Deposition of the banded silica and iron formations, which are typical of the middle "Huronian" rocks of the Lake Superior district, can be accounted for as follows. The region was in a very mature stage of geomorphic development, and the climate of the region was sub-tropical to warm-temperate, with moderate to high rainfall. Iron and silica were the principal products of weathering, being derived alternately during the cooler and warmer parts of the year. The environment of deposition was a large fresh-water lake, low in nutrients and thus having a relatively low organic productivity, and it was of sufficient depth to permit development of density stratification of the water during the summer. The lower water layer, in summer, was isolated from the atmosphere and had a slightly reducing and acid condition; iron was kept in solution in a reduced state but silica was deposited. During the winter the entire column of water in the lake circulated and it was oxidizing and alkaline, causing precipitation of the iron. Any of the iron minerals common to the banded iron formations could be deposited in this environment, because the stability fields of these minerals include the ranges of Eh and pH which occur in this type of lake.