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The word geochronology is derived from H. S. Williams (1893), use of geochrone which was a unit of time (equivalent to the duration of the Eocene period) whereby estimated ages were quantified. His estimate of ages back to Cambrian time, done before the discovery of radioactivity, were remarkably accurate. Since the use of radiometric methods, the term geochronology, for most scientists, has referred to the measurement of time in units of a year, generally measured radio- metrically but interpolated by thickness and fossil content of strata.

However, another scale was conceived for the natural chapters of earth history, and was calibrated by successive creations of life with intervening revolutions. That was refined into a bio- stratigraphic scale until, in recent years, it became clear that standardization for international use was impossible without stratotype boundaries. So a new scale is in the process of definition. The International Subcommission of Stratigraphic Classification has referred to the time element of these global chronostratigraphic divisions as geochronologic.

Geochronology might in any case be thought to signify geologic time. Several scales, different in their construction, that apply to geologic time have been proposed. Consequently, geochronology is now a general term referring to the whole science of geologic time, and it is an essential discipline in stratigraphy.

Time correlation is the primary activity to improve the time-space framework. To achieve international correlation a degree of standardization is necessary so that divisions shall have agreed names and definitions. Two kinds of scales are used to provide a time frame to support, and be supported by geological events.

One is the obvious chronometric scale of the physicist in caesium seconds or of the astronomer (and seemingly geologist) in earth-years. The other will be the chronostratic scale. Each provides a common language to express the time relationships of geological phenomena. The symposium was our effort to calibrate the chronostratic scale in boundary stratotypes against the chronometric scale.

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