THE CRETACEOUS ROCKS OF BRITAIN. 
no locality, only stating that it “ occurs in grey lumps at the foot of 
the Wiltshire Downs”: — 
Finely divided silica, soluble in caustic potash - 
Insoluble siliceous matter - 
Alumina and oxide of iron - 
Lime. 
Magnesia. 
Potash and soda - ,, - 
Phosphoric acid - » 
Sulphuric acid - • ■ 
Carbonic acid and loss ■ 
16-710 
6'090 
*780 
40*757 
•825 
traces 
•242 
1*546 
33-050 
100*000 
Assuming that all the lime is in combination with the Phosphoric, 
Sulphuric, and Carbonic acids, and that the Magnesia is present 
as a carbonate, the following may be the mineralogical composition 
of this marl: — 
Soluble silica (chiefly as free silica) - - - - 16 "71 
Silica (as quartz and clay) - - - - - 6*09 
Alumina and oxide of iron - - *78 
Carbonate of lime.. - 70*35 
Carbonate of magnesia - - - - ■ - - 1’73 
Sulphate of lime - - - - - - - > .2*62 
Phosphate of lime.*52 
Loss (by difference) - - - - - - - - 1*20 
100*00 
Professor J. B. Harrison has also been kind enough to make 
partial analyses of two samples from the zone of Am. varians in 
Wiltshire A being a somewhat siliceous chalk from a quarry 
north-east of Clay Hill, near Warminster, and B a hard grey 
shelly chalk, somewhat resembling Totternhoe Stone, from the 
railway cutting at Upton Scudamore, in the same district. The 
results are as follow :— 
A. 
B. 
Water - - 
‘25 
•04 
Colloid silica - 
6-68 
2*28 
Quartz sand ----- 
6*42 
3*47 
Argillaceous matter (clay and glau- 
conite) ----- 
19-07 
6-22 
Calcium carbonate (by difference)*- 
67'58 
87*99 
100-00 
100-00 
Mr. Hill cut and examined a slice of B. (see p. 279) and found 
no globular colloid silica in it but many sponge-spicules, the 
Avails of which still consist of the original colloid silica, so that it 
is evidently to these spicules that the 2-28 per cent, of colloid 
silica found by analysis is due. Grains of quartz and of 
glauconite are fairly numerous, and will account for the quartz 
and some of the argillaceous matter in the analysis. The 
sample A contains a small amount of globular colloid silica, as 
Avell as some sponge-spicules, sand and glauconite. 
* This will include magnesium carbonate, if any was present. 
