WATER. 643 



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

 that have been exhumed by the waters. 



The fine earthy material deposited by streams, or their sediment, is 

 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 be- 

 yond calculation. Streams are everywhere at work, rivers with 

 their large tributaries and their thousand little ones spreading 

 among all the hills and to the summits of every mountain. 

 And thus the whole surface of a continent is on the move towards 

 the oceans. In the rainy seasons the streams increase immensely 

 their force. Streamlets in the mountains that are almost dry in 

 summer become destructive torrents 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 mountains, — running waters being the moving-power, and 

 the mutual friction 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 decomposition 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 l-1500th the weight of the water, or l-2900th its bulk ; equi- 

 valent for an average year to 812,500,000,000 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 ob- 

 tained 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., , 51- , 52. 



" " " 1 : 1449 12 mos., '52-'53. 



" Columbus, " 1 : 1321 9 mos., '58. 



" Mouths, by Mr. Meade, 1 : 1256 2 mos., '38. 



" " " Mr. Sidell, 1 : 1724 1838. 



" Various places, Prof.RiddelLl : 1245 14 days, summer of 1843. 



New Orleans, " 1 : 1155 35 days, summer of 1846. 



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



" " Aries, Messrs. Gorsse & Subours, 1 : 2000 4 mos., 1808-9. 

 " in Delta, Mr. Surell, 1 : 2500 



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



