M. JlSert — Comparison of Eocene Strata. 227 



a gulf dependent on the North Sea, the opening to which was 

 between the Ardennes and the Boulonnais, already elevated above the 

 sea level. It is thus natural, as we advance to the North Sea, that the 

 lacunae will be less. Belgium would furnish an intermediate term, 

 and it is one of these terms that MM. Comet and Briart have 

 described. If we could explore the bottom of the North Sea, other 

 terms would appear, for Belgium itself only increases the number of 

 lacunas. 



But that which we call the North Sea is still only a gulf, having a 

 communication with the ocean between the Shetland Isles and Norway; 

 this gulf has undergone oscillations, and has perhaps been sometimes 

 dry land. The complete continuity between these deposits cannot 

 be well recognized, and it will be necessary to seek them in the 

 great oceanic depressions. 



However it may be, we conceive that certain species of the pisolitic 

 limestone, the Voluta robusta for example, have continued to live 

 in the North Sea, and returned to live in Belgium with new species 

 at the period of the Mons limestone. The sea at this time, or a 

 little after, penetrated into the Paris basin, leaving there some 

 slight marine sediments, as those at the base of the Eilly sands ; and 

 after its departure, when the basin was transformed into a lake, 

 some species, as Cerithium inopinatum, and a genus near to Liotia, 

 lived in the brackish waters of the environs of Paris at the time 

 when the eastern part of the basin became a lake, the lake of 

 PJiysa gigantea and Faludina aspersa. 



It is scarcely possible, in our present state of knowledge, to 

 believe otherwise. We see the intimate relations there are between 

 the Eilly limestone and that of Mons. 



The basin of Mons has doubtless been emerged, and the limestone 

 denuded before the incoming of the fresh water, which is shown 

 by the older Heersien deposits. It remains to be proved if the 

 fauna of these waters was similar to that of Eilly. These have 

 been replaced by marine waters at a time which appears to cor- 

 respond to the formation of the conglomerate of Meudon ; and 

 during the deposition of the white calcareous marls with Pholadomya 

 cuneata, the white calcareous marls of Dormans without fossils 

 would be deposited in the Paris basin. 



It is only after this succession of deposits that the invasion of 

 the sea of Bracheux occurred, with its sands and rolled pebbles, 

 eroding the Heersien system and the more ancient deposits of the 

 Mons basin, as the marls of Dormans, the Eilly limestone and the 

 underlying beds in the Paris basin, and forming an horizon well 

 defined, above which the stratification becomes clear and undoubted. 



"In returning to this subject," says M. Hebert, "after an interval 

 of twenty years, I can only confirm mj'' former opinion respecting 

 the Eilly limestone, and surely if the facts discovered during this 

 period tend to prove I was then in error, I should readily recognize 

 it, as I prefer to correct myself rather than be corrected by others ; 

 but before all I prefer to see truth established on a solid base, even 

 if it should be at my expense." J. M. 



