398 SCIENCE PROGRESS 



The Chalk has long been divided into three members — the 

 Upper Chalk (with many flints), the Middle Chalk (with few 

 flints), and the Lower Chalk (with no flints). As a rough-and- 

 ready arrangement that is easy to remember, this system has 

 been universally accepted. 



But within the last few decades it was realised that a more 

 scientific and less empirical arrangement was desirable, more 

 especially as in Devonshire the base of the Middle Chalk is 

 crowded with flints, and the Upper Chalk of Thanet is almost 

 flintless. 



--•D'Orbigny showed that these three divisions correspond 

 approximately with the changes in the Fauna, and that each 

 division is characterised by a special assemblage of fossils. He 

 gave them respectively the names Senonian, Turonian, and 

 Cenomanian. 



A little later Charles Barrois, after a meteor-like traverse 

 of the Chalk of Great Britain and Ireland, established the 

 foundations of the Zonal System, which has since been per- 

 fected by Dr. Arthur Rowe. The Chalk is divided into a number 

 of zones, of varying thickness, each zone being characterised 

 by a certain set of associated fossils. Each zone is named 

 after some one species, which is also characteristic of the zone 

 by being confined to it, or by being more abundant there than 

 elsewhere. 



These zones are as follows : 



Senonian (Upper Chalk). 

 Zone of : 



Ostrea lunata 1 



Belemnitella mucronata L absent in Kent. 



Actinocamax quadratus J 



Marsupites testudinarius 142 ft. thick. 



Micraster cor-anguinum 250 ,, 



„ cor-testudinarium 56 „ 



Holaster planus 35! „ 



Turonian (Middle Chalk). 



Terebratulina gracilis 161 ,, 



Rhynchonella cuvieri 70 ,, 



Cenomanian (Lower Chalk). 



Holaster subglobosus 146 „ 



Ammonites varians 54 



» 



We will now briefly consider the characteristics of each zone 

 separately. 



