SHARPE DEVONIAN ROCKS. 25 



The Systeme gedinnien is the lowest division of M. Dumont's 

 Terrain Bhenan, and the lowest fossiliferous rock found in Belgium. 

 This division must be separated from the Devonian series, with which 

 M. Dumont's classification connects it, since it does not contain a 

 single species found in the " coblenzian " or " ahrian " divisions 

 above it. On the other hand we observed in M. Dumont's collection 

 from this division so many forms with which we were familiar in the 

 Upper Silurian beds, that Prof. E. Forbes and myself referred it with 

 little doubt to the Tile-stone division of the Upper Ludlow rock. It 

 rests, according to M. Dumont, unconformably on the unfossiliferous 

 slate series of the Terraiti Ardennais, which forms the great axis of 

 the Ardennes. 



The minor divisions adopted by M. Dumont all appear consistent 

 with nature ; but the larger groups must be modified, to bring them 

 into harmony with the great divisions now established on the ground 

 of containing the same organic remains. 



The Terrain anthraxifere should surely begin at the Systeme con- 

 drusien or Carboniferous Series, which is well distinguished by its 

 fauna from the "Eifel system" below it : but the continuance through- 

 out of a number of fossil species connects together the Systemes 

 eifelien, ahrien, and coblentzien, which, taken together, constitute all 

 that ought to be included in the " Devonian system," from which, as 

 stated before, the Barnstaple and South Petherwin beds must be 

 removed. 



The red sandstones are not naturally the base of the Eifel series, 

 but rather the top of the Rhenane series, being formed at the end of 

 a long period, in which the sea was becoming shallower, and the de- 

 posits both more arenaceous and more ferruginous ; both of which 

 characters were gradually increasing during the Rhenane period, after 

 which the true Eifel series commences with grey shales deposited 

 apparently in deep water. 



The "Gedinnian system" must be entirely separated from the 

 Rhenane series, with which, as already stated, it has no organic re- 

 mains in common, and should stand alone as the sole representative 

 in Belgium of the Silurian system*. 



The Table of Belgian formations, thus modified, will compare as 

 follows with our English strata. 



In the following scheme, an entire separation is established 

 between the Old Red Sandstone and the true South Devon or Eifel 

 series of limestones and schists, but it may admit a doubt whether 

 the lower members of the Old Red Sandstone series of South Wales 

 may not have been synchronous vkdth the Ilfracombe and Linton 



* The principles which have led M. Dumont to adopt his present arrangement 

 of greater groups, or Terrains, will be found in an article " Surla valeur du carac- 

 tere paleontologique en Geologic," in the Bulletin de 1' Academic Royale de Bel- 

 gique, torn. xiv. No. 4. Every great deposit of conglomerate is considered to have 

 been the result of a period of physical disturbance, which broke the series of de- 

 posits and caused the destruction of previously existing animals. The remains of 

 these animals will be found among the transported materials forming the first beds 

 of the new formation, therefore " les divisions paleontologiques nepeuvent concorder 

 exactement avec les divisions geologiques fondees sur les revolutions du globe." 



