DIMFTIXG SAND. 



25 



more nearly in ;i])i)roximation to its erosive force tlian to 

 its lifting* force. With changes in velocities the latter 

 varies as the sixth power, while the erosive force varies as 

 the square. It is therefore much easier for the coarser 

 ingredients to be rolled along- with the dune sand than it 

 is for the dune sand to be picked up and carried away 

 with the finer ingredients. The wind much more rapidly 



Table XV. Mechanical Composition of Sand heaped up by Incipient Drifting. 



Length of 



diameter in 



mm. 



59 



From a 

 drifting field, 

 Kansas. 



On a snow drift 

 in Maryland. 



61 



From a gutter In 

 Baltimore, Md. 



Beach, 

 St. AtigustiDe.Fla. 



63 



Koad-bed, 

 Carman, Ills. 



16—8 

 8-4 

 4—2 

 2—1 



i- 



-i 



i~ 



1 



s 



1 

 s 



-xV 



iw- 



-ttV 



■J? 



I 



1 _ 

 TiTs 



_ 1 

 TTS 



_ 1 



.6 



2.6 

 0.2 

 44.6 

 1.4 

 .5 

 tr. 



I 



tr. 



.3 



U7.0 



2.9 



T 



16.8 



29.6 



51.4 



.8 



tr. 



}• 



ceases to lift sand grains exceeding one eigth of a milli- 

 meter in diameter than it ceases to roll grains which be- 

 come larger thtin one fourth of a millimeter. The opera- 

 tion of this principle is more or less evident in all the 

 samples, but it is best seen in such as have been taken 

 from the surface of the highest ridge and the rear slope of 

 a dune. It is most conspicuous in the general average of 

 the averages of the sands from each locality. 



