254 
THE CRETACEOUS ROCKS OF BRITAIN. 
ft. in. 
Layer of brown chert, continuous - - - - - 0 1 
Yellowisli-grey chalk, slightly glauconitic, with many 
layers and concretions of brown or black chert, seen for a 
depth of about - - - - - - - - -40 0 
About 60 0 *7 
The total thickness at Rouen is believed to be more than 1G0 feet. 
The layer of green-coated nodules near the top may indicate the 
erosion and removal of a certain thickness of the chalk which is 
shown in the cliff at St. Jouin. 
Orne and Sarthe .—Passing now to the departments of the Cal¬ 
vados and Orne, there are sections near Blangy, Lisieux, and Orbi- 
quet which expose the lower part of Cenomanian series, cor¬ 
responding with Beds 1 to 12 of the section at St. Jouin. The 
beds seen at these localities consist of yellowish-grey or buff-coloured 
chalk, more or less glauconitic, with grey cherts and often hard 
lumps of silicified rock. Mr. Hill found by examination of samples 
under the microscope that the material is really a siliceous chalk, 
sometimes containing 50 per cent, of colloid silica with a certain 
amount of fine quartz-sand. 
There are good sections of the whole formation at Yimou tiers in 
the Orne, and in this district it has been divided by local geologists 
into two zones, that of Am.[Acanth.] rotomagenis for the upper part, 
and that of Am. Mantelli below. From some fresh observations 
recently communicated to me by M. Lecoeur, of Yimoutiers, and 
combining the sections seen in several quarries, the complete suc¬ 
cession seems to be as follows, the thickness being greater than he 
formerly supposed : — 
Feet. 
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'Alternating beds of soft grey glauconitic marl and 
hard yellowish crystalline siliceous limestone, with a 
bed of the latter at the base. Fossils are numerous, 
and many are in light brown phosphate - 
Fine glauconitic sandy chalk, with courses of rough 
siliceous limestone and occasional lumps of chert - 
Bed of grey glauconitic sand with many fossils, 
including teeth and vertebrae of fish - 
Greyish marly glauconitic sand, with some rough and 
harder beds, many cherts occur, both scattered 
and in layers - - - - , - 
Buff-coloured glauconitic and sandy chalk, consisting 
largely of hard irregular siliceous lumps with soft 
silty chalk between ; also two layers of large cherts 
and some scattered cherts ----- 
A course of large detached masses of sandy glauco- 
\ nitic limestone. 
Greenish sandy marl, passing down into dark green 
glauconitic sand -. 
30 
60 
1J 
28 
22 
10 
About If 2 
