228 MESSES. W. HILL AND A. J. JTJKES-BROWNE ON 



effected by tlie action of a current whicTi was strong enough to sift 

 and wash away the finer parts of the chalky sediment, leaving only 

 the heavier particles to form the laminated marls ; where this ero- 

 sion was deepest we find the marls resting on the whiter and harder 

 chalk ; where it was least we find something like a passage from the 

 higher grey chalk into the marl above. The nodule-bed which is 

 sometimes found below the white part of the Lower Chalk, seems to 

 mark a time when nodules, green grains, and fish-teeth were allowed 

 to accumulate on the sea-bottom, and when very little of the finer 

 kind of chalky matter was deposited over the area of its occurrence. 



§ 5. Minute Stetjctuee of the Beds. 



Lower or Grey ChalJc. — Between the Totternhoe Stone and the 

 Belemnite Marls there exists a thick bed of chalk, which is part of 

 the so-called Grey Chalk. As above mentioned the material com- 

 posing the lower part of this zone is of a greyish colour and of a 

 tough close nature ; but it gradually becomes paler, and at a certain 

 horizon, the distance of which above the base of the Totternhoe 

 Stone may be variable, it changes to a white chalk. In thin slices 

 seen under the microscope the grey-coloured chalk presents a con- 

 stant character. It consists almost entirely of fine amorphous mate- 

 rial in which recognizable atoms (probably of shell) may be seen in 

 varying, but never in great, abundance ; here and there specimens of 

 Foraminifera may also be found. But about the horizon where the 

 chalk becomes white, small spherical bodies, usually referred to as the 

 disunited cells of Globic/erince or other Poraminifera, appear in some 

 abundance, and the chalk, losing its tough character, becomes more 

 brittle, and breaks with a smooth, clean fracture. Where this chalk 

 can be seen in its greatest development (Cherry Hinton) the Glohi- 

 gerina-Q,Q^B> in about 20 to 25 feet gradually disappear, and the chalk, 

 though white, assumes something like the tough character noted at 

 its base. At Cherry Hinton this white chalk extends from 50 to 55 feet 

 below the Belemnite Marls ; at Ashwell and Hitchin it is not more 

 than 25 feet thick. At Chalkshire the white chalk is reduced to a 

 thin bed about 12 feet thick, and this agrees closely with the base of 

 the ^hite chalk at Cherry Hinton both in structure and character. 

 Grey-coloured chalk, also agreeing in character and structure with 

 that at Cherry Hinton, Ashwell, and Hitchin, is seen at Chalkshire 

 within 13 or 14 feet of the Belemnite Marls. 



Tlie Gritty Beds. — The hard nodular gritty beds which appear to 

 occur at uncertain horizons in the Lower Chalk seem to consist almost 

 entirely of shell-fragments with scarcely any of the finer material 

 which constitutes the bulk of most chalk. Scattered through the 

 m ass may be seen large specimens of several species of Foraminifera, 

 and many fragments of teeth or bone in which the minute structure 

 is sometimes shown ; grains of a clear green glauconitic mineral are 

 also frequent. The brown nodules which occur abundantly in these 

 beds have in structure the character of the chalk which lies 

 immediately below them, while the tubular holes by which all are 



