With respect to the uncertainties of the method, measurements on glauconites of the Paleogene of western Europe effected during the last decade provide isotopic dates within a margin of error of 5%, that is about 2 m.y. (Table 2). A discussion of the age of the main limits: Cretaceous-Paleocene (63 to 65 m.y.), Paleocene-Eocene (53 to 55 m.y.), Cuisian-Lutetian (47 to 49 m.y.), Lutetian-Bartonian (42 to 44 m.y.), Eocene-Oligocene 34 to 36 m.y.), Oligocene-Miocene (22 to 24 m.y.) is presented. The middle-late Eocene boundary may be placed either between the Lutetian and the Bartonian (42 to 44 m.y.) or between the Bartonian and the Priabonian (39 to 41 m.y.).

During the last decade several hundred datings have been made in Europe on Paleogene glauconites, principally from France, England, Belgium, Germany, and the U.S.S.R. Others, fewer in number, have been obtained in the Mediterranean region and in central Europe. Before a presentation of the results, the inherent uncertainties of the method are briefly discussed. These originate either in the specimen, in the mode of operation, or in the choice of physical "constants."