Transcriber’s Notes:
Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
Equal signs “=” before and after a word or phrase indicate =bold=
in the original text.
Small capitals have been converted to SOLID capitals.
Illustrations have been moved so they do not break up paragraphs.
Deprecated spellings have been preserved.
Typographical and punctuation errors have been silently corrected.
HARPER’S LIBRARY _of_ LIVING THOUGHT
[Illustration]
[Illustration]
[Illustration: _Fig. 1._—Haloes in cleavage flake of Biotite. ×70.]
[Illustration: _Fig. 2._—Radium Halo (lower part of field) and Thorium
Halo (upper part of field) in Biotite cut across cleavage. ×100.]
[Illustration: _Fig. 3._—Haloes in cleavage flake of Biotite. ×85]
[Illustration: _Fig. 4._—Haloes due to Ra. emanation (inner disc), Ra.
A and Ra. B. ×450.
PLEOCHROIC HALOES]
THE AGE OF THE EARTH
BY
ARTHUR HOLMES
B.Sc., A.R.C.S.
MEMBER OF THE IMPERIAL COLLEGE
FELLOW OF THE GEOLOGICAL SOCIETY OF LONDON
FELLOW OF THE ROYAL GEOGRAPHICAL SOCIETY
ILLUSTRATED WITH TWENTY FIGURES AND DIAGRAMS
LONDON AND NEW YORK
HARPER & BROTHERS
45 ALBEMARLE STREET, W.
1913
_Published March, 1913_
TO
PROFESSOR THE HON. R. J. STRUTT
AND
PROFESSOR W. W. WATTS
IN GRATITUDE
FOR EARLY TRAINING
AND ADVICE
PREFACE
It is perhaps a little indelicate to ask of our Mother Earth her age,
but Science acknowledges no shame and from time to time has boldly
attempted to wrest from her a secret which is proverbially well
guarded. On January 30th, 1911, I placed before the Natural History
Society of the Royal College of Science a brief account of some of
these attempts, and out of that paper this little book has gradually
developed. In the present survey of the subject—the measurement of
geological time—I have endeavoured to give as full and complete
an account of the chief methods of attack as space would allow. My
particular object has been to draw attention to their respective
limitations and to test the validity of the various assumptions on
which they are based. So far, the most remarkable feature of the
problem lies in the extraordinary discrepancy between the conclusions
drawn from the two most prominent methods of dealing with it. I have
sought to mitigate the severity of this disagreement by discussing the
possibility and consequences of reconciliation and by suggesting a
path along which reconciliation may be found. If my treatment should
stimulate a greater interest in the time problem, or provide material
for further discussion, it cannot fail to bring nearer the ultimate
solution of a question which for more than a century has had an
unrivalled notoriety for provoking controversy.
I desire to acknowledge my gratitude to Prof. J. Joly and the Council
of the Royal Dublin Society for permission to use the admirable series
of micro-photographs which constitute the Frontispiece; to Prof. P.
Lowell for his photographs of the Polar Caps of Mars; and to Sir Wm.
Crookes and the Council of the Royal Society for the photograph and
radiograph of a piece of pitchblende.
To a number of friends who have helped me in revising the proof-sheets,
and to others who have turned my rough sketches into finished diagrams,
I wish to tender my thanks.
Finally, I have a special pleasure in expressing my gratitude for
the valuable assistance which has been afforded me by my friend Mr.
R. W. Lawson, of Armstrong College, Newcastle-upon-Tyne. While the
manuscript was in preparation I had the benefit of his suggestions and
friendly criticism, and at a later stage he kindly verified many of my
calculations. I am indebted to him in particular for the mathematical
treatment of the final section of Appendix A (dealing with the uranium
time-average) and for the two diagrams with which it is illustrated.
ARTHUR HOLMES.
GEOLOGICAL DEPARTMENT,
IMPERIAL COLLEGE, S.W.
_December 1st, 1912._
CONTENTS
PAGE
PREFACE ix
LIST OF ILLUSTRATIONS xii
CHAP.
