LOWER CHALK—NORTH WEST OE FRANCE. 
259 
Very use of the word f craie ’ indicates the difference, and is quite 
correct in the department of the Seine Inferieure, where the matrix 
does consist of fine chalky matter. ... If we examine the 
Chalk Marl of the Isle of Wight under the microscope we find that 
it actually is a craie glauconieuse ; it consists essentially of minute 
grains of quartz and glauconite and of shell fragments embedded in a 
fine chalky matrix. The material of the corresponding part of 
the Cenomanian at Havre has less of the chalky matrix and more 
of the inorganic constituents, quartz and glauconite ”• [but is other¬ 
wise similar]. 
“ (2) The occurrence of layers and lumps of true chert does un¬ 
doubtedly create a superficial resemblance between the two sets 
of strata ; but the researches of Dr. Hinde have so completely proved 
the connection between the formation of cherts and the existence 
of siliceous sponges, that the occurrence of cherts can only be taken 
as evidence of the conditions being locally favourable to the growth 
of such sponges.” 
It should have been added that siliceous concretions of a char¬ 
acter intermediate between cherts and flints occur in the Lower 
Chalk of England (Wiltshire and Dorset), and that whether a sili¬ 
ceous concretion develops into a chert or a flint seems to depend 
very largely on the texture of the deposit in which it is formed ; 
those occurring in deposits which are sufficiently coarse or sandy 
in texture tending to become nodules of clear chalcedonic chert, 
while those in a material of such fine texture as ordinary chalk 
are always flints. 
“ (3) The presence of Pecten asper and other fossils, which in 
England are chiefly found in the Upper Greensand, is the sole argu¬ 
ment which requires serious consideration. Let us first of all see 
what it amounts to when admitted without any qualification. 
Where does this special fauna exist in England ? Certainly not 
in the Chert Beds either of Wiltshire, Dorset, or the Isle of Wight, 
which have everywhere a very limited fauna. The fossils of the 
Warminster Greensand [which is the fauna in question] have mostly 
been obtained from a bed of bright greensand lying above the Chert 
Beds and passing up into Chloritic Marl [see p. 156]. 
“ It is quite a mistake to suppose that this is a typical Upper 
Greensand fauna : it is only that of the very highest bed, * so that if 
two-thirds of the French Cenomanian are to be correlated with 
the thin bed of greensand near Warminster on the strength of 
the similarity between the fauna, we must imagine that this thin 
bed [varying from 5 to 10 feet] has expanded to a thickness of 80 
feet or more in France, although the beds both above and below 
have very greatly diminished in thickness. 
* Since the above was written we have come to the conclusion that 
the fossiliferous part of the Rye Hill Sand (a thickness of only 2 feet) 
should be included in the Cenomanian zone of Am. varians. 
4219. S. 
