THE FORMS OF IRON AND MANGANESE DEPOSITED AT ORDINARY TEMPERATURES. The mineralogical forms in which iron and manganese are deposited from solution in nature at ordinary temperatures depend on the conditions of air and water, whether of an oxidizing or a reducing nature, and on the character of the associated organic and inorganic matter either in solution or on the floor of the sea, lagoon or bog in which the deposition occurs.[18] There are four principal methods by which iron and manganese are precipitated in nature from surface waters: (1) By oxidation, as in the case of the precipitation of hydrous oxides and in the precipitation of the carbonate by the partial oxidation of more complex organic salts.[19] (2) By reduction, as in the precipitation of sulphide of iron by the reduction of sulphate of iron. (3) By gaseous or soluble precipitants, as in the precipitation of sulphide of iron by the action of sulphuretted hydrogen or a soluble sulphide on a soluble salt of iron, and as in other cases to be mentioned later. (4) By replacement of carbonate of lime or some other substance. Different forms are precipitated by these different methods. Iron at ordinary temperatures is usually deposited from solution as the hydrous sesquioxide, the carbonate, the sulphide or the hydrous silicate of iron and potash known as glauconite. Manganese under similar conditions is deposited as the hydrous oxide[20] or as the carbonate, and possibly sometimes, though very rarely, as sulphide. When solutions of organic or inorganic salts of iron and manganese are freely exposed to the action of air, as in shallow or rapidly moving streams, or in lakes and some bogs, they are quickly oxidized and both may be deposited as more or less hydrous oxides. In many bogs, however, the metals may be precipitated as hydrous oxide on the surface where oxidizing agencies predominate, but when these oxides sink and come into contact with decaying organic matter, free from the active oxidizing influences of the air, they may be reduced to carbonates.