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Title: The age of the Earth
Author: Arthur Holmes
Release date: March 19, 2026 [eBook #78241]
Language: English
Original publication: London: Harper & Brothers, 1913
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Transcriber’s Notes:
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in the original text.
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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.
CHAPTER II
TIME AND TIDE IN THE SOLAR SYSTEM
Early ideas of cosmogony—The Laplacian hypothesis—Tidal
friction in the earth-moon system—Its application by Kelvin
to the question of the earth’s antiquity—Its application
by Sir G. Darwin to the history of the moon and the date
of its birth—Difficulties in the way of the Laplacian
hypothesis—The Planetesimal hypothesis—The origin of the
ancestral solar nebula and its transformation into the solar
system—Early stages of the earth’s history.
Speculative fancies concerning the origin of the world form the subject
matter of many of the earliest writings on record, and throughout the
intellectual history of mankind the problem has proved to be one of
supreme fascination. It was not, however, until quite recent times
that the efforts of imagination gave place to reasoned hypotheses,
tempered by a more sober regard for physical probabilities. At first,
on having attained the status of a science, geology steadfastly
refused to consider seriously the cosmogonic fantasies then current.
It was Hutton, who by advocating the direct observation of nature in
place of the old scholastic arguments, first delivered geology from
the inevitable wranglings that would necessarily have arisen from so
premature a discussion of the beginning of things. Cosmogony, in spite
of this, continued to receive attention from workers in other sciences,
and while to-day we are still unable from geological facts alone to
trace back with confidence the details of the earth’s beginning, yet
the uncertainty which justified Hutton in entirely disregarding the
earth’s genesis no longer exists. Astronomy, physics, and chemistry
have all contributed to the elucidation of what may be called the
prehistoric period, and have done much to remove our modern ideas from
the dangerous quicksands of speculation.
It is becoming more and more evident that many of the fundamental
problems of geology can be solved only with reference to the processes
involved in the making of the earth and in its subsequent evolution.
The dynamic agencies at work to-day are genetically the outcome of the
ancestral forces which first moulded our planet, and theoretically, the
tectonics and constitution of the earth’s crust should lead back to
a more complete understanding of its initial condition. But the mind
of man is impatient of delay; shrewd guesses are made and gradually
adjusted to known facts, with the result that many geological doctrines
are founded not on observation alone, but also in part on fundamental
hypotheses.
During the 19th century scientific thought was powerfully influenced
by the achievement of Laplace, for it was he who first presented the
famous Nebular hypothesis in a consistent and acceptable form. Previous
thinkers, notably Buffon in 1745, and Kant in 1755, had put forward
similar generalisations, but they fell into certain errors which
Laplace escaped, and their expositions lacked the completeness and
authority which his mathematical genius guaranteed. In the _Système
du Monde_, published in 1796, Laplace attempted to trace the gradual
evolution of the solar system from a spheroidal nebula, under the
normal operation of natural laws. He started with a hot gaseous nebula
of lens-like shape extending beyond the present orbit of the outermost
planet and rotating slowly in the same direction as that of the sun.
The average density of such a nebula would be about ¹/₂₅₀,₀₀₀,₀₀₀
that of ordinary air. As Laplace explicitly states that the sun was
already foreshadowed in the nebula as a strongly condensed central
nucleus, the outer atmosphere must have been of inconceivable tenuity,
a condition maintained only by intense heat. Through the loss of this
heat by radiation, and under the influence of its own gravitation, the
nebula would slowly contract. In accordance with the laws of dynamics,
contraction would necessarily be accompanied by a compensating increase
in the rate of rotation. At a certain stage, the centrifugal force at
the Equator would balance the attraction of gravity, and a ring of
gas would be left behind. The same process of ring separation would
afterwards be repeated from time to time as contraction progressed,
and each ring being inherently unstable, would rupture, ultimately
forming a spheroidal mass with the requisite directions of rotation and
revolution. The Asteroids, a group of tiny planets revolving in the
zone between Mars and Jupiter, probably represent a ring which failed
to coalesce into a single planet. In their newly-born condition the
planets were still hot and gaseous, and by the operation of the same
mechanical agencies on a smaller scale most of them detached a second
generation of rings, and these having aggregated in turn became the
satellites. The great central nucleus, continuing to contract after the
last planetary ring had been abandoned, became the sun.
On this hypothesis the earth was originally a hot fluid globe with
a heavy atmosphere consisting of the vapours of its more volatile
constituents. As it contracted it rotated faster and faster, until a
limiting velocity was attained which allowed the separation of the
ring which ultimately condensed to form the moon. At a later stage the
earth began to solidify and the crust first made its appearance. The
lighter igneous rocks, the granites and corresponding lavas, presumably
formed the outer layers, while beneath this zone the more basic magmas
arranged themselves in the order of their density. When cooling had
sufficiently progressed, the waters condensed to form the oceans, and
with the emergence of the lands, denudation began for the first time
and the earth’s historical period was inaugurated.
At the time of the birth of the moon the rotatory velocity of the
earth must have been very rapid compared with that of the present day,
and an obvious deduction is that the earth has gradually slowed down
in the course of its history—in spite of the accelerative effect of
contraction—and is probably still being retarded. From a comparative
study of ancient and modern eclipses, Dunthorne demonstrated in 1749
that the earth appeared to be losing time. More than a century later,
Adams showed from a revision of all the data, that the loss amounted to
22 seconds in a century, although he was careful to point out that the
assumptions on which his calculation was based were themselves not
securely founded. It was Kant who started the idea that the ceaseless
operation of tidal friction would tend to bring about this result. Long
afterwards Kelvin indicated the way in which tidal retardation might be
applied to estimate the date of the earth’s solidification. He supposed
that the form then assumed by the earth had survived in its essential
features throughout geological history. Subsequent diminution of the
rate of rotation was not considered to be attended by a corresponding
change of oblateness. Granting this assumption, the present figure
of the earth ought to inform us of the rate of rotation under which
it would be in fluid equilibrium at the time of consolidation. After
making these concessions to speculation, Kelvin safeguarded himself by
assigning wide limits to the earth’s age. His maximum estimate was a
liberal 1000 million years, but he further stated his opinion that if a
much higher antiquity than 100 million years were demanded, the polar
flattening and equatorial bulging should be more marked than we find
them.
These conclusions were necessarily somewhat vague, and based as they
were on a very questionable assumption, they were readily susceptible
to damaging criticism. It is known from the concordant evidence
afforded by seismic, geo-physical, and astronomical phenomena that
the earth is essentially solid throughout with a rigidity more than
twice that of steel. So far our conception of the earth is similar to
Kelvin’s, but the postulate that such a body could resist rotational
changes without modification in shape, cannot now be granted. As a
whole the earth tends to conform to the laws of fluids, though its
response may be tardy and the alteration of form may lag considerably
behind the ultimate cause. This being so, it would be more correct to
refer Kelvin’s time limits to the period of “lag” rather than to the
period of the earth’s history as a solid globe.
Sir G. Darwin’s departure from the ring conception of Laplace in the
case of the moon, and his alternative theory of the moon’s origin and
history are well known. Tracing back to their logical conclusions the
effects of tidal friction in the earth-moon system, he has developed
one of the most fascinating romances in the domain of cosmogony.
Unfortunately it is impossible to apply any definite dates to the
critical stages of this history, if for no other reason than the
imperfection of our present lunar and tidal theories. His lower
estimate of the time which has elapsed since the disruptive catastrophe
lies between 50 and 60 million years, but the actual time would
certainly be much greater. Indeed, Darwin himself, referring to the
problem of cosmical time after the advent of radium and the overthrow
of the older standards, wrote, “I feel with some degree of confidence
that if the tidal theory shall ultimately be condemned it will not meet
its execution on the score of lack of time.” A recent discussion of
the problem by Prof. Chamberlin and his colleagues is unfavourable to
Darwin’s view. After a careful study of the rate of tidal retardation
they show that the lengthening of the day is of the order of one second
in 500,000 years, and that if the physical condition of the earth has
remained essentially as at present, at least 220,000 million years must
have elapsed since the day occupied 20 of our present hours and the
month 29 such days. Yet just after the birth of the moon, as outlined
by Darwin, day and month were alike in duration, each being equal to
about three of our present hours. It must be confessed that, after all,
we know but little of the moon’s history with certainty.
At the time of its announcement to the world the Laplacian theory was
remarkably in accordance with the knowledge of the time. But as further
discoveries were made, numerous inconsistencies and contradictions
arose. Not only were certain of the movements of the planets and
satellites found to be in a retrograde direction, but, besides these
awkward facts, at every stage of the postulated development of the
solar system insuperable mechanical difficulties appear which render
the truth of the hypothesis highly improbable. Finally, in the dynamics
of the system there are so many discrepancies between the requirements
of theory and the actual circumstances that Prof. Moulton and Dr. See
both insist that it could not have originated in the way that Laplace
imagined. The heavens have been diligently searched for nebulæ of
the Laplacian type, but no certain case has been observed. The vast
majority conform to the spiral type and to a nebula of this kind appeal
has been made in the recently developed Planetesimal hypothesis of
Chamberlin and Moulton.
[Illustration: FIG. 5.
Symmetrical Nebula in Piscium, M 74.]
[Illustration: FIG. 6.
Whirlpool Nebula in Canes Venatici, M 51.
SPIRAL NEBULÆ.]
The general structure of a spiral nebula is immediately suggestive
of tidal action. From a central nucleus two spiral arms emerge at
diametrically opposite points. Often they are beautifully defined
(Figs. 5, 6), but seen from the side they appear as discs of misty
light. In the arms nebulous knots and irregularities are generally
apparent, the precursors, perhaps, of a system of bodies analogous in
their distribution to the planets. To account for the existence of a
spiral form, conditions of extreme tidal distortion are suggested. In
the case of our ancestral sun such conditions would be brought about
by the close approach of another celestial body. As the stranger
drew near, the tidal stresses set up would gradually increase until
explosive outbursts of matter from the sun were projected in the
plane of attraction, one on the near side, another on the far side.
The result of the combined attractions of the two suns on the ejected
material would be to develop a spiral structure. An enormous number of
tiny planets or planetesimals would thus begin to circulate about the
sun, associated with scattered knots of larger dimensions which would
form the nuclei of the future planets and satellites. These knots would
at once begin to grow by coalescence at the expense of the vast numbers
of planetesimals associated with them in adjacent zones. Dynamically
the scheme is sound, and the many suggestive consequences which unfold
themselves explain away most of the difficulties which proved so
embarrassing to the Laplacian hypothesis. Into a discussion of these
advantages space does not allow us to enter, but it may be said that of
all the attempts to grapple with the fundamental problem of the genesis
of the solar system, the Planetesimal hypothesis appears to be the
most successful. It is to be welcomed, apart from its many convincing
features, if only because of its stimulation to the further study of
the early stages of earth history.
The earth began on this hypothesis as a nebular knot, and it has since
grown up to its present mass by the capture of outside planetesimals.
It is very unlikely that it was ever in a molten condition. Internal
heat arose in part from the condensation of the mass during the
period of its growth. The temperature would slowly rise until the
fusion point of certain of the constituents was reached and the
liquid tongues and pockets thus formed would then tend to move away
from the centre—the lighter and less viscous stony material being
squeezed upwards relatively to a network of the heavier and more rigid
metallic material. Once vulcanism had been initiated in this way the
process would continue until a highly metallic nucleus had collected.
Surrounding it there would gradually form a thick zone of silicate
rocks, the differentiation from the original heterogeneous mixture of
stony and metallic constituents being due to the selective fusion of
the former. In dealing with the question of the earth’s heat in a later
chapter, the value, and indeed the necessity of this conception will be
realised.
The atmosphere and oceans must have been derived from the planetesimals
themselves, and on analogy with rocks and meteorites, the planetesimals
would not be lacking in the raw material from which to evolve them.
Even now, the outer 70 miles of the earth’s crust would be competent to
supply all the nitrogen of the atmosphere, the water of the oceans
and the vast quantity of carbon-dioxide represented by limestones and
carbonaceous deposits. With the existence of an ocean and atmosphere,
a new factor in surface differentiation arose. Mechanical and chemical
denudation became possible, and the first sediments were deposited.
Although the earth’s growth had not yet ceased, all the agents occupied
in its subsequent development were now at work and its geological
history may be said to have definitely commenced.
CHAPTER III
ASTRONOMICAL CONSIDERATIONS
The Great Ice Age—Extension of glaciation—Croll’s
ingenious hypothesis—Inadequacy of the explanation—The
ice caps of Mars—De Geer’s discovery of the annual layers
of glacial clay in Sweden—Application to the measurement
of time—Sederholm’s observations in Finland—Cyclic
sedimentation in the Cretaceous strata of Colorado—Its
correlation by Gilbert with an astronomical time unit.
In the last chapter mention was made of Kelvin’s work on tidal
friction and of Sir G. Darwin’s speculations as to the early history
of the earth-moon system. Their conclusions depended very largely on
a cosmogony which has failed to meet with general acceptance. A very
different appeal to astronomical causes was made by Croll in his famous
attempt to account for the anomalous conditions of the glacial period.
Here we may also consider the correlation by Baron De Geer and Mr. G.
K. Gilbert of certain unusual phases of sedimentation with the seasonal
and climatic changes brought about respectively by the earth’s motion
and its periodic fluctuations. All of these investigations find a
common basis in their direct reference to astronomical considerations.
In the course of its history the earth has undoubtedly passed through
several periods of extreme cold, periods when Arctic conditions swept
down from their polar strongholds and invaded the temperate zones and
even the tropics. Ice and snow have left their traces in many a grooved
and striated surface, and in gently rounded outlines the landscape
often betrays their former presence. Erratics and perched blocks,
terminal and lateral moraines, lakes and alluvial terraces, U-shaped
and hanging valleys all bear witness to the wide extension of the
vanished glaciers of the past. Relics of glaciation have been preserved
in the Cambrian or pre-Cambrian rocks of Norway, China, Australia
and South Africa. Still more remarkable are the records of a Permian
ice age found in the rocks of South America, South Africa, India and
Australia. No unequivocal evidence of glaciation in later periods is
forthcoming until the advent of the Pleistocene. The Great Ice Age
through which the earth has so recently passed was not, however, a
single epoch of glacial extension. Four episodes in the climatic cycle
can be recognised—a genial episode, a period of falling temperature
and glacial advance, a glacial episode, and finally, a period of rising
temperature and glacial retreat. That this cycle has been four times
repeated is the testimony of the Eastern Alps as interpreted by Prof.
Penck. In the British area the ebb and flow of temperature has not left
so clear a record, and a more continuous and persistent glaciation
appears to be indicated.
At the time of the maximum advance of the ice, all northern Europe lay
buried beneath an immense ice sheet, which was fed by enormous glaciers
slowly creeping down from the uplands. Curiously enough, Siberia, which
in parts experiences the coldest winter in the world, is not now and
was not then covered by the ice-field. Farther south the ice collected
in the Pyrenees, Alps, Caucasus and in the great Asiatic ranges
and descended in all directions far below the level of the present
snow-line. In Canada and the United States the ice mantle proceeded
from three great centres—from Labrador along the Atlantic coast,
from the Keewatin district by Hudson’s Bay and from the Cordilleras
along the Pacific coast. In the tropics and in the southern hemisphere
the story is the same. Down the slopes of Kenia, Kilima N’jaro and
Ruwenzori, the towering peaks of Central East Africa, the glaciers
descended 5000 feet below their present termination. Kosciusko in
New South Wales bears conspicuous traces of a former ice-cap which
completely shrouded all but the highest peaks of the plateau. From the
highlands of Tasmania and New Zealand, and from the Cordilleras and
Andes of Patagonia and Chili the evidences of a prolonged glaciation
are equally clear.
No satisfactory theory of climate has yet been propounded which affords
an adequate explanation of this universal lowering of the snow-line.
Whether or not the temperature fluctuations were synchronous all
over the earth is difficult to decide, but that a colder climate
characterised the southern no less than the northern hemisphere cannot
be doubted. The causes to which appeal has been made in accounting for
climatic changes are of three classes,—astronomical, geographical and
atmospheric. In general the various tendencies will be opposed to one
another and by their interference prevent the attainment of universal
extremities of climate. Occasionally, however, circumstances may
arise when their joint action will lead in a single direction. A wide
extension of tropical or polar conditions would then be expected to
follow.
Of the three contributory causes, only one, the astronomical, bears
any determinable relation with time. The theory that secular variation
of terrestrial climates results from the changing eccentricity of the
earth’s orbit, was first proposed by Adhémar. In 1868 James Croll
greatly elaborated and extended this theory, presenting it with such
a formidable array of quantitative data and yet in such an attractive
form that it exerted a considerable influence on the geological thought
of the day. Not only did it seem to offer an acceptable means of escape
from the maze of difficulties by which the problem was shrouded, but in
addition it promised a faithful chronology fixing the date and duration
of the Ice Age with almost ideal precision.
Laplace established the fact that the ellipticity of the earth’s
orbit is subject to periodic oscillations between certain limits.
Sometimes the path is nearly circular but at other times it becomes a
more flattened ellipse. If the earth were the only planet, its orbit
would suffer no change; that it does is due to the attractions of the
sister planets. Nevertheless, the orbit is essentially stable, and the
yearly journey always occupies the same time. Formulæ were devised by
Leverrier from which it was possible to calculate with some accuracy,
the actual value of the eccentricity at any given period in past or
future within a few million years of the present. Croll utilised these
formulæ to compute the dates of maximum and minimum eccentricity for
the past three million years. He found three important periods when
that factor was specially high, betraying a type of orbit more than
usually flattened. These were:
(_a_) from 2,500,000 to 2,600,000 years ago.
(_b_) ” 720,000 to 980,000 ” ”
(_c_) ” 80,000 to 240,000 ” ”
It was to the last of these that the Glacial Period was assigned.
When the earth is at perihelion, i.e. in that part of its orbit which
lies nearest to the sun, it enjoys a more generous radiation than
falls upon it at aphelion, when it is farthest from the sun. But this
alone does not determine the time of summer and winter. At present
the northern winter and southern summer occur when the earth is in
perihelion. As is well known, this is owing to the inclination of
the earth’s axis and the blanketing effect of the atmosphere. In the
northern hemisphere, the greater thickness of air presented to the
sun’s rays during the winter keeps out more heat than is gained by
the relative closeness to the sun. However, this condition is not
permanent. As discovered by Hipparchus in the year 134 B.C.,
the positions of summer and winter and of the equinoxes on the ecliptic
are subject to a slow forward movement. In 26,000 years they make the
complete circuit, and so in the course of time the relation of the
seasons to perihelion is slowly altered.
The _precession of the equinoxes_ was shown by Newton to be a
dynamical consequence of the spinning of the earth about a tilted axis.
While this change is progressing, the position of perihelion is also
shifting, and the resultant period is thereby reduced on the average
to 21,000 years. Thus, in about 10,000 years from now the northern
hemisphere will enjoy summer at perihelion. At the same time, the
northern winter will occur at aphelion. The cold season will then be
longer and more severe than now, and the annual accumulation of snow
correspondingly increased. If, in addition, we suppose the orbital
eccentricity to approach its maximum value, the northern hemisphere
would then be in the grip of winter for nearly four months of the year,
and the cold would become still more bitter than before. As a slight
compensation for the rigours of the winter, the summer, though short,
would be very much hotter. It was extremes such as these, accompanied
perhaps by favourable geographical conditions, that Croll postulated
for his glacial period. He believed that the snow and ice which would
collect during a long frosty winter would successfully resist the
evaporative powers of the summer, and that permanent snow-fields would
therefore arise. Once started, the snowy mantle would tend to continue.
A great deal of heat could be absorbed without raising the temperature
above freezing-point, and the result of evaporation would be the
creation of a thick blanket of fog, an effective agent in guarding the
ice against the ravages of the summer radiance.
A peculiarity of Croll’s hypothesis is that glacial epochs could not
exist in both hemispheres at the same time. South of the Equator the
summer would be longer than it is at present, and the winter would not
only be short, but also comparatively mild. The climate would be that
of a genial interglacial period. The alternation of glacial epochs
between north and south is a necessary consequence of Croll’s view, but
when the facts are examined they are found to be unfavourable to this
assumption. In Sweden the last period of extension of the ice appears
to have been synchronous with that in New South Wales, the close of
each being dated, if our present time-scale is reliable, at 15,000
to 20,000 years ago. Both date and coincidence are decisive against
the theory. Although Croll’s hypothesis achieved a great popularity,
geologists were not wanting who considered the suggested causes to
be utterly inadequate to produce so radical a change of climate. It
happened that during the penultimate period of extreme eccentricity,
which began nearly a million years ago, the astronomical conditions
were more favourable to glaciation than they have been since. The
question therefore arose why no traces had been preserved in the
deposits of that time. Indeed, since the Cambrian, many hundreds of
glacial periods should have come and gone. Had this been the case, and
corroborative evidence sufficiently convincing, the earth’s chronology
would have been written boldly in its rocks. But Croll’s brave attempt
to number the ages was unsuccessful, and his attractive theory no
longer holds the field.
[Illustration: FIG. 7.
At maximum: full extent of white.
At minimum: inner circle.
NORTH POLAR CAP.]
[Illustration: FIG. 8.
At maximum: full extent of white.
At minimum: disappears entirely.
SOUTH POLAR CAP.
MARS.]
A gigantic experiment illustrating the very conditions which Croll
postulated has been recently shown to us by Prof. Lowell. For his
working model we must look to the sky and carefully watch the changing
seasons of Mars. The eccentricity of the orbit of Mars is much higher
than that of the earth could ever have been, and moreover, the southern
winter falls near aphelion. The conditions for a permanent ice-cap
over the south polar regions are therefore ideal. The actual facts are
surprising, and on the accompanying plate Prof. Lowell has depicted
them admirably. During the winter a large snow-cap collects around the
south pole, but in summer it is entirely dispersed. Around the north
pole the winter snow-cap is less extensive, but all through the summer
it never quite disappears. While the long southern winter undoubtedly
makes possible a greater accumulation of snow, the hot short summer
more than compensates by its superior powers of evaporation. A
permanent and widely extended ice-field evidently could not originate.
This demonstration of Croll’s hypothesis in actual practice does not
present an altogether complete analogy to terrestrial conditions.
The surface temperature of Mars and the thermal properties of its
atmosphere may be widely different from ours, and the excessive rate
at which the polar caps diminish in the spring clearly indicates the
comparative thinness of the deposit. But in spite of these differences,
the remarkable conclusion stands unassailed—that the evaporative power
of the short hot summer of the south exceeds that of the long but
cooler summer of the north.
[Illustration: FIG. 9.
Directions of Glacial Movement in Southern Scandinavia.]
It was stated above that since the culmination of the last glacial
epoch more than 15,000 years have elapsed. Of the several methods which
have been employed to determine this period only one can be dealt with
here. The present genial climate has not greatly varied during the last
7000 years. As we trace back the record of temperature still farther a
gradual fall can be discerned, accompanied by a wide extension of the
ice. Fig. 9 depicts the directions of movement of the last continental
glacier of Scandinavia and its southern boundary across Jutland and
the Baltic provinces. As it retreated, it left the terminal moraine
known as the Baltic ridge, and its southern limit was gradually pushed
back till it extended to what is now the coast of Scania. From this
point its recession has been followed in great detail by De Geer, who
has made a careful study of the deposits which, extending from the
Baltic up to the Scandinavian ice-shed, mark the progress of its annual
retreat. Upon his observations he has founded a system of geological
chronology which is of the greatest importance in that it marks the
first effort towards absolute accuracy.
Each spring and summer, as the glacier thawed, a great deal of sand
and clay was set free and carried away in suspension by the numerous
streams which flowed from under the melting ice. The coarser material,
on reaching the sea, settled down almost at once, but the finest
particles of clay, able to remain in suspension much longer, were not
completely deposited. Then came the autumn and winter, and the freezing
of the streams. The sea received no further supply of sediment, and the
load of fine mud slowly settled on the sea-bottom to form a thin layer
of pure clay, sharply differentiated from the coarser bed below. The
following year the glacier retreated a few hundred feet to the north,
and the material then liberated was sorted out as before and again
deposited in two well-marked seasonal layers. As this process continued
year after year the area of deposit moved northwards with the ice, and
the annual layers of sediment thus became superimposed one upon the
other like wedge-shaped tiles on a roof. The width of each bed is
generally less than thirty miles, for even the finest mud cannot be
traced beyond that distance. This being the case, no vertical section
contains all the layers, and it is rarely that more than a hundred can
be counted in one place. The total thickness of the recessional deposit
seldom exceeds thirty feet.
De Geer successfully attempted the difficult task of counting the
annual bands of glacial clay deposited throughout the period of retreat
from the Scanian coast. Fortunately, any given set of layers can be
traced from one locality to another, and as each ribbon of sediment
dies out the higher beds are followed up in the same way until the
whole series has been examined from bottom to top. The late-glacial
beds number about 5000, and the time which has elapsed since the
ice border reached the eastern coast of Scania is therefore 12,000
years.[2] The time of recession from the Baltic ridge to the Scanian
coast remains to be estimated. Much of the record is hidden beneath the
waters of the Baltic, and in North Germany De Geer’s method has not yet
been applied. It appears, however, that the withdrawal of the ice was
not uniform. It began slowly and reluctantly, but towards the north
became more rapid. In the region of Stockholm the retreat was five
times as fast as in Scania. We may therefore assume that in the still
earlier stages the time taken was considerably more than that required
for a retreat over an equal distance in Scania. The latter would
have occupied rather more than 2500 years, and on this basis Sollas
provisionally accepts 5000 years as the period during which the ice
front was driven back to the south coast of Sweden. In this way the
whole interval which has passed since the culmination of the last
glacial episode is determined to be greater than 15,000 years, with
17,000 years as a probable value.
[Footnote 2: The post-glacial layers number about 7000.]
Amongst the Archean rocks of N.W. Finland, Prof. Sederholm has found
in the Bottnian schists and phyllites primary characters which are
strikingly similar to those of the banded glacial clays. Under the
microscope their textures are distinctly clastic, and each composite
stratum is sharply divided into two thin bands, the coarser one
originally of sand and marking the beginning of a new year of
deposition, the finer one originally of clay. Sederholm interprets
the phenomenon on a uniformitarian basis as indicating that, even in
those remote times, there was a marked difference in the seasons. By
measuring the thickness of many thousands of annual layers, he finds
their mean thickness to be about five inches. The total thickness of
the banded phyllites amounts to 10,000 feet and the time they appear to
represent is therefore only 24,000 years. This conclusion is probably
far from the truth, for, as Sederholm particularly emphasises, it is
difficult to know what is meant by the _thickness_ of a deposit,
so many arbitrary and misleading elements enter into its determination.
