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 _débris_—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 infer something as to
the past history of our Globe. Nothing is, on the other hand,
known respecting the origin of uranium or thorium—the parent
radioactive bodies. And while not questioning the law
239
and regularity which undoubtedly prevail in the periods of the
members of the radioactive families, it appears to me that it is
allowable to ask if the change rate of uranium has been always
what we now believe it to be. This comes to much the same thing
as supposing that atoms possessing a faster change rate once were
associated with it which were capable of yielding both helium and
lead to the rocks. Such atoms might have been collateral in
origin with uranium from some antecedent element. Like helium,
lead may be a derivative from more than one sequence of
radioactive changes. In the present state of our knowledge the
possibilities are many. The rate of change is known to be
connected with the range of the alpha ray expelled by the
transforming element; and the conformity of the halo with our
existing knowledge of the ranges is reason for assuming that,
whatever the origin of the more active associate of uranium, this
passed through similar elemental changes in the progress of its
disintegration. There may, however, have been differences in the
ranges which the halo would not reveal. It is remarkable that
uranium at the present time is apparently responsible for two
alpha rays of very different ranges. If these proceed from
different elements, one should be faster in its change rate than
the other. Some guidance may yet be forthcoming from the study of
the more obscure problems of radioactivity.
Now it is not improbable that the halo may contribute directly to
this discussion. We can evidently attack
240
the biotite with a known number of alpha rays and determine how
many are required to produce a certain intensity of darkening,
corresponding to that of a halo with a nucleus of measurable
dimensions. On certain assumptions, which are correct within
defined limits, we can calculate, as I have done above, the
number of rays concerned in forming the halo. In doing so we
assume some value for the age of the halo. Let us take the
maximum radioactive value. A halo originating in Devonian times
may attain a certain central blackening from the effects of, say,
rob rays. But now suppose we find that we cannot produce the same
degree of blackening with this number of rays applied in the
laboratory. What are we to conclude? I think there is only the
one conclusion open to us; that some other source of alpha rays,
or a faster rate of supply, existed in the past. And this
conclusion would explain the absence of haloes from the younger
rocks; which, in view of the vast range of effects possible in
the development of haloes, is, otherwise, not easy to account
for. It is apparent that the experiment on the biotite has a
direct bearing on the validity of the radioactive method of
estimating the age of the rocks. It is now being carried out by
Professor Rutherford under reliable conditions.
Finally, there is one very certain and valuable fact to be
learned from the halo. The halo has established the extreme
rarity of radioactivity as an atomic phenomenon. One and all of
the speculations as to
241