348 
THE CRETACEOUS ROCKS OF BRITAIN. 
and Stauronema Carteri, and so far as at present known these do not 
extend westward into Dorset nor farther north than Berkshire, but 
spread eastward through Hants and Sussex to Folkestone and 
southward to the mouth of the Seine, and probably beneath the 
Paris basin, though that has not yet been proved. 
The surfaces of erosion which have been indicated above are 
not proofs of emergence above the sea-level of the period as many 
French geologists have imagined, but are only signs of some geo¬ 
graphical change which for a certain time greatly increased the 
strength of the marine currents and prevented the deposition 
of much sediment, even where it did not cause actual erosion of 
the deposits already accumulated. That this change was due 
to a partial or general subsidence may be regarded as fairly certain, 
because the succeeding deposit is known to have extended beyond 
the limits of the underlying greensands and because this deposit 
is a chalky one containing much less terrigenous material than those 
of Selbornian age. 
Limits of the Lower Chalk Sea. 
So far as we can judge from the evidence at present avail¬ 
able, it would appear that this subsidence was one affect 
ing the whole area of deposition, so that the shore lines 
were everywhere carried back both on the eastern and 
on the western - side of the Anglo-Gallic sea. On the western 
side shallow-water deposits of this age are preserved in France, 
Devonshire, the North of Ireland, and the West of Scotland. An 
outlier in the Cotentin, where five feet of Cenomanian sand rest 
directly on Palaeozoic rocks, shows that they overlapped the beds 
which correspond to our Upper Greensand. In Ireland, also, 
the Glauconitic Sands which appear to represent the highest paid 
of the Selbornian stage are very thin, and there seems reason to 
believe that they are overlapped by the “ Yellow Sandstones ” 
which apparently correspond to the Lower Chalk of England.* 
Evidence of similar overlap is found again in the north-east of 
France, where the beds described by Professor Barrois extend 
north-eastward beyond the Gaize, and are themselves overlapped 
by the zone of Actinocamax plenus (see ante, p. 244). 
Not only was the sea widened and deepened, but all the inlets 
of the western land must have been enlarged and lengthened, and 
it is probable that portions of this land were isolated from the rest. 
Thus the central part of France seems to have become an island 
in consequence of the Cenomanian sea passing between it and 
Brittany.f How far the inlet between Dartmoor and Wales was 
carried westward we have no means of knowing, nor can we say 
what has happened to the west of Wales, for there are no traces 
* See “ The Cretaceous Strata of County Antrim,” by W. F. Hume, 
D.Sc., Quart. Journ. Geol. Soc., Yol. liii. p. 540. For my own view of the 
age of the beds mentioned, see Students’ Handbook of Stratigraphical 
Geology, p. 447, 1902. 
| See “ Etude paHogeogr sur le Detroit de Poitiers,” par A. de Grossouvre, 
Comptes Rend, de 1 Assoc. Franc, pour Av. Sci. 1901, p. 401. 
