76 
LAMPLUGH : NOTES ON THE WHITE CHALK OF YORKSHIRE. 
It is .ilmost certain that botli in the Norfolk and the Isle of 
Wight secti()nj5, higher zones of the Chalk are represented than are 
known to occur in Yorkshire. In these areas the higher beds are 
characterised by the presence of Belemnitella mucronata, while in 
the highest known part of the Yorkshire sections Belemnitella quad- 
rata, which marks an inferior horizon, alone has as yet been recognized. 
The higher zone mayperhaps exist beneath some portion of drift-covered 
Holderness, and the greater thickness which the Upper Chalk appears 
to possess in this area may thus be accounted for. It is at any rate 
quite evident that in the Flamborough and Bridlington district a 
considerable but unknown quantity of rock has been stripped off by 
denudation, and that the original thickness of the Chalk formation 
has been much greater than it now is. 
That portion of the Chalk which is present in the Flamborough 
sections quite probably, however, attains its maximum thickness in 
this district. With the insufficient evidence in hand it is not indeed 
possible to institute a direct comparison of the thickness of any par- 
ticular portion in these sections with its thickness in the regions to 
the westward and southward, except the lowermost part of the series. 
But the general tendency of the whole of the Secondary rocks of 
Yorkshire is to reach their greatest development in the eastern part of 
the area, and to become thinner towards the west and south, and 
this is probably shared by the Upper Cretaceous strata. In the 
Red Chalk this tendency is strikingly evident, and, as Blake and 
Hill have demonstrated, it is also very decided in the Low^er Chalk. It 
is however scarcely likely, from the uniform character of the deposit, 
that the westerly decrease is maintained at the same high proportion 
in the Middle and Upper Chalk as in the lower divisions. Yet it is 
probably sufficiently great to reduce considerably the aggregate 
thickness of the Upper Cretaceous rocks in that quarter. It will be 
no easy matter to gather the data necessary for the elucidation of 
this point, but I think that much might be done by a careful 
study of the fine sections opened out within the last few years in 
the construction of the railway across the Wolds from Driffield to 
Market Weighton, and by the comparison of these exposures with 
the above described cliff-sections. 
