Vol. 52.] A DELIMITATION OF THE CENOMANIAN. 139 



The rock, as a whole, when viewed in thin section under the 

 microscope, resembles more nearly the Cambridge Green sand than 

 any of our specimens of Chloritic Marl. 



Bed 6. 



Seen from a short distance, the yellowish- white chalk into which 

 the Chloritic Marl quickly passes, and which forms the major part 

 of the Cenomanian in the cliffs between St. Jouin and Cape La Heve, 

 looks like the rock we are accustomed to see on this side of the 

 Channel ; but a striking difference is seen in the lines of massive 

 cherts which occur at irregular intervals from near the base to the 

 summit. 



A more critical examination shows that the lower part of the 

 Cenomanian, all that included in Bed 6, possesses peculiarities not 

 found in the Chalk of England. These are comparative lightness in 

 the hand, a certain amount of gritty mealiness to the touch ; and 

 it is everywhere speckled by glauconitic grains, and a few minute 

 mica-flakes sparkle on its fracture. 



These peculiarities extend through Bed 6, and then there is a 

 gradual passage to that white pulverulent limestone with which we 

 are familiar. 



When a thin section of Bed 6 is examined under the microscope, 

 the matrix is found to be made up partly of fine, amorphous, cal- 

 careous matter, mixed probably with a small proportion of fine 

 inorganic material, and partly of calcite in the condition of definite 

 though minute crystals. The rock is full of shelly fragments, the 

 derivation of the larger part of which is uncertain, but prisms of 

 Jnoceramus-shell are clearly less numerous than in shelly English 

 chalk ; such fragments as show structure are more frequently those 

 of Pecten or portions of the spines or tests of echinodermata. 



Eoraminifera are present, but are not very numerous, the forms 

 most in evideuee being Textularia, Oristellaria, and. Rotalia ; Globi- 

 gerina is rare, and spheres do not occur at all. 



Intermingling with these calcareous elements are many sponge- 

 spicules whose silica is invariably in the colloid state. These are 

 sometimes closely packed, sometimes widely separate, but they are 

 always present in greater number than in any English Chalk, 

 except, perhaps, selected specimens of siliceous Chalk Marl. 



Permeating the whole mass in single globules or aggregations of 

 globules, is a large amount of colloid silica. 



Glauconitic grains of large size, compared with those of our Lower 

 €halk, are seen in all specimens, sometimes in great abundance, and 

 glauconite may also be seen infilling the spicular canals and the 

 chambers of foraminifera. 



Quartz-grains are common, but the quantity varies much in 

 different specimens ; they are everywhere more numerous and larger 

 than in the English Chalk Marl, and some are angular. 



This general description will apply to the whole of Bed 6 between 

 St. Jouin and Cape La Heve. Sometimes the fine material prepon- 

 derates, there are fewer quartz- and glauconite -grains, and the 



