GEOLOGICAL SOCIETY OF LONDON. 



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large masses from a varying source. The sinking of the 

 smaller fragments into the ice, the author shows, is depen- 

 dent upon the greater power of absorbing solar heat pos- 

 sessed by snow than ice ; and as the stone gives out this heat 

 to the ice below nearly as fast as it is received, a greater 

 depth of ice is melted in a given time beneath the stone 

 than in the surrounding part of the glacier (vide p, 9.) The 

 existence of the large blocks on the pedestals of ice, is due to 

 the same cause. The example is thus afforded of the succes- 

 sive effects of heat and cold upon a block of stone in such a 

 situation, during each twent5^-four hours — evidencing that the 

 heat imparted diminishes with much greater rapidity as it 

 penetrates deeper, as it is imparted to the portions above on 

 their becoming cold during night, as well as those below, and 

 beyond the point where the two waves of heat and cold are 

 reduced to a mean quantity, the temperature of the stone is 

 constantly at or about 32^, as the heat perforates feebly and 

 uniformly. The surrounding portions of the glacier, how- 

 ever, being undefended by the stone, experiencing during 

 the day the direct radiation of the sun, melt and run off. At 

 night the surface cools by radiation, the cold being propagated 

 downwards ; but, on the return of day, the melting process is 

 renewed, and the degradation of the general surface of the 

 glacier is thus effected, the amount of ice dissolved being in 

 proportion to the direct intensity of the sun's rays, and the 

 time they shine ; while the surface of the ice beneath the 

 stone will be dissolved only in proportion to the excess of the 

 mean temperature of day and night, above 32^, diminished 

 by the effect of the thickness of the stone. One curious cir- 

 cumstance, observes Sir John Herschel, seems to follow from 

 this reasoning; namely, that the ice of a glacier, or other 

 great accumulation of the kind, may, at some depth beneath 

 the surface, have a permanent temperature much below freez- 

 ing, though in a situation whose mean amount of tempera- 



