Читать книгу The Birth-Time of the World and Other Scientific Essays - John Joly - Страница 5
THE BIRTH-TIME OF THE WORLD [1]
ОглавлениеLONG ago Lucretius wrote: "For lack of power to solve the
question troubles the mind with doubts, whether there was ever a
birth-time of the world and whether likewise there is to be any
end." "And if" (he says in answer) "there was no birth-time of
earth and heaven and they have been from everlasting, why before
the Theban war and the destruction of Troy have not other poets
as well sung other themes? Whither have so many deeds of men so
often passed away, why live they nowhere embodied in lasting
records of fame? The truth methinks is that the sum has but a
recent date, and the nature of the world is new and has but
lately had its commencement."[2]
Thus spake Lucretius nearly 2,000 years ago. Since then we have
attained another standpoint and found very different limitations.
To Lucretius the world commenced with man, and the answer he
would give to his questions was in accord with his philosophy: he
would date the birth-time of the world from the time when
[1] A lecture delivered before the Royal Dublin Society, February
6th, 1914. _Science Progress_, vol. ix., p. 37
[2] _De Rerum Natura_, translated by H. A. J. Munro (Cambridge,
1886).
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poets first sang upon the earth. Modern Science has along with
the theory that the Earth dated its beginning with the advent of
man, swept utterly away this beautiful imagining. We can, indeed,
find no beginning of the world. We trace back events and come to
barriers which close our vista—barriers which, for all we know,
may for ever close it. They stand like the gates of ivory and of
horn; portals from which only dreams proceed; and Science cannot
as yet say of this or that dream if it proceeds from the gate of
horn or from that of ivory.
In short, of the Earth's origin we have no certain knowledge; nor
can we assign any date to it. Possibly its formation was an event
so gradual that the beginning was spread over immense periods. We
can only trace the history back to certain events which may with
considerable certainty be regarded as ushering in our geological
era.
Notwithstanding our limitations, the date of the birth-time of
our geological era is the most important date in Science. For in
taking into our minds the spacious history of the universe, the
world's age must play the part of time-unit upon which all our
conceptions depend. If we date the geological history of the
Earth by thousands of years, as did our forerunners, we must
shape our ideas of planetary time accordingly; and the duration
of our solar system, and of the heavens, becomes comparable with
that of the dynasties of ancient nations. If by millions of
years, the sun and stars are proportionately venerable. If by
hundreds or thousands of millions of
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years the human mind must consent to correspondingly vast epochs
for the duration of material changes. The geological age plays
the same part in our views of the duration of the universe as the
Earth's orbital radius does in our views of the immensity of
space. Lucretius knew nothing of our time-unit: his unit was the
life of a man. So also he knew nothing of our space-unit, and he
marvels that so small a body as the sun can shed so much, heat
and light upon the Earth.
A study of the rocks shows us that the world was not always what
it now is and long has been. We live in an epoch of denudation.
The rains and frosts disintegrate the hills; and the rivers roll
to the sea the finely divided particles into which they have been
resolved; as well as the salts which have been leached from them.
The sediments collect near the coasts of the continents; the
dissolved matter mingles with the general ocean. The geologist
has measured and mapped these deposits and traced them back into
the past, layer by layer. He finds them ever the same;
sandstones, slates, limestones, etc. But one thing is not the
same. _Life_ grows ever less diversified in character as the
sediments are traced downwards. Mammals and birds, reptiles,
amphibians, fishes, die out successively in the past; and barren
sediments ultimately succeed, leaving the first beginnings of
life undecipherable. Beneath these barren sediments lie rocks
collectively differing in character from those above: mainly
volcanic or poured out from fissures in
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the early crust of the Earth. Sediments are scarce among these
materials.[1]
There can be little doubt that in this underlying floor of
igneous and metamorphic rocks we have reached those surface
materials of the earth which existed before the long epoch of
sedimentation began, and before the seas came into being. They
formed the floor of a vaporised ocean upon which the waters
condensed here and there from the hot and heavy atmosphere. Such
were the probable conditions which preceded the birth-time of the
ocean and of our era of life and its evolution.
It is from this epoch we date our geological age. Our next
purpose is to consider how long ago, measured in years, that
birth-time was.
