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Bible Encyclopedias
Arenig Group
1911 Encyclopedia Britannica
or In geology, the name now applied by British geologists to the lowest stage of the Ordovician System in Britain. The term was first used by Adam Sedgwick in 1847 with reference to the "Arenig Ashes and Porphyries" in the neighbourhood of Arenig Fawr, in Merioneth, North Wales.
The rock-succession in the Arenig district has been recognized by W. G. Fearnsides ("On the Geology of Arenig Fawr and Moel Llanfnant," Q.J.G.S. vol. lxi., 5905, pp. 608-640, with maps) as follows: - o 5 Dicranograptus - shales. Derfel or Orthis - limestone. U Rhyolitic ashes = Upper Upper Ashes Massive ashes = Middle of. a Acid andesitic ashes =Lower Arenig.
z Daerfawr Shales. Zone of Didymograptus Murchisoni. u Platy ashes Lower Ashes of Arenig - Great Agglomerate (Hypersthene Andesites). Olchfa or Bifidus - shales ( Didymograptus bifidus). Filltirgerig or Hirundo Beds Didymograptuslairundo. Erwent or Ogygia - limestone Henllan or Calymene - ashes Didymograptus Llyfnant or Extensus - flags Basal Grit extensus. (unconformity) The above succession is divisible into: (I) a lower series of gritty and calcareous sediments, the "Arenig Series," as it is now understood; (2) a middle series, mainly volcanic, with shales, the "Llandeilo Series"; and (3) the shales and limestones of the Bala or Caradoc Stage. It was to the middle series (2) that Sedgwick first applied the term "Arenig." In the typical region and in North Wales generally the Arenig series appears to be unconformable upon the Cambrian rocks; this is not the case in South Wales. The Arenig series is represented in North Wales by the Garth grit and Ty-Obry beds, by the Shelve series of the Corndon district, the Skiddaw slates of the Lake District, the Ballantrae group of Ayrshire, and by the Ribband series of slates and shales in Wicklow and Wexford. It may be mentioned here that the "Llanvirn" Series of H. Hicks was equivalent to the bifidus-shales and the Lower Llandeilo Series.
REFERENCES. Sedgwick, Synopsis of the Classification of the British Palaeozoic Rocks (1885); Sir A. Ramsay, "North Wales," Geol. Survey Memoir, vol. iii.; C. Lapworth, Ann. Mag. Nat. Hist. vol. vi., 1880; G. A. J. Cole and C. V. Jennings, Q.J.G.S. vol. xlv., 1889; C. V. Jennings and G. J. Williams, ibid. vol. xlvii., 1891; Messrs Crosfield and Skeat, ibid. vol. lii., 1896; G. L. Elles,. Geol. Mag., 1904; J. E. Marr and T. Roberts, Q.J.G.S., 1885; H. Hicks, ibid. vol. xxxi., 1875. See also ORDOVICIAN. (J. A. H.)
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Chisholm, Hugh, General Editor. Entry for 'Arenig Group'. 1911 Encyclopedia Britanica. https://www.studylight.org/encyclopedias/eng/bri/a/arenig-group.html. 1910.
