188 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 30, 



taken place, and after the Baculites, Belemnites, and Ammonites had 

 disappeared*. The Lavershie and Vigny beds seem to have preceded 

 the Farringdon gravels, inasmuch as the Chalk veas less elevated 

 and less denuded when those beds were deposited than when the 

 gravel in question was accumulated ; although this difference might be 

 due to local causes. 



Throughout this memoir, I have taken it for granted that all the 

 fossiliferous beds usually referred to the Upper green sand really 

 belong to that formation. If future examination should show that 

 the sands of Essen on the Ruhr, or of any of the other localities here 

 quoted, belong to a later period than the Upper green sand, the argu- 

 ment in favour of the Danien date of the Farringdon gravel will be 

 so much strengthened. But no such change of classification is ne- 

 cessary for my argument, as the more extended vertical range here 

 claimed for many species is in harmony with all later observations, 

 which show that the duration of species at all geological periods has 

 been longer than was formerly suspected ; and that the same species 

 reappear at various levels in beds of similar character, notwithstand- 

 ing that those beds may be separated by others deposited under con- 

 ditions unfit for the habitation of the species in question. The most 

 mischievous heresy which has grown up in modern geology, is that 

 which attempts to limit the existence of species to individual beds or 

 deposits. The definitions of species, which ought to rest on zoolo- 

 gical grounds only, are marred by warping them to fit geological 

 views ; and geological divisions are established or strengthened on 

 lists of species, many of which are only founded on the evidence of 

 those very divisions, by the ingenious process of reasoning in a circle. 

 Fortunately such views have found little favour with English geolo- 

 gists, but we suffer from them indirectly, whenever they affect 

 foreign works on Palaeontology which we have occasion to consult. 



[P.S. "When the preceding memoir was written I had not had an 

 opportunity of consulting M. Graves's admirable "Essai sur la Topo- 

 graphic geognostique du De'partement de I'Oise," which contains a 

 description of the Pisolitic limestone of Laversine.— AprU 22, 1854.] 



* Vide d'Archiac, Histoire des Progres de la Geologic, vol. iv. p. 375. 



