ICE AND GLACIERS 251 



If particles be detached from its corners they are seen to 

 consist of these angular granules. Glacier ice, when it 

 begins to melt, is seen to possess the same structure, except 

 that the pieces of which it consists are mostly larger than 

 in artificial ice, attaining the size of a pigeon's egg. 



Glacier ice and compressed ice are thus seen to be sub- 

 stances of a granular structure, in opposition to regularly 

 crystallised ice, such as is formed on the surface of still 

 water. We here meet with the same differences as between 

 calcareous spar and marble, both of which consist of carbon- 

 ate of lime ; but while the former is in large, regular crystals, 

 the latter is made up of irregularly agglomerated crystalline 

 grains. In calcareous spar, as well as in crystallised ice, the 

 cracks produced by inserting the point of a knife extend 

 through the mass, while in granular ice a crack which arises 

 in one of the bodies where it must yield does not necessarily 

 spread beyond the limits of the granule. 



Ice which has been compressed from snow, and has thus 

 from the outset consisted of innumerable very fine crystal- 

 line needles, is seen to be particularly plastic. Yet in ap- 

 pearance it materially differs from glacier ice, for it is very 

 opaque, owing to the great quantity of air which was origi- 

 nally inclosed in the flaky mass of snow, and which remains 

 there as extremely minute bubbles. It can be made clearer 

 by pressing a cylinder of such ice between wooden boards; 

 the air-bubbles appear then on the top of the cylinder as a 

 light foam. If the discs are again broken, placed in the 

 mould, and pressed into a cylinder, the air may gradually 

 be more and more eliminated, and the ice be made clearer. 

 No doubt in glaciers the originally whitish mass of neve is 

 thus gradually transformed into the clear, transparent ice 

 of the glacier. 



Lastly, when streaked cylinders of ice formed from pieces 

 of snow and ice are pressed into discs, they become finely 

 streaked, for both their clear and their opaque layers are 

 uniformly extended. 



Ice thus striated occurs in numerous glaciers, and is no 

 doubt caused, as Tyndall maintains, by snow falling between 

 the blocks of ice; this mixture of snow and clear ice is 

 again compressed in the subsequent path of the glacier, and 



