Most of the iron deposits of Martin and Greene Counties, Ind. , are within the Mansfield formation (lower Pennsylvanian), but small deposits also occur in the Beech Creek limestone (upper Mississippian). Goethite, the major iron mineral, occurs as cementing material, concretions, bands along bedding planes, veins, irregular open-space fillings, and replacements of plants in sandstone and shale. Small replacement bodies of goethite are common in the limestone. Magnetite occurs as finely disseminated grains within the interior shells of concretions. Iron deposits in Indiana probably were precipitated in swamps and lagoons in early Pennsylvanian time. The belief that the iron was deposited in bogs is substantiated by the following evidence: (1) the irregular and local distribution of the iron deposits in Indiana is characteristic of bog iron deposits; (2) the relative abundance of those plant fossils associated with iron deposits is similar for both the iron deposits in Indiana and bog iron deposits; (3) the chemical analyses of the iron deposits in Indiana are similar to other iron deposits that are known to be of bog origin; (4) clay that is similar in composition to underclay (indicative of swamp conditions) is present in the Indiana deposits; and (5) the syngenetic nature of the concretions and the abundance of concretions of the Indiana deposits are like those of other iron deposits that are known to be of bog origin. Shrinkage cracks and colloform texture indicate that the iron was largely a colloidal precipitate. Electrolytes, micro-organisms, oxidation, and supersaturation may have been active in precipitation of the iron. Iron content is too low and phosphorus content is too high to consider the deposits in Indiana as being of commercial grade for most purposes.