J 20 DEPOSITS IN ESTUARIES. 
gradual accumulation during a long series of 
years. 
In the case of deposits formed in estuaries, the 
admixture and alternation of the remains of flu- 
viatile and lacustrine shells with marine Exuviae, 
indicate conditions analogous to those under 
which we observe the inhabitants both of the 
sea and rivers existing together in brackish 
water near the Deltas of the Nile,* and other 
great rivers. Thus, we find a stratum of oyster 
shells, that indicate the presence either of salt or 
brackish water, interposed between limestone 
strata filled with freshwater shells among the 
Purbeck formations; so also in the sands and 
clays of the Wealden formation of Tilgate forest, 
we have freshwater and lacustrine shells inter- 
mixed with remains of large terrestrial reptiles, 
e. g. Megalosaurus, Iguanodon, and Hylaeo- 
saurus ; with these we find also the bones of 
the marine reptiles Plesiosaurus, and from this 
admixture we infer that the former were drifted 
from the land into an estuary which the Plesio- 
saurus also having entered from the sea, left its 
bones in this common receptacle of the animal 
and mineral exuviae of some not far distant 
land.t 
Another condition of or2:anic remains is that 
»' 
* See Maddea's Travels in Egypt, vol. ii. p. 171-175. 
t For the detailed history of the organic remains of the 
Wealden formation, see Mr. Mantell's highly instructive and 
accurate volumes on the geology of Sussex. 
