LOWER CHALK—MICROGRAPHIC STRUCTURE. 277 
From Mupe Bay, South Dorset, to Kent, the lowest beds are a 
bluish or brownish grey, passing upward to a pale grey at the 
summit of the zone. In North Dorset the lowest beds are pale grey, 
the higher ones almost white, but as one proceeds along the northern 
outcrop to the north-east in Wiltshire, Berkshire, and Oxfordshire 
they pass to a bluish grey, the upper part being of a lighter grey 
colour. This continues through Buckingham, but in Bedfordshire 
the marl immediately above the Cambridge Greensand is again 
pale grey, the centre dark grey, passing to greyish-wliite at the 
summit. Still further east the colour gradually dies away, and the 
equivalent to the Chalk Mail of the more eastern counties is a rock 
nearly white in colour. This continues through Lincolnshire and 
and Yorkshire, till at Speeton the lowest bed is again grey. 
Examples of the Chalk at Mupe Bay or Lulworth are hard and 
semi-crystalline, the rock being indurated with ealcite ; in North 
Dorset it is a softish marly chalk. Alternating beds of marl and 
soft chalk occur in this division in the Isle of Wight, Wiltshire, 
Berkshire, Oxfordshire, and Buckinghamshire. To the northward 
through thecounties of Bedford and Cambridge it is a soft marl, break¬ 
ing down rapidly under the infiutnce of weather to a marly clay. 
These conditions die away gradually to the east-ward, till in North¬ 
west Norfolk it is a compact hard chalk, a condition which ccntinres 
through Lincolnshire and Yorkshire, but as one progresses 
through these two latter counties the Chalk Mail is not a smooth 
limestone but rough, nodular, and veiny, and being usually 
thoroughly indurated by caleite it forms a very hard tough rock. 
Local variations, such as the siliceous beds of Wiltshire and 
Berkshire and the Marl Bock of Bucks, will be noticed in due 
course. 
Microscopic Aspect of Thin Sections. 
Viewed in thin seoti< ns with an objective of two-thirds of an 
inch focus, the chalk of this zone is sec n to consist of a fine cal- 
careo-siliceous groi nd mass, the. constitution of which is more par¬ 
ticularly described on p. 290, and in this are scattered mineral grains 
and grains of glauconite, with Foraminifera, shell-fragments, and 
other recognisable organic materials. The proportions of these 
ingredients vary great 13 ' in different localities and at differing 
horizons in the zone. 
The following is a general description of the leading features seen 
in a series of thin slices of the chalk of the zone of A. varians from 
various horizons, progress'ng from west to east, 
Dorset.— All specimens from Dorset centan a large proportion 
of recognisable organic fragments. These consist of comminuted 
shell-fragments, with a coarser piece here and there. The greater 
part of them can generally be identified as prisms of Inoceramus 
shells, 1 ut many are doubtless derived- from other species of 
Mollusca. ‘‘Spheres" are very abundant n some examples* while 
others (as at Batcombe) ccnta’n comparatively few. 
4219 , T 2 - 
