246 prvOCEEDiiirGS of the geological society. [March. 20, 



ditious, and by the interchange of such, conditions over portions of 

 the same area, supplemented by an occasional intrusion of the ocean, 

 that I would account for the alternation of freshwater, marine, and 

 hrackish-water fossils in the Purbeck strata ; for, as I shall endea- 

 vour to show more fully with, respect to the Wealden, neither the 

 conditions of accumulation of the sedimentary strata, nor the life- 

 conditions of its fauna appear to be suflticiently in accordance with 

 an estuarine position. 



It is not yet known with certainty whether the English Purbeck 

 strata are anywhere in direct sequence with the marine Portlandian 

 series, unless such proves to be the case at Swindon — or whether, 

 as in the Isles of Purbeck and Portland and apparently in the Vale 

 of Wardou:r, the Portland beds had been everywhere elevated before 

 the commencement of the lower Purbecks ; but we do know (and 

 this is an important point in their history) that the Purbeck beds lie 

 almost entirely within the former boundary of the Portlandian ocean. 

 This is to me an important fact, as it tends to prove what I have 

 already suggested, that the so-called " area of subsidence" in which 

 I suppose the Purbeck strata to have originated was itself a portion 

 of an older, larger, but equally subsiding area of the Portlandian ocean. 



Now there would appear to bo certain partially understood laws 

 in operation in nature in relation to areas of subsidence which seem 

 to have a bearing on the present subject, the general results of which 

 may be stated as foUows : — first, that the central portion of any sub- 

 siding area will sink faster than the parts nearer to its margins ; and 

 secondly, that such subsiding areas have a tendency to diminish in 

 extent. If, therefore, as I have supposed, the Purbeck beds origi- 

 nally occupied such an area of subsidence, one might expect to find 

 that the extent of the succeeding Wealden basin would be less than 

 that at one time covered by the Purbeck watei's ; and such, I believe, 

 is in reality the case. 



Purbeck strata are described by Fitton as occurring at Coombe 

 Wood in Oxfordshire*, at Long Crendon towards Thame f, and in 

 other places where they are covered by the higher beds of the Neo- 

 comian. 



The point of junction of the Purbeck and Wealden beds is unfor- 

 tunately nowhere very clearly observable ; yet, as the two formations 

 have been always taken as continuous, I shall so consider them. 



In turning from the Purbeck to the Wealden, one finds immediate 

 evidence of the increase of fluviatile action. The abundant frag- 

 mentary remains of ferns and other terrestrial vegetation in the 

 Hastings Sands and lower portion of the Wealden beds, the tree- 

 stems, and the bones of reptiles were doubtless all brought down by 

 rivers to their present resting-places. The sand and clay, the pebbles 

 of " quartz and jasper " J may also all be traced to river-action. It is 

 clear, however, on examination of these strata, that such action was 

 intermittent, and that its efiects varied greatly at different times over 

 different portions of the AVealden area. There is throughout the 



* Geol. Trans. 2 ser. iv. p. 275. t Ihid. p. 281. 



X Tilgate-beds. Mantell's Geol. Suss. (1827), p. 29. 



