190 



DYNAMICAL GEOLOGY. 



animals, that have been surprised and drowned by freshets, or bones that 

 have been exhumed by the waters. 



The amount of transportation going on over a continent, especially in 

 seasons of floods, is beyond calculation. Streams are everywhere at work, 

 rivers with their large tributaries, and their thousands of little ones spreading 

 among all the hills and to the summit of every mountain; and thus the 

 whole surface of a continent is on the move toward the oceans. The amount 

 transported is a measure of the amount 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 and Abbot, is about 



the weight of the water, or 



Ratio. 



Time. 





1808 



12 mos., 1851-1852. 





1449 



12mos., 1852-1853. 





1.S21 



9 mos., 1858. 





1256 



2 mos., 1838. 





: 1724 



1838. 





: 1245 



14 days, summer of 1843, 





: 1155 



35 days, summer of 1846. 





: 17000 



1844. 





2000 



4 mos., 1808-1809. 





2500. 







858, 



at flood-time. 



150 ""^ .. v^igjiiu yjj. U.U.V.. yyuuy^j., Kji. 2 9 0¥ ^^^ 



bulk; equivalent for an average year to 812,500,000,000,000 pounds, or a 

 mass one square mile in area and 241 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. 



Mississippi Kiver, at CarroUton, by Delta Survey, 

 Mississippi River, at CarroUton, by Delta Survey, 

 Mississippi River, at Columbus, by Delta Survey, 

 Mississippi River, at Moutlis, by Mr. Meade, 

 Mississippi River, at Mouths, by Mr. Sidell, 

 Mississippi River, at various places, by Prof. Riddell, 

 Mississippi River, at New Orleans, by Prof. Riddell, 

 Rhone, at Lyons, by Mr. Surell, 

 Rhone, at Aries, by Messrs. Gorsse and Subours, 

 Rhone, in Delta, by Mr. Surell, 

 -Ganges, 



For the Danube, the ratio at low water is 1 : 33,000 ; at flood, 1 : 2400; for the Po, at 

 flood, 1 : 300 (Lombardini) ; for the Meuse, at low water, 1 : 71,420 ; at flood, 1 : 2100 

 (Chandellon) ; for the Irrawaddy, at low water, 1 : 5725 ; at flood, 1 : 1700 (Login) ; for 

 the La Plata at Buenos Ayres, 1 : 7752, at which rate it carries seaward about 224,000 tons 

 of sediment each 24 hours, but dropping part of it along the 100 miles before it reaches 

 the sea (Higgin). 



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

 cubic feet, or 378,100,000 tons. The Nile brings down annually nearly 150,000,000 tons. 

 The bulk may be calculated, by taking 1-9 as the specific gravity of the material. 



Besides the material held in suspension, the Mississippi pushes along into 

 the Gulf large quantities of earthy matter ; and the annual amount thus 

 contributed to the Gulf is estimated 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 mentioned, makes the total 268 feet. 



This amount is equivalent to an average of -^^-^ of a foot 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 4920 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 1880 years. All the rivers that enter 

 the ocean or the seas over the land, are working in the same way, and with 

 results to the continental surface mostly between these two extremes. 



