200 
DR. MORISOT-NOTES ON THE CHALK ROCK. 
TJnweathered Chalk Eock soils the fingers very slightly, and if 
looked at through a good lens is seen to contain a number of small 
crystals. Indeed I think the hardness of the rock is mainly due to 
the development in it of crystals of calcium carbonate, to the con¬ 
version in fact of the original soft chalk into something like crystal¬ 
line limestone. Sometimes crystallisation proceeds a stage further, 
and we have large crystals of calcite formed. This is well seen in 
a small pit on the top of the Dunstable Downs. 
The Chalk Eock differs very markedly in chemical composition 
from soft chalk. I here give two analyses of the Chalk Eock of 
Eoxmoor which I owe to the kindness of Mr. Whitaker. 
Analysis by C. W. Mean well, from 1 London Geological Eield 
Club : Excursions during 1888/ p. 19 (1889) :— 
Silicates of iron and of alumina . -97 
Phosphoric acid. ’61 
Oxides of lime, iron, and of manganese . I’ll 
Carbonate of lime .96*77 
Loss on ignition, other than carbonic acid . • 11 
Sulphuric acid . trace 
99-57 
the same work. 
. 1-003 
. -128 
. 54-900 
. -374 
. -347 
•465 
. -713 
. 42 101 
100-031 
It therefore contains nearly twice as much magnesium carbo¬ 
nate, nearly twice as much silica, and more than three times 
as much sulphuric acid (in the form mainly of sulphate of magne¬ 
sium and sulphate of calcium) as ordinary chalk does. The most 
marked peculiarity is, however, the relatively very large amount 
(viz. 0*6 to 0-7 per cent.) of phosphoric acid in the form of phos¬ 
phates which it contains, only a mere trace being found in chalk 
of the ordinary type. No doubt the peculiarity of its chemical 
composition has much to do with the hardness as well as with the 
yellowish tinge of the rock. 
The Chalk Eock comes to the surface in various parts of Hert¬ 
fordshire ; we notice it in the valley of the Lea near Harpenden; 
at various points in the neighbourhood of Baldock ; near Mark- 
yate Street, Berkhamsted, and Boxmoor, etc. At Boxmoor it is 
well seen in the large chalk-pit close to the railway-station. But by 
far the best section in this neighbourhood, and perhaps in this 
country, occurs in a cutting on the Midland Eailway between 
Another analysis, by P. G. Sanford, from 
Specimen dried at 100° C :— 
Insoluble residue, sand, silica, etc. 
Soluble silica . 
Lime . . 
Iron and alumina (Fe 20 3 -hAl 2 03 ) 
Sulphuric acid . 
Phosphoric acid. 
Carbonic acid .. 
Total calcium carbonate 97*001 
,, magnesium „ *785 
