ROUNDING OF SAND GRAINS 255 



sand grains weigh less in water than in air, a corresponding correction 

 must be made (47), the formula being according to Mackie for water: 



R^ xHspg.-l)d 

 mh 

 On making a comparison (Mackie-47: J-'^o) of the relative efficiency 

 of wind and water as rounding agents, we have from the formulas 



X^ g-rjnf Q v2 (g-Qor 1^ r^ 



for wind Roc ^-^^ and for water Roc — -^-^ — - — 



mh mh 



the following relationships: 



wind agency spg. , . , ^ . 2.61; 



?2 £. = __.' » which for quartz = 



water agency spg.-l i . 65 



or wind = 1.6 times water, but since average wind velocities are 

 always greater than average water velocities, it follows that the result 

 attained by wind will be much greater than that given. Mackie 

 has calculated that with a wind velocity of 8 miles an hour, and a 

 water velocity of 2 miles an hour, the ability of the wind to round 

 a given quartz particle (sp. 2.65) is nearly 29 times as great as that 

 of water. Mackie concludes that "under the conditions stated, 

 particles less than one-fifth the diameter of those rounded by water 

 will be rounded to an equal degree by wind" (47 :jio). Mackie has 

 calculated the values of the coefficient of roundability or psephicity 

 for various minerals a few of which are given (Mackie— 47:502). 



Tabic showing coefficient of psephicity of iiiinerals in air and ivater. 

 In air In water 



Examination of a sample of sand from the River Spey at 

 Craigellachie, Scotland, at a point where it could not be contami- 

 nated by admixture of particles derived from the sandstones 

 showed, according to Mackie (47: jod), no appreciable rounding of 

 the quartz grains, "though they certainly did not all of them pre- 

 sent the marked angularity of the quartz grains in most boulder 

 clays." The feldspars also were angular, though some of them, be- 



