58 



perceptible alteration in the sea-level in a moderate space of 

 time. 



The mere consideration of the number of cubic feet of detritus 

 annually removed from any tract of land by its rivers, does not 

 produce so striking an impression upon the mind as the state- 

 ment of how much the mean surface level of the district in 

 question vcould be reduced by such a removal. This informa- 

 tion 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 laud 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 (supei-ficial 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 Ganges. The number 

 of cubic feet of detritus discharged annually by that river is 

 6,368,677,400. (See Lyell's Principles.) 



6,368,677,400 1 .i .i i r 



T-- I -.r-, ^r.r, r.r.r. ,„^r. = T^£^ \ conscquently the reduction 

 11,151,860,000,000 1751 ' i 



of the mean level of the Ganges district is , ^,^ of a foot annu- 

 ls ol 



ally, or 1 foot in 1751 years. 



6,368,677,400 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. 



ON THE CHANGES OF THE SEA-LEVEL. [1854. 



Mexico were not replaced from foreign sources, the levels of f 

 the rivers, lakes, and springs must rapidly fall. 



5,444,074,288,640 



= about i a foot, so that the mean 



11,151,360,000,000 

 annual 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 computed 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 =:r 

 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 supei-ficial feet contained in the area of 1,100,000 square 

 miles drained solely by the Mississippi. 



11,108,275,200,000 



foot = J foot nearly. Consequently the 



30,666,240,000,000 

 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 are the same in this district at the same period of two 

 consecutive years, the water sufficient to produce the above- 

 mentioned 4 inches of the total of rain-fall upon the whole of 

 this district must have been annually derived from clouds 

 which have been charged with vapor in parts of the earth 

 beyond the confines of the tract of country under consideration ; 

 since if the 4 inches of rain annually carried into the Gulf of 



The estimate of denudation obtained from these countries 

 may be incorrect when applied to other lands differing in alti- 

 tude and receipt of rain. Besides, many rivers empty them- 

 selves into lakes and inland seas, and other extensive tracts are 

 entirely without rain. Since there must be extensive districts 

 which contribute no detritus whatever to rivers, I propose to 

 assume that one half the earth's surfice only is drained by 

 rivers flowing directly into the sea,* and that the average 

 supply of detritus does not exceed that afforded by the district 

 through which the Mississippi flows (a country where there 

 are no very high mountains, and only a moderate quantity of 

 rain). 



The quantity of soluble salts annually carried into the ocean 

 must amount to a very large volume, particularly as river- 

 water always contains matter in solution, while it is only 

 during two or three months of the year that alluvium in sus- 

 pension is carried down in large quantities. The proportion of 

 soluble salts in the water of the Thames is 17 to 70,000, or 1 

 to 4117 ; while the proportion of alluvium srrspended in the 

 water of the Mississippi is as 1 to 3000. y 



The level of the land is as much reduced by what is carried 

 away in solution, as if this were mud and sand removed in 

 suspension; and, a submarine deposit formed from materials 

 brought into the sea in solution will displace a volume of water 

 equal to their former bulk ; and therefore, when the annual 

 supply of soluble salts to the ocean does not exceed the quan- 

 tity separated from solution, the same effect will be produced 

 upon the sea-level by matter introduced, whether it be in 

 solution or suspension. While the proportion of the land to 

 the ocean remains as 1 to 3, J it is evident, that a reduction of 

 3 feet in 'the mean surface-level of the land must take place by 

 denudation before a volume of detritus would be conveyed into 

 the sea sufficient to displace enough water to occasion an ele- 

 vation of one foot on the ocean-level. 



There is great need of further information respecting the 

 amount of sediment carried down by other rivei's besides those 

 mentioned ; yet if the rate of denudation obtained from the 

 statistics of the Ganges and Mississippi be any guide to what 

 is occurring on the remainder of the globe, we cannot suppose 

 that an indefinite time would be required for the performance 

 of a denudation, which should reduce the mean surface-level of 

 the land 3 feet and raise that of the ocean 1 foot. It was 

 during the contemplation of the changes of level that might 

 have been produced by the operations of ordinary physical 

 agents upon the surface of the earth, that Hutton was led to 

 remark that it was not necessary to suppose the area of the 

 land always maintained the same extent, but that from time to 

 time new land would be formed by the elevatory movements 

 of the sea-bottom to compensate for what had been carried into 



* By reference to Johnston's Physical Atlas, the calculated pro- 

 portion of land drained by rivers running into European lakes and 

 inland seas may be seen. 



f For the statistics of the Jlississippi Eiver, see Sir Charles Lyell's 

 Second Visit to the United States, edit. 1S47, vol. ii, p. 249 to 253 and 

 other places. 



X M. Balbi shows (Atlas, Soc. Difif. Useful Knowledge, 1844) that 

 the land on the globe equals 37,647,000 square geographical miles, the 

 sea equals 110,875,000 square geographical miles. 



