Isle of Mull, Scotland; (38) London Clay, 
England; (39) Paris Basin, France; (40) Celas, France; (41) Sezanne, France; (42) 
Bavarian Alps; (43) Jesuitengraben, Bohemia; (44) Kiev, Ukraine; (45) Elisabethgrad, 
Ukraine; (46) Eroilan-duz, Turkestan; (47) Kasauli, India; (48) Deccan Plateau, 
India; (49) Assam, India; (50) Burma, Further India; (51) Na-giao, Indo-China; (52) 
Takashima, Kyushu, Japan; (53) Steel's Crossing, Washington; (54) Comstock, Oregon; 
(55) Goshen, Oregon; (56) Ashland, Oregon; (57) Weaverville, California; (58) Chalk 
Bluffs, California; (59) Clarno, John Day Basin, Oregon; (60) Swauk, Washington; 
(61) Calgary, Alberta; (62) Red Deer River, Alberta; (63) Upper Ravenscrag, Sas- 
katchewan; (64) Fort Union, from Yellowstone Park to South Dakota; (65) Wind 
River, Wyoming; (66) Green River, Wyoming; (67) Roche Percee, Saskatchewan; 
(68) Denver Beds, Colorado; (69) Raton, Colorado and New Mexico; (70) Wilcox, 
Claiborne and Jackson, southeastern United States; (71) Brandon, Vermont. 



Rock Magnetism 

• S. K. RUNCORN 



POLAR WANDERING, AS A GEOLOGICAL HYPOTHESIS, SEEMS 
to have been first mentioned in correspondence between Halley and Hooke. 
It is interesting that it was then invoked as an explanation of the occur- 
rence of marine fossils in sedimentary rocks well above sea level! In the 
early days of geology, Buffon and the "catastrophic school" were advocates 
of the shifting pole hypothesis as an essential element in the evolution 
of the earth's crust. Apparently Francis Bacon first suggested that con- 
tinental drift had occurred when he noticed the similarity of the Atlantic 
coast lines of South Africa and South America. 

Wegener gave the first thorough discussion of these hypotheses, open- 
ing a lively geological and geophysical discussion which reached its height 
in the 1920's. Of late, these important hypotheses have been discounted, 
partly because the geological data were complicated and by no means con- 
clusively in favor of them and partly for the less legitimate reason that a 
tenable explanation of the supposed phenomena had not been put for- 
ward. Darwin's famous paper on polar wandering was thought to have 
disposed of the possibility. The suggested explanations of continental drift 
were shown by Jeffreys and others to be incompatible with the inferences 
successfully drawn by geophysicists on the strength of the earth's interior. 
Yet Wegener's book, though dated, makes a strong case for continental 
drift. Later writers, such as Du Toit, amassed a great deal of information 
from structural geology and paleontology which, by its nature, could 
hardly appear decisive to the scientists in other fields and which perhaps 
unintentionally obscures some of the simpler and very persuasive reasons 
for serious consideration of continental drift. Moreover, these arguments 
are essentially qualitative, and their various presuppositions are open to 
criticism. They were therefore, perhaps unfortunately, not widely 
considered. 

Recently, renewed interest in the problem of polar wandering and con- 
tinental drift has resulted from paleomagnetic measurements. The direc- 
tions of the permanent magnetization of certain sedimentary and igneous 
rocks of many ages from various parts of the world have now been deter- 

• From Science (Apr. 17, 1959) pp. 1002-12. 

271 



272 S. K. RUNCORN 

mined. Most of the rocks studied have been well-bedded red sandstones 
and basaltic lavas. These rocks often possess a high degree of magnetic 
stability and have consistent directions of magnetization over consider- 
able distances within one continent in any one geological period. Other 
rocks are known to be permanently magnetized, but have not yet been so 
extensively studied. Basaltic lavas are found to have a strong permanent 
magnetization (with intensity of 10""^ to 10~^ electromagnetic units); red 
sandstones, a less strong magnetization (with intensity of from 10~^ to 
10"''' electromagnetic units). Except in Cenozoic times (the last 60 mil- 
lion years) these magnetizations are in different directions from the mag- 
netization induced by the present geomagnetic field. 

The possibility that the magnetization of rocks could be used in the 
investigation of polar wandering and continental drift has long been 
recognized. This follows from the supposition that the nondipole and 
equatorial dipole components of the geomagnetic field are oscillatory 
phenomena, and indeed changes in these components have been observed 
in recent centuries. This "geomagnetic secular variation" occurs because 
the field originates in the earth's fluid core (only a negligible amount 
arises from the f erromagnetism of the crust ) . Averaged over periods of the 
order of the free-decay time of electric currents in the core (a few thou- 
sand years), the field is reasonably expected, on theoretical grounds, to be 
that of a dipole at the geocenter oriented along the axis of rotation. If, 
therefore, mean directions of magnetization of a rock series, based on 
samples sufficiently spread stratigraphically to eliminate the secular varia- 
tion, are found to be different from the present mean field, there is a 
strong indication that those rocks were magnetized when they were in a 
different orientation with respect to, and at a different angular distance 
from, the axis of the earth's rotation at that particular geological time. 

It is interesting to note that William Gilbert, who was unaware of the 
existence of secular variation when he published his great work De Magnete 
in 1600, concluded (J) that "unless there should be a great dissolution of 
a continent and a subsidence of the land such as there was of the region 
Atlantis of which Plato and the ancients tell, the variation (i.e. the declina- 
tion) will continue perpetually immutable (in any one place)." As will be 
seen later, it appears that Gilbert's words were somewhat prophetic. 

Physical process of magnetization of lavas 

The magnetic mineral in a basaltic lava is usually a member of the mag- 
netite-ulvospinel solid-solution series. The Curie point of these is a maxi- 
mum (575°C) for pure magnetite and decreases with increasing titanium 
content. The process of magnetization of a lava has been very carefully 
studied by reheating samples of lava in the laboratory in zero magnetic 
field until the Curie point is reached and then cooling them in fields of 
about Vi gauss, meanwhile observing the magnetic moment of the sample 
at different temperatures (see, for example, 2). In principle, the process 



ROCK MAGNETISM 273 

by which magnetization is acquired on cooling is now well understood, 
from the standpoint of both experiment and theory. Normally, the coer- 
cive force and the intensity of magnetization decrease with temperature, 
the decrease being particularly rapid just below the Curie point. Conse- 
quently, in the presence of the geomagnetic field, the lava becomes 
strongly magnetized as it cools below the Curie point when its coercive 
force is low. On cooling to ordinary temperature, the coercive force rises 
to about 50 gauss, and subsequent changes in the direction of the geomag- 
netic field have no further influence on the magnetization. 

In some cases, however, the magnetization of the iron-oxide minerals is 
anomalous. Nagata (3) has found and carefully studied a pumice which 
becomes magnetized in a direction opposite to the field in which it cools, 
thus verifying a remarkable prediction made by Neel (4). This process 
occurs because the magnetic minerals are tiny intergrowths of two ferri- 
ilmenites. The component of higher Curie point becomes magnetized first 
as the pumice cools, but when the Curie point of the other component is 
reached, the geomagnetic field within the mineral has been overwhelmed 
by a field in the opposite direction due to the magnetization of the 
former component. Under certain conditions the "reversed magnetization" 
of the second component may outweigh the first. Such intergrowths are 
a common feature of the iron-oxide minerals in rocks, and so, although 
no other example of the phenomenon discovered by Nagata has yet been 
found in lavas, it has been held that similar processes are responsible for 
natural magnetizations with polarities opposite to that of the present geo- 
magnetic field. Such reversals have been found in the Tertiary lavas of the 
Columbia River plateau (Fig. 1), Iceland, Japan, and the Central Massif 



Fig. 1. Directions of 
magnetization of Co- 
lumbia River lavas: 
(solid circle) plotted on 
lower hemisphere; 
(open circle) plotted on 
upper hemisphere. 
(Cross) Field directions 
corresponding to geo- 
centric dipole along 
present geographical 
axis. [From measure- 
ments in C. D. Camp- 
bell and S. K. Runcorn. 
J. Geophys. Research 
61, 449 (1956)]. 




274 S. K. RUNCORN 

of France. They are also common in sediments and occur at all times in 
the geological column, though apparently with varying frequencies. The 
alternative explanation of these widespread reversals of magnetization 
throughout the geological column is, of course, that the geomagnetic field 
has, every few millions of years, reversed its polarity. 

The fact that Tertiary lavas, when examined today, have not been found 
to possess the self-reversal property which Nagata discovered cannot be 
held to exclude completely the possibility that they did not possess this 
property at the time of their cooling and magnetization, for slow changes 
take place in the iron oxide minerals with time. However, the natural 
occurrence of reverse magnetization is so widespread that it would be 
exceedingly strange if reversals are to be attributed mainly to these anoma- 
lous processes rather than to real and frequent reversals of the polarity of 
the field. However, nature can, on occasion, cover its tracks very well, and 
it may be said that the decisive experiment on this problem is yet to be 
performed. One test, however, has now been made in a large number of 
cases, and provides strong evidence in favor of real reversals of the geo- 
magnetic field. 

Many workers who have measured the magnetization of dykes and lava 
flows have also measured the magnetization of the country rock at small 
distances from the point of contact with the lava or dyke. The sampled 
country rock was heated, during the intrusion of the dyke or the extrusion 
of the lava, above its Curie point and so lost its original magnetization 
and acquired a thermoremanent magnetization at the same time as the 
lava or dyke. In every case so far reported the magnetization acquired by 
the country rock is in the same sense as that of the lava or dyke. Cases of 
dykes in contact with older lava flows were reported by Hospers in the 
lava flows in Iceland. Cases of lavas in contact with underlying sediments 
which were baked red were reported by Roche in the Central Massif of 
France and by Opdyke and Runcorn (5) in the lava fields near Flagstaff, 
Arizona. If it is supposed that reversals of the geomagnetic field do not 
occur and that the reversed magnetizations which are observed in approxi- 
mately 50 percent of Cenozoic lavas are due to the self-reversal property 
of the iron oxide minerals they contain, then one should, in about 50 per- 
cent of the cases studied, find the country rock and the igneous rock 
which bakes it having magnetizations in opposite senses. Although these 
contact-zone observations require further careful documentation, I feel 
that they exclude the possibility of any widespread "self-reversal" in nature. 

There is another piece of evidence which bears on this question of 
reversal. No lava of Recent times (that is, since the last ice age) has been 
observed to acquire a reversed magnetization. This fact again seems to 
me to exclude the possibility of the widespread occurrence of a self- 
reversal of the Nagata type, although it does not exclude the widespread 
occurrence of a reversal which occurs through slow chemical change or 



ROCK MAGNETISM 275 

through exsolution processes which might take a length of time of the 
order of a geological period to occur. 

Physical process of magnetization of sediments 

The first careful examination of the magnetization of sediments was 
made on the varved clays of New England and Sweden, which have been 
deposited in glacial lakes in the last several thousand years. There seems 
little doubt that the remanent magnetization of these clays arises from 
the magnetic orientation of the iron oxide grains, which retain some of 
the magnetization originally acquired in the igneous rocks from which 
the clays were derived by erosion. The varved clays may easily be dis- 
persed and redeposited in the laboratory under magnetic fields of various 
strengths and orientations, and it has been proved by Johnson, Murphy, 
and Torreson (6) and by Griffiths and King (7) that the clays become 
magnetized roughly in the direction of the field but with an "inclination 
error." This error arises from the tendency of the elongated or discoidal 
grains to lie parallel to the bottom. Since the particles will usually be mag- 
netized along a long axis, the permanent magnetization of the clay has 
a lower angle of magnetic inclination than the field in which the particles 
are deposited. Griffiths and King have also shown that currents in the 
water may affect the direction in which the elongated particles settle and 
hence may affect the direction of acquired magnetization. 

However, the study of varved clays has only limited application in paleo- 
magnetic studies, for these clays are of very infrequent occurrence in the 
geological column, and it is unwise to infer from these studies the process 
by which other sediments, particularly red sandstones, acquired their mag- 
netization. Laborator}^ experiments have only limited application to this 
subject, as it is impossible to infer or to reproduce exactly the physical 
and chemical conditions in which rock is laid down. It is possible that 
the remanent magnetization of varves may give a more correct value for 
the direction of the field at the time of magnetization than the laboratory 
experiments suggest, for it has been shown experimentally that, even after 
deposition, the water in the pores between the grains of the sediment 
enables the denser and smaller iron oxide grains to rotate in the direction 
of the field, and this process would appear not to be subject to the two 
causes of misalignment described above. As the varves are the only de- 
posits showing annual layers, they would appear to be ideal for the care- 
ful study of the short-term changes of the earth's magnetic field, known 
as the secular variation. 

By far the most widely studied of other sediments are the red sandstones 
and shales; it is an observed fact that red sandstones and shales are fre- 
quently much more strongly magnetized than other sediments and can 
very often be shown to possess "magnetic stability." By this is meant that 



276 S. K. RUNCORN 

they acquired a permanent magnetization early in their geological history 
and have retained it unaltered (at least within a few degrees) since. This 
important fact has been determined by the use of a "field" test of stabihty, 
first suggested by Graham (S). By finding pebbles in a conglomerate bed 
which were derived from the rock formation under study and determining 
the directions of magnetization which they have at present, information 
about the stability of the rock since the conglomerate bed was formed can 
be obtained. For example, pebbles of Torridonian (late Precambrian) sand- 
stone in a Triassic conglomerate are seen to have directions of magnetiza- 
tion in random directions in space, as they must have had when the 
conglomerate was formed. This is evidence that the Torridonian sand- 
stone has had magnetic stability over the last 150 million years. Similarly, 
folded or tilted beds which have directions of magnetization which agree 
with those of flat-lying beds of the same geological formation elsewhere 
only after allowance has been made for the geological dip, must have 
been magnetically stable since the tectonic movements took place. Had 
the original magnetization acquired when the beds were formed been 
unstable and a new magnetization been imposed by some agency after the 
rocks had attained their new positions, then the present directions of the 
magnetizations of the various samples would be more nearly parallel. 

Even when the red sandstones are not completely magnetically stable, 
the instability often takes a simple form : The rocks acquire a component, 
of intensity var}ang from specimen to specimen, directed along the mean 
geomagnetic field in recent times, which is known to be that of a dipole 
orientated along the present axis of rotation of the earth. Thus, the re- 
sultant directions of magnetization of samples from such a formation form 
a streak, rather than a well-grouped set, in a plane containing the present 
"dipole" direction and the direction of the field at the time of the for- 
mation of the rocks. Some information about the latter direction can 
therefore be obtained even from unstable rocks. An example is shown in 
Fig. 3. The cause of such a magnetization has not yet been established, 
although it is known that iron oxide minerals of a certain grain size cannot 
retain magnetization for long periods, and presumably an appreciable com- 
ponent of iron oxide of the critical grain size is present in some samples 
of the sediments. Such grains would slowly pick up a magnetization from 
the ambient magnetic field and so produce the above-mentioned eflPect. 

The comparative magnetic stability of the red sandstones can reason- 
ably be ascribed to the high coercive force (many thousands of gauss) of 
the hematite grains which they contain. The small grains forming the red 
coating of the quartz grains, of which the sandstones are mainly composed, 
and the black detrital iron oxide grains, which are usually present to the 
extent of about 1 percent of the whole rock, are usually found to be hema- 
tite. Miller and Folk (9) point out that red beds, in contrast to grey-green 
and white sediments, usually contain abundant detrital iron oxides, but 



ROCK MAGNETISM 277 




Fig. 3 (left). Streaking in Chinle formation (Upper Triassic) from 
Moab, Utah. Solid square indicates present dipole field. Solid line and 
solid circles are on the lower hemispheres of the projection; dashed line 
and open circles are on the upper hemispheres of the projection. Fig. 4 
(right). Magnetic directions in Triassic beds, Frenchtown, New Jersey. 
Solid square indicates present dipole field. Solid circles lie on the lower 
hemispheres of the projection; open circles lie on the upper hemispheres 
of the projection. 



they incorrectly describe these black detrital grains as magnetite and ilme- 
nite. It is not known which component carries the remanent magnetiza- 
tion, and it is likely that in some rocks it is the coating and, in some, the 
detrital minerals. It has, however, been shown by laboratory experiments 
that the crystallization of hematite from iron hydroxide soaking into a 
pure quartz sand in the earth's magnetic field leaves the sample perma- 
nently magnetized in the direction of the geomagnetic field. This phe- 
nomenon, called "chemical magnetization," deserves further study, but 
it seems reasonable to assume that, in the course of a chemical change 
producing a ferromagnetic mineral (even at ordinary temperature) the 
iron ions will become free to turn into the direction of a weak field and 
that, on the completion of the chemical change, the material will remain 
permanently magnetized unless it is exposed to a field of very much greater 
intensity. Another possibility is that the hematite grain grows beyond a 
critical size below which it has a very low coercive force but above which 
it is very stable. Such a process is suggested by Neel's theory of the mag- 
netization of single-domain grains {10). The directions of magnetization 
of sediments which acquired their magnetization in this way would not, 
of course, be expected to possess inclination error. 

It has been mentioned that the commonest recognizable form of mag- 
netization acquired by rocks since deposition is magnetization directed 
along the earth's present field, and one possible mechanism by which this 



278 S. K. RUNCORN 

is obtained has been described. Such magnetization is not uncommon in 
rocks now exposed in the southwestern United States, and it is also pos- 
sible that it is a recent chemical magnetization. In a hot climate in which 
there are at times heavy rains, it is possible that in the surface layers some 
of the hematite becomes hydrolyzed. Later on, the hydroxide formed de- 
composes to hematite again, which then picks up a magnetization parallel 
to the present field. This process would be expected to be particularly 
important in porous sediments. 

Other sources of secondary magnetization 

In the early days of study of rock magnetism, any anomalous mag- 
netizations found in rocks were usually ascribed to very special causes (see 
11), as it had not then been understood that rock formations usually 
possess a reasonably uniform magnetization over considerable areas. 
Lightning, in particular, was cited as a source of magnetizations in rocks, 
and effects of this kind were demonstrated by placing lava samples around 
the bottom of lightning conductors. There seems little doubt that lavas 
in exposed positions may get strong, but very localized, magnetizations in 
this way, though sufEcient studies do not seem to have been made of such 
anomalies. 

Recently the effect of mechanical stress on the direction of remanent 
magnetization has been discussed. Graham (12) has shown by laboratory 
experiments that the direction of magnetization of lavas and metamorphic 
rocks changes appreciably under uniaxial stresses of an order which might 
be produced by burial beneath some thousands of feet of rock. Although 
he is not able to show that such effects have irreversible characteristics, 
it is probably true that, over long periods of time, irreversible changes in 
the magnetization of rocks might occur in this way. In some rock forma- 
tions the agreement in the fault patterns over large distances suggests that 
stress systems are more than a local effect. Thus, it appears desirable to 
entertain the possibility that magnetostriction effects could alter the origi- 
nal magnetization of rocks in such a confusing way as to prevent the rema- 
nent magnetization of rocks under study from throwing light on major 
geophysical problems. This indeed seems to be what Graham suggests. 

However, although laborator}' experiments suggest ways in which the 
magnetization of rocks could be produced at or after deposition and could 
later be altered, deductions from them have little direct relevance to the 
interpretation of the remanent magnetization of rocks. This is a surpris- 
ing point of view only to those who imagine that the physics of the proc- 
esses by which rocks are formed and the history of the rocks are known in 
quantitative detail. 

There will be those who hold that if this is true we might as well aban- 
don the subject; however, this does not seem to be the way the scientist 



ROCK MAGNETISM 279 

works— he tries to make sense of those observations of the physical world 
which can be made. Therefore, while laboratory experiments on the mag- 
netic properties of rocks are interesting for their own sake and need to be 
pursued extensively, carefully analyzed field measurements are more 
likely to reveal how any particular rock formation became magnetized. 

Results of paleomagnetic surveys 

A typical example of a well-grouped set of paleomagnetic directions is 
given in Fig. 4. Some of the scatter may represent the effect of the wob- 
bling of the geomagnetic field about its mean position over the period 
of time represented by the rock series. The mean direction of such a set 
of measurements is assumed to correspond to the field which would be 
produced at that place by a geocentric dipole oriented along the earth's 
axis of rotation at that time. A simple formula of spherical trigonometry 
makes it possible to calculate the position of the poles for that geological 
time on the present globe, that is, assuming for the moment that the 
present distribution of the continents has remained unchanged. 

In the last few years there has been a very rapid increase in the accumu- 
lation of paleomagnetic data. The initial aim has been to trace in outhne 
the changes in the earth's magnetic field throughout geological time, as 
determined by rocks from different continents. For this purpose the sam- 
pling has been restricted in certain ways: 

1 ) Study has so far been restricted to rather strongly magnetized rocks. 
Because initial surveys showed that, in general, among igneous rocks, lavas, 
and, among sediments, red sandstones, are most strongly magnetized, the 
study has largely been restricted to these. It is not known that weakly mag- 
netized rocks are intrinsically less useful for purposes of this study, but 
strongly magnetized rocks can be more easily measured, and consequently 
changes in their magnetization in the course of the laboratory processes 
can be more easily observed. 

2) The main sampling has been carried out in those areas where very 
little tectonic movement has occurred, on the grounds that stress and 
rise of temperature might irreversibly affect the original direction of 
magnetization. 

3) Surveys throughout the geological column have been made, rather 
than very extensive collections of rocks from one particular period, al- 
though certain rock series, such as the Torridonian sandstone, have been 
studied in great detail. 

In the interpretation of paleomagnetic data it has been assumed, on the 
basis of theory, that the geomagnetic field, when averaged over some thou- 
sands of years, is a dipole directed along the axis of rotation. This theory 
has experimental support in that it accords with paleomagnetic observa- 
tions for late Tertiary and Quaternary times in different areas of the world. 



'280 S. K. RUNCORN 

Collections of samples from rock formations selected in the way described 
above have been measured on an astatic magnetometer, and their direc- 
tions of magnetization have been checked in some cases with a spinner 
magnetometer. 

We have taken steps to eliminate, or allow for, the effect of magnetiza- 
tion acquired in recent times along the present direction of the earth's 
magnetic field. Where possible, the field tests of stabihty of magnetization 
of folded beds and conglomerates have been used. A degree of stability is 
invariably found in such rocks. In the vast majority of cases a geological 
formation gives a well-grouped set of directions of magnetization, from 
which the mean can be calculated. The mean has been designated as being 
the direction of the magnetic field of a given geological period (or part 
of a period) minus the effect of the geomagnetic secular variation. The 
pole position calculated from this direction and from the present geo- 
graphical latitude and longitude of the site is not only the mean magnetic 




Fig. 5. Polar wandering paths for North America and Europe. ( — • — ) 
Path inferred from British rocks, plotted in northern hemisphere; (~o— ) 
path inferred from British rocks, plotted in southern hemisphere; ( — A — ) 
path inferred from American rocks, plotted in northern hemisphere; 

( ) plotted in southern hemisphere. (M) Miocene; (E) Eocene; 

(K) Cretaceous; (Tj.) Triassic; (P) Permian; (Cp) Cambrian, Pennsyl- 
vanian; (D) Devonian; (S) Silurian; (O) Ordovician; (e) Cambrian. 
(A) Algonkian (late Precambrian of the United States) : (A^) Hakatai 
shales (Doell's measurements) ; (A^) Hakatai (south rim of Grand Can- 
yon) ; (Pre-€) Late Precambrian of Great Britain: (Pre-c^) lower Torri- 
donian; (Pre-c^) Langmyndian; (Pre-c*) Upper Torridonian. 



ROCK MAGNETISM 281 

pole for that period of time but is assumed to be the pole of rotation of 
the earth relative to the continent in question. 

From Precambrian times to the present, pole positions have been deter- 
mined relative to Great Britain, North America, and Australia (Figs. 5 
and 6). The following features of these pole positions, or "polar wander- 
ing curves," as they are called, have been found: 




Fig. 6. Stereographic projection showing position of Australia relative 
to the pole. (PPR) Pliocene, Pleistocene, and Recent (newer volcanics 
of Victoria) ; (E) Lower Tertiary, probably Eocene (older volcanics of 
Victoria) ; (J) Mesozoic, probably Jurassic (Dolerite sills of Tasmania) ; 
(Tj.) Triassic, probably lower Triassic (Brisbane tuff) ; (P2) Permian, 
Upper Marine Series (volcanics of Illawarra coast); (Pi) Permian, 
Lower Marine Series (volcanics of Hunter Valley); (C) Upper Car- 
boniferous (Kuttung red varvoid sediments and Kuttung lavas) ; (D) 
Devonian, probably Lower Devonian (Ainslie volcanics); (S) Upper 
Silurian (Mugga porphyry); (€2) Middle Cambrian (Elder Mountain 
sandstone); («i) Lower Cambrian (Antrim plateau basalts); (Pre-cs) 
Top of Upper Proterozoic (Buldiva quartzite) ; (Pre-C2) Upper Pro- 
terozoic (Mallagine lavas); (Pre-€i) lower part of Upper Proterozoic 
(Edith River volcanics). 



1) Pole positions of successive geological periods lie on a reasonably 
smooth curve, and they lie successively nearer the present pole as their 
age diminishes. 

2) The curves drawn through these pole positions for the two conti- 
nents of Europe and North America are of roughly similar shape, whereas 
that for Australia is different. 



282 S. K. RUNCORN 

3) There is systematic displacement between the curves for Europe 
and North America which has been interpreted by Runcorn (13) as show- 
ing that, after Triassic times, a relative motion of North America and 
Europe took place. It is not by any means easy to be specific about the 
value of this displacement, but estimates range from a value of about 24° 
(see Figs. 7 and 8) to 45° (see 14). 




Fig. 7. Upper Triassic 
pole positions for the 
United States and Great 
Britain. (1) Springdale 
sandstone, Utah ; (2) 
lavas near Holyoke, 
Massachusetts ; (3) lavas 
and sediments of Con- 
necticut; (4) Newmark 
series, New Jersey; (5) 
Keuper marls, England. 
[P. M. du Bois, E. Irv- 
ing, N. D. Opdyke, S. 
K. Runcorn, M. Banks, 
Nature 180, 1186 (1957)] 



Fig. 8. Wind directions 
and equator for Paleo- 
zoic times. Solid circle, 
Carboniferous pole po- 
sition; arrows, paleo- 
wind directions in Per- 
mocarboniferous times. 




ROCK MAGNETISM 283 

4) Results obtained in Australia (IS), South America {16), and South 
Africa [17] lead one to suppose that a very considerable amount of con- 
tinental drift occurred in the Southern Hemisphere in Mesozoic times. 

Statistical methods in measuring rock magnetism 

It is, perhaps, at first sight surprising that measurements of the paleo- 
magnetic directions of, say, a dozen samples from a rock formation hun- 
dreds of feet thick and covering hundreds of square miles may provide 
an adequate estimate of the direction of the earth's field during the epoch 
in which these rocks were laid down. To the geologist a rock formation 
is a series of rocks, whose lithological character enables them to be traced 
and, in consequence, mapped, over a considerable area of country. The 
rocks comprising the formation will be laid down in similar environments 
or in a series of alternating environments. A formation usually spans a 
fraction of a geological period— perhaps some million years. In the case of 
basaltic flows, a single flow may be traced over many tens of miles, over 
which its thickness remains remarkably constant. Therefore it must have 
flowed out, solidified, and cooled below the Curie point within a few 
months. Consequently, in a single flow one might expect the lava to record 
the direction of the earth's field at a point of time. The flow lying upon 
it will likewise provide a record of the value for the field at another point 
of time, perhaps many hundreds or thousands of years later. In practice 
it seems that the directions of magnetization of samples from a single 
lava flow are scattered because of the magnetic disturbances produced by 
neighboring flows, but this problem has not yet been studied carefully. 

In a sedimentary formation the time relations between different sam- 
ples present a difficult problem. Commonly, a sediment possesses innumer- 
able bedding planes, recognizable today as planes of weakness which are 
revealed by erosion. Such planes represent surfaces on which the rate or 
type of deposition changed, or down to which erosion removed previously 
deposited sediment. Such bedding planes may therefore represent long 
intervals of time. Between successive bedding planes the sedimentary 
material may be deposited rapidly; these may become magnetized in a 
time much less than that in which the magnetic field can alter by a few 
degrees— that is, in a time much less than the time scale of the secular 
variation (18). Further, in lacustrine, deltaic, and marine sediments de- 
posited offshore in a transgressing sea, sedimentation is not continuous 
over the entire area now represented by these rocks. Consequently the 
sediments are in the form of wedges, which disappear when traced later- 
ally. Similarly, the bottom and top of a sedimentary formation at one 
place will not represent the same time-span as analogous horizons of the 
same rock formation in a different place; a time line running through the 
formation will therefore, in general, make an angle with the bedding 
planes. 



284 S. K. RUNCORN 

The above theory therefore suggests that if samples are selected from 
different horizons spanning a considerable thickness of the formation, the 
mean direction should effectively average out the secular variation and 
any deviations due to polar wandering during the time represented by 
the formation. 

It is found that the directions of such samples are scattered randomly 
about a mean direction, and Fisher (19) has suggested that the relative 
frequency of directions at an angle q with this mean is given by ircose, 
where K is a measure of the precision. If each of the N directions is rep- 
resented by a unit vector, then the magnitude of the vector sum R will be 
much less than N if there is great scatter and will be nearly equal to N in 
the case of a close grouping of directions. Fisher shows that an estimate 
of K is provided by (N — J)/(N — R) and that the best estimate of the 
mean direction is the vector mean. I have given the approximate formula 
that 63 percent of the directions make an angle with the mean direction of 
less than 81 /VK degrees (J8). I have also shown that the angular radius 
of the cone of confidence which, described about the calculated mean 
direction, includes the true mean direction with a probability of 95 per- 
cent equals approximately 140 VKN in degrees. It can therefore be readily 
seen that if K is 100, 63 percent of the directions he within a cone of 
semiangle of 8° described about the vector mean direction, and the 
angle of the cone of confidence can be reduced to within 5° by taking 
about ten samples. 

Just as there are local magnetic anomalies on the earth's surface today 
which alter the direction of the geomagnetic field (for example, at Kursk, 
U.S.S.R.), so there will undoubtedly be found anomalous paleomagnetic 
directions. 

It may be asked whether it is possible to show that over very consider- 
able areas the direction of the magnetic field deduced from the paleomag- 
netic measurements is consistent. There are not yet as many measurements 
relating to this point as one would like. But almost every rock formation 
which has been studied extends over hundreds of miles, and there is cer- 
tainly consistency in the paleomagnetic directions to this extent. It is much 
more interesting, however, to consider whether the paleomagnetic meas- 
urements of rock formations of the same age across an entire continent 
give poles vv'hich are in the same place. In this connection it must be noted 
that the polar-wandering curve indicates a mean movement of the pole of 
about one-third of a degree per million years, and consequently it is quite 
possible that, during a geological period, the polar motion (apart from the 
secular variation which is assumed to be smoothed out in all cases) could 
lead to discrepancies of up to 20 or 30 degrees in the paleomagnetic direc- 
tions of rocks of the same geological period. Unfortunately the rocks which 
have been used so far in studies of paleomagnetism are, of course, those in 
which fossils are most scarce, and consequently the determination of the 



ROCK MAGNETISM 285 

geological age to any accuracy very much shorter than a geological period 
seems rather difficult. However, the Upper Triassic of the United States 
furnishes an example of the good agreement between pole positions from 
widely different areas, as is shown in Fig. 7. 

Paleowind directions 

For independent evidence of polar wandering, recourse must be had to 
the evidence of paleoclimatology. The methods geologists have used in 
such investigations are not quantitative and are open to various objections. 
It is of interest to consider whether there are more physical methods of 
determining the latitude and orientation with respect to the axis of rota- 
tion of land masses at different geological times. 

The explanation of the deflection to the east of the winds blowing 
toward the equator in the trade-wind zones was given long ago by Hadley 
and concerns the deflecting action of the Coriolis force on air drawn to 
the equator. Consequently, it is probable that through geological time 
there has always been a trade-wind belt, although its extent in latitude may 
have altered. Recently, Opdyke and Runcorn {20) have examined the 
question of whether the winds in ancient geological time were appropri- 
ately orientated relative to the equator of that time. That the direction 
of the wind which transported sand in the accumulation of certain aeolian 
deposits may be determined by measurements of the direction of the line 
of greatest dip in cross-laminated rocks is a theory that has been developed 
by Reiche (21) and Shotton {22). These authors showed that the Coco- 
nino sandstone of Arizona and the New Red Sandstone of Great Britain 
represent the accumulation of many crescentic or barchan dunes, traces 
of the lee slopes of which are revealed in exposures of these rocks as cross 
laminations of large size. Modern barchan dunes have been carefully 
studied by Bagnold in the Libyan Desert and by many other workers. 
Steady wind blows sand up the gently sloping windward side of the dune, 
the sand falling on the lee slope at its angle of repose, about 32^/2°. The 
laminations of the lee slope are consequently protected from erosion, and 
apparently may be preserved (perhaps truncated) if the dune sea consoli- 
dates into rock. 

