onclude? 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

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the slow breakdown of the commoner elements may be dismissed. The
halo shows that the mica of the rocks is radioactively sensitive.
The fundamental criterion of radioactive change is the expulsion
of the alpha ray. The molecular system of the mica and of many
other minerals is unstable in presence of these rays, just as a
photographic plate is unstable in presence of light. Moreover,
the mineral integrates the radioactive effects in the same way as
a photographic salt integrates the effects of light. In both
cases the feeblest activities become ultimately apparent to our
inspection. We have seen that one ray in each year since the
Devonian period will build the fully formed halo: an object
unlike any other appearance in the rocks. And we have been able
to allocate all the haloes so far investigated to one or the
other of the known radioactive families. We are evidently
justified in the belief that had other elements been radioactive
we must either find characteristic haloes produced by them, or
else find a complete darkening of the mica. The feeblest alpha
rays emitted by the relatively enormous quantities of the
prevailing elements, acting over the whole duration of geological
time--and it must be remembered that the haloes we have been
studying are comparatively young--must have registered their
effects on the sensitive minerals. And thus we are safe in
concluding that the common elements, and, indeed, many which
would be called rare, are possessed of a degree of stability
which has preserved them un

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changed since the beginning of geological time. Each unaffected
flake of mica is, thus, unassailable proof of a fact which but