THE CAMBRIDGE GAULT AND GKEENSAND. 



273 



than the beds below, apparently marking the cessation of current- 

 action and the commencement of deep-sea conditions, I am inclined 

 to believe that a stronger line of division will have to be drawn at 

 its base than has hitherto been thought necessary. 



(2) With regard to the denudation of the Gault, it is veiy difficult 

 to know how much of the diminution in thickness through Cam- 

 bridgeshire is due to denudation, and how much to thinning out. 

 There can be no doubt that the formation does become thinner both 

 to the north and the west. At Harwich it is only 40 feet thick, at 

 Norwich 36 ; and in West Norfolk, near Lynn, it disappears alto- 

 gether*. But by taking a line on Norwich and constructing the 

 following diagram (fig. 4), we may gain some idea of the amount of 

 rock removed by denudation in excess of the normal thinning-out. 



Fig. 4. — Diagram representing the Erosion of the Gault over 

 Cambridgeshire. 

 (Horizontal scale, 20 miles =1 inch ; vertical, 300 feet =1 inch.) 

 A 



E. Eddlesborough. A. Arlesey. C. Cambridge. N. Norwich. 



The line AN representing the original surface of the Gault, and 

 ACN the present denuded surface, the area enclosed between them 

 may be taken as a measure of the amount of clay thus removed, 

 amounting over Cambridge to 60 feet. How far the coprolite-bed 

 extends under the chalk towards Norwich cannot of course be known 

 until more deep wells have been sunk. 



(3) I see no reason to suppose that the Upper Greensand ever ex- 

 tended further north-eastward than Buckinghamshire ; and the follow- 

 ing diagrammatic section (fig. 5) represents my view of the relations of 



s.w. 



Fig. 5. — Diagrammatic Section through Bedfordshire. 



N.E. 



Buckinghamshire. 



a. Lower Gault. 



b. Upper Gault. 



c. Upper Greensand. 



Bedfordshire. Cambridgeshire. 



d. Chalk-marl. 



e. Coprolite-bed at base of Chalk-marl. 



* I am inclined to think that the beds described as Upper Greensand at 

 Harwich and Norwich are more probably referable to the sandy upper portion 

 of the Gault ; if so, the numbers above given would stand corrected thus — 

 Harwich 61, Norwich 42. 



