In the Cuilcagh mountains, northwest Ireland, three formations can be distinguished lithologically: the Yoredale sandstone, Yoredale shale, and Millstone grit, all of Carboniferous age, and together having a thickness of more than 1,100 feet. The Yoredale shale, 700 feet thick, contains a goniatite fauna that indicates the presence of the P 1 , P 2 , and E 1 stages and also makes possible the refined subdivision of the lower part of the formation into zones.

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