The term Tertiary was first used by
Giovanni Arduino during the mid-18th century. He classified geologic time into primitive (or primary), secondary, and tertiary periods based on observations of geology in
Northern Italy. Later a fourth period, the
Quaternary, was applied. In the early development of the study of geology, the periods were thought by
scriptural geologists to correspond to the Biblical narrative, the rocks of the Tertiary being thought to be associated with the
Great Flood. In 1833,
Charles Lyell incorporated a Tertiary Period into his own, far more detailed system of classification, based on
fossil mollusks he collected in Italy and Sicily in 1828–1829. He subdivided the Tertiary Period into four epochs according to the percentage of fossil mollusks resembling modern species found in those
strata. He used
Greek names: Eocene, Miocene, Older Pliocene, and Newer Pliocene. Although these divisions seemed adequate for the region to which the designations were originally applied (parts of the
Alps and plains of Italy), when the same system was later extended to other parts of Europe and to America, it proved to be inapplicable. Therefore, the use of mollusks was abandoned from the definition and the epochs were renamed and redefined. For much of the time during which the term 'Tertiary' was in formal use, it referred to the span of time between 65 and 1.8 million years ago. The end date of the Cretaceous and the start date of the Quaternary were subsequently redefined at c. 66 and 2.6 million years ago respectively. In 1989, the
International Commission on Stratigraphy (ICS) eliminated the use of "Tertiary" on their stratigraphic charts, instead dividing this timespan into the
Paleogene and
Neogene periods), which in previous stratigraphic charts were treated as subdivisions of the Tertiary. but this was not widely followed and the ICS later restored the Quaternary to full period status by 2009. In 2006 it was remarked that despite the ICS elimination of the term over a decade earlier, at that time, "Tertiary" was still more common than Paleogene and Neogene. ==Modern equivalents==