LOWER CHALK—DIVISION INTO ZONES, 
17 
2. Division of the Lower Chalk into Zones. 
Since therefore none of the lithological names hitherto applied 
to portions of the Lower Chalk can be used without confusion or 
inconsistency, we are compelled to seek a more satisfactory method 
of subdivision in the guidance of fossils, that is to say, in the limita¬ 
tion of the range of certain species. A study of the distribution 
of the fossils of the Lower Chalk by various observers at different 
places has certainly established the fact that some species are either 
confined to or are much more common in the lower part; while 
others are common in the upper part, and in some areas are con¬ 
fined to that part. In this way we get a useful subdivision of the 
whole stage into two broad zones, or “ assises ,” as the French would 
call them ; the lowest zone is characterised by the presence of Ammo¬ 
nites [Schloenbachia] varians and Rhynchonella grasiana , the second 
one by Holaster subglobosus, Holaster trecensis, and Actinocamax 
plenus ; Ammonites [ Haploceras] Austeni also occurs. 
The investigations made for this Memoir have proved that the 
guidance of these fossils is satisfactory over a large part of England, 
and further, that over these areas the limits of these two zones 
coincide with the more marked lithological differences in the 
character of the Chalk. 
The range of Ammonites [Schloenbachia] varians, appears to be 
co-extensive over the whole of the English Cretaceous area, i.e., 
the species appears to have died out or to have become very rare 
at about the same horizon over the whole region, and probably 
throughout the whole Anglo-Parisian basin, for so far as we can 
ascertain its range is the same over all the northern part of France. 
With Holaster subglobosus, however, the case is different: its 
range is not everywhere the same. In the south-east of England, 
and thence westward to Wiltshire and northwards through the 
counties of Berkshire, Oxford, Bucks, Bedford, Herts, Cambridge, 
and Suffolk, it is very rarely found in association with Am. [Schloen¬ 
bachia] varians; but is often abundant in the massive grey chalk 
which forms the central part of the Lower Chalk. Further north, 
however, it ranges downward to the base of the Chalk, and 
consequently ceases to be of any value as an index-fossil. In Dorset 
and Devon, again, it is also found at the very base of the formation 
in association with Am. [Schloenbachia] varians and other fossils of 
the lower zone. 
In the Isle of Wight it occurs occasionally in the lower beds, 
but much more frequently in the higher zone, so that it can still 
be taken as an index-fossil. 
So far as we know at present there is no species which is restricted 
to the upper part of the Lower Chalk, and is at the same time com¬ 
mon enough in every district to be a useful index. It is more con- 
4219, B 
