48 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



series of the Boulonnais district present the clearest evidences of 

 great variations in the depth of the sea in which their several com- 

 ponent sedimentary beds were formed ; and that the sea-bed at several 

 successive periods had become terrestrial surface ; yet for all this, 

 such movements had not produced any sensible unconformity between 

 different parts of the series. To this it may be added that the re- 

 currence of a terrestrial vegetation in parts of the series where it is 

 not usually found, — as first beneath the Ferques limestone, next in 

 the yellow sandstone above, and again subordinately to the mountain- 

 limestone, — would indicate, that such local phaenomena must have 

 been dependent on the permanent presence of an area of dry land, at 

 no great distance and throughout the whole of the middle and upper 

 palaeozoic periods ; such an area could not have found place to the 

 east of the Boulonnais ; and, for reasons which will more fully appear 

 in the sequel, it may be fixed somewhere to the west, within the space 

 now occupied by the English Channel, — an extension of that m.ineral 

 axis which on other considerations has been shown to have occupied 

 the lower extremity of that area. 



Franco-Belgic Sections. 



In the Bellignies section, distant about 100 miles to the east of 

 that of the Boulonnais, there is a like concordance, extending down to 

 the sandstone and conglomerate group which underlies the Stringoce- 

 phalus-limestone ; and a like general concordance holds true over the 

 whole area from the Ardennes to the Belgian coal-field. 



In the Pepinster section the series is a continuous one from the 

 slates in the neighbourhood of Spa up to the highest portion of the 

 coal-measures. 



The Eifel series is again in perfect sequence from the Goniatite- 

 shales, above the great calcareous group, down to the slates of the 

 valley of the Lieser. 



It was at one time supposed that the slates of the north-east of the 

 Ardennes, as from Spa to Stavelot and Viel-Salm, which are un- 

 doubtedly of great antiquity, were unconformable beneath the series 

 of the Pepinster section ; but no such unconformity exists, nor are 

 the lowest strata there older than the slate-rocks of the Middle 

 Rhine (Coblentzien of Bumont). 



The first question which arises out of these consideration is, 

 whether the Stringocephalus-limestone, and its equivalent beds in the 

 valley of the Vesdre, at one time formed a continuous mass with the 

 Eifel series. An examination of the conglomerate beds which extend 

 from Malmedy to Stavelot is conclusive on this point. The materials 

 of this conglomerate have been derived either from the Eifel limestones, 

 or from the red arenaceous beds which next underlie them, — all the 

 Eifel species might be collected out of it. The condition of the 

 rolled corals shows that the limestone masses from which they have 

 been derived more resembled such as occur on the Eifel side of the 

 Ardennes than on the Belgian, consisting of friable beds, from which 



