LOWEtl CHALK—CHEMICAL COMPOSITION. 333 
A. B. 
Soluble silica - - - - - v -2*16 2*11 
Carbonic acid - - - - - - - 29*96 36*73 
Sulphuric acid - - *. - , - , - - - ‘21 *06 
Phosphoric acid - - „ - . - - - '21 *05 
Lime . - - - - - . - - - 41*52 49*16 
Magnesia - _ - - „ - - - - , - *30 1*18 
Potash and soda - - - - - - 1*90 1*47 
Alumina - - - - - - - — * 20 
Oxides of iron - - - - - - 2*20 1*74 
Insoluble matter - .. - - - - - 21 *35 7*26 
99*81 99*96 
If all the carbonic acid found by these analyses is combined with 
lime the amount of carbonate of lime in A is 08 "09 per cent., and 
in B 83*47 per cent., while in each case there is an excess of lime, 
amounting to between 2 and 3 per cent, beyond the amounts re¬ 
quired to combine with the carbonic, phosphoric, and sulphuric 
acids. This excess was probably present as silicate of lime in com¬ 
bination with other silicates. 
Both samples were probably taken from the lower part of the 
Lower Chalk, i.c., from the zone of Ammonites varians, and, judging 
from the large quantity of insoluble matter in A, it must have come 
from near the base of that zone. 
In Wiltshire and Berkshire certain beds in the lower and middle 
parts of the Lower Chalk, but probably all included in the zone of 
Ammonites varians, contain considerable amounts of colloid silica 
which is soluble in a boiling solution of caustic potash. In Wilt¬ 
shire, moreover, siliceous nodules of a dull grey colour often occur 
in some of these siliceous chalks. Several analyses have been made 
by Professoi* J. B. Harrison (now the Government Chemist of 
British Guiana) from samples sent to him ; two of these analyses 
have already been published and are reproduced below, A being a 
sample from near Collingbourne Kingston in Wiltshire, and B from 
bed at Chilton in Berkshire : — 
A. 
B. 
Moisture -------- 
1 
*08 
Combined water and organic matter - 
l 
*69 
3 
*18 
Colloid silica - - 
38 
*69 
19 
*08 
Quartz - 
13 
*41 
11 
*23 
Silicates (clay and glauconite) - 
14 
■75 
*47 
Calcium carbonate ------ 
30 
*771 
64 
.96 
Calcium sulphate ------ 
*49/ 
. - ----- - . - . 
99* 
80 
0 
0 
r**4 
*00 
In the case of the Collingbourne material it 
will 
. be seen 
that 
can hardly be called a chalk, for two-thirds of it consist of siliceous 
matter, less than one-third being pure chalk or carbonate of lime. 
In the other sample the proportions are reversed, and this is a more 
usual composition, two other partial analyses of Wiltshire siliceous 
chalks showing totals of 34 and 35 per cent, of siliceous matter, 15 
to 20 per cent, of it being soluble colloid silica. 
A chalk-marl from Wiltshire analysed by Professor Voelcker 
in 1858* was evidently one of these siliceous marls, but he gives 
* Jo urn. of Bath and W. Eng. 8oc., Ser. 2, Vol. vi., p. 240. 
