UPPER AND LOWER CHALK OF DOYEPv, ETC. 243 



part of the zone of H. planus^ or the horizon of the Micrasters and 

 Gasteropods, is reached that this peculiar structure is fully attained. 

 This passage maybe traced in Hertfordshire and at Dover, fragments 

 of the hard rock in which Micrasters and Gasteropods occur being very 

 similar in both localities. There is one remarkable difference, how- 

 ever : green grains which are abundant in all Chalk Rock between 

 Cambridge and the Thames are very rare at Dover ; and the green - 

 coated nodules which form one of the features in many exposures of 

 the beds known as Chalk Rock in the Midlands do not occur at 

 Dover at all. It may be noted, however, that brownish phosphatic (?) 

 nodules are not uncommon. From such fragments as I have examined 

 of the upper portion of the rocky beds, it appears that the peculiar 

 structure already described passes gradually away in the hard yel- 

 lowish rock, and is lost at a higher horizon. 



Summary and Conclusions. 



It will be seen from the foregoing remarks that there exist in 

 the Chalk of Dover two well-marked bands having peculiar litholo- 

 gical characters, enclosing about 150 feet of softer chalk which con- 

 tains but few flints. Both these bands appear to attain a greater 

 development at Dover than in Cambridgeshire. The lower has all the 

 characters of the Melhoum RocJc^ and, as in Cambridgeshire, it overlies 

 a thin bed of marly chalk containing: Belemnitella plena. Its deve- 

 lopment is, however, much greater, its peculiar character extending 

 almost through the zone marked by Rhynclionella Cuvieri, from 

 which zone it can hardly be separated as in Cambridgeshire. This 

 fossil, accompanied by Ecliinoconus subrotunclus, is characteristic of 

 the basal 60 or 70 feet of the Middle Chalk in both localities. 

 Above the nodular chalk the zone of Terehratulina gracilis is equally 

 well marked in all respects. But at the summit of this zone there 

 occurs at Dover a bed in which Holaster planus occurs almost alone, 

 other fossils, except sponges and Ventriculites, being scarce ; in it 

 the chalk passes from a sottish material to beds of rocky chalk com- 

 posed of crystalline lumps contained in a softer matrix. 



This zone of Holaster planus is not definitely recognized in the 

 Survey Memoir on Cambridge ; but referring to the zone of Terehra- 

 tulina gracilis, the authors say (on page 63) : — " The question, too, 

 of further division of the Middle Chalk is one that cannot be solved 

 in Cambridgeshire, though there would certainly appear to be palae- 

 ontological evidence for separating off the upper 50 feet of this zone 

 and constituting them into another division." 



Mr. Jukes-Browne, however, in his paper * on " The Subdivisions 

 of the Chalk," recognizes a zone of Holaster planus underlying the 

 " Chalh Roclc,^' and regards this rock as the topmost bed of the zone. 



The upper limit of the Middle Chalk of Cambridgeshire is described 

 in the Memoir as a thin baud of crystalline rock containing certain 

 fossils which may be peculiar to it ; but at Dover there is a succession 

 of bands of hard crystalhne chalk to which the term " rock " may be 



* Geol. Mag. dec. ii. vol. vii, p. 253, 

 aJ.O.S. No. 166. 8 



