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Ulsterian The basal series in the Devonian of N. America, underlain by rocks of the Silurian System and overlain by the Erian Series. It comprises the Helderbergian, Deerparkian (Oriskanyan), and Onesquethawian (Onondagan) Stages, and is roughly contemporaneous with the Lower Devonian Series and Eifelian Stage (Middle Devonian) of Europe.
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