xiii Action of Water on the Land 253 



that the " American " falls have receded 30 feet, and the 

 "Horse-Shoe" falls 104 feet in the last 48 years. If the 

 structure of the rocks is the same all the way even at this 

 rate the time, geologically speaking, is close at hand when 

 the river-bed will be lowered along its whole length and 

 Lake Erie will be drained. If the Niagara River had been 

 muddy instead of being exceptionally clear, its erosive power 

 would have been greater, and the falls would have been 

 worked out long ago. The falls of St. Anthony on the 

 Mississippi, for example, have been cut back about 900 feet 

 since they were discovered in 1680. 



331. The Work of Rivers. The amount of sediment 

 and of dissolved solids in the water of rivers gives a clue 

 to their average effect in lowering the whole surface of their 

 basins. From calculations of this kind it appears that in 

 order to lower the average level of their basins by i foot 

 the Danube must work for nearly 7000 years, the Missis- 

 sippi for 6000 years, the Yellow River for 1500 years, the 

 Upper Ganges for 800 years, and the Po only for 700 

 years. Dr. John Murray calculates that in 6,000,000 years 

 river erosion at the present rate would reduce all the land 

 of the globe to sea-level, and M. de Lapparent, observing that 

 the deposit of sediment at the same time raises the level of 

 the ocean, shows that at the present rate of surface erosion 

 4,500,000 years would suffice to equalise the level of land and 

 sea. 



332. Lakes are bodies of water occupying hollows of 

 the land. As contrasted with rivers ( 330) they are 

 transitory features of a region, being subject to considerable 

 fluctuations in extent and destined ultimately to disappear. 

 Lakes often originate in the obstruction of a river valley. 

 If blocked at a narrow gorge by drifting ice or an avalanche 

 the river-bed below runs dry, and the water above rises, 

 flooding the valley until it reaches the lip of the ice-wall. 

 Ultimately the pressure of the accumulated water bursts 

 the ice-barrier, and a terrific flood suddenly desolates the 

 valley below. The famous parallel roads of Glen Roy in 

 Scotland are believed to be beaches etched out at succes- 

 sive levels by the water of a glacieji^festryGt^d lake, the 



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