MacMillan: OCCURRENCE OF SPHAGNUM ATOLLS. 3 



form width of about ten feet. On the west its continuity is 

 broken by a channel, twelve feet across, which furnishes com- 

 munication between the waters of the intra-insular lagoon and 

 those of the pond outside of the atoll. 



Measurements of Ballard's atoll and pond. Measure- 

 ments of the pond in which Ballard's atoll is situated 

 showed its dimensions to be as given above. The depth of the 

 water is nowhere great. The greatest depth is about twelve 

 feet and this maximum of depth, as is usually the case with the 

 glacial lakes and ponds of Minnesota i is in the middle, 

 consequently within the lagoon. At the shore there is no 

 definite beach line. From it the water increases gradually in 

 depth until within about six feet of the outer aspect of the 

 atoll -ring. At this point it measures but about four and one 

 half feet. The transition to the emergent surface of the atoll 

 is therefore somewhat abrupt, but in this character not 

 equaling the still more abrupt shelving off from the inner 

 aspect of the atoll to the maximum depth of the lagoon. No- 

 where outside of the atoll, so far as our measurements indi- 

 cated, was the depth of water half so great as within the 

 lagoon. The bottom from shore to atoll is very soft, consist- 

 ing of black vegetable muck with a considerable intermixture 

 of decaying sphagnum. Within the lagoon, the decayed 

 sphagnum is not so abundantly present. 



Vegetation of the pond outside of Bollard's atoll. When 



observed in the early part of July the waters of the pond pre- 

 sented a considerable though not luxuriant growth of aquatic 

 and semi- aquatic vegetation. The following plants were most 

 conspicuous: 



Carex pseudocyperus Linn. var. americana Hochst. — Sedge. 



Carex lupulina Muhl. — Sedge. 



Carex retrorsa Schwein. — Sedge. 



Scirpus fluviatilis (Torr. ) Gray. — Bulrush. • 



Juncus effusus Linn. — Rush. 



Iris versicolor Linn. — Pleur de Lis. 



Potentilla palustris (Linn. ) Scop. — Marsh Five finger. 



Utricularia vulgaris Linn. — Bladderwort. 



Utricularia intermedia Hayne. — Bladderwort. 



At one point in the pond a small island had been formed upon 

 which a clump of Betula pumila Linn, (low birch) had estab- 



1. UaU. C. W. Fortnation andj Defjrniation of Minn. Lakes. Sciencz. 21 : 314 

 1893. 



