On the Causes of Glacier- Motion. 



211 



less than 300 feet thick, and which at no part of their course have a 

 slope anything approaching 1 in 4. 



• We have now to show that the theories generally propounded for 

 glacier action are all of them negatived by some of the foregoing con- 

 siderations. These theories may be stated as follows : — 



(1.) The glacier simply slides over its bed as a solid body. This is 

 negatived by the fact that some parts move faster than others. 



(2.) The glacier flows under the action of its own weight, exactly 

 as a viscous body flows. This is the theory of Forbes. It is dis- 

 proved by the facts given above, which show that even on a slope of 

 1 in 4 a glacier would not flow unless it was at least 300 feet thick. 



(3.) The glacier moves by the crushing of its base. This has been 

 disproved by Moseley's experiments, which showed that the crushing 

 resistance of ice was considerably higher than the shearing resistance. 



(4.) The glacier moves by the melting of its base. This is the 

 theory of Hopkins. He placed a block of ice at 32° F. on a slab at a 

 small angle, and found that it slowly descended as it melted f On this 

 view the bottom of the glacier must always be in a melting state. 

 But glaciers are of all sizes and thicknesses, and they move in winter as 

 well as summer. Bessels (" Die Amerikanische Nordpol Expedition," 

 p. 398) measured the motion of an Arctic glacier (not apparently very 

 thick), in the month of April, which is just when the winter cold 

 would have sunk deepest, and found it considerable. Again in the 

 " Zeitschrift des deutschen Greologischen Gresellschaft," vol. 33, p. 693, 

 is an account of measurements of a Greenland glacier, both in winter 

 and summer, which show that the motion in winter is only 20 per 

 cent, less than in summer. It has been suggested to me that the 

 interior heat of the earth may be sufficient to keep the bottom of the 

 ice from freezing ; but this cannot apply near the sides, where the ice 

 is shallow, and the freezing of a very small strip on each side would 

 be sufficient to keep the whole mass from descending. Moreover, this 

 cause would apply to masses of snow as much as to ice. But it is 

 known that masses of snow, though lying on steep slopes, do not 

 descend in this way, even in summer, but melt away where they lie. 



(5.) According to the theories of Tyndall, Croll, and others, the 

 glacier moves not in the form of ice, but of water. These theories are 

 based on the known fact that the freezing point of ice is lowered by 

 pressure. Hence it is supposed that certain parts of a glacier are con- 

 tinually being exposed to so much pressure that they melt. The water 

 escapes downward, and the pressure being relieved, it freezes again. 

 The continuity of the glacier is further kept up by the process of 

 regelation, according to which two pieces of ice if placed in contact, 

 form into one solid mass. 



The advocates of this theory hardly seem to consider how very 

 small the lowering of the freezing point is for any ordinary pressure. 



