existing Physical Causes during stated Periods of Time. 261 



tion would be reduced by such a removal. This information 

 may be obtained by calculation from the published accounts 

 of the quantity of mud annually abstracted from districts of 

 known dimensions by their rivers. In this manner it is found 

 that the Ganges would in about 1751 years, at its present annual 

 rate, carry away from the land it drains (which is supposed to be 

 about 400,000 square miles) as much detritus as would cover 

 that area to the depth of one foot, as the following calculation 

 will show : — 



Thus, 27,870,400 (superficial feet in a mile) x 400,000= 

 11,151,360,000,000, the number of superficial feet in the area of 

 400,000 square miles drained by the Ganged. The number of 

 cubic feet of detritus discharged annually by that river is 

 6,368,677,400. (See LyelFs Principles.) 



6,368,677,400 1 ^.i .i. ^ .• c 



11,151,360,000,000 =1751; ^'^''^^'fy '^' ^^^^^^^^^ «f 

 the mean level of the Ganges district is -. of a foot annually, 



or 1 foot in 1751 years. 



6,368,677,440 cubic feet of mud discharged x 856 water to 

 mud =5,444,074,288,640= the number of cubic feet of water 

 annually discharged by the Ganges. 



5,444,074,288,640 i. . i r , .i. ..i. i 



11,J51,360,00 Q;00^ = "^"^^ * " ^""^^ '^ '^^^ '^' ^""^ ""^^"^ 

 discharge of water is equal to about 6 inches of rain on the whole 

 area of 400,000 square miles. 



The Mississippi, on the other hand, would occupy 9000 years 

 at its present annual rate in reducing to the amount of one foot 

 the mean surface-level of the district it drains, which is com- 

 puted at eleven hundred thousand square miles. The result is 

 obtained as follows : — 



If 3,702,758,400 cubic feet of mud are annually carried down 

 by the Mississippi (since the mud is to the water as 1 to 3000), 

 3,702,758,400 x 3000 = 11,108,275,200,000 = the number 

 of cubic feet of water annually carried by the river into the 

 Gulf of Mexico. The area of district drained by this river is 

 stated at 1,100,000 square miles-5280 x 5280=27,878,400 = 

 the number of superficial feet in a mile— 27,878,400 x 1,100,000 

 = 30,666,240,000,000 = the number of superficial feet con- 

 tained in the area of 1,100,000 square miles drained solely by 

 the Mississippi. 



11,108,275,200,000, , i, , , r .i .i, 



30,666,240,000,000 ^^^^ = * ^^^* ^'"'^y* Consequently the 

 water carried down by the river is equal to about 4 inches of 

 rain over the surface of land drained. 



If it be assumed that the levels of the rivers, lakes, and springs 



