This primary source comes from the Records of the Office of the Chief of Engineers.
Full Citation: Coal Producing Districts and Fields- State of Illinois; 1917; United States; War Department Map Collection, 1939 - 1942; Records of the Office of the Chief of Engineers, Record Group 77; National Archives at College Park, College Park, MD. [Online Version, https://www.docsteach.org/documents/document/coal-producing-districts-and-fields-state-of-illinois, May 14, 2024]