278 GEOLOGY 



Though the effect of this is slight, it is to be correlated with the 

 much more important fact that compression produces heat which may 

 raise the temperature of the ice to the melting-point at some points, 

 while tension may reduce the temperature to or below freezing, at 

 other adjacent points. There is therefore a tendency for the ice to 

 melt at the points of contact and compression, and for the water so pro- 

 duced to refreeze at adjacent points where the surface is under tension. 

 This process becomes effective beneath a considerable body of snow, 

 and here the granules gradually lose the spheroidal form assumed 

 in the early stages of granulation, and become irregular polyhedrons, 

 interlocked into a mass of more or less solid ice. 



The moisture of the air may be of importance in the process, 

 even far below the surface. Under severe wind pressure, air pene- 

 trates porous bodies appreciably, as the " breathing" of soils, "blow- 

 ing-wells," and "blowing-caves" teach us, for all these things are 

 expressions of the effective penetration and extrusion of the air, 

 under variations of barometric pressure. In the snow-fields, and 

 in the more granular portions of glaciers near their heads, the po- 

 rosity is doubtless sufficient to make this process effective. The 

 probable effect is two-fold: (1) The condensation within the ice 

 of moisture from the air at some times, and (2) escape of moisture 

 from within the ice at others. These alternating processes are 

 attended by oscillations of temperature which may involve melting 

 and freezing, and these processes produce granular change. 



Whether these processes furnish an adequate explanation of the 

 changes or not, all gradations may be observed from snowflakes 

 into granular neve, and thence into the granules of glacier ice, 

 ranging in size up to that of walnuts, and even beyond. In co- 

 herence, these aggregations vary from the ne've' (coarse-grained 

 snow) stage where the grains are small and spheroidal, to the ice 

 stage where the cohesion is strong through the interlocking growths 

 of the large granules. 



Structure and arrangement of the crystals in glacier ice. A 

 crystal of ice is made up of a series of plates arrange 1 at riirht angles 

 to the principal axis of the crystal. These plates may be likened to 

 ji pile of cards, the principal axis being represented by a line vertical 

 to them. If a cube be cut from a large crystal of ice, it will behave 



