Page:Popular Science Monthly Volume 79.djvu/307

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F one should see in a story the statement that a floating island 100 feet square, upon which were growing trees thirty feet in height, was used by the hero as a place of refuge, and that the island traveled 1,000 miles out to sea, he would probably accuse the author of an abuse of the imagination. But such an island—without the hero—was seen off the coast of North America and is known to have traveled at least 1,000 miles. Floating islands also occasionally occur in the lakes of some of our northern states. It is the purpose of this article to point out the location of some of these islands and to explain their origin.

In order to understand the formation of a floating island let us imagine a pond on the edges of which rushes and grasses are growing. These gradually push their way out into the deeper water, leaving a mass of decaying vegetable matter upon which, in time, mosses such as sphagnum may secure a foothold, and start a shelf which will extend out into the water. As soon as the sphagnum has become well established, water-loving plants and shrubs such as alders, sheep laurel and sweet gale will grow with the moss. Cranberries and pitcher plants may also aid in the formation of the mat, forming the familiar cranberry bog. This shelf will be attached to the shore for several feet, the distance depending on the depth of the water, but the peat will seldom attain a thickness of more than three feet. After the mat has become firm, black spruces and larches may grow upon it, often anchoring it and always making it more compact by means of their roots. Such a mat is illustrated by the drawing on page 304.

After the shelf has extended itself some distance into the pond, if the water level is raised unusually high by excessive rainfall or the construction of a dam across the outlet, the mat may break off and form a floating island. This island will either become attached near its former position by roots extending underneath it, or it will float around the lake.

In case the pond is small and nearly circular the mat may surround it. If the water in the pond is raised, at the time when all but the center of the pond is overgrown, an atoll or ring-shaped island may be formed. For the persistence of this atoll the water must remain at its high level and the atoll must become anchored.

Finally, let us suppose that the vegetation completely covered the lake before the water rose. There is sufficient elasticity in such a mat to permit it to rise slightly with a rise in the water level. The mat, however, is not sufficiently elastic to permit its center to rise more than