Thus, the Paleozoic Era refers to ‘‘ancient life,’’ the Mesozoic Era to ‘‘medieval life’’ and the Cenozoic Era to ‘‘modern life.’’ Rocks older than Paleozoic generally lack diagnostic fossils and are widely known as belonging to the Precambrian Era.

radiometric dating means placing events-9

The scale, developed before absolute dating techniques were discovered, is a relative geologic scale that provides a standard of reference for dating rocks throughout the world.

It lists the succession of rock depositions that are recognized on and immediately beneath the Earth's surface.

The standard stratigraphic column, based on fossil plant and animal assemblages from different European strata, is used to date fossils in strata from other parts of the Earth and is the foundation of the geologic time scale.

The breaks were thought to have resulted from worldwide events of mountain building during which no sedimentation occurred and left gaps in the record of rocks, that is, an unconformity.

It is now realized that mountain building events were not necessarily worldwide, but may be limited to a single continent or even part of a continent during one interval of geologic time.

The longest divisions of geologic time are the eras.Most geologic time scales recognize four eras, three of which have been named for the fossils in the associated strata.The application of radiometric dating techniques began early in the 20th century.The quantitative methods provided by these techniques had the potential for dating divisions of the geologic time scale and for estimating the age of the Earth itself.The age of the Earth now is estimated to be between 5 billion and 4.7 billion years and estimates of the duration of the geologic time scale divisions have been made.Originally, geologic time scale divisions were based on the natural breaks in the stratigraphic column.