00 DYNAMICAL GEOLOGY. 



different velocities, and therefore slide on each other. 

 This is called differential motion, and is characteristic of 

 fluid, as distinguished from solid motion. But the differ- 

 ential motion of glaciers is not free, like that of water, 

 but reluctant, and with much -resistance, like that of very 

 stiff pitch. In small masses, and under quickly applied 

 force, it breaks like a solid ; but in large masses, and 

 under heavy, slowly applied force, it behaves like a stiffly 

 viscous fluid. 



Thus, like rivers, glaciers move much faster in the 

 middle than on the margins, and on the top than near the 

 bottom. Like rivers, also, they move faster on steep than 

 on gentle slopes. Like rivers, also, the velocity increases 

 with the depth of the stream. For example, the Mer de 

 Glace is 350 feet deep, and moves a foot and a half per 

 day, while some of the great Greenland glaciers, 2,000 to 

 3,000 feet deep, with less slope, run sixty feet per day. 

 Like rivers, also, glaciers conform to the inequalities of 

 their bed and banks, but, as it were, reluctantly i. e., 

 they conform to large and gentle inequalities, but not to 

 the small and sharp ones. In a word, glaciers, like rivers, 

 have their narrows and their lakes, their shallows and 

 their deeps, their cascades and their level reaches. They 

 are truly ice-rivers. 



Glaciers as a Geological Agent. 



te structure, the properties, and the cause of the 

 motion of glaciers are questions of deepest interest to 

 the physicist, but it is their geological agency i. e., their 

 agency now, and still more in former times, in sculptur- 

 ing the earth's surface which chiefly concerns the geol- 

 ogist. 



We have seen that a glacier may be regarded as an ice- 

 river. Like rivers, therefore, glaciers erode their beds and 

 banks, transport materials, and make deposits. But in 