From De Geer’s work, it might have been said at first that a thickness
of thirty feet was deposited in a hundred years, but as the deposit
was traced over the country, it became equally evident, that with no
apparent thickening of the formation, deposition had gone on for 5000
years, and considering the growth northwards from the Baltic ridge,
perhaps for 10,000 years. The whole difficulty lies in determining
which parts of a formation are strictly contemporaneous. A measurement
of thickness is significant only in relation to the immediate area of
deposit and to the rate at which it moves landwards or seawards. The
_thickness_ of a formation and the _maximum thickness_ of the
layers formed in successive years may differ enormously, as De Geer’s
researches have made so evident. For the same reason the term _rate
of deposit_ is loose and misleading unless it is clear to what it
refers.
Another astronomical method of estimating time, though embracing much
longer periods, has been applied by Gilbert to certain formations
in Colorado. The basin of the Arkansas River is largely occupied by
Cretaceous sediments, a succession of which is given in the adjoining
diagram. At the four stages marked A, B, C and D the argillaceous
shales give place to a calcareous type, and in these there is a regular
alternation of thin layers of limestone and of calcareous shale. At A
the average thickness of a pair of beds is 1·5 feet, and the number
of repetitions is 15. At B the limestones are more massive, but the
parting shales are very thin. Here again the average thickness of two
adjacent beds is 1·5 feet. The limestones at C are less pure, and each,
with its associated layer of shale, amounts on the average to about 2·7
feet. At D the succession is similar.
[Illustration: FIG. 10.
Vertical Section of the Upper Cretaceous strata of Colorado. Below are
the Comanchian or Lower Cretaceous beds; above are the Transition beds
to the Eocene.]
To explain the remarkably regular alternation of conditions which
determined this uniform rhythm of sedimentation, purely terrestrial
causes appear to be insufficient. Upheaval and subsidence of the
earth’s crust, and the changing distribution of land and sea, are
characterised rather by their irregularity than by any rhythmic
sequence. While there is undoubtedly a rough periodicity in earth
movements, yet it is discernible only on the broadest scale and is out
of all proportion to the requirements of this case. Gilbert therefore
suggests an astronomical cause. Of the several cyclic changes to which
it is reasonable to appeal, the annual revolution of the earth, and
the variation of the eccentricity of its orbit, demand periods which
are in the first case too short (one year), and in the second too long
(91,000 years). As we have already seen, the relation of the seasons
to the position of perihelion repeats itself about every 21,000 years,
and this astronomical cycle seems better adapted to meet the case.
The climatic changes which accompany the precession of the equinoxes
might influence the character of sedimentation in many ways. With a
changing circulation of winds and currents, argillaceous material might
be transported and deposited at one time and calcareous at another. On
land, vegetation might predominate during part of the cycle, and the
surface waters would then dissolve more calcium carbonate than during
a period when vegetation became sparse. At the same time mechanical
erosion would be impeded in the first
instance, but would be more active in the second.
It is evident that the shale was deposited more rapidly than the
limestone, for when the principal deposit was calcareous the thickness
is 1·5 feet, whereas, in the case of the less calcareous beds, it
rises to 2·7 feet. In the normal shales a conservative estimate of the
equivalent thickness would be 4 feet. Adopting the astronomical time
unit of 21,000 years, the rate of deposit would then be of the order:
Limestone, 1 foot in 14,000 years.
Shale, 1 foot in 5,000 years.
On this basis the 3900 feet of shale in the Benton, Niobrara and Pierre
formations represent about 20 million years, for in this example
the term _thickness_ seems to have a definite meaning. If the
assumptions are correct, the duration of the whole Cretaceous period
must therefore be considerably greater than this.
CHAPTER IV
THE WORK OF DENUDATION
Transference of material from land to sea—The denudation
ratio—Weathering of rocks—The work of chemical
denudation—Summary of the data—Composition of the saline
matter in the oceans and of that annually carried to the
oceans—The work of mechanical denudation—Suspended
and bottom loads of rivers—Mississippi not a good
average case—Dole and Stabler’s work in the United
States—Application to the whole land area—Total
material removed and rate of degradation of land—Marine
erosion—Types and quantities of sediments annually produced.
The purely geological methods which have been devised to investigate
our problem are of two kinds. The first attempts to apply a time-scale
to the sedimentary rocks and was, historically, the earliest to be
proposed; the second, due to Joly, deals with the accumulation of salt
in the oceans. The one is concerned with material carried away from the
land mechanically; the other with the material removed in solution. The
various agents of weathering, of which rain and frost are the chief,
disintegrate the surface rocks and supply the rivers with their load of
detritus. The turbid condition of rivers when in flood, heavily charged
with alluvial matter, is a familiar and convincing proof that the
effect of erosion in conjunction with the transporting power of running
water must always be to wear down the land areas. In the dynamical
study of denudation and sedimentation the first essential is to know
the rate at which the rivers are working. Measurements of their load of
silt and dissolved salts and of their annual discharge to the sea make
it possible to arrive at reliable estimates of their activity.
Incidentally, it is useful to determine the ratio which solvent
denudation bears to the whole. The _denudation ratio_, as it may
conveniently be called, is the ratio of the load of dissolved material
to the total load carried both in solution and in suspension.
The amount of material removed in solution from the surface rocks is
not quite the same as that which is carried to the oceans. A small
proportion is abstracted from the over-ground circulation by the waters
which sink below the surface, and while some of this is undoubtedly
brought up again through the agency of springs, it seems possible, as
Prof. Schwarz has boldly suggested, that certain constituents, such as
iron and magnesium, may be permanently removed from the earth’s crust
by downward migration. In the denudation ratio this possibility is left
out of account as having no bearing on the study of sedimentation.
A rough estimate of the denudation ratio may be made by considering the
weathering and decay of rocks _in situ_. Soluble constituents are
withdrawn by leaching and a residue of the more stable minerals and
alteration products is left behind. Weathering involves not only the
abstraction of material but also the introduction of fresh material
from external sources. Oxidation, hydration, and carbonatisation are
the most typical reactions, and they must be allowed for in determining
the proportion of the original rock lost by solution. This can be done
approximately by assuming that some element—aluminium being usually
chosen—has remained invariable during the course of decomposition.
From the analysis of a large number of fresh rocks and of their altered
equivalents, it is found that on an average 30% is dissolved, leaving a
residue of 70%. The denudation ratio ought therefore to be about 0·3.
The direct determination of the work of chemical denudation requires
three distinct sets of measurements: (_a_) the annual discharge
of rivers into the oceans; (_b_) the analysis of their waters;
(_c_) the measurement of their drainage areas. Mellard Reade was
the first to point out the importance of these factors in the study of
dynamical geology. In 1879 he collected such information as was then
available and deduced from it the quantity of rock material annually
removed from the whole land area, his estimate being 5280 million tons.
Sir John Murray’s corresponding estimate of 1887, which was based on
analyses of the waters of nineteen of the world’s principal rivers, was
4975 million tons.
Until 1909 this figure could not be improved upon, but in that year
there was published by the United States Geological Survey the results
of the detailed and systematic work carried out by R. B. Dole and H.
Stabler. For the first time an attempt had been made to measure the
discharge, drainage areas, salinity, and suspended load of all the
important rivers of a large continental area. The estimates represent
the averages of observations made daily for a year or longer, and in
the case of the discharge, the measurements extended over at least
seven years.
Seasonal variations and the effects of floods are apt to be misleading,
and a long continued series of observations is necessary if their
relative importance is not to be over—or under—estimated. The amount
of material in solution varies but slightly from year to year, and the
average of one year’s results is within 10% of the true mean value. The
annual discharge varies much more than this, and still more inconstant
is the load of suspended material, which, in certain years, may differ
from the average value by 50%. These figures indicate how difficult
it is to introduce exact measurements into geology with any hope of
finality.
A summary of all the best data now available has recently been given
by Dr. F. W. Clarke, and covers about 28 million square miles of the
drainage areas of the earth. The details are given in the following
table:
-----------+---------------+------------------------
| | SOLVENT DENUDATION.
| DRAINAGE AREA | TONS ANNUALLY REMOVED.
CONTINENT. | IN SQ. MILES. +---------+--------------
| | PER SQ. | FROM WHOLE
| | MILE. | AREA.
-----------+---------------+---------+--------------
N. America | 6,000,000 | 70·5 | 423,000,000
S. America | 4,000,000 | 45·5 | 182,000,000
Europe | 3,000,000 | 90·0 | 270,000,000
Asia | 7,000,000 | 75·0 | 525,000,000
Africa | 8,000,000 | 40·0 | 320,000,000
| ---------- | ---- | -------------
| 28,000,000 | 61·0 | 1,708,000,000
-----------+---------------+---------+--------------
The total land area of the globe is estimated by Murray at 55·7
million square miles, but of this 11·5 million square miles are areas
of internal drainage, such as the Great Basin of the United States
and the Asiatic depressions, which contribute nothing to the ocean.
The circumpolar regions, representing 4·5 million square miles, must
also be left out of account, and the remaining 39·7, or say, 40
million square miles, is that from which the oceans are fed. If the
figures given above be accepted as typical, then the annual addition
of material to the oceans by solution amounts to 2440 million tons.
Some of this, however, is derived from the atmosphere—chiefly as
carbon-dioxide. Applying the necessary correction of nearly 10%, there
remains 2220 million tons as the amount actually derived from the
rocks. Murray’s estimate, it will be noticed, is almost exactly twice
that of Clarke.
The composition of the saline matter carried to the oceans may be found
by suitably weighing each analysis of river water according to the
discharge of the latter. The general mean of all such results is given
in the table opposite, together with the total amount of each substance
in the ocean. The small traces of elements other than those listed are
quite insignificant.
---------------------+----------------------+--------------------
| ANNUAL ADDITION | TOTAL SALINE MATTER
CONSTITUENT. | OF SALINE MATTER | OF THE OCEANS IN
| IN MILLIONS OF TONS .| MILLIONS OF TONS.
---------------------+----------------------+--------------------
SiO₂ | 284 |
Fe₂O₃Al₂O₃ | 67 |
Mg | 83 | 1,535,000,000
Ca | 497 | 490,000,000
Na | 156 | 12,616,000,000
K | 37 | 454,000,000
Cl | 138 | 22,800,000,000
Br | | 78,000,000
CO₃ | 857 | 80,300,000
NO₃ | 22 |
SO | 299 | 3,172,000,000
---------------------+----------------------+--------------------
Total | 2440 | 41,230,000,000
---------------------+----------------------+--------------------
Annual discharge of river water
into ocean = 24·3 × 10¹² tons
Volume of ocean water = 307,496,000 cubic miles
Density of ocean water = 1·026 (mean)
Mass of ocean water = 1,178,270 × 10¹² tons
-----------------------------------------------------------------
It is obvious from a comparison of these two columns that the annual
increment of dissolved matter is not permanently retained by the
oceans. The greater proportion is precipitated by chemical and organic
agencies, and either becomes incorporated with detrital material or
goes to form individual sediments. The chief substances produced are
calcium and magnesium carbonate, gypsum, limonite, and silica. Rather
more than one-third of the carbonates appear to associate themselves
intimately with sands and muds. The greater part of the remaining
two-thirds is deposited as limestone on the continental shelves (after
being used by various organisms in shell-making) in waters which are
comparatively free from terrigenous sediment. The abstraction of gypsum
from the ocean takes place at irregular intervals under suitable
conditions of concentration. Of the limonite and silica, the chief
precipitation takes place on the continental shelves where they
associate themselves with the detrital sediments. The history of
potassium is rather obscure, but on the contrary, that of sodium
appears to be the simplest of all. That it is stored up in the oceans
is an assumption which is granted as justifiable by most geologists.
The only new factor required in order to estimate the mechanical work
of denudation is the load of material carried by the rivers. Besides
the silt transported in suspension, larger fragments are carried by
rolling along the stream bottom. Measurements of the bottom load are
lacking except in a solitary case—that of the Mississippi—in which
it amounted to about 10% of the whole. It is difficult to define any
precise difference between bottom load and suspended load, the former
being only a limiting case of the latter. When the water is fully
charged with rock debris the highest proportions are found near the
bottom and sides, and at a point in mid-stream at about one-third the
depth from the surface—the position of the stream lines of maximum
velocity. In clear water the rolling power reaches its maximum value,
for apart from fluid friction, energy is expended in no other way. Most
rivers fall between these extremes. In making actual measurements,
samples are taken from representative points in the river and used to
give the average over the whole section. In the final estimate it seems
probable that a large proportion of the bottom load is accounted for.
Even if a correction ought to be applied it would be pedantic to do
so except for rivers which have been under observation for several
consecutive years, since the variation from the mean annual load is
very great from year to year. The Nile varies by 40% and the Potomac
by as much as 100%. In the United States the mean variation is about
50%. With uncertain data of this kind a correction of less that 10% may
safely be disregarded.
Of all the rivers of the world, the Mississippi has been most favoured
by measurements of the kinds required, and many estimates of the rate
of continental degradation, of the rate of deposition of sediments and
of the age of the earth have been based upon them. There is no doubt,
however, that the Mississippi is working more rapidly than any other
river of importance in North America, except, perhaps, the Colorado
River. The high declivity in the west, the Tertiary elevation of the
plains to which the streams are not yet adjusted, and the abundance
of easily eroded glacial drift are all factors which promote this
activity. In the case of rivers other than those of North America for
which data are available, the same high rate of denudation obtains; the
Rhone and the Po, for example, being amongst the most energetic workers
in the world. Generalising for the whole earth from these rivers alone,
would obviously give misleading results. The work of Dole and Stabler
again comes to our aid, and in the following table their aggregate
measurements for the whole area of the United States are tabulated.
Similar evidence for four widely separated rivers is also given:
----------------+-----------+----------------------+----------
| | MILLIONS OF TONS OF |
| | MATERIAL REMOVED |
DRAINAGE BASINS.| AREA IN | PER YEAR. | DENUDATION
| SQ. MILES.+-----------+----------+ RATIO.
| |IN SOLUTION| IN |
| | |SUSPENSION|
----------------+-----------+-----------+----------+----------
United States | 3,088,500 | 241·5 | 468 | 0·34
Mississippi | 1,265,000 | 122 | 304 | 0·29
Nile | 1,100,000 | 21 | 52 | 0·29
Uruguay | 150,000 | 7·5 | 15 | 0·33
Rhone | 34,800 | 8·5 | 36 | 0·19
----------------+-----------+-----------+----------+----------
Leaving out the Mississippi because of its inclusion in the United
States and weighing each result according to the area over which it
holds, the mean denudation ratio is 0·31, a figure which agrees very
well with our previous estimate. If now we use the denudation ratio to
calculate the material removed by mechanical denudation over the whole
land surface, we should not be far from the truth. It is clear that
if from the 40 million square miles which drain into the oceans the
quantity of material carried in solution represents 0·3 of the total
material removed, then, as the former amounts to 2440 millions of tons
annually, the quantity carried away mechanically must be 5700 million
tons. That this figure is of the right order is favoured by another
consideration. The mean elevation of North America is very nearly that
of all the land areas of the earth. Moreover, according to Clarke’s
figures the rate of denudation over North America is slightly higher
than the average for all the lands, but more closely approaches it than
does that of any other continental area. We may therefore take the rate
of denudation of North America as a fair average and apply it with some
confidence to all the drainage areas of the globe. Doing this, the
total amount of suspended material annually discharged into the oceans
is computed to be 6000 million tons.
We may sum up the work of denudation in round figures as follows:
Material annually removed in solution 2500 million tons
Material annually removed in suspension 6000 ” ”
-----------------
Total 8500 million tons
These figures may also be expressed in terms of the rate at which the
land areas are being worn down. By solvent denudation a degradation
of one foot in 30,000 years is implied, and by mechanical denudation,
one foot in 12,000 years. Taking both together the average rate of
denudation is found to be _one foot in 8600 years_. It should
be clearly understood that individual areas may be lowered at rates
very different from this. The maximum rate is attained in the Irawadi
basin, one foot of which is removed in 400 years. The Po is also an
exceptional river, and lowers its basin by one foot in 850 years.
On the other hand, in the Hudson Bay district of North America the
drainage only carries away one foot in 47,000 years.
No minimum figure can be given, for wherever deposition of sediment
takes place on the land areas the temporary rate of denudation locally
becomes negative. In making these calculations, the density of rock
material is taken as 2·6; the weight of a cubic foot as 165 lbs;
and the weight of a cubic mile as 10,800 million tons. Although the
surface covering of loam or earth weighs only about 100 lbs per cubic
foot, the denser and more closely packed underlying rock need alone be
considered, for it is by its decay and expansion that the superficial
blanket above is produced.
Our final problem is to determine the nature and quantity of the
sediments which are ultimately formed on the continental shelves. This
can only be done roughly, but the results will suffice to serve our
purpose. First of all, two serious difficulties must be met before the
way is open to take this step. So far, marine denudation has been left
out of account. It is not yet possible to make a wholly satisfactory
estimate of the relative magnitude of the supply of detritus captured
directly by the sea. The unknown factor is the average encroachment
of the sea upon the coasts. For the British Isles, Croll suggested
an average of three feet per century, and the figure assumed by Sir
A. Geikie about the same time was ten feet per century. A much later
estimate by Prof. Watts places the average retreat of the English
coast at a hundred feet per century. Along parts of our East coast
marine erosion is still more rapid than this, the conditions being
exceptionally favourable. On the other hand, Geikie considers that all
the force of the Atlantic beating upon the N.W. coast of Scotland may
not wear it away at more than one foot per century. What the average
between these extremes may be can only be guessed at. If for
convenience we accept Geikie’s figure as affording a likely average for
all the coast lines of the earth (125,000 miles), and if the average
height of the cliffs be taken as 150 feet, then the mass of material
annually removed will be about 700 million tons.
The other difficulty is concerned with the annual amount of material
which remains in the oceans in solution, and also of that which is
deposited on the ocean floor outside the limits of the continental
shelves. For the former, a knowledge of the age of the oceans is
necessary. Considering all the evidence, the amount retained at the
present day seems to be about 200 million tons, but this is certainly
too high as a figure representing the average increase throughout the
history of the oceans.
For the deep-sea deposits little more than a guess is possible,
although we can now approximate to the right order of magnitude by
considering the circulation of radium. The radium in the material
removed from the lands is redistributed between the sediments on the
continental shelves, the deep-sea deposits and the water of the oceans.
Applying our present knowledge of the distribution of radium (see p.
131) the annual mass of the deep-sea deposits is found to be about ¹/₃₀
of the whole, i.e. about 300 million tons.
The difficulties can scarcely be avoided by balancing them against
each other. There still remain 200 million tons (700 - 300 - 200)
to be added to the 8500 million tons already found as the total for
sub-aerial denudation. This gives us 8700, or as an extra safeguard,
say 9000 million, tons as the mass of sediment annually deposited on
the continental shelves. It is unfortunate that to arrive at this
figure an element of doubt should be introduced by associating the
results of careful experimental work with the vague conclusions just
arrived at. The bugbear of the whole investigation is marine erosion;
but if it is remembered that the figures given in that connection are
meant to be suggestive rather than final, no erroneous impression need
be carried away.
If the sediments ultimately formed are shales (20% quartz), sandstones
(75% quartz) and limestones (75% calcium carbonate), their proportions
will be as follows:
Shales 70% or 6300 million tons.
Sandstones 16% “ 1440 ” ”
Limestones 14% ” 1260 ” ”
--------------------------
Total 100% = 9000 million tons.
In the two following chapters the application of denudational
statistics to the measurement of geological time will be considered.
CHAPTER V
THE SALINITY AND AGE OF THE OCEANS
Halley’s proposal in 1715—Joly’s application of modern
data in 1899—Further studies by Sollas, Clarke, and
Becker—Data of the problem—Corrections for disseminated
sodium, wind-borne sodium, human agencies, and marine
erosion—Uniformity not capable of proof—Importance of
cyclic circulation of sodium—Origin of chlorine now fixed
in salt—Hour-glass method applied—Land areas of the past
and their elevation compared with those of to-day.
In 1715 the famous astronomer Edmund Halley published a paper which he
entitled, _A short Account of the Cause of the Saltness of the Ocean,
and of the several Lakes that emit no rivers; with a Proposal by help
thereof to discover the Age of the World_. He showed that since the
water removed from lakes by evaporation is perfectly fresh, “the saline
particles brought in by the rivers remain behind, while the fresh
evaporate; and hence it is evident that the salt in the lakes will be
continually augmented and the water grow salter and salter.” Applying
the same principle to the oceans, he wrote, “It is not improbable but
that the ocean itself is become salt from the same cause, and we are
thereby furnished with an argument for estimating the duration of
all things.” Two hundred years ago it seemed hopeless to attempt to
determine the annual increment of salt added to the oceans, but Halley
used his argument “to refute the ancient notion some have of late
entertained of the eternity of the world.” The paper was completely
forgotten, until Dr. G. F. Becker again drew attention to it a year or
two ago.
As we saw in the last chapter, the analysis of river waters has now
made possible a determination of the annual amount of material carried
in solution by the rivers into the oceans. T. Mellard Reade was the
first to contemplate the application of solvent denudation to the
measurement of geological time, and the data he gathered together
in support of his arguments came as a great surprise to those who
had concentrated attention merely on the mechanical work of erosion.
No independent advance, however, was possible until 1899, when Joly
pointed out that of the many elements which enter into the composition
of sea water, sodium alone tends to accumulate. All the others are
sooner or later rejected, associating themselves with the detrital
sediments, or forming chemical or organic sediments by their ultimate
precipitation. Joly then proceeded to use sodium as the age-index of
the oceans. He assumed that the annual increment Naᵣ of sodium added
to the oceans by all the rivers of the world has remained practically
constant throughout geological time. If Naₒ represents the total amount
of sodium now accumulated, the ratio Naₒ/Naᵣ gives the time which has
elapsed since the oceans first existed and denudation began to wear
down the lands. Joly’s first estimate was from 80 to 90 million years,
and shortly afterwards he increased this to 100 million years. Sollas
attacked the problemafresh in 1909, and from a most careful survey of
all the data and a detailed enquiry into every phase of the subject,
he concluded that the most probable estimate of the age of the oceans
would appear to lie between 80 and 150 million years.
In 1910 a further study was made by Clarke and Becker. The latter
departed from the uniformitarian basis on which all the other
calculations had been founded. He inferred that sodium accumulation
progressed more rapidly in the past than at present. All the original
sodium must have been derived from igneous rocks, and Becker considers
that at the time when the oceans were first possible the surface of the
earth must have consisted exclusively of such rocks. At the present
day three-quarters of the land areas are covered by sedimentary rocks
which can supply no further important additions to the sodium content
of the ocean. Assuming that the production of sodium has been always
proportional to the area of igneous exposures, and that the total land
area of the globe has averaged 80% of the present area, he finds that
the age indicated is about 70 million years.
We may now proceed to examine the problem in detail. The fundamental
data on which the method is based may be summarised as follows:
Mean density of the ocean (Murray) 1·026
Volume of the ocean (Murray) 323,800,000 cu. miles
” ” (Joly) 339,248,000 ”
” ” (Clarke) 302,000,000 ”
” ” (Karsten) 307,496,000 ”
Clarke now accepts the latter as being the best estimate, and we
therefore calculate that the
Mass of the ocean = 1,178,270 × 10¹² tons.
Total salinity of the ocean (Dittmar) 3·5%
Sodium of the ocean (Dittmar) 1·08%
Accumulated sodium, Naₒ = 12,600 × 10¹² tons.
Annual increment, Naᵣ = 156,000,000 tons.
From these values the quotient Naₒ/Naᵣ gives the age of the oceans to a
first approximation as 80·8 million years. This figure, however, cannot
be regarded as final. Two of the assumptions on which it is based are
that all the sodium liberated from igneous rocks is contained in the
ocean, and that all the sodium carried annually to the ocean has been
liberated from such rocks for the first time. The obvious corrections
to be applied will increase the numerator of the ratio and decrease
the denominator, thereby increasing the age estimate. The degree to
which the numerator must be augmented is, as far as we know, relatively
insignificant. Saline deposits, such as those of the Stassfurt
district, are only of trivial importance. The salt of the ocean, if
spread regularly over the whole land areas, would form a layer about
120 feet thick, and all the beds of rock salt which have accumulated by
evaporation become negligible beside this vast quantity. The amount of
sodium in ground waters and disseminated through the sedimentary rocks
is to be taken into consideration more carefully. Marine sediments at
the time of their formation are saturated with sea water, and, when
raised up to form land, they must, therefore, be strongly charged with
salt. The actual proportion of sodium abstracted from the ocean in this
way cannot at one time be very great, probably not more than 1% of the
whole. Throughout geological time, however, a certain amount of sodium
has been in cyclic circulation between land and sea. A rough idea of
the influence of this circulation on the age estimate may be gained
from a knowledge of the total volume of the sedimentary rocks, and of
their average pore space.
Neither of these factors is yet known with the desirable accuracy, but
as far as present needs are concerned, the total bulk of the sediments,
excluding deep-sea deposits, may be placed at 70 million cubic miles,
and their pore space at 10%. The latter figure is probably too low,
for fresh sediments have a pore space of 40% to 50% of their volume,
and in consolidated sandstones 20% is common. Many of the sediments
have been denuded and re-deposited time after time; on the average,
probably three times. On the other hand, if the ocean has progressively
increased in salinity, its average sodium content must have been
about half that of the present. Basing our calculations on these
considerations, the total volume of sediments which have ever existed
is about 210 million cubic miles; the pore space to be filled by sea
water would then be 21 million cubic miles. The total amount of sodium
precipitated within the rocks by evaporation could not therefore exceed
420 million million tons, and at the present rate of denudation its
removal would require nearly three million years. To what extent this
result would be increased by absorption effects, whereby interstitial
salt solutions are concentrated in order that they may be in
equilibrium with normal sea water, cannot be estimated. This corrective
factor would probably not be large.
A still more important cyclic circulation is brought about by
wind-borne sea salt. Fine particles of spray are swept from the foaming
crests of waves by the wind and carried often many miles inland. Near
the coast the salt blown over the land is naturally most abundant. It
falls in sea fret and rain, and is ultimately returned to the sea in
the drainage from the land. Fortunately, the amount of sodium supplied
in this way can be estimated by analyses of rainwater and a measurement
of the rainfall. As before, the cyclic sodium must be excluded from
that entering into the denominator as primary sodium. Joly allows an
additive time correction of 10%, Clarke of 7%, and Becker of 6%. The
time estimate is therefore increased by a further five or six million
years.
