the unknown, are experienced, whether we endeavour
to review those activities which slowly laid the
foundations of mountains, uplifted and again destroyed
them ; or whether we contemplate the biological past
which stands to our brief length of days as the integral
P 2
212 RADIOACTIVITY AND GEOLOGY.
life to the life of the individual. The primary question
of historical geology is the age which has encompassed
the collective events since the beginning.
The age of the earth may indeed claim an importance
outside of geological science. For is it not the clue to
the chronological scale of our solar system and for
want of a better even of our universe ? Distinct from
the age of the earth, the geological age, or period since
denudation began and life became possible, if of lesser
cosmic importance, is still a minor limit of priceless
value. If geological time had been as once was
believed almost eternal in duration, then must our
conception of solar chronology be enlarged accordingly.
If, on the other hand, Kelvin's twenty millions of years
could be established, then must solar events be, com-
paratively speaking, short-lived, and the whole cosmos
may have entered on its present phase of evolution
within a period correspondingly recent.
It is, in a word, the primary chronological fact of
inorganic evolution, as it is of organic evolution ; and
to it our fundamental ideas of duration must be re-
ferred, as the depths of space are referred to the
distance separating earth and sun.
The discoveries of radioactive science early suggested
the possibility of approaching the problem of the geo-
logical age in a new way. Uranium is constantly
breaking down into an element of less atomic weight ;
and this transformation progresses alike within the
hardest rocks and in the waters of the ocean. The
derived element changes to one of yet lower atomic
weight, and so this material degradation proceeds until a
URANIUM AND THE AGE OF THE EARTH. 213
stable substance is reached at which the change ceases ;
or ceases for the present time. Now, what becomes of
the lost mass ? We have no reason to believe that any
part of it is destroyed. Indeed, we know that it is not
destroyed; we know that it is radiated in the alpha
particles, the energy of which has been so important
a factor in geological dynamics. The process of trans-
formation is thus attended with the rejection of portions
of the atom at every change of atomic weight. The
discarded particles are physically alike in every case,
so far as we know ; and, as Kutherf ord long ago pre-
dicted, are in themselves a recognized kind of matter
helium.
This substance, radiated in an ionic or electrified
state, accumulates amidst the changing matter which
gives rise to it, unless there be some avenue of escape.
In the case of the uranium family the number of alpha
rays or helium atoms, which the entire sequence of
changes involves, is fairly well known. If we know
the rate at which these changes progress and this
knowledge is also ours to a sufficient degree of
accuracy and in any particular case determine both
the amounts of radioactive substances present and the
quantity of accumulated helium, we are evidently in
possession of all the data required to determine the
period during which the changes have been going on.
The identity of the alpha ray with helium is a *
fundamental point in this procedure. The evidence
on which this identification is founded may be taken
as conclusive. Rutherford and Geiger 1 have shown
. U.S., 81 A, p. 162.
214 RADIOACTIVITY AND GEOLOGY.
by a very beautiful investigation that the charge on
a single alpha particle is about twice the charge carried
by an ionized atom of hydrogen ; and hence by a know-
ledge of the ratio of the charge to the mass, the mass
is found to be 3*84 times that of the hydrogen atom ;
in other words, the atomic weight is 3 '84. Now, quite
independently it is known that the atomic weight of
helium is 3 P 96. The agreement is remarkable, consider-
ing the delicate nature of the measurements involved.
In a second investigation 1 the purified emanation of
radium is introduced into a glass tube so thin in the
walls that the alpha rays pass through, while the
emanation and its transformation products remain
enclosed. This tube is placed within a larger one
rendered highly vacuous. A spectroscopic examination
of the residual gas in this outer tube at first shows
no helium, but as time progresses the helium spectrum
appears. This shows that the alpha rays, passing
through the walls of the inner tube and trapped within
the outer one, are helium. Here we are secure from
the possibility that the helium known to come into
existence in the transformations of radium and its
descendants, is some resultant of the changes other
than the alpha ray itself. Finally, if any further
proof was required, Dewar 8 has measured the rate of
production of helium from radium, and finds that this
is in close agreement with that which Eutherford had
predicted on a count of the emitted alpha particles : that
is 0*37 cubic millimetres per gram of radium per day.
