198 J. S. GARDNER ON THE LOWER LONDON TERTIARIES. 



juxtaposition. It has come to be recognized that these interlacing 

 faunas can be referred to two distinct types — the one perhaps best 

 known as the fauna of the " Calcaire grossier," and the other most 

 readily definable as that of the London Clay. Further, it is admitted 

 by nearly every writer on the Eocene that the latter fauna is of a 

 more temperate or northern type than the former ; and the conclu- 

 sion has been repeatedly drawn that it must have belonged to a more 

 northerly sea, completely shut off from direct communication with 

 that sea in which the " Galcaire-grossier" fauna lived. The " Calcaire- 

 grossier " fauna is, in England, peculiarly and wholly distinctive of the 

 Bracklesham period, and also seems to appear first in Erance at about 

 the same time ; it was therefore for a long time held to be cha- 

 racteristic of the Middle Eocene and of nothing else. The fallacy 

 of such views was signally demonstrated by the discovery of the 

 " Montian system " in Belgium*, containing at the very base of the 

 Eocene a fauna allied to it in the closest possible manner. Since 

 the abandonment of the belief in special creations it is perfectly 

 obvious that slight but persistent modifications in groups of genera 

 must be the ultimate test of the relative age of strata over the whole 

 earth, the incoming of new types being only of value when limited 

 areas are compared. Correlation by zones over wide areas, although 

 extremely valuable, is misleading if perfect contemporaneity for the 

 whole zone is thought to be implied, as zones can only represent the 

 migrations of species following the successive spread or shifting of 

 favouring conditions. Much of the difficulty in correlating the 

 formations in England and America, for instance, has arisen from a 

 disregard of these considerations. 



Starting with the Calcaire de Mons at the base of the Eocene, we 

 find the southern sea with its distinctive fauna occupying parts of 

 Belgium, while the next fact of which we have cognizance is an 

 extension of the northern sea, with its distinctive fauna 4 occupying 

 the same ground and spreading south into Erance. It deposited the 

 " Heersian," " Landenian," " Sables de Bracheux," &c, whose faunas 

 are all intimately related to those of the Thanet Sands. There is no 

 trace of the " Montian " in England ; but the rest of the Eocenes are 

 more or less represented here, and it is with them in England only 

 that I am able to deal. The deposits of the northern sea occupied 

 first a limited portion of the east of the London basin only, and then, 

 during the London Clay, extended to the utmost confines of the 

 Hampshire basin. It is important to notice, as corroborative of the 

 complete separation of the two seas, that though a great increase in 

 temperature took place at some time between the Thanet Sands and the 

 London Clay, none of the warmer " Calcaire-grossier " species found 

 their way into the London- Clay sea, but an immigration instead took 

 place of quite other species of Cowries, Volutes, Nautili, and heat- 

 loving genera. Next, omitting freshwater formations, the southern 



* The appearance of this fauna, "anticipating its normal epoch of appari- 

 tion," is called by M. Barrande a "colony," by M. Marcou a " centre d 'apparition 

 d'etres precursews" " It constitutes an exception to the laws of palaeontology " 

 (Geol. Belgique : Mourlon, 1880, vol. i. p. 193). 



