324 The Geology of Devizes. 
Roundway, Etchilhampton, and near Eastcott. At the last place it 
contains further evidence of its siliceous character in yielding hard 
cherty nodules which may be regarded as imperfectly formed flints. 
These, too, correspond closely with nodules that are found in the 
Malmstone of Surrey, but no such objects have ever before been 
found in Chalk, so that Wiltshire can claim them as a unique 
phenomenon. 
The structure of this siliceous Chalk is illustrated in Fig. 3 ; 
which is made up of two parts of a slide of such Chalk, and shows 
the various states of mineralization exhibited by the sponge spicules. 
Fig. 3. Structure of Siliceous Chalk (magnified about one hundred times). 
The general ground mass is seen to consist partly of globular silica, 
partly of shell fragments (/) with still finer calcareous dust between 
them; a is a sponge spicule the central part of which is filled with 
globular silica; 4 4 are spicules of unaltered silica cut through 
obliquely and showing the axial canal filled with Glauconite; ¢ is 
a partly dissolved spicule—its outline only shown by the globular 
silica into which it seems to have passed ; and d is another partly 
destroyed spicule; ¢ is a grain of Glauconite, of which this slide 
