400 CRETACEOUS. 



stone. Dr. Percy has detected no chemical difference between white and black 

 flint ; though some observers state that tliere is occasionally a little carbonate of 

 lime in the white coating. Prof. Hughes is inclined to believe that in the flmt in 

 the Chalk this white exterior vi^ould, from its manner of occurrence, often appear 

 to be due not to vi'cathering but to the imperfect surface of the flint in process of 

 formation, and hence the carbonate of lime has not been entirely replaced by 

 silica.' In flints obtained from the Drift the white coating is, however, chiefly due 

 to decay. 



Another peculiar variety is the Banded flint so frequently to be seen among the 

 stones of a gravel pit, or on the sea-shore where the shingle is largely composed of 

 flint. This banding, as pointed out by Dr. S. P. Woodward, is due to infiltration, 

 and is not in itself organic, but at the same time the banding may have sometimes 

 been produced by the infilling with siliceous material of cavities which have 

 resulted from the decomposition or decay of organic remains.- 



The Chalk is rich in organic remains, although most speci- 

 mens are usually procured rather from the quarrymen than by 

 the geologist himself. Some Plant-remains have been recorded, but 

 on the whole there are few remains of land-organisms. Of Fora- 

 minifera, about one hundred species and notable varieties occur. 

 Sponges are also abundant, and many of the Ventriculites described 

 by Mr. J. Toulmin Smith were obtained from Burham, near 

 Chatham.^ The more conspicuous fossils are the Echinoderms, 

 Ananchytes, Micraste?', Cardiaster, etc., and the Mollusca, Ammonites, 

 Tunili/i's, Scaphites, Baculites, Ostrea, Inoceramus, Pecten, etc., 

 Hippurites also occurs ; while the Brachiopoda include species 

 of Rhynchonella, Terebratula, etc. The more important species will 

 be noticed subsequently. Polyzoa, Annelidcs, and Crustacea are 

 met with ; and among Crustacea the remarkable Cirripede, Loricula 

 pu/chella, first found in the Lower Chalk of Cuxton, near Rochester, 

 has also been met with near Norwich ; many Ostracoda also occur. 

 The Corals include species of Caryophyllia, Trochos?nilia, and 

 Parasmilia. (See p. 353.) Remains of Fishes are not uncommon, 

 they include Notidanus microdon, Corax falcatus, Otodiis appendi- 

 culatus, Lanuta subulata, Beryx, Ptychodiis mammiUa7-is, and P. 

 dccunens (allied to the Cesitacioti or Port Jackson Shark). The 

 Reptiles include Mosasaurus, Ptcrodactyhis, Acanthopholis, Cheloiic, 

 etc. Remains also of Birds have been found in the ' Cambridge 

 Greensand,' and near Maidstone.'^ 



The Chalk is generally divided as follows : — 



Upper. ^Chalk with flints. 



Middle. — Chalk with few flints. 



Lower. — Chalk without flints, Grey Chalk, Chalk Marl, Chloritic Marl, etc. 



The division often made into Chalk with flints and Chalk without flints, is 

 at best a very rough and general classification, derived from the character of the 

 beds in the south-east of England ; and it is now generally admitted that no 



' Proc. Cambr. Phil. Soc. iii. 12; Geol. and Nat. Hist. Repertory, ii. 126. 



- Geol. Mag. 1S64, p. 145 ; also M. H. Johnson, Proc. Geol. Assoc, ii. 264 ; 

 R. Mortimer, Ibid. v. 353; T. R. Jones, Ibid. iv. 453 ; Wetherell, Q. J. xv. 193. 



3 Ann. Nat. Hist. 1848. 



* T. R. Jones, Geol. Mag. 1870, p. 74. 



^ For figures of Chalk fossils see Dixon's Geology of Sussex, ed. 2 ; also 

 references, p. 353. 



