Page:Popular Science Monthly Volume 79.djvu/308

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PSM V79 D308 Possible formation of a floating island.png
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 disappear 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 disappear. 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 periodic 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 hypothesis 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

PSM V79 D308 Floating island of sadawga lake whitingham vt.png
Cross-section of 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.