24 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The Terrain TlMnan of M. Dumont comprises three divisions, of 

 which the lowest or Systeme gedinnien appears to belong to the top 

 of the Silurian s^'stem. The two upper divisions, called Ahrien and 

 Coblentzien, which are consequently intermediate between the Old 

 Red Sandstone and the Silurian system, have not yet been recognized 

 in England : the few species of organic remains found in them con- 

 nect them with the Eifel series, since they all differ from those of the 

 Systhne gedinnien below, while nearly half are found in the Eifel 

 series above the Old Red Sandstone * : the most characteristic and 

 abundant species seem to be Sj)irifer ostiolatus, S. macroptei'iis, and 

 Pleurodictyimi problematicum. 



The uppermost division, or Systhne ahrien, is described as consist- 

 ing principally o{ '' yres, psatmnites et schistes gris-hleudtres :" where 

 we saw it on the line of our section, we found the following descending 

 series : — 



Grey grits, with divisions of grey shale. 



Red grits, schist, and wacke. 



Indurated greenish-grey shale. 



Sandstone passing into quartz-rock. 



The upper and middle portions of the " Ahrian system" resemble 

 the grey schistose and sandstone series of Ilfracombe and the Valley 

 of Rocks, resting on the harder red sandstone series of Linton and 

 the Foreland, in which Spirifer ostiolatus is the prevailing shell. 

 With these we must also connect the rocks between the limestone of 

 Plymouth and the mica-schists of Bolt Head f, consisting of grey, 

 chloritic, schistose, and arenaceous beds, of great thickness, overlying 

 red sandstones, which last are stated by the authors quoted to bear 

 the closest resemblance to the red arenaceous rocks east of Combe 

 Martin J : the red sandstone south of Plymouth is said to overlie 

 the Plymouth limestone, in which statement there must be an error, 

 as it leads to inextricable confusion in our classification of the rocks, 

 from which we can escape by supposing that the Plymouth limestone, 

 being the younger rock, dips towards the older sandstones south of 

 it ; these, having also a dip southward, appear to overlie the lime- 

 stone, from which they must really be separated by a great fault 

 running east and west. 



Systeme coblentzien, comprising Gres et phyllades gris-bleudtres. — 

 Our excursion did not extend to the rocks of this series. The organic 

 remains found in it are more numerous than those in the "ahrian 

 system " above it. In M. Dumont' s collection w^e observed several 

 large Devonian species of Spirifer, and also several large species of 

 Or this and Leptcena, yet unknown in this country. No part of De- 

 vonshire can be referred to this division, but perhaps the Systeme 

 coblentzien. may be found in the south of Cornwall, where large 

 districts still belong to the " terra incognita" of geology. 



* Lists of the species will be found in M. Duniont's Memoire siir les Terrains 

 Ardennais et Rhenan, p. 227 ; and his collection contains others not included in 

 those lists. 



t Sedgwick and Muvchison on Devonshire, Geol. Trans. 2nd series, vol. v. 

 p. 657, and pi. 51, fig. 5. 



t hoc. cit. p. 657 and p. 663. 