That the geological age of the Earth is very great appears from
what we have already reviewed. The sediments of the past are many
miles in collective thickness: yet the feeble silt of the rivers
built them all from base to summit. They have been uplifted from
the seas and piled into mountains by movements so slow that
during all the time man has been upon the Earth but little change
would have been visible. The mountains have again been worn down
into the ocean by denudation and again younger mountains built
out of their redeposited materials. The contemplation of such
vast events
[1] For a description of these early rocks, see especially the
monograph of Van Hise and Leith on the pre-Cambrian Geology of
North America (Bulletin 360, U.S. Geol. Survey).
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prepares our minds to accept many scores of millions of years or
hundreds of millions of years, if such be yielded by our
calculations.
THE AGE AS INFERRED FROM THE THICKNESS OF THE SEDIMENTS
The earliest recognised method of arriving at an estimate of the
Earth's geological age is based upon the measurement of the
collective sediments of geological periods. The method has
undergone much revision from time to time. Let us briefly review
it on the latest data.
The method consists in measuring the depths of all the successive
sedimentary deposits where these are best developed. We go all
over the explored world, recognising the successive deposits by
their fossils and by their stratigraphical relations, measuring
their thickness and selecting as part of the data required those
beds which we believe to most completely represent each
formation. The total of these measurements would tell us the age
of the Earth if their tale was indeed complete, and if we knew
the average rate at which they have been deposited. We soon,
however, find difficulties in arriving at the quantities we
require. Thus it is not easy to measure the real thickness of a
deposit. It may be folded back upon itself, and so we may measure
it twice over. We may exaggerate its thickness by measuring it
not quite straight across the bedding or by unwittingly including
volcanic materials. On the other hand, there
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may be deposits which are inaccessible to us; or, again, an
entire absence of deposits; either because not laid down in the
areas we examine, or, if laid down, again washed into the sea.
These sources of error in part neutralise one another. Some make
our resulting age too long, others make it out too short. But we
do not know if a balance of error does not still remain. Here,
however, is a table of deposits which summarises a great deal of
our knowledge of the thickness of the stratigraphical
accumulations. It is due to Sollas.[1]
Feet.
Recent and Pleistocene - - 4,000
Pliocene - - 13,000
Miocene - - 14,000
Oligocene - - 2,000
Eocene - - 20,000
63,000
Upper Cretaceous - - 24,000
Lower Cretaceous - - 20,000
Jurassic - - 8,000
Trias - - 7,000
69,000
Permian - - 2,000
Carboniferous - - 29,000
Devonian - - 22,000
63,000
Silurian - - 15,000
Ordovician - - 17,000
Cambrian - - 6,000
58,000
Algonkian—Keeweenawan - - 50,000
Algonkian—Animikian - - 14,000
Algonkian—Huronian - - 18,000
82,000
Archæan - - ?
Total - - 335,000 feet.
[1] Address to the Geol. Soc. of London, 1509.
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In the next place we require to know the average rate at which
these rocks were laid down. This is really the weakest link in
the chain. The most diverse results have been arrived at, which
space does not permit us to consider. The value required is most
difficult to determine, for it is different for the different
classes of material, and varies from river to river according to
the conditions of discharge to the sea. We may probably take it
as between two and six inches in a century.
Now the total depth of the sediments as we see is about 335,000
feet (or 64 miles), and if we take the rate of collecting as
three inches in a hundred years we get the time for all to
collect as 134 millions of years. If the rate be four inches, the
time is soo millions of years, which is the figure Geikie
favoured, although his result was based on somewhat different
data. Sollas most recently finds 80 millions of years.[1]
THE AGE AS INFERRED FROM THE MASS OF THE SEDIMENTS
In the above method we obtain our result by the measurement of
the linear dimensions of the sediments. These measurements, as we
have seen, are difficult to arrive at. We may, however, proceed
by measurements of the mass of the sediments, and then the method
becomes more definite. The new method is pursued as follows:
[1] Geikie, _Text Book of Geology_ (Macmillan, 1903), vol. i., p.
73, _et seq._ Sollas, _loc. cit._ Joly, _Radioactivity and Geology_
(Constable, 1909), and Phil. Mag., Sept. 1911.
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The total mass of the sediments formed since denudation began may
be ascertained with comparative accuracy by a study of the
chemical composition of the waters of the ocean. The salts in the
ocean are undoubtedly derived from the rocks; increasing age by
age as the latter are degraded from their original character
under the action of the weather, etc., and converted to the
sedimentary form. By comparing the average chemical composition
of these two classes of material—the primary or igneous rocks and
the sedimentary—it is easy to arrive at a knowledge of how much
of this or that constituent was given to the ocean by each ton of
primary rock which was denuded to the sedimentary form. This,
however, will not assist us to our object unless the ocean has
retained the salts shed into it. It has not generally done so. In
the case of every substance but one the ocean continually gives
up again more or less of the salts supplied to it by the rivers.
