It is concluded from the present-day geochemistry of iron, and its probable paleogeochemistry that a primary accumulation of this element cannot arise in a marine environment. The Precambrian banded iron formations are, therefore, suggested to have been formed in a lacustrine or closed-basin environment. Known limnological processes are invoked to provide a mechanism for the common rhythmic banding of iron and silica. The difference between the dominantly nondetrital, low-alumina, banded Early-Middle Precambrian and the dominantly detrital, high-alumina, oölitic Late Precambrian and post-Precambrian iron formations is linked to the effects upon weathering processes of a change in the character of the earth's crust and of the development of biotic systems. Hematite is considered to be the only primary iron mineral in sedimentary rocks; other iron minerals, particularly the ferrous silicates, are regarded as essentially diagenetic (although probably preconsolidation) in origin. Organic matter has an important influence on both weathering and diagenetic processes.