Vol. 52.] A DELIMITATION OP THE CENOMANIAN". 177 



intermediate depths where glauconitic marls and sands were accu- 

 mulating, and the comparatively deep water of the Chalk Marl area ? 



Further, as the period was one of continued subsidence, and as 

 the formation of the Chalk Marl was preceded by that of glauconitic 

 sand in a shallower sea, what is more likely than that the creatures 

 which lived in the shallower sea should gradually migrate to other 

 tracts where the same conditions prevailed, as the area of the sea 

 grew larger, and the central portion of it grew deeper ? 



Expressed in a few words, our belief is that the * Cenomanien ' 

 of Havre and Rouen is simply a southern extension of our Lower 

 Chalk, formed in a somewhat shallower part of the sea, rather 

 nearer to a coast-line, and in a locality where conditions were more 

 favourable to the growth of siliceous sponges. Hence it consists 

 of a rather more sandy and glauconitic chalk, with a larger amount 

 of siliceous matter in the form of sponge-spicules and chert-nodules. 



We would point out that English geologists have hitherto been 

 obliged to accept the idea of a ' Cenomanien' which could not be corre- 

 lated with any one or two divisions of the English Cretaceous series, 

 but was supposed to include a Pecten asper-zone which, in England 

 at any rate, had no definite base. Our enquiry relieves them from 

 the necessity of adopting so unsatisfactory a correlation, and sub- 

 stitutes a Cenomanian with a clearly-detined base both in England 

 and France. 



As regards what has been termed ' the zone of Pecten asper ' in 

 England, we think that some other fossil should be chosen as an 

 index of the zone, so as to avoid the confusion which has arisen 

 from the presence of Pecten asper at higher horizons in France. 

 "Unfortunately cephalopoda are so rare in this zone that it is impos- 

 sible to select one of that class, and we think that it is best to 

 leave the matter for future consideration. 



PLATE V. 

 Cenomanian Ammonites. For Explanation, see text, pp. 156, 157. 



Discussion". 



The Pkesident said that any attempt such as the Authors of the 

 present paper have made to correlate any part of the Cretaceous 

 beds of this country with those of the Continent must be hailed with 

 satisfaction by all students of geology ; and as the present Authors 

 have given especial attention to this subject, their views deserve the 

 most careful attention from geologists. He invited discussion on 

 the paper that they had just heard read, and asked Mr. Hill 

 whether the floating cephalopoda referred to by him included tetra- 

 branchiata, or were confined to dibranchiata. 



Dr. W. F. Hume, in congratulating the Authors on the paper, 

 expressed himself in complete agreement with their conclusions, and 

 remarked on the extreme variation in thickness displayed by the 

 Cenomanian beds exposed in Beer Bay and Hooken Cliff respec- 

 tively. He also asked whether the Authors would consider the 



