190 



PHYSICAL GEOLOGY 



will not yield to pressure without breaking, even when the pressure is 

 very slowly applied. If this is true under all conditions ice is not a 

 viscous substance as has been supposed, and the theory fails. 



(2) Expansion and Contraction. — According to this theory a glacier 



moves downhill as 

 a solid body, simply 

 through alternations 

 of temperature. 

 When a mass suffers 

 a rise in temperature 

 it expands, the mo- 

 tion taking place in 

 the direction of least 

 resistance, that is, 

 down the bed. When 

 the temperature 

 falls, contraction will 

 ensue; but since gravity opposes a backward movement a gradual 

 creeping down the bed occurs. The creep of sheet lead on a roof 

 illustrates this action. Since ice is a poor conductor of heat, it is 



Fig. 181. 



An iceberg. The vessel gives an idea 

 of the size. 



FlG. 182. — Iceberg, Labrador. The dark bands of debris were probaLly 

 horizontal in the glacier. (Photo. F. B. Sayre.) 



evident that such rapid movement as is often observed could not 

 result from this cause. 



(j) Regelation. — A theory (Tyndall) based upon the fact that 

 broken ice lu;ils under pressure, even at melting temperatures, holds 

 that the movement of glaciers is accomplished by the repeated frac- 



