240 
THE CRETACEOUS ROCKS OF BRITAIN. 
ft. in. 
3. Whitish marly glauconitic chalk, resting irregularly on 
the bed below ------ seen f or 3 6 
2 . Glauconitic sand, with many phosphatic nodules and 
some pebbles at the base, which rests on an eroded sur¬ 
face of No. 1 ----- - about 1 G 
1 . Yellowish-grey clay, with some glauconitic layers, con¬ 
taining many pebbles of older rocks, and piped into the 
underlying Devonian schists - - - - 1 to 1 6 
The highest bed here is identified as the zone of Belemnites 
plenus, and this species, together with Nautilus subradiatus, occurs 
in the glauconitic sand below, which he appears to regard as a thin 
littoral representative of the zone of Holaster subglobosus. As 
Terebratula tornacensis is particularly abundant in this sand, 
M. Parent calls it “ les sables a T. tornacensis." 
Passing still farther east to the country between Valenciennes 
and Tournay and Mons, on the borders of Belgium, we enter the 
typical region of the Tourtia rendered classical by the memoirs of 
d’Archiac, Dumont, and Messrs. Briart and Cornet. Tourtia is a 
name given by the miners to all the loose sands and sandy marls 
which rest on the surface of the Palaeozoic rocks. These beds differ 
much in character and fossil contents at different places, and many 
attempts have been made to distinguish different horizons and to 
correlate the “ Tourtias " of different localities. 
It can hardly be said that the classification of the Tourtias is 
yet settled, but Professor Barrois has given reasons in his “ Terrain 
Cretace des Ardennes” (1878) for regarding the Tourtia de Mons 
as the equivalent of his zone of Belemnites plenus—i.e., as repre¬ 
senting the upper part of the Lower Chalk. I 11 the northern paid 
of the area this marl with Bel. plenus rests directly on the Palaeozoic 
rocks, but further south beds come in between them (the Tourtia 
de Montignies) which he refers to the zone of Pecten asper ; and in 
1878 he was of opinion that the zone of Holaster subglobosus was 
unrepresented in this Department (Op. cit., p. 368). He admitted, 
however, on p. 348 that there is a general analogy between the 
fauna of. the Tourtia de Montigny and that of the Gres du Maine, 
and he has. informed me that he now agrees with M. Parent, who in 
1893 suggested that this Tourtia, like the Greensand of Aix-en- 
Gohelle, was a littoral representative of the zone of Hoi. subglobosus. * 
Recurring, however, to the views expressed by Professor Barrois 
in 1878, he was then of opinion that the zone of Hoi. subglobosus was 
absent not only in the Nord, but also throughout the Departments 
of Aisne and Ardennes, yet he identifies everywhere a “ niveau a 
Ammonites laticlavius ” as forming the upper part of his zone of 
Pecten asper. Consequently he assumed that the highest and the 
lowest zones of what he calls the “ Cenomanien Superieur ” are 
* Still more recently M. J. Cornet lias described (Ann. Soc. Geol. de Belg. 
T. xxviii. p. 53) the beds traversed by some new shafts near Mons, which 
have disclosed beds of glauconitic sand between the Tourtia de Mons and 
the Meule de Bracquegnies; these contain Am. [*SVA7] variant. Am. 
\Acanth .] rotomagensis , Inoceramus striatus , Pecten asper , and other fossils. 
They appear to represent the zone of Am. variant. 