Clarke has suggested that the present rate of sodium accumulation has
been accelerated by human agencies. Nearly six million tons of sodium
are annually produced as common salt, and, as sewage and chemical
refuse, much of this is again returned to the oceans. Subtracting this
from the denominator, the age is increased by other three million years.
A number of corrections may be made to decrease the first rough
estimate of the ocean’s age. Solvent denudation due to marine erosion
must not be forgotten. Experiments by Joly have shown that sea water
is several times (2½-14) more active as a solvent than fresh water. He
attributes to its action over the tide-swept strand—covering a total
area of about 60,000 square miles—a supply of sodium amounting to 3%
of that derived from the normal drainage areas. This is a very liberal
estimate, and in applying a negative correction of three million years
we are granting all that can reasonably be claimed.
Finally, there are other possibilities of uncertain magnitude which
deserve mention. The ocean may conceivably have contained sodium
before the fluviatile contributions commenced. By volcanic eruptions,
sodium-bearing materials may be cast widespread over the sea.
_Juvenile_ waters expelled from igneous magmas may presumably
afford a supply of sodium. The importance of saline waters derived from
the heated interior of the earth has been particularly insisted upon
by the great geologist Suess. As a source of sodium, such waters are
probably of little moment, for the whole amount of sodium in the ocean
is already more than explained by the erosion of igneous rocks. Solvent
denudation involves a loss from average igneous rock of 1·57% due to
sodium abstraction alone. According to Clarke, the average sodium
content of igneous rocks is 2·52%, and the amount retained by the
detrital material which goes to make up the sediments is 0·95% of the
original rock. Now, if the total mass of sodium which has accumulated
in geological time is 12·6 × 10¹⁵ tons and this represents 1·57% of the
igneous rock in which it originally occurred, it is evident that the
mass of the igneous rocks which have been denuded away amounts to 800
× 10¹⁵ tons. The mass of one cubic mile of rock may be taken as 10,800
million tons, and hence the volume of igneous rock which has been
removed is 74 million cubic miles. The sediments derived from the
latter ought therefore to occupy a volume of the same order. A number
of independent estimates of the volume of sediments existing on the
land areas have been attempted, and while no great accuracy can be
hoped for, it is significant that all agree in assigning a volume
somewhat greater than 74 million cubic miles. The sediments hidden
under the oceans would add still further to the total. If any reliance
at all is to be placed on these estimates, it is clear that the sodium
in the ocean ought to be more than it is; at any rate, there would
appear to be little room for sodium derived from other sources, such as
the interior of the earth.
We may sum up the results of this discussion as follows:
Approximate age, Naₒ/Naᵣ = 81 million years.
Correction for
(_a_) Disseminated Sodium = 3 million years.
(_b_) Wind-borne Sodium = 6 ” ”
(_c_) Human agencies = 3 ” ”
(_d_) Marine erosion = -3 ” ”
--------------------
Age of the oceans 90 million years.
Another assumption on which this computation is founded must now be
examined; that is, the practical uniformity of the annual addition of
sodium throughout the period involved. Little weight can be attached to
the argument that the source of supply has been gradually impoverished
in alkalies, for the soils from limestones, which are among the poorest
of the alkali-bearing rocks, may contain more sodium than do the soils
derived from granites. The sedimentary rocks being less able to resist
erosion are more rapidly disintegrated and removed than are the igneous
rocks. The composition of streams considered in relation to the rocks
which they drain is of interest here. Hanamann has shown that in the
Elbe and its tributaries three times more sodium is carried away from a
square mile of Cretaceous sediments than from an equal area of granite.
In the same connection, it is well known that the waters flowing
through stratified rocks carry more salt than those from igneous rocks.
The greater exposure of the latter in former land areas, if such were
the case, would certainly not contribute a greater sodium income to
the ocean than if sedimentary rocks prevailed. Joly considers that an
approximate uniformity has been preserved whatever the distribution of
rock types may have been. It would not, however, be difficult to prove
by actual analyses that a greater area of igneous rocks would result in
a reduction of the sodium increment, and that the present rate may be
too high because of the predominance of sedimentaries.
This unexpected conclusion raises the question of the validity of the
method at its most critical point. Can we be sure that the cyclic
circulation of sodium has been taken sufficiently into consideration?
The only source to which the anomalous proportion of sodium from
sedimentary rocks can be traced, apart from the factors already dealt
with, is constituted by the ground waters. These waters often contain
minute quantities of salt of which the origin is very obscure. Whence
came the chlorine? Igneous rocks themselves only contain about 2% of
the chlorine necessary to convert the liberated sodium into salt. The
emission of chlorine from volcanoes and solfataras probably affords
the necessary supply, but most of this never reaches the ground waters
directly. Becker, dealing with the same difficulty in the case of river
water, calculates that 40% of the annual sodium increment is combined
with chlorine of which the origin cannot be traced. He accordingly
rejects this amount as not being of normal or primary origin. If it
has all been derived from the oceans, which he regards as an extreme
possibility, it must be subtracted from the apparent income, since it
is simply a circulation of capital.
A _maximum_ figure for the age of the oceans (still on the
assumption of uniformity) ought theoretically to be attainable by
accepting the whole of the sodium-chloride of the rivers as cyclic. We
have the following figures:
Chlorine = 138 million tons.
Sodium combined with chlorine = 87 ” ”
Sodium uncombined with chlorine = 69 ” ”
-----------------
Total Sodium, Naᵣ = 156 million tons.
The unchloridised sodium implies an age of 180 million years, an
estimate which would in general be condemned as excessive.
The difficulties presented by sediments and ground waters are almost
insuperable, and it may be safer to adopt a means by which they can
be entirely disregarded. The sodium lost from igneous rocks amounts
by mass to 1·57% of the latter. Igneous and metamorphic rocks are at
present exposed over one-fifth of the drainage areas, or 8,000,000
square miles. Many of the metamorphic rocks are partially or wholly
of sedimentary origin, and ought not to enter into the computation.
We have already seen that the average rate of denudation is one foot
in 8600 years, and at this rate, one cubic mile of primary rock, or
10,800 million tons, would be denuded away in 4·54 years. In one year
the supply of sodium would therefore be 37 million tons, and the time
required for the accumulation of the existing store of sodium in the
oceans would be 340 million years.
We meet with a curious discrepancy at this point. The actual amount
of unchloridised sodium is 69 million tons, while that which is drawn
from the igneous rocks cannot exceed 36 million tons. In each case
sub-aerial denudation alone is being considered, so that the failure
to take marine denudation into account does not affect the matter.
The figures imply that a great deal of unchloridised sodium is drawn
from the sedimentaries. Even if the extreme assumption is made for the
moment, that the igneous rocks lose all their sodium, 2·52%, the annual
supply could only reach 60 million tons. The discrepancy is evidently
due to the facts that sediments freshly formed from primary rocks
retain a considerable percentage of sodium, and that it is only after
the material has been re-assorted, perhaps several times, that the
element is withdrawn to the extent indicated by Clarke’s average
figures. At the present day, even when the igneous rocks have been
deprived of 37 million tons of sodium, the sedimentary rocks, to bring
up the total to 69 million tons, must lose more than half of their
primary sodium. It is interesting to observe that if all the sodium
now carried each year to the oceans—156 million tons—were derived
directly from the rocks, igneous and sedimentary, the latter would be
obliged to lose nearly twice as much sodium as they actually contain!
This remarkable conclusion indicates either that our statistics of
the annual production of sediments are hopelessly wrong, or else that
the chloridised sodium is almost wholly cyclic. The latter inference,
supported as it is by the impossibility of otherwise accounting for the
chlorine, thus receives further support.
In the application of the above data to the age question, it has been
tacitly assumed that the present exposures of primary rocks are neither
greater nor less in area than the average of such exposures since the
oceans began.
It is impossible to know what the average may have been, for while
the original lands must have been entirely formed of igneous or
pseudo-igneous planetesimal material, it is equally certain that
their area was but a fraction of that of the present land areas. The
evolution of the earth’s surface features has tended to intensify the
difference between the ocean depths and the mountain heights. The ocean
basins have gradually deepened and become more stable, and their
relative permanence is generally admitted. The lands are now more
extensive than ever they have been; initially it is probable that only
low and scattered islands emerged from the shallow primitive seas. We
are faced here with many vague problems. The volume of the seas may
have been less than it is to-day; the original rocks were probably not
granitic in type, and were certainly poor in sodium; carbon-dioxide
seems to have been the chief atmospheric companion of nitrogen, and
solvent denudation would be correspondingly more vigorous. How far
all these considerations affect the point at issue cannot be decided.
To discuss them would lead us into a complex maze of speculation. The
original basis of our calculation may afford as near an approach to
the truth as could be arrived at by any other reasonable hypothesis of
changing land areas, and of their composition.
On this view it is possible to calculate a _minimum_ figure for
the age of the oceans, but it must be remembered that the results
based on an assumption, which is merely a uniformitarian guess, are
themselves equally uncertain. If the igneous rocks had lost all their
sodium, we have already found that the annual supply would be 60
million tons, and the sediments would be destitute of that element.
The age would then be at least 210 million years. The contradiction
between this minimum figure and the maximum deduced from the total
unchloridised sodium, viz. 180 million years, is explicable on one or
both of two further possibilities, that the average exposure of igneous
rocks in the past has been greater than that of the present day,
or that the amount of primary sodium now being derived from the
sedimentary rocks is considerably above the average for the past. Our
interpretation of the data is made particularly difficult because
of the widespread covering of glacial detritus, rich in felspathic
constituents and easily eroded.
The contradictory results become still worse when it is remembered that
the rate of denudation—one foot in 8600 years—is probably too high to
be safely applied to areas of igneous rocks. Analysis of river waters
draining such areas indicate a rate which is only about a half of the
above. But even this correction is surpassed in importance by one
arising out of the possibility that the present standard of measurement
may be misleading in not being typical of geological time. The mean
height of the drainage areas is subject to much fluctuation. By
denudation it is steadily reduced; by earth movements, and particularly
during the periods of intense diastrophism, it may become unusually
great. It seems not unlikely that we are now near a period of extreme
continental elevation, and that the geological processes which are
thereby quickened up, cannot be accepted as affording a true standard.
This suggestion will be dealt with further in the next chapter.
The high figures—210-340 million years—given above, must not be
supposed to possess any serious value. The whole discussion merely
serves to betray the uncertainty of the method and the doubtful
applicability of even the most accurate data. For the present we can
only conclude that our knowledge of the part played by sodium and
chlorine in the constant redistribution of the materials of the earth’s
crust is still lamentably imperfect, and that quantitative deductions
drawn from it must be regarded as being purely provisional.
CHAPTER VI
SEDIMENTATION AND GEOLOGICAL TIME
The maximum thickness of the sedimentary rocks—Rate
of deposit—Uniformitarian basis of the method as
usually applied—Arguments against the validity of this
assumption—Present a period of land extension and
continental elevation—Present geological rates not
true standards—Ideal sedimentation curve and rates
of deposit—Difficulties in the application of the
data—Summary of time estimates based on this method—The
hour-glass method applied to the accumulation of sediments
and of calcium carbonate.
The most familiar method of estimating geological time is based upon
the total observed thickness of stratified rocks and the rate at which
they may have been deposited. Our knowledge of the so-called maximum
thickness of each of the stratigraphical systems has been carefully
summarised by Sollas, to whom we owe the following table.
_Maximum Thickness of the Geological Systems._
FEET
Recent and Pleistocene 4,000
Pliocene 13,000
Miocene 14,000
Oligocene 12,000
Eocene 20,000
------
63,000 feet.
Cretaceous 44,000
Jurassic 8,000
Triassic 17,000
------
69,000 ”
Permian 12,000
Carboniferous 29,000
Devonian 22,000
------
63,000 ”
Silurian 15,000
Ordivician 17,000
Cambrian 26,000
------
58,000 ”
Algonkian 82,000 82,000 ”
Archean ? ?
-------------
Total 335,000 feet.
-------------
The rate of deposit of sediment on the ocean floor is a factor over
which there has been much dispute. The rate varies between very wide
limits, according to distance from the shore and from the mouths
of great and active rivers. What is generally sought is to give an
estimate which will correspond to the maximum thickness of accumulated
material, i.e. an estimate of the average maximum rate of deposit. The
same rate is then applied in turn to the whole of the geological column
in the hope that no serious error will be introduced by the assumption
of uniformity. This attitude was taken up in defence against the
attacks of Kelvin and his followers. The tendency to invoke more active
geological agencies in the past, greater floods and tidal waves, a more
stupendous upheaval of mountains and more violent volcanic eruptions,
did not commend itself to most geologists. Geikie stated the case very
clearly in his eloquent address of 1892, when he affirmed that “the
geological record furnishes a mass of evidence which no arguments drawn
from other departments of Nature can explain away, and which, it seems
to me, cannot be satisfactorily interpreted save with an allowance of
time much beyond the narrow limits which recent physical speculation
would concede.” But while the conception of greater activity in the
past met with little favour, the assertion of uniformity was as far as
geologists dared to go. No one suggested that we might be living in an
age of more than average activity. Yet there are many reasons which
favour this hypothesis in preference to the alternative views.
That the average land area of the past was less than that of to-day has
already been stated. According to the palæo-geographical researches of
Mr. C. Schuchert, the mean area of North America since Cambrian times
has been four-fifths of its present area. In the case of the other
continents a smaller fraction would probably be more representative. Of
very much greater importance is the fact that the average height of the
land areas above sea-level has often been less than it is, so that the
present average is excessive when viewed from the broader standpoint of
geological time. The chief defect in the time estimates based on the
rate of sedimentation lies, according to Chamberlin, in the too full
dependence on standards derived from the geological processes now in
action. It is tacitly assumed that current rates are representative,
or that the departure from the mean rate is not such as to involve any
grave error. Joly, for example, in discussing the divergent evidence of
geological processes and radioactive minerals, points out that to bring
the different methods into agreement we must assume “that the rivers
are now bearing to the sea about 14 times the average percentage of
the past—_not less_ than 9 times.” Then he says, “It seems quite
impossible to find any explanation of such an increase.”
In the present high relief of the earth’s surface at least a partial
explanation may be found. Prof. Chamberlin writes in a private
communication to the author: “Because of the relatively high gradients,
the wash of clastic material from the slopes and its deposition in
the basins, as well as the transfer of salts to the sea, are to-day
more rapid than in average times. We seem to be at, or near, one of
the great extremes of intensification of the processes of solution and
degradation. And so, whether conclusions are based upon degradation
and clastic deposition, or upon solvent action and the accumulation of
solutes in the sea, the present rates are high rates, and if these are
made the basis of time estimates, the estimates are minimum ones. There
are abundant evidences that periods of base-levelling have occupied
a notable part of geological time. There is cogent evidence that the
Archean and Proterozoic (Algonkian) terranes were reduced well towards
base-level in pre-Cambrian times, and that subsequently extensive
base-levelling clearly seems to have intervened at repeated intervals.
To me the evidence seems to support the existence of a dozen or a score
of stages of peneplanation, some of which appear to have made a notable
advance toward complete base-levelling.”
During those intervals, when the average continental height was low,
denudation and deposition would proceed very slowly. How slowly,
we have no adequate means for determining. A very careful study of
drainage basins with reference to their mean elevation would be a step
towards a sounder method than it has yet been possible to apply. That
the departure from present rates, in past time, may have been very
considerable, is indicated crudely by a very simple calculation. It
has been found experimentally that the carrying power of water varies
as the sixth power of its velocity. Roughly, we may say it varies as
the sixth power of the mean square of the heights of the drainage
basin from which it finds its way to the sea. If this were strictly
true, then, if the present day contours of the land were reduced to
half their value, the power of removing material would be reduced to
less than one four-thousandth. Of course, other factors would begin to
operate which would prevent the attainment of any variation as extreme
as this. Nevertheless, our faith in the value of present standards as
applied to geological time cannot but be seriously shaken. Chamberlin
feels warranted in thinking “that the substitution of mean velocities
of denudation, deposition, and saline accumulation, if it could be made
to approach the realities of the case, would have the effect of
multiplying by a considerable figure the best estimates that have been
made on the basis of current velocities.” Other factors tending to
increase the age estimate are numerous, but comparatively insignificant
beside those already briefly discussed.
In the last chapter we concluded with an estimate of the quantity of
different types of sediment which are now annually deposited.
Shales 6300 million tons.
Sandstones 1440 ” ”
Limestones 1260 ” ”
------------------
9000 million tons.
If it be assumed that all the arenaceous sediments form littoral,
deltaic and estuarine types, i.e. that they are concentrated over an
area of say 100,000 square miles, the rate of accumulation would be one
foot in 150 years, or dealing only with fresh unconsolidated sediment,
of one foot in about 100 years. But these figures are obviously too
high, for a great deal of sandstone comes under shallow water marine
types. Fortunately, in the deltaic deposits of two important rivers
human remains of recognised age are found buried; and from measurements
of the thickness in each case it is known that the Nile has deposited
loose sediment at one foot in 320 years, and the Po at one foot in 174
years.
We have here, as in all deltaic deposits, a mode of growth analogous to
that of the glacial clays studied by De Geer. It by no means follows
that these rivers will continue to raise their beds at the same rates,
for the main growth of deposit is not upwards, but seawards.
To get a rough idea of the rate of sedimentation, the ideal section
opposite has been constructed. The continental shelves are assumed
on the average to be representable as a band of 100 miles in width
fringing a coastline of 100,000 miles. The average thickness of a
deposit laid down according to this plan would be about 0·4 of the
greatest thickness, the latter, on the scale of the diagram, having
reached 400 feet. In the particular case illustrated the land is slowly
sinking, and as the sea encroaches upon it the deposits gradually
overlap.
If deposited over one square mile, as consolidated sediments of density
2·5, our 9000 million tons of material would form a rectangular mass
4570 feet in thickness. Being deposited over the ten million square
miles of the continental shelf, the average thickness of the layer is
0·000457 feet, corresponding to a rate of deposit of one foot in 2200
years. The maximum rate is therefore one foot in 880 years. If this
figure be applied to the total thickness of the geological column,
then, disregarding its imperfection, the time implied would be about
300 million years.
The distribution of conglomerate, sandstone and shale is sufficiently
indicated in the diagram. It is difficult to know how to distribute the
argillaceous and calcareous types. They are mutually exclusive, for
limestone cannot form by organic agencies in places where mechanical
detritus is being deposited. They are therefore taken together, on the
basis that where one is not being formed the other is.
[Illustration: FIG. 11.
Section illustrating the formation of Sediments on the Continental
Shelf while the latter is being slowly depressed.]
The following table gives the rates of deposit at various distances
from the shore, for the case illustrated in Fig. 11:
-------------------------+--------------------------------
| YEARS FOR DEPOSIT OF ONE FOOT.
DISTANCE FROM +----------------+---------------
SHORE IN MILES. | FRESH | CONSOLIDATED
| DENSITY = 1·8. | DENSITY = 2·5.
-------------------------+----------------+---------------
{ 0 | |
{ 10 | 2,780 | 2,000
Sandstones { 20 } | 1,670 | 1,200
{ 30 } | 1,120 | 880
{ 40 } | 1,430 | 1,030
50 } | 2,280 | 1,660
60 } Shales | 5,210 | 3,750[3]
70 } | 10,420 | 7,500[4]
80 } | 13,900 | 10,000[5]
90 } | 20,850 | 15,000[6]
100 } | 41,700 | 30,000
-------------------------+----------------+---------------
[Footnote 3: Cf. Rates of deposit of the Cretaceous sediments of
Colorado, p. 47.]
[Footnote 4: Cf. Rates of deposit of the Cretaceous sediments of
Colorado, p. 47.]
[Footnote 5: Cf. Rates of deposit of the Cretaceous sediments of
Colorado, p. 47.]
[Footnote 6: Cf. Rates of deposit of the Cretaceous sediments of
Colorado, p. 47.]
These figures, however, have but little value, for there is no
single law of deposition. The effects of ocean currents, of earth
movement, and of the presence or absence of great rivers should all
be considered, and they provide a problem so complex that as yet it
is hopelessly beyond a general solution. Difficulties are encountered
at every stage; not only are the estimated rates of doubtful value,
but their application is discredited by our ignorance as to what
constitutes the real maximum thickness of sediments.
As already indicated, the latter difficulty is due to the tendency for
successive beds to overlap while the cycle of deposition is running
its course. As was admirably stated by Prof. Watts in his Presidential
address to the Geological Society in 1911, deltaic deposits gradually
extending seawards are more characteristic during periods when the
land is being elevated relative to sea-level. On the contrary, during
periods of depression estuarine conditions prevail and the beds grow
landwards.
The same problem has been attacked by Sederholm from a rather different
point of view. He says, “As the layers successively formed cover each
other like scales or roof-tiles, no vertical section contains them all.
If we mean by maximum thickness the sum of the maxima of the layers
formed in successive years, it certainly measures millions of feet.”
In this case the rates of deposit as ordinarily found would not, of
course, be applicable.
Before proceeding farther, it may be well to review the various
estimates of time which have been founded upon the geological method.
The earlier geologists believed that the sediments were deposited
widespread over the ocean floor and the rate of deposition was
therefore taken as even less than that of denudation. Then came the
_Challenger_ expedition in 1872-5, and it became certain that
the formation of all except the less important deep-sea deposits
takes place almost entirely on the submarine continental shelves. A
considerable modification of the estimated rates of deposition was then
made necessary and the time periods were correspondingly shortened.
------+--------------+------------+-----------+----------
| | | RATE OF |
| | MAXIMUM | DEPOSIT | TIME IN
DATE. | AUTHOR. | THICKNESS | YEARS FOR | MILLIONS
| | IN FEET. | ONE FOOT. | OF YEARS.
------+--------------+------------+-----------+----------
1860 | Phillips | 72,000 | 1332 | 96
1869 | Huxley | 100,000 | 1000 | 100
1871 | Haughton | 177,200 | 8616 | 1526
1878 | Haughton | 177,200 | ? | 200
1883 | Winchell | — | — | 3
1889 | Croll | 12,000[7] | 6000[8] | 72
1890 | de Lapparent | 150,000 | 600 | 90
1892 | Wallace | 177,200 | 158 | 28
1892 | Geikie | 100,000 | 730-6800 | 73-680
1893 | McGee | 264,000 | 6000 | 1584
1893 | Upham | 264,000 | 316 | 100
1893 | Walcott | — | — | 45-70
1893 | Reade | 31,680[9] | 3000[10]| 95
1895 | Sollas | 164,000 | 100 | 17
1897 | Sederholm | — | — | 35-40
1899 | Geikie | — | — | 100
1900 | Sollas | 265,000 | 100 | 26·5
1908 | Joly | 265,000 | 300 | 80
1909 | Sollas | 335,800 | 100 | 80
------+--------------+------------+-----------+----------
[Footnote 7: Spread evenly over the land areas.]
[Footnote 8: Rate of denudation.]
[Footnote 9: Spread evenly over the land areas.]
[Footnote 10: Rate of denudation.]
Most of these estimates are little more than rough guesses. We do not
know how much of the story is lost to us, or how much is hidden away.
The time which has usually been regarded as expressing the geological
requirements most adequately is 100 million years. The fanciful figures
arrived at by Winchell, and McGee (who suggested a probable age of 6000
million years) are merely illustrations of how the data could be
twisted to produce impossibly extreme results. The latest estimate, due
to Sollas, includes an allowance of 25·4 million years for the duration
of pre-Cambrian time—the same period as that which has apparently
elapsed since. A further allowance is made for unconformities, those
gaps in the sequence which are unrepresented by sediment. Taking the
great unconformities as probably numbering six, each being equivalent
to 40,000 feet of sediment, 24 million years are added. For minor
unconformities and interruptions in the record other 5 millions are
granted and the total is thus brought up to 80 million years.
In an attempt to free ourselves from the difficulties with which
this method is beset, we may adopt a mode of procedure similar to
that followed in the last chapter. It was there assumed that the
total volume of the sediments which have ever existed amounts to some
210 million cubic miles. The present annual supply of sediment when
ultimately compressed and consolidated would occupy 0·83 of a cubic
mile. If the present rate of accumulation were reliable, geological
time would then be of the order 250 million years. A still nearer
approach to the truth may be made by calculating on a uniformitarian
basis how long the existing sediments, which we placed at 70 cubic
miles, have taken to form. From the 8,000,000 square miles of igneous
and metamorphic rocks, one cubic mile would be denuded away in 4·54
years, which implies that one cubic mile of consolidated sedimentaries
would be formed in about five years. The age then works out at 350
million years. Making a further correction for the slower rate of
denudation of igneous rocks, this figure may perhaps be doubled.
Finally, there is the correction for average rate in place of present
rate, and to what extent this would increase the estimate it is
impossible to say.
We may revert to the maximum thickness of the sedimentary rocks to
support the estimate of the total volume of sediments which has ever
existed. On the basis of the sedimentation curve, the sediments have
been deposited on an area of 10 million square miles, and if laid down
everywhere at their average maximum thickness, 60 miles, they would
cover about 0·4 of that area. The total volume which can ever have
existed, leaving unconformities out of the question, is therefore, 60 ×
10,000,000 × 0·4 cubic miles, or 240 million cubic miles.
Finally, a crude estimate may be based on the amount of calcium
carbonate which has accumulated in geological time. Several estimates
of the volume of limestones in existence have been made, e.g.:
Dana 18·40 million cubic miles.
Reade 10·00 ” ” ”
Van Hise 6·25 ” ” ”
The limestones now forming make up 14% of the total sediments which
collect on the continental shelves. On the land the proportion must be
lower than this, because limestone is denuded at a rate well above the
average. If we take limestone formations at 10%, the volume would be
about 7 million cubic miles, a figure not far from that of Van Hise.
The calcium carbonate may now be estimated. Limestones contain on an
average 75% and shales and sandstones together about 7%. The total
volume, calculated at density 2·6, would therefore be in round figures
10 million cubic miles.[11] Igneous rock contains 3·43% of calcium, and
if in the process of denudation all of this is dissolved and removed,
the present rate of production of calcium would be equivalent to 1
cubic mile of calcium carbonate in 32 years, on the same basis as in
previous calculations. The time estimate at this rate would be 320
million years.
[Footnote 11: Leith finds 12·5-22 million cubic miles.]
We may now sum up our various results as follows:
1. _Accumulation of Sodium._ MILLION YEARS.
