Page:The Journal of geology (IA journalofgeology11893univ).pdf/379

tion 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^[1] 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.

The carbonates of iron and manganese may be precipitated when the solutions containing them are protected from oxidation by a reducing agent, such as decaying organic matter, or by being far removed from the air. Carbonate of manganese, however, is a much more stable compound than carbonate of iron, and the oxidizing conditions are often sufficiently strong to cause the deposition of iron as hydrous sesquioxide and not strong enough to change the manganese from its carbonate form. It is not uncommon, therefore, to have iron deposited in one place as hydrous sesquioxide, and manganese carried further on and deposited as carbonate, or even under special conditions deposited as carbonate with the hydrous sesquioxide of iron. Fresenius^[2] has shown that the warm springs of Wiesbaden, which contain iron and manganese among their other mineral constituents, deposit iron in the form of hydrous sesquioxide, while manganese is carried on further in solution and deposited as carbonate. In this behavior, therefore, we have the first striking difference in the deposition of iron and manganese, and it will be further discussed later on.