120 PROQEjaJl^Gj^: OF THE GEOLOGICAL SOCIETY. 



indicate that the depression was effected gradually, since their occur- 

 rence can only be explained on the supposition that portions of the 

 continent remained above the waters, during the accumulation of 

 many feet of the oceanic beds, which, if the evidence afforded by the 

 exclusively marine character of the remains entombed in all but the 

 lowest members of the cretaceous system is to be trusted, ultimately 

 covered it. In the course of such a subsidence, the several rocks 

 constituting the surface of the continent would, on their successive 

 submersion, be enveloped in marine beds, which, although not strictly 

 speaking contemporary, must, from the similarity of the circum- 

 stances prevalent during their accumulation, be almost identical with 

 respect to their organic contents. The oceanic strata deposited at 

 the period referred to will thus be found in contact with a variety of 

 the older masses, and certainly the lower greensand fulfils this con- 

 dition, since, in the region under consideration, it rests on nume- 

 rous members of the preceding systems. 



When the elevation of the Portland stone happened, the con- 

 cavities of the ocean in which it was deposited, having no outlet, 

 would retain their salt water, and form inland seas inhabited by many 

 of the same animals as those which dwelt in it before the event which 

 led to their isolation. It is into these basins that I conceive the river, 

 which produced the Wealden of the south-east of England, north of 

 France and Hanover, must have flowed. There is no reason to suppose 

 that the elevation and extension of the land would destroy the inhabit- 

 ants of its surface ; and it is, therefore, not surprising to find that the 

 Megalosaurus and Poikilopleuron of the oolites also occur in the 

 Wealden beds which rest upon the Portland stone. The identity of 

 the fishes in the two groups seems, at first, less easily accounted for 

 by the hypothesis now proposed, but we know that several species 

 now flourish in the Caspian, as well as in the seas with which it was 

 formerly united^. Thus among others it contains two species of 

 Sturgeon (Acipenser Huso and A, Ruthenus), both of which also 

 inhabit the Black and neighbouring seas, and the latter even the 

 Arctic ocean. The common salmon, too, so universal in all the 

 seas and rivers of Europe, is equally abundant in the Caspian. The 

 Mollusca of this modern basin are, I believe, in a great measure 

 peculiar to itself, and so must those of its prototypes have been, 

 since, except the Ostrea distorta, none of the few shells referable to 

 marine genera, which the Wealden contains, are found either in the 

 Portland stone or lower greensand. 



The want of correspondence in the number of the " dirt beds," 

 and in the nature of the strata associated with them, at the va- 

 rious localities where they have been observed, shows that the 

 vegetables which gave rise to them, although they must have 

 flourished contemporaneously or nearly so, were not distributed over 

 a continuous surface. They probably grew on low grounds near 

 the mouths of the rivers, or on islets on their deltas. In such situa- 

 tions they would, from time to time, be inundated by the rivers and 

 buried under their deposits, on the one hand, and suffer from the 