The one exception is the element sodium. The great solubility of
its salts has protected it from abstraction, and it has gone on
collecting during geological time, practically in its entirety.
This gives us the clue to the denudative history of the
Earth.[1]
The process is now simple. We estimate by chemical examination of
igneous and sedimentary rocks the amount of sodium which has been
supplied to the ocean per ton of sediment produced by denudation.
We also calculate
[1] _Trans. R.D.S._, May, 1899.
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the amount of sodium contained in the ocean. We divide the one
into the other (stated, of course, in the same units of mass),
and the quotient gives us the number of tons of sediment. The
most recent estimate of the sediments made in this manner affords
56 x 1016 tonnes.[1]
Now we are assured that all this sediment was transported by the
rivers to the sea during geological time. Thus it follows that,
if we can estimate the average annual rate of the river supply of
sediments to the ocean over the past, we can calculate the
required age. The land surface is at present largely covered with
the sedimentary rocks themselves. Sediment derived from these
rocks must be regarded as, for the most part, purely cyclical;
that is, circulating from the sea to the land and back again. It
does not go to increase the great body of detrital deposits. We
cannot, therefore, take the present river supply of sediment as
representing that obtaining over the long past. If the land was
all covered still with primary rocks we might do so. It has been
estimated that about 25 per cent. of the existing continental
area is covered with archæan and igneous rocks, the remainder
being sediments.[2] On this estimate we may find valuable
[1] Clarke, _A Preliminary Study of Chemical Denudation_
(Washington, 1910). My own estimate in 1899 (_loc. cit._) made as a
test of yet another method of finding the age, showed that the
sediments may be taken as sufficient to form a layer 1.1 mile
deep if spread uniformly over the continents; and would amount to
64 x 1018 tons.
[2] Van Tillo, _Comptes Rendues_ (Paris), vol. cxiv., 1892.
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major and minor limits to the geological age. If we take 25 per
cent. only of the present river supply of sediment, we evidently
fix a major limit to the age, for it is certain that over the
past there must have been on the average a faster supply. If we
take the entire river supply, on similar reasoning we have what
is undoubtedly a minor limit to the age.
The river supply of detrital sediment has not been very
extensively investigated, although the quantities involved may be
found with comparative ease and accuracy. The following table
embodies the results obtained for some of the leading rivers.[1]
Mean annual Total annual Ratio of
discharge in sediment in sediment
cubic feet thousands to water
per second. of tons. by weight.
Potomac - 20,160 5,557 1 : 3.575
Mississippi - 610,000 406,250 1 : 1,500
Rio Grande - 1,700 3,830 1 : 291
Uruguay - 150,000 14,782 1 : 10,000
Rhone - 65,850 36,000 1 : 1,775
Po - 62,200 67,000 1 : 900
Danube - 315,200 108,000 1 : 2,880
Nile - 113,000 54,000 1 : 2,050
Irrawaddy - 475,000 291,430 1 : 1,610
Mean - 201,468 109,650 1 : 2,731
We see that the ratio of the weight of water to the
[1] Russell, _River Development_ (John Murray, 1888).
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weight of transported sediment in six out of the nine rivers does
not vary widely. The mean is 2,730 to 1. But this is not the
required average. The water-discharge of each river has to be
taken into account. If we ascribe to the ratio given for each
river the weight proper to the amount of water it discharges, the
proportion of weight of water to weight of sediment, for the
whole quantity of water involved, comes out as 2,520 to 1.
Now if this proportion holds for all the rivers of the
world—which collectively discharge about 27 x 1012 tonnes of
water per annum—the river-born detritus is 1.07 x 1010 tonnes. To
this an addition of 11 per cent. has to be made for silt pushed
along the river-bed.[1] On these figures the minor limit to the
age comes out as 47 millions of years, and the major limit as 188
millions. We are here going on rather deficient estimates, the
rivers involved representing only some 6 per cent. of the total
river supply of water to the ocean. But the result is probably
not very far out.
