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 archaean 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