Suppose the metalliferous solutions to be confined in a shallow basin, or, at least, to pass through it so slowly that they become thoroughly oxidized. Under such conditions the deposition of iron and manganese would go on continuously, and so nearly on the same spot that a comparatively homogeneous manganiferous iron ore would be formed. If the supply of metalliferous solutions were not continuous, but were intermittent, as is sometimes the case in local basins, such as coastal lagoons, which are often dependent for their supply of water on the changes of season and the sudden fluctuations of weather, then interstratified layers of iron and manganese ore might be produced. The iron, becoming oxidized on the surface, sinks to the bottom, possibly in some cases to be converted there to the simple carbonate by organic matter. Further oxidation precipitates hydrous oxide or carbonate of manganese on top of the iron. A renewed supply of surface waters brings more solutions of iron and manganese, or else the evaporation of the water in the closed basin concentrates the materials which have not yet been precipitated. In either case there is a further alternate deposition of the two ores.[30] Another process of separation of iron and manganese in nature might take place by the formation of sulphide of iron. It has already been shown that iron is sometimes deposited as sulphide and later oxidized in the same manner as the carbonate. Manganese, on the other hand, is rarely found as sulphide, and there is reason to think that the sulphide never represented the original form of any large sedimentary deposits of manganese ore (see pages 364 to 365). It seems probable, therefore, that from a solution of iron and manganese in surface waters the iron might, where the conditions are favorable, be precipitated as sulphide (FeS₂) and the manganese might be carried on in solution to be deposited somewhere else as oxide or carbonate. Subsequently the oxidation of the ores would give rise to oxide of iron from the sulphide and oxide of manganese from the carbonate; and the two ores, though occurring at the same horizon, would be separated by a greater or less distance.