We may arrive at a probable age lying between the major and minor
limits. If, first, we take the arithmetic mean of these limits,
we get 117 millions of years. Now this is almost certainly
excessive, for we here assume that the rate of covering of the
primary rocks by sediments was uniform. It would not be so,
however, for the rate of supply of original sediment must have
been continually diminishing
[1] According to observations made on the Mississippi (Russell,
_loc. cit._).
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during geological time, and hence we may assume that the rate of
advance of the sediments on the primary rocks has also been
diminishing. Now we may probably take, as a fair assumption, that
the sediment-covered area was at any instant increasing at a rate
proportionate to the rate of supply of sediment; that is, to the
area of primary rocks then exposed. On this assumption the age is
found to be 87 millions of years.
THE AGE BY THE SODIUM OF THE OCEAN
I have next to lay before you a quite different method. I have
already touched upon the chemistry of the ocean, and on the
remarkable fact that the sodium contained in it has been
preserved, practically, in its entirety from the beginning of
geological time.
That the sea is one of the most beautiful and magnificent sights
in Nature, all admit. But, I think, to those who know its story
its beauty and magnificence are ten-fold increased. Its saltness
it due to no magic mill. It is the dissolved rocks of the Earth
which give it at once its brine, its strength, and its buoyancy.
The rivers which we say flow with "fresh" water to the sea
nevertheless contain those traces of salt which, collected over
the long ages, occasion the saltness of the ocean. Each gallon of
river water contributes to the final result; and this has been
going on since the beginning of our era. The mighty total of the
rivers is 6,500 cubic miles of water in the year!
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There is little doubt that the primeval ocean was in the
condition of a fresh-water lake. It can be shown that a primitive
and more rapid solution of the original crust of the Earth by the
slowly cooling ocean would have given rise to relatively small
salinity. The fact is, the quantity of salts in the ocean is
enormous. We are only now concerned with the sodium; but if we
could extract all the rock-salt (the chloride of sodium) from the
ocean we should have enough to cover the entire dry land of the
Earth to a depth of 400 feet. It is this gigantic quantity which
is going to enter into our estimate of the Earth's age. The
calculated mass of sodium contained in this rock-salt is 14,130
million million tonnes.
If now we can determine the rate at which the rivers supply
sodium to the ocean, we can determine the age.[1] As the result
of many thousands of river analyses, the total amount of sodium
annually discharged to the ocean
[1] _Trans. R.D.S._, 1899. A paper by Edmund Halley, the
astronomer, in the _Philosophical Transactions of the Royal
Society_ for 1715, contains a suggestion for finding the age of
the world by the following procedure. He proposes to make
observations on the saltness of the seas and ocean at intervals
of one or more centuries, and from the increment of saltness
arrive at their age. The measurements, as a matter of fact, are
impracticable. The salinity would only gain (if all remained in
solution) one millionth part in Too years; and, of course, the
continuous rejection of salts by the ocean would invalidate the
method. The last objection also invalidates the calculation by T.
Mellard Reade (_Proc. Liverpool Geol. Soc._, 1876) of a minor limit
to the age by the calcium sulphate in the ocean. Both papers were
quite unknown to me when working out my method. Halley's paper
was, I think, only brought to light in 1908.
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by all the rivers of the world is found to be probably not far
from 175 million tonnes.[1] Dividing this into the mass of
oceanic sodium we get the age as 80.7 millions of years. Certain
corrections have to be applied to this figure which result in
raising it to a little over 90 millions of years. Sollas, as the
result of a careful review of the data, gets the age as between
80 and 150 millions of years. My own result[2] was between 80 and
90 millions of years; but I subsequently found that upon certain
extreme assumptions a maximum age might be arrived at of 105
millions of years.[3] Clarke regards the 80.7 millions of years
as certainly a maximum in the light of certain calculations by
Becker.[4]
The order of magnitude of these results cannot be shaken unless
on the assumption that there is something entirely misleading in
the existing rate of solvent denudation. On the strength of the
results of another and
[1] F. W. Clarke, _A Preliminary Study of Chemical Denudation_
(Smithsonian Miscellaneous Collections, 1910).
[2] _Loc. cit._
[3] "The Circulation of Salt and Geological Time" (Geol. Mag.,
1901, p. 350).
[4] Becker (loc. cit.), assuming that the exposed igneous and
archæan rocks alone are responsible for the supply of sodium to
the ocean, arrives at 74 millions of years as the geological age.
