262 
THE CRETACEOUS ROCKS OF BRITAIN. 
east and Speeton on the north. These have been examined zone 
by zone and county by county, and their leading characters will 
be described in the following pages. The figures in brackets which 
^follow certain detailed descriptions of the micro-sections have 
reference to those specimens which have been subject to further 
critical examination, (See the tables of analyses.) 
2. The Preparation and Examination of Chemical Residues.— 
In order to obtain an idea of the micro-mineralogical contents of 
the Chalk from various horizons it is necessary to get rid of the 
chalky matter. For this purpose a series of specimens from each 
zone was taken, chiefly from localities in Devon, Dorset, Isle of 
Wight, Kent, Buckinghamshire, Hertfordshire, Cambridgeshire, 
Norfolk, Lincolnshire, and Yorkshire, and from a few intermediate 
points. 
Portions of the specimens selected were treated by the method 
employed by Dr. Hume. * The chalk was broken into fragments not 
larger than a good-sized pea, and dried at a temperature of not less 
than 100° C. for at least two and a half hours. A portion was then 
weighed, the quantity taken depending on the size of the speci¬ 
men. This dried chalk was then treated with a 20 per cent, solu¬ 
tion of hydrochloric acid (1*60 sp. gr. B.P.), the resulting solution 
(with the suspended insoluble matter) being poured off from time 
to time. After complete solution the whole was allowed to settle 
in a large beaker, and the clear fluid after passing through a 
filter was thrown away. Hot water was added to the remainder, 
and after twenty had bee n slowly counted, about half the fluid with 
such material as remained suspended was poured into a second 
beaker. This process was repeated until the coarser particles were 
as far as possible separated from the finer. To remove the cal¬ 
cium chloride the whole of the residue was washed with hot water, 
and the fluid poured off through a filter until there was little or no 
reaction with a solution of nitrate of silver in the filtrate. Both 
the finer and coarser residues were dried and weighed separately, 
and from these weighings the percentages are calculated. 
The coarse residue, when the quantity was sufficiently large, 
was further subdivided by sifting through three sieves of 40, 60, 
and 90 meshes to the inch. This created four divisions with par¬ 
ticles of comparatively relative size ; each division was weighed 
separately. The proportions of the various constituents in each 
division were then estimated by the eye, a little of the material being 
spread thinly on a slide and viewed under the microscope, the 
percentage being calculated in relation to the weight. 
From 10 to 100 of the mineral grains in each of the four divisions 
were measured in their longest diameter, and the average taken. 
Where the amount of sand was considerable the average size of the 
grains was calculated in relation to the weight of the respective 
divisions. 
Chemical and Micro-Mineralogical Researches, p. 11. 
