{mountain (pcitrfo 



may smother them and reclaim the temporarily 

 lost opening. 



But, though only one tree may grow, this in 

 due time shades the grass, a circle of young 

 trees rise around it, and these in turn carry for- 

 ward the work of winning territory. At last the 

 park is overgrown with trees ! 



Glacier meadows may be seen in all stages of 

 evolution. The lake-basin gouged by a glacier 

 goes through many changes before it is cov- 

 ered by a forest and forgotten. No sooner does 

 ice vanish and a glacier lake appear than its 

 filling-in is commenced. Landslips and snow- 

 slides thrust boulders and cliff-fragments into 

 it; running water is constantly depositing sand 

 and sediment upon its bottom. Sedge and moss 

 commence covering its surface as soon as its 

 water becomes shallow. In due time it becomes 

 a bog with a thick covering like a wet mattress, 

 composed of the matted roots of sedge and grass. 

 Over this, wind and water deposit earthy mat- 

 ter, but centuries may pass before the bog is 

 filled in sufificiently to have a dry surface and 

 produce grass and flowers and finally trees. 



237 



