APPENDIX. 11 



thence up the Red RiVer to Cotile, and down Cotile and Bayou 

 Bluff to the Cocodrie, and with its riglit bank and the Zeche and 

 Vermilion to the Gulf of Mexico, longitude 92° west. 



The eastern limit on the Gulf is at longitude 89°, and the width 

 of the basin on the Gulf front is nearly 180 miles, while at 

 Natchez it is 28 miles; at Gaines' Landing, lat. 33° 30', it is 

 90 miles, and but little less above that point to near the mouth 

 of Ohio River. 



Exceptions. — Within this area — nearly all below the level of 

 the Mississippi's high water, there are various areas that are 

 not al'luvial. Some ridges have considerable length, such as the 

 Bloomfield in Missouri, and the Crowley Ridge in Arkansas, 

 more than 100 miles long and very narrow, lying between the 

 St. Francis and Black rivers ; also the Bastrap and Macon Hills 

 in north Louisiana, and the Sicily Island and Avoyilles on 

 Ouachita and Red Rivers. All these are subtracted in com- 

 puting the area of the alluvial basin, still leaving a tract of land 

 with an average of 12 feet below the highest water mark of tile 

 Mississippi River at points opposite, and measuring 88,706 

 square miles. 



This is the extent in greatest floods of the Fresh water Delta 

 Sea. 



Apparent Geological History. — The testimony left on the 

 features of the land shows the drift beds in the form of bluffs on 

 both sides the basin, and of indefinite lateral extent. The drain- 

 age of the great hydrographic basin has unquestionably eroded 

 the bed now occupied by the alluvion; and whether in concen- 

 trated form as now, or in several or many parallel streams, the 

 Mississippi has cut out for itself the delta basin, and tlirown 

 down upon its lap the varied detritus brought from the abrasions 

 of its many tributaries. 



The diluvial beds thus worn away lie upon a tertiary, at various 

 points exposed on either side of the basin. The depth to which 

 the bed has been cut (the thickness of the alluvion) is ascertained 

 by only a very few sections, but sufficient to warrant the conclu- 

 sion for all its northern half, say above lat. 32° 30', that this 

 depth is rarely greater than the deepest portions of the present 

 shifting chatmel, about 100 feet. 



Ample testimony exists, and it is everywhere observable, that 

 the river changes its bed with ease and rapidity, showing that 

 the material removed to give the channel-way is of recent alluvial 

 deposit, and that at least so far as these changes have occurred, 

 the depth of the alluvion has for a minimum 100 feet or more. 



At points further down, we know not at what limit, a bay from 

 the Gulf has been gradually filled with alluvial material, the 

 marine waters being driven back, and the deposits cast on the 

 bottom of the sea, until at the present age, the alluvion has; 

 greatly invaded the basin proper of the Gulf of Mexico. 



(11) 



