PART II. CHAPTER XVI. 



211 



Relative Age of the Wealden Group. 



green-sand, are not only conformable, but of similar mineral 

 composition. 



It is also a remarkable fact, that the same Iguanodon Man- 

 telli which is so conspicuous a fossil in the Wealden, has recently 

 been discovered near Maidstone, in the overlying Kentish rag, or 

 marine limestone of the lower green-sand. Hence we may infer 

 that some of the saurians which inhabited the country of the 

 great river, continued to live when part of the country had 

 become submerged beneath the sea. Thus, in our own times, 

 we may suppose the bones of large alligators to be frequently 

 entombed in recent freshwater strata in the delta of the Ganges. 

 But if part of that delta should sink down so as to be covered 

 by the sea, marine formations might begin to accumulate in the 

 same space where freshwater beds had previously been formed; 

 and yet the Ganges might still pour down its turbid waters in 

 the same direction, and carry the carcasses of the same species 

 of alligator to the sea, in which case their bones might be in- 

 cluded in marine as well as in subjacent freshwater strata. 



Age of the Wealden. — Some geologists have classed the 

 Wealden as a member of the cretaceous group, while others have 

 considered it as more nearly connected with the antecedent 

 oolitic deposits ; nor is it easy to decide which opinion is prefer- 

 able, because the organic remains of the cretaceous and oolitic 

 groups are marine, while those of the interposed Wealden are 

 almost all freshwater. The testacea and plants of the latter 

 appear as yet to be specifically distinct from those of any other 

 formation ; but if we examine the reptiles, it appears that the 

 Megalosaurus Buchlandi is common to the Oolite and Wealden, 

 the teeth and bones of this great Saurian occurring both in the 

 limestone of Stonesfield and in the Hastings sand. 



There are also some generic forms, both of repliles and fish, 

 common to the Oolite and Wealden, and not yet discovered in 

 the Chalk. Vertebrse, for example, of the Plesiosaurus are not 

 confined to the oolite and lias, but have been also found in the 

 Wealden ; and the Lepidotus, a genus of fish very characteristic 

 of the Wealden, is unknown in the cretaceous group, while it is 

 abundant in the oolitic series. 



On the other hand, the same species of Iguanodon has been 

 already, mentioned as decidedly common to the Wealden and 

 green-sand. 



In Scotland, and in different parts of the Continent, marine 

 deposits have been found which are supposed to have been coe- 

 val with the Wealden, and which are intermediate in fossil cha- 

 racters as in position between the Cretaceous and Oolitic sys- 