I. THE TIME PROBLEM AND ITS HISTORY 1
II. TIME AND TIDE IN THE SOLAR SYSTEM 22
III. ASTRONOMICAL CONSIDERATIONS 32
IV. THE WORK OF DENUDATION 48
V. THE SALINITY AND AGE OF THE OCEANS 61
VI. SEDIMENTATION AND GEOLOGICAL TIME 76
VII. RADIOACTIVITY 91
VIII. THE THERMAL ENERGY OF THE SUN 110
IX. THE THERMAL ENERGY OF THE EARTH 122
X. RADIOACTIVE MINERALS AND THEIR AGES 137
XI. REVIEW OF THE EVIDENCE 166
APPENDIX A 177
APPENDIX B 184
INDEX 191
LIST OF ILLUSTRATIONS
_Figures printed as plates_
FIG. 1}
FIG. 2} Pleochroic Haloes _Frontispiece_
FIG. 3}
FIG. 4}
FACING PAGE
FIG. 5} Spiral Nebulæ 28
FIG. 6}
FIG. 7} Polar Caps of Mars 39
FIG. 8}
FIG. 12} Photographs of Pitchblende 92
FIG. 13}
_Figures printed in text_
PAGE
FIG. 9. Glacial Movement in S. Scandinavia 40
FIG. 10. Upper Cretaceous Strata of Colorado 45
FIG. 11. Section illustrating the Formation
of Sediments on Continental Shelf 83
FIG. 14. Apparatus for Estimating Radium 106
FIG. 15. Pleochroic Haloes of the Uranium Family 108
FIG. 16. Temperature Gradients 127
FIG. 17. Geological Time Scales 172
APPENDIX A—
Graph I 180
Graph II 181
Table of Radioactive Elements 190
THE AGE OF THE EARTH
CHAPTER I
THE TIME PROBLEM AND ITS HISTORY
Primitive races and their creation myths—The chronologies
of Eastern sages—The demands of geology opposed by
theological prejudice—Bishop Ussher’s date of the
creation of the world—The Deluge and the Doctrine of
Catastrophism—Ludicrous conceptions of fossils—The rise
and eventual success of Uniformitarianism—Leonardo da
Vinci—Steno—Generelli—Desmarest—Hutton—Scrope—Lyell—Ussher’s
chronology rejected—Geological time no longer limited—The
hour-glass of denudation and deposition—Darwin’s excessive
drafts on time—The conservation of energy—Geology
limited once again—Thomson on the age of the earth and
sun, 1862—The great controversy—Huxley, 1869—Perry and
the correspondence in Nature, 1895—Sir G. Darwin and the
birth of the moon—Clarence King’s estimate of the age
of the earth—Time implied by the stratified rocks—Joly
on the salinity and age of the oceans—The significance
of radioactivity—Emission of energy by radium and its
widespread distribution—Radioactivity provides a new
measure of time—Controversy again—Methods of dealing with
the problem—The inadequate testimony of biology.
The most primitive races of mankind, strenuously engaged in the daily
struggle for existence, appear to have given little thought to the
antiquity of the world on which they lived. Even at the present day
there exist barbaric tribes to whom it has never occurred that the
earth may have had a beginning. The conception of creation, of the
production of order from chaos, indicates a marked intellectual
advance, but into the myths and legends of which this idea was so often
the inspiration, the element of time did not usually enter.
The East African natives have traditions of the upheaval of mountains
and of the freeing of the earth from bondage, legends which may have
had a foundation in the phenomena exhibited by volcanoes and glaciers.
If the question “When?” is asked, the natural reply is merely a look of
astonishment, and persistent enquiry elicits nothing more definite than
a vague “Long ago.” Anything else, of course, could not be expected
from a people whose conceptions of the development of the universe
are limited to a recognition of the alternation of seasons and the
insistent mysteries of life and death.
The priests and philosophers who flourished during the ancient
civilizations of the East, speculated in greater detail and devoted
some attention to the elaboration of a chronology of earth history.
