AQUEOUS AGENCIES. 19 



floods of the rivers. Another part sinks into the earth, 

 and, after doing its appointed work of soil-making, re- 

 appears on the surface as springs, and forms the ordinary 

 flow of rivers in dry times. This part joins the surface 

 drainage, and together they concentrate their work along 

 certain lines, and thus produce stream-erosion. A third 

 portion never reappears on the surface, but finds its way, 

 by subterranean passages, to the sea. 



By the continued action of rain and rivers all lands 

 (except some rainless deserts) are being cut away and 

 carried to the sea. Every one, each in his own vicinity, 

 may see this process going on. The soil of the hillsides 

 is everywhere being washed away by rain, and carried off 

 in the muddy streams. At what average rate is this wash- 

 ing process going on ? This is a question of extreme 

 importance. 



Average Rate of Erosion. By observations made on 

 rivers in all parts of the world it has been estimated that 

 all land-surfaces are being cut away at a rate of about 

 one foot in 3,000 to 5,000 years. The Mississippi cuts 

 down its whole drainage-basin one foot in 5,000 years, the 

 Ganges one foot in 2,000 years. Some rivers cut still 

 more rapidly, but most less rapidly than these. The rate 

 differs in different parts of the same basin. In mountain- 

 regions the rate is at least three times the average given 

 above, and on steeper slopes still greater. On the lower 

 plains the erosion is small, and in many places there is 

 deposit instead of erosion. Making due allowance for all 

 these variations, it is probable that all land-surfaces are 

 being cut down and lowered by rain and river erosion at 

 a rate of one foot in 5,000 years. At this rate, if we take 

 the mean height of lands as 1,200 feet, and there be no 

 antagonistic agency at work raising the land, all lands 

 would be cut down to the sea-level and disappear in 

 6,000,000 years. 



This universal cutting away of land-surfaces we have 



