82 
THE CRETACEOUS ROCKS OF BRITAIN. 
phosphatised to the depth of half an inch or more, and are 
frequently bored by lithopliagous Mollusca; these lumps are seldom 
larger than a cricket ball, but generally oval, and mingled with 
them are pliospliatic nodules, phosphatised sponges belonging chiefly 
to Jerea and Hallirhoa, and broken shells of Pecten asper, with 
occasionally other fossils. It is this bed which rests on an eroded 
surface in the more western sections at Eocken End, Brook, and 
Compton Bay, and it was in this bed that Mr. Ehodes found 
Stauronema Carteri at Compton Bay. 
Further, Mr. Hill ascertained that the bed which overlies it at 
Ventnor (No. 4) is the most variable bed in the whole group, being 
sometimes as much as 3 feet thick, and sometimes thinning out 
altogether. In this bed fragments of Stauronema have also been 
found by Mr. Hue, of Ventnor. Am. [Schloenbachia] varians also 
occurs but rarely, and, indeed, fossils are always rare in this dis¬ 
continuous intermediate bed. 
In consequence of these fresh discoveries we have modified the 
opinion expressed in the paper above mentioned, and, finding that 
the nodule-bed coincides with the downward range of Stauronema 
Carteri, we have adopted it as the basement bed of the Chalk in 
the Isle of Wight. At the same time, what was then said about 
the sudden incoming of an abundant Ammonite fauna at a higher 
horizon (Bed 5) is perfectly time, for it is only Am. [Schl.] varians 
that occurs in the lower beds, and only in Bed 5 that the other 
species of Ammonites and Turrilites make their appearance. Even 
should one or two be hereafter found, below, they can only occur 
as rare fossils, and the contrast between the beds will remain as 
strong as before. 
We continue to use the name Chloritic Marl because it is a 
convenient designation and has been so long in use, but it is a bad 
name for the bed, not only because the green grains are not 
chlorite, but because the matrix is not a marl; the lower beds are 
glauconitic sands, the highest bed is a hard glauconitic sandy 
chalk, with but a small quantity of argillaceous matter. This bed 
is sometimes overlain by a layer of grey marl containing glau¬ 
conite grains, and in other localities it passes up into grey glauconitic 
and sandy chalk, in which Plocoscyphia labrosa is abundant and 
Turrilites tuberculatus is not uncommon. 
The Plocoscyphia beds are succeeded by alternating beds of hard 
grey chalk and soft marly chalk, the marly beds becoming thinner 
and less conspicuous in the upper part of the series. These beds 
contain many fossils, and form the mass of the zone of Ammonites 
varians, that species being associated with Am. [ Acanth .] rotoma- 
yensis and Am. [Acanth.] Mantelli. 
The higher part of the Lower Chalk consists of nearly white 
chalk in massive beds, extending, with little change, up to the Belem- 
nite Marl, and having a thickness of 85 to 90 feet. This white 
chalk is lithologically comparable to the whitish chalk which forms 
the upper 50 feet of the Folkestone section, which it also resembles 
