Ordinary granite, containing $\mathrm{ca}$. 3×${10}^{-6\mathrm{}}$ g U per g and $\mathrm{ca}$. 6×${10}^{-6\mathrm{}}$ g Th per g of rock, will support an ionization current of $\mathrm{ca}$. ${10}^{-15\mathrm{}}$ ampere per 100 ${\mathrm{cm}}^2$ of polished rock surface. About 90 percent of this ionization is due to alpha-rays from the rock, and about 10 percent to beta-rays. From hand specimens the gamma-radiation is too feeble to operate an ionization chamber, but can be measured with a Geiger-Müller tube-counter. Observations of the surface ionization due to alpha-rays, or of the gamma-ray activity with the tube-counter, represent the combined effect of the uranium, actinium and thorium series. They can be corrected for the effects of the uranium and actinium series by employing emanation measurements, and thus the thorium content of ordinary rocks may be obtained. Important applications lie in the field of geophysics and in certain cosmic-ray problems.

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