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
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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.
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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.
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