re: geological periods

[Scott Horton <HORTONS@EM.AGR.CA>]

The reason periods (eg. Maastrichtian) are used instead of dates is
because the dates will change as better radiometric (or other)
techniques are developed. Therefore don't worry about the dates. Every
author can use a different set of dates, as long as they reference time to
the standard stratigraphic periods (this IS standardized, fortunately, with
minor differences across the Atlantic not worth worrying about). If you
need a more specific time reference, use Upper, Middle, and Lower. eg.
Upper Maastrichtian.