no oftener than once in a
century; and possibly at still longer intervals.

The equilibrium amount of radium contained in some nuclei may
amount to only a few atoms. Even in the case of the larger nuclei
and more perfectly developed haloes the quantity of radium
involved is many millions of times less than the least amount we
can recognise by any other means. But the delicacy of the
observation is not adequately set forth in this statement. We can
not only tell the nature of the radioactive family with which we
are dealing; but we can recognise the presence of some of its
constituent members. I may say that it is not probable the
zircons are richer in radium than I have assumed. My assumption
involves about 3 per cent. of uranium. I know of no analyses
ascribing so great an amount of uranium to zircon. The variety
cyrtolite has been found to contain half this amount, about. But
even if we doubled our estimate of radium content, the remarkable
nature of our conclusions is hardly lessened.

It may appear strange that the ever-interesting question of the
Earth's age should find elucidation from the

238

study of haloes. Nevertheless the subjects are closely connected.
The circumstances are as follows. Geologists have estimated the
age of the Earth since denudation began, by measurements of the
integral effects of denudation. These methods agree in showing an
age of about rob years. On the other hand, measurements have been
made of the accumulation in minerals of radioactive _debris_--the
helium and lead--and results obtained which, although they do not
agree very well among themselves, are concordant in assigning a
very much greater age to the rocks. If the radioactive estimate
is correct, then we are now living in a time when the denudative
forces of the Earth are about eight or nine times as active as
they have been on the average over the past. Such a state of
things is absolutely unaccountable. And all the more
unaccountable because from all we know we would expect a somewhat
_lesser_ rate of solvent denudation as the world gets older and the
land gets more and more loaded with the washed-out materials of
the rocks.

Both the methods referred to of finding the age assume the
principle of uniformity. The geologist contends for uniformity
throughout the past physical history of the Earth. The physicist
claims the like for the change-rates of the radioactive elements.
Now the study of the rocks enables us to inf