12 FOURTH REPORT — 1834. 



of shells, belonging, as far as examined, almost, though not 

 exclusively, to species inhabiting the adjoining coast. The 

 mass is divided, by thin scams of some foreign matter, into 

 layers from 1 to 18 inches thick, and is so soft before exposure 

 to the air, that it is easily cut by a tool into slabs of any re- 

 quired size, and in this form is extensively used for building. 

 Near the surface the fragments of the shells, generally speak- 

 ing, are the smallest; but they occur of various sizes, and 

 frequently in the same layer the shells are entire. Much of 

 this rock, especially the more comminuted kind, exhibits not 

 unfrequently a confused crystallization ; this process having 

 gone so far as to present the fragments in an almost obliter- 

 ated state. The coarse varieties are composed of some frag- 

 ments evidently thus altered, and of others which have not 

 yet lost their colouring matter. The shells belong principally 

 to the genus Area ; they are A. pexata, A. jwnderosa, A. in- 

 congrua, A. transversa ; also Lutraria canaliculata, all of 

 Say; besides a Mactra, a Donax, a Crepidula, a Lucina, and 

 another species of Area, which is probably either extinct upon 

 our coast, or extremely rare. Natica, Oliva, and N^assa tri- 

 villata, of Say, are also mentioned. Mr. Dietz attributes the 

 formation of this island to the agitation of the tides and winds, 

 conceiving the shells to be driven first towards the shore, and 

 deposited afterwards at their present distance from the beach 

 by the retiring tide. But such an explanation seems not alto- 

 gether satisfactory, for I cannot learn that this heaping-up of 

 shells from beneath the water is anywhere noticed upon our sea- 

 islands at present. The winds do indeed drive the sands from 

 the beach, and the shoals which are laid bare at low water, upon 

 them, but mingled with hardly any shells, while the rock of 

 Anastasia Island is made tip of shells exclusively. Such agita- 

 tion wovild seem incompatible with the accumulation of so ho- 

 mogeneous a mass, which is found to contain neither pebbles, 

 sand, nor other transported matter of any sort. My own pre- 

 sent conviction regarding these coast-islands is. That they are 

 all the portions of a range of shoals or bars formed along the 

 line of junction of the turbid waters from our rivers, and the 

 great in-setting currents connected with the gulf-stream; — that 

 since the existence of the gulf-stream and the present drainage 

 of the Atlantic plain, this growth of sediment opposite the coast 

 has been going on ; — that in the more tranquil places upon these 

 bars, vast colonies of shell-fish planted themselves ; — and that 

 the whole line of shoals has been lifted, vvith part of the adjacent 

 continent, by the force of an earthquake or earthquakes, to 

 their present small elevation above tlie waves. Traces of more 



