3°4 



THE POPULAR SCIENCE MONTHLY 



Cross-section of a Lake, showing a mat of vegetation growing out from both sides. 

 A rise in water level might break off this shelf, and thus form a floating island. 



a few feet, if the sides remain attached, and consequently it may dis- 

 appear with unusually high water. In order for it to reappear either 

 the water must go down or some agency must push the center up higher. 

 This agency is marsh gas. This gas with its light specific gravity 

 could exert such a force on the center of the mat that it would he 

 buoyed up. In order for the gas to act in such a manner it must be 

 present in large quantities and must not escape until that mat has 

 reached the surface. When the gas escapes the mat will again disap- 

 pear. This alternate appearance and disappearance of the mat makes 

 it a periodic island. A true periodic island would not be attached to 

 the sides. Such islands are rare, and conclusive evidence is lacking to 

 show that they are not attached. 



The various ways in which floating islands, floating atolls and peri- 

 odic islands may originate are as follows : 



I. Floating Islands are divided into — (A) natural islands and 

 (B) artificial islands. 



(A) Natural islands may be formed by : 



1. The coming together of floating vegetable masses. This hypoth- 

 esis demands sufficient floating material upon which there is plant life 

 of a suitable kind, or upon which plant life may start. There must 

 also be some favorable agency to collect this material. In a large lake 

 where high waves could break off pieces of sphagnum from the shore, 

 the waves might collect the pieces so as to form a floating island. In 

 small lakes, cat-tails or other rushes form a favorable place for such 

 material to lodge. 



2. The action of waves beating against a mat of vegetation may by 

 their force break off large islands. This could happen only in the 

 case of a large body of water and would probably account for the origin 

 of the floating island mentioned in the introduction which was seen in 

 the Atlantic Ocean in 1892. When first noticed in July in latitude 



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Cross-section op Sadawga Lake, Whitingham, Vt., showing two floating islands. 



The large island grew out from the shore and was broken off by high water. 



It is now attached at the east side. 



