404 CRETACEOUS. 



Chalk conditions may have come on earHer in the east of England 

 than in the west, for in Dorsetshire and Devonshire we have 

 evidence of shallow-water conditions in the lowest beds of Chalk, 

 while the Upper Chalk, as Mr. Whitaker has pointed out, overlaps 

 the Lower. Moreover, the occurrence in the Chloritic Marl of the 

 west of England of Holaster suhglobosus, Ammonites Rothojiiagensis, 

 Scaphiies cvqualis, and Tefebrattila sefuiglobosa, is noteworthy.^ S. 

 (Equalis does not occur in the Chloritic Marl of the Isle of Wight. 



The Red Chalk and the so-called ' Cambridge Greensand ' will 

 be described subsequently. 



Chalk J\IarI. — The Lower Chalk includes the Chalk Marl (more 

 or less argillaceous chalky beds with hard stony layers), and 

 above it the Grey Chalk, divisions which are locally separated 

 by the Totternhoe Stone, but are not always to be distinguished, 

 especially in the west of England. Among the fossils of this 

 formation we find Turrilites costafns, T. tiiberculatus, Hajnites 

 attenuatus, H. armatus, Bacidites anceps, Belemnitella plena. Ammonites 

 Rothomagensis, A. varians, A. Mantelli, A. Sussexiensis, Nautilus 

 Deshngchampsianus, N. elegans, Pecten Beaveri, P. orbicularis, 

 Spondylus st rial us, Lima globosa, Inoceramus latus, Plicatula inflata, 

 Rhynchonella Mantelliana, R. Cuvieri, R. Martini, Terebratula 

 semiglobosa, T. biplicata, Terebratulina striata, Discoidca cylindrica, 

 Holaster subglobostis, Ptychodus decurrens, Chelone Benstedi, etc. 



Totternhoe Stone. — This bed consists of hard grey and sandy 

 chalk, containing about eight per cent, of silica, and charged in 

 places with glauconitic grains. It takes its name from Totternhoe, 

 about three miles west of Dunstable, in Bedfordshire. It contains 

 Anunonites Rothomagensis, A. varians. Nautilus elegans, Lima globosa, 

 Ostrea vesieularis, Pecten fissicosta, P. orbicularis, P. Beaveri, Rhyn- 

 chonella Mantelliana, Terebratula biplicata, T. semiglobosa, etc. 



Middle Chalk. 



This division is locally marked by the occurrence of the Mel- 

 bourn Rock at its base, and by the Chalk Rock at the top. It 

 contains Ammonites peramplus, Lnoceramus mytiloides (^labiatus), L. 

 Brongniarti, /. Cuvieri, Terebratula semiglobosa, Terebratulina gracilis, 

 Rhvnchojiella Cuvieri, Galerites subrotimdus, Cardiaster pygmcBa, 

 Holaster planus, Discoidea minima, etc. 



Melbourn Rock. — This rock, named from INIelbourn, between 

 Cambridge and Royston, was described by ]\Ir. Jukes-Browne 

 in 1880.- It is a hard yellow and white nodular stone, locally 

 known as * rag,' and from eight to ten feet thick. It contains 

 Rhynchonella Cuvieri, Terebratulina striata, etc. 



Chalk Rock. — This bed was so named by INIr. Whitaker in 1859, 

 originally as the topmost bed of the Lower Chalk in Wiltshire, 



' Q. J. xxxiii. 446, 447. 



'^ See Penning and Jukes-Browne, Geol. Cambridge, p. 55. 



