PLEOCHROIC HALOES 157 



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 be- 

 come ultimately apparent to our inspection. We 

 have seen that one ray in each year since the 

 Devonian 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 radioactive families. We are evidently 

 justified in the belief that had other elements 

 been radioactive we must either find characteris- 

 tic haloes produced by them, or else find a com- 

 plete 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 re- 

 membered 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 unchanged since the 

 beginning of geological time. Each unaffected 

 flake of mica is, thus, unassailable proof of a fact 

 which but for the halo would, probably, have 

 been for ever beyond our cognisance. 



