SHARPE BOULONNAIS. 247 



set us by M. Dumont, to which I lately called the attention of the 

 Society, and to endeavour to divide into natural groups this vast 

 series of strata hitherto thrown together almost indiscriminately. 



The nearest resemblance in this country to the Ferques limestones 

 is found in the beds of South Petherwin and Tintagel in Cornwall, 

 Barnstable, Pilton, and Croyde in North Devon (especially the 

 former), in which we find the same abundance of Sjnrifer disjimctiis 

 (_S. Verneuilii) and Sp. calcaratus {Sp. Archiaci) in all their nume- 

 rous varieties, together with many other species in common. These 

 therefore may be taken together as a well-marked group of beds, 

 which for the sake of convenience may be called the Petherwin Group, 

 that name having already been adopted by Professor Sedgwick. 

 The Cucullgea sandstone must be regarded as a subordinate member 

 of this group, since it occurs above the limestone of Ferques and 

 below the Barnstable and Pilton beds. The English deposits classed 

 vdth the Ferques limestones differ from the latter in wanting the 

 Devonian and Silurian corals which form so marked a feature at 

 Ferques ; but coral-reefs are everywhere so local that no argument 

 can be drawn from their absence. 



On comparing the organic remains of the Petherwin groixp with 

 the next group below it (which is the Eifelian series of M. Dumont, 

 including the limestones of the Eifel and of S. Devon), we jfind most 

 of the corals and many of the shells of Ferques occurring also in the 

 Eifelian group, but there are also many of the most abundant species 

 at Ferques which are not found in the Eifelian bed. And if we look 

 over the Eifelian species enumerated in the list of MM. d'Archiac 

 and de Verneuil, we shall find that not one-seventh are continued 

 into the Petherwin group of beds, and that the most characteristic 

 of the Eifel species are not found in the upper deposit, viz. Spirifer 

 aperturatus, S. ostiolatus, and S. speciosus, Terebratida ferita, 

 Calceola, Strigocephalus, &c. 



Many of the shells, but few of the corals, of Ferques are found in 

 the Carboniferous Limestone *, so that from the organic remains alone 

 it might be doubtful whether we should refer the Petherwin series to 

 the Carboniferous or the Devonian system, though the balance would 

 probably incline in favour of the former classification. > 



But the conformable stratification of the Ferques beds with the 

 overlying Carboniferous series of the Boulonnais, and the similar con- 

 formity of the Petherwin and Pilton groups to the Culm-measures of 

 Devonshire, connect the Petherwin group with the Carboniferous 

 series, while the complete break between the Petherwin and Pilton 

 groups and the true Devonian rocks below separates them from the 

 latter system. 



Therefore, while admitting that the change from the Devonian to 

 the Carboniferous series has been very gradual, I think that the 

 balance of evidence places the Petherwin group as a lower member 

 of the Carboniferous series. 



* I do not include Prof. M'Coy's Irish localities in this calculation, as they 

 probably refer to beds of the Petherwin age, and not to the typical Carboniferous 

 Limestone. 



