286 THE EARTH'S SURFACE AND PLANT LIFE 



the summer is not sufficient to melt all the snow which 

 falls in the winter, then the layers of snow will increase 

 from year .to year. 



To have this occur the temperature for the whole year 

 need not be below the freezing point, but the heat of the 

 summer must not be sufficient to melt all the snow which 





MOUNT HOOD, CASCADE RANGE, OREGON 

 A beautiful old volcanic cone which is continually covered with snow. 



fell in the colder season. Lofty mountains, even in the trop- 

 ics, have their upper parts snow-covered. In the far north 

 and the far south the line of perpetual snow falls to sea 

 level, inclosing the mighty expanse of the Arctic and the 

 Antarctic snow fields. 



Glaciers. Wherever there is not enough heat in the 

 warm season to melt the snow which accumulates during 



