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.