IGCP-18 Address at Budapest on "Standard Definitions for the Principle Units of the Worldwide Chronostratigraphic Scale", Sept. 11, 1969, Box: 92, Folder: 20. Hollis Hedberg papers, RH MS 546. University of Kansas. Kenneth Spencer Research Library. https://archives.lib.ku.edu/repositories/3/archival_objects/151714 Accessed February 16, 2025.