Pleochroic halos are dark rings of various radii seen in mica and other minerals. There is general agreement that alpha particles emitted by radioactive isotopes create the halos. The radii of the rings are proportional to the alpha particle energy, and can thus identify the isotopes in the mineral. Some halos, however, are apparently formed by very short-lived polonium isotopes without any trace of parent uranium isotopes. How can polonium isotopes with half-lives only seconds long get into geologically old mica sans parents?
York argues the case for selective local chemical concentration of polonium from fluids in the surrounding rocks. The captured polonium atoms decay almost immediately while the fluid containing the parent atoms passes on. R.V. Gentry objects that mica is almost im permeable and that we must consider the possibility that our concepts of geological time are grotesquely wrong. York energetically defends established Geology using radioactive dating and paleontological arguments. His contempt of Gentry's position is scarcely veiled. This paper is an excellent review of the piechroic halo problem as well as a classic defense of the scientific status quo.
(York, Derek; "Polonium Halos and Geochronology," EOS, 60:617, 1979.)
Comment. York does not mention Gentry's years of careful work that led him to his heresy, nor are the many objections to radioactive dating discussed. It reminds one of the confident assertions of the permanency of the ocean basins made only a few years ago. York might be correct, but the "we now know that" approach is disturbing.