As we have said, the entire area of Illinois seems at one period to have been an ocean-bed, which has not since been disturbed by any considerable upheaval.  The present irregularities of the surface are clearly traceable to the washing out and carrying away of the earth.  The Illinois River has washed out a valley about two hundred and fifty feet deep, and from one and a half to six miles wide.  The perfect regularity of the beds of mountain limestone, sandstone, and coal, as they are found protruding from the bluffs on each side of this valley, on the same levels, is pretty conclusive evidence that the valley itself owes its existence to the action of water.  That the channels of the rivers have been gradually sunken, we may distinctly see by the shores of the Upper Mississippi, where are walls of rock, rising perpendicularly, which extend from Lake Pepin to below the mouth of the Wisconsin, as if they were walls built of equal height by the hand of man.  Wherever the river describes a curve, walls may be found on the convex side of it.

The upper coal formation occupies three-fifths of the State, commencing at 41 deg. 12’ North latitude, where, as also along the Mississippi, whose banks it touches between the places of its junction with the Illinois and Missouri rivers, it is enclosed by a narrow layer of calcareous coal.  The shores of Lake Michigan, and that narrow strip of land, which, commencing near them, runs along the northern bank of the Illinois towards its southwestern bend, until it meets Rock River at its mouth, belong to the Devonian system.  The residue of the northern part of the State consists of Silurian strata, which, containing the rich lead mines of Galena in the northwest corner of the State, rise at intervals into conical hills, giving the landscape a character different from that of the middle or southern portion.  Scattered along the banks of rivers, and in the middle of prairies, are frequently found large masses of granite and other primitive rocks.  Since the nearest beds of primitive rocks first appear in Minnesota and the northern part of Wisconsin, their presence here can be accounted for only by assuming that at the time this region was covered with water they were floated down from the North, enclosed and supported in masses of ice, which, melting, allowed the rocks to sink to the bottom.  A still further proof of the presence of the ocean here in former times is to be found in the sea-shells which occur upon many of the higher knolls and bluffs west of the Mississippi in Iowa.

Illinois contains probably more coal than any other State in the Union.  It is mined at a small depth below the surface, and crops out upon the banks of most of the streams in the middle of the State.  These mines have been very imperfectly worked till within a few years; but it is found, that, as the work goes deeper, the quality of the coal improves, and in some of the later excavations is equal to the best coals of Ohio and Pennsylvania, and will undoubtedly prove a source of immense wealth to the State.