CHAP. XXVIII.] HEAPS OF GNATHODOX. 135 



high water, and two feet below the lowest water of 

 the Mississippi. It is said to have become sensibly 

 shallower in the last forty years, its depth being now 

 fourteen or fifteen feet only, for it receives annual 

 supplies of mud from the Mississippi, poured into it 

 by one of its mouths, called the Iberville river. 



The south-east wind sometimes drives the salt 

 water into the great lagoon, and raises its level from 

 five to ten feet. On a mud-bank near the shore 

 I observed the living Gnathodon, accompanied by a 

 modiola (Dreissena ? ), and -there was a small bank 

 of dead shells on the southern borders of the lake, 

 which may have been thrown up by the waves in a 

 storm, the valves of most of them being separate. 

 I learnt that the road materials before spoken of 

 were procured from the east end, where there is an 

 enormous mound of dead shells, a mile long, fifteen 

 feet high, and from twenty to sixty yards broad. 

 Dr. Riddell, Director of the Mint at New Orleans, 

 estimates the height of some of these shell-banks 

 north of the lake, at twenty feet above its level ; yet 

 he thinks they may have been washed up by the 

 waves during storms. I suspect, however, that some 

 change in the relative level of land and sea has taken 

 place since their accumulation. Dr. M Cormac in 

 formed me that he had observed heaps of these same 

 shells recently cast up along the margin of the bay 

 called the Sabine Lake, where the waters of the 

 delta are brackish. 



Returning to the bayou, we passed a splendid 

 grove of live oaks on the Metairie ridge, supposed 

 by some to be an old bank of the Mississippi. These 



