394 Professor E. Forbes on Purbecks. 



water and brackish strata of the Purbeck series, must be sought for, 

 not simply in either a rapid or a sudden change of their area into 

 land or sea, but in the great lapse of time which intervened between 

 the epochs of deposition at certain periods during their formation. 



A most striking feature of the MoUuscan fauna of the Purbecks 

 is this ; — so similar are the generic types of these Mollusca to those 

 of tertiary freshwater strata and those now existing, that, had we 

 only such fossils before us, and no evidence of the infra-position of 

 the rocks in which they are found, we should be wholly unable to 

 assign them a definite geological epoch. 



An examination and comparison of these Purbeck fossils with tho 

 collections from the Hastings sands, and Wealden in the Museum of 

 the Geological Society (to which they were chiefly presented by Dr 

 Fitton), and in the cabinet of Dr Mantell, leads the author to be- 

 lieve that the fauna of the middle and upper divisions of the Weal- 

 den series is, so far as species are concerned, almost entirely distinct 

 from that of the lower or Purbeck division. Many of the species 

 reputed to be common to the whole series, are found on inquiry to 

 include more species than one under one name, whilst some other 

 forms recorded as Wealden, but, so far as the author has observed, 

 peculiar to the upper Purbecks, and occupying only a limited horizon 

 in that part of the series, are derived from certain anomalous strata 

 near Tunbridge Wells, which the author believes will prove, on 

 closer examination and accurate survey, to be Purbeck strata brought 

 up among the true Wealden by faults. The excellent monograph on 

 the Wealdens of North Germany by Dunker and Von Mayer, in 

 which a vast number of species of animals and plants are described 

 and accurately figured, affords the strongest confirmation of this 

 view, and shows that while the fauna of the German Weald clay 

 and Hastings sands corresponds in essentials with that of the same 

 formations in Britain, the Purbecks of the Continent, just as here, 

 differ from the superior beds almost entirely in their organic con- 

 tents, and correspond with similar beds in our own series. 



The marine or brackish-water bands in Germany, containing 

 Ostrea Fittoniana, appear to be represented in England by cor- 

 responding bands with the same fossils, and accompanied by species 

 of Corbula, Cardium, and Melania^ in the upper part of the Hast- 

 ings sand at Swanage. All the investigations of the author so far, 

 have gone to indicate the probability of the presence of several dis- 

 tinct assemblages of organic remains (similar to those which he has 

 shewn to exist in the Purbecks), in the higher portions of the Weal- 

 den series of formations, whilst the true position of these strata is 

 shewn, without a question, to be in connection with the Oolitic 

 or lower, and not with the Cretaceous or upper division of the 

 secondary rocks. 



