486 
THE CRETACEOUS ROCKS OF BRITAIN. 
lower beds was 7. Guvieri, and that I. mytiloides occurred at a 
higher horizon in the chalk with flints. * The real facts seem to 
be that I. mytiloides occurs through a greater thickness of chalk 
than Professor Barrois at first supposed, ranging up into chalk with 
Hints, but that it nowhere occurs abundantly above the chalk 
with 7. Cuvieri , as Mr. Blake supposed; that the commonest 
forms in the Terebratulina zone are those which have been 
referred to 7. Cuvieri and I. Brongniarti ; but that 7. mytiloides and 
a species resembling I. undulatus also occur. 
Mr. Blake divided the lower paid of the flint-bearing chalk into 
(1) creamy chalk with nodular flints, and (2) slaty chalk with 
thin flints, grouping both into a zone of Inoceramus mytiloides. 
This we cannot agree with, for most of it undoubtedly belongs to 
the Terebratulina zone, and in all probability much of the slaty 
chalk belongs to that of Holaster planus. 
Mi*. Blake estimated the thickness of his zone of Inoc. mytiloides 
at 200 feet, and, if we subtract 50 feet for the “ slaty chalk,” we shall 
have 150 feet for the thickness of the Middle Chalk in the northern 
part of the area ; according to Mr. Lamplugh’s estimates, however, 
it is more likely to be 200 feet, for he considers the total thickness 
of the flint-bearing chalk in the northern cliffs to be 460 feet, and 
it is not likely that more than 250 feet of this thickness is assign 
able to the Upper Chalk (zones of 77. planus and Micrasters). 
Stratigraphical Details. 
There is an excellent section of the lower part of the Middle 
Chalk in the large quarries west of Hessle Station. The following 
notes of the section were made by mvself in 1880 : — 
ft. in. 
Chalk rubble .2 0 
Pure white chalk, rather hard,with a layer of large 
nodular grey flints ------ 6 0 
Continuous layer of greyish-white flint, stained 
yellowish in places by iron - - - - 0 4 
Hard compact semi-crvstalline limestone, tinged 
with yellow ------- 2 0 
Soft chalk, with horizontal strings of grey marl 0 9 
Massive white chalk with large flints near the base 6 0 
Thin seam of clay. 
White chalk with flints.2 0 
Thin seam of clay. 
White chalk with flints of irregular elongate 
shapes, Inoceramus Brongniarti - - - 6 0 
Layer of dark clay -.02 
White chalk without flints, in beds from 2 to 3 
feet thick, Inoceramus mytiloides - - - 26 O 
Rough yellowish chalk in thinner beds, with 
Inoceramus mytiloides , seen in excavation 
below the level of the rails, but base not ex¬ 
posed - 10 0 
61 3 
* Proc. GeoL Assoc., V*ol. v. p. 251. (1878), 
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