MIDDLE CHALK—GENERAL ACCOUNT. 
363 
Cidaris, Kirudo, but Dr. Barrois* * * § in 1873, preferred to call it by 
the name of Inocemmus labiates, and it is so designated in his 
“ Recherches sur le Terrain Cretace Superieur.” (1876.) This 
species, however, is more correctly called I. mytiloides (see Pig. 57). 
When describing the Chalk of Cambridgeshire in 1880 and 1881f, 
I proposed to take Rhynchonella Cuvieri (Fig. 62) for the index 
species, as the typical form of this shell was generally abundant 
in the zone. This has been accepted by subsequent writers, and it 
is now usually known in England by this name. 
It coincides with the “ chalk without flints, but with many thin 
beds of organic remains ” (nodules) of W. Phillips’ description of the 
Dover Chalk J; and with Beds viii. and ix. of Mr. Price’s section 
in 1877. § It is also to be identified with the Upper Marden 
Park Beds of Mr. Caleb Evans in Surrey.|| 
From its more or less nodular character and its creamy white 
or yellowish tint, it forms a contrast to the grey Belemnite marl at 
the top of the Lower Chalk and to the compact white chalk 
which generally underlies that band. This nodular character 
is most marked at the base of the zone, and the lowest beds over 
the greater part of England form a band of hard nodular rock, 
to which the name of Melbourn Rock was given in 1880, from 
the village of Melbourn, near Royston, in Cambridgeshire. 
The Melbourn Rock may be described as a hard nodular chalky 
limestone, forming three or four massive beds when quarried, 
but weathering into a series of nodular and less nodular layers. 
The rock consists of a compact matrix in which small lumps or 
nodules of hard white chalk are arranged in planes or layers. The 
thickness of such rocky chalk varies in different places, but in 
the southern and midland counties it is generally from 8 to 10 feet; 
it passes up, however, into the hard bedded shelly chalk of the over • 
lying zone, the small nodules becoming fewer and fewer and the chalk 
more compact, but more rough and shelly. The lower part of 
the rock often has a greenish-white tint and a marbled appear¬ 
ance from the presence of a greenish-grey marl which is arranged 
in wavy layers round the nodules ; this part generally forms one 
massive bed 3 or 4 feet thick, and the nodules are less regularly 
arranged than in the higher part, which generally splits into two 
or three beds and is of a yellowish-white colour. 
Mr. Hill remarks,<1 “ There seems to be every gradation in the 
definition of the individual nodule from a well-defined sharply- 
angular fragment to the nodule whose shadowy outline blends 
with and is scarcely traceable in the mass of the chalk. In size 
* Note sur le Terr. Cret. entre Saint Omer et Boulogne, Mem. Soc. des 
Sciences, Lille, Tom. xi. 
f See Geol. Mag., Dec. 2, Vol. vii. p. 248, arid the Geology of the Neigh¬ 
bourhood of Cambridge, Mem. Geol. Survey, 1881, p. 60. 
+ Trans. Geol. Soc., Ser. 1, Vol. v. p. 16. 
§ Quart. Journ. Geol. Soc., Vol. xxxiii. p. 431. 
II On some sections of Chalk between Croydon and Oxted, Geol. Assoc., 
Separate Paper. 
M In “ Geology of London,” Mem. Geol. Survey, 1889, Vol. i., p. 522. 
