The twelve currently recognised periods of the present eon – the Phanerozoic – are defined by the International Commission on Stratigraphy (ICS) by reference to the stratigraphy at particular locations around the world.In 2004 the Ediacaran Period of the latest Precambrian was defined in similar fashion (and was the first such newly designated period in 130 years) but earlier periods are simply defined by age.By 1830 Lyell's famous textbook, Principles of Geology, came out. Such was the age of the great creationist geologists!The principle of faunal succession in the geologic record was established by direct observation as early as 1799 by William Smith.The goal of this lecture is come to come to a scientific understanding of geologic time and the age of the Earth.
The geologic column was not composed by assembling a chronology of "periods," "eras" or other supposed measures of time, but by superposition of objectively defined sequences of sedimentary strata called "systems." The "periods" and "eras" were later appended to the system nomenclature of the "geologic Column" transforming it into a "geologic time scale." The notion that the earth's crust has on "onion skin" structure with successive layers containing all strata systems distributed on a global scale is not according to the facts.Eons and eras are larger subdivisions than periods while periods themselves may be divided into epochs and ages.The rocks formed during a period belong to a stratigraphic unit called a system.Then the paleontologists use the geologists' dates as evidence for the age of the fossils! The relative order of the strata was first determined by the principles of stratification. Beagle, a very strong Bible believer, made it a point to have a copy of Lyell's book for the ship's library.(The principle of superposition was recognized as early as 1669 by Steno.) Reverend Benjamin Richardson and Reverend Joseph Townsend were a couple of early geologists involved in this work. Obviously, even Lyell was not pushing evolution at the time.