IOO RUNNING WATER 



of many streams and quite revolutionized the drainage of certain 

 districts. Since that time the displaced streams have cut out 

 new channels for themselves, often through hard rocks, and 

 many now flow in quite deep gorges, with nearly vertical walls. 

 Au Sable Chasm, New York, is an example of these geologically 

 modern river gorges, the atmosphere not having had time to 

 widen it. 



3. The Niagara is an exceptional case, the gorge being cut, 

 not only by the direct abrasion of the running water, but also by 

 the action of the spray and frost at the falls. In the ravine the 

 upper rock is a hard, massive limestone, which is underlaid by a 

 soft clay-shale. The latter is continually disintegrated by the 

 spray of the cataract and by the severe winter frosts, undermin- 

 ing the limestone, which, when no longer able to bear its own 

 weight, breaks off in tabular masses. Thus the falls are steadily 

 receding, leaving behind them a gorge, which is deepened by the 

 river. 



4. The most remarkable known examples of river erosion are 

 the canons of the Colorado. The Grand Canon is over 200 miles 

 long and from 4000 to 6500 feet deep, with precipitous walls. It 

 is extremely probable that the river has been rendered able to cut 

 to such profound depths by the gradual uplifting of the whole 

 region, which is now a lofty plateau, in places more than 8000 

 feet above the sea. The erosive power of the river has thus been 

 continually renewed and a more or less uniform rate of excavation 

 secured. (See Frontispiece.) 



Transportation by Rivers. — The main importance of rivers as 

 geological agents is not their work of erosion, but lies rather in 

 what they accomplish as carriers of the results of their own destruc- 

 tive activity and that of the atmosphere, comprising both the 

 materials which are mechanically swept along in suspension and 

 those which are carried in solution. 



Materials in Suspension. — The transporting power of running 

 water is dependent upon the velocity of the current, and both 

 mathematical and experimental treatment of the problem brings 

 out the surprising result that the transporting power varies directly 



