on 
THE CRETACEOUS ROCKS OF BRITAIN. 
to Norfolk, but with much variation in the relative thickness 
of the differently tinted portions, as well as in the total thickness 
of the zone. When it is traced to the south-west, however, and 
studied in the fine cliff sections of the Isle of Wight, we find that 
the whole of this zone of Holaster subglobosus consists of white or 
whitish chalk; moreover H. subglobosus and H. trecensis may be 
found readily up to the very top of it. We did not observe any 
change nor any means of separating the lower from the upper part. 
This massive white chalk extends westward through Dorset, 
but at the same time becomes much less fossiliferous, though 
an occasional Holaster trecensis may be found, and here the zone 
is best indicated by this species, for Holaster subglobosus is much 
more abundant at the base of the Chalk in Dorset. In West Dorset, 
where the chalk of the Am. various zone also becomes white and 
massive, it is quite impossible to draw any line between one zone 
and the other, though we believe both are represented. 
In Lincolnshire and Yorkshire this part of the Lower Chalk has 
a rather different aspect. The lower part of it consists of rather 
hard, rough and lumpy, greyish-white chalk in regular beds, sepa¬ 
rated by thin layers or seams of grey marl, and in some places a 
portion of about 6 or 7 feet in thickness is coloured bright pink. 
The upper beds consist of smoother and more compact whitish 
chalk, and a similar portion near Louth is coloured pink or yel¬ 
lowish pink. 
Holaster subglobosus is not uncommon in these beds both in York¬ 
shire and Lincolnshire, but is equally common in the zone of Am¬ 
monites varians, so that it will be more convenient to take Off aster 
sphcericus as the index of the upper zone in these counties ; we may 
then speak of the zone of Off aster splicericus as the equivalent of 
the zone of Holaster subglobosus. 
At the top of the zone of Holaster subglobosus there are almost 
everywhere some beds of grey mail which frequently contain Actino- 
camax plenus, and have sometimes been called a zone, but, strictly 
speaking, they do not constitute a zone, and have no distinct zonal 
fauna. They may be regarded as a subzone, but it is seldom 
that many fossils occur in them, and we shall prefer to speak of 
them as the Belemnite Beds or Marls. 
This band is generally a very narrow one, often only 3 or 4 feet 
thick, but in some places, as at Beachy Head and in South Dorset, 
it expands to 15 or 16 feet. Its lithological characters are remark¬ 
ably constant, and palaeontologically it is well characterised by 
the presence of Actinocamax plenus. It is traceable from Dover in 
the south-east to Cheddington in the west of Dorset, and from the 
Isle of Wight in the south to Speeton in Yorkshire in the north ; 
the only exception to its continuity being the north-west of Nor¬ 
folk, where it cannot be distinguished with certainty. 
