THE WORK OF SNOW AND ICE. 



263 



has its origin in the snows collected on the mountain heights seen in 

 the distance, and it works its way down the valley in a manner which, 

 in the aggregate, is similar to the movement of a stiff hquid. The 



Fig. 237.^White glacier (central background) joining a larger glacier (foreground), 



Alaska. (Reid.) 



likeness to a river extends to many details. Not only does the center 



move faster than the sides, and the upper part faster than the bottom, 



as in the case of streams, but the movement is more rapid in constricted 



portions of the valley and slower in the broader parts. These and 



other Hkenesses, some of which are apparent rather than real, have 



given origin to the view that glacier ice moves like a stiff viscous Hquid. 



But while the points of likeness between glaciers and rivers are 



several, their differences are at least equally numerous and significant. 



The trains of debris on the surface (the dark bands in the illustration), 



like the central currents of streams, pass nearer the projecting points 



of the valley walls and farther from the receding bends; but beyond 



this point the analogy fails, for the trains of debris on the ice do not 



conform in detail to the courses of the currents of a winding stream, 



nor is there evidence of the rotatory motion that characterizes river 



water. Furthermore, the glacier is readily fractured, as the numerous 



