Field work by the writer during 1939, 1940, and 1941 resulted in the discovery of Paleocene mammals only 50 feet above the highest remains of Triceratops in the Denver Basin, Colorado. This new evidence checks that of the floras, and thus indicates within relatively narrow limits the position of the Cretaceous-Tertiary boundary in that area. As this boundary definitely places the basal portion of the Denver formation and Dawson arkose in the Upper Cretaceous sequence, some changes in the nomenclature of these and other strata adjacent to the boundary become necessary. Stratigraphic and paleontologic evidence shows that the Laramie formation, Arapahoe formation, and the Cretaceous parts of the Denver formation and Dawson arkose comprise a unit correlative with the Lance formation and its equivalents. Therefore, the writer proposes that the Laramie be redefined to include all the Upper Cretaceous sequence between the top of the Fox Hills and the base of the Paleocene in the Denver Basin; that the term Arapahoe as Arapahoe conglomerate member be retained for the conglomerate immediately overlying the present Laramie formation; and that the terms Denver formation and Dawson arkose be restricted to the Tertiary strata.

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