Several years ago while studying the coal measures of Iowa and the neighboring states, it was suggested in a general way that the so-called Lower Coal Measures and the so-called Upper Coal Measures eventually might prove to be essentially contemporaneous.
Proceedings of the Iowa Academy of Science
©1906 Iowa Academy of Science, Inc.
Keyes, Charles R.
"Alternation of Fossil Faunas,"
Proceedings of the Iowa Academy of Science, 13(1), 199-201.
Available at: https://scholarworks.uni.edu/pias/vol13/iss1/25