1 Rutherford and Royd, Proc. Manchester Lit. and Phil. Soc., 53, p. 1.
2 Sir James Dewar, Proc. E. S. t 81 A, p. 280.
URANIUM AND THE AGE OF THE EARTH. 215
The diagram of the radioactive elements given in
Chapter I shows that in the direct line of descent, the
atom of uranium emits two alpha particles ; and
ionium, radium, emanation, radium A, radium (7, and
polonium, each emit one particle. When therefore
these substances exist together in equilibrium, as
they would in an old mineral, every atom of the
parent substance which transforms involves, on the
simplest assumption, the production of a total of 8
atoms of helium. Additional to this source of helium,
the actinium branch supplies a small quota. But this
must be a comparatively unimportant addition, the
activity of actinium in uranium-bearing minerals being
only a feeble part of the whole. 1
At the time when Eutherford 2 first applied this
method to determine the age of radioactive minerals,
the constants were not so well known as they are now.
The original calculations are accordingly in need of
correction. He cites the case of a specimen of fergu-
sonite which Eamsay and Travers found to contain
1*81 cubic centimetres of helium per gram, and about
7 per cent, of uranium. We now know that the ratio
of uranium to radium in minerals must be closely as
1 is to 34 x 10~ 7 . The quantity of radium present
must, therefore, be 23'8 x 10~ 9 grams per gram. Now,
3 '4 x 10 10 alpha particles are expelled per second per
gram of radium, 3 or if we count up the eight rays which
appear involved in the theory of radioactive change,
1 Boltwood, American Journ. Sc. t April, 1908, p. 291.
2 Rutherford, Phil. Mag., Oct., 1906, p. 368.
3 Rutherford and Geiger, loc. cit.
216 RADIOACTIVITY AND GEOLOGY.
for each atom of radium breaking up, 27'2 x 10 l
alpha particles are emitted ; and this is the number
expelled per second per gram of radium. Taking now
2*72 x 10 19 molecules as the number of molecules in one
cubic centimetre of gas at standard pressure and tem-
perature, we would have 10~ 8 cubic centimetres of
helium evolved per second in a hypothetical mineral
containing one gram of radium. This is equal to 316
cubic millimetres per year. But in the fergusonite
the amount of radium is only 23*8 x 10~ 9 , and there-
fore the annual evolution of helium is 0*75 x 10~ 8 cubic
centimetres per gram. We can from this calculate
the age. The helium in the mineral was found to be,
as stated, T81 cubic centimetres per gram : dividing
this number by the amount of helium evolved per
year in one gram of this mineral, we find the duration
required for the accumulation of the helium was 241
millions of years. Nothing is said about the probable
geological position of this mineral. It may possibly
date from a time before our epoch of denudative
history begins. Kutherford, however, suggests that
the age of the geological strata might also be inves-
tigated in this manner.
This calculation may be taken as typical of the
steps involved. It will be seen that we have to
measure not only the helium, but also the uranium
or the radium. If we take the radium and of
necessity we must in general do so we may proceed
to find the age on the rate of production of helium
which the presence of the observed amount of radium
URANIUM AND THE AGE OF THE EARTH. 217
involves. Or we may make a similar calculation,
working directly from a knowledge of the uranium
present.
According to the experimental value found by Sir
James Dewar for the rate of evolution of helium,
the quantity is 135 cubic millimetres per year per v
gram of radium. This, however, is only for radium in
equilibrium with its products of rapid change. It
therefore only includes four out of the eight alpha
particles which in an old mineral containing the
complete series would be emitted. Doubling his
figure, we find the constant is 270 cubic milli- y
metres per annum. This would add about seventeen
per cent, to the age as determined on the data cited
above.
