4o8 CRETACEOUS. 



ncUa Cuvicri, Tcrehrafula semiglobosa, T. siihifera, Inoceraynns tenuis, 

 etc. The Red Chalk was penetrated in a well-boring at Holkham, 

 near Wells, in Norfolk. 



Analysis of the Hunstanton Rock showed the following con- 

 stituents : — 



Carbonate of lime with a little alumina 82-3 



Peroxide of iron 6 '4 



Silica 1 1 -^ 



Hence the deposit is rather an impure Chalk, or Chalk marl, coloured red by 

 peroxide of iron. The amount of iron-oxide varies, as one analysis showed 9'6o 

 per cent.i_ As remarked by Mr. David Forbes, it appears that the Red Chalk does 

 not contain quite so much iron as does the Gault ; so that if the latter rock were 

 subjected to any oxidizing influences, it would assume the red colour of the 

 Hunstanton rock, as it does by burning. The term ' ferruginous,' therefore, 

 as applied to rocks, is by no means a necessary indication that they contain any 

 larger proportion of iron than do many rocks that have no ferruginous appearance. 



In West Norfolk there is a deposit of marly clay 20 to 70 feet thick, 

 which rests on the Lower Greensand, and contains at its base a Coprolite-bed 

 (nine inches), which is largely made up of phosphatized Gault fossils. This 

 Coprolite-bed has been worked at West Dereham ; but while it has been regarded 

 as the base of the Chalk Marl, it is now included by Messrs. Jukes-Browne and 

 Hill with the Gault.- 



Near Lynn Mr. Whitaker identifies the Chalk Marl with a thickness of twenty 

 feet, but it disappears near Sandringham ; in places it contains layers of a pale 

 pink colour. The absence of this clayey division at Hunstanton, as well as the 

 absence of the Gault clay, may in Mr. Whitaker's opinion have had something 

 to do with the colouration of the Red Chalk, the peroxidation of the iron-ore 

 being made more easy by freedom for the percolation of water. 



Above the Red Chalk at Hunstanton is a layer about eighteen 

 inches in thickness called the 'Sponge Bed,' in which the branching 

 or ramose markings termed ' Spongia paradoxica' are found. These 

 markings have been shown by Prof. Hughes to be inorganic con- 

 cretionary structures.^ This 'Sponge Bed' has sometimes been 

 included in the Hunstanton Series. 



The Hard Chalk has been largely quarried at Stoke Ferry and Whittington ; 

 it has yielded Iclithyosattnis cai7tpylodon ; also Animoniies pera7nphis, and A. 

 Ajistenii, each about two feet in diameter. Many fossils have also been obtained 

 at Marham and Shouldham. Mr. Jukes-Browne informs me that the ' Hard 

 Chalk' includes representatives of both Lower and Middle Chalk. Totternhoe 

 Stone occurs at Stoke Ferry, and near Roydon.* West of Harpley layers 

 of tabular flint occur in the Middle Chalk, and these sometimes enclose 

 lenticular masses of Chalk. Here also, and at Massingham and Wells, clayey 

 seams occur in the Chalk ; while at Trimingham a sandy bed has been noticed 

 by Mr. C. Reid. At Hillington curious concretionary structures affect the beds, 

 presenting anticlinal and other appearances. ' Curved joints ' have also been 

 observed in the Chalk of Cherry Hhiton, near Cambridge, which give an 

 appearance of false-bedding to the rock. 



^ R. C. Clapham, Geologist, vi. 29 ; Bonney, Cambridgeshire Geology, p. 77. 



2 C. Reid and G. Sharman, G. Mag. 1886, p. 55. 



■' Q. J. xl. 273. 



* Whitaker, Proc. Norwich Geol. Soc. i. 238. 