The Chaldeans had a well-deserved reputation for astronomy, and their
known observations go back for more than 6000 years. Cicero relates
that their venerable priesthood had records of stellar observations
stretching back for 470,000 years; a fanciful period which tallies with
the date assigned by the Chaldeans to the origin of Man. According
to the same remarkable system, the earth had already existed for 215
myriads[1] of years. The Persian sages, led by Zoroaster believed that
the total duration of the world’s existence was limited to 12,000
years. The Hebrew chronological tables are familiar to everyone, and
the restricted interpretation which was placed upon them during the
Middle Ages, when they affected European beliefs so powerfully, will be
presently referred to. Opposed to these ideas of a definite beginning
were the more abstract conceptions of Indian philosophers, who regarded
Time and the Earth as eternal.
[Footnote 1: A myriad = 10,000.]
To determine the age of the earth, or to express the actual problem
with more accuracy, to measure the duration of geological time, became
a definite scientific aspiration only during the last century. The
ultimate purpose of Geology is essentially to elucidate the history of
the earth, a record of which is imperfectly written in the stratified
and igneous rocks to which access is possible. As the characters and
sequence of the various formations gradually became better understood,
it was found that the story they disclosed was one of successive
changes of life and scene of the most impressive kind. The immensity
of time which seemed to be indicated was at first a fruitful source
of confusion and prejudice, for it brought geology into disrepute at
an early period, owing to the widely prevalent idea that the writings
of Moses fixed the antiquity of the universe beyond dispute. It is
indicated by a marginal reference in most English Bibles that the
creation of the world took place in the year 4004 B.C. This
famous estimate, which probably represents the most limited period ever
assigned to the past duration of our planet, was put forward in 1650 by
Bishop Ussher. Some such date as this had been generally believed
in during the Middle Ages as marking the epoch of transition from
chaos to an ordered world. The corresponding Byzantine date was 5509
B.C. The whole of geological history had therefore to be
squeezed into about six or seven thousand years, and this limitation
naturally demanded some extraordinary hypotheses to uphold it. As
Prof. Sollas says, “In the days when Geology was young, it found a
careful foster-mother in Theology, who watched over its early growth
with anxious solicitude, and stored its receptive mind with the most
beautiful stories which the young science never tired of transforming
into curious fancies of its own, which it usually styled ‘Theories of
the Earth.’”
At the time of the revival of learning at the close of the 15th
century, men’s ideas of the earth’s past history were largely dominated
by the exaggerated effects ascribed to the Noachian Deluge. This
devastating catastrophe was the type of a succession of destructive
cataclysms which was believed to have preceded it. Supported by Jewish
cosmogony, and in harmony with the scriptures, this view prevailed as
the Doctrine of Catastrophism until well into the 19th century. It
was believed by some writers that all sediments were deposited during
the Flood; others, impressed by the succession of different types of
deposits found far from the present sea-level, and often containing
fossils, considered that one flood was insufficient. As far back as the
9th century Rhabanus Maurus, in explaining this phenomenon, called to
his aid three great universal floods, the two later ones being
contemporaneous with Jacob and Moses respectively, but these floods
apparently were not as convincing as that of the time of Noah, for
they appear no more in geological literature. Marine sedimentation of
antediluvial times was believed by some to have been extraordinarily
active, and this was added to the effects of the Deluge to relieve the
latter of the enormous amount of work for which it was held responsible.
Fossils were regarded with suspicion and jealousy, and most of the
early naturalists resolutely set themselves against the obvious
deduction to be drawn from them. About 300 B.C., Theophrastus had
failed to see in them the evidence of past life, and according to
this philosopher a “plastic virtue latent in the earth” was supposed
in some mysterious way to have given them birth. The process was
occasionally explained as being akin to crystallisation, but other
authors, gifted with more fascinating powers of imagination though
with less philosophic insight, called to their aid the occult powers
of “lapidifying juice” and the obscure consequences of “tumultuous
movements of terrestrial exhalations.” There were even those who
thought that fossils were the work of the devil, subtly designed to
draw believers away from the faith.
