5.38 
THE CRETACEOUS ROCKS OF BRITAIN. 
chalk. We suggested that the reason why “ deposits of a chalky 
character were not common among the Challenger dredgings ” 
was that these dredgings came almost entirely from the surface- 
layer ; and that the deeper layers might have “ as close a similarity 
to chalk as the surface ooze has in chemical composition.” 
When, therefore, we consider that the superficial layer of Globi- 
gerina ooze is almost certainly the mere raw material out of which 
the mass of the underlying deposit is formed, and that in all 
probability this final product is not a granular ooze but a fine 
compact chalky mud ; when also we bear in mind the Tertiary 
chalk of Barbados, and recollect the further alteration and con¬ 
solidation which Cretaceous chalk must have undergone, we 
may say safely that very little force remains in the objection 
against the comparison of chalk and foraminiferal ooze on the 
score of the rarity of perfect pelagic Eoraminifera in the former. 
When further exploration is made of the deeper layers of the 
calcareous oceanic deposits, it is probable that these will be found 
to be quite comparable with chalk in this respect; and for present 
purposes, if it can be shown that in all other respects there is a 
close resemblance between any part of the English Chalk and 
recent Globigerina ooze, I see no reason why the conditions under 
which the latter is now being formed should not be taken as a 
general index of those under which the former was accumulated. 
At the same time it must be distinctlv understood that we 
i «/ 
are far from making the assertion that Cretaceous chalk was 
originally a Globigerina ooze. On this point Mr. Hill sends me 
the following remarks, “ I agree that the chalk is largely a forami¬ 
niferal deposit, and that in this sense it may be compared with a 
foraminiferal ooze, such as that which is known as ‘ Globi¬ 
gerina ooze/ but I see no evidence that our chalk has ever been 
a Globigerina ooze in the sense that tests of Globigerina were its 
principal organic constituent.” He continues,— 
“ If Globigerina was originally the dominant form, how is it 
that so few tests have escaped destruction as compared with 
those of other genera. In most parts of the Upper Chalk Globi¬ 
gerina is absolutely rare, while in the same washings other genera 
occur by the hundred, and though in the Middle Chalk Globi¬ 
gerina is sometimes fairly common, yet other forms are collectively 
more numerous. This seems inexplicable on the supposition 
that Globigerina were originally the most numerous, for such 
large and strong-tested forms surely had an equal chance of es¬ 
caping disintegration, and even a better chance than some of 
the delicate forms which do occur. 
“ The Barbadian chalk illustrates the force of the argument, 
for though it is true that Globigerince are not of more frequent 
occurrence in it than in some parts of the English chalk, they are 
the only Eoraminifera that are common in the rock. Moreover, 
their thick shells and their general resemblance to those of the 
