BASIN OF LAKE MICHIGAN.--This basin, stretching from the north to the south nearly four hundred miles, lies deeply in the series of formation of limestones, sandstone, and schists, to which we apply the term of the Michilimackinac system. Its north and west shores are skirted from Green Bay to a point north of the Sheboygan, with the calcareous stratum. At this point, the ancient drift, the lacustrine clay of Milwaukie and the prairie diluvium of Chicago, constitute a succession, of which the surface is a slightly waving line of the most fertile soils. Among the pebbles cast ashore at the southern head of this lake I observed slaty coal. It seems, indeed, the only one of the lakes which reaches south into the coal basin of Illinois. If the level at which coal is found on the Illinois were followed through, it would issue in the basin of the lake below low-water mark. Digging for this mineral on the Chicago summit, promises indeed not to be unsupported by sound hypothesis. After passing Chicago, of which a sketch is added, the sands which begin to accumulate at the Konamik, the River du Chemin, and the St. Joseph's River,[233] appear in still more prominent ridges, skirting the eastern coasts to and beyond Grand River. These sands, which are the accumulations of winds, are cast on the arable land, much in the manner that has been noticed at the Grand Sable on Lake Superior, and reach the character of striking dunes at the coast denominated the Sleeping Bear. The winds which periodically set from the western shore, produce continual abrasions of its softer materials, and are the sole cause of these intrusive sand-hills. Pent up behind them, the water is a cause of malaria to local districts of country, and many of the small rivers upon this side are periodically choked with sand. The sketch transmitted of this bleak dune-coast (omitted here), as it is seen at the mouth of Maskigon Lake, will convey a false idea of the value of this coast, even half a mile from the spot where the surf beats. It is designed to show the air of aridity which the mere coast line presents. The stratification regains its ordinary level and appearance before reaching the Plate or Omicomico River, and the peninsula of the Grand Traverse Bay, and the settlements of the Ottawa Indians on Little Traverse Bay, afford tracts of fertile lands. Point Wagonshonce consists of a stratum of limestone of little elevation, which constitutes the southeast cape of the strait. Here a lighthouse is needed to direct the mariner. [233] The subjoined petrifaction of a leaf, apparently a species of betula, was obtained on this river. See _ante_, p. 206.