156 GORGE OF THE RHINE. 



of the Falls of Niagara, which appear to have been pushed back 

 several miles, at the rate of 40 or 50 yards in fifty years, to the pos- 

 sible discharge hereafter, through the St. Lawrence, of the waters of 

 Lake Erie. Such a discharge, if it were brought about suddenly, 

 would occasion a local deluge ; but the lake is so rapidly filled up by 

 sediment, that it is a question whether it will not become dry ground 

 before the Falls of Niagara shall have been pushed back so far as to 

 be capable of emptying it. Excavation at the rate of a yard a year 

 would require twelve thousand years for the formation of the Niagara 

 gorge, which is seven miles long. The Falls of St. Anthony, near the 

 junction of the Minnesota with the Mississippi, have formed a gorge 

 eight miles long to Fort Snelling. The falls have been receding since 

 1680 at an average rate of five feet a year, so that they may have 

 required between eight and nine thousand years to excavate the valley, 

 according to Prof. Winchell. 1 The Fall of the Rhine at Schaffhausen is 

 a grand exhibition of the erosive power of water, particularly the wear- 

 ing of the base of the island pinnacles of limestone, which now stand 

 proudly in the midst of the currents, but will eventually be hurled 

 down the thundering cataracts. The gorge of the Rhine, sixty miles 



Fig. 52. Gorge of the Rhine. 



long from Eingen to Rolandseck, has been cut by an ancient waterfall, 

 long since passed away, which has lowered the level of the Rhine and 

 its tributaries, and drained lakes in its course, which are now small 

 plains. 2 



Transporting Power of Streams. In considering now the trans- 

 porting action of streams, we may distinguish between such as flow 

 through valleys of uniform declivity without lakes, and such as pass 

 through broad receptacles of water before arriving at the sea, as is the 

 case with several rivers of England, Wales, and Scotland, and streams 

 which flow down from the Alps. 



Elvers without Lakes. A certain velocity of current is requisite 

 for the transport of every kind of earthy matter ; the finer the matter, 

 the less the force required to move it along. Hence in the lower parts 

 of rivers, where their course slackens, and they approach the sea, though 

 they can no longer remove rocks and transport loads of sediment, 

 their waters are muddy, and their channels and sides receive continual 

 augmentation. Such a river as the Yorkshire Ouse is very instruc- 



x Quart. Jour. Geol. Soc., vol. xxxiv. p. 886. 

 2 Ramsay, Quart. Jour. Geol. Soc., vol. xxx. p. Si. 



