100 DEFOSITES IN ESTUARIES. 



and sandy slate not six feet thick, contains an admixture of 

 terrestrial animals and plants with shells that are decidedly- 

 marine; the bones of Didelphys, Megalosaurus, and Ptero- 

 dactyle are so mixed with Ammonites, Nautili, and Belem- 

 nites, and many other species of marine shells, that there 

 can be little doubt that this formation was deposited at the 

 bottom of the sea not far distant from some ancient shore. 

 We may account for the presence of remains of terrestrial 

 animals in such a situation by supposing their carcasses to 

 have been floated from land at no great distance from their 

 place of submarine interment. 



A similar explanation may be given of the mixture of the 

 bones of large terrestrial mammalia with marine shells, in 

 the Miocene Tertiary formations of Touraine, and in the 

 Crag of Norfolk. 



Cases of Animals destroyed suddenly^ 



The cases hitherto examined, are examples of the pro- 

 cesses of slow and gradual accumulations in which are pre- 

 served the remains of marine, lacustrine, and terrestriial 

 animals that perished daring extended periods of time, by 

 natural death. It remains to state that other causes seem 

 to have operated occasionally, and at distant intervals, to 

 produce a rapid accumulation of certain strata, accom- 

 panied by the sudden destruction, not only of testacea, but 

 also of the higher classes of the then existing inhabitants of 

 the seas. We have analogous instances of sudden destruc- 

 tion operating locally at the present time, in the case of 

 fishes that perish from an excessive admixture of mud with 

 the water of the sea, during extraordinary tempests; and 

 also from the sudden imparting of heat, and noxious gases, 

 to water in immediate contact with the site of submarine 

 volcanoes. A sudden irruption of salt water into lakes or 

 estuaries, previously occupied by fresh-water, or the sudden 

 occupation of a portion of the sea, by an immense body of 



