The Ordovician period

Abstract

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.