168 L. Dudley Stamp — Limit het%veen 



Gedinnian of the Ardennes with the English Downtonian^ was 

 formed before seeing their paper, or knowing of the succession in 

 North France, and it has been amply confirmed by the wonderful 

 series there exhibited. 



III. WHERE SHOULD THE BOUNDARY BETWEEN THE 

 SILURIAN AND DEVONIAN SYSTEMS BE PLACED ? 



(1) Pal^ontological Evidence. 



The Silurian and Devonian systems were originally described from 

 their development in England, where there is nowhere a super- 

 position of typical marine Devonian on typical marine Silurian. 

 The later detailed classification of the Upper Silurian and 

 " Downtonian " has been formed on the supposition that the 

 upper limit of the Silurian coincided with a change of facies 

 from marine to lacustrine — a truly unsatisfactory basis for inter- 

 national correlation. - 



The j)alaeontological affinities of the fauna of the Gedinnian are 

 briefly noted above. Those geologists who have advocated the 

 placing of the Gedinnian in the Silurian have undoubtedly been 

 correct in recognizing the close connexion of Gedinnian and 

 Downtonian.^ The difficulties of correlation largely disappear 

 when Downtonian is considered as Devonian, since the supposed 

 Silurian character of the Gedinnian lamellibranch fauna cannot be 

 considered as of great importance. Palaeontologists are generally 

 agreed as to the unsatisfactory nature of lamellibranchs as zonal 

 forms, and in any case our Silurian species are very imperfectly 

 known. 



In North France the percentages of Silurian and Devonian species 

 in each bed have been worked out, and on this basis the summit 

 of the Silurian should be taken at the top of the Grauwacke de 

 Drocourt. 



In Britain it is necessary to rely principally on fish remains, 

 which, with one or two exceptions, are first found in the Ludlow 

 Bone-bed. The Downtonian and Lower Old Red Sandstone are 

 united by several genera and even species of Ostracoderms. 



(2) Stratigraphical Evidence. 



The Silurian sea of Upper Ludlow times was of comparatively 

 limited extent. In Lower Gedinnian times on the continent of 

 Europe the sea spread over a vast tract of country in the Ardennes ^ — • 



^ Throughout this note the term "Downtonian" is used in its restricted 

 sense, applying only to beds above the horizon of the Ludlow Bone-bed. 



2 See Evans, Geol. Mag., Vol. LVI, 1919, p. 549. 



^ Leriche, " Poissons fossiles du Nord de la France " : Mem. Soc. geol. du 

 Nord, vol. V, 1906, pp. 13-21. 



'' Compare South Devon and Cornwall, Green and Sherborn, Geol. Mag., 

 1913, p. 70. 