The crescentic shape of the dune causes the direction of the line of 
greatest dip of the cross lamination to be spread over about a right angle, 
so that the wind direction at one locality is the mean of these directions 
obtained from a number of cross-laminated units, each of which repre- 
sents a different part of the dune. 

Cross laminations can arise from deposition in rivers and in beach de- 
posits but are usually of smaller scale. There is, however, no single criterion 
which permits classification of a cross-bedded sandstone as aeolian or not 
aeolian (23). Opdyke and Runcorn {20) show that certain parts of the 



286 S. K. RUNCORN 

Tensleep, Casper, and Weber sandstones, of Wyoming and Utah, of Penn- 
sylvania age are likely to be aeolian. They show that the wind which de- 
posited these sandstones came from the northeast quadrant, as is true 
also in the case of the Coconino sandstone of similar late Paleozoic age, 
studied by Reiche {21). The consistency of these wind directions over a 
large area is shown in Fig. 9. 




Fig. 9. Paleowind di- 
rections. 



It is, of course, true that the wind today is affected by topography, the 
planetary wind system being considerably distorted in certain areas. The 
consistency of the wind directions described above, however, indicates 
that this wind is probably a planetary wind and not one affected decisively 
by local geography. It must be remembered that the present time is one 
of unusually high relief, and it may be that the planetary wind system 
was less distorted in remote geological time. Again, it must be remem- 
bered that a rock series represents a long period of time during which 
local effects may be expected to average out, to some extent. There is an 
analogy here with rock magnetism, in which the mean direction of mag- 



ROCK MAGNETISM 287 

netization of a geological period apparently averages out the nondipole 
parts of the geomagnetic field which are of importance at any one instant 
of time. 

It will probably not be possible to map the directions of the ancient 
winds in the detail in which the ancient magnetic field can be mapped, 
unless some method apart from the study of aeolian sandstones, which 
appear to occur infrequently in the geological column, can be found. But 
it is interesting to see from Fig. 8 how the late Paleozoic wind directions 
of North America and Great Britain fit in as the northeast trade winds 
relative to the late Paleozoic equator, derived from paleomagnetic studies. 

Geological evidence of paleoclimates 

The traditional method of inferring the climates of a geological period 
depends on the type of sediment and on the fossil record. It cannot be 
said that most of the evidence is of a type which can be interpreted un- 
ambiguously. For an explanatory comparison of the paleomagnetic and 
paleoclimatic evidence, we use two of the least disputable inferences from 
the geological record. 

1 ) Evidence of glaciation over considerable areas in Permocarboniferous 
times has been found in Australia, South Africa, South America, and India. 
Unless there has been radical change in the climate of the globe as a 
whole, we can infer that such glaciations were restricted to the then polar 
regions. Simpson (24) suggests that extensive sea-level glaciation could 
not have occurred at latitudes of less than about 50°. The paleomagnetic 
observations show that Australia was in high latitudes in Permocarbon- 
iferous times and also in late Precambrian times when there is ajso evidence 
of glaciation in Australia. Paleomagnetic surveys of South Africa, South 
America, and India for Permocarboniferous times are of key importance. 

2) Occurrence of extensive red beds suggests either a hot, humid or a 
hot, arid climate. It is difficult to see how such conditions could occur 
except near the equator if the axis of rotation of the earth is nearly per- 
pendicular to the ecliptic. Similarly, dune sandstones and evaporites indi- 
cate a position close to the equator. Abundant beds of the type described 
are typical of northern Europe and Great Britain from the Devonian to 
the Triassic, of western United States between Pennsylvania and Jurassic 
times, and of eastern United States between the Silurian and Triassic. 
The paleomagnetic determinations put Great Britain and the United 
States in low latitudes during Paleozoic and early Mesozoic times. Divid- 
ing the values of the paleomagnetic angles of inchnation less than about 
30° by 2 gives the corresponding latitudes quite accurately {2S). 

Hypothesis of polar wandering 

The evidence of paleomagnetism, with which that of paleoclimates does 
not conflict, suggests that the poles of rotation of the earth and the land 



288 S. K. RUNCORN 

masses have gradually changed their relative positions. We must therefore 
briefly consider the mechanism by which polar wandering and continental 
drift could have been brought about. The latter involves more degrees of 
freedom than the former, but, fundamentally, both require that the earth 
be able to flow if subjected to steady stresses over millions of years, and 
both require that there be internal movements of some kind. Recently 
the mechanics of polar wandering has been discussed in outline. Clearly, 
what is required is that if the axis of figure of the earth is displaced from 
the axis of rotation by an infinitesimal amount, the stresses due to the 
centrifugal forces will cause the earth to flow so that the equatorial bulge 
will return to a plane perpendicular to the new axis of rotation. The time 
constant of this process appears to be between a few hundred thousand 
years and a few million years. The physical cause which displaces the two 
axes in the first place is a matter for conjecture. Random disturbances in 
the crust or processes in the mantle are possibilities. Mountain building 
and convection currents in the mantle have been shown to be adequate 
causes. It should perhaps be emphasized that no change in the direction 
of the axis of rotation in space, that is, no change in the angular momen- 
tum of the earth, is involved in these processes. 

Hypothesis of continental displacements 

Probably most geologists and geophysicists feel reluctant to admit the 
possibilitv of relative displacements of the continental masses in the recent 
history of the earth. It is often stated that a sound reason for such skep- 
ticism is the absence of any adequate theory of the mechanism by which 
such continental displacement could have taken place. This is an argu- 
ment which should not be given much weight. Not until the last few 
years has there been an adequate theory for the existence of the geomag- 
netic field, but scientists did not previously disbelieve in the existence of 
the field for this reason. 

That the coast line of much of South Africa and South America fits 
together is of course a fact which the exponents of continental drift have 
thought very significant. JeflFreys' {26) statement that the fit is a poor one 
has recently been shown to be untrue by Carey {27). It is significant also 
that the mid-Atlantic ridge follows a line parallel to these two coast lines. 

It is perhaps significant that the continental displacements of thousands 
of miles since the late Mesozoic represent an annual rate of movement of 
the same order as that occurring along the San Andreas fault {28). By 
geodetic observations this has been determined to be 1 centimeter per year 
at the present time. Geological correlation suggests that there has been a 
displacement of possibly 350 miles in 100 million years, or 0.6 centimeter 
per year. The existence of this relative motion in the earth's crust today 
implies that movements deeper in the crust are taking place for which we 



ROCK MAGNETISM 289 

have no adequate theory. We have no means of knowing whether such 
movements are capable of causing relative movements of larger areas of 
continental material. 

Perhaps thermal convection in the mantle is occurring, and this may be 
the explanation of continental drift. It is well known that the present dis- 
tribution of continents and oceans has certain regularities. The oceans and 
continents are diametrically opposite, and only 3 percent of the area of the 
continents has land antipodal. Prey and Vening Meinesz have expressed 
this fact mathematically by showing that if the height or depth of the rock 
surface is expressed as a series of spherical harmonics, the first, third, 
fourth, and fifth harmonics are predominant. Vening Meinesz draws the 
inference that the present distribution of the continents is fixed by the 
presence in the mantle of convection currents with a certain number of 
cells. One would infer that the continental rafts would be drawn toward 
those parts of the world where the convection currents are falling. At first 
sight it appears strange that the dispersion of the continents occurred so 
late in the history of the earth. If the above argument is accepted, then 
the dispersion of the continents at the end of Mesozoic time must reflect 
a change in the convection patterns in the mantle at that time. 

It is not easy to suggest a reason for a change in the convection pattern 
so late in geological time, but it may be the result of a gradually growing 
core, which, as its radius increased, would favor convection with a higher 
number of cells. It has been suggested that the present concentration of 
the land masses in one hemisphere is the result of a primevil convection 
current consisting of a single cell which swept the continental material to 
one area. Such a single cell convection pattern would, however, be set up 
only if the heavy iron core was then very small. The idea of a core growing 
through geological time, rather than one formed initially, has been postu- 
lated by H. C. Urey in recent years, and may now receive support from 
continental drift. 



REFERENCES AND NOTES 

1. W . Gilbert, De Magnete (1600), book 4, chap. 3. 

2. T. Nagata, Rock Magnetism (Maruzen, Tokyo, Japan, 1953) . 

3. ,Naturel7S, 35 (1955). 

4. L. Ned, Ann. geophys. 7, 90 (1951 ) . 

5. N. D. Opdyke and S. K. Runcorn, Plateau 29, 1 (1956) . 

6. E. A. Johnson, Murphy, O. W. Torreson, Terrestrial Magnetism and Atmospheric 
EZec. 53, 349 (1948). 

7. D. H. Griffiths and R. F. King, Monthly Notices Roy. Astron. Soc. Geophys. Suppl. 
7, 103 (1957). 

8. J. W. Graham, J. Geophys. Research 54, 131 (1949). 

9. D. N. Miller and R. L. Folk, Bull. Am. Assoc. Petrol. Geologists 39, 338 (1955). 

10. L.Neel, Advances mPhys. 4. 191 (1955). 

11. C. A. Heiland, Geophysical Exploration (Prentice-Hall, Englewood Cliffs, N. J., 
1940). 



290 S. K. RUNCORN 

12. J. W. Graham, /. Geophys. Research 61, 735 (1956); Advances in Phys. 6, 362 
(1957). 

13. S. K. Runcorn, Proc. Geol. Assoc. Can. 8, 77 (1956) . 

14. P. M. du Bois, Advances in Phys. 6, 177 (1957) . 

15. E. Irving and R. Green, Geophys. /. J, 64 (1958) . 

16. K.M. Creer, Ann. geophys. J4, 373 (1958). 

17. A.E.M. Nairn, NdfuT-e 178,935 (1956). 

18. S. K. Runcorn, Advances in Phys. 6, 169 (1957) . 

19. R. A. Fisher, Proc. Roy. Soc. (London) 217 A, 29S (1953). 

20. N. D. Opdyke and S. K. Runcorn, Bull Geol. Soc. Am., in press. 

21. P. Reiche, /. GeoZ. 46, 905 (1938). 

22. F. W. Shotton, Geol. Mag. 74, 534 (1937); Liverpool Manchester Geol. J. 1, 450 
(1956). 

23. For discussion of this subject, see W. H. Twenhofel, A Treatise on Sedimentation 
(McGraw-Hill, New York), p. 610. 

24. G. C. Simpson, Quart. J. Roy. Meteorol. Soc. 83, 459 (1957). 

25. D. W. Collinson, K. M. Creer, E. Irving, S. K. Runcorn, Phil. Trans. Roy. Soc. 
(London) ISO, 13 (1957). 

26. H. Jeffreys, The Earth (Cambridge Univ. Press, ed. 3, 1952), p. 392. 

27. S. W. Carey, Geo/. Mdg. 92, 196 (1955). 

28. K. L. Hill and T. W. Dibblee, Bull. Geol. Soc. Am. 64, 443 (1953). 



THE X • .. • , , . . 

i am, in point of fact, a particularly haughty 
"HTQT"OR V ^"^ exclusive person, of pre-Adamite 

ancestral descent. You will understand this 
when I tell you that I can trace my ancestry 
^^ back to a protoplasmal primordial atomic 

slobule. — w. s. GILBERT, The Mikado 

LIFE 



The Course of Evolution 

• GEORGE GAYLORD SIMPSON 



THE MEANING OF HUMAN LIFE AND THE DESTINY OF MAN 
cannot be separated from the meaning and destiny of hfe in general. 
"What is man?" is a special case of "What is Hfe?" The extent to which 
we can hope to understand ourselves and to plan our future depends in 
some measure on our ability to read the riddles of the past. The present, 
for all its awesome importance to us who chance to dwell in it, is only a 
random point in the long flow of time. Life is one and continuous in space 
and in time. The processes of life can be adequately displayed only in the 
course of life throughout the long ages of its existence. 

How old is life? We do not know, but we have some interesting clues. 
Aside from fantastic fiction, life can be no older than the earth. Measure- 
ments of the results of radioactivity in certain minerals have established 
that some rocks in the earth's crust are about 2,000,000,000 years old. There 
is evidence that even these astonishingly ancient rocks were formed long 
after the planet earth came into existence. The whole age of the earth is 
probably on the order of 3,000,000,000 years. That we judge to be the pos- 
sible span of life on the earth, although a billion years or more may have 
passed after the earth was formed and before life arose. 

How did life arise? Again, the honest answer is that we do not know but 
that we have some good clues. This ultimate mystery is more and more 
nearly approached by recent studies on the chemical activity of living par- 
ticles, of viruses and of genes, the submicroscopic determiners of heredity 
and growth. The most fundamental properties of life are reproduction and 
change (or mutation). Particles with these properties would be, in essence, 

• From The Meaning of Evolution, pp. 13-26. Published as a Mentor Book by arrange- 
ment with Yale University Press, 1951. 

291 



292 GEORGE GAYLORD SIMPSON 

alive, and from them all more complex forms of life could readily arise. 
Current studies suggest that it would be no miracle, nor even a great statis- 
tical improbability, if living molecules appeared spontaneously under spe- 
cial conditions of surface waters rich in the carbon compounds that are the 
food and substance of life. And the occurrence of such waters at early 
stages of the planet's evolution is more probable than not. This is not to 
say that the origin of hfe was by chance or by supernatural intervention, 
but that it was in accordance with the grand, eternal physical laws of the 
universe. It need not have been miraculous, except as the existence of the 
physical universe may be considered a miracle. 

Fossils, primary documents of the historians of life, can tell us nothing 
of the very earliest stages. Truly primeval life was tiny, fragile, soft-bodied, 
without resistant parts that can have endured such long burial in rocks 
heated and cooled, deep-sunk and upflung in the slow down-warpings and 
upheavals of the planet's crust. The oldest fossils surely recognized are sim- 
ple water plants, algae, primitive enough, to be sure, and yet already several 
strides along the road of evolution. Their age is at least 1,000,000,000 years, 
perhaps more. Even after these first fossils, a tremendous time elapsed be- 
fore life became highly varied or began to leave a fairly good and continu- 
ous fossil record. Evolution is a cumulative process and in it, as usual in 
such processes, there is an effect of acceleration. Early stages were aeon- 
long and slow almost beyond imagination. They built a basis on which, 
finally, more rapid evolution occurred. 

One of the episodes of rapid evolution, and apparently the most funda- 
mental of all, occurred some 500,000,000 years or more ago, around the 
beginning of the. Cambrian period of the geologists. At this point it is best 
to introduce the geologic time scale, by which the sequence and relative 
timing of events in the history of life are most readily dated, a sort of chron- 
ological shorthand convenient both for writer and for reader. . . . 

At the beginning of the Cambrian, fossils became abundant, and their 
basic diversity increased rapidly through the Cambrian and the following 
period, the Ordovician. "Rapidly" must, to be sure, be considered relatively 
in this connection. The length of time involved was on the order of 100 to 
150 million years, which is no short span even to a geologist. Yet it is only 
about a tenth of the length of the long pre-Cambrian preliminaries, and 
the evolutionary divergence of the organisms now appearing in the fossil 
record is really very profound and fundamental. Indeed during the Cam- 
brian and Ordovician all the really important and really basic types of ani- 
mal structure appear in the fossil record, although each was at first repre- 
sented by relatively few and extremely primitive forms. 

Most zoologists classify animals into about twenty major groups, called 
phyla (singular: phylum), each representing a fundamental anatomical 
plan. Some students recognize more than twenty phyla and some fewer, 
but the differences of opinion relate almost entirely to a small number of 



THE COURSE OF EVOLUTION 293 

peculiar, soft-bodied living animals of uncertain origin, of no real impor- 
tance in the modern fauna and practically without fossil records. Animals 
of real importance today or in the history of life may all be referred to only 
fifteen basic phyla. Five of these are collectively called "worms" and have 
poor fossil records. The other ten have, by and large, good fossil records 
and their histories since the Cambrian or Ordovician can be followed satis- 
factorily in broad outline, although it hardly needs saying that innumerable 
details need to be filled in. 

The most important, broadest groups of animals in the history of life are 
as follows: 

1. Protozoa. These animals have no differentiation of their substance 
into separate cells, each individual consisting of a single mass of protoplasm 
analogous to one cell of higher animals, all of which are many-celled or 
metazoan. The relatively simple protozoan structure can function only in 
very small animals and most of them are microscopic. Among the many 
sorts of protozoans, the Foraminifera, secreting tiny limy supports or skele- 
tons, are particularly abundant as fossils. 

2. Porifera. These are the sponges. They are many-celled and some of the 
cells have differentiated functions, but they are not clearly arranged in 
definite layers or well divided into separate sorts of tissues. Many sponges 
have had internal supports which have left a long, although not a particu- 
larly continuous or impressive, fossil record. 

3. Coelenterata. This is a large and highly varied phylum of aquatic, 
mostly marine, animals with the cells differentiated into two distinct tissue 
layers. The most familiar coelenterates, the corals, have left one of the best 
known fossil records. 

4. Graptolithina. This is an extinct group of puzzling nature which prob- 
ably does not merit recognition as a truly basic phylum. The trouble is that 
the preserved parts leave doubt as to which phylum should contain them. 
They are listed because they have left a rich fossil record and were evi- 
dently important denizens of the Paleozoic seas. 

5.-9. Platyhelminthes. Nemertinea. Nemathelminthes. Trochelminthes. 
Annelida. These are mainly "worms" in popular conception, but their basic 
anatomical characters are so varied that separation into five phyla is war- 
ranted. Collectively, the "worms" are now extremely important in the 
economy of nature, but they are mostly small, soft-bodied forms and their 
fossil record is poor. 

10. Bryozoa. These are colonial, aquatic, mostly marine forms sometimes 
called "moss animals." They resemble some corals, but are more complex 
in having three, rather than two, primary tissue layers and a body cavity 
(coelom) in the middle layer. The colonies secrete skeletal supports which 
are easily fossilized and have given this group an unusually good fossil 
record. 



294 GEORGE GAYLORD SIMPSON 




Protozoa 



Porifera Coelenterata 




Annelida 



Brachiopoda 



Bryozoa 




Echinodermata 



^ 



MoUusca Arthropoda 




Chorda to 



Fig. 1. Examples of the 
most important basic 
types, phyla, of animals. 
All the phyla include 
large numbers of ex- 
tremely diverse animals 
many of which look 
radically different from 
these examples. For 
characterizations of 
each group see the text. 
The specimens are 
drawn to different 
scales. 



11. Brachiopoda. These forms, the marine lamp shells, resemble clams in 
having two external shells. Unlike clams, however, each shell is usually sym- 
metrical around a midline, and the internal anatomy is quite different from 
clams. Like all shells, they fossilize easily and have left an excellent histori- 
cal record. 

12. Echinodermata. This an unusually varied phylum, including such 
forms as starfishes, sea urchins, and sea lilies. The tissue and organ differ- 
entiation is of advanced type. There is an underlying bilateral symmetry, 
but this is usually obscured by a secondary, five-rayed, radial symmetry. A 
complex skeleton of limy plates develops in the middle tissue layer and has 
resulted in a long and rich fossil record. 

13. Mollusca. This is the most dominant and successful of the mainly 
aquatic phyla at present and through much of the history of life. Repre- 
sentatives include clams, snails, octopuses, and others. The fossil record is 
outstanding, probably better than for any other phylum. 

14. Arthropoda. In sheer weight of numbers, this is the most successful 
of all the phyla, for it includes the insects, now several times more varied 
than all other animals put together. Other representatives are crabs, scor- 
pions, spiders, and the extinct trilobites. Characteristics include a jointed 
body with legs and with a hard outer coating. The fossil record is good for 
some groups but it is deficient indication of the probable abundance of 
others, especially of the insects in later geologic times. 

15. Chordata. Like some other advanced phyla, the chordates have com- 
plex tissue and organ differentiation and bilateral symmetry, to which they 
add basic mobility promoted by an internal longitudinal rod, jointed except 
in the most backward types. The jointed-backbone forms, comprising the 
bulk of the phylum, are the Vertebrata or vertebrates. Vertebrates in- 
clude "fishes" (really four different aquatic groups), amphibians, reptiles, 



THE COURSE OF EVOLUTION 295 

birds, and mammals. To us, they are the most interesting of animals be- 
cause they include ourselves and the most familiar and conspicuous do- 
mestic and wild animals. The fossil record is generally good and is justifiably 
emphasized because of its special interest and application to man. 

In figure 2 the relative variety of these major phyla today and the extent 
of their fossil records are shown. This is a picture, in broad strokes, of what 
is known of the history of life for the last 500,000,000 years. The notorious 
incompleteness of the fossil record, and the less publicized shortcomings of 
students of that record, keep the diagram from representing exactly the 
picture of life as it really existed. Yet the imperfections could be exagger- 
ated, and it may fairly be claimed that this is strongly suggestive, at least, of 
the general course of events. 



40,000 
30,000 
20,000 ■■ 
10,000 . • 



Approximate Numbers of Species 
in Recent Fauna 



nil ^ - n 



fl 




Duration and Diversity of the principal groups of animals ( 



ainly on counts of genera and higher groups ) 



Fig. 2. The broad outlines of the historical record of life. In the lower 
figure the phyla of animals are represented by vertical bands or path- 
ways the widths of which are proportional to the known variety (espe- 
cially in terms of genera) of the phylum in each of the geological 
periods since the pre- Cambrian. The upper figure represents the ap- 
proximate variety (in species) of each at the present time. 



Several striking facts fundamental for the history of life appear in this 
diagram. First, all the phyla are of great antiquity. All date from the Cam- 
brian or Ordovician. (Remember, however, that this does not mean that 
they all appeared suddenly or at the same time except in a very loose sense; 
Cambrian and Ordovician cover at least 100,000,000 years.) Since some- 
time in the Ordovician, around 400,000,000 years ago, no new major type 



296 GEORGE GAYLORD SIMPSON 

of animal has appeared on earth. It would appear that the fundamental 
possibilities of animal structure had then all been developed, although truly 
profound changes and progressive developments were yet to occur within 
each type. The profundity of such changes is exemplified by the difference 
between a jawless "fish" (such as a lamprey) and man— both vertebrates, 
but how importantly different! 

Note, second, that none of the basic types has become extinct. (The 
graptolites are only an apparent exception; it has been noted that it is im- 
probable that they merit rank as a truly basic type.) Of the lesser types 
within phyla many, indeed most, have become extinct, but the major grades 
of organization persist. This extraordinary fact bothered Sigmund Freud, 
who could not see why all ancient forms have not yielded to a death wish, 
and it has bothered some others who feel that progressive evolution should 
imply constant replacement of all lower forms by higher. The explanation 
is really quite simple. In the filling of the earth with life, some broad spaces 
were filled first, filled well and adequately, leaving neither reason nor pos- 
sibility for refilling by types of later development. A protozoan, because 
ancient and relatively simple, is not therefore an imperfect type destined 
for replacement within its own sphere. It is a fully adequate answer to the 
problems of life in that particular sphere. The sphere persists, and so do the 
protozoans. Other phyla represent, not advances over the protozoans for 
life as protozoans live it, but the development of other possibilities, other 
ways of life, and filling of other spheres in the economy of nature. There 
has, indeed, been replacement within given ways of life and expansion 
within types to more varied ways of life. Therefore new types of protozoans, 
or of vertebrates, have arisen and some have replaced earlier types, but the 
fundamental patterns of the phyla continue without extinction or replace- 
ment. 

The third major generalization reflected in the diagram of figure 2 is that 
on the whole life has tended to increase in variety. The usual pattern for 
any phylum, or for life as a whole, is to appear in relatively few forms and 
later to become vastly more diversified. The pattern is quite irregular in 
many cases. Some phyla, like the sponges (Porifera), have expanded slowly 
and in no spectacular way. The Bryozoa expanded first with unusual rapid- 
ity, then declined, then expanded again. The chordates have fluctuated, 
with three fairly distinct expansions, each greater than the last. Marine 
animals as a whole lost ground, became less varied, around the Permian 
and Triassic when there was a great crisis for life in the seas. There is, 
nevertheless, an unmistakable general tendency for life to become more 
varied in the course of its history. 

How this increase in variety has come about and what it means are best 
seen within the histories of separate phyla, and the vertebrates (in the 
Chordata) provide the best analyzed and most interesting example. Their 
history is summarized in figure 3. There are eight distinct types, classes in 



THE COURSE OF EVOLUTION 297 

technical classification, of vertebrates. The first four are primarily aquatic 
and are usually lumped popularly as fishes: the jawless fishes (Agnatha, 
with lampreys and hagfishes as living examples), the placoderms (Placo- 
dermi, no living examples ) , the cartilage fishes (Chondrichthyes, including 
sharks and rays among others), and the bony fishes (Osteichthyes, with 
trout, perch, cod, herrings, and a whole host of others— most of the fishes 
familiar to us today). The other four classes are primarily terrestrial and 
are more commonly distinguished: amphibians, reptiles, birds, and mam- 
mals, or Amphibia, Reptilia, Aves, and Mammalia in technical nomen- 
clature. 




Based on Numbers of Known Genera 



Fig. 3. The broad outlines of the historical record of the vertebrates. 
For each vertebrate class the width of the pathway is proportional to its 
known variety in each of the geological periods in which it lived. 



These classes, too, as broad structural types, subordinate only to those of 
the phyla, have tended to persist. Only one of the eight (Placodermi) is 
extinct. Yet most of them, five of the eight, are less varied now than they 
were relatively early in their histories and indeed only two, the bony fishes 
and the birds, may now be at their peak. Among the vertebrates there has 
been a succession from lower (or at least earlier) to higher (or later), and 
partial replacement of an ancestral by a descendant type is common. In the 
aquatic habitat, the bony fishes, among the last to arise, have replaced 
most other types. The reptiles replaced most sorts of amphibians and most 
reptiles were in turn replaced by mammals and, to lesser extent, by birds. 
Birds and mammals have hardly any tendency to replace each other, for 
their ways of life are too radically different. Nor is there any strong tendency 
for birds and mammals to replace fishes, in spite of the fact that fishes are 
much older, because, again, the ways of life have little overlap. 



298 GEORGE GAYLORD SIMPSON 

Thus the net total expansion of the chordates, seen in figure 1, does not 
result from uniform expansion within the phylum, but from a complex 
sequence of origins of new types, some replacing older types and some ex- 
panding into quite new spheres of existence. 

TO DINOSAURS TO MAMMALS 



Pterodactyl 




Plesiosaur 



Cynodont 



Ichtyyosaur 



Turtle 



Cotylosaur 

Fig. 4. Basic adaptive radiation in the reptiles. Only a few examples 
of the more widely divergent lines are shown. For dinosaur radiation 
see Fig. 5. 



The same sorts of events have occurred within each class, and here may 
be seen still more clearly how a new type, once it has originated, tends to 
spread and to become diversified in adaptation to a variety of environ- 
mental conditions and of ways of life. This process is known as "adaptive 
radiation" and is particularly well shown by the reptiles, living and extinct. 
As shown in figure 4, the reptiles started with primitive, four-footed, long- 
tailed forms. From these, in different lines, arose immensely diverse reptiles: 
paddle-swimmers like the plesiosaurs; fish-like swimmers such as the ichthy- 
osaurs; legless crawlers like the snakes; fliers, the pterodactyls; armored 
tanks like the turtles, and many others. The mammals are also an outcome 
of the reptilian radiation, but they represent an adaptation so potent and 
so superior in most spheres available to land-dwellers that they radiated 
in their turn on a grand scale and eventually largely replaced the reptiles. 

Even more limited groups of reptiles, such as the dinosaurs, have di- 
versified by adaptive radiation in a striking way, although with far less scope 
than for reptiles as a whole. Something of the diversity arising at this level 



THE COURSE OF EVOLUTION 299 



is suggested by figure 5. And so it goes, on down the line in successively 
smaller groups, each radiating within its scope. 

To man, the mammal, mammals are preeminent subjects of study. Space 
permits no details here, but it must be added that mammals, too, have 



Tricerotops 




Fig. 5. Adaptive radiation of the dinosaurs. Only a few of the many 
divergent lines are shown. 



diversified enormously and something must be said briefly as to man's place 
in this diversity. The extent of mammalian adaptive radiation can be 
glimpsed by considering kangaroos, moles, bats, monkeys, armadillos, rab- 
bits, mice, whales, cats, elephants, seacows, horses, and giraffes— all mam- 
mals along with us. In technical classification the next grade recognized 
below a class is an order, and conservative students count no fewer than 
thirty-two orders in the class Mammalia, of which fourteen are extinct and 
eighteen survive today. 

Among these numerous orders is that of the Primates, to which we be- 
long. This order is so primitive in many ways that it is hard to characterize, 
but even unprogressive members show a tendency toward using the hands 
to manipulate objects in coordination with the sense of vision, and this is 
among the basic primate characters. Oddly enough— or, at least, man is likely 
to think it odd— the early primates and their least modified modern de- 
scendants are not particularly intelligent. Yet there is a tendency toward high 
intelligence in the more progressive lines of primates, and the order does 
include the highest brain development yet attained by any form of life. 



300 GEORGE GAYLORD SIMPSON 




Chimpanzee 



Lemur 



Man 



Fig. 6. Representatives 
of the main groups of 
primates. The lemur 
represents the prosim- 
ians; the capuchin mon- 
key, the South American 
monkeys or ceboids; 
the macaque, the Old 
World monkeys or cer- 
copithecoids; the chim- 
panzee, an ape, and the 
man both represent 
broadly manlike or 
hominoid group. Each 
group includes a vari- 
ety of different forms, 
only one of which is 
shown as an example. 



Broadly speaking, there are four major sorts of primates, exemplified in 
figure 6. The primates, too, have undergone adaptive radiation, indeed a 
whole series of radiations. The prosimians, first to arise, radiated early in 
the Cenozoic, the Age of Mammals. Of these early lines, many became 
extinct. A few survived without deep change and represent the prosimian 



SURVIVING: 



PROSIMIANS 



NEW WORLD 
MONKEYS 



OLD WORLD 
MONKEYS 




Fig. 7. A representation of primate history and the origin of man as a 
series of adaptive radiations in space and time. 



THEORIES OF EVOLUTION 301 

type today. Others progressed in various ways and gave rise to higher forms, 
monkeys, apes, and men, at different levels of progression and in various 
geographic regions. The general pattern of this history is suggested by 
figure 7. 

Looking back over the whole, tremendous panorama of the history of 
life, now so briefly reviewed from the dim origins down to man, certain 
broad processes and impressions become evident. There is a trend toward 
increase and radiation into all possible ways of life, a principle of addition 
and multiplication. Within the phyla and increasingly within smaller 
groups there is frequent replacement of older groups by younger, a prin- 
ciple of substitution. There is also extinction; a principle of subtraction, 
almost universal among detailed types, less frequent for broader grades of 
organization and practically non-operative at highest levels. 

All of these definable processes are evident in the record, and still there 
is also an impression of a certain disorder or, at least, lack of uniform plan. 
Addition, multiplication, substitution, and subtraction do not appear con- 
stantly or interact to produce clear patterns. There is an odd randomness 
in the record, a suggestion that it involves a sort of insensate opportunism. 
There is a lack of fixed plan in detail, but a tendency to spread and fill the 
earth with life whenever and however this chanced to become possible. 

Yet not all is random. We know (or let us grant such knowledge at this 
point) that change must have direction and cause, and we feel a need to 
evaluate changes and their causes. From this brief and superficial examina- 
tion of the record of life let us pass to an attempt to interpret it more pro- 
foundly, and first to the question as to what sorts of causes may be oper- 
ating in the tangled fabric of evolution. 



Theories of Evolution 

• GEORGE GAYLORD SIMPSON 



EARLY IN THE 19TH CENTURY IT WAS WELL KNOWN THAT 
fossils occur in a definite sequence and characterize different periods in 
the history of the earth. The idea had been suggested long before, and 
after the work of Smith, Brogniart, and Brocchi no one in a position 
to judge seriously doubted it. The idea of evolution was also familiar 

• From Life of the Past: An Introduction to Paleontology (New Haven: Yale Uni- 
versity Press, 1953), pp. 140-50. 



