242 



ME. W. HILL ON THE BEDS BETWEEN THE 



Pleurotomaria depressa, Mant 



perspectiva, Mant 



sp. ( a deep form, ornamented) . . 



Turbo gemmatus, Sby 



(species) 



Cerithium (species) , 



Troehus Marcaisi? {d'Orb. pi. 186 bis.) 

 Aporrhais (Eostellaria) stenoptera 



{Goldf.) 



Solarium, sp 



Ammonites Mantelli ?, Sby 



obtectus, Sharps 



peramplus, Mant. 



Scapbites sequalis ?, Sby 



Nautilus Fie uriausianus, d'Orb 



Ptychodus? 



Otodus? 



Dover. 



Chalk-rock ' 



of Cam- 

 bridgeshire. 



Structure of the Hard Beds which marlc the TJ^jyer and Lower Limits 

 of the Middle ChaXk. 



Although the structure of small thin slices of chalk as seen 

 under the microscope cannot perhaps be trusted as a means of 

 testing its horizon, yet the similarity in the structure of the chalk in 

 the hard beds which define and mark the limits of the Middle Chalk 

 both in Hertfordshire and Cambridgeshii^e, and also at Dover, can 

 hardly be passed without remark. 



The white chalk at the base of the zone of Belemnitella plena is 

 fairly constant in its microscopical characters between Cambridge 

 and Tring, and at Dover these characters are in no way changed. 

 The small round bodies referred to as the single ceUs of Glohigerina 

 become as suddenly abundant in the upper part of the yellowish 

 chalk in the zone of Belemnitella plena at Dover as they do in the 

 upper part of that zone in Cambridgeshire and Hertfordshire. 



The Grit bed (bed 8 of Mr. Price), which forms the base of the 

 Middle Chalk of Dover, exhibits the same nodular peculiarities as may 

 be seen in the Melhourn Boch, which occupies the same horizon in 

 Cambridgeshire and Hertfordshire. 



The microscopical structure of the rocky beds which mark the 

 upper limit of the Middle Chalk is peculiar. The usual amorphous 

 matrix is crowded with shelly fragments, many of which exhibit 

 a structure similar to that of the test or spines of Echinoderms ; 

 these with the sponge-spicules, Eoraminifera, prisms of Inoceramus- 

 shells, green grains, &c., give the chalk a marked character. At 

 the summit of the zone of Terebratulina gracilis the chalk appears 

 to be composed of Ohhigerina-celh, a few shell-fragments, and 

 a rather large proportion of the amorphous matrix ; from this 

 there seems to be a gradual passage, and it is not till the upper 



