38o The American Geologist. December, i904. 
group, or the Upper Marshall, and the lower portion was called 
the Marshall group, equivalent to the Waverly in Ohio, and 
the Kinderhook of Illinois. 
Above this series, in the vicinity of Grand Rapids, is a group 
of shales, limestone, and gypsum layers, called by Winchell the 
Michigan Salt group. This formation has been shown by Rom- 
inger and Lane to be destitute of salt beds, and the Saginaw 
valley and principal Michigan brines come from below this hor- 
izon, so that it seems advisable to follow Lane* and call it 
merely the Michigan group. 
Above the Michigan group comes the Carboniferous lime- 
stone of Winchell, exposed at Grand Rapids and other places 
around the border of the Coal Measure basin. It is equivalent 
to the Bayport limestone of eastern Michigan, to the INIaxville 
limestone of Perry and Muskingum counties in Ohio, and to 
the upper part of the St. Louis limestone of the Mississippi 
valley. Over the Carboniferous limestone, the Saginaw Coal 
Measures are found forming the interior basin. The Waver- 
ly group of Michigan, including the rocks described up to 
the Carboniferous limestone, according to Rominger,t "forms 
underneath the drift, the surface rocks over half the extent of 
the Peninsula, but its natural outcrops are very limited, either 
horizontally or vertically," 
The Mississippian series in Michigan forms a basin-shaped 
fold, and in the center of the Peninsula it is overlain by the 
Coal M'easures, and can only be mapped in such sections by 
the aid of well records. 
The whole series of Michigan presents more or less irreg- 
ularity, in places represented by shales, and again by sandstone 
apparently contemporaneous. The Michigan group in places 
is cut out entirely on the border of the Coal Measures, and 
again the Bayport limestone is present and the lower gypsum 
beds are gone. This limestone at Grand Rapids is about 50 feet 
thick and rests on the gypsum formation. 
In the interpretation of the geological history revealed by 
these rocks and their relations, the writer wishes to acknowl- 
edge his indebtedness to the various papers of Weller. Lane, 
and Keyes. 
* Micb. Geol. Survey, vol. vii, part IJ. p. IS. 
t Mich. Geol. Survey, vol. iii, part I, p. 69. 