(_a_) Uncorrected quotient Naₒ/Naᵣ 80·8
(_b_) Partially corrected Naₒ/Naᵣ 90
(_c_) Unchloridised sodium alone 180
(_d_) Primary sodium alone 210-340
2. _Accumulation of Sediments._
(_a_) Maximum thickness 300
(_b_) Total volume which has ever existed 250
(_c_) Total volume now existing 350
3. _Accumulation of CaCo₃._
(_a_) Total volume now existing 320
Not one of these estimates is to be regarded as final; the
uncertainties are too many and too great. The whole trend of this
chapter has been to show that whatever may be the true reading of the
hour-glass of denudation and deposition, it ought probably to be very
much higher than has been generally assumed.
CHAPTER VII
RADIOACTIVITY
Geological application—Röntgen rays—Becquerel
rays—Mme. Curie and the discovery of Radium—The
gold-leaf electroscope—The nature of α-, β- and
γ-rays—Helium and its production from radium—The
heat output of radium—Transformation of the radium
atom—Disintegration theory—The half-life period of
radium and its generation from uranium—The uranium,
thorium and actinium families—Direct measurement of
the heat output—Radioactivity independent of external
influences—Pleochroic haloes—Measurement of radium in
rocks and minerals.
In the limited space at our disposal, only a brief outline of the
salient facts of the new science of radioactivity can be given. It is
necessary to give some account of these recent discoveries because
of their immediate application to the study of the earth itself.
In a science like geology, the function of which is to study the
consequences of energy transformations in the earth’s crust during the
long ages of her gyrations through space, the recognition of sources of
energy previously undreamed of is a stimulant to research which must
profoundly affect the interpretations of the phenomena we observe. For
our present purpose it will be necessary to review only those facts
which have a bearing on the question of the earth’s age, and of its
thermal history.
In 1895 Röntgen gave his first account of _A New Kind of
Radiation_. This extraordinary radiation, now familiar to us as
the X, or Röntgen rays, revealed itself by its remarkable property of
passing through objects which are entirely opaque to ordinary light.
It was found, for example, that a photographic plate was not protected
from its influence by being wrapped in black paper, but was affected
just as though it had been exposed.
This discovery awakened the deepest interest amongst all scientific
workers, and the idea arose that phosphorescence and the Röntgen
rays might in some way be vitally connected. Certain substances,
after being exposed to sunlight, are able to shine in the dark with
a phosphorescent light, and it was this radiation which was examined
from the new point of view. Working on these lines, with uranium salts,
Becquerel found, almost by accident, that they gave out rays which
were capable of penetrating black paper and making themselves evident
by their effect upon a photographic plate wrapped within it. This
observation in 1896 marks the commencement of the harvest of wonderful
discoveries which have revolutionised our conception of the structure
of matter.
[Illustration: FIG. 12.
Taken in the dark by its own rays.]
[Illustration: FIG. 13.
Taken in ordinary daylight.
PHOTOGRAPHS OF PITCHBLENDE.]
The emission of rays was found to be an inherent property of uranium
and entirely independent of the phosphorescence exhibited by certain
of its compounds. Physical and chemical changes made no difference;
the radioactive properties evidently belonged to the atom itself, for
they could be detected under whatever conditions uranium happened to
be present. Mme. Curie at once began an exhaustive examination of all
the other known elements, and came to the conclusion that thorium alone
possessed radioactive properties similar to those of uranium. In the
course of her researches, she found that uranium-bearing minerals such
as pitchblende (uraninite) were far more active in their emission of
rays than could be accounted for by the uranium present. Systematically
following up so significant a fact, she traced the activity to the
bismuth and barium separated from these minerals. In 1898 two new
elements were announced. One, associated with bismuth, was called
Polonium; the other, found in minute quantities with the barium, was
appropriately named Radium.
The spontaneous radiations of these elements, collectively known as the
Becquerel rays, were soon distinguished from the Röntgen rays by their
greater complexity. Their analysis is largely due to the scientific
genius of Rutherford, who has classed them into three well-marked
types, the Alpha or α-, Beta or β-, and Gamma or γ-rays. The instrument
chiefly employed in their detection is the gold-leaf electroscope,
which not only is simple in construction, but is capable of extreme
sensitiveness. Under ordinary conditions the air is a good insulator,
and a charged electroscope loses its charge only very slowly. However,
if radium is brought near the instrument, the discharge proceeds much
more rapidly, the effect of the Becquerel rays being to ionise the
air and so render it conductive to electricity. In the electroscope
this effect can be readily observed and accurately measured, for the
leaf falls at a regular rate, which depends on the intensity of the
radiation. The sensitiveness of this method so far surpasses that of
the most delicately adjusted balance that by its means a quantity of
radium can be detected which would need to be multiplied thousands of
millions of times before the balance would turn to its weight.
It is easy—by interposing between the electroscope and the radium a
gradually increasing thickness of suitable material such as thin sheet
aluminium—to establish the existence of three kinds of rays which
differ greatly in their power of penetrating matter. The γ-rays are, on
the average, about a hundred times more penetrating than the β-rays,
and these in turn are equally the superior of the α-rays. In each case
the penetration in different materials depends approximately on their
density. The three types can also be distinguished by their behaviour
in a strong magnetic field. Giesel showed that the β-rays were
deflected in such a way as to indicate that they consisted of particles
carrying a negative charge of electricity. The α-rays were much more
difficult to deflect, but Rutherford successfully turned them from
their path by applying a very intense field, and from the direction
of their deflection he concluded that they consisted of positively
charged particles. The γ-rays have, however, resisted all attempts made
to alter their course. Their real nature is not yet beyond dispute,
and from the great similarity between their properties and those of
the Röntgen rays they are generally considered to be, like the latter,
either electro-magnetic pulses in the ether, or uncharged corpuscles of
a kind not yet understood.
When an electric discharge is passed through a high-vacuum tube, rays
are emitted from the cathode which, on reaching the walls of the
tube, produce a yellowish-green phosphorescence. The cathode rays
are identical with β-rays in every essential respect, and both are
proved to be tiny, negatively charged particles, called electrons. The
velocities of both cathode and β-particles are inconceivably high;
in the latter case even approaching that of light. Now Röntgen rays,
whatever may be their true nature, are set up by the sudden stoppage of
electrons when they impinge upon the anti-cathode of a vacuum tube. In
the same way it is supposed that the β-rays, by their sudden expulsion,
give rise at the same moment to γ-rays. It is a significant fact in
this connection that the β- and γ-rays are always associated together,
whereas the α-ray may be quite independent.
The story of the brilliant researches which culminated in the
measurement of the mass of an electron, and of the charge it carries,
is one of the most fascinating in the annals of science. But here we
must be content with the bare statement that the electron has only
¹/₁₇₀₀ of the mass of a hydrogen atom. The application of similar
methods to the α-particle has demonstrated that it is of atomic
dimensions, and consists either of a hydrogen atom with a single ionic
charge, or of a helium atom with twice that charge. Other evidence
decides conclusively in favour of the second alternative.
Of all the elements, helium has probably the most interesting history.
It was first discovered in the sun during the eclipse of 1868, but it
was not until 1895 that it was detected in terrestrial materials. In
that year Sir W. Ramsay identified it in uranium- and thorium-bearing
minerals by means of its highly characteristic spectrum. Even then,
before radioactivity had been recognised, the association of helium
with these elements had forced itself upon his notice. In 1902 the
genetic relationship between helium and certain of the radioactive
elements was predicted by Rutherford and Soddy. During the following
year Ramsay and Soddy working in conjunction sought for, and actually
observed, its generation from radium. Using a radium preparation from
which helium was at first entirely absent they were able to prove,
with the aid of the spectroscope, that in the course of a few months
sufficient of the gas had been generated to be identified. This
important discovery has received abundant confirmation during the last
decade, and to-day we even know the number of helium atoms which a gram
of radium emits in a given time. Moreover, the production of helium has
been demonstrated from uranium, thorium and actinium, and from most of
the other radioactive elements which are characterised by the emission
of α-rays. The rate of generation of helium will be considered in more
detail in Chapter X, where it finds an important application.
An observation of fundamental importance was made by Curie and Laborde
in 1903 when they discovered that radium is able to maintain a
temperature above that of its environment. The source of this emission
of heat lies in the kinetic energy of the Becquerel rays. Projected
from radium, they are sooner or later completely absorbed by the
matter through which they penetrate. Simultaneously, their energy is
given up to the molecules with which they collide, and it therefore
reappears as heat. By far the greater proportion—over 85%—of the
energy is carried by the relatively heavy α-particles which move with
velocities nearly one-tenth that of light. Knowing their masses and
velocities, and the number liberated per hour from a gram of radium, it
is a simple dynamical problem to calculate their total kinetic energy.
Expressing this quantity in heat units, it is found to be equivalent
to an hourly production of 113 calories. (Ap. A, p. 177.) As we shall
see later, this figure is in close agreement with the results of direct
measurements.
The most remarkable feature about these phenomena is that they appear
to continue unceasingly. Year after year the spontaneous production of
helium goes on, accompanied by a steady evolution of heat. But what
of the atoms that remain? They evidently cannot continue to be the
same element after having given up part of their energy and expelled
from themselves the material particles of the α- and β-rays. A close
examination reveals the genesis of a new element—a gaseous substance
known as radium emanation. Its atom represents the residual fraction
of the radium atom after the separation of a single α-particle. The
emanation is highly unstable; so rapidly does it give up energy and
liberate helium that its original intense activity decreases by half
every four days. In its place arises another product, radium A, of
still more transitory existence. A long succession of similar
transmutations has been traced, each accompanied by a setting free of
energy.
In 1902 Rutherford and Soddy advanced the theory of the disintegration
of the radioactive atoms. The radium atom is essentially unstable and
ultimately breaks up, explosively emitting α- and β-particles. Each
atom has a certain expectation of life which distinguishes it from the
atoms of other radioactive substances; but such is the distribution
of instability that a constant proportion of the total number of
atoms present is always breaking up. Out of a million atoms of radium
a definite number disintegrates each second, and the proportion has
been proved to be invariable in every case it has been possible to
test. From element to element it is this proportion, and therefore the
average promise of life, which varies. An equilibrium between formation
and transformation is gradually attained, and once it is established
a given quantity of radium produces as many atoms of emanation as the
emanation produces of radium A, and so on. When in equilibrium the
longer-lived radio-elements are present in greater quantity than those
of shorter lives in order to compensate for their comparatively slow
rate of decay, and so to enable them to keep pace with their more
rapidly changing associates.
We may now calculate the average life period of radium. The volume of
emanation in equilibrium with one gram of radium has been measured and
found to be O·58 cubic millimetres—less than the volume of a pin’s
head. The rate of change into radium A also lends itself to direct
measurement, and from these two quantities it can be shown (see Ap. A,
p. 179) that in one year ¹/₂₆₀₀ of the radium originally in existence
must undergo transformation. The same result can be arrived at more
directly. We know the number of atoms in a gram of radium and also the
number of helium atoms expelled in the course of a year. The proportion
which has suffered disruption follows at once, and is found to be
¹/₂₅₀₀.
From evidence of this kind it is concluded that after about 1850 years
only half of any given quantity of radium then maintains its identity
as that element. Now it is clear that radium must have a progenitor by
which it is formed as fast as it decays, or otherwise there could now,
after the lapse of millions of years, be none in existence.
That progenitor is undoubtedly uranium—an element which disintegrates
so slowly that its half-life period is three million times as long
as that of radium. This association is revealed by the study of
radioactive minerals. For every gram of uranium there exists in
equilibrium with it, 3·4 × 10⁻⁷ grams of radium. The constancy of
this proportion points to a genetic connection and admits of no other
interpretation. The generation of radium has been experimentally
verified by Soddy, who observed it after a number of years in a uranium
preparation originally quite free from it. In practice this work is
complicated, and the desired effects are retarded, by the intermediate
production of another radio-element, ionium, which has an average life
many times longer than that of radium. The time taken to establish
equilibrium is therefore very long. The primary minerals from which the
equilibrium ratio of radium to uranium is deduced, have existed
for hundreds of millions of years and so far as they have remained
unaltered by percolating waters they afford a safe guide. But in
secondary minerals, like autunite, the age of which is often to
be reckoned only in thousands of years, the equilibrium stage has
not always been reached, and the ratio between the two elements is
consequently somewhat lower than that found in the older minerals.
In the accompanying diagram the complete family of uranium is
summarised together with the half-life period of each member and the
rays it emits. Another series of elements related in the same way
is derived from thorium which is a parent of even longer life than
uranium. As to the final products of transformation, which, being
stable, ought to survive and in the course of time to accumulate,
helium is the only one which has been directly observed. That lead is
the end product of the uranium series is most probable, as we shall see
in a later chapter. Of the stable element to which the thorium series
ultimately leads we are still in ignorance. Neither lead nor bismuth,
which suggest themselves by their appropriate atomic weights, satisfy
the conditions, for their presence in a mineral bears no relation to
the quantity of thorium (see p. 190).
The actinium group of radio-elements calls for little notice here.
Boltwood finds a fixed ratio between actinium and uranium in minerals,
and the former would therefore appear to be a descendant of uranium
just as radium is. Actinium, however, does not fall in the main line of
descent. There is some probability that it marks the beginning of a
collateral series, also springing from uranium. The proportion of
uranium which disintegrates in this direction rather than towards
radium can only be very small.[12] It is possible that uranium atoms, or
those of one of its products, may be of two different types, according
to their internal structure, and that on disruption each may give rise
to an independent chain of elements.
[Footnote 12: About 8%.]
The heat output of radium and its relation to the energy of the
α-particle has already been mentioned. The first direct measurement
of the heat generated was made by Curie and Laborde. Using suitably
constructed calorimeters, they were able to demonstrate a regular
evolution of about 100 calories in an hour from a gram of radium—heat
enough to raise the temperature of an equal mass of water from freezing
to boiling-point. The radium in these experiments was in radioactive
equilibrium with the emanation and radium A, B and C, so that
altogether four α-particles were disengaged for each atom of radium
which transformed. The approximate correctness of this first estimate
will be seen from the following table, in which the best results up to
date are quoted:
1903. Curie and Laborde 100 }
1904. Rutherford and Barnes 110 } gram calories
1904. Runge and Precht 105 } emitted per
1908. v. Schweidler and Hess 118 } hour per
1909. Duane 120 } gram
1911. Duane 117 } of radium
1912. St. Meyer and Hess 132 }
The correspondence of these results with the theoretical figure, 113
calories, which represents about 85% of the energy,[13] is eminently
satisfactory (see Ap. A, p. 177).
[Footnote 13: Atomic recoil 6%, β-rays 4%, γ-rays 5%, approximately.]
The disintegration theory affords a simple explanation of the origin
of this energy. From the new point of view the atom is no longer
indivisible, no longer the ultimate foundation of all matter; rather is
it regarded as a tiny universe of electrons, a whirling assemblage of
charged particles. These minute entities move with very high velocities
within the atom itself. In general the energy is locked up, but in the
radioactive elements the atomic structure is unstable and from time
to time some of the energy can escape. The intrinsic energy of the
atom itself is the real source of the heat liberated by radium, and
the apparent permanence of its output is simply an expression of the
immense supplies of energy which an atom can store within itself.
The essential features of radioactivity are two: the spontaneous
emission of Becquerel rays, and the atomic character of the change. The
radiation from a radioactive substance is proportional to the quantity
of the substance and to nothing else. Temperature, being a function of
molecular movement, is without influence upon atomic transformation.
Experiments made at temperatures ranging between that of liquid air and
1600° C. show that the same law of disintegration holds throughout.
Similarly, the effect of very high pressures indicates that these
inter-atomic processes are quite independent of any external influence.
Whether in solution or in chemical combination, whether taking part
in energetic chemical reactions or suffering bombardment by its own
radiations, the atom continues to obey the law which determines its
life period, unmoved by any experiences through which we can oblige
it to pass. The radioactive properties are simply superimposed upon
the chemical and other properties of the substance, and as far as
laboratory conditions are concerned, they are entirely spontaneous and
can in no way be affected.
Whether or not radioactivity is a general property of atomic
matter cannot yet be announced. Potassium and rubidium are feebly
radioactive—a thousand times less than uranium—and emit β-rays. It
has been thought that some other metals may share in a minor degree the
same attributes, but up to the present no reliable results have been
forthcoming.
In its geological applications radioactivity possesses a great
interest. The most important bearing of all lies in the part it
plays as a source of terrestrial heat. Radium is widespread amongst
the surface materials of the earth’s crust. Rocks and soils, and
natural waters all contain radium as a constituent, widely diffused
but always present. In the following two chapters the influence of
radio-thermal action in the earth and sun, and on speculations as to
their ages, will be dealt with. Before passing on to these cosmic
aspects of the subject, a brief account may be given of the way in
which minute quantities of radium can be detected and accurately
measured—quantities which in rocks average little more than a
billionth of a gram in each gram of the rock.
Radium itself is not directly dealt with; it is the emanation in
equilibrium with it which is actually measured. The emanation is
separated from a suitable amount of rock, mixed with air, and
introduced into an electroscope. After about three hours, the emanation
has generated radium A, B and C in their equilibrium amounts, and the
emission of three sets of α-particles then steadily proceeds. In the
electroscope, the rate of fall of the changed leaf depends directly on
the conductivity of the air or on the number of ions which are utilised
as carriers of electricity. Now each α-particle, in virtue of its
enormous energy, is capable of producing in air about 100,000 ions, and
the total number present is therefore determined almost wholly by the
number of α-particles. The β- and γ-rays certainly have an ionising
effect, but so small that in this connection it may be ignored. To be
quite sure that all the ions are taking an active part in the discharge
and collapse of the leaf, it is only necessary to observe the latter
before its divergence from the central support falls to less than, say,
10°. The rate of collapse can easily be gauged by observing the leaf
through a microscope with a divided scale attached to the eyepiece,
the time during which the leaf passes over a given number of divisions
being taken with a stop watch. The rate of fall is proportional to
the conductivity, which depends in turn on the number of α-particles
expelled, and therefore on the quantity of emanation present in
the electroscope; this quantity, provided that it was initially in
equilibrium with the radium in the original rock, determines the amount
of that element which was present. A constant can be determined once
for all by experiment with known materials, which directly connects the
rate of fall in scale divisions per hour with the equivalent amount of
radium or uranium in grams. Some idea of the delicacy of the method may
be gained from a particular example. In the apparatus devised by Prof.
Strutt and used both in his own investigations and those of the present
writer, a leakage of one scale division per hour corresponds to 9 ×
10⁻¹³ grams of radium.
To use this apparatus it is necessary to bring the rock into solution
before the emanation can be extracted. The solution is stored up for a
few weeks until the equilibrium amount has been generated and the flask
A containing it is then attached to the water condenser B (Fig. 14).
The emanation can be expelled by vigorous boiling, and greatly diluted
with air, it passes out into the gas-holder C. The stream condenses in
B and drops back. At the end of an hour the cooling water is run out of
B and the steam then drives all the air charged with emanation into C.
In order that none should be allowed to escape back, the connection at
D is closed. Meanwhile the electroscope F has been exhausted and the
emanation is made to pass into it through the tap at E, the gas-holder
being slowly filled with water. After three hours the measurements may
then be made.
[Illustration: FIG. 14.
Apparatus for estimating Radium by its Emanation.]
Joly has recently varied this method in order to save the labour
attending the process of getting a rock into clear solution. The
mixture of finely-powdered rock and fusion mixture is heated in an
electric tube-furnace, and the expelled gases containing the emanation
are drawn straight from the furnace. Carbon-dioxide is absorbed by
soda-lime, and the remaining gas is collected and finally passed
into the electroscope as before. The absence or presence of thorium
makes no appreciable difference in these measurements, for the
life of its emanation is many thousands of times less than that of
radium emanation. Joly has utilised this fact in his solution-method
for determining minute quantities of thorium in rocks and minerals—a
method similar in principle to that described above for radium.
The presence of radioactive elements in rocks sometimes reveals itself
in a most beautiful way. In mica, cordierite, hornblende, chlorite,
tourmaline, and a few other minerals, small circular spots known as
pleochroic haloes are sometimes seen. A tiny crystal can generally be
detected in the centre, usually of zircon, but sometimes of apatite,
epidote, rutile, or sphene. Until quite lately the nature and origin of
these intensely pleochroic spots was entirely unknown. Joly showed in
1907 that they are due to the radioactivity of tiny inclusions around
which they spread spherically outwards. The α-rays discharged from
the minute central crystals are able to ionise the biotite (or other
mineral) and this effect, to which the colouring action is due, spreads
just as far as the α-particle can penetrate. The range of the different
α-particles from the uranium family is given overleaf, both for air and
for biotite.
-----------------------+-----------------------
| RANGE IN MILLIMETRES.
+---------+-------------
SOURCE OF Α-PARTICLE. | |
| IN AIR. | IN BIOTITE.
-----------------------+---------+-------------
Radium C | 70·6 | 0·033
Radium A | 48·3 | 0·023
Ra. Emanation | 42·3 | 0·020
Radium F | 38·6 | 0·018
Radium | 35·4 | 0·017
Ionium | 28·0 | 0·013
Uranium 1 and 2 | 27·0 | 0·013
-----------------------+---------+-------------
Now Bragg has shown that the ionising effect of an α-particle is
greatest just before it comes to rest. We should therefore expect to
find a number of intensely coloured spherical shells existing around
the central crystal and corresponding in each case to one or other of
the range limits here given. In section, and magnified 700 diameters,
the appearance of the shells would be as in the following diagram.
[Illustration: FIG. 15.
Pleochroic Haloes in Biotite due to Uranium and its transformation
products. Magnified 700 diameters.]
The dimensions of the actual haloes are exactly in agreement with
the distances to which the different α-particles can travel, and
occasionally the successive spheres of ionisation and colouring are
beautifully developed. In the biotite of Co. Carlow granite, Joly has
found several very perfect examples, and by his courtesy four of his
photographs are reproduced in the Frontispiece. Not only is the proof
that the haloes are due to α-rays conclusive beyond question, but
the correctness of Bragg’s laws from which the range in biotite is
calculated are established in an unexpected way. Thorium haloes are
also found (as in Fig. 2, upper left-hand part of the field), and these
again have diameters in perfect accord with the demands of theory.
A point of interest in the history of igneous rocks is that the haloes
disappear when the rock is heated. The presence of well-formed haloes,
therefore, implies the maintenance for very long periods of fairly
uniform conditions. So slowly do these haloes form that it may become
possible, when they have been further investigated, to give a rough
minimum estimate of the age of the minerals in which they occur. The
radioactivity of a zircon is much more intense than that of the rock
enclosing it, but the smallness of the quantities involved is such
that two or three weeks may elapse between the expulsion of successive
α-particles or helium atoms. By the accumulative effects of millions
of these atoms a spherical halo, faint in youth, but assuming a deeper
tint with age, is gradually produced.
CHAPTER VIII
THE THERMAL ENERGY OF THE SUN
The sun’s heat and the conservation of energy—Mayer’s
meteoric hypothesis—Helmholtz and contraction under
gravity—The earth’s dependency upon solar radiation—The
work of Kelvin and Ritter—The insufficient contributions of
atomic disintegration—Arrhenius’ view of the importance of
molecular energy—Cyclic development of the universe.
Before the doctrine of the conservation of energy was established, the
steady radiation of solar light and heat was not, in its quantitative
aspect, a phenomenon to be wondered at. Regarding the sun merely as a
gigantic fire, philosophers such as Leibnitz and Kant were satisfied
that the intense emission of energy was sustained simply by combustion.
As soon as the chemistry of combustibles came to be studied, it was
at once evident that the energy derived from burning alone would be
hopelessly insufficient. If the sun had been originally a colossal
mass of the most powerful explosives known to us, then, under the
most favourable conditions for maintaining the present output of
heat, all would have been at an end within a few thousand years. The
total amount of available energy would not have sufficed even for the
historical period—a period which is merely a ripple in the vast ocean
of geological time.
What, then, can be the source of the energy which for millions of years
has enabled the sun to bathe the earth in a welcome glow of light and
heat? How is the loss to be accounted for? For how long can the sun
continue to radiate its energy without becoming perceptibly colder?
These critical questions must have appealed to Mayer with some force
when he recognised the truth that energy could neither be created nor
destroyed. For the first time a sound explanation became an imperative
demand. Mayer realised that in the collision and friction of bodies,
heat energy is acquired in strict equivalence to the energy of motion
which has apparently disappeared. He saw that the mechanical generation
of heat would be of vastly greater importance in cosmic evolution than
the limited possibilities of combustion. A piece of coal falling into
the sun from infinite space would yield, by the stoppage of its motion,
six thousand times as much heat as it could provide by burning.
Applying these principles, Mayer thought that the sun’s heat might be
traced to the kinetic energy of swarms of meteorites. These bodies
falling into the sun with enormous velocities, would be competent, if
only the supply were ample, to generate the heat annually required.
Kelvin at first also advocated this view, but he was soon compelled to
relinquish it in favour of another explanation less at variance with
known facts. The observations of astronomers were all against there
being a circulation and influx of meteorites to the extent required by
the sun if income and expenditure of heat were to balance. Comets would
suffer resistance in their passage round the sun. The rotational
velocity of the sun would be constantly impeded, and probably, far back
in the past, it would have been brought to a standstill. The sun’s mass
would be appreciably increased every year, and an immediate effect
of this would be to hurry up the earth in its orbit, so that each
year would be notably shorter than the preceding one. Happily for the
stability of the solar system, there is no evidence for an infalling of
meteorites on the scale first contemplated.
In 1856, Helmholtz, another early worker in the domain of energy, found
a more satisfactory escape from the dilemma. Instead of looking outside
the sun for the origin of the heat supply, he sought for an internal
source, and found one—certainly a more efficient substitute—in the
contraction of the sun’s diameter under its own strong gravitation.
Knowing the amount of heat annually radiated, it is easy to calculate
that a shrinkage of 1000 feet would make up the loss for five years.
The decrease in the apparent diameter would, at this rate, never become
detectable in human experience. Helmholtz imagined a time when the
sun existed as a nebula spreading far out into space. As it slowly
cooled and contracted, the mechanical work of shrinkage would reappear
as heat. Assuming the present sun to be a globe of uniform density,
Helmholtz calculated that its past history must have been restricted to
about 20 million years.
The annual output of heat had been determined by Pouillet, and his
result, which was too low, was used by Helmholtz in this estimate. The
solar constant of radiation is measured by the heat in calories, which
would be absorbed in one minute by a surface of one square centimetre
placed outside the earth’s atmosphere at right angles to the sun’s
rays. Allowance is made in this way for the absorptive effects of gases
and of the load of dust held by the lower strata of the atmosphere.
