114 



HOBBS— CHARACTERISTICS OF THE 



[April 22, 



earlier observed almost identically the same phenomena along the 

 line of his route. The intricate ramifications of the superglacial 

 rivers and the occupation of almost the entire remaining surface of 

 the ice by shallow ice wells and basins along his route are shown 

 in Figs. 34 and 2,2.^^ These ice wells are in no wise restricted to 

 inland-ice but are found in mountain glaciers as well, and repre- 

 sent but one of a series of allied phenomena dependent upon differ- 

 ential melting due to the presence of fine rock particles upon the ice. 

 They were quite thoroughly described by Agassiz in his " Systeme 



Kilomefers 



Fig. 34. Map showing the superglacial streams within the marginal zone 

 of the inland-ice of Greenland (after Nordenskiold). 



Glaciere." The particles of rock if not contiguous upon the ice sur- 

 face absorb the sun's rays and cause excessive melting of the ice 

 above and beneath them. They thus sink down into the ice and form 

 dust wells (Fig. 35, a). The thin walls which separate those wells 

 which are close together, being now attacked by the warm air on 

 their sides instead of on the top only, they in their turn melt away 

 to form a small basin, which soon either wholly or in part fills 

 with water (Fig. 35, b). Where in contact with their neighbors 

 and where of such thickness of accumulation as not to be heated 

 through by the sun's rays, these rock particles behave in quite a 



^^ A. E. Nordenskiold, " Gronland," pp. 197-204, map 3. 



