Lake Michigan, one of the five great lakes of the United States, and the only one which is entirely included in these states. It lies in a N. and S. direction, extending from the N. W. corner of Indiana and the N. part of Illinois about 320 m. to Mackinaw, where it communicates with Lake Huron by a strait 4 m. wide in its narrowest part. The lake is bounded E. by the lower peninsula of Michigan. The upper peninsula bounds it N. W. In this portion is Green bay, which extends S. into Wisconsin; this state and Illinois complete the western boundary of the lake. The following are its dimensions as given by Dr. Douglass Houghton: length, 320 m.; mean breadth, 70 m.; mean depth, 1,000 ft.; elevation above the sea level, 578 ft.; area, 22,400 sq. m., exceeding the area of Lake Huron by nearly 2,000 sq. m. The country around Lake Michigan is for the most part low and sandy; on the E. side particularly the sands thrown up by the waves are blown inland and form bills, which sometimes are 150 ft. high. The rocks are the limestones and sandstones of the sub-carboniferous groups, lying in horizontal strata, and never rising into bold cliffs. On the Michigan side they belong chiefly to the Portage and Chemung groups, and on the Illinois side to the Helderberg limestone.

Along the southern shores are post-tertiary beds of clay and sand lying a few feet above the level of the lake, and containing fresh-water shells like those living in its waters. This fact and the low watershed that separates the lake from the valley of the Illinois river, together with the great capacity of this valley, which appears as if worn by a mighty river, render it probable that the waters of Lake Michigan at some period found their way by the valley of the Mississippi into the gulf of Mexico. The lake at present is believed to be moving westward, gradually encroaching on the shores of Wisconsin and leaving those of Michigan. The existence of a lunar tidal wave was determined by the observations of Lieut. Col. James D. Graham at Chicago in 1858. The mean of 340 observations shows a difference of elevation of the lake surface between high and low water of 153 thousandths of afoot; and the mean of 24 semi-diurnal spring tides (i. e., one day before and two days after new or full moon) gives a difference of elevation of 245 thousandths of a foot, or a little over 3 inches. High water occurs half an hour after the meridian passage or southing of the moon. - This lake has few harbors and bays, and the only islands it contains are at its N. E. extremity.

It is not therefore very safe to navigate, especially as it is subject to severe storms at different seasons. But there is a large traffic on it, between Chicago and the lower lake ports. The straits of Mackinaw, which longest retain the ice, are usually open between May 1 and Dec. 1. The fish of the lake are like those found in Lake Huron, and the fisheries are for the most part concentrated about Mackinaw. The best harbors are at Little Traverse bay, and at Grand Haven at the mouth of Grand river on the E. shore of the lake. Chicago, near the head of the lake, has but an indifferent harbor, and the same may be said of those of Milwaukee and Sheboygan on the W. side.