Quite recently Soddy 1 has measured the helium
evolved by uranium itself, and obtained quantities
which suggest that Boltwood's view, that two alpha
rays are evolved from each transforming atom of
uranium, is incorrect. This observation, however, does
not, probably, point to a lessened rate of production of
helium, but rather to the existence of some unrecog-
nized element in the uranium series. The inferred
rate of production of helium, in the case of uranium
and some other of the radioactive elements, rests upon
measurements of the relative ionizing activity ; the
helium out-put in the case of radium and its products
of rapid change being, as we have seen, directly deter-
mined.
1 Soddy, Nature, Dec. 3, 1908, p. 129.
218 RADIOACTIVITY AND GEOLOGY.
In a series of papers appearing in the Proceedings
of the Eoyal Society within the last couple of years,
Strutt has gone into the question of the occurrence of
helium as a widespread constituent of the earth's crust,
more completely than any of his predecessors. Sub-
sequent to Kamsay's discovery of helium in cleveite
and other minerals, Lord Eayleigh 1 in 1896 showed
that helium was a constituent of the hot springs
at Bath ; and Moureu 2 found that, in hot springs
occurring in France, helium was a common ingredient ;
and this observation has since been confirmed for
thermal springs in various parts of the world.
Natural gases obtained from the earth have also
been shown to contain helium as a general consti-
tuent. Thus Cady and McFarland, 8 in forty-one
sources of natural gas in America, found in all cases
helium present from traces up to as much as 1*84
per cent.
It would seem from Strutt's observations that we
need look no further than the radioactivity of common
rocks and minerals for these supplies of helium. In a
large number of rare and common minerals, in rocks
both igneous and sedimentary, helium occurs ; and
most generally in quantities which might be referred
to the uranium-radium elements present. 4 Exceptions
to this have, however, been observed. For instance,
L Lord Rayleigb, Proc. R. S., 59, p. 198.
2 Cotnpte Rendu, 1906. v. 152, p. 1155.
3 Annual Report of Chemical Soc. on Radioactivity, by F. Soddy,
1007, p. 323.
4 Strutt, Proc. R. S., 80 A, p. 572.
URANIUM AND THE AGE OF THE EARTH. 219
a fluorite wherein a large quantity of helium can only
be ascribed to thorium, 1 and beryl, wherein the helium
occurs without any adequate quantity of radioactive
elements to account for its presence. Again, Strutt 2
finds reason to believe that in the Stassfurt salt
deposits, helium is liberated by the element potassium ;
the radioactive properties of which have lately been
shown by the researches of Campbell and Wood. 3
Throughout the results obtained by Strutt there is
a very wide variation in the ratio of the quantity of
helium to the quantity of uranium oxide as calculated
from the radium. This wide variation seems impos-
sible to account for on the score of the age of the
substance; indeed, as Strutt points out, we find in
the Cornish granites a considerably smaller ratio than
obtains in the minerals of the veins traversing it, and
which are necessarily younger in age. 4
More recently Strutt has approached the problem
of the geological age directly ; taking as materials
for examination substances which must be contem-
poraneous in age with the strata in which they are
found. More especially he makes use of phosphatic
nodules and phosphatized bones, which he finds to
contain an unusually large amount of radinm ; some-
times up to fifty times that found in common rocks.
The following table summarizes his results. 5
1 Strutt, Proc. K. S., 80 A, p. 56.
2 Ibid., 81 A, p. 278.
3 Campbell and Wood, Camb. Phil. Soc. Proc., vol. 19, p. 15.
4 Strutt, Proc. R. S., 80 A, p. 582.
5 Ibid., 81 A, p. 272.
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URANIUM AND THE AGE OF THE EARTH. 221
The last column has been deduced on data similar
to those already discussed in relation to the age of a
specimen of fergusonite, but calculating, for convenience
in dealing with the data used by Strutt, the annual
production of helium from one gram of uranium oxide
(U 3 8 ). This constant is found to be 913 x 10' 8 cubic
centimetres.
The variations of apparent age, irrespective of the
order which must be assigned from the geological
position, are very considerable. Strutt considers this
is best explained as due to escape of helium, regarding
the deduced age as a minor limit.
