1847] PRESTWICH ON THE LONDON CLAY. 357 



of Barton and Highgate, and the discovery of several species common 

 to the two localities, led to the belief that they were contemporaneous. 

 The more material differences in the fauna were then and afterwards 

 attributed to differences dependent upon distance, and on the sepa- 

 ration of the two seas at the period of the deposition of the strata ; 

 and the finding, at Kew, of several species common, some to High- 

 gate, and others to Barton, was supposed to remove any doubts as to 

 the identity of the two groups. 



Having thus associated the London clay with the Barton beds, and 

 afterwards, by the same reasoning, with the beds at Stubbington and 

 Bracklesham, the organic remains occurring at these various places 

 were taken as belonging to one and the same deposit ; and they were 

 then compared *' en masse"" with those of the Calcaire grossier. The 

 consequence was, that the supposed relation of the London clay and 

 the Calcaire grossier appeared to be confirmed by the discovery of a 

 considerable number of French species at Barton and at Bracklesham : 

 the beds at these localities were therefore supposed to present an 

 intermediate fauna corresponding with their intermediate geographical 

 position, and thereby gave some verisimilitude to the hypothesis of 

 the synchronism of those divisions of the London, Hampshire, and 

 Paris groups. Let us now inquire how far, independently of the aid 

 it has received from Barton and Bracklesham, the London clay of 

 the London district * agrees in its organic remains with the Calcaire 

 grossier. 



The Testacea of the Calcaire grossier have been variously estimated 

 at from about 800 to 1000 species ; whereas the London clay proper, 

 apart from Barton and Bracklesham, exhibits apparently less than 

 250 species f. I am however inclined to think that the number of 

 the Testacea of the Calcaire grossier has been over-estimated, from 

 the circumstance of specimens from beds both above and below it, 

 and cropping out at the same localities, having been, before the di- 

 stinction of the beds was well ascertained, quoted indiscriminately as 

 Calcaire grossier fossils. There is unfortunately no complete recent 

 list published ; and the same fact, which causes doubt as to the num- 

 ber of species, renders also their chronological value problematical. 



We have however the excellent, but only local, lists of the Eocene 

 fossils of the Department de I'Aisne given by M. D'Archiac^:, and 

 his shorter but more general lists in his paper on the relations of the 

 French, Belgian and English Tertiaries§, There are also numerous 

 papers by Constant Prevost, Elie de Beaumont, M. C. D'Orbigny 



* I have used the terms London or Hampshire tertiary districts, in preference to 

 London basin or Hampshire basin, as denoting the areas occupied by the tertiary- 

 deposits. The map in Plate XIV. shows the relative position of the two districts, 

 t Of these not three-fifths are described. (See list at the end.) 

 X In this district M. d'Archiac enumerates from the Sables et Gres moyens, or 

 Gres de Beauchamp 61 species of Testacea. 



Calcaire grossier jJ^P^^;;;;;;;;;;;;;;;; ^^J 



Lits Coquilliers 190 „ 



Etages des Lignites 40 „ 



All these divisions have many species in common. — Memoires de la Societe Geolo- 



gique de France, vol. v, part 2. 



§ Bulletin de la Societe Geologique de France, vol. x, p. 168. 



