MacMUlan: OCCURRENCE OF SPHAGNUM ATOLLS. 7 



Hypothesis of changes in pond level. The explanation of 

 the presence of sphagnum atolls may be derived from assumed 

 changes in level of the pond water, and indeed their presence 

 may, conversely, be held to indicate or to demonstrate fluctua- 

 tions in the pond level. If it be possible to conceive that in 

 these two atoll- producing ponds there has been, during the 

 course of years, a gradual diminution in size followed by a 

 rather rapid increase in diameter and depth, I believe the for- 

 mation of the atolls would become a phenomenon readily com- 

 prehensible. It might be shown that the sequence of events 

 was somewhat as follows: The ponds through those gradual 

 and rather complicated changes in drainage, rate of silt-depo- 

 sition, annual rain fall, bottom physiognomy and evaporation 

 which are known to affect the dimensions of bodies of fresh 

 water in a glacial area, slowly diminished in size until their 

 shore lines were approximately coincident in position with the 

 inner aspect of the modern atoll ring. The whole diameter of 

 the pond, at this stage of its development as a geographical 

 feature, would have been approximately equal to the diameter 

 of the present intra insular lagoon. Concomitantly with such 

 diminution in size, doubtless extending over a term of years, 

 vegetation of the shoreward area would have established itself 

 in characteristic zones ^ . The littoral flora and the submerged 

 plants just outside the shores would have formed a loose turf 

 lining the ed,i4es of the pond. This turf would have become 

 gradually more solid as it extended farther landward and 

 would at a little distance from the water's edge have become 

 modified in character and in vegetation, giving a foot-hold for 

 plants of larger growth. It is not imperative to assume that 

 this shore- lining formation was necessarily of a sphagnum 

 type, although in fact it might very well have been of such a 

 nature. When, subsequently to this epoch of gradual diminu- 

 tion, the ponds began to increase again, the effect of the rise in 

 level of the w^ater was to detach from the shore a ring of the 

 loose littoral turf and .this mass of vegetation with its attend- 

 ant soil, buoyed up at first as a circular floating bog, appears 

 to-day as the characteristic sphagnum- atoll. That the atolls 

 should be of such regular width, varying but a foot or two on 

 any line from outer to inner aspect, indicates the regularity of 

 slope, on all sides, of the pond-bottom towards the shore. Ap- 

 parently the line along which the floatable portion of the shore - 



3. Jlagnln, Ant. Recherch. sur la vegetation des Lacs du Jura. Rev. Gen. de Botan. 

 5:241,a03. 1893. 