Pouillet’s value in these units was 1·76 and the results of subsequent
experiments, made up to 1905, varied between his figure and 4·1.
This was unsatisfactory, and, under the auspices of the Smithsonian
Institution, work has recently been done to clear up the discrepancy.
The constant is now known with some certainty to be 1·95. Using this
figure the duration of the sun’s heat would, according to Helmholtz, be
limited to 18 million years.
The surface temperature of the earth can owe but little to its internal
energy. Taking the temperature gradient at 1° C. in 32 metres, and the
average conductivity of rock as 0·004, the temperature maintained by
this flow of heat alone would reach only 34° Absolute (239° C. below
zero). It is evident then that the genial warmth of the greater part
of the earth’s surface is maintained almost wholly by the absorption
of solar radiation. For this reason the active life of our planet is
intimately bound up with that of the sun, and any age limit assigned
to the latter becomes a still more embarrassing restriction in its
application to the earth.
We may now return to our discussion of the sun’s vast expenditure of
energy, armed with data worthy of confident acceptance, and with the
knowledge that, for at least as long as the earth has been a habitable
globe, so long has the sun emitted its life-giving rays at a rate
not very different from that of the present. The evidence of geology
is clear on this point. The geographical distribution of plants and
coral reefs in past ages betrays no sign of a steadily cooling sun. In
some of the oldest sedimentary rocks which are known, the imprints of
raindrops have been found, and the size and force of the latter were
evidently not very different from those which fall to-day by the shores
of seas and lakes. The intensity of climatic forces has remained, on
the average, unchanged.
Kelvin somewhat mitigated the consequences of Helmholtz’s extreme view
in his later treatment of the problem. Helmholtz had assumed a sun of
uniform density; but Kelvin pointed out that, as the density probably
increases enormously towards the centre, the amount of heat which
has been already available may have been very much greater than that
previously calculated. Kelvin’s cautious spirit was not shared by his
contemporaries, who readily accepted the smaller estimates. The more
daring investigations of Ritter, and of other physicists who followed
his lead, did not support any period which exceeded 12 million years.
Ritter showed that as the sun contracted from the nebulous state its
temperature would at first begin to rise. Not only would contraction
supply the energy necessary to sustain radiation, but an even greater
quantity of energy would be available for heating purposes. An
interesting summary of Ritter’s work will be found in the second of
the fascinating little volumes by Arrhenius on _The Life of the
Universe_.
Geologists found no consolation in these speculative studies, and
even from Kelvin’s more liberal allowance of time, an element of
embarrassment was not absent. While an annual shrinkage of the sun’s
diameter by 200 feet would suffice for the present, yet, unless at
some time the sun’s temperature begins to fall, it is not clear why
shrinkage should continue. Cooling is none the less certain because it
is temporarily delayed, nor because its rate is for a time diminished.
Increasing density would gradually put an end to effective contraction,
and the sun would then cool as a white-hot ball would do—its capacity
for replenishing its losses having been exhausted for ever. If this
were all, then, in the course of a few more million years, an icy
death would overtake the earth. In the last gleams of the fading solar
twilight our planet would disappear—a barren and frozen world.
More recent views lead to a less pessimistic outlook, and the
twilight of the sun, though ultimately inevitable, is removed to
an indeterminately remote future. Gravitation is manifestly an
insufficient cause to maintain the sun’s heat for the periods required.
We need a supply not for less than a dozen million years, nor even for
the 100 million years which would have satisfied geologists a decade
ago. Some source a hundred times as fruitful as contraction under
gravity is required. Happily there is no longer any need to regard the
sun as a serious difficulty, for there are locked within its atoms and
molecules stores of potential energy capable of fulfilling every
terrestrial requirement.
The importance of radio-thermal action in the sun was pointed out by
Rutherford and Soddy in 1903. A month or two later, W. E. Wilson made
a calculation of the amount of radium which, if distributed throughout
the sun, would entirely explain the evolution of its radiant energy.
He found that 2·5 parts of radium in every million of the sun’s mass
would be necessary. In uranium the equilibrium amount of radium is only
0·34 part in a million, so that, even if the sun consisted entirely
of uranium and its disintegration products, the heat generated would
account for only one-seventh of the total expenditure. Wilson used in
his calculation a solar constant of three calories per minute, which
is certainly too high. His result must also be modified to allow for
the heating effects of the other radioactive bodies. According to the
most recent determinations, a million grams of uranium would give out
77 calories per hour, and in the same time a million grams of thorium
would give out one-third as much, each element being in radioactive
equilibrium. The energy from the sun when divided throughout its mass
averages 300 calories per hour per cubic metre. To sustain this steady
evolution of heat four million grams of uranium would be required. The
average weight of a cubic metre of solar matter is only 1·44 million
grams, so that by no possibility could more than one-third of the sun’s
heat be accounted for by radioactivity.
That radioactive bodies do exist in the sun can admit of little doubt.
Helium was first known as a solar element and its abundance suggested
the presence of those radioactive elements from which it could have
been generated. Direct spectroscopic evidence has not yet revealed any
traces of radium, and, indeed, so minute are the quantities involved
that until recently there was little hope of detecting it in this way.
Uranium lines have now been found in the sun’s spectrum, and unless the
laws of radioactivity are totally different at the temperature of the
sun, we may safely assume that uranium exists in equilibrium with its
associated elements. The sun’s radiation is destitute of the Becquerel
rays, but this in no way denies their emission from solar matter.
Before reaching the earth, the rays would be obliged to pass through
both solar and terrestrial atmospheres, and the latter alone would be
more than sufficient to absorb them completely.
In the materials of the earth’s crust, uranium only averages about one
part in 150,000, and thorium one part in 50,000. If the proportions of
these elements which enter into the constitution of the sun are of the
same order, their contributions to the sun’s energy can only be very
small. The importance of radio-thermal phenomena is not felt until
cooling has progressed to a more advanced stage, as exemplified by the
earth, when the heat lost is balanced against that set free by atomic
disintegration. We must therefore find some other means of escape from
the embarrassment of a rapidly cooling sun.
Arrhenius has made a bold attempt in this direction. He calls to his
aid a universal law, first enunciated by Le Chatelier, which may be
stated as follows: _If a system in equilibrium is subjected to
external influences which disturb its equilibrium, then the internal
reaction within the system will be such as to oppose the external
influences_, i.e. the normal effects of the latter will be partially
overcome. This general statement may be illustrated by the particular
example to which the chief appeal is made. In the chemical changes
which constitute combustion, heat is evolved, and the reaction is said
to be exothermic. But the heat so liberated tends all the while to
prevent the reaction from proceeding, and were an external source of
heat of sufficient intensity applied so as to raise the temperature,
the compounds previously formed would be again separated into their
constituent elements. The liberated elements possess a greater quantity
of intrinsic energy than when they are united to form a compound. That
is to say, not all the heat supplied is able to exercise its normal
effect of raising temperature; the internal reaction is responsible
for the withdrawal and absorption of _part_ of the energy. If
now still more heat is applied the same opposing tendency continues,
and the elements will again combine, this time forming endothermic
compounds characterised by further absorption of heat, and consequently
by higher intrinsic energy than that which the free elements possessed.
The system shows a conservative disinclination to be made hotter, and
as more and more heat is supplied, enormous quantities of energy are
accumulated in the recesses of the molecule itself. In the case of
water, the general tendency is well illustrated. Ice at the freezing
point absorbs 80 calories and becomes water at the same temperature.
Water at boiling-point absorbs 540 calories and becomes steam. This
in turn, when raised to about 3000° C., dissociates into hydrogen
and oxygen, the absorption of energy being 3800 calories. Although
laboratory conditions do not allow us to experiment further, there are
no grounds on which to suggest that this is the end of the process.
While matter thus opposes an increasing temperature by its internal
reactions, it nevertheless resists the reverse change quite as
actively, and for the same reason. If an intensely energetic exothermic
compound be allowed to cool, it will strenuously refuse to do so at
numerous stages. The energy previously gained is emitted steadily or
explosively according to the thermal environment.
In discussing the constitution of the sun and the source of its
powerful radiation, this _Law of Reaction_, as it is called,
finds a pertinent application. With its help Arrhenius has pointed
out the path which appears to lead us safely out of the difficulty in
which we were left by Helmholtz. The chromosphere which, disregarding
the mysterious corona, constitutes the outermost strata of the solar
atmosphere, is largely composed of free elements at a temperature of
6000°-7000° C. Lower down in the photosphere, 9000° C. is probably
attained. Temperature and pressure both increase enormously with
depth, and indeed, the average solar temperature has been estimated at
a thousand times that of the chromosphere. Under these conditions it
seems reasonable to suppose that the sun’s interior is characterised
by compounds charged with a high concentration of energy. Brought by
convection currents towards the surface such highly explosive compounds
would dissociate with expansion and an immense evolution of heat. It
may be to explosions of this sort that the prominences are due. These
violently projected gaseous tongues are shot out with velocities which
sometimes reach a thousand times that of the swiftest rifle bullet.
Since energy is proportional to the square of the velocity, it would
appear from this that solar energies are at least a million times
greater than those of our most powerful explosives. It has already been
stated that if the life of the sun depended solely on the latter it
would endure for only a few thousand years. As it is, the energy seems
amply sufficient to last a million times as long. Here then, furnishing
a regular and sufficient income from within, we have found an almost
inexhaustible source of heat, which is competent to maintain the sun’s
present expenditure for inconceivably long ages, as most probably it
already has done in the past.
True, we are in the face of a new difficulty. Whence arose this
absorption and concentration of energy in the first place? It is
evident that once extinct, our sun could not be re-awakened to the
warmth of its former activity merely by collision. Gravitational energy
alone affords no escape from the ultimate _Wärmetod_, the thermal
extinction towards which the universe would appear to be tending. If
the development of the universe be everywhere toward the equalisation of
temperature implied by the laws of thermo-dynamics, the question
arises—Why, in the abundance of past time, has this melancholy
state not already overtaken us? Either we must believe in a definite
beginning, in the creation of a universe furiously ablaze with energy,
or else we must assume that the phenomena which we have studied simply
reflect our limited experience. Toward the latter alternative we
readily incline, the more so because of the hint it affords of cyclic
processes in the scheme of Nature. Not only is energy being diffused;
somewhere, our hazy conception tells us, energy is being elevated and
stored up. With profound insight, Spencer pointed out in 1864 that it
is to the attenuated nebulæ that we should look for the absorption and
concentration of energy. In the universe nothing is lost, and perhaps
its perfect mechanism is the solitary and only possible example of
perpetual motion. In its cyclic development we may find the secret of
its eternity and discover that the dismal theory of thermal extinction
is, after all, but a limited truth.
CHAPTER IX
THE THERMAL ENERGY OF THE EARTH
Temperature gradients, conductivity, and the rate at which
the earth loses energy—Kelvin’s attempt to estimate the age
of the earth—King’s treatment of the problem—Estimates by
Becker and Suzuki—The distribution of radium in rocks—The
thermal equilibrium of the earth—Concentration of radium
towards the surface—Constitution of the earth.
As the earth’s crust is penetrated by bore-holes, tunnels and mines, a
steady increase of temperature with depth is encountered. The rate of
increase varies greatly from place to place as the following records
show:
LOCALITY. DEPTH. TEMP. GRADIENT.
Anzin, France — 1° C. in 15·3 metres
Wigan 750 metres 1° C. “ 30·0 ”
Sperenberg 1700 ” 1° C. ” 36·5 ”
Mt. Cenis Tunnel 1600 ” 1° C. “ 43·0 ”
Minas Geraes, Brazil — 1° C. ” 86·0 ”
Calumet, Michigan 1430 ” 1° C. “ 12·20 ”
Such a variation would naturally be expected when the chemical
reactions of weathering, cementation and metamorphism are remembered,
and the unequal distribution of radioactive elements in the earth’s
crust. The solution of silicates by ground waters takes place with
liberation of heat, and it has been estimated that 120 calories are
released when one gram of rock is decomposed by weathering. The
processes of metamorphism take place at the expense of the earth’s
heat, and the net result of the complete cycle in which (a) an igneous
rock is eroded, (b) the resulting sediments buried and transformed
into schists and (c) the latter brought again to the surface by earth
movement and denudation, is a running down of energy involving a
permanent loss. Moreover, the earth loses heat from the interior, not
only by conduction, but also by convection. The circulation of ground
waters and the activities of vulcanism—which include the upward
movement of molten magmas, heated waters and gases—all result in the
transference of heat from the interior to the outer zones of the crust.
It is generally accepted, that when cooling by convection is left out
of account, the temperature gradient due to conduction alone is of the
order of 1° C. in 32 metres. If this estimate should be in error, it is
more likely to be too high than too low.
To calculate the rate at which heat escapes from the earth by
conduction from the interior, it is necessary to know the average
conductivity of rocks. This factor, in turn, varies greatly in
different materials, but in the case of the most predominant rocks the
conductivity is accurately known, and the value _k_ = 0·004 may
be accepted as very close to the true average value. The variation of
conductivity with increasing temperature and pressure scarcely affects
the problem. The former tends to diminish the conductivity, the latter
to augment it. As far as our present knowledge goes, these two effects
almost exactly balance each other, and the assumption that
conductivity remains fairly constant with depth is therefore justified.
If _r_ represent the earth’s radius, and _d_θ/_dr_ the
temperature gradient, θ being the temperature, then the quantity of
heat which passes from the surface per second is given by
_d_θ
4π_r_² × _k_ × ——— and can be readily calculated.
_dr_
According to the Laplacian hypothesis, the earth’s original store of
heat was derived from the nebula from which it separated. Kelvin made
the assumption that as the molten globe cooled down it was preserved
by convection currents at a temperature nearly uniform from centre to
surface. On attaining the point of solidification, it would gradually
become solid throughout, most probably starting from the centre, and
only when solidification was complete could the surface continue to
cool further. The problem which Kelvin set himself to solve was this:
Given a solid globe originally at a uniform temperature of 7000° F.
(3871° C.), and subsequently cooling down, to calculate the time which
would be required for the establishment of the present surface gradient.
Taking the most probable values for conductivity _k_, density
of rock ρ, specific heat of rock σ, and temperature gradient
_d_θ/_dr_, he applied his data to Fourier’s differential
equation for the linear conduction of heat—
_d_θ _k_ _d_²θ
——— = ——— · ———
_dt_ ρσ _dr_²
Solving this equation for the unknown factor _t_, the required
time period was found. The high initial temperature was chosen from
very meagre data, as representing a maximum figure for the
melting-point of rock. Kelvin was particularly anxious that his
treatment should provide an over- rather than an under-estimate of
time. The experiments of Dr. C. Barus have shown that diabase, a good
typical rock, becomes thoroughly liquid at 1200° C. If Kelvin had
used this temperature instead of his arbitrary 3871° C., his periods
of cooling would have been reduced to less than one-tenth of those
actually arrived at.
For enormous periods of time, the development of a temperature gradient
would be restricted to the earth’s outer zones. The interior, in
complete thermal isolation, would remain unaffected, its loss of heat
being quite insensible. The limited thickness of the outer shell, in
which cooling would make itself felt, is made clear by the following
figures:
160 miles in 100 million years.
240 ” ” 240 ” ”
320 ” ” 600 ” ”
570 ” ” 1000 ” ”
The different periods which Kelvin favoured in his famous contributions
to this problem have already been mentioned, and are tabulated below.
1862. 96 million years (limits 20-400).
1876. 50-90 ” ”
1897. 20-40 ” ”
Clarence King, in 1893, applied a new criterion to the subject, taking
into consideration the effect of pressure in raising the melting-point
of rocks, and the necessity for an earth which should be stable under
the influence of tidal stresses. Barus had measured the melting-point
of diabase at various pressures, and a law of variation of
melting-point with depth was formulated on his experimental results.
If this law were to hold as far as the centre, diabase would there be
able to exist in the solid state at any temperature below 76,000° C.
Thus the hypothesis arose that solidification would begin at the centre
owing to the high pressure obtaining there. Under these conditions,
however, a temperature gradient was already developed. If the gradient
were to exceed the rate by which the fusion point of rocks is raised
by pressure, the former would catch up, and at a certain depth the
temperature would reach the fusion point and a zone of fluid rock would
be inevitable.
King accepted diabase as a representative rock, and rejected any
distribution of heat which would demand a liquid zone in that part of
the earth’s crust where diabase or similar rocks would be expected to
prevail. This procedure is justified by the consideration that were
such a zone to exist, the earth would be incapable of maintaining tidal
stability, and the crust would break down. King found that in the
admissible cases, the initial temperature of crustal solidification
would not exceed 2000° C., and that the period of cooling, which
would reduce the gradient to that of the present was limited by 24
million years. Higher initial temperature would involve fluidity, and
superior age necessitate a lower surface gradient. The following curves
represent the gradients of Kelvin’s earth of 100 million years, and
King’s earth of 20 million years, in relation to the diabase
fusion point curve. It will be seen that according to King’s argument,
Kelvin’s earth implies the impossible condition of a liquid zone from A
to B.
[Illustration: FIG. 16.
Temperature Gradients in relation to the Fusion point of Diabase.]
In 1910, Becker attempted to deal with the same problem without relying
on the temperature gradient, it being considered that owing to the
presence of radium in rocks the gradient could not be trusted. In its
place he assumes that the crustal strains associated with upheaval
and subsidence are completely relieved at the surface of easiest
fusion—and that according to the calculations of Hayford on isostatic
compensation, the present depth of that surface is 71 miles. At that
depth, therefore, the temperature curve and the diabase curve approach
most closely, so that the additional temperature required to produce
fusion and relief of strain there becomes a minimum. In the above
diagram, C would represent the point of easiest fusion. Becker
justifies his choice of diabase by showing that on the Laplacian law
of density, rocks of this type become predominant at depths greater
than 40 miles, the more acid rocks, lying above, being more refractory.
Tidal stability is provided for by rejecting any temperature curve
which crosses the diabase line in the zone of basic rocks. The most
probable earth, according to Becker, is one with an initial temperature
of 1300° C., which would attain a surface gradient of 1° C. in 42
metres in 60 million years. He concludes that only “a tenth of the heat
emitted by the earth can be ascribed to radioactivity plus all other
exothermic chemical transformations.”
A Japanese estimate of the time elapsed since the molten surface of the
earth began to solidify appeared in 1912. Suzuki makes the assumption
that a thin solid crust has gradually increased in thickness, so that
the latent heat of fusion liberated at the junction of solid and liquid
rock is equal to the heat lost at the surface. The present thickness
of the crust is assumed, on the authority of Milne and Arrhenius to
lie between 30 and 40 miles. Granting these postulates, the thermal
constants for basalt and granite lead to an age of 20 to 60 million
years according to the thickness of the crust and the material of which
it is composed.
It is surprising that Becker and Suzuki should have treated the
problem in this restricted way. The heat evolution attending atomic
disintegration was established in 1903, and in dealing with the earth
this phenomenon must be regarded as one of fundamental importance.
To ignore the significance of radio-thermal energy is to reduce the
problem to a mathematical exercise, interesting, no doubt, but with
little value in its geological application. Let us make a simple
calculation of the quantity of radium, which, if uniformly distributed
throughout the earth, would make good the loss of heat. If Q is the
heat generated per second by the radium in each cubic centimetre, then
we have
_d_θ
4π_r_² × _k_ × ———— = ⁴/₃ × π_r_³Q,
_dr_
whence, Q = 6 × 10⁻¹⁵ calories per second.
= 2·16 × 10⁻¹¹ calories per hour.
Now 1 gram of radium in complete radioactive equilibrium emits 216
calories per hour and consequently all the heat would be supplied by
10⁻¹³ grams per cubic centimetre, or 1·8 × 10⁻¹⁴ grams per gram of
earth material.
RADIUM PER GRAM OF IGNEOUS ROCK IN BILLIONTHS (10⁻¹²) OF A GRAM.
+-----------------+------------+-------------+----------+-----------+
| OBSERVER. | ACID. |INTERMEDIATE.| BASIC. |ULTRABASIC.|
+-----------------+------+-----+-----+-------+----+-----+----+------+
|Strutt |11[14]| 2·59| 4 | 2·25 | 9 | 0·52| 4 | 0.46 |
|Farr and Florence| 3 | 1·83| 4 | 1·68 | 6 | 0·54| | |
|Buchner | 8 | 2·61| 15 | 1·64 | 4 | 0·73| | |
|Fletcher | 4 | 0·85| 20 | 0·85 | 5 | 0·71| | |
|Holmes | 8 | 2·80| | | 4 | 0·85| 10 | 0·51 |
+-----------------+------+----+------+-------+----+-----+----+------+
|Mean |34 | 2·63| 43 | 1·28 | 28 | 0·66| 14 | 0·50 |
|Joly |86 | 3·01| 48 | 2·57 | 31 | 1·28| | |
+------------------+-----+-----+-----+-------+----+-----+----+------+
[Footnote 14: The number of rocks examined in each case is given in the
first of each pair of columns.]
Turning to the rocks themselves, the actual amount of radium is found
to be a hundred times more than we want. Strutt was the first to
discover this embarrassing richness, and his results, with those of
later investigators, are summarised in the adjoining table. Joly’s
results are given apart from those of other observers, for they were
arrived at by the fusion method, and, moreover, separate rocks were
not examined. A composite mixture of typical rock specimens was made
up and a single analysis then sufficed to determine the average radium
content. It will be noticed that Joly’s results are consistently
higher than those found by the solution-method. Joly claims that his
own procedure is more reliable than that followed in the solution
method. Up to 1909, Joly had himself employed the latter method in
the examination of 126 igneous rocks. In striking disagreement with
the results of other workers, he found an average radium content of 7
× 10⁻¹² grams per gram of rock. An explanation of the discrepancy is
not yet forthcoming, but in the light of his most recent work, which
gives an average of 2·5 × 10⁻¹², he has now suggested that his earlier
results be set aside. Measurements of thorium in rocks are not yet
so plentiful as those of radium, and most of our present knowledge
of the distribution of this element is due to Joly and Fletcher. The
most probable averages of the data now available may be summarised as
follows:
------------------+-----------------------------------------
| AVERAGE PER GRAM OF ROCK.
TYPE OF ROCK. +---------------------+-------------------
| RADIUM. | THORIUM.
------------------+---------------------+-------------------
Igneous | 2·5 × 10⁻¹² grs. | 2·0 × 10⁻⁵ grs.
Sedimentary | 1·5 × 10⁻¹² ” | 1·0 × 10⁻⁵ ”
Metamorphic | 2·0 × 10⁻¹² ” | 1·5 × 10⁻⁵ ”
Deep-sea deposits | 5·0 × 10⁻¹² ” |
------------------+---------------------+-------------------
Accepting these figures for igneous rocks provisionally and combining
them with the respective heat emission of radium and thorium in
complete radioactive equilibrium, viz.,
Radium[15] per gram 6 × 10⁻² calories per second.
Thorium ” ” 7·5 × 10⁻⁹ ” ” ”
[Footnote 15: Equivalent to 3,000,000 grs. of uranium.]
it is clear that each gram of the earth’s crust is a source of heat
supplying on an average 15 × 10⁻¹⁴ calories per second on account of
its radium content, and 15 × 10⁻¹⁴ calories on account of its thorium
content. The total heat emission is therefore of the same order in each
case, and amounts altogether to 30 × 10⁻¹⁴ calories per second. The
whole mass of the earth is 6 × 10²⁷ grams, and if this were the source
of as much radio-thermal energy throughout, the supply of heat in 1000
million years would have been sufficient to raise its temperature to
about 40,000° C., and the present gradient should be many times greater
than it is. This conclusion cannot be reconciled with the evidence
afforded by the crustal rocks, both their structure and temperature
gradients being decisively against any such possibility.
There are three cases which may be considered. The earth may be in
thermal equilibrium, gaining as much heat as it loses and cooling only
as the slow decay of the radio-elements permits; or it may be growing
hotter, or, which is very unlikely, it may be cooling more rapidly than
it would do if in radio-thermal equilibrium. The first case is the one
now regarded with most favour. If the earth has cooled at all, and
there seems to be no sound reason why we should altogether abandon that
venerable conception, it must at some time have attained a condition
of equilibrium. With the slow march of atomic disintegration its own
rate of cooling would then keep time. The temperature gradient would be
maintained solely by radioactivity for an immeasurably long period.
The superabundance of radium which seemed to be implied by Strutt’s
original work is certainly, as he then suggested, restricted to the
surface rocks. The interior of the earth must be relatively free from
radium and thorium. It is easy to calculate the thickness of the outer
zone of the earth’s crust, which would suffice to supply the stream
of heat passing to the surface. The temperature θᵣ at any distance
_r_ from the surface is given by the following equation, where
_k_ is conductivity, _h_ the heat production of radium and
thorium per second, in each gram of rock, ρ the density of the rock,
and D the total depth of the radioactive layer:
_hr_ρ _r_
θᵣ = ————(D - ———)
_κ_ 2
At the base _r_ becomes equal to D and the temperature θᴰ is therefore
given by
_h_ρD²
θᴰ = —————
2_κ_
Using the figures given above, the thickness of the radioactive layer
would be restricted to about ten miles and the basal temperature would
reach only 250° C. This result cannot be held to express the facts, for
there can be no doubt that the radium and thorium content decreases
with depth for the same reason that the type of rock varies with
depth. A glance at the table on p. 130 will show that there is a rough
proportionality between the acidity or percentage of silica of a rock
and its radium content. The more basic rocks are much poorer in radium,
and, as would be expected, in thorium also. Now we have good reason to
suppose that the more deep-seated rocks of the earth’s crust are of
basic and ultra-basic composition, and that below the 30-mile crustal
zone they are exclusively ultra-basic, perhaps similar in composition
to the material of stony meteorites, with which they agree almost
exactly in density (3·4). This information, which it might be thought
would be for ever withheld from us, is derived from the study of
earthquake waves. The latter in passing through the earth’s interior
carry with them a record of the type of material they have penetrated.
Within the stony zone, which extends downwards for several hundred
miles, and separated from it somewhat sharply, lies the heavy core of
the earth (density about 7·8), probably of metallic composition, like
the iron meteorites. If we may judge from the latter, this nucleus is
entirely free from radium, and that there is safety in this analogy is
indicated by the very low radium content of such native iron as has
found its way in basaltic magmas up to the surface.
We have already seen (p. 30) that a first differentiation of the
original heterogeneous material from which the earth was built would
result in the formation of a metallic core surrounded by a stony zone.
