TIIK CRETACEOUS SYSTEM 507 



In the north-west of Norfolk the Middle Chalk appears to be 

 only about 100 feet thick. The Melbourn rock continues at the 

 base of the zone of Rhynchonella Cuvieri, but the beds above are 

 less nodular than tlu-y are farther south, and the zone of Terebratu- 

 lina lata consists of hard white chalk with many layers of flints. 



In Lincolnshire there is no definite Melbourn rock, and the zone 

 of Rhynch. Cuvieri is reduced to a thickness of 10 or 15 feet, con- 

 sisting of yellowish -grey chalk full of fragments of Inoceramus 

 shell and containing Inoc. mytiloides, Rhynch. Cuvieri, and Terebratula 

 semiglobosa. At the top is a layer of grey shale, and the succeeding 

 zone of Ter. lata consists of firm white chalk with frequent layers 

 of grey flints. Inoceramus Cuvieri is the only common fossil, and 

 the thickness of the zone may be 80 to 100 feet. 



In South Yorkshire both zones are exposed in some large 

 quarries near Hessle Station and have yielded a larger number of 

 fossils, but little is known of the Middle Chalk in its course through 

 Yorkshire except that the whole of it becomes very hard. Near 

 Speeton, on the coast, the Middle Chalk is exposed in a fine range 

 of cliffs and has been described by Mr. A. W. Rowe. 21 The zone of 

 Rh. Cuvieri is, as usual, in the north very thin, being less than 12 feet, 

 but the overlying Ter. lata zone has expanded to a thickness of 210 

 feet, that fossil with small Rhynch. Cuvieri, Inoceramus Urongniarti, 

 and Holaster planus being common throughout the whole of it 



Upper Chalk. The chalk of this division has been divided 

 into seven zones (see p. 491), but the higher zones are not every- 

 where present, having been removed from large areas in the south 

 of England during the upheaval which preceded the formation of 

 the Eocene deposits. These seven zones may be grouped into two 

 sub-stages, the lower of which, including the zones of Hoi. planus, 

 Micraster cortestudinarium, and M. coranguinum, may be termed the 

 Micraster chalk, while the higher zones may be called the Beleinnite 

 chalk until more definite names are proposed. 



In Kent the Upper Chalk forms the cliffs from St Margaret's 

 Bay near Dover to Walmer and Deal, and again from Ramsgate 

 round the Isle of Thanet to Margate and Reculvers, but the 

 highest zone found in the county is that of Marsupites. 



Mr. W. Hill finds a convenient base for the zone of Holaster 

 planus in a bed which is full of scattered flints, and in which 

 Echinocorys vulgaris makes its appearance. 22 Measured from this 

 the thickness of the zone is about 48 feet, and it consists of a 

 succession of beds of rough lumpy chalk, the lumps being very 

 hard but embedded in a softer matrix, and the top is marked by a 

 coarse hard nodular rocky[chalk. In these beds Micraster Leskei, 

 M. precursor, and Hoi. planus are common. 



