312 



GEOLOGY. 



from the air, and during another part, evaporation from the ice. These 

 alternating processes are attended by oscillations of temperature. While 

 the balance between loss and gain of substance may be immaterial, 

 the oscillating nature of the process and the fluctuations of temper- 

 ture are probably favorable to granular change. 



Whether these processes furnish an adequate explanation of the 

 changes or not, the observed fact is that there are all gradations from 

 snowflakes and pellets into granular neve, and thence into glacier granules 

 {Gletscherhorner) , varying in size up to that of filberts and walnuts, and 

 even beyond. In coherence, these aggregations may vary from the early 

 slightly coherent granular stage, where the grains are small and sphe- 

 roidal, to the ice stage, where the cohesion has become strong through 

 the interlocking growths of the large granules. Even when the mass 

 has become seemingly solid ice, sufficient space is usually left between 

 the granules to give the dispersive reflection to light which imparts 

 to glacier ice its distinctive whitish color. 



The arrangement of the crystal axes. — The most radical difference 

 between glacier ice and ice formed directly from water is in the 

 arrangement (orientation) of the crystals. In the ice formed on undis- 

 turbed water, the bases of the crystals are at the surface and their prin- 

 cipal axes are vertical, as shown by Miigge.^ As they grow, the crystal 



Fig. 291a. Fig. 291b. Fig. 291c. 



Figures to illustrate the method of deformation of ice crystals. 



prisms extend downwards. This gives a columnar or prismatic struc- 

 ture to the ice, well seen when it is ''honeycombed'' by partial melting. 

 In the glacier, on the other hand, the crystals, starting from snowflakes, 

 have their axes turned in various directions according to the accidents 

 of their fall; and as the snow develops into ice, the principal axes of the 



^Ueber die Plasticitat der Eiskrystalle. Neues Jahrbuch fiir Mineralogie, etc., 

 1895, Bd. II, p. 211. 