This matter was discussed by me formerly (Trans. R.D.S., 1899,
pp. 54 _et seq._). The assumption made is, I believe, inadmissible.
It is not supported by river analyses, or by the chemical
character of residual soils from sedimentary rocks. There may be
some convergence in the rate of solvent denudation, but—as I
think on the evidence—in our time unimportant.
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entirely different method of approaching the question of the
Earth's age (which shall be presently referred to), it has been
contended that it is too low. It is even asserted that it is from
nine to fourteen times too low. We have then to consider whether
such an enormous error can enter into the method. The
measurements involved cannot be seriously impugned. Corrections
for possible errors applied to the quantities entering into this
method have been considered by various writers. My own original
corrections have been generally confirmed. I think the only point
left open for discussion is the principle of uniformitarianism
involved in this method and in the methods previously discussed.
In order to appreciate the force of the evidence for uniformity
in the geological history of the Earth, it is, of course,
necessary to possess some acquaintance with geological science.
Some of the most eminent geologists, among whom Lyell and
Geikie[1] may be mentioned, have upheld the doctrine of
uniformity. It must here suffice to dwell upon a few points
having special reference to the matter under discussion.
The mere extent of the land surface does not, within limits,
affect the question of the rate of denudation. This arises from
the fact that the rain supply is quite insufficient to denude the
whole existing land surface. About 30 per cent. of it does not,
in fact, drain to the
[1] See especially Geikie's Address to Sect. C., Brit. Assoc.
Rep., 1399.
15
ocean. If the continents become invaded by a great transgression
of the ocean, this "rainless" area diminishes: and the denuded
area advances inwards without diminution. If the ocean recedes
from the present strand lines, the "rainless" area advances
outwards, but, the rain supply being sensibly constant, no change
in the river supply of salts is to be expected.
Age-long submergence of the entire land, or of any very large
proportion of what now exists, is negatived by the continuous
sequence of vast areas of sediment in every geologic age from the
earliest times. Now sediment-receiving areas always are but a
small fraction of those exposed areas whence the sediments are
supplied.[1] Hence in the continuous records of the sediments we
have assurance of the continuous exposure of the continents above
the ocean surface. The doctrine of the permanency of the
continents has in its main features been accepted by the most
eminent authorities. As to the actual amount of land which was
exposed during past times to denudative effects, no data exist to
show it was very different from what is now exposed. It has been
estimated that the average area of the North American continent
over geologic time was about eight-tenths of its existing
area.[2] Restorations of other continents, so far as they have
been attempted, would not
[1] On the strength of the Mississippi measurements about 1 to 18
(Magee, _Am. Jour. of Sc._, 1892, p. 188).
[2] Schuchert, _Bull. Geol. Soc. Am._, vol. xx., 1910.
16
suggest any more serious divergency one way or the other.
That climate in the oceans and upon the land was throughout much
as it is now, the continuous chain of teeming life and the
sensitive temperature limits of protoplasmic existence are
sufficient evidence.[1] The influence at once of climate and of
elevation of the land may be appraised at their true value by the
ascertained facts of solvent denudation, as the following table
shows.
Tonnes removed in Mean elevation.
solution per square Metres.
mile per annum.
North America - 79 700
South America - 50 650
Europe - 100 300
Asia - 84 950
Africa - 44 650
In this table the estimated number of tonnes of matter in
solution, which for every square mile of area the rivers convey
to the ocean in one year, is given in the first column. These
results are compiled by Clarke from a very large number of
analyses of river waters. The second column of the table gives
the mean heights in metres above sea level of the several
continents, as cited by Arrhenius.[2]
Of all the denudation results given in the table, those relating
to North America and to Europe are far the
[1] See also Poulton, Address to Sect. D., Brit. Assoc. Rep.,
1896.
[2] _Lehybuch dev Kosmischen Physik_, vol. i., p. 347.
17
most reliable. Indeed these may be described as highly reliable,
being founded on some thousands of analyses, many of which have
been systematically pursued through every season of the year.
These show that Europe with a mean altitude of less than half
that of North America sheds to the ocean 25 per cent. more salts.
A result which is to be expected when the more important factors
of solvent denudation are given intelligent consideration and we
discriminate between conditions favouring solvent and detrital
denudation respectively: conditions in many cases
antagonistic.[1] Hence if it is true, as has been stated, that we
now live in a period of exceptionally high continental elevation,
we must infer that the average supply of salts to the ocean by
the rivers of the world is less than over the long past, and
that, therefore, our estimate of the age of the Earth as already
given is excessive.