302 GEORGE GAYLORD SIMPSON 

to all the scientists of that day, but its acceptance was by no means so gen- 
eral. It now seems to us obvious that the two ideas reinforce and comple- 
ment each other: the fossil sequence is the record of evolution. Yet this 
connection was not accepted by the students of fossils in that period. Al- 
most to a man they denied the truth of evolution, and they cited the fossil 
record as evidence for their stand. Knowledge of that record was still very 
incomplete, and we have seen that the record itself is incomplete. What 
the early paleontologists and geologists thought they saw was a sequence 
of quite distinct faunas and floras. They had not yet found or they over- 
looked evidence of evolutionary transition from one stage to the next. 

The most distinguished paleontologist of the time was Cuvier (1769- 
1832) . He is, indeed, often hailed as founder of the science of paleontology, 
although he had many predecessors in the study of fossils and was pro- 
fessor of anatomy, not of paleontology, at the National Museum of Nat- 
ural History in Paris. (A detailed study of the history of any field of science 
gives the impression that no specialties and no theories appear full blown 
or have a precisely defined time of origin.) It was Cuvier's view that each 
successive fauna, as known to him, was the result of repeopling of the earth 
after a great catastrophe that had wiped out many previous species. The 
last of these catastrophes was of course the Biblical deluge. As to where 
new species came from after a catastrophe, Cuvier was quite positive that 
they did not evolve from older species, but otherwise he hedged. He was 
inclined to believe that they had existed all along and that when they 
appeared in the fossil record "they must have come from elsewhere." This 
is, indeed, commonly true of new groups that appear suddenly in the record, 
but as a general explanation of the origin of new species it seems curiously 
evasive. 

Alcide d'Orbigny (1802-57), first professor of paleontology at the Paris 
museum, squarely faced the problem and came up with an answer even 
further from the truth. He taught that life on the earth was completely 
wiped out in each catastrophe and an entirely new set of species specially 
created in each successive stage. 

Cuvier was an unusual combination of spellbinder and scientist. When 
his evidence was thin he made up for it by oratory. He produced a great 
body of work still valid and valuable today, but he also effectively silenced 
opposition to his entirely wrong theory of catastrophes or revolutions, and 
he retarded acceptance of the truth of evolution. Almost all students of 
fossils, of whom there were many, during the first half of the 19th century 
accepted his authority and followed his views. Most of them also accepted 
d'Orbigny's modification of Cuvierian theory. "Successive creations" were 
for a time the easy way out of increasing embarrassment as they came to 
see that the fossil record does show a progressive development of life not 
accounted for by the Book of Genesis. 

In the meantime and even at the institution of Cuvier and d'Orbigny, 



I THEORIES OF EVOLUTION 303 

heretical views were being voiced by nonpaleontologists. Lamarck (Jean- 
Baptiste Demonet, Chevalier de Lamarck, 1744-1829, to give him his full 
style) was professor of invertebrate zoology there. Particularly in his Zoo- 
logical Philosophy, published in 1809, he came out wholeheartedly for evo- 
lution as the general explanation of the history of life. His associate in 
vertebrate zoology, Etienne Geoffroy Saint-Hilaire (1772-1844), accepted 
this view, with differences of opinion as to details of the process. Cuvier 
scorned to reply publicly to Lamarck's arguments, but he debated Saint- 
Hilaire at the Academy in 1830 and scored such an oratorical victory that 
httle more was heard of evolution in France for another generation. 

Lamarck's literary style was not brilliant, and his remarks on anatomy 
and physiology include much that was even then recognizable as nonsense. 
This helps to explain why his influence on his contemporaries was virtually 
nil. He was nevertheless the first really important figure in the development 
of modern evolutionary theory. His particular theory of how evolution 
occurs was, however, quite different from that later called "neo-Lamarck- 
ian." He believed, first of all, that there is some mysterious, inherent 
tendency for life to progress from the simple to the complex, from the less 
to the more perfect. This is a very old idea, foreshadowed by Aristotle. It is 
now known to be incorrect, and yet it became so confused with the whole 
concept of evolution that it still exerts a sort of vestigial, hidden effect on 
some students of the subject. 

Lamarck was acute enough to observe that life does not really form such 
a progression. He explained away this inconvenient fact by saying that the 
true course of evolution is perturbed by local adaptations. Adaptation was 
said to result from the activities and habits of organisms, which modify 
their anatomy. He assumed, as did almost everyone from the dawn of 
history down to and including Darwin, that such modifications would be 
inherited in like form by offspring. 

The place of Charles Darwin (1809-82) in the history of evolutionary 
theory is known to everyone, although not always quite correctly known. 
It is, again, typical of the history of science that there is practically nothing 
in Darwin's theories that had not been expressed by others long before 
him. His predecessors, however, were long on speculation and short on 
facts, and much that they said impressed their contemporaries as silly— as, 
in most cases, it does us today. Darwin brought together an enormous body 
of solid, pertinent fact, he reduced speculation to a minimum, and nothing 
he wrote (even though some of it later proved to be quite wrong) could be 
characterized as nonsense. He demonstrated to the satisfaction of the whole 
scientific world that evolution has, in fact, occurred. He also produced a 
particular theory as to how it occurred, of which more later. 

From our present point of view an especially interesting thing about 
Darwin's principal work. The Origin of Species (1859), is that it devoted 
two chapters to explaining why paleontology and paleontologists up to that 



304 GEORGE GAYLORD SIMPSON 

time did not support the truth of evolution. The most objective proof of 
that truth was to come from the fossil record, but it was clear that further 
search and study from this point of view were necessary. Some of the old- 
timers, such as Owen or Agassiz, were unable to adjust to this revolution in 
thought. Almost immediately after publication of The Origin of Species, 
however, a large number of evolutionary paleontologists was at work. 
Among them were T. H. Huxley (1825-95) in England, Cope (1840-97) 
and Marsh (1831-99) in the United States, Gaudry (1827-1908) in France, 
Kovalevsky (1842-83) in Russia, and Riitimeyer (1825-95) in Switzerland, 
to mention only a few. 

This renewed work rapidly proved that evolution is a fact. Accumulated 
series of ancestors and descendants and discovery of numerous transitional 
forms among fossils left no reasonable doubt. Achievement of adequate 
proof was gradual of course, but two landmarks may be mentioned: Marsh's 
completed demonstration of the essential stages in the evolution of the 
horse (1879), and the review of the evolution of all groups of fossils then 
known in volumes of the great handbook by Karl von Zittel (1839-1904) 
of Munich, beginning publication in 1876. 

Adequate proof that evolution did occur was a first necessity and a great 
achievement. Much more confirmation has piled up since, but more was 
hardly needed after about 1880. In the meantime attention was directed to 
the next and more difficult problem: how and why has evolution occurred? 
Lamarck had made a rather primitive attempt to cope with this question, 
and Darwin had made a much more substantial but still incomplete con- 
tribution to its solution. In the later years of the 19th century and early in 
the 20th many different views were aired, with paleontologists taking prom- 
inent parts in the arguments. Some of the theories ascribed evolution 
vaguely to a life force of some kind or to other virtually unknowable meta- 
physical factors. Such theories are nonexplanatory and stultifying. Theyi 
also seem to me, and to most other students of the question, quite incon- 
sistent with the fossil record, and no further discussion of them is needed 
here. 

Two opposing schools of theory were based realistically and naturalist- 
ically on the material facts of life and its record: neo-Lamarckism and Dar- j 
winism. The so-called neo-Lamarckian school was only in part derived from 
Lamarck and, indeed, it conflicted with much of his doctrine. It contended 
that materials for evolution were individual modifications caused by the 
reactions of organisms (a point really Lamarckian) and by action of the 
environment on organisms (a point flatly denied by Lamarck). It neces- 
sarily insisted that such modifications were heritable; otherwise they could 
have no direct influence on evolution. (Lamarck believed this, but so did 
Darwin and most other students from antiquity to about 1900.) It omitted 
Lamarck's main thesis: that of an inherent progression in evolution. That 
idea was taken over by some of the metaphysical theories. 



THEORIES OF EVOLUTION 305 

Neo-Lamarckism stressed interaction of organism and environment. 
Adaptation was its keynote, and it professed to explain adaptation in a 
particularly direct and simple way. Individuals adapt themselves, and, said 
the neo-Lamarckians, that is all there is to it. Paleontologists were then, 
as they are still, especially impressed by adaptation and by its slow and 
progressive development through time. They were not then (but they are 
now) particularly concerned with the actual mechanism of heredity and so 
did not observe that this was a crucial difficulty for neo-Lamarckism. Many 
paleontologists were neo-Lamarckians, and that theory seemed for a time 
to draw substantial support from the fossil record. The work of Cope, an 
exceptionally able paleontological theoretician, is an example. 

Even if true, neo-Lamarckism could not be a general explanation of evo- 
lution. Many evolutionary events known positively to have occurred could 
not conceivably be explained in this way. For instance, the nonreproducing 
castes of insects cannot have inherited their characteristics from ancestors 
in which those same characteristics arose as modifications. Neo-Lamarckism 
finally came to an end with definite establishment of the fact that the sort 
of inheritance required by that theory cannot occur. There are still a hand- 
ful of neo-Lamarckians in countries where known truths, possibilities, and 
known errors may be expressed with equal freedom. The only significant 
support for neo-Lamarckism now, however, is in the U.S.S.R., where only 
error may be expressed if the political bosses so decree. They have decreed 
that Soviet biologists must be "Michurinists." Michurinism, put over by 
Lvsenko, is a reactionan' form of neo-Lamarckism. It should be added that 
before this dictatorial action Soviet scientists were making excellent and 
substantial contributions to modern evolutionary theory. 

Darwin himself accepted what was later called neo-Lamarckism as a sub- 
sidiary factor in evolution. His followers, the neo-Darwinians, rejected it 
and concentrated attention on what Darwin had designated as the main 
factor: natural selection. In natural populations it is usual for more young 
to be born than can survive to reproduce in turn. On the whole, those that 
do survive are better fitted to their conditions of life. Since some, at least, 
of the characteristics making them more fit are hereditary, changes in 
hereditv of the group through the generations are in the direction of greater 
fitness. This factor, inherentlv so reasonable and probable, would evidently 
tend to produce progressive adaptation. Ever since its first clear and wholly 
logical formulation bv Darwin (and Wallace, 1858, and more extensively 
by Darwin alone in 1859), manv students have accepted it as the explana- 
tion of adaptation or of evolutional change in general. 

There were nevertheless strong objections to the theory of natural selec- 
tion, in which many paleontologists joined. Experimental proof of the 
operation of natural selection was slow in being achieved. It has now been 
amplv demonstrated, as well as the actually observed operation of natural 
selection in nature. A whole series of counterarguments was based on judg- 



I 



-> 



306 GEORGE GAYLORD SIMPSON 

ment that distinctions so slight as to be ineffective for natural selection 
were nevertheless involved in evolution. These arguments have also been 
entirely controverted in more recent years. It has been demonstrated that 
natural selection is much more subtle and powerful than at first appeared. 
Any variation observable by us is under favorable circumstances sufficient 
for the action of natural selection. Another series of arguments was based 
on the claim that many changes in evolution are nonadaptive and would 
not be favored or might even be opposed by natural selection. The sup- 
posed examples are complex, but in general they have turned out to 
have one of three explanations. In some cases the claimed phenomena 
are not real, for instance those of nonadaptive orthogenesis. In others 
the changes were really adaptive or may most reasonably be so re- 
garded; adaptation is an extremely intricate process and human judgment 
of what is or is not adaptive is often fallacious. Finally, it is entirely pos- 
sible that some truly nonadaptive change does occur, because natural 
selection is not necessarily effective under all conditions. That natural 
selection causes adaptation is not at all controverted if adaptation is found 
not to be perfect or universal. 

Most of the objections to natural selection that formerly loomed large 
and that long spurred a search for alternative explanations have thus been 
fully removed. To that extent the original Darwinian theory has been sub- 
stantiated. There were, however, two much more serious objections, and 
these have led to essential modifications in the theory. In its original form 
the theory took the existence of heritable variations more or less for granted. 
It therefore was ver}/ incomplete in not accounting for the origin of such 
variation. Darwin did attempt to account for this but failed. A related prob- 
lem is that Darwinian natural selection seems to explain only the elimina- 
tion of the unfit and not really the rise of the fit. The existence of a third 
and at least equally serious problem was hardly realized until it was solved: 
Darwin assumed that inheritance is blending, that a large and a small par- 
ent, for instance, would always have offspring of intermediate size. If that 
were true progressive change bv natural selection could not occur. At about 
the time when this objection became apparent, it was found that inherit- 
ance does not blend in this way. 

Around 1900 and thereafter geneticists began to learn just how variations 
do arise and are inherited. For present purposes the essential point is that 
inheritance is dominated by a developmental system controlled by definite 
structural and chemical units in the reproductive cells. Those units are the 
chromosomes. From time to time the units undergo changes of various 
sorts: increase or decrease in number, changes in arrangement, or chemical 
changes within the chromosomes among the still smaller units, the genes, 
that occur in them. Such changes, called in general mutations, produce 
through the developmental system new characteristics in the organism as a 
whole. The result is the rise of characteristics not inherited from the par- 



THEORIES OF EVOLUTION 307 

ents and yet heritable ty the offspring. That is the ultimate source of 
hereditary variation. 

When these facts were being discovered some geneticists at first believed 
that mutation was the whole story of evolution and that natural selection, 
along with neo-Lamarckism, was supplanted. It seemed to them that new 
sorts of plants and animals simply arose by mutation and that selection 
had no role beyond the gross fact that the new organisms must be capable 
of survival. It is a peculiarity of mutations that they show no special tend- 
ency to occur in the direction of past or current evolutionary change or of 
increased adaptation and are to that extent random. For the mutationists, 
evolution as a whole was therefore an essentially random process. 

Paleontologists knew that the early mutationist theory could not possibly 
be true. Trends may continue slowly in the same direction for millions of 
years. Most of the changes observed in fossils are clearly nonrandom and 
adaptive throughout. Abrupt origin of new groups by mutation is not sub- 
stantiated by the fossil record, while gradual change in varying populations 
is at least common. The known features of the history of life certainly 
cannot be accounted for by random mutation alone. 

This radical disagreement had disastrous effects on the study of evolu- 
tionary theory, although fortunately they did not long endure. Some stu- 
dents despaired of finding any explanation for evolution, or turned again to 
metaphysical pseudo explanations that did not really explain anything. On 
the whole, antagonism developed between geneticists and paleontologists, 
along with many neontologists. For a time each side was so sure the other 
was wrong that they went their own ways without consideration of the 
whole picture of which each saw a part. Yet along with their separate 
theories, which were incomplete and partly wrong on each side, each had 
quite incontrovertible facts. That the facts seemed to conflict was the fault 
of the students and their theories, not of the facts, after all. 

It is the great achievement of the present generation of students of 
evolution that the conflict has been fully resolved. A theory has been de- 
veloped that takes into account the pertinent facts of paleontology, neon- 
tology, and genetics and that is consistent with all. Because the theory 
involves natural selection as an essential factor it is sometimes called neo- 
Darwinism. That is something of a misnomer. As between Darwin and 
Lamarck, the theory owes much to Darwin and little or nothing to La- 
marck. Nevertheless its genetical side, which is at least as important as 
selection, was wholly lacking or wrong in Darwin's own theory, and even 
selection is given a broader and somewhat different meaning from Dar- 
win's. Since the theory is a synthesis from many forerunners and from 
many fields of biological science, it is often and less misleadingly called the 
synthetic theory of evolution or the modern evolutionary synthesis. 

Early geneticists— in the young science of genetics "early" means roughly 
from 1900 to 1930— were necessarily concerned mainly with particular 



308 GEORGE GAYLORD SIMPSON 

mutations and the inheritance of these by individuals. Such an ap- 
proach is similar to that of the old typological systematics, which 
studied single characters and their combinations in "types" as abstractions 
and did not consider their real rise, variation, and change in populations. 
The first abortive attempts to reconcile paleontology and genetics were 
made on this basis. New types were considered as mutant forms arising at 
one jump and as such in individuals. We have seen that this view is really 
inconsistent with much that is known from the fossil record. It is also in- 
consistent with the most reasonable interpretations of living populations 
in nature. Finally it turned out also to be inconsistent with the findings of 
genetics as that science matured. 

Truly fruitful synthesis could be achieved only when both genetics and 
paleontology advanced beyond the typological stage. Beginning in about 
1930 and in full swing today has been the development of population 
genetics. More or less simultaneous has been the development of what 
might well be called population paleontology and population systematics. 
It is through these movements that the varied approaches to the problem 
of evolution through paleontologv, systematics, and genetics have turned 
out really to lead to the same result. 

The core of the synthesis is not particularly complex or esoteric. New 
organic characters, variants and new structures, arise by mutations in 
populations. The frequencies of particular characteristics and, what is even 
more important, of their combinations result not only from mutation but 
to even greater extent from processes involved in the reproduction of the 
population. In sexual reproduction there is a constant shuffling of genetic 
combinations. Certain changes in frequencies of mutations and their com- 
binations occur at random with respect to adaptation. (That they are in 
this sense "random" does not mean that they have no determinable ma- 
terial causes.) Other changes are systematic. There may be a consistent 
tendency in reproduction for offspring in each generation to have more of 
certain mutations and combinations, less of others, than the last generation. 
From one generation to the next the changes in such frequencies are usu- 
ally very slight, or indeed practically indistinguishable from small random 
fluctuations. In the long run, however, the cumulative effect becomes quite 
appreciable— evolution is indeed a slow process as the fossil record shows. 
This consistent differential in reproduction is what is meant by "selection" 
in the modern theory. If adaptation is understood in a broad sense, the 
differential tends always in the direction of adaptation. 

Darwinian natural selection, death or survival of certain sorts of indi- 
viduals, may and usually does lead to reproductive or genetical selection. 
When it does not, it has no real effect on evolution. When it does, it is a 
special case of genetical selection. Genetical selection is more general and 
does not necessarily involve Darwinian natural selection. To take only one 
simple case, it is evident that two individuals that survive equally long may 



THEORIES OF EVOLUTION 309 

nevertheless have quite different numbers of offspring. Then genetical but 
not Darwinian selection has occurred. 

Genetical selection meets the old objection that Darwin's natural selec- 
tion was not "creative," that it eliminates and does not originate, Genetical 
selection determines what mutations will, in fact, spread in populations and 
how they will be combined. This is decidedly a creative role. In much the 
same sense an architect is creative. He does not make building materials, 
but he determines what materials shall be used and how they shall be put 
together to produce an organized result. 

In populations that do not reproduce sexually the basic situation is even 
simpler. For the most part their evolution is an interplay of mutation and 
Darwinian selection. Organisms reproducing asexually are less common 
than those reproducing sexually and have played a lesser role in progressive 
evolution. Sexual reproduction must have arisen very early in the history of 
life, and it now occurs even in very lowly organisms. It has recently been 
found that in some, at least, of the bacteria a form of sexual reproduction 
occurs, and this is widespread and well known in many other protistans. 

There are numerous different factors involved in these processes. Each 
has many variations of kind, intensity, and direction. Their interactions are 
extremely complex, and the study of particular phases and aspects of evolu- 
tion is almost incredibly intricate. Yet all involve the relatively simple basic 
processes that have now been summarized. In detailed, technical studies it 
has been established that these processes are not only consistent with the 
fossil record but are also adequate to explain it. 

You will have noticed that this explanation is complete at a certain level 
or up to a certain point but that it still leaves deeper problems unsolved. 
Most importantlv, it does not explain why mutations arise or why and how 
they produce their particular effects. These problems have not yet been 
solved, although progress is being made and there is every reason to think 
that they are soluble. It seems unlikely that their solution will have much 
effect on current interpretations of the fossil record, even though it will 
surely deepen our understanding of the whole history of life. 



Origin of the Amniote Egg 

• ALFRED S. ROMER 



ONE OF THE MOST IMPORTANT STEPS IN THE EVOLUTION 

of vertebrates was the "invention" of the amniote egg, which, with asso- 
ciated developmental processes, is characteristic of the higher vertebrate 
classes. Its appearance marks the beginnings of the history of the reptiles 
and the potentialities of evolution of the great groups that are dominant 
today, the birds and mammals. The evolution of the amniote type of devel- 
opment was a necessary antecedent to the true conquest of the land. 

Amphibian versus reptilian reproduction 

As it is seen today on the breakfast table, the amniote egg is a familiar, 
commonplace, and hence seemingly prosaic object. It is, however, marvel- 
ously well adapted for the reproduction of terrestrial animal types and 
permits a developmental history of quite a different sort from that found in 
lower vertebrates. Among fishes, the eggs are laid in the water, and the re- 
sulting young remain there as persistently gill-breathing water-dwellers. 

The amphibians (of which the frogs, toads, newts, and salamanders are 
the common modern types) developed limbs, and thus the ability to walk 
abroad upon the land, many millions of years ago, in Paleozoic times (Fig. 
1). But even today, the most familiar North Temperate Zone representa- 
tives of that group have hardly changed a whit in their reproductive proc- 
esses. No matter how far common frogs, toads, or newts may have wandered 
during the year, every spring sees them returning to ponds and streams. 
There they lay unprotected clusters or strings of eggs similar to those of 
their fish ancestors. There is little nourishing yolk in this typical amphibian 
egg, and consequently the tiny creature which hatches from it must, from 
an early stage, be highly adapted to an active, food-seeking life as a water- 
dwelling, gill-breathing, essentially fishlike larva. After a considerable period 
of feeding and growth, there takes place a radical change in structure and 
mode of life— a change which is most strikingly seen in the rapid meta- 
morphosis of tadpole into frog or toad. Gills atrophy; lungs expand, and air 
takes the place of water as an oxygen source; the tail fin is reduced; legs 
develop, and the amphibian is freed to walk out onto the land. 

• From Scientific Monthly (Aug. 1957), pp. 57-63. 

310 



ORIGIN OF THE AMNIOTE EGG 311 




Fig. 1. Eryops, an example of a Paleozoic amphibian well equipped for 
terrestrial life, which quite surely reproduced in typical amphibian 
fashion. 



This necessity of leading a "double life" is a serious handicap to the de- 
velopment of the individual; it must, at successive stages, be structurally 
and functionally fitted for two very different modes of life and, in conse- 
quence, falls far short of perfection in adaptation for either. And even if 
the end-product is a terrestrial, or potentially terrestrial, adult, the release 
from the water is never complete, for every spring the typical amphibian 
must return to its natal element to deposit eggs and initiate the next life- 
cycle. Such an amphibian is bound to the water; its release is never com- 
plete. 

Quite in contrast (Figs. 2, 3) is the mode of reproduction seen in typical 




shell — 
chorion 

ollantois 
amnion 



yolk in 
yolk sac 



Fig. 2. The classic contrast. (Left) A simplified version of the familiar 
Leuckhart wall chart, as it appears in many a textbook, showing the 
"typical" amphibian mode of development through a series of water 
dwelling tadpole stages to final metamorphosis into an adult frog. 
(Right) Diagrammatic representation of the development of an amniote 
egg, which shows the growing embryo protected by shell, chorion, and 
amnion, supplied with an embryonic lung (the allantois), and a food 
supply of yolk. 




312 ALFRED S. ROMER 

reptiles (and in birds and the most primitive mammals as well). The 
amniote egg can be laid on land; neither young nor adult need ever enter 
the water. The young amphibian must be prepared to make its own living 
while still of very small size; the amniote egg is richly supplied with nutri- 
tious yolk, which enables the young to attain considerable growth before 
birth. If they were exposed to air, the delicate tissues of a developing em- 
bryo would be subject to fatal desiccation; early in its development the 
amniote embryo is surrounded by a continuous membrane, the amnion (to 
which this developmental type owes its name). 

Within the liquid-filled amniotic sac, the developing embryo is in a 
miniature replica of its ancestral pond. Protection against mechanical in- 
jury is afforded by the shell. Extending out from the body of the embryo is 
a sac— the allantois— which expands beneath the shell and serves two 
further vital functions. The growing embryo, in which metabolic processes 
are proceeding at a rapid rate, must breathe. The shell is porous; the 
allantois beneath it forms an embryonic lung, receiving oxygen from the 
air and giving off carbon dioxide waste. A result of the rapid metabolism of 
the growing embryo is the accumulation of nitrogenous waste— an embr}'- 
onic urine which must be stored, until hatching, within the compass of the 
egg. The cavity of the allantois also serves this purpose, acting as a tem- 
porary bladder. 

As a result of this complex but efficient series of amniote adaptations, 
the animal is completely freed from an aquatic life. No longer is the adult 
compelled to return to the water for reproductive purposes. The young. 




Fig. 3. The oldest 
known amniote egg, 
from the Lower Per- 
mian of Texas. 



ORIGIN OF THE AMNIOTE EGG 313 

within the protection of its shell and membranes, is freed from the neces- 
sity of undergoing a fishlike larval life; nourished by the abundant yolk, it 
can hatch directly as a vigorous little replica of its parent, fully and directly 
equipped for terrestrial existence. 

Once this new amniote pattern of development had evolved, in Upper 
Paleozoic days ( J ) , there began the great radiation of reptiles that is char- 
acteristic of the Mesozoic "Age of Reptiles," during which period the rela- 
tively unprogressive amphibians were reduced to their present insignificance. 
And from this reptilian radiation there presently emerged the still more 
progressive lines which gave rise to the birds and mammals. As far as can be 
told from the fossil record, the adult structure of the very earliest reptiles 
showed little if any advance over that of their amphibian relatives and con- 
temporaries. It was solely owing to the amniote mode of development that 
the evolution of higher vertebrates was made possible. 

Aquatic nature of the oldest amniotes 

How, when, at what stage did this crucial reproductive improvement 
appear? The story once seemed clear to me, in a form in which I told it to 
many a student audience. Well before the close of the Carboniferous pe- 
riod, the fossil record shows us, there had appeared advanced amphibian 
types with well-developed limbs and other features indicating that, as 
adults, they could be, and were, mainly terrestrial forms rather than water- 
dwellers. A sole obstacle lay in the path of their conquest of the land — their 
mode of development, through which they were chained to the water (a 
lovely and dramatic phrase! ) . At long last there came the final stage in their 
release— the development of the terrestrial amniote egg. Their bonds were 
broken, and, as true terrestrial forms, the early reptiles swept on to a con- 
quest of the earth! 

This is a fine story. However, I now suspect that it is far from the truth. 
It assumes that the adult first became a land-dweller and that terrestrial re- 
production was a later development. It now seems to me more probable 
that the reverse was the case— that the egg came ashore first and that the 
adult tardily followed. 

My skepticism arose from a study of the oldest adequately known rep- 




Fig. 4. Limnoscelis, a primitive reptile which was contemporaneous 
with amphibians such as Eryops and was still amphibious in habits but 
which had quite surely attained the amniote type of development. 



I 



314 ALFRED S. ROMER 

tilian faunas, those of the early part of the Permian period. Some years ago 
I restudied the remains of Limnoscelis palustris (Fig. 4) from the Permian 
of New Mexico (2) . This is quite surely a reptile, although a very primitive 
one, with a terrestrial amniote mode of reproduction. But the adult was, 
quite certainly, far from being a fully developed land-dweller. Williston (as 
can be seen by the scientific names he applied to the animal) was im- 
pressed, as I was later, by the fact that its habitat appears to have been 
essentially aquatic. Can this have been a reversion from the purely terrestrial 
existence which we had assumed to be characteristic of the ancestral rep- 
tiles? This is very doubtful. Limnoscelis is such an early and primitive rep- 1 
tile that it is much more probable that its ancestors had never abandoned 
an aquatic life. Limnoscelis surely laid its eggs ashore, but the adult, it ap- 
pears, still remained happily in its ancestral waters. I 

Still stronger skepticism is induced by a study of Permian pelycosaurs 
(3). This group consists of early forms which are not merely reptiles but 
reptiles that are already separated from other major lines and on the way to 
becoming the ancestors of mammals. Here, in this progressive group, one 
would think that we would be dealing with purely terrestrial amniotes. 
Some pelycosaurs are reptiles of this nature. But the more primitive pelyco- 




Fig. 5. Ophiacodon, an early reptile with amniote development, which 
had already advanced in certain respects toward the mammals but was 
still essentially a water-dweller. 

saurs (of which Ophiacodon, Fig. 5, is best known) were not, to any degree, 
terrestrial. They were aquatic fish-eaters; they possessed limbs which would 
enable them to climb the banks, but their home, like that of their am-} 
phibian and fish ancestors, lay in the Permian streams and ponds. 



ORIGIN OF THE AMNIOTE EGG 315 
Can this be a secondary reversion to the water? Again (as in the case of 
Limnoscelis) , this is highly improbable. This type of pelycosaur is known 
well back into the Carboniferous period, and the only obvious conclusion 
from the facts is that, despite the phylogenetic position of these pelyco- 
saurs— well advanced up one major branch of the reptilian family tree— they 
had never left the water. 

The fossil evidence, then, strongly suggests that, although the terrestrial 
egg-laying habit evolved at the beginning of reptilian evolution, adult rep- 
tiles at that stage were still essentially aquatic forms, and many remained 
aquatic or amphibious long after the amniote egg opened up to them the 
full potentialities of terrestrial existence (4). It was the egg which came 
ashore first; the adult followed. 

Why the amniote egg? 

If we accept this as a reasonable conclusion from the paleontological evi- 
dence, we are, nevertheless, faced with a major puzzle. In the light of the 
earlier point of view, one could readily account for the success of the 
amniote type of development as being strongly favored by selective process 
in animals which were otherwise terrestrial in habits. But what strong ad- 
vantage could there be in terrestrial embryonic development in the case 
of forms which were still aquatic or, at the most, amphibious in adult life? 

To attempt a solution, let us review reproduction in modern amphibians. 
I have cited the reproductive habits of familiar North Temperate frogs, 
toads, and newts. But if we examine the developmental histories of the 
modern orders as a whole— and particularly the varied tropical anurans — 
we gain quite another picture. The fishlike mode of development I have 
described is, to be sure, primitive, but so many modern amphibians have 
departed from it that it can hardly be regarded as typical of the group as 
a whole. 

In a large proportion of modern forms, the eggs are not laid in the 
water in ancestral fashion. In fact, these amphibians may go to any ex- 
treme to avoid this (5). The eggs may be laid on the bank near the water, 
under logs or stones or in a cavity in the earth, in a hollow stump, or in 
a "nest" of leaves in a tree. They may be carried about on land, placed 
in pockets on the back of one or the other parent, kept (curiously) in the 
vocal pouch of the male, or, in the case of the "obstetrical toad," wrapped 
clumsily around the father's legs. 

The "typical" amphibian egg, like that of the fish ancestors, is small in 
size, with only a modest amount of yolk, and, except for the presence of a 
surrounding jelly, there is no development of membranes or other pro- 
tective devices for the embryo. This is quite in contrast to the amniote 
egg; but, in one modern amphibian or another, we find a variety of modifi- 
cations which parallel those of amniotes in most respects. In some in- 
stances the amount of yolk is greatly increased, the developing embryo is 



316 ALFRED S. ROMER 

perched above a distended yolk sac, much as in an amniote, and the neces- 
sity of larval feeding is done away with. There is no expansion of an 
allantoic "bladder" to function as a lung, but comparable air-breathing or- 
gans may be formed by expansion of a highly vascular tail or by the de- 
velopment of broad, thin sheets of superficial tissue extending out from 
the gill region. There is no development of a complete amnion as a pro- 
tection against desiccation, but in some forms there is a nearly complete 
covering of the embryo by somewhat comparable sheets of tissue. In fact, 
the only amniote structure that is not parallel is the shell— a relatively 
minor part of the whole complex. 

In sum, many modern amphibians have developed, to varied degrees 
and in varied fashion, adaptations which, like those of amniotes, tend to 
reduce or eliminate the water-dwelling larval stage. What is the signifi- 
cance of this series of adaptations? Not any "urge" toward a purely terres- 
trial existence, for the amphibians which show these trends toward direct 
development are as varied in adult habits as are amphibians as a whole. 

There appear to be two major advantages (i) Eggs and young in a pond 
form a tempting food supply, an amphibian "caviar," open to attack by 
a variety of hungry animals, ranging from insects to other vertebrates; fur- 
thermore, the larvae are in heavy competition for food with other small 
water-dwellers. If eggs are laid in less obvious places, the chance of 
survival is greatly increased; if guarded or carried by a parent, they are 
under protection, (ii) In some regions there are annual dry seasons, when 
the ponds and pools in which "normal" amphibians would lay their eggs 
tend to dry up. Reduction or elimination of the water stage increases 
the chances of survival of the young, which might be destroyed if they 
were living as tadpoles in a drying pond. 

