The writer agrees with Schuchert in referring to the Silurian system only the strata that occur between the top of the Richmond group of the Ordovician and the base of the Manlius formation of the Devonian. A marked sedimentary break is present at the top of the Richmond everywhere in Illinois, and with the exception of the long-lived Leptœna rhomboidalis, not a single one of the Richmond species continues into the overlying Silurian strata. As thus defined, Silurian rocks are exposed in Illinois in four rather widely separated areas, which may be referred to respectively as the Southwestern area, the Central-western area, the Northeastern area, and the Northwestern area.
The oldest formations of Silurian rocks in Illinois, and indeed the oldest known in the Mississippi Valley, are the Girardeau limestone and Orchard Creek shale, which are exposed in a few places . . .