The relation between the field unit of gamma radiation intensity, microroentgen per hour, and the laboratory units, expressed in concentration of radioactive elements, was determined by measuring the radiation intensity over a level rock surface and measuring the gamma radioactivity of the same rocks in the laboratory. The available data indicate that 1%K equivalent of gamma radioactivity is 1.25X10 (super -12) Ra equivalents, 3.5X10 (super -6) K equivalents, 4.4X10 (super -6) Th equivalents, and produces a gamma radiation intensity of 0.57 microroentgen per hour over a flat surface. Measurements of volcanic sediments of upper Eocene and Miocene age in Brazos County, Texas, show gamma-ray emissivities of 4.5 to 10.9% K equivalents of gamma-ray emissivity. This is about the same as ordinary shales. The mixture of volcanic particles with clastic sediments in these samples suggests that the radioactivity of the original volcanic matter may possibly have been reduced by weathering.