We will doubtless soon have more to go upon in
judging of the possibilities of this method according as
the errors affecting it and causing the variations are
investigated. We cannot, however, discuss its impor-
tance without indicating the directions in which error
may lessen its value.
The assumptions involved are that at the time of
formation of the mineral substance a certain amount of
uranium was introduced, which continued in its present
position, only diminishing by transformation ; and that
all its products of change also remained, neither increased
nor diminished, preserved in a similar manner within
the substance. The gravest objection which can be
brought against the method is, of course, the risk that
these conditions are not likely all to be fulfilled in
any one case, and that we are in ignorance of the fact
whether they are fulfilled or not. Only in the event
of arriving at concordant results under different
circumstances of material and preservation would
222 RADIOACTIVITY AND GEOLOGY.
there be any assurance that some one or other of the
conditions had not broken down during the long
pa^.
These doubts arise in view of the known potency of
underground waters in producing changes, either of sub-
tractive, additive, or metasomatic character, throughout
even hundreds of feet of rock. When along with this
fact the geological history of even the most undisturbed
strata is considered the changes of temperature and
pressure which must have assailed them and recol-
lecting that many millions of years are involved in the
case of even comparatively modern rocks the chances
of finding helium, uranium, and radium, and all the
chain of elements neither increased nor diminished
must be remote. The retentive and stable nature of
the material selected for investigation will be in this
connexion an element of importance.
On the other hand might be urged the fact that in
uranium ores we find a satisfactory degree of uniformity
in the ratio of uranium to radium. It would appear from
Boltwood's observations, however, that the constancy
of the helium ratio is less in evidence, and, indeed,
widely departed from. 1 Again, it should be borne
in mind as generally true, that great losses in
the past, of uranium or radium, or even gains of
these substances, might occur; but if the conditions
of change had been so far mitigated, or had so far ex-
hausted themselves for the last two thousand or three
thousand years, as to suffer equilibrium to be estab-
lished, we would remain in ignorance of the losses
1 Boltwood, Am. Journ. Science, Feb., 1907, p. 77.
URANIUM AND THE AGE OF THE EARTH. 223
or gains of the millions of antecedent years. The
possibilities are indicated in the formation of secondary
ores such as gummite, autunite, uranophane, and secon-
dary pitchblende ; all fairly common in association
with uranium minerals, and many found deposited
in comparatively recent geological times. And with
reference to even the harder crystalline minerals, it
must be noted that Jaquerod and Perrot have shown
that helium permeates quartz at temperatures above
220 C, and with a velocity which increases with the
temperature. 1 This would seem to render untrust-
worthy, readings obtained from metamorphic rocks, or
rocks which had taken part in great earth movements.
The removal and redeposition of radium are not
merely hypothetical. Danne records a remarkable
case. 2 The radium was found in deposits of phosphate
of lead (pyromorphite), in clays containing lead, and in
pegmatite, but most abundantly in the phosphate.
No trace of uranium was present. To reach its
present position in the pyromorphite, the radium must
penetrate veins of quartz and felspar moisture due to
neighbouring springs being the probable carrier. The
quantity of radium varied considerably, and Darme
believes that about a hundredth part of a gram per
tonne must be present. This is, of course, a very large
amount. The source appears indicated in the occur-
rence of autunite in this region, and it is stated as
probable that other occurrences of uranium exist. The
storage of radium is in this case traceable, very
1 Nature, Oct. 22, 1908, p. 635.
4 See Hutherfoid's Radioactivity, p. 465.
224 RADIOACTIVITY AND GEOLOGY.
probably, to carriage by water in past ages. The
concentration in the phosphate is remarkable.
