1913] COOPER—ISLE ROYALE 119 
phytic mosses, principally Grimmia ovata Web. and Moore. A 
brilliant orange band of the lichen Placodium is a prominent feature 
of cliffs, its lower edge 2m. above the water. This band is also 
distinguishable, though much less prominent, on the sloping shores. 
Next come foliose lichens, with the mosses Hedwigia albicans (Web.) 
Lindb. and Orthotrichum anoma- 
lum Hedw., and finally appear the 
fruticose forms, prominent among 
them the three large species of 
Cladonia, C. rangiferina (C.) 
Web., C. sylvatica (C.) Hoffm., 
and C. alpestris L.; also species 
of Stereocaulon (fig. 17). 
The conditions on these bare 
surfaces are obviously severe, 
and without the aid of crevice 
plants vegetation advances very 
slowly upon them. Below they 
are frequently swept clean by 
the waves, and above, the wash 
of the rain keeps them clear of all 
débris except that of large size. 
Only in hollows can any gravel 
or humus accumulate, and only 
in such places does any moisture 
remain. As the lichen vegeta- 
tion increases in bulk, soil and 
j Fic. 17.—Rock surface at Chippewa 
moisture are more and more con- Harbor partially covered with a mat of 
served, and occasional herbs and  cladonias; paucity of crevices resulting in 
low shrubs, and even trees, come gba slow invasion; jack pines, wey 
nth 
in. Where there are few or no pees as the elk 
crevices, however, the process is 
so slow that such areas frequently become surrounded by mature for- 
est, though the vegetation upon them may not have advanced as far 
as the heath-mat stage. Such “‘rock openings” are common in the 
forest upon the lower ridges. One of them, near Siskowit Lake (Sec. 
32, T. 65 N., R. 35 W.), deserves description and illustration (fig. 18). 
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