p.8

So little is accurately known of these
azoic rocks that it is entirely impossible
to represent them in detail upon the map;
and hence one color only is used to represent
the whole.

They form the floor upon which in
the southern and eastern parts of the state is
built up the later formations hereafter to
be described; so that an Arterian well sunk
in these rocks would reach them at a greater
or less depth. Taking into account the
thickness of the several overlying strata, and
the elevation of the different parts of the state
above Lake Michigan, an attempt has been
made (for the first time in this country) to show
by a system of lines, the probable depth at
which these rocks will be found at any given
point.