21 
LOWER CHALK—DIVISION INTO ZONES, 
It consists either entirely or very largely of grey shaly marl, some¬ 
times greenish-grey and sometimes yellowish or buff coloured; 
but over a large area this marl is divided into two layers or 
courses by a bed of hard white compact chalk, and in some few 
places it contains a kind of chalk conglomerate which consists of 
rounded lumps of hard white chalk embedded in a grey marly 
matrix. 
The fauna of these beds is not a large one, and the fossils are a 
* mixture of Lower and Middle Chalk species, making it evident 
that the deposits were formed during an epoch of transition from 
one set of conditions to another. One species ( Rhynchonella pli- 
catilis ) has not yet been found below the Chalk Rock except at 
this horizon ; the typical form of Inocerarrms mytiloides makes its 
appearance here also, but Hoi aster trecensis is present, ticaphites 
cvqualis has been found in Dorset, and Actinocamax plenus is a Lower 
Chalk species which here attains its largest size and most abundant 
development. This Belenmite is not to be found in every small 
exposure, but is fairly common in some localities, especially in 
the southern and midland counties. 
The lower band of shaly marl sometimes rests on a slightly uneven 
surface of hard white chalk, and in some localities a certain amount 
of chalk seems to have been removed by erosion before the deposi¬ 
tion of the marl. It might, therefore, seem more natural to regard 
this zone as the base of the Middle Chalk instead of the uppermost 
part of the Lower Chalk'; but too much weight must not be given 
to small physical breaks of this kind, especially when they are found 
to be local, and are not accompanied by a great change in the fauna. 
There are large areas in which there is a complete passage from the 
firm white beds of the Lower Chalk, through grey marly chalk, into 
the Belemnite Marls. Further, in its minute structure, the white 
chalk of this zone resembles that of the Chalk below, and differs 
from that which overlies it. 
The lithological characters, the fossil contents and the strati- 
graphical relations of this zone combine to prove that it was formed 
at a time of change when erosion was more prevalent than deposit 
tion ; when, in fact, the bottom of the Cretaceous sea was swept 
by currents which were in some places strong enough to break up 
and wash away parts of previously existing beds. 
Reviewing the English Lower Chalk as a whole, we may remark 
in the first place that it is a clearly defined formation, well marked 
off in most districts both from the underlying and overlying strata. 
In several areas its basement bed rests on an eroded surface of the 
Gault or Greensand, and even where there is a passage from Green¬ 
sand to Chalk, the beds about the grouping of which any doubt can 
be felt are only 2 or 3 feet thick. 
In respect of thickness, the Lower Chalk varies much ; it attains 
its greatest development in Wiltshire, Where it is not leSvS than 
