LOWER CHALlv—MICROGRAPH LC STRUCTURE. 
279 
Can be traced here and there, are to be seen. There is a large 
admixture of inorganic matter. 
Another taken from above the “ Cast Bed ” (16) exhibits little else 
than fine amorphous material, with very finely comminuted shell- 
fragments and minute mineral grains scattered through it. 
“ Spheres ’are again absent; one or two arenaceous Foraminifera can 
be seen in outline, but no others. 
Wiltshire and Berkshire.— All specimens from the zone of Am. 
varians in these counties contain a large proportion of finely com¬ 
minuted shell-fragments ; they are small and of an even size, and 
form not less than 40 per cent, of the rock, and occasionally more. 
The chief feature is the presence of thread-like sponge-spicules, which 
are common and often abundant, especially in the harder examples; 
but another feature is the comparative rarity of “ spheres.” These 
occur sparingly in a few examples near the top of the zone, but in 
the majority of cases are rare. Mineral grains are present in all 
specimens, their size and shape corresponding to that of shell-frag¬ 
ments ; they are not well seen until a section is viewed with crossed 
Nicols. The grains are distinctly more numerous than in the marl 
of North Dorset. In specimens from the lower beds the presence 
of fine siliceous inorganic material is evident. 
Two samples taken from the railway cutting by Upton Scuda¬ 
more, near Warminster (see p. 152) showed exceptional characters. 
The exposure is probably from 50 to 60 feet above the base of the 
zone ; the one was a hard, grey chalk, the other a tough brownish- 
grey sandy chalk. The first proved to be full of recognisable 
organisms and organic debris, such as Foraminifera, shell-frag¬ 
ments, spheres, sponge-spicules, a few Badiolaria, and many grains 
of glauconite and quartz. The coarser ingredients apparently 
make up nearly 60 per cent, of this rock. There is no globular 
colloid silica, but unaltered siliceous sponge-spicules are common. 
The matrix too is very full of definite calcitic crystals. The other 
sample is really a sandy chalk, and contains a considerable 
quantity of fine quartz-sand mixed with a few angular shell-frag¬ 
ments and a very few Foraminifera. The percentage of fine 
inorganic material in this bed must be large. 
In North Wilts and through Berkshire beds of a peculiar siliceous 
chalk occur at various horizons. These contain besides the ordinary 
ingredients of the zone a certain amount of organic silica, partly 
in the form of sponge spicules, and partly as globules or discs like 
those which occur in the Malmstone. (See Vol. I. of this memoir.) 
The beds in which this colloid silica occurs are generally harder 
than the rest, and are sometimes very hard and compact. These 
siliceous beds appear to be local and to form lenticular masses which 
occur at different horizons and at different places. It is notice¬ 
able, too, that those beds which are less siliceous are more purely 
calcareous than the generality of chalk of this zone elsewhere. The 