Nevertheless, there were, from time to time, more rational thinkers
to whom such ideas were both repugnant and untenable. Endowed with
a keener perception than their fellows, and with a more critical
temperament, they felt compelled to regard fossils as organic remains.
Leonardo da Vinci (1452-1519) and Steno (1631-1686) were amongst the
most illustrious of these early observers. The work of Steno, published
in 1669, shows how seriously he was handicapped by the orthodox belief
that only a few thousand years had elapsed since the beginning of
the world. Any ideas which demanded longer periods were contrary to
revealed truth and were therefore bitterly opposed. Steno demonstrated
not only the true nature of fossils, but also the orderly succession
of stratified deposits. He traced the structure of the rocks to their
subsequent movements, and their surface features to the destructive
agencies of denudation. These remarkable investigations distinguished
Steno as an observer far in advance of his age. The revelation of a
tiny chapter of the earth’s past history which was thus afforded him
must surely have suggested the necessity of calling more liberally upon
time. However, whether or not he recognised how hopelessly inadequate
was the period at his disposal, he remained the victim of theological
prejudice, and cautiously avoided speculations as to the antiquity of
the earth.
Gradually, as the result of careful and patient work like that of
Steno, the foundation stones of modern geology were laid in the face
of a bitter antagonism. Generelli in Italy in 1749, and Desmarest in
France in 1777 established the importance of the slow but ever active
processes at work in the evolution of the earth’s surface features. It
was found no longer legitimate to evoke forces more intense, upheavals
more violent, or catastrophes more devastating than those of present
experience.
Closely following these courageous authors, and independently of their
influence, came in 1785 the _Theory of the Earth_ of Hutton. In
this epoch-making work the principle was defined which made dynamical
geology possible and which has proved to be of the greatest assistance
in wresting from the rocks their history. It was asserted that the
activity of the agencies which had moulded the earth’s surface in
the past and brought it to its present condition still remained
undiminished in intensity. In existing causes lay the key with which to
unlock the secrets of the past.
The doctrine of Uniformitarianism, as it came to be called, naturally
implied that the earth’s age should be restricted no longer by dogmatic
obstinacy. To Hutton time scarcely presented a difficulty. He found it
at his unlimited disposal both in past and future, and he concluded
his enquiries with the assurance that he found “no vestige of a
beginning—no prospect of an end.” He did not, however, infer that
the world had neither beginning nor end, a view for which some of his
critics held him responsible. On the contrary, he carefully pointed out
that in tracing back the course of events we are at last limited in
our retrospect, and that beyond the dim horizon of those early times
stretches an unknown past. Concerning this past, Hutton preserves an
open mind. It is not time which fails but data; and as he says “to
reason without data is nothing but delusion.”
Hutton’s convictions were regarded with righteous horror by the
official leaders of the day, most of whom combined the study of theology
with that of their favourite science, and demanded in the latter a
harmonious agreement with the scriptures. From their point of view
Catastrophism had the advantage, and they were firmly persuaded of its
truth. Fifty years had yet to elapse before the superiority of many of
Hutton’s opinions came to be generally recognised, and even then the
fallacies underlying the earlier doctrines were but grudgingly admitted.
The opening years of the 19th century were signalised by the work
of Lamarck and Cuvier in palæontology. Lamarck recognised that
fossil shells were most commonly of marine types and that in the
gently accumulating sediments of the sea-bottom they were buried and
preserved. In the succession of faunas which he studied, and in his
belief that by some law of development they were all related, he found
a cogent argument in favour of the great antiquity of the earth.
Cuvier, however, was led by his researches to extend the conception
of great world convulsions. Time after time the earth’s inhabitants
had been destroyed and entombed, only to be replaced by the creation
of fresh types after the force of the cataclysm had subsided. The
Deluge was the last of these upheavals, and a scientific proof of its
widespread effects was drawn from the superficial deposits found in so
many parts of the globe.
