EROSION BY RIVERS 247 



The distance necessary for rock fragments to travel before they 

 become completely destroyed varies with the character of the rock, 

 as shown in the following table : 



Rhaetic sandstone (Average weight, 40 grams) 15 km. 



Clay slate (Average weight, 24 grams) 42 km. 



Orthoceras limestone . . (Average weight, 61 grams) 64 km. 



Granular limestone. . . . (Average weight, 40 grams) 85 km. 



Granite (Average weight, 36 grams) 278 km. 



Rate of erosion. From the known area of the hydrographic 

 basin of a river, and the measured amount of transportation of the 

 material in the river, it is possible to arrive at a reasonably accurate 

 estimate of the rate of erosion of the river system in question. 

 Thus the Mississippi system, with a hydrographic basin 1,244,000 

 square miles in area and an annual discharge of sediment of 

 7,471,411,200 cubic feet of sediment, erodes at the rate of one foot 

 in 4,640 years ; while the Ganges system, with a hydrographic basin 

 of only 400,000 square miles and an estimated annual discharge of 

 sediment of 6,368,000,000 cubic feet, erodes its basin one foot in 

 1,751 years. Other estimates make it i meter in 7,781 years, or i 

 foot in about 2,628 years. The greater efficiency of the Ganges is 

 attributable in large part to heavy rainfall during six months of the 

 year, and to the steeper slopes of the basin. 



Le Conte (43: 11), using the JMississippi basin as more typical 

 than the Ganges for the earth's surface as a whole, concludes that 

 the continent is probably lowered at the rate of one foot in 5,000 

 years. 



Transporting Poiver of River Currents. Streams move solid 

 material either by rolling it along the bottom or by carrying it in 

 suspension. Suspension of fine material is favored by minor up- 

 ward currents in the main current, while others carry it down again 

 or against the side of the channel. Suspended material is repeat- 

 edly dropped and picked up again, the solid particles making their 

 journey down the river with many interruptions. The total amount 

 of material transported by rivers is often very great. Thus the 

 Mississippi River carries more than 400,000,000 tons of sediment 

 each year to the Gulf of Mexico, or more than a million tons a day. 

 The exact volume, according to the measurements of Humphreys 

 and Abbot { ^^-.148-1 50), is 7,471,411,200 culjic feet, a mass suffi- 

 cient to cover an area of one square mile to a depth of 268 feet. 

 The amount carried to the sea by all the rivers of the earth has been 

 estimated at i)erhaps 40 times this quantity. (Salisbury-62 : /_'_'.) 



The following table (Kayser-39 : 575) gives the amounts of ma- 



