188 AGE OF DELTA OF MISSISSIPPI. [CHAP. XXXIV. 



tity of mud held in suspension, increases regularly with the in 

 creased height arid velocity of the stream. On the whole, com 

 paring the flood season with that of clearest water, his experi 

 ments, continued down to 1849, give an average annual quantity 

 of solid matter somewhat less than his first estimate, but riot va 

 rying materially from it. From these observations, and those of 

 Dr. Carpenter and Mr. Forshey (an eminent engineer, to whom 

 I have before alluded), on the average width, depth, and velocity 

 of the Mississippi, the mean annual discharge of water and sedi 

 ment was deduced. I then assumed 528 feet, or the tenth of a 

 mile, as the probable thickness of the deposit of mud and sand in 

 the delta ; founding my conjecture chiefly on the depth of the 

 Gulf of Mexico between the southern point of Florida and the 

 Balizc, which equals, on an average, 100 fathoms, and partly 

 on some borings, 600 feet deep, in the delta near Lake Pont- 

 chartrain, north of New Orleans, in which the bottom of the 

 alluvial matter is said not to have been reached. The area 

 of the delta being about 13.600 square statute miles, and the 

 quantity of solid matter annually brought down by the river 

 3,702.758,400 cubic feet, it must have taken 67,000 years for 

 the formation of the whole ; and if the alluvial matter of the 

 plain above be 264 feet deep, or half that of the delta, it must 

 have required 33,500 more years for its accumulation, even if its 

 area be estimated as only equal to that of the delta, whereas it is 

 in fact larger. 



From information since received, I think it not improbable 

 that the quantity of water may have been underrated in this 

 estimate ;* and, if so, a larger amount of sediment would have 



* I allude chiefly to the observations and experiments, on the velocity of 

 the Mississippi at various depths, made by Mr. W. H. Sidell. during a Gov 

 ernment survey, communicated to me through the kindness of Mr. Ruggles, 

 of New York, which, if correct, would lead to the inference that the average 

 number of cubic feet of water discharged into the Gulf per second, is con 

 siderably greater than Mr. Forshey and Dr. Carpenter deduced from their 

 observations on the velocity of the stream at different depths. If, as I un 

 derstand, there exist documents in the hydrographer s office at Washington, 

 which would afford more ample data for such calculations, the Government 

 would confer a boon on the scientific world by publishing them without 



