584 MESSES. JUKES-BROWNE AND W. HILL ON THE LOWER PAET 



base are not shelly, but agree in character with the underlying 

 limestone. 



This bed was certainly passed through in the boring at Dersing- 

 ham, and at this place, as well as at Snettisham and Heacham, it 

 gradually passes up into hard, white Chalk-marl, which, though less 

 gritty, is full of shell-fragments and Foraminifera. The structure of 

 the Inoceramus-hed at Hunstanton is almost identical with that 

 of the hard, gritty chalk at Roydon. The shell-fragments are 

 much coarser in its basal two or three feet, but it passes up into 

 similar material and, finally, into the hard, white Chalk-marl, 

 identical with that at Dersingham. 



The Totternhoe Stone. — The beds usually referred to as the 

 Totternhoe Stone vary a little in structure. That most in request 

 for building-purposes, and which is quarried at Totternhoe and other 

 places, and may be described as the typical stone, is seen, under the 

 microscope, to consist of from 60 to 70 per cent, of shell-fragments, 

 remarkably uniform in size, many glauconitic grains, which are 

 frequently of large size and often in the form of beautifully perfect 

 casts of Tbraminifera, and a small percentage of fine quartz-sand. 

 Both above and below the typical Totternhoe Stone are beds which 

 present no such regularity in the assortment of the shell-fragments 

 composing them, and the beds of the stone itself vary in thickness, 

 and are often separated from one another by layers of less shelly 

 chalk. 



The " Brassil," which underlies the Stone at Bur well, consists 

 mainly of very coarse shell-fragments ; but the green-coated nodules 

 contained in it are not shelly, their microscopic structure being 

 comparable with that of the Chalk-marl beneath. 



At Isleham, Beck How, and Stoke Ferry we are able to identify 

 the Totternhoe Stone by its microscopical characters. It presents 

 in all these exposures the same appearance as in specimens from 

 the less defined beds of Hertfordshire and Cambridge, being rather 

 irregular in grain. 



At Sandringham, Dersingham, and all exposures beyond, the 

 massively bedded layer at the top of the hard Chalk-marl possesses 

 the same shelly character as its equivalent at the top of the Marl 

 in Hertfordshire and Cambridge. Specimens from most of these 

 exposures show some little irregularity in the size of the comminuted 

 fragments of shell of which they are largely composed ; but that 

 from the cliff of Hunstanton is very like the upper part of the stone 

 exposed in the Totternhoe quarries. 



It must be added, however, that the fine quartz-sand, which at 

 Totternhoe forms a part of its constituent material, is almost 

 absent at Hunstanton. The gradual diminution in the proportion 

 of this can be followed along the line of the outcrop of the Stone. 



The Grey Chalk. — Above the Totternhoe Stone the change 

 in the character of the deposit is usually abrupt, but not 

 invariably so. Thin sections from the lower part of the Grey 

 Chalk between Newmarket and Shouldham present exactly the 

 same characters as those which we have before described in Cam- 



