1847.] 



PRESTWICH ON THE LONDON CLAY. 



359 



are extremely rare. The per-centage of species in all these classes 

 common to the two deposits will not, I think, average more than in 

 the Testacea. 



Where therefore is the affinity of these formations ? Where the 

 proofs of their synchronism ? Are we to be satisfied with an assumed 

 analogy of position, — with a commmiity of apparently not more than 

 forty or fifty species of organic remains, out of probably 1000 to 1200 

 or more, and attribute so wide a variation of fauna merely to geogra- 

 phical position and diiferences of conditions ? — or is not rather the 

 'prima facie evidence sufficient to render it probable that they differ in 

 age as well as in origin, and to lead us to investigate more closely their 

 relative superposition? If then the hypothesis which has referred 

 them both to the same period is insufficiently grounded, the question 

 arises, in what relation do they stand one to the other ? 



For an answer to this question, it is necessary to recur to the 

 Hampshire tertiary series, and to search there for the intermediate 

 and connecting types, without which it would be difficult to institute 

 a comparison. 



Divisions of the Tertiary Series. 



Lowest — Sands and Mottled Clays. 



In determining the synchronism of any portion of the London and 

 Hampshire series, we should endeavour to establish by a good base- 

 line the earliest point of contemporaneity. The lower part of the 

 Hampshire tertiaries presenting at first sight considerable apparent 

 differences from those of London, has caused the base-line, if it may 

 be so termed, to be placed near the top of the series, viz. in the clays 

 of Barton, making them parallel with the London clay, and thus 

 forcing the strata above and below into an order bearing the same 

 relation to the one as to the other*. 



* The following is a sketch of the present arrangement of the Tertiary series of 

 _ England and France, with the order of superposition and co-relation of the strata. 



Hampshire Basin. 



London Basin. 



?Bagshot / P?i*^^5 with regard to 

 ^^ N the Hampshire series 

 sanas: i uncertain. 



200. Upper freshwater formations. 

 20. Upper marine formations. 

 170. Lower freshwater formations. 



Paris Basin. 



Feet. 



40. Calcaire lacustre suj)^rieure. 



80. Sables marins superieurs. 

 150. Calcaire lacustre inferieure. 



50. Gr^s de BeauchampS. 



00. London clay : Highgate, Sheppey. 



350. London clay: Barton, 

 lesham and Bognor. 



Brack- I 100. Calcaire grossier. 



80. Plastic clays : pebble-beds, fluvia- 

 tile beds of Woolwich, and sands 

 below them to the chalk , 



100. Lignites and Argile plastique. 



1 100. Sands and plastic clays 4: varie- 

 gat|d sands and clays, andhg- 

 nites, forming the central and 

 lower strata of Alum Bay, in- 

 cluding thefossiliferous brown 

 clay 5 (stratum "rf" of Mr. 

 Webster), and the red clays 

 below it. 



1 These numbers refer to a roughly approximate average thickness of the deposits. 



2 These three formations have of late generally been considered as one. 



3 M. d'Archiac and others refer the Gr^s de Beauchamp to the thick stratum of sand imme^ 

 diately overlying the London clay at Alum Bay, and which they also consider the equivalents of 

 the Bagshot sands. 



4 Sometimes this distinction has not been admitted, and the series from the freshwater beds 

 to the chalk has been considered as all belonging to the London clay series. 



6 The presumed marine equivalents of the fluviatile beds of Woolvnch.— Sejo^. 1847, 



