PLEOCHROIC HALOES—JOLY. Bat 
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, 10° rays. But now suppose we find that we can not 
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 Prof. 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 specu- 
lations as to the slow breakdown of the commoner elements may be 
dismissed. The halo shows that the mica of the rocks is radio- 
actively 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 De- 
vonian period will build the fully formed halo, 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 radio- 
active 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 regis- 
tered 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 unchanged since the beginning of geo- 
logical time. Each unaffected flake of mica is, thus, unassailable 
proof of a fact which but for the halo would, probably, have been 
forever beyond our cognizance. 
