536 
THE CRETACEOUS ROCKS OF BRITAIN. 
part of the Upper Chalk, and further, that in the Trimmingham 
Chalk, they are much more abundant, its apparent grittiness being 
due to the quantity of such fragments and of Foraminifera. 
If we compare the shell-fragments of the chalk with those 
of recent calcareous oozes we find one conspicuous difference, 
and this is the absence of prismatic shell-debris. Since, however, 
the genus Inoceramus is extinct, and its nearest living representa¬ 
tive, Perna, has a restricted range both in space and in depth, the 
absence of such fragments in modern Pteropod and Globigerina 
oozes is easily accounted for. Chalky oozes that are being accumu¬ 
lated at no great distance from land, and in less than 800 fathoms, 
often contain much shell-debris derived from the shells of Gastero¬ 
poda, Lamellibranchiata, Pteropoda, and Heteropoda, fragments 
of Echinoderm tests and spines, Bryozoa, Ostracods, and calcareous 
Algae ; but in the deposits which are found in deeper water such 
fragments are much rarer. 
Viewed as a whole, therefore, modern oc^nic deposits exhibit 
as great differences in the quality of shell-debris which they con¬ 
tain as do the different portions of the great Cretaceous deposits 
of chalk ; but the differences observable in the Cretaceous deposits 
may not have been due so directly to the bathymetric conditions, 
because it is quite possible that many of the Cretaceous shells, 
such as species of Inoceramus , Ostrea, and Spondylus , lived in 
deeper water than any recent species of these genera seem to 
frequent. 
2. Is White Chalk a Foraminiferal Deposit ? 
A cursory consideration of the differences above mentioned 
might lead to the conclusion that very little of the chalk is really 
a foraminiferal deposit, and that few parts of it can be compared 
at all with Globigerina ooze. There are reasons, however, for 
thinking that we shall be greatly mistaken if we suppose that 
the surface layer of a calcareous oceanic ooze furnishes us with 
anything like a sample of the deposit as a mass ; and yet the pub¬ 
lished descriptions of these oozes relate almost entirely to this 
surface laver. 
mJ 
In the greater depths, where little calcareous material seems 
able to accumulate and where red clay is the prevalent deposit, 
it is quite possible that the lower layers do not differ much from 
that at the surface. But in the lesser depths, between 2,000 
and 500 fathoms, where the rate of deposition is much more 
rapid and where there is a constant fall, or “ rain/’ of pelagic 
Foraminifera and Mollusca, it is surely most probable that the 
lower layers differ considerably from that which lies on the surface. 
The very influence, supposed to be the decomposing and solvent 
action of sea water, which causes the disappearance of Pteropod 
shells at about 1,400 fathoms, and that of Foraminifera at about 
2,500 fathoms, must have some effect on the constituents of the 
