Distinguished for pioneering work on K-Ar dating of young basalts and its use for determining the geomagnetic reversal time-scale and the relative motion of the Pacific oceanic plate and underlying mantle. These have proved fundamental to theories of global tectonics and mantle conviction. Also distinguished for the full realization, in terrestrial rocks, of Ar-Ar step-heating analysis to identify Ar inheritance in minerals and to relate this to specific geological processes. Recently he solved an important controversy over hominid evolution by providing a comprehensive and precise time-frame for the East African lake deposits.