Page:VCH Sussex 1.djvu/50

 A HISTORY OF SUSSEX links are discovered species pass by imperceptible gradations into each other, so that the ordinary rules of zoological nomenclature break down, and it is exceedingly difficult to know what to call any but the extreme forms. Space will not permit any discussion of these variable fossils ; we can only observe that they are particularly abundant in the lower part of the Upper Chalk, Micraster cor-bovis being confined to the Holaster planus zone and the Chalk below ; while Micraster cor-testu- dinarium and M. cor-anguinum give their names to the next two zones, respectively no and 240 feet in thickness. Though still-varying micrasters extend throughout the Upper Chalk, other more abundant fossils have been selected to give their names to the higher zones. Next comes softer chalk with fewer flints, at any rate inland, belonging to the zone of Marsupites, curious purse-like echinoderms, two species of which are not uncommon, associated with JJintacrinus and the small sea urchin Cardtaster pilula. On the coast the thickness of this zone is nearly 80 feet. The next zone, that of Actinocamax quadratus, is more flinty, and this superposition of flinty chalk on soft, easily eroded chalk with few flints has led to the formation of a series of isolated, flat-topped hills, with steep scarps towards the north and gentler slopes to the south, which form a chain extending east and west through the middle of the Downs. Each of these hills was fortified in prehistoric times, the best known of the fortresses being the conspicuous camp of Cissbury, where also the flints of this zone were mined for the manufacture of implements. According to Dr. Rowe, a thickness of 170 feet of Chalk belonging to this zone can be measured in the cliffs. It is doubtful whether there is more than this anywhere in the county, for the higher part of the zone and the whole of the zones above appear to be missing. The best places to study the Chalk zones in Sussex are, for the Middle and Upper Chalk, the cliffs between Eastbourne and Brighton ; but for the Lower Chalk the large pits near Lewes are more satisfactory, for at Eastbourne the Lower Chalk is exceptionally hardened and dis- turbed. With regard to the zones into which the Chalk is divided, it may be remarked that these are belts corresponding roughly with the occurrence of certain faunas, and that the name of a particular fossil is only given to a zone as a matter of convenience. The name-fossil may be entirely absent from a particular district, or it may there range above or below the zone to which it gives its name ; and usually, as might be expected, most of the other species overlap towards the border and are found also in the zone above or below. Hard and fast boundaries are no more to be found between life-zones, except in rare cases, than they are to be found between species ; it is only our imperfect knowledge that in each case has allowed us to draw sharp lines. Between the Chalk of Sussex and the oldest of the overlying Tertiary deposits there exists an enormous gap. Not only is great part of the Chalk missing, but several early Eocene deposits elsewhere well developed and containing peculiar fiunas are absent ; even the Thanet 12