The well-logging instrument, comprising a scintillometer, used to measure the natural radioactivity of the rocks through which the drill hole passes. Potassium (40K) is the most abundant radioactive element and occurs in clays (especially the mineral illite and micas). The record is a gamma-ray log, expressed in API units (a gravity scale devised by the American Petroleum Institute and applied mainly to measurements of crude oil). The log is particularly useful for delineating the alternation of clay-rich and clay-poor lithologies, e.g. claystones interbedded with limestones or sandstones. Conventionally, claystone horizons yield API values of more than 75. High gamma-ray log values will also be recorded from organic-rich shales which also concentrate other radioactive elements, e.g. uranium and thorium. In addition, glauconitic sands, volcaniclastic sands, zircon-rich sands, and clay-matrix-rich sands produce gamma-active sediments. See also photon log.