526 
THE CRETACEOUS ROCKS OF BRITAIN. 
CHAPTER XLV. 
THE CHEMICAL COMPOSITION OF MIDDLE CHALK. 
Analyses of Middle Chalk do not bring out much variety of 
chemical composition. The hardness of the Melbourn Rock and 
of certain beds-in the Terebratulina zone appears to he due partly 
to the slow accumulation of the material of which they are formed, 
and partly to subsequent changes by which these particular beds 
were rendered denser by the infiltration of crystalline carbonate 
of lime or. cal cite; the amount of this calcite being greater in 
some places than in others. 
The Chalk of the Rhynchonella Cuvieri Zone. 
The following is an analysis of Melbourn Rock taken from the 
cutting on the railway one mile south of Burghclere, Hants, and 
made by Mr. J. T. Hewitt (late of St. John’s, Cambridge) ; pub¬ 
lished in Papers of the Hants Field Club, No. ii., p. 72 (1888). 
Silica - 
- 
- 
- 
7-99 
Lime 
- 49-07 
Magnesia 
- 
- 
- 
- 
trace 
Peroxide of iron - 
- 
- 
- 
- 
•33 
Carbonic acid 
- 
- 
- 
- 38*53 
Phosphoric acid - 
“ 
“ 
- 
4*15 
100-07 
In this analysis the amount of silica is unusually large and the propor¬ 
tion of lime is not sufficient to be in combination with both the phosphoric 
and the carbonic acids, so that it does not seem to be reliable. 
Messrs. Way and Paine give an analysis of a “ very hard yellowish 
chalk, mixed with broken chalk fossils, and lying between the 
last marl (i.e., Grey Chalk), and the chalk with flints.”* This 
sample was doubtless from near the lower part of the Rhyn¬ 
chonella Cuvieri zone. They also give one of the lower portion 
of the Chalk with flints (? upper part of Middle Chalk). Both were 
obtained near Farnham. These two analyses are copied below : — 
1. 
2. 
Clay and sand, insoluble in acid - 
- 
- 
- 
2-04 
•66 
Carbonic acid 
” N 
- 
- 
- 
42-14 
42 * 98 
Sulphuric acid - 
- 
- 
- 
- 
•31 
trace 
Phosphoric acid - 
- 
- 
- 
- 
•07 
•08 
Lime - 
- 
- 
- 
- 
54*37 
55 * 24 
Magnesia 
- 
- 
- 
- 
•25 
*10 
Potash 
_ 1 
•08 
•06 
Soda - 
- 
- 
- 
- 
•19 
•14 
Oxides of iron - 
- - - 
- 
- 
- 
•55 
•74 
100-00 
100-00 
The amount of carbonate of lime in the first sample will be over 95 per 
cent., and in the second about 98 per cent. 
*Journ. Roy. Agric. Soc., Yol. xii., p. 552 (1852). 
