hills. Near the banks of the principal
rivers the ground is elevated into bluffs, on which may be still found
the traces left by water, which was evidently once much higher than
it now is; whence it is inferred, that, where the fertile plains of
Illinois now extend, there must once have been a vast sheet of water,
the mud deposited by which formed the soil, thus accounting for the
great fertility of the prairies.

* * * * *

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 ro