M. Clark — Silurian Fossils of North-East Ireland. 497 



and over the greater part of the Eussian platform, the base of the 

 overlapping Cenomanian does not show any trace of the beds with 

 Am. inflatus." 



By the " free Cenomanian " he clearly means the upper or true 

 Cenomanian, free from the zone of Am. rostratus (inflatus) ; the zone 

 of Am. varians having overlapped that of Am. rostratus, just as the 

 latter has overlapped the Lower Gault with its zones of ^m. interruptiis 

 and Am. mammillaris. He concludes by showing that the generality 

 of the special Cenomanian transgression has been much exaggerated ; 

 that Cenomanian deposits are absent in some parts of France, 

 Switzerland, Spain, North Africa, and America ; and that subsidence 

 of one region has generally been compensated by the upheaval of 

 some neighbouring region. 



We may hope that this vigorous protest will go far toward dis- 

 pelling the false conception of a special Cenomanian subsidence and 

 transgression which has so long been one of the fixed opinions of 

 Continental geologists. The transgression exists, but should be 

 described as the Cretaceous, not the Cenomanian, transgression. 



It may well be that the extent of surface gained by the sea of the 

 Cenomanian or Lower Chalk age in Europe was greater than that 

 gained during any previous stage of the Cretaceous period, but the 

 extent of such an invasion of the sea depends not only on the amount 

 of vertical subsidence but also on the slope of the area invaded ; 

 thus a subsidence which would carry the sea only 20 yards up 

 a steep slope might carry it 20 miles over a fairly level plain. 

 It is therefore very probable that those regions where Cenomanian 

 deposits form the basement-beds of the Cretaceous Series over 

 a large extent of country were broad terrestrial plains during the 

 formation of the earlier Cretaceous strata ; and that the subsidence 

 which carried the sea over them was not more rapid nor vertically 

 greater than the subsidence which occurred in the preceding 

 Selbornian stage or in that of the succeeding Turonian. 



V. — Notes on the Fossils of the Silurian Area of 



North-East Ireland.^ 



By E. Clark, Esq. 



IT would be difficult after the closest investigation to add much 

 to the exhaustive list of species given in the excellent paper 

 on this subject which was read by Mr. Swanston before the 

 Belfast Naturalists Field Club some twenty - five years ago. 

 In that paper reference is made to the classification by the 

 Geological Survey of the rocks of the district, under the general 

 heading of Lower Silurian. Subsequently to the publication of 

 the maps of the area, the information as to the Silurian fauna 

 was greatly extended by the labours of Professor Lapworth and 

 Mr. Swanston, to whom science is so largely indebted. The 



1 Eead before the British Association, Belfast, Sept. 1902, in Section C (Geology). 

 Communicated by pemiission of the Director of the Geological Survey. 



DECADE IV. — VOL. IX. — NO. XI. 32 



