ABSTRACT
Determination of the ages of rocks by radioactive methods calls for a knowledge of the rate of disintegration of the uranium or thorium into helium and lead. This disintegration rate as it now exists can be measured very accurately. Further more, observations on the rates of disintegration of the radio-active elements have been made under the highest attainable temperatures, pressures and magnetic fields with no noticeable effect. Pleochroic halos provide authors with information as to the constancy of radioactive rates of decomposition throughout geologic time. Flakes of brown mica contained in certain granites show a sprinkling of tiny spots under the microscope. Thus pleochroic halos provide independent and direct testimony to the constancy of the rate of disintegration of radioactive elements and give to the geologist great confidence in methods of dating the rocks based upon radioactive decomposition.