Until the barren ideas of Catastrophism had been abandoned, the past
provided little more than an exercise for the imagination. But the
immature days of geology were passing away. The solid work of William
Smith prepared the way for an accurate historical development of the
science. Scrope drew attention to the importance of studying geological
processes actually in operation, and arrived at conclusions essentially
the same as those of Hutton. Finally, during the years 1830-1833, the
first edition of Lyell’s _Principles_ was issued, a work which set
flowing the full tide of Uniformitarianism. Ten years previously Lyell
had felt compelled to renounce the unsound doctrines of his teachers,
and with the publication of these carefully reasoned volumes he became
the champion of the rival position. He denied the former existence
of catastrophes of an order of magnitude different from those of the
present. In their place he demanded only time. A short but convulsive
past was to be exchanged for a longer one, uniform and comparatively
tranquil throughout.
Lyell’s views did not find many ready adherents among the older
geologists. Buckland at Oxford and Sedgwick at Cambridge had long
taught a system of geology which they believed to be in accordance with
the first few chapters of Genesis. Eventually, however, they found
that under more careful examination the evidence of a universal flood
faded away, and with them and their generation the tendency to demand a
literal agreement between geological theory and the writings of Moses
gradually disappeared. A new fraternity of geologists arose whose ideas
were limited by fewer prejudices, and who found the time barriers
raised against them no longer.
Geology was now able to stand firm on its own evidence, and to insist
without fear of contradiction that long periods must have elapsed during
the slow accumulation of strata. The burden of reconciliation now fell
upon the theologians. Happily, the days of malice and persecution had
gone by, and in the light of a more broad-minded criticism it was
found that the Church had nothing to lose by the rejection of Bishop
Ussher’s chronology. The first three words of the Bible, “In the
beginning,” were interpreted afresh, and from the indefinite lapse of
time which they seemed to imply the geologist was allowed to draw at
will.
From this period until 1862, when Thomson (Lord Kelvin) attacked the
problem in an entirely new way, there was no necessity to apologise
for the most extensive drafts on the bank of time, and no further
restraint was felt in estimating the antiquity of the earth. But the
data were still inadequate, and many mistakes were made. Most of the
estimates put forward were afterwards regarded as wildly extravagant,
and naturally, they have no intrinsic value to-day. The stratified
rocks, representing the aggregate accumulation of material denuded from
the lands, afforded a valuable time-index. It was recognised that the
processes of denudation and deposition, like a gigantic hour-glass,
had been recording time since land and sea were first defined. But the
record was perplexingly difficult to read, and the time units, based on
estimates of present rates of erosion and sedimentation, were little
more than guesses, uncertain and even misleading.
The only interpretation worthy of serious consideration was that of
John Phillips, who concluded in 1860 that the time required for the
deposition of the complete succession of strata lay between 38 and 96
million years. As an example of one of the more extravagant demands,
mention may be made of Darwin’s estimate in 1859 of the time elapsed
since the latter part of the Cretaceous period. From the rate of chalk
erosion in Kent, he deduced that the excavation of the Wealden valleys
had required a period of 300 million years. Referring to this estimate,
Jukes gave his opinion that while it might be a hundred times too
great, it was equally probable that it was a hundred times too small.
Evidently 30,000 million years, in the absence of more reliable data,
was not considered an absurdly excessive period for the erosion of the
Weald. It should be noticed, however, that after the publication of
Thomson’s views, Darwin withdrew this estimate from the _Origin of
Species_.
We must now return to consider the problem as it appeared to the
leading physicists of half a century ago. In 1842 Mayer, and during the
following year Joule, had placed the conception of the conservation
of energy on a firm experimental basis, and started the principles of
thermo-dynamics on their brilliant career through the sciences. Twenty
years later, Thomson invaded the domain of Geology, hoping to reform
its speculations and bring them into accordance with the doctrines
of the conservation and degradation of energy, doctrines which were
apparently set in defiance by the orthodox tenets of the science.
As early as 1852 he had shown that under the laws to which matter and
energy are subject, the earth within a finite period of past time must
have been unfit as a habitation for life. This general conclusion he
afterwards worked out in detail, determined to protest against what he
considered the immoderate application of the principle of Uniformity.
