270 WAVES OF SAND AND SNOW 



flat crests had a very smooth surface of close-lying, 

 fine-grained, clean yellow sand. The slow current, 

 flowing with a close approximation to absolute 

 steadiness, both of velocity and direction, could 

 only move material on the nearly flat summits or 

 crests by pushing it along the surface. Now, it is 

 an accepted fact that the film of water next a solid 

 is almost stationary, and such must be nearly the 

 state in this case where the surface of the san,d 

 is so smooth and compact. Only larger grains 

 projecting from the surface into the region of 

 appreciable current would be pushed along. This 

 process I conceive to have already gone on until 

 all the available larger grains had been rolled over 

 the cliff, where they had formed a covering to 

 the surface of the succeeding pool. 



Now, in the pool to leeward of the cliff the 

 straightforward movement of the current is stopped, 

 being replaced by a gentle pumping action. The 

 power of the eddy to remove the sand depends, 

 therefore, upon the slowness with which the sand- 

 grains subside — i.e ., for any particular kind of sand 

 upon its fineness. The eddy could not toss away 

 the coarser materials which the direct current had 

 rolled over the cliff, nor even raise them sufficiently 

 to get at the underlying finer material. The very 

 slow current appears therefore to be only able to 



