382 



Ecology. 



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. 481. 

 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. 482. 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 longer un- 

 covered by ice were being covered by plant life. One of the 

 Greenland rock lichens is shown in fig. 483. 



