EROSION OF RAIN AND RIVERS. 11 



basin in one year. To this must be added the soluble matters, which 

 are about one sixth as much as the insoluble. 



Estimates of this kind have been made for two great rivers, viz., the 

 Ganges and the Mississippi. The whole amount of sediment annually 

 carried to the sea by the Ganges has been estimated as 6,368,000,000 

 cubic feet. This amount, spread over the whole basin of the Ganges 

 (400,000 square miles), would make a layer y^y of a foot thick. The 

 Ganges, therefore, erodes its basin one foot in 1,751 years.* The area 

 of the Mississippi basin is 1,244,000 square miles. The annual dis- 

 charge of sediment, according to the recent and accurate experiments 

 of Humphrey and Abbot, is 7,471,411,200 cubic feet, a mass sufficient 

 to cover an area of one square mile, 268 feet deep.f This spread over 

 the whole basin would cover it ygVo °^ a ^ 00 ^ Therefore, this river 

 removes from its basin a thickness of one foot in 4,640 years. The 

 cause of the great difference in favor of the Ganges is, that this river 

 is situated in a country subject to very great annual fall of water, the 

 whole of which falls during a rainy season of six months. The rains 

 are therefore very heavy, and the floods and consequent erosion propor- 

 tionately great. The erosive power of this river is still further increased 

 by the great slope of the basin, as it takes its rise in the Himalaya, the 

 highest mountains in the world. 



Now, since continents may be regarded as made up of hydrographi- 

 cal basins, the average rate of their erosion may be determined either by 

 making similar experiments on all the rivers of the world, or, since this 

 is impracticable, by taking some river as an average. We believe the 

 Mississippi is much nearer an average river than the Ganges. It can 

 hardly be less than the average, for a considerable portion of the earth 

 — as rainless deserts — is not subject to any erosion. It is probable, 

 therefore, that the whole surface of continents is eroded at a rate not 

 exceeding one foot in 4,640 years. For convenience we will call it one 

 foot in 5,000 years. We will use this estimate when we come to speak 

 of the actual erosion which has occurred in geological times. 



Law of Variation of Erosive Power. — The erosive power of water, or 

 its power of overcoming cohesion, varies as the square of the velocity of 

 the current (p oc v" 2 ). The velocity depends upon the slope of the bed, 

 the depth of the water, and many other circumstances, so numerous and 

 complicated that it has been found impossible to reduce it to any simple 

 law. The angle of slope, however, is evidently the most important cir- 

 cumstance which controls velocity, and therefore erosive power. In the 

 upper portions of great rivers, like the Mississippi, the erosion is very 

 great ; while in the plains near the mouth there may be no erosion, but, 



* Philosophical Magazine, vol. v, p. 261. 



f Humphrey and Abbot, Report on Mississippi River, pp. 148-150. 



