34 
THE CRETACEOUS ROCKS OE BftITAtN. 
thickness allowed for the Grey Chalk ” by Phillips, and thinks that 
“ more than the true ‘ Chalk Marl ’ may have been included under 
that name.” 
In 1875 Messrs. Potier and de Lapparent examined the chalk 
cliffs on each side of the Straits of Dover for the Channel Tunnel 
Company. In their report* they adopt Phillips’s sub-divisions 
and estimates of thicknesses, but his “ Grey Chalk ” they divide into 
two “ assises ; ” the upper assise they number YI. and describe as 
“ argillaceous chalk of a dark bluish-grey colour, rather pyritous, 
compact; the upper part is rather hard but cuts smooth under the 
hammer (se polit sous le marteau) ; some yellowish beds.” The 
lower assise (VII.) is described as “chalk of a rather lighter bluish- 
grey, argillaceous throughout,with some harder sandy beds.” 
They give 45 metres as the thickness of No. VI., and assign 
15 metres to No. VII., but it is evident that these figures are 
merely estimated proportions of Phillips’s 200 feet, and that they 
did not measure the beds themselves. 
Professor Cli. Barrois briefly described the Folkestone section 
in 187G,| but with the exception of separating what he then con¬ 
sidered to be “ Chloritic Marl,” he does not make any division of the 
Lower Chalk, classing all as the “ assise a Holaster subglobosus.” 
He refers, however, to the divisions of Messrs. Potier and de Lap- 
parent and compares these with the palaeontological zones which 
he had liimeslf established in the Boulonnais, correlating them as 
follows : — 
V. of Potier and de Lapparent—zone of Belemnites plenus. 
VI. =zone of Am. rotomagensis. 
VII. „ „ = zone of Am. varians. 
In 1877 Mr. F. G. H. Price made a still more detailed study of 
the section and employed Griffiths, the local collector of fossils, 
to collect systematically from every portion of the Lower Chalk. 
Mr. Price remeasured the section, and found the total thickness 
from the base of the so-called “ Greensand ” to the top of the “ Belem- 
nite Marl ” to be 197 feet. He divided the whole into seven “ beds,” 
which are essentially palaeontological subdivisions based on the 
results of his fossil-collecting, but are not subzones of equal palaeon¬ 
tological value. 
In this paper Mr. Price proposes to make a distinction between 
Chalk Marl and Grey Chalk, but he only takes the lowest 10 feet 
of the Grey Chalk, and, grouping them with the underlying marly 
Greensand, calls the ensemble Chalk Marl. On the other hand, he 
takes the Grey Chalk up to the top of the Belemnite band, and 
thus includes much more than either Phillips or Whitaker had 
regarded as Grey Chalk. 
* Rapports sur les Explorations Geologiques (Chemiii de Fer Sous- 
Marin),Paris, 1875. Republished in 1877. 
f Recherches sur le Terrain Crdtace Superieur, p. 130. 
