‘The only chronometric scale applicable in geological history for a stratigraphic classification of rocks and for dating geological events exactly is furnished by the fossils. Owing to the irreversibility of evolution, they offer an unambiguous time scale for relative age determination and world wide correlations of rocks.’

Carl O. Dunbar, Ph.D. (Geology) (Professor of Paleontology and Stratigraphy, Yale University and formerly Assistant Editor, American Journal of Science) in ‘Historical Geology’, John Wiley and Sons, New York, 1949, p.52.