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The Base of the Devonian, with special reference to the Welsh Borderland

  • L. Dudley Stamp

Extract

The main thesis of this paper is to show that the Ludlow Bone-bed forms the true base of the Devonian System. This is urged on several grounds.

(a) Priority. It was the limit originally used by Murchison. Although subsequently altered by him, one finds if one attempts to use his later limit:—

(i) That the Old Red Sandstone was not re-defined accordingly.

(ii) That Dumont's Rhénan (Devonian) System, defined in 1845, has priority over Murchison's later definition (1848).

(iii) That the later limit adopted by Murchison does not form a true horizon and has been very differently interpreted by later writers.

(b) Palæontology.

(i) There is a marked faunal change at the Ludlow Bone-bed. The Bone-bed marks the first appearance of fossil fishes, and thus the Devonian marks the dawn of an age of vertebrates. There is only one old record of a pre-Devonian fish in the British Isles; one doubtful one in Gotland, and an anomalous one in the Ordovician of America. The Shropshire succession forms a standard of comparison for all areas of the Old Red Sandstone type of deposit.

(ii) The limit adopted after a detailed study by other writers of an uninterrupted marine series is on the same horizon. The succession at Liévin (Northern France) forms a standard for comparison of all areas of the Devonian (marine) type of deposit.

(c) Stratigraphical. In England the Ludlow Bone-bed marks also a change in physical conditions, more noticeable elsewhere as an unconformity. Physical breaks or unconformities on the same horizon occur in Scotland, Belgium, Brittany (slight), Norway, Sweden, and Spitsbergen.

(d) Practical Considerations. The Downtonian rocks, i.e. the strata from the Ludlow Bone-bed horizon upwards, form a stratigraphical and palæontological entity, incapable of broad or even of rough separation below the Dittonian. The Ludlow Bone-bed, on the other hand, forms an horizon which can be recognized and mapped all over North-Western Europe. In England there appears to be a slight faunal break between the Downtonian and Dittonian, but this is less marked elsewhere.

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page 388 note 1 Phillips, , op. cit., p. 178.

page 387 note 1 Gardiner, , “The Silurian Rocks of May Hill”: Proc. Cotteswold Nat. F.C., vol. xx, pt. iii, 1920, pp. 185222.

page 388 note 1 Reed, & Reynolds, , “On the Fossiliferous Silurian Rocks of the Southern Half of the Tortworth Inlier”: Q.J.G.S., vol. lxiv, 1908, pp. 512–43.

page 388 note 2 Op. oit., p. 197.

page 388 note 3 Reynolds, , “A Silurian Inlier in the Eastern Mendips”: Q.J.G.S., vol. lxiii, 1907, pp. 217–38; Further work on the Silurian Rocks of the Eastern Mendips”: Proc. Bristol Nat. Soc., ser. rv, vol. iii, 1912, issued for 1911, pp. 7682.

page 388 note 4 Gardiner, , Proc. Cotteswold Nat. F.C., vol. xix, 1916, p. 129; Strahan and others, “Geology of the South Wales Coalfield: Part I. The Country around Newport, Mon., 1899, 2nd ed., 1909; Part II. The Country around Abergavenny, 1909”: Mem. Geol. Surv.

page 388 note 5 Geological Survey Maps, 1 inch, N.S., Sheets 232 and 249.

page 388 note 6 “Country around Newport”: op. cit., pp. 15, 18.

page 389 note 1 Sollas, , “On the Silurian District of Rhymney and Pen-y-lan, Cardiff”: Q.J.G.S., vol. xxxv, 1879, p. 475.

page 389 note 2 Cox, , “The Geology of the Cardiff District”: Proc. Geol. Assoc., vol. xxxi, 1920, p. 51.

page 389 note 3 Banks, , “On the Tilestones, or Downton Sandstones, in the neighbourhood of Kington, and their contents”: Q.J.G.S., vol. xii, 1856, p. 93.

