In the future, this Web site may provide for your reference a substantial list of time scales in use within the United States and internationally. At present, we provide two significant ones:
Geolex's content is drawn from the literature published since the late 1800's. Throughout that time, scientific knowledge has increased, thereby leading to modifications to the geologic time scale. Because the definitions of geologic time intervals (e.g., the Ordovician) have been modified as more information is gathered, the geologic age of a unit as stated in a report published in, for example, 1950, may be different according to today's time scale. It is critical to be aware of this when studying older reports.
In Geolex, the orginally-reported ages as shown in the summary paragraphs on the "Significant Publications" pages are being "migrated" to the modern time scale, to be shown on the "Unit Summary" pages. This is an arduous task, especially when considering that during the past 10-20 years, portions of the geologic time scale have been subject to extensive revision, and certain of these scientific debates have not yet yielded consensus (e.g., on the two charts listed above, compare subdivisions of the Cenozoic). Time scale revisions have been extensive for Epochs of the Ordovician, Mississippian, and Permian Periods -- as noted on the Geolex Search page, these Epochs are not yet searchable, until the time scale "migration" can be completed.