JURASSIC OF THE PARIS BASIN. 



567 



Discussion of Results. 



From the above observations it is proposed to classify the rocks 

 under consideration as follows : — 



1. PoitTLANDIAN. 



Upper => Purbeck. 



Lower = Portland Limestone. 



2. BOLONIAN. 



Upper = " Middle Portland." 

 Lower = " Lower " Portland. 



3. KlMMERIDGIAN. 



Virgulian. 



Pterocerian. 



Astartian. 



4. Corallian. 



Supracoralline. 

 Coral Rag. 

 Coralline Oolite. 



5. Oxfordian. 



The first point to be considered is the upper limit of the 

 Oxfordian group. When the English classification is compared 

 with the French, serious discrepancies are found to exist ; for 

 oar Lower Calcareous Grit, and part at least of our Coralline 

 Oolite, are in France universally placed in the Oxfordian. In the 

 Ardennes department it has been seen that grits which underlie the 

 Neuvizy ironstone correspond both in character and contents with 

 the JSTothe Grits and the Calcareous Grits of Yorkshire up to the 

 Passage-beds ; yet these are placed as Middle Oxfordian by Sauvage 

 and Buvignier(2). The richly fossiliferous ironstone forms the type 

 of the Upper Oxfordian for all French geologists ; and when we trace 

 its range and that of the rocks which overlie it, this seems justified 

 stratigraphically. For above it lies a series of very changeable lime- 

 stones, contrasting in this respect with the uniformity of the iron- 

 stone ; and these same limestones, after the ironstone has died out, 

 are carried on in a recognizable form over other Oxfordian beds. 

 Thus the independence of the two groups is most marked where 

 the dividing line is drawn. The Palaeontological separation is also 

 well marked ; for out of the 154 species recordedby Buvignier from 

 the ironstone in the Meuse, only 45 pass upwards to any member 

 of the Coral Rag. Excluding the Creue limestone, which forms a 

 kind of passage-rock, the difference is equally marked wherever the 

 fossils have been sufficiently studied, as in the Yonne, where Cotteau 

 shows (16) that, out of 323 species in the lowest Corallian beds, only 

 17 occur in the Oxfordian, if we draw the line between the two 

 groups as proposed ; and even where the lithology is the same, 



Lower = Oxford Clay. 



