292 
THE CRETACEOUS ROCKS OF BRITAIN.- 
and Chalks become hardened seems to be due to the gradual 
growth of these minute crystals. In many samples the arrange¬ 
ment of these minute crystals suggests that they have continued 
to increase until they have enclosed the coarser ingredients of 
the mass, and interlocking with each other have converted the 
whole into a compact semi-crystalline rock. The crystalline grains 
vary in size from very minute particles to such as pass into broader 
crystals. When developed in abundance, as in some of the 
harder and purer chalks, they give a granular aspect to a slice of 
the rock under the microscope, and this material will hereafter be 
described as “ granular crystalline calcite." 
(3) The exceedingly fine inorganic material (clay) present in 
considerable proportions in the Chloritic and Chalk Marls can 
scarcely be recognised when a portion of the finest washings are 
distributed on a slide and mounted in Canada Balsam, though its 
presence gives a greyish tint and a general appearance of dirtiness 
to the deposit. In the finest part of the residue which results in 
the treatment of specimens of this division by dilute acid it can be 
seen as structureless amorphous material. It resembles the 
amorphous material of the Gault already described,* but differs in 
one important particular. It is neutral when viewed with 
polarised light like that of the Gault, but the “ many points of 
light,” the “ ill-defined crypto-crystalline appearance, as though 
particles of quartz had become entangled in the amorphous matter/' 
are not so evident, and though the material does appear to contain 
within it minute mineral particles, these are far less numerous than 
in the Gault clays. 
Tabulation of Results. 
In the annexed table will be found the results of the treatment of 
nineteen specimens of this zone by the acid solution. Two of these 
are from notes which Dr. Hume has kindly allowed us to make 
use of, and two others are extracted from the published work, 
“ Chemical and Micro-Mineralogical Researches,” by Dr. Hume. 
We have to thank Mr. Chapman for kindly sending us a specimen 
from 10 feet above the Chloritic Marl, Dover, and Mr. Lamplugh 
for the specimens from Hunstanton ; the remainder were obtained 
by ourselves. 
* Cretaceous Rocks of Britain, Mem. Geol. Survey, Vol. i. p. 355, 
