336 WAVES OF SAND AND SNOW 



crests, and remain there until the progression of 

 the reef once more exposes them to the direct 

 current. But the upward currents on the lee of 

 the ridges carries away the lighter materials in 

 suspension. 



These conclusions at which the engineers of the 

 Mississippi Commission have arrived help us to 

 understand how the size and weight of the particles 

 limit the height of sand-waves. 



General Conclusions as to the Conditions of the 

 Formation and Growth of Sub-aqueous Sand- 

 waves. 



Let us suppose waves in sand of uniform size 

 to grow in a stream of sufficient depth under the 

 action of a current of a certain velocity. What 

 property of the sand itself will ultimately stop the 

 growth of the waves ? If the uniform sand be 

 light — i.e., the particles be small — the limit will 

 be reached owing to the inability of the upper 

 layers at the crest of the waves to withstand the 

 horizontal thrust to which they are subject between 

 the direct current behind them and the eddy in 

 front. Growth will therefore cease when the crests 

 are ploughed off. 



On the other hand, suppose the uniform sand to 

 be heavy — i.e., the particles large — then for a 