There is, however, one condition which will operate to unduly
diminish our estimate of geologic time, and it is a condition
which may possibly obtain at the present time. If the land is, on
the whole, now sinking relatively to the ocean level, the
denudation area tends, as we have seen, to move inwards. It will
thus encroach upon regions which have not for long periods
drained to the ocean. On such areas there is an accumulation of
soluble salts which the deficient rivers have not been able to
carry to the ocean. Thus the salt content of certain of
[1] See the essay on Denudation.
18
the rivers draining to the ocean will be influenced not only by
present denudative effects, but also by the stored results of
past effects. Certain rivers appear to reveal this unduly
increased salt supply those which flow through comparatively arid
areas. However, the flowoff of such tributaries is relatively
small and the final effects on the great rivers apparently
unimportant—a result which might have been anticipated when the
extremely slow rate of the land movements is taken into account.
The difficulty of effecting any reconciliation of the methods
already described and that now to be given increases the interest
both of the former and the latter.
THE AGE BY RADIOACTIVE TRANSFORMATIONS
Rutherford suggested in 1905 that as helium was continually being
evolved at a uniform rate by radioactive substances (in the form
of the alpha rays) a determination of the age of minerals
containing the radioactive elements might be made by measurements
of the amount of the stored helium and of the radioactive
elements giving rise to it, The parent radioactive substances
are—according to present knowledge—uranium and thorium. An
estimate of the amounts of these elements present enables the
rate of production of the helium to be calculated. Rutherford
shortly afterwards found by this method an age of 240 millions of
years for a radioactive mineral of presumably remote age. Strutt,
who carried
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his measurements to a wonderful degree of refinement, found the
following ages for mineral substances originating in different
geological ages:
Oligocene - 8.4 millions of years.
Eocene - 31 millions of years.
Lower Carboniferous - 150 millions of years.
Archæan - 750 millions of years.
Periods of time much less than, and very inconsistent with, these
were also found. The lower results are, however, easily explained
if we assume that the helium—which is a gas under prevailing
conditions—escapes in many cases slowly from the mineral.
Another product of radioactive origin is lead. The suggestion
that this substance might be made available to determine the age
of the Earth also originated with Rutherford. We are at least
assured that this element cannot escape by gaseous diffusion from
the minerals. Boltwood's results on the amount of lead contained
in minerals of various ages, taken in conjunction with the amount
of uranium or parent substance present, afforded ages rising to
1,640 millions of years for archæan and 1,200 millions for
Algonkian time. Becker, applying the same method, obtained
results rising to quite incredible periods: from 1,671 to 11,470
millions of years. Becker maintained that original lead rendered
the determinations indefinite. The more recent results of Mr. A.
Holmes support the conclusion that "original" lead may be present
and may completely falsify results derived
20
from minerals of low radioactivity in which the derived lead
would be small in amount. By rejecting such results as appeared
to be of this character, he arrives at 370 millions of years as
the age of the Devonian.
I must now describe a very recent method of estimating the age of
the Earth. There are, in certain rock-forming minerals,
colour-changes set up by radioactive causes. The minute and
curious marks so produced are known as haloes; for they surround,
in ringlike forms, minute particles of included substances which
contain radioactive elements. It is now well known how these
haloes are formed. The particle in the centre of the halo
contains uranium or thorium, and, necessarily, along with the
parent substance, the various elements derived from it. In the
process of transformation giving rise to these several derived
substances, atoms of helium—the alpha rays—projected with great
velocity into the surrounding mineral, occasion the colour
changes referred to. These changes are limited to the distance to
which the alpha rays penetrate; hence the halo is a spherical
volume surrounding the central substance.[1]
The time required to form a halo could be found if on the one
hand we could ascertain the number of alpha rays ejected from the
nucleus of the halo in, say, one year, and, on the other, if we
determined by experiment just how many alpha rays were required
to produce the same
[1] _Phil. Mag._, March, 1907 and February, 1910; also _Bedrock_,
January, 1913. See _Pleochroic Haloes_ in this volume.
21
amount of colour alteration as we perceive to extend around the
nucleus.
