96 RECENT FORMATIONS. 



semi -indurated rock, consisting wholly of fragments of shells, 

 belonging, as far as examined, almost, though not exclu- 

 sively, to species inhabiting the adjoining coast." There are 

 similar islands and beaches on the coasts of Georgia and 

 Florida ; and Dr. Rogers believes, that most of the other 

 sand beaches and islands which lie along the coast of North 

 America, as far as Long Island, have the same origin. We 

 do not say that these shells, by their aggregation, gave of 

 themselves origin to the islands in question, but it is very 

 evident that they have materially contributed to it: pro- 

 bably the layers of shell-rock are the result of vast colonies 

 of bivalve and other shell-fish which had settled on sand- 

 banks, and been lifted up afterwards above the waters by the 

 agency of earthquakes. A deposit, composed entirely of two 

 existing shells, in a subfossil state, the Cyrena carolinensis, 

 and more especially the Rangia cyrenoides of Des Moulins, 

 extends along the whole shore of the Gulf of Mexico from 

 Pensacola to Franklin in Louisiana, bends round Mobile 

 Bay, Lake Poutchantrain, and ranges across the delta of the 

 Mississippi immediately above its marshes, a total distance 

 of nearly three hundred miles, and probably much further ! 

 It is remarkable that the shells " occur in beds with scarcely 

 any admixture of sand or earth, and they are consequently 

 found extremely useful in repairing roads, and paving the 

 streets of the city. They are dug from the surface of the 

 soil, both on the main shore and the islands of the bay. 

 These deposits border the bays of the Gulf of Mexico be- 

 tween Mobile and New Orleans, and they occur in the vici- 

 nity of Franklin, Louisiana. The Ohandeleur Isles, between 

 Mobile Bay and the delta of the Mississippi, consist of depo- 

 sits of these shells covered by a fertile soil. The Rangia 

 lives in vast numbers in the extensive flats below Mobile, 

 burrowing three or four inches beneath the surface of the 

 sand, in which numerous depressions indicate where they 

 are to be found." * 



There is reason, therefore, to believe that the exuvial 

 coverings of the mollusca continue to be heaped up in the 

 dark unfathomed caves of ocean, to become, at some future 

 period, and under the pressure and influence of conjectural 

 agencies, the material constituents of calcareous banks or 



* Fourth Report of Brit. Association, 14, 30. Near Valparaiso, there are 

 great beds of shells, which are elevated some yards above the level of the sea. 

 ' They nearly all consist of one species of Erycina ; and these shells at the 

 present day live together in great numbers on the sandy flats. So wonderfully 

 numerous are those forming the beds, that for years they have been quarried 

 and burnt for the lime, with which the large town of Valparaiso is supplied." 

 C. DARWIN, Voy. of Adv. and Beagle, iii. 310. See also vol. ii. p. 421. 



