MacMillan: OCCURRENCE of sphagnum atolls. 11 



floating upon the water, be seriously distorted in shape, and 

 very probably broken in pieces. The regularity and central 

 position of the observed atolls indicates that they have been 

 sheltered from such a destructive force and have been permit 

 ted to anchor themselves firmly and permanently. 



Size and shape of the ponds. Another important condition 

 for the successful development of an atoll may be the limited 

 size and regular shape of the parent ponds. In a pond large 

 enough for the excitation of considerable wave movement this 

 destruction of the atoll would be readily brought about; and in 

 a long or irregular pond the floating, littoral bog, after separa- 

 tion from shore would by slight pressure of the wind tend to 

 break at the ends or corners and would become disintegrated. 

 A regularly annular and not too large bog might however, upon 

 the principle of the arch retain its shape although subjected to 

 considerable lateral pressure by the wind. In this connection 

 it is interesting to notice that Anderson's atoll, much the smaller 

 of the two and perfectly sheltered by high forest-clad hills on 

 every side, is perfect in outline, without a break. Ballard's 

 atoll, on the other hand, situated in a considerably larger pond, 

 less adequately sheltered, is not perfect in outline but is broken 

 at one point and slightly irregular at others. 



Regular slope of the pond bottom. Another condition of 

 successful atoll formation would seem to be the regularity of 

 the slant of the pond bottom on all sides. Were it not for such 

 very perfect regularity around the pond, the detachment of an 

 annular bog would not be easy. At some point where the deep 

 water came closer to the shore than at others it is probable 

 that the atoll ring would be broken in detachment; and at an- 

 other i)oint where the water was much shallower than else- 

 where, the littoral zone of vegetation would perhaps be too 

 firmly attached to be removed by the rising of the pond. 



Original character of littoral vegetation. Still another and 

 probably a very important condition of atoll formation would 

 be the original distinctive character of the shore vegetation of 

 the pond. If plants of robust growth, sending their roots deep 

 into the ground had early established themselves by the wa- 

 ter's edge, they might have served to anchor the whole area 

 about them to such an extent that no floating bog could have 

 been formed, or if formed it must have been fragmentary in 

 character, being derived from areas where such strong plants had 

 not become established. Therefore great homogeneity of vege- 



