CHALK. 401 



trustwortliy divisions can be based on the abundance, rarity, or even absence of 

 flint-nodules. 



The researches of Prof. E. Hebert, Mr. Caleb Evans, and particularly of Dr. 

 C. Barrois, have shown that the Chalk may be divided into certain palaontological 

 zones, and these are indicated in the accompanying Table. ^ (See p. 402.) 



The zones have been followed out in various parts of England by Mr. Meyer, 

 Mr. Jukes-Browne, Mr. W. Hill, and others ; but Mr. Whitaker has remarked 

 that although they are very valuable when applied to particular sections, yet their 

 application to great inland stretches of country without continuous sections, or 

 when the structure of the deposits could be seen only in occasional pits, was by 

 no means safe.- Lithological divisions, however, such as the Chalk Rock, the 

 Melbourn Rock, and the Totternhoe Stone, appear to mark certain zones, and 

 these rocks can be traced over considerable areas. As with other palasontological 

 zones, the species taken as indices may range above or below the zones, and while 

 these divisions mark the general succession of life-forms in different areas, they 

 are not likely to be strictly contemporaneous : a few species, however, appear to 

 be confined within narrow limits. 



Lower Chalk. 



Chloritic Marl. — The term Chloritic Marl, applied to beds 

 between the Upper Greensand and Chalk Marl, was introduced 

 in 1848 by Edward Forbes and Captain L. L. Boscawen Ibbetson.' 

 The term Craie Chloritee was previously used by French geologists 

 for the calcareous beds of the Upper Greensand. 



The Chloritic Marl consists of white or pale-yellow marl, some- 

 times indurated, with dark green glauconitic grains, phosphatic 

 nodules, and iron-pyrites. Among its fossils Scaphites cvqualis is 

 locally conspicuous ; hence it has been termed the Scaphite-bed. 

 Other species include Ammonites Rothomagensis (which takes its 

 name from Rothomagum, Rouen), A. Mantelli, A. Coiipei, A. varians, 

 Turrilites Wiestii, Nautilus expansus, N. lavigatus, Ostrea vesicularis, 

 Tercbratula biplicata, RhyncJionella Majitclliana, Holaster (^Ananchytes) 

 subglobosus, Staurofiema Carteri, etc. 



In the Wealden district, according to Mr. Topley, the Chloritic 

 Marl appears to be more closely allied to the Upper Greensand,^ 

 while in the west of England it constitutes the basement-bed of 

 the Chalk. Such being the case, it may very naturally be considered 

 as a passage-bed, but it is now usually regarded as the base of the 

 Chalk, as Edward Forbes originally advocated. If, however, we 

 regard it as ushering in the Chalk conditions, the Chloritic Marl 

 may very well be of slightly different ages in different localities. 



^ The Upper Chalk is equivalent to the Senonian of D'Orbigny (so named from 

 Sens, in the Department of Yonne), the Middle Chalk is equivalent to his 

 Turonian (from Touraine), and the Lower Chalk and Upper Greensand together 

 are equivalent to the Cenomanian (from Csenomanum or Mans in Sarthe). A. 

 Geikie, Text-book of Geology, ed. 2, p. 825 ; see also Davidson, G. Mag. 1869, 

 p. 200 ; and Barrois, Recherches sur le Terrain Cretace Superieur de I'Angleterre 

 et de ITrlande, 1876. 



* Q. J. xxxiii. 446. 



^ Ibbetson, Notes on Geol. I. of Wight, 1849, p. 21 ; see also A. J. Jukes- 

 Browne, G. Mag. 1877, p. 355. 



* Geol. Weald, p. 153. 



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