THE MELBOURN ROCK, ETC. 225 



ft. in. 



Soil and broken ciialk 2 



Marlv chalk with some nodules enclosing a 



band of hard white smooth rock 1 6 



/'Hard yellowish rock with rough fracture 2 



Laminated marly chalk, greenish grey with 



Melbourn J small nodules 3 



Rock. I Hard compact yellowish rock 1 



I Laminated marly chalk 3 



l^ Hard rock with nodules in three massive beds. 5 

 „ f f ^oft grey marly chalk enclosing a lenticular 



Zone ot I ^^^^^^ ^^ j^.^ j.^^ ^^j^ -^^ pj^^^^j. 2^ to 4 



HeL plena. 1^ jia^a grey-and- white mottled chalk 2 



„ f { passing down into smooth, hard, white 



Zoneot I chalk, greyer near the base 14 



Hard grey sandy chalk with green-coated 



nodules and green grains 2 6 



Holaster 

 subglobosus 



§ 4. RELATIO^fS BETWEEN THE BeLEM^'ITE-MaRLS AND THE ChALK 



BELOW. 



The difference in the nature of the chalk below the Belemnite-marls 

 in different places led ns to examine the upper part of the Lower Chalk 

 more closely, and we found that a certain portion of it exhibited dif- 

 ferent characters from the rest, both in general external appearance 

 and in microscopical structure. At Cherry Hinton, near Cambridge, 

 the chalk between the Totternhoe Stone and the Belemnite-marls is 

 about 80 feet thick ; the lower part of this is ordinary grey chalk, 

 but about bb feet from the base of the marls the rock becomes 

 w'hiter and harder, breaking with a smooth clean fracture ; this firm 

 white chalk continues for some 15 feet and then passes up into softer 

 and more marly chalk, which, however, is still much whiter than 

 that at the base of the zone. The harder white chalk is found by- 

 microscopical examination to contain a much larger number of the 

 small round spherical bodies usually regarded as separate Globige- 

 rina-cells, than the softer chalk above, while in the grey chalk below 

 there are very few indeed. 



Similar firm white chalk is found at all exposures below the 

 Belemnite-marls ; at AshwcU and Hitchin, however, there is much 

 less of it than near Cambridge, its thickness at these places being 

 only from 20 to 25 feet ; but the change from grey to white is so 

 abrupt that a hand-specimen will show the two kinds of chalk. 



At Totternhoe the section is somewhat anomalous ; for the Tot- 

 ternhoe Stone here is overlain by a band of very light-coloured chalk, 

 8 or 10 feet thick ; this is succeeded by nearly 30 feet of the usual 

 grey-coloured chalk, which passes abruptly into whiter chalk above ; 

 of the last some 20 feet are shown, and there may be 15 or 20 feet 

 more below the Melbourn llock which caps the hill. Here there- 

 fore the upper white chalk must be more than 30 feet thick, and 

 perhaps 40. Fj-om this place, however, it seems again to diminish 

 in thickness : in the railway-cutting at Tring there appears to be 

 only 18 feet, and at Chalkshire, near AVendover, there is not more 

 than 10 (fig. 3); but this 10 feet consists of very firm white chalk, 



