M. Sebert — Comparison of Eocene Strata. 225 



during the return of the sea after an emersion of greater or less 

 duration. 



The Oldhaven beds which underlie the London Clay are also 

 wanting in the Paris basin, nor have they any representative in the 

 Belgian area. 



2. (c). Upper Tpresien and Paniselien. — Sands of Cuise. — Lower 

 Bagshot Sands. 



Above the Clay of Flanders (Lower Ypresien) are the argillaceous 

 sands of the Upper Ypresien, terminated by fossiliferous beds which 

 are characterized by species of the sands of Cuise, among which are 

 Num. planulatus, Br. ; Turr. edita, Sow. ; T. hybrida, Desh. ; Crassa- 

 tella propinqua, Wat. ; Lucina sqnamula, Desli. It is above the last 

 beds with Num. planulatus that M. Hebert defines the upper limit of 

 the Lower Eocene. There is also so much analogy both minera- 

 logically and palEeontologically between the Upper Ypresien and 

 Paniselien, that he considers they ought to be united in a single 

 group. 



This reunion into a single stage of the Upper Ypresien and 

 Paniselien, and the general correspondence with the part of the sands 

 of Soissons which is superior to the lignites, that is, the sands of 

 Cuise, has long since been made by Mr. Prestwich,^ and although 

 some doubt, and even difference of opinion, has been expressed by 

 the Belgian geologists on this point, M. Hebert is of the same 

 opinion as Mr. Prestwich. 



Thus there are three great marine faunas, that of Bracheux, that 

 of the London Clay, and that of the Cuise Sands ; these, however, 

 are not equally developed throughout the whole area, in consequence 

 of the physical changes and oscillations of level which the surface 

 has undergone during the whole period of their formation. 



3. Strata comprised betioeen the Bracheux Sands and the Chalk. 



These are divided into difi"erent beds by M. Hebert, as shown in 

 the Table below, commencing with the lowest or Calcaire de Mons 

 reposing on the eroded surface of the White Chalk, and containing 

 a rich marine fauna resembling, but differing from specifically, that 

 of the Calcaire grassier, as well as some land and freshwater genera; 

 its equivalent is wanting in the Paris basin. These beds underwent 

 denudation prior to the deposition of the Lower Heersien sands, 

 during which the conglomerates of Nemours and Killy were formed. 



The Lower Heersien sands of Belgium are considered to be 

 equivalent to the Eilly sands of France, and the Upper Heersien 

 sands as equal to the Killy limestone, so rich in land and freshwater 

 mollusca, and the strontian-bearing marls of Meudon. The over- 

 lying Heersien marine marls of Belgium are represented in the 

 Paris basin by the conglomerate of Meudon and the marls of 

 Dormans ; these marls and all the underlying strata, according to 

 M. Hebert, being below the Bracheux sands, and forming the lower 

 group of the Lower Eocene strata. 



' Quart. Journ. Geol. Soc. Yol. ii. 1855. 



DECADE II. — YOL. I. — NO. Y. IS 