page 392 note 1 Measurements of the whole section have been given by H. T. De la Beche (On the formation of the Rocks of South Wales and South-Western England”: Mem. Geol. Surv., vol. i, 1846, p. 23). In the absence of any lists of fossils it is difficult to interpret his section, but apparently the beds just below the bridge are those indicated as “fossiliferous”.

page 393 note 1 Strahan, . Cantrill, , Dixon, , & Thomas, , “The Geology of the South Wales Coalfield. Part VII. The Country around Ammonford (Sheet 230)”: Mem. Geol. Surv., 1907.

page 394 note 1 The. Geology of the. South Wales Coalfield: Part X. “The Country around Carmarthen,” 1909; Part XI. “The Country around Haverfordwest,” 1914; Part XII. “The Country around Milford,” 1916.

page 394 note 2 The presence of Pteraspis in the red marls indicates the presence of Dittonian rocks, but these remains have not been found in the basal beds. See Mem. Geol. Surv., Haverfordwest, p. 113, etc.; Carmarthen, p. 63; compare Abergavenny, pp. 1617; Newport, 2nd ed., pp. 1718.

page 395 note 1 Abs. Proc. Geol. Soc., No. 1075, 19th 11, 1921, p. 7.

page 395 note 2 Traquair, , “Report on the Fossil Fishes collected by the Geological Survey of Scotland in the Silurian Bocks of the South of Scotland”: Trans. Roy. Soc. Edin., vol. xxxix, 1899, pp. 827–64.

page 396 note 1 Cf. Woodward, Smith, “Notes on the Geology and Fossils of the Ludlow District”: Proc. Geol. Assoc., vol. xviii, 1904, pp. 438–9. The truth of these statements is still more apparent from the recent work of Mr. King. It is true that the more advanced forms, such as Cyathaspis, do occur even in the Ludlow Bone-bed, but they are relatively scarce.

page 396 note 2 Hinde, , “The Bone-bed in the Upper Ludlow Formation”: Proc. Geol. Assoc., vol. xviii, 19031904, p. 443; Harley, , “The Ludlow Bone-bed and its Crustacean remains”: Q.J.G.S., vol. xvii, 1861, pp. 542–52, pl. xiii; the supposed Astacoderma described in this paper are in reality fragments of other organisms and are not included in the present lists.

page 396 note 3 Rohon, , “Die obersilurischen Fische von Oesel”: Mém. Acad. Set. St. Petersbourg, ser. vii, vol. xli, No. 5, 1893.

page 399 note 1 Peach, & Horne, , “The Silurian Rocks of Britain: Vol. I. Scotland”: Mem. Geol. Surv., 1899, pp. 568–9. It is important to note that the term Downtonian is used in the same sense in the memoir as in this paper, viz. as the beds between the Ludlovian and the Old Red Sandstone.

page 400 note 1 King, , Abs. Proc. Geol. Soc., No. 1075, 19th 11, 1921, p. 7.

page 400 note 2 Campbell, , “The Geology of South-Eastern Kincardineshire”: Proc. Geol. Assoc., vol. xxiii, 1912, p. 295; Trans. Roy. Soc. Edin., vol. xlviii, 1913, p. 923; Geol. Mag., 1911, pp. 63–9.

page 401 note 1 Evans, , “Devonian, Sedimentary Rooks”: Handb. d. Reg. Geol., Band iii, Abt. 1, 19161917, pp. 126–8.

page 401 note 2 Cyathaspis is mentioned by Symonds, Records of the Rocks, 1872, in connexion with Cornwall, but apparently in error.

page 401 note 3 Gosselet, , Barrois, , Leriche, , Crepin, , Pruvost, , & Dubois, , “Description de la Faune siluro-dévonienne de Liévin”: Mém. Soc. géol. Nord., vol. vi (ii), 19121922. See review in Geol. Mag., Vol. LX, 1923, p. 92.

page 401 note 4 Stamp, , op. cit., 1920.

page 401 note 5 Leriche, , “Contribution à l'Étude des Poissons Fossiles du Nord de la France et des Régions Voisines”: Mém. Soc. géol. Nord., vol. v, 1906, pp. 1339.

