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This book is the planned successor to A Geologic Time Scale by W. B. Harland, A. V. Cox, P. G. Llewellyn, C. A. G. Pickton, A. G. Smith and R. Walters published in 1982. It adopts the same style and employs and develops similar methods, but it has been entirely reworked. The state of the art 1989 is thus presented; but the data assembled provide a source of reference which will serve for some years. The work develops and assesses a new calibration of the geologic time scale employing a new database. The new scale is summarized on the book covers and on a wall chart to be published separately. The work has been coordinated in detail throughout; nevertheless each chapter can stand on its own.