CHALK. 397 



Certain striated or fibrous appearances in Chalk, which in some instances may 

 be due to slickensides, have in otlier cases been attributed to chemical action 

 inducing an incipient formation of aragonite. ^ 



Chalk has been shown to consist to a large extent of fine amorphous particles of 

 carbonate of lime, together with the shells of Foraminifera {Globigerina, etc.), 

 valves of Ostracoda, fragments of Corals, Polyzoa and Sponges ; and more rarely 

 Radiolaria {Folycistiiia) and frustules of Diatomacea;. 



Some exceedingly minute bodies termed Morpholites, Coccoliths, and Rhab- 

 doliths, also help to form the mass of the Chalk, and they have proved 

 to be of organic origin, some belonging to protozoan structures and some 

 to seaweeds. Other and more conspicuous organic remains help to form the 

 Chalk, such as fragments of Mollusca (especially I)iocera?mts) and of Echino- 

 dermata ; as well as calcareous matter voided by Fishes and other animals. 

 Sometimes, however, about 90 per cent, of the Chalk is composed of shells and 

 fragments of Globigeri/uv. Its organic origin was pointed out by Lonsdale in 

 1835-' 



The Chalk is considered to have been formed in a deep and 

 open sea, and indeed the researches which have been carried on in 

 the North Atlantic Ocean show that materials for a continuous bed 

 of limestone, with flint-nodules, are now being deposited there, 

 at depths of from 400 to 2000 fathoms, while many forms of life 

 met with there are analogous to those of the Chalk. ^ 



The Chalk itself, judging from its method of formation and 

 general purity, probably extended at one time over the greater part 

 of England and Wales. Chalk occurs at Antrim, which was no 

 doubt connected with the Chalk of England ; hence it may be that 

 over about two-thirds of the country the Chalk has been entirely 

 removed by denudation, and the extensive deposits of flint-gravel 

 that occur, especially in the Thames Valley and the Eastern 

 Counties, are but feeble records of the destruction. It is possible 

 that some of the granitic peaks of Dartmoor (then a much loftier 

 range than now) supplied the Chalk with the quartz grains in 

 Devonshire ; and this is about all one can say of the former exist- 

 ence of land in the area of England and Wales during the Chalk 

 period. In Scotland there are certain estuarine beds, perhaps of 

 late Cretaceous age, which point to the existence of land in the 

 Hebrides at this epoch. ^ 



Flints. — The flints are nodules of amorphous silica, which usually occur in 

 approximately horizontal (though interrupted) layers from one to six feet apart, and 

 these appear to correspond with the lines of bedding. Sometimes, however, 

 they have no connection with the divisional lines present, and even the flint-layers 

 themselves are often somewhat irregular when looked at in detail, isolated flints 

 occurring above or below the general line. 



That the flints are mineral aggregations is proved by their irregular and often 

 fantastic shape, and the fact that they have been formed round various organisms 



^ See R. Mortimer and Dr. O. Ward, Q. J. xxxi. iii. 1 13 ; Judd, Q. J. xxix. 418. 



^ Lyell, Elements of Geology, 1841, vol. i. p. 56 ; see also H. C. Sorby, Address 

 to Geol. Soc. 1879 ; Dixon's Geol. Sussex, edit. 2, by T. R. Jones, pp. 123, 2S3 ; 

 Jones, Trans. Hertfordsh. Nat. Hist. Soc. iii. 152 ; Morris, Lecture on Geology 

 of Croydon, and P. Geol. Assoc, viii. 212. 



^ Sir C. Wyville Thomson, Voyage of the "Challenger," The Atlantic, 

 vol. ii. 1877, pp. 291-300 ; J. Prestwich, Address to Geol. Soc. 1871 ; W. H. 

 Hudleston, P. Geol. Assoc, vii. 245. 



* Judd, Q.J. xxxiv. 733, 738. 



