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.