So profoundly did he influence geological opinion, that for nearly
fifty years the question of the earth’s antiquity centred around his
name. A short account of his contributions to the physical aspect of
the subject, and of the famous controversy which they aroused, cannot
therefore be dispensed with.
In 1855 Thomson indicated the way in which observations of underground
temperatures might be applied to the determination of absolute dates
in geological history. Already the origin and maintenance of the sun’s
heat had been discussed by Mayer and Helmholtz, and during the years
1860 and 1861 Thomson dealt with their respective suggestions. During
the following year, 1862, came the epoch-making papers _On the Age of
the Sun’s Heat_, and _On the Secular Cooling of the Earth_.
The age of the sun’s heat evidently restricts geological time in a
very decided way, and Thomson drew attention to the necessity of the
fact that unless the sun were a perpetual miracle, it could not have
continued to radiate heat-energy for an unlimited period. His guarded
conclusion was that the sun most probably has not illuminated the earth
for 100 million years, and almost certainly not for 500 million years.
Regarding the earth as a globe which had gradually cooled down, he
tried to show that the principles of thermo-dynamics had been
disregarded by geologists. From the earth’s present store of heat, as
revealed by underground temperature gradients, he calculated that the
consolidation of the crust took place about 100 million years ago.
Owing to the uncertainty of much of the data on which this estimate was
based, he allowed wide limits. Had a solid crust formed permanently
less than 20 million years ago, underground heat should be greater
than is actually observed. Had it formed at a date more remote than
400 million years ago, then the temperature gradient near the surface
should have been notably less than it is.
In 1865 appeared a remarkable little paper in which attention was
drawn to the earth’s thermal history, and particularly to its more
energetic youth, the object again being to refute the doctrine of
Uniformity. During the next year Thomson delivered the Rede Lecture
on _The Dissipation of Energy_, and showed the importance of the
tides in terrestrial dynamics. All over the ocean the effect of the
friction set up by moving water is either to accelerate or to retard
the earth’s rotation. The final result was, he asserted, to retard the
earth as though a gigantic friction brake were being slowly applied.
The ultimate tendency was towards a state when relative motion between
earth and moon should be no more.
In 1868, in an address on _Geological Time_, Thomson gathered
together his three limiting criteria, and further developed the
application of tidal friction to the question. In his discussion
the earth’s figure was supposed to be a survival from the period of
consolidation; a permanent record of terrestrial conditions at that
time. This being so, the rate of rotation could be computed under which
that form would be in fluid equilibrium.
The three maximum time limits which Thomson drew from his arguments
were respectively 500, 400, and 1000 million years, with a lower limit
of 20 million years. The final conclusion was “that the existing state
of things on the earth ... must be limited within some such period of
past time as one hundred million years.”
The reply to this series of attacks on what was supposed to be
orthodox geological opinion, came from Huxley in 1869. He did not deny
the consequences of the dissipation of energy in the cases to which
attention had been directed, but decided in favour of the geological
evidence, which indicates, as time has more and more substantiated,
that “they have made no practical difference to the earth during the
period of which a record is preserved in the stratified rocks.”
Thomson returned to the attack a month or two later, protesting once
more against this attitude. At the close of his address, he said: “A
large proportion of British popular geologists of the present day have
been longer contented than other scientific men to look upon the sun
as Fontenelle’s roses looked upon their gardener. ‘Our gardener,’ say
they, ‘must be a very old man; within the memory of roses he is the
same as he has always been; it is impossible he can ever die, or be
other than he is.’”
There were no further contributions to the problem until 1876, when
Thomson revised the former conclusion which he had based on the
distribution of underground heat, and narrowed his limits to 50 and
90 million years. In later years he reduced both these limits still
further, though he was always more prudent than Tait, who with little
justification cut down the time allowance in a most alarming way. Tait
wrote in 1875, “Ten million years is about the utmost that can be
allowed from the physical point of view for all the changes that have
taken place on the earth’s surface since vegetable life of the lowest
known form was capable of existing there.” Assertions such as this
were among the most embarrassing circumstances that geologists had to
face. The late Sir George Darwin was more kindly disposed, although his
contributions to the question were considered to support the physical
arguments. Believing that the moon had been derived from the earth by
the separation of an enormous tidal wave, he calculated that since this
stupendous event at least 56 million years must have passed.
