The Silurian rocks of northeastern Illinois and eastern Wisconsin occur over a belt 350 miles long and from 20 or less to 50 miles wide, extending from below Kankakee, Illinois, northward to the extremity of Green Bay Peninsula, in Wisconsin. This area is on the east side of the La Salle anticline and the highland of central Wisconsin, away from which the strata dip gently toward the east; so that the older Silurian rocks reach the surface at intervals only along the west border.
The most southern exposure of Alexandrian rocks in this region is in the banks of Horse Creek, 1½ miles east of the town of Essex and about 14 miles west of Kankakee. These strata consist of a thickness of 12 to 15 feet of brown magnesian limestone, in layers 3 to 6 inches thick, that are now known . . .