The further differentiation of the latter, whereby the crust with its
abundant variety of acid and basic rocks was developed, is of too
complex a nature to be considered here. It is very probable that as the
more siliceous constituents separated and became concentrated towards
the surface, they carried with them their store of radio-elements.
In this way, basing our ideas on evidence quite independent of the
temperature gradient, we are led to the remarkable conclusion that the
radium and thorium of the earth are to be found almost exclusively in
the earth’s crust. The most probable depth of the radioactive layer
may therefore be placed at 30 miles and the basal temperature in this
case would be about 750° C., which would be more in accordance with the
requirements of volcanic phenomena. Moreover, it must not be forgotten
that the heat lost by the upward movement and convection currents
of rock magmas, heated waters and gases, has also to be accounted
for. The radio-thermal equivalent must be substantially increased to
include this phase of the subject. The basal temperature of 750° C.
is only a minimum, and the higher temperatures demanded by geology
are not therefore inconsistent with the facts. However, until more
data are accumulated, it would be rash to attempt to deduce the exact
distribution of the radio-elements in the crust, but already we may
assert with confidence that the crustal average is somewhat lower than
that of the surface rocks in which granitic types form so large a
proportion. What the average actually may be cannot yet be decided.
Kelvin’s problem must now be reversed. It is impossible to deduce the
earth’s age from its thermal condition. We can only say that the age
must be very much greater than Kelvin calculated. The new problem which
presents itself is to determine the thermal history of the earth,
accepting its antiquity as a known or partially known factor. The
science of radioactivity is a welcome addition to the tools which the
geologist employs in his difficult task of elucidating the earth’s
history, and it is peculiarly valuable in helping him just where he has
hitherto had most cause for despair.
CHAPTER X
RADIOACTIVE MINERALS AND THEIR AGES
The rate of helium production from uranium and thorium—Lead
the final product of the uranium family—Its accumulation
in geological time—Lead and helium ratios as a measure
of geological time—Assumptions to be granted—Value of
analyses in deciding quality of material for estimating
ratios—Necessity for fresh, stable, primary rock
minerals—Strutt’s work on the helium ratio—The lead
ratio—Boltwood’s collection of analyses—Examples, with
geological data—The author’s work on the Devonian minerals
of Norway.
As we saw in Chapter VII the α-particles which are emitted at certain
points in the line of descent of the radioactive elements have been
identified with helium. Fortunately the evidence is conclusive, for
upon this identification depends the latest and most elegant method yet
devised of measuring geological time.
When all the members of a genetically related series of radio-elements
are in equilibrium, the transformation proceeds in such a way that an
equal number of atoms of each element disintegrates in the same time.
When an atom of uranium disintegrates it does so with the production
of two atoms of helium. At the same time, as indicated in the diagram,
page 190, each of six other members of the family also emit a single
atom of helium. Consequently, the total number of helium atoms which
are liberated in the course of the complete transformation of a
single atom of uranium is eight. From this result and the remarkable
measurement made by Rutherford and Geiger—that 3·4 × 10¹⁰ α-particles
or atoms of helium are expelled per second from a gram of radium—it
is possible to calculate that the annual production of helium from
a gram of uranium in equilibrium with all the other products of the
series is 10·7 × 10⁻⁸ cubic centimetres. Measurements of the ionising
power of thorium and its chain of dependent elements may be utilised to
calculate the same rate in the case of the thorium family. It is found
that one gram of thorium is equivalent in helium generation to 0·26
gram of uranium. The details of these calculations will be found in
Appendix A.
While there is little uncertainty in these indirect results, it is
gratifying to know that Prof. Strutt, during 1909-10, verified them
both by a direct appeal to experiment. In certain minerals which
contain radioactive constituents, the evolution and accumulation of
helium must have been steadily proceeding for very long periods. Before
a direct determination of the rate of helium production can be made, it
is evidently necessary to use material entirely free from that element.
Strutt worked with richly radioactive minerals, from large quantities
of which he expelled completely the accumulated store of helium. This
was done by preparing solutions with every precaution to avoid the
presence of undissolved particles of mineral, and afterwards boiling
them till the helium was removed. The solutions were then put aside
until a fresh supply of helium had been generated in sufficient
quantity to be detected and measured. To isolate the gas from the
solutions and accurately to determine its volume was obviously a matter
of great experimental difficulty, so minute were the volumes dealt
with. However, as the result of the exquisite delicacy of his methods,
Strutt brought his experiments to a successful issue.
The minerals used were pitchblende and thorianite, the former
containing uranium alone, and the latter both uranium and thorium.
The annual production of helium per gram of the parent element (in
radioactive equilibrium with its respective family) was found to be:
(_a_) in the case of uranium:
10·6 × 10⁻⁸ ccs. or 1·88 × 10⁻¹¹ grams.
10·7 × 10⁻⁸ ccs. was the calculated estimate.
1 cc. would therefore be formed in 9,600,000 years.
(_b_) in the case of thorium: 2·4 × 10⁻⁸ ccs.
1 gram of thorium is therefore equivalent in its
rate of helium generation to 0·23 grams of uranium,
the calculated estimate being 0·26 grams.
The remarkable concordance of these results with the theoretical
requirements is an eloquent tribute to the refined methods and
experimental skill with which the measurements were carried out.
Experiments carried out by Boltwood and Rutherford in 1911 afford
an equally striking confirmation of the conclusions on which the
theoretical results were based. They measured the rate of production
of helium from radium, the latter being in equilibrium with its
early disintegration products, three of which also emit α-particles.
Radio-lead and polonium were completely removed. The annual evolution
of helium to be expected was 158 cubic millimetres. The first direct
determinations were made by Sir James Dewar in 1908, and his best
results corresponded to 169 cubic millimetres. Boltwood and Rutherford
arrived at a much closer agreement, their figure being 156 cubic
millimetres.
These results cannot fail to inspire the conviction that our atomic
theory of matter is essentially correct. We are in possession of two
experimental facts. The number of helium atoms expelled per second from
a gram of radium has been directly counted, and the volume of helium
accumulated in a year has been directly measured. The number of atoms
in a given volume of helium (at N.P.T.) can be deduced at once, and the
calculation is independent of any underlying theory. Calculation gives
2·69 × 10¹⁹ atoms per cubic centimetre; the atomic theory demands 2·72
× 10¹⁹.
Although there can now be no doubt that helium is one of the stable
disintegration products, yet there is no direct evidence as to the
identity of the ultimate products in the direct line of descent. In the
uranium series indirect evidence points to lead with a considerable
degree of certainty, but the end product of the thorium family is still
unrecognised. In every uranium-bearing mineral the parent element
slowly breaks down, while the final product of the transformation
accumulates at its expense. Hence, if lead is the favoured element it
ought to be found in association with uranium in all minerals which
contain the latter. Moreover, in minerals which can be proved to be
of the same antiquity, the amount of lead per gram of uranium should
be constant; further, in minerals of various geological ages the
proportion of lead should vary according to the latter. A mineral which
began its accumulation of lead in pre-Cambrian times should certainly
contain more at the present time than one in which lead has been
collecting only since, say, the Tertiary outburst of igneous activity.
The same statements apply equally well to the case of helium.
In so far as these principles may be used conversely to test the
identity of lead with the ultimate product, they lend every support to
that important conclusion. Dr. Hillebrand, the leading authority on the
analysis of uranium-bearing minerals, has never in the course of a long
experience found uranium unaccompanied by lead. It was this constant
association which led Boltwood, in 1905, to suggest the probability of
a genetic relationship existing between these two elements. In 1907,
Boltwood went farther and showed that for minerals of the same age the
amount of lead for each gram of uranium, or the ratio Pb/U, was, in
general, nearly constant. He collected all the best analyses of primary
uranium minerals, but unfortunately he omitted to give the geological
details of their occurrence. As will be seen in the present chapter,
when the relative ages of the minerals are compared with their lead
ratios, a striking proportionality discloses itself.
The evidence of atomic weights is also favourable to lead. The complete
disintegration of an original atom of uranium may be expressed as
follows:
U ➜ 3He + Ra ➜ 8He + Pb
238·5 3·994 226·36 3·994 207·08
———————————— —————————————
238·5 238·34 239·03
Atomic weights are appended to each symbol, and the totals, which
should be equal, are placed underneath. The agreement is close, but
not as convincing as one could desire. Two alternative explanations of
the discrepancies are suggested. Either lead is not the final product,
or the atomic weights of both radium and uranium are too low by about
0·5. Neither alternative can readily be granted, but it may be pointed
out that since uranium is the heaviest known element and radium follows
not far behind, any impurities whatever, with the exception of thorium,
would have the effect of lowering the observed atomic weights.
Accepting the above equation as substantially correct, the mass of
lead generated in one year from a gram of uranium can now easily be
calculated. For eight atoms of helium, one of lead is produced, or,
mass for mass, six and a half times as much.
Consequently, as a gram of uranium involves the annual production of
1·88 × 10⁻¹¹ grams of helium, the associated lead which remains must
amount to 1·22 = 10⁻¹⁰ grams. If this rate were constant we could find
how long it would take for any mass of uranium to become completely
converted into helium and lead. However, the rate is not constant, but
is proportional at every moment to the quantity of uranium remaining
unchanged. As the parent element becomes exhausted, it disintegrates
more and more slowly.
Now in very considerable periods amounting to hundreds of millions of
years only a very small fraction of the uranium originally in existence
is decayed. For this reason, if only a small proportion of lead or
helium has collected in a mineral since it began its life-history, then
no serious error will be made in assuming their rates of evolution to
have been constant. In minerals which have been in existence for 400
million years the slowing down is only about 5%. If an appreciable
error should arise in ignoring this decline, then in place of the
present day percentage of uranium the time-average must be substituted
(see Ap. A, p. 179); that is to say, the amount of uranium which, if it
did break up at a regular rate, would evolve the same quantities of the
ultimate products.
Having calculated this uranium average, Uₘ, with the necessary
approximation, the total quantity of lead or helium accumulated in a
mineral would then provide a direct measure of its age.
Using lead as the age-index, and knowing its percentage, Pbₜ, in the
mineral, then the time it has taken to collect, i.e. the age of the
mineral, Pbₜ, is given by Pbₜ/Uₘ × 8200 million years.
Using helium as the age-index, both thorium and uranium must be
estimated. The amount of thorium may be conveniently expressed in terms
of uranium, since the latter is four times as active in its helium
production as thorium. The total equivalent quantity of uranium, Uₑ,
is thus known, and thorium then need play no further part in the
calculations. It is found most convenient to measure the amount of
helium, Heₜ, as the volume in cubic centimetres per gram of mineral. In
this notation the age is given by Heₜ/Uₑ × 9·6 million years.
The validity of this procedure evidently demands the granting of
certain obvious assumptions. Our choice of suitable minerals will
not only be limited by these considerations, but the reason for a
particular choice will be justified. The assumptions fall under the
following headings:
(_a_) That no appreciable amount of lead or helium was present
at the genesis of the mineral.
(_b_) That no lead, helium, or uranium has subsequently been
added or removed by external agencies.
(_c_) That no lead or helium has originated by any other
radioactive process than those already suggested.
The first and second suppositions bring up the whole problem of the
origin of minerals. It is possible, by means of a physical examination,
to decide whether a mineral is of the same age as the rock in which it
occurs, or whether it is older or younger. In igneous rocks, such as a
granite, the majority of the minerals are of the same antiquity as the
rock, that is, they date from the period of consolidation of the rock
magma. The component mineral particles of most sedimentary and detrital
rocks existed long before the strata were laid down, whereas the
cementing materials by which they are consolidated are partly furnished
by percolating solutions and therefore may be subsequent to the period
of deposition. In the same category come those ore deposits which
occupy the fissures and crevices of pre-existing formations.
In whatever way a mineral may occur, its history can always be
traced back either directly or by conjecture to an igneous rock, and
it is rarely that it is possible to go beyond the magma from which
such a rock must have consolidated. And even if this can be done in
exceptional cases, there lies behind still another magma to which the
material can be referred. Consequently, the minerals of igneous rocks
are regarded as primary or original, and here we may briefly consider a
few facts relative to their crystallisation from a molten condition.
A molten rock is regarded as a solution in which the numerous
constituents are dissolved one in another. Now certain of the
constituents can only remain in solution provided the latter is, with
regard to them, very dilute. That is to say, they are only slightly
soluble in a solvent composed of the rest of the rock material.
Consequently, substances of which these constituents form an essential
part will, as a general rule, be the first to crystallise; as examples,
zircon, sphene, and apatite may be cited. For the same reason it
happens that the magma does not remain homogeneous, but rejects certain
of the rarer elements which collect together in a subsidiary magma of
peculiar composition. To this concentrate of exceptional constituents,
the gases and water vapour expelled during solidification are also
added, and serve to maintain it in a state of æquo-igneous fusion,
even when the bulk of the rock has already crystallised. The residual
liquors yield the minerals of pegmatites and of drusy cavities. Certain
minerals which are conspicuously rare in the body of the normal rock,
are often developed on a large scale in pegmatite dykes, and it is from
these that the most perfect and beautiful crystal forms are generally
obtained.
Amongst the elements of limited solubility in a rock magma, uranium
and thorium must be placed. The accessory minerals of ordinary
igneous rocks, such as those already mentioned—zircon, sphene, and
apatite—are rich in the radioactive elements when compared with
commoner minerals like felspar and hornblende. In general, the richness
of a mineral in uranium seems to depend on its position in the order of
consolidation. The minerals first to be formed claim the greater part
of the available store. The original surplus, unable to dissolve in the
magma, is held over, and, should it be rich in radioactive ingredients,
uranium- and thorium-bearing minerals may be formed during the later
stage of pegmatitic intrusions. It is a striking fact that, as primary
constituents, these minerals invariably occur in pegmatites associated
with granite or syenite. As examples, pitchblende or uraninite,
thorite, thorianite, and monazite may be mentioned.
We must now consider the part played by lead and helium during the
genesis of minerals. Before the consolidation of the magma, both
these elements must, of course, have been generated within it for
an unknown period. As to the effect of physical conditions upon
radioactive transformations it has already been shown (p. 102) that
all the evidence points to the conclusion that these atomic changes
are independent of the temperatures and pressures under which a molten
magma exists. The helium already present at the time of crystallisation
appears to behave physically in no way different from the other gases.
There is no evidence that it tends to congregate in any particular
mineral. A small proportion may be distributed through the resulting
rock, but probably the larger share is expelled.
The lead which may be originally present follows a similar course.
The metal is rejected, not only by the primary magma but, with rare
exceptions, by the residual magma also. It finds no definite place
in igneous rocks. Doubtless a certain amount of lead is retained in
the molecular network of crystals, but that amount is not high. In
the rocks of Leadville, Colorado, Hillebrand found an average of less
than 0·002 per cent of lead. In the nepheline syenite of Southern
Norway, using specimens free from minerals which one would expect to
be comparatively rich in accumulated lead, the present writer was able
to determine a percentage of only 0·0004. If, then, there should be
initially a greater quantity of original lead, where are we to look for
it? Probably the most of it goes to form lead-ores, such as galena.
Separated from the pegmatites it appears in the later phases of
ore deposition which follow on the heels of igneous activity. In
company with hot gases, sulphide solutions, and a number of metallic
companions, our lead is carried away and deposited in the fissures
encountered by the mineralised waters. If the agency of magmatic gases
appears to have been an important factor in the production of ore
bodies, the origin of the latter is said to be pneumatolytic. Brögger
has shown that in Southern Norway galena was one of the minerals to be
formed in this way.
In the last phase of this complex series of operations, magmatic waters
contribute their share to the filling of mineral veins, and it is
amongst these hydatogenetic ores that galena is most usually found.
The important point is that lead, for the most part, is drawn from the
primary magma at, or perhaps before, the time of crystallisation, and
it is not until the igneous activities have declined that it again
appears in an active rôle.
Let us now consider the effect of the original distribution of lead and
helium in a newly-formed rock. It will be clear that an analysis of
the rock as a whole would give values of Pb/U and of He/U much higher
than those corresponding to the period since consolidation. Of the
total amounts of lead and helium, part would be originally segregated
in the rock and part would be due to subsequent genesis. In most rocks,
the former part is of sufficient magnitude altogether to invalidate
the use of the ratios as age indices. This difficulty can be avoided
by confining attention to particular minerals—indeed, to just those
minerals which concentrate within themselves the radioactive parent
elements. Within them lead and helium may accumulate to such a degree
that the amount initially present becomes negligible. Zircon from the
Devonian syenites of Southern Norway contains more than twenty times
as much lead as the rock in which it occurs. Roughly, we may say that
since the zircon came into being its content of lead has multiplied
twenty times. Thorite may accumulate a hundred or a thousand times as
much lead as it possessed at first. For the same reason, minerals like
zircon and sphene often contain hundreds of times as much helium as
the rock from which they are taken, and there is little possibility
of error in assuming that they have themselves generated the whole of
their supply.
Another difficulty, and a more serious one, must now be faced. Can we
be sure that for periods of hundreds of thousands years a mineral has
remained comparatively unaltered by external agencies? With regard
to helium there is undoubtedly a tendency to escape. Strutt has
demonstrated that when a radioactive mineral has been powdered, helium
begins to leak, rapidly at first, then at a diminishing rate. Even
crystals washed out of their original matrix showed a considerable
leakage of helium. The observed rate of escape always exceeds the rate
of generation, and it therefore follows that during the life-history of
a mineral the conditions must be specially favourable to the retention
of helium, for otherwise the latter could not have accumulated.
Nevertheless, these experiments prove conclusively that the majority of
minerals do not contain their full store of helium, and it is a matter
for surprise that they contain so much. Consequently, ages deduced from
the helium content of minerals can be regarded only as a fraction of
the true age.
Dealing with lead and uranium, we must consider the tendency to
alteration of the minerals in which they occur. From the surface down
to the permanent level of the ground waters, rock material is subject
to weathering. The more soluble constituents are leached out and
complex silicate minerals are decomposed by the combined action of
water, oxygen, and carbon-dioxide. It is in this belt of weathering
that igneous rocks suffer most change. Many of the primary minerals
are broken down and alteration products take their place. All the
reactions involve considerable increase in volume, and not only are
minerals altered in place, but material is carried away and deposited
elsewhere as secondary minerals. Can we be sure that lead and uranium
have remained untouched during this redistribution? In some cases we
cannot, but fortunately for our purpose many of the most valuable
minerals, like zircon, are dense and exceptionally stable. A mineral is
only stable over a limited range of conditions. It adapts itself more
or less readily to its physical environment. Certain minerals, however,
are much more capable than others of withstanding great changes without
undergoing metamorphism or alteration, and amongst these are many of
the uranium-bearing minerals.
When there has been a migration of lead or uranium, an appeal to
analysis will rarely fail to dispel the difficulty by disclosing the
fact. It is inconsistent with the chemical properties of these elements
that both should have been affected in the same proportion, and hence
the ratio of lead to uranium obtained from different minerals of the
same geological age affords an immediate test of the extent to which
they have suffered from alteration in the course of their history.
If the analyses give consistent results, it can be safely assumed
that the effects of alteration have been inconsiderable; if there are
marked discrepancies the results must be rejected as valueless from
a chronological point of view. A microscopical examination of the
minerals before analysis is a useful safeguard, for in this way altered
material can often be detected. It is clear that reliable conclusions
can only be drawn from minerals which are undoubtedly _fresh_.
Becker has criticised the method by directing attention to a suite of
minerals from Llano Co., Texas. Their geological age is well defined.
The Burnet granites with which they are associated are intrusive into
a series of schists and quartzites, metamorphosed sediments of late
Algonkian time. The Cambrian rocks lie upon this complex and the period
of intrusion is therefore between two limits which are not very far
apart. The lead ratios of these minerals are far from being constant,
as the following examples show:
Yttrialite 1·15
Yttrialite 0·51
Mackintoshite 0·39
Uraninite 0·17
Fergusonite 1·04
Fergusonite 0·30
Boltwood found a satisfactory agreement in four cases, the ratio being
0·17, but, as he pointed out himself, most of the minerals from this
locality are unsuitable, because of incipient or advanced alteration.
The quartzose pegmatites in which the minerals occur are riddled with
alteration products and secondary minerals, and the whole series is
altogether unfavourable to accurate age determination. It is doubtful
whether the apparent agreement of the ratios quoted by Boltwood ought
to be accepted without further verification; for the present they
cannot be regarded without suspicion.
This example shows how the actual results indicate the vicissitudes,
varying from mineral to mineral, which the lead and uranium contents
may have undergone. The method confirms or denies the validity of its
application in every case. Judging from the relative solubilities of
the constituents in question, uranium is likely to be abstracted from
a mineral during the process of weathering more readily than lead,
and consequently the age deduced from a weathered or altered specimen
should in general be too high. A differential effect of this kind would
account for the high ratios given by the Llano Co. minerals.
Strict attention must be paid to the question of origin, and secondary
minerals avoided as carefully as altered primary minerals. Pitchblende
is often secondary, e.g. when it occurs in veins with metalliferous
sulphides. Other examples, of a rather different type, are autunite
and carnotite. Secondary minerals are necessarily more recent than the
rocks in which they occur, and many of them date back to no very remote
period. Autunite is sometimes formed quite near the surface, within
a few inches in fact. Its antiquity cannot therefore be more than a
few thousand years, and in this time a detectable quantity of lead
could not be generated. The traces actually found were probably in the
original possession of the mineral. In an analysis made by the writer,
only 0·06% of lead was found in specimens of autunite from Mozambique,
where it occurs in bright green flakes attached to the large biotite
crystals of pegmatitic dykes. In keeping with the age of the pegmatites
and the high proportion of uranium—45%—it should have contained a
hundred times as much had it been a primary mineral. The paucity of
lead in autunite has even been put forward as an argument against the
contention that lead is the ultimate product of disintegration of the
uranium family. We now see how baseless is this argument when the
origin of the mineral is remembered. It is not surprising that autunite
should contain so little lead; on the contrary, it contains much more
than the uranium can account for in the time at its disposal.
From these considerations it will be obvious that the only minerals
to be chosen as material from which to determine the lead-ratio are
fresh, stable primary rock-minerals. Having decided this, there remains
a third possibility which might cast doubt upon the method. It can be
objected that lead may originate as a product of some element other
than uranium. Analytical results show clearly that thorium cannot give
rise to lead, or a more proportionate relationship between these two
elements would have announced the fact. There is also a possibility
that certain of the longer-lived members of the uranium family may
themselves be segregated in a mineral, independently of uranium. If
so, they would gradually disintegrate, leaving no trace of themselves
other than the residual helium and lead. In a magma containing 10%
uranium, the radium would amount only to 0·0000034%. In actual magmas,
even of pegmatites, the quantity present is always much less than
this, and even if it be allowed that such tiny quantities may saturate
the magmatic solution, the precipitation and concentration in any
particular mineral would not be sufficient to leave an appreciable
residue of lead.
The application of the accumulation of helium in minerals to the
measurement of geological time, was first suggested by Rutherford in
1905, when he wrote: “I think that, when the constants required for
these calculations are more definitely fixed, this method will probably
give fairly trustworthy information as to the probable age of some of
the radioactive minerals of the earth’s crust, and indirectly as to the
age of the rocks in which they are found.”
During the years 1908-10, Strutt examined a great number of minerals,
and determined the helium ratio whenever practicable. His first set
of experiments dealt with phosphatic nodules and phosphatised bones.
These may sometimes contain fifty times as much uranium as average rock
material, and they have a further advantage in that they can be found
in strata of nearly every age. As they frequently consist of fossils
characteristic of the formations in which they occur, their age is
well defined. However, the power of retaining helium is both poor
and variable in the case of these phosphates, and the time relation
is therefore obscured. Such materials never retain more than a small
fraction of the helium which has been generated within them.
More suitable in their power of retention are certain iron ores,
from which significant results were obtained. The helium ratio, and
therefore the numerical age derived from it, showed a marked dependency
upon the geological age of the mineral, as the following examples
illustrate:
---------+---------------------+--------------
MINERAL. | GEOLOGICAL AGE. | MILLIONS OF
| | YEARS.
---------+---------------------+--------------
Siderite | Upper Oligocene | 8·4
Hæmatite | Eocene | 30·8
Hæmatite | Upper Carboniferous | 141·9
Hæmatite | Devonian | 145·2
---------+---------------------+--------------
Strutt next investigated the more compact minerals of igneous rocks,
notably zircon and sphene. Zircon can be obtained from rocks belonging
to several periods of igneous activity, and being a durable and stable
mineral it is peculiarly fitted to retain the helium generated within
it. Even allowing that the helium found does not represent the whole
amount generated, it is unlikely that the fraction lost will vary as
conspicuously as in the case of phosphates. In so far as that fraction
depends on the structure of the mineral it is probably more uniform for
zircon than for most other minerals. The helium ratio ought therefore
to stand in a close relation to the geological age of the specimen.
That it does so is clearly demonstrated by Strutt’s results, which are
given below. The geological ages have been taken from the most recent
literature, and are given in greater detail than those published in the
original paper. Where two periods are bracketed together they are to
be understood as referring to the limits between which the age of the
igneous rock may fall.
-------------------------+---------------------+--------------
LOCALITY. | GEOLOGICAL AGE. | MILLIONS
| | OF YEARS.
-------------------------+---------------------+--------------
Mt. Somma, Vesuvius | { Recent |
| { Pleistocene | 0·1
| |
Mayen, Eifel | Pleistocene | 1·0
| |
Campbell I., N.Z. | Pliocene | 2·5
| |
Expailly, Auvergne | Miocene | 6·3
| |
Brevig, Norway | Devonian | 54
| |
Cheyenne Canon, | { Upper Cambrian |
Colorado | { Archean | 141
| |
Green River, | { Carboniferous |
N. Carolina | { Archean | 147
| |
Ural Mts. | Pre-Devonian | 209
| |
Ceylon | Archean | 286
| |
_Blue Ground_, Kimberley | Archean | 321
| |
Sebastopol, Ontario | Archean | 622
-------------------------+---------------------+--------------
The results for sphene add but little to the above table, for most of
the rocks from which workable quantities of sphene can be obtained are
of pre-Cambrian age. The most notable helium ratio was from a specimen
occurring in the Archean rocks of Ontario, and corresponded to an age
of 715 million years. Summarising all the data afforded by Strutt’s
work, we may graduate the geological column with a time-scale. It must
be clearly understood, however, that the ages as expressed in years
are, in the case of the helium ratio, minimum values only. How much
greater the time represented by the geological periods actually is will
appear from the ages as deduced from the lead-ratio. A few of these are
placed in the table below for comparison.
