50 MAN AND THE GLACIAL PERIOD. 



change of position, regulation instantly takes place, and 

 thus the phenomena which simulate plasticity are pro- 

 duced. As the freezing-point of water is, within a narrow 

 range, determined by the amount of pressure to which it 

 is subjected, it is not difficult to see how these changes 

 may occur. Pressure slightly lowers the freezing-point, 

 and so would liquefy the portions of ice subjected to great- 

 est pressure, wherever that might be in the mass of the 

 glacier, and thus permit a momentary movement of the 

 particles, until they should recongeal in adjusting them- 

 selves to spaces of less pressure.* This is the theory by 

 which Professor James Thompson would account for the 

 apparent plasticity of glacial ice. 



* Forms of Water, p. 168. 



