284 GEOLOGY 



It is probable, also, that sharp differential strains and shearing 

 are developed at the level where the surface water of the warm 

 season, descending into the ice, reaches the zone of freezing; for the 

 expanding of the freezing water at the upper limit of the cold zone 

 may cause the layer expanded by it to shear over that below. As 

 the level of freezing descends with the advance of the warm season, 

 the zone of shearing sinks. This is a phase of shearing developed 

 in a special horizon relatively near the surface. It may be noted 

 that expansion at the zone where descending water freezes, not only 

 leads to shear, but to the development of surface cracks, for the 

 surface is stretched as the zone below expands. In the course of 

 years, the cracks developed in this way may become wide crevasses, 

 limited below by the depth of the zone of freezing in summer. 



High temperature and water. In the zone of waste, the higher 

 temperature and the greater abundance of water lend their aid to 

 the fundamental agencies of movement ; and there is need for these 

 aids to promote a proportionate movement, for here the granules 

 are more intimately interlocked, and the ice more compact and in- 

 herently more solid and rigid. The average temperature is near the 

 melting-point (p. 247), and during the warm season the ice is 

 bathed in water all the time, so that the necessary changes in the 

 crystals are facilitated. Under these conditions, movement takes 

 place more readily than in the more open granular ice of lower 

 temperature and drier state. 



Application. The co-operation of the auxiliary agencies of 

 motion with the fundamental ones appears to explain the peculiar- 

 ities of glacial movement. In regions of intense cold. where a dry 

 state and low temperature prevail, as in the heart of Greenland, 

 the snow-ice mass may attain an extraordinary thickness. Here 

 the burden of movement seems to be thrown almost wholly upon 

 compression, with the slight aid of molecular changes due to in- 

 ternal evaporation and allied inefficient processes. Since the 

 temperature in the upper part of the ice is very adverse (p. 245), 

 the compression must be great before it becomes effective in melting 

 the ice, and hence the great thickness of the snow before motion is 

 considerable. -Similar conditions more or less affect the heads of 

 Alpine glaciers, though here the high .irnidients favor motion with 



