74 BELT ON PRODUCTION OF LAKES BY ICE ACTION. 



tity throughout the winter." * Most of the water issuing from the 

 bottom of glaciers proceeds from the melting of the ice at the upper 

 surface of the glacier, finding its way to the bottom through 

 crevices and channels in the ice ; but a not inconsiderable portion is 

 produced by the melting of the lower surface next the earth. 

 Professor Forbes made some careful observations on this point, and 

 found that in summer the glacier wasted away by melting at the 

 surface 3.62 inches daily, and by subsidence or wasting at the bot- 

 tom 1.63 inches daily. The water that issues from beneath the ice 

 in Greenland throughout the long and severe winter, can only pro- 

 ceed from land springs and from the melting of the ice next the 

 earth. The only example of glaciers that do not give off water 

 during the winter that I have been able to find, are some of those 

 small ones on the higher parts of the Alps that have been called 

 " glaciers of the second class," on which, from their altitude, the 

 effect of the earth's heat must be very small. 



Let us apply these facts to the consideration of the question of 

 a depression in the pathway of a glacier, which has reached such a 

 depth that the ice is not bodily discharged from it, but simply fills 

 it, the glacier passing over the choked up hollow. We have seen 

 that at the bottom and sides of the hollow, the ice would be slowly 

 melted by the earth's heat, increasing with the depth of the basin ; 

 as the ice at the loiver end of the basin melted the whole mass 

 would be pushed along by the thrust of the moving glacier above 

 it. Into the crevice at the upper end would pour the water coming 

 down the bottom of the glacier from above the basin, which would 

 pass underneath and be forced out at the lower end, carrying with 

 it the mud produced by the crushing down of the ice as it melted 

 at the bottom, and by the grinding along its floor as it melted at 

 the lower end of the basin. The water coming from above would 

 assist in melting the ice, especially in summer, but its most import- 

 ant eifect would be the scouring out of the bottom of the basin, so 

 that an ever clean face of rock would be presented to the huge tool 

 operating upon it. That such an action, or a somewhat similar one, 

 would take place at the bottom of an ice filled basin, with a glacier 

 passing over it, and that it would be effective in deepening it, I 

 cannot doubt. It would in some measure resemble the action of a 



Jour. Geographical Soc. Vol. 23, p. 153. 



