80 
THE CRETACEOUS ROCKS OF BRITAIN. 
We shall therefore regard the Lower Chalk of the island as com¬ 
prising only two zones, regarding the beds known as the Chloritic 
Marl and the Belemnite Marl as subsidiary divisions or sub¬ 
zones. Eventually some other sub-zones may be established, such as 
that of Plocoscyphia labrosa, proposed by Professor Barrois, and 
possibly a higher one characterised by abundance of Scaphites 
(Equalis and TurrHites costatus. The following table indicates 
the scheme of arrangement which may perhaps be found appli¬ 
cable : — 
Zone of Holaster 
subglobosus 
Zone of Ammonites 
varians 
Marl with Actinocamax plenus. 
Clmlk with few fossils, Holaster trecensis 
and Discoidea cylindrica. 
Beds with Turrilites costatus. 
Beds with Scaphites cequalis. 
Beds with Plocoscyphia labrosa. 
Beds with Stanronema Carteri. 
The basement beds with Stauronema Carteri are those which 
have usually been known as the Chloritic Marl, and have been the 
subject of much discussion. Some account of this discussion has 
been given in Chapter II., and much difficulty has been experi¬ 
enced in determining at what particular horizon the line between 
the Upper Greensand and the Chalk should be drawn. There is 
undoubtedly a series of passage beds, and if they are regarded merely 
as a set of beds, without reference to the distribution and range of 
fossils, different observers would be likely to draw the line at different 
horizons, and even when the fossils have been carefully collected 
it is not very easy to decide what beds should be included in the 
Chloritic Marl or zone of Stauronema Carteri. 
Moreover the sections to which previous descriptions have referred 
are those of the Undercliff, and no attempt has hitherto been made 
to correlate the exposures at each end of the Undercliff with one 
another and with those in Compton Bay and Culver cliffs. All 
these differ considerably in detail, and no such attempt would be 
successful unless the fossils obtainable from each bed were care¬ 
fully collected at the same time. 
An endeavour was made to do this in 1897, and after a careful 
examination of the principal sections, Mr. Rhodes was directed to 
collect carefully from the several beds at the different localities. 
We have thus obtained better palaeontological evidence than any 
geologist has hitherto had at his disposal, but nevertheless we have 
not found it easy to make up our minds as to the exact horizon 
which should be taken as the base of the Chloritic Marl. In such a 
matter the result must very much depend on the value which is 
put upon the evidence of particular fossils. We have elsewhere 
given reasons for considering Pecten asper as an unreliable guide ; as 
a matter of fact in the Isle of Wight it does not occur in the Chert 
Beds of the Selborman, but is found in a perfect state of preserva¬ 
tion throughout the Chloritic Marl. Again, Am. [ Schloenbachia ] 
varians, though generally a good guide, is known to occur both in 
Wiltshire and North Dorset below what must be regarded as the 
