Silurian and Devonian Systems. 171 



known for his work on tlie Devonian, has traced fossiliferous marine 

 bands in the Lower Coblentzian of the South of Belgium (Bassin 

 d'Eifel)/ and is of the opinion that the Old Red Sandstone (lacustrine) 

 facies does not exist at all in the Ardennes. The existence of shallow- 

 water and sparsely fossiliferous rocks in the Upper Gedinnian and 

 Lower Coblentzian is only the natural consequence of the huge 

 transgression and attendant shallowing of the sea in Lower 

 Gedinnian times, and cannot be regarded as representing a change 

 separating two periods — the Silurian and Devonian. 



(3) CONSIDEEATIONS OF PRIORITY. 



Murchison originally drew the line of division between his Silurian 

 and his Old Red Sandstone systems in Shropshire just above, the 

 Ludlow Bone-bed. It was only the occurrence of many of the 

 Upper Ludlow fossils in the " Tilestones " (^ Downton Castle 

 Sandstone) that induced him to include the latter in the Silurian.^ 

 For reasons of priority it would be preferable to include the Bone- 

 bed still in the Silurian, but there is evidence to show that bone- 

 beds tend to occur at the base rather than at the summit of systems. 

 The writer considers the Ludlow Bone-bed as an " organic " basal 

 conglomerate marking the base of the Devonian system. 



IV. CONCLUSION. 



The absence of a clearly defined limit between the Silurian and 

 Devonian in England has long been a great hindrance to workers 

 both here and abroad. The result has been apparent in recent years 

 in the so-called " Gedinnian problem ", i.e. as to whether the 

 Gedinnian is to be considered as Silurian or Devonian.^ It is at 

 least desirable that a statement of our present knowledge should be 

 clearly outlined. After the publication of the detailed account of 

 the succession in North-East France, and also of further work on 

 the Dovv^ntonian fish faunas of England,^ it is hoped that the whole 

 subject will be treated more fully by Dr. Pierre Pruvost and myself. 



^ Mem. de I'lnst. geol. de I'Univ. de Louvain, vol. i, 1913. 



2 Siluria, 4th ed., 1867, pp. 134-5. 



^ See De Dorlodot, Bull. Soc. beige Geol., vol. xxvi, 1912, Proc. verb. 

 pp. 17-39, 62-5; Ann. Soc. geol. Belgique, vol. xxxix, 1912, pp. M 291- 

 M371. Mailleux, " Le texte explicatif du leve geol. de la plan, de Couvin," 

 1912 (Serv. geol. Belgique): Bull. Soc. beige Geol., vol. xxvi, 1912, Proc. verb. 

 p. 139. Fourmarier, Ann. Soc. geol. Belgique, vol. xxxix, 1912, p. 233. 

 Lericlae, Bull. Soc. beige Geol., vol. xxv, 1911, Proc. verb. p. 327. 



'' Indicated bj^ King & Lewis, Proc. Birmingham Nat. Hist, and Phil. 

 Soc, vol. xiv, 1917, p. 90. 



