II. ON THE OCCURRENCE OF SPHACtNUM ATOLLS 

 IN CENTRAL MINNESOTA. 



Conway MacMillan. 



Location of the atolls. "While at the Gull lake Bio- 

 logical Station during the summer of 1893 two interesting 

 examples of a peculiar and, I believe, hitherto unrecorded 

 peat-moss formation were noted and studied somewhat in de- 

 tail. From their position in the middle of ponds of consider- 

 able size and from theii annular shape I have named them 

 sphagnum atolls. Both of the atolls were discovered in the wil- 

 derness of western Cass county, in small lakes or ponds tribu- 

 tary to larger bodies of water. The larger atoll of the two is 

 situated in a pond tributary to lake Whitman, in the n. w. \ of 

 sec. 17, T. 135, R. 29 w. of the 5th meridian. This will be known 

 here as Ballard's atoll. The smaller lies in a pond of somewhat 

 less extent, tributary indirectly to the north bay of lower Gull 

 lake, in sec. 22 of the same township. This will be referred to 

 as Anderson's atoll. From its proximity to the buildings of the 

 biological station, Ballard's atoll was examined somewhat more 

 particularly and may receive the more extended description. 



Description of Ballard's atoll. The pond in which this 

 atoll has been formed is about one hundred and fifty yards 

 across and almost circular in shape. It is surrounded ex- 

 cept for a short distance on the west, by rather precipitous 

 morainic hills, 50 — 75 feet in height. On the west two low bars 

 jut out, leaving a short channel not more than six feet in width, 

 through which the waters of the pond when at high level drain 

 oif into lake Whitman. The channel is choked with Carex, 

 Typha and Scirpus. The bars and the hills are clothed with an 

 abundant growth of young hardwood timber, intermixed with 

 coniferous trees and a sparse underbrush. The latter reaches, 

 on all sides, the edges of the pond. From the tops of the sur- 

 rounding hills, as one looks down upon the pond, Ballard's atoll 

 — a ring of green — is at once conspicuous and very sharply de- 

 limited. It is about seventy -five feet in diameter and of a uni- 



