LOWER CHALK—CONDITIONS OF DEPOSITION. 
349 
of Cretaceous deposits on the borders of St. George’s Channel, and 
the history of the Irish Sea has yet to be written. It is very prob¬ 
able, however, that in the Jurassic and Lower Cretaceous periods 
there were deep and broad valleys between Wales and Ireland, 
draining northwards into a northern gulf, and it is possible that 
during the subsidence of the Lower Chalk age a communication 
may have been established between the northern gulf and the 
inlet to the south of Wales, in which case Wales would have 
been converted into an island. 
When discussing the probable geography of the Selbornian age 
in a previous volume we saw reason to think that the Selbornian 
deposits once extended over the Cheshire plains and across the 
basin of the Irish Sea to the north-eastern corner of Ireland. If 
such a gulf existed at that time it must have been enlarged and 
extended during the subsequent Cenomanian subsidence. The 
glauconitic sands, which occur at the base of the Cretaceous series 
in Mull and Morvern and have been described by Professor Judd,* 
seem to indicate that this gulf extended northward into that 
district, so that Scotland was at any rate partially divided from 
Ireland, though there may still have been a land connection to 
the west of it. 
In the region of the Pennine promontory it is not likely that there 
was much geographical change, for we should not suppose that a 
subsidence of 200 or even 300 feet would have narrowed it to any 
great extent; nor that such a subsidence would have carried the 
sea of the Lower Chalk very far northward in Yorkshire, but the 
absence of any Cretaceous outlier on the Yorkshire Moors leaves 
us absolutely without smy evidence in that direction. 
Thus so far as we can form an opinion the geography of the sea 
in which the Lower Chalk was accumulated did not differ very 
greatly from that of the preceding Selbornian epoch, except in 
being wider and deeper. All the surrounding tracts of land were 
reduced both in size and in elevation, and as a consequence less 
terrigenous material was carried into the adjoining seas. 
Sedimentation. 
Let us now consider the disposition of the sediments which 
were deposited in this Cenomanian sea after the disturbing 
and erosive influences had ceased and only the finest kind 
of terrigenous sediment was carried by the currents to be mingled 
on the sea floor with the calcareous ooze which is derived from the 
decay and disintegration of calcareous shells. 
The chalk of the Am. varians Zone.—Ii in the interim above men¬ 
tioned any change took place in the direction of the marine cur¬ 
rents, it would appear that they subsequently resumed their former 
courses, for it is interesting to find that the more argillaceous facies 
of the Lower Chalk coincides very nearly with the area over which 
the Selbornian is most argillaceous. In other words, the Chalk Marl 
* Quart. Journ. Geol. Soc., Vol. xxxiv. p. 729, 1878. 
