240 Professor Forbes on the Leading Phenomena of Glaciers. 



From this we might expect that 

 we should have a separation of 

 the ice particles, a rupture or fis- 

 suring of the substance of the 

 glacier every where parallel to 

 the resisting walls or bottom? 

 producing a cross section, as in 

 the annexed figure. 



This is what actually takes place, and especially in glaciers 

 of the second order, where the retardation being almost en- 

 tirely due to the action of the bottom, the successive layers slip 

 over one another with increasing velocity towards the surface. 



But the question arises, how does this action produce the 

 frontal dip of the figure at page 237 ^ Why does not the 

 canal shaped glacier present a series of trough-shaped layers, 

 as in the figure above, whose bottom remains parallel to 

 the bottom or bed of the glacier ? The reason appears to 

 be this. The fluid is urged on (in the centre of the gla- 

 cier especially) by its own weight. It is falling down an 

 inclined plane by the force of gravity. It is, however, pre- 

 vented from moving by the intense friction of the whole 

 of the lower part of the glacier upon its bed. If the gla- 

 cier be solid, there can be no motion, unless there be sufli- 

 cient force to overcome this friction ; and this we have seen 

 to be one great (and we think insurmountable) difficulty, both 

 of the hypothesis of De Saussuro and that of De Charpentier. 

 But the semifluid has another mode of progression, — the 

 pressure 7nay not overcome the friction of the bed, or else 

 the fluid pressure at the lower end mai/ be dragging the 

 whole glacier over its bed, that is immaterial ; but any par- 

 ticle in a fluid or semifluid mass, urged by a force from above, 

 does not necessarily move in the direction in which the force 

 impels it, it moves diagonally ; forwards, in consequence of the 

 impulse ; upwards, in consequence of the resistance directly in 

 front. Hence a series of surfaces of separation shaped (to use 

 familiar illustrations) somewhat like the mouth of a coal-scut- 

 tle, or of a sugar-scoop, will rise towards the surface, varied 

 in curvature by the law of velocity of the difl^erent layers of 

 the glacier. Near the head or origin of the glacier, where 



