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rail road between Milwaukee and Madison; and similar occurrences have taken place in Ohio and Michigan.

Along the margin of some streams, and of lakes subject to fluctuations of level in the surface of the water the turf often becomes broken into irregular masses, which separate, and being carried away by the current, or by the force of the wind form little floating islands, that are freqently drifted to great distances from the place where they [grew]. were formed. The roots take but slight hold of the soft mud beneath the water; so that a rise of the water easily lifts the sod, [with it]; which rises and falls with the fluctuation of the water. These floating masses if obstructed in their course down the current may collect forming a dam and a pond or lake above it.

The marshes and turf-covered lakes become the appropriate habitat of a great number of mollusks with their covering of calcareous shells. These shells [gradually] continually accumulate at the bottom, and as time rolls away, extensive beds are formed of what is known as "shell marl." Mingled with the decayed vegetable matter the marl forms one of the most valuable

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