MIDDLE CHALK—CHEMICAL COMPOSITION. 
527 
The following samples, taken from different parts of the Middle 
Chalk, were sent to Professor J. B. Harrison, the Government 
Chemist of British Guiana, who kindly undertook to have them 
analysed in the Laboratory at Georgetown under his own superin¬ 
tendence, the method employed being that devised by him for 
separating the different siliceous ingredients. The horizons and 
localities of the samples are : — 
No. 1. Melbourn Bock, Okeford Fitzpaine, Dorset, lower paid. 
No. 2. Melbourn Rock from the same place, upper part. 
No. 3. Chalk, 50 feet above Melbourn Rock, Hitchin, Herts. 
1. 
2. 
3. 
Organic matter, etc. 
'57 
•50 
*10 
Quartz - . 
'55 
•30 
•15 
Colloid silica - 
'65 
•45 
•35 
Combined silica - 
1'25 
*29 
•23 
Iron peroxide - 
*40 
•26 
*28 
£ 
o3 
Alumina - 
*39 
•39 
*77 
zn 
0} rr! v 
Inorganic Oxide - 
*28 
•05 
•23 
^5 J2 N 
O 
Calcium oxide 
— 
•05 
— 
o 
• r-H 
Magnesia - 
•02 
•35 
traces 
m 
Potash - - - 
•16 
•04 
•14 
, Soda ----- 
trace 
trace 
•13 
Calcium sulphate - 
*17 
•41 
•29 
Calcium carbonate - 
95'20 
97-02 
96*15 
Magnesium carbonate - 
•68 
— 
1-34 
100-32 
\ 
100*11 
100*16 
Traces of chlorine were found in all of them. 
Summarising the results of the above analyses, and arranging 
the ingredients more nearly in order of relative importance, they 
may be expressed as below : — 
1. 
2. 
3. 
Calcium carbonate - 
- 
- 
95-20 
97-02 
96*15 
Magnesium carbonate 
- 
•68 
— 
1*34 
Calcium sulphate - 
- 
- 
*17 
•41 
•29 
Silicates - 
- 
- 
2-50 
1*43 
1*78 
Colloid silica - 
- 
- 
•65 
•45 
*35 
Quartz - 
- 
- 
•55 
*30 
•15 
Organic matter 
- 
- 
•57 
•50 
•10 
100-32 
100*11 
100-16 
It will be noticed that these are all very pure limestones, and 
that the amount of crystalline silica or quartz in them is very 
small indeed. The amount of clay in the Melbourn Rock varied 
only from 1*4 to 2*5 per cent., and the total amount of siliceous 
matter in the lower sample of this rock is only 3'70. 
