Mass-balance geochemistry of thick sequences of fossil soils (Oligocene, John Day Formation, central Oregon) show trends similar to those produced by modern Andisol soil-forming processes. These andic trends include phosphorus accumulation, potassium retention, and alumina and iron depletion. Identification of these trends benefited from the geochemical comparison of numerous fossil soil horizons from two paleosol sequences: one andic and one non-andic. The comparison of two paleosol sequences is justified because both sequences developed from the same type of parent material in nearly identical tectonic-depositional settings. The two sequences are: (1) the Early Oligocene Big Basin Member, dominated by clayey, Alfisol-like paleosols, and (2) the mid-Oligocene Turtle Cove Member, dominated by ash-rich, Andisol-like paleosols with calcareous horizons. The change in paleosol type occurred circa 30 Ma throughout the John Day Basin, and is most likely due to global cooling which changed climatic conditions in central Oregon from humid to subhumid. Drier climate is interpreted to have caused a decrease in weathering intensity so that amorphous weathering products of volcanic glass accumulated in soils to form Andisols. Prior to this circa 30 Ma transition, weathering produced crystalline clays from volcaniclastic detritus to form Alfisols rather than amorphous weathering products.