Yol. 52.] A DELIMITATION OF THE CENOMANIAN. 101 



in the midst of the Gres du Maine, and that the ' Eotomagien ' or 

 Rouen Chalk is merely the chalky facies of the upper part of the 

 Cenomanian of the Orne and Sarthe. This view has been accepted 

 by Prof. A. de Lapparent and by the officers of the Service de )a 

 Carte geologique de France. 



French geologists, however, have not reached the end of the 

 difficulties and controversies to which the local and isolated facies 

 of their typical Cenomanian has given rise. They are even now at 

 variance with regard to the line of separation between the Albien 

 and Cenomanian stages. The Albien fauna of d'Orbigny was mainly 

 that of the Lower Gault ; the fauna of what we know as the Upper 

 Gault and Blackdown Beds was by d'Orbigny included partly in 

 the Albien and partly in the Cenomanian, under the mistaken 

 impression that our Blackdown Beds represented a part of his 

 Cenomanian. 



Gradually, however, it became known that between the typical 

 Albien and Cenomanian faunas there was a distinct zonal assemblage 

 in the Upper Gault of Wissant, in the Gaize of the Ardennes, and in 

 the Yraconnien of the Jura. The question then arose as to whether 

 this zone of Ammonites inflatus should be included in the Albien 

 or in the Cenomanian, and on this question French geologists differ 

 to the present day, some thinking with Prof, de Lapparent that it 

 should be classed as part of the Albien, others agreeing with Prof. 

 Hebert and Dr. Barrois in regarding it as Cenomanian. 



Further, Dr. Barrois's researches in England and in the North- 

 east of France disclosed the existence of beds containing Pecten 

 asper between the zone of Ammonites inflatus and the base of the 

 Chalk in those regions. Now Pecten asper is a common shell in the 

 Cenomanian of the Sarthe, and consequently French geologists are all 

 of opinion that the true Cenomanian contains an equivalent of this 

 zone of Pecten asper. 



In England, as Dr. Barrois has shown, the Upper Greensand of 

 Wiltshire, Hampshire, and Dorset may be divided into two zones, 

 the lower being his zone of Ammonites inflatus, the upper being his 

 zone of Pecten asper, which includes the chert-beds and green sands 

 of Warminster and other places. The result of French investi- 

 gation, therefore, has been to tell us that our subdivisions into 

 Gault, Upper Greensand, and Lower Chalk do not tally in any way 

 with their Albien and Cenomanian stages, and that if we wished 

 to adopt the French nomenclature we should have to draw a hard- 

 and-fast line in the middle of our Upper Greensand. 



Having thus briefly indicated the history of French opinion, and 

 mentioned the difficulties which have arisen in comparing the 

 sections on each side of the Channel with the beds in the area 

 which was selected as the typical facies of the Cenomanian stage, 

 let us now as briefly indicate the progress of English studies in the 

 same field. 



It is to William Smith that we owe the nomenclature and 

 primary classification of the English Cretaceous strata : he found 

 in Wiltshire and elsewhere a succession of (1) clay, (2) and, 



i 2 



