One of the problems of the wartime program of studies of domestic manganese deposits concerned the identification of, and modes of origin of the manganese oxide minerals. Of the hundreds of specimens of the oxides collected in the United States, the minerals of about 250 specimens were identified by X-ray analysis; complete chemical analyses were made of about 35 specimens and partial analyses of about 150 specimens. This report presents the conclusions that arise out of a review of the geologic environment under which the specimens were found. One conclusion of this review concerns the supergene vs. hypogene origin of the oxides. In order to reach conclusions concerning the supergene and hypogene origin of the 33 oxides of manganese recognized thus far, it was necessary to define the criteria that seemed usable.One group of oxides appears to be persistently supergene: groutite, hydrohausmannite, lithiophorite, rancieite, hetaerolite, hydrohetaerolite, chalcophanite, crednerite, woodruffite, and wad. Another group of oxides appears to have been formed only by hypogene processes: manganosite, hausmannite, pyrochroite, bixbyite, galaxite, jacobsite, franklinite, pyrophanite, and ilmenite. A third group of oxides appear to have been formed by supergene processes in some places and by hypogene processes in other places: manganite, pyrolusite, ramsdellite, cryptomelane, psilomelane, hollandite, braunite, and coronadite.Another conclusion concerns a genetic relation between: (1) veins of manganese oxides in the southwest, largely in Tertiary volcanic rocks, (2) bodies of oxides in travertine aprons near active hot springs, and inactive Pleistocene springs, and (3) stratified oxides, largely in late Tertiary sedimentary rocks in the southwest. From the features of these three groups of deposits of oxides and their geologic and geographic distribution, it appears that hot water from great depth rose on fractures in areas of volcanic activity, deposited oxides in the fractures, appeared at the surface as hot springs, deposited oxides in the aprons near the springs and continuing to local basins, deposited manganese oxides with local debris as persistent beds in sediments, partly or wholly of volcanic origin.