This paper is a summary of work of Yagi and the writer, much of which has been published in Japanese. Glauconite seems to be formed under marine conditions by a process of hydration of silica and subsequent absorption of bases and loss of alumina. In the glauconite studied, some 12 molecular per cent of SiO₂, 22.5 per cent of MgO, FeO, and Fe₂O₃, and 10 per cent of K₂O are "soluble" in acids or replaceable by bases. The removal or exchange of these bases does not essentially alter the optical properties of the mineral. Glauconite may originate from a number of mother materials, such as faecal pellets, clayey substances filling cavities of foraminifera, radiolaria, and tests of other marine organisms, or from silicate mineral sub tances, such as volcanic glass, feldspar, mica, or pyroxene. The presence of organic matter seems to facilitate the formation of glauconite. In salt water, as indicated by the sediments of Aomori Bay, the mother substances during glauconitization lose alumina, silica, and alkalies except potash, and gain ferric iron and potash. In Kasumiga-uri Lagoon, whose water is nearly fresh, the mother substances lose silica and gain ferrous iron, but the other constituents do not vary greatly. Sea water, therefore, seems essential. The need for sea water is further indicated by the greater absorption of potash by hydrated silicates in normal sea water than in fresh water containing potassium and magnesium chloride in the same concentration as in sea water.