262 Mr. A. Tylor on Changes of the Sea-level effected by 



ai*e the same in this district at the same period of two cousecU" 

 tive 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 vapour in parts of the earth beyond the confines 

 of the tract of country under consideration ; since, if the 4 inches 

 of rain annually cai-ried into the Gulf of Mexico were not replaced 

 from foreign sources, the levels of the rivers, lakes, and springs 

 must rapidly fall. 



The estimate of denudation obtained from these countries may 

 be incorrect when applied to other lands differing in altitude and 

 receipt of rain. Besides, many rivers empty themselves into 

 lakes and inland seas, and other extensive tracts are entirely 

 without rain. Since there must thus be extensive districts which 

 contribute no detritus whatever to rivers, I propose to assume 

 that one-half the earth's surface 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 moun- 

 tains, and only a moderate quantity of rain). 



The quantity of soluble salts annually carried into the ocean 

 must amount to a veiy 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 suspension 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 suspended in the water of the Mississippi 

 is as 1 to SOOOf. 



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 sus- 

 pension ; 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 quantity separated 

 from solution, the same eff*ect wiU be produced upon the sea-levei 

 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 



* Bv reference to Johnston's Physical Atlas, the calculated proportion 

 of land drained by rivers running into European lakes and inland seas may 

 be seen. 



t For the statistics of the Mississippi River see Sir Charles Lyell's Second 

 Visit to the United States, edit. 1847, vol. ii. p. 249 to 253, and other 

 places. 



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

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

 equals 1 10,875,000 square geographical miles. 



