498 
THE CRETACEOUS ROCKS OF BRITAIN. 
oysters. The glauconitic chalk passes up into the white marly 
chalk of the zone of Inoceramus labiatus, which contains a few 
scattered flints and many concretions of iron pyrites, but is no¬ 
where more than 30 feet thick in this district. Analyses given by 
M. Guillier show that it varies considerably in lithological composi¬ 
tion, some samples containing 85 per cent, of calcium carbonate, 
others only from 66 to 74, with 24 to 28 per cent, of insoluble 
siliceous matter. 
The I. labiatus marls are succeeded by beds which were described 
by Guillier as the chalk with Terebratella Bourgeoisi, but this 
has recently been separated into two zones by M. de Grossouvre, * 
a lower zone of yellowish sandy chalk or tuffeau, which contains 
many species of Ammonites , especially per ampins, Woolgari, 
deverioides, and Galliennei. This is generally 6 or 7 feet thick, 
and is well exposed at Ponce, Mezieres-sur-Ballon, and Bourre, 
M. de Grossouvre regards it as equivalent to the zone of Ter. 
gracilis. The succeeding beds are still more sandy and micaceous 
and some of them contain concretions of granular sandy chert 
enclosing sponge remains; Bryozoa also are numerous, and among 
other fossils may be mentioned Terebratella Bourgeoisi, Micraster 
Mickelini, Ostrea turonensis, and Gyprina Noueliana. These beds 
vary much in thickness, but are sometimes as much as 30 or 40 
feet, and are regarded by M. de Grossouvre as the equivalent of 
the eastern zone of Micraster breviporus , and the craie de Vervins. 
It would appear therefore that in the Departments of the 
Orne and the Sarthe, the Turonian is much thinner than it is 
to the eastward, and that it consists of the following beds :— 
Yellowish sandy chalk, with Am. peramplus - - 6 
White marly chalk, with Inoc. mytiloides - - 30 
Glauconitic chalk, with Ter. carantonensis - 6 
Sand and sandstone (in Sarthe only"! - - 3 to 30 
45 to 70 
* Bull. Soc. Geol. de France, Ser. 3, Tom. xvii. p. 505, et seq. 
