Two samples of glauconite from the Upper Cambrian Franconia formation in Goodhue County, Minnesota, give an average K-Ar age of 440 million years. Illite from the Siyeh limestone, Belt series, Glacier Park, Montana, is dated at 740 million years - by K-Ar and 780 million years by Rb-Sr techniques. Pipestone from the Sioux quartzite at Pipestone, Minnesota, gives a K-Ar age of 1.20 billion years, and the time of deposition of the Sioux formation is placed in the early middle Keweenawan or lower Keweenawan. Geologic history of sediments subsequent to deposition is most important in attempting to date the time of deposition. X-ray crystallographic studies coupled with radioactivity-dating should lead to a better understanding of the history of sediments and of the changes that commonly are attributed to diagenesis.