IOWER CHALK—BERKSHIRE. 
165 
CHAPTER XIII. 
THE LOWER CHALK IN BERKSHIRE. 
General Description. 
In the western part of Berkshire the Lower Chalk is probably 
about as thick as it is in the adjoining part of Wiltshire, namely, 
240 feet, but in the north-eastern part near the river Thames its 
thickness has been calculated at not more than 220 feet, 
It is in this county that the Totternhoe Stone makes its first 
appearance as a recognisable bed, but it has not yet been identified 
further west than the village of Chilton. The stratigraphical im¬ 
portance of this bed is that it occurs just at the horizon where Am¬ 
monites mrians dies out, and so enables us to draw a line between 
the zones of Am. mrians and Holaster subglobosus. 
Thus in the northern part of Berkshire the succession and rela¬ 
tive thicknesses of the beds are as follow : — 
ft. 
5. Grey Marls with a bed of white chalk - - - 2-3 
4. White'and grey chalk .about 75 
3. Totternhoe Stone - ----- o 
2. Marly and blocky chalk - - about 135 
1. Glauconitic marl - - 2-5 
About 220 
Nos. 1 and 2 make up the zone of Ammonites various, Nos. 3, 
4 and 5 form the zone of Holaster subglobosus, No. 5 being the 
subzone of Actinocamax plenus . 
Above the Chloritic Marl there is some thickness of grey chalk 
marl in beds alternating with courses of harder grey chalk; higher 
up there is greyish-white, marly chalk, not very clearly bedded, 
and the upper 60 feet of the Am. various zone consists of marly chalk 
with courses of hard siliceous chalk characterised by the presence 
of sponge spicules and globular silica.* 
The Totternhoe Stone has been described on p. 15, and the small 
representative of it in Berkshire agrees with tlxat description in all 
essential particulars. 
* See Quart. Journ. Geol. Soc., Vol. xlv., p. 403. 
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