The latter estimate is fairly easily and surely made. But to know
the number of rays leaving the central particle in unit time we
require to know the quantity of radioactive material in the
nucleus. This cannot be directly determined. We can only, from
known results obtained with larger specimens of just such a
mineral substance as composes the nucleus, guess at the amount of
uranium, or it may be thorium, which may be present.
This method has been applied to the uranium haloes of the mica of
County Carlow.[1] Results for the age of the halo of from 20 to
400 millions of years have been obtained. This mica was probably
formed in the granite of Leinster in late Silurian or in Devonian
times.
The higher results are probably the least in error, upon the data
involved; for the assumption made as to the amount of uranium in
the nuclei of the haloes was such as to render the higher results
the more reliable.
This method is, of course, a radioactive method, and similar to
the method by helium storage, save that it is free of the risk of
error by escape of the helium, the effects of which are, as it
were, registered at the moment of its production, so that its
subsequent escape is of no moment.
[1] Joly and Rutherford, _Phil. Mag._, April, 1913.
22
REVIEW OF THE RESULTS
We shall now briefly review the results on the geological age of
the Earth.
By methods based on the approximate uniformity of denudative
effects in the past, a period of the order of 100 millions of
years has been obtained as the duration of our geological age;
and consistently whether we accept for measurement the sediments
or the dissolved sodium. We can give reasons why these
measurements might afford too great an age, but we can find
absolutely no good reason why they should give one much too low.
By measuring radioactive products ages have been found which,
while they vary widely among themselves, yet claim to possess
accuracy in their superior limits, and exceed those derived from
denudation from nine to fourteen times.
In this difficulty let us consider the claims of the radioactive
method in any of its forms. In order to be trustworthy it must be
true; (1) that the rate of transformation now shown by the parent
substance has obtained throughout the entire past, and (2) that
there were no other radioactive substances, either now or
formerly existing, except uranium, which gave rise to lead. As
regards methods based on the production of helium, what we have
to say will largely apply to it also. If some unknown source of
these elements exists we, of course, on our assumption
overestimate the age.
23
As regards the first point: In ascribing a constant rate of
change to the parent substance—which Becker (loc. cit.) describes
as "a simple though tremendous extrapolation"—we reason upon
analogy with the constant rate of decay observed in the derived
radioactive bodies. If uranium and thorium are really primary
elements, however, the analogy relied on may be misleading; at
least, it is obviously incomplete. It is incomplete in a
particular which may be very important: the mode of origin of
these parent bodies—whatever it may have been—is different to
that of the secondary elements with which we compare them. A
convergence in their rate of transformation is not impossible, or
even improbable, so far as we known.
As regards the second point: It is assumed that uranium alone of
the elements in radioactive minerals is ultimately transformed to
lead by radioactive changes. We must consider this assumption.
Recent advances in the chemistry of the radioactive elements has
brought out evidence that all three lines of radioactive descent
known to us—_i.e._ those beginning with uranium, with thorium,
and with actinium—alike converge to lead.[1] There are
difficulties in the way of believing that all the lead-like atoms
so produced ("isotopes" of lead, as Soddy proposes to call them)
actually remain as stable lead in the minerals. For one
[1] See Soddy's _Chemistry of the Radioactive Elements_ (Longmans,
Green & Co.).
24
thing there is sometimes, along with very large amounts of
thorium, an almost entire absence of lead in thorianites and
thorites. And in some urano—thorites the lead may be noticed to
follow the uranium in approximate proportionality,
notwithstanding the presence of large amounts of thorium.[1] This
is in favour of the assumption that all the lead present is
derived from the uranium. The actinium is present in negligibly
small amounts.
On the other hand, there is evidence arising from the atomic
weight of lead which seems to involve some other parent than
uranium. Soddy, in the work referred to, points this out. The
atomic weight of radium is well known, and uranium in its descent
has to change to this element. The loss of mass between radium
and uranium-derived lead can be accurately estimated by the
number of alpha rays given off. From this we get the atomic
weight of uranium-derived lead as closely 206. Now the best
determinations of the atomic weight of normal lead assign to this
element an atomic weight of closely
[1] It seems very difficult at present to suggest an end product
for thorium, unless we assume that, by loss of electrons, thorium
E, or thorium-lead, reverts to a substance chemically identical
with thorium itself. Such a change—whether considered from the
point of view of the periodic law or of the radioactive theory
would involve many interesting consequences. It is, of course,
quite possible that the nature of the conditions attending the
deposition of the uranium ores, many of which are comparatively
recent, are responsible for the difficulties observed. The
thorium and uranium ores are, again, specially prone to
alteration.
