Granulated samples of acidic igneous rocks have been found to give a much higher rate of emission of alpha particles than corresponds to the known total contents of uranium and thorium. Abnormally large residual ranges of the alpha particles suggest a surficial distribution of the radioactive elements on the surfaces of the granules in the form of secondary mineral coatings. Most of the alpha-particle activity can be removed by dilute hydrochloric acid, leaving an activity corresponding to the low content of uranium and thorium found in sandstones and arkoses, in which the essential mineral grains of granites have been rounded and cleaned by attrition. The proportion of surficial activity appears to be less for granite samples taken from considerable depth below the surface than for samples from the zone of surface-water saturation. Losses of activity by acid treatment appear to be related to the degree of retentivity of helium in different igneous rock types. Determination of the helium age ratio before and after acid treatment suggests that the essential rock minerals contain as much helium as they should commensurate with their age and with the quantity of radioelements uniformly distributed within them, and not removable by acid. The low ratio of helium to radioactivity for the rock as a whole may be due either to loss of helium from highly radioactive areas that are easily affected by acid and thus also probably by ground waters, or to actual supergene enrichment of radioelements late in the history of the rock.

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