One of the features of the existing geological time scale about which Vail et al. (1977) expressed concern is that “the boundaries of the global cycles in several cases do not match the standard epoch and period boundaries.” The cycles in the Exxon global cycle chart are bounded by unconformities, and traditional stratigraphic methods also made use of marked breaks in sedimentation, including angular unconformities, as boundaries between many stratigraphic units. Hancock (1977) has described the historical development of the time scale, including the biostratigraphic methods developed by Oppel and D’Orbigny during the nineteenth and early twentieth century. Many of the major boundaries in the Phanerozoic time scale were established in the nineteenth century on the basis of gross litho-stratigraphic or paleontological changes, not on detailed zonal studies.