620 THE JOURNAL OF GEOLOGY. 



other part. Wherever it has not, it is because local condi- 

 tions, such as extensive crevassing, have prevented its retention 

 at the surface. 



Superglacial drift of eolian origin. — Another sort of super- 

 glacial material sometimes arises through the agency of the 

 wind. As glaciers advance into regions which are free from 

 snow and ice, they advance into regions whence dust and sand 

 may be blown upon them. Once lodged upon the ice, dust is 

 not likely to be carried farther by the wind, since there is suf- 

 ficient moisture to hold it. Once moistened, too, it is likely to 

 freeze to the surface of the ice. Such dust is liable to removal 

 by superglacial waters. If it escapes them, it is likely to remain 

 upon the ice so long as the latter remains unmelted. It is believed 

 that considerable quantities of dust reach the surface of existing 

 glaciers in this way. Such dust as is blown upon the surface 

 of the ice, within the zone of wastage, is superglacial from the 

 beginning. 



This process of dust accumulation goes on most actively near 

 the ends of glaciers, since the surroundings here are best adapted 

 to furnishing the dust. But the same process must go on to 

 some slight extent throughout the whole gathering ground of 

 existing glaciers. As the snow accumulates year by year, it 

 contains a modicum of dust blown upon the snow-field. This 

 dust becomes englacial. The embedding snow is presently con- 

 verted into ice. As the ice moves toward the end of the glacier, 

 passing from the zone of accumulation to the zone of wastage, 

 its surface melts, and the dust contained in the part which is 

 melted, appears at the surface. Some of it is doubtless washed 

 away by the superglacial drainage resulting from rain and from 

 surface ablation. Such as escapes this fate may remain upon the 

 surface of the ice. The amount of dust which shows itself on the 

 surface of a snow-field at the end of a melting season is some- 

 times considerable. As seen in section in a snow-field, the snow- 

 falls of successive winters are seen to be clearly defined by the 

 presence of these bands of earthy matter ("dirty ice") between 

 them; these bands indicate the condition in which the surface of 



