Abstract
The pleochroic halo has been regarded as the index of the geologic age of rocks, especially of granite. The autoradiographic studies now enable us to measure the alpha radioactivity even of a single grain of minute mineral and to determine more perfectly the evolution of the halo produced around it. Thus the geologic age of a rock can be determined more easily. In this paper are treated the development of haloes and the radioactive homogeneity of the minute minerals in their nuclei.
