By the use of beta-ray counters a method has been developed for the determination of the three radioactive constituents in ancient sediments—potassium, thorium, and uranium. Owing to the high efficiency of the counters to beta radiation only 3 grams of sample are required. The apparatus is calibrated with radioactive standards. Over 300 samples representing several distinct types of sedimentary rocks have been measured. The highest concentration of all three radioactive constituents was found in the Antrim formation, an organic black shale of Mississippian-Devonian age in Michigan. Similar characteristics were observed in the Chattanooga formation from wells in Oklahoma. These black shales show maxima of organic matter, uranium, thorium, potassium, colloidal size grades, and thorium/uranium ratio. The radioactivity measurements are complicated by the high emanating power of these shales. Logs of beta-ray activity, alpha-ray activity, and estimated per cent black shale have been made on cuttings from Pure No. 3 Gingrich, Osceola County, Michigan, between depths of 1600 and 3200 feet. These logs include the Marshall and Ellsworth formations, the Antrim shale, and the Traverse limestone. The results indicate significant variations in the distribution of radioactive elements and organic materials in these sedimentary rocks. It is concluded that the principal loci of radioactivity are (1) in heavy minerals in sands and sandstones, (2) in potassium-bearing sediments, and (3) in sediments of colloidal deposition. These relationships are of possible importance in defining the source beds of petroleum.