Geographical Distribution 



159 



which soon join them, and by the more powerful action of 

 the frost, and the ever-wearing action of the dust blast driven 

 by the wind, the rock is gradually crumbled into dust, which 

 together with the decay of moss and lichen forms a little 

 soil, in which other mosses may take root ; and by and by the 

 seeds of a few hardy grasses find a place, and by their roots 

 the wind and rain-borne dust is caught and soon a little turf 



A Lichen Society 



Lichens are among the earliest invaders of a rocky surface, preparing 

 the way for higher plants to follow. 



Courtesy of the Con/rad Lantern Slide Company, Chicago. 



is formed. Now some hardy perennials wander in, and their 

 stouter roots serve as wedges to pry apart the smaller chips 

 of rock and aid in making new soil. And in course of time 

 a heather society is formed, made up of grasses and the low- 

 growing matted bodies of perennial herbs, mingled with a 

 few annuals, which live but a single year. And now some 

 hardy shrub, perchance a mountain willow, or it may be a 

 seedling pine comes in, and ever and anon new trees take 

 root and grow, decay of root and leaf and fallen stem adds 

 to the humus soil, and in the shadow of the growing trees 

 sun-loving plants die out and shade-lovers take their place; 

 and now a forest stands where once was barren rock. 



