1847.] PRESTWICH ON THE LONDON CLAY. 355 



Old terms are altered with difficulty ; they have the sanction of 

 usage and weight of successive authorities, and are in geology occa- 

 sionally apt, by their theoretical signification, to stand in lieu of proof. 

 In the case before us, considering up to a late period the contempo- 

 raneity of the marine strata of London, Bracklesham, Barton, and 

 Paris well established, I thought it scarcely necessary to investigate 

 the data on which their identification rested ; or rather, proceeding 

 under the full impression of its correctness, I yielded to it facts, and 

 endeavoured to conform to it phsenomena, which, had I allowed them 

 their due influence, would have earlier led to independent inferences. 

 The differences known to exist in the fauna of these groups had been 

 attributed solely to changes of condition and distance — causes, which 

 as they produce effects whose limits are not easily assignable, some- 

 times prevent inquiry by offering an apparently available solution to 

 anomalies in the faunae, that might otherwise lead to further and more 

 correct investigation. 



I allude more particularly to this point, because in my paper on 

 the Tertiaries of the Isle of Wight, following the usually received 

 nomenclature, I applied the term ^'London clay'^ to the clays of 

 Barton, the sands of Bracklesham, and their associated strata, and 

 designated the group below them the ^"^ Bognor beds,^' in consequence 

 of the typical fauna of the Bognor rocks occurring in this portion of the 

 Hampshire series. If however the evidence which I shall lay before 

 the Society this evening be conclusive, these terms must be changed. 



Either the clays of Barton and Bracklesham are the London clay, 

 and the clays of Highgate and London are not the London clay ; or else 

 we must restrict the term to the marine clay of the neighbourhood 

 of London and its equivalent elsewhere : in which case its application 

 to the clays of Barton and the clays and sands of Bracklesham must, 

 as I shall endeavour to show, be discontinued. 



With this exception, I adhere to the arrangement introduced in my 

 former paper. In lieu of the term London clay in the sections of 

 Alum and White-Cliff Bay, I propose that the terms Barton clays 

 and Bracklesham sands be used. Below these divisions are the Bog- 

 nor beds, — the strata which I conceive to be the sole equivalent of the 

 London clay ; and then the mottled clays and sands, the latter rest- 

 ing immediately upon the chalk. (See tabular arrangement of the 

 strata at the end of these papers, and Plate XIV.) 



Comparison with the " Calcaire gj^ossier." „ 



Let us first examine the present received hypothesis, that the beds 

 of Barton, Bracklesham and London are synchronous with one 

 another as well as with the Calcaire grossier, and see upon what 

 grounds it is established. 



In the immediate neighbourhoods of Paris and of London the ter- 

 tiary series commences with variable strata of sands and clays reposing 

 upon the chalk ; these are for the most part unfossiliferous, but con- 

 tain occasionally subordinate beds with fluviatile and estuary shells*. 



* In France, and more especially in England, these are only local conditions ; 

 the lowermost beds of the series are as frequently marine. 



