ABSTRACT
Haloes are found in which the effects of other and less penetrating alpha rays of the uranium and thorium families may be clearly shown, and in some the quantity of radioactive material is so small that the halo may be described as 'under-exposed'. More especially haloes of true spherical form are necessarily formed around very minute nuclei, so that even if these were entirely composed of a parent radioactive element, it is only by the integration of effects over a very prolonged period that any results are brought about. Haloes are sometimes found linearly arranged along the veins, showing that they partook of the radioactive substances contained in them. The haloes may most safely be regarded as of early Devonian age. IT is now well established that the minute circular marks seen in sections of certain coloured rock minerals-notably the coloured micas-are due to the effects produced by the alpha radiation of a central radioactive particle.
