A biochemical mode of formation, controlled by the configuration of the sea floor, is here advocated for bedded deposits of marine phosphates.Decaying organisms on the bottom of depressions of an otherwise open sea produce ammonium phosphate and ammonia. Nitration bacteria oxidize ammonia into nitrates that act as fertilizers for the phytoplanktons on which other nektons and planktons flourish. On death, the biota fall to the bottoms of the sea-floor depressions, where the amounts of soluble phosphates increase in due course beyond the needs of the animals and plants living above. The sheltered depressions preserve the solutions from dissipation in the open sea. The ammonium phosphate reacts with Ca ions or with precipitating CaCO 3 to give calcium phosphate.Two general kinds of phosphates are recognized: a dark-colored, finegrained type contains pyrite and bituminous material, and its grains are elongated and angular. It is formed in the deeper, more sheltered sea-floor depressions with a negative Eh. A light-colored, coarse-grained type contains little pyrite and organic material, and is at least partly oxidized. The grains are rounded and the deposits contain detrital quartz. They are formed in relatively shallow depressions, where they are brecciated by strong waves that also roll the fragments into rounded pellets and nodules.It is suggested that phosphates are deposited in a medium whose pH is around 7.8, the limestone fence.