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proportion, thus giving rise to the common low-manganese iron ores.

The hydrous oxides of iron and manganese, however, are not isomorphous,^[1] and, therefore, when they are precipitated together, as in bog-deposits, the association is often much less intimate than in the cases just mentioned, and is simply due to the fact that, under certain conditions, the oxides of both metals are precipitated in the same place.

When iron and manganese ores occur in more or less separate deposits, it becomes necessary to suppose the action of influences different from those which cause the deposition of both together, and such influences are to be found in the different modes of precipitation, under certain conditions, of the two metals. It has been shown by Fresenius^[2] that certain warm springs, on reaching the surface, first deposit hydrous sesquioxide of iron, and farther on carbonate of manganese. This not only points to the well-known fact that carbonate of iron is more easily oxidized than carbonate of manganese, but it also leads to the belief that the bicarbonate or other salt of iron in the water is more easily oxidized than the manganese salt.

An action somewhat similar to that described by Fresenius readily explains the occurrence of manganese sometimes in entirely separate deposits, sometimes in distinct but closely alternating deposits.^[3] Under certain conditions, if the waters from which the precipitation took place were moving, the iron and manganese, owing to the difference in oxidability, as stated above, would be laid down in different places, resulting in the formation of deposits of iron ore free from manganese, and manganese ore free from iron in different positions along the plane of the same geologic horizon. Such occurrences are often seen in the iron