248 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



In Devonshire we find the top of the Petherwin series well-defined 

 by the shales containing Posidonia Becheri in great abundance. 

 Shales crowded with the same Posidonia guide us in separating the 

 Petherwin series from the Carboniferous Limestone in Northumber- 

 land. The limestone of Hook Point, Waterford, undoubtedly belongs 

 to the same group, as also many beds in other parts of Ireland classed 

 by Mr. Griffiths and Prof. M'Coy in the Carboniferous Limestone ; 

 and also the limestone of Senzelles in the Province of Namur, 

 Belgium. 



When we compare together the organic remains of the Silurian, 

 Devonian, and Carboniferous periods, we find that many species of 

 corals lived on from the Silurian through the Devonian epochs ; but 

 although the Carboniferous Limestones are often full of corals, these 

 are of species diiferent from the Devonian corals : thus there was a 

 complete change of the species of corals between the end of the 

 Petherwin and the beginning of the Carboniferous period*. But 

 the greatest change in the species of Mollusca was between the Silu- 

 rian and the Devonian periods, and many species lived on from the 

 Devonian to the Carboniferous periods. Corals appear so sensitive 

 to changes of climate, that the above facts make it probable that 

 there was no material change of climate during the long period of 

 time in which the Silurian and Devonian rocks were deposited, and 

 the complete extinction of the then existing corals at the close of the 

 Devonian and Petherwin period may have been caused by a change 

 of climate. 



A change of climate sufficient to destroy the Devonian corals 

 might have left uninjured the Mollusca inhabiting deeper waters ; 

 but it is difficult to find a cause for the extinction of so many Silu- 

 rian Mollusca in seas which continued to support the same species 

 of Corals. 



But let us return to the Boulonnais. I stated above that all the 

 lower beds which come to the surface contain nearly the same fossils 

 as the Ferques limestone ; but the shaft sunk at Caffiers in a fruitless 

 search for coal passed into still lower beds of dark grey schists con- 

 taining a fossil which at the Boulogne meeting was pronounced to be 

 a Graptolitef, and which in the Museum of that town is labelled 

 Graptolithus Sagittarius. 



As the Graptolithus Sagittarius has hitherto been only found in 

 the Lower Silurian series, its presence at Caffiers would show that 

 the Ferques beds rested immediately on Lower Silurian rocks ; or if 

 the specific name is abandoned, as Graptolites have only been found 

 in the Silurian system, the Ferques beds must at least rest on Silurian 

 strata, without the intervention of any portion of that enormous 

 series of Eifelian and Rhenane rocks which both on the Belgian side 

 of the Ardennes and in Devonshire separate the Petherwin series 

 from the Silurian formations : therefore the correct identification of 

 the Caffiers fossil becomes of great geological interest. 



* For these facts we are mainly indebted to the labours of Mr. Lonsdale. 

 t Bull. Soe. Geol. Fr. vol. x. pp. 398, 401, & 416, and vol. xi. p. 250. 