May not the amniote type of development have been similarly evolved 
to gain some immediate advantage rather than as any sort of "preadapta- 
tion" for land life? For modern amphibians, protection of the eggs from 
enemies is by far the more important of the two major advantages that 
are gained by changes in reproductive methods (although, in certain in- 
stances, adaptations which shorten larval life appear to be related to pro- 
tection against potential drouth conditions). For the Paleozoic reptile 
ancestors, the reverse was probably the case. Potential egg devourers were 
then presumably less abundant, but danger of desiccation was far greater. 

Today there are only limited regions of the tropics in which the annual 
weather cycle is one of seasons of heavy rains alternating with drouths. 
But as Barrell first pointed out, large areas of the earth in late Paleozoic 
days appear to have been subject to marked seasonal drouth (the pres- 
ence of numerous red-bed deposits in the Upper Paleozoic appears to be 
correlated in great measure with drouth phenomena). Under such condi- 
tions, the life of the amphibious vertebrates of the day was a hazardous 
one. Particularly hazardous was the developmental process. If the old- 



ORIGIN OF THE AMNIOTE EGG 317 

fashioned methods were retained, and the young must, perforce, spend a 
long period of time as gill-breathing larvae, they were in grave danger 
of being overtaken by the oncoming of the rainless season and of being 
killed in their drying natal ponds. Any reproductive improvement which 
would reduce or eliminate this danger had a strong survival value. It is 
probable that various essays in this direction were made. The one truly 
successful one was that which led to the development of the amniote egg 
and the resultant origin of the reptiles, which, from that time on, be- 
came increasingly successful over their less progressive amphibian relatives. 
Today, a variety of amphibians are struggling (so to speak) to attain some 
type of development comparable to that which the reptile ancestors 
achieved eons ago, but their efforts are too little and too late. 

Deductions from the study of climatic history are thus consonant with 
the facts of the fossil record. The fine story of the reptile ancestor as an 
animal which had become fully terrestrial 'in adult life and needed only, 
as a final step, to improve its reproductive habits in order to conquer the 
earth, is, apparently, pure myth. It was the egg which came ashore first; 
the adult followed later. 

We may picture the ancestral reptile type as merely one among a va- 
riety of amphibious dwellers in the streams of late Paleozoic days. All 
were basically water-dwellers. All, alike, found their living in the water, 
with fishes and invertebrates as the food supply, for there was, at first, 
little animal life on land to tempt them. In most respects the early reptile 
had no advantage over its amphibian contemporaries. Only in its new 
type of development was the reptile better off. This advantage, however, 
did not at first imply the necessity of any trend toward increased adult 
life on land. And it was only slowly, toward the close of the Paleozoic Era, 
that many (but not all) of the reptiles took advantage of the new oppor- 
tunities which amniote development offered them and became terrestrial 
types, initiating the major evolutionary reptilian radiation in the Meso- 
zoic— the Age of Reptiles. This potentiality of conquest of the earth by 
the reptiles was not the result of "design." Rather, it was the result of a 
happy accident— the further utilization of potentialities that had been at- 
tained as an adaptation of immediate value to their amphibious ancestors 

Teleology and tetrapod evolution- 
It is sometimes said that the evolution of terrestrial vertebrates from 
fish ancestors cannot be explained by purely natural processes. How, it is 
asked, could there have been effected in the ancestral fish the whole series 
of structural and functional changes which were necessary for a success- 
ful existence on land but which appear to have been of no immediate value 
to the animal in its piscine existence? How could these changes have oc- 
curred unless there were some supernatural directive force behind the 
process, some mysterious "urge" that made for "preadaptation"? So runs 



318 ALFRED S. ROMER 

the argument, for example, in that recent popular work, Human Destiny, 
by Lecomte du Noiiy. 

In this case, as in other cases where the evolutionary history seems so 
complex that natural explanations seem at first to be improbable, the story 
can, I think, be explained on quite natural grounds, with involvement of 
only the simplest of recognized evolutionary principles— selection for char- 
acters that are immediately useful to the animal in its actual environment 
without reference to their possible use in some future mode of life. 

The complete transition from water to land involves a long series of 
structural and functional changes. For present purposes, however, we may 
select three of the most striking and outstanding changes that are neces- 
sary to enable a fish ancestor to become a successful terrestrial animal: (i) 
the development of lungs for air-breathing; (ii) the development, from 
fish fins, of limbs capable of supporting it on land; and (iii) the develop- 
ment (as discussed earlier in this article) of a type of egg which will free 
the reproductive processes from the water. 

How could these changes be of use to a water-dweller? The clue, I be- 
lieve, lies largely in the environmental factor already noted— widespread 
seasonal drouth in late Paleozoic times. Let us discuss these three factors 
in turn. 

Lungs. We think of lungs as essentially characteristic of land animals. 
What need does a fish, breathing with gills, have for such structures? 
Today there are only a very few fish which possess lungs, and one tends to 
assume from this fact that lung-bearing fishes are merely "poor relations" 
of the land vertebrates, back eddies in the stream of evolution, and that 
lungs, here, have little meaning. 

But this is not at all the case. If we look into the life and habits of 
these fishes, we find that lungs are highly advantageous in the peculiar 
conditions under which they live. Three of them are members of the 
lungfish group proper— the Dipnoi, one genus each being found in Aus- 
traha, Africa, and South America. The others {Polypterus and a relative) 
are African members of quite another series of bony fishes— the Actinop- 
terygii, or ray-finned fishes— but differ from the normal members of this 
group in possessing lungs. All five are tropical forms, and, more particu- 
larly, all live in regions of seasonal drouth. In such areas, lungs are of no 
advantage in the rainy season. But when the rains cease, the streams slow 
down and cease running, the remaining ponds and pools become stagnant, 
with a lowered oxygen content, and the situation is very different. 

Ordinary fishes, which depend on gill-breathing for their oxygen supply, 
die in enormous numbers in drouth conditions and can continue to survive 
as species only because the few survivors spawn large numbers of young, 
which can soon restore the populations. In the case of the lung-bearing 
fishes, which lay a relatively modest number of eggs, survival of the indi- 
vidual is more important. It is the presence of lungs which enables the 



ORIGIN OF THE AMNIOTE EGG 319 

fish to survive, for, in default of sufficient oxygen in the water, it can rise 
to the surface and take in a supply of atmospheric oxygen. 

Lungs, then, may be highly useful to a fish— crs a fish— under drouth 
conditions. And, as I have said, such drouth conditions appear to have 
been exceedingly common in late Paleozoic times, including the Devonian 
period, when fish evolution first attained a high degree of development and 
amphibian evolution began. Under such circumstances, we would expect 
that— quite in contrast to present times— lung-bearing fishes would have 
abounded in Devonian fresh waters. This was definitely the case. 

The higher bony fishes are (and were then) arrayed in three major sub- 
divisions— lungfishes, or Dipnoi; Crossopterygii, the group from which the 
land vertebrates are derived; and the Actinopterygii, the ray-finned fishes 
that are dominant today. The Devonian dipnoans were, quite surely, lung- 
bearing, as are their modern descendants. The crossopterygians also un- 
questionably had lungs (6). These two groups made up the greater part 
of the Devonian bony fish population. The ray-finned forms, most of 
which today lack lungs, were then insignificant in numbers, but it is pos- 
sible that even in this case lungs were present in their Devonian repre- 
sentatives, for they are retained in Polypterus, a primitive living member 
of this group. 

But for lung-bearing fishes in the Devonian, we need not stop with a 
consideration of the higher bony types. Much of the remainder of the 
fish population of the time was comprised of members of the Placodermi, 
a primitive fish group that is now quite extinct. It is difficult to discover 
the nature of the soft structures of fossil forms, but in the one placoderm 
in which exceptional preservation has revealed internal structures, it was 
found, surprisingly, that even this lowly type of ancient fish was a lung- 
breather (7). 

Lungs in fishes are today the exception. In Devonian fresh waters, lung- 
bearing was the rule; lungless forms were in a strong minority. In such a 
typical Devonian fish deposit as that at Scaumenac Bay, Canada, at least 
95 percent— and probably more— of the specimens in a collection will be 
found to be lung-bearers. These fishes were not air-breathers because of 
any mysterious preadaptation or "predetermination" toward land. Under 
drouth conditions, lungs were structures of immediate and vital importance 
to them as fish. 

Limbs. How can limbs of a terrestrial type be of use to a fish? The an- 
swer (which was first suggested by Watson, and on which I have later 
elaborated) is the seemingly paradoxical one that fishes ancestral to am- 
phibians came to walk on land so that they could continue to live in the 
water. 

Devonian drouth is, again, the clue to the situation. We find, in many 
late Paleozoic fresh-water deposits, remains of amphibians intermingled 
with those of the Crossopterygii. Many of the amphibians led much the 



320 ALFRED S. ROMER 

same sort of life as did their first relatives. Like them, they were car- 
nivores, making a living on smaller fishes and invertebrates. The amphib- 
ians, like the fishes, normally lived in the water; for, in early days, there 
was little suitable food on land. Under "normal" conditions the fish was 
rather better adapted to its existence than was the amphibian, whose 
limbs were actually a bit of a hindrance in swimming. If drouth conditions 
arose, the fish was still under no handicap, for, if the water became stag- 
nant, the fish could come to the surface for oxygen as readily as the 
amphibian. 

But if conditions became still worse, if the pool in which these animals 
lived dried up completely, what then? The fish would be, quite literally, 
stuck— immobilized in the mud and doomed to death if the water did not 
soon return. At this point, limbs show their advantage. For the amphibian 
could leave the drying pool, could crawl, slowly and painfully, up or down 
the stream bed, find some pool that had not dried up, and enter it to 
resume its normal hfe in the water (8). 

Limbs, to an early amphibian, were quite surely not a mysterious pre- 
adaptation for a life on land; such an existence, in the first stages of limb 
development, was (so to speak) the last thing it thought of or desired. 
Limbs were an immediately useful adaptation for life in the water; only 
gradually, as a terrestrial food supply developed, would the amphibians 
take advantage of the potentialities of becoming land-dwellers. 

The land egg. The preceding discussion has, I hope, been sufficent to 
show that we do not have to account for the origin of the amniote egg by 
assuming any sort of mysterious "urge" toward a more completely terres- 
trial existence. For an amphibious animal capable of emerging onto land, 
laying eggs safely ashore would be immediately advantageous. 

We have thus seen that some of the most prominent characteristics of 
land vertebrates can be accounted for as a series of adaptations that were 
of practical advantage as soon as they were acquired, while the animal 
was still partially or even entirely aquatic in its mode of life. The entire 
major evolutionary progression from fish, through amphibian, to terrestrial 
reptile— seemingly mysterious— can be interpreted in simple, natural terms. 
And it is probable that many another evolutionary development which 
appears difficult to understand without the introduction of teleology will 
likewise prove, when sufficiently investigated and studied, to be inter- 
pretable in the accepted framework of current evolutionary theory. 

REFERENCES AND NOTES 

1. Collections of the Museum of Comparative Zoology at Harvard College, Cambridge, 
Mass., contain remains of the oldest fossil amniote egg (Fig. 3), a rather battered- 
looking shell, from early Permian rocks, about twice the age of the famous dinosaur 
eggs. It was laid by one of the archaic reptile types then recently evolved, but by 
which one of them we cannot, of course, say. The egg was x-rayed in the wistful hope 



ORIGIN OF THE PACIFIC ISLAND MOLLUSCAN FAUNA 321 

that it might reveal an embryo. Because of the considerable amount of iron that it 
contained, it was necessary to use a powerful instrument, ordinarily used for testing 
armor plate, to penetrate it. As might have been expected, the x-ray plate showed 
nothing at all; had it not been addled, the egg would probably have hatched, and 
the shell would have been destroyed in the process. 

2. S. W. Williston, Am. ]. Sci. 31, 378 (1911) and 34, 457 (1912); A. S. Romer, 
ibid., 244, 149 (1946). 

3. A. S. Romer and L. I. Price, Geol. Soc. Amer. Spec. Paper 28 (1940), pp. 172-173. 

4. It is generally assumed that the great marine reptiles of the Mesozoic — the ichthyo- 
saurs and plesiosaurs — and the amphibious-to-aquatic chelonians were descended from 
the terrestrial ancestors. It is, however, quite possible that these ancestors had never 
fully abandoned water-dwelling and that this "reversion" was but a partial one. 

5. Many of the adaptations noted here are described by G. K. Noble, The Biology of 
the Amphibia (New York, 1931); see also B. Lutz, Copeia 4, 242 (1947) and 
Evolution 2, 29 (1948); G. L. Orton, Ann. Carnegie Museum 31, 257 (1949). 

6. J. Millot, Nature 174, 426 (1954). It is of interest to note that the sole living 
crossopterygian, Latimeria, still retains vestigial lungs, although it has shifted from 
fresh waters to the deep sea, where lungs are now functionless. 

7. R. H. Denison, /. Paleontol. 15, 553 (1941). 

8. For various recent emendations and elaborations of this suggestion, see C. J. Goin and 
O. B. Goin, Evolution 10, 440 (1956). 



Origin of the Pacific Island MoUuscan Fauna 

• HARRY S. LADD 



Abstract. It has long been recognized that the marine mollusks of the Pacific 
islands are related to the fauna of Indonesia, there being few ties between the 
mollusks of the islands and those of western America. Indonesia has been re- 
garded as the center of dispersal of the "Indo-Pacific fauna," yet prevailing 
winds, currents and even the major storm tracks trend from the islands toward 
Indonesia and the west. 

Until recently all of the islands of the Pacific Basin were thought to be 
geologically very young, but data otbained from submarine mapping, dredging, 
and drilling now indicate that: (1) a shallow water fauna, including mollusks, 
was present in the area as early as middle Cretaceous; (2) rich Tertiary mol- 
luscan faunas (possibly richer than those of today) were widespread both inside 
and outside the Pacific Basin proper; (3) many, if not all, of the hundreds of 
atolls that lie between Hawaii and Indonesia stood above the sea (some as high 
islands) at intervals during the Tertiary; (4) some 50 guyots, now far beneath 
the sea, projected into shallow water as additional stepping stones for the dis- 
tribution of marine life, many of them in the broad gap southward of Hawaii. 

• From American Journal of Science (Bradley Volume, 1960), pp. 137-50. Publica- 
tion authorized by the Director, U. S. Geological Survey. 



322 HARRY S. LADD 

These discoveries suggest that the Pacific islands once formed a giant archi- 
pelago and that the islands could have been the home of many elements of the 
Indo-Pacific fauna. Faunal migration, favored by winds and currents, was toward 
Indonesia rather than from it. 

With some islands or parts of islands projecting above the sea at all times 
since the Cretaceous there would be a reasonable explanation for the ancient 
stocks of land shells and plants found in Hawaii and other volcanic islands. 
The newly discovered stepping stones appear to be satisfactory replacements for 
the land bridges once called for by many biologists. 

INTRODUCTION 
Area 

The Pacific Ocean, exclusive of adjacent seas, covers 64 million square 
miles (Shepard, 1948, p. 281) or about one-third of the earth's surface. 



80° 




30» 



Fig. 1. Map of Pacific showing boundaries of Polynesia, Micronesia, 
Melanesia; andesite line shown by dashes. Base map from data fur- 
nished by Hydrographic Office. 



ORIGIN OF THE PACIFIC ISLAND MOLLUSCAN FAUNA 323 

Pacific islands, including New Guinea, the large continental islands of In- 
donesia and the "island continent" of New Zealand cover only about 
1,400,000 square miles or less than 3 percent of the ocean area. Many of 
the islands, including most of the large ones, are concentrated in the 
southwest quadrant. To the north and east of centrally located Hawaii 
there are wide areas of unbroken ocean (fig. 1). 




80° 



Fig. 2. Prevailing winds in and near island area (dashed line). Figure 
shows pattern in January; in July there is a shift to the north but no 
essential changes are involved. Data from Tannehill, 1952, with permis- 
sion of Princeton University Press. 



The island area, as here described (fig. 2), includes a generous south- 
west one-quarter of the Pacific basin. It extends from Hawaii to the west, 
southwest and south to include almost all of Polynesia, Micronesia and 
Melanesia. Under existing conditions this area cannot be looked upon as a 
gigantic archipelago because it includes groups of islands that are sepa- 
rated by hundreds of miles of ocean containing neither islands nor reefs. 



324 HARRY S. LADD 

Such gaps, however, did not exist in past times when hundreds of islands 
that are now atolls projected above the sea and volcanic mounds (guyots) 
that are now deeply submerged projected into shallow water to form banks 
and reefs. 

Fauna! relations 

Intensive studies of the distribution of life in the Pacific date back to 
Charles Darwin and Alfred Russel Wallace one hundred years ago. Each 
of these co-discoverers of the theory of natural selection obtained support 
from island studies, Darwin in the Galapagos (1839) and Wallace in the 
East Indies (1860). Both concerned themselves chiefly with vertebrates 
but their interests included insects and other invertebrates. The patterns 
of distribution that they observed were explained by evolutionary theory 
and strongly supported it. The unique advantages of islands in evolu- 
tionary studies were recognized in their classic studies. 

The general patterns of the distribution of mollusks in the Pacific, par- 
ticularly those of the terrestrial forms, aroused attention because of the 
difhculties involved in transporting such forms to small and widely scat- 
tered islands. Suggested dispersal agents have included land connections, 
drifting vegetation, typhoons and migratory birds. The use of islands as 
stepping stones, including those now buried beneath the sea, was sug- 
gested by Wallace in 1881 (p. 270). In Wallace's time there was little 
geological evidence to support the idea of submerged islands. As late as 
1950 it was pointed out that complete proof for island distribution was 
"hopelessly buried in the geological past." In recent years, however, new 
data bearing on the problems of island distribution have been obtained 
from 3 sources— submarine mapping, dredging and drilling. Much is be- 
ing learned about the distribution of molluscan life in the Pacific from 
the Cretaceous to Recent times. The present article attempts to sum- 
marize these findings and offers a hypothesis of dispersal for the marine 
mollusks that is contrary to the generally accepted conception. 

Indo-PaciRc marine fauna.— The Indo-Pacific aspect of the Pacific is- 
land molluscan faunas has long been recognized and Indonesia has been 
regarded as the center of dispersal. It has also long been known that the 
prevailing winds, the main surface currents and even the typhoon tracks 
are in the opposite direction, but these apparently discordant facts have 
often been ignored. It has been pointed out that there are high level winds 
above the prevailing winds that blow in different directions at greater 
speeds (Darhngton, 1957, p. 20), Such winds may be called upon to 
explain the anomalous distribution of certain types of insects or even 
minute terrestrial mollusks but are of little help as regards marine mollusks. 

Many students of Pacific invertebrates believe that the fauna of Indo- 
nesia and the Philippines is a rich one while that of the islands of the 
open Pacific is comparatively poor. Indeed, they feel that the faunas be- 



ORIGIN OF THE PACIFIC ISLAND MOLLUSCAN FAUNA 325 

come increasingly impoverished with increased distance from Indonesia. 
Actually, the case may not be as strong as it appears for it must be borne 
in mind that the fauna of the Indonesia area, both living and fossil, has 
been more intensively collected than that of any individual island or is- 
land group. 

Consider, for example, the reef-building corals. A map showing the dis- 
tribution of genera in 39 localities was published by Wells (1954, pi. 186). 
More than 50 genera have been reported from the much studied Great 
Barrier Reef, part of Indonesia (Celebes) and nearby Palau. To the 
north and east the numbers of genera drop, Hawaii having only 15. It is 
notable, however, that a prominent bulge in the isopangeneric lines had to 
be drawn to take in the Marshall Islands where 52 genera were found, 
mostly from intensively collected Bikini. If all islands from Indonesia to 
Hawaii were as closely collected as was Bikini the pattern of distribution 
as shown by numbers of genera might be greatly altered. 

The marine molluscan fauna of the Indo-Pacific is known to be large. 
Ekman (1953, p. 13) estimated it at 6,000 species. The known faunas of 
individual island groups are appreciably smaller. A total of about 1,000 
shelled forms were listed for French Oceania (Dautzenberg and Bouge, 
1938). The known Hawaiian fauna is probably close to 1,500 species. 
Solem thinks it probable that the list for the New Hebrides will even- 
tually be well over 2,000 species (1959, p. 267). Actually the fauna of 
most groups is incompletely known and this is particularly true of the 
micro-mollusca. 

Information about the recently discovered shallow water Cretaceous 
fauna dredged from 2 guyots in the now submerged Mid-Pacific Moun- 
tains between Hawaii and the Marshall Islands (fig. 1) is given by Hamil- 
ton. The assemblage is made up of reef corals, stromatoporoids, rudistids 
and other mollusks (Hamilton, 1956). During the Eocene, shallow water 
limestones containing numerous benthonic larger Foraminifera were laid 
down on widely separated islands— in Tonga, the Marshalls, the Marianas 
and Palau. Mollusks also occur in many of these limestones but well pre- 
served material has not yet been found. In the Miocene sediments, the 
molluscan records are much more complete and preservation is excellent. 
Abundant and varied molluscan assemblages have been collected from the 
Marshalls, Fiji, the Marianas and Palau, though most of these have not 
yet been described. The faunas from the lagoonal beds drilled in the 
Marshalls appear to be richer in numbers of species and individuals than 
those dredged from the same lagoons today. The lagoonal faunas, both 
living and fossil, are dominated by the gastropods but in other areas where 
volcanic muds are found there is a rich fauna of pelecypods as well. It is 
possible that the molluscan fauna of the existing sea in many parts of the 
island area is poorer than it was during parts of the Miocene, although 
this cannot as yet be clearly demonstrated. 



326 HARRY S. LADD 

The observed richness of the marine fauna in Indonesia may be partly 
due, as Fenner A, Chace, Jr. suggested in his review of the present paper, 
to the accumulation there of species that arose in the island area and 
drifted west and southwest. All those that found a suitable niche in 
hospitable Indonesia would survive after the parent stock in the ever- 
changing islands became extinct. 

Terrestrial fauna and /Zora.— Interpretations involving the terrestrial fauna 
and flora of the islands differ greatly from those dealing with the marine 
fauna. Students of the varied land shell faunas that are found on all of the 
existing high islands from Hawaii to Indonesia recognized at an early 
date that these faunas appear to have been derived from ancient stocks. 
Pilsbry pointed out long ago that the faunas, though containing many 
endemic species, were nearly homogeneous over wide areas and contained 
no admixture of the great series of modern families that characterize Ter- 
tiary and Recent continental faunas. Pilsbry felt it necessary to postulate 
a late Paleozoic or early Mesozoic continent upon which he superposed 
the present island masses of volcanic and coral rock (Pilsbry, 1900, p. 
581). Adamson in a review dealing primarily with the terrestrial fauna of 
the Marquesas agreed with Pilsbry's appraisal of ancient lineage, stating 
pointedly that no modern family of land snails reached the central Pa- 
cific until brought by man (1939, p. 15). The high degree of endemism in 
the Marquesas fauna suggested that the islands had been an isolated 
archipelago since early Tertiary, if not earlier (p. 75). Adamson favored 
wind distribution rather than land connections. 

Paleontological evidence bearing on the history of Pacific land shells is 
meager but suggestive. Two species of typical high island shells have been 
described from the Miocene of the Marshall Island drill holes (Ladd, 
1958). One of these species is from the lower Miocene, so it is certain 
that some land shells were present in the islands in middle Tertiary times. 
In Indonesia, on the other hand, the richly fossiliferous and much studied 
Tertiary sections of Java have yielded no land shells below the upper 
Pliocene (Jutting, 1937, p. 171). 

Studies of Pacific island insects have revealed some unusual patterns 
and have resulted in a variety of interpretations. Some have called for 
land bridges (Meyrick, E., 1899, p. 132), others for island stepping stones 
(Zimmerman, 1948, p. 49-52), still others for continental drift (Britton, 
1957, p. 1383-1389). Usinger noted that in many groups of insects ex- 
isting today there appears to be a progressive diminution in the number 
and variety of forms from the rich epicontinental areas of the south- 
west Pacific to the eastern Polynesian islands. He favored transportation 
via stepping stones, in both present and past times, but he noted that the 
progressive reduction breaks down when applied to Hawaii (1940, p. 
314). 



ORIGIN OF THE PACIFIC ISLAND MOLLUSCAN FAUNA 327 

Botanists likewise have long recognized that certain of the floral ele- 
ments found on existing Pacific islands represent fairly ancient stocks. 
Skottsberg (1928, p. 914) pointed out that because of the assumed newness 
of the islands they have been regarded as biological dependencies of Asia, 
America, or Australia. He contended that the island floras do not consist 
of scraps easily traced to surrounding lands. The high incidence of endem- 
ism in oceanic islands like Hawaii suggests that the flora is as old as that 
found on continental islands such as the Solomons and Fiji. 

There is to date no paleontological evidence of great age from the large 
and strategically located Hawaiian islands themselves, though shallow 
water Cretaceous fossils have been dredged from submerged mountains 
less than 500 miles to the southwest. Fossils of equal age may lie buried 
in Hawaii. The Hawaiian mountain chain has been built up from the sea 
floor and it apparently has been built slowly, flow by flow, as the island of 
Hawaii is being built up today. This process of slow accretion is of par- 
ticular interest because throughout the history of any given island most of 
the land is available to terrestrial hfe. 

Centers of dispersal 

A voluminous literature deals with the dispersal of marine invertebrate 
life in the Pacific, A variety of faunal provinces based, in most instances, 
on the distribution of some particular group of organisms has been sug- 
gested. These arrangements cannot be reviewed here, but it should be 
pointed out that most writers agree on two basic conceptions: (1) that 
connections between the Pacific and Europe via ancient Tethys existed 
during the Cretaceous and into Eocene time; (2) that the existing marine 
fauna in the islands is closely related to that of Indonesia (Edmondson, 
1940; Domantay, 1953; Powell, 1958; Ekman, 1953). Many workers be- 
lieve that Indonesia was the main center of dispersal; sometimes the 
postulated routes of migration to the islands are direct, sometimes in- 
direct. 

MEANS OF DISPERSAL 

Pelagic types of marine invertebrates can drift or swim almost any- 
where that currents and land barriers permit, and the same is true of 
organisms that encrust seaweed or other types of floating vegetation; these 
are the cosmopolites of the oceans. Bottom dwelling inhabitants of shal- 
low water, on the other hand, are, in general, limited by the duration of 
their free-swimming larval stages. To such forms wide stretches of deep 
ocean may be an effective barrier. The distribution of shallow water forms 
among widely separated islands is, thus, not easily explained. Several 
agents of dispersal are considered below. 



328 HARRY S. LADD 

Land connections 

The existing Pacific basin has been bridged by a great variety of imagi- 
nary continental lands and shallow seas, the first of which were supposed 
to have existed in Cambrian time (Gregory, 1930). Paleontologists were 
responsible for many of the ancient connections but biologists have de- 
manded similar structures, citing the peculiarities of the existing fauna and 
flora. Continental drift has also been called upon to explain the existing 
ocean basin. In the present paper no attempt is made to go back beyond 
the middle Cretaceous as that is the age of the oldest fossils yet found in 
the island area here considered. 

Data obtained by soundings, dredging, and drilling in recent years have 
shown that there were many more shallow banks, reefs, and high islands 
in the southwest part of the Pacific basin in Cretaceous and Tertiary times 
than there are today. Chief reason for the reduction in the numbers of 
these stepping stones has been subsidence that has affected large areas 
in the Pacific Basin proper, that is, that part of the basin within the 
circum-Pacific andesite line (fig. 1; see Schmidt, 1957, p. 172-173, and 
Cloud, et al., 1956, p. 19, fig. 1, for discussion). 

In 1946 Harold Stearns called for a considerable subsidence of the 
Hawaiian area, citing the occurrence of coralliferous limestone in wells on 
Oahu more than 1,000 feet below sea level (p. 253). Also in 1946 Hess 
made his first report on guyots, interpreting them as wave truncated vol- 
canoes that now lie thousands of feet below sea level. In 1954 Emery, 
Tracey, and Ladd (p. 152-154) summarized the evidence for subsidence in 
Micronesia and Pohnesia. In 1956 Hamilton published evidence to show 
that the subsidence of some 4,000 feet took place in the area of the Mid- 
Pacific Mountains to the southwest of Hawaii (p. 48). Subsidence in this 
area led to the wide island-free gap between Hawaii and the Marshall 
Islands (fig. 1). 

Island stepping stones 

The idea that islands now sunk beneath the sea once served as stepping 
stones in the distribution of life to existing islands is not new. In dis- 
cussing the terrestrial molluscan fauna of the Galapagos Islands, Wallace 
(1881, p. 270) speculated that during the long history of the existing 
islands some other islands may have existed between the Galapagos and 
the coast to serve as "stepping stones" in the distribution of life. He 
noted that sunken banks, the relics of such islands, are known from many 
parts of the ocean and he stated that countless others no doubt remain 
undiscovered. Since Wallace's time, many authors, in considering the 
central and western Pacific, have stressed either the need for stepping 
stones or have given evidence for their probable existence (Vaughan, 
1933, p. 933, 935; Zimmerman, 1942, p. 283; Mayr, 1953, p. 8; Hamilton, 



ORIGIN OF THE PACIFIC ISLAND MOLLUSCAN FAUNA 329 



ENIWETOK 



•-^^ 




n6NGERIK 



Fig. 3. Stepping stones. Atolls and guyots in the Marshall Islands. Tops 
of guyots, now some 4,000 feet below sea level, were truncated by marine 
erosion in earlier times when they projected into shallow water and 
aided in the distribution of marine invertebrates. Many structures of this 
type are present in the Mid-Pacific Mountains area (Fig. 1). (After von 
Arx, 1954.) 



1953, p. 206; Ladd 1958, p. 194-196; Menard, 1959, p. 213). 

Evidence from submarine mapping and dredging. — The discovery of a 
large number of flat-topped seamounts (guyots) scattered over millions of 
square miles in the western Pacific (Hess, 1946) paved the way for ex- 
tensive mapping of the ocean floor and for dredging operations. One of 
the objectives was to test Hess' conclusion (based on configuration only) 
that the structures represent ancient volcanic islands truncated by wave 
action. Dredging on the flat summits produced rounded cobbles of basalt 
and a variety of shallow water fossils dating back to Tertiary and Creta- 
ceous times (Emery, et al. 1954, p. 129; Hamilton, 1956). The pattern of 
distribution of the guyots is significant. A total of 50 have been confirmed 
by survey in the island area here considered (Menard, 1956) but only two 
have been discovered in the wide island-free sea that separates Hawaii and 
America. These two lie about 600 and 800 miles off the west coast of 
America (Carsola and Dietz, 1952). In this same island-free belt many 
volcanic seamounts are known, but none apparently projected into shal- 
low water so as to function as stepping stones in the distribution of 
marine life. 

Evidence from drilling.— Deep drilling on several existing atolls has dem- 
onstrated that during Tertiary and later times these structures stood well 
above the sea for appreciable lengths of time and could have, served as 



330 HARRY S. LADD 

stepping stones for the distribution of terrestrial mollusks, while the reefs 
and shallows around them continued to serve this same function for 
shallow water marine mollusks. 

The evidence for the periodic emergence is partly petrologic, involving 
leaching of the limestone and the alteration of organic aragonite to calcite 
under atmospheric conditions (Emery, et al. 1954, p. 132); additional 
support is given by the occurrence of fossil land shells and concentrations 
of pollens and spores of land plants (Ladd, 1958). In the area to the 
south and west of the Hawaiian Group there are about 250 atolls plus 
table reefs and shallow banks that bring the total close to 300 (Bryan, 
1953; Cloud, 1958). 

Paleontological studies of drill cores and cuttings from the Marshall 
Islands show that a number of the Tertiary species (algae, Foraminifera, 
corals, and mollusks) that occur in outcrops above sea level outside the 
andesite line also lived within the Pacific Basin where they are now 
deeply buried below sea level.^ 

Currents 

Direct transport.— In the island area outlined in figure 1 the prevailing 
current north of the Equator (North Equatorial Current) is to the west. 
South of the Equator the prevailing current (South Equatorial Current) 
is to the southwest and south. These two broad bands are separated by 
the Equatorial Counter Current flowing east. This current is stronger 
(more than 1 knot) but it affects only a narrow band. (Sverdrup, Johnson 
and Fleming, 1946, Chart 7).^ The prevailing winds that are responsible 
for the currents are the Northeast Trades and the Southeast Trades, sep- 
arated by a narrow, shifting belt of calms near the Equator, (fig. 2). 

