8 MINNESOTA BOTANICAL STUDIES. 



vegetation separated from the firmly anchored portion was 

 very nearly concentric with the shore line itself. Having, by 

 its rise, thus detached a floating bog differing from the innu- 

 merable other floating bogs of the Minnesota lakes only in its 

 regular and annular outline, the pond continued to increase and 

 spread over the greater part of its original bed. This increase 

 in size left the annular bog far out in the waters of the pond 

 which had formed it by its fluctuations in level, and as the 

 mass of vegetation and soil became thoroughly saturated with 

 the water below, its character may. gradually have changed 

 until only the sphagnum plants retained vitality, Generations 

 of these, succeeding each other contributed to the weight of 

 the ring and finally pressed it down upon the bottom of the 

 pond, forming the anchored atoll of the present. As the 

 texture of the atoll became firmer — at first along a line about 

 equi-distant from its outer and inner faces — new plants estab- 

 lished themselves, their seeds and fruits having been carried 

 there by the winds and by birds, or having perhaps lain dor- 

 mant a long time after the original detachment of the bog. 

 Ballard's atoll with its Sarracenia, Eriophorum and Kalmia 

 vegetation and its spongy texture appears to be less fully de- 

 veloped than Anderson's with its firmer structure and its 

 growth of Ledum and Picea. Both, however, might have been 

 formed synchronously, but the smaller, situated in the shallower 

 pond might be supposed to have developed its peculiarities 

 more rapidly than the larger. 



Fluctuations in lake and pond levels. Such fluctuations 

 in the level of ponds and lakes are by no means unknown^ , and 

 may very properly be assumed in the case of the two ponds in 

 question. Mr. Warren Upham very kindly calls my attention 

 to the remarkable case of Stump lake in North Dakota as evi- 

 dence, and with his permission I offer here, from his forth- 

 coming work on the glacial lake Agassiz, a somewhat extended 

 quotation, the whole of which has direct bearing on the subject 

 discussed in the present paper: 



"Devil's lake and Stump lake, situated near together in 

 North Dakota, were found by my levelling in August, 1887, re- 

 spectively 1,432 and 1,417 feet above the sea. Devil's lake at- 

 tains a maximum depth of 75 or 80 feet in the eastern portion 

 of its broadest area, and the northeast arm of Stump lake 

 is said to be in some places 100 feet deep. Both lakes are now 



4. Wliittesly, Charles. On Fluctuations of Level in the North American Lakes 

 Smithson. Contr. Knowl. 13: pp. 25. 1860. 



