OP THE UPPER CRETACEOUS US WEST SUPPOLK AM NORFOLK. 593 



to have thinned out, so that there is a direct transition from the 

 Gault-marl to the Chalk-marl. It is hardly too much to say that the 

 structure of the Totternhoe district affords the key to that of West 

 Xorfolk, because it explains that passage from Gault to Chalk which 

 has been such a puzzle to those who have examined the Norfolk 

 sections. It is now evident that such a complete transition from 

 one formation to the other is the normal condition of things in that 

 part of England which lies to the north-east of Buckinghamshire. 



This being so, we are completely relieved from the necessity of 

 finding an equivalent of the Upper Greensand as distinct from the 

 Upper Gault in the Hunstanton section, and it only remains to 

 decide whether the Bed Rock in that section is the equivalent of the 

 Xorfolk Gault or of the Norfolk Chalk-marl. This question seems 

 to find a decisive answer in the following considerations : — 



1. That the Xorfolk Gault becomes increasingly calcareous towards 

 the north, till at Dersingham it passes into 7 feet of marly and 

 chalky material, the lower portion of which is coloured red. 



2. That the microscopical stiucture of the Hunstanton rock bears 

 the same relation to the red and yellow marls of Dersingham that 

 the hard Chalk-marl of Norfolk does to the softer Chalk-marl of 

 Cambridgeshire. 



3. That the hard whitish limestone which overlies the represen- 

 tative of the Gault from Grimston to Dersingham is identical, in 

 our opinion, with the so-called " sponge-bed which overlies the 

 Eed Bock at Hunstanton. 



4. That the fossils are chiefly Gault species, and are such as 

 would constitute a deep-sea fauna contemporaneous with that of the 

 shallower and muddier water in which the Gault of South England 

 was formed. 



Erom these premises we come to the inevitable conclusion that 

 the Eed Eock of Hunstanton must be the equivalent of the Gault, 

 and not of its upper division only, but that it is a condensed repre- 

 sentative of both Lower and Upper Gault, formed outside the limits 

 of the area reached by mud-bearing currents. 



Cambridge Greensand. — It has been stated above that the transi- 

 tion from Gault to Chalk-marl must be regarded as the normal state 

 of things in the counties of Bedford, Hertford, Cambridge, Suffolk, 

 and Norfolk. Xow it is true that, over a large part of this area, 

 there is no such transition, but, on the contrary, a plane of erosion 

 between the two formations, and a nodule-bed at the base of the 

 Chalk-marl, containing fossils and phosphates derived from the 

 Gault. Bat this must be regarded as an abnormal condition of 

 things ; as explained by one of us twelve years ago*, the Hertford 

 and Cambridge area was part of a tract of the sea-bottom which was 

 invaded by rapid currents, and over this tract a certain portion of 

 the Gault was sifted and swept away. In the south of England the 

 interval is represented by certain glauconitic sands and marls ; but in 

 Norfolk there seems to have been simply an absence of deposition, 



* Jutes-Browne on " The Relations of the Cambridge Gault and Greensand." 

 Quart. Journ. Geol. Soc. vol. xxxi. p. 256 et seq. 



