OF ROCKY PLACES. 



603 



plants stand farther from the substratum and gain an advantage 

 in light relation above the earlier ones. In case the creviced rock 

 is where soil may be washed over the rock occasionally in high 

 flood, an older type of plant formation may begin at once, since 



Fig. 525- 



Edge of glacier in Greenland, showing freshly deposited rocks. (From Prof. 

 R. S. Tarr.) 



the crevices are apt to be filled with soil left by the flood. 

 Eventually the accumulation of soil may be so great as to encour- 

 age a close or climatic formation. 



1097. Retention of made soil. In the struggle of plants for 

 existence, there are a number of species which stand ready to 



