The iron formation of the Iron River district is part of a Precambrian sequence of strata characterized by a high iron content and varied mineralogy. The iron formation, where unoxidized, consists largely of interlaminated chert and siderite. It is underlain by a graphitic slate containing about 20 per cent iron in the form of very finely disseminated pyrite. Graywacke overlies the iron formation with at least local disconformity; this rock consists of clastic grains in a matrix of siderite or iron-rich chlorite. Above the gray-wacke is a magnetic ironstone, a laminated rock that now consists of iron-rich chlorite, magnetite, chert, and siderite, each layer being an intermixture of two or more of these constituents. The average iron content of the entire sequence, including the iron formation, is approximately 20 per cent. The rocks are only slightly metamorphosed, although the area is one of intense structural deformation.

The origin of the rocks is discussed, and it is concluded that the high iron content is primary. The rocks are believed to be the products of an era of iron-rich sedimentation in which the specific iron mineral formed —sulfide, carbonate, or silicate—depended upon the immediate depositional environment. Inasmuch as the formation of iron-rich minerals continued despite extensive changes in both the basin of deposition and the adjacent land areas, the ultimate cause for such an epoch is one that transcended such factors. Evidence is presented to show that the climate of the era, when linked with certain other factors, is entirely adequate to explain the formation of these iron-rich rocks.