4 SOILS. 



and shoved by it, exert upon the rocky channels in which the 

 ice stream moves, as well as upon each other. The product 

 of this grinding process is largely very fine (hence " glacier 

 flour"), so that it remains suspended in the water of the 

 glacier-streams until their velocity is permanently checked 

 when reaching a plain or lake. This suspended stone-flour 

 imparts to the glacier streams their distinctive character of 

 " white rivers," as contradistinguished from the clear, dark 

 " green rivers " that have their origin outside of glaciated 



FIG. i. Zermatt Glacier (Agassiz). 



areas. This difference can be readily observed in traveling 

 along any of the glacier-bearing mountain chains of the world, 

 and is frequently expressed in the names, of the- streams. 



The physical analysis of mud from the foot of Muir glacier, 1 

 Alaska, at its sea front, made by Professor Loughridge, shows 

 the prevalent fineness of the materials brought down by the 

 glacier waters. 



1 Collected by Dr. W. E. Ritter of the University of California. 



