A. J. JukeS' Broivne — Two Stages in Tipper Chalk. 369 



With regard to the plane of division between them, the weight of 

 evidence indicates that it should be drawn at the top of the zone of 

 Offaster pilula and Scaphites binodosus. This line seems to be clear 

 enough in Germany, but has not yet been accurately determined 

 in England or France. 



NOMENCLATUKE . 



Saving demonstrated the existence of two different faunas, and 

 consequently of two different stages, in what we have hitherto been 

 accustomed to call the Upper Chalk, it becomes necessary to consider 

 by what names they should be called. The old-fashioned names 

 Lower Middle and Upper Chalk may still be used for divisions which 

 can be shown on geological maps, because they have usually well- 

 marked lithological boundaries, but we cannot continue to employ 

 them as stage-names, since the Chalk is really divisible into four 

 stages, not three as was formerly supposed. 



There can be little doubt that we must adopt the French method 

 of nomenclature, and it will also be convenient to employ the actual 

 French names for these stages as far as possible, because these names 

 are in general use on the Continent. Unfortunately, however, the 

 French geologists are not yet agreed on the question of simplifying 

 their nomenclature and making it accord with the stratigraphical 

 value of their divisions. 



The French terminology originated with d'Orbigny in 1843, and at 

 first he only recognized two divisions or stages in the Chalk, giving 

 these the names of Turonien and Senonien. Subsequently he became 

 aware that his Turonien included two distinct faunas and also that 

 there was another fauna above his Senonien in Denmark. Consequently 

 in 1852 he established four stages under the names of Cenomanten, 

 Turomen, Senonien, and DanifAi. These names have been in use ever 

 since that date, and the definitions of them given by d'Orbigny in 

 his Geologie Stratigraphique of 1852 show that the first three 

 correspond very closely with our Lower, Middle, and Upper Chalk. 



As time went on, however, French geologists found that the 

 Senonian was divisible into two or more parts, which they regarded 

 as stages or sub-stages. Thus the Senonian of the Paris Basin was 

 divided by Lambert in 1876 into Lower and Upper divisions, while 

 that of Aquitaine was divided by Coquand into four parts, to which 

 he gave the names of Coniacien, Santonien, Campanien, andDordonien.^ 



In 1879 de Mercey pointed out that there were really only two 

 separate stages in the Senonian of d'Orbigny, but that these ought to 

 be recognized and that one of them should receive a new name while 

 Senonian was retained for the other; just as d'Orbigny himself had 

 given a new name to part of his original Turonian, but retained that 

 name for a restricted Turonian. De Mercey advocated the retention 

 of the name Senonian for the upper part and the adoption of Coquaud's 

 name Santonian for the lower part.^ 



The correlation of the successive zones was not, however, fully 

 established in 1879. Coquand's stages were really sub-stages, but they 



■^ Coquand, Bull. Soc. geol. France, ser. il, t. xiv, p. 746, 1857. 

 - Bull. Soc. geol. France, ser. in, t. vii, p. 355, 1879. 



DECADE V. — VOL. IX. — NO. VIU. 24 



