GEOLOGY 



white and with many flints in some beds and few in others. But these 

 characters are found to be more or less impersistent when a wide area 

 is examined, and it is now recognized that the fossils afford a more 

 satisfactory basis for classification. During the vast period represented 

 by the Chalk, the fauna inhabiting the sea was steadily changing, most 

 of its species being gradually modified, or extinguished and replaced 

 by others. The shells and other hard parts of many of these organisms 

 were embedded in the slowly accumulating mud of the sea-floor ; and 

 by the succession of these fossil species synchronous divisions may be 

 recognized in widely separated districts, even where the enclosing rock- 

 substance has lost its distinguishing peculiarities. On this basis, by a 

 close study of its fossils, the Chalk of Kent has been recently re-classified, 

 divided into ' zones ' like the Gault, and correlated zone by zone with 

 the Chalk of other districts.^ For this purpose the tests of the sea- 

 urchins of the genera Micraster and Holaster, which along with other 

 genera are among the commonest fossils of the Chalk, have been found 

 especially serviceable; while certain other zones are distinguished by the 

 presence of another echinoderm, Marsupites ; by the different species 

 of the extinct cuttle-fish, Beleninitella and Actinocamax ; and by species 

 of brachiopods, Terebratulina and Rhynchonella. Besides the fossils which 

 have been selected as ' zonal ' indicators, the Chalk abounds in other 

 organic remains, including sponges of great variety ; small corals ; a few 

 univalve and many bivalve shells, the latter including characteristic 

 species of Inoceramus and Spondylus ; a few crustaceans ; many cephalo- 

 pods of the genera Ammonites^ Scaphites, Baculites, Nautilus, etc. ; and the 

 teeth, bones and other hard parts of numerous fish and reptiles.^ 



Owing to the prevalence of a covering of clayey earth, and in part 

 also to the lower average elevation of the hills, the Kentish Downs 

 present a more varied aspect than is usual in Chalk uplands. Instead 

 of a dry thin soil and treeless surface covered only with smooth short 

 turf, the Chalk in this county more frequently sustains a deep productive 

 loam, with cultivated tracts and park-lands in which the beech and 

 other trees thrive well. 



The Chalk is extensively quarried in many places, especially along 

 the margin of the Thames valley, for burning into lime and for the 

 preparation of whiting. Mixed with clayey material it is also largely 

 used in the manufacture of Portland cement. As a water-bearing forma- 

 tion its economic importance is very great, the rainfall upon its surface 



I Dr. A. W. Rowe, 'An Analysis of the genus Micraster,' Quart. Journ. Geo!. Soc. (1899) Iv. 

 494-544 ; and ' Zones of the White Challc of the English Coast, pt. I, Kent and Sussex,' Proc. Geol. 

 Assoc. (1900) xvi. 289-368 ; and ' pt. 2, Dorset,' ibid. xvii. 1-76. G. E. Dibley, 'Zonal Features 

 of the Chalk Pits in the Rochester, Gravesend and Croydon Areas,' Proc. Geol. Assoc. (1900) xvi. 

 484-99. The earlier work of Dr. C. Barrois, ' Recherches sur le terrain cretace superieur de 

 I'Angleterre et de I'lrlande,' Memoires de la Soc. Geol. du Nord, tome i. (1876), should also be referred 

 to. 



« For the latest fossil lists see the papers of Dr. Rowe and Mr. Dibley above quoted and the Mem. 

 Geol. Survey, 'The Cretaceous Rocks of Britain, vol. ii. The Chalk' (1902). For description of the 

 numerous vertebrate remains obtained from the Kentish Chalk, see article ' Palaeontology,' p. 31. 



