312 



ECOLOGY. 



the lichens lay hold on them and invest them with fantastic 

 figures of varied color. Disintegrating rock, debris of plants 

 and animals, join to form the virgin soil. Certain of the blue- 

 green algae, as well as some of the mosses, are able to gain a 

 foothold on rocks and assist in this process of soil formation. 



Fig. 260. 

 Rock lichen (Parmelia contigua). 



A view of rocks thrown down by the melting and retreating 

 edge of a glacier in Greenland is shown in fig. 261. These 

 rocks at the time the photograph was taken had no plant life 

 on them. At other places in the vicinity of this glacier, rocks 



