ICE AND SNOW. 99 



vertical prisms, penetrating its whole thickness, 

 and standing side by side like the columns of a 

 basaltic cliff; which, in their mode of formation, 

 have, 1 imagine, a close analogy. Dr. Slagintweit 

 informed me that neither the ice nor the basalt 

 forms exact prisms, the angles never having the 

 precise measurements of true crystals. In this 

 condition the ice may be strong enough to sup- 

 port a considerable weight; and I have travelled 

 over it with a large party on several occasions 

 when the prisms on which the foot rested were 

 depressed at every step, and a pointed stick could 

 be driven through the whole thickness into the 

 water beneath, with as much ease as into a bank 

 of snow. The ice then, in fact, presents the phy- 

 sical characters of a semi-fluid mass, as pointed 

 out by Professor Forbes, its parts being moveable 

 on each other, not only vertically, but as in the 

 case of travelling glaciers, capable of gliding past 

 one another horizontally. 



In spring, when the action of the sun-light is 

 very powerful, an incipient thaw takes place at 

 mid-day on the surface of the snow, which, on 

 freezing again, acquires a glassy crust. As the 

 season advances, but while the temperature of the 

 air is still even at noon far below the freezing 

 point, the crust in clear weather becomes pene- 

 trated in the direction in which it is struck by the 



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