Some of the many writers who have recognized the Indo-Pacific aspects 
of the island faunas have recognized also that the winds and currents set 
from America and cannot be called upon to support dispersal from Indo- 
nesia. W. A. Br)'an pointed this out specifically for the Hawaiian marine 
fauna in 1921. He stated that we must look back beyond the Cretaceous 
for the origin of at least part of the Hawaiian fauna and flora, and he 
expressed the opinion that lands and currents were different in those times 
(p. 154-155). Edmondson hkewise noted that present currents precluded 
direct migration from the southwest Pacific to Hawaii (1940, p. 595). Ek- 

1 At a time when additional occurrences of Tertiary rocks are being discovered in the 
western Pacific, an earher occurrence should be deleted as erroneous. In 1922 Yabe and 
Aoki described Tertiary Foraminifera from pebbles in a reef conglomerate from Jaluit 
Atoll in the Marshalls, an occurrence that has been repeatedly cited in subsequent litera- 
ture. Hanzawa could find no additional material on Jaluit and when he reexamined 
Yabe's thin sections, found two kinds of limestone. He concluded that Aoki's Recent 
material from Jaluit had inadvertently been mixed with Tertiary material from Saipan 
(Hanzawa, 1957, p. 36-37 and oral communication, 1959). 

2 This current, however, is probably largely responsible for the Indo-Pacific aspect of 
the fauna of isolated Clipperton Island (Hertlein and Emerson, 1953, p. 353). 



ORIGIN OF THE PACIFIC ISLAND MOLLUSCAN FAUNA 331 

man noted that the wide stretch of the eastern Pacific, bare of islands, 
clearly forms an effective barrier to dispersal in spite of favorable currents. 
He also expressed the opinion that even the small and widely dispersed 
oceanic islands between Hawaii and Asia played a part in populating 
Hawaii. (Ekman, 1953, p. 21-22). Cloud, in a paper deahng with the 
shoal-water ecology of Saipan in the Marianas, referred to radial migration 
from Indonesia but pointed out that current maps for the existing ocean 
are not clearly reflected by biogeographic patterns. He concluded (1959, 
p. 396) 

Either current movements have changed recently, or are very complex or 
round-about in detail, or wind-induced surface movements that run counter to 
or across main trends are more important than deep flow in some plankton 
movement. Depending on breeding seasons and wind patterns, wind-driven 
surface flow could be of major importance in dispersal of marine biotas. 

The distribution of shallow water marine mollusks depend importantly 
on the duration of pelagic larval stages. Gunnar Thorson is at present 
engaged in a comprehensive study of this method of transport. His find- 
ings to date were summarized in a paper delivered to the International 
Oceanographic Congress in New York on September 9th, 1959. He re- 
ported wide variation among invertebrate groups. Among the mollusks, 
the pelecypods in general have a short larval period and are unsuited to 
long distance transport. Pelagic larvae of some prosobranch gastropods, 
on the other hand, have been taken alive in midocean and some of them, 
apparently, spend as much as 6 months drifting with the current.^ Thorson 
states that 5.5 percent of mollusk larvae remain longer than 3 months in 
the plankton and many others remain up to 2 months. Under favorable 
temperature conditions the periods may be doubled. 

In his discussion of distribution Thorson expressed the belief that cur- 
rents may vary somewhat from century to century and that a maximum 
time for larval existence in the plankton might occur only once in 100 
years. 

The average speed of the equatorial currents is only about one-third 
of a knot (John Lyman, oral communication, 1959).^ This amounts to 9 
miles per day or a drift of some 700 miles for pelagic larvae spending 10 
weeks in the plankton. Such a period would seem adequate to transport 
some mollusks among the stepping stones that existed in past times. 

Much is still to be learned about the transportation of many pelagic 
larvae, but the fact remains that a variety of mollusks did spread over the 

3 All but 1 of the 27 mollusks listed by Hertlein (1937, p. 305-309) as occurring both 
in Polynesia and western America are prosobranch gastropods. 

^ The Panamanian land bridge that now separates the Atlantic and Pacific Oceans did 
not come into existence until the middle Pliocene (Woodring, 1949) and it is possible 
that the speed of the west setting currents in the Pacific might have been higher during 
the Cretaceous and early Tertiary. 



332 HARRY S. LADD 

scattered Pacific islands. Difficulties in moving short-lived forms are ap- 
parent but these difficulties are not nearly so great if the move is from 
the islands toward Indonesia rather than in the opposite direction. 

Rtiftmg.— Rafting by drifting logs may be an efficient means of dis- 
pensing marine wood-boring mollusks, and some benthonic mollusks may 
be distributed when the holdfasts of kelp and other sea weeds are set 
adrift by storms. Emery and Tschudy (1941) reviewed the literature on 
this subject. Mollusks and other invertebrates that cling to or are ce- 
mented to rocks may be transported long distances on floating pumice. 
Hedley (1899, p. 412-413) cited examples of the transportation of living 
reef corals by this means. Terrestrial mollusks may be rafted if, as some- 
times happen, logs with attached branches, or even entire trees are up- 
rooted and carried to sea (Ladd, 1958, p. 193-194). 

Migratory birds 

The migratory habits of many Pacific birds are well known (references 
cited in Ladd, 1958). Traffic is in both directions as far as the islands are 
concerned. Some birds that breed in Alaska and other boreal areas winter 
in the islands or in New Zealand and Australia, others that breed in New 
Zealand and such austral areas winter among the Pacific islands. The birds 
do use the islands as stepping stones and small mollusks no doubt are 
carried entire in particles of mud that may adhere to the feet of a bird in 
its flight from one island to another. Operculate snails may become locked 
to the feather of a bird or be eaten by a bird or fish and then regurgitated 
or passed with the feces (Bondesen and Kaiser, 1949, p. 268-270). The 
importance of such means of dispersal is difficult to evaluate but would, in 
any case, apply only to minute mollusks, particularly those living in shallow 
waters. 

Typhoons 

The extraordinary lifting and transporting power of typhoon winds is 
probably a factor of considerable importance in the distribution of small 
terrestrial mollusks and many types of insects (Darlington, 1957; Ladd, 
1958, p. 194). The effects of such winds in strengthening prevailing ocean j 
currents or in developing abnormal currents may be appreciable but is 
difficult to evaluate. 

In the northwest Pacific, tropical cyclones (including typhoons) origi- 
nate north of the Equator and travel westward toward Asia where they 
turn northward. Similar storms in the southwest Pacific head southwest- 
ward from the equatorial regions, curving toward Australia and New Zea- 
land (Dunn, 1951, p. 895). In 1957 there were 17 storms of typhoon in- 
tensity in the northwest Pacific (an unusually large number), one origi- 
nating near Hawaii, the others in lower latitudes. If only a fourth of this 



ORIGIN OF THE PACIFIC ISLAND MOLLUSCAN FAUNA 333 

number occur in the western Pacific in an average year there would have 
been more than half a billion typhoons in the island since the close of the 
Cretaceous. 

SUMMARY INTERPRETATION 

The Pacific Ocean probably existed in much its present form at least as 
far back as the middle Cretaceous. Evidence recently obtained from 
soundings, dredging and drilling suggests that the southwest part of the 
ocean basin, in those early times, contained many more islands and shal- 
low water banks than it does today. The shallow water marine inver- 
tebrates, including mollusks, that lived among the islands and banks could 
have migrated with the aid of prevailing winds and currents from the 
island area to the west and southwest. These early migrants may have in- 
cluded many elements of what is now known as the Indo-Pacific fauna. 

Shallow water sediments as old as Cretaceous have recently been dis- 
covered in the Central Pacific and it is believed that existing volcanic 
oceanic islands, such as Hawaii, have had longer geological histories than 
is indicated by their meager fossil record. If, throughout their long history, 
they have been built up by local volcanic eruptions, there may always have 
been some intervening land on which ancient stocks of terrestrial faunas 
and floras could endure. 

The Pacific island area may have been, in effect, a giant archipelago in 
Cretaceous and Tertiary times. Its marine fauna could migrate toward In- 
donesia, its terrestrial life could persist. Two heretofore conflicting lines of 
evidence would, thus, be reconciled— geological evidence that suggested 
until recently only youthful islands, and evidence from terrestrial inver- 
tebrates and plants that has long pointed to greater age. 

ACKNOWLEDGMENTS 

I am indebted to Harald A. Rehder and Fenner A. Chace, Jr., of the 
U. S. National Museum and to Preston E. Cloud, Jr. of the U. S. Geo- 
logical Survey who read the manuscript critically and offered valuable 
suggestions. 

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its origin: Bernice P. Bishop Mus., Bull. 159, 93 p. 
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Mus. Special Pub. 7, pt. 1, p. 153-158. 



334 HARRY S. LADD 

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Holothuroidea: 8th Pacific Sci. Cong. Proc, v. 3, p. 417-455. 
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Soe., p. 887-901. 
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atolls, pt. 1: U. S. Geol. Survey, Prof. Paper 260-A, 265 p. 
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Quart. Jour., v. 86, p. 72-136. 
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64, 97 p. 
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Mem. 66, 163 p. 
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South Wales Proc, v. 24, pt. 3, p. 391-417. 
Hertlein, L. G., 1937, A note on some species of marine mollusks occurring in both 

Polynesia and the western Americas: Am. Phil. Soc. Proc, v. 78, no. 2, p. 303-312. 
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Pacific): San Diego Soc. Nat. Hist. Trans., v, 11, no. 13, p. 345-364. 
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faunas in the inner Pacific: 7th Pacific Sci. Cong. Proc, v. 4, Zoology, Auckland, New 

Zealand, p. 5-9. 
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ORIGIN OF LIFE 335 

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Origin of Life 

• ELSO S. BARGHOORN 



INTEREST IN ORIGINS, WHETHER OF PEOPLES, OF CUS- 
toms, or of the earth and its physical features, have exercised man's imagi- 
nation and invited speculation throughout the course of human history. 
Scientific scrutiny of the origin and timing of events in the history of the 
earth, however, have awaited slow accumulation of observation and knowl- 
edge regarding the nature of the physical and biological world. It is per- 
haps true that much, if not most, of what constitutes the achievements 
of science, in contrast to the applications of science, during the past 300 
years has been built of an unending intellectual curiosity concerning the 
manifestations of matter and its relation to the environment. It is scarcely 
to be wondered, therefore, that interest in the problem of the origin of 
life has taunted the curiosity of scientists of the past and will continue to 
in the future. Our present century, however, perhaps because of a rapid 

• From Treatise on Marine Ecology and Paleoecology, II, ed. H. S. Ladd (New York: 
Geological Society of America, Memoir 67, 1957), pp. 75-86. 



*> 



336 ELSO S. BARGHOORN 

and continuing advance along so many frontiers of quantitative science, 
has been featured by rejuvenated interest and, indeed, almost an absorp- 
tion in problems of the origins and dates of events in the earth's history. 
Discovery of radioactivity in the late nineteenth century and consequent 
radical modification of ideas on what had previously been considered the 
immutable state of the atom has brought all manner of new methods of 
inquiry into play, ranging from means of dating the earth's crust and 
specific events in its geologic history to methods of studying the intricate 
course of molecules in complex biological systems. 

From these many diverse and unrelated fields of research attention 
focuses from time to time on the ancient problem of the origin of living 
matter. It is the purpose of this discussion to show how various areas of 
science bear on the question and to point out the need for greater integra- 
tion of what may appear to be seemingly unrelated facets of science on 
this problem of paramount intellectual interest. This discussion contains 
no new theories and no tours de force of pure speculation. It is an attempt 
to summarize ideas on a problem which is almost indefinable. The author 
feels no special qualifications for the task except perhaps a lasting interest 
in the problem. As Bernal (1951) has aptly pointed out, "it is probable 
that even the formulation of this problem is beyond the reach of any one 
scientist." 

The question of the origin of life is both a philosophical and a scien- 
tific one. It is philosophical in that its solution and ultimate analysis re- 
main an enigma in the present state of knowledge of chemical processes 
and physical order in living systems. It is scientific in that it can be ap- 
proached and analyzed with techniques of modern science, ranging from 
those of cosmography and astronomy to the refinements of physical chem- 
istry. As a cosmic event the origin of life is an epiphenomenon of the 
origin of the earth and should be set in its appropriate astronomical set- 
ting. But the problem compounds at a disconcerting rate, because inquiry 
as to how the earth came into being and into its position in space— a 
highly critical physical setting for the creation of protoplasm— becomes 
involved in the still larger question of the origin of the solar system. In- 
deed the mystery may be extended from there to speculation on the origin 
and structure of the universe. 

Analysis of the problem of the origin of life, well supplemented with 
unhampered speculation, has been attempted by an increasing number of 
workers in rather diverse fields of science within the past few decades, es- 
pecially in the physical sciences. A strong motivating stimulus during this 
period and one lending scientific dignity to the problem doubtless was the 
pubhcation in 1936 (English translation in 1938) of The Origin of Life 
by the Russian biochemist A. I. Oparin. Oparin's ideas on the origin of 
life are deeply embedded in subsequent theories. The reader is urged to 
review his treatise. Oparin develops a forcibly clear and compelhng argu- 



ORIGIN OF LIFE 337 

ment explaining the origin of primary organic molecules, in particular 
proteins, and of primary colloidal systems. Oparin's assumptions concern- 
ing the probable reducing nature of the primitive atmosphere, essential to 
his theory, have been indirectly corroborated by new information con- 
cerning the atmospheres of other planets, in particular the outer planets 
(Kuiper, 1951). The abundance of both methane and ammonia in the 
atmospheres of Jupiter and Saturn has been regarded as presumptive evi- 
dence of their occurrence in the primitive atmosphere of the earth. 

With the gradual breaking down of the classical distinction between 
organic and inorganic chemistry during the past half century, the way has 
been paved for closer integration of biological and physical chemistry. 
Perhaps no more striking illustration of this, in connection with our prob- 
lem, can be found that the simple but ingenious experiments of Miller 
(1953; 1955), a student of Harold Urey, who succeeded in producing a 
whole series of the structural units of protein, the amino acids, in a purely 
"inorganic" system. The experiments are almost unique in the inverse 
ratio of their simplicity to their fundamental significance. For the first time, 
and under controlled laboratory conditions, the synthesis of numerous 
complex organic molecules, arising from relatively simple compounds 
of widespread terrestrial occurrence, was achieved. These experiments 
and their biosynthetic implications have opened up numerous pos- 
sibilities of biochemical study, but most significantly they provide a tan- 
gible basis for analyzing the critical and, as yet, hypothetical initial steps 
in primary syntheses. The physical conditions of Miller's system— the pres- 
ence of methane, ammonia, hydrogen, and water — are certainly within the 
conceivable conditions of the primitive atmosphere of the earth, as is also 
the energy source, an electrical discharge. Interestingly enough, it should 
be noted that Bernal (1951), without benefit of experiments! evidence, 
postulated the probable formation of nitrogenous compounds, including 
the amino acids, in the chemical system of the primitive earth through the 
action of high energy ultraviolet radiation (2000 A or less). This energy 
influx no longer exists on the earth's surface because of the shielding ef- 
fect of the ozone layer in the upper atmosphere. 

There exists what might be called a great chemical void between the 
production of simple amino acids and their union into complex proteins, 
much less that of living substance. Recently, Wald (1954) had discussed 
this problem in a highly provocative and interestingly written essay. The 
Origin of Life. Following a modified Oparin scheme, Wald assumes the 
existence of an oxygen-free primitive atmosphere featured by the presence 
of highly reduced organic molecules, such as hydrocarbons, produced from 
metallic carbides reacting with water vapor. In the absence of enzymes, the 
formation of more complex organic molecules proceeds at an exceedingly 
slow rate. The important distinction is drawn between the possibility or 
probability of a reaction and the rate of reaction. Enzymes in living sys- 



338 ELSO S. BARGHOORN 

terns vastly accelerate the rate of reactions but are not essential to the 
possibility of their occurrence. In the Wald scheme organic complexes, J 
"spontaneously" formed, are protected from destruction through absence « 
in the environment of the two current forces of destruction: free oxygen and 
living organisms. The inherent tendency for complex molecules to undergo 
dissolution rather than integration Wald regards as the greatest obstacle 
in the scheme of synthesis. To preserve the entity of large organic mole- 
cules requires a generous supply of both material and energy. The dis- 
integrative tendencies, however, he points out, are offset by certain forces 
of integration, only in part understood, such as the tendency of very large 
molecules to remain intact simply through their size. Also there is an 
integrative force in certain organic molecules leading toward aggregates 
of oriented structures. Briefly, the system proposed is "amino acids ^ 
protein —> aggregate." The aggregates are essential intermediates between 
organic molecules and organisms. Interaction of aggregates forms more 
and more complex structures, and the system ultimately leads to living 
matter and the initiation of organic evolution. 

The physical setting of most of the postulated schemes of the origin of 
life has been assumed to be that of the sea. The nature of the primaeval 
sea has been the subject of conjecture by a number of authors, most re- 
cently by Rubey (1949) who considers the geological history of sea water 
from geochemical, paleontological, and biogeochemical evidence and deduc- 
tions. His arguments, to a great extent based on geologic observation, lead 
to the conclusion that the sea and atmosphere insofar as the amount of 
CO2 is concerned, has been essentially comparable to that of today 
throughout much of geologic time. How far back into the early Pre- 
cambrian these conditions may be postulated is largely speculative. It is of 
interest that most biochemical assumptions regarding the ecology of pri- 
mary organisms tacitly imply the saline environment of the sea. Oparin 
(1938) conceives the aggregation of organic complexes through the proc- 
ess of co-acervation, a phenomenon of large molecules whereby they re- 
duce their state of hydration and coagulate into a system presenting an 
interface with the environment. Bernal (1951) doubts such spontaneous 
settling out of proteins from a dilute suspension such as that of the open 
sea, but considers the possibility of molecular agglomeration in a lagoonal 
environment. Wald (1954) regards the sea, with its necessary salts, as the 
aqueous mixing medium and means of coming together of organic mole- 
cules in the necessary combinations leading to complex aggregates. He 
postulates that the sea gradually became a "dilute broth, sterile and oxygen- 
free." This is the "original soup" to which the many discussions regarding 
the origin of life finally turn. Urey (1952) proposes that the concentration 
of organic matter in the primitive seas reached the order of I per cent. 

The creation of order in molecular arrangements, in particular the 
preferential asymmetric isomerization which characterizes the organic 



ORIGIN OF LIFE 339 

molecules produced in vital processes, is suggested by Bernal (1951) to 
have begun in adsorption on mineral particles, particularly the clay min- 
erals and quartz. Fine-grained clays have been found preferentially to ad- 
sorb organic molecules in a regular manner, especially the montmorillon- 
ite clay minerals. This unusual property of the clay minerals was also em- 
phasized by McElroy {in Woodring, 1954), in his discussion of the origin 
of life; he pointed out the possible role of clay as an inorganic catalyst in 
the absence of enzymes. Phosphate, methyl, and other energy sources could 
thus be transferred without loss of bond energy. Hendricks, in the same 
discussion (Woodring, 1954), also brought out the fact that clay would 
combine with purines, pyramidines, and phosphates. Hence several of 
the components of nucleic acid could adsorb on clay particles. Under 
present natural conditions such organo-inorgano combinations would of 
course be impossible since the incessant activities of saprophytic organ- 
isms, such as bacteria, would destroy them. 

Bernal (1951) discusses the possible role of quartz, as a locus of ad- 
sorption in the origin of primary molecules, and in particular as a possible 
means of origin of optical activity and asymmetrical structure in organic 
molecules. Quartz is the only common and ubiquitous mineral exhibiting 
asymmetry. Asymmetric isomers once formed would produce conditions 
favoring only their kind. Oparin (1938) discusses the subject of prefer- 
ential isomerism and its origin and proposes several rather implausible 
theories. It should be noted that in living systems enzymes themselves 
are asymmetric, and hence aid in the formation of asymmetric molecules. 
This fact is of interest in considering the possible role of inorganic cataly- 
sis in the initial synthesis of enzymes and the origin of their isomerism. 

In discussing the manifestations of molecular order in organic systems 
Wald (1954) has called attention to the quasicrystallinity, in effect ap- 
proaching true cr}'stallinity, which may be observed, and in part artificially 
reproduced, in complex molecular aggregates. Many large protein and 
nucleic acid molecules, while in aqueous solution, are so large that they 
tend to align with respect to each other as a result of electric charges 
distributed on their surfaces, hence, forming "hquid crystals." As an ex- 
ample of this innate tendency toward organization Wald illustrates 
the reactions of collagen from cartilage and muscle. Collagen, dispersed 
in dilute acetic acid and dried, shows a complex but diffuse filamentous 
structure but when treated, after acidified dispersion, with a I per cent 
solution of sodium chloride coagulates into the highly organized fibrillar 
state comparable to that before dispersion. The significance of this re- 
crystallization of collagen is very considerable since it demonstrates an in- 
termolecular capacity for complex orientation from the random dispersed 
condition in an aqueous phase. It adds support to the concept of an in- 
herent capacity of organic molecules to assume preferred orientation in 
biologic systems. Whether or not such transitions of a simple type exist 



I 



340 ELSO S. BARGHOORN 

in living cells is not known, but it is of great significance that they occur 
and may be produced under controlled conditions. 

Perhaps the most critical condition in the various schemes of the origin 
of life lies in the composition of the primitive atmosphere. Following 
Oparin, most biochemical and geochemical theories are based on assump- 
tions of an anoxic atmosphere, a reducing environment in which early 
life began as a crude fermentative system, initially implemented by influx 
of high-energy ultraviolet radiation (2000 A and below). Accordingly, 
nearly all if not all free oxygen in the atmosphere is of biological, photo- 
synethetic origin. A somewhat modified hypothesis, offered by Urey (1952), 
proposes that organic compounds were produced photochemically at a 
time before carbon and nitrogen were oxidized by atmospheric oxygen, the 
oxygen (hydroxyl) being produced by the photolysis of water in the 
primitive atmosphere. Hydrogen is lost from the upper atmosphere. Ac- 
cording to this scheme, life as a manifestation of protoplasmic activity 
began at an undetermined, and probably geologically undeterminable time, 
before free oxygen began to accumulate. The biochemical consequences 
of this hypothesis are far reaching because it would imply a possibly very 
brief period for development of respiratory metabolism— if fermentative 
life were its prelude. In addition it would somewhat modify the time con- 
cept inherent in the theory of very gradual accumulation of primary or- 
ganic molecules in an anaerobic atmosphere. On geologic grounds it 
should be noted, however, that the highly oxidized state of iron in certain 
Archean rocks would seem to necessitate a very remote time for the 
oxidizing environment to come into operation. 

An interesting corollary to the idea that free oxygen was absent in the 
primitive atmosphere is presented in a recent paper by Gulick (1955) who 
calls attention to the significance of the biochemistry of phosphorus in the 
origin of life. If there is need for dissolved phosphorus for life to make a 
start it would most probably be present in low-oxygen compounds such as 
phosphites and hypophosphites. The energy-transfer role of these low- 
oxygen phosphorus compounds requires extremely anaerobic conditions. 
The quantity of phosphorus compounds available under present natural 
conditions would be too low for primaeval organisms to utilize. Gulick 
states, "It is inconceivable that the earliest organisms could have been very 
adept in a biochemical adjustment such as gathering in a rare nutrient" 
{i.e., phosphorus). 

One feature of the primitive earth often postulated in the effluence of 
hydrocarbons into the atmosphere from the interior of the earth by the 
action of water on metallic carbides. Oparin reviews the earlier observa- 
tions of the chemistry of this reaction which was known over a century 
ago. On geologic grounds the argument is difficult to defend since naturally 
occurring metallic carbides are unknown as minerals in the earth's crust 
today and do not occur in magmatic extrusions. The concept of abundant 



ORIGIN OF LIFE 341 

metallic carbides existing in quantities great enough fundamentally to 
affect the planetar}^ atmosphere is a theoretical assumption which certainly 
has no geologic substantiation. However, of course, we have no way of 
sampling a portion of the interior of the earth for substantiating or refuting 
the assumption. Metal carbides, though unknown on the earth, are found 
in nature only in meteorites. 

Life, once having come into existence on the earth, must have left 
some record in the geologic past. This is an assumption of paleontology 
which, owing to imperfections and gaps in the record, is perhaps more 
theoretical than practical. However, since formulation of our current un- 
derstanding of major events in organic evolution and the achievement of 
a general framework of absolute chronology in which it is set, search for 
ancient and primaeval life has been pressed hard and with meager, yet 
notable, success. The transition from speculation and theory on the origin 
of life to examination of the tangible evidence of life is a difficult one. The 
hampering influence of dealing with visible manifestations of phenomena in 
earth history leads to a certain skepticism about theories dealing with condi- 
tions preceding those which have left any decipherable geologic imprint 
on the earth. Perhaps it is for this reason, at least in part, that relatively 
few geologists have joined the ranks of those who have theorized on the 
enigma of the origin of life. Yet it is geological observation, coupled with 
geochemical analysis, which ultimately places control on theories of the 
paleochemistry of ancient sedimentary rocks, those which may contain 
the earliest record of life. The morphological paleontologic record is ordi- 
narily inaccessible in metamorphosed rocks, and unfortunately the ma- 
jority of the oldest sediments are so metamorphosed as to obliterate all 
traces of possible contained organic structure. The fossil record of life 
may, however, be preserved through its geochemical manifestations, ir- 
respective of form, such as, for example, in the biochemical accumulation 
or concentration of certain minerals. The vast accumulations of iron in 
rocks of Huronian or equivalent Precambrian age in various parts of the 
world seem most reasonably to have accumulated through biogeochemical 
processes. The calcareous stromatolites and reeflike structures known 
widely from Precambrian rocks are most probably the result of calcareous 
algae even though organic form is poorly defined and organic matter is 
absent. 

The general skepticism of paleontologists toward Precambrian fossils is 
well justified since so many alleged Precambrian animal and plant fossils 
have proven to be artifacts of sedimentary and metamorphic processes. 
However, logical deduction yields strong presumptive evidence that plant 
life must have been present well back in the Precambrian, The geochemi- 
cal balances in the earth's atmosphere today are to a great extent under 
biological control. It has been estimated that the atmospheric oxygen of 
the earth is renewed each 2000 years by photosynthetic exchange, and the 



I 



342 ELSO S. BARGHOORN 

carbon dioxide of the atmosphere each 300 years (Wald, 1954). These 
two gases alone affect all manner of controls on the nature of weathering 
and the character of sediments formed. Rubey (1951) discusses this ques- 
tion in relation to the probable ranges in partial pressure of carbon dioxide 
during geologic time. His review of the problem is comprehensive and 
weighs the evidence from paleontology, geochemistry, sedimentary pe- 
trology, and biology. His general conclusions are that (p. 1134) "for a 
large part of geologic time, carbon dioxide has been supplied to the at- 
mosphere and ocean gradually and at about the same rate that it has been 
subtracted by sedimentation." Had this not been the case the character of 
calcium and magnesium deposition and its relative extent would show 
major changes in geologic time. There is no evidence of this in comparing 
Precambrian with post-Precambrian sediments. Unfortunately, there is 
paucity of data regarding the chemical composition of ancient sedimentary 
rocks. The role of living organisms in circulating carbon dioxide, although 
highly efficient, is of course small in relation to the total carbon budget- 
that which is supplied by weathering of limestones (and from igneous 
sources) and that which is subtracted by sedimentation. How important 
biological systems were in the early genesis of carbon dioxide in the 
early atmosphere (fermentative reactions) cannot be determined, but at 
the present time the concentration of this gas is remarkably influenced by 
photosynthesis, the rate of which process is very probably controlled in 
nature by the amount present in the atmosphere. Small increases in car- 
bon dioxide would probably be fairly rapidly eliminated by photosynthetic 
subtraction. A substantial decrease would be disastrous for autotrophic 
plant life. 

The problem of oxygen in the history of the atmosphere is perhaps 
even more complex than that of carbon dioxide. Oxygen has tremendous ] 
combining capacity, and its current high and sustained concentration in 
the atmosphere is almost entirely the result of photosynthesis, past and 
present. The time at which the atmosphere became strongly oxidizing 
would presumably approximate the time of perfection of a system of 
photosynthesis operating in the visible light range. The oldest lithologic 
evidence of red beds, deposits which seem assuredly to have been produced 
under conditions of high oxidation, hence becomes of critical importance. 
Red beds of Huronian age are known, though earlier Precambrian red 
beds have not been observed, insofar as the author is aware. However, 
iron ores of Archean age such as those of the Vermilhon range of Minne- 
sota and adjacent Canada, are difficult to explain except under conditions 
of a highly oxidative atmosphere. Urey (1952) has offered an alternative 
explanation of their possible source, involving oxidation of ferrous iron 
to ferric oxide at high temperatures, but considers that it would probably 
be a rare event. It seems more likely on geologic grounds that highly 



ORIGIN OF LIFE 343 

oxidized iron on the large scale of the Archean ore deposits has resulted 
from the effects of an oxidizing atmosphere of biogenic origin. 

If we follow the few threads of geochemical reasoning offered here, in 
relation to the paleochemistry of oxygen and carbon dioxide, we are led to 
the conclusion that life must have existed and, in fact, must have been 
abundant in the Precambrian. The existence of oxygen in the atmosphere, 
quite certainly biologically produced, can be traced far back in the Pre- 
cambrian through its expression in oxidized sediments. The abundance 
of highly oxidized carbon in the form of limestone and indirect evidence 
deduced from the probable paleochemistry of sea water and Archean 
sediments are indicative of an oxygen-carbon dioxide balance which did 
not deviate markedly from that of today. 

The demonstration of tangible evidence, in contrast to indirect geo- 
chemical evidence of Precambrian life, therefore assumes great importance, 
not only as a clue to the evolution of life but as a guide to delineating the 
probable biogeochemical history of the earth, both atmosphere and litho- 
sphere. It is of interest therefore to examine briefly some of the more 
cogent evidence of Precambrian organisms. The author makes a plea to 
be spared from a general review of Precambrian fossils and the volumi- 
nous literature which has accumulated during the past century. 

Graphite and graphitic carbon in ancient Precambrian sediments and 
metamorphic rocks provides no certain evidence of its biogenic origin. 
In a series of papers Rankama (1948; 1954a; 1954b) has proposed a geo- 
chemical argument, based on the isotopic ratios of Carbon^^ and Car- 
bon^^ in sediments of varying age, that early Precambrian carbon in phyl- 
lites and schists from northern Finland and Canada are of biogenic origin. 
The occurrence of problematical fossils {Corycium enigmaticum) has been 
proposed to support the biological origin of the carbon (Sederholm, 1897; 
1924; Rankama, 1950). Rankama's arguments, however, are primarily geo- 
chemical, not paleontological. Craig (1953, and unpublished) has vigor- 
ously criticized Rankama's arguments on the basis of his mass-spectro- 
metric studies of the stable carbon isotope ratios in a wide range of marine 
and terrestrial sediments and including living plants. He finds no evidence 
of isotopic fractionation in the course of geologic time, hence no age 
effects on the relative abundance of the two isotopes in nature. His data 
also show no correlation in Carbon^^ ^^d Carbon^^ ratios in known bio- 
genic carbon— wood, coal and fossiliferous limestone— with geologic age. 
It would be of interest to study, by Craig's techniques, the isotopic ratios 
in carbonaceous sediments of increasing Precambrian age, from Keweena- 
wan to early Archean, and to compare these with lithologically com- 
parable carbonaceous sediments of more recent, post-Paleozoic age. 

It is theoretically possible that the most ancient carbon-rich meta- 
morphosed sedimentary rocks are holdovers or "fossils" derived from the 
primitive fermentative metabolism of early life. The paleoecological or 



344 ELSO S. BARGHOORN 

paleochemical evidence of reducing conditions in the environment of 
deposition would then seem logical and understandable. If this interpre- 
tation were to be accepted, organic sediments of early Precambrian age 
would indeed contain the original carbon of the primaeval organisms, the 
"organic broth" of biochemical theories of the origin of life. 