page 402 note 1 Stamp, and others, “The Geology of Belgium”: Proc. Geol. Assoc., vol. xxxiii, 1922.

page 402 note 2 Leriche, , “La Faune du Gedinnien inférieur de l'Ardenne”: Mém. Mus. r. Nat. Hist. Belg., vol. vi, 1912, pp. 158.

page 402 note 3 Op. cit., 1912, pp. 212–16.

page 402 note 4 Leriche, , “Observations sur la limite entre le Silurien et le Dévonien à propos d'un note de M. Stamp”: Bull. Soc. belge Geol., vol. xxxi, 1921–2, pp. 158–66; Stamp, … “Réponse à M. Leriche”: Ibid., (in the press).

page 403 note 1 Barrois, , “Reunion extraordinaire … à Pinist”: Bull. Soc. géol. France, ser. III, vol. xiv, 1886, p. 42.

page 403 note 2 Oehlert, , Cong. Géol. Inter., viii, 1900, Livret Guide, Guide No. vi: Livret Guide de la Réunion extraordinaire de la Société géol. France dans la Mavenne et la Sarthe, 1909.

page 403 note 3 Strahan, & Pringle, , Sum. Prog. Geol. Surv. for 1915, 1916, pp. 43–6.

page 403 note 4 Previously unrecorded. The writer is much indebted to the officers of the Geological Survey, especially to Mr. J. Pringle, for permission to examine the cores from this boring. They are preserved in the Museum of the Survey.

page 404 note 1 Pruvost, , “Observations sur les Terrains dévoniens et carbonifères du Portugal et sur leur faune”: Communieaçoes do Serv. Geol. Port., vol. x, 1914, pp. 121.

page 404 note 2 Priem, , “Sur les poissons et autres fossiles du Silurien supérieur du Portugal”: Ibid., vol. viii, 1910–11, pp. 1–10, pls. i–ii.

page 404 note 3 Kiaer, , Das Obersilur im Kristianiagebiete, 1908.

page 404 note 4 Kiaer, , “A new Downtonian fauna in the Sandstone Series of the Kristiania area”: Videnskapsselkapets Skrifter I, Mat.-nat. Klasse, 1911, No. 7.

page 405 note 1 Munthe, , “The sequence of Strata in Southern Gotland”: Geol. Fören. i Stockholm Förhandl, Bd. 32, H. 5, pp. 13971453, 1910; also Exc. Pamph. 19, xi, Cong. Geol. Inter., 1910.

page 405 note 2 Lindström, , “On Remains of a Cyathaspis from the Silurian Strata of Gotland”: K. Vet. Akad. Handl, Bd. 21, 1895; see also review in Geol. Mag., 1895, pp 170–4.

page 405 note 3 Summary by Moberg, , “The Silurian of Sweden”: Sveriyes Geol. Undersökning, No. 229, 1910.

page 405 note 4 Schmidt, , “On the Silurian … of the Baltic Provinces of Russia”: Q.J.G.S., vol. xxxviii, 1882, pp. 514–35.

page 406 note 1 Kiaer, , “Spitsbergens devoniske faunaer”: Förh. 16 Skand. Naturforsk., 1916.

page 407 note 1 Schuchert, , American Geologist, 03, 1913, pp. 1617–18.

page 407 note 2 For details see Hartnagel, “Preliminary Observations on the Cobleskill (‘Coralline’) Limestone of New York”: New York State Mus. Bull. for 1902, 1903, p. 1109.

page 407 note 3 Chamberlin, and Salisbury, , “Geology”: Earth History, vol. ii, 1906, p. 376.

page 407 note 4 Schuchert, , “The Boundary between the Silurian and Devonian in Shrop shire and France”: Science, N.S., vol. lii, 8th 10, 1920, p. 340.

page 408 note 1 Evans, , Proc. Geol. Assoc., vol. xxxiii, 1922.

The Base of the Devonian, with special reference to the Welsh Borderland

  • L. Dudley Stamp

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