656 



DYNAMICAL GEOLOGY. 



lost by the land, as well as of that gained by the river plains, lakes, 



and seas. 



The amount of silt carried to the Mexican Gulf by the Mississippi, 



according to the Delta Survey under Humphreys & Abbot, is about 



l-1500th the weight of the water, or l-2900th its bulk ; equivalent for 



an average year to 812,500,000,000,000 pounds, or a mass one square 



mile in area and two hundred and forty-one feet deep. 



The following table contains the ratio of sediment to water by weight, as obtained 

 by the Delta Survey, and also the results of other investigations. It is from Humphreys 

 & Abbot's Report (p. 148) : — 



Ratio. Time. 



12 mos., 1851-1852. 

 12 mos., 1852-1853. 

 9 mos., 1858. 

 2 mos., 1838. 

 1838. 



14 days, summer of 1843. 

 35 days, summer of 1846. 

 17000 1844. 

 2000 4 mos. 

 2500 

 858 



1808 

 1449 

 1321 

 1256 

 1724 

 1245 

 1155 



1808-1809. 



Mississippi R., at Carrollton, by Delta Survey. 

 Mississippi R., at Carrollton, by Delta Survey, 

 Mississippi R., at Columbus, by Delta Survey 

 Mississippi R., at Mouths, by Mr. Meade, 

 Mississippi R., at Mouths, by Mr. Sidell, 

 Mississippi R., at Various places, Prof. Riddell, 1 

 Mississippi R., at New Orleans, Prof. Riddell, 1 

 Rhone, at Lyons, by Mr. Surell, 1 



Rhone, at Aries, Messrs. Gorsse & Subours, 1 

 Rhone, in Delta, Mr. Surell, 1 



Ganges, 1 



The bulk may be calculated, by taking 1.9 as the specific gravity of the material. 



The annual discharge of sediment from the Ganges has been estimated at 6,369,000,000 

 cubic feet, or 756,200,000,000 pounds. The Nile brings down annually nearly 300,000,- 

 000,000 pounds. 



Besides the material held in suspension, as these authors observe, 

 the Mississippi pushes along into the Gulf large quantities of earthy 

 matter ; and, from observations made by them, they estimate the an- 

 nual amount thus contributed to the Gulf to be about 750,000,000 

 cubic feet, — which would cover a square mile 27 feet deep ; and this, 

 added to the 241 feet above, makes the total 268 feet. 



This amount is equivalent to an average of 4^0 °f a f° ot annually 

 from the whole drainage area of the river ; or, in other words, the area 

 would be lowered by it, on an average, one foot in 4,920 years. The 

 Ganges works faster, the amount it transports to the sea being such as 

 would lower its drainage area, on an average, a foot in 1 ,880 years. 

 All the rivers that enter the ocean or the seas over the land, are work- 

 ing in the same way, and with results to the continental surface mostly 

 between these two extremes. 



Mr. T. Mellard Reade makes out that the water which annually runs off from the 

 area of England and Wales, about 68,451,000,000 tons, carries to the sea 8,370,630 tons 

 of solids in solution, or 1,223 parts in every 10,000 of water, consisting of about 0.95 of 

 calcium and magnesium carbonates and sulphates, 0.166 of sodium chlorid, and the 

 rest nitrates, sodium carbonate, alkaline sulphates, silica and iron sesquioxyd ; and at 

 15 cubic feet to the ton, the denudations thus occasioned would equal one foot in 12,978 

 years. Prestwich obtained in a similar calculation, one foot in 13,000 years for the cal- 

 cium carbonate carried off by the Thames from the chalk, greensand, and oolitic for- 

 mation. The total annual denudation for England, from this source alone, is thus esti- 



