Ill APPENDIX. 



The protrusion along the margin of the Gulf is manifestly 

 eighty miles or more, if we assume the line of the convex shore 

 of the Gulf to be continued from west of Vermilion Bay to the 

 shore of the Mississippi Sound at Bay St. Louis. This line 

 passes through the city of New Orleans. 



In the progress of the stupendous work of filling the whole 

 Gulf area* of 700,000 square miles, and of average depth of 

 3000 feet or more, by the Mississippi and its auxiliary streams, 

 the delta we are now considering has projected beneath the pre- 

 sent Gulf surface an enormous plateau with less than ten fathoms, 

 water soundings, which extends from five miles outside the pre- 

 sent passes of the Mississippi, westward, and has a breadth in 

 front of the Caillon and Atchafalaya of more than sixty miles, 

 and only approaches the shore west of the Vermilion Bay, the 

 western limit of the delta proper. 



Judging from the bottom samples brought up by the sound- 

 ings, the alluvial materials characteristic of the Mississippi's con- 

 tributions lie upon and form the bottom of the Gulf for some 

 seventy miles still further advanced, in a crescent southward and 

 eastward of this ten-fathom curve, even to where the soundings 

 reach beyond 500 fathoms. 



This, then, is the provisional southward or Gulf limit of the 

 great alluvion. 



Topography op the Basin. — The river's banks are everywhere 

 the highest ground within the basin, including in this description 

 of the banks all the shores of the Old River lakes which are so 

 numerous along its tortuous course. 



And as the river has elevated or built up the alluvion by drop- 

 ping upon its lap the sediraentar}'- and transported contributions 

 eroded from the great hydrographic basin, it will be obvious to 

 the student that the highest portions of these banks are found to 

 be nearly on the level (a little below) the greatest high-water 

 marks, as we shall see below from the chapter on the physics of 

 the river. 



A line of levels transverse to the current, which runs mainly 

 southward, shows everywhere an undulating surface to tiie ground ; 

 for "the delta is everywhere threaded by interlocking bayous 

 and navigable channels, placing every cultivable acre of land 

 immediately upon or very near to steamboat navigation. In this 



* Accordins: to Humphreys and Abbot, the contributions of the river, 

 as held in susf)ensioii, would raise a square mile 241 feet liigh. Add five 

 times this aitioiint for material pnslied forward on the river bottom, and we 

 liave 241 X 6 = 1446' as the height of the one mile block. With a gulf 

 depth of 3000 feet our contributions would fill half a square mile per year, 

 and would fill the (Julf area of 700,000 square miles in 1,400,000 years. But 

 as the other streams discharging into the Gulf bear probably one-fourth 

 the amount of sediment furnished by the Mississippi, this period would b© 

 reduced to nearly a million years. 



(12) 



