In introducing this symposium on a critical point in geologic time emphasis must first be laid on the fact that the Periods were defined during the last century by European paleontologists, and that American events can be dated only by comparison of American with European faunas and floras, unless simultaneous and world-wide diastrophic movements can be demonstrated to have occurred.
The demarcation between the Cretaceous and the Eocene periods of Europe rested first on the work of Deshayes on the extinct molluscan fauna of the Paris basin. It gradually developed in definition and clearness under Lyell, D’Orbigny, Mayer, and Eymar. Gradually also vertebrate reptiles and mammals entered into the problem, and the formations along the northern coast of Europe and Belgium, with their contained marine fossils, served to define the Mæstrichtien stage (Dumont, 1849), while in the north another late Cretaceous phase typified the Danien stage (Desor, 1846). According to . . .