120 LEWIS G. WESTGATE 
similar effect may be produced on the sea-cliff in time of heavy storms. 
But the impact of storm-waves even would seem to be less effective than 
the pressure of glacial ice, and the fact that the cliff base is exposed 
to the air, and is often, and in some cases always, water-soaked, 
indicates that the loosening of the rock fragments which are finally 
dislodged by the waves is in reality the result of weathering. In the 
case of streams flowing over jointed coherent rock, it is difficult to 
believe that even the swift currents of flood seasons are able to dis- 
lodge rocks from the stream-beds. The impact of the water is too 
slight, and is exerted on the nearly flat stream-bed at a great disad- 
vantage. The stream is able to sweep away blocks, not too large, 
which have been loosened and partly dislodged by weathering; but 
it is not easy to believe that the stream is the dislodging agent. 
If, as seems certain in the case of stream-erosion,and as seems 
probable in case of wave-erosion, the loosening of the rock fragments 
is the result of weathering, then plucking becomes merely the first 
step in the transportation of débris, and is reduced to a vanishing 
quantity between weathering and transportation. In that case the 
stream becomes a transporting and not a corrading agent; weather- 
ing becomes the important factor in valley-deepening as in valley- 
widening; while the stream acting as a transporting agent prevents 
the process from clogging. To the extent that weathering replaces 
plucking by wave-action, the same thing is true of shore erosion. 
In glacial erosion only is plucking left as a large factor, and even here 
it is not certain that it is the only factor in joint-block removal. 
