The mineralogy of specimens from seven manganese deposits in the Olympic Peninsula has been studied by microscope and x-ray diffraction methods. Primary minerals are chiefly silicates and oxides. Among the silicates bementite is predominant, and minor minerals include hibschite, rhodonite, johannsenite, tephroite, inesite, grossularite, and alleghanyite. Hausmannite is the chief oxide mineral, but in some deposits braunite and jacobsite are common. Crednerite is present in trace amounts, as is hematite. Carbonates, including rhodochrosite, manganoan calcite, and calcite, are widespread but are nowhere abundant. Alabandite, barite, quartz, and native copper, with associated cuprite, are rare.Interpretation of microscopic textural relationships leads to the recognition of three general periods in the primary paragenetic sequence. Pre-bementite minerals include hibschite, rhodonite, tephroite, johannsenite, and alleghanyite. Minerals more or less contemporaneous with bementite are braunite, hausmannite, jacobsite, crednerite, alabandite, barite, copper, manganiferous carbonate, and probably grossularite. Post-bementite minerals include manganiferous carbonate and calcite, inesite, and quartz.Supergene minerals are widespread but not abundant. They generally form thin veinlets in the primary material and consist chiefly of hydrous calcium-manganese oxides (rancieite, todorokite, and birnessite). Nsutite is the only manganese dioxide mineral recognized. Neotocite accompanies most of the supergene oxides.The primary deposits show microscopic features consistent with deposition in a sedimentary environment. These include micro-conglomerates, tuffaceous breccias, probable fossils, crystalline micro-pellets, and rare stratification. Some hausmannite specimens show nodular colloform structure that is suggestive of accretionary growth similar to that of modern marine manganese nodules.It is concluded that the mineralogy is consistent with a hypothesis of marine origin similar to that proposed by Park (38). Evidence is lacking for appreciable post-diagenetic change or for later introduction of the manganese and silica. The ultimate source of these constituents, however, is uncertain. Some connection with submarine vulcanism, however, seems probable.