Much more difficult to explain, except on the basis of well-organized 
photosynthetic life, however, is the occurrence of relatively thick sediments 
of very high carbon content, virtually conforming to coal, in rocks of 
Huronian age. In both Finland and the United States Precambrian 
"coals" occur which seem most certainly to be of organic origin. These 
are highly carbonaceous members of sedimentary sequences showing vary- 
ing range in carbon content up to 99.77 pure carbon in the case of sedi- 
ments from Finland (Marmo, 1953; Metzger, 1924). Precambrian "coals" 
of northern Michigan (Michigamme shale) yield 78-80 per cent carbon, 
the mineral fraction being of the order of 98 per cent silica (S. A. Tyler, 
E. S. Barghoorn, and L. P. Barrett, unpublished). The Michigan sedi- 
ments are characterized occasionally, in their shale members, by oval- 
shaped graphitized structures, conspicuously displayed along the bedding 
planes. These may reasonably be interpreted as the graphitized compres- 
sions of organisms. Their forms and orientation are indicative of origin 
from avoid colonial blue-green algae of a type occurring today (e.g., the 
genus Nostoc). Sediments of very high carbon content, interbedded with 
highly carbonaceous black shales and featured by intermediate facies be- 
tween shale and nearly pure carbon, seem virtually impossible to explain 
except by processes of organic deposition. Precambrian coals, in the 
opinion of the author, are impressive evidence of the abundant and wide- 
spread occurrence of organisms, capable of growth and accumulation to 
the extent of forming nearly pure organic sediments. Photosynthetic origin 
of their carbon seems certain but unfortunately cannot be proved. 

Conclusive evidence that organisms of simple organization existed in 
relatively early Precambrian time has recently been secured from rocks 
of Lower Middle Huronian age (Tyler and Barghoorn, 1954). The or- 
ganisms, comparable to blue-green algae and aquatic fungi, are structurally 
preserved in dense, nonferruginous cherts of the basal members of the 
Gunflint iron formation exposed along the north shore of Lake Superior 
in Ontario. The plants, as they may best be called on the basis of morphol- 
ogy, are primarily filamentous structures organized in colonies varying 
from actinomorphic aggregates to random groups of unbranched, septate 
filaments. Spherical sporelike bodies are abundant. Their preservation in 
a hyaline unrecr^stallized chert renders visible, in transmitted light, a 
remarkable degree of original structure and orientation. Both colonial 
aggregates and free filaments may be traced in three dimensions. The 
truly organic composition of the microscopic organisms may be demon- 
strated by their release from embedding silica by dissolution in hydro- 



ORIGIN OF LIFE 345 

fluoric acid. This process can be observed under the microscope and 
demonstrates also the presence of a large amount of amorphous attrital 
material of organic origin, probably representing the degraded remnants 
of pre-existing colonies. 

Samples of the Gunflint chert have been subjected to rigorous analysis 
by Abelson (Personal communication) for their possible amino acid 
content. Employing a meticulous method to eliminate the possibility of 
surface contamination, he has demonstrated chromatographically the 
presence in the chert of eight amino acids: alanine, proline, glycine, glu- 
tamic acid, valine, leucine or isoleucine, aspartic aid, and lysine. The latter 
two were present in trace amounts. The concentration of original amino 
acids in the cherts was 2.0 micromoles per gram. Abelson (Personal com- 
munication) has quite reasonably raised some question as to the geologic 
age of the chert, although in the opinion of the writer this can be readily 
answered on the basis of field relations. The Gunflint iron formation, of 
which the chert is a basal member, is generally regarded as Middle Hu- 
ronian (Leith, Lund, and Leith, 1935), of an age well over 1000 million 
years. 

The Gunflint plants are apparently the oldest organisms showing defi- 
nitive biological structure which have yet been found. Their occurrence 
suggests a re-appraisal of the doubts which have been cast upon the prob- 
able nature of Corycium enigmaticum (Rankama, 1948) and indeed in- 
vite speculation on the possibility that the "plants" described by Gruner 
(1925) from the Archean are actually organic, rather than phenomena of 
crystallization. 

The most recent evidence, derived from paleontologic sources, hence, is 
assuring that organized and primitive, yet well differentiated, life existed 
well back in Precambrian time. It seems no longer necessary to speculate 
on the possibility of life in the Precambrian; rather it seems more neces- 
sary to pursue the quest for ancient life more vigorously in the field. 

Consideration of the theoretical basis for the origin of life and the origin 
of primary systems of organic substance necessarily leads to speculation on 
the nature of living substance, and how it differs from the nonliving. Ber- 
nal (1951) discusses this problem in a most illuminating way, though per- 
haps abstruse to the biologist and paleontologist who deals constantly 
with the extraordinary differences between "living" and "dead" matter. 
The weaknesses in comprehending and explaining biological processes, 
despite their extreme complexity, are perhaps more easily formulated by 
the physicist than the biologist. The basic laws governing living matter are 
certainly more easily approached by avenues of physical science than 
through the ruck of complexities and apparent contradictions presented 
by biological systems. Brillouin (1949) presents a curious and whimsical 
analogy between the problem of living and nonliving in his comparison of 
the reactive potential of a living organism and that most familiar example 



346 ELSO S. BARGHOORN 

of the nonliving— the automobile. In considering the commonplace, but 
extraordinary fact that the organism heals its own wounds, he contrasts 
it with that of an automobile with a flat tire, whereupon we wait until 
the tire mends itself. The example is constructive, though far-fetched, but 
perhaps demonstrates how little we really appreciate the extraordinary 
properties of living matter. 

Schrodinger (1945) in a memorable essay on the problem of life, as 
seen from the physicists' point of view, comments on the difficulty of 
transferring knowledge of physical, inorganic systems to understanding of 
biological systems. He traverses the steps of understanding of biological 
integration through the genetic stages of organismal complexity to the 
point at which the biologist may be inclined to believe the physical in- 
evitability of life. The simple interpretation of vital phenomena in terms 
of modern physics, however, is not so convincing as it once was. Modern 
developments in genetics and cytogenetics have added new complexities to 
the explanation of vital phenomena, in particular the role of genes which 
apparently aflPect the rate of mutation in specific environments. Organ- 
ismic innovations, the more they are investigated, seem to add to con- 
tinually rather than detract from the complexity of interpreting life in 
terms of orthodox physical models. 

Any effort to survey current ideas on the origin and early development 
of life necessarily leads to a realization of how frustratingly complex and 
diverse modern science has become. The problem ramifies into almost all 
aspects of science from astronomy to nuclear chemistry. One comes away 
with the clear appreciation that the border lines of departmentalized fields 
in the sciences must be bridged by some means of communication, lest 
the progress of thinking becomes inhibited by scientific regionalism. There 
is always the gap between the so-called experimental sciences and the so- 
called observational sciences, a distinction between those who experiment 
with and those who simply observe natural phenomena. It is essential that 
this gap — perhaps it is more apparent than real— be closed. This is par- 
ticularly necessary in correlating realms of pure speculation with those of 
mundane but careful observation of natural events. Observational science 
may range from study of extragalatic stars to the visually observable prop- 
erties of the cell in the microscope. Experimental science may range from 
mathematical theory to study of the properties of subatomic particles in 
an atomic pile. A meeting ground between the two avenues of science- 
observation and experimentation— certainly is to be found in the prob- 
lem of the origin of life. It is one which requires constant interplay be- 
tween remote areas of science. It is a subject rife with speculation, yet 
one on which logical controls may be placed, controls derived from ob- 
servable phenomena as well as theory, and one which requires continued 
integration of theory with observational facts. The enigma appeals to all 
those concerned with the expression of life, particularly to biologists, 



ORIGIN OF LIFE 347 

paleontologists, and those interested in the paleochemistry and paleontol- 
ogy of the earth. To many scientists, however, the varied and incredible 
manifestations of the complexity of living systems are more remarkable 
and intriguing than the origin of life itself. To them there is a greater 
appeal in the panorama of organic evolution— that difficult sequel to the 
mystery of the origin of life. 

REFERENCES 

Bernal, J. D. The physical basis of hfe: 80 pp., London, Routledge and Paul, 1951. 
Brillouin, L. Life, thermodynamics and cybernetics: Am. Scientist, vol. 37, pp. 554-568, 

1949. 
Craig, Harmon. The geochemistry of the stable carbon isotopes: Geochem. et Cos- 

mochem.. Acta. vol. 3, pp. 53-92, 1953. 
Gruner, J. W. Discovery of hfe in the Archaean: Jour. Geol., vol. 33, pp. 151-152, 1925. 
Gulick, Addison. Phosphorus as a factor in the origin of life: Am. Scientist, vol. 43, pp. 

479-489, 1955. 
Kuiper, Gerard P., editor. The atmospheres of the earth and planets: 434 pp., Univ. of 

Chicago Press, 1951. 
Leith, C. K., Lund, R. J. and Leith, A. Pre-Cambrian rocks of the Lake Superior region: 

U. S. Geol. Survey Prof. Paper 184, 34 pp., 2 pis., 1935. 
Marmo, Vladi. Schungite — a pre-Cambrian carbon: Geo. Foren in Stockholm Forh, vol. 

75, pp. 89-96, 1953. 
Metzger, Adolph A. Th. Die jatulischen Bildungen von Suojarvi in Ost Finnland: Bull. 

Comm. Geology, Finlande, pp. 64-80, 1924. 
Miller, Stanley L. A production of amino acids under possible primitive earth conditions: 

Science, vol. 117, pp. 528-529, 1953. 
. Production of some organic compounds under possible primitive earth condi- 
tions: Jour. Am. Chem. Society, vol. 77, pp. 2351-2361, 1955. 
Oparin, A. 1. The origin of life: Trans, from the Russian text by Sergius Morgulis, viii 4- 

270 pp. New York, Macmillan Company. 
Rankama, Kalervo. New Evidence of the origin of pre-Cambrian carbon: Geo. Society 

of America Bull., vol. 59, no. 5, pp. 389-416, 4 figs., 6 pis., 1948. 
. Origin of carbon in some early pre-Cambrian carbonaceous slates from south- 
eastern Manitoba, Canada: Comptes Rend, de la Soc. Geol. de Finlande, vol. 27, pp. 

5-20, 1954a. 

The isotopic composition of carbon in ancient rocks as an indicator of its bio- 



genic or non-biogenic origin: Geochem. et Cosmochem. Acta, vol. 5, pp. 142-152, 

1954b. 
Rubey, W. W. Geologic history of sea water. An attempt to state the problem: Geol, 

Society of America Bull., vol. 62, pp. 1117-1147, 1951. 
Schrodinger, Erwin. What is life? The physical aspect of the living cell: viii + 91 pp.. 

New York, Macmillan Company, 1945. 
Sederholm, J. J. Uber eine archaischen Sediment formation im siidwestlichen Finland 

und hire Bedeutung fiir die Erklarung der Entstehungsweise des Grundgebirges: 

Comm. Geol. Finlande Bull. no. 6, 1897. 
. Uber die primare Natur des Coryciums Centralblatt f. Mineral: Jahrg., p. 717, 

1924. 
Tyler, S. A. and Barghoorn, E. S. Occurrence of structurally preserved plants in pre- 
Cambrian rocks of the Canadian shield: Science, vol. 119, pp. 606-608, 1954. 
Urey, Harold. On the early chemical history of the earth and the origin of life: Proc. 

Nat'l. Acad. Sci., vol. 38, pp. 351-363, 1952. 
Wald, George. The origin of hfe: Scientific American, vol. 191, no. 2, pp. 44-53, 1954. 
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Acad. Sci., pp. 219-224, 1954. 



ORIGIN 

AND 
EVOLUTION 

OF 
THE EARTH 



There's nothing constant in the universe, 
All ebb and flow, and every shape that's born 
Bears in its womb the seeds of change. 
— OVID, Metamorphoses, XV 



The Origin of Continents, Mountain Ranges, 
and Ocean Basins 



• GEORGE C. KENNEDY 



ONE OF THE UNEXPECTED DISCOVERIES IN EARTH Sci- 
ence in the previous century was that of a fundamental difference between 
continents and ocean basins. Ocean basins are not merely the low lying 
parts of the Earth's surface Hooded by salt water but are great, relatively 
steep-sided, structural depressions. In fact, there is too much water for the 
size of the ocean basins, and parts of the continents are now flooded and 
probably have been flooded through a great deal of the geologic past. A 
typical continental mass with adjacent ocean basins is shown in Figure 
lA. 

Precise measurements of gravitation attraction in major mountain 
ranges, continental areas, and over the ocean basins showed an even more 
unexpected feature. The continents and mountain ranges do not represent 
extra loads of rock superimposed upon the Earth's crust, but are masses 
of lighter rock floating in a denser substrate. An iceberg floats above the 
water much in the same fashion, buoyed up by deep submerged roots. 
The great mountain ranges of the world and the continental masses sim- 
ilarly have deep roots of light rock penetrating down into the denser 
crust, and thus the mountain ranges and continents float at elevations 
appropriate to the depth and size of these submerged light roots. Thus, 

• From American Scientist (Dec, 1959), pp. 491-504. A Sigma Xi-RESA National 
Lecture, 1958-59. Publication #122, Institute of Geophysics, University of California, 
Los Angeles 24, California. 

349 



350 GEORGE C. KENNEDY 

all the major features of relief of the surface of the Earth show mirror 
image features within the crust, much as is indicated in Figure IB. The 
major mountain ranges float at high elevations because they are buoyed 
up by light rocks. The continents float at intermediate elevations with 
roots of intermediate depth, and the deep oceans are underlain by thin 
layers of light rock. 

MOUNTAIN RANGE 
SEA LEVEL ../^^^ 



a: 



Fig. 1. Profiles through the earth's crust. 

Seismologists, from the study of earthquake waves, have shown that the 
Earth's mantle is solid to the depth of the outer core, some 2900 kilo- 
meters. The observation that large mountain ranges and continental 
masses float on the crust of the Earth at elevations appropriate to the 
size and density of their roots implies that rocks at shallow depths in the 
Earth's mantle, although solid, have little strength and can flow in re- 
sponse to small stresses given sufficient time. This deduction is strength- 
ened by the observation that rocks in deep, eroded, old mountain chains 
are intensely contorted and folded, plain evidence that, at high pressures, 
solid rocks can flow readily and do not have great strength. 

Recognition that continental rocks are lighter and more buoyant than 
oceanic rocks gave rise to the concept that the crust of the Earth is made 
of two contrasting materials: sialic material, rich in silicon, the alkalies 
and aluminum, making up the continents; and simatic material, richer in 
iron and magnesium, making up the denser rocks below the floor of the 
ocean and lying under the sial of the continents. The sial is assumed to be 
granite or granodiorite in composition, and the sima is assumed to be 
basaltic in composition. 

Early in this century, the Yugoslav seismologist, Mohorovicic, obtained 
evidence from seismograms that earthquake waves, traveling a few tens of 
kilometers below the surface of the Earth, gave records showing sharply 



CONTINENTS, MOUNTAIN RANGES, AND OCEAN BASINS 351 

higher speeds for both shear and compressional waves than earthquake 
waves travehng near the surface. This indicated an abrupt change in rock 
types at a few tens of kilometers under the continents and at a few kilo- 
meters under the oceans. In recent years, extensive studies have produced 
a fairly clear general picture of the nature and depth of this level of 
change, or discontinuity, under the continents, mountain ranges, and 
ocean basins. This discontinuity, called the Mohorovicic or M disconti- 
nuity, is at a general depth of 30 to 40 kilometers under the continents, but 
may be as deep as 60 kilometers under the roots of major mountain chains. 
It is as shallow as 4 to 5 kilometers below the floor of the deeper parts of 
the ocean. The discovery of the M discontinuity seemed to confirm the 
notion that the crust is fundamentally made up of two different kinds of 
rock material. The discontinuity itself appears to be the boundary between 
these, the sialic rocks above and the simatic rocks below. The rocks be- 
low the M discontinuity have seismic velocities and densities which sug- 
gest that they may be even richer in magnesium and iron than normal 
sima of basaltic composition. Consequently, they are, by some, called ul- 
trasima. However, throughout this paper the word sial will be applied to 
the lower velocity rocks above the M discontinuity, including the range 
of basalts to granites, and the word sima will be applied to the denser 
rocks below the M discontinuity. 

Prior to and along this general picture, the concept of isostasy developed. 
This is the notion, previously discussed, that the lighter continental rocks 
float at an appropriate depth, depending on their mass and mean density, 
in a denser substratum. As rock is eroded from the tops of continents and 
mountain ranges they tend to float up higher and higher, renewing their 
relief, permitting erosion to continue. 

Four facts, however, sharply contradict this picture of a sialic crust of 
varying thickness floating on a simatic substratum of different chemical 
composition and different density. 

1. Large areas of continents, long near sea level, have been uplifted 
many thousands of feet in the air. Further, this uplift seems to have taken 
place rather rapidly in terms of geologic time. 

2. Sediments of low density, filling troughs along the margins of conti- 
nents, apparently are able to subside into this higher density substratum. 

3. Inasmuch as radioactive, heat-producing elements are associated with 
sialic rocks, one might expect heat flow through the thicker parts of the 
Earth's crust to be much greater than through the thinner parts of the 
Earth's crust. However, as a first approximation, heat flow through the 
crust of the Earth is approximately the same through continents, moun- 
tain ranges, and ocean basins. 

4. The lifetime of continents and mountain ranges is vastly greater than 
rates of erosion would suggest. 

Let us examine each of these apparent facts and their consequences on 



352 GEORGE C. KENNEDY 

the hypothesis of siahc continents floating on a simatic basin. The prob- 
lem of the upHft of large plateau areas is one which has puzzled students 
of the Earth's crust for a very long time. Regions which are at sea level, 
or near sea level, may, over a relatively short geologic time span, such as a 
few million years, be uplifted several thousands of feet. The Colorado 
plateau and adjacent highlands is an example. Here, in an area of ap- 
proximately 250,000 square miles that apparently stood at sea level for 
several hundreds of millions of years was uplifted approximately one mile 
vertically some 40,000,000 years ago in early mid-Tertiary time and is still 
a high plateau. The Grand Canyon of the Colorado has been carved 
through this great uplifted plateau. 

Given an Earth with sialic continents floating in a denser simatic sub- 
stratum, what mechanism would cause a large volume of low standing 
continents to rise rapidly a mile in the air? Furthermore, evidence from 
gravity surveys suggest that the rocks underlying the Colorado plateau are 
in isostatic balance, that is, this large area is floating at its correct eleva- 
tion in view of its mass and density. Recent seismic evidence confirms 
this, in that the depth to the M discontinuity under the Colorado plateau 
is approximately 10 kilometers greater than over most of continental North 
America. Thus, appropriate roots of light rock extend into the dense 
substratum to account for the higher elevation of the Colorado plateau. 
We have then a double-ended mystery, for the Colorado plateau seems to 
have grown downward at the same time that its emerged part rose up- 
ward. This is just as startling as it would be to see a floating cork suddenly 
rise and float a half inch higher in a pan of water. To date, the only 
hypothesis to explain the upward motion of large regions like the Colo- 
rado plateau is that of convection currents. Slowly moving convection 
currents in the solid rock, some 40 to 50 kilometers below the surface of 
the Earth, are presumed to have swept a great volume of light rock from 
some unidentified place and to have deposited it underneath the Colo- 
rado plateau. A total volume of approximately 2,500,000 cubic miles of 
sialic rock is necessary to account for the uplift of the Colorado plateau. 
While it is not hard to visualize rocks as having no great strength at the 
high pressures and temperatures existing at depths of 40 to 50 kilometers, 
it is quite another matter to visualize currents in solid rock of sufficient 
magnitude to bring in and deposit this quantity of light material in a 
relatively uniform layer underneath the entire Colorado plateau region. 

The Tibetan plateaus present a similar problem, but on a vastly larger 
scale. There, an area of 750,000 square miles has been uplifted from ap- 
proximately sea level to a mean elevation of roughly three miles, and the 
Himalayan mountain chain bordering this region has floated upward some 
five miles, and rather late in geologic time, probably within the last 
20,000,000 years. The quantity of light rock which would need to be swept 
underneath these plateaus by convection currents to produce the effects 



CONTINENTS, MOUNTAIN RANGES, AND OCEAN BASINS 353 

noted would be an order of magnitude greater than that needed to uplift 
the Colorado plateau, that is approximately 25,000,000 cubic miles. Even 
more troublesome than the method of transporting all this light rock at 
shallow depths below the surface of the Earth is the problem of its source. 
The region from which the light rock was moved should have experienced 
spectacular subsidence, but no giant neighboring depressions are known. 
A lesser but large problem is how such enormous quantities of light rock 
can be dispersed so uniformly over so large an area. 

This evidence of uplift and downsinking of various crustal blocks, with 
the blocks always remaining in approximate isostatic balance, does not 
seem to harmonize with the view of a floating sialic continent on a denser 
substratum where one might expect to find little variation in elevation 
with time. 

The second problem, that of the subsidence of troughs, is of equal diffi- 
culty. The rivers of the world carry enormous quantities of sediments sea- 
ward. Most of this sedimentary burden is deposited within a few tens or 
hundreds of kilometers of the shore line and little is transported to the 
deep ocean basins. Thus, elongate prisms of sediments are built up paral- 
lel to the shores of certain regions where great quantities of sediments are 
transported to the sea. The crust, in response to this added load of sedi- 
ments, begins to buckle downward. Troughs filled with sediments appear, 
paralleling the coast line. The chicken and the egg argument enters here, 
for it is not entirely clear whether deposition of sediments generates the 
troughs or whether the troughs are formed first and are later filled with 
sediments. However this may be, one such trough now in the making is 
along the coast of the Gulf of Mexico on both sides of the mouth of the 
Mississippi River. Surprisingly enough, this trough deepens at about the 
rate new sediments are added to it. Thus, the sediments are always de- 
posited in relatively shallow water. 

Fundamental laws of physics are violated and on a large scale if this 
downwarping is produced directly by continued loading of sediments. 
These deep troughs filled with sediments may contain 50,000 to 100,000 
feet of sediments and may be 1000 or more miles long and 100 miles in 
width. The mean density of the sediments, even compacted under a load 
of 10,000 feet of other sediments, is approximately 2.4 to 2.5. The rocks 
displaced in the downwarping trough are known to be denser, with a mean 
density of 2.8 to 2.9. By what mechanism do light sediments displace 
denser, crystalline rock? These troughs of sediments, like the plateaus con- 
sidered earlier, always appear to be in isostatic balance. If the conven- 
tional is to be sustained, dense rock must automatically be removed from 
below the bottoms of these sedimentary troughs at approximately the 
same rate that they receive sediments from the rivers which feed them so 
that the troughs balance and float with their upper layers of sediments 
under a few tens or hundreds of feet of water most of the time. 



354 GEORGE C. KENNEDY 

The problem of the mechanics of the formation of deep troughs of low 
density sediments is heightened when their full history is considered. 
Many are known in the geologic record. In most, sediments accumulate for 
perhaps a hundred million years and reach a total thickness of as much as 
100,000 feet. These thick, highly elongate lenses of sediments may then 
be slowly folded and uplifted to form mountain ranges which may initi- 
ally stand as much as 20,000 feet high. Surprisingly, the geologic record 
shows that a large fraction of the mountain ranges of the world have been 
formed from rocks of these thick, geosynclinal troughs. Extensive volcanic 
activity may accompany and continue beyond the time of the formation 
of the mountain ranges. The mystery, then, of the downsinking of the sedi- 
mentary troughs, in which low density sediments apparently displace 
higher density rocks, is heightened when we note that these narrow elon- 
gate zones in the Earth's crust, downwarped the most, with the greatest 
accumulation of rock debris, shed by the higher portions of the continents, 
become in turn the mountain ranges and the highest portions of the 
continents. 

The third of the major problems connected with the postulated sialic 
continental area and simatic oceanic region is that pointed out by recent 
measurements of flow of heat through the crust of the Earth. 

A considerable number of recent measurements have been made of 
temperature gradients and rock conductivities within the outer part of the 
Earth's crust. Careful temperature profiles have been made within many 
of the accessible deep mines and in numerous wells and tunnels. From 
these data, a fairly reliable picture has developed of heat flow within the 
Earth's outer crust, although measurements are not nearly so detailed or 
as numerous as is to be desired. The rate of escape of heat through most 
continental areas appears to be approximately 1.2 microcalories. per centi- 
meter per second. It has been known for many years that most of the 
heat escaping from the Earth is radiogenic heat, generated in the Earth 
by decay of radioactive isotopes of uranium, thorium, and potassium. 
Little or none of the heat escaping from the Earth is primary heat, in- 
herited from an initially hot Earth. In fact, there is no compelhng evi- 
dence that the Earth was molten in its youth or even formed from hot 
material. We know that the rocks near surface today appear to be in 
fairly reasonable thermal balance. The rate of heat escaping from them 
to the surface of the Earth is very close to the rate at which heat is gen- 
erated in them by radioactive decay of certain elements. 

Over the last twenty years, extensive data have been accumulated con- 
cerning the distribution of the radioactive elements. Uranium, thorium, 
and potassium are 10 to 100-fold as abundant in the light silica-rich rocks 
as they are in denser simatic material, rich in magnesium and iron, and 
low in silica. Consequently, we might expect that radiogenic heat in the 
thick sialic continents should be vastly greater than the heat generated 



CONTINENTS, MOUNTAIN RANGES, AND OCEAN BASINS 355 

in the presumably radioactive-poor simatic material underlying the ocean 
floors. Further, we would expect heat flow to be greatest in the thickest 
parts of the continents, that is, in mountainous regions buoyed up by thick 
roots of sial rich in radioactive elements. A number of studies of heat flow 
through the continents have been made over the last two decades. These 
studies have been made by examining the distribution of temperatures 
and rock conductivities down deep wells and along tunnels. Surprisingly, 
these studies show almost no correlation between mean elevation of land 
mass and heat flow through the Earth's crust. This was most unexpected 
because all the broader regions of higher elevation are presumably under- 
lain by thick zones of light rock which, from all determinations, should be 
richer in radioactive elements. 

Nonetheless, it was confidently expected that heat flow through the floor 
of the ocean would be a fraction of that observed in the continental land 
masses. The first measurements of heat flow through the floors of the ocean 
were reported in 1952 by Sir Edward Bullard. These determinations of 
heat flow were ingeniously made by inserting probes containing ther- 
misters into the muds on the floors of the oceans. Startlingly, the heat 
flow determined by these measurements through the floor of the ocean 
was almost identical with that measured in continental and mountainous 
areas. Later results by Revelle and Maxwell (1952 and unpublished), al- 
though indicating wide ranges of heat flow from place to place in the 
oceans, have only affirmed the earlier observation that heat flow through 
the ocean floor is essentially the same as that on the continents. 

There seem to be only two possible explanations for this most unex- 
pected discovery: either the concentration of radioactive elements in the 
rocks below the floor of the ocean is the same as that in rocks which make 
up the continents or else heat is transferred by some special mechanism 
from deeper down in the Earth to near-surface sites underneath the oceans. 
If the concentration of radioactive materials in the few tens of miles be- 
low the floor of the ocean is the same as in a few tens of miles below 
the continents, then our previous view that the floors of the oceans are 
underlined by radioactive-poor sima and the continents were underlain by 
radioactive sial certainly cannot be right. The alternative explanation, 
equally difficult, is that high temperature rocks from deeper down in the 
Earth are convectively carried up to near-surface environments below the 
oceans. Thus, heat escape through the floor of radioactive-poor oceans is 
fortuitously approximately the same as heat escape through the radio- 
active-rich continents. 

The fourth problem, that of the long lifetime of continents and moun- 
tain ranges, is perhaps the most difficult of all. The rivers of the world 
strip tremendous quantities of rock debris off the continents each year and 
deposit it in the oceans. The Mississippi, for example, contains about one- 
half weight per cent of solids as it flows into the Gulf of Mexico. Each 



356 GEORGE C. KENNEDY 

year, it brings to the Gulf of Mexico approximately 750 million tons of 
dissolved and solid material. The great rivers are steadily wearing down 
their basins. Calculations show that the Missouri River lowers its drain- 
age basin about one foot in each eight thousand years, and that the rate 
of erosion for the entire United States approximates one foot in 10,000 
years (Gilluly, Waters, Woodford, 1952). At this rate, all the land masses 
of the world would be eroded to sea level in something of the order of 
10-25 million years. This is particularly surprising in view of the fossil 
record. Land animals and plants have been known on the surface of the 
Earth for well over 300 million years, and the sedimentary record indicates 
high land masses extending back at least two billion years. Much geo- 
logical evidence indicates that the ancient continents were in approxi- 
mately the same place as the present continents and that continents have 
existed more or less as they are today and for a period of at least two bil- 
lion years. How do we reconcile an erosional lifetime for continents of 
something like 25 million years with a known lifetime of something of the 
order of two billion? Why has not all the continental sial been uniformly 
distributed through the ocean basins? 

The mountain ranges bordering the continents and interior to the con- 
tinents present an even more difficult problem. The rates of erosion along 
the slopes of steep mountains are many times those of lower lying conti- 
nental land masses. The lifetime of mountains, therefore, must be far less 
than the 25 million years estimated for continents. In contrast to this 
reasoning, however, is the geologic record which strongly suggests that the 
Appalachian Mountain Range has existed more or less where it is today 
and, as far as we know, with reasonably similar relief for the last 200 
million years, shedding sediments both to interior valleys and coastward. 
Thus, we see orders of magnitude discrepancy between lifetimes of moun- 
tain ranges and continents, estimated on the basis of known rates of ero- 
sion, and the lifetimes of the mountains and continents as indicated by the 
geologic record. Even though we assume that mountain ranges and conti- 
nents are somewhat analogous to icebergs that float up as their exposed 
portions are melted away, the presumed depth of roots of the mountain 
ranges and thicknesses of the light continental rocks permit extension of 
the estimated lifetime of continents by no more than tenfold that based 
on present erosion rates and mean elevations. 

Thus again, the notion that the rocks which make up the continents 
are grossly different in composition from those underlying the ocean basin 
does not seem to hold up, for we would expect that the rain waters wash- 
ing over the continents would have long ago dispersed the continental 
rocks into the oceans. 

These four major observations then— persistence of continents and 
mountain ranges in spite of high erosion rates, the relatively uniform values 
for heat flow in continents and ocean basins, subsidence of marginal 



CONTINENTS, MOUNTAIN RANGES, AND OCEAN BASINS 357 

troughs in response to loading by low density sediments, and uplift of 
plateaus once worn to sea level— suggest the inadequacy of the traditional 
view that continents represent masses of low density silica and alumina- 
rich rock floating in the denser media of sima, iron, and magnesium-rich 
rock. 

Recent theoretical studies by Gordon J. F. MacDonald and experimental 
work by Robertson, Birch, and MacDonald (1957) and by the writer, as 
well as interpretation by J. F. Lovering (1958), suggest a different struc- 
ture of continents, a structure which simultaneously explains most of the 
observed phenomena associated with continents, mountain ranges, and 
ocean basins and accounts for the four major stumbling blocks in existing 
theory. This new model of the Earth's crust stems from theoretical con- 
siderations largely confirmed by recent experimental work in the field of 
high pressures. 

Very many crystalline solids undergo polymorphic inversions to denser 
phases when subjected to high pressures. The behavior of matter at high 
pressures has been extensively investigated by P. W. Bridgman (1952) 
who has demonstrated literally hundreds of polymorphic inversions among 
common substances in the pressure range 0-100,000 atmospheres. Graph- 
ite and diamond form, for example, a familiar polymorphic pair. At 
suflBciently high pressures and temperatures graphite may be converted to 
diamond. A temperature of 1500 K and a pressure of 100,000 atmospheres 
is sufficient for the conversion, and, indeed, many thousands of carats of 
diamonds are now being made annually by General Electric Company by 
subjecting carbonaceous material to high temperatures and pressures 
(Bundy, Hall, Strong, and Wentorf, 1955). 

It has long been noted (see, recently, MacDonald, 1959) that basalts 
and eclogites, rocks with sharply contrasting mineralogy, have essentially 
identical chemical composition (see Table I). 





Eclogite 


Plateau Basalt 




{MacDonald, J 959) 


{Daly, 1933) 


Si02 


48.12 


48.80 


TiOa 


.85 


2.19 


AI2O3 


10.42 


13.98 


CaO 


9.99 


9.38 


MgO 


14.22 


6.70 


FeO 


13.92 


13.60 


NaaO 


1.45 


2.59 


K2O 


.58 


.69 



Eclogite, however, contains no feldspar; instead, it is made up of jadeitic 
pyroxene and garnet. The mean density of eclogite is 3.3 gm/cc, that of 
gabbro is 2.95 gm/cc. As eclogite is the denser of the two phase assem- 
blages, it is the rock which must exist at the higher pressures. 



358 GEORGE C. KENNEDY 

The density contrast, about 10%, between gabbro and eclogite is almost 
the same density contrast believed from seismic evidence to exist at the 
M discontinuity, although the contrast at the discontinuity has usually 
been assumed to be a chemical contrast rather than a phase contrast. 

