596 MESSES. JUKES-BROWNE AND W. HILL ON THE LOWER PART 



horizontal line, but it would not make any difference to this calcula- 

 tion if it were a curved one. It is quite possible that the Gault 

 below Arlesey fills up a hollow in the surface of the Lower Green- 

 sand, and that its upper surface is nearly level, but the thicknesses 

 would of course remain the same. 



It might be said that the assumed thickness of the Gault at Cam- 

 bridge (170 feet) is too great, and that the Lower Gault may have 

 thinned out more rapidly northward of Arlesey than it does south- 

 ward ; but we reply that on the other hand it may have continued 

 to thicken for some distance north of Arlesey, for we have no real 

 evidence to the contrary. "We think that, considering all the cir- 

 cumstances, the thickness we have assigned is a reasonable one, and 

 that the projection as shown in the diagram has a more natural 

 appearance than if the original thickness at Cambridge were taken 

 to be less. 



A further inference may be drawn from this diagram, for if it is 

 a fairly accurate representation of the facts, it will enable us to 

 predicate the probable limit of the Cambridge Greensand in a 

 northerly direction. This will be indicated by the point where the 

 line representing the original slope of the Gault meets the line 

 representing its present surface ; now the prolongation of the 

 present slope north of Soham meets the line for the base of the 

 Upper Gault at a point about seven miles south of Stoke Perry, and 

 hence it may be expected that the Cambridge Coprolite-bed will be 

 found to extend beneath the fens to this point, or at any rate for a 

 distance of 10 miles beyond Soham. It is probable that the number 

 of derived nodules and fossils diminished gradually to the north- 

 ward, and that the bed passes gradually into the glauconitic marl of 

 Stoke Eerry : and further, since we found a derived phosphatic 

 specimen of Avicula grypliceoicles in that marl, it is evident that 

 Stoke cannot be far beyond the limits of the eroded area. 



Totternhoe Stone. — The stratigraphical importance of this band 

 lies chiefly in its affording the means of fixing the upper limit of 

 the Chalk-marl : for if this stone had not been recognizable, it would 

 have been very difficult to draw any line between the Grey Chalk 

 and the still harder chalk representing the Chalk-marl ; but, as a 

 consequence of tracing this stone through the west of Norfolk, we 

 were able to recognize the horizon in the Hunstanton cliffs, and 

 thus to give a more satisfactory reading of that section than any 

 which has hitherto been proposed. 



The Totternhoe Stone partakes in the universal attenuation of 

 the Lower-Chalk strata in West Norfolk, diminishing from 20 feet 

 at Burwell to 4 feet at Stoke Terry, and 2 feet at Hunstanton, but 

 its lithological characters undergo very little alteration. 



Grey Chalk. — It is interesting to observe that the chalk over- 

 lying the Totternhoe Stone exhibits the same separation into a lower 

 grey and an upper white portion that we have observed in this 

 division elsewhere ; the characters of the upper white portion are, 

 indeed, more distinctly marked than they are further south, the rock 

 being so much harder than the underlying grey chalk that in old 



