136 



PHYSICAL GEOLOGY 



years. The Ganges Basin is being reduced by the removal of 

 material in the solid form alone, at the rate of a foot in less 

 than 2000 years ; and the Danube Basin a foot in approxi- 

 mately 6800 years. In some parts of each basin the rate is far 

 more rapid. Figure 124 shows the results of recent estimates 

 of the rate of land reduction throughout the United States. 



FIG. 124. Rates of land reduction by stream erosion in the United States. 

 The figures are the number of years required for one inch of denudation. 

 (After National Conservation Commission.) 



The average elevation of the continents above sea level is 

 about 2300 feet. If they were being degraded by streams at 

 the average rate of the Mississippi Basin, and continued to be 

 cut down at that rate to sea level, and nothing occurred to off- 

 set the work of the streams, the lands would be destroyed in 

 about 8,000,000 years. Probably, however, the average 

 rate of land reduction is less than that assumed. Nor could 

 the present rate, whatever it may be, continue till the land 

 was at sea level, for as it gets lower, the streams would flow 

 more slowly, and therefore degrade less rapidly. Again, judg- 

 ing by the past, diastrophism and vulcanism would intervene 

 to maintain the land masses. Still other factors would modify 



