270 



GEOLOGY. 



wind with absolute equality, and the surface drainage of the ice tends 

 to aggregate it. Every dust particle acts Hke a small stone, and where 

 aggregations of dust occur, they melt their way clown into the ice, 

 developing holes or ^^ dust -wells '' (Fig. 245). These wells rarely reach 

 a depth of more than a few inches, but they may be so numerous that 

 the pedestrian is obHged to watch his steps. This is especially true 

 near the edge of the large ice-caps. It is evident that the depth of 



Fig. 244. — Bowlder on ice pinnacle. Forno glacier, Switzerland. (Reid.) 



these dust wells must be slight, for so soon as they are deep enough 

 to cut off the sun's rays from the dust at the bottom, the deepening 

 ceases. Other things being equal, they are deeper in low latitudes 

 than in high. 



