122 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



course the Portland stone ; the overlying strata marked by horizontal 

 shading, the Wealden, occupying the former site of an inland sea ; and 

 the dotted portion, the lower greensand. The letters aaa denote the 

 littoral face of the continent, and h the summit of the barrier which 

 divided the ocean from the inland sea. Let it be supposed that, either 

 during the period in which the continent remained stationary in its 

 elevated position, or at any stage of its subsidence, the lower green- 

 sand had covered the submerged surface to the extent indicated by 

 the dotted line 1, while at the same epoch, the Wealden had accumu- 

 lated in the inland sea to the height of 1'; further, that at a later date 

 the two series had respectively reached 2 and 2', and subsequently 3 

 and 3'. It is evident, according to this hypothesis, that the horizontal 

 extension of the lower greensand from 1 to 2 and from 2 to 3 took 

 place contemporaneously with the vertical accumulation of the Weal- 

 den from r to 2' and from 2' to 3'. At length however, in conse- 

 quence of the continued depression of the continent, the ocean would 

 overcome the barrier 6, and, by the change of circumstances thus 

 occasioned, lead to the extinction of the inhabitants of the inland sea ; 

 while it would, at the same time, begin to extend over the now 

 completed Wealden a series of strata similar, as regards the fossils 

 entombed in them, to those with which it had continuously enveloped 

 the previously submerged portion of the surface of the subsiding land. 

 The flexures of the strata are, of course, very much exaggerated in 

 the figure ; on the scale of nature, and seen in such limited sec- 

 tions as are usually exposed to geological observation, the several 

 deposits would appear strictly conformable to each other. 



If this division of the Wealden, as has been generally conjectured, 

 was deposited in an estuary, the latter must have communicated 

 with the ocean, during part of both the Jurassic and Cretaceous pe- 

 riods, — the organic remains enumerated (see p. 116) connecting 

 it with the one, and the Iguanodon and Lonchopteris with the other. 

 To the hypothesis, that the estuary opened into the Jurassic ocean, 

 it may be answered, that the Portland stone, beyond whose limits, 

 in the south of England at least, the Wealden does not extend*, ap- 

 pears to be quite as fully developed where overlaid by the latter as 

 in other localities ; which could not have happened under the cir- 

 cumstances alluded to. Moreover, as already stated, there is no 

 transition or alternation between the Portland stone and Purbeck 

 beds. We might also expect, in such a case, to find a greater pro- 

 portion of marine moUusca and other invertebrata of the Jurassic 

 period, either mingled with the freshwater remains, or in separate 

 layers intercalated between the fluviatile strata, than actually occurs. 

 The only shell (^Ostrea distorta, Sow,) hitherto identified as common 

 to the Portland stone and Wealden, belongs to a genus whose species 

 can resist such changes as would prove fatal to most others ; there 

 is, therefore, nothing improbable in assuming, that it survived the 

 separation of the inland sea from the ocean. 



If identity of fossils proves the near relationship which exists be- 



* Dr. Fitton in Trans. Geol. Soc. 2nd series, vol. iv. p. 332. 



