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How did Charles Lyell divide the Tertiary Period in 1833?
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Tertiary Period
Gypsum cones, which resulted from the evaporation of the Mediterranean Sea during the Messinian Salinity Crisis, in the Sorbas basin, Spain.
In 1833, Scottish geologist Charles Lyell divided the Tertiary Period into four subdivisions based on the relative percentages of living mollusk species found in fossiliferous rock layers. These subdivisions, listed from oldest to youngest, were:1
- Eocene: Contained approximately 3 percent of living mollusk species.1
- Miocene: Contained about 20 percent of living mollusk species.1
- Older Pliocene: Contained more than one-third and often over 50 percent of living mollusk species.1
- Newer Pliocene: Contained about 90 percent of living mollusk species.1
Lyell’s statistical approach utilized data compiled by his colleague Gérard-Paul Deshayes. While these categories were originally intended as biostratigraphic units, they laid the groundwork for the modern geologic time scale. The "newer Pliocene" was later renamed the Pleistocene in 1839.1