253 
The Origin of Sphagnum Atolls. 
till. A very narrow trench varying in depth from a few inches to 
four feet completely surrounds the hog. Inside the trench where 
Sphagnum has been killed, Typha and sedges have become estab¬ 
lished. The trench in the shallower places is being invaded by 
Typha, species of Carex, Sagittaria, and Alisina. The maximum 
depth found in the peat is about eight feet, and there is no remnant 
of the parent pond in the centre. 
The fact that the change of vegetation of the Ballard and 
Anderson atolls was evidently due to burning, suggested that 
perhaps the cause of the atoll’s outer zone of water was due to 
fire. In pursuit of this study evidences of fire were searched for 
in the surface layer of all the atolls and in the Anderson and 
Glenwood atolls at various depths beneath the surface. The 
character of the peat beneath the surface was explored by studying 
the samples taken with a peat-sampler at various intervals in a line 
across the bog. In all the atolls studied fire scars were found in 
or near the trench on stumps and fallen logs. In samples taken 
from the Anderson bog charcoal was found at two, three, four, five, 
and six feet below the surface, which indicates that fires occurred at 
those levels. By digging with a spade in the trench of the Glen¬ 
wood atoll quantities of charcoal were found, and upon inquiry of 
the park attendants it was learned that the bog had been burned 
several times. The burning of the Hubert atoll was so superficial 
around the inner edge of the trench and so recent that surface fire 
scars indicated clearly the course and extent of the fire. 
After finding that the marginal zone of water of the atolls just 
mentioned is beyond any doubt due to deep burning during dry 
seasons, the question arose as to whether it is ever formed as the 
result of other causes, such as the method suggested by MacMillan, 
or by shade, or by such processes as the inwashing of toxic 
substances from the banks. 
In considering the conditions assumed by MacMillan, it is 
true that the water-level does rise and fall, but the vegetation 
of the shore is so firmly anchored by roots and rhizomes that only 
on windward shores of moderately large lakes is it ever detached. 
Furthermore, if the turf did float upward with the rise of water, 
which in small ponds perhaps never happens, the slight wave and 
wind action would inevitably set the debris further shoreward and not 
toward the centre. In the Anderson and Ballard atolls an excessive 
rise of water-level can only be temporary since both drain rapidly 
by direct outlets. All the conditions prescribed in the hypothesis 
