18 
THE CRETACEOUS ROCKS OF BRITAIN. 
venient, therefore, to retain Holaster subglobosus as the index of the 
normal or more prevalent aspect of the zone and to choose other 
species for indices in the other areas. In the counties of Lincoln¬ 
shire and Yorkshire we may adopt Off aster sphcericus ,* which is 
not uncommon in the higher beds, is rare in the lower, and has 
not yet been found in any other part of England. 
In Dorset fossils are very scarce in the higher beds, and Holaster 
subglobosus has not been found in them, but a few specimens of 
Hoi. trecensis have been obtained, and in this county the zone may 
temporarily be indicated under the name of H. trecensis. 
We are now in a position to give a general account of the two 
primary subdivisions or zones of the Lower Chalk. 
1. Zone of Ammonites varians.— This zone varies con¬ 
siderably in lithological character ; in some districts it consists 
largely of soft marly and more or less argillaceous chalk (the Chalk 
Marl of older writers), but generally these marly beds alternate 
with courses of hard, grey chalk, and in the more northern counties 
it consists entirely of more or less hard chalk with only thin seams 
of marl. In Berkshire and Wiltshire it contains beds of compact 
siliceous chalk, which sometimes contain nodules or concretions 
of impure flint; while in Dorset it appears to pass into a firm homo¬ 
geneous and whitish chalk which encloses siliceous concretions 
with nuclei of true flint. 
Where the zone of Am. varians is well developed and well ex¬ 
posed, as at Folkestone and in the Isle of Wight, certain subzones 
may be distinguished in it. In these localities the basement bed, 
commonly called the Chloritic Marl, but more accurately the 
“ band ” of Stauronema Carteri, is really a subzone of the more com¬ 
prehensive zone of Ammonites varians. Above this some writers 
have recognised a band of Plocoscyphia labrosa (=P. mceandrina). 
Locally also certain species, such as Scaphites cequalis and Tere- 
bratulina nodulosa, are confined to, or specially abundant at, cer¬ 
tain higher horizons, and might be used as indices of local bands 
or subzones ; the geographical extension of such subdivisions being, 
of course, very much more limited than that of the inclusive zone. 
Over a certain portion of the British area, i.e., from Yorkshire 
southward as far as the Thames Valley and a little way into Berk¬ 
shire, this zone is marked off from that above by a bed which has 
received a special name, the Totternhoe Stone (see p. 187). Where 
this stone does not exist there is a gradual passage from the one 
zone to the other, but wherever the fossils have been carefully col¬ 
lected from below upward, there is always an horizon beyond which 
* This Echinoderm was described under the name of Holaster rotundus 
in 1888 (see Quart. Journ. Geol. Soc., Vol. xliv. p. 364), but it proves 
to be identical with that described by Schluter in 1869, and named 
Off aster sphcericus , 
