412 STUART WELLER 



northern extension of the basin was early in Mississippian time, 

 the successive reoccupations of the region perhaps falHng a Httle 

 short each time of the previous one, and the successive withdrawals 

 perhaps being a little greater each time until the final withdrawal 

 at the close of Ste. Genevieve time. With each readvance of 

 the sea to the north, however, essentially the same basin was 

 reoccupied, so that the sea-pattern during all of this time, in the 

 periods of greater submergence, remained practically the same, 

 differing only in more or less minor details. During each interval 

 of submergence in this period, the sea spread far to the north in 

 Illinois, and westward in Iowa, and doubtless also in northern 

 Missouri, surrounding and perhaps completely submerging at 

 times the Ozark Island, and doubtless was connected with the seas 

 which extended westward to the Rocky Mountain land. 



With the return of the seats into the Illinois basin at the begin- 

 ning of Upper Mississippian or Chester time, the conditions 

 indicated by the sedimentary record were very different from those 

 that had existed earlier. In the earlier period almost no sand 

 deposits were accumulated anywhere in the region except in 

 Kinderhook time, during the initial submergence of the basin, the 

 only exception to this being the presence of the thin Rosiclare 

 sandstone layer that is present in the Ste. Genevieve limestone in 

 most sections. In Chester time, as has already been shown in 

 the discussion of the successive formations, thick deposits of sand 

 were accumulated, and most of the limestones were associated with 

 large amounts of clastic material in the form of shales, these shaly 

 Chester limestones being in strong contrast with the great thick- 

 nesses of nearly pure, massively bedded limestones of the Lower Mis- 

 sissippian. The original source of all these accumulations of Chester 

 ■ sand is not entirely clear. The Aux Vases sandstone at the base, 

 which is thickest toward the eastern shore of Ozarkia and thins 

 southeastwardly to nothing, may have been derived from the wear- 

 ing away of some of the more ancient sandstones of Ozarkia, but 

 such sandstones as the Bethel, Cypress, Hardinsburg, and Tar 

 Springs, which are thick formations in southeastern Illinois and 

 beyond in Kentucky, becoming much reduced or even absent 

 altogether toward the shore of Ozarkia, could not have had such 



