DISTRIBUTION FROM A GROWTH OF THE SANDHILLS. M5 



curve of the wind and grows both horizontally and vertically, 

 as indicated in fig. 4, but as it grows the space to be filled by 

 sand increases, till the supply drifted by the wind no longer suffices 

 to fill the void, and, after drifting along and covering the windward 



Fig. 4.- 



-Longitudinal section of a sandhill further advanced in growth. 



part of the sandhill, it forms a slope to leeward, down which the 

 sand grains fall. In the hollow so produced an eddy of the wind 

 is formed, which helps to keep the hollow open, and, as the sand- 

 hill grows in height, this eddy increases in force. 



As the sandhill grows in height the volume of the space to be 

 filled by sand increases, and in an increasing ratio ; consequently the 

 proportion of what may be regarded as the unfilled space in the 

 hollow, under the upward bend of the wind, to the filled up 

 portion increases as the sandhill grows ; for the supply of sand may 



Fig. 5. — Longitudinal section of a fully developed sandhill. 



be regarded as appreciably constant, the amount of space it will fill in 

 a given time is constant, and the amount of unfilled space will vary 

 from nothing in the incipient stage of the formation of the sandhill 

 to quite one-third, on the central section, when the sandhill has 

 reached a height of 10 or 12 feet and may be regarded as fully formed. 

 So far we have only considered the vertical section, along the direc- 

 tion of the wind and through the axis of the sandhill. We must now 

 turn to what takes place on either side of this line. In the earliest 

 stage the general form of the ground plan is shown in fig. 6. 



Fig. 6. — Ground plan of incipient sandhill. 



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