39 



mud is furnished from a grindstone wlien a knife or scythe is being 

 sharpened on it. On the pebbly beaclies of the sea or lakeshore 

 much the same thing may be seen ; and here also the constant grind- 

 ing of the rocks together wears off the edges until tiie pebbles 

 become smooth and round. 



Supplied with l)its of rock from the soil, or ground from the peb- 

 bles and rocks along its course, the stream carries its load onward, 



16.— The bed of a stream at low water, revealing the rounded pebbles that havebeen 

 worn and smoothed by being rolled about, thus grinding off tiny bits which 

 later are built into the flood-plains. 



perhaps to a lake, which it commences to fill, forming a broad delta 

 of level and fertile land, near where the stream enters the lake. 

 Or, possibly, the stream enters the sea and builds a delta there, as 

 the Mississippi river has done. 



But much of the mud does not reach the sea. The greatest sup- 

 ply comes when the streams are so flooded by heavy rains or melt- 

 ing snows that the river channel is no longer able to hold the water, 



431 



