178 THE FORMS OF WATER IK 



Grorner glaciers, the ice does not possess the trans- 

 parency which it exhibits near the ends of the glaciers. 

 It is white, or whitish. Why? Examination shows 

 it to be filled with small air-bubbles ; and these, as we 

 now learn, are the cause of its whiteness. 



452. They are the residue of the air originally en- 

 tangled in the snow, and connected, as before stated, 

 with the whiteness of the snow. During the descent of 

 the glacier, the bubbles are gradually expelled by the 

 enormous pressures brought to bear upon the ice. Not 

 only is the expulsion caused by the mechanical yielding 

 of the soft thawing ice, but the liquefaction of the sub- 

 stance at places of violent pressure, opening, as it does, 

 fissures for the escape of the air, must play an import- 

 ant part in the consolidation of the glacier. 



453. The expulsion of the bubbles is, however, not 

 uniform; for neither ice nor any other substance offers 

 an absolutely uniform resistance to pressure. At the 

 base of every cascade that we have visited, and on the 

 walls of the crevasses there formed, we have noticed in- 

 numerable blue streaks drawn through the white trans- 

 lucent ice, and giving the whole mass the appearance 

 of lamination. These blue veins turned out upon ex- 

 amination to be spaces from which the air-bubbles had 

 been almost wholly expelled, translucency being thus 

 converted into transparency. 



454. This is the veined or ribboned structure of gla- 

 ciers, regarding the origin of which diverse opinions are 

 now entertained. 



