548 
THE CRETACEOUS ROCKS OF BRITAIN. 
during the formation of our Middle Chalk was not very different 
from that which existed during the time of the Lower Chalk. 
This may be accounted for in two ways : either the movement 
of subsidence was much less in the Atlantic region than in the 
European, or, if the subsidence was of nearly equal amount in 
both regions, the western land was so much higher and steeper 
that only small parts of it were actually submerged by a depression 
which carried the sea over large parts of the European land. 
Evidence of Current-Action. 
The Middle Chalk nearly everywhere presents clear indications 
of the action of strong currents, and not only of surface currents, 
but also at one time of currents that swept over the floor of the 
sea. Some alteration in the strength and direction of the 
marine currents may very probably have been produced by the 
opening or deepening of the straits of Poitou, between Brittany 
and the Plateau of Central France, which thus probably became 
an island. 
The evidence for the bottom current above mentioned is found 
in the nodular character of the Melbourn Bock, and in the nodular 
structure of nearly the whole of the Middle Chalk in South and 
West Dorset. In Lincolnshire also the whole zone of Rhynchonella 
Cuvieri is of a nature that suggests the action of a strong current, 
for it is condensed into a small thickness, consists of rough shelly 
and nodular chalk, and terminates in a layer of shalv marl, which 
marks it off sharply from the pure white, fine-grained chalk of 
the overlying zone. 
All who have discussed the nodular structure of these beds 
agree in regarding it as due to slow accumulation under the in¬ 
fluence of a strong current. Thus, describing the Melbourn Bock 
in 1886, Mr. Hill and I wrote as follows It may also be noticed 
that the atoms of shell are sometimes arranged round the nodule 
in a manner which suggests either the gradual sinking of the 
nodule into the soft mud, or that the atoms of shell were deflected 
from their course by the nodule, while travelling over the sea-bed 
under the action of a current/’* 
Dr. W. F. Hume, writing in 1894, explained the nodular con¬ 
dition of the Rhynchonella Cuvieri zone as “ directly due to the 
currents set up by the sudden alteration in physical conditions,” 
and inferred that “ most chalky nodular beds are thus formed, 
whether under the influence of elevation or depression.'”! 
* Quart. Journ. Geol. Soc., Vol. xlii., p. 230, 
t Proc. Geol. Assoc., Vol. xiii., p. 230. 
