RECENT FORMATIONS. 95 



found scattered through the world, the mass being the 

 remains of some gregarious species, such as the oysters, 

 wliich the sea has left uncovered in its retreat, or which, 

 by their increase near the mouths of rivers, have forced the 

 stream to seek another entrance into the ocean. For proofs 

 of their efficacy to this purpose, we need not travel abroad, 

 though undoubtedly the more remarkable of them are, as 

 you might anticipate, to be found in warm and tropical 

 climates, where life is more prolific than in the moderate 

 temperature of Europe, and productive of larger growths. 

 In Senegal, Adanson mentions that in the tide-way of the 

 Del, not far from the mouth of the Niger, the village of 

 Del is built on the extremity of a bank of shells, which 

 extends nearly a league to the north ; all this enormous 

 bank being solely composed of the valves of the tree-oyster 

 that had once lived there pendant from the roots of the 

 mangol-trees, but had been left dry by a change in the course 

 of the river, that change effected by their own natural in- 

 crease.* The same traveller describes another bank formed 

 in a similar manner, by the same species of oyster, which 

 is of still greater extent, and gives name to a district of 

 Senegal — " le quartier de la Chaux," — because the whole 

 country is thence furnished with all its lime.f America sup- 

 plies us with some remarkable examples of the same kind, 

 where at the mouth of many of its large rivers, a little 

 elevated above the tide, there are extensive beds of the 

 Ostrea virginica mixed with some littoral shells in a sub- 

 fossil state. I Anastasia Island, upon the eastern coast of 

 Florida, which is about ten or twelve miles long, and one 

 and a half broad, "is composed of horizontal layers of a 



* Voy. ail Senega], 128. t Ibid. 147. 



X Dr. Rogers adds — " The position in which these beds of shells are 

 invariably seen, is upon the low level plains adjacent to the tide-creeks of 

 our rivers, where they appear to have dwelt in colonics in the slieltered 

 bays at a time when these jdains were at a small de])tli beneath the water, 

 and to have been lifted with them by, perhaps, the last shock which has 

 changed the level of the coast. These shells, in a sub-fossil state, occur 

 in Cumberland county. New Jersey, on the bank of Stow Creek, at Egg 

 Harbour, on the Severn, at Euston, in Maryland ; again, upon the York 

 River in Virginia, and indeed upon many others of the southern rivers. 

 They occur at the mouth of the Potomac, resting upon the beds of marine 

 shells, which were originally described in the Journal of the Academy of 

 Natural Sciences, by Mr. Conrad, and considered by him as referable to 

 tlie newest of our fossiliferous formations. In the same locality, these beds 

 of fossil Ostrea virginica are seen to be covered by tlie diluvium, so that 

 there can be no question of their origin having been during the latest stage, 

 as it were, of the tertiary period, and not connected, as imagined by the 

 vulgar, with human agency." — Fourth Report of Brit. Assoc. 17. — See also 

 Bosc's Coquil. ii. 29.5. 



