1856.] PRESTWICH BRITISH XND FOREIGN TERTIARIES. 391 



of the Paris basin ; whilst the upper, on the contrary, are thickest to 

 the eastward. The total thickness of the deposit, therefore, rarely at 

 any one place exceeds 1 00 feet, whilst the Upper Bracklesham series, 

 with which it corresponds, is more than 500 feet thick. This dif- 

 ference the author attributed to a more rapid subsidence of the 

 English area than of the French at that geological period. This, he 

 showed, was accompanied by more marine conditions prevailing all 

 through the English deposit, and by the continuance throughout of 

 the same green sands which in France were confined to the lower 

 division. That the whole series was, however, synchronous with the 

 Calcaire grossier he considered proved by the circumstance, that, 

 although the freshwater beds which existed in France did not extend 

 to this country, yet the organic remains of some of the beds of the 

 Bracklesham series gave evidence of an upper division higher than 

 the beds with the Venericardia pLanicosta dioA Cerithiumgiganteumoi 

 Bracklesham, for at the latter place the proportion of shells ranging 

 up into the overlying Barton series was 30 per ce?i^., whereas in some 

 beds recently discovered by Mr. F. Edwards at Bramshaw, and ap- 

 parently at the top of the Bracklesham series, the proportion is 

 46 per cent. The middle beds of the upper Bracklesham series show 

 the closest affinity with the Middle Calcaire grossier, although there 

 are only 140 species in common. The lowest division of this series 

 is more fossiliferous in England than in France, showing a closer re- 

 lation (43 per cent.) with the underlying beds than does the mass of 

 the Calcaire grossier, in which the proportion is as 28 per cent. 

 The total number of Molluscs in the Calcaire grossier of the Oise 

 is 651, and in the Bracklesham series of Hampshire 368. 



Above this zone is the series of the Sables moyens in France and 

 Barton clays in England. Owing to the number of Calcaire gras- 

 sier fossils which had been found at Barton, these beds had been 

 considered synchronous with the Calcaire grossier, a view which 

 the author himself had formerly adopted wdth reserve. Seeing, 

 however, that the Bracklesham series probably represents all the 

 divisions of the Calcaire grossier, and that the distinction between 

 the Bracklesham and Barton series was of equal value to that be- 

 tween the Calcaire grossier and the Sables moyens, the author now 

 correlated the Barton clays with the Sables moyens, as suggested by 

 M. Graves, M. Dumont, Sir Charles Lyell, and M. He'bert. He, 

 however, alluded to the difficulty of doing this upon the evidence of 

 any small number of organic remains, or even of a few species con- 

 sidered characteristic in one area ; and he showed that in the Barton 

 clay itself, although there were many Sables moyens species (63), still 

 there were a greater number of Calcaire grossier species (69). In 

 the same way in the Laeckenian system of Belgium, which overlies 

 the Bruxellian system (the equivalent of the Calcaire grossier), 

 there are forty-five Calcaire grossier and Bracklesham sand species, 

 and only forty-four Barton and Sables moyens species. But Mr. 

 Prestwich showed that, taking the per-centage of species which 

 range from the lower to the higher series, each area offered nearly 

 an equal amount of distinction ; as out of 1 00 species of the lower 



