GENERAL ACCOUNT OF THE CHALK. 
9 
recognised at Dover by Mr. W. Hill in 1886,* and more recently 
Mr. Hill has traced the Melbourn Eock through Kent and Surrey 
in the course of explorations specially made for the Geological Survey 
during the years 1896 and 1897. 
It should be mentioned, however, that while the Melbourn Eock 
maintains its characters and position throughout these south-eastern 
counties, there is no bed of solid limestone with green-coated 
nodules like the Chalk Eock of more western counties, its place 
being taken by a considerable thickness of nodular chalk, in which 
harder rocky beds occur at irregular intervals. 
It has indeed become apparent to Mr. Hill and myself, during 
the course of our joint and separate explorations, that the bed 
originally described by Mr. Whitaker as the “ Chalk Eock/' is only 
one of several beds of chalky limestone which make their appearance 
in and above the zone of Holaster planus, and also that layers of 
green-coated nodules are not confined to this one horizon. In 
tracing the base of this zone it is therefore necessary to pay more 
attention to fossils than to lithological characters. 
In conclusion, it has been shown by the testimony of many ob¬ 
servers that the Chalk of England can be divided into three parts or 
stages, Lower, Middle, and Upper, which correspond roughly 
with the French stages known as Cenomanian, Turonian, and 
Senonian. Each of these stages is capable of further subdivision 
into zones, as will be pointed out in the sequel. 
The Lower Chalk comprises the beds which were formerly 
known as Chalk Marl and Crey Chalk, as well as a certain thick¬ 
ness of the “ white chalk without flints.” Over the greater part of 
England, its basement bed is a sandy glauconitic marl, often con¬ 
taining phosphatic nodules ; in certain areas this has been called 
the Chloritic Marl, in others the Cambridge Greensand; but over 
the area of the Eed Chalk its lowest bed is a pure limestone without 
any admixture of sand. Its summit is generally defined by a band 
of grey marl, in which the Belemnite Adinocamax plenus is fre¬ 
quently to be found. 
The total thickness of the Lower Chalk varies from 250 to about 
60 feet. In Devon its place is taken by calcareous sandstone and 
quartziferous limestone, varying from 1 to 40 feet in thickness. 
The Middle Chalk consists partly of hard rocky or nodular 
chalk and partly of firm white chalk. Its basement-bed is the hard 
nodular chalk to which the name of Melbourn Eock has been given. 
Flints are generally absent from the lower part, are sparingly dis¬ 
tributed in its middle portion, but are sometimes numerous in the 
higher beds. 
As defined in this Memoir the Middle Chalk does not include the 
Chalk Eock of the Midland and Eastern counties, but does include 
the bed to which that name has been applied in Dorset and in the 
* On the Beds between the Upper and Lower Chalk of Dover, Quart* 
Journ. Geol. Soc., Vol. xlii. p. 232. 
