260 WEALD CLAY. [Oh. XVIII 



The Wealden is divisible into two minor groups : 



Thickness. 

 1st. "Weald Clay, chiefly argillaceous, but sometimes including 



thin beds of sand and shelly limestone "with Paludina 140 to 280 ft. 

 2d Hastings Sand, chiefly arenaceous, but in which occur some 



clays and calcareous grits* - - - 400 to 1000 ft. 



Another freshwater formation, called the Purbeck, consisting of various 

 limestones and marls, containing distinct species of mollusks, Cyprides, 

 and other fossils, lies immediately beneath the "Wealden in the southeast 

 of England. As it is now found to be more nearly related, by its organic 

 remains, to the Oolitic than to the Cretaceous series, it will be treated of 

 in the 20 th chapter. 



Weald Clay. 



The upper division, or "Weald Clay, is of purely freshwater origin. Its 

 highest beds are not only conformable, as Dr. Fitton observes, to the 

 inferior strata of the Lower Greensand, but of similar mineral composition. 

 To explain this, we may suppose, that, as the delta of a great river was 

 tranquilly subsiding, so as to allow the sea to encroach upon the space 

 previously occupied by fresh water, the river still continued to carry down 

 the same sediment into the sea. In confirmation of this view it may be 

 stated, that the remains of the Iguanodon Mantelli, a gigantic terrestrial 

 reptile, very characteristic of the Wealden, has been discovered near 

 Maidstone, in the overlying Kentish rag, or Marine limestone of the 

 Lower Greensand. 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 en- 

 tombed 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 seaward the car- 

 cases of the same species of alligator, in which case their bones might be 

 included in marine as well as in subjacent freshwater strata. 



The Iguanodon, first discovered by Dr. Mantell, has left more of its 

 remains in the "Wealden strata of the southeastern counties and Isle of 

 Wight than has any other genus of associated saurians. It was an her- 

 bivorous reptile, and regarded by Cuvier as more extraordinary than any 

 with which he was acquainted ; for the teeth, though bearing a great 

 analogy, in their general form and crenated edges (see figs. 303, or, 

 303, 5), to the modern Iguanas which now frequent the tropical woods 

 of America and the "West Indies, exhibit many striking and important 

 differences. It appears that they have often been worn by the process of 

 mastication ; whereas the existing herbivorous reptiles clip and gnaw off 



* Dr. Fitton, GeoL Trans. Second Series, vol iv. p. 320. 



