SNOW-WAVES AND SNOW-RIPPLES 187 



The surface layers of a deposit of such material also 

 drift superficially before even a very slow current. 

 In the case of aeolian transportation this third ot 

 most mobile class of particles is dust. In the case 

 of the movement of detritus by water it is mud. 



The middle, or second, term of the series com- 

 prises particles which subside at a moderate rate, 

 and material of this order is called sand, whether 

 it be found on land or under water. It has in 

 dififerent places a different mineral composition, but 

 on the whole its chief component is quartz, whose 

 hardness and texture are such that when the 

 diameter of the particles is from about 5V to 

 about 200^ of an inch their mutual attrition by 

 collisions when drifting is very slow. 



•When dry sand is drifting before the wind on a 

 beach where stones lie scattered on the surface, 

 a single tongue of loose sand deposits behind each 

 projecting stone, with a sharp crest or ridge exactly 

 in a line with the centre of the stone. Each side 

 of the ridge has a concave surface., It fills the 

 space of dead air between the right and left- 

 handed halves of the vortex or eddy produced by 

 the stone. Much sand is whirled in the eddy, but 

 some of it works its way out and drops in the dead- 

 air space. A similar ridge of sand collects behind 

 a walking-stick thrust into the beach. Neither 