-------------------------+-----------------------------------
| TIME-SCALE IN MILLIONS OF YEARS.
THE GEOLOGICAL SYSTEMS. +------------------+----------------
| HELIUM RATIO. | LEAD RATIO.
-------------------------+------------------+----------------
Pleistocene | 1 | --
Pliocene | 2·5 | --
Miocene | 6·3 | --
Oligocene | 8·4 | --
Eocene | 30·8 | --
Cretaceous | -- | --
Jurassic | -- | --
Triassic | -- | --
Permian | -- | --
Carboniferous | 146 | 340
Devonian | 145 | 370
Silurian | } | }
Ordivician | } | } 430
Cambrian | } 209 | --
Algonkian | } | 1000-1200
Archean | 710 | 1400-1600
-------------------------+------------------+----------------
The application of the lead-ratio to the measurement of the antiquity
of minerals was first due to Boltwood. He tested its reliability by an
appeal to the best analyses published up to 1907. Some of these, with
geological details, will now be given.
In Glastonbury, and also in Portland, Connecticut, primary uraninite
is found in the felspar quarries. The pegmatite in which the mineral
occurs is associated with a granite which intrudes Lower Carboniferous
strata. It is probably to be referred to the close of the Carboniferous
period, and is certainly pre-Triassic. Five different specimens gave
lead ratios in striking agreement, corresponding to an age of 340
million years.
It should be observed in all the following tables that in calculating
the lead ratios the time-average of uranium has been used, and not the
amount actually present. The statement of analyses is, of course, in
percentage.
----------+-------+--------
URANIUM. | LEAD. | RATIO.
----------+-------+--------
70 | 2·9 | 0·041
70 | 3·0 | 0·042
70 | 2·8 | 0·039
72 | 3·0 | 0·041
72 | 2·9 | 0·040
----------+-------+--------
Similar crystals of uraninite have been furnished by the pegmatites of
Branchville, Connecticut. The intruded strata are of either Silurian or
Ordivician age, and the evidence suggests that the period of intrusion
is possibly coincident with that of the earth movements which commenced
at the close of the Ordivician. Here again the lead ratios closely
agree, the age being 430 million years:
----------+-------+--------
URANIUM. | LEAD. | RATIO.
----------+-------+--------
74 | 4·0 | 0·052
75 | 4·0 | 0·051
74 | 4·0 | 0·052
66 | 3·5 | 0·051
----------+-------+--------
In North Carolina uraninite occurs in coarse pegmatites which are mined
for their large flakes of mica. In this instance a good agreement is
scarcely to be expected, as secondary products abound, and the three
specimens from Spruce Pine which were examined by Hillebrand showed
signs of incipient alteration. The fourth specimen was from South
Carolina and this again lacked the freshness which is so essential.
Zircon is also found in the North Carolina pegmatites, and the writer
has examined two sets of specimens with the results given below. The
material in this case appeared to be quite fresh.
The geological period is difficult to establish with certainty. The
relations of the Appalachian rocks of Carolina are so obscure that
the required age may be anywhere from pre-Cambrian to Carboniferous.
Judging from the lead ratios, one is tempted to suggest that the age
is not far from the Silurian, but until our time-scale is better
determined one must be chary in this converse application of the
method. It may confidently be hoped that in this way the geologist will
be greatly helped in his attempt to unravel the history of igneous
activity in the earth’s crust. But the time is not yet, for the
accumulation of facts and data has only just commenced.
-----------+----------+--------+--------------
MINERAL. | URANIUM. | LEAD. | RATIO Pb/U.
-----------+----------+--------+--------------
Uraninite | 77 | 3·9 | 0·049
Uraninite | 77 | 4·2 | 0·052
Uraninite | 67 | 3·3 | 0·047
Uraninite | 71 | 3·3 | 0·045
Zircon | 0·076 | 0·0036 | 0·046
Zircon | 0·130 | 0·0055 | 0·041
-----------+----------+--------+--------------
Other series of minerals could be given from the pre-Cambrian rocks
of North America, Ceylon, and Mozambique, but it is unnecessary to
multiply details further. In all cases the Archean seems to date from
1200-1600 million years.
We now turn to the pre-Cambrian rocks of Scandinavia and Finland, and
for comparison with those of N. America the following classifications
and correlations may be given. The chief unconformities are indicated
by wavy lines. It should be pointed out that the correlation of the
pre-Cambrian rocks over wide areas is, perhaps, the most difficult task
the geologist has to attempt, and the scheme given opposite is based
more on analogy than direct evidence.
_Fennoscandia._ _North America._
Jotnian Keweenawan }
}
Jatulian Upper Huronian }
}
Upper Kalevian Mid. Huronian } Algonkian
}
Lower Kalevian }
Lower Huronian }
Bottnian }
Ladogian Laurentian }
Katarchean Keewatin } Archean
It is not yet possible to support this correlation by a concordant
system of chronology. Igneous rocks occur at most of the horizons, but
it is extremely difficult to get samples of suitable mineral species
for analysis. However, there is no doubt that in the future a definite
time estimate will be attached to each of the above periods. When
we are in full possession of this knowledge, and only then, will a
reliable correlation of these rocks be possible.
The analyses collected by Boltwood include two groups which are of
minerals taken from the pegmatites of Southern Norway, rocks famous
for the occurrence of rare minerals. The first group, from the igneous
complex of the Moss district, of which the average age is 1000 million
years, is as follows:
------------+----------+-------+--------
MINERAL. | URANIUM. | LEAD. | RATIO.
------------+----------+-------+--------
Uraninite | 66 | 8·4 | 0·12
Uraninite | 68 | 7·8 | 0·11
Annerdödite | 15 | 2·2 | 0·135
Uraninite | 66 | 9·3 | 0·13
Uraninite | 57 | 8·0 | 0·13
Uraninite | 65 | 8·8 | 0·135
Uraninite | 68 | 8·8 | 0·12
Uraninite | 76 | 9·0 | 0·11
Thorite | 8·2 | 1·2 | 0·13
------------+----------+-------+--------
The second group, from the complex of Arendal, demands an age of 1200
million years. The Scandinavian geologists believe that both groups of
rocks are younger than the quartzites and other metamorphic sediments
with which they are always associated, and that the latter rocks are of
late Archean age. Sederholm, however, thinks they may be equivalent to
his Kalevian division. The geological evidence, as far as it goes, does
not point to any difference in the ages of these two sets of Norwegian
minerals, but, on the other hand, there is no positive evidence that
the ages are the same. An analogy made in ignorance cannot be held
to constitute a proof. Field work in this case does not disqualify
the testimony of the radioactive minerals; it rather invites their
co-operation in the perplexing task of disentangling the intricate
structural relations of the rocks.
----------+----------+-------+-------------
MINERAL. | URANIUM. | LEAD. | RATIO Pb/U.
----------+----------+-------+-------------
Uraninite | 56 | 9·8 | 0·14
Uraninite | 61 | 10·2 | 0·15
Uraninite | 56 | 9·4 | 0·15
Thorite | 9 | 1·5 | 0·16
Orangite | 7·5 | 1·2 | 0·15
Xenotime | 2·9 | 0·62 | 0·19
----------+----------+-------+-------------
Amongst these rocks correlation is exceedingly difficult, and even
their relative ages are hidden in obscurity. Högbom holds that the
massifs of Moss and Arendal are contemporaneous with the Ser-archean
granites of Sweden. Similar hyperites and quartzites are found in
nearly every locality and the granites always appear to be younger than
these. After their intrusion, an enormous thickness of rock was denuded
away before the Jatulian sediments were laid down. The physical break
here indicated is one of the greatest in the history of the earth,
and undoubtedly represents an immense lapse of time. The granites and
pegmatites must therefore be considerably older than the Jatulian rocks.
With a view to testing the constancy of the lead-ratio in a series of
minerals from a single igneous complex, the author, in 1911, made a
number of experiments on carefully chosen material. There occurs in the
Christiania district of Norway, a geologically depressed area of nearly
4000 square miles, which is separated by faults from the surrounding
pre-Cambrian rocks on every side. Within this area there is a nearly
complete sequence of early Palæozoic rocks, surmounted by a few beds of
red sandstone of Lower Devonian age. Over these beds and intercalated
with them are lava flows; and finally, penetrating the whole mass, and
representing a later phase of the same period of igneous activity,
are great intrusions of plutonic rocks. Amongst the earliest of the
intrusions is a series of thorite-bearing nepheline-syenites. Brögger
believes them to be of Middle or Lower Devonian age, most probably the
latter. The minerals occurring in them are, in many instances, notably
radioactive, and thus they afford an admirable series in which to
investigate the consanguinity of lead and uranium. A suite of minerals
was obtained from Brevig, and estimations of these elements made, with
the following results:
--------------+--------------+--------------+-------
| URANIUM. | LEAD. |
MINERAL. | GRS. PER 100 | GRS. PER 100 | Pb/U
| GRS. MINERAL | GRS. MINERAL |
--------------+--------------+--------------+-------
Thorite | 10·1040 | 0·4279 | 0·042
Orangite | 1·2437 | 0·0570 | 0·046
Orangite | 1·1825 | 0·0542 | 0·046
Thorite | 0·4072 | 0·0196 | 0·048
Homelite | 0·2442 | 0·0121 | 0·049
Zircon | 0·1941 | 0·0085 | 0·044
Pyrochlore | 0·1923 | 0·0120 | 0·062
Pyrochlore | 0·1855 | 0·0093 | 0·050
Biotite | 0·1602 | 0·0069 | 0·043
Tritomite | 0·0631 | 0·0026 | 0·041
Freyalite | 0·0526 | 0·0028 | 0·053
Mosandrite | 0·0432 | 0·0024 | 0·056
Aegerine | 0·0253 | 0·0015 | 0·060
Astrophyllite | 0·0140 | 0·0007 | 0·050
Catapleite | 0·0132 | 0·0009 | 0·068
Nepheline | 0·0010 | 0·0004 | 0·400
Felspar | 0·0006 | 0·0003 | 0·500
--------------+--------------+--------------+-------
It will be noticed, that with a few exceptions, the value of the ratio
increases as the percentage of uranium diminishes. This is probably
due to the relative importance of lead originally entangled in the
minerals at the period of their crystallisation. Thus it would seem in
the case of nepheline and felspar that almost the whole of the lead
found was originally present, while that which has since been generated
is very small in comparison. Minerals with so little uranium contain
too much occluded lead to be reliable, and are, of course, valueless in
age-estimations. When sufficient uranium is held by a mineral, the lead
generated becomes increasingly important, until the original amount
is of negligible consequence. There is always the possibility that in
some of the richer minerals larger quantities of lead were occluded
than in felspar, but the agreement among the ratios renders this
improbable. Rejecting all the results after that of biotite because of
the low percentage of uranium, and omitting that of the first specimen
of pyrochlore because the estimation could not be verified owing to
lack of material, the mean ratio is 0·046. Replacing the uranium
percentage by its time-average value, the ratio becomes 0·045 and
the corresponding age 370 million years. It may be thought somewhat
arbitrary to select certain results preferentially, but in view of the
interpretation placed upon them the choice is not unfair, nor without
justification.
Most of the available evidence drawn from radioactive minerals has now
been passed in review. As yet it is a meagre record, but, nevertheless,
a record brimful of promise. Radioactive minerals, for the geologist,
are clocks wound up at the time of their origin. After a few years’
preliminary work, we are now confident that the means of reading these
time-keepers is in our possession. Not only can we read them, but if
they have been tampered with and are recording time incorrectly, we
can, in most cases, detect the error and so safeguard ourselves against
false conclusions.
CHAPTER XI
REVIEW OF THE EVIDENCE
The discrepancy between the geological and radioactive
methods of estimating time—Uniformity of the rate of
decay of uranium—Joly’s criticism—Comparison of the two
time-scales—Doubtful assumptions made in the geological
arguments—Possibility of reconciliation no longer hopeless.
Of the various methods which have been devised to solve the problem of
the earth’s age, only two, the geological and the radioactive, have
successfully withstood the force of destructive criticism. The other
arguments may be dismissed without further discussion, as in every
case their cogency has been vitiated by the detection of a fundamental
error. From the mists of controversy which for half a century have
hung over the subject, the two _hour-glass_ methods alone emerge,
and the final issue must be fought out between them. In the one the
world itself is the hour-glass, and the accumulating materials are
salt, the sedimentary rocks and calcium carbonate. Three concordant
sets of results may be drawn from this triple scheme of measurement,
but it must not be supposed that they are altogether independent. Each
set of data is intimately related to the others and all stand or fall
together. In the other case the accumulating materials are helium and
lead, and the hour-glass is constituted by the minerals in which they
collect. Provided that the field-evidence is clear and convincing
and that the relative geological age of a mineral specimen can be
determined, the construction of an exact and precise time-scale is
a task which can be dealt with successfully in the laboratory. The
problem has advanced from the qualitative to the quantitative stage,
and for the first time in historical geology accurate measurement
founded on delicate experimental work has become possible.
It is a matter for regret that confidence in this pioneer work has
been shaken by the advocates of the geological methods of attack. The
surprises which radioactivity had in store for us have not always been
received as hospitably as they deserved. With the advent of radium
geologists were put under a great obligation, for the old controversy
was settled overwhelmingly in their favour. But the pendulum has swung
too far, and many geologists feel it impossible to accept what they
consider the excessive periods of time which seem to be inferred.
That there exists a serious discrepancy obviously points to a flaw
in the underlying assumptions of one or the other or both of the
methods. Evidently we are at the parting of the ways. The fundamental
assumptions on which the arguments are based cannot both be right.
One of them must be rejected. Which is it to be? Let us consider each
in turn, and discuss the consequences of the two possible forms of
reconciliation.
The only assumption which can reasonably be called into question is
that of uniformity, and it is involved equally in both calculations. It
is here, at the root of the problem, that the discrepancy really lies.
If we favour the uniformity of geological processes—a well-worn
doctrine which has done good service—then we must reject uniformity
of radioactive disintegration. Joly has drawn attention to the latter
possibility. He asks: Is it assured that the parent substance, uranium,
has always in the past disintegrated at the rate determined by its
present average life period? As far as we know, the rate of decay
for substances of rapid transformation is constant, and independent
of temperature and pressure changes. On the grounds that a large
number of radioactive bodies decay at a constant rate, it is believed
that this constancy is a definite attribute of all the radioactive
elements. In the case of uranium this assumption cannot be proved for
periods commensurate with its half-life period. On analogy with the
behaviour of the shorter lived elements, it is probable that had we
lived in Cambrian times and experimented with Archean uranium-bearing
minerals just as has been done during the last decade, the half-life
period would then have been exactly the same as we now find it—about
5400 million years. In the case of radium emanation there can be no
doubt that experiments in Cambrian times would have given results
concordant with ours. It would be as unphilosophic to doubt this as
to believe that the laws of physics and chemistry vary with time. The
difference between uranium and its daughter elements, the difference
which suggests to Joly a possible distinction, is simply one of origin.
We are in complete ignorance of the genesis of uranium. It is not
impossible that, owing its origin to some process other than atomic
transformation, the particular distribution of intrinsic energy among
its atoms may not be such as to maintain a constant rate of decay.
At the moment of its birth every radioactive atom has a definite
expectancy of life, and when a sufficiently large number of atoms is
under observation a definite fraction disintegrates every second. There
is this difference in the case of parent elements. As they become aged
with reference to the time of their origin, they are not reinforced
by the addition of fresh, newly-born atoms, as are the other members
of each series. Joly’s supposition seems to be that in the absence
of this reinforcement the uranium in its early stages may possibly
disintegrate more rapidly than it does now. However, it is not found
that the younger atoms of the short-lived elements are, on an average,
more prone to rapid decay than are their older companions. Whether an
element is in equilibrium with the higher members of its family, or
whether it is separated from them, its transformation proceeds with
unaffected regularity.
It is very improbable that reconciliation will be found in the
supposition of a progressive retardation of the rate of decay of
uranium. There are three possibilities. Uranium may have disintegrated
in the past exactly as it now does; or it may have decayed more slowly
or more rapidly. The latter two alternatives do not favourably commend
themselves. There is no evidence which can be cited in their support.
On the other hand, the hypothesis of _constant_ change is deduced
from a well-established series of experimental facts, and is remarkably
in accordance with the general phenomena of radioactivity. Uranium,
in other respects, does not present any anomaly, and with regard
to the mechanism of its decay physicists are not likely to regard
it as an exceptional case without very definite reasons for doing
so. The discordance between the time estimates drawn from the rates
of geological and radioactive changes cannot be held to constitute
a sufficient reason for rejecting current opinions unless it is
conclusively demonstrated that the geological estimates are beyond
question. In the future the case for uranium may be established
more securely, when the dynamics of atomic disintegration, and the
conditions upon which the distribution of unstable atoms depends,
becomes more intimately understood. At present there is only one
means of testing the constancy of uranium decay, but unfortunately
it affords only negative evidence. The range of α-particles from a
radioactive element is connected in some way with its rate of decay.
If then, uranium in the past disintegrated more rapidly, the radius of
its particular pleochroic halo ought to record the difference. It is
improbable that any variation—assuming for the moment that there were
a variation—could be detected even in the most favourable cases.
We now turn with a double interest to the geological estimates. If it
can be shown that they ought to be largely increased, as Chamberlin
and a few other geologists believe, then not only is a reconciliation
at once made possible, but, in turn, the constancy of uranium decay is
placed beyond doubt. Little need be added to the discussion in Chapter
VI. It was there indicated that one factor previously over-looked—the
average height of the continents in geological time—very largely
controls the rate of denudation and therefore of sedimentation. Let us
make an attempt to discover how past rates must be related to those
of the present to make possible a complete reconciliation. In Fig. 17
the discrepancy is illustrated graphically by comparing the respective
time scales from the close of the Archean (gneiss and granite phase) to
the present day. All the sediments of which relics have remained to us
are in this way taken into consideration. Lying buried in the Archean,
the base of the record is obscured beyond recognition by the prevalence
of metamorphic and plutonic igneous rocks. The extent to which the
earliest sediments have been lost in the evolution of the earth’s
crust, and the part they have played in the genesis of granites and
gneisses are questions which betray our ignorance and offer food merely
for wild speculations. These possibilities, however, do not touch the
immediate point at issue, for in their time relations they lie outside
the limits to which this discussion is restricted.
The curve A is plotted strictly against the maximum observed thickness
of sediments, and corresponding to it is the sedimentation line A′
to which is granted 300 million years. According to these two graphs
the greatest error lies beyond the Cambrian. The average rate of
denudation and of sediment accumulation must now be nine times that
of the pre-Cambrian periods, but if post-Cambrian is compared with
present the ratio is reduced to two-and-a-half. In the B series, the
lead ratios are plotted in a straight line and the stratigraphical
column is extended in accordance with the palæontological evidence that
pre-Cambrian time is at least as long as that which has elapsed since
the beginning of the Cambrian. On this basis present rates are four
times the average for post-Archean time.
[Illustration: FIG. 17.
Geological Time Scales.]
Assuming that the true time-scale lies somewhere between the extremes
of A and B, we are led to two conclusions which, if accepted, greatly
lessen the severity of the discordance between A and A′ and B and
B′. From A and A′ it appears that pre-Cambrian denudation took place
much more slowly than has since been typical. This proposition is in
complete accordance with the view that the pre-Cambrian continents,
when viewed in the light of the reconstructed geographies of the later
periods, were of limited area and restricted elevation. From B and
B′ the broad time conception of palæontology gains further support,
and the existence of great gaps in the pre-Cambrian succession is
suggested. It is well known that the most important unconformities of
the whole geological record are to be found in the imperfect succession
of the earliest formations. It is impossible to do more than guess
at the duration of time periods which are without their sedimentary
equivalents. On the most extreme assumption, the Algonkian sediments
should be represented not by 82,000 feet, but by more than 300,000
feet. Unwilling though we may be to consider the record imperfect to
this incredible degree, it is of importance to point out that had such
an immense thickness existed in successive periods of time, the sodium
content of the ocean would not necessarily be in any way different
from what it is. The same amount of primary rock material may have
been broken up, but instead of a reassorting of the materials to form
three successive sets of sediments (on a rough average) we would be
obliged to postulate six or more repetitions of the sorting process.
The question need not be pursued farther. The suggestion here put
forward is, that in the limitations of pre-Cambrian geography and in
the imperfection of the sedimentary relics of those remote times, the
discrepancy which is peculiar to the pre-Cambrian finds an adequate
explanation.
If this be allowed, all that remains is to decide whether it is
inconsistent with geological principles to assert that the modern
hour-glass is running at two-and-a-half to four times its average
rate. The decision depends largely on the broad point of view from
which geological interpretation proceeds. From the standpoint of
Catastrophism little progress was made. Uniformity proved a great
advance, but in detail it is apt to lead us astray if applied too
dogmatically. Modern interpretation is based on the more philosophic
conception of Evolution, and in place of the earlier idea which was
insisted upon by the older physicists—that changes have been such as
would accompany a gradual running down of the earth’s internal kinetic
energy—the _form_ of development now favoured is that of cycles
of phenomena, recurring in their broad features again and again and not
necessarily hampered in their activity by any progressive diminution in
the store of available energy. Igneous action, deposition of sediments,
marine transgression and recession, are all rhymic phenomena and
the factor common to each one, whether as cause or effect, is
earth-movement.
The conditions of the present day cannot then be accepted as
representing average conditions, unless it were by a happy accident.
Amidst all the details of earthquakes and volcanic eruptions some great
cycle is now running its course, and only in relation to the particular
phase of the cycle under which we happen to pursue our investigations
will our conclusions be strictly tenable. Although we cannot hope to
judge the exact place which the present takes in the larger scheme of
terrestrial activity, yet in comparison with the past, the present
epoch would seem to approach just those extremes most favourable to
a high rate of denudation, and to a rapid accumulation of sediments.
Marine recession, brought about by deepening of the ocean basins, and
raising of the land areas have together brought about continental
expansion and elevation. The vulcanism of the present day, whether
regarded as a closing phase of a period of igneous action, or as the
initiation of a new cycle, probably affords an example of more than
average intensity and violence. The weathering capacity of rain must be
enhanced in proportion to its content of dissolved acid gases, and this
in turn is conditioned by the prevalence of vulcanism. Still another
factor leading to higher rates, though of a different category, is
due to recent glaciation. Over wide areas easily eroded deposits are
exposed, the areas being generally those which would resist denudation
most successfully. In Fennoscandia, for example, four-fifths of the
pre-Cambrian shield is buried beneath a thin covering of moraine.
It is not suggested that present rates have never before been reached,
but only that they are characteristic of the more intense phases of
denudation rather than of average conditions. If this be granted,
reconciliation of the rival time estimates is no longer hopeless. There
can be no doubt that agreement will never be brought about by the more
convincing testimony of experimental demonstration. It must be almost
entirely a question of interpretation. An attempt has been made to show
that in the geological evidence there is nothing impossibly at variance
with the dictates of the radioactive minerals. With the acceptance of
a reliable time-scale, geology will have gained an invaluable key to
further discovery. In every branch of the science its mission will be
to unify and correlate, and with its help a fresh light will be thrown
on the more fascinating problems of the Earth and its Past.
APPENDIX A
(_a_) _Kinetic Energy of α-particles_
1 gram of radium in equilibrium with emanation,
Ra. A B and C generates heat at the rate of =132
calories per hour= (85% due to α-particles).
e = charge on α-particle = 9·3 × 10⁻¹⁰ E.S. units.
= 3·1 x 10⁻²⁰ E.M. ”
m = mass of α-particle }
v = velocity of α-particle } see Table below.
N = number of α-particles liberated from 1 gram of
radium = 3·4 × 10¹⁰ per second.
Energy E transformed per second is given by—
Nmv²
E = ½∑ ——— × e
e
Ne mv²
= —— ∑ ——
2 e
+---------------+---------------+----------------+
| | v | mv²/e |
| Element. | Cms. per sec. | E.M. Units. |
+---------------+---------------+----------------+
| Radium | 1·56 × 10⁹ | 4·78 × 10¹⁴ |
| Ra. emanation | 1·70 × 10⁹ | 5·65 × 10¹⁴ |
| Ra. A | 1·77 × 10⁹ | 6·12 × 10¹⁴ |
| Ra. C | 2·06 × 10⁹ | 8·37 × 10¹⁴ |
+---------------+---------------+----------------+
Substituting these values, we have—
Ne = 3·4 × 10¹⁰ × 3·1 × 10⁻²⁰ = 10·5 × 10⁻¹⁰ E.M. units.
mv²
∑ —— = 10¹⁴(4·78 + 5·65 + 6·12 + 8·37)
e
= 24·9 × 10¹⁴ E.M. units;
whence E = 13·1 × 10⁵ ergs per second
= 4·73 × 10⁹ ergs per hour.
Now 4·19 × 10⁷ ergs = 1 gram-calorie.
∴ E = 113 calories per hour.
(_b_) _Production of helium from Uranium and Thorium in
equilibrium with all their disintegration products._
_Uranium_—
N = number of helium atoms liberated from 1 gram
of radium alone = 3·4 × 10¹⁰ per second.
(_Rutherford and Geiger_, 1908).
The equilibrium ratio of radium to uranium is
3·4 × 10⁻⁷. Hence for each gram of uranium in
equilibrium the number of atoms produced amounts to—
3·4 × 10¹⁰ × 3·4 × 10⁻⁷ × 8 per second = 29·1 × 10¹¹ per year.
Now the number of helium molecules, and therefore of atoms,
in 1 cc. of the gas at N.P.T. is 2·72 × 10¹⁹.
The annual production of helium must consequently be
29·1 × 10¹¹
———————————— ccs.,
2·72 × 10¹⁹
i.e. 10·7 × 10⁻⁸ ccs., or 1·88 × 10⁻¹¹ grs. per gram of uranium.
An experimental determination gave 10·6 × 10⁻⁸ ccs.
(_Strutt_, 1910).
_Thorium_—
The ionising power, or the energy of the α-particles from 1 gram of
thorium, is 0·325 of that from 1 gram of uranium, each element being in
complete equilibrium.
Average range of α-particles from thorium and its products = 5·4 cms.
Average range of α-particles from uranium and its products = 4·3 cms.
The average thorium α-particle is therefore 1·25 times as energetic as
the average uranium α-particle.
Hence the actual production of α-particles or helium atoms from
thorium is only 0·325/1·25 = 0·26 of that of uranium.
Experimental determinations 0·23 (_Strutt_, 1910),
0·27 (_Rutherford and Geiger_, 1910).
