588 



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



The amount of carbonate of lime in A may be taken as 95*5 per 

 cent., and in B as 92 per cent. ; for although the Inoceramus-bed is 

 so gritty to the touch, it is really a pure limestone, and does not 

 contain a larger quantity of silica than the compact limestone upon 

 which it rests, its grittiness being entirely due to the angular 

 fragments of hard Inoceramus-shell, of which it is so largely 

 composed. 



We were also desirous of knowing the chemical composition of 

 the red marls, which occur at Grimston, Roydon, and Dersingham, 

 and to see whether the last was more closely related in this respect 

 to the Roydon marl or to the Red Chalk of Hunstanton. By the 

 kindness of Dr. Johnstone, to whom we sent samples, we have 

 been furnished with the following analyses of (A) the Grimston, (B) 

 the Roydon, and (C) the Dersingham Marl : — 





A. 



B. 



c. 





22-60 



24-13 



25-70 





69-50 



64-46 



64-49 



Carbonate of magnesia .... 



•90 



•90 



1-32 





•66 



•36 



•33 





3-40 



6-00 



4-16 



Alumina and phosphoric acid 



1-60 



•90 



•80 





trace 



traces 



traces 





1-34 



3-25 



3-20 





100-00 



100-00 



100-00 



It will be seen that the three are very similar in their com- 

 position, the marls from Roydon and Dersingham being almost 

 identical, although the localities are more than four miles apart. 

 A comparison of the above analyses with that of the overlying 

 grey marl at Roydon (p. 586) shows that they may be regarded 

 as the same marl, coloured red by peroxide of iron, the proportions 

 of siliceous matters and carbonate of lime being almost the same, 

 while the proportions of iron and alumina are in an inverse ratio. 



They all differ from the Red Chalk, an analysis of which has 

 been given by Mr. Wiltshire* ; and the following is that of a pink 

 sample from Hunstanton, by Dr. Johnstone : — 



Silica, &c. 7-50 



Carbonate of lime 83*81 



Alumina 1*67 



Peroxide of iron 5*72 



Manganese. . . *58 



Magnesia . . . *62 



99-90 



This is, lithologically, a chalk ; but its being so does not prove it 

 to be stratigraphically Chalk, any more than the composition of the 

 Roydon marl proves that to be Chalk- marl. 



* Quart. Journ. Geol. Soc. vol. xxv. p. 185. 