The physical evidence appeared at first to be irrefutable, and the
estimates based upon it equally certain. Yet earth history could
not comfortably be squeezed into less than 100 million years. The
stratified rocks were there in undoubted succession; mile after mile
of thickness with no indication of more rapid accumulation than that
of modern deposits. In spite of this, however, a prejudice in favour
of short estimates was gradually aroused, and some attempt was made
to hurry up geological activities in the past in renunciation of the
principles of Uniformitarianism. But many geologists refused to give
way and vigorously attacked the physical stronghold, searching out and
exposing all the assumptions, and noting with satisfaction the
uncertainty of much of the data and its doubtful applicability. A few
physicists themselves denied that the limited estimates were securely
founded.
In 1895 the controversy was re-opened by the publication in
_Nature_ of a correspondence initiated by Prof. Perry. While Perry
had previously accepted Kelvin’s conclusions, he now challenged the
validity of all three. The strongest criticism was directed against
Kelvin’s assumption of a homogeneous earth. It was shown that if the
rocks of the interior, under high temperatures and pressures, were to
conduct heat ten times as well as those near the surface, then Kelvin’s
figures would need to be increased fifty-six times. To settle this
point adequate data were not then forthcoming, but Perry stated his
belief that, if geologists had sound reasons for demanding long periods
of time, he saw nothing which denied them four times the greatest (1000
million years) of Kelvin’s estimates.
Kelvin wrote that he would rather know the date of the _consistentior
status_ than that of the Norman Conquest, so interesting did he
find the subject. After remarking that 100 million years were ample to
satisfy Geikie, he said, “I should be exceedingly frightened to meet
him now with only 20 million in my mouth.” However, Kelvin carefully
examined the data referring to the conductivities of rocks and decided
that they were not favourable to Perry’s suggestion. Thus fortified, he
lent his support to the independent estimate of 24 million years which
had been arrived at in 1893 by Clarence King.
Kelvin’s last pronouncement of his views was in 1897, when he delivered
an address on _The Age of the Earth as an Abode Fitted for Life_.
He then narrowed down his earlier estimates to 20 and 40 million years.
To most geologists these limits were seriously in conflict with the
requirements of their science. A feeble attempt was made to force an
agreement, but it was generally held that the sedimentary succession
implied a period three times as long. An independent mode of reckoning
geological time was developed by Prof. Joly in 1899. He made a careful
study of the amount of sodium annually removed from the land by solvent
denudation, and calculated the time during which the total quantity
of sodium now held by the oceans could have accumulated. The period
closely approached 100 million years, and thus further support was
added to the testimony of the rocks, for with the birth of the ocean
their deposition must have commenced.
Among many of the more optimistic geologists there was a prevalent
expectation that some flaw would ultimately be found in the physical
arguments. Their anticipation was realised ten years ago in the most
surprising and unexpected way. In 1903 came the discovery by Curie
and Laborde that radium maintains a temperature above that of its
environment owing to the spontaneous evolution of heat involved in
its disintegration. Other investigators found that radium and its
radioactive associates were widely distributed in the earth’s surface
materials. In all waters and gases of natural origin, and all rocks and
soils, traces of these elements have been detected. We owe a great deal
to the careful researches of Prof. Strutt, which have shown
conclusively that the earth can no longer be regarded merely as a
cooling body. A newly recognised source of heat must now be taken into
account, and indeed, so relatively abundant is the supply, that our
present difficulty is to understand why the earth is not hotter than we
actually find it.
With these discoveries the long controversy was finally buried, and
Kelvin’s treatment of the problem was proved to have been fallacious.
