Chert-hematite iron formation, remarkably similar to the Lake Superior iron formations, is interbedded with and grades laterally into lower Carboniferous limestone. Samples representing 78 feet of interbedded ironstone (65 percent) and limestone (35 percent) averaged 32.8 percent soluble Fe. Diagenetic minerals include chlorite, maghemite, magnetite, stilpnomelane, and minnesotaite. The iron formation is believed to have been formed by chemical or biochemical precipitation of iron oxide and silica in a basin protected from extensive clastic sedimentation. There is overlap, in time, of ironstone deposition and volcanic activity, but no convincing evidence for a genetic relationship. The similarity to Precambrian iron formation casts doubt on some arguments for a reducing atmosphere in Precambrian time.