Geologic Time Scale:
Applying Steno’s Laws and the Principle of Faunal Succession led to the creation of a relative chronology of Earth history by William Smith, termed the geologic time scale. The names on the scale come from the places where rocks of this age were first described. For example, the Cambrian Period is based on the Latin name for Wales (“Cambria”) where these rocks, bearing primitive trilobites and graptolites, were described by William Smith. Likewise, the Devonian Period is based on the region of England called Devonshire where rocks of this age are found. The roots of some names are not so obvious. The Silurian Period comes from the Latin name, “Silurs” for a Celtic tribe that lived in a region that is now part of Wales.
The ages (in millions of years) found on the geologic time scale came long after William Smith and his colleagues established it, and these ages are based on the radiometric dating of igneous rocks that are associated with fossil-bearing sedimentary rocks. The ages of the various units on the geologic time scale are frequently revised, as geologists discover new locations where rocks that can be dated radiometrically are found with sedimentary rocks containing different types of fossils. In this way the geologic time scale is the synthesis of radiometric dating, Steno’s Laws and the Principle of Faunal Succession.
Last modified October 02 2008 03:13 PM