Page:Popular Science Monthly Volume 79.djvu/310

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THE POPULAR SCIENCE MONTHLY

waters of the ultra-insular lagoon and those of the pond outside of the atoll. The depth of the water is nowhere great. The greatest depth is about twelve feet and this maximum of depth is in the middle, within the lagoon. . . . The general texture of the atoll was loose, so that one standing anywhere upon it soon sank into the soft and spongy mass up to the knees. . . .

[The smaller] pond is barely fifty yards across, with high banks, and the atoll ring is within a foot or two of twenty yards in diameter. Its breadth, however, is greater than that of [the other] atoll, being twelve feet on the average from the outer to the inner aspect. The lagoon, then, is slightly less than fifty feet across. . . . The water of the pond was shallow, averaging four feet, just outside the atoll ring. . . . In general the texture of the atoll was much firmer than that of [the other] atoll. One could stand anywhere upon it without sinking in above the insteps.

The explanation of the presence of sphagnum atolls may be derived from the assumed changes in level of the pond water, and indeed their presence may, conversely, be held to indicate, or to demonstrate, fluctuations in the pond level. If it be possible to conceive that in these two atoll-producing ponds there has been, during the course of years, a gradual diminution in size followed by a rather rapid increase in diameter and depth, I believe the formation of the atolls would become a phenomenon readily comprehensible. . . . Concomitantly with the diminution in size, doubtless extending over a term of years, vegetation of the shoreward area would have established itself in characteristic zones. The littoral flora and the submerged plants just outside the shores would have formed a loose turf lining the edges of the pond. This turf would have gradually become more solid as it extended landward and would therefore at a little distance from the water's edge have become modified in character, giving a foothold for plants of larger growth. . . . When, subsequently to this epoch of gradual diminution, the ponds began to increase again, the effect of the rise in level of the water was to detach from the shore a ring of the loose littoral turf, and this mass of vegetation with its attendant soil, buoyed up at first as a circular floating bog, appears to-day as the characteristic sphagnum atoll. . . . The increase in size [of the pond] left the annular ring far out in the waters of the pond. . . and as the mass of vegetation and soil became thoroughly saturated with the water below, its character may gradually have changed until

PSM V79 D310 Series of floating islands on sadawga lake whitingham vt.png

A View of Sadawga Lake, Whitingham, Vt. Two floating islands are shown in the central part of the picture, and behind them is the main floating island from which they have become detached.