66 THE WORLD'S LUMBER ROOM. 



white* only because they do not lie perfectly close ; for when 

 the air is squeezed out they adhere together and form a lump of 

 transparent ice. The snow which collects on the heel of one's 

 boot is converted into ice by pressure ; and the vast quanti- 

 ties of snow which fall and accumulate among the mountains 

 are similarly converted into ice by their own weight, which 

 also causes them to creep slowly down the mountain sides 

 into the valleys, where the warmer air changes them once 

 more into water. If the glaciers, as these rivers of ice are 

 called, remained stationary high up among the mountains, 

 they would go on increasing in thickness year by year, as 

 they received fresh additions of snow, and year by year, as 

 its waters were locked up in the form of ice, instead of being 

 returned to it, the ocean would sink lower and lower. 



Glaciers, then, may be called rivers of ice, but unlike 

 other rivers, they are able to move uphill as well as down, 

 and while at one time they descend into deep basins, at 

 another they ascend hills several hundred feet high. Their 

 rate of motion is very slow, and slower in winter than in 

 summer, being sometimes only a few inches, and sometimes 

 two feet or more in the course of the day, but it never ceases 

 entirely (Fig. 15). 



The fresh additions of snow which it is constantly receiv- 

 ing at its upper end are for ever pushing it on, urging it 

 down the steep slopes and more slowly up the hills, and the 

 motion is helped by the expansion and contraction of 

 the ice with each variation of temperature, day and night, 

 summer and winter. Every time it expands it must creep 

 onward, be it ever so little, and when it contracts again it 



* Powdered glass looks white for the same reason, 



