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288 Lake Michigan

strong impulsive action upon the bottom of the lake from the motion of the sea towards the shore."

This explanation it is believed would account very satisfactorly for the formation of a single beach line, and perhaps of an extensive flat of level sandy prairie, but it is difficult to conceive how it should produce a regular succession of such [ridges] beaches separated by intervening valleys; so that this matter remains to be hereafter explained.

Lake Michigan is destitute of islands, except a few near its northern extremity, and it is unfortunate for the interests of navigation that it does not present along its shores many deep and narrow bays or other places of security for vessels during heavy winds. In this respect however its character is quite similar to the other lakes. Navigation usually commences in the spring about the middle or last of March on this lake; but as the straits of Mackana remain closed with ice after that

288 Lake Michigan

strong impulsive action upon the bottom of the lake from the motion of the sea towards the shore."

This explanation it is believed would account very satisfactorly for the formation of a single beach line, and perhaps of an extensive flat of level sandy prairie, but it is difficult to conceive how it should produce a regular succession of such [ridges] beaches separated by intervening valleys; so that this matter remains to be hereafter explained.

Lake Michigan is destitute of islands, except a few near its northern extremity, and it is unfortunate for the interests of navigation that it does not present along its shores many deep and narrow bays or other places of security for vessels during heavy winds. In this respect however its character is quite similar to the other lakes. Navigation usually commences in the spring about the middle or last of March on this lake; but as the straits of Mackana remain closed with ice after that