1913] COOPER—ISLE ROYALE 129 
ucts retard it. On a steep slope soil materials and seeds as well 
are washed away to a greater extent than on a gentle one. The 
effect of wind in opposing invasion consists in the drying out and 
blowing away of the small but valuable deposits of humus which 
accumulate upon the rocks, in the uprooting of scattered trees 
feebly anchored in the shallow soil, in the breaking off of those more 
firmly established, and in the increase of the evaporation rate. 
The effects of these factors are seldom or never strictly separable, 
- but in various combinations they result in the production of three 
ray 
‘ Day iA i 
Fic. 22.—Three phases of rock shore vegetation; see text 
phases of rock shore vegetation, differing from each other in respect 
to the position of the limit of forest extension and in the extent 
to which the forestable territory has been occupied. These are 
illustrated in the diagram (fig. 22). Both sides of the island repre- 
sented are assumed to be equally exposed to wind and waves. 
A represents an island which is effectually sheltered from the 
lake storms. Shores such as these are characteristic of the inner 
coves and harbors of Isle Royale. Upon the gentle slope at the 
right the climax forest in full development has reached the water’s 
