Figure 1.
Advantage of defining chronostratigraphic units (stages) by lower boundary-stratotypes rather than by unit-stratotypes. Under boundary-stratotypes a specific level (horizontal dashed line) within a stratotype section (solid vertical line) serves to define the base of the superjacent unit and the top of the subjacent unit. Capital letters refer to widely separated type localities. Modified from Salvador (1994, their fig. 14).