The writers have long been concerned with the Cambrian stratigraphy of Wisconsin and the Upper Mississippi Valley. Thwaites has studied the Wisconsin sequence for more than a quarter of a century, and his work on well cuttings, done under the auspices of the Wisconsin Geological and Natural History Survey, has contributed extensively to an understanding of the subsurface distribution of the Cambrian strata. Raasch has been a close student of the Wisconsin Cambrian for over a decade, and Twenhofel has been interested in the sequence since 1916. All of the type sections of the formations and members have been studied by one or more of the writers.

Conferences on the Cambrian sequence and nomenclature have been held with A. C. Trowbridge and G. I. Atwater, of the University of Iowa, and with Ira Edwards, of the Milwaukee Public Museum. These discussions have revealed a gratifying unanimity of opinion among . . .

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