312 A. J. Jukes-Broicne — On the term Neocomian. 



Southern Area. Northern Area. 



[ Chalk White Chalk. 



Upper Cretaceous. < Upper Greensand ) -r, ■, chalk 



iv™™™,-™ ^ I Lower Greensand Upper Is eocomian. 



JNeocomian or \ i^-iji at 



Middle Neocomian. 



"Wealden Lower Neocomian. 



Lower Cretaceous 



Now in order to ascertain whether this use of the name Neocomian 

 was in any way desirable or justifiable, it is necessary to make some 

 inquiry into the history of the name and its usual acceptation by the 

 majority of geologists on the continent. 



The primary groups or divisions of the Lower Cretaceous series 

 usually recognized in France and Switzerland are those proposed by 

 D'Orbigny, namely : — (1) Neocomien; (2) Urgonien ; and (8) Aptien, 

 and the history of these names is as follows. Previous to 1835 the 

 rocks included in the first two groups were known as the " Jura- 

 Cretacee " group, and in that year Thurman proposed to call them 

 Neocomien, from their development in the neighbourhood of 

 Neuchatel ; this name was adopted by Marcou and others, but it is 

 important to remember that this Neocomien series did not include 

 the beds now known as Aptien ; thus we find Prof. Marcou in his 

 " Lettres sur les roches du Jura " writing in 1856 of Lower, Middle, 

 and Upper Neocomien, but the summit of his Upper stage is the 

 white limestone of Neuchatel (Urgonien). 



Meantime D'Orbigny had been making his palasontological investi- 

 gations, and had in 1843 described a series of beds lying between the 

 Gault and what was then called "Neocomian" in the south-east of 

 France ; these beds he proposed to call Aptien from Apt in Vaucluse. 

 The relation of the Neocomian as understood by Thurman and 

 Marcou to the Aptien and Lower Greensand is clearly stated by 

 Marcou in his first letter on the rocks of the Jura (p. 14) thus, " It 

 is clear in fact that the Lower Greensand of England is in no way 

 the equivalent of the Neocomian, and is hardly perhaps to be corre- 

 lated with the upper part of the Neocomian . . . The blue marls 

 which I have called the Marnes d'Hauterive, and which contain so 

 many fossils, the Calcaire jaune inferieur, etc., that is to say, the 

 Lower and Middle Neocomian, have no marine representatives in 

 England." 



He appends a table showing that these Neocomian strata are the 

 marine representatives of the English Wealden, with possibly the 

 lowest beds of the Lower Greensand, and he recommends the adop- 

 tion of the name as indicating the normal marine type to which the 

 freshwater Wealden is an exception. 



Marcou, however, does not look with favour upon D'Orbigny's 

 name of Urgonien, which was proposed in 1850, D'Orbigny then 

 suggesting a new classification for the whole series, separating the 

 upper half of the beds called Neocomian by Marcou under the name 

 of Urgonien (from Orgon, Pouches des Ehone), and thus limiting 

 the name Neocomian to the lower half of Marcou's series, that is, 

 from the horizon of the yellow stone of Neuchatel downwards. 



