314 
ELEMENTS OE GEOLOGY; 
w s.w. Fig. 288. E.N.E. 
1. Tertiary. 2. Chalk and Ganlt. 3. Upper Neocomian (or Lower Greensand). 
4. Wealden (Weald Clay and Hastings Sands). 
contains beds of oysters and other marine shells which indb 
cate fluvio-marine conditions. The uppermost beds are not 
only conformable, as Dr. Fitton observes, to the inferior stra¬ 
ta of the overlying Neocomian, but of similar mineral com¬ 
position. 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 sedi¬ 
ment into the sea. In confirmation of this view it may be 
stated that the remains of the Iguanodon Mantelli^ a gigan¬ 
tic' terrestrial reptile, very characteristic of the Wealden, has 
been discovered near Maidstone, in the overlying Kentish 
Rag, or marine limestone of the Upper Neocomian. Hence 
we may infer that some of the saurians which inhabited the 
country of the great river continued to live when part of 
the district had become submerged beneath the sea. Thus, 
in our own times, we may suppose the bones of large alliga¬ 
tors to be frequently entombed in recent fresh-water 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 fresh¬ 
water beds had previously been formed; and yet the Gan¬ 
ges might still pour down its turbid waters in the same di¬ 
rection, and carry seaward the carcasses of the same species 
of alligator, in which case their bones might be included in 
marine as well as in subjacent fresh-water strata. 
The Iguanodon, first discovered by Dr. Mantell, was an 
herbivorous reptile, of which the teeth, though bearing a 
great analogy, in their general form and crenated edges (see 
Figs. 289, 289, 5, p. 315), to the modern Iguanas which now 
frequent the tropical woods of America and the West In¬ 
dies, exhibit many important differences. It appears that 
they have often been worn by the process of mastication; 
whereas the existing herbivorous reptiles clip and gnaw oft’ 
the vegetable productions on which they feed, but do not 
chew them. Their teeth frequently present an appearance 
