1013] COOPER—ISLE ROYALE 131 
the intervening spaces are covered with heath mat or are partially 
bare. The outermost pioneers of the forest may or may not have 
reached the limit of its possible extension. Such a shore is illus- 
trated in fig. 24. 
Although as we pass outward the trees become less frequent and 
smaller, there is no corresponding decrease in age. The outermost 
individuals are often as old as any in the fully developed forest. 
The process of invasion in a given spot is a matter of much longer 
duration than a generation or two. ‘Tree after tree lives its life 
in the same crevice and finally succumbs to the severity of the 
Fic. 24.—Shore of type B: gradual invasion; island near Blake Point 
conditions, each contributing toward the development of the 
future forest only the small remnant of humus resulting from its 
decay that is not blown or washed away. Through successive 
generations the number of individuals living at the same time slowly 
increases, until the assemblage of trees attains the character of true 
forest. 
Certain of the larger rock openings upon the forested ridges, 
now 20 m. or more above the water, have not yet lost their resem- 
blance to the shores, in spite of the dozens of centuries that have 
passed since the lake stood at their level. Such an opening (Sec. 
