A. J. Jukes- Browne — On the term Neocomian. 315 



The two lowest Neocomian groups have been termed by some 

 Valenginien and the two upper Hauterivien ; the lowest rests on 

 beds of Purbeck age, containing Planorbis Loryi, and other freshwater 

 shells. 



Dr. Barrois informs me that the total thickness of the three 

 divisions in the Jura is from 200 to 300 metres (650 to 990 feet). 



In the north of France there is only one locality where anything 

 like a complete section of the series is found, and this is at Yassy in 

 the north-eastern department of Haute-Marne. The succession here 

 as described by M. Cornuel is as follows : — 



Greensand with Ostrea arduennensis (? base of Gault), 28 feet. 



f Sand and Sandstone with Ostrea aquila and O. arduennensis. 

 Plicatula clays — in three divisions, the middle containing Ammonites 

 Aptien. J msus and Jissicostatus, the lower Ostrea aquila and Ter. sella, 



97 feet. | besides Plicatula placunea. 



Eed marl with Orbitolites lenticulata, Heteraster oblongus and other 

 >■ fossils. 



! Oolitic ironstone and ferruginous sandstone, with Unio and fresh- 

 water fossils. 

 Mottled clays and sands (freshwater). 

 Clay with Ostrea Leymerii. 



I Yellow marly clay. 

 w . 1 Spatangus limestone (Toxaster, etc.) 



^eocomien, l White sand _ 



ee • J Ferruginous sands and ironstone. 

 \ Dark marly clay. 



Prof. Hebert prefers to group the Oyster-clay as Neocomien, and 

 to place the red marl at the top of the Urgonien, commencing the 

 Aptien with the Plicatula-clays. 



In passing westward from the Haute Marne, the Neocomien and 

 Urgonien beds appear to thin out, and are overlapped by the Aptien, 

 which in Ardennes is reduced to a few feet of glauconitic clay with 

 marly ironstone at the base containing fossils and pebbles derived 

 from Palaeozoic rocks like the " pebble beds " of Godalming and 

 Faringdon (Barrois, Terr. Cret. des Ardennes, pp. 252, 263). 



It is interesting to notice that in the Vassy section the marine 

 Neocomian beds are succeeded by a group the greater part of which 

 consists of freshwater beds resembling our Wealden strata. More- 

 over, the fauna of the overlying red bed has been identified by 

 Renevier as that of his Bhodanien, and he finds the same fossils in 

 the Atherfield beds of the Isle of Wight ; here therefore we seem 

 to have a basis of correlation, and it becomes important to decide 

 whether this Bhodanien horizon should be grouped with the Urgonien 

 or the Aptien, for it is nowhere of sufficient stratigraphical impor- 

 tance to rank as a separate primary division. 



Two of the best modern authorities on the Cretaceous rocks of 

 France, namely, Prof. Hebert and Dr. Ch. Barrois, agree in placing 

 the " couche rouge cle Vassy " in the Urgonien, but the difference of 

 opinion which exists on this point may be held to prove that the 

 Bhodanien is really a passage group, and that no very decided line 

 can be drawn between the Aptien and Urgonien divisions of the 

 continental series. 



