4 oo SCIENCE PROGRESS 



About iS ft. above the base of the zone there is a bed of 

 tabular flint, which is familiar to visitors to St. Margaret's Bay. 

 This is known as the Micraster cor-anguinum tabular, and is 

 found in the pits inland. 



Zone of Micraster cor-testudinarium. 



This zone presents a marked contrast to the one above. 

 Flints are numerous, but scattered : it is 56 ft. thick in Kent, the 

 chalk is distinctly harder, discoloured with yellow remains of 

 sponges, and weathers out into hard irregular nodules. 



The fossils are abundant, this affording a great contrast to 

 the zone above. The most characteristic kinds are a special 

 form of Micraster cor-testudinarium and of Micraster precursor, 

 Holaster placenta (a large, depressed, thin-shelled urchin), a 

 gibbous variety of Echinocorys scutatus and Scrpula ilium, a little 

 convoluted worm tube. 



Zone of Holaster planus. 



This zone, only 35! ft. thick, is perhaps the most interesting 

 of the series. The chalk is hard, sometimes intensely so. It 

 is greyish with marl staining ; flints are abundant but scattered. 

 In its upper portion there is a narrow band of intensely hard 

 nodules of silicious chalk embedded in a soft marly matrix ; 

 this weathers out inland into a breccia by the removal of the 

 marl. In places the hard band is an unbroken sheet of nodular 

 chalk, so hard that it can scarcely be broken with the hammer, 

 under which it rings like granite. 



About the middle of the zone there is a band, 9 ft. thick, 

 of hard chalk with green grains and phosphatic nodules, and 

 a peculiar fauna, for this is the only bed above the Cenomanian 

 in which Gasteropods and Cephalopods occur commonly; of the 

 former Turbo gemmatus, Pleurotomaria perspectiva, and Trochus 

 schluteri occur as casts ; and of the latter Hcteroceras reussianum 

 and Scaphites geinitzi are the most characteristic. 



This band is the palaeontological representative of the Chalk 

 Rock of the Midlands. The indurated silicious bands mentioned 

 above are its physical representatives. These are so hard that 

 they form a feature which can be detected at its outcrop, and 

 so is most useful for mapping. 



The junction between this zone and the underlying chalk 

 is marked by a prominent band of marl situated between two 



