Page:Popular Science Monthly Volume 79.djvu/311

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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.

[The essential requirements in the formation of atolls are] a small parent pond, height and regularity of banks, regular and gentle slope of the bottom, suitable original littoral vegetation, small lateral pressure and tension of winter ice, and comparatively prompt anchoring of the bottom.

As previously explained, periodic islands may rise without the formation of gas, but as gas usually escapes from them after they have risen, irrespective of the force causing them to rise, it is a factor which demands attention. Under the average floating island there is little gas, owing to the loose texture of the peat. Consequently, in order to have gas support islands it is necessary that an upper layer of the bog be made of dense material, thus allowing the gas to escape but slowly. The most favorable place for it to collect is between the layers of peat. This is illustrated by experiments in digging peat from a bog filled with water. Holes were dug in the evening of one day and the next morning they were found to be filled with peat masses in the form of domes, cleft in the middle. These domes were formed of one of the lower layers of the peat. The gas under this layer combined with lateral pressure forced it up.

Periodic islands usually rise in spring and sink in fall, owing to the activity of gas-producing organisms in warm water. Some periodic islands have been reported which rose for only a few days. Because of the short time which most of these islands have been known, it has been impossible to study them. Attention is now being given to one in this country and it is to be hoped that through its study valuable information will be obtained regarding this type of island.