^80 



GEOLOGY. 



ently composed of dust blown upon the ice. It is supposed to imply 

 aridity in the region about the ice. If a sufficient mantle of dust were 

 spread over the border zone of the ice, and if the air were very dry, nearly 

 all the water melted on the surface of the ice might be held back by the 

 dust-wells until the water was evaporated or absorbed. 



Fig. 251. — Spouting stream. Glacier south side of Olriks Bay, North 



Greenland. 



Drainage. — Some of the water produced by surface melting forms 

 little streams on the ice. Sooner or later they plunge into crevasses 

 or over the sides and ends of the glacier. In the former case, they may 

 melt or wear out well-like passages (moulins) in the ice, and even in the 

 rock beneath. Much of the surface water sinks into the ice. Its ready 

 penetration is aided by the ^^ dust-wells " which mark the surface of many 

 glaciers. In north Greenland wells which contain six or eight inches 

 of water at the end of a warm day are often dry in the morning. The 

 water has leaked out and passed to lower levels. From these and other 

 harmonious observations it is inferred that the superficial part of a 

 glacier at least is readily penetrated by water. The depth to which 