Kelvin lived just long enough to know something of the first attempts
to utilise the principles of radioactivity in solving the vexed problem
of time. The discovery of radium did not only destroy the validity
of the older thermal arguments; but also, it led directly to the
elaboration of a new and more refined method. As we shall see in the
sequel, every radioactive mineral can be regarded as a chronometer
registering its own age with exquisite accuracy. The record is not
always completely preserved, but a few attempts have been made to read
it, and in the more favourable cases, periods of enormous duration have
been revealed. Indeed, if our interpretation is correct, some of the
oldest Archean rocks must date back 1600 million years.
Not many years ago geologists were dissatisfied with the shortness of
their time allowance; to-day they are confronted with an embarrassing
superabundance. Certainly, it has been dogmatically denied that
radioactive minerals can have the great ages which have been attributed
to them, or that they can account for more than a small proportion of
the earth’s loss of heat. But such denials do not help to remove the
difficulty; they rather tend to aggravate it. In the face of two rival
and mutually inconsistent systems of earth chronology, neither of
which can be rejected with impunity, it becomes essential to examine
most carefully the fundamental assumptions underlying each method, in
the hope of detecting the subtle errors which are responsible for so
glaring a disagreement. In this way alone can a means of reform be
indicated and the road made clear for the construction of a time-scale
which will be acceptable to all.
It is obvious that as yet we cannot measure the earth’s absolute age if
by that expression is meant the time which has elapsed since our planet
first existed. Kelvin’s work most nearly approached this desideratum,
but, as we have seen, it was doomed to ultimate failure, though not
indeed, to discredit. Sir George Darwin’s calculation of the lapse of
time since the birth of the moon stands apart; his results are limited
only by a definite minimum, and otherwise are sufficiently elastic to
meet any reasonable demand. But this convenience, added to the doubtful
validity of the hypothesis on which his estimates were based, detracts
from their value. It would be unfair to expect too much of them, for
they were only put forward in support of a hypothesis which would have
been immediately disproved if ridiculously long or short time periods
had been involved.
As we shall see in the following chapter we are still far from
understanding the sequence of events which led up to the origin of the
earth. All we can hope to do is to fix the dates of critical periods
of its history and assign its origin to a point still more remote. The
different methods which have been elaborated to deal with the problem
are all based on a common principle. The rates of certain changes
at the present day are determined as accurately as possible, and in
imagination, the respective processes are traced backward in time,
until limiting conditions are arrived at. Thus, Kelvin takes us back
to a time when the earth was not yet a solid globe; Darwin traces back
the moon’s history until he finds it revolving close to the earth; Joly
bids us imagine the oceans in their original freshness, free, or nearly
so, from salt; Geikie finds an end at last to the long succession of
stratified rocks and seeks to estimate the time they represent. Last
of all, and most brimful of promise, there lies in the mechanism of
radioactivity an elegant method for assigning a date to the period of
crystallisation of every igneous rock in which suitable minerals can be
found.
The testimony of biology scarcely calls for consideration here, for
as Huxley pointed out, biology must take its time-scale from geology.
The procession of life forms shows broadly that the time involved must
have been very great; and, moreover, where we first meet it in the
Cambrian rocks, it is already far on its journey. This is usually taken
to signify that pre-Cambrian time must be at least as long as that
which has elapsed since. Such a statement can only be regarded as very
conservative.
Prof. Poulton considers that the process of evolution must have
required much longer periods of time than those estimated by the
geologists. Sollas, on the contrary, is satisfied that 26 millions of
years would be ample to meet all the demands of biology. It is obvious
that a numerical estimate cannot be derived from the succession of
organic forms, for the rate of change of species is as yet known only
relatively. The influence of changing environment is complex beyond the
possibility of exact analysis, and any method of chronology based on
the development and ultimate extinction of past types would necessarily
give results of very unequal value. The little shell _Lingula_ has
retained its individuality with but little deviation from its original
form throughout the time represented by the fossiliferous strata, but
the same record is eloquent of the gradual unfolding of fauna after
fauna, culminating at last in the highly specialised organisms of
to-day.