1913] COOPER—ISLE ROYALE 201 
The next important fact is that the sphagnum does not make 
its first growth at the extreme edge of the bog area and from here 
works its way centerward only. On the contrary, it begins its 
growth some distance within the bog margin and works both ways; 
very slowly toward the margin, faster toward the center. Proof 
of this course of events is seen in the entire absence of sphagnum 
from the marginal zone, except in certain parts where it is mani- 
festly a recent invader. Soil samples taken at various depths in 
Fic. 38.—Sedge zone of Carex limosa type; Raspberry Island 
the marginal zone, examined microscopically, failed to show the 
slightest trace of sphagnum remains, although these are long pre- 
served and readily recognized. Another proof is found in the form 
of the sphagnum accumulation, which is that of a ridge parallel 
to the bog margin and at a somewhat constant distance from it. 
This ridge usually has its greatest thickness close to the outer 
(marginal) side, doubtless marking here the region of first growth. 
It will be remembered too that in the primitive stage observed 
at Amygdaloid Lake the sphagnum was seen to be spreading both 
ways. The face toward the bog margin (on Raspberry Island) is 
usually rather abrupt, forming a prominent wall which bounds the 
marginal zone. Occasionally a thin layer of sphagnum is found 
