Variations of Sierra Glaciers. 21 



various glaciers of Europe and Asia, and the fuller 

 records of Switzerland and other inhabited mountain- 

 lands show that the front of a glacier will often advance 

 continuously for several decades, and then for another 

 long period continuously retreat. 



It is easy to get some inkling of the cause of these 

 changes if we consider the essential nature of a glacier. 

 It originates in a mountain valley where the accumulation 

 of winter snow is greater than the summer heat can dis- 

 sipate. The excess of snow piles up year after year, is 

 compacted into ice, and creeps down the slope, consti- 

 tuting a slow but continuous stream of ice. As it de- 

 scends it meets new climatic conditions, the winter snow 

 becoming gradually less and the summer melting gradu- 

 ally greater, until at length a region is reached where there 

 is an annual loss of material, instead of an annual gain. 

 In that region it wastes away and comes to an end. So 

 long as there is a balance at the lower end between the 

 supply, through forward motion, and the loss, through 

 melting (or, more strictly, melting and evaporation), the 

 end of the glacier is constant in position ; but whenever 

 one of these factors overpowers the other the glacier 

 either grows longer, when it is said to advance^ or grows 

 shorter, when it is said to retreat. But while these ele- 

 mentary propositions are simple and easily understood, 

 there are various modifying factors as to which much less 

 is known, and the full theory of glacier variation has not 

 yet been reached, although it has received much attention 

 from geographers. The problem is of such interest that 

 records of glacier variation are carefully made in most 

 regions of ready access, and the reports of changes are 

 annually summarized. In this country the summary is 