There is some reason to fear that phosphatic sub-
stances, as repositories of radioactive elements and
their products of change, may prove unreliable. In
the first place, it is evident that at some time in the
history of the phosphatic nodules there must have
been an accretion or absorption of uranium or radium,
or of both. For, in fact, these bodies are of organic
origin, and probably would not originally have
possessed their radioactive materials. When did this
accretion occur? The most probable answer is that
it was effected upon the sea-floor. But are we justified
in assuming that it did not continue for a very long
time after ? Knowing from the facts of their genesis
that accretion or absorption of radioactive elements
occurred at one time in their career, and also knowing
that underground waters are not infrequently charged
with small quantities of radium, and often with helium,
why should we limit the absorption to the geological
period in which the nodules are f oimd ? If this reason-
ing is correct, there is possibility of results either
unduly great or unduly little, according to very com-
plex cr A id'tions in the long past.
The history of these phosphatic bodies shows that
they have had a chequered career. They originated as
alteration products of organic remains thrown down
on the floor of the ocean, where they probably
gathered radium or uranium, and, indeed, almost
certainly both, the surroundings being highly radio-
active. Their sojourn under these conditions may
URANIUM AND THE AGE OF THE EARTH. 225
have been thousands of years : possibly in some cases
much longer. If during this period radium was
directly absorbed, a positive source of error in the
accumulated helium is incurred to start with.
The phosphatization of these bodies is by no means
completed in their original surroundings. A further
concentration is effected after the sediments have been
elevated into land surfaces ; when, by leaching with
meteoric or subterranean waters, the excess of calcium
carbonate is washed away. 1
The phosphorites of Tennessee are cited by Clarke
as " an excellent illustration of the several processes,
chemical and mechanical, which have taken part in
their formation." These were partly Ordovician and
partly Devonian, and as phosphatic limestones deriving
their phosphates from bones, shells, and teeth, were
first laid down in a shallow sea. There was then
leaching of the limestones, which removed carbonates,
leaving a mixture of phosphates, clay, and iron
hydroxide. The soil thus formed was again concen-
trated by mechanical washing, the moving waters
carrying away the clay and leaving the heavy phos-
phatic nodules. Some of the phosphates were actually
dissolved by percolating waters and precipitated as
secondary deposits in the underlying limestones. All
this indicates that in these substances we deal with
bodies specially liable to alteration. That these altera-
tions may take the form of the addition of substances
to the phosphate, appears from the following facts :
"In certain Cretaceous sandstones of Eussia calcium
- ] Clarke, The Data of Geochemistry, p. 448.
Q
226 RADIOACTIVITY AND GEOLOGY.
phosphate occurs as a cement for the sand grains, and
also in the form of fossil-bones and fossil wood. The
wood has been completely replaced by the phos-
phate. Although bone is itself largely composed of
calcium phosphate, fossil bones are not identical
chemically with recent bones. The fossils show an
enrichment in calcium carbonate, iron oxide, and
fluorine, as A. Carnot has shown, and especially in
fluorine. Modern bones from various animals were
found by Carnot to contain a minimum proportion of
fluorine ; Tertiary bones were much richer ; Triassic
and Cretaceous bones still more so; and in bones from
Devonian and Silurian formations the ratio of fluoride
to phosphate was nearly that of apatite. This progres-
sive enrichment in fluorine Carnot attributes to the
% . agency of percolating waters, carrying small quantities
of fluorides in solution." 1 Bischof kne'w of the absorp-
tion by fossil bones of the element fluorine, and suggests
a chemical explanation. 2
In short, here are cumulative actions of just the kind
we previously referred to as possibly resulting in the
accretion of radioactive materials, unless we suppose
the fluorine generated within the phosphate. The
conclusion seems to be that a steady, mutual substan-
tiation must be required .of results which involve, to
so great a degree, unaltered relations having existed
between the mineral and its surroundings ; and the
further back we seek to extend our explorations, the
more desirable is the assurance arising from mutual
1 Clarke, loc. cit., p. 447.
2 Bischof, Chemical and Physical Geology, ii., p. 33.
URANIUM AND THE AGE OF THE EARTH. 227
support. That greater security might sometimes be
gained by direct determination of the uranium is sug-
gested by the presumptive evidence in favour of
radium being captured by phosphates without the