The flame spectrometer permits rapid determination of potassium in rocks, but requires long and delicate manipulations to obtain suitable results. Highly sensitive methods of detecting natural radioactivity suggested the possibility of determining potassium by its gamma radioactivity. Total radioactivity would not give the desired results because potassium normally is accompanied by thorium or uranium in rocks. Gamma spectrometry would permit establishing the proportion between these. The theory, equipment, and methods are discussed in detail and results obtained are good approximations.