THE REVOLUTIONS OF THE CRUST OF THE EARTH. 305 



products, as well as by free carbouic acid, and by other substances less 

 prevalent in the atmosphere. 



The debris of these rocks were carried into the sea by the rains, pro- 

 ceeding from the condensation of vapors. The waters charged with 

 material in suspension were united into streams and rivers, and emptied 

 into lakes and seas. The denudation of the islands and continents, 

 under the influence of atmospheric agents, is considerable. M. Croll * 

 has calculated how much the American continent has been denuded in 

 the entire Mississippi region. Knowing the quantity of material in sus- 

 pension in the water (y-Vu of its weight), the amount of water annually 

 emptied into the Gulf of Mexico, and the whole extent of the basin of 

 which the waters empty into this river, this savant found that in 4,566 

 years 1 foot (304 millimeters) of earth is carried into the ocean. Taking 

 into account the specific weight of the sediments (1.9) and of the crys- 

 talline rocks (2.5), we ought to bring the number of years to 6,000. 

 As the mean height of the American continent is estimated by Hum- 

 boldt at 748 feet (227 meters), the entire continent must at this rate be 

 leveled and buried in the sea in 4,000,000 years ; that is, if no ulterior 

 uprising takes place. This denudation is not everywhere the same. It 

 depends in general upon the mean elevation of the continent above the 

 level of the sea. Thus the denudation of India, at least three times more 

 rapid than that of !North America, is easily accounted for by the great 

 elevation of the Himalayan regions, which send down their waters 

 through the tributary rivers of the Ganges and the Indus, and by the 

 very frequent atmospherical precipitations which are the consequence 

 of this elevation. The sources of the Ganges are found at a height of 

 3,962 meters, (2£ miles). 



If we consider the effects of denudation during a time which is only a 

 small fraction of the geological epochs, we may well ask how many 

 times whole continents have been carried into the sea, to be sent forth 

 again by the central forces of the earth's nucleus. We readily perceive 

 that the various sediments have been several times destroyed and re- 

 formed by the effect of denudation ; those primitive beds which have 

 remained uninterrupted in the succession of their deposits owe their 

 preservation to several circumstances, particularly to their being sub- 

 merged, until the more recent epochs. 



Speaking in general terms, we may say that the entire mass consti- 

 tuting all the mountains and continents, of a mean elevation of 307 

 meters (1,000 feet) above the level of the sea,t has been diminished by 

 an amount of material sufficient to fill up an abyss six times the depth 

 of the ocean. It must not be forgotten, however, that the continents 

 are continually rising, and that certain parts of the ocean may, by up- 

 heaval, become land ; likewise the continents may have been lowered at 

 certain points, and submerged for a time, to appear again regenerated 

 and enriched with new formations of strata. 



* On Geological Time, etc., Philosoph. Magazine, vol. xxxv, 1668, i>. 379. 

 t Humboldt, dsie Centrale, t. 1, pp. 182, 1S7. 

 S 20 



