358 
THE CRETACEOUS ROCKS OF BRITAIN. 
the foraminiferal fauna of the Chalk Marl and the assemblage 
found near Culebra Island, in the West Indies, at 390 fathoms ; 
thus in the Chalk Marl there are three common species of Textu- 
laria (T . turris, T. trochus, and T. agglutinans), and the only locality 
where these three have been found existing together is Culebra. 
T. turris, indeed, has only been found at two other places—viz., in 
the Bahama Channel at 438 fathoms, and off Pernambuco at 350 
fathoms. Other species common to the Chalk Marl and the Culebra 
station are Gaudryina pupoides, Bulimina Presli, Haplophragmium 
latidorsatum, and Ammodiscus incertus. 
Again, with reference to the Miliolines we find another significant 
coincidence, Dr. W. B. Carpenter having long ago pointed out that 
the Miliolines of the Chalk Marl are small and delicate, like those 
which now occur in depths between 300 and 500 fathoms. 
The above mentioned considerations certainly make it probable 
that the Chalk Marl of the south-eastern and south-central counties 
was formed at a depth approaching but possibly rather less than 
400 fathoms. This conclusion is greatly strengthened by the 
contrast exhibited in the foraminiferal fauna of the upper part of 
the Lower Chalk. On this point Dr. Hume makes the following 
remarks.: "In the more central portions of the mass of the Lower 
Chalk it is of the greatest interest to note that all the Textularians 
disappear, except the deep-sea form ( T . agglutinans), and a very 
minute species which only occurs in the higher beds and 
in the Middle Chalk. . . . The upper beds of the Lower 
Chalk, both in the Isle of Wight and near Lewes, are chiefly charac¬ 
terised by the presence of a particular Bulimine ( B . variabilis), 
together with some of the above typical deep-sea forms.”* * 
Finally, he comes to the conclusion that “ a depth of over 400 
fathoms was attained before the Lower Chalk period closed ; ” in 
other words, it would seem that in passing upward from the Chalk 
Marl to the beds of nearly white chalk which underlie the Belem- 
nite marls, we are tracing the effects of a subsidence which carried 
the area through the bathymetrical limit of 400 fathoms, and that 
the zone of Holaster subglobosus was formed in water which finally 
approximated to a depth of 500 fathoms. 
We think that many parts of the Lower Chalk must, when 
deposited, have been similar to the calcareous oozes which are now 
found off the coasts of Yucatan and Florida in depths of from 
350 to 860 fathoms. These deposits contain from 68 to 83 per 
cent, of calcium carbonate, with from 15 to 7 per cent, of soluble 
siliceous organisms, and the mineral particles in them average 
about ‘07 mm. in size, rarely exceeding T mm. 
____ 
* The Genesis of the Chalk, Proc. Geof. Soc., Yol. xiii., p. 211. 
