14 FOURTH REPORT — 1834. 



rolinensis, and Rangia cyrenoides of Des Moulins ( Clathrodon 



cuneatus, Gray) ; the former, however, is rare, the deposit con- 

 sisting almost entirely of the latter shell. In the vicinity of 

 Mobile, which is built on a sandy flat, very little elevated above 

 the tide, the beds in question are superficial, although co- 

 vered by a vegetable mould bearing a forest of gigantic pines. 

 When one of the trees is prostrated by the wind, the decom- 

 posing shells are seen adhering to the roots, but beneath they 

 are entire, and nearly as hard, when dry, as the recent species. 

 It is remarkable that they occur in beds with scarcely any ad- 

 mixture of sand or earth, and they are consequently found 

 extremely useful in repairing roads, and paving the streets of 

 the city. Thejr 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 between Mobile and 

 New Orleans, and they occur in the vicinity of Franklin, 

 Louisiana. The Chandeleur Isles, between Mobile Bay and 

 the delta of the Mississippi, consist of deposits of these shells 

 cov^ered 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 numei'ous de- 

 pressions indicate where they are to be found." According to 

 Mr. Conrad, the Rangia was first seen in a sub-fossil state in 

 the newer Pleiocene, at the mouth of the Potomac, where, how- 

 ever, it is rare. Though it there occurs in a deposit of marine 

 shells, the sea appears not to be the usual resort of the species; 

 and it is only in the brackish water in the bays and estuaries 

 that it is abundant. He is therefore inclined to regard the few 

 found in marine deposits as coming from some neighbouring 

 estuary. As it abounds in the recent state in the present shel- 

 tered sounds which fringe the Gulf of Mexico, the presumption 

 is very strong that the fossil beds above described are colonies 

 which, previous to the change of level of the land, flourished in 

 precisely similar situations. This would account satisfactorily 

 for the narrow and very long belts in which they run, skirting 

 round the bays and the coast above its present marshes, from 

 Pensacola, in Florida, to near Franklin, in Louisiana. 



Diluvial Actio7i over North America. — Almost the whole 

 surface of North America, as far as examined, may be said to 

 be covered with an investment of earth, pebbles, and boulders, 

 obviously of diluvial origin. The thickness of this deposit varies, 

 though its average depth may be said to be from ten to twenty 

 feet. All that low and level tract described as the Atlantic 

 plain, and also the lower sections of the great valley of the Mis- 

 sissippi, appear to be the districts where it conceals the under- 



