o02 the cretaceous rocics of Britain. 
are chiefly prisms of Inoceramus, or fragments of these shells 
showing the prismatic structure. The quantity of shell-particles 
seen in different specimens varies considerably. In some cases 
they form 60 per cent, of the rock-material, or even more, and 
where shell-fragments are a less important constituent, large and 
coarse prisms or pieces of Inoceramus shell are often a marked 
feature in thin sections of the rock. The interstices between the 
shell-fragments are often crowded with Spheres, and where 
shell-fragments are not abundant the Spheres frequently con¬ 
stitute the major part of the deposit, while here and there a large 
Grlobigerina or Textularia occurs. 
In the counties of Buckingham, Bedford, Hertford, and Cam¬ 
bridge the Mel bo urn Rock consists of two well-defined layers—a 
lower nodular bed, white in colour, and an upper yellowish layer. 
In the lower layer (Plate VII. Pig. 1) the nodules are thickly packed, 
sometimes well defined, but sometimes difficult to distinguish. Under 
the microscope they are seen to consist of more evenly constituted 
chalk than the surrounding matrix, and frequently contain traces 
of Radiolaria. The matrix usually consists of amorphous material 
and fine calcareous particles, but occasionally large fragments of 
Inoceramus shell occur, while Spheres are abundant, and form 
a large part of the rock. The upper yellower part (Pig. 2) is 
smoother, the nodules often widely separated and better defined 
than in the lower layer. Coarse fragments of Inoceramus- shell 
are a feature in the amorphous matrix, and Spheres are not 
so important a constituent of the deposit. 
In specimens from well-marked nodular rock it will be seen that 
the nodules have a different structure to the surrounding chalk. 
They consist of a fine, somewhat dense calcareous paste, now 
hardened by the infiltration of calcite ; Spheres are moderately 
abundant, and a feiv Grlobigerina and Textularia can be seen, but 
shell-fragments are not numerous (see Plate VI.). As a rule they 
resemble the upper part of the Lower Chalk and the hard 
white chalk which often occurs in the Belemnite Marls. 
The material surrounding the well-marked nodule is usually 
crowded with coarse shell-fragments ; a few Globigerince occur, 
while “ Spheres ” often occupy all the other available space, the 
whole being cemented by amorphous calcareous paste, now finely 
granular crystalline calcite. In some cases where the whole of 
the amorphous paste is removed, the matrix is practically a shell 
sind, and the cementing material is infiltrated calcite (Plate VI. 
Pig. 1). The difference in the character of the nodule and that 
of the surrounding matrix is often striking, and in many examples 
of this rock a distinct arrangement of the particles of shell can 
ba traced round the nodule, strongly suggesting that they had 
l)3en deflected from their course by the nodule while travelling 
over the sea-bed under the influence of a gentle current.* From 
* The nodular character of the Melbourn Rock is often very well 
shown in rolled fragments on the sea-shore, as near Dover and Beachy 
Hud. 
