Concretions of manganese and iron oxides are widely distributed at the surface of the sediments of the 3 major oceans of the world. These nodules form by the agglomeration of colloidal precipitates of Mn and Fe from seawater. A total of 10 12 metric tons of Nb nodules is estimated to be presently at the surface of the Pacific Ocean sediments. The nodules are indicated to be forming at an annual rate of 6X10 6 metric tons in this ocean. Chemical analyses have been made of samples of Mn nodules from about 110 stations in the Pacific Ocean. Within a local area, the composition of the nodules is quite uniform. Over the whole of the Pacific Ocean, the chemical composition of the nodules varies markedly. Compositional regions of the nodules in this ocean have been defined.