Indeed, Fermor (1914), Holmes (1927), and Goldschmidt (1922) sug- 
gested that M discontinuity might be a phase contrast and that the rocks 
below it are eclogite. Their suggestion received Httle discussion or ac- 
ceptance but has been recently revised by G. J. F. MacDonald on the basis 
of calculations of the pressure-temperature conditions controlling the 
phase change of nepheline plus albite to jade and of albite to jade plus 
quartz. The calculations of MacDonald (1954) were based on new thermo- 
chemical values for heat capacity at low temperatures and heats of solution 
of nepheline, albite and jade by K. K. Kelley and his colleagues (1953). 
Similar calculations, Kelley et al. (1953) and Adams (1953), have firmly 
established the slope of the transition in a pressure-temperature plane of 
the reaction, nepheline plus albite = 2 jade, and that of the reaction, 
albite == jade plus quartz. 

These thermochemical calculations have been confirmed by experimental 
work of Robertson, Birch, and MacDonald (1957) and by the writer. 
These two experimental studies, though in disagreement in detail, confirm 
the calculations based on thermochemical data that, at pressures of 15,000 
to 25,000 atmospheres, depending on temperature, the nepheline plus al- 
bite undergoes a polymorphic change to jade, and albite undergoes an 
inversion to jade plus quartz at slightly higher pressures. Further, an ex- 
periment made by me on basalt glass showed that, at 500° and pressures 
below 10,000 bars, basalt glass crystalhzed as gabbro. The major mineral 
component is feldspar. At pressures above 10 kilobars and at a tempera- 
ture of 500°, the amount of feldspar decreases and, finally, basalt glass 
crystallizes directly to a rock made up dominantly of jadeitic pyroxene. 
Identification of phases were by X-ray. Significantly, 500° and 10 kilobars 
are approximately the temperatures and pressures estimated at the M dis- 
continuity underneath the continents. It thus appears that the M dis- 
continuity may reflect a phase change from gabbro to eclogite rather than 
a change in chemical composition. This phase change will account for 
the observed change in seismic velocity from approximately 6.5 kilometers 
per second to 8.1 kilometers per second and a change in density from 2.9 
to approximately 3.23. Thus, the older suggestions of Fermor, Holmes, 
and Goldschmidt are supported by field measurements, theoretical calcula- 
tions, and recent experimental work. 

If the discontinuity caused by a phase change takes place at a depth of 
30 kilometers, a depth equivalent to a pressure of approximately 10 kilo- 
bars under the continent, how do we account for the much greater depth 
to the discontinuity under mountain ranges and the much shallower depth 
to the discontinuity under the oceans? The answer lies in the fact that 



CONTINENTS, MOUNTAIN RANGES, AND OCEAN BASINS 359 
the change takes place at a different pressure for a different temperature 
(see Fig. 2). As near as can be told from the computations and from the 
experimental data, the slope of this phase change is approximately the 
same as the Earth's pressure-temperature gradient, as indicated in Figure 



TEMPERATURE 
200 300 400 



"C 
500 



Fig. 2, Postulated tern- ^ 
perature gradients un- t ^^ 
der mountain ranges, ° 
continental areas, and 
oceanic regions. en 



=J^ 


1 1— ■ ■ I I 1 


1 




^ 


/^^. 






^ 


\a >sS. 




- 


- 






- 


- 


b\J\. 




- 


« 


1 1 1 1 1 


\ 


^ 



2} Consequently, if it is assumed that the Earth's temperature increases 
a little more rapidly per foot of depth under mountain ranges than under 
continents generally, the transition will take place at a vastly greater 
depth (Depth C in Fig. 2). If it is assumed that the Earth's temperature 
increases with depth a little more slowly under the oceans than under the 
mountains and continents, the transition is at shallow depths (Depth A 
in Fig. 2). Thus, the single transition explains the varying depths to the 
M discontinuity under the oceans, mountain ranges, and continents. 

We assume that there are variations in temperature from continents to 
ocean basins to mountain ranges, and, consequently, we would expect 
variations in heat flow. However, the necessary variations in heat flow to 
account for these different depths of intersection are exceedingly small, 
well within the range of observations and are certainly not the threefold 
variations in heat flow that we would expect if the continents and moun- 
tain ranges were thick zones of radioactive-rich sial and the ocean was 
underlain by radioactive-poor sima. 

It is interesting to note in Figure 2 that, within the assumptions used 
in drawing this graph, the Earth's pressure-temperature gradient is almost 
the same under the oceans as is the slope of the phase change. The inter- 
section here is assumed to be at low pressures and temperatures (Point A 

^ The pressure-temperature gradients of Figure 2 are approximately the same as those 
computed on the assumption that mean heat flow is approximately 1.2 X 10"^ cal/cm/sec 
and that half the heat is radiogenic heat, generated in the upper 40 kilometers of crust. 
The remaining half is from below. 



I 



360 GEORGE C. KENNEDY 

in Fig. 2). Because the temperature is very low, reaction rate of the phase 
change might be expected to be very slow and the response of the dis- 
continuity position under the oceans might be extremely sluggish to small 
changes in temperature and pressure. Thus, we may not always have 
thermodynamic equilibrium under the oceans. 

Early in this discussion it was noted that the relief of the Earth's crust 
is a direct function of the thickness of the zone of light rock. If the thick- 
ness of the zone of light rock reflects the depth to the M discontinuity, 
which it almost certainly does, the relief of the Earth's crust can be in- 
terpreted as mirroring the various temperature gradients in the upper part 
of the mantle. 

The four major problems of the surface of the Earth, discussed earlier, 
seem satisfactorily explained by phase transition. A chemical contrast at 
the discontinuity is unnecessary. The rocks on both sides of the M discon- 
tinuity may thus be of the same composition and the depth to the dis- 
continuity may be a function of very slight temperature variations from 
place to place in the Earth's crust. 

The uplift of continents, once at sea level, to high plateaus would be a 
consequence of warming the rocks near the M discontinuity a few tens 
of degrees. When this happened, the phase change would migrate down- 
ward to much greater depths. The dense rock below the discontinuity 
would become light rock and the volume increment would float the con- 
tinents to higher levels. Thus, convection currents are no longer needed to 
transport millions of cubic miles of light material underneath the con- 
tinents in order to float them higher into the air. 

Similarly, the long lives of mountain ranges are explained. As the tops 
of mountains are eroded away, pressure at the discontinuity deep below the 
mountains decreases. Dense rock at the discontinuity would be converted 
to light rock, so light roots underneath the mountains would be recreated 
to keep them floated to high elevations. 

The downsinking of sediments in troughs is also explained by the phase 
transition. If sediments from a mountain range were rapidly removed and 
deposited in troughs, the first effect of loading would be to increase the 
pressure at the base of the trough with very little change in the tempera- 
ture. Consequently, the discontinuity would migrate toward the surface. 
The trough would sink, not only because of the added load of rock at the 
surface, but because light rock would be converted into dense rock at the 
discontinuity below the trough with a consequent decrement in volume 
of material below. Thus, the short-time eflfect of rapid sedimentation is 
one of sinking. A most interesting long-time effect appears. The added 
new sediments filling the trough are of low thermoconductivity and pos- 
sibly richer in radioactive material than the surrounding rock. Conse- 
quently, given sufficient time, the temperature would slowly rise at the 



CONTINENTS, MOUNTAIN RANGES, AND OCEAN BASINS 361 

bottom of the trough, and, although the discontinuity would first migrate 
surfaceward under response to loading, it would ultimately migrate down- 
ward under response to the rise in temperature owing to the blanket of 
poorly conducting sediments rich in radioactive elements deposited in the 
trough. Thus, troughs might sink for considerable time and then be up- 
lifted to form mountain ranges as the roots of the trough deepen with 
warming of the base. 

This implies that mountains are generated largely because of vertical 
motion and not lateral thrust. A good deal of the faulting and folding 
of rocks in mountain ranges is assumed to be the result of load. By this 
thesis, the major folds and faults associated with mountain chains are 
gravitational in origin, though concomitant lateral thrust of other origin 
is not excluded. 

The problem of the relatively uniform heat flow to the surface of the 
Earth is readily explained by the phase transition concept. The earlier 
crustal models assumed continents were made up of silica-rich and radio- 
active element-rich rocks. Thus, continents should, but do not, show heat 
flows several times that of oceanic areas. If the bulk composition of conti- 
nental rocks were not vastly different from the bulk composition of oceanic 
rocks, we would expect relatively uniform heat flow from place to place in 
the Earth's crust. This is indeed what we do find. The precision, however, 
of measuring heat flow is not sufficiently great to exclude the possibility 
that minor variations in temperature do exist from place to place in the 
Earth's crust. In fact, it is necessary to appeal to these minor variations to 
account for the existence of ocean basins, mountain ranges and continents 
on the basis of a phase change as discussed here. 

If we assume the M discontinuity to be a phase change, many questions 
are answered, but other questions are also raised. The phase change can- 
not be a simple solid-solid phase change inasmuch as the major minerals 
involved are of variable composition. Consequently, the change must take 
place over a considerable depth interval and should not be a sharp change 
taking place at a fixed depth. The data of seismology bear on this prob- 
lem. They permit the interpretation that the discontinuity may take place, 
instead of at a given depth, over an interval of as much as 10 kilometers 
under the continents (Frank Press, oral communication). This is within 
the requirements of the change. However, more difficult problems emerge 
when oceanic areas are considered. The discontinuity under the oceans is 
very shallow and apparently takes place over a very narrow depth interval. 
In fact, the pressure interval seems much too narrow for it to represent the 
gabbro-eclogite change. However, further experimental work needs to be 
done to measure precisely the required pressure interval and more refined 
seismic work will be necessary before we know exactly the distribution of 
seismic velocities below both the oceans and the continents. 



362 GEORGE C. KENNEDY 



ACKNOWLEDGMENTS 

Gordon J. F. MacDonald first brought to the writer's attention the sug- 
gestion that the Mohorovicic discontinuity was a phase change. The ex- 
perimental confirmation of the reahty of the phase change in the labora- 
tory would not have been undertaken without his stimulation. D. T. 
Griggs has contributed much to the author's understanding of the prob- 
lems involved. G. D. Robinson, V. E. McKelvey, and D. M. Hopkins 
have critically reviewed this manuscript and many thanks are due them. 

REFERENCES 

1. Adams, L. H., A note on the stability of jadeite, American Journal of Science, 2S1, 
299-308 (1953). 

2. Birch, F., Flow of heat in the Front Range, Colorado, Bulletin, Geological Society 
of America, 61, 567-630 (1950). 

3. Bridgman, P. W., The physics of high pressure, London (G. Bell and Sons, Ltd.), 
1952. 

4. Bullard, E. C, Heat flow through the floor of the eastern North Pacific Ocean, 
Nature, J70, 202-203 (1952). 

5. Bundy, F. P., Hall, H. T., Strong, H. M., and Wentorf, R. H., Man-made diamonds, 
Nature, 176, 51-55. 

6. Daly, R. A., Igneous rocks and the depth of the Earth, New York and London 
(McGraw-Hill Book Co.), 1933. 

7. Fermor, L. L., The relationship of isostasy, earthquakes, and volcanicity to the 
Earth's infra-plutonic shell, Geological Magazine, SI, 65-67 (1914). 

8. Gilluly, James, Waters, A. C, and Woodford, A. O., Principles of Geology, San 
Francisco (W. H. Freeman & Co.), 1952. 

9. Goldschmidt, V. M., Uber die Massenverteilung im Erdinneren, v^rglichen mit der 
Struktur gewisser Meteoriten, Naturwissenshaften, 10, 918-920 (1922). 

10. Holmes, A., Some problems of physical geology in the Earth's thermal history, 
Geological Magazine, 64, 263-278 (1927). 

11. Hubbert, M. K., and Willis, D. G., Mechanics of hydraulic fracturing, AIME 
Petroleum Transactions, 210, 153-168 (1957). 

12. Kelley, K. K., Todd, S. S., Orr, R. L., King, E. G., and Bonnickson, K. R., Thermo- 
dynamic properties of sodium-aluminum and potassium-aluminum silicates, U. S. 
Bureau of Mines Report of Investigations, 4955 (1953). 

13. MacDonald, G. J. F., A critical review of geologically important thermochemical 
data, Doctoral dissertation. Harvard University (1954). 

14. MacDonald, G. J. F., Chondrites and the chemical composition of the Earth, Re- 
search in Geochemistry, New York (P. H. Ableson, J. Wiley), 1959. 

15. Nettleton, L. L., Fluid Mechanics of salt domes in Gulf Coast oil fields. Bulletin, 
American Association of Petroleum Geologists, 18, 1175-1204 (1934). 

16. Revelle, R., and Maxwell, A. E., Heat flow through the floor of the eastern North 
Pacific Ocean, Nature, 170, 199-202 (1952). 

17. Robertson, E. C, Birch, F., and MacDonald, G. J. F., Experimental determination 
of jadeite stability relations to 25,000 bars, American Journal of Science, 2SS, 
115-137(1957). 

18. Yoder, H. S., Jr., The jadeite problem. Part II, American Journal of Science, 248, 
225-248, 312-334 (1950). 



Development of the Hydrosphere 
and Atmosphere 

with special reference to probable composition 
of the early atmosphere 



WILLIAM W. RUBEY 



Abstract. A satisfactory hypothesis of the development of the hydrosphere and 
atmosphere depends upon evidence from many sciences and the solution of 
many other fundamental problems of earth history. But because it is so closely 
related to many other problems, any progress toward unravelling the history of 
the hydrosphere and atmosphere limits the range of permissible speculation 
about such distantly related questions as the origin of the solar system, con- 
tinents, mountains, and living organisms. Several hypotheses of the source of 
the earth's air and waters are examined for their consistency with established 
principles and observed geologic evidence, and special attention is given to the 
probable composition of the early atmosphere. 

Hypotheses of the origin of the atmosphere and hydrosphere fall into two 
chief categories: (1) that all air and water of the earth are residual from a 
dense primitive atmosphere that once enveloped a molten globe; or (2) that 
they have accumulated at the earth's surface by leakage from the interior. 

The quantities of water, carbon dioxide, organic carbon, nitrogen, sulfur, etc., 
that have been or are now part of the earth's atmosphere and hydrosphere may 
be estimated within reasonable limits of uncertainty and these "excess" 
volatiles afford a basis for testing chemical consequences of the alternative 
hypotheses. Several writers have suggested that the primitive atmosphere may 
have been composed largely of CH4 and NH3. However, the equilibrium con- 
stants for reactions of these and other gases, combined with the evidence of 
the "excess" volatiles, indicate that CO2 and N2 are much more likely. The 
stabilities of methane and ammonia depend upon the presence of free hydrogen; 
and the escape rate of hydrogen from the earth is such that methane probably 
could have persisted in significant amounts in the early atmosphere no more 
than 10' to 10* years. For all but a relatively brief period at the very beginning 
of earth history, the atmosphere probably contained CO2 and N^ rather than 
CH4 and NH3. 

When the consequences of a dense atmosphere of CO2 and N2 (but with 

• From Crust of the Earth, ed. A. W. Poldervaart (New York: Geological Society of 
America, 1955), Special Paper 62, pp. 631-50. Publication authorized by the Director, 
U. S. Geological Survey. 

363 



364 WILLIAM W. RUBEY 

almost no free O2 or H2) are examined, it is found that several chemical effects 
(such as the quantity of rocks that would have to be weathered, of sodium dis- 
solved in sea water, and of CaCOs deposited on the sea floor very early in 
early history) are not borne out by the observed geologic record. From this 
and other lines of evidence it seems extremely improbable that the present 
atmosphere and hydrosphere are residual from any such dense primitive atmos- 
phere. Instead, it seems likely that the atmosphere and hydrosphere have ac- 
cumulated gradually during geologic time by the escape of water vapor, CO2, 
CO, N2, and other volatiles from intrusive and extrusive rocks that have risen 
more or less continuously from the deep interior of the earth. 

The amount of free oxygen in the early atmosphere is a separate problem 
that cannot be solved until the evidence of the earliest rocks has been appraised 
more fully. Current hypotheses of the origin of life appear to require a reducing 
atmosphere, yet it seems likely that oxygen has been accumulating from the 
photodissociation of water vapor ever since the earth was formed. The oxidation 
of ferrous iron and sulfides in the earliest sediments may have kept the oxygen 
content very low, and life may have begun in local reducing environments. 

INTRODUCTION 

In a symposium on the crust of the earth no apologies are necessary for 
considering the probable history of air and water, even though they are 
not hard rocks. If we could be sure that the earth was somehow born fully 
equipped with ready-made continental masses and a crustal layer, then 
the hydrosphere and atmosphere would probably have influenced the 
rocks of the crust only superficially, by way of such processes as the ero- 
sion of uplifts, the deposition of sediments, and the development of living 
organisms. But if, as many geologists and others think likely, the crust, the 
continents, and the ocean basins have evolved from changes and dynamic 
processes that have operated through the geologic past, then the hydro- 
sphere and atmosphere are almost certainly inter-related in origin and sub- 
sequent history with the deeper-seated processes of petrogenesis, epeirogeny, 
and mountain making. Just what the mechanism of this inter-relationship 
may be is to the writer a fascinating problem that touches many aspects 
of earth history (Rubey, 1951, p. 1139-1143; 1953, p. 350), but its dis- 
cussion will not be attempted in this paper. 

The origin and subsequent history of the hydrosphere and atmosphere 
are still unsolved problems and probably will long remain so. The ques- 
tions involved in these problems depend for their answers upon evidence 
from a surprising range of scientific disciplines— not only the fields of 
geology, geochemistr}^, meteorology, oceanography, and hydrology, which 
come readily to mind, but also such seemingly unrelated subjects as as- 
tronomy, seismology, nuclear physics, biochemistry, and others. A truly 
satisfactory hypothesis of the development of the hydrosphere and at- 
mosphere must wait until nearly all other broader problems of earth his- 



DEVELOPMENT OF HYDROSPHERE AND ATMOSPHERE 365 

tory and constitution have been solved. But precisely because it is so 
closely related to many other problems, any progress that can be made 
toward unravelling the histor}' of the earth's air and waters touches also 
these many other problems and thereby limits, at least to some degree, 
the range of permissible speculation regarding them. 

It would be difficult if not impossible to make specific acknowledgments 
to the many colleagues who in informal discussions have contributed at 
least indirectlv many of the ideas presented in this paper. The writer is 
especiallv indebted to his friends G. P. Kuiper and H. C. Urey of the 
University of Chicago and G. C. Kennedy and George Tunell of the Uni- 
versity of California at Los Angeles for reading the manuscript and offer- 
ing many helpful suggestions and criticism. None of the anonymous col- 
leagues or four reviewers, however, are to be held responsible for any of the 
views expressed, as one of them has kindly been at some pains to make 
clear. 

ALTERNATIVE THEORIES OF ORIGIN OF THE 
HYDROSPHERE AND ATMOSPHERE 

Several years ago the writer reviewed the available evidence that bears 
on the origin of sea water (Rubey, 1951, hereafter referred to as "Sea 
water"). This review sought to assemble critical evidence, to find or for- 
mulate alternative hypotheses that appear consistent with the facts, and 
then to examine these hypotheses for consequences that can be tested by 
the actual geologic record. The present paper outlines those portions of 
this earlier review that bear upon the probable chemical composition of 
the earth's early atmosphere and seem to merit more detailed considera- 
tion at this time. 

Source of "excess" volatiles.— When the probable quantities of rocks 
weathered and sediments deposited during geologic time are compared, it 
seems evident that most of the major rock-forming elements (Si, Al, Fe, 
Ca, Mg, Na, K, etc.) in sedimentary rocks and all the dissolved bases in 
sea water have been derived from the weathering of earlier rocks through- 
out the past. This is not, however, an adequate source for a group of other 
materials (H2O, CO2, CI, N, S, and several others), all of which are much 
too abundant in the present atmosphere and hydrosphere and in ancient 
sedimentary rocks to be accounted for solely by rock weathering (Table 1). 
For these materials (which for convenience may be called the "excess" 
volatiles) some other source is required. In seeking this other source, we 
encounter head-on the central problem of the origin of the hydrosphere 
and atmosphere. 

Only two possible sources of these "excess" materials appear to have 
been suggested. Either the waters of the present ocean and all the other 
"excess" volatiles have been inherited from a primitive ocean and atmos- 



I 



366 WILLIAM W. RUBEY 

phere, or they have risen to the surface from the earth's interior during 
the course of geologic time. 

The first possibihty is sometimes stated about as follows: If the earth 
were once molten throughout, then much of the water and many mate- 
rials were at one time volatilized in a hot primitive atmosphere. Later, as 
the earth cooled, the water vapor condensed and formed a primitive ocean. 
According to this hypothesis, the present atmosphere and ocean are simply 
residual from a hot primitive atmosphere. 

The alternative hypothesis lacks the simplicity and vividness of the first 
one. This explanation depends upon some complex and relatively un- 
familiar process or processes of "degassing" of the rocks of the earth's 
interior, and these complex processes get the hypothesis deep into prob- 
lems of physical chemistry and petrogenesis. Two distinct variants of this 
second hypothesis may be recognized: (a) that the water and other vola- 
tiles came from the earth's interior within some brief period very early in 

Table 1. — "Excess" volatile materials in present atmosphere and hydrosphere 
and in buried sedimentary rocks 

After correction for quantities released by rock weathering ("Sea water," p. 1116) 

(in units of lO^" grams) 

H2O 16,600 

Total C as CO2* 910 

CI 300 

N 42 

S 22 

H 10 

B, Br, A, F, etc 4 

* The detailed estimates total 248 units of "excess" C and 506 units of "excess" O, 
but for reasons indicated elsewhere ("Sea water," p. 1116-1117; Kuiper, 1952c, p. 419- 
420), the C is listed here as though it were all CO2. Only the C portion of this estimate 
is used in the calculations of the present paper. 

earth history; or (b) that these volatile substances have been escaping 
from the earth's interior gradually and at about the same rate throughout 
much of geologic time. 

Chemical consequences of dense primitive atmosphere. — In the review 
paper previously mentioned, an effort was made to follow through, in a 
semiquantitative way, some of the consequences of these alternative hy- 
potheses, and to find corollaries of one or the other that would permit 
critical tests by field or laboratory evidence. One of these several lines of 
consequences from alternative hypotheses was an effort to estimate the 
chemical effects that might be expected (a) if all the "excess" volatiles 
were present at one time in an early ocean and atmosphere (regardless of 
whether inherited from a primitive atmosphere or erupted suddenly from 
the interior early in earth history) or (b) if, on the other hand, the vola- 
tiles accumulated gradually to form the present ocean and atmosphere. 



DEVELOPMENT OF HYDROSPHERE AND ATMOSPHERE 367 

With a primitive ocean and atmosphere made up of all the "excess" vola- 
tiles indicated in Table 1, the different elements and compounds would 
dispose themselves between solution in the ocean water and as gases in the 
atmosphere, depending on their individual chemical properties and their 
quantities. Under the postulated conditions and with the given gross com- 
position (which would permit essentially no free oxygen or hydrogen), 
it was assumed that, at the outset before rock weathering became impor- 
tant, the predominant gases would be CO2, N2, and H2S, rather than 
CH4, NH3, and other possibilities ("Sea water," p. 1121). With an 
assumed temperature of 30°C as a basis for estimating the distribution of 
materials between ocean and atmosphere, it becomes possible to work out 
the sequence of chemical changes that would be expected. 

On this hypothesis the ocean and rain water would at first be intensely 
acid. This acid water would decompose bare rock with which it came in 
contact and in so doing would dissolve some of the bases from these rocks. 
As a result, the acidity of the water would decrease, and the chemical 
effects of dissolved gases would come into play. As decomposition of rocks 
proceeded, more and more dissolved bases, chiefly Ca, Mg, Na, K, and Fe, 
would be carried to the sea; as soon as the appropriate solubility products 
were exceeded, CaCOs, FeS2, and other compounds would be precipitated 
on the sea floor. 

Under the postulated conditions the atmosphere would have been made 
up largely of CO2 at the outset, but as carbonates continued to precipitate 
from the ocean more and more of the original CO2 would be subtracted 
from the atmosphere. Eventually the concentration of this gas would be de- 
creased to some point at which the most primitive forms of life might con- 
ceivably have existed. The highest partial pressure of CO2 that any living 
organism is known to tolerate (more than 3000 times that in today's 
atmosphere) may be selected, rather arbitrarily, as something that might 
have approximated the CO2 content of an early Precambrian atmosphere. 
From this train of argument it becomes possible to deduce, as chemical 
consequences of a dense residual atmosphere, the quantities of rocks that 
would have to be decomposed, of bases that would have to be dissolved in 
sea water, and of CaCOs that would have to be deposited in order to 
reduce the original CO2 of the atmosphere to a concentration at which 
primitive hfe might have begun ("Sea water," p. 1120-1123). 

The results of this calculation appear distinctly unfavorable to the hy- 
pothesis from which we started. The amount of rock that would have to 
be decomposed under this hypothesis before life began would be very 
large— considerably larger in fact than seems, from independent estimates, 
to have been decomposed in all of geologic time, both before and after 
the advent of living organisms. Similarly the total amount of Na dissolved 
in sea water would have been much greater, even before life began, than 
the amount which from other evidence appears to have been dissolved in 



368 WILLIAM W. RUBEY 

all earth history— that is to say, the hypothesis leaves no place for solution 
of any additional Na after early Precambrian time. Furthermore, the quan- 
tity of carbonate deposits required means that the early Precambrian 
should have been a time of abnormally great carbonate deposition through- 
out the world; more limestone and dolomite should have been deposited 
then than now remains in rocks of all ages. The known geologic record 
indicates relatively few thick limestones of early Precambrian age; the 
record suggests instead that the bulk of the earth's limestone was deposited 
much later, during Paleozoic time ("Sea water," p. 1122-1123). 

Chemical consequences of gradual accumulation.— U the same type of 
reasoning is now applied to the alternative hypothesis (i.e., that the "ex- 
cess" volatiles have accumulated gradually to form the present ocean and 
atmosphere), the results are significantly different. The same assumptions 
may be made regarding which gases would be stable, the decomposition 
of rocks, and the solution of bases by acid waters. But where for the other 
hypothesis an upper tolerance limit of COo in the atmosphere was as- 
sumed to measure the quantity of rocks decomposed, Na dissolved, and 
carbonates deposited before life became possible, for this alternative hy- 
pothesis we may assume that the COo content of the atmosphere never 
rose above this same limit. From these assumptions calculations show that 
carbonates, for example, would begin to precipitate from the sea water 
when less than one-tenth of the total water and other "excess" volatiles 
had accumulated in the atmosphere-ocean system. Thereafter, if COo 
continued to be released into the system at a relatively constant rate, rock 
decomposition would proceed, and limestones would be deposited at a 
more or less constant rate down to the present time. That is to say, with 
this alternative hypothesis we are not led to expect embarrassingly large 
quantities of rocks decomposed, Na dissolved, and carbonates deposited 
in the early Precambrian, as we were with the first hypothesis ("Sea 
water," p. 1122-1124). 

The train of chemical consequences just outlined is but one of several 
independent corollaries that mav be deduced from the opposed hypotheses 
("Sea water," p. 1117-1120, 1124-1134, 1140-1143). These will not be dis- 
cussed here, except to mention that all such corollaries that the writer 
has considered and has tried to follow through semiquantitatively seem 
to lead rather compellinglv to the same conclusion— namely, that the 
geologic evidence appears distinctly to favor the hypothesis that the atmos- 
phere and ocean have accumulated gradually from some process of "de- 
gassing" of the earth's interior and that this process must have operated 
at a more or less steady rate through geologic time. 



DEVELOPMENT OF HYDROSPHERE AND ATMOSPHERE 369 

POSSIBILITY THAT METHANE AND AMMONIA WERE 
DOMINANT GASES IN THE EARLY ATMOSPHERE 

In ttying to estimate the initial chemical character of the postulated 
earlv atmosphere and ocean, the writer had followed ("Sea water," p. 
1121) traditional theory (Kelvin, 1899, p. 86; and others later) in taking 
carbon dioxide, free nitrogen, and hydrogen sulfide as the principal gases 
likely to be stable under the assumed conditions. However, a number of 
investigators in various fields have recently considered somewhat the same 
general problem (Oparin, 1938, p. 104; Conway, 1943, p. 173; Bernal, 
1951, p. 32-33; Urey, 1952a, p. 353; Miller, 1953, p. 528) and have taken 
other gases, particularly methane or ammonia, or both, as more likely to 
have been the dominant constituents of the earth's early atmosphere. 
Poole also at one time advocated the prevalence of methane in the early 
atmosphere (1941, p. 350), but more recently he has rejected this view 
and concluded instead that a primitive atmosphere of carbon dioxide, free 
nitrogen, and water vapor seems more probable (1951, p. 203). 

The reasons that have led these writers to consider methane or ammonia, 
or both, as major constituents of the early atmosphere are probably sev- 
eral, but they may include one or more of the following considerations: 

First, we know that hydrogen and helium greatly exceed in abundance 
all other chemical elements in the spectra of the sun and stars (Brown, 
1949, p. 628; Rankama and Sahama, 1950, p. 39-40; Kuiper, 1952a, p. 329; 
Urey, 1952b, p. 252; 1952c, p. 231-233). For significant downward revision 
of previous estimates of the relative abundance of helium, see Neven 
(1954, p. 371-372) and Underbill (1954, p. 381-382). If hydrogen were 
at one time very abundant in the atmosphere of the earth, then methane 
and ammonia, rather than carbon dioxide and nitrogen, should have been 
the dominant gases. 

A second consideration is the fact that methane and ammonia are the 
most abundant gases in the atmospheres of the major planets, Jupiter, 
Saturn, Uranus, and Neptune (Kuiper, 1952a, p. 374). These planets are 
all more distant from the sun and more massive than the earth, and it does 
not necessarily follow that these gases were once present in the same 
abundance on earth. 

Third, the hypothesis of Oparin (1938) and Horowitz (1945) is widely 
attractive to scientists in many fields (Woodring, 1954). This postulates 
that before ozone became a significant constituent of the earth's atmos- 
phere, complex organic compounds were svnthesized by photochemical 
processes; that the most primitive forms of life originated in this way; and 
that these first self-duplicating molecules evolved into more specialized 
organisms as they consumed the supply of previously formed organic com- 
pounds. This hypothesis seems to require a reducing atmosphere, and 



370 WILLIAM W. RUBEY 

some of its advocates have held that it also requires the presence of either 
methane or ammonia, or both. Others (Wildt, 1944, p. 3; Rabinowitch, 
1945, p. 82-83, 124-126; Rankama and Sahama, 1950, p. 321; Bernal, 1951, 
p. 29-31, 33; Blum, 1951, p. 166-168; Wald, 1954, p. 39), however, seem 
to think that the same general principle could operate quite as well in an 
atmosphere of carbon dioxide, provided no free oxygen were present. 

Finally, Miller (1953) has succeeded in synthesizing two amino acids, 
the essential substances of proteins, by passing an electric discharge (the 
effects of which would be comparable to lightning) through a mixture of 
water vapor, methane, ammonia, and hydrogen. Insofar as known, this 
experiment has not yet been tried with other combinations of gases. 

The chemical character of the early ocean would of course have de- 
pended greatly upon what gases were dissolved in it. If the calculations 
in the writer's "Sea water" paper were based on the wrong gases in the 
atmosphere, then the specific conclusions reached regarding the probable 
chemical effects of a dense residual atmosphere and ocean would require 
modification. It should be mentioned, however, that the over-all conclu- 
sion reached there— that the earth's atmosphere and hydrosphere are prob- 
ably secondary and have accumulated gradually from "degassing" of the 
interior— is not affected by this question of which gases were dominant in 
the early atmosphere, because it rests upon several independent lines of 
evidence, such as the mineralogical and paleontological evidence that CO2 
must have been supplied to the atmosphere at a fairly constant rate 
throughout the past ("Sea water," p. 1127-1134). 

As indicated by the list of writers who have discussed various aspects of 
the subject recently, the question of the probable composition of the 
earth's early atmosphere is of considerable interest in a number of fields 
of science. The evidence afforded by the "excess" volatiles appears to bear 
directly on this question and to place some restrictions on the range of 
permissible speculation regarding it. Inasmuch as this evidence of the 
"excess" volatiles seems not to have been considered as fully as its impor- 
tance warrants, the writer has presumed to wander far outside his own 
field of science in order to present at least the high lights of it. 



EQUILIBRIUM CONCENTRATIONS OF DIFFERENT GASES 

Gross composition of dense primitive atmosphere from "excess" vola- 
tiles.— The significance of the "excess" volatiles for this problem may be 
seen most readily if one tries to estimate the probable composition of an 
early atmosphere under the two alternative hypotheses previously de- 
scribed: (a) residual from a dense primitive atmosphere, or (b) by grad- 
ual accumulation. The basic assumption is that the "excess" volatiles (that 
is, those materials present in today's atmosphere, hydrosphere, and bio- 



DEVELOPMENT OF HYDROSPHERE AND ATMOSPHERE 371 

sphere and entombed in ancient sediments that cannot be accounted for 
by rock weathering) afford the best, and in fact almost the only, direct 
evidence of the probable composition of the early atmosphere, whatever 
may have been its mode of origin. 

In order to learn which gases would be stable under various conditions, 
the writer's earlier estimates of "excess" volatiles may be taken as a start- 
ing point (Table 1). Perhaps these earher estimates could now be im- 
proved, but they are at best only estimates, and these readily available 
figures are used for the present calculations in the belief that any conclu- 
sions based upon them will still be valid qualitatively even when these 
estimates are improved by new data. 

Equations were then assembled of a number of gas reactions (Table 2) 

Table 2. — Chemical reactions considered 
(g) indicates the gas phase 



H20 (g) 


= H2 (g) + V2O2 (g) 




CH4 (g) -f 2H20 (g) 


= CO2 (g) + 4H2 (g) 




CH4 (g) + H20 (g) 


= CO (g) + 3H2 (g) 




CH4 (g) 


= C (sohd) + 2H2 (g) 




NH3 (g) 


= 1/2N2 (g) + r2H2 (g) 




2HC1 (g) 


= H2 (g) + C12 (g) 




H2S (g) 


= S (sohd) -f H2 (g) 





that seem likely to be important for this gross composition and for mod- 
erate temperatures and pressures. To keep the total number of compo- 
nents small, the possible oxidized gases of sulfur are neglected. This neglect 
probably introduces no significant errors: of the four principal gas-forming 
elements to be considered, sulfur is the least abundant among the "excess" 
volatiles; furthermore, with the gross composition given, little if any free 
oxygen is possible, and the sulfur oxides would therefore be negligible in 
amount. 

The mean temperature of the present atmosphere, considering the dis- 
tribution of its mass and temperature with height (Gregg, Samuels, and 
Stevens, 1931, p. 117, 125) is approximately — 20°C. The conditions of the 
present inquiry require an average surface temperature consistent with a 
liquid ocean but are not otherwise more narrowly specified. Experimental 
data for oxidation potentials, free energies of formation, and equilibrium 
constants are conveniently tabulated in reference books for the standard 
laboratory temperature of 25°C. If this be taken as the mean temperature 
of the postulated early atmosphere, it means, by comparison with the 
present earth, a climate tropical even at the poles. In view of numerous 
other sources of uncertainty in the calculations and estimates that follow, 
it seems scarcely worthwhile to attempt the thermal corrections necessary to 
convert the equilibrium constants to a mean temperature near that of 



372 WILLIAM W. RUBEY 

the earth's atmosphere at the present time. For these reasons 25°C has 
been used as the basis for calculation. 

The equilibrium constants for 25°C in Table 3 (calculated from data in 
Latimer, 1952, p. 30, 39, 54, 72, 91, 128) are for the seven equations hsted 
in Table 2. The several degrees of dependence of these equilibrium mol 
fractions upon pressure, here measured by the exponents of the symbol P, 
are based on the ideal gas laws (Rossini, 1950, p. 353). These pressure 
relationships would not hold strictly true, of course, for actual gases, but 
they serve as a useful basis for approximation. 

Table 3. — Equilibrium constants at 2S°C 

!"-^ ^^-1 —gov 10-41 V p-i/2 
(H2O) - ^-u X lU X f 

(C02)-(H2)4 - 1 3 V 10-20 V p-2 
(CH4)-(H20)2 - ^-^ X ^u X f 

(CO)-(H2)3 _ _,, 

(CH4)-(H20) ~ ^-^ X ^^ X ^ 



(Csol)-(H2)2 

(CH4) 

(N2)1^2.(H2)3/2 

(NH3) 

(H2)-(Cl2) 

(HC1)2 



1.3 X 10-9 X P-1 
1.2 X 10-3 X P-1 
2.0 X 10-" X P*^ 



l^^^MM^l ^ 1.6 X 10-0 X PO 

(hl2b) 



P =z sum of partial pressures of gaseous components participating in given reaction. 

With the seven equilibrium constants and the six totals of C, CI, N, S, 
O, and H from the estimates of "excess" volatiles (Table 1), we have 13 
equations and 13 components of unknown concentration participating in 
the seven reactions. Theoretically at least, this problem can be solved. In 
fact, however, the writer found it intractable. The number of unknowns 
was therefore reduced to 12 by assuming that an early atmosphere in con- 
tact with an extensive ocean would be saturated with water vapor. For the 
assumed temperature of 25°C and the total pressure of all gases (which is 
to be found later in the calculation), this gives the number of mols of 
water vapor in the atmosphere. The scarcely perceptible effects of this 
arbitrary assumption upon the numerical values of the final results are 
mentioned more explicitly below. 

In preparing the list of "excess" volatiles given in Table 1, one finds that 
some of the chemical elements are more readily tallied in such an inventory 
than others. Carbon and chlorine, for example, are readily enough recog- 
nized as such wherever they occur, but the "excess" oxygen and "excess" 
hydrogen are relatively subtle concepts, and these materials are thus more 



DEVELOPMENT OF HYDROSPHERE AND ATMOSPHERE 373 

difficult to identify with reasonable assurance. Strictly speaking, it is not 
quite logical to accept the estimates of both "excess" hydrogen and 
"excess" oxygen, for it would be simpler and possibly more accurate to 
assume that all the "excess" hydrogen was originally combined as HoO, 
thus leaving no "excess" hydrogen. But to make this assumption would 
prejudice the outcome of the present inquir}' as it would mean almost no 
possibility of any CH4 or NH3 in the early atmosphere. It was therefore 
decided to use for these calculations all the 10 X 10-^ g of estimated 
"excess" hydrogen. As the number of unknowns had, for reasons men- 
tioned above, been reduced from 13 to 12, the number of equations re- 
quired for solution of the problem was also reduced to 12; because of the 
uncertainties involved in it, the separate estimate of 506 X 10-" g of 
"excess" oxygen was not used in these calculations. 

One still needs to assemble the experimentally determined solubilities 
of each of these gases in water. When these solubility data and the equa- 
tions are all duly tabulated, only the work of solving the problem remains. 
This proved to be a formidable chore. The amount of a gas dissolved 
depends upon the quantity of water, the solubility of the gas, and its par- 
tial pressure in the atmosphere. But the partial pressure of the gas depends 
in turn upon how much of the total amount is dissolved in water and how 
the amount that still remains in the atmosphere compares with the total 
of all other gases present. And so on, almost ad infinitum. However, with 
sufficient patience, the problem can be solved numerically by a series of 
successive approximations. 

When all the paper work is done, we have an estimate of the amounts 
of the different gases under equilibrium conditions (Table 4). It is, of 
course, only an estimate. Perfect equilibrium might never be attained, even 
with geologic time. Ammonia would react with CO2 and HCl to form 
carbonates, chlorides, and other compounds; these would dissolve in water 
but scarcely affect the distribution of the gases between atmosphere and 
ocean. Furthermore, no account has been taken of photochemical processes 
that would have operated on the gases. Such photochemical reactions 
would doubtless form other and more complex compounds. But from 
what is known regarding the photochemical destruction of ammonia and 
methane (Wildt, 1937, p. 325-334; Hutchinson, 1949a, p. 30-32; 1949b, 
p. 596; Kuiper, 1952a, p. 316-317; 1952c, p. 420-421), the net effect of 
such processes would be to decrease rather than to increase the proportion 
of these two gases. Insofar as the results have significance then, they mean 
that, under the assumed conditions, free nitrogen in the atmosphere would 
at the outset tend to be about 490 times more abundant than ammonia, 
and carbon dioxide about 3800 times more abundant than methane. 

The arbitrar}/ assumption of a water-saturated atmosphere made to 
facilitate the computations scarcely affects the numerical value of the 
final results. Table 4 shows that, even if fully saturated with water vapor, 



374 WILLIAM W. RUBEY 



Table 4. — Composition of early atmosphere and ocean by residual hypothesis 
(All "excess" volatiles in atmosphere or ocean, and before rock weathering begins) 



CO2 

N2 

H2S 

HGl 

H2O (vapor) 

NH3 

H3BO3 .... 

HBr 

HF 

CH4 

CI2 

CO 

H2 



Atmosphere 
(per cent by volume] 



91 
6.4 
2.0 
0.2 
0.2 
0.01 



0.02 
0.01 



Total atmospheric pres- 
sure — 14.1 kg/cm2 



Dissolved in ocean 
(units of 1020 gm) 



240 
0.2 
12 
310 



14 
7.4 
1.0 
0.3 
0.004 
0.2 

t 

f 



Mass of ocean water — 

16.6 X 1020 kg 
pH of ocean water — 0.3 



* Less than 1 ppm. 

t Less than 1 ppm of total dissolved substances. 

the total atmosphere required by this hypothesis is so large that water 
vapor can make up only a very small part of it. For this reason the effects 
of this arbitrary assumption upon the final results are negligible. To ap- 
praise these effects, one may assume an atmosphere only one-tenth (in- 
stead of fully) saturated with water vapor and then carry through the 
same calculations as before. When this is done it is found that, of the 
volume percentages listed in the center column of Table 4, only that of 
water vapor itself is modified by this change: it becomes 0.02 instead of 
0,2 per cent; the others are changed by amounts too small to show in the 
figures listed. 

It is important to note that the composition indicated in Table 4 is that 
of the postulated early atmosphere while still uncontaminated by the effects 
of rock weathering. As mentioned above, this intensely acid water of the 
ocean (and also of rain from an atmosphere of this composition) would 
promptly begin to decompose bare rock with which it came in contact. 
The bases thus dissolved from silicate minerals would accumulate in the 
sea until they reached concentrations sufficient to cause precipitation of 
CaCOs, FeS2, and other compounds. Such precipitation would subtract 
acid radicals from the sea water, and consequently the composition of the 
atmosphere would in time be modified significantly. With an atmosphere 
of this original composition the most notable effect would be the sub- 
traction of large quantities of CO2 ("Sea water," p. 1120-1123). 

The results of these calculations thus seem to confirm what was simply 



DEVELOPMENT OF HYDROSPHERE AND ATMOSPHERE 375 

assumed in the previous paper ("Sea water," p. 1121)— namely, that, with 
a gross composition equivalent to that of all the "excess" volatiles, the 
postulated dense primitive atmosphere would at the outset have been 
made up dominantly of carbon dioxide and nitrogen. 

Gross composition from gradual accumulation of "excess" volatiles.— It 
is scarcely necessary to carry through the laborious calculations still another 
time in order to see which gases would be the dominant ones under the 
alternative hypothesis of gradual escape of the "excess" volatiles from the 
earth's interior. Under this hypothesis the total atmospheric pressure 
would never have been great at any time. Neglecting for the moment the 
effects of rock weathering, the principal differences in relative abundances 
of the several gases in the atmosphere would be those due to the smaller 
volume of ocean water and to the effects of lower partial pressures of the 
different gases. Under these conditions smaller quantities of all gases would 
be dissolved in ocean water, and relatively more would therefore remain 
in the atmosphere. For example, carbon dioxide, which is more soluble 
than methane, would be relatively more abundant in the atmosphere under 
this gradual-accumulation hypothesis than under the alternative one of 
higher partial pressures of the gases and greater volume of ocean water. 
A second and quantitatively more important difference depends upon the 
fact that the equilibria of those gas reactions that are pressure dependent 
would, under the gradual-accumulation hypothesis, be shifted toward the 
components that occupy more volume (Tables 2 and 3). If, for example, 
the total atmospheric pressure were 1 kg/cm^ instead of 14.1 kg/cm^, the 
equilibrium ratio of COo to CH4 would be (14.1)^ = 200 times greater 
than under the first hypothesis, other things being equal. Thus it seems 
clear that, under the hypothesis of gradual escape of volatiles from the 
interior, even more than under the other hypothesis of a dense primitive 
atmosphere, CO2 and N2 would be the dominant gases to be expected 
initially. 

As volatiles continued to leak upward gradually from the "degassing" 
of the earth's interior, they would become mixed with an atmosphere and 
ocean already somewhat modified in composition by the effects of earlier 
rock weathering and deposition of carbonates. With gradual addition of 
more volatiles and especially if free oxygen began to accumulate from the 
photodissociation of water vapor in the upper atmosphere (Poole, 1941, 
p. 346-347, 359) and from the photosynthesis of carbon from CO2 by 
plants, the compositions of both atmosphere and ocean water would ap- 
proach ever more closely those of today ("Sea water," p. 1122-1124). 

OTHER LINES OF EVIDENCE 

The present inquiry might be terminated at this point with the conclu- 
sion that the hypothesis of gradual "degassing" of the earth's interior 



•'/ 



376 WILLIAM W. RUBEY 

leads to chemical consequences at the surface that appear entirely con- 
sistent with the observed geologic record. But to close the discussion here 
would be to neglect other considerations and lines of evidence that deserve 
attention. It is obvious that some gross composition of the atmosphere and 
hydrosphere very different from that of the "excess" volatiles would lead 
by this method of calculation to a very different composition of the early 
atmosphere. For example, one might assume, entirely ad hoc, an original 
source of methane and ammonia and thereby reach entirely different con- 
clusions about the atmospheric composition. However, without independ- 
ent evidence to support it, such an assumption would seem unwarranted. 
A brief survey is therefore attempted to see if lines of evidence neglected 
in the above discussion may point to some gross composition very different 
from that of the "excess" volatiles. 

If, as seems probable, the atmosphere has grown by "degassing" of the 
earth's interior, one might look for evidence of some different gross com- 
position of the early atmosphere in the gases now escaping from volcanoes 
and hot springs or in the gases occluded in igneous rocks and meteorites. 
Reliable data on the composition of such volcanic and occluded gases are 
scanty. In most gases from volcanoes and hot springs and in gases occluded 
in igneous rocks that have been analyzed (Chamberlin, 1908, p. 23-24; 
Allen, 1922, p. 33-41, 70-80; Naughton and Terada, 1954; summary of 
selected analyses in "Sea water/' p. 1136-1137), CO2, CO, and N2 greatly 
predominate over CH4 and NH3, and the average composition of such 
gases agrees reasonably well with the gross composition of the "excess" 
volatiles ("Sea water," p. 1135-1137). 

This evidence is not so conclusive as it sounds, however, because it is 
uncertain to what extent such gases may have been contaminated by pre- 
vious contact with the atmosphere or with sedimentary rocks and then 
recycled through a later stage of molten rock. Because of this possibility 
of contamination and the common view that meteorites probably afford 
the best available clues about the composition of the earth's interior, the 
occluded gas content of meteorites may be particularly significant, for 
meteorites presumably have not been subjected to previous contact with 
the earth's atmosphere or to subsequent recycling. The scanty data avail- 
able indicate that the occluded gases in stony and iron meteorites are like- 
wise relatively high in COo, CO, and N2 and low in CH^ and NH3 
(Merrill, 1926, p. 123; Nash'and Baxter, 1947, p. 2541-2543). Admittedly 
the analysis of occluded gases is difficult and the results uncertain, because 
the combination of chemical elements can be modified significantly by 
the analytic extraction procedures used (Shepherd, 1938, p. 316-320). But 
even with these uncertainties in mind, no important sources of methane 
and ammonia are indicated. In the gases of stony meteorites, the tvpe 
presumably most nearly representative of the silicate rocks of the deep 
interior, the atomic volumes of oxygen exceed the combined totals of 



DEVELOPMENT OF HYDROSPHERE AND ATMOSPHERE 377 

carbon, hydrogen, and nitrogen, and oxygen exceeds hydrogen by an aver- 
age factor of about four (Chamberhn, 1908, p. 26). The available evidence 
therefore suggests that the gases now escaping from the earth's interior 
and also the occluded gases as an index of those most likely to escape, are 
of approximately the composition of the "excess" volatiles and that CH4 
and NH3 are relatively uncommon. 

If no reason seems evident for assuming an important source of CH^ and 
NH3 in the gases from the earth's interior, one may look for some process 
that might have produced these gases at the earth's surface or within the 
atmosphere. Ammonia has not been found (Migeotte and Chapman, 
1949), but small quantities of methane and h}drogen have been detected 
in the present atmosphere. Both these gases are subject to known dis- 
sipative processes. Methane undergoes decomposition by normal oxidation 
and by photochemical, biological, and other processes (Wildt, 1937, p. 
325-330; Hutchinson, 1949a, p.30-32; 1949b, p. 596; Kuiper, 1952a, 316- 
317, 343; 1952c, p. 420-421); and hydrogen is so light an element that it 
is escaping from the earth's gravitational field. It seems reasonable to as- 
sume that both gases are in a steady state and are being continually pro- 
duced by processes that operate today and thus may possibly have operated 
more actively in the past. 

The small amount of methane in the present atmosphere (Migeotte, 
1948) seems fully accounted for by strictly biological processes (Hutchin- 
son, 1949a, p. 30-32), and it therefore affords no evidence of an important 
generative process that might have operated before the advent of life. 

The hydrogen in today's atmosphere is of interest in the present inquiry 
because, if the amount were much greater sometime in the past, the chemi- 
cal equilibria are such that CH4 and NH3 would also have been relatively 
abundant. According to Paneth (1939, p. 304) the atmosphere now con- 
tains about 5 X 10"^ parts by volume of H^ or approximately 1.8 X 10^^ g 
in the total atmosphere. This amount may be compared with the yield of 
the only natural process that is thought to produce hydrogen in significant 
quantities today. In the upper limits of the atmosphere, HoO vapor is 
probably undergoing continual decomposition by ultraviolet radiation 
(Poole, 1941, p. 346-347, 359; Harteck and Jensen, 1948; Dole, 1949). 
After reviewing the pertinent data, Kuiper (1952a, p. 310-313) revised and 
greatly lowered previous estimates of the rate at which this process oper- 
ates. He concluded that approximately 7.7 X 10"^^ mol HoO/cm.^ sec are 
being decomposed, which would mean the production of 2.5 X 10^^ g 
Ho/yr. 

From Spitzer's modification (1952, p. 239-244) of Jeans' fundamental 
equation for the escape rates of lighter atoms and gases from a planet, 
one may estimate that this quantity of hydrogen would escape from the 
1.8 X 10^* g of the present atmosphere each year if the kinetic tempera- 
ture of the upper atmosphere were approximately 3500°K. However, the 



378 WILLIAM W. RUBEY 

temperatures thought to prevail in the upper atmosphere today, 500 km 
or so above the earth's surface, are around 1500°K, with occasional bursts 
of radiation producing a maximum of something like 2500°K (Spitzer, 
1952, p. 228-239, 247; Kuiper, 1952b, p. 418). Taken at face value, there- 
fore, this calculation suggests that hydrogen is now being produced more 
rapidly than it is escaping— in other words that it is accumulating toward 
some concentration at which the escape rate can eventually equal the pro- 
duction. It is much more probable, however, that the discrepancy between 
the calculated and actual temperatures of the upper atmosphere is attribut- 
able solely to uncertainties in the data used for this calculation. If the rate 
of photodissociation is less than Kuiper estimates or if the hydrogen con- 
tent of the uppermost atmosphere is greater than Paneth's estimate, the 
discrepancy disappears. 

The results of this calculation are merely suggestive of course. Spitzer 
points out (1952, p. 243) that his equation is not reliable for very rapid 
escape rates, because with such rates diffusion of the gas becomes the con- 
trolling factor. However, the available information indicates that the pres- 
ent rate of photodissociation of water vapor in the upper atmosphere is 
probably quite adequate to explain the small amount of H2 observed in 
today's atmosphere. 

If for any reason the rate of photodissociation of water vapor were much 
greater sometime in the past while the escape rate of hydrogen remained 
the same, or if the upper atmospheric temperature which controls escape 
rates were much lower while the photodissociation rate remained the same, 
then the concentration of hydrogen in the atmosphere might have been 
greater than it is today. Presumably, however, the rates of the two proc- 
esses vary together with any changes in upper atmospheric temperature, 
and, without convincing reasons why some difference in the balance 
between the two processes should be expected in the past, it is unwar- 
ranted to assume that photodissociation of water vapor has ever caused a 
significantly higher concentration of hydrogen in the atmosphere than it 
does today. 

The gases escaping from the earth's interior and those being produced 
at the surface afford no strong evidence of important sources of CH4 and 
NH3 in the past. But when we turn to consider possible sources from out 
in interplanetary or interstellar space, a gross composition ver}' different 
from that of the "excess" volatiles is clearly suggested. The question then 
becomes whether such a possible atmosphere from outer space could have 
persisted long enough after the earth was first formed to leave chemical 
evidence of itself in the earliest rocks and in the subsequent history of the 
earth. 

The hypothesis that the present atmosphere and ocean are largely resid- 
ual from a dense primitive atmosphere fits particularly well with the pos- 



DEVELOPMENT OF HYDROSPHERE AND ATMOSPHERE 379 

sibility that the earth may have brought with its sohd substances from 
outer space an initial atmosphere ver)' different in composition from that 
calculated above. This supposed initial atmosphere might have consisted 
not only of the "excess" volatiles as postulated above but also of all other 
chemical elements in the proportions observed in the solar and stellar 
atmospheres today. This would mean a very high concentration of hydro- 
gen, which in turn would greatly favor the occurrence of CH4 and NH3 
rather than CO2 and No. 

To make this assumption explicit, one might start with a primeval earth 
containing the present percentages of Si, Fe, and other heavier elements 
(Brown, 1949, p. 626; Kuiper, 1952a, p. 330) and calculate from cosmic 
abundances the quantities of lighter elements it should have contained 
originally. This would give a protoearth approximately 190 times as heavy 
as the present earth and consisting of 80 to 85 per cent by weight of 
hydrogen. After allowing for oxygen to balance Si, Mg, etc., in the rock- 
forming oxides, that remaining affords a measure of the huge volume of 
water to be expected on such a planet. To this set of assumed conditions 
the various gas-reaction constants and solubilities in sea water may then 
be applied as in the calculation described above for the "excess" volatiles. 
This yields an initial atmosphere of about two-thirds helium and one-third 
hvdrogen bv volume, and minor amounts of other gases, among which 
CH4 and NH3 would be much more abundant than CO2 and N2. 

Whether or not this exceedingly hypothetical set of conditions could 
have persisted long enough to have chemical effects still perceptible in the 
earliest rocks of the earth depends largely on the rate at which the lighter 
gases and especiallv hydrogen would escape from the planet's gravitational 
field. If the kinetic temperature at the upper limits of. this initial atmos- 
phere could be estimated, the escape rates might be calculated from 
Spitzer's equation. Assuming a temperature of 1500°K, as today, the ini- 
tially tremendous quantities of hydrogen would decrease to present atmos- 
pheric concentration in only 3 X 10^ years. However, the upper 
atmospheric temperature today depends largely on the properties of the 
two dominant gases, N2 and O2, and the temperature in an He-Ho atmos- 
phere would be different and difficult to estimate. It would probably 
(Kuiper, 1952a, p. 311; Urey, 1952a, p. 354) be as high as or even higher 
than that of the present N2-O2 atmosphere, and if higher the escape rate 
would be more rapid. 

By an argument quite independent of the temperature or even of the 
exact mechanism of escape— i.e., by comparing the energy needed to drive 
off the lighter elements with that received from the sun— Kuiper (1951, 
p. 389, 392) estimates that a period of about 10^ years would be required 
in the early stages of earth formation for this escape. This would mean 
only a relatively brief introductor\^ chapter at the beginning of Precam- 
brian history. Or if this initial atmosphere escaped before the earth con- 



380 WILLIAM W. RUBEY 

densed to its present size, the escape period would be merely the preface 
before the first chapter of the Precambrian volume. 

The details of the above calculations based on a protoearth composed 
largely of hydrogen and helium are not given here because, when one 
looks at this model critically, it is quite incredible. Modern views of the 
origin of the solar system and earth (Kuiper, 1952a, p. 322-325, 342) are 
that the great bulk of the lighter elements, and perhaps much of the 
heavier elements as well, escaped long before the earth condensed to any- 
thing approaching its present dimensions. The results of these calculations 
are mentioned here merely as something in the nature of a limiting case. 
The admittedly incredible model, being very high in hydrogen at the 
outset, calls for an extremely high escape rate at the beginning; neverthe- 
less, it is more favorable to the presence of large amounts of CH4 and 
NH3 in an initial atmosphere than most other assumptions one might 
make. With lapse of time and depletion of hydrogen, the escape rate 
would become slower; and this extreme assumption then becomes approxi- 
mately equivalent to other far more moderate assumptions that might be 
made about the original atmospheric composition. 

Among recent discussions of the composition of the early atmosphere, 
two contributions by H. C. Urey (1952a; 1952c, p. 142-157) are especially 
prominent. He concludes that methane and ammonia have probably been 
important constituents of the atmosphere for a large part of geologic time. 
Many of the arguments he presents in support of this conclusion have been 
discussed in the paragraphs above. However some of his points seem less 
than convincing to the writer, and these deserve separate consideration. 

In discussing the high temperatures to be expected as stony and iron 
meteorites accumulated to form the earth, he lists several chemical reac- 
tions that should take place under these conditions and gives the equi- 
librium constants of these reactions. He then remarks: "From these 
equilibrium constants one sees that hydrogen was a prominent constituent 
of the primitive atmosphere and hence that methane was as well" (1952a, 
p. 353). It is perfectly valid procedure to deduce quantitative corollaries 
from an assumption and then to see whether or not these corollaries are 
confirmed by observed facts; but here, unless the writer misinterprets the 
passage, Urey seems to accept the deduced corollaries as in themselves facts. 

Urey merits a salutation from geologists for the equation (1952a, p. 354), 

CaSiO^ -f CO^ = CaCO^ -f SiO^ 

which he uses to show how limestone is formed from rock silicates and 
why the concentration of CO2 in the atmosphere must have remained very 
low at all times. This equation, hterally wollastonite + carbon dioxide = 
calcite + quartz, is so condensed and dramatic a metaphor that it is 
scarcely recognizable to geologists as a shorthand statement of the familiar 
over-all process by which rocks are decomposed and limestone deposits 



DEVELOPMENT OF HYDROSPHERE AND ATMOSPHERE 381 

formed. It is nonetheless a useful thumb-nail summary of the long and 
complex series of genetic relationships usually spelled out, laboriously but 
in a still oversimplified version, somewhat as follows: (a) Carbon dioxide, 
sunlight, and water favor the growth of plants on the land and in the sea; 
(b) plants support the growth of all other organisms; (c) decaying or- 
ganic matter yields CO2 which, dissolved in ground water, forms carbonic 
acid; (d) calcic feldspars, pyroxenes, and amphiboles in crystalline rocks 
and carbonates in sedimentary rocks are decomposed by weak solutions of 
carbonic acid, with the formation of clay minerals and the release of Ca 
and other bases into aqueous solution; (e) ground waters carrying Ca and 
other dissolved materials discharge into streams and so into the ocean 
where solar evaporation causes gradual concentration; (f) CO2 in the 
atmosphere produces a small but significant concentration of COg"" ions 
in sea water; and (g) when the solubility product of CA++ ions and CO3"" 
ions exceeds a certain value, CaCOg is deposited. 

It so happens that an increase in dissolved CO^ decreases the concen- 
tration of dissolved COg"" ions rather than the reverse (Harvey, 1945, p. 
52-74; Sverdrup, Johnson, and Fleming, 1946, p. 192-210; "Sea water," 
p. 1127), so that an increase of atmospheric CO2 would cause solution 
rather than precipitation of CaCOa, thereby allowing Ca++ ions to accu- 
mulate in the sulfate-bearing sea water until gypsum as well as calcite 
would become insoluble. Consequently, at first glance, Urey's chemical 
shorthand looks exactly backwards for the deposition of limestone. But 
looked at more carefully it is seen to be entirely valid, because in the long 
run the formation of carbonate deposits in the sea depends upon the sup- 
ply of calcium from rock weathering on the lands; and rock weathering 
in turn depends, by way of decaying organic matter, upon the concentra- 
tion of carbon dioxide in the atmosphere. 

Properly interpreted, Urey's equation is a brilliant and useful generaliza- 
tion regarding the net effect of two basic and highly complex processes 
in the chemistry of the earth. Read literally, however, it certainly does not 
accurately represent the way limestone deposits are formed in nature. 
Even the broader relationship it expresses so succinctly between atmos- 
pheric CO2 and rock weathering must be viewed with circumspection, be- 
cause silicate rocks decompose very slowly in pure rain water that is 
uncontaminated by decaying organic matter. In the writer's opinion, this 
equation cannot properly be used to estimate even approximately the 
maximum concentration of CO, in the early atmosphere before life had 
appeared on earth. 

This brief survey of the evidence from volcanic and occluded gases, from 
present-day processes by which CH^ and H, are being formed, and from 
the cosmic abundances of elements has turned up no positive evidence of 
a gross composition of the atmosphere and hydrosphere that was signifi- 
cantly different any time in the past from that indicated by the "excess" 



382 WILLIAM W. RUBEY 

volatiles. Calculations indicate that, with this gross composition, CO2 and 
N2 wei;e probably the dominant gases in the earth's early atmosphere. No 
positive evidence is known, yet the observed facts do not contradict the 
possibility that the earth may have had, for a relatively brief period after 
it was first formed, a very different initial atmosphere which was soon lost 
or greatly modified by escape of the lighter elements. The very fact that 
hydrogen is escaping from the upper atmosphere today precludes us from 
saying that hydrogen may not have been, even for a considerable time, 
mucli more abundant than it is today; and having escaped, it has left no 
record of itself in the "excess" volatiles that remain. If hydrogen were 
once abundant, then so also must have been methane and ammonia. This 
view is held by some writers, but from the available evidence the writer 
considers this possibility exceedingly remote. 

PROBLEM OF FREE OXYGEN IN THE EARLY ATMOSPHERE 

Little mention has been made thus far of the presence or absence of 
free oxygen in the earliest atmospheres. This is in itself a difficult problem 
distinct from those discussed above, but a few remarks regarding it seem 
appropriate here. The gases escaping from volcanoes, fumaroles, and hot 
springs and those occluded in igneous rocks and meteorites contain no 
free oxygen, except small amounts that seem clearly the result of contami- 
nation from other sources. Likewise the estimates of "excess" volatile mate- 
rials in the present atmosphere and hydrosphere and in ancient sedimentary 
rocks afford no evidence of any original free oxygen; in fact, they indicate 
less oxygen than enough to balance the "excess" carbon as CO2 ("Sea 
water," p. 1116-1117). Thus whether the earliest atmosphere was residual 
from a dense primitive atmosphere or accumulated gradually by escape 
from the interior, no reason is known to expect free oxygen from such 
sources when the earth was first formed. 

Just how soon thereafter oxygen began to accumulate is a problem about 
which it seems advisable to withhold judgment until the evidence from 
the earliest rocks has been appraised more extensively. The presence of 
somewhat higher ratios of ferric to ferrous iron in Precambrian slates 
(Nanz, 1953, p. 57) than in average igneous rocks; of sedimentary iron 
oxides in the Precambrian iron formations (James, 1954, p. 238, 245-247, 
257) of this and other continents; and of anhydrite and gypsum inter- 
bedded with limestone in the Grenville series (Engel and Engel, 1953, 
p. 1033, 1043) suggests that at least some oxygen must have been available 
fairly early in geologic time. 

Biologists have long been interested in whether free oxygen appeared 
before or after the advent of living organisms. The oxygen in today's at- 
mosphere is being formed continuously by its release from CO2 and HgO 
during photosynthetic fixation of carbon by green plants. Green plants 



DEVELOPMENT OF HYDROSPHERE AND ATMOSPHERE 383 

require free oxygen in order to survive in the dark, but certain primitive 
types of bacteria are capable of fixing carbon in the presence of hydrogen 
sulfide, hydrogen, or other compounds (Rabinowitch, 1945, p. 101-102, 
124-125). Thus living organisms may have come into existence long be- 
fore free oxygen became available, a conclusion that is consistent with the 
apparent requirements of the Oparin-Horowitz theory of the origin of life. 

On the other hand, free oxygen is apparently being produced continu- 
ously in the upper atmosphere by the photodissociation of water vapor 
and the resultant escape of hydrogen from the earth's field. From the best 
evidence available this process seems to be operating at a rate which, con- 
tinued throughout geologic time, would yield about 5 times as much oxy- 
gen as that in today's atmosphere (Kuiper, 1952a, p. 310-314). However, 
this quantity is only about one third to one sixth the total oxygen that 
has been removed from circulation by 'the oxidation of materials now 
burie