Vol. 52.] A DELIMITATION OF THE CENOMANIAN. 141 



Calvados and Orne. 



South of the Seine we confined our examination of the litho- 

 logical characters of the Cenomanian to Beds 5 and 6. In this 

 direction we saw nothing corresponding to the ' Gaize,' and where 

 the top of Bed 4 was exposed the material was a soft and exceed- 

 ingly glauconitic sand. A specimen from Bed 5, near Honfleur, 

 compares with the base of Bed 6 at La Heve rather than the Chloritic 

 Marl (Bed 5), except that it contains so many large glauconitic 

 grains, quartz-grains being in rather small quantity. Neither do 

 sponge-spicules or colloid silica occur commonly in this specimen. 

 The cementing-material of the hard lumps found in this bed is 

 crystalline calcite. 



Southward of Honfleur we find a steady increase in the amount 

 of quartz-sand and organic silica in Beds 5 and 6. 



Thus, at Blangy, the matrix of a specimen from 10 or 12 feet 

 from the base of the Cenomanian consists — judging by the eye — 

 of at least 50 °/ of globular colloid silica, and the amount of sand- 

 grains (quartz) mingled with this is greater than in any specimen from 

 La Heve. Mica- flakes can also be recognized, and among the sand 

 are grains other than those of quartz. Glauconite is present as usual, 

 there are a few shell-fragments and much amorphous calcareous 

 matter ; sponge-spicules are common. 



Specimens from Orbiquet taken at a higher horizon, one about 

 20 feet and another from about 45 feet above the base of Bed 6, show 

 a similar increase in the amount of colloid silica and terrigenous 

 material, the relative proportion of this decreasing upwards. 



"We regret that we omitted to secure a series of specimens from 

 Bed 6 at Yimoutiers. Our knowledge of the minute structure of 

 the Chalk here is obtained from that adhering to our fossil speci- 

 mens, their height from the base of the bed being uncertain. Such 

 specimens show that the conditions noted at Orbiquet continue to 

 this point without much alteration. 



One specimen showing a structure somewhat similar to that of 

 Bed 11 has already been alluded to. 



Still farther south we find that the base of Bed 6 has completely 

 changed from a calcareous to a siliceous rock, comparable with the 

 Malm of our Upper Greensand. 



At Gace, in the hard bed at the base of the Cenomanian, the 

 equivalent of Bed 5 at Cape La Heve, the cementing-material is not 

 calcite but silica, derived probably from the abundant spicules in 

 the deposit, and much of this silica has passed from the colloid to 

 the chalcedonic condition. The so-called ' Craie ' which overlies 

 the basement-bed is so completely siliceous that the reaction in 

 acid is of the slightest, while at still higher horizons the deposit is 

 not a chalk at all, but a sandy, micaceous, glauconitic, calcareous 

 silt, with many sponge-spicules, amongst which the reniform 

 spicules of Geodia x are common. 



1 Identified by Dr. G. J. Hinde. 



