

176 THE WORLD OF LIFE 



CHAP. 



portion of lowland 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 consequence lowers the land perhaps more 

 rapidly than any important river on the globe. On the 

 whole, we may take these rivers as fairly representative. 

 Their mean rate of denudation 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 we can even see and hear the process con- 

 tinually going on. Under every precipice there is a more 

 or less extensive 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' down- 

 pour of rain, innumerable trickles of muddy water course 

 down in every direction ; while every streamlet or brook — 

 though usually of water as clear as crystal — becomes a rapid 

 torrent of mud-laden water. It is by a consideration of 

 these every-day phenomena in operation over every square 

 yard of thousands of square miles of surface that we are able 

 to understand and appreciate the tremendous power of rain 

 and rivers, greatly assisted by frost, in the disintegration of 

 rocks, which lower the whole surface of the land at such a 

 rate that, if we had means of accurate comparison with its 

 condition a few thousand years ago, we should see that in 

 many places the whole contour and appearance of the 

 surface was changed. 