(_c_) _Half-life Period of Radium._
(1) The number of α-particles emitted from 1 gram of radium per second
(n = 3·4 × 10¹⁰) is equal to the number of atoms disintegrating per
second.
If N is the number of atoms in 1 gr. radium, then λ, the fraction which
transforms per second, is given by—
n
λ = ——.
N
The number of atoms in 1 gr. hydrogen is 6·24 × 10²³,
and as the atomic weight of radium is 226 times that
of hydrogen,
N = 2·76 × 10²¹.
∴ λ = 1·25 × 10⁻¹¹ gr. per sec.
= 3·94 × 10⁻⁴ gr. per year.
=Half-Life period= = 0·69315/λ = =1760 years=.
(2) 1 gr. of radium is in radioactive equilibrium with 0·58 cubic
millimetre, or 5·7 × 10⁻⁶ grs. of emanation (atomic wt. = 222).
If λ₁ = 2·085 × 10⁻⁶ is the fraction of emanation transforming per
second, we have—
λ = 5·7 × 10⁻⁶ × λ₁
= 1·19 × 10¹¹ gr. per sec.;
whence—=Half-Life period = 1850 years.=
The earlier values given for the half-life period were 1760 and 2000
years, but the lower figure seems most accurate, with 1850 years as a
probable value.
The half-life of uranium would then be—
1850
—————————— = =5400 million years=.
3·4 × 10⁻⁷
(_d_) _Time-Average of Uranium_
Uₜ = Quantity of uranium remaining after a time t.
Uₒ = Quantity of uranium originally present (t = o).
Uₘ = Time-average of uranium during time t.
λ = Disintegration constant of uranium.
Pbₜ = Lead accumulated during time t.
Heₜ = Helium ” ” time t.
[Illustration: Graph I]
Graph I represents the rate of decay of uranium—according to the
exponential law—
( -λₜ)
Uₜ = Uₒ ( e ).
There is one rate of decay which, if it remained constant throughout
the time t, would have a total effect equivalent to that produced by
the actual slowly decreasing rate of decay. This average rate is
represented by some point on the curve, and the corresponding quantity
of uranium, Uₘ, is the time-average. Equating the amount of uranium
transformed in each case, we have—
Uₒ - Uₗ = λUₘt
Uₒ - Uₜ
whence Uₘ = ———————— (_a_).
λt
[Illustration: Graph II]
For periods less than 2000 million years, the time-average is nearly
equal to the arithmetic mean of Uₒ and Uₗ.
Uₒ + Uₗ
Uₘ = ———————— (_b_).
2
The value of Uₘ for 2000 million years is,
according to equation (_a_), equal to 0·874 Uₒ
and ” ” ” (_b_) ” “ 0·879 Uₒ.
The ages of minerals rarely exceed 1500 million years, and therefore
the error involved by using equation (_a_) in preference to
(_b_) is quite negligible.
In Graph II the ratio Uₘ/Uₜ, i.e. the factor by which the present
uranium content of a mineral must be multiplied in order to obtain the
true time-average, is plotted against time.
An approximation to the age, t, of a mineral is afforded by the ratio
Pbₜ/Uₜ. From the graph the factor corresponding to this time can be
obtained, and thence the time-average. This in turn can be utilised to
give the more correct age represented by Pbₜ/Uₘ.
A more straightforward method of correction is as follows:
Uₒ can be determined from known quantities according to the following
equation:
Uₔ = Uₜ + Pbₜ + Heₜ
= Uₜ + 1·15 Pbₜ;
whence, =Uₘ = Uₗ + 0·575 Pbₜ=.
The age of the mineral is then given by the ratio =Pbₜ/Uₘ=, or
directly from Graph II.
(_e_) _Analyses made by the Author of Radium in Igneous Rocks_
(_cited on p. 130_)
_Acid Rocks_— _Ra. per gram of rock_
Granite, Mozambique 5·84 × 10⁻¹² grs.
” ” 2·61 ”
” ” 1·77 ”
” N. Nigeria 3·09 ”
” Rhodesia 2·43 ”
” Transvaal 2·12 ”
” South Africa 1·81 ”
” ” ” 2·73 ”
_Basic Rocks_—
Basalt, Mozambique 0·94 × 10⁻¹² grs.
Dolerite ” 0·85 ”
Gabbro ” 1·07 ”
Norite ” 0·54 ”
_Ultrabasic Rocks_—
Composite analysis of 10 specimens from
Scotland, New Zealand, Africa,
and Canada 0·51 × 10⁻¹² grs.
APPENDIX B
BIBLIOGRAPHY
CHAPTER I
Thomson (Kelvin). _Rep. Brit. Ass._, p. 1819. 1855.
—— _Proc. Glasgow Phil. Soc._, Vol. IV, p. 272. 1860.
—— _Phil. Mag._, February, 1862, p. 158. 1861.
—— _Popular Lectures and Addresses_
[_P.L.A._], Vol. I, p. 349. 1862.
—— _Natural Philosophy_, Appendix D.
—— _P.L.A._, Vol. II, p. 6. 1865.
—— _P.L.A._, Vol. II, p. 10. 1868.
Huxley. _Presidential Address, Quart. Journ. Geol. Soc. 1869._
Thomson. _P.L.A._, Vol. II, p. 73. 1869.
—— _P.L.A._, Vol. II, p. 238. 1876.
—— _P.L.A._, Vol. I, p. 369. 1887.
King. _Am. Journ. Science_, p. 1. 1893.
Correspondence in _Nature_. Perry, Kelvin, Tait.
January 3rd, March 7th, April 18th. 1895.
Poulton. _Rep. Brit. Ass._, p. 808. 1896.
Kelvin. _Phil. Mag._, January, 1899, p. 66. 1897.
Correspondence in _Nature_. 1903.
Wilson. July 9th.
Darwin. September 24th.
Joly. October 1st.
Correspondence in _Nature_.
Kelvin, Strutt, Lodge, etc. September 20th. 1906.
For other references see below.
CHAPTER II
Chamberlin and Salisbury. _Geology_, Vol. II, Chaps. I and II. 1909.
T. C. Chamberlin and other Writers. _The Tidal Problem._
Carnegie Inst. of Washington. Pub. No. 107. 1909.
Darwin. _The Tides._ 1911.
Poincaré. _Hypothèses cosmogoniques._ Paris. 1911.
CHAPTER III
Croll. _Phil. Mag._ May, 1868.
—— _Climate and Time._ 1875.
De Geer. _Geol. Fören i Stockholm Föhr Band 32._ 1910.
Gilbert. _Journ. of Geol._, Vol. III, p. 121. 1895.
Lowell. _Evolution of Worlds_, p. 197. 1910.
Sederholm. _Bull. Comm. Géol. de Finlande_, No. 30, pp. 7-15. 1911.
Sollas. _Ancient Hunters._ 1911.
CHAPTER IV
Babb. _Science_, Vol. XXI, p. 343. 1893.
Clarke. _Data of Geo. Chemistry. Geol. Sur., U.S.A. Bull. 491._
—— _Smith. Misc. Collect._, Vol. LVI, No. 5. 1910.
Dittmar. _Challenger_ Report, Vol. I, p. 203. 1884.
Dole and Stabler. _Water Supply Papers. Geol. Sur., U.S.A._,
Nos. 234 and 236. 1910.
Geikie. _Trans. Geol. Soc., Glasgow_, p. 153. 1868.
Humphreys and Abbott. _Physics and Hydraulics of Mississippi River_,
p. 148. 1876.
Merrill. _Rocks, Rock weathering and Soils._ 1897.
Murray. _Scot. Geog. Mag._, Vol. III, p. 65. 1887.
—— _Scot. Geog. Mag._, Vol. IV, p. 38. 1888.
Reade. _Chemical Denudation in Relation to Geological Time._ 1901.
Russell. _River Development._ 1898.
Schwarz. _Causal Geology._ 1910.
Tylor. _Phil. Mag._, April, p. 258. 1853.
CHAPTER V
Ackroyd. _Chem. News_, p. 265. 1901.
—— _Geol. Mag._, p. 445 and p. 558. 1901.
Becker. _Science_, Vol. XXXI, p. 459. 1910.
—— _Smith. Misc. Collect._, Vol. LVI, No. 6. 1910.
Clarke. _Smith. Misc. Collect._, Vol. LVI, No. 5. 1910.
—— _Proc. Am. Phil. Soc._, p. 214. 1912.
Fisher. _Geol. Mag._, p. 124 and p. 132. 1900.
Halley. _Phil. Trans._, Vol XXIX, p. 296. 1715.
Hanamann. _Landesdurchforschung Böhmens. Archiv.
Natur._, Vol. IX. Vol. X. 1894. 1898.
—— _Cited in Clarke’s Data_, 2nd ed., p. 92.
Joly. _Trans. Roy. Soc., Dublin_, Vol. VII, p. 23. 1899.
—— _Rep. Brit. Ass._, p. 369. 1900.
—— _Geol. Mag._, p. 344 and p. 504. 1901.
—— _Radioactivity and Geology_, p. 236. 1909.
—— _Phil. Mag._, p. 357. September, 1911.
Karsten. _Inaug. Diss., Kiel._ 1894.
Rudzki. _Bull. Acad. Sci., Cracovie._ February, 1901.
Sollas. _Presidential Address, Quar. Journ. Geol. Soc._,
Vol. LXV. 1909.
CHAPTER VI
Blake. _Geol. Mag._, p. 72. 1903.
Croll. _Stellar Evolution_, p. 48. 1889.
Geikie. _Rep. Brit. Ass._ 1892.
—— _Nature._ August 4th, 1892.
—— _Rep. Brit. Ass._, p. 727. 1899.
Haughton. _Manual of Geology_, p. 101. 1871.
—— _Nature_, p. 266. 1878.
Holmes. _Nature._ July 6th, 1911.
Joly. _Radioactivity and Geology._ 1909.
de Lapparent. _Bull. Soc. Géol. de France_,
Vol. XVIII, p. 351. 1890.
—— _Géologie_, p. 225, p. 261, and p. 1958. 1906.
Lyell. _Principles._ Tenth Edn., Vol. I, p. 301. 1867.
McGee. _Science_, Vol. XXI, p. 309. 1893.
Phillips. _Life on the Earth_, p. 119. 1860.
Reade. _Geol. Mag._, p. 99. 1893.
Sederholm. _Naturen, Helsingfors_, Nos. 33 and 34. 1897.
Sollas. _Nature._ April 4th, 1895.
—— _Rep. Brit. Ass._, p. 711. 1900.
—— _The Age of the Earth and other Geological Studies._ 1905.
—— _Presidential Address, Quar. Journ. Geol. Soc._,
Vol. LXV. 1909.
Upham. _Am. Journ. Science_, Vol. XLV, p. 217. 1893.
Walcott. _Journ. Geol._, Vol. I, p. 675. 1893.
Watts. _Presidential Address, Quar. Journ. Geol. Soc._,
Vol. LXVII. 1911.
Wallace. _Island Life_, p. 222. 1892.
Winchell. _World Life_, Chicago, p. 378. 1883.
CHAPTER VII
Mme. Curie. _Traité de Radioactivité._ 1910.
Ramsay and Soddy. _Proc. Roy. Soc. A._, Vol. LXXII, p. 204. 1903.
Russell. _Proc. Roy. Soc. A._, Vol. LXXXVI, p. 240. 1912.
Rutherford. _Radioactive Substances._ 1912.
Rutherford and Soddy. _Phil. Mag._, Vol. IV, p. 582. 1902.
Rutherford and Geiger. _Proc. Roy. Soc. A._,
Vol. LXXXI, p. 151. 1908.
Rutherford and Royds. _Phil. Mag._, Vol. XVII, p. 281. 1909.
Whytlaw-Gray and Ramsay. _Proc. Roy. Soc. A._,
Vol. LXXXVI, p. 270. 1912.
CHAPTER VIII
Abbott. _The Sun._ 1910.
Arrhenius. _The Life of the Universe_, Vol. II, p. 193.
Helmholtz. _Phil. Mag._, p. 516. 1856.
Rutherford and Soddy. _Phil. Mag._ May, 1903.
CHAPTER IX
Becker. _Bull. Geol. Soc. Am._, p. 113. 1908.
—— _Smith. Inst. Misc. Collect._, Vol. LVI, No. 6.
Buchner. _Proc. Konink. Akad. van Wetensch. te Amsterdam._
October, 1910, February, 1911, and April, 1912.
Chamberlin. _Journ. Geol._, p. 674. 1911.
Eve and McIntosh. _Phil. Mag._ August, 1907.
—— _Trans. Roy. Soc. Canada_, p. 69. 1910.
Farr and Florance. _Phil. Mag._ November, 1909.
Fletcher. _Phil. Mag._ July, 1910.
—— _Phil. Mag._ January, 1911.
—— _Phil. Mag._ June, 1911.
—— _Phil. Mag._ February. 1912.
Joly. _Rep. Brit. Ass._, p. 677. 1908.
—— _Radioactivity and Geology_, 1909.
—— _Phil. Mag._ October, 1909.
—— _Phil. Mag._ July and August, 1910.
—— _Phil. Mag._ July, 1911.
—— _Cong. Internat. de Rad. and d’Elec._, p. 370. 1911.
—— _Phil. Mag._ February, 1912.
—— _Phil. Mag._ October, 1912.
King. _Am. Journ. Science_, p. 1. 1893.
Strutt. _Proc. Roy. Soc. A._, Vol. LXXIX, p. 472. 1906.
—— _Proc. Roy. Soc. A._, Vol. LXXXIV, p. 377. 1910.
Suzuki. _Proc. Math.-Phys. Soc., Tokyo_, p. 204. 1912.
CHAPTER X
Becker. _Bull. Geol. Soc., Am._, Vol. XIX, p. 113. 1908.
Boltwood. _Am. Journ. Science_, p. 260. 1905.
—— _Am. Journ. Science_, Vol. XXIII, p. 77. 1907.
Boltwood and Rutherford. _Phil. Mag._, Vol. XXII, p. 586. 1911.
Dewar. _Proc. Roy. Soc. A._, Vol. LXXXI, p. 280. 1908.
—— _Proc. Roy. Soc. A._, Vol. LXXXIII, p. 404, 1910.
Holmes. _Proc. Roy. Soc. A._, Vol. LXXXV, p. 248, 1911.
Joly. _Radioactivity and Geology_, p. 211. 1909.
—— _Phil. Mag._ September, 1911.
Kœnigsberger. _Geol. Rundschau_, Vol. I, p. 245. 1910.
Rutherford. _Radioactivity_, p. 486. 1905.
Strutt. _The Accumulation of Helium._
—— _Proc. Roy. Soc. A._, Vol. LXXXI, p. 272. 1908.
—— —— Vol. LXXXII, p. 166. 1909.
—— —— Vol. LXXXIII, p. 96. 1910.
—— —— Vol. LXXXIII, p. 298. 1910.
—— —— Vol. LXXXIV, p. 195. 1910.
—— —— Vol. LXXXIV, p. 380. 1910.
[Illustration: THE RADIOACTIVE ELEMENTS
URANIUM FAMILY
ACTINIUM FAMILY
THORIUM FAMILY]
INDEX
α-particles, deflection of, 94
— nature of, 95
— penetration of, 94, 107, 108
— heating effect of, 96, 97
— kinetic energy of, 177
— atoms of helium, 96
Actinium and uranium, 100
Adams, 25
Age of earth, 86, 89, 125, 157
— minerals, 154-64
— moon, 15, 27
— oceans, 68, 89
— sediments, 86, 89
— sun, 112, 113, 120
Arrhenius, energy of sun, 117
Atmosphere, origin of, 25, 30
— as thermal blanket, 36
Atomic theory, 40
Atoms in 1 cc. helium, 140
— in 1 gr. hydrogen, 179
— in 1 gr. radium, 179
— disintegration of, 98, 102
— constitution of, 102
Autunite, age of, 100, 153
— radium-ratio in, 100
— lead-ratio in, 153
β-particles, deflection of, 94
— nature of, 95
— from potassium, 103
Barus, diabase fusion point, 125, 126
Barnes and Rutherford, heat output of radium, 101
Basic rocks, radium in, 130
— distribution of, 128, 134
Becker, age of oceans, 62, 63, 70
— age of earth, 127
Becquerel, discovery of radioactivity, 92
Becquerel rays, 93
Biology and the earth’s age, 20, 21
Boltwood,constancy of radium-ratio, 99
— origin of actinium, 100
— test of lead-ratio, 158
Boltwood and Rutherford, production of helium, 139
Bragg, ionisation, 107
Bückner, radium in rocks, 130
Buffon, 23
Calcium carbonate, accumulation of, 88, 89
Catastrophism, 4, 8
Chaldean chronology, 2
Chamberlin, planetismal hypothesis, 28-30
— tidal theory, 27
— rate of denudation, 79, 80, 81
_Challenger_ expedition, 85
Chlorine in rain, 66
— in rivers, 53, 70
— in sea, 53
— in rocks, 70
Chronology, Chaldean, 2
— Persian, 3
— Hebrew, 3
— Indian, 3
— Byzantine, 4
Clarke, sodium in rocks, 67, 70, 71
— chemical denudation, 51-3
— volume of oceans, 63
— age of oceans, 63, 66
Cosmical energy, 120
Croll, glacial theory, 35-9
— marine erosion, 58
— age of earth, 86
Crust of earth, thickness of, 128, 134
— evolution of, 30, 134
Crystallisation of rock magmas, 145-8
Curie, Mme., radioactivity of thorium, 92
— discovery of radium, 93
Curie and Laborde, heating effect of radium, 96, 101
Cuvier, 8
Cycles of denudation and deposition, 85, 174, 175
Cyclic sedimentation, 44
— circulation of salt, 66-70
Dana, volume of limestone, 88
Darwin, geological time, 11
Darwin, Sir G., moon’s history, 15, 19
— age of moon, 27
Decay, radioactive, 98
— physical independence of, 102
— possible variation with time, 168
Deep-sea deposits, 59
Deluge, 4, 8
Denudation, initiation of, 25, 31
Denudation, chemical, 50, 54
— mechanical, 54
— marine, 58, 66
— present rates of, 78, 80
Deposition, 47, 81-5
Dewar, production of helium, 140
Diabase, fusion point, 125, 126, 128
Dittmar, salinity of oceans, 64
Dole and Stabler, denudation of N. America, 50, 55
Duane, heating effect of radium, 101
Earth, origin of, 22 _et seq._
— early history of, 25, 29, 73
— heat of, 12-16, 30, 122
— interior of, 134
— distribution of radium, 131-5
Earthquakes, 134
Electroscope, 93, 104
Emanation of radium, 97, 104
End products, 100, 140
Energy, conservation of, 11, 111
Eve and McIntosh, radium in rocks, 130
Farr and Florance, radium in rocks, 130
Fletcher, radium in rocks, 130
Fossils, early ideas, 5
γ-rays, nature of, 94
Geer, De, glacial clays, 41-4
Geiger and Rutherford, counting of α-particles, 138
Geikie, age of earth, 86
— marine erosion, 58
— on geological time, 78
Glacial periods, 33
Glacial periods, chronology of, 36 _et seq._
Glacial clays in Sweden, 41
Glaciation, indications of, 33, 34
— cause of, 35
Gilbert, Cretaceous sediments of Colorado, 44
Haloes, pleochroic, 107-9
Halley, salinity of oceans, 61
Heat, emission of, by radium, 96, 97, 101
— — by uranium, 116, 131, 132
— — by thorium, 116, 132
— escaping from earth, 122-4, 132
— of the sun, 110 _et seq._
Helium, discovery of, 95, 96
— from radio-elements, 96
— rate of evolution of, 138
— leakage of, 149
— as end product, 149
— accumulation of, 154
— — in phosphates, 154
— — in iron ores, 155
— — in zircons, 156
— — in sphenes, 156
Helium-ratio, as age-index, 143
Helmholtz, 12, 112
Hutton, 7, 22
Huxley, reply to Kelvin, 14, 20
Igneous rocks, exposures of, 71, 72, 73
— sodium in, 67
— radium in, 131
— thorium in, 131
Ionisation, 93, 104
Ionium, 99
Iron meteorites, 134
Iron ores, helium in, 155
Joly, estimation of radium, 105
— — thorium, 107
— radium in rocks, 131
— thorium in rocks, 131
— age of the earth, 86
— age of the oceans, 17, 20, 62
— on radioactive decay, 168
Kant, 23, 26, 110
Kelvin, duration of earth’s heat, 12-17, 124, 125
— — sun’s heat, 12-14, 111
— tidal retardation, 13, 26, 27
Lamarck, 8
Land, area of, 52
— degradation of, 57
Laplace, 23, 28, 35
Laplacian hypothesis, 23-25
— difficulties of, 28
Lapparent, de, 86
Lead in igneous rocks, 147
— genetic connection with uranium, 100, 140, 141
— in uranium minerals, 158-164
— rate of production of, 142
Lead-ratio as age-index, 143
Lowell, Polar caps of Mars, 39
Lyell, _Principles_, 9
Mars, Polar caps of, 39
Mass of electron, 95
— of hydrogen atom, 179
Mayer, sun’s heat, 12, 111
Meteorites and sun’s heat, 111
— radium in, 134
Mica, pleochroic haloes in, 109
Minerals, radium-ratio in, 99, 100
— actinium-ratio in, 100
— lead-ratio in, 141, 143
— helium-ratio in, 143
— choice of, 153
Mississippi as denuding agent, 55
— bottom load of, 54
Moon, 15, 19, 20, 27, 28
Moses and geology, 3, 9
Moulton, 28
Murray, area of land, 52
— salinity of rivers, 50
— volume of ocean, 63
Nebulæ, Laplacian, 24, 28
— spiral, 28, 29
Nebular hypothesis, 23-25, 28
Nile, annual deposit of, 81
North America, denudation of, 50, 56
Oceans, origin of, 25, 50
— mass of, 53, 64
— volume of, 53, 63
— saline content of, 53
— age of, 68, 89
Origin of earth, 22, _et seq._
Perry, 16
Phillips, 10
Phosphates, helium in, 154
Phosphorescence of uranium salts, 92
Pitchblende. _See_ Uraninite
Pleochroic haloes, 107-109, 170
Polonium, discovery of, 93
Poulton, biology and geological time, 21
Products, final radioactive, 100, 140
Radioactive disintegration, 98
— — physical independence of, 102
Radioactive layer, 133
Radiation from uranium, 92
Radium, discovery of, 93
— detection of, 93, 103-107
— emanation from, 98, 104
— helium from, 96, 138-140
— α-rays from, 96, 138
— half-life of, 98, 99, 179
— atomic weight of, 142
— genesis from uranium, 99
— heating effect of, 96, 101, 129, 131
— end products of, 100
— in earth’s crust, 103, 131
— in igneous rocks, 131
— in sediments, 131
— in sun, 117
Ramsay, helium in minerals, 96
Reade, chemical denudation, 50, 62
— age of earth, 86
— volume of limestone, 88
River water, volume of, 53
— sediment in, 56, 57
— salinity of, 53
Rocks, radium in, 130, 131
— thorium in, 131
— chlorine in, 70
— sodium in, 67
Röntgen rays, discovery of, 92
— nature of, 94
Runge and Precht, heat output of radium, 101
Rutherford, age of minerals, 154
— Becquerel rays, 93
Rutherford and Barnes, heat output of radium, 101
Rutherford and Geiger, counting of α-particles, 138
Rutherford and Soddy, disintegration hypothesis, 96, 98
— radium and sun’s heat, 116
Schwarz, 49
Schuchert, 78
Schweidler v. and Hess, heat output of radium, 101
Sederholm, annual deposits, 43
— thickness of sediments, 43, 85
Sediment carried by rivers, 57
Sediments, volume of, 65, 68, 88
— pore space of, 65
— thickness of, 43, 44, 76, 77
— rate of deposit of, 81, 84
— rate of accumulation of, 47
— radium in, 131
— thorium in, 131
— sodium in, 67
— on Continental shelf, 83
— oceanic, 59
— time represented by, 86
Sedimentation, cyclic, 41, 43, 44
Soddy, generation of helium, 96
— — radium, 99
Sodium in rivers, 53, 64
— in ocean, 53, 64
— in igneous rocks, 67, 71
— in sediments, 67
— age of ocean, 64
— wind-borne, 66
— in sewage, 66
Solar system, origin of, 23 _et seq._
Solar constant, 113
Sollas, thickness of sediments, 76
— age of earth, 86, 87
— age of oceans, 62, 63
— biology and time, 21
— glacial retreat, 43
Steno, 5, 6
St. Meyer and Hess, heat output of radium, 101
Strutt, determination of radium, 105
— radium in rocks, 129, 130
— rate of generation of helium, 139
— leakage of helium, 149
— accumulation of helium, 154-157
Sun, origin of, 25
— helium in, 95, 117
— radium in, 117
— temperature of, 119
— duration of heat of, 12, 14, 112, 114, 120
Suzuki, age of earth, 128
Temperature—gradient of earth’s crust, 122
— due to radium, 128, 132
Thermal conductivity of rocks, 123
Thomson. _See_ Kelvin
Thorianite, generation of helium by, 139
Thorium, discovery as a radio-element, 92
— family of elements, 100, 190
— end product of, 100
— heat output of, 131
— distribution of, in rocks, 131
Tidal retardation, 13, 26, 27
Transformation, radioactive, 97, 98
— end products of, 100
Unconformities, 160
Underground temperature, 12, 13, 122-8
Uniformitarianism, rise of, 7, 9
— criticised, 12, 78, 174
Uraninite, generation of helium by, 139
— lead-ratio of, 151, 158-62
Uranium, atomic weight of, 142
— discovery as a radio-element, 92
— phosphorescence of salts 91, 92
— range of α-rays from, 178
— family of, 100, 190
— end products of, 100, 140-142
— generation of radium from, 99
— — helium from, 142
— — lead from, 142
— heat output of, 116, 131
— half-life period of, 179
— time-average of, 143, 179-182
Uranium-bearing minerals—
Llano Co., 151, 152
Connecticut, 158, 159,
Carolina, 159, 160
S. Norway, 161, 162
Brevig, 163, 164
Mozambique, 153, 160
— alteration of, 150-153
Ussher, chronology of, 3, 10
Van Hise, 88
Velocity of α-particles, 177
Watts, marine erosion, 58
— cycle of deposition, 85
Weathering of rocks, 48, 49
— of minerals, 150
Wilson, W. E., sun’s heat, 116
Zircon, helium-ratio in, 156
— lead-ratio in, 160, 164
Zoroaster, chronology of, 3
WILLIAM BRENDON AND SON, LTD.
PRINTERS, PLYMOUTH
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