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Geological Society, London, Special Publications; 1964; v. 1; p. 241-254;
DOI: 10.1144/GSL.SP.1964.001.01.24
© 1964 Geological Society of London

Part 4: Stratigraphical Review

The Ordovician period

H. B. Whittington, D.SC., PH.D., A.M., F.G.S., Professor

Museum of Comparative Zoology, Harvard University, Cambridge 38, Mass.

A. Williams, PH.D., F.R.S.E., M.RI.A., F.G.S., Professor

Department of Geology, The Queen’ University, Belfast
Department of Geology, The Queen’ University, Belfast

Stratigraphical and palaeontological evidence for the location of the boundaries of the Ordovician System is examined and considered to be less important than historical priority, so that the Tremadoc is excluded from the system. Reasons are also given for reverting to the twofold subsystemic division advocated by Lapworth in which the base of the Nema-graptus gracilis Zone (or its correlatives) is taken as the lower limit of the Upper Ordovician. The only reliable radiometric age-determination is that of 445 m.y. for the early Upper Ordovician; an estimate of 500 m.y. for the base of the Ordovician appears reasonable.