280 THE CRETACEOUS ROCKS OF BRtTAlK. 
sponge-spicules are usually present either in a siliceous condition 
or as calcite casts. 
So thoroughly indurated are these beds with organic silica that 
when a fragment of the rock is treated with hydrochloric acid it does 
not fall to powder, but is held together by a network or mesh of 
silica. These beds have been previously described by us, and to 
that account the reader may be referred.* 
db 
Fig. 55 — Structure of Siliceous Chalk from Collingbourn Kingston in 
Wiltshire. (From the outside of a nodule.) 
The ground mass is partly a fine calcareous mud, and partly globular silica, the latter 
be'ng most abundant in the upper part of the slide. It includes ; cut quartz-grains ; bb 
glauconite grains ; cc shell-fragments ; dd cavities filled with silica which is partly 
colloid and partly crystalline ; ee sponge-spicules replaced by chalcedonic silica, (x 50 
diam). 
Oxfordshire and Buckinghamshire .—Examples of this zone 
coming from Chilton in Berks and from Oxfordshire present 
uniform characters according to the horizon from which they were 
taken. Finely comminuted shell-fragments form from 25 to 50 
per cent, of the material. “Spheres” are rare on the whole, 
but are more numerous in the upper part of the zone; Foramin- 
ifera occur very sparingly. Sponge-spicules of the usual thin and 
thread like character are to be seen in the Chilton examples. Small 
grains of glauconite occur in all, but are not abundant. Mineral 
grains are evenly distributed through the deposit; they are larger 
and more numerous near the base, w r here much line inorganic 
* The Occurrence of Colloid Silica in the Lower Chalk of Berkshire and 
Wiltshire, Quart. Journ. Geol. Soc., Yol. xlv. p. 403, 1889. 