25
207. By a somewhat similar calculation it is deduced that
thorium-derived lead would possess the atomic weight of 208. Thus
normal lead might be an admixture of uranium- and thorium-derived
lead. However, as we have seen, the view that thorium gives rise
to stable lead is beset with some difficulties.
If we are going upon reliable facts and figures, we must, then,
assume: (a) That some other element than uranium, and genetically
connected with it (probably as parent substance), gives rise, or
formerly gave rise, to lead of heavier atomic weight than normal
lead. It may be observed respecting this theory that there is
some support for the view that a parent substance both to uranium
and thorium has existed or possibly exists. The evidence is found
in the proportionality frequently observed between the amounts of
thorium and uranium in the primary rocks.[1] Or: (b) We may meet
the difficulties in a simpler way, which may be stated as
follows: If we assume that all stable lead is derived from
uranium, and at the same time recognise that lead is not
perfectly homogeneous in atomic weight, we must, of necessity,
ascribe to uranium a similar want of homogeneity; heavy atoms of
uranium giving rise to heavy
[1] Compare results for the thorium content of such rocks
(appearing in a paper by the author Cong. Int. _de Radiologie et
d'Electricité_, vol. i., 1910, p. 373), and those for the radium
content, as collected in _Phil. Mag._, October, 1912, p. 697.
Also A. L. Fletcher, _Phil. Mag._, July, 1910; January, 1911, and
June, 1911. J. H. J. Poole, _Phil. Mag._, April, 1915
26
atoms of lead and light atoms of uranium generating light atoms
of lead. This assumption seems to be involved in the figures
upon, which we are going. Still relying on these figures, we
find, however, that existing uranium cannot give rise to lead of
normal atomic weight. We can only conclude that the heavier atoms
of uranium have decayed more rapidly than the lighter ones. In
this connection it is of interest to note the complexity of
uranium as recently established by Geiger, although in this case
it is assumed that the shorter-lived isotope bears the relation
of offspring to the longer-lived and largely preponderating
constituent. However, there does not seem to be any direct proof
of this as yet.
From these considerations it would seem that unless the atomic
weight of lead in uraninites, etc., is 206, the former complexity
and more accelerated decay of uranium are indicated in the data
respecting the atomic weights of radium and lead[1]. As an
alternative view, we may assume, as in our first hypothesis, that
some elementally different but genetically connected substance,
decaying along branching lines of descent at a rate sufficient to
practically remove the whole of it during geological time,
formerly existed. Whichever hypothesis we adopt
[1] Later investigation has shown that the atomic weight of lead
in uranium-bearing ores is about 206.6 (see Richards and Lembert,
_Journ. of Am. Claem. Soc._, July, 1914). This result gives support
to the view expressed above.
27
we are confronted by probabilities which invalidate
time-measurements based on the lead and helium ratio in minerals.
We have, in short, grave reason to question the measure of
uniformitarianism postulated in finding the age by any of the
known radioactive methods.
That we have much to learn respecting our assumptions, whether we
pursue the geological or the radioactive methods of approaching
the age of our era, is, indeed, probable. Whatever the issue it
is certain that the reconciling facts will leave us with much
more light than we at present possess either as respects the
Earth's history or the history of the radioactive elements. With
this necessary admission we leave our study of the Birth-Time of
the World.
It has led us a long way from Lucretius. We do not ask if other
Iliads have perished; or if poets before Homer have vainly sung,
becoming a prey to all-consuming time. We move in a greater
history, the landmarks of which are not the birth and death of
kings and poets, but of species, genera, orders. And we set out
these organic events not according to the passing generations of
man, but over scores or hundreds of millions of years.
How much Lucretius has lost, and how much we have gained, is
bound up with the question of the intrinsic value of knowledge
and great ideas. Let us appraise knowledge as we would the
Homeric poems, as some-
28
thing which ennobles life and makes it happier. Well, then, we
are, as I think, in possession today of some of those lost Iliads
and Odysseys for which Lucretius looked in vain.[1]
[1] The duration in the past of Solar heat is necessarily bound
up with the geological age. There is no known means (outside
speculative science) of accounting for more than about 30 million
years of the existing solar temperature in the past. In this
direction the age seems certainly limited to 100 million years.
See a review of the question by Dr. Lindemann in Nature, April
5th, 1915.
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