648 DYNAMICAL GEOLOGY. 



called silt, or detritus. In accordance with the law with regard to the 

 transporting power of water, stones and pebbles make the bed of rapid 

 streams, and in general earth or silt, where the current is slow. 



The amount of transportation going on over a continent is beyond 

 calculation. Streams are everywhere at work, rivers with their large 

 tributaries and their thousand 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. In the rainy sea- 

 sons, the streams increase immensely their force, streamlets in the 

 mountains, that are almost dry in summer, becoming destructive tor- 

 rents during the rains. 



The process of transportation is also one of wear. The stones are 

 reduced to sand and fine earth, by the friction. The silt is nothing but 

 the coarse material of the upper waters, ground up. The soil of the 

 plains and sand of the sea-shore are the pulverized rocks of the moun- 

 tains, — running. waters being the moving-power, and the mutual fric- 

 tion of stone upon stone, or grain of sand upon grain, the means of 

 grinding. The word detritus means worn out, and is well applied to 

 river-depositions. On large rivers, stones and pebbles disappear from 

 the alluvium, long before they reach the sea, and partly for the reason 

 here mentioned. The process is sometimes aided by the partial de- 

 composition of the rocks. 



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

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

 1-1 500th the weight of the water, or l-2900th its bulk ; equivalent 

 for an average year to 812,500,000,000 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. 



Mississippi R., at Carrollton, by Delta Survey, 1 : 1808 12 mos., 1851-1852. 

 Mississippi R., at Carrollton, by Delta Survey, 1 : 1419 12 mos., 1852-1853. 

 Mississippi R., at Columbus, by Delta Survey, 1 : 1321 9 mos., 1858. 

 Mississippi R., at Mouths, by Mr. Meade, 1 : 1256 2 mos., 1838. 



Mississippi R., at Mouths, by Mr. Sidell, 1 : 1724 1838. 



Mississippi R., at Various -places, Prof. Riddell, 1 : 1245 14 days, summer of 1843. 

 Mississippi R., at New Orleans, Prof. Riddell, 1 : 1155 35 days, summer of 1846. 

 Rhone, at Lyons, by Mr. Surell, 1 : 17000 1844. 



Rhone, at Aries, Messrs. Gorsse & Subours, 1 : 2000 4 mos., 1808-1809. 



Rhone, in Delta, Mr. Surell, 1 : 2500 



Ganges, by Mr. Everest, 1 : 510 12 mos. 



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



The total annual discharge of sediment from the Ganges has been estimated at 

 6,368,000,000 cubic feet. 



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



