.260 THE CRETACEOUS ROCKS OF BRITAIN. 
“ Our lists, however, show that of the fossils which may be col¬ 
lected from these beds near Ha we in a few days’ time; quite as 
many occur in our Chalk Marl as in the Warminster Beds.” 
We also pointed out in 1896 that it was unsafe to depend on 
Ecliinoderms or ground-feeding Molluscs in correlating two different 
deposits at some distance apart, because the conditions of life may 
have been different in the two areas. We believe also that the 
reason why Pecten asper and some other members of the 
Warminster fauna do not occur in the Chalk Marl is simply 
because the water became too deep for them, and not because 
they had everywhere ceased to exist. 
We have shown that the Cenomanian of Havre was probably 
formed in greater proximity to land than the Lower Chalk of our 
southern coasts, and also that in passing from Havre toward the 
south-west we reach shallow-water sands and sandstones. It is 
not surprising, therefore, that there should be a certain correspond¬ 
ing change in the fauna. Is it likely that the same assemblage; 
of creatures would inhabit, the littoral sandy floors, as well as the 
intermediate depths where glauconitic marls and sands were being 
formed, and the comparatively deep water of the chalk-marl area ? 
“ Further, as the period was one of continued subsidence, and 
as the formation of the Chalk Marl was preceded by that of glau¬ 
conitic sand in a shallower sea, what is more likely than That the 
creatures which lived in that shallower sea should gradually migrate 
to other tracts where the same conditions prevailed, as the area 
of the sea grew larger and the central portion of it grew deeper ? 
“ The case is quite different with the Cephalopoda, for they could 
move freely from place to place, and were not directly dependent 
on depth of water or on the nature of the sea floor. They must 
therefore be much more trustworthy chronological guides [than 
echinoderms and bivalve molluscs]; andfff we rely upon their guid¬ 
ance in the present case we find that they lead us to the very conclu¬ 
sion which we regard as correct, for the Cephalopoda of the lower 
part of the Cenomanian of Havre are identical with those of our 
Chloritic Marl and Chalk Marl, and they do not occur in the Chert 
Beds of our Upper Greensand. 
“ To sum up, therefore, our belief is that the Cenomanian deposits 
of the Seine Inferieur, Calvados, and Orne, above that horizon 
which we have taken as its base, were originally continuous with 
our Lower Chalk, but were formed jin $ shallower part of the sea, 
nearer to a coast line and in a region more favourableto the growth of 
siliceous sponges. Hence they consist of a more sandy material, 
with a larger amount of siliceous matter in the form of sponge- 
spicules, this latter leading to the frequent development of chert 
nodules.” 
