LOWER CHALK—DIVISION INTO ZONES. 
19 
Am. [Schloenbachia] varians does not seem to occur, and in those 
districts where the lower beds consist mainly of marly chalk (or 
“ Chalk Marl ”) the disappearance of Am. varians coincides with 
the horizon at which the Chalk Marl passes into more massive 
Grey Chalk. 
The above description applies only to those districts where the 
zone of Ammonites varians really consists of chalk, or chalk-marl 
On the coast of Devon, however, we find a deposit which occupies 
the place of this zone, and contains most of its characteristic fossils, 
with many others which are peculiar to it, but this deposit is litho¬ 
logically very different, being’ partly a calcareous grit or quartz- 
sand cemented by calcite and partly a quartzifers limestone. The 
palaeontological relations of these remarkable beds were discussed 
in 1896,* when it was suggested that they might be known 
as the zone of Ammonites Mantelli, a species which is rather more 
common in them than Am. varians. We regard them as bearing 
the same relation to the Chalk Marl of eastern England as the typical 
Cenomanian of La Sartlie does to the Rotomagian and “craie glau- 
conieuse,” of northern and north-eastern France. 
2. Zone of Holaster subglobosus— The upper part of the 
Lower Chalk maintains a certain lithological type over a larger 
area than the lower zone of Ammonites varians does, although 
some differences make themselves apparent when it is traced into 
Lincolnshire on the one hand and into the Isle of Wight on the other. 
Where the Totternhoe Stone is present, we regard it as the base¬ 
ment of this zone, but in the south of England, where the Tottern¬ 
hoe Stone is absent, the lower limit of this zone is somewhat indefi¬ 
nite ; the marly chalk of the Ammonites varians zone passes 
gradually into firmer and more massive or blocky grey chalk, and 
where this change takes place Am. varians becomes rare, while 
Holaster subglobosus generally becomes common together with 
Holaster trecensis, Discoidea cylindrica, and spines of Cidaris 
Bowerbanki. 
This grey chalk is succeeded by firm blocky chalk, which is nearly 
white, and in which fossils are so rare that in W. Phillips’s excel¬ 
lent description of the Chalk of Dover, written in 1818, it is denomi¬ 
nated as “ chalk without flints and with few organic remains.” The 
change from grey to white is always rapid, even when there is no 
marked plane of division between the two ; but sometimes they are 
divided by a bed with special lithological characters, such as a layer 
of hard yellowish nodular lumps, or by a bed of hard grey chalk 
somewhat resembling Totternhoe Stone. Such a bed of grey chalk 
occurs near Tring and is locally known as “ rag ” 
This type of partly grey and partly white chalk extends over the 
whole of eastern England from Kent to Wiltshire and from Wilts 
* See Quart. Jouru. Geol, Soc., Vol. lii. p. 99. 
4219. R 2 
