452 STKATIGRArHICAL GEOLOGY 



Clay is an exceptional argillaceous facies, and that it is represented 

 in France and in Germany by thick masses of limestone and marl 

 which belong to several sub-stages or zones. Even in Normandy 

 and the Boulonnais the highest part of our Kimeridge Clay is 

 represented by calcareous sandstones which are faunistically 

 linked to the Portlandian stage. 



Lastly, French geologists regard the Purbeck Beds as a subordinate 

 portion or sub-stage of an expanded Portlandian stage, and they 

 have good reason for this because the marine equivalents of the 

 Purbeck Group contain a fauna which is closely related to that of 

 the Portlandian. 



The student will now be able to understand the following 

 tabular correlation of the succession found in France and in the 

 Jura Mountains with the English subdivisions, these and the. 

 French divisions having the value of sub-stages. 



England. Paris Basin. Jura. 



C Purbeck Beds Purbeckian Marls and limestones. 



4.] Portland Beds Bononian Massive limestones. 

 ^Clays with DiscmaJ 



( Clays with Ex. virgula Virguliau Compact limestones. 



3. -j Clays with A. alternans Pterocerian Pale limestones. 



(Sandsfoot Beds, etc. Astartiau Grey limestones and marls. 



2 /Corallian limestone Corallian Corallian limestones. 



'\LowerCalcareousGrit Oxfordian Oxfordian limestones. 



, /Oxford Clay Divesian Ironstone and clay 



'\Kellaways Beds Callovian(restr.) Rough limestone. 



It will be seen that a good correlative grouping of these 

 sub-stages into four stages would be made in accordance with the 

 brackets, and that these stages could be called respectively (1) 

 Oxfordian, (2) Corallian, (3) Kimeridgian, (4) Portlandian. The 

 total thickness in the Jura is not more than 1200 feet. 



The most important and interesting groups are the Corallian, 

 the Pterocerian, and the Purbeckian. The Corallian swells out in 

 Lorraine, Burgundy, and the Jura, and includes thick masses of 

 coralliferous limestone, often over 300 feet thick, which the French 

 regard as true coral-reefs; they contain Diceras arietinum, D. 

 eximium, with Comoseris mceandroides, and other corals. 



The Pterocerian in the Ardennes, Meuse, and Aube consists of 

 clays, marls, and limestones with Perisphinctes decipiens and Exoyyra 

 virgula, not very different from the corresponding part of our 

 Kimeridge Clay ; but southward they are replaced by massive 

 limestones and marls containing a different set of fossils, especially 

 Harpagodes Oceani, Natica gigas, Pholadomya Protei, and Hemi- 

 cidaris Thurmanni. 



The Purbeckian is divisible into two parts, the lower, about 150 



