362 
THE CRETACEOUS ROCKS OF BRITAIN. 
that I am inclined to think the chief faunal difference between 
them is the absence in the latter of those species which have hitherto 
been only found in the Chalk Rock. Consequently, I am driven 
to the conclusion that if the zone of Holaster planus has to be in¬ 
cluded in the Middle Chalk, the zone of M. cortestudinarium must 
also be taken into the same stage. This is the conclusion to which 
Schlliter came when dealing with the Chalk of Westphalia,* and 
to which Professor Barrois also came with respect to the Chalk of 
France in 1878.t 
But if we adopted this arrangement we should be as far as ever 
from having a satisfactory base for our Upper Chalk, for though 
many species which are common in the zones of Hoi. planus and 
Micr. cortestudinarium die out in the zone of M. coranguinum they 
seem to do so gradually, and we have not been able to discover any 
horizon which can be followed through the country as a line of division 
between the zones of M. cortestudinarium and M. coranguinum. 
On the other hand, there are few places where there is much 
difficulty in drawing a line at the base of the Chalk Rock or of the 
beds which correspond with it; this line has indeed been actually 
drawn over a large part of England, it is engraved on many of 
the maps of the Geological Survey, and the chalk above it has 
generally been described in Survey publications as Upper Chalk. 
It has consequently been decided to continue this arrangement 
and nomenclature. In this memoir, therefore, the Middle Chalk 
will be regarded as consisting of two zones only, namely : 
2. The zone of Terebratulina. 
1. The zone of Rhunchonella Cuvieri. 
«j 
It must be remembered, however, that the Terebratulina zone of 
this memoir will include all the chalk which lies below the Chalk 
Rock of Oxfordshire and Buckinghamshire, and consequently takes 
in that which was described as the zone of Holaster planus in the 
memoir on the Geology of London (1889); it also includes some 
beds in Dorset and in the Isle of Wight which have been regarded 
hitherto as Upper Chalk. 
The Middle Chalk is thickest in the south-east of England, where 
it has a maximum of 240 feet, thence it diminishes westward till it 
is only about 130 feet in South Dorset and less than 100 feet in 
North Dorset, but increases again in Devon to 140 feet. In parts 
of Wiltshire it is only 80 or 90 feet thick, but thickens north¬ 
ward till it reaches a maximum of 230 feet in Cambridgeshire, 
but again thins to little over 100 in Norfolk. 
Zones of the Middle Chalk. 
Zone of Rhynchonella Cuvieri,— This zone was first 
established in France by Professor Hebert (1866) as the zone of 
* Verhandl. des naturh. vereins der Preuss. u. Westf. xxxiii., Jahrg. 4 
folge iii. Band. p. 330. 
t Terr. Cret. des Ardenneg, Ann. Soc. Geoi. Nord. Tom. v., p. 434. 
