FLOATING ISLANDS 



3°5 





1'39'n 



39.5° N., longitude 65° W. the island was about 9,000 square feet in 

 extent, with trees thirty feet in height upon it, which made it visible for 

 seven miles. It had apparently become detached from the coast of 

 this country and been can-ied out to sea by the Gulf Stream. It was 

 again seen in September in latitude 45° 29' K, longitude 42° 39' W., 

 after it had passed through a severe storm. By this time it must have 

 traveled over 1,000 nautical miles, and it may have eventually arrived 

 at the coast of Europe. 



3. The raising of the water 

 level separates mats from the 

 shore, as previously explained. 

 An interesting case under this 

 heading is that in which there are 

 two kinds of peat in an overflowed 

 bog. If the peat is arranged in 

 layers, the bottom layer being- 

 heavy, since it is formed by the 

 decompositon of woody material, 

 may separate from the top layer 

 which is light on account of its 

 sphagnum content. In this cir- 

 cumstance the bottom layer re- 

 mains in its former position, the 

 upper layer breaking away to 

 form a floating island. 



4. The action of ice in the 

 northern climates sometimes 

 separates masses of vegetation 

 from the shore. This expansion and contraction of ice is often an 

 important factor in forming shore topography and an overgrowing mat 

 could offer but feeble resistance to this force. 



(B) Artificial islands may be formed by the damming of ponds 

 or lakes, which is a very common mode of origin of floating islands in 

 this country. An example of this type of island is that in Sadawga 

 Lake shown in cross-section in the second sketch. 



II. Atolls 



Two atolls in central Minnesota, which are characteristic of all such 

 atolls, have been described by Conway MacMillan in the Minnesota 

 Botanical Studies, 1 and the following description of them is taken from 

 that article. 



The [larger] pond is about 150 yards across and almost circular in shape. 

 It is surrounded, except for a short distance on the west, by rather precipitous 

 morainic hills 50-75 feet in height. . . . [The atoll] is about 75 feet in diameter 

 and of uniform width of about ten feet. On the west its continuity is broken 

 by a channel, twelve feet across, which furnishes communication between the 



M894, Bui. 9, Vol. 1. 



vor.. lxxix. — 21. 



A Section of an Outline Map of 

 North America, showing the course of a 

 floating island which was seen floating in 

 the Atlantic Ocean in 1892. 



