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Eocene
The Eocene (symbol E ) Epoch, lasting from , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope 13C in the atmosphere was exceptionally low in comparison with the more common isotope 12C. The end is set at a major extinction event called the ''Grande Coupure'' (the "Great Break" in continuity) or the Eocene–Oligocene extinction event, which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay. As with other geologic periods, the strata that define the start and end of the epoch are well identified,〔The extinction of the Hantkeninidae, a planktonic family of foraminifera became generally accepted as marking the Eocene-Oligocene boundary; in 1998 Massignano in Umbria, central Italy, was designated the Global Boundary Stratotype Section and Point (GSSP).〕 though their exact dates are slightly uncertain.
The name ''Eocene'' comes from the Greek (''eos'', dawn) and (''kainos'', new) and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.〔(【引用サイトリンク】url=http://www.etymonline.com/index.php?term=Eocene&allowed_in_frame=0 )〕
==Subdivisions==
The Eocene epoch is usually divided into early, middle, and late subdivisions. The corresponding rocks are referred to as lower, middle, and upper Eocene. The Ypresian stage constitutes the lower, the Priabonian stage the upper; and the Lutetian and Bartonian stages are united as the middle Eocene.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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