190 THE WORLD OF LIFE 



We can easily see here that the rapidity of denudation is 

 proportionate to the height and extent of the mountain-ranges 

 in which the river has its sources, combined with the amount of 

 the average rainfall, and the proportion of plains to uplands 

 in its whole basin. The Ganges has a large proportion of low- 

 land plain in its area; the Hoang-Ho has less, and therefore 

 denudes more rapidly. The Danube and the Mississippi both 

 drain an enormous area of lowlands where denudation is slight, 

 and the rainfall of both is moderate; they therefore lower 

 their basins slowly. The Po drains an enormous extent of 

 snowy Alps in proportion to its whole basin, and in conse- 

 quence lowers the land perhaps more rapidly than any impor- 

 tant river on the globe. On the whole, we may take these 

 rivers as fairly representative. Their mean rate of denuda- 

 tion is very nearly one foot in three thousand years, and we 

 may therefore, till more complete observations are made, take 

 this as a measure of the average rate of denudation of most of 

 the great continents. 



Of course, the rate of lowering will be extremely unequal, 

 being at a maximum in the mountains and a minimum in 

 the plains, where it may not only be nothing at all, but if 

 they are flooded annually they may be raised instead of 

 lowered. In the loftier mountains with numerous peaks and 

 precipitous slopes the average lowering may often be ten times, 

 and sometimes even a hundred times, the mean amount. In 

 such districts w^e can even see and hear the process continually 

 going on. Under every precipice there is a more or less ex- 

 tensive mass of debris — the " screes " of our lake district ; 

 and every winter, chiefly through the action of rain and frost, 

 the rocks above are split off, and can be heard or seen to fall. 

 Even on grassy hills after a few hours' downpour of rain, in- 

 numerable trickles of muddy water course down in every di- 

 rection ; while every streamlet or brook — though usually of 

 water as clear as crystal — becomes a rapid torrent of mud- 

 laden w^ater. It is by a consideration of these every-day phe- 

 nomena in operation over every square